Chemical vapor deposition of transparent electrically conducting layers of indium oxide doped with tin

Thin Solid Films, 29 (1975) 155-163 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland

155

CHEMICAL VAPOR DEPOSITION OF TRANSPARENT ELECTRICALLY C O N D U C T I N G LAYERS OF I N D I U M OXIDE DOPED WITH TIN

J. KANE AND H. P. SCHWEIZER Laboratories RCA Ltd., Zurich (Switzerland) W . KERN

RCA Laboratories, Princeton, N. J. (U.S.A.) Received January 15, 1975; accepted February 21, 1975)

The use of the indium chelate derived from dipivaloyl methane, in combination with dibutyl tin diacetate, has been investigated as a CVD technique for the preparation of transparent highly conducting layers of tin-doped indium oxide. A set of optimized deposition conditions is presented together with important optical, electrical, structural and chemical properties of the films. Typical films of the doped indium oxide have n-type resistivity in the range 2.2 x 10 -4 to 7.1 x 10-4 f~ cm. Films with the lowest resistivity exhibit an optimum dopant concentration of around 8 at.% tin in InzO3:Sn. Coatings have been prepared on quartz or glass substrates with sheet resistances of 10-50 f~/I--] and optical transmission in excess of 80 % net throughout the visible spectrum.

1. INTRODUCTION Indium oxide doped with tin is widely accepted as the best available material for the fabrication of transparent conducting films for device applications in which low resistivity is of prime importance 1-3. Tin oxide doped with antimony is also widely used as a transparent conductor but the sheet resistivities attainable with this material are not as low as those obtainable with tin-doped indium oxide. Resistivity values reported for tin-doped indium oxide are in the range 1.8 × 10 -4 to 6.3 × l 0 - 4 ~ cm 4-1°. Coatings of tin-doped indium oxide have been prepared by r.f. sputtering of a mixed oxide target 7-13 or by reactive sputtering in oxygen of an indium/tin alloy target 4" 5, 14-21. Excellent coatings can be produced by this technique but as a commercial process it has the severe disadvantage of high equipment cost and relatively low production rate. Other techniques, including spray hydrolysis of halides 6' 2a-2s and reactive evaporation of metals 26, have been used with varying degrees of success. Chemical vapor deposition (CVD) techniques, have not been reported except for the preparatio n of undoped indium oxide films 27' 28. The object of the present investigation was to devise a CVD technique for the preparation of tin-doped indium oxide coatings based on the use of

156

J. KANE, H. P. SCHWEIZER, W. KERN

volatile metallo-organic or organometallic compounds of tin and indium. Such compounds of indium have not been extensively studied but it is clear from the available literature that volatile thermally stable compounds of indium are relatively uncommon. The chelates of 13-diketones are about the only materials readily available which fulfill the requirements of volatility, thermal stability at a temperature sufficiently high to produce an adequate vapor pressure, and thermal instability in the CVD higher temperature range. Since the 13-diketonate chelates derived from dipivaloyl methane (2,2,6,6,-tetramethylheptane-3,5-dione) are known to be thermally stable below 400 °C and sufficiently volatile, it was decided to base the investigation on this particular chelate of indium. Large numbers of volatile tin organometallic compounds are readily available. For the present investigation dibutyl tin diacetate was used since we had previously used it successfully in CVD work for the preparation of conductive transparent films of tin oxide 29' 30. 2. FILM DEPOSITION

