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Effects of residual gases and rf power on ITO rf sputtered thin films
Vacuum/volume 37/numbers Printed in Great Britain
5/6/pages
0042-207X/87$3.00+ Pergamon Journals
447 to 44911987
Effects of residual gases and rf power sputtered thin films M Clement,
J Santamaria,
lJ Complutense,
Madrid,
E lborra
and G Gonzhlez-Diaz,
Dpto de Electricidad
on IT0 y Electronica,
.OO Ltd
rf
F Fisicas,
Spain
A study of the influence of gas composition and applied rf power on the sputtering of an IT0 compound target in Ar atmosphere is shown. The use of glow discharge optical and mass spectroscopies has allowed us to avoid the lack of reproducibility in film properties and deposition rate due to water desorption from the target surface. Hence, the study of rf power influence on film properties is made more easily. The influence of both power related effects namely heating and electron bombardment is analysed. Optimization of the process allowed us to obtain resistivities down to 5 x 10e4 f2 cm, mobilities up to 35 cm2 (V s) -’ and average transmittance of 95% in films 0.5pm thick.
1. Introduction Indium tin oxide (ITO) is a transparent conducting oxide extensively studied in recent years because of its potential applications in optoelectronic technology. IT0 thin films are frequently used as transparent electrodes in various electrooptical devices, solar cells and reflective window planes. Among the usual growth techniques (evaporation, ion plating, CVD, spray . .) sputtering in pure Ar and in reactive atmospheres is one of the most widely used’. However, there are few studies of the process appearing in the literature. Particularly, the influence of the sputtering gas composition on film properties when compound targets are sputtered in pure Ar, has been barely studied. A lack of reproducibility in film properties, which disappears with long conditioning periods (up to 3 h) has been often reported. In this paper we present a study of the evolution of sputtering gas composition during the sputtering of compound IT0 targets, and its influence on film properties. 2. Experimental The study was conducted using the previously described rf diode sputtering apparatus (GCA Vacuum Industries)3. The IT0 target was 5 in. in diameter and 90% In,O,-10% SnO, composition having been supplied by Cerac. The chamber was pumped to a base pressure of 1 x lo-’ torr before the sputtering gas (99.999% pure Ar) was admitted and the pressure controlled with a MKS Baratron automatic system. RGA experiences allowed us to ensure a constant water content in the chamber before plasma ignition. Glow discharge optical and mass spectroscopic (GDOS, GDMS) measurements were performed with systems described elsewhere4. Ionized and non-ionized Ar, In and water GDOS and GDMS signals were monitored with time. The substrate temperature was measured with a thermocouple directly attached onto the
film surface. Resistivity and Hall effect measurements were performed using the van der Pauw method. The transmittance in the visible and near ir range was measured using a Cary 17 and a Perkin Elmer spectrophotometer. Thickness was measured by means of a Sloan-Dektak profile analyser.
3. Results and discussion A lack of reproducibility in electro-optical properties and deposition rate was found in films produced with an identical set of accessible parameters (rf power, Ar pressure, base pressure, substrate temperature). These irreproducibilities were attributed to water desorption from the target (due to the hygroscopic character of its surface) and chamber walls. This water, hardly evacuated during the sputtering run, probably acted as a source of oxygen, altering the film composition. In order to control the process we investigated the evolution of the water content by monitoring water-related GDMS and GDOS signals (H20+, H30+, Hz, H+ GDMS and H,O GDOS). As already discussed in previous work GDOS and GDMS peaks were normalized to Ar signals to make the measurements insensitive to changes in detection conditions. Relative variations of GDOS and GDMS signals correlated very well showing deviations less than 3%. Figure 1 shows relative variations of the water signal with time. Experiences with nonhygroscopic targets (Cu) allowed us to distinguish between water coming from the IT0 target (0) and chamber walls (A ). It can be seen that the major contribution to water content comes from target desorption during the first 30 min. This water can either be extracted by ionic impact or, more likely, desorbed as a result of the heating of the target surface due to ionic bombardment. On the other hand, the chamber wall contribution is probably due to desorption caused by energetic electrons scattered in the gas atmosphere. Figure 1 shows also the evolution of the GDOS In0 signal. The first decay and subsequent increase should be explained in terms of a decrease in the sputtering yield caused by 447
M Clement
et a/: IT0
rf sputtered
thin
films
.
. .
0
0
0 0 0
.
0 l
0
0
0 0
-10
. . 1)
p20
10
30
40
A.
