WO2007061273A1

WO2007061273A1 - Method of forming silicon film by two step deposition

Info

Publication number: WO2007061273A1
Application number: PCT/KR2006/005064
Authority: WO
Inventors: Won-Jun Lee; Kwang-Chul Park; Sang-Ho Son; Jae-Kyun Park; Jong-Moon Choi; Sa-Kyun Rha
Original assignee: Aet Co., Ltd.
Priority date: 2005-11-28
Filing date: 2006-11-28
Publication date: 2007-05-31
Also published as: KR20070055899A; KR100773123B1; TW200727345A

Abstract

Disclosed is a method for forming a polycrystalline silicon through two-step deposition: supplying silicon precursor and plasma alternately to the inside of a plasma reaction tube to forming a crystalline silicon thin film of an atom layer at a lower temperature than 5000C; and forming a separate upper silicon thin film with the crystalline silicon thin film as a seed layer. The present invention can form a polycrystalline silicon thin film at relatively low temperature by the two-step deposition. The method of forming a polycrystalline silicon thin film by two-step deposition according to the present invention, comprises: sequentially supplying silicon precursors and reductive plasma into the inside of a reaction tube having a substrate placed therein to deposit a crystalline silicon thin film of atom layer having a predetermined thickness on the substrate; and depositing an upper silicon thin film on the crystalline silicon thin film functioning as a seed layer.

Description

METHOD OF FORMING SILICON FILM BY TWO STEP

DEPOSITION

Technical Field

[1] The present invention relates to a method for forming a poly crystalline silicon thin film, and more particularly, to a method for forming a polycrystalline silicon by two- step deposition: supplying silicon precursors and plasma alternately to the inside of a plasma reaction tube to forming a crystalline silicon thin film of atom layer at a lower temperature than 500⁰C; and forming a separate upper silicon thin film on the crystalline silicon thin film functioning as a seed layer. The present invention can form a polycrystalline silicon thin film at relatively low temperature by such two-step deposition. Background Art

[2] Usually, elements employing polycrystalline silicon thin films are used for active elements for active matrix liquid crystal displays and switching elements and peripheral circuits for electro-luminescence elements.

[3] Currently, a polycrystalline silicon thin film can be obtained by applying a method such as a solid phase crystallization method(SPC), a rapid thermal annealing method(RTA), a continuous wave Ar laser annealing method, an excimer laser annealing method(ELA), etc. on a substrate deposited with amorphous silicon.

[4] The solid phase crystallization method carries out a heat treatment at a high temperature more than 600⁰C for a long time to form a polycrystalline silicon thin film. However, the method using a high temperature heat treatment like this requires a long and high temperature heat treatment. Furthermore, there occur many defects in the grains crystallized by this method, thus making it difficult to make an element, and a substrate made of glass can not be employed due to high crystallization temperature.

[5] The rapid thermal annealing method and continuous wave Ar laser annealing method also require a process temperature more than 500⁰C, thereby making it impossible to be applicable to a glass substrate, which can cause thermal deformation.

[6] On the other hand, the excimer laser annealing method has been widely applied for making a polycrystalline silicon thin film. This excimer laser annealing method will now be described with reference to the accompanying drawings.

[7] FIG. 1 is a flow chart forming a polycrystalline silicon thin film using an excimer laser annealing method according to the prior art, and FIG. 2 is a process flow diagram for illustrating the flow chart of FIG. 1.

[8] Firstly, an amorphous silicon thin film is deposited on a substrate using a plasma- enhanced chemical vapor deposition (PECVD) method (SlO). For this purpose, supplying SiH molecules 3 and H molecules 4 to the inside of a reaction tube 1 and generating plasma at a predetermined condition make silicon atoms 3a and hydrogen atoms 3B deposited on the substrate 2, thus forming the amorphous silicon thin film as shown in FIG. 2(a).

[9] After the amorphous silicon thin film has been formed on the substrate 2, dehy- drogenation process is conducted to remove hydrogen atoms 3b contained in the amorphous silicon thin film (S20). Referring to FIG. 2(b), after the dehydrogenation process, hydrogen 3b escapes from silicon atoms 3a.

