Process control of RF plasma assisted surface cleaning

Thin Solid Films 283 (1996) 158-164

ELSEVIER

Process control of RF plasma assisted surface cleaning H. S t e f f e n *, J. S c h w a r z , H. K e r s t e n , J.F. B e h n k e , C . E g g s E.-M.-Arndt-University, Department of Physics, Domstr. lOa, 17487 Greifswald, Germany

Received 4 August 1995;accepted 4 January 1996

Abstract Aluminum plates contaminated with hydrocarbon containing compounds (lubricants) were treated in an argon, oxygen and hydrogen plasma generated by a capacitively coupled 13.56 MHz rf discharge. Power and gas pressure have been varied. During the process the plasma was monitored by optical emission spectroscopy. The spectral line intensities of several species have been measured as a function of the process duration and of the macroscopic parameters. Typical time constants for the cleaning process have been obtained. In order to determine thicknesses and optical properties of the contamination, spectroscopic ellipsometry was used whereas the removal of the contamination layers was observed by means of in situ kinetic ellipsometry. The determined cleaning rates per discharge power are 0.1 nm/Ws for 02 and about 0.01 nm/Ws for Ar and H2 which emphasizes the efficiency of oxygen plasma treatment. Keywords: Aluminium;EllipsomeWy;Glow discharge; Plasma processingand deposition

1. Introduction Plasma cleaning belongs to the most important applications of non-isothermal plasmas in dry surface processing, which has been developed primarily for the surface preparation of large vacuum vessels used by accelerators and fusion devices, as well as for optical and microelectronics industry, as the first step to thin film deposition [ 1-4]. In order to obtain good adhesion between a coating and a metal it is often necessary to remove surface contaminations [5-7]. Conceming plasma cleaning of large metal surfaces interest is turned to more efficient and environmentally compatible processes such as priming or varnishing. If integrated into a production line cleaning and activation by plasma can be an effective pretreatment for painting. This procedure may substitute commonly used precleaning processes which are often dangerous. For an optimization of the plasma cleaning process a better understanding of the involved mechanisms requires the knowledge of the correlation between plasma, surface, and technological parameters. The cleaning process is a combination of chemical reactions of the surface contaminants with radicals formed in the discharge volume as well as at the surface and sputtering effects by ion bombardment [3,8,10,11 ]. The efficiency depends on the gas mixture (Ar, 02, He), the properties of the surface which has to be cleaned, *Correspondingauthor. 0040-60901961515.00 © 1996 Elsevier ScienceS.A. All rights reserved Pi!$0040.6090(96)085 35-5

and the excitation mode (DC, RF, ECR). Process control and end point detection are important tools for monitoring the plasma treatment. They have been extensively employed in plasma etching and deposition [ 12-14]. In previous investigations aluminum plates have been treated by low pressure oxygen dc discharges which are shown to be very effective in removing carbonaceous contaminations [ 10]. In the present study the removal of lubricants (oil, grease) from the aluminum surfaces by a capacitively coupled low pressure rfdischarge under different experimental conditions has been investigated. Several analytical methods can be used for obtaining information on the cleaning mechanisms: optical emission spectroscopy (OES) of excited species (atoms, molecules) and kinetic as well as spectroscopic ellipsometry. As criteria for end point detection the constant average emission intensity of typical spectral lines and bands has been taken. Conventional in situ surface analysis is often incompatible with plasma activated methods. In contrast, when coupled to a plasma chamber, ellipsometry is a nondestructive technique which does not disturb the surface processes. By ellipsemetrical measurements film thickness of the contaminating layers and etching rates could be determined in situ very precisely.

2. Experimental The measurements have been performed in a plasmacleaning reactor (PCR) of aluminum (diameter: 280 ram, height:

