Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATON OF THIN FILMS FOR VARIOUS APPLICATIONS"

Abstract

The present invention reports a modified pulse plasma process for preparation of thin films for various applications which comprises placing a substrate in a conventional glow discharge reactor, maintaining the substrate temperature in the range of room temperature to 300°C, passing source gases and if required, diluting gases over the said substrate in the said reactor, at a pressure in the range of 0.05-0.7 torr, applying voltage to the said reactor using a pulse power supply capable of power modulation (pulse mode operation), operating in the frequency range of 10MHz to 150MHz, having modulation frequency in the range of 1-10Hz, power level of at least two watts, modulation depth in the range of 80-99%, duty cycle in the range of 2-25%, to create a plasma atmosphere around the said substrate being maintained at the said temperature, in the presence of source gases and diluting gases, if required, maintained at the said pressure, in the said reactor to get the thin films deposited uniformly on the substrate taken

Full Text

The present invention relates to a modified pulse plasma process for preparation of thin films for various applications The present invention particluarly relates to an improved process using very high frequency(VHF) discharges for the deposition of thin film materials such as silicon, carbon, germanium and their alloys such as silicon carbide, silicon nitride, etc useful for electronic, optical, tribological applications. The thin film materials produced by the process of the present invention will find usage in photovoltaic cells, thin film transistors, photoconductors, thin film insulators and devices incorporating films of several micron thickness such as photoreceptors, xerox copiers, radiation and particle detectors, Optical Spatial Light Modulators, tribological coatings and coatings for optical applications. Since the deposition rate is high, shorter deposition times saves energy and time and since the process efficiently utilises the source gases it saves material consumption as well. Thus cost of production especially for thick films is reduced significantly. Device quality hydrogenated amorphous silicon (a-Si:H) is normally deposited by decomposing silane in a conventional radio frequency(RF) plasma chemical vapour deposition(CVD) reactor operating at 13.56 MHz in a parallel plate capacitor geometry. In this mode of operation the r.f power is applied in a continuous wave (CW) form. Over a period of time, a set of parameters have been realised under which the properties of the material deposited satisfy more or less the needs of the photovoltaic/device industry. The set of parameters as described by Y.Hishikawa.S.Tsuda, K.Wakisaka and Y. Kuwano, in Journal of Appl.Phys. V73,4227(1993) are, for a standard parallel plate capacitor glow discharge reactor with 2-4 cm interelectrode separation, 100% silane at a pressure of 100-300 mTorr, substrate temperature in the range of 150-250°C and the r.f. power - 25mW/cm2, applied in the continues wave(CW) mode. Under these conditions the deposition rate obtainable is very low 1-2 A/sec and the material posses typically, a dark conductivity of 10~10 -10"11 ( ohm"1 cm"1 ) ', a photoconductivity of 10"4 -10"5 (ohm'1cm"1) at an illumination 100 mW/cm2 with spectral distribution equivalent to an air mass 1.5, and a hydrogen concentration , (10 at% and has an optical bandgap of 1.75 eV (0.05 eV with midgap density of states(DOS) ~1015/cm3 /eV. Attempts to increase the growth rate beyond this value by applying higher r.f. power has been found to degrade the film's optoelectronic properties rendering the material useless as an electronic material. This was mainly attributed to particle formation via gas phase reactions and secondary plasma reactions which leads to the incorporation of more polyhydride groups in the film. Reference in this connection may be made to A.Bouchoule, A.PIain, L.Boufendi, J.P.BIondeau and C.Laure in Journal of Applied Physics, 69,1991,1991. Efforts that were directed to deposit device quality a-Si:H films at higher rates include use of higher silanes as described by B.A.Scott, M.Brodsky, D.c.Green, P.B.Kirgy, R.M.Phecenick and E.E.Simonyi, in Applied Physics Letters, V37.725.1980 and by K.M.H.Maessen, MJ.M.Pruppers, F.H.P.M.Habraken.J.Bezemer and W.F.Van der Weg as described in Journal of Non Crystalline