[10] Then, pulsed excimer laser beams 5 are illuminated to the dehydrogenated silicon atoms 3 a to locally heat and crystallize only the surface of the amorphous silicon thin film on the substrate 2, thus allowing the silicon thin film to be crystallized while minimizing the damage to the glass substrate (S30) (see FIG. 2(c)).

[11] However, in the case where the amorphous silicon thin film is formed on the substrate by the excimer-laser crystallization, a high-priced excimer laser is required. Additionally, since the substrate of a large area is scanned using the laser beam, a process time becomes longer, and the size of grain of the silicon becomes non- uniformity.

[12] Furthermore, the process may become complicated since the laser crystallization should be proceeded under vacuum after the deposition of the amorphous silicon thin film has been complete and hydrogen have been removed in the thin film using the rapid thermal annealing method(RTA). Disclosure of Invention

Technical Problem

[13] Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a method for forming a polycrystalline silicon through two-step deposition: supplying silicon precursor and plasma alternately to the inside of a plasma reaction tube to forming a crystalline silicon thin film of an atom layer at a lower temperature than 500⁰C; and forming a separate upper silicon thin film with the crystalline silicon thin film as a seed layer. The present invention can form a polycrystalline silicon thin film at relatively low temperature by the two-step deposition. Technical Solution

[14] According to an aspect of the present invention, a method of forming a polycrystalline silicon thin film by two-step deposition, which includes: sequentially supplying silicon precursors and reductive plasma into the inside of a reaction tube having a substrate placed therein to deposit a crystalline silicon thin film of atom layer having a predetermined thickness on the substrate; and depositing an upper silicon thin film on the crystalline silicon thin film functioning as a seed layer.

[15] Said depositing the crystalline silicon thin film of atom layer may includes: (a) supplying silicon precursors into the inside of a reaction tube having a substrate placed therein to make the silicon precursors react with the substrate while maintaining the inside of the reaction tube at a predetermined pressure and a temperature; (b) pumping and purging to remove silicon precursors not having reacted and reaction byproducts remaining inside the reaction tube; (c) supplying reductive plasma into the inside of the reaction tube to form solid state silicon by reducing the silicon precursors deposited on the substrate; and (d) pumping and purging to remove various ions not having reacted and reaction byproducts remaining inside the reaction tube.

[16] The steps (a) to (d) may be repeated to form the crystalline silicon thin film having the predetermined thickness on the substrate.

[17] The silicon precursors each may be a halogen compound, and the silicon precursors each may ^J be any ^J one of SiCl 4 , SiF 4 , SiH 2 Cl 2 , SiHCl 3 , SiH 3 Cl, SiI 4 , Si 2 Cl 6 , and Si 2 F 6.

[18] A pressure inside the reaction tube may range from lOmTorr to lOTorr to cause the silicon precursors to react, and the silicon precursors may be supplied into the inside of the reaction tube during a time from 1 second to 10 minutes.

[19] H or D may be used as a gas to form the reductive plasma. And, any one of ca- pacitively-coupled plasma(CCP), inductively-coupled plasma(ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma may be used as a source for the reductive plasma.

[20] When the ICP is used as the source for the reductive plasma, a plasma power may be maintained between 100 watts and 20 kirowatts, a pressure may be maintained between 5mTorr and 5Torr, and a flow rate of a gas for generating the reductive plasma may be maintained between 100 Seem and 50 Slpm.

[21] An inert gas or H may be used for said purging, and temperature of the substrate may be maintained between 300 and 500⁰C. And, the substrate may be heated using any one of a resistance heating technique, an induction heating technique, and a ramp heating technique.

[22] A thickness of the crystalline silicon thin film of atom layer may range from lnm to lOOnm.

[23] The upper silicon thin film may be formed using any one of a PECVD method, a

LPCVD method, an APCVD method, and a sputtering method.

[24] When the upper silicon thin film is deposited using the PECVD method, silicon precursors, H , and inert gases may be simultaneously supplied into the inside of the reaction tube, plasma may be applied to the inside of the reaction tube, and a temperature of the substrate may be maintained between 200 and 500⁰C. [25] The silicon precursors each may be any one of SiH , Si H , SiCl , SiF , SiH Cl ,

SiHCl , SiH Cl, SiI , Si Cl and Si F , and any one of capacitively-coupled plasma(CCP), inductively-coupled plasma(ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma may be used as a source for generating the plasma.

