Eur. Phys. J. D 60, 653–660 (2010)DOI: 10.1140/epjd/e2010-00245-x

Regular Article

THE EUROPEANPHYSICAL JOURNAL D

Characterization of an atmospheric pressure plasma jetfor surface modification and thin film deposition

S. Bornholdt, M. Wolter, and H. Kerstena

Institute of Experimental and Applied Physics, Christian-Albrechts-Universitat zu Kiel, 24098 Kiel, Germany

Received 12 May 2010 / Received in final form 29 July 2010Published online 28 September 2010 – c© EDP Sciences, Societa Italiana di Fisica, Springer-Verlag 2010

Abstract. In this paper, an experimental study is presented to characterize a commercially available atmo-spheric pressure plasma jet (APPJ) kINPen which can be used for local surface modification, e.g. changingthe wettability as well as for thin film deposition with silicon-organic and metal-organic precursors toenhance scratch resistance or to lower the gas permeability. Characterization of the jet discharge has beencarried out by three methods: (i) measurement of the energy influx from the jet plasma to a substrate by acalorimetric probe, (ii) spatial resolved investigation of the plasma beam by optical emission spectroscopy(OES) and (iii) observation of the plasma jet by video imaging. The deposited SiOx and AlOx films wereanalyzed by XPS measurements.

1 Introduction

Atmospheric pressure plasma jets have been established assuitable sources of low-temperature and non-equilibriumatmospheric pressure plasmas [1–3]. The main distinctivefeature of this kind of plasma tools is that the jet is notconfined by electrodes and its dimension can be adjustedin a wide range allowing local treatment of 3D surfaces,e.g. the inner walls of wells, trenches or cavities [2]. Amongother applications, the treatment of temperature-sensitivesurfaces such as polymers or biomedical tissues is of inter-est. For example, Kuchenbecker et al. [4] and Rajasekaranet al. [5] used a dielectric barrier discharge for medical ap-plication on human body, whereas Bibinov et al. [6] useda dc atmospheric pressure plasma jet. Mostly nitrogen,oxygen or a mixture of these gases are used to operatethe discharge. The authors characterized the discharge bymeasurement and simulation of the spatial distribution ofnitrogen and oxide gas density. The diagnostics are mainlybased on UV radiation and emission spectroscopy of ni-trogen or the flux of ozone.

In this paper a low-temperature, rf-powered (1.7 MHz,2.5–3.5 kVpp) capillary APPJ (kINPen) is studied, whichis operated with argon in air at a constant argon gas flow of5 L/min. Related experiments and previous measurementshave been published elsewhere [7].

The investigations are focused on the energy influxfrom the APPJ to a substrate, e.g. glass and metal. Ofspecial interest in respect to the energy influx is the contri-bution of charge carriers which interact with the surface.The final surface temperature TS of treated substratesduring plasma processing effects elementary surface pro-cesses like adsorption, desorption, and diffusion as well as

a e-mail: kersten@physik.uni-kiel.de

chemical surface reactions [8–11]. On the other hand, es-pecially in the case of thin film deposition, the structureand morphology as well as the stoichiometry of the filmdepend strongly on the energetic conditions at the sur-face [8,12–14]. For example, surface diffusion of adsorbedatoms can be enhanced, which results in a rearrangementof deposited atoms [15,16]. A bombardment of growingfilm with low-energy ions from the surrounding plasmaresults in a modification of its properties, too [17].

Since the surface temperature is essentially influencedby the energy fluxes resulting from energetic particle bom-bardment, chemical surface reactions and plasma radia-tion [18,19], the different contributions to substrate heat-ing are separated and studied independently by a suitablevariation of the experimental conditions [20].

The experimental determination of the energy influxin atmospheric pressure plasma processes, especially forAPPJ operation is only rarely done, because measuringthe energy influx at atmospheric pressure is challenging.In contrast to low pressure plasmas (LPP) atmosphericpressure plasmas (APP) are often more inhomogeneous,e.g. they form tiny, filamentary discharge channels or havea small size like micro hollow cathodes [21,22]. In the at-mospheric environment the mean free path and the kineticenergy of the plasma species (electrons, ions, radicals)are different from LPP. From calorimetric probe measure-ments it is possible to obtain information on the energyinflux as well as on the real temperature evolution on thesurface of the substrate. This is a crucial point for thetreatment of sensitive biomedical material and of special,temperature-sensitive synthetic materials (e.g. polyethy-lene, polymethyl methacrylate).

