Thin film deposition by reactive magnetron sputtering: On the influence of target oxidation and its effect on surface properties

ELSEVIER

Thin Solid Films 305 (1997) 164-171

Thin film deposition by reactive magnetron sputtering: On the influence of target oxidation and its effect on surface properties D. Rohde *, H. Kersten, C. Eggs, R. Hippler Institut fiir Physik der Ernst-Moritz-Arndt-Universitg~tGreifswald, Domstrasse lOa, 17489 Greifswald, German), Received 9 October 1996; accepted 13 March 1997

Abstract

The application of X-ray photoelectron spectroscopy (XPS) for the characterization of deposited thin indium tin oxide (ITO) layers by reactive direct-current magnetron sputtering in Ar:O2 gas mixtures is performed. The influence of the gas mixture and the sputter-process duration on the discharge power and oxidation state of the target implies that the target state is an essential parameter for the production of ITO films. For understanding the plasma-surface interaction, the plasma has been monitored in front of the target and near the substrate by means of Langmuir-probe diagnostics. The internal plasma parameters, namely n, and k bT~, at the target also at the substrate have been determined as functions of discharge power and radial position. XPS analyses indicate a "'selective oxidation" of tin compared to indium as well as a change of the surface composition during the sputter process. © 1997 Elsevier Science S.A. Ke)words: X-ray photoelectron spectroscopy (XPS); Thin film deposition

1. Introduction

Transparent conductive oxide films (TCO), including indium tin oxide (ITO), have useful and technologically important properties, such as high optical transmittance in the visible range and high reflectance in the infrared as well as good electrical conductivity. ITO films have, for a long time, attracted much attention from the technological point of view for windows in solar cells, transparent heaters for windows, electrodes for liquid crystal displays, etc. [1]. These films have been prepared on various substrates with several deposition methods: chemical vapour deposition (CVD) [2], thermal evaporation, ion plating [3], ion beam assistant deposition (IBAD) [4], and reactive magnetron sputtering [5]. Reactive magnetron sputtering is an important tool for the deposition of thin films on the surface of metals or other materials. Among the several commercially available sputtering systems, planar sources are often employed. The magnetic confinement of the plasma makes the ionization process very efficient, and the discharge power can be used to a rather high percentage for the sputtering process [2-5]. The optical and electrical properties of the ITO

* Corresponding author.

0040-6090/97/$t7.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 1 5 7 - 0

films can be influenced by the discharge conditions of the magnetron sputtering source. However, one of the main problems in reactive sputter deposition is the run-to-run reproducibility of the film properties. The difficulties are mainly due to poisoning of the target, while sputtering in reactive atmosphere is due to target erosion. As mentioned, the present study reports the deposition of ITO on a Si (111) substrate by reactive magnetron sputtering in an Ar:O 2 discharge. One of the most important and fundamental parameters in reactive magnetron sputtering is the target composition which influences the arrival rate ratio O:In and O:Sn, and the quality of the deposited films [6]. For the characterization and optimization of the deposition process, the dependence of the internal plasma parameters (electron density, electron temperature) on the external deposition parameters (discharge power, sputter time, gas mixture) has been analysed. To obtain the internal plasma parameters, Langmuir-probe measurements were employed. Simultaneous to these measurements, the time behaviour of the discharge, the neutral gas mixture and the target oxidation state have been monitored by mass spectroscopy and the evaluation of the I - V discharge characteristics (I: discharge current, V: discharge voltage). The prepared films were analysed with X-ray photoelectron spectroscopy (XPS).

165

D. Rohde er a l . / T h i n Solid Films 305 (1997) 164-171

The study includes an analysis of the In-3d, Sn-3d, and O-ls XPS spectra as a function of each deposition parameter to obtain information on the oxidation state of In and Sn and the composition of the corresponding film. The effect of the target oxidation state could be clearly shown by the XPS measurements which support ellipsometric studies performed elsewhere [7].

rotary forepump. Typical operation conditions for reactive magnetron sputtering are given in Table 1. A shutter placed in front of the target cathode, which was opened to start the deposition process, has been used to ensure that the measurements were carried out only if the plasma discharge was stable.