[26] When the upper silicon thin film is deposited using the LPCVD method, silicon precursors, H , and inert gases may be simultaneously supplied into the inside of the reaction tube, and a temperature of the substrate may be maintained between 300 and 500⁰C.

[27] The silicon precursors each may be any one of SiH , Si H , SiCl , SiF , SiH Cl ,

SiHCl 3 , SiH 3 Cl, SiI 4 , Si 2 Cl 6 and Si 2 F 6.

Brief Description of the Drawings

[28] The above aspects of the present invention will be more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which:

[29] FIG. 1 is a flow chart for forming a silicon thin film using an excimer laser annealing method;

[30] FIG. 2 is a process flow diagram for forming a silicon thin film using an excimer laser annealing method;

[31] FIG. 3 is a process flow diagram for forming a polycrystalline silicon thin film through two-step deposition according to an embodiment of the present invention;

[32] FIG. 4 is a flow chart for forming a silicon thin film of atom layer according to an embodiment of the present invention;

[33] FIG. 5 is a process flow diagram for forming a silicon thin film of atom layer according to an embodiment of the present invention; and

[34] FIG. 6 is a graph showing a degree of crystallinity for a polycrystalline silicon thin film formed according to an embodiment of the present invention. Mode for the Invention

[35] Preferred embodiments according to the present invention and operations thereof will be described with reference to the accompanying drawings.

[36] The present invention is a technique of depositing a silicon thin film, which decides the properties of TFTs, using an atomic layer deposition method(ALD) called a base technology of next generation flat panel display apparatuses. The present invention can conduct a process at lower temperature and improve the properties of TFTs, thus making it possible to manufacture high capability display devices. The present invention can be applicable to other display devices such as flexible displays as well as flat panel displays, and also applicable to the energy fields such as solar cells.

[37] Referring to FIG. 3(a), firstly, a crystalline silicon thin film 150 of atom layer having a predetermined thickness is deposed on a substrate 110 to make a poly- crystalline silicon thin film according to a method of the present invention. More specifically, the crystalline silicon thin film 150 is firstly deposited on the substrate 110 placed inside a reaction tube. A silicon precursor and reductive plasma need to be sequentially or repeatedly supplied alternately to the inside of the reaction tube so that the crystalline silicon thin film 150 of atom layer can be deposited on the substrate 110.

[38] Referring to FIG. 3(b), after the crystalline silicon thin film 150 of atom film has been deposited on the substrate 110, a separate upper silicon thin film 160 is deposited on the crystalline silicon thin film 150 of atom film. The upper silicon thin film 160 is deposited using an existing deposition method. Because the crystalline silicon thin film 150 deposited prior to the deposition of the upper silicon thin film 160 functions as a seed layer, however, the upper silicon thin film 160 starts to be crystallized by the seed layer, i.e. crystalline silicon thin film 150.

[39] Hereafter, forming the crystalline silicon thin film of atom layer will be firstly described and then depositing the upper silicon thin film on the crystalline silicon thin film of atom layer and crystallizing the upper silicon thin film will be described to avoid any possible confusion in understanding the present invention and provide consistent descriptions.

[40] Referring to FIG. 4, in order to deposit the crystalline silicon thin film on the substrate, the substrate is placed inside a reaction tube and reacted with a silicon precursor supplied into the reaction tube with the inside of the reaction tube maintained at a given pressure and a temperature (Sl 10).

[41] Supplying the silicon precursor 120 into the inside of the reaction tube 100 and maintaining the inside of the reaction tube 100 at the given pressure and temperature lead to a chemical reaction of the silicon precursor 120 and the substrate 110 to thereby cause silicon atoms 121 to be deposited on the substrate 110 (refer to FIG. 5 (a)).

[42] The silicon precursor 120 is preferably a halogen compound of silicon. For example, the silicon precursor 120 is any one of SiCl , SiF , SiH Cl , SiHCl , SiH Cl,

4 4 2 2 3 3

SiI 4 , Si 2 Cl 6 , and Si 2 F 6.

