Title of Invention

A DEVICE FORE THE DEPOSITION OF SEMICONDUCTER THIN FILMS AND A PROCESS THEREFOR

Abstract

A device for the deposition of semiconductor thin films which comprises an open ended cylindrical reactor chamber (1), the said chamber being held vertically by clamps (11 & 12), characterized in that a cylindrical furnace (2) being provided at middle periphery of the said chamber, the said chamber being provided with a spray gun (3) passing through sealed cap (13) fitted to its bottom, the said spray gun being connected through a tube (4) to a funnel (5) containing a reactant solution, the said spray gun being also connected to another tube (14) which delivers the carrier gas through a rotameter (9), the top of the said chamber being provided with a thermocouple (6) and a holder (7) for holding a substrate (8), temperature indicator (10) being connected to the said thermocouple (6).

Full Text

The present invention relates to a device for the deposition of semiconductor thin films and a process therefor. The device of the present invention is particularly useful for depositing ultra thin film compound semiconductor materials by low cost techniques having a fast growth rate, These films should have uniform edge to edge thickness with acceptable electrical, structural and optical properties for the production of opto-electronic devices. Categorically the methodologies of thin film deposition can be either (a) Physical deposition techniques (b) Chemical deposition techniques. It is well known that the former involves capital intensive infrastructure and hence is not suitable for utilisation in low cost thin film devices. The second category involves mainly three possible methods, (i) Electrochemical (ii) Solution growth or chemical bath deposition and (iii) Spray pyrolysis techniques, which are all low cost propositions. However, the methods (i) and (ii) suffer from the difficulty that these are essentially slow deposition processes. The history of spray pyrolysis can be traced back as early as 1910 when transparent conducting oxides was achieved by spray pyrolysis of SnCl4 solution, as reported by Foex, Bull, Soc, Ch£m. Fr 11, 6(1944), in his review. The extension of this technique to sulphide and selenide films took place in the 1960's by R.R. Chamber I in, WBAFB Contract No, AF331657-7919 (1962), J.E. A Hill & R.R, Chamber 1 in, U.S. Patent 3, 148,064 (1964), R,R, Cham-berlin fc. J.S, Skarman, J. Electrochem, Sbc. 113, 86 (1966), J,S. Skarman, Solid State Electron, 8, 17 (1965), R.R. Chamberlin, Ceram. Bull. 15, 698 (1966), and R.R. Chamberlin, J.S. Skarman, Solid State Electronics, 9, 819 (1966). Spray pyrolysis (SP) involves spraying a solution, usually aqueous, containing soluble salts of the constituent atoms of the desired compound on to a heated substrate. Droplets reaching the hot substrate undergoes pyrolytic (endothermic) decomposition and forms a single crystallite or a cluster of crystallites of the products. The other volatile by-products and excess solvent escape as vapour. The substrate provides the thermal energy for the thermal decomposition and subsequent recombination of the constituent species, followed by sintering and re-crystallisation of the clusters of crystallites. The result is a coherent film. The chemical solution is atomised into a spray of fine droplets by a spray nozzle with the help of a carrier gas which may or may not play an active role in the pyrolytic reaction involved, A block diagram of a typical spray pyrolysis set up is shown in Fig.1 of the drawings accompanying this specification. The various parts are (1) Carrier gas, (2) Gas flow meter (3) Filtered solution reservoir (4) Solution flow control (5) Spray gun (6) Substrate, (7) Substrate heater (8) Radiation heater (9) Exhaust (10) Radiation shield gas enclosure (11) Thermocouple, The filtered carrier gas and solution are fed into a spray nozzle at a predetermined constant pressure and flow rate. The substrates are placed on the top of a heater over which the sprayed droplets strike in the form of a vertical cone, from the tip of the spray nozzle. Large area uniform coverage of the substrate may be obtained by employing semiautomatic arrangements. The set-up is generally enclosed in a chamber provided with an exhaust duct to remove the vaporised constituents and to provide a stable flow pattern. There are two recent approaches which has modified the aforesaid technique, M.E, Doty and T.V, Bernitsky, U.S. Patent 4, 659, 247 (1957), P.V, Meyers, Polycrystal 1 ine CdTe n-.i-p Solar Cells, SERl/STR-221-3519 DE 39009437, prepared under sub contract No. ZL-7-06031-2, This is the narrow-gap inverted reactor design which utilises gravity sorting of heavier droplets. However, the substrates in these two processes are