2.2. Target state and conditioning 2. Experimental

During the thin film deposition, the variation of current-voltage characteristics was controlled by examination of their time behaviour. Mass spectrometry has been implemented for characterising the time dependence of the oxygen concentration in the discharge volume. In Fig. 3, the time variation of discharge current and the spectrometers' signal intensity of oxygen is plotted. In agreement with previous measurements [7], it shows distincdy that the oxidation state of the target and the oxygen supply in the discharge volume is quite sensitively reflected by the discharge parameters. After ignition of the plasma in a pure Ar atmosphere, the discharge current decreases slowly because of target conditioning. As the sputter target is poisoned with oxygen, the gas composition consists, in the first minutes, mainly of the basic Ar gas and oxygen species released from the target. These species contribute to the formation of additional charge carriers due to ionising electron collisions leading to an increase of the electron density. As a consequence, the discharge current is higher than it usually is under similar conditions in a pure argon discharge. With increasing time, target poisoning decreases as the discharge current decreases. Finally, the target is reduced and the discharge mechanism is only controlled by the argon. The discharge adjusts itself to a value smaller than in the initial oxidised phase. The reduction condition-

2.1. Sputter equipment The effect of magnetron sputtering is based on the principle of a gas discharge enhanced by a magnetic field [8]. Permanent magnets, which are placed below the target (diameter = 90 mm), generate a magnetic field of about 0.035 Tesla, which causes in combination with the electric field at the cathode, the secondary electrons drifting in a circuit in front of the target [9]. The electron confinement increases the sputter sources efficiency compared to ordinary ones [5]. The current density at the cathode shows a maximum where the magnetic field lines are parallel to the surface. These regions of high densities can be observed as gleaming rings and two erosion trenches in the indium-tin target which occur there. A transition from one to two burning rings induces a sudden increase of the I - V discharge characteristic (Fig. 1). A schematic view of the experimental setup used for magnetron sputtering is given in Fig. 2. Opposite to the planar target cathode, the wafers were mounted in a distance of 50 mm on a sample holder. Silicon wafers of 40 mm diameter served as substrates. The pumps are connected to the reactor vessel by a diaphragm valve. The pumping unit consists of a turbomolecular pump and a

120

I

i

1 00 8O

--E

60

t

I

L

I

5 sccm oxygen; argon : .....

5o

~ " . ~ -

~

--o- 7s sccm

j

~100 sccm - - - 125 sccm - - 1 5 o sccm

v

~ F

~"

"

/J

/~.-"

40 "I

o 2O 0 300

350

400

450

500

550

600

650

voltage [V] Fig. 1. Current-voltage characteristics of the magnetron discharge for several Ar:O: mixtures.

166

D. Rohde et al. / T h i n Solid Films 305 (1997) ]64--171

water, power, gas - 600 V target shieldt

L

~

shutter

magnetron

• t

probe ~

subslTate

pumps

mass spectrometer

illllllll Fig. 2. Experimental set-up.

hag of the target is completed when the current is steady. A defined oxygen gas supply for reactive sputtering results in a remarkable increase of discharge current. The oxygen is consumed (incorporation in target surface) at the target again, and the target saturation is completed when the discharge current is constant in time. There exists a balance of oxidation and reduction at the target surface. If the discharge is switched off ( I = 0), a further increase of the oxygen mass spectrometer signal will be observed. This is due to the oxygen which has been consumed previously at the target and which is now left in the volume. Finally, when the reactor is vented, the target becomes totally oxidised and a preventional conditioning in the next run is necessary.