H. Steffen et al. / Thin Solid Films 283 (1996) 158-164

200 mm) with windows for OES, ellipsometry, and substrate handling. The samples which had a surface area of 80 × 150 mm or 50 X 50 mm for ellipsometric investigation, respectively, were mounted on the powered electrode. The gas flow input was realized by a lot of tiny nozzles right above the powered electrode (substrate). The substrate holder consists of a large solid aluminum block with a high heat capacity for an effective cooling of the samples during plasma treatment. The block was electrically isolated from the top flange. The discharge is located within the volume between the plane powered rf electrode with the substrate and the U-like formed ground electrode. A schematic view of the experimental setup is given in Fig. 1. Argon, oxygen, and hydrogen, respectively, were used as process gases. The gas pressure could be varied between 4 and 50 Pa. The pumps are connected to the reactor vessel by a sliding valve. The pumping unit consists of a turbomolecular pump (Aleatel ATS 200-1000) and an oilfree membrane forepump (Alcatel 2MZA). The gas flow was controlled by using a MKS 260 flow controller. "lhe 13.56 MHz rf power which was varied between 2 and 225 ~V was supplied by an rf generator OEM-12 A (Eni power systems) and the matching was optimized by a control unit (Dressier). Summarizing, the typical operation conditions during plasma treatment were as follows: rf power: 2-225 W power density: 0.02-1.9 W/cm 2 frequency: 13.56 MHz background pressure: 10- 2 Pa process pressure: 4-50 Pa substrate temperature: 20-150 °C treatment time: 1-10 rain Wisura-Akamin and Wisura-Zimetol served as contaminations, respectively, dissolved in Toluol (1:9) to achieve a better spreading on the substrate surfaces. These oi!s used are typical lubricants in handling and industrial treatment of aluminum sheet metals.

matching unit I

II

,,J

subsU~te

II.

.......

I

b

UVISEL

pumping system Fig. 1. Schematicview of the experimentalsetup,

159

An optical multichannel analyzer (OSMA, SI) was used for optical emission spectroscopy (OES) for a real-timediagnostic of the discharge during the plasma process. A lot of spectral lines and bands can be obtained during the reactive removal of hydrocarbons by a plasma. They disappear after a certain time which is characteristic of the surface cleaning [ 10,15]. In particular, the time behaviour of the H~-line (656.3 rim) and two differentCO-bands (519.8 nm and451.1 nm) were examined. The progress of the plasma cleaning at the surface was monitored by ellipsometry [ 15,16]. A spectroscopic polarization modulation ellipsometer (PME, Uvisel Jobin Yvon) was employed for characterization of the contaminated and cleaned substrates and for in situ monitoring of the plasma treatment process. However, for the ellipsometrie measurements the design of the reactor had to be modified in order to realize an angle of about 70° of the ellipsometer's light beam with respect to the sample surface. This experimental setup sometimes caused unstable discharge conditions and unstable light emission at higher discharge power. Therefore the rf power was varied in these cases only between 2 and 20 W and the pressure was chosen to be 10 Pa for oxygen and 50 Pa for hydrogen during ellipsometric investigations. Unfortunately, with these discharge parameters the intensity of plasma radiation was not high enough for the OES device. Thus, the results of ellipsometry could not be directly compared with those obtained by OES. For the ellipsometric measurements not only polished AI rlates, but also Si wafers and AI coated Si wafers were used. In addition, the surface chemical composition of the substrates before and after plasma treatment was studied by AES and XPS by means of a Fisons MT 500 equipment.

3. Results and discussion 3.1. Optical emission spectroscopy

Optical emission spectroscopy (OES) from a gas discharge is often employed as a qualitative diagnostic of plasma physics and chemistry. One of the large variety of applications of optical spectroscopy to glow discharges is the monitoring of etching rates and the determination of process endpoints. The advantage of OES is that several lines (working gas and products) can be monitored simultaneously and the signal intensity can be taken a~ a gauge for the concentration of the involved species in the discharge volume. A certain part of these species results from the etching or cleaning process at the substrate. However, the relationship between the emission intensity and the cleaning rate is a very complicated function of discharge conditions, geometry and the composition of the surface to be cleaned. Assuming excitation due to inelastic electron collisions the intensity l ( t ) of a line corresponding to the electronic tran-

160

H, Steffen eta I, /Thin Solid Films 283 (1996) 158-164

sition (i ~ j ) is related to the density n(x) of particles etched ~ m the surface:

~(x)

=

n(x)P~(x) ~'~g(A)

