Advanced coatings through pulsed magnetron sputtering

Kelly PJ Hisek J Zhou Y Pilkington RD and Arnell RD

httpdxdoiorg101179026708404225010702

Title Advanced coatings through pulsed magnetron sputtering

Authors Kelly PJ Hisek J Zhou Y Pilkington RD and Arnell RD

Type Article

URL This version is available at httpusirsalfordacuk1562

Published Date 2004

USIR is a digital collection of the research output of the University of Salford Where copyright permits full text material held in the repository is made freely available online and can be read downloaded and copied for nonshycommercial private study or research purposes Please check the manuscript for any further copyright restrictions

For more information including our policy and submission procedure pleasecontact the Repository Team at usirsalfordacuk

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

ADVANCED COATINGS THROUGH PULSEDMAGNETRON SPUTTERING

P J Kelly J Hisek Y Zhou R D Pilkington and R D Arnell

Pulsed magnetron sputtering (PMS) has becomeestablished as the process of choice for the depositionof dielectric materials for many applications Theprocess is attractive because it offers stable arc freeoperating conditions during the deposition of forexample functional films on architectural and auto-motive glass or antireflectiveantistatic coatings ondisplays Recent studies have shown that pulsing themagnetron discharge also leads to hotter and moreenergetic plasmas in comparison with continuous dcdischarges with increased ion energy fluxes delivered tothe substrate As such the PMS process offers benefitsin the deposition of a wide range of materials Thepresent paper describes three examples where PMS hasled to either significant enhancement in film propertiesor enhanced process flexibility in low friction titaniumnitride coatings in Al doped zinc oxide transparentconductive oxide coatings sputtered directly frompowder targets and in thin film photovoltaic devicesbased on copper (indiumgallium) diselenide These

examples demonstrate the versatility of PMS and openup new opportunities for the production of advancedcoatings using this technique SE499

The authors are in the School of Computing Scienceand Engineering University of Salford SalfordM5 4WT UK (pKellysalfordacuk) Based on apresentation at the meeting on lsquoPulsed plasma process-ingrsquo held at Salford University on 17 September 2003Accepted 3 December 2003

Keywords Pulsed magnetron sputtering Titaniumnitride Al doped zinc oxide Thin film photovoltaicdevices Copper (indiumgallium) diselenide

2004 IoM Communications Ltd Published by Maney for

the Institute of Materials Minerals and Mining

INTRODUCTION

Pulsed magnetron sputtering (PMS) is widely recog-nised as an enabling technology particularly for thedeposition of dielectric materials1 ndash 8 Pulsing themagnetron discharge in the midfrequency range(20 ndash 350 kHz) alleviates the chief problem associatedwith the continuous dc reactive sputtering of suchmaterials namely the occurrence of arc events at thetarget This is achieved through the discharging of thepoisoned regions on the target during the reversevoltage or lsquopulse offrsquo phase The correct selection ofpulse parameters (frequency duty reverse voltage)can result in extended arc free operating conditionseven during the deposition of highly insulatingmaterials4 The suppression of arcs stabilises thedeposition process and reduces the incidence ofdefects in the film Consequently films of forexample alumina titania and silica can be producedby pulsed sputtering with very much enhancedstructural electrical and optical properties in com-parison with films produced by continuous dcprocessing56 The pulsed sputtering technique isnow being exploited commercially in large areamultiple magnetron systems for many applicationsincluding solar control and low emissivity coatingsbarrier layers on packaging flat panel displays andsolar cells Again very long term process stabilityreduced defect densities improved film propertiesand enhanced dynamic deposition rates have beenreported for these systems78

Recent Langmuir probe studies have shown thatpulsing the magnetron discharge also significantlymodifies the characteristics of the depositionplasma910 For example increased plasma densitiesand electron temperatures have been measuredadjacent to the substrate in pulsed discharges

Therefore higher ion energy fluxes can be transportedto the growing film Additionally mass spectrometrystudies of the distribution of ion energies at thesubstrate in an asymmetric bipolar pulsed dc magne-tron discharge have identified the existence of popu-lations of ions whose energies can be directly relatedto distinct phases or features within the target voltagewaveform11 Clearly the increased flux and energy ofthe particles incident at the substrate may also contri-bute to the observed improvements in the structureand properties of films produced by pulsed process-ing Furthermore these benefits are not limited todielectrics but can be exploited in the deposition ofnew advanced coating materials The present paperdescribes three examples where use of the PMSprocess has led to either significant enhancement infilm properties or enhanced process flexibility

(i) low friction titanium nitride (TiN) coatings(ii) Al doped zinc oxide transparent conductive

oxide (TCO) coatings sputtered directly fromblended powder targets

(iii) thin film photovoltaic devices based oncopper indium diselenide (CIS)

Low friction TiN coatings

TiN has long been a workhorse hard coating for thecutting tool industry It has though never beenregarded as a low friction coating However asignificant enhancement in tribological propertieshas been observed for TiN coatings deposited ontotool steel by PMS in comparison with films grown bycontinuous dc sputtering High resolution SEMmicrographs revealed major structural differences inthe two film types which may account for thevariation in film properties12

DOI 101179026708404225010702 Surface Engineering 2004 Vol 20 No 3 157

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

Al doped zinc oxide TCO coatings sputtered directlyfrom blended powder targets

Mixed zinc oxide and alumina powder targets havebeen used to produce transparent semiconductivefilms on glass by PMS Resistivities of the order of1023 V cm were obtained after annealing withoutdeterioration of the coating to substrate adhesionThe results to date demonstrate that the PMS ofdoped ZnO films from powder targets is a flexiblenovel technique for the deposition of high qualityTCO materials1314

Thin film photovoltaic devices based on CIS

These have demonstrated exceptional energy conver-sion efficiencies and a high tolerance to radiationdamage The opto-electronic properties of thiscomplex material are a function of its defect structureand therefore the material growth parameters Todate the reproducible growth of CIS has beenproblematic it has always been necessary to usepost-deposition processing to obtain material withthe required properties Initial work with PMShas produced single phase stoichiometric p-typefilms with resistivities in the range 5 V cm andexcellent optical absorption properties

EXPERIMENTAL

TiN coatings were deposited by continuous and pulsedreactive magnetron sputtering in a Teer Coatings LtdUDP250 rig12 Sputtering took place from a single3006100 mm 995 pure titanium target The reac-tive sputtering process was controlled by opticalemissions monitoring (OEM) using conditions selectedto produce stoichiometric TiN coatings based onprevious experience12 The magnetron was driven by a5 kW Advanced Energy1 MDX dc power supplyWhen operating in pulsed mode this supply wasconnected in series with an Advanced EnergySPARC-LE 20 pulse unit The SPARC-LE 20 con-verts the dc input from the MDX into an asymmetricbipolar pulsed dc output to the magnetron with thepulse frequency fixed at 20 kHz The SPARC-LE 20has a duty cycle of 90 ie each pulse on cycleduring which sputtering takes place has a duration of45 ms and each pulse off cycle during which thetarget voltage is reversed has a duration of 5 msDuring pulse off the target voltage is reversed to 10of the nominal pulse on voltage

Coatings were deposited onto various substratematerials including silicon wafers and tool steel tosuit different analytical techniques All substrateswere ultrasonically precleaned in propanol Prior tothe deposition of the TiN coatings the substrateswere also dc sputter cleaned at 21000 V for 15 min