Solids, V77&78.785.1985 using silane diluted with hydrogen. Similar process has been reported in use by J.C.Knights, R.A.Lujian, M.P.Rosenblum, R.A.Street, D.K.Beigleson and R.A.Reimer as described in Applied Physics Letters, V38,331(1981) using helium diluted silane. Also, the modified electrode geometry as described by T.Hamasaki, M.Ueda, A.Chayahara, M.Hirose and Y.Osaka and given in Applied Physics Letters V44,1049(1984) and the technique using VHP discharges in the 100MHz range as described by H.Chatham,P.Bhat, A.Benson and C.Matovich in the Journal of Non Crystalline Solids, V115,201,1989 and that of H.Curtins, N.Wyrsch and A.V.Shah as described in Electron Letters, V23.228.1987 and also use of ECR discharges as described by S.Kato and T.Aoki, in Jurnal Non Crystalline Solids,V77&78, 813,1985 have been used to prepare this films. The properties of the films thus deposited at rates in the range of 10-20 A/sec in some ways were found to be comparable to films deposited at very low rates. All these discharges employed the CW mode of operation. Efforts as described by T.Hamasaki, M.Ueda, M.Hirose and Y.Osaka in Journal of Non Crystalline Solids,V59&60,679,1983, by G.Scanbrook.l.P.Liewellyn.S.M.Ojha and RAHeinecke in Vacuum 38,627,1988, by Y.Watanabe, M.Shiratani, Y.Kubo.l.Ogawa and S.Ogi, Applied Physics Letters V53,1263(1988), by LJ.Overzet and J.T.Verdeyen, Applied Physics Letters, V48,695(1986) and by Y.Watanabe, M.Shiratani and H.Makino, Applied Physics Letters, V57,1616(1990) were directed towards the pulsed mode of operation of 13.56MHz wherein the r.f. power was applied in square wave modulated mode with 100% modulation and employed very high r.f. power levels range with "ON" periods in the range of few milliseconds. In these efforts the time averaged deposition rate was reported to be higher. Some of these pulsed techniques use 100% modulated KW of R F Power of few milliseconds duration and /or gas delivery systems which let in the gases the short bursts. The main objective of the present invention is to provide an improved process for the preparation of thin film materials useful for electronic, optical and tribological applications which obviates the draw backs of low productivity i.e low deposition rate. Another objective of the present invention is to provide an improved process using very high frequency(VHF) pulse mode discharges for the deposition of thin film materials. Accordingly, the present invention provides a modified pulse plasma process for preparation of thin films for various applications which comprises placing a substrate in a conventional glow discharge reactor, maintaining the substrate temperature in the range of room temperature to 300°C, passing source gases and if required, diluting gases over the said substrate in the said reactor, at a pressure in the range of 0.05-0.7 torr, applying voltage to the said reactor using a pulse power supply capable of power modulation (pulse mode operation), operating in the frequency range of 10MHz to 150MHz, having modulation frequency in the range of 1-10Hz, power level of about two watts, modulation depth in the range of 80-99%, duty cycle in the range of 2-25%, to create a plasma atmosphere around the said substrate being maintained at the said temperature, in the presence of source gases and diluting gases, if required, maintained at the said pressure, in the said reactor to get the thin films deposited uniformly on the substrate taken In an embodiment of the present invention the substrates used for deposition of the thin film may be such as glass, glass coated with transparent conducting oxides, silicon wafers and other semiconducting materials, plastics, paper and metals. In yet another embodiment of the present invention the source gases used for preparation of the thin film materials may be such as silane.disilane, methane, acetylene, benzene vapours, germane, nitrogen, ammonia. In still another embodiment of the present invention the diluting gases used for diluting the source gases may be such as hydrogen, helium and other inert gases. In the present invention no elaborate changes in the design of the Plasma Enhanced Chemical Vapour Deposition(PECVD) reactor or the power supply is required. The power is modulated at low frequencies with less than 100% power modulation with the high and low powers properly chosen. The "ON" period of the high power is changed upto several tens of milliseconds. The source gases are fed into the