2.1. Apparatus The deposition apparatus was a rotating hotplate reactor of the type used for the CVD of oxide and silicate glass films on semiconductor device wafers 31, 32 The gas inlet system was modified for the use of the rather mildly volatile indium compound. The indium and tin compounds were placed in closed containers maintained at controlled temperatures in the range 180°-220 °C and 80°-120 °C, respectively. Connections between the source chambers and the main reaction chamber were heated to prevent condensation of the organometallic compounds during transport. A schematic diagram of the deposition apparatus showing the points of entry of the reactant gas streams is presented in Fig. I. 2.2. Chemicals Both organometallic compounds employed are available commercially but the indium chelate is rather expensive. It may be conveniently prepared by the method of Eisentraut and Sievers 33 using hydrated indium chloride and the organic ligand, dipivaloyl methane. The yield in this reaction is approximately 80 ~ and the product is purified by recrystallization or by sublimation. The vapor pressure of the chelate 34 as a function of temperature is shown graphically in Fig. 2. 2.3. Study of deposition parameters The parameters of gas flow, gas composition, substrate temperature and geometry of the reaction chamber were adjusted empirically in a large number of experiments, until uniform indium oxide coatings with the optimum tin doping level were obtained. The optimum source temperature for the indium chelate was found to be 220 °C, at which temperature the vapor pressure of the compound is approximately 10 torr, as seen from Fig. 2. The metallo-organic indium compound was transported to the reaction chamber by a stream of heated inert carrier gas through gas lines heated to 250 °C to prevent condensation.

157

TRANSPARENT ELECTRICALLY CONDUCTING I n 2 0 3 : S n FILMS

OXYGEN ~,,,-

N2

N2 CARRIER GAS

N2 CARRIER GAS

'~ '

FURNACE

re;: INDIUM

....

;' COMPOUND

H20

H20

TIN COMPOUND SUBSTRATE ~;!

"

:,

: ; I Ilnnmu|]lH ]

'6

HOTPLATE REACTOR

Fig. 1, Schematic representation oftheCVDapparatus. •

,

,

,

,

,

•

.,

,

.~O

-,°\ IO

. . . . . . .

r,c

Fig. 2. Vapor pressure of indium chelate as a function of temperature.

158

J. KANE, H. P. SCHWEIZER, W. KERN

A dopant q,uantity of the volatile tin compound was introduced into the gas stream prior to its entry into the main reaction chamber. The tin organometallic source chamber was maintained at 98 °C in a boiling water bath and the rate of introduction of the dopant was controlled by the rate of flow of the carrier gas. The reactant gas, which was normally an oxygen-nitrogen mixture or nitrogen saturated with water vapor, was pre-heated and introduced into the reaction chamber through a separate inlet. At deposition temperatures in excess of 37Y the In203:Sn films began to form but the growth rate at temperatures below 450 °C was too low to be practical. At temperatures above 450 °C the growth rate was higher and the electrical properties of the films were superior. Using quartz substrates, the films with the lowest resistivities were produced at the highest deposition temperature (555 °C). Such a high deposition temperature is not practical as it is above the softening point of soda lime glass, which is the substrate material of most commercial interest. A high deposition temperature also increases the rate of diffusion of alkali cations from the substrate into the oxide layer. These cations act as a p-type doping agent in the n-type oxide film, thus neutralizing some of the charge carriers and reducing the conductivity. High deposition temperatures are therefore undesirable in processing glasses of high alkali content. A third undesirable aspect of a high deposition temperature is the greater surface roughness of the films, as is shown by the scanning electron micrographs in Fig. 3. From the point of view of overall film quality a deposition temperature of 500 °C is the best compromise. At this temperature films have been prepared with a resistivity of 5 x 10 -4 ~ cm, of excellent thickness uniformity and with acceptable surface quality. 3.

FILM PROPERTIES

3.1. Electrical and chemical properties The electrical resistance of the films was measured using a conventional four-point probe. Measurements of film thickness were made using a Sloan Dektak equipped with a 25 nm stylus. For the measurements a step was etched in the film with warm 37 ~ hydrochloric acid. The resistivities of several coatings on fused quartz substrates together with corresponding tin dopant concentrations expressed as a tin-to-indium weight ratio are presented in Table I. The tin concentrations were measured by atomic absorption analysis with an error limit of about 5 ~. The concentrations investigated ranged from 0.045 to 0.12 in terms of the Sn/In weight ratio. These results indicate that a Sn/In weight ratio of 0.045 yields films with the lowest resistivity. A 0.045 weight ratio of Sn/In is equivalent to a Sn dopant concentration of about 8.0 at.~,/, (or approximately 3.6 wt. ~ Sn) in the In203:Sn film. Identical and closely similar results to those shown in Table I were obtained with substrate plates of polished silicon, sapphire and various commercial silicate glasses. Spark-source mass spectrographic analysis was used to examine for the presence of trace impurities in the films. Iron (5.4 ppma) and chlorine (1.4 ppma) were the only impurities present above the 1 ppm level.