( 1)

where P~(x) is the probability of exciting a particle of state i, ~'~ is the probability for deexcitation back to the original state by radiation, and g(A) is the fraction of emitted photons corresponding to this transition. The excitation probability itself depends on the electron concentration in the plasma and the excitation cross-section, which is an energy-dependent function too. However, in this investigation only relative changes in the signal intensity of the involved species are of interest. For a given set of discharge conditions for surface cleaning the emission intensity from the removed impurities as a function of time yields a signal which is proportional to the amount of material removed from the surface. Thus, the use of a timedependent optical signal gives a feedback control for the course of the plasma process. Because the hydrocarbon contaminants contain mainly hydrogen, carbon and oxygen the time behaviour of spectral lines of such characteristic species formed by electron impact dissociation were examined. In particular the Ha-line and COband intensities can be used as a gauge for impt:rities. They increase strongly after the discharge is ignited and reach their raa:~imum dependence on the rf power and the amount of cont~'~ination at the surface. Finally, the spectral intensities decrease to their initial value. There have also been observed lines c,~rresponding to CH species, that means CH- and CH + bands. However, they were not taken as a gauge because their time behaviour was very similar to CO. Their intensities were rather low in comparison to CO. The time dependent intensity curve l(t) of a spectral line has been fitted by a function consisting of two exponentially decaying parts:

l( t) = loo + iole-"" + lo2e -'/'2

(2)

The rel~ation time can be taken as a measure of the duration of the cleaning process and the intensity decay of the radicals can be described with two time constants (~-i, %). The first and shorter one (1"1) does not essentially depend on pressure and rf power. It is in the order of about 30 s, which is the time needed for the matching unit to adjust the set parameters for input and reflected power. In the first few seconds a lot of volatile components of the lubricants are removed from the substrate. This results in an immediate increase of pressure and ionization rate which makes it difficult for the matching control unit to hold the fixed power parameters. Therefore the second time constant ~'2 is taken into consideration to compare the cleaning process after stabilization of the rf discharge. The successful end of the plasma treatment has been assumed if the emission intensity reaches the offset value 1oo (average intensity without contamination) within a tolerance of about 5%. A remarkable difference has been observed between the time constants determined from the shape of the H~-line and

the CO-bands, respectively. The constants ~'2 determiaed from the CO-bands are significantly smaller than those which have been calculated from the H,cline. In respect to the total number of particles in the contamination layer the fraction of hydrogen is higher than the fraction of carbon atoms and, additionally, the hydrogen is pumped with a smaller flow rate. Therefore one should choose the H,, intensity as suitable for end point detection of the plasma cleaning process. Fig. 2 shows examples for the time dependence of the H~-Iine intensity given for two different discharge powers, whereas in Fig. 3 the curves of CO band intensity at different gas pressures are presented. For the measurements represented in Fig. 2 the aluminum substrates have been contaminated with Wisura-Akamin. However, in the case of Fig. 3 WisuraZimetol has been used. Since T2 describes a characteristic time regime for the plasma treatment it depends on discharge power, gas pressure, gas composition, and the chemical composition of the original contamination. In Table 1 some values of ~'2 for an oxygen rf discharge for different powers at a gas pressure of 10 Pa and for different pressures at P,y= 50 W, respectively, 2500 r

'

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P =lOOW

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•

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I

[

100

I

150

I / 2OO

I 250

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Fig. 2. Curve of H,, intensity for plasma treatment of Wisura-Akamin contaminated AI substrates (p,, l0 Pa. oxygen). i

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Time [s] Fig.3. TimedependenceoftheCOintensity( A= 451 rim)for 100Woxyg.~n discharge(Wisura-ZimetolcontaminatedA! substrates).

H. Steffen et al. / Thin Solid Films 283 (1996) 158-164

161

Table l Time constants I"2for the Ho line as a functionof the rf power (p = 10 Pa, O2) and the oxygengas pressure(P = 50 W), Wisura-Akamincontaminated AI ~ubstrates

Table 2 Timeconstants¢2forthe Haline and the CO line (451 nm) as a functionof the. oxygen gas pressure (P= 100 W), Wisura-ZimetolcontaminatedAI s~Jbstrates

P (W)

~2(s)

p (Pa)

~z(s)

p (Pa)

Ha r2(s)

CO ~2(s)

