Wet Etching of AI203 for Selective Patterning ofMicrostructures Using Ar Ion Implantation and H3P04

R Levy; M. Bianconi, and L CorreraGIVE - Istituto LIAMEL, 1- 40129 Bologna, Italy

ABSTRACT

A new etching method for single-crystal A1505 is proposed using ArH implantation and H5P04 chemical etching.Photolitographically defined patterns are transferred into sharp structures on the wafer surface by selectively removingthe damaged material. A double implantation of Ar* at 50 key followed by 180 key was performed on all samples, usinga dose of 2 >< 1015 ions/cm2 for the low-energy implant. Doses of 2 x 10u, 2 X 1016, and 4 >< 1016 ions/cm2 were used for the180 keV implant. Scanning electron microscopy and atomic force microscopy were used to characterize the patternobtained, while transmission electron microscopy and Rutherford backscattering channeling analysis were employed tostudy the implanted layer. Under the conditions investigated, both the etchable range (90 to 160 nm) and the etching rate(1 to 7 nm/mm) were found to be strongly correlated with the implanted dose. Sharp and uniform interfaces betweenetched and unetched regions were obtained, forming steps with an average slope of 30 to 45°. The proposed method offershigh selectivity, lack of contamination of the substrate, and compatibility with standard processing.

IntroductionOver the last several decades A1505 maintained the

interest of researchers because of its distinctive mechani-cal, thermal, optical, and electrical properties. Al503 filmshave been widely used as insulating layers or coatingsbecause of their chemical stability and hardness.Moreover, silicon-on-sapphire (SOS) devices attract con-siderable attention on A1505 from a technological point ofview, particularly due to their true compatibility with theplanar technology of silicon. Al503 thin films have alsobeen used for heteroepitaxially grown Si-on-insulator(SOl) structures for sensor applications,1'2 such as stopetching layers,5'4 or as protective layers for reactive sput-ter etching.5 Finally, the successful use of A1505 singlecrystals as substrates for high-critical-temperature super-conductor film growth6 envisages a link between semicon-ductor and superconductor technologies.

For these reasons, selective etching of single crystallineA1505 is an important technological issue in many areas.Patterning of Al505 thin films has been reported using areactive ion beam system7 operating with CH5F2, CH3F,and CHF5 gases. The aim of this work is to study an alter-native way for patterning A1505 using techniques compat-ible with those of the planar technology.

As for A1505 chemical etching, H5P06 at high tempera-ture selectively etches defective regions of the crystal, andits use at boiling temperature was reported by Scheupleinand Gibbs5 as an etch-pit technique to identify disloca-tions. H5P04 begins to homogeneously etch sapphire onlyat much higher temperatures (42 5°C) and, indeed, has beenemployed for chemical polishing of the surface.5

In this work, Ar implantation is proposed for creatinga layer of damaged material which is then selectivelyremoved by H5P04 wet etching. This technique wasalready employed to pattern silicon carbide, using Arimplantation followed by chemical etching with a 1:1 mix-ture of HF:HNO5'° The use of an inert element as ionspecies is necessary for setting up a procedure free ofresidual contamination. Thus, the etching chemistry isonly related to the amount of damage introduced in thecrystal and not to the formation of intermediate com-pounds, as in the case of implantation of reactant species.Actually, Ar was chosen because it is a well-known inertgas widely used in ion implantation. Recently, a methodfor etching A1505 using Si ion implantation and bufferedhydrofluoric acid has been reported.11 That proceduretakes advantage of formation of easily etchable Al-Sicompounds, but the authors do not comment about resid-ual contamination of the A1505.

Photolithography was used to pattern on the Al503 sur-face suitable masks preventing damage in selected areas.Micrometric structures with sharp interfaces were definedon the wafer surface using the proposed process.

ExperimentalThe present results refer to sapphire samples cut from

3 in. wafers, (1102) oriented (r-plane), with surface rough-ness around 2 nm. Photoresist 600 nm thick allowed selec-tive damaging of the substrate. Two different patterns wereused, one formed by 50 >< 50 p.m areas separated by 200jim, the other consisting of 50 p.m lines separated by 5 p.m.