In addition to the energy influx optical emission spec-troscopy (OES) in the spectral range between 280 and1100 nm is performed. Spectra of the APPJ plasma

654 The European Physical Journal D

Fig. 1. The operating scheme (top) and a photograph (bot-tom) of the used plasma jet “kINPen 08”.

radiation (e.g. argon and nitrogen lines) were measuredin different axial distances from the plasma jet orifice. Bythe OES it is possible to get a spatially resolved map forthe evolution of the gas mixtures in front of the plasma jet.Besides OES the plasma jet has been observed by videoimaging. From high speed video imaging one obtains in-formation on the time-resolved behavior of the plasma jetin front of different substrate materials, especially on theformation of filamentary discharge channels. Observationby video imaging is a rather new technique for the char-acterization of the APPJ kINPen providing data on thelight intensity and inhomogeneities in the jet.

The surfaces of the modified substrates are studiedby X-ray photoelectron spectroscopy (XPS) in order toproof the deposition by using a silicon-organic precursor(HMDSO) or an aluminum-organic precursor (ATI), re-spectively, in the process plasma.

2 Experimental method

2.1 Plasma jet

For the experiments presented in this paper a miniaturizedcapillary APPJ was used. This plasma jet “kINPen 08”isa commercially available system, distributed by the INPGreifswald [23].

The general working principle of the APPJ is shownschematically in Figure 1, which shows a photograph ofthe plasma jet, too.

The rf-power supply (1.7 MHz) is connected to thecenter rod electrode by a matching network. The systempower is 65 W (250 V) and the peak to peak voltage isbetween 2.5 and 3.5 kV. The electrode is surrounded by aceramic cap which forms the gas stream and a ring elec-trode which is connected to ground. The ring electrodestabilizes the operation of the jet. The gas pressure is con-stant at 1.5 bar and the resulting gas flow through the jet

is constant 5 L/min. The working gas inside the jet is ar-gon. Other process gases (precursors) are supplied to theplasma beam outside the nozzle through a small gas inletby forming an own precursor atmosphere. The length ofthe plasma jet depends on the rf power and is between1 and 14 mm. The diameter of the plasma jet is about1 mm. The gas temperature has been measured to be inthe range from 30 to 95 ◦C [24].

2.2 Calorimetric probe measurement

The integral energy flux from the plasma towards thesubstrate has been measured by a compact calorimetricprobe [19,25].

The probe is mounted on a manipulator arm to allowhorizontal and vertical motions in front of the plasma jet.In these experiments, the energy flux measurements arecarried out by observing the rate of temperature changedTS/dt of the probe which is brazed to a thermocouple(type K) and placed within a solid shield. The probe is athin metal plate which serves as a kind of substrate. Theprobe is connected only by the thermocouple and a wirefor additional biasing. Due to its large heat capacity theshield is at a constant environmental temperature Tenv

during the measurement. The technical reliability of thiscalorimetric probe setup was successfully demonstrated [7,20,26]. The measurement of the total energy flux Qin isbased on the determination of the difference between thetime derivatives of the substrate temperature Ts duringheating (which means the plasma-on phase) and cooling(plasma-off) multiplied with the heat capacity of the probehas to be determined by calibration [27,28].

The schematic setup for the different experiment se-ries is shown in Figure 2. For the thin film deposition thecalorimetric probe was replaced by a “real” substrate.