50 100 150 200

127.4 110.2 81.6 46.8

4 5 8 10

131.5 96.6 120.8 105.8

5 8 10 12

420.5 374A 219.0 202.7

80 74 67 52

are summarized. The aluminum substrates have been contaminated here with Wisura-Akamin. With increasing rf power the cleaning process becomes more effective. The self-bias at the electrode increases with power, too. Thus one can also provide higher ion energies with increasing power. The influence of energetic ions in connection with a reactive gas is a very important effect in material removal [8,17,18]. However, in technological applications it must be ensured that the power density does not exceed a critical value at which the thermal stress of the suhstrates treated becomes too high. In our experiments this value was about 2 W / c m 2. With increasing pressure the time constant ¢2 decreases weakly. The most efficient range of pressure depends on the gas used. It was in the range of 10-15 Pa for oxygen, 5-8 Pa for argon and about 20 Pa for hydrogen. Besides the discharge conditions the time constant is also influenced by the properties of the contamination layer. For Wisura-Zimetolm, which is a more solid lubricant with scarcely volatile components the time constants for the Ha-line under comparable discharge parameters are three to four times ~arger than for the more fluid Wisura-Akamin contamination. In addition, in such cases ~'2 shows a stronger dependence on pressure, see Table 2. As mentioned above one can also recognize from Table 2 the large difference between ¢2 for H~- and CO-line, respectively. A part of the impurities are already pumped away because of their high vapour pressure, and the amount of hard contamination which can only be removed by the following plasma treatment depends on the nature of the lubricant originally used. For the end point detection of CO it was found that the band heads completely disappear faster than the H,-line. However, this is not a solid proof for a totally cleaned surface. Sometimes if the lubricant film was very thick polymerization layers on the A1 surfaces have been observed. These polymers could not be removed by oxygen and not even by argon and hydrogen. In those cases no CO-bands could be observed at the plasma process. Nevertheless, a higher power input and increased cleaning time could remove those polymers, too. The detailed structure of the polymers was not examined. Also measurements of the O and O~ behaviour were taken to compare the times for reaching the maximum and the stationary level with the time constants given in Table 2. T~e durations to reach the maximum intensity were found to be

25 s and 62 s for O and about 50 s for O~-, which is in good agreement with the time constant ~'~and its behaviour related to the matching problem. Vice versa, the time for O or O~concentration to reach a stationary value is between 150 and 250 s for all different disch~ge conditions. This result correlates with the CO behaviour and the Ha cleaning times for Akamin-contaminated aluminum substrates. The same process durations could be found by mass spectrometric measurements of the several species in dc plasma cleaning [ 15 ]. The typical mass numbers of radicals formed by oil components (e.g. 76, 93, 147) disappear completely after about 150 s denoting the end point of the cleaning process. In the same time the oxygen species reach their maximum stationary level. 3.2. Surface analytical investigations

By means of a modular surface analysing system the surfaces of non-treated and plasma cleaned AI substrates were examined by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) [8,10,19]. AES spectra were obtained by using a 5 keV electron gun whereas the photoelectron spectra were excited with a Mg K , X-ray source ( 1253.6 eV). Unfortunately, the analysis chamber (base pressure: 5 . 1 0 - s Pa) was not connected to the plasma treatment vessel. Thus, the A! substrates had to be transferred at atmosphere and a new native o:dde layer might grow during the transport as well as a recontamination with atmospheric carbon occurring. Typical XPS survey spectra are given in Fig. 4. The main core-level peaks observed are due to C ls and O Is. Two small peaks due to the AI 2s and Ai 2p core levels are also observed. The AES and XPS spectra of non-treated aluminum substrates show a high carbon peak. Due to the high degree of carbon contamination the AI peaks are hardly visible in the XPS spectra. Plasma cleaning of hydrocarb~ contaminated AI substrates in an argon or oxygen discharge, respectively, reduces the C peak remarkably and in addition the AI peak increases weakly as shown in XPS measurements. However, the oxygen signal, which indicates the native oxide layer, remains as high as before. By means of ex situ XPS it was not possible to distinguish between carbon remaining on the sample, due to poor cleaning results and carbon coming

H. Steffen et al. /Thin Solid Films 283 (1996) 158-164

162 I

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1200

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I 1400

E wan (e~O Fig. 4. XPS survey spectra of nontreated and plasma cleaned aluminum substrates.

from atmospheric recontamination. Nevertheless, the tremendous decrease of the C signal in the XPS spectra after plasma treatment demonstrates the success of plasma cleaning. Because of both the combined chemical and sputtering action oxygen can be favoured as most efficient. The OES and surface analytical investigations indicate clearly that the main cleaning effect must be the reaction of hydrocarbon contamination with excited oxygen species to form CO and CO2 volatile products.