The coatings were characterised in terms of theirstructures and properties using a range of analyticaland measurement techniques including SEM elec-tron probe microanalysis (EPMA) X-ray diffraction(XRD) microhardness testing and surface profilo-metry The tribological properties of the TiN filmswere investigated by thrust washer friction and weartesting15 and scratch adhesion testing

The ZnO Al and CIS coatings were both depositedin a rig specifically designed for powder target use13

A single 180 mm dia unbalanced magnetron wasinstalled in the base plate of the chamber in the

lsquosputter uprsquo configuration The magnetron was builtat Salford and utilises rare earth magnets to give highfield strengths at the target (22 kG maximum) Thesubstrate holder was positioned directly above themagnetron at a separation of 120 mm The substrateholder could be rf biased if required A 500 W radiantheater was also installed in the rig which could bepositioned post-deposition to face the coated sub-strate This allowed annealing of the coatings to takeplace in controlled atmospheres at temperatures of upto 500uC Finally a dummy magnetron was installedin the chamber roof vertically opposed to the magne-tron This dummy device only included an outer ringof magnets and was installed to produce a closedmagnetic field across the chamber maximising the ionto atom ratio incident at the substrate16 The rig isshown schematically in Fig 1

The Al doped ndash ZnO powder blends were madeby mixing appropriate quantities of zinc oxide andaluminium oxide powders in a rotating drum forseveral hours For each material the average particlesize was 5 mm and the purity was 9999 Initialbatches were produced with a dopant concentrationof 4 wt-alumina Following blending approxi-mately 60 g of powder was evenly distributed acrossthe surface of a copper backing plate on the magne-tron to form a target The backing plate had beenrecessed to a depth of 2 mm to allow a reasonabletarget thickness to be produced The powder waslightly tamped down to produce a uniform thicknessand surface to the target No further processes wereinvolved in target production To demonstrate theflexibility of this technique subsequent experimentshave been carried out in the same manner using zincoxide blended with oxides of tin antimony indiumand gallium14 Tin doped indium oxide (ITO) blendshave also been produced and tested

In all cases coatings were deposited onto glassmicroscope slides by PMS using an Advanced EnergyPinnacle Plus magnetron driver supply This unitwhich is also an asymmetric bipolar supply can

1 Schematic representation of powder target rig

158 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

operate at pulse frequencies of up to 350 kHz atduties in the range 50 ndash 100 The glass substrateswere rf sputter cleaned prior to deposition (see Kellyet al13 and Zhou et al14 for full run details) Thecoatings were subsequently analysed by SEM EPMAand XRD The electrical properties were investigatedusing a four point probe and the optical propertieswere investigated using an Aquila Instrumentsnkd8000 spectrophotometer

The starting material for the CIS coatings waspowder prepared from crushed stoichiometric singleand polycrystalline CIS ingots which were grown inwhat is a standard technique at Salford Univer-sity1718 This powder was sieved to particle sizes inthe range 005 ndash 1 mm and was formed into a targetin the manner described above The composition ofthe starting powder was 29Cu ndash 17In ndash 54Se (at-) asdetermined by EPMA Coatings were again depositedonto glass substrates by PMS using the Pinnacle Plussupply at a range of frequencies and duties Thecoatings were analysed using the techniques describedpreviously

RESULTS

TiN coatings

EPMA analysis of the continuous and pulsed dc TiNcoatings confirmed that both sets of coatings hadstoichiometric compositions Both sets of coatingshad similar hardness values as measured using aFischerscope H100 with a 50 mN load (typicallyy26 GPa) and similar roughness values as mea-sured using a Talysurf 10 (Ra~030 ndash 038 mm forcoatings deposited onto tool steel) XRD analysisindicated that the strong (111) texture of thecontinuous films was shifted to a weaker (002) texturein the pulsed films12

The most interesting results though for the TiNfilms were the relative tribological properties ofthe continuous and pulsed films Figure 2 comparesthe friction response of these films in unlubricatedthrust washer tests running against phosphated shimcounterfaces (full test conditions are summarised inthe figure caption) In these tests the pulsed filmsrepeatedly gave significantly lower coefficients offriction than did the continuous films Indeed in theexample shown the average coefficient of the pulsed

film is 009 compared with 034 for the continuousfilm No measurable wear was observed on the surfaceof either film after a test duration of 60 min Thepulsed films also performed notably better than thecontinuous films during scratch adhesion testing Acomparison of the friction force v normal load for anexample of each film type is shown in Fig 3 Thecritical load for the continuous film was 24 Nwhereas the pulsed film did not fail until thenormal load reached 65 N (again full test conditionsare summarised in the figure caption)

Examination of these films by high resolutionSEM revealed significant structural differences in thepulsed and continuous films Figure 4 shows exam-ples of micrographs of the surfaces of TiN coatingsdeposited onto silicon wafers The pulsed films aredenser smoother and have fewer voids than thecontinuous films

Doped ZnO coatings

The magnetron discharge readily ignited under theoperating conditions chosen for the deposition of theZnO Al coatings (pulse frequency 350 kHz duty62 target current 2 A pressure 02 Pa) The processproved stable with no problems such as outgassingor arcing being observed Figure 5 shows a SEMmicrograph of the fracture section of a typicalZnO Al coating deposited using the conditionsdescribed above The coating has a dense columnarstructure and appears to be defect free Based onFig 5 the coating deposition rate was estimated to be500 nm h21 EPMA analysis indicated that the coat-ings contained approximately 2 wt-Al ie close tothe composition of the target (4 wt-Al2O3)

The coatings were post-deposition annealed atreduced pressure in nitrogen at 470uC for 2 h Priorto annealing the coatings were highly insulatingHowever following this process resistivities ofv361023 V cm were recorded XRD analysis indi-cated that the annealed coatings had strong (002)textures Optical transmission spectra for examplesof the ZnO Al coatings are shown in Fig 6 It isapparent from this figure that annealing the coatingshas shifted the absorption edge towards shorter wave-lengths ie higher band gap energies This processhas also increased the average and peak visible

2 Frictional response from unlubricated thrust washertesting of TiN coatings deposited by continuous dc andpulsed dc reactive magnetron sputtering (normal load100 N rotation speed 30 rev min21 counterfacephosphated shim 55Rockwell C)

3 Scratch adhesion test results for TiN coatings depositedby continuous dc and pulsed dc reactive magnetron sput-tering (initial load 10 N loading rate 100 N min21velocity 10 N min21)

Kelly et al Advanced coatings through PMS 159

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

transmission of the coatings The average visibletransmission of the as deposited coatings was 862(90 peak) compared with 887 (92 peak) for theannealed coatings

CIS coatings

Once again no problems were observed sputteringthe powder targets in pulsed dc mode with stable

discharges being achieved over a wide range offrequencies and duties SEM examination of thecoatings showed they were uniform fully dense andpinhole free By way of example Fig 7 is a SEMmicrograph showing the fracture section of a CIScoating deposited onto a glass substrate As one of themajor issues in the fabrication of CIS thin films thecomposition of the samples has been analysed byEPMA All samples showed near stoichiometry withthe average composition being 245 ndash Cu ndash 245In ndash51Se (at-)

Analysis of the electrical properties of these coat-ings showed that they were p-type semiconductorswith resistivity values of the order of 5 V cmOptically these coatings are highly absorbing acrossthe visible spectrum and into the IR spectrum This isevidenced in Fig 8 which shows the transmissionspectra from 350 to 1700 nm for a typical example ofthe CIS coatings deposited by PMS from a powdertarget