In general, the power balance at the substrate is givenby [28,29]:

Qin = HS + Qout (1)

where Qin marks the energy gain by the several energeticcontributions from the plasma, HS = mc(dTS/dt) denotesthe temporal rate of enthalpy change of the substrate andQout summarizes the heat losses by radiation and thermalconduction by the gas and the substrate. Due to the con-struction of the calorimetric probe (shielding) the lossesby radiation are always small in comparison with the in-coming fluxes due to the plasma process. HS is deter-mined by Qin −Qout. During the heating phase (Qin > 0)HS(heat) is given by Qin − Qout and during the coolingphase (Qin = 0) it is HS(cool) = −Qout. Inserting theseexpressions into equation (1) yields:

Qin = HS(heat) − HS(cool) (2)

= mc

[(dTS

dt

)heat

−(

dTS

dt

)cool

]Tenv

. (3)

Assuming no change of the environmental temperatureTenv, which is achieved by short measurement times and

S. Bornholdt et al.: Characterization of an APPJ for surface modification and thin film deposition 655

Fig. 2. Schema of the experimental setup for the calorimet-ric probe measurements (a) and the optical emission spec-troscopy (b). The calorimetric probe as well as the optical fibrefor OES could be moved in axial direction. For observation byvideo imaging (c) the camera was mounted side-on. For the de-position experiments the precursor was mixed into the plasmajet by using a small gas box surrounding the jet (d).

the large heat capacity of the shield, the expression withinthe brackets of equation (3) is a quantity proportional tothe thermal power at the substrate, if the slopes dTS/dtare determined at the same environmental temperatureTenv. In order to obtain absolute values of Qin the specificheat capacity of the calorimetric probe has been deter-mined by a known thermal power, e.g. diode laser radia-tion. For the used calorimetric probe (copper) we obtaineda value for CS = mc of 0.16 J/K.

2.3 Optical emission spectroscopy

For the purpose of the optical diagnostic of the plasmajet a commercial spectrometer (HVR 2000+) was used.The emitted light from the plasma jet is collected by anoptical fibre. This apparatus allows to study emission linesin the range between 280 and 1100 nm. The sensitivityis 75 photons/count at 400 nm and the optical resolutionis 1 nm FWHM. The used spectrometer and the analysissoftware is a commercial equipment by Ocean Optics. Thefiber optics is mounted side-on by a movable holder. Bythis construction it is possible to measure the emissionlines at different vertical and horizontal distances fromthe jet, see Figure 2b. Only relative intensities of relatedlines have been measured in order to compare the axialdistribution of the several gas species.

2.4 Video imaging

In order to observe the behavior of the plasma jet in frontof substrates (metal plate, insulating glass) a high speedcamera (PixeLINK) which was mounted side-on has beenused (Fig. 2c). The frame rate was 1000 frames per seconds

(fps) and the exposure time was 0.5 ms. The observed areawas a field of 72 × 96 pixel, e.g. 72 pixels in vertical (ax-ial) direction and 96 pixels in radial (horizontal) direction.The calibration of this field of view centered around thejet results in an observation window of 5.14 × 6.86 mm.This area is large enough to monitor the plasma jet out-side the nozzle and in front of an insulating (glass plate) orconductive (copper plate) material, respectively. By thisway it is possible to study the formation and expansionof filaments in front of the surface material. The recordedvideo file was cut into single frames and analyzed with aMatLab c© program to observe the motion of the plasmajet and the processed surface in detail.

2.5 Sample preparation and XPS analysis

The precursors (HMDSO and ATI) were heated in a wa-ter or oil bath at 63 ◦C and 155 ◦C, respectively, and thenfed via an argon flow into the cap, which was mountedon top of the jet-system (see Fig. 2d). The chemical com-position in case of thin film deposition (SiOx, AlOx) hasbeen studied by XPS [30]. For the XPS measurements thesubstrates were cut into square pieces with a edge lengthof 1 by 1 cm. After plasma deposition the probes wereanalyzed with a XPS spectrometer which used a Al-K-X-ray source (1486.6 eV). The spectrometer energy scalewas calibrated in respect to the C1S (C–C, C–H) compo-nents set at 285 eV. For the analysis of the XPS spectrathe CASA XSP software [31] was employed.

3 Results and discussion

3.1 Calorimetric probe

The energy influx from the plasma jet to a substrate hasbeen measured by the calorimetric probe.

The integral energy influx Qin obtained by the proce-dure described above has been measured at different axialdistances from the nozzle, see Figure 3 (top).