[p 2.55 mm, diameter d v = 100 p,m) was used. The probe measurements were recorded using an automatic Siemens device which allows a maximal probe potential =

3,0 e=-.i 'm

,i,,,,,,+,,++,,,,,,,,l,,,,,,,,,l,,,+,,+,+l,,,,+++,~l+

oxygenconcenb'at]on increases

~

2,5 2,0

09 =]¢-=.1 o-c~J

0

1,5

valveopen; inletof argon

\,++\

1,0 0,5

dischargevoltage

( - -

0,0

2.3. Probe measurements

smirchoff

oxygen

++l,,,+,,,,,l,,,,,,,,,l,,,,+,,,,l,,,,,,,,,l,,,+,,,,+

150

In order to determine the internal plasma parameters, a horizontally movable cylindrical Langmuir probe (length

Table 1 Typical operation conditions for reactive magnetron sputtering Discharge current Discharge voltage Discharge power Deposition pressure Background pressure Argon gas flow Oxygen gas flow Magnetic field of magnetron Target material Distance target cathode substrate

10-120 mA 250-600 V 2.5-72 W 1.1-1.4 Pa 2. t0 .+ Pa 25-150 sccm 0 - 1 0 sccm 0.035 Tesla In:Sn = 90:10 50 m m

ox~d~.ed

reduced ( tarot

"~ 'm Q.~O

50

ignNon

\t t-CJ

0

o~gen

_.__.~ emittance

._m

0

500

1000

1500

2000

2500

time [s] Fig. 3. Time behaviour of discharge current and signal intensity of mass

spectrometer, which depends on partial pressure, for the Oz peak.

D. Rohde et aI. / Thin Solid Films 305 (19577) 254~171

A Up

variation of = 60 V. The evaluation of the characteristics was done by a procedure using the second derivative of the electron current, based on the following assumptions: I. Electron and ion concentration are equal (n e = ni); 2. Maxwellian energy distributions with the temperatures kBTe and kBTi with kBT~ >> kBTi; 3. Probe dimension: smaller than the mean free path of electrons and larger than the screening length around the probe. The probe characteristics have been measured in front of the target and near the substrate. To minimise the influence of the magnetic field on the probe current, the probe was always directed perpendicular to the magnetic field lines [10]. The shape of the electron energy distribution function (EEDF) has been derived from the second derivative of the electron probe current vs. the probe voltage: 1

Table 2 XPS line positions for In, Sn and O, measured with Mg K c~ X-ray source In-3d-5/2-peak Sn-3d-5/2-peak

O-Is-peak

}Ze

dVp 2

.

(1)

e o Ap,

(2)

716

3000 2500

718

720

722

724

726

O l s - Peak

!/

1500

I.L!

eV eV eV eV eV eV eV eV

728

730

In203

1000

LI.! Z

809,6 810.4 766.9 767.7 769.3 720,4 722.3 724.0

02 SnOx

2OOO

t--

Eki n = E kin = Eki n = Eki n = Eki n = Eki n = Evn = Eki n =

ORIGINAL model function

,

d

In (InzO 3) In (In) Sn (SnO2) Sn (SnO) Sn (Sn) O (0 2) O (SnO.~) O (In20 3)

The surfaces of the deposited thin ITO films have been studied by XPS with a conventional surface analysis equipment MT 500 (VG Microtech). Unfortunately, the samples had to be transferred between the preparation sputter plant and the analysis chamber at air. For the XPS measurements, the native oxide layer on top of the films had to be removed. Ar + ions were used at an energy of 5 keV for the samples' sputter cleaning before XPS analysis. The In, In203, Sn, SnO x and oxygen peaks of the XPS spectra were used for quantitative film analysis. Line position for the investigated In, Sn and O peaks are summarised in Table 2. The well-known sensitivity factors [12] were used for a quantitative analysis. An example for the peak-fitting procedure is Fig. 4, which shows the result for the O-ls peak.

~/8kBTe - -

1 2 1 2 3 I 2 3

2.4. Smface diagnostics by XPS

Assuming a Maxwellian energy distribution, the electron temperature kBTe has been evaluated from the slope of the second derivative of the probe current Ip vs. the voltage Vp in a semi-logarithmic plot in the electron retarding regime [11]. The electron density G was derived from the measured electron saturation current I~, sat towards the probe at the plasma potential Vp = Vpl. In that case yields: 1

Peak Peak Peak Peak Peak Peak Peak Peak

where Ap denotes the probe surface, m e the electron mass, and e o the electron charge.

d2Ie

f ( e o V ) ~ - - e~o-jV

16T

/' 500

] -500

.....