DISCUSSION

PMS has become well established as a highly effectivedeposition technique with numerous commercialapplications To date though most of these applica-tions involve the deposition of dielectric materials

5 SEM micrograph of fracture section of ZnOndash 2Al(wt-) coating deposited on glass substrate by PMSfrom blended powder target (ZnOndash 4 wt-Al2O3)

6 Optical transmission spectra for ZnOndash 2Al (wt-)coatings before and after annealing (pure nitrogen at420uC for 1 h)

a b

4 High resolution SEM micrographs showing surface topography of TiN coating deposited onto silicon wafer by a con-tinuous dc reactive magnetron sputtering b pulsed dc reactive magnetron sputtering

7 SEM micrograph of fracture section of CIS coatingdeposited onto glass substrate by PMS from CISpowder target

160 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

However as discussed in the present paper thebenefits of utilising this technology extend wellbeyond this category of materials

Taking the first example the production of TiNcoatings with coefficients of friction v01 in un-lubricated tests is a remarkable result and demon-strates the enhancement in properties that can beachieved through the use of pulsed processing Asstated earlier TiN and its derivatives (TiAlN etc) arenot known as low friction materials19 Coefficients offriction would not normally be lower than say 03for these materials in any form of unlubricated testThe enhancement in the tribological properties of thefilms grown by pulsed processing can be attributed tothe significant structural modifications observed inthese coatings The exact mechanisms by which thesechanges have been brought about are not yet knownbut may perhaps be related to the increased ionenergy flux known to be delivered to the substrateduring pulsed processing11 The shift to a (200) tex-ture in the pulsed TiN films supports this hypothesisas this has been observed to occur elsewhere followinghigh energy bombardment and has also been associ-ated with enhanced mechanical properties20

The deposition of doped zinc oxide coatings fromblended powder targets is an example of how pulsedprocessing can bring enhanced process flexibility As aconsequence of using pulsed dc power to sputteroxide targets directly no reactive sputtering controlsystem is required and the complexities of rfmatching networks are avoided These targets willnot actually sputter using continuous dc power as itis not possible to initiate a discharge In fact it isdifficult to initiate a discharge even using pulsed dcat pulse frequencies below approximately 200 kHzHowever above this frequency a stable discharge canbe readily ignited and sputtering can take placedirectly from the oxide blends with no evidence ofarcing or outgassing The reason for the existenceof an apparent lsquocut offrsquo frequency is not knownHowever it is believed to be related to the natureof the target voltage waveforms A feature of thePinnacle Plus supply is a positive voltage overshoot atthe beginning of the pulse off period The magnitudeof this voltage increases with pulse frequency as canbe seen in Fig 9 which compares the target voltagewaveform from this supply at both 140 kHz and350 kHz pulse frequencies At the lower frequencythe magnitude of the overshoot voltage is approxi-mately 300 V whereas at 350 kHz this reaches600 V It is postulated that these significant voltage

overshoots at the higher frequency range of the powersupply (ie w200 kHz) aid plasma ignition withpoorly conducting targets

Doped zinc oxide coatings are attracting consider-able interest as competitors for the most widely usedTCO coating namely ITO21 ndash 24 The dopant materialsare used to modify the electrical and optical propertiesof the TCO coatings The choice and concentration ofdopant are critical to the film performance While solidtargets are limited to one composition per targetpowder targets offer an infinite variety of composi-tions Furthermore multiple dopant compositionsgiving tailored film properties are readily achievableAlthough doped zinc oxide coatings have beenproduced elsewhere by pulsed sputtering2124 andfrom sintered powder targets22 the authors believethis is the first time that blended oxide powder targetsand pulsed dc processing have been used in combina-tion The film properties presented here bear compar-ison with published data for ZnO Al coatingsproduced by other techniques21 ndash 24 and while thismay not be a production technique this approachprovides an ideal means of screening candidatematerials and identifying optimum compositions

The deposition of CIS coatings directly from a CISpowder target using PMS is also unique CIS and therelated copper indium gallium diselenide (CIGS) arevery promising absorbent semiconductors for use inhigh efficiency photovoltaic applications They pos-sess the highest absorption coefficients known25 Todate solar panels using these materials reached astabilised efficiency of 10 ndash 12 However there is agreat performance discrepancy between laboratorysolar cells and commercial modules Several researchgroups have achieved individual solar cell deviceswith efficiencies over 18 This mismatch is mainlythe result of the complexity of the material and theresulting requirements for its fabrication To depositstoichiometric thin films and to control the crucialproperties research and module fabrication tend toconcentrate on a three stage evaporation process Amajor problem is the incorporation of selenium toachieve stoichiometry and the desired crystal struc-ture and further annealing steps are always neces-sary The ability to deposit stoichiometric coatingsdirectly from a powder target in a single stage processis a major step forward in this area Furthermorethe coatings have dense defect free structures andthe necessary electrical and optical properties to beincorporated into high performance photovoltaicdevices Their high resistance to radiation damage

8 Optical transmission spectra for CIS coating depositedonto glass substrate by PMS from CIS powder target

9 Comparison of target voltage waveforms operating atpulse frequencies of 140 kHz and 350 kHz

Kelly et al Advanced coatings through PMS 161

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

Al doped zinc oxide TCO coatings sputtered directlyfrom blended powder targets

Mixed zinc oxide and alumina powder targets havebeen used to produce transparent semiconductivefilms on glass by PMS Resistivities of the order of1023 V cm were obtained after annealing withoutdeterioration of the coating to substrate adhesionThe results to date demonstrate that the PMS ofdoped ZnO films from powder targets is a flexiblenovel technique for the deposition of high qualityTCO materials1314

Thin film photovoltaic devices based on CIS

These have demonstrated exceptional energy conver-sion efficiencies and a high tolerance to radiationdamage The opto-electronic properties of thiscomplex material are a function of its defect structureand therefore the material growth parameters Todate the reproducible growth of CIS has beenproblematic it has always been necessary to usepost-deposition processing to obtain material withthe required properties Initial work with PMShas produced single phase stoichiometric p-typefilms with resistivities in the range 5 V cm andexcellent optical absorption properties

EXPERIMENTAL

TiN coatings were deposited by continuous and pulsedreactive magnetron sputtering in a Teer Coatings LtdUDP250 rig12 Sputtering took place from a single3006100 mm 995 pure titanium target The reac-tive sputtering process was controlled by opticalemissions monitoring (OEM) using conditions selectedto produce stoichiometric TiN coatings based onprevious experience12 The magnetron was driven by a5 kW Advanced Energy1 MDX dc power supplyWhen operating in pulsed mode this supply wasconnected in series with an Advanced EnergySPARC-LE 20 pulse unit The SPARC-LE 20 con-verts the dc input from the MDX into an asymmetricbipolar pulsed dc output to the magnetron with thepulse frequency fixed at 20 kHz The SPARC-LE 20has a duty cycle of 90 ie each pulse on cycleduring which sputtering takes place has a duration of45 ms and each pulse off cycle during which thetarget voltage is reversed has a duration of 5 msDuring pulse off the target voltage is reversed to 10of the nominal pulse on voltage

Coatings were deposited onto various substratematerials including silicon wafers and tool steel tosuit different analytical techniques All substrateswere ultrasonically precleaned in propanol Prior tothe deposition of the TiN coatings the substrateswere also dc sputter cleaned at 21000 V for 15 min

The coatings were characterised in terms of theirstructures and properties using a range of analyticaland measurement techniques including SEM elec-tron probe microanalysis (EPMA) X-ray diffraction(XRD) microhardness testing and surface profilo-metry The tribological properties of the TiN filmswere investigated by thrust washer friction and weartesting15 and scratch adhesion testing