The measurements of the total energy influx startedat a distance of 25 mm from the nozzle. In this rather fardistance the plume of the plasma jet could not been de-tected by naked eyes and an energy influx was just able tomeasure. About 14 mm from the orifice the visible plasmajet ends (see Fig. 1). Up to this position the total energyinflux is in the order of about 20 mW/cm2. When thecalorimetric probe was moved closer to the nozzle alongthe plasma jet the total energy influx slowly increases andat a distance of 11 mm the mean value of the energy influxis ca. 50 mW/cm2.

In order to asses and to measure the effect of freecharge carriers (electrons, ions) which may exist in theplasma jet plume and which contribute to the energy bal-ance the calorimetric probe has been biased (70 V, 0 V,−70 V). By probe biasing either the electrons (+70 V) areaccelerated and the ions are repelled or vice versa (−70 V).We assume that in a rather long distance from the noz-zle only very few free electrons and ions exist. However,

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Fig. 3. Total energy influx (top) and probe current (bottom)in dependence on the distance from the nozzle along the plasmajet for three different bias voltages of the probe.

if the calorimetric probe is positively or negatively biasedeither the few electrons or ions are repelled. The trans-ferred kinetic energy is rather low since the charge carriersare located due to the high pressure and the cannot gainmuch kinetic energy as in low pressure plasmas. In caseof grounded probe (0 V) both types of charge carriers canreach the probe with higher probability and they can re-combine at the copper surface of the calorimetric probe.This recombination might be the reason for the constantlyhigher energy influx for 0 V than for biased operation atdistances larger than 12 mm.

Depending on the bias voltage of the probe a fast in-crease of the energy flux starts between 10 and 12 mm infront of the nozzle. Again, three mm closer to the noz-zle the energy influx reaches a maximum for all bias volt-ages. For the negative voltage (−70 V) the highest value is250 mW/cm2, for the positive voltage (+70 V) this max-imum is about 210 mW/cm2 and for the grounded probethe maximal energy influx is 230 mW/cm2. When thecalorimetric probe comes still closer to the nozzle the en-ergy influx is almost constant. At a distance less than6 mm it was not possible to perform any measurements.The reason for that was the strong interaction betweenthe plasma and the probe. Many filaments are generatedand reasonable measurements are not possible. Obviously,in distances smaller than 12 mm from the nozzle there is a

higher density of charge carriers which cause the remark-able increase in heating the calorimetric probe.

In addition to the energy influx the current onto thecalorimetric probe was simultaneously measured, too. InFigure 3 (bottom) the measured electrical currents areplotted. If the probe is far away from the plasma jet (e.g.25 mm from the nozzle) no current is detectable. By mov-ing the probe along the direction of the jet there is almostno current detectable up to a distance of about 13 mm.This observation corresponds with the results for the en-ergy influx. When the probe moves closer to the nozzlea different behavior is observed. The grounded and neg-atively biased probe show an increase of the current upto 2 mA. The behavior of the probe current for a posi-tively biased calorimetric probe is similar to the negativelybiased probe, but the maximum value now is −2 mA. Un-fortunately, we had only the possibility to put a bias volt-age of ±70 V. The fact that the positive deviation is asbig as the negative could be an indication that the ionsaturation current is not attained.

When the bias voltage of the calorimetric probe ismore positive than the floating potential of the surround-ing plasma more electrons are collected and the resultingcurrent to the probe is negative. On the opposite, if thebias voltage is more negative than the floating potentialthe current to the probe is positive due to the collectionof ions. In general, the current to the calorimetric probeis very low in comparison to experiments in low pressureplasmas which deliver an energy influx in the same orderof magnitude as for the APPJ [20]. One reason is that theplasma jet is operated at atmospheric pressure. Therefore,we assume that the most part of the measured energy in-flux stems from the UV-radiation of the plasma jet. Due tothe short mean free path the contribution of the chargedplasma species do not play an important role. We esti-mated this part to be less than 10% of the total energyinflux. If the charged species would play a dominant rolefor the energy influx, a dramatic increase for higher nega-tive bias voltages should be observed due to positive ions.But this is obviously not the case. Also we have no indi-cation of extraordinary high ion density from the spectralobservations: no ArII (Ar+) lines are detected.