kept directly in contact with the heater banks, (as in the standard SP technique) thereby a possibility of contamination of the film cannot be ruled out. The atomisation process and the growth kinetics have been studied in detail by several workers viz. A, Banerjee, Ph.D. Thesis, I IT Delhi (1973), A, Banerjee, S,R. Das, A,P. Thakoor, H.S. Randhawa and K.L. Chopra, Solid State Electronics, 22, 495 (1979) and E. Shanthi, V, Dutta, A. Banerjee and K.L. Chopra, J. Appl. Phys., 51, 6243 (1950), They have found that the deposition process is the net result of (a) spreading of a drop into a disc, (b) pyrolytic reaction between the decomposed reactants, (c) evaporation of the solvent and (d) repetition of the preceding process with succeeding drop rates. Consequently, the film generally contains overlapping discs, each disc corresponding to a single droplet, THE DEFECTS/DRAWBACKS OF PRIOR ART DEVICES & PROCESSES ARE 1. In the standard spray technique the solution droplets directly hit the substrates resulting in two processes of heat transfer : (a) heat required for the chemicals to undergo the reaction, (b) heat taken up by the solvent to go to the gaseous state. These two heat loss mechanisms from the substrate surface may cause thermal shock to the glass substrates resulting in possible cracks. 2. Since the substrates are in direct contact with the heater and the heater is exposed to the reacting chemicals there is a possible chance of contamination of the deposited films from the heater. 3, The sprayed solutions form a conical shape while depositing on the substrates due to which, the outer edges of the deposited films are likely to be thinner. Although the technology involved in references M.E, Doty and T.V. Bernitsky, U.S. Patent 4, 689, 247 (1987) and P.V, Meyers, Polycrystalline CdTe n-i-p Solar Cells, SERI/STR-221-3519 DE 89009437, prepared under sub contract No. ZL-7-06031-2 do not conform to the conical shape of the sprayed solution, non-uniformity in the film edges have also,been reported as drawback by these authors. 4, The films are grown by the overlapping of several discs, hence, there is a high probability that voids are likely to occur in between. Hence ultra thin films with long range continuity cannot be obtained by this process. The object of the present invention is to provide a device useful for the deposition of thin films and a process therefor. in the device & process of the present invention the droplets/mist are first vaporized in a preheated zone. The vapour thus formed inside a vertical glass tube condenses in thin film form on the substrate. As a result, there is no possibility of droplets directly reaching the substrate. Therefore, the growth kinetics of the overlapping discs does not arise in this case. Figs.2(a) & 2(b) accompanying this specification represent the device of the present invention useful for the growing of thin semiconductor films. Accordingly, the present invention provides a device for the deposition of semiconductor thin films which comprises an open ended cylindrical reactor chamber (1), the said chamber being held vertically by clamps (11 & 12), characterized in that a cylindrical furnace (2) being provided at middle periphery of the said chamber, the said chamber being provided with a spray gun (3) passing through sealed cap (13) fitted to its bottom, the said spray gun being connected through a tube (4) to a funnel (5) containing a reactant solution, the said spray gun being also connected to another tube (14) which delivers the carrier gas through a rotameter (9), the top of the said chamber being provided with a thermo-couple (6) and a holder (7) for holding a substrate (8), temperature indicator (10) being connected to the said thermocouple (6). The thermocouple passes through a hole drilled in the graphite strip. The graphite substrate holder is clamped at a position very close to the upper end of the tube furnace. The mist/droplets while passing through the heated portion of the reactor tube get converted to the vapour state. This vapour, then travels vertically upward and strikes the substrate surface. The necessary pyrolysis reaction is accomplished on the substrate surface to generate the desired film. The residual byproduct passes out as gas through an exhaust outlet. Accordingly, the present invention provides a process for the deposition of semiconductor thin films which comprises placing a substrate above the central heating zone of the reactor chamber, maintaining the temperature of the substrate in the range of 350-400°C, vapour spraying the substrate with a reactant solution using an inert gas at a flow rate in the range of 4 to 6 Iitres/min. Deposition of thin films of CdS A spray solution containing CdCl2(0.01M) and thiourea (0.011M) dissolved in