~ .........

716

J .........

718

i .........

I .........

720

722 Eki

n

J .........

724

I .........

726

I .........

728

H

730

[eV]

Fig. 4. Example of fitted peaks; N(E) quantity of photoelectrons emitted from surface, T(E); transmission function which depends on electron energy.

168

D. Rohde et al. / Thin Solid Films 305 (1997) 164-171 5,5

3. Results and discussion

i

5,0

3.1. Plasma analysis

~

Of special interest are the results of probe measurements in the substrate region. Because the plasma conditions near the substrate influence the deposition, the layer properties are very sensitive. The electron density ne in the substrate region was found to be smaller than in the target region. It was about 10% of n e in the target region. The electron temperatures kBT~ in both regions were .comparable. The dependence of k~T~ and n~ on the radial probe position in front of the substrate is shown in Fig. 5 and Fig. 6. Different discharge powers were used as parameters. Correlating to the magnetic field, a maximum of both n~ and kBT~ can be observed at a probe position of r = 15 ram. Next to the target, the maximum is even more pronounced. Electron temperature slightly depends on the discharge power, whereas the electron density rises monotonously. The flux of sputtered particles depends on the plasma density as well as on the sputter yield. The yield is mainly affected by the ion energy. Therefore, the deposition rate and the oxidation state of the films are strongly influenced by the argon ions. The ion density is comparable with the electron density which is in the range of 2.5 • t0-~3-5.2 • 10 -14 m - 3 (Fig. 6). The voltage drop at the target is about 400 V resulting in the sputtering of the target. TRIM [13] calculations of In, Sn, SnO, and InzO 3 using argon ions of this energy delivered the following sputter yields: In, 2.1; Sn, 1.6; SnO, 0.52 for Sn component and 0.44 for O component; and, In203, 0.43 for In component and 0.57 for O component. The plasma potential was measured to be about 3 V and the floating potential was about - 1 2 V. The difference between the potentials of the plasma and the substrate essentially determines the kinetic energy of the ions and

~,~ 3,5

*:' .,;.

1,2

E

.£

:"

o~

t-.

2,5

"~ t-

2,0

,. V . = .

-¢'..::" ... ~:::::: ::.~::j, ~i:::::: . . . . . . .

.

"',..,

,

-,

£

1,5

.

,.

,,'.

,, ,

/ L :/'?. ::iiii~

0,5 0,0

;

' ' 8; radial probe position [mm]

'

20

Fig. 6. n= for severaldischargepowers in argon atmosphere. electrons striking, thereby influencing the energy balance at the surface [14].

3.2. Surface analysis 3.2.L Investigation of the influence of pre-sputter time (target conditioning) In earlier ellipsometric studies of target conditioning, it could be shown that the target oxide will not be completely decomposed to metal atoms and oxygen during the pre-sputter process [7]. In order to verify the influence of the target oxidation state on the properties of the deposited thin films, examinations were carried out with respect to target pre-sputter time. The duration of pre-sputtefing were 0, 30, or 60 rain. After that time, the shutter was opened and the deposition started for 3 rain at 65 W. By XPS, the ratio of the fitted peak areas for the metal and the metal oxide were determined. The ratio of indium oxide compared to pure indium decreases continuously with increasing pre-sputtering as seen in Fig. 7. As the sputtering was carried out in pure argon, the oxide can come only from the target itself. With ,

0,90 -

,

)(4",,..

'.2 ' +

..

'"..

,

'~"

0,85"' '"" ,,.-.+

/ sputter time: 3 min t power: 65 W

.o

''9, •

0,8

.,"

"5 _.e 1,o

O . , : : : ' / i , = . . + , -. .-.Ik " ' " ' , . tl, . . . . . . . . .