The investigations are further experimental proof thatobviously free charge carriers can exist in the plasma jetat a certain distance from the nozzle even at atmosphericpressure. In distances larger than 12 mm no electrons orions, respectively, could be detected. The energy influxwhich still can be measured at this long distance andwhich increases slowly by approaching the nozzle is mainlydue to radiation. The influence of free charge carriers inthe jet can be slightly observed by the difference in thestrong increase of Qin at about 11 mm (Fig. 3 top). Justin this region an electric current is detectable. Unfortu-nately, it cannot be distinguished if the charge carriers arecoming from the plasma inside the nozzle or if they areproduced by collisions or photo-ionization in this regionof the jet.

Spectroscopic measurements by Foest et al. [2] yieldenergy influxes due to VUV/UV radiation between 15 and

S. Bornholdt et al.: Characterization of an APPJ for surface modification and thin film deposition 657

0 2 4 6 8 10 12 14 16101

102

103

104

distance from nozzle [mm]

inte

nsity

[arb

. uni

ts]

Ar (763 nm)O (777 nm)OH (308 nm)N2 (337 nm)

Fig. 4. Relative intensities of selected species, e.g. Ar, O, OHand N2 in dependence on the distance between the spectrom-eter and the nozzle of the plasma jet.

50 mW/cm2 for a comparable plasma jet. For wave-lengths between 115 and 200 nm a spectral radiance upto 880 µW/mm2 sr was reported. With these results wecalculated an energy influx by the VUV/UV radiation tothe calorimetric probe of 17 mW for the used probe areaof 0.19 cm2. The comparison between the values by Foestand our measurements shows a good agreement.

The maximum temperature which has been obtainedat the calorimetric probe under our experimental condi-tions was 55 ◦C. The knowledge of the maximum tem-perature is very important for applications of the plasmajet, e.g. modification of temperature sensitive surfaces orbiomedical materials.

3.2 Optical emission spectroscopy (OES)

A lot of effort has already been made in spectral charac-terization of the discharge [2,7]. The most intensive argonlines can be found in the spectral region between 670 and970 nm. Few argon lines with a much lower intensity inthe spectra can be found at 415 and 420 nm. In additionto the argon lines spectral lines from nitrogen, oxygen,hydrogen and hydroxyl groups can be identified, e.g. theHα line at 656.28 nm or the hydroxyl group at 308 nm, re-spectively. These lines are resulting from dissociation andionization (due to the plasma jet) of the surrounding at-mosphere which contains nitrogen, oxygen and water. Thereference lines (or wavenumbers) are taken from the NISTdatabase [32].

As expected the intensity of the spectral lines variesalong the plasma jet. In Figure 4 the most intensive linesfor argon, nitrogen, oxygen and the hydroxyl group in de-pendence on the distance between the spectrometer andthe plasma jet nozzle are shown.

The Hα line is not plotted in Figure 4 because of itsmarginal intensity in front of the plasma jet orifice. At alarger distance this line cannot be identified clearly.

A detailed description of the profiles and the con-volution procedure may be found in [33,34]. In general,two tendencies can be observed. The maximum of ar-gon, oxygen and OH lines is immediately at the nozzleof the plasma jet. With increasing distance these sig-nals decrease. The signal of O and OH lines vanishes atca. 11 mm. In comparison with visual observation (Fig. 1)also a change in the light of the plasma jet is observableat that position. The beam becomes smaller and the colorchanges from white to light blue.

The nitrogen lines show a quite different behavior.At the nozzle the nitrogen intensity is in the order of40 counts ±20. The signal increases with increasing dis-tance and reaches a maximum of ca. 1000 counts at about9 mm from the nozzle. At this point the signal also de-creases as well as any other intensities. The detectableintensity for argon and nitrogen lines ends at a distanceof about 14 mm. We assume that the excited (metastable)argon atoms and the UV-radiation interact with the mole-cules of the surrounding air. The excitation and dissoci-ation of oxygen and water molecules is due to the UV-radiation similar to the formation of the ozonosphere onearth [35]. In contrast, the nitrogen molecules are mainlyexcited due to collisions with the metastable argon atoms.Hence, the nitrogen molecules have to diffuse into theplasma jet from the surrounding air. Because of thesedifferent mechanisms the different evolution of the axialline intensities can be explained. The analysis of the ar-gon line intensity confirmed the measurements by Foestet al. [2] where the formation of an argon channel is de-scribed which hardly interacts with the environment.