TRANSPARENT ELECTRICALLY CONDUCTING I n 2 0 3 :Sn FILMS Sample (!)

159 Sample (2)

3750 ×

7500 x

15000x Fig. 3. Scanning electron micrographs of two samples of indium oxide:tin deposited on vitreous fused quartz plates. Sample (1): deposition temperature 530 °C, sheet resistance 14.0 tl/D, Sn/In weight ratio 0.0857. Sample (2): deposition temperature 555 °C, sheet resistance 10.0 tl/V1, Sn/In weight ratio 0.0494.

0.0494 0.0448 0.0744

0.0926 0.0857

-

0.118

90.7 17.0 21.0 43.0 74.0 390.0 28.5 14.0 9.0 39.4 10.0 2.95 9.5

Sheet resistance (f2/I-])

Deposition time 10 min (except 30 min for 500 °C samples). Substr~tes: plates of fused quartz.

1160 2500 2500 2500 2500 2500 1160 1160 1160 420 1160 2040 1160

2200 2500 2700 1650 5050 4900 2300 3800 8500 2000 4800 ~ 7500 ~ 7500

420 1500 1000 550 180 0 420 420 1160 1160 420 420 1160

1 2 3 4 5 6 7 8 9 10 11 12 13

475 500 500 500 500 500 505 530 530 530 555 555 555

Film thickness (/~)

Deposition Deposition S n - N 2 1n-N2 Sn/ln run no. temperature (cm 3 m i n - 1) (cm 3 min 1) weight ratio (°C) 20 4.3 5.7 7.1 37 190 6.6 5.3 7.7 6.9 4.8 ~2.2 ~ 7.1

Film resistivity (10-4~cm) 74 86 81 89 65 70 74 70 61 78 85 57 52

4000 79 80 81 82 92 75 89 94 78 78 79 64 67

4400 86 89 94 80 80 93 99 80 80 82 99 68 72

4800 95 97 99 84 99 84 95 92 87 89 84 71 73

5200

Net °/o o transmission (,~)

C HE M IC AL C O M P O S I T I O N A N D P H Y S I C A L P R O P E R T I E S OF T I N - D O P E D I N D I U M OXIDE FILMS F O R V A R I O U S D E P O S I T I O N C O N D I T I O N S

TABLE I

100 96 93 90 89 83 88 100 82 98 87 74 73

5600

100 89 87 96 82 94 84 89 88 100 99 76 76

6000

89 89 89 87 85 83 88 87 79 88 89 68 67

avg.

z ,v

TRANSPARENT ELECTRICALLY CONDUCTING

In203 : S n

161

FILMS

3.2. Optical properties Good optical transmission throughout the visible spectrum is usually an important requirement of transparent conducting films. An average transmission in excess of 80 % net between 4000 and 6000/k was taken to be acceptable for most device applications. The transmission data were measured with an uncoated substrate in the reference beam. Due to interference effects the light transmission can be peaked at any given wavelength in the visible spectrum by a correct choice of film thickness. An average value for the transmission throughout the visible region is therefore a better measure of the optical quality of the films. The transmission values presented in Table I show average values ranging from 67 to 89 %. The films with the best overall qualities (deposited at 500 °C) have an average transmission of 89 %, a resistivity of 4.9 x 10 -4 f~ cm and a Snfln weight ratio of 0.0926. The films with the lowest resistivity (2.2 x 10 -4 f~ era) deposited at 555 °C have an average transmission of only 68%, which may be marginal for some applications. Infrared absorption spectra in the range 2000-667 cm- 1 for films on specially prepared silicon substrate wafers exhibited strong absorbance without specific bands. The refractive index of the tin-doped indium oxide films was measured ellipsometrically for films on silicon substrates which had been deposited simultaneously with the films on quartz substrates. The results are recorded in Table II. The wide variation of the measured refractive index (1.67-2.48) is not fully understood. However, in previous work with CVD-deposited tin oxide, similar variations of refractive index with deposition conditions were f o u n d 29. Refractive index values of 2.03 and 1.95, measured at 480 and 620 nm, were reported by Fraser and C o o k 9 for sputtered films of tin-doped indium oxide.