',,.+ 0-

.~_>~ 3,0

0,95

1,4

t,o

'

4.0

1,6

•=

,'

4,5

0,80

"9"°

,

-

' ....

0,75-

,t"-0 0 II1

OH 0,70-

0,6 0,4

;

'

1'0 2'0 ' go radial probe position from]

2o

Fig. 5. kbTe near the substrate for several discharge powers in argon atmosphere.

6

1'o

2'o 3'o 4b pre-sputter time [min]

~b

6b

Fig. 7. Ratio of InzO3:In as a function of pre-sputter time in argon atmosphere.

D. Rohde et al. /Thb~ Solid Films 305 (1997) 164-171

increasing sputter time, the target becomes more reduced, hence, the contribution of metal oxide decreases whereas the pure metal becomes more significant. Former investigations at deposited ITO films, which have been analysed by ellipsometry, show very similar results [7,15]. The ratio of SnO:Sn shows the same qualitative behaviour, depending on target conditioning time (Fig. 8). However, it was discovered that oxygen is preferentially bounded to Sn rather than In. After any pre-sputtering time (0, 30, 60 rain), the metal oxide to pure metal ratio will always be greater for Sn than for In (compare Figs. 7 and 8). The oxygen reacts favorably with metallic tin atoms or clusters than with indium. In Fig. 9, the change of the In:Sn ratio with increasing pre-sputter time is plotted. At a completely oxidised target (t = 0), the relative In:Sn ratio is very high. This is due to the fact that the most tin is oxidised as SnO or S n Q and does not occur as pure metal. Indium is not oxidised in such a large extent as tin (see Fig. 7 and Fig. 8), hence, the ratio of pure indium to pure tin is rather high. With increasing pre-sputtering (t >_ 30 rain), an equilibrium is reached. That means that after a longer pre-sputtering, the target is reduced and there is no more oxygen for a complete tin oxidation at the target. Now there is pure tin at the target too, and the ratio between both metals remains constant. One can conclude that tin oxidation at the target (also at the substrate) is more probable than indium oxidation. This behaviour might be called "selective oxidation". 3.2.2. Examination o f the influence o f the gas mixture on fibn composition Another essential parameter for layer forming is the supply of reactive gas, here oxygen, in the argon atmosphere. After target conditioning as described above, ITO films have been deposited at different Ar:O z mixtures. The deposition time was 3 rain and the discharge power was 55 W. Again, the deposited layers have been examined by XPS (Fig. 10). The flow ratio of A r : Q has been varied between 0.4 and 1.6.

3,0

20

~

,

0

16.

"'"',,,

r-"

44e-

12. 10

1'0

2LO

3'0

4~0

5tO

6~0

pre-sputter time [min]

Fig. 9. Ratio of pure indium to pure tin as a function of pre-sputter time in argon atmosphere•

If the flow ratio A r : O 2 is smaller than 0.8, an oxygen saturation in the layer is obtained. Even an additional oxygen supply (Ar:O 2 ~ 0) does not change the peak ratios, which is a clear evidence for the saturation. The "selective oxidation" of tin can be observed here too. There is significantly more indium than tin in the film. As the target alloy consists of In and 10% Sn but the ratio between In and Sn in the deposited layer is much larger, we conclude that a larger amount of tin is oxidised compared to indium. 3.2.3. The influence o f Also, the influence gas mixtures of O2:Ar The power was varied

35 30

discharge p o w e r of the discharge power using two (0:25 and 15:10) has been studied. between 20 and 70 W. In Fig. 11,

0

I ..o.. SntoO i

'

/

25 g~)

15

X>"

10

02

5

/'"'"

,,"

20

// / /" ,'

0" ~

~ ///'

ii

, [Tll

...........

I ""~'" In to 0 i

m 4000 = 03 'S o

t~3 2,00

o co 1,5.

sputter time: 3 min power: 65 W

'",,

--= 5000

sputter time: 3 min power. 65 W

2~5 -

,

18-

0

0

169

3000 2000

,' ,,/ "

",

1000

b~

...-~ 0

1.0

~ ......... ~---~

....

