Title of Invention

"AN IMPROVED RF ELECTRODE SYSTEM FOR DEPOSITING LARGE AREA UNIFORM HYDROGENATED SILICON FILMS"

Abstract

An improved RF electrode system for depositing large area uniform hydrogenated silicon films comprising a RF electrode chamber; a set of electrodes which serve as cathodes as well gas injections ; the RF electrode being placed vertically within the chamber ; one substrate on either side of the electrode followed by; one anode after each substrate followed by the system, thus having within the chamber a pan of central electrodes having gas injection holes from both sides with the substrate, anode and heater on either side of each electrode to ensure uniform distribution of the gas mixture within the chamber.

Full Text

AN IMPROVED DESIGN FOR'fiF ELECTRODE SYSTEM FOR DEPOSITING LARGE AREA UNIFORM HYDROGENATED SILICON FILMS *, FIELD OF INVENTION The invention relates to the gas injection cum RF (Radio Frequency) electrode which serves as the cathode in Plasma Enhanced Chemical Vapor Deposition (PECVD) system. The invention has resulted in making highly uniform thin films of different types of hydrogenated silicon films (Si:H) required for the fabrication of amorphous silicon thin film solar cells. The deposition rates of the film are also higher than those by conventional gas injection system. BACKGROUND OF THE INVENTION In an RF Plasma Enhanced Chemical Vapor Deposition equipment the basic materials which are undoped and doped amorphous hydrogenated silicon (a-Si:H) films the RF electrode serves as the cathode to which the RF power supply is connected. The substrate on which the a-Si:H film is deposited by the decomposition of silane gas (SiH») at elevated temperature (200 - 250°C). There are two types of configurations for the RF electrode system viz: (1) horizontal electrode system with horizontal substrate in between (2) Vertical electrode system with substrate in vertical position. There are two ways of injecting the gas between the electrode system viz: (1) laminar flow and (2) shower type in which gases are ejected from the hole in cathode of the RF electrode system. The vertical electrode system with shower type gas injection system is at present the preferred one in the production line of a-Si solar modules. The design of the RF electrode system (cathode) which also serves as gas injector is crucial for the uniformity of the film over large area which is essential for the fabrication of large area a-Si solar modules. In the design of the RF electrode system which also serves as the gas injector it is necessary to maintain uniformity over large area of the substrate on which the Si:H films are deposited. The present invention is intended to improve the uniformity of the film deposited on large area substrate. DETAILS OF PRIOR ART The design of the Radio Frequency (RF) electrode system is crucial to performance of the production line for a-Si solar modules using RF glow discharge method. The RF electrode is also used as the cathode to which RF power supply is connected. The anode is grounded and the substrate is put between the cathode and the anode. In the prior art two configurations for the RF electrode system are used viz horizontal and vertical. In the horizontal configuration the cathode and the anode are horizontal and parallel to each other. The gases, which are/ decomposed flow horizontally between the electrodes. In this type of configurations the uniformity of the gas distribution between the electrodes is not good and there is considerable wastage of gas. More recently specially in the production line vertical configuration of the RF electrode system is used. However the uniformity of the gas distribution over large area is still a problem leading to lowering of conversion efficiency of a-Si modules the layers have to made thicker than their optimized value. We have invented a new design for the RF electrode, which serves as the cathode as well as shower type gas injection system. The improvement mainly comprises the following two factors: (1) variations in the dimension in the holes through which gas is ejected over the area of the electrode and (2) insertion of two new type of gas distribution arrangement inside the RF electrode chamber. The latter factor ensures uniform gas distribution inside the RF electrode chamber so that more uniformity in gas distribution between the RF electrodes is achieved. rief Statement of the Invention: Thus, according to the invention, there is provided an improved RF electrode system for depositing large area uniform hydrogenated silicon films comprising: i. a RF electrode chamber, ii. a set of electrodes which serve as cathodes as well gas injectors, iii. the RF electrode being placed vertically within the chamber, iv. one substrate on either side of the electrode followed by, v. one anode after each substrate followed by the system, thus having within the chamber a pan of central electrodes having gas injection holes from both sides with the substrate, anode and heater on either side of each