All samples were double implanted at room temperaturewith Ar at 50 and 180 key (Table I). While the firstimplantation principally affects the near surface of thewafer and allows the first stages of the chemical reactionto proceed, the second one damages the inner layer. Thedose rate was estimated to be around 1015 ions/cm2 s. Theimplantation angle was 8° from normal to the surface toavoid channeling. Due to shadowing effects, a differencebetween opposite sides of patterned structures is expected,as shown in Fig. 1.

After implantation the resist was removed from the sur-face using boiling acetone followed by plasma etching orchemical etching using H5P04 at room temperature. Thiscleaning procedure was necessary because the ion beamproduces hardening of the photoresist. Finally, the sampleswere wet etched using H5P04 at 200°C.

The height of the resulting step was routinely measuredby means of a mechanical stylus profilometer with 5 nmresolution. Scanning electron microscopy (SEM) with X-ray microanalysis, and atomic force microscopy (AFM)were used to characterize the patterned regions after theetching. The damage profile was measured by means of theRutherford backscattering (RBS) channeling techniqueusing 2 MeV He ions. Transmission electron microscopy(TEM) analyses were performed on cross-sectioned sam-ples using a Philips CM 30 microscope operating at 300 kV.

Results and DiscussionEtching profiles—The etching process causes the

removal of the damaged layer and turns out to be dose

Table I. Dose and energies used to implant the samples with Ar°at room temperature. The measured maximum etchable depth

and the mean etching rate are also given.

50 keV,dose

Sample (ions/cm2)

180 key,dose

(ions/cm2)

Saturationdepth(nm)

Etchingrate

(nm/mm)

° Permaoent address: Departmento de Fisica CNEA, (1429)CF,Argentina.

A 2.0 )< 1011 2.0 x io'B 2.0 x 1011 2.2 x 1016C 2.0 >< 1011 4.0 x 1016

344 J Electrochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc.

90 1150 2.5160 7

J. Elect rochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc. 345

unshadowed shadowed

implantedlayer

Fig. 1. Sketch of the implantation geometry.

dependent. Figure 2 shows a SEM micrograph of the Al,03surface after 40 mm etching for sample B. The sample istilted in order to better evidence the surface features. Thestep produced by etching is clearly visible and appears tobe straight and uniform. The region where implantationwas masked remains unetched (top side of the picture)while the implanted area was etched away, thus forming a150 nm height step. At this final stage of the process, allsamples show the same morphology.

The sample surface was studied by AFM. Figure 3 showsthe effect of the implantation angle on the final structure,resulting in different slopes of the edges at opposite sidesof the pattern. Numerical differentiation of the profilesalso revealed this difference.

Typically the side corresponding to the resist step fac-ing the ion beam (Fig. 3a) appears sharper than the shad-owed one (Fig. 3b). The maximum boundary width (meas-ured between 10 and 90% of the full height) was around300 and 500 nm for the unshadowed and shadowed sides,respectively. For the former, the maximum slope found is40 to 45°. At the end of the whole process, the surfaceroughness at both sides of the step is that of the virgin sub-strate (—2 nm).

Figure 4 displays the etched depth vs. time f'ir three dif-ferent 180 keV implantation doses. Both tI' saturationdepth and the average etching rate increase with increas-ing Ar dose. For the lower dose used the n aximum etch-able depth is around 90 nm, while for a se an order ofmagnitude greater it reaches 150 nm. Fo" '.further twofoldincrease in dose, the corresponding rcchable depth is160 nm, showing a trend toward satir AtjOfl.

Fig. 2. SEM micrograph of the fransferred pattern onto an A1203surface for sample B after 40 mm etching. The arrow indicates thehigher edge of the step.

x (nm)Fig. 3. AFM profile (—) across the (a) unshadowed and (b) shad-

owed interface for sample B after 40 mm etching; numerical deriv-ative analysis of profiles (---) is also shown.

The average etching rate varies from 1 nm/mm for adose of 2 X iO ions/cm2 to 7 nm/mm for 4 X 1016 ions/cm2.The unimplanted areas of A1203 remain unetched at leastfor etching times of the order of 1 h, as shown by the timedependence in the saturated region for all samples. As noevident rounding of the corners appears before the satura-tion level is reached, extremely high selectivity is intro-duced by this method.

Damage profiles.—RBS-channeling analyses indicatethat the damage mainly consists of point defects or amor-

'V

•0•A1203

i1

IN

0 500 1000 1500

160

140

120I100

80

60

40

20

0 40 80 120 160 200etching time (mm)

Fig. 4. Etched depth vs. time for samples A, B, and C.