3.3. EUipsometricmeasurements Ellipsometry is an optical, nondestructive technique for determining film thickness, refractive index and other surface properties. In any ellipsometric experiment the object is to measure the change of the state of polarization of totally polarized light after reflection at the substrate/thin film system. The reflection properties of such a system are described by the complex ratio of Fresnel reflection coefficients p, which is given by PfrPffitan qt" e-ta r~

(3)

where rp(r~) is the complex Fresnel reflection coefficient for light polarized parallel (perpendicular) to the plane of incidence. Often p is expressed by the angles qt and A. In order to relate the measured parameters with actual characteristics of the near-surface region, a model must be constructed, from which the Fresnd reflection coefficients can be derived [9]. In situ kinetic ellipsometry can be employed to control the decrease of the lubricant thickness during plasma cleaning. However, for this purpose the dielectric functions ofsubstrate material and lubricant layer must be known. In this study the dielectric function of the lubricant layer was determined by spectroscopic ellipsometry. For the ellipsometric studies Si wafer, AI coated Si wafer and polished AI plates were used. One of the important properties of silicon substrates is their very good surface quality and the knowledge of the optical constants which makes it easier to compare the experimental data with the theoretical

model or to compare non-treated wafer with contaminated ones. By using silicon wafer only the properties of the contamination layer are unknown for the optical model. Thus, the number of free parameters is reduced in comparison with substrate materials not so extensively documented. In contrast to earlier investigations [ 10,11], it was now possible to separate the optical properties of the contamination. By means of the determined refractive index of the contamination film thicknesses and etching rates could be obtained not only on Si wafers but also on A! films and A! plates. The etching rates are about the same on all substrates and therefore the results on Si wafer can be generalized. Contaminated, evacuated and plasma etched substrates were characterized by means of ex situ spectroscopic measurements. The pumping and cleaning processes were observed in situ by kinetic ellipsometry at a wavelength of A= 633 nm. For the separation of the refractive index of the contamination the dispersion theory of Cauchy was used: B

C

n(A) =A +-~+-~

E + F-~ k(~)=D +-~

(4)

(n: refractive index, k: absorption coefficient, A, B, C, D, E, F: constants). The contamination was assumed to be one homogeneous layer. This assumption was supported by the good agreement between measurement and the homogeneous etching model used in the evaluation of the plasma cleaning process, see Fig. 5. In Fig. 6 a typical plot for the ellipsometric angles qfand A versus treatment time is given, which was monitored during the evacuation of the chamber and following oxygen plasma cleaning. The plot in Fig. 5 represents the same measurement as the qt-A plot compared with the simulation of the cleaning process. 180

i

~'t

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measurement . . . . . simulation

140

<~

120

plasma on

100

80

eO

.

§

s 10

,

I 15

i

I 20

i

I 25

, 30

Fig. 5. Comparison between experiment and simulation of e]lipsometri¢ ~-A plot for a cleaning procedure (10 Pa oxyge, gas pressure, 5 W dis-

charge power).

H. Steffen et aL / Thin Solid Films 283 (1996) 158-164 30

25

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Treatmenttime[si

Photonenergy[eV1

Fig. 6. The time behavior of ~ a n d A during an entire cleaning process run

( 10 Paoxygen,5 W).

Fig. 7. Comparison between spectroscopic measurement and simulation of the contamination layer after evacuation but before plasma treatment.