,,i ......... i ......... i ......... t ......... t ......... i ........

1'0

20

3'0

4'0

5'0

6'0

pre-sputter time [min]

Fig. 8. Ratio of SnO:Sn as a function of pre-sputter time in argon atmosphere.

0,4

0,6

0,8

1,0

1,2

1,4

,6

argon to oxygen gas flow

Fig. 10. Ratio of tin (a), and indium (b) to oxygen in the films as a function of Ar:O; gas mixture in discharge volume.

170

D. Rohde et a l . / Thin Solid Films 305 (]997) 164-171

160. s,5-

O

5,04,54,o-

E

<

120-140"

..... .-~ O

---~--- in Ar/02 atmosphere ---&--- in Ar atmosphere I

3,52

3,0-

I00.

e-

80-

co

2,5-

2,02 1,5 1,0. 2O

".=

""~-'_.A...ininArArtatmosphere O2 atmosphere

/

....~

.'*'•

.j'

i

E

"

,,

//

40. S'"

20.

"A •

2~5

3'0

3'5

'

' 4'5 5~0 power [W]

4b

'

5'5

'

6}3

'

6'5

70

0

20

)1(

_..._...._.&

& ..................• ........... ~

2's ' a'o

3'~ ' 4'0 ' i s

&""~ . . . . .

~'0 ' 5'5 '6b

' 6'~ ' 70

power [W]

Fig. 11. Ratio of In203:In as a function of the discharge power for two gas mixtures: "=- 25 sccm Ar, * = 15 sccm Ar, 10 sccm O 2.

Fig. 13. Ratio of In:Sn as a functionof the dischargepower for two gas mixtures. (See above).

Fig. 12 and Fig. t3, the ratios of indium oxide:indium, tin oxide:tin, and indium:tin, respectively, are plotted as dependent on the discharge power supplied for the deposition process. In all three graphs, one can recognise that for sputtering in a pure argon discharge, the ratios are independent on power. Since the target was reduced in each run, only a small amount of oxygen was found in the films. The In:Sn ratio (Fig. 13) of the deposited layers reflects the ratio between both metals at the target (90:10). There is a uniform deposition of the alloy. In the reactive sputtering mode, where oxygen is supplied to the discharge especially the tin, oxidation shows a significant dependence on power. The contribution of fin oxide compared to tin increases continuously with increasing discharge power (Fig. 12). A higher discharge power results in a higher electron density (Fig. 6), which causes a higher dissociation degree of the oxygen molecules and an increase in reactivity. Because the tin reacts more likely with the produced oxygen radicals than indium, the oxygen is bound to the tin atoms of the film as long as there are still free tin particles. That is also the reason for the observation that the indium oxide remains constant with chanNng discharge power (Fig. 11). This observation emphasise the "selective oxidation" again.

Although the free energy of In203 ( - 8 3 1 ld tool -1) is much more negative than for SnO ( - 2 5 2 kJ tool - I ) and SnO 2 ( - 5 1 6 lc~ tool-t), one cannot conclude that the formation of indium oxide has the largest probability under the non-equilibrium conditions of plasma processing [16]. In equilibrium thermochemistry under atmospheric pressure, In203 formation would be the preferred reaction because of its tow free energy and exothermic formation heat. However, tdnetic considerations must be considered here. A stepwise formation of In203 using InO as first step will not occur, because the free energy for InO is positive (364.4 kJ tool-I). The direct formation of In203 is a many-body reaction though much less probable than the formation of SnO, consisting of only two atoms. Due to the low reactant fluxes and the surface conditions, a single SnO formation will most probably occur instead of in203 formation. Later, if all Sn bonds are saturated, the oxygen atoms may diffuse onto the surface, without trapping, by tin and they can react with indium. Fig. 13 also shows the results pointed out. The tn:Sn ratio increases with increasing power at reactive sputtering because at higher discharge power, a large amount of tin is already oxidised, therefore less pure tin is available, which yields to an increasing In:Sn ratio.