electrode to ensure uniform distribution of the gas mixture within the chamber. It is to be remembered that the dimensions of the holes are not uniform throughout the electrode and are varied over the height of the electrode face to ensure uniformity of the deposited film. Brief Summary of the Invention: In accordance with the 1st embodiment of the invention, there is disclosed herein a design of the RF electrode system which serves as cathode as well as gas injector. The RF electrode is placed vertically at the center of the two anodes so that two substrates can be coated simultaneously. In the 2nd embodiment of the invention a method is disclosed which along with the 1** embodiment of the invention produce highly uniform p-type, intrinsic and n-type amorphous hydrogenated silicon films over a large area. The films also shows uniform characteristics for the fabrication of a-Si solar cells. BRIEF DESCRIPTION OF THE DRAWINGS In Figure 1 a schematic view of the RF electrode is given. Injection of gases from both sides of the electrode is provided through holes of proper dimension. The dimensions of the holes are also varied over the height of the electrode face. This ensures uniformity of the deposited film. The special feature of the construction is the two injection systems fitted inside the electrode chamber that ensures uniform distribution of the gas mixture within the electrode chamber. In Figure 2 the schematic of the deposition chamber in which the RF electrode system is fixed has been described. In Figure 3 the position of the small substrate (Corning 7059 glass substrate) mounted on different position of the substrate holder (size 30 cm x 30 cm) have been shown. The positions have also been marked numerically. DESCRIPTION OF THE PREFERRED EMBODIMENT 1. Description of p-type a-SiC:H layer These are deposited by RF glow discharge between the RF cathode and the two anodes with the substrate holders in between. The p-type is deposited in one chamber of the RFPHCVD system which is evacuated initially by Turbomoleculer pump. The flow ratios of the different source gas mixture are given in Table I along with the deposition parameters. The substrate temperature was 200°C. The thickness and the properties of p-a-SiC:H layer with substrate at different positions are given in Table 2. It is seen that the deposition over the whole area of the substrate is uniform to within 3%. II. Deposition of Intrinsic and n-type a-Si:H layer Results similar to those reported under Item I were obtained for intrinsic and n-type a-Si:H layer The characteristics of the different intrinsic and n-type film are given in Table 1. III. Fabrication of Double Junction a-Si Solar Cell Double junction a-Si solar cells have been fabricated at a time at the different position of the substrate and the PV parameters are given in Table 3. The results clearly indicates the uniformity of the deposition over the whole substrate (30 cm x 30 cm) Table 1: The flow rates of the different source gases for deposition of different layers along with the deposition parameters. Substrate temperature 200°C. (Table Removed) Table 2: Variation of thickness and optoelectronic properties with the position of the substrate (Table Removed) Table 3: PV parameters of double junction a-Si solar cells fabricated at different position of the substrate holder (30 cm x 30 cm) (Table Removed) WE CLAIM 1. An improved RF electrode system for depositing large area uniform hydrogenated silicon films comprising : a RF electrode chamber i. a set of electrodes which serve as cathodes as well gas injections ii. the RF electrode being placed vertically within the chamber v. one substrate on either side of the electrode followed by v. one anode after each substrate followed by the system, thus having within the chamber a pan of central electrodes having gas injection holes from both sides with the substrate, anode and heater on either side of each electrode to ensure uniform distribution of the gas mixture within the chamber. 2. An improved RF electrode system as claimed in claim 1, wherein the dimensions of the holes are not uniform throughout the electrode and are varied over the height of the electrode face to ensure uniformity of the deposited film. 3. An improved RF electrode system for depositing large area uniform hydrogenated silicon films substantially as herein described with reference to the accompanying drawings.

Documents:

1078-DEL-2002-Abstract-(12-12-2008).pdf

1078-del-2002-abstract.pdf

1078-DEL-2002-Claims-(12-12-2008).pdf

1078-del-2002-claims.pdf

1078-DEL-2002-Correspondence-Others-(12-12-2008).pdf

1078-DEL-2002-Correspondence-Others-(19-03-2010).pdf

1078-del-2002-correspondence-others.pdf

1078-del-2002-correspondence-po.pdf

1078-del-2002-description (complete).pdf

1078-DEL-2002-Drawings-(12-12-2008).pdf

1078-del-2002-drawings.pdf

1078-del-2002-form-1.pdf

1078-del-2002-form-13.pdf

1078-del-2002-form-18.pdf

1078-DEL-2002-Form-2-(12-12-2008).pdf

1078-del-2002-form-2.pdf

1078-del-2002-form-3.pdf

1078-del-2002-gpa.pdf