[43] If SiH Cl is used as the silicon precursor 120 in FIG. 5(a), silicon precursors, each of which is composed of a silicon atom 121 and two Cl atoms 123 and two H atoms 125 bonded to the silicon atom 121, are supplied to the inside of the reaction tube 100 and react with the substrate 110 at a predetermined pressure and a temperature, so that silicon atoms 121, halogen atoms, and Cl atoms 123 are deposited on the substrate 110 and part of the silicon precursors 120 and reaction byproducts 130 remains inside the reaction tube 100.

[44] Pressure of the inside of the reaction tube 100 should be more than lOmTorr, most preferably in the range from lOmTorr to lOTorr, to cause a reaction on the precursors 120. It is desirable that the silicon precursors 120 are supplied into the inside of the reaction tube 100 for 1 second- 10 minutes.

[45] Referring to FIG. 5(b), after the silicon atoms have been deposited on the substrate

110, silicon precursors 120 not having reacted and reaction byproducts 130 inside the reaction tube 100 are removed (S 120). Pumping and purging are performed to remove the silicon precursors 120 not having reacted and reaction byproducts 130. It is desirable to use inert gases such as N , Ar, He, etc., or H gas as a purge gas.

[46] After the silicon precursors 120 not having reacted and reaction byproducts 130 have been removed from the reaction tube 100, reductive plasma is supplied to the inside of the reaction tube 100 so that the silicon precursors deposited on the substrate 110 are reduced to form solid state silicon (S 130).

[47] It is desirable to use H or D as a gas to form the reductive plasma, and it is desirable to use any one of capacitively-coupled plasma (CCP), inductively-coupled plasma (ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma as a source for the reductive plasma.

[48] In the case where the inductively-coupled plasma is used, it is desirable to maintain the plasma power between 100 watts and 20 kirowatts, the pressure between 5mTorr and 5Torr, and the flow rate of gas for generating reductive plasma between 100 Seem to 50 Slpm.

[49] If H gases are supplied into the inside of the reaction tube 100 and a plasma process is conducted to cause reductive plasma, then the halogen Cl atoms deposited on the substrate 121 are bonded with H to leave reaction byproducts 140 and the reacting hydrogen 125 are bonded with silicon atoms deposited on the substrate 121 (refer to FIG. 5(c)).

[50] Referring to FIG. 5(d), after the reductive plasma have been supplied to the inside of the reaction tube 100 to reduce the silicon precursors deposited on the substrate 110 and form solid state silicon, various ions not having reacted and reaction byproducts 140 remaining inside the reaction tube 100 are removed (S 140).

[51] Pumping and purging are performed to remove the various ions not having reacted and reaction byproducts 140 remaining inside the reaction tube 100. It is desirable to use inert gases such as N , Ar, He, etc., or H gas as a purge gas.

[52] It is desirable to heat the substrate 110 to maintain a temperature between 300 and

500⁰C so that a crystalline silicon thin film of atom layer may be formed by the aforementioned processes. As described above, the present invention makes a difference from a prior art that directly deposits a crystalline silicon thin film at a high temperature more than 600⁰C. While various heating techniques can be employed to heat the substrate, it is desirable to use any one of a resistance heating technique, an induction heating technique, and a ramp heating technique.

[53] And, the steps Sl 10-S140 can be repeatedly performed several times. That is, the steps Sl 10-S140 can be repeated several times to form a crystalline silicon thin film 150 of atom layer having a desired thickness on a substrate as shown in FIG. 5(e). Assuming that a crystalline silicon thin film of ID atom layer is formed in case of carrying out the steps Sl 10-S140, it is enough to conduct the steps Sl 10-S140 five times in order to deposit a crystalline silicon thin film of 5D thickness of atom layer.

[54] In the case of repeating the steps SI lO-S 140, the silicon precursors used, the conditions to cause the silicon precursors to react, the gases for reductive plasma, the source for reductive plasma, and conditions to process the plasma are the same as the case of performing the step SI lO-S 140.

[55] While the thickness of the crystalline silicon thin film of atom layer deposited by the above processes can be changed variously, it is desirable to be in the range between lnm and lOOnm. That is, it is desirable that the crystalline silicon thin film of atom layer is formed to have a thickness within the above range by either performing the steps SI lO-S 140 or repeating the steps SI lO-S 140 several times.

[56] Next, depositing an upper silicon thin film taking the crystalline silicon thin film of atom layer as a seed layer will be described in detail.