(min)
Figure 1. (0) H,O signal (water coming from IT0 target and chamber walls). (A) H,O signal (water coming from chamber walls (Cu target)). (0) GDOS In signal. the presence of light H+ and Hz ions coming from water molecule dissociation which do not contribute to the extraction. This effect explains the lack of reproducibility observed in the deposition rate. In fact, films produced under the same water content conditions, and after all the water was desorbed from the target, no longer exhibited such lack of reproducibility. Figure 2 shows the mobility, average transmittance and resistivity of films produced in the same sputtering conditions (70 W, 2 x lo-’ torr) vs relative water content measured by GDMS. A strong correlation between both magnitudes can be seen. The resistivity also showed an increase due to the dependence of the carrier density on the oxygen vacancy concentration. Part of these vacancies otherwise present in the films are probably annihilated either by 0 or by OH groups coming from water dissociation. The variation observed in mobility and transmittance is probably associated with defect centres associated with excess metal (nonbonded In or Sn atoms) which act as scattering centres. A similar kind of defect has been reported in ref 6. The presence of large amounts of water in the discharge therefore reduces the number of these defects. Measurements of mobility at variable temperature allowed us to eliminate grain boundaries as scattering centres. In fact,films grown with high values of substrate bias (small grain size) exhibited high values of mobility.
.
.
x’
-m
60
50 TIME
A.
1102O POWER
(W)
Figure 3. Transmittance (O), mobility ( x ) and carrier density (A) YSrf incident power of films 0.5 pm thick produced at 2 x lo-’ torr. The dependence of film properties on the rf incident also studied. This study was performed in a very sphere, after all the target water had been eliminated. the variations of film average transmittance, mobility concentration are shown. The observed minimum presence of two competitive mechanisms.
(aI
power were clean atmoIn Figure 3 and carrier suggests the
,o) XP A
n
200 POWER
(W)
X
:”
50
Figure 2. Transmittance (0 ), mobility ( x ) and resistivity (A ) vs relative water content in arbitrary units in 0.5 pm thick produced at 70 W and 2 x lo-* torr. 448
100
150 200 TEMPERATURE
GO i’C)
Figure 4. (a) Mobility ( x ) and carrier density (A) vs rf incident power of films produced at 2 x 10m2torr keeping temperature constant at 200°C with external heater. (b) Mobility ( x ) and carrier density (A) vs substrate temperature of films produced at 2 x lo-* torr keeping constant the rf incident power at 50 W.
M Clement et al: IT0 rf sputtered thin films
Experiments performed at fixed rf power and varying temperature, and at fixed temperature (with an external heater) and varying rf power, allowed us to distinguish between the effect caused by secondary electron bombardment and the heating effect associated with this bombardment. Figure 4(a) shows the variation of mobility and carrier concentration with rf power at 200°C. Carrier concentration remains roughly constant. The decrease in the mobility should be associated with scattering by defect centres (quoted above) generated by electronic impact. Figure 4(b) shows variations of mobility and carrier concentration of films produced at 50 W rf power with substrate temperature. An increase in the growth temperature enhances atom mobility and the defect concentration decreases because of a more-ordered growth or because of metallic atom re-evaporation. The occurrence of these two competitive mechanisms explains the behaviour shown in Figure 3. The electronic impact related effect dominates over the heating one and both mobility and transmittance decrease due to defect generation. Above 70 W the heating effect becomes dominant and mobility and transmittance increase because the defects are annihilated or annealed out. This experience was reproduced for several different water contents in the discharge resulting in the same qualitative behaviour. The same effect of defect elimination can be achieved by biasing the substrate. Ion bombardment causes resputtering of non bonded metallic-related species probably responsible for the carrier scattering. Combining high power levels and high bias voltages we
obtained films with resistivity values of 5 x 10m4 0 cm mobilities up to 35 cm’ (V s)- ’ and average optical transmittance of 95% (thickness = 0.5 pm) in a reproducible way.
4. Conclusions The use of GDOS and GDMS to study the evolution of sputtering gas composition has allowed us to determine the origin of the lack of reproducibility in film properties and deposition rate which appear in the sputtering of compound IT0 targets. Large amounts of water adsorbed in the target during exposure to the ambient are desorbed during the first 30 min. Small variations in water concentration in the sputtering gas cause great variations in film properties and sputtering yields. After all the water from the target was desorbed, the effect of rf power was also studied. Two effects have been found: one is due to electron impact and the other to the substrate heating associated to electronic bombardment. Defect generation and annihilation due to these effects explain the observed behaviour.