The ZnO Al and CIS coatings were both depositedin a rig specifically designed for powder target use13

A single 180 mm dia unbalanced magnetron wasinstalled in the base plate of the chamber in the

lsquosputter uprsquo configuration The magnetron was builtat Salford and utilises rare earth magnets to give highfield strengths at the target (22 kG maximum) Thesubstrate holder was positioned directly above themagnetron at a separation of 120 mm The substrateholder could be rf biased if required A 500 W radiantheater was also installed in the rig which could bepositioned post-deposition to face the coated sub-strate This allowed annealing of the coatings to takeplace in controlled atmospheres at temperatures of upto 500uC Finally a dummy magnetron was installedin the chamber roof vertically opposed to the magne-tron This dummy device only included an outer ringof magnets and was installed to produce a closedmagnetic field across the chamber maximising the ionto atom ratio incident at the substrate16 The rig isshown schematically in Fig 1

The Al doped ndash ZnO powder blends were madeby mixing appropriate quantities of zinc oxide andaluminium oxide powders in a rotating drum forseveral hours For each material the average particlesize was 5 mm and the purity was 9999 Initialbatches were produced with a dopant concentrationof 4 wt-alumina Following blending approxi-mately 60 g of powder was evenly distributed acrossthe surface of a copper backing plate on the magne-tron to form a target The backing plate had beenrecessed to a depth of 2 mm to allow a reasonabletarget thickness to be produced The powder waslightly tamped down to produce a uniform thicknessand surface to the target No further processes wereinvolved in target production To demonstrate theflexibility of this technique subsequent experimentshave been carried out in the same manner using zincoxide blended with oxides of tin antimony indiumand gallium14 Tin doped indium oxide (ITO) blendshave also been produced and tested

In all cases coatings were deposited onto glassmicroscope slides by PMS using an Advanced EnergyPinnacle Plus magnetron driver supply This unitwhich is also an asymmetric bipolar supply can

1 Schematic representation of powder target rig

158 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

operate at pulse frequencies of up to 350 kHz atduties in the range 50 ndash 100 The glass substrateswere rf sputter cleaned prior to deposition (see Kellyet al13 and Zhou et al14 for full run details) Thecoatings were subsequently analysed by SEM EPMAand XRD The electrical properties were investigatedusing a four point probe and the optical propertieswere investigated using an Aquila Instrumentsnkd8000 spectrophotometer

The starting material for the CIS coatings waspowder prepared from crushed stoichiometric singleand polycrystalline CIS ingots which were grown inwhat is a standard technique at Salford Univer-sity1718 This powder was sieved to particle sizes inthe range 005 ndash 1 mm and was formed into a targetin the manner described above The composition ofthe starting powder was 29Cu ndash 17In ndash 54Se (at-) asdetermined by EPMA Coatings were again depositedonto glass substrates by PMS using the Pinnacle Plussupply at a range of frequencies and duties Thecoatings were analysed using the techniques describedpreviously

RESULTS

TiN coatings

EPMA analysis of the continuous and pulsed dc TiNcoatings confirmed that both sets of coatings hadstoichiometric compositions Both sets of coatingshad similar hardness values as measured using aFischerscope H100 with a 50 mN load (typicallyy26 GPa) and similar roughness values as mea-sured using a Talysurf 10 (Ra~030 ndash 038 mm forcoatings deposited onto tool steel) XRD analysisindicated that the strong (111) texture of thecontinuous films was shifted to a weaker (002) texturein the pulsed films12

The most interesting results though for the TiNfilms were the relative tribological properties ofthe continuous and pulsed films Figure 2 comparesthe friction response of these films in unlubricatedthrust washer tests running against phosphated shimcounterfaces (full test conditions are summarised inthe figure caption) In these tests the pulsed filmsrepeatedly gave significantly lower coefficients offriction than did the continuous films Indeed in theexample shown the average coefficient of the pulsed

film is 009 compared with 034 for the continuousfilm No measurable wear was observed on the surfaceof either film after a test duration of 60 min Thepulsed films also performed notably better than thecontinuous films during scratch adhesion testing Acomparison of the friction force v normal load for anexample of each film type is shown in Fig 3 Thecritical load for the continuous film was 24 Nwhereas the pulsed film did not fail until thenormal load reached 65 N (again full test conditionsare summarised in the figure caption)

Examination of these films by high resolutionSEM revealed significant structural differences in thepulsed and continuous films Figure 4 shows exam-ples of micrographs of the surfaces of TiN coatingsdeposited onto silicon wafers The pulsed films aredenser smoother and have fewer voids than thecontinuous films

Doped ZnO coatings

The magnetron discharge readily ignited under theoperating conditions chosen for the deposition of theZnO Al coatings (pulse frequency 350 kHz duty62 target current 2 A pressure 02 Pa) The processproved stable with no problems such as outgassingor arcing being observed Figure 5 shows a SEMmicrograph of the fracture section of a typicalZnO Al coating deposited using the conditionsdescribed above The coating has a dense columnarstructure and appears to be defect free Based onFig 5 the coating deposition rate was estimated to be500 nm h21 EPMA analysis indicated that the coat-ings contained approximately 2 wt-Al ie close tothe composition of the target (4 wt-Al2O3)

The coatings were post-deposition annealed atreduced pressure in nitrogen at 470uC for 2 h Priorto annealing the coatings were highly insulatingHowever following this process resistivities ofv361023 V cm were recorded XRD analysis indi-cated that the annealed coatings had strong (002)textures Optical transmission spectra for examplesof the ZnO Al coatings are shown in Fig 6 It isapparent from this figure that annealing the coatingshas shifted the absorption edge towards shorter wave-lengths ie higher band gap energies This processhas also increased the average and peak visible

2 Frictional response from unlubricated thrust washertesting of TiN coatings deposited by continuous dc andpulsed dc reactive magnetron sputtering (normal load100 N rotation speed 30 rev min21 counterfacephosphated shim 55Rockwell C)

3 Scratch adhesion test results for TiN coatings depositedby continuous dc and pulsed dc reactive magnetron sput-tering (initial load 10 N loading rate 100 N min21velocity 10 N min21)

Kelly et al Advanced coatings through PMS 159

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

transmission of the coatings The average visibletransmission of the as deposited coatings was 862(90 peak) compared with 887 (92 peak) for theannealed coatings

CIS coatings

Once again no problems were observed sputteringthe powder targets in pulsed dc mode with stable

discharges being achieved over a wide range offrequencies and duties SEM examination of thecoatings showed they were uniform fully dense andpinhole free By way of example Fig 7 is a SEMmicrograph showing the fracture section of a CIScoating deposited onto a glass substrate As one of themajor issues in the fabrication of CIS thin films thecomposition of the samples has been analysed byEPMA All samples showed near stoichiometry withthe average composition being 245 ndash Cu ndash 245In ndash51Se (at-)

Analysis of the electrical properties of these coat-ings showed that they were p-type semiconductorswith resistivity values of the order of 5 V cmOptically these coatings are highly absorbing acrossthe visible spectrum and into the IR spectrum This isevidenced in Fig 8 which shows the transmissionspectra from 350 to 1700 nm for a typical example ofthe CIS coatings deposited by PMS from a powdertarget

DISCUSSION

PMS has become well established as a highly effectivedeposition technique with numerous commercialapplications To date though most of these applica-tions involve the deposition of dielectric materials