3.3. Surface morphology Films deposited under optimum conditions exhibited very smooth structureless surfaces. If deposited at higher substrate temperatures the film surfaces TABLE II REFRACTIVE INDEX AND THICKNESSOF TIN-DOPED INDIUMOXIDEFILMSON SILICONBY ELLIPSOMETRY

Deposition run no.

1 2 3 4 5 6 7 8 11 12 13

Deposition Film thickness temperature (°C) (A)

475 500 500 500 500 500 505 530 555 555 555

2216 2383 2287 2096 5375 4615 2349 3837 4818 6251 6148

Refractive index at 5461 n

Confidence limits o f n

1.5812 2.0159 2.0484 1.7172 1.9984 1.6788 2.0345 2.1168 2.0148 2.4809 2.2766

Not computed 2.0147-2.0171 2.0473-2.0495 1.6977-1.7367 1.9974-1.9995 1.6776-1.6801 2.0333-2.0357 2.1143-2.1194 2.0023-2.0269 2.4760-2.4855 2.2736-2.2796

162

J. KANE, H. P. SCHWEIZER, W. KERN

became progressively rougher with the increasing size of the crystallites composing the film. As grown at relatively low substrate temperatures of 400°-450 °C the films showed no evidence of crystallinity and the surfaces were smooth. Scanning electron micrographs of two samples deposited at 530° and 550 °C are shown in Fig. 3. These samples have a distinctly grainy appearance at 20 000 x magnification, especially sample (2) which was deposited at the higher temperature. This degree of surface roughness is still acceptable for device applications. 4. DISCUSSION It has been established that high quality tin-doped indium oxide films can be deposited by a CVD process based on the use of volatile organometallic compounds. At present the technique is only of laboratory interest since the indium compound is too expensive to be used commercially. The project has demonstrated, however, that a CVD technique is capable of producing films of quality comparable with those produced commercially by spray hydrolysis or a sputtering technique. To adapt the process for commercial production a cheap volatile indium compound needs to be found. This source material would ideally be more volatile and it would be a liquid at room temperature, since this would greatly simplify the vapor transport procedure. The compound used at present is a solid of low volatility which is difficult to transport in a reproducible manner. The high temperature (250 °C) used in the volatilization also causes slight decomposition of the compound and this produces additional difficulties in the transport process. Unfortunately there are no indium organometallics available at present which fulfill these requirements. 5. CONCLUSIONS

It has been shown that highly conducting coatings of tin-doped indium oxide (10-50 ~ / [ ] ) with a net optical transmission in excess of 80% can be prepared by a CVD process based on the co-oxidation of the volatile indium chelate derived from dipivaloyl methane and dibutyl tin diacetate. The tin concentration measured in films deposited under widely varying temperature and reactant flow conditions ranged from 0.045 to 0.118, expressed as a tin-to-indium weight ratio. The ratio yielding the lowest film resistivity was found to be 0.045 or slightly below. This value corresponds to a concentration of approximately 8.0 at. % or about 3.6 wt.% of Sn in the In2Oa:Sn film. Specific resistivities as low as 2.2 × 10 -4 f~ cm have been measured with average transmission greater than 80%, but typical samples are in the range 4.8 x 10-4-5.8 x 10 -4 ~ cm. For comparison, undoped In20 3 films deposited at 425 °C by this method had a resistivity of 30 x 10 -4 f~ cm. The optimum substrate temperature of deposition is to a certain extent dependent on the thermal stability of the substrate material but for inert materials like vitreous quartz a deposition temperature of 555 °C is considered to be optimum. Soda glass requires a lower processing temperature (500 °C) due to the migration of alkali metal ions into the transparent coating which tends to depress the electrical conductivity.