References
1J L Vossen, RCA Rev, 32,269 (1971). * J Szczyrtbowski et al, Physica Status Solidi (a), 78, 243 (1983). 3 I Mar&l et al, Thin SolidFilms, 114, 327 (1984). 4 E Iborra, PhD Thesis Madrid (1986). ’ W D Westwood and R J Boy&, J &pl Phys, 44,261O (1973)
449
5/6/pages
0042-207X/87$3.00+ Pergamon Journals
447 to 44911987
Effects of residual gases and rf power sputtered thin films M Clement,
J Santamaria,
lJ Complutense,
Madrid,
E lborra
and G Gonzhlez-Diaz,
Dpto de Electricidad
on IT0 y Electronica,
.OO Ltd
rf
F Fisicas,
Spain
A study of the influence of gas composition and applied rf power on the sputtering of an IT0 compound target in Ar atmosphere is shown. The use of glow discharge optical and mass spectroscopies has allowed us to avoid the lack of reproducibility in film properties and deposition rate due to water desorption from the target surface. Hence, the study of rf power influence on film properties is made more easily. The influence of both power related effects namely heating and electron bombardment is analysed. Optimization of the process allowed us to obtain resistivities down to 5 x 10e4 f2 cm, mobilities up to 35 cm2 (V s) -’ and average transmittance of 95% in films 0.5pm thick.
1. Introduction Indium tin oxide (ITO) is a transparent conducting oxide extensively studied in recent years because of its potential applications in optoelectronic technology. IT0 thin films are frequently used as transparent electrodes in various electrooptical devices, solar cells and reflective window planes. Among the usual growth techniques (evaporation, ion plating, CVD, spray . .) sputtering in pure Ar and in reactive atmospheres is one of the most widely used’. However, there are few studies of the process appearing in the literature. Particularly, the influence of the sputtering gas composition on film properties when compound targets are sputtered in pure Ar, has been barely studied. A lack of reproducibility in film properties, which disappears with long conditioning periods (up to 3 h) has been often reported. In this paper we present a study of the evolution of sputtering gas composition during the sputtering of compound IT0 targets, and its influence on film properties. 2. Experimental The study was conducted using the previously described rf diode sputtering apparatus (GCA Vacuum Industries)3. The IT0 target was 5 in. in diameter and 90% In,O,-10% SnO, composition having been supplied by Cerac. The chamber was pumped to a base pressure of 1 x lo-’ torr before the sputtering gas (99.999% pure Ar) was admitted and the pressure controlled with a MKS Baratron automatic system. RGA experiences allowed us to ensure a constant water content in the chamber before plasma ignition. Glow discharge optical and mass spectroscopic (GDOS, GDMS) measurements were performed with systems described elsewhere4. Ionized and non-ionized Ar, In and water GDOS and GDMS signals were monitored with time. The substrate temperature was measured with a thermocouple directly attached onto the
film surface. Resistivity and Hall effect measurements were performed using the van der Pauw method. The transmittance in the visible and near ir range was measured using a Cary 17 and a Perkin Elmer spectrophotometer. Thickness was measured by means of a Sloan-Dektak profile analyser.
3. Results and discussion A lack of reproducibility in electro-optical properties and deposition rate was found in films produced with an identical set of accessible parameters (rf power, Ar pressure, base pressure, substrate temperature). These irreproducibilities were attributed to water desorption from the target (due to the hygroscopic character of its surface) and chamber walls. This water, hardly evacuated during the sputtering run, probably acted as a source of oxygen, altering the film composition. In order to control the process we investigated the evolution of the water content by monitoring water-related GDMS and GDOS signals (H20+, H30+, Hz, H+ GDMS and H,O GDOS). As already discussed in previous work GDOS and GDMS peaks were normalized to Ar signals to make the measurements insensitive to changes in detection conditions. Relative variations of GDOS and GDMS signals correlated very well showing deviations less than 3%. Figure 1 shows relative variations of the water signal with time. Experiences with nonhygroscopic targets (Cu) allowed us to distinguish between water coming from the IT0 target (0) and chamber walls (A ). It can be seen that the major contribution to water content comes from target desorption during the first 30 min. This water can either be extracted by ionic impact or, more likely, desorbed as a result of the heating of the target surface due to ionic bombardment. On the other hand, the chamber wall contribution is probably due to desorption caused by energetic electrons scattered in the gas atmosphere. Figure 1 shows also the evolution of the GDOS In0 signal. The first decay and subsequent increase should be explained in terms of a decrease in the sputtering yield caused by 447
M Clement
et a/: IT0
rf sputtered
thin
films
.
. .
0
0
0 0 0
.
0 l
0
0
0 0
-10
. . 1)
p20
10
30
40
A.