The initial thicknesses estimated from spectroscopic ellipsometric measurements are between 40 and 120 nm. These variations of the initial contamination thickness are the reason for the reproducibility problems during OES measurements. The contaminating lubricant films are already partially reduced if the evacuation of the reactor chamber is started. During pumping down to the base pressure the layer thicknesses were reduced to about 15 to 40 nm depending on the initial state. The considerable decrease of the layer thickness during the evacuation of the vessel could be proved in situ only by ellipsometry, which emphasizes the ability of this powerful method. The optical constants of the layer change a little during the evacuation of the reactor, which could be due to selective pumping, especially the absorption increases for photon energies above 3 eV. Selective pumping means that volatile parts of the impurities are removed during the evacuation process because of their high vapour pressure. Impurities are pumped away to a certain degree which can be shown by ellipsometry when the curves reach the plateau in Fig. 6. When the pumping procedure is completed the optical properties of the remaining lubricant films remained constant and reproducible. From different simulations an error of only 2% for refractive index and thickness has been estimated. Fig. 7 shows the good correspondence between simulation and experimental data for a typical measurement after chamber evacuation. The following plasma treatment results in a further purification which is shown in Fig. 6 by drastic changes of gt and A. The etching process was simulated with the refractive index extracted from the spectroscopic ellipsometric measurement. The model represented in Fig. 5 leads to a constant etching rate of 0.45 nm s-~ for Akamin. There is a good agreement between model and measurement for the plasma cleaning process, but there exists a remarkable mismatch at the beginning of the evacuation process. This can be explained by the variation of the optical constants during the evacuation of the chamber. Of course the state of the surface after plasma treatment was ef special interest. The data for the initial state of a silicon

wafer without contamination could not be reached completely. Moreover A decreases again after reaching a maximum. Several interpretations are possible. Because of the maximum in A as compared with a clean silicon wafer it might be that the lubricant has not been removed totally. But the decrease of A in Fig. 6 after reaching this maximum at about 170° has to be interpreted as an increase of the lubricant thickness once more. This observation could be explained by oxide or roughness growing after the surface has been completely cleaned. Because this curve of A has been observed in oxygen as well as in H2 and Ar growing roughness seems to be more likely. The thickness of such a roughness which was simulated by a layer consisting of 60% a-Si and 40% voids would be between 1.5 al,d 3 nm. Similar results have been obtained for AI substrates due to the end phase of the etching process. Nevertheless, further investigations are necessary for a clear interpretation of these eilipsometric measurements. Although there are still some difficulties concerning the cleaning state of the substrate after plasma treatment the etching rates could be determined with a good reproducibility. The etching rates in relation to the discharge conditions are presented in Fig. 8. For the gases used the etching rates show a linear dependence on the supplied rf power. By taking into account the different gas pressures the etching rates obtained per discharge power were 0.1 nm/Ws for 02 and about 0.01 nm/Ws for Ar and H2, respectively. In accordance with the results of optical spectroscopy one can conclude that plasma cleaning with oxygen is much more efficient for the removal of hydrocarbon lubricants than etching with Ar or H2. For the necessity of rapid carbon removal or in cases of large hydrocarbon contamination pure oxygen plasma cleaning is most effective. But one has to consider the disadvantage of increased surface oxidation, sputtering of the base material and possibly enhanced outgassing of oxygen-containing species [3]. However, in a large variety of applications these side effects are not disturbing and they are even sometimes desired.

164

H. Steffen et al. / Thin Solid Films 283 (1996) 158-164 !

Laboratory studies on small vessels and the in situ measurements of plasma and surface parameters have demonstrated that glow discharge cleaning can produce quite clean surfaces on technical materials.

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Acknowledgements

4

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This study was supported by the Deutsche Airbus Bremen under Grant 0249880593488 and by the Deutsche Forschungsgemeinschaft under SFB 198. The authors would like to thank E. Eich for his technical support.

[W]

Fig. S. Etching rates as a function of the supplied rf power.

References 4. Summary This study reveals that the removal of thin hydrocarbon contaminations on aluminum substrates by rf discharges is a reliable and effective process. For plasma monitoring optical emission spectroscopy and for surface monitoring ellipsometry are suitable tools. These methods give a quite complete impression of the progress of the treatment and the state of the substrate surface. Whereas surface analysis and ellipsometry are more sophisticated methods for basic research OES seems to be useful even f. industrial applications. It is a relatively fast method which does not require any specific reactor setups. By evaluating the curve of spectral line intensities of the characteristic species, information on time constants of the reactor system and on the process itself can be obtained. By means of the time dependent curves of the ellipsometric angles ~' and A the whole cleaning process was also monitored. Moreover, layer thicknesses and optical constants of the contaminations were determined by spectroscopic ellipsometry, and etching rates are obtained by in situ eUipsometric analysis. The determined cleaning rates per discharge power are 0.1 nm/Ws for O2 and about 0.01 nm/Ws for Ar and H2, which emphasizes the efficiency of oxygen plasma treatment.

[ 1] [2] [3] [4]

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