TRANSPARENT ELECTRICALLY CONDUCTING I n 2 0 3 : S n FILMS

163

ACKNOWLEDGMENTS

The authors would like to thank the following persons for their contributions in the instrumental analysis part of the project: E. Meier, H. Meier and D. P. Bortfeld at Zurich; E. M. Botnick, E. Choo, B. J. Seabury and J. M. Shaw at Princeton. REFERENCES 1 R.D. King, Aircr. Eng., 44 (1972) 4. 2 R.D. King, Mar. Eng. Rev., May 1973. 3 R.D. King, Automotive Engineering Congress, 1974, Society of Automotive Engineers, Detroit, Mich., U.S.A., 1974. 4 Y.T. Sihvonen and D. R. Boyd, Rev. Sci. lnstrum., 31 (1960) 992. 5 U.S. Patent 3,235,476 (1966), to D. R. Boyd, Y. T. Sihvonen and C. D. Woelke. 6 R. Groth, Phys. Status Solidi, 14 (1966) 69. 7 J.L. Vossen, RCA Rev., 32 (1971) 289. 8 J.L. Vossen and E. S. Poliniak, Thin Solid Films, 13 (1972) 281. 9 D.B. Fraser and H. D. Cook, J. Electrochem. Soc., 119 (1972) 1368. 10 U.S. Patent 3,749,658 (1973), to J. L. Vossen. 11 J.R. Bosnell and R. Wagnorne, Thin Solid Films, 15 (1973) 141. 12 M. Hecq, A. DuBois and J. Van Cakenberghe, Thin Solid Films, 18 (1973) 117. 13 M. Hecq, A. DuBois and J. Van Cakenberghe, C. R. du let Colloq. Int. de Pulv~risation Cathodique, Montpellier, 1973, Soci6t6 Fran~aise du Vide, Paris, p. 151. 14 V.A. Williams, J. Electrochem. Soc., 113 (1966) 234. 15 V.M. Vainshtein and V. I. Fistul', Soy. Phys.-Semicond., 1 (1967) 104. 16 V.I. Fistul' and V. M. Vainshtein, Soy. Phys. Solid State, 8 (1967) 2769. 17 German Patent 1,909,869 (1969), to F. H. Gillery and J. P. Pressau. 18 German Patent 1,909,910 (1969), to F. H. Gillery and J. P. Pressau. 19 J . M . Pankratz, J. Electron. Mater., 1 (1972) 182. 20 R.R. Mehta and S. F. Vogel, J. Electrochem. Soc., 119 (1972) 752. 21 M, M. Bonnet and Marchal, C. R. du ler Colloq. Int. de Pulv~risation Cathodique, Montpellier, 1973, Soci6t6 Franctaise du Vide, Paris, p. 157. 22 U.S. Patent 2,516,663 (1950), to M. J. Zunick. 23 R.E. Aitchison, J. Aust. Appl. Sci., 5 (1954) 10. 24 U.S. Patent 2,694,649 (1954), to M. S. Tarnopol. 25 I. Imai, J. Phys. Soc. Jpn, 15 (1960) 937. 26 D, Furuuchi, Proc. Japan Microeleetronics Soc. Meeting, November 20, 1974, p. 1. 27 V.F. Korzo and L. A. Ryabova, Soy. Phys. Solid State, 9 (1967) 745. 28 L.A. Ryabova and Ya. S. Savitskaya, J. Vac. Sci. TeehnoL, 6 (1969) 934. 29 J. Kane, H. P. Schweizer and W. Kern, J. Electrochem. Soc., 121 (1974) 206c, 15 RNP. 30 J. Kane, H. P. Sehweizer and W. Kern, J. Electrochem. Soc., 121 (1974) 376c, 277 RNP. 31 W. Kern, RCA Rev., 29 (1968) 525. 32 W. Kern and A. W. Fisher, RCA Rev., 31 (1970) 715. 33 K.J. Eisentraut and R. E. Sievers, J. Am. Chem. Soc., 87 (1965) 5254. 34 B, J. Curtis and H. R. Brunner, J. Therm. Anal., 5 (1973) 111.