The behavior of excitation and interaction of the am-bient nitrogen with the Ar metastables has to be consid-ered for the injection of other molecular gases/precursorsinto the plasma jet regarding technological application,e.g. thin film deposition. Obviously, it is possible to injectsuch species into the jet at a certain distance from thenozzle to prevent the electrode from erosion and keep thecapillary free from film growth, which may happen by adirect injection together with the working gas. This factcan enhance the life time and reduces maintenance costs,which is desired for industrial use.

3.3 Video imaging

During the visual observation of the plasma jet it was fig-ured out that the beam looks quite homogeneously whenthe free jet does not interact with any surfaces. In contrast,when the plasma jet interacts with a surface a broaden-ing of the plasma plume was observed. Figure 5 illustratessuch behavior of the plasma jet. In case of a free jet (leftphotograph) a small plasma needle is formed. In case ofsubstrate material in front of the plasma jet (right photo-graph) it grows wider and the formation of filaments canbe observed. Note that the photos taken with a commonphoto camera.

For better visibility the region of the interaction be-tween the plasma and the surface has been observed with

658 The European Physical Journal D

Fig. 5. Comparison of the behavior of the plasma jet withand without a substrate in front of the plasma. On the leftphotograph the “free” plasma jet is shown. The interactionwith a metal plate is shown on the right hand side of thisphotograph.

a high speed camera. The left series in Figure 6 showsthe time evolution of the plasma beam in front of a metal(copper) plate. The formation of some discharge channels(filaments) can be clearly observed. In the right series inFigure 6 the behavior of the plasma jet in front of an insu-lating material (glass) is shown. Here we can find a ratherquiet discharge without visible filaments. It looks like theundisturbed free plasma jet.

For deeper understanding of the plasma discharge theimages from the movies were analyzed by a MatLab c© pro-gram and plotted in color code diagrams. To get an im-pression of the temporal evolution the single photographsare combined to a series of e.g. 60 frames, respectively60 ms. The intensity distribution at every millisecond hasbeen recorded along the line indicated in Figure 6. Bythis procedure the differences in the time-dependent be-havior of the plasma jet in front of the surfaces can beidentified very easily. In Figure 7a the plasma jet inter-acts with a glass surface. The relative light intensity ofthe jet is about 40% (normalized to the maximum lightintensity for the metal plate). Therefore, the photographis not so bright than for the case in front of a metal plate(Fig. 7b). The light intensity is mainly distributed between25 and 55 pixels, respectively between 1.8 and 3.9 mm inhorizontal plane. This region corresponds with the regionwhere the plasma beam is located (center of the jet is at40 pixels). In contrast, in front of the copper plate (seeFig. 7b) the maximum relative intensity is 100% (normal-ized). The position of the maximum peaks is more ran-domly distributed in time.

The intensity fluctuations and position of the plasmajet in front of the copper plate are justified by the forma-tion of small filaments (discharge channels) which “jump”

Fig. 6. Temporal evolution of the plasma jet in front of aconductor (copper plate) on the left and in front of an insulator(glass wall) on the right. The substrate is always located atthe bottom of the photographs. The dashed line indicates theposition of the measured light intensities evaluated in Figure 7.

in a larger horizontal plane, e.g. between 5 and 80 pixels.The formation of filaments is a common feature in opera-tion of microplasmas [36].

3.4 Thin film deposition in precursor-containingatmosphere

The APPJ has been designed for surface treatment, e.g.for change of wettability etc. An interesting feature is theuse of the APPJ for thin film deposition of slica or alu-mina layers, respectively, in order to protect polymer sub-strates. For this purpose, silicon-containing (HMDSO) oraluminum-containing (ATI) precursors have been addedto the jet plasma.