346 J. Electrochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc.

phous clusters. To further elucidate the relationshipbetween dose and damage induced by Ar implantation, thedefective fraction was calculated from RBS spectra(Fig. 5). The effect of the 50 keV implantation stage, com-mon to all samples, is to produce a defective fraction ofabout 0.7 in the first 30 nm (a defective fraction equal to 1corresponds to a fully damaged or amorphous layer). Asfor the second implantation, increasing doses produce anincrease of the defective fraction in the 50 to 200 nmregion up to a saturation value of about 0.7, thereby f ore-ing the damage profile to expand deeper into the sample.Figure 6 shows a TEM cross-sectional micrograph forsample B. As indicated by selected area diffraction analy-ses, the TEM profile evidences two highly defectiveregions (dark areas). The relative position of the damagedlayers is well correlated with the defective fraction profileof Fig. 5, therefore confirming the RBS results. Both theoverall behavior of the damage profile and the saturationvalue of the accumulated damage are in agreement withpreviously reported results for A1203 implantation with Crat room temperature.'2

The comparison between the damage profiles and theetching vs. time data produces three relevant features ofthe process: (i) the etching becomes effective only when thedefective fraction exceeds 0.4, below which the etchingrate is negligible, at least in the time scale investigated; (ii)the increase in the etchable range is a consequence of thewidening of the damaged profile; and (iii) the etching rateincreases with the amount of damage. Thus, to obtainoptimal etching parameters, both the energy and the doseof the incident beam should be tuned.

Etching mechanisms—During the process of ion implan-tation, collisions with the lattice produce disorder in thecrystal. As a result, the implanted layer is highly defective,including distortion and breaking of the A1203 bonds. Theweakening of the structure allows the selective etching ofthe crystal, which otherwise is very stable against anychemical etchant. It is known that H3P04 is able to evi-dence dislocations and other defects in sapphire,8 and itwas very effective in etching the implanted layer. The fastkinetic of the etching process is probably due to the sameoxidation state displayed by P and Al, permitting theimmediate formation of Al salts. Indeed, SEM microanaly-sis of partially etched samples clearly showed the presenceof Al-P-O compounds located at the border of the im-planted virgin A1203 regions.

1,0

0,8

.—

' 0,6

0,4

0,2

0,00 50 100 150 200

depth (nm)Fig. 5. Calculated defective fraction from RBS spectra as a func-

tion of depth for samples A and B. Arrows mark the saturationdepth. The solid line indicates the threshold value for the etchingprocess.

Some relevant features of the etching process come intoview from the study of the boundary between implantedand unimplanted areas. Surface rougimess measurementsevidence the swelling produced by the Ar implantation,resulting in a step of about 30 nm for 4 X 1016 Ar7cm2.Assuming a uniform ion distribution in the implantedlayer, a volume expansion of about 15% can be estimated.Therefore, the etching process is expected to be positivelyaffected by the stress profile located at the interface. Onthe other hand, symmetry considerations suggest that thelattice rearrangement during the etching is less favored atthe interface, where the atoms on the virgin side arealready in a stable configuration. Consequently, the avail-able number of distorted or broken bonds at the boundaryis higher than in the bulk of the implanted region, thusenhancing the etching rate at the border. Therefore, bothphysical and chemical reasons call for an enhanced etch-ing rate at the implanted virgin interface. This effect wasevidenced by AFM profiles of samples at early stages ofthe process, where an increase in the etching rate up to afactor of 3 for the 4 x 1016 Ar dose was measured. Thespatial extension of this zone is about 5 p.m. As the processcontinues, the saturation depth is reached in the remain-ing damaged region.

The enhanced etching effects are particularly evidentwhen patterned lines are close enough to allow fast etch-ing regions to overlap (i.e., closer than 10 p.m, as in thecase of the 5 p.m-implanted lines). Figure 7 shows a tape ofdamaged material abruptly detached from the substrate,after 5 mm etching. In this case, the measured etchedrange close to the interface is about 120 nm, and 40 nm inthe central part of the removed zone. The typical longitu-dinal extension of the tapes is about 100 p.m. Actually,tape detachment could be attributed to stress release andlocalized fast etching rate.