5 SEM micrograph of fracture section of ZnOndash 2Al(wt-) coating deposited on glass substrate by PMSfrom blended powder target (ZnOndash 4 wt-Al2O3)

6 Optical transmission spectra for ZnOndash 2Al (wt-)coatings before and after annealing (pure nitrogen at420uC for 1 h)

a b

4 High resolution SEM micrographs showing surface topography of TiN coating deposited onto silicon wafer by a con-tinuous dc reactive magnetron sputtering b pulsed dc reactive magnetron sputtering

7 SEM micrograph of fracture section of CIS coatingdeposited onto glass substrate by PMS from CISpowder target

160 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

However as discussed in the present paper thebenefits of utilising this technology extend wellbeyond this category of materials

Taking the first example the production of TiNcoatings with coefficients of friction v01 in un-lubricated tests is a remarkable result and demon-strates the enhancement in properties that can beachieved through the use of pulsed processing Asstated earlier TiN and its derivatives (TiAlN etc) arenot known as low friction materials19 Coefficients offriction would not normally be lower than say 03for these materials in any form of unlubricated testThe enhancement in the tribological properties of thefilms grown by pulsed processing can be attributed tothe significant structural modifications observed inthese coatings The exact mechanisms by which thesechanges have been brought about are not yet knownbut may perhaps be related to the increased ionenergy flux known to be delivered to the substrateduring pulsed processing11 The shift to a (200) tex-ture in the pulsed TiN films supports this hypothesisas this has been observed to occur elsewhere followinghigh energy bombardment and has also been associ-ated with enhanced mechanical properties20

The deposition of doped zinc oxide coatings fromblended powder targets is an example of how pulsedprocessing can bring enhanced process flexibility As aconsequence of using pulsed dc power to sputteroxide targets directly no reactive sputtering controlsystem is required and the complexities of rfmatching networks are avoided These targets willnot actually sputter using continuous dc power as itis not possible to initiate a discharge In fact it isdifficult to initiate a discharge even using pulsed dcat pulse frequencies below approximately 200 kHzHowever above this frequency a stable discharge canbe readily ignited and sputtering can take placedirectly from the oxide blends with no evidence ofarcing or outgassing The reason for the existenceof an apparent lsquocut offrsquo frequency is not knownHowever it is believed to be related to the natureof the target voltage waveforms A feature of thePinnacle Plus supply is a positive voltage overshoot atthe beginning of the pulse off period The magnitudeof this voltage increases with pulse frequency as canbe seen in Fig 9 which compares the target voltagewaveform from this supply at both 140 kHz and350 kHz pulse frequencies At the lower frequencythe magnitude of the overshoot voltage is approxi-mately 300 V whereas at 350 kHz this reaches600 V It is postulated that these significant voltage

overshoots at the higher frequency range of the powersupply (ie w200 kHz) aid plasma ignition withpoorly conducting targets

Doped zinc oxide coatings are attracting consider-able interest as competitors for the most widely usedTCO coating namely ITO21 ndash 24 The dopant materialsare used to modify the electrical and optical propertiesof the TCO coatings The choice and concentration ofdopant are critical to the film performance While solidtargets are limited to one composition per targetpowder targets offer an infinite variety of composi-tions Furthermore multiple dopant compositionsgiving tailored film properties are readily achievableAlthough doped zinc oxide coatings have beenproduced elsewhere by pulsed sputtering2124 andfrom sintered powder targets22 the authors believethis is the first time that blended oxide powder targetsand pulsed dc processing have been used in combina-tion The film properties presented here bear compar-ison with published data for ZnO Al coatingsproduced by other techniques21 ndash 24 and while thismay not be a production technique this approachprovides an ideal means of screening candidatematerials and identifying optimum compositions

The deposition of CIS coatings directly from a CISpowder target using PMS is also unique CIS and therelated copper indium gallium diselenide (CIGS) arevery promising absorbent semiconductors for use inhigh efficiency photovoltaic applications They pos-sess the highest absorption coefficients known25 Todate solar panels using these materials reached astabilised efficiency of 10 ndash 12 However there is agreat performance discrepancy between laboratorysolar cells and commercial modules Several researchgroups have achieved individual solar cell deviceswith efficiencies over 18 This mismatch is mainlythe result of the complexity of the material and theresulting requirements for its fabrication To depositstoichiometric thin films and to control the crucialproperties research and module fabrication tend toconcentrate on a three stage evaporation process Amajor problem is the incorporation of selenium toachieve stoichiometry and the desired crystal struc-ture and further annealing steps are always neces-sary The ability to deposit stoichiometric coatingsdirectly from a powder target in a single stage processis a major step forward in this area Furthermorethe coatings have dense defect free structures andthe necessary electrical and optical properties to beincorporated into high performance photovoltaicdevices Their high resistance to radiation damage

8 Optical transmission spectra for CIS coating depositedonto glass substrate by PMS from CIS powder target

9 Comparison of target voltage waveforms operating atpulse frequencies of 140 kHz and 350 kHz

Kelly et al Advanced coatings through PMS 161

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

operate at pulse frequencies of up to 350 kHz atduties in the range 50 ndash 100 The glass substrateswere rf sputter cleaned prior to deposition (see Kellyet al13 and Zhou et al14 for full run details) Thecoatings were subsequently analysed by SEM EPMAand XRD The electrical properties were investigatedusing a four point probe and the optical propertieswere investigated using an Aquila Instrumentsnkd8000 spectrophotometer

The starting material for the CIS coatings waspowder prepared from crushed stoichiometric singleand polycrystalline CIS ingots which were grown inwhat is a standard technique at Salford Univer-sity1718 This powder was sieved to particle sizes inthe range 005 ndash 1 mm and was formed into a targetin the manner described above The composition ofthe starting powder was 29Cu ndash 17In ndash 54Se (at-) asdetermined by EPMA Coatings were again depositedonto glass substrates by PMS using the Pinnacle Plussupply at a range of frequencies and duties Thecoatings were analysed using the techniques describedpreviously

RESULTS

TiN coatings

EPMA analysis of the continuous and pulsed dc TiNcoatings confirmed that both sets of coatings hadstoichiometric compositions Both sets of coatingshad similar hardness values as measured using aFischerscope H100 with a 50 mN load (typicallyy26 GPa) and similar roughness values as mea-sured using a Talysurf 10 (Ra~030 ndash 038 mm forcoatings deposited onto tool steel) XRD analysisindicated that the strong (111) texture of thecontinuous films was shifted to a weaker (002) texturein the pulsed films12

The most interesting results though for the TiNfilms were the relative tribological properties ofthe continuous and pulsed films Figure 2 comparesthe friction response of these films in unlubricatedthrust washer tests running against phosphated shimcounterfaces (full test conditions are summarised inthe figure caption) In these tests the pulsed filmsrepeatedly gave significantly lower coefficients offriction than did the continuous films Indeed in theexample shown the average coefficient of the pulsed

film is 009 compared with 034 for the continuousfilm No measurable wear was observed on the surfaceof either film after a test duration of 60 min Thepulsed films also performed notably better than thecontinuous films during scratch adhesion testing Acomparison of the friction force v normal load for anexample of each film type is shown in Fig 3 Thecritical load for the continuous film was 24 Nwhereas the pulsed film did not fail until thenormal load reached 65 N (again full test conditionsare summarised in the figure caption)

Examination of these films by high resolutionSEM revealed significant structural differences in thepulsed and continuous films Figure 4 shows exam-ples of micrographs of the surfaces of TiN coatingsdeposited onto silicon wafers The pulsed films aredenser smoother and have fewer voids than thecontinuous films