i

,

i

,

1

i

J

~

,

60-

y¢ ---~--- in Ar 1 02 atmosphere l / . , ~,•,'/'

4. Conclusion

50-

---A.- in Ar atmosphere

The influence of deposition conditions in a magnetron sputter source on the produced ITO films concerning their chemical composition was examined. Discharge power and gas composition were varied and the effect of pre-sputtering (target cleaning) with regard to the layer qualities were investigated. Information about the appearance of chemical compounds and their ratio in regions near the surface of ITO films were obtained by XPS. Especially, a selective oxidation due to the non-equilibrium surface chemistry of tin, which is dependent on the discharge power and the reactive gas composition, was observed. Under our experimental conditions, the metal

7"

O

40-

t-

03

30

..f

O

o* l-

03

,/

20 100

20

,t ................. ,i. ..........................

A.......................... .41,

2s

50

at

as

40

4s

ss

6'o

a's

70

power [W] Fig. 12. Ratio of SnOx:Sn as a function of the discharge power for two gas mixtures. (See above).

D. Rohde et al./ Thin SoI~d Films 305 (1997) 164-171

oxide of the target is not decomposed completely, while the metal oxide particles are sputtered. On the other hand, the oxygen partial pressure increases during sputtering of the oxidised target, which can also be seen in the film composition. The combination of mass spectrometry, the measurements of the time behaviour of I - V characteristics and the Langmuir-probe measurements, yield information about the target state and its influence on the plasma at the ITO deposition in an Ar:O 2 magnetron discharge. The increase of electron density with g o w i n g discharge power results in a higher sputter yield of target material and a rising oxygen dissociation. The discharge power determines the sputter rate at the target surface and its temperature, whereas the oxygen partial pressure determines the oxidation rate at the target. The ratio of oxidation rate:sputter rate determines the target state. However, the sputter rate at the magnetron target depends on the radius, and the target material will not become completely reduced. In a certain area of the target, mainly oxide sputtering may occur. The distribution of regions with different sputtering and oxidation behaviour depends on the magnetic field and can be seen easily in the radial dependence of the electron density. The influence on the growth rate and the oxidation rate of the ITO layers can be directly shown in layer composition by XPS measurements.

171

References [1] A.K. Ghosh, C. Fishman, T. Feng, J. Appl. Phys. 49 (1978) 3490. [2] T. Maruyama, T. Kitamura, Jpn. J. Appl. Phys. 28 (I989) L1096. [3] M.I. Ridge, R.P. Howson and Ch.A. Bishop, New techniques for roll coating of optical thin films, 47 (1982) SPIE 325. [4] J.C. Manifacier, L. Szepessy, J.F. Bresse, M. Perotin, R. Stuck, Mater. Res. Bull. 14 (1979) 163. [5] J.A. Thornton and A. Penfold, in: Thin Film Processes, J.L. Vossen and W. Kern (Eds.), Academic Press, New York, San Francisco, London, I978. [6] W. Fukarek, PhD thesis, University of Greifswald, I99I. [7] W. Fukarek, H. Kersten, J. Vac. Sci. Technol. A12 (2) (1994) 523. [8] S.M. Rossnagel, H.R. Kaufman, J. Vac. Sci. Technol. A6 (2) (1988) 223. [9] J.L. Vossen and J.J. Cuomo, in: Thin Film Processes, J.L. Vossen and W. Kern, (Eds.), Academic Press, New York, San Francisco, London, 1978. [10] M. Tichy, to be published. [11] F. Adler, H. Kersten and H. Steffen, Contrib. Plasma Phys. 35 (1995). [12] J.C. Riviere, Surface Analytical Techniques, Clarendon Press, Oxford, 1990, 214. [13] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [14] H. Kersten, G.M.W. Kroesen, Contrib. Plasma Phys. 30 (6) (1990) 725-731. [I5] F. Adler, H. Kersten and H. Steffen, Escampig XII, Noordwijkerhout (1994) 388. [16] M. Henzler and W. G~Spel, Oberfliichenphysik des Festk~Srpers, Teubner Verlag, Stuttgart, 199I.