methanol is taken in the separating funnel. The liquid is allowed to pass through the tube by opening the stopcock (15). A certain height of the tube is completely filled with the solution and maintained constant throughout the deposition time. This helps 'to maintain a constant rate of flow of the solution. Nitrogen gas coming through the rotameter into the spray gun forces the solution vertically upwards inside the reactor tube in the form of a mist. The substrate on which the film is to be coated is kept at a temperature around 360o-400°C, which is approximately 1.5 inch away from the central heating zone. The temperature of the central heating zone is about 50°C more than the substrate temperature. Several process parameters are important for uniform film deposition. These are, a) the distance separating the tip of the spray nozzle from the lower end of the tube furnace, b) length of the heated section of the reactor tube inside the tube furnace, c) temperature of the central heating zone, d) gas flow rate, e) height of the liquid in the polythene tube. For a typical CdS film deposition the following specifications with respect to the aforesaid parameters are important :- a) Distance separating the tip of the spray nozzle from the lower end of the tube furnace : 2 inches. b) Length of the heated zone of the reactor tube: 4 inches. c) Temperature of the central heating zone; 430-450°C. d) Gas flow rate: 4 to 6 litres/minute. e) Height of the liquid in the polythene tube: 4.5 inches. A uniform CdS film thickness in the range of 500 to 1000 Angstrom can be deposited within 10 to 15 minutes by maintaining the above parameters. ADVANTAGES OF THE INVENTION 1, In the present invention, the deposition is carried out against the force of gravity of sorting out the heavier droplets. 2, The sprayed droplets are converted to the vapour state in a preheated zone in between the spray nozzle and the substrate. Thus, the droplets do not strike the substrate directly as in all other spray deposition techniques. This prevents the mechanism of film growth by overlapping discs. The resulting vapour passing through the vertical reactor tube strikes the substrate uniformly to generate a unif orm f i Im thickness from edge to edge, 3, In this process the chemical reaction occurs inside the glass reactor tube, a section of which is surrounded from outside by a small tube furnace to raise the required temperature. Thus, the question of contamination of the film by the heater does not arise at all. This is a very important parameter for growing high purity thin film semiconductors. 4, The thermodynamics of heat exchanges in this process are also different from all other spray techniques, where the glass substrates are subjected to thermal shock, as described earlier. In this technique, no heat loss from the glass substrates occurs for conversion of the droplets/mist to the gaseous form. 5, This technique enables the growth of ultra thin films of uniform thickness, since film formation does not occur through overlapping discs. We Claim: 1. A device for the deposition of semiconductor thin films which comprises an open ended cylindrical reactor chamber (1), the said chamber being held vertically by clamps (11 & 12), characterized in that a cylindrical furnace (2) being provided at middle periphery of the said chamber, the said chamber being provided with a spray gun (3) passing through sealed cap (13) fitted to its bottom, the said spray gun being connected through a tube (4) to a funnel (5) containing a reactant solution, the said spray gun being also connected to another tube (14) which delivers the carrier gas through a rotameter (9), the top of the said chamber being provided with a thermo-couple (6) and a holder (7) for holding a substrate (8), temperature indicator (10) being connected to the said thermocouple (6). 2. A process for the deposition of semiconductor thin films using the device as claimed in claim 1 which comprises placing a substrate in the said holder (7) at the central heating zone of the said reactor chamber (1), maintaining the substrate temperature in the range of 380-400°C, spraying the vapour on substrate by the ...; spray gun (3) with a reactant solution using an inert gas at a flow rate in the range of 4 to 6 litres/min. 3. A device for the deposition of semiconductor thin films substantially as herein described with reference to figs.2(a) & 2(b) of the drawings accompanying this specification.

Documents:

544-del-1995-abstract.pdf

544-del-1995-claims.pdf

544-del-1995-correspondence-others.pdf

544-del-1995-correspondence-po.pdf

544-del-1995-description (complete).pdf

544-del-1995-drawings.pdf

544-del-1995-form-1.pdf

544-del-1995-form-2.pdf

544-del-1995-form-4.pdf

544-del-1995-form-9.pdf

abstract.jpg