Doped ZnO coatings

The magnetron discharge readily ignited under theoperating conditions chosen for the deposition of theZnO Al coatings (pulse frequency 350 kHz duty62 target current 2 A pressure 02 Pa) The processproved stable with no problems such as outgassingor arcing being observed Figure 5 shows a SEMmicrograph of the fracture section of a typicalZnO Al coating deposited using the conditionsdescribed above The coating has a dense columnarstructure and appears to be defect free Based onFig 5 the coating deposition rate was estimated to be500 nm h21 EPMA analysis indicated that the coat-ings contained approximately 2 wt-Al ie close tothe composition of the target (4 wt-Al2O3)

The coatings were post-deposition annealed atreduced pressure in nitrogen at 470uC for 2 h Priorto annealing the coatings were highly insulatingHowever following this process resistivities ofv361023 V cm were recorded XRD analysis indi-cated that the annealed coatings had strong (002)textures Optical transmission spectra for examplesof the ZnO Al coatings are shown in Fig 6 It isapparent from this figure that annealing the coatingshas shifted the absorption edge towards shorter wave-lengths ie higher band gap energies This processhas also increased the average and peak visible

2 Frictional response from unlubricated thrust washertesting of TiN coatings deposited by continuous dc andpulsed dc reactive magnetron sputtering (normal load100 N rotation speed 30 rev min21 counterfacephosphated shim 55Rockwell C)

3 Scratch adhesion test results for TiN coatings depositedby continuous dc and pulsed dc reactive magnetron sput-tering (initial load 10 N loading rate 100 N min21velocity 10 N min21)

Kelly et al Advanced coatings through PMS 159

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

transmission of the coatings The average visibletransmission of the as deposited coatings was 862(90 peak) compared with 887 (92 peak) for theannealed coatings

CIS coatings

Once again no problems were observed sputteringthe powder targets in pulsed dc mode with stable

discharges being achieved over a wide range offrequencies and duties SEM examination of thecoatings showed they were uniform fully dense andpinhole free By way of example Fig 7 is a SEMmicrograph showing the fracture section of a CIScoating deposited onto a glass substrate As one of themajor issues in the fabrication of CIS thin films thecomposition of the samples has been analysed byEPMA All samples showed near stoichiometry withthe average composition being 245 ndash Cu ndash 245In ndash51Se (at-)

Analysis of the electrical properties of these coat-ings showed that they were p-type semiconductorswith resistivity values of the order of 5 V cmOptically these coatings are highly absorbing acrossthe visible spectrum and into the IR spectrum This isevidenced in Fig 8 which shows the transmissionspectra from 350 to 1700 nm for a typical example ofthe CIS coatings deposited by PMS from a powdertarget

DISCUSSION

PMS has become well established as a highly effectivedeposition technique with numerous commercialapplications To date though most of these applica-tions involve the deposition of dielectric materials

5 SEM micrograph of fracture section of ZnOndash 2Al(wt-) coating deposited on glass substrate by PMSfrom blended powder target (ZnOndash 4 wt-Al2O3)

6 Optical transmission spectra for ZnOndash 2Al (wt-)coatings before and after annealing (pure nitrogen at420uC for 1 h)

a b

4 High resolution SEM micrographs showing surface topography of TiN coating deposited onto silicon wafer by a con-tinuous dc reactive magnetron sputtering b pulsed dc reactive magnetron sputtering

7 SEM micrograph of fracture section of CIS coatingdeposited onto glass substrate by PMS from CISpowder target

160 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

However as discussed in the present paper thebenefits of utilising this technology extend wellbeyond this category of materials

Taking the first example the production of TiNcoatings with coefficients of friction v01 in un-lubricated tests is a remarkable result and demon-strates the enhancement in properties that can beachieved through the use of pulsed processing Asstated earlier TiN and its derivatives (TiAlN etc) arenot known as low friction materials19 Coefficients offriction would not normally be lower than say 03for these materials in any form of unlubricated testThe enhancement in the tribological properties of thefilms grown by pulsed processing can be attributed tothe significant structural modifications observed inthese coatings The exact mechanisms by which thesechanges have been brought about are not yet knownbut may perhaps be related to the increased ionenergy flux known to be delivered to the substrateduring pulsed processing11 The shift to a (200) tex-ture in the pulsed TiN films supports this hypothesisas this has been observed to occur elsewhere followinghigh energy bombardment and has also been associ-ated with enhanced mechanical properties20

The deposition of doped zinc oxide coatings fromblended powder targets is an example of how pulsedprocessing can bring enhanced process flexibility As aconsequence of using pulsed dc power to sputteroxide targets directly no reactive sputtering controlsystem is required and the complexities of rfmatching networks are avoided These targets willnot actually sputter using continuous dc power as itis not possible to initiate a discharge In fact it isdifficult to initiate a discharge even using pulsed dcat pulse frequencies below approximately 200 kHzHowever above this frequency a stable discharge canbe readily ignited and sputtering can take placedirectly from the oxide blends with no evidence ofarcing or outgassing The reason for the existenceof an apparent lsquocut offrsquo frequency is not knownHowever it is believed to be related to the natureof the target voltage waveforms A feature of thePinnacle Plus supply is a positive voltage overshoot atthe beginning of the pulse off period The magnitudeof this voltage increases with pulse frequency as canbe seen in Fig 9 which compares the target voltagewaveform from this supply at both 140 kHz and350 kHz pulse frequencies At the lower frequencythe magnitude of the overshoot voltage is approxi-mately 300 V whereas at 350 kHz this reaches600 V It is postulated that these significant voltage

overshoots at the higher frequency range of the powersupply (ie w200 kHz) aid plasma ignition withpoorly conducting targets

Doped zinc oxide coatings are attracting consider-able interest as competitors for the most widely usedTCO coating namely ITO21 ndash 24 The dopant materialsare used to modify the electrical and optical propertiesof the TCO coatings The choice and concentration ofdopant are critical to the film performance While solidtargets are limited to one composition per targetpowder targets offer an infinite variety of composi-tions Furthermore multiple dopant compositionsgiving tailored film properties are readily achievableAlthough doped zinc oxide coatings have beenproduced elsewhere by pulsed sputtering2124 andfrom sintered powder targets22 the authors believethis is the first time that blended oxide powder targetsand pulsed dc processing have been used in combina-tion The film properties presented here bear compar-ison with published data for ZnO Al coatingsproduced by other techniques21 ndash 24 and while thismay not be a production technique this approachprovides an ideal means of screening candidatematerials and identifying optimum compositions

The deposition of CIS coatings directly from a CISpowder target using PMS is also unique CIS and therelated copper indium gallium diselenide (CIGS) arevery promising absorbent semiconductors for use inhigh efficiency photovoltaic applications They pos-sess the highest absorption coefficients known25 Todate solar panels using these materials reached astabilised efficiency of 10 ndash 12 However there is agreat performance discrepancy between laboratorysolar cells and commercial modules Several researchgroups have achieved individual solar cell deviceswith efficiencies over 18 This mismatch is mainlythe result of the complexity of the material and theresulting requirements for its fabrication To depositstoichiometric thin films and to control the crucialproperties research and module fabrication tend toconcentrate on a three stage evaporation process Amajor problem is the incorporation of selenium toachieve stoichiometry and the desired crystal struc-ture and further annealing steps are always neces-sary The ability to deposit stoichiometric coatingsdirectly from a powder target in a single stage processis a major step forward in this area Furthermorethe coatings have dense defect free structures andthe necessary electrical and optical properties to beincorporated into high performance photovoltaicdevices Their high resistance to radiation damage