chamber by controlling the flow by mass flow controllers and the pressure is controlled by the combination of a throttle valve and a baratron pressure sensor. The temperature of the substrates is controlled by a temperature controller. The effects of pulse parameters on the properties of the material have been systematically studied. Films were deposited using silane, disilane & hydrogen & helium diluted silane using r.f. & VHF discharges. Using this Modified Pulsed Plasma Deposition (MPPD) method, a-Si:H films were deposited at rates higher than the CW values with optoelectronic properties comparable to or better than those deposited by the conventional CW technique. The effects are much more pronounced at VHF frequencies and under moderately diluted silane discharges(20-80%). The improved process of the present invention allows deposition such as of hydrogenated amorphous silicon at deposition rates > 10A/sec without deteriorating the optoelectronic and other material properties. In this process films from less than a micron to several micron thick can be economically deposited. This invention is described in detail in the following examples which are provided by way of illustration only and therefore should not be construed to limit the scope of the invention. Example-1 In one deposition silicon substrates were cleaned and placed in the PECVD reactor and the reactor was pumped to a base pressure better than 10 -6 torr. The substrates were heated to 275°C. The source gas silane and the diluting gas helium were let into the reactor through mass flow controllers. The silane flow was 24 sccm and helium flow was 8 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 13A/sec. have a hydrogen content of 3at% and showed a photoconductivity of 10~5 and a photosensitivity of 105 Example-2 In one deposition glass substrates coated with conducting oxide were cleaned and placed in the PECVD reactor and the reactor was pumped to a base pressure better than 10~6 torr. The substrates were heated to 275°C . The source gas silane and the diluting gas helium were let into the reactor through mass flow controllers. The silane flow was 24 sccm and helium flow was 8 seem and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 13A/sec. have a hydrogen content of 3at% and showed a photoconductivity of 10"5 and a photosensitivity of 105. Example-3 In one deposition glass substrates were cleaned and placed in the PECVD reactor. The reactor was pumped to a base pressure better than 10~6 torr. The substrates were heated to 275°C . The source gas silane and the diluting gas helium were let into the reactor through mass flow controllers. The silane flow was 24 sccm and helium flow was 8 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 13A /sec. have a hydrogen content of 3at% and showed a photoconductivity of 10"5 and a photosensitivity of 105. Example-4 In another deposition silicon substrates were cleaned and placed in the PECVD reactor. The reactor was pumped to a base pressure better than 10"6 torr. The substrates were heated to 275°C. The source gas silane and the diluting gas hydrogen were let into the reactor through mass flow controllers. The silane flow was 24 sccm and hydrogen flow was 8 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 12.5A/sec. have a hydrogen content of 6at% and a photoconductivity of 2x10"5 and a photosensitivity of 4x105. Example-5 In another deposition glass substrates were cleaned and placed in the PECVD reactor. The reactor was pumped to a base pressure better than 10~6 torr. The substrates were heated to 275°C . The source gas silane and the diluting gas hydrogen were let into the reactor through mass flow controllers. The silane flow was 24 sccm and hydrogen flow was 8 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 12.5A/sec. have a hydrogen content of 6at% and a photoconductivity of 2x10"5 and a photosensitivity of 4x10 5. Example- 6 In another deposition glass substrates coated with conducting oxide were cleaned and placed in the PECVD reactor. The reactor was pumped to a base pressure better than 10~6 torr. The substrates were heated to 275°C . The source gas silane and the diluting gas hydrogen were let into the reactor through mass flow controllers. The silane flow was 24 sccm and hydrogen flow was 8 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 60W and 10W at a modulation frequency of 2Hz with a dwell time of 25milliseconds. The films, deposited at the rate