ConclusionsIn this work, we have shown that it is possible to intro-

duce high selectivity in (102) A1203 using conventionaltechniques like ion implantation and standard photolith-ography followed by wet etching with H3P04 at 200°C.The proposed technique can be easily employed in Al203technology where micromachining is required for devicefabrication.

This method is truly contamination-free, as only Ar wasused, although it is not a room temperature process. It

Fig. 6. TEM micrograph cross section for sample B showing twolayers of homogeneously damaged material (dark areas) corre-sponding to the first and second implantation stages. The arrowindicates the sample surface.

I • I • I

DA• •B..• •.• •••.ri U .

nOn UDu 0 •

tODDU.

I

J. Electrochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc. 347

offers the advantage of being easy to implement and doesnot affect the original roughness of the substrate. Whencompared with a reactive ion beam etching process, themain drawback is its low etching rate; however, it offersthe possibility of producing structures oriented at an angleto the surface.

The procedure has been demonstrated to be suitable forpatterning well-defined micrometric structures. Choosingappropriate etching parameters, sharp edges and slopes of40 to 45° can be obtained. As selectivity and etching rateare strictly related to the amount of damage introduced,the full capabilities of this method could be investigated byemploying different implantation conditions (ion species,multiple energies, doses, implantation angle, and substratetemperature) to achieve the desired pattern definition.

AcknowledgmentsThe authors acknowledge F Biscarini for his assistance

with the AFM measurements, F Corticelli for the SEManalyses, A. Migliori for TEM observations, E. Gabilli andR.Lotti for the ion implantation, L. Buldini and S.Nicoletti for helpful discussions on the process, and M.Cervera for help during the RBS analyses. One of us (FL.)was partially supported by CONICE Argentina, duringhis stay at Lamel.

Manuscript submitted March 31, 1997; revised manu-script received September 15, 1997.

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Toward a Unified Reaction Mechanism for Chemical Vapor

Deposition of CopperNarendra S. Borgharkar and Gregory 1. Griffin

Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803-73 03, USA

ABSTRACT

We report the reaction kinetics of copper chemical vapor deposition using 112 reduction of Cu(hfac)2 H(hfac) =1,1,1,5,5,5-hexafluoro-2,4-pentanedione]. We observe a growth rate of 0.48 0.07 mg cm2 h' (i.e., 9 nm min) at refer-ence conditions of 300°C substrate temperature, 40 Ton H2 partial pressure, and 1.8 Torr Cu(hfac).,. The dependence ofgrowth rate on operating conditions can be described using a Langmuir-Hinshelwood rate expression with a numeratorthat is first-order in Cu(hfac)9 and half-order in 112, and a third-order denominator that includes adsorption terms forCu(hfac)2, H2, and H(hfac). Optimized values for the surface reaction rate constant and the reactant adsorption coefficientsare determined using nonlinear regression. On the basis of the optimized rate expression, we propose a minimum-para-meter reaction mechanism for Cu(hfac)2 reduction. The proposed rate limiting step is the surface reaction betweenadsorbed H and the first dissociated (hfac) ligand from adsorbed Cu(hfac)2, leading to the desorption of H(hfac) product.The proposed mechanism also includes the major elementary steps needed to describe the competing disproportionationreaction. In particular, the rate constant for recombinative desorption of Cu(hfac)2 obtained in the present work is remark-ably consistent with previous kinetic results for Cu(hfac)(2-butyne) disproportionation.

InfroductionChemical vapor deposition (CVD) of copper thin films is

being intensively studied as an alternative fabricationtechnique for interconnect metallization in future genera-tions of deep submicron integrated circuits.'3 Copper offersseveral intrinsic property advantages over aluminum, thecurrent material of choice for metallization; these includelower resistivity, improved electromigration resistance, and

* Electrochemical Society Active Member.

increased resistance to stress-induced voidage. The CVDtechnique has traditionally offered several advantages overcompeting physical deposition methods, including the pro-mise of improved film conformality over high-aspect ratiofeatures and the possibility of performing selective areadeposition.

Two reaction chemistries have emerged for performingcopper CVD. The highest deposition rates to date havebeen achieved using the disproportionation of various CuWmono-13-diketonates, notably Cu(hfac)(VTMS)1

Fig. 7. SEM micrograph ot a m width A1203 tape partiallydetached from the surface (sample C). Etching time, 5 mm. SmallAl-P.o particles are visible at the edges.