During the plasma process the HMDSO molecules dis-sociate into C5H15OSi+2 radicals and CH3 groups [3]. Theradicals can be deposited onto the substrate surface andform thin films. The properties of the films (hardness,transparency etc.) depend on the stoichiometry, e.g. onthe composition and the percentage of SiOx. During thefilm deposition hydrocarbon groups and carbon atoms arealso incorporated into the films. The reason for this is thatthe precursor is commonly only partly dissociated. Withadditional oxygen the ratio between silicon and oxygenin the SiOx film can be varied from polymer-like (1:1)

S. Bornholdt et al.: Characterization of an APPJ for surface modification and thin film deposition 659

Fig. 7. Relative distribution of light intensities of the plasmajet in horizontal plane in front of an insulator (glass) (a) andin front of a conductor (copper) (b). The dashed line indicatesthe photograph at 10 ms, as indicated in Figure 6.

to quartz-like (1:2) structures. The XPS-spectra for thecoated and uncoated PMMA substrates in Ar/HMDSOplasma are plotted in Figure 8. The diameter of the ringof highest deposition rate corresponds to the capillary andjet diameter.

The XPS spectra of the deposited sample show typi-cal peaks of Si2s (154.5 eV), Si2p (103.5 eV) states and anincrease of the oxygen (533.5 eV) peaks which are clearindications for SiOx deposition. The static “footprint” ofthe deposited film has a crater-like shape, see inset in Fig-ure 8. This structure is due to the operation of the jet: inthe centre the layer thickness is smaller compared to theedge of the jet. This observation, which has also been re-ported in literature [37], is caused by transport phenomenaat the interaction zone between jet and substrate surface.

Furthermore, the APPJ has been operated in ATI-containing atmosphere to deposit AlOx thin films. Sim-ilar to the HMDSO-containing plasma the ATI moleculesare fragmented by dissociation of methyl groups and thestronger Al–O bindings are preserved, resulting in the for-mation of alumina films. In order to proof the mechanism,we deposited the films onto silicon wafers. After examina-tion by XPS not only the Si peaks of the substrate butalso the Al peaks of the deposited layers can be clearly rec-ognized (Fig. 9). To our knowledge, this experiment has

Fig. 8. XPS overview spectra for untreated (grey line) PMMAsample and for the sample coated by the plasma jet inAr/HMDSO atmosphere (black line). The inset shows a typical“footprint” of the deposited film.

Fig. 9. XPS overview spectra of an alumina layer depositedby the APPJ onto a glass substrate in an ATI-containing at-mosphere. The Al peaks are clearly visible.

been demonstrated for the first time the possibility of alu-mina layer deposition by the atmospheric pressure plasmajet “kINPen”. A local deposition of thin AlOx films un-der atmospheric conditions might be highly desirable forcorrosion protection.

4 Summary

The atmospheric pressure plasma jet (APPJ, kINPen) hasbeen characterized by different types of diagnostics.

By using the calorimetric probe we were able to mea-sure the energy influx by the plasma jet at different dis-tances between nozzle and substrate surface. With thisknowledge the optimal position for surface modification ofdifferent materials can be determined. It was found thatthe VUV/UV radiation from the plasma jet provides themain contribution of the energy influx to the substrate

660 The European Physical Journal D

surface. In addition we can estimate the real surfacetemperate which plays an important role for activation oftemperature sensitive surfaces or organic/biomedical ma-terials. By comparing the determined energy influx withthe measured electric probe currents the influence of freecharged carriers in the plasma jet could be demonstrated.

By optical emission spectroscopy the composition ofthe plasma jet could be identified. It was shown that inthe plasma not only the inert gas (argon) is included. Inaddition, some species from the ambientatmosphere alsointeracts with the jet. This is an important fact for surfacemodification or injection of precursors into the plasma forsurface treatment.

With the method of video imaging it was possible to in-vestigate the behavior of the plasma jet in front of differentsubstrate materials. If the substrate is a non-conductivematerial, interaction of the plasma with the substrate sur-face is rather homogeneous. In case of a conductive sur-face many filaments and randomly distributed spots onthe substrate surfaces are formed. Under these conditionsan inhomogeneous surface modification takes place.

Successful deposition of thin SiOx films by using anadditional precursor HMDSO as well as the deposition ofthin ALOx layers by using ATI as precursor in the jetcould be proofed by XPS measurement.

The authors wish to thank the Leibniz-Institut fur Plas-maforschung und Technologie e.V. (INP) and neoplas GmbH -especially K.D. Weltmann and M. Hackel - for their encourag-ing support.

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