8 Optical transmission spectra for CIS coating depositedonto glass substrate by PMS from CIS powder target

9 Comparison of target voltage waveforms operating atpulse frequencies of 140 kHz and 350 kHz

Kelly et al Advanced coatings through PMS 161

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

transmission of the coatings The average visibletransmission of the as deposited coatings was 862(90 peak) compared with 887 (92 peak) for theannealed coatings

CIS coatings

Once again no problems were observed sputteringthe powder targets in pulsed dc mode with stable

discharges being achieved over a wide range offrequencies and duties SEM examination of thecoatings showed they were uniform fully dense andpinhole free By way of example Fig 7 is a SEMmicrograph showing the fracture section of a CIScoating deposited onto a glass substrate As one of themajor issues in the fabrication of CIS thin films thecomposition of the samples has been analysed byEPMA All samples showed near stoichiometry withthe average composition being 245 ndash Cu ndash 245In ndash51Se (at-)

Analysis of the electrical properties of these coat-ings showed that they were p-type semiconductorswith resistivity values of the order of 5 V cmOptically these coatings are highly absorbing acrossthe visible spectrum and into the IR spectrum This isevidenced in Fig 8 which shows the transmissionspectra from 350 to 1700 nm for a typical example ofthe CIS coatings deposited by PMS from a powdertarget

DISCUSSION

PMS has become well established as a highly effectivedeposition technique with numerous commercialapplications To date though most of these applica-tions involve the deposition of dielectric materials

5 SEM micrograph of fracture section of ZnOndash 2Al(wt-) coating deposited on glass substrate by PMSfrom blended powder target (ZnOndash 4 wt-Al2O3)

6 Optical transmission spectra for ZnOndash 2Al (wt-)coatings before and after annealing (pure nitrogen at420uC for 1 h)

a b

4 High resolution SEM micrographs showing surface topography of TiN coating deposited onto silicon wafer by a con-tinuous dc reactive magnetron sputtering b pulsed dc reactive magnetron sputtering

7 SEM micrograph of fracture section of CIS coatingdeposited onto glass substrate by PMS from CISpowder target

160 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

However as discussed in the present paper thebenefits of utilising this technology extend wellbeyond this category of materials

Taking the first example the production of TiNcoatings with coefficients of friction v01 in un-lubricated tests is a remarkable result and demon-strates the enhancement in properties that can beachieved through the use of pulsed processing Asstated earlier TiN and its derivatives (TiAlN etc) arenot known as low friction materials19 Coefficients offriction would not normally be lower than say 03for these materials in any form of unlubricated testThe enhancement in the tribological properties of thefilms grown by pulsed processing can be attributed tothe significant structural modifications observed inthese coatings The exact mechanisms by which thesechanges have been brought about are not yet knownbut may perhaps be related to the increased ionenergy flux known to be delivered to the substrateduring pulsed processing11 The shift to a (200) tex-ture in the pulsed TiN films supports this hypothesisas this has been observed to occur elsewhere followinghigh energy bombardment and has also been associ-ated with enhanced mechanical properties20

The deposition of doped zinc oxide coatings fromblended powder targets is an example of how pulsedprocessing can bring enhanced process flexibility As aconsequence of using pulsed dc power to sputteroxide targets directly no reactive sputtering controlsystem is required and the complexities of rfmatching networks are avoided These targets willnot actually sputter using continuous dc power as itis not possible to initiate a discharge In fact it isdifficult to initiate a discharge even using pulsed dcat pulse frequencies below approximately 200 kHzHowever above this frequency a stable discharge canbe readily ignited and sputtering can take placedirectly from the oxide blends with no evidence ofarcing or outgassing The reason for the existenceof an apparent lsquocut offrsquo frequency is not knownHowever it is believed to be related to the natureof the target voltage waveforms A feature of thePinnacle Plus supply is a positive voltage overshoot atthe beginning of the pulse off period The magnitudeof this voltage increases with pulse frequency as canbe seen in Fig 9 which compares the target voltagewaveform from this supply at both 140 kHz and350 kHz pulse frequencies At the lower frequencythe magnitude of the overshoot voltage is approxi-mately 300 V whereas at 350 kHz this reaches600 V It is postulated that these significant voltage

overshoots at the higher frequency range of the powersupply (ie w200 kHz) aid plasma ignition withpoorly conducting targets

Doped zinc oxide coatings are attracting consider-able interest as competitors for the most widely usedTCO coating namely ITO21 ndash 24 The dopant materialsare used to modify the electrical and optical propertiesof the TCO coatings The choice and concentration ofdopant are critical to the film performance While solidtargets are limited to one composition per targetpowder targets offer an infinite variety of composi-tions Furthermore multiple dopant compositionsgiving tailored film properties are readily achievableAlthough doped zinc oxide coatings have beenproduced elsewhere by pulsed sputtering2124 andfrom sintered powder targets22 the authors believethis is the first time that blended oxide powder targetsand pulsed dc processing have been used in combina-tion The film properties presented here bear compar-ison with published data for ZnO Al coatingsproduced by other techniques21 ndash 24 and while thismay not be a production technique this approachprovides an ideal means of screening candidatematerials and identifying optimum compositions

The deposition of CIS coatings directly from a CISpowder target using PMS is also unique CIS and therelated copper indium gallium diselenide (CIGS) arevery promising absorbent semiconductors for use inhigh efficiency photovoltaic applications They pos-sess the highest absorption coefficients known25 Todate solar panels using these materials reached astabilised efficiency of 10 ndash 12 However there is agreat performance discrepancy between laboratorysolar cells and commercial modules Several researchgroups have achieved individual solar cell deviceswith efficiencies over 18 This mismatch is mainlythe result of the complexity of the material and theresulting requirements for its fabrication To depositstoichiometric thin films and to control the crucialproperties research and module fabrication tend toconcentrate on a three stage evaporation process Amajor problem is the incorporation of selenium toachieve stoichiometry and the desired crystal struc-ture and further annealing steps are always neces-sary The ability to deposit stoichiometric coatingsdirectly from a powder target in a single stage processis a major step forward in this area Furthermorethe coatings have dense defect free structures andthe necessary electrical and optical properties to beincorporated into high performance photovoltaicdevices Their high resistance to radiation damage

8 Optical transmission spectra for CIS coating depositedonto glass substrate by PMS from CIS powder target

9 Comparison of target voltage waveforms operating atpulse frequencies of 140 kHz and 350 kHz

Kelly et al Advanced coatings through PMS 161

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

However as discussed in the present paper thebenefits of utilising this technology extend wellbeyond this category of materials

Taking the first example the production of TiNcoatings with coefficients of friction v01 in un-lubricated tests is a remarkable result and demon-strates the enhancement in properties that can beachieved through the use of pulsed processing Asstated earlier TiN and its derivatives (TiAlN etc) arenot known as low friction materials19 Coefficients offriction would not normally be lower than say 03for these materials in any form of unlubricated testThe enhancement in the tribological properties of thefilms grown by pulsed processing can be attributed tothe significant structural modifications observed inthese coatings The exact mechanisms by which thesechanges have been brought about are not yet knownbut may perhaps be related to the increased ionenergy flux known to be delivered to the substrateduring pulsed processing11 The shift to a (200) tex-ture in the pulsed TiN films supports this hypothesisas this has been observed to occur elsewhere followinghigh energy bombardment and has also been associ-ated with enhanced mechanical properties20