(min)
Figure 1. (0) H,O signal (water coming from IT0 target and chamber walls). (A) H,O signal (water coming from chamber walls (Cu target)). (0) GDOS In signal. the presence of light H+ and Hz ions coming from water molecule dissociation which do not contribute to the extraction. This effect explains the lack of reproducibility observed in the deposition rate. In fact, films produced under the same water content conditions, and after all the water was desorbed from the target, no longer exhibited such lack of reproducibility. Figure 2 shows the mobility, average transmittance and resistivity of films produced in the same sputtering conditions (70 W, 2 x lo-’ torr) vs relative water content measured by GDMS. A strong correlation between both magnitudes can be seen. The resistivity also showed an increase due to the dependence of the carrier density on the oxygen vacancy concentration. Part of these vacancies otherwise present in the films are probably annihilated either by 0 or by OH groups coming from water dissociation. The variation observed in mobility and transmittance is probably associated with defect centres associated with excess metal (nonbonded In or Sn atoms) which act as scattering centres. A similar kind of defect has been reported in ref 6. The presence of large amounts of water in the discharge therefore reduces the number of these defects. Measurements of mobility at variable temperature allowed us to eliminate grain boundaries as scattering centres. In fact,films grown with high values of substrate bias (small grain size) exhibited high values of mobility.
.
.
x’
-m
60
50 TIME
A.
1102O POWER
(W)
Figure 3. Transmittance (O), mobility ( x ) and carrier density (A) YSrf incident power of films 0.5 pm thick produced at 2 x lo-’ torr. The dependence of film properties on the rf incident also studied. This study was performed in a very sphere, after all the target water had been eliminated. the variations of film average transmittance, mobility concentration are shown. The observed minimum presence of two competitive mechanisms.
(aI
power were clean atmoIn Figure 3 and carrier suggests the
,o) XP A
n
200 POWER
(W)
X
:”
50
Figure 2. Transmittance (0 ), mobility ( x ) and resistivity (A ) vs relative water content in arbitrary units in 0.5 pm thick produced at 70 W and 2 x lo-* torr. 448
100
150 200 TEMPERATURE
GO i’C)
Figure 4. (a) Mobility ( x ) and carrier density (A) vs rf incident power of films produced at 2 x 10m2torr keeping temperature constant at 200°C with external heater. (b) Mobility ( x ) and carrier density (A) vs substrate temperature of films produced at 2 x lo-* torr keeping constant the rf incident power at 50 W.
M Clement et al: IT0 rf sputtered thin films
Experiments performed at fixed rf power and varying temperature, and at fixed temperature (with an external heater) and varying rf power, allowed us to distinguish between the effect caused by secondary electron bombardment and the heating effect associated with this bombardment. Figure 4(a) shows the variation of mobility and carrier concentration with rf power at 200°C. Carrier concentration remains roughly constant. The decrease in the mobility should be associated with scattering by defect centres (quoted above) generated by electronic impact. Figure 4(b) shows variations of mobility and carrier concentration of films produced at 50 W rf power with substrate temperature. An increase in the growth temperature enhances atom mobility and the defect concentration decreases because of a more-ordered growth or because of metallic atom re-evaporation. The occurrence of these two competitive mechanisms explains the behaviour shown in Figure 3. The electronic impact related effect dominates over the heating one and both mobility and transmittance decrease due to defect generation. Above 70 W the heating effect becomes dominant and mobility and transmittance increase because the defects are annihilated or annealed out. This experience was reproduced for several different water contents in the discharge resulting in the same qualitative behaviour. The same effect of defect elimination can be achieved by biasing the substrate. Ion bombardment causes resputtering of non bonded metallic-related species probably responsible for the carrier scattering. Combining high power levels and high bias voltages we
obtained films with resistivity values of 5 x 10m4 0 cm mobilities up to 35 cm’ (V s)- ’ and average optical transmittance of 95% (thickness = 0.5 pm) in a reproducible way.
4. Conclusions The use of GDOS and GDMS to study the evolution of sputtering gas composition has allowed us to determine the origin of the lack of reproducibility in film properties and deposition rate which appear in the sputtering of compound IT0 targets. Large amounts of water adsorbed in the target during exposure to the ambient are desorbed during the first 30 min. Small variations in water concentration in the sputtering gas cause great variations in film properties and sputtering yields. After all the water from the target was desorbed, the effect of rf power was also studied. Two effects have been found: one is due to electron impact and the other to the substrate heating associated to electronic bombardment. Defect generation and annihilation due to these effects explain the observed behaviour.
References
1J L Vossen, RCA Rev, 32,269 (1971). * J Szczyrtbowski et al, Physica Status Solidi (a), 78, 243 (1983). 3 I Mar&l et al, Thin SolidFilms, 114, 327 (1984). 4 E Iborra, PhD Thesis Madrid (1986). ’ W D Westwood and R J Boy&, J &pl Phys, 44,261O (1973)
449