of 12.5A/sec. have a hydrogen content of 6at% and a photoconductivity of 2x10"5 and a photosensitivity of 4x10 5. Example-7 In another deposition the substrates such as glass coated with conducting oxide were cleaned and placed in the PECVD reactor. The reactor was pumped to a base pressure better than 10"6 torr. The substrates were heated to 275°C . The source gas silane was let into the reactor through mass flow controllers. The silane flow was 24 sccm and the reactor pressure was maintained at 0.3torr using baratron and throtlle valve and their controllers. The VHF discharge was created by applying the voltage using a 100MHz power source. The power was modulated between 75W and 10W at a modulation frequency of 2Hz with a dwell time of 50milliseconds. The films, deposited at the rate of 15A/sec. have a hydrogen content of 13at% and a photoconductivity of 2x10"7 and a photosensitivity of 4x104 . A representative set of data is presented for the photoconductivity,ph, optical gap,Eg, hydrogen concentration,CH in atomic percentage, the microstructure factor, R etc. in tables I and II to support this claim.lt can be seen that in VHF MPPD the hydrogen concentration,CH & microstructure factor, R are lower than CW values. Also, the optical bandgap,Eg is not very much affected by deposition conditions. TABLE-I; Comparision of a-Si:H films deposited at different conditions on all substrates. (Table Removed) TABLE-II: Disorder parameters of a-Si:H deposited under different conditions on all substrates: (Table Removed) The main advantages of the improved process of the present invention are, i it provides a method to deposit a-Si:H at higher rates with optoelectronic and other properties comparable to that obtainable with conventional CW method for use in thin film devices such as photovoltaic cells, thin film transistors, photoconductors, thin film insulators and devices incorporating films of several micron thickness such as photoreceptors, xerox copiers, radiation and particle detectors, Optical Spatial Light Modulators, etc. ii) It offers digital processing capability by way of operating at two diffrerent power levels in the deposition. iii) Also, one can control the magnitude of the self bias generated in a PECVD process and its duration by controlling the HPL and its dutcycle, pulse width and modulation frequency. This will be useful where controlled ion bombardment of the growth surface is required to control adhesion, stress and other properties of thin films. We Claim: 1 An improved process for preparation of thin films for various applications which comprises, a. placing a substrate in a conventional glow discharge reactor, maintaining the substrate temperature in the range of room temperature to 300°C, b. passing source gases and if required, diluting gases over the said substrate in the said reactor, at a pressure in the range of 0.05-0.7 torr, c. applying voltage to the said reactor using a pulse power supply capable of power modulation (pulse mode operation), operating in the frequency range of 10MHz to 150MHz, having modulation frequency in the range of 1-1 OHz, power level of about two watts, modulation depth in the range of 80-99%, duty cycle in the range of 2-25%, to create a plasma atmosphere around the said substrate being maintained at the said temperature, in the presence of source gases and diluting gases, if required, maintained at the said pressure, in the said reactor to get the thin films deposited uniformly on the substrate taken 2 A process as claimed in claim 1, wherein the substrates used for deposition of the thin film are glass, glass coated with transparent conducting oxides, silicon wafers and other semi conducting materials, plastics, paper and metals. 3 A process as claimed in claims 1-2, wherein the source gases used for preparation of the thin film are silane, disilane, methane, acetylene, benzene vapours, germane, nitrogen, ammonia. 4 A process as claimed in claims 1-3, wherein the diluting gases used for diluting the source gases are hydrogen, helium and other inert gases. 5 An improved process for preparation of thin films for various applications substantially as herein described with reference to the examples.

Documents:

2373-del-1998-abstract.pdf

2373-del-1998-claims.pdf

2373-del-1998-correspondence-others.pdf

2373-del-1998-correspondence-po.pdf

2373-del-1998-description (complete).pdf

2373-del-1998-form-1.pdf

2373-del-1998-form-19.pdf

2373-del-1998-form-2.pdf