The deposition of doped zinc oxide coatings fromblended powder targets is an example of how pulsedprocessing can bring enhanced process flexibility As aconsequence of using pulsed dc power to sputteroxide targets directly no reactive sputtering controlsystem is required and the complexities of rfmatching networks are avoided These targets willnot actually sputter using continuous dc power as itis not possible to initiate a discharge In fact it isdifficult to initiate a discharge even using pulsed dcat pulse frequencies below approximately 200 kHzHowever above this frequency a stable discharge canbe readily ignited and sputtering can take placedirectly from the oxide blends with no evidence ofarcing or outgassing The reason for the existenceof an apparent lsquocut offrsquo frequency is not knownHowever it is believed to be related to the natureof the target voltage waveforms A feature of thePinnacle Plus supply is a positive voltage overshoot atthe beginning of the pulse off period The magnitudeof this voltage increases with pulse frequency as canbe seen in Fig 9 which compares the target voltagewaveform from this supply at both 140 kHz and350 kHz pulse frequencies At the lower frequencythe magnitude of the overshoot voltage is approxi-mately 300 V whereas at 350 kHz this reaches600 V It is postulated that these significant voltage

overshoots at the higher frequency range of the powersupply (ie w200 kHz) aid plasma ignition withpoorly conducting targets

Doped zinc oxide coatings are attracting consider-able interest as competitors for the most widely usedTCO coating namely ITO21 ndash 24 The dopant materialsare used to modify the electrical and optical propertiesof the TCO coatings The choice and concentration ofdopant are critical to the film performance While solidtargets are limited to one composition per targetpowder targets offer an infinite variety of composi-tions Furthermore multiple dopant compositionsgiving tailored film properties are readily achievableAlthough doped zinc oxide coatings have beenproduced elsewhere by pulsed sputtering2124 andfrom sintered powder targets22 the authors believethis is the first time that blended oxide powder targetsand pulsed dc processing have been used in combina-tion The film properties presented here bear compar-ison with published data for ZnO Al coatingsproduced by other techniques21 ndash 24 and while thismay not be a production technique this approachprovides an ideal means of screening candidatematerials and identifying optimum compositions

The deposition of CIS coatings directly from a CISpowder target using PMS is also unique CIS and therelated copper indium gallium diselenide (CIGS) arevery promising absorbent semiconductors for use inhigh efficiency photovoltaic applications They pos-sess the highest absorption coefficients known25 Todate solar panels using these materials reached astabilised efficiency of 10 ndash 12 However there is agreat performance discrepancy between laboratorysolar cells and commercial modules Several researchgroups have achieved individual solar cell deviceswith efficiencies over 18 This mismatch is mainlythe result of the complexity of the material and theresulting requirements for its fabrication To depositstoichiometric thin films and to control the crucialproperties research and module fabrication tend toconcentrate on a three stage evaporation process Amajor problem is the incorporation of selenium toachieve stoichiometry and the desired crystal struc-ture and further annealing steps are always neces-sary The ability to deposit stoichiometric coatingsdirectly from a powder target in a single stage processis a major step forward in this area Furthermorethe coatings have dense defect free structures andthe necessary electrical and optical properties to beincorporated into high performance photovoltaicdevices Their high resistance to radiation damage

8 Optical transmission spectra for CIS coating depositedonto glass substrate by PMS from CIS powder target

9 Comparison of target voltage waveforms operating atpulse frequencies of 140 kHz and 350 kHz

Kelly et al Advanced coatings through PMS 161

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3

Pub

lishe

d by

Man

ey P

ublis

hing

(c)

IOM

Com

mun

icat

ions

Ltd

makes them particularly attractive for use in spaceapplications

CONCLUSIONS

Although PMS has tended until now to be associatedwith the deposition of dielectric materials the benefitsoffered by pulsed processing in comparison withcontinuous processing extend well beyond this classof materials Examples presented here demonstratethe enhanced film properties and enhanced processflexibility that can be achieved through the use ofpulsed processing The production of low friction TiNcoatings doped zinc oxide coatings from blendedoxide powder targets and dense stoichiometric CIScoatings directly from a CIS powder target allrepresent important steps forward in coating techno-logy Furthermore they point the way to theproduction of new generations of advanced coatingmaterials through the use of pulsed processing

ACKNOWLEDGEMENTS

The present paper combines the results of severalprojects The authors would like to acknowledge thecontributions made by current and former colleaguesat Salford University including Dr C F Beevers DrP S Henderson Mr Geoff France and Dr ChesterFaunce Furthermore we should like to acknowledgethat the high resolution microscopy presented in thepresent paper was carried out in the Center forMicroanalysis of Materials University of Illinoiswhich is partially supported by the US Department ofEnergy under grant DEFG02-91-ER45439

REFERENCES1 s schiller k goedicke j reschke v kirchoff s schneider

and f milde Surf Coat Technol 1993 61 3312 p j kelly o a abu-zeid r d arnell and j tong Surf

Coat Technol 1996 86 ndash 87 28

3 r a scholl Surf Coat Technol 1998 98 8234 p j kelly p s henderson r d arnell g a roche and

d carter J Vac Sci Technol 2000 A18 28905 j orsquobrien and p j kelly Surf Coat Technol 2001 142 ndash 144

6216 j orsquobrien p j kelly j w bradley r hall and r d arnell

Proc 45th SVC Tech Conf Florida 306 ndash 311 2002 Societyof Vacuum Coaters Albuquerque New Mexico

7 g brauer j szczyrbowski and g teschner Surf CoatTechnol 94 ndash 95 658

8 k suzuki Thin Solid Films 1999 351 89 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2000 135 22110 j w bradley h bAcker p j kelly and r d arnell Surf

Coat Technol 2001 142 ndash 144 33711 j w bradley h bAcker y aranda-gonzalez p j kelly

and r d arnell Plasma Sources Sci Technol 2002 11 (2)165

12 p j kelly c f beevers p s henderson r d arnell j w

bradley and h bAcker Surf Coat Technol 2003 174 ndash 175779

13 p j kelly y zhou and a postill Thin Solid Films 2003 426111

14 y zhou p j kelly and a postill Proc 7th Int Symp onlsquoSputtering and Plasma processes ISSPrsquo Kanazawa JapanJune 145 ndash 148 2003 The Vacuum Society Japan

15 p j kelly r d arnell m d hudson a e j wilson andg jones Vacuum 2001 61 6

16 p j kelly and r d arnell Surf Coat Technol 1998108 ndash 109 317

17 j parkes r d tomlinson and m j hampshire J CrystGrowth 1973 20 315

18 r d tomlinson Solar Cells 1986 16 1719 p j kelly and r d arnell Vacuum 2000 56 15920 p patsalas c charitidis and s logothetidis Surf Coat

Technol 2000 125 33521 b szyszka Thin Solid Films 351 16422 t minami s suzuki and t miyata Thin Solid Films 2001

398 ndash 399 5323 p nunes d costa e fortunato and r martins Vacuum

2002 64 29324 r hong x jiang v sittinger b szyszka t hoing

g brauer g heide and g h frischat J Vac Sci Technol2002 A20 900

25 m a contreras b eggas k r ramanathan j hiltners schwartzlander f hasoon and r noufi Prog Photovolt1999 7 311

162 Kelly et al Advanced coatings through PMS

Surface Engineering 2004 Vol 20 No 3