[57] As described above, after the crystalline silicon thin film 150 of atom layer having a given thickness has been deposited on the substrate 110, a separate upper silicon thin film 160 is formed by various deposition methods using the crystalline silicon thin film 150 as a seed layer.

[58] The upper silicon thin film 160 to be deposited on the crystalline silicon thin film

150 can be formed by various existing deposition methods. That is, the upper silicon thin film 160 can be deposited on the crystalline silicon thin film 150 as a seed layer using any one of a PECVD method, a LPCVD method, an APCVD method, and a sputtering method.

[59] In the case that the upper silicon thin film 160 is deposited by the PECVD method, plasma having a predetermined condition is applied to the reaction tube filled with silicon precursors, H , and inert gases. At this time, the temperature of the substrate is preferably maintained in the range from 200 to 500⁰C.

[60] It is desirable to use any one of SiH , Si H , SiCl , SiF , SiH Cl , SiHCl , SiH Cl,

SiI , Si Cl , and Si F as the precursors supplied into the inside of the reaction tube, and it is desirable to use any one of capacitively-coupled plasma (CCP), inductively- coupled plasma (ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma as a source for causing the plasma.

[61] In the case that the upper silicon thin film 160 is deposited by the LPCVD method, it is desirable to maintain the temperature of the substrate between 300 and 500⁰C, with the reaction tube filled with silicon precursors, H , and inert gases. [62] At this time, it is desirable to use any one of SiH , Si H , SiCl , SiF , SiH Cl , SiHCl

4 2 6 4 4 2 2

, SiH Cl, SiI , Si Cl , and Si F as the silicon precursors similarly to the case of using

3 3 4 2 6 2 6 the PECVD method.

[63] The above-mentioned processes for the upper silicon thin film proceed without any exposure to the atmosphere of the upper silicon thin film. That is, it is desirable not to allow any exposure to air during a time from the deposition of the crystalline silicon thin film 150 to the deposition of the upper silicon thin film 160.

[64] As mentioned above, the present invention is a novel low-temperature crystallization method, which can directly form a crystalline silicon thin film even at the temperature range where existing methods can not form crystalline thin films by firstly forming a crystalline silicon thin film as a seed layer and secondly growing up an upper silicon thin film on the crystalline silicon thin film.

[65] Because the crystalline silicon thin film of atom layer is firstly deposited on the substrate to function as a seed layer, the upper silicon thin film on the seed layer has an improved degree of crystallization accordingly.

[66] FIG. 6 is a graph showing a degree of crystallinity of a silicon thin film using

Raman spectroscopy. FIG. 6(a) is a graph showing a degree of crystallinity of a silicon thin film formed by an existing PECVD method, and FIG. 6(b) is a graph showing a degree of crystallinity of a silicon thin film formed by sequentially applying an ALD method and a PECVD method according to the present invention; the former, the ALD method, for forming a crystalline silicon thin film of atom layer as a seed layer, and the latter, the PECVD method, for forming an upper silicon thin film to be crystallized. As can be seen from the graphs, the degree of crystallinity of the crystalline silicon thin film formed by the present invention is even much better than that of the crystalline silicon thin film by the conventional method. Industrial Applicability

[67] As mentioned above, the method of forming a polycrystalline silicon thin film by two-step deposition according to the present invention produces the following effects.

[68] First, the crystalline silicon thin film of atom layer even at the temperature lower than 500⁰C, at which silicon thin films can not be formed by existing methods, can be directly formed.

[69] Second, the crystallization of the upper silicon thin film by the crystalline silicon thin film of atom layer deposited as a seed layer of the upper silicon thin film can be effectively induced.

[70] Third, the productivity of low-temperature polycrystalline silicon TFTs can be dramatically enhanced because the method of the present invention has simpler processes than the existing methods of forming the polycrystalline silicon thin films using excimer laser apparatuses.

[71] Fourth, the silicon thin film formed by the present invention has higher electric field mobility and driving current than an amorphous silicon thin film, thereby improving the reliability as well as making LCD drivers built in a TFT panel when being applied to the manufacturing process of TFTs.

Claims

[1] A method of forming a poly crystalline silicon thin film by two-step deposition, comprising: sequentially supplying silicon precursors and reductive plasma into the inside of a reaction tube having a substrate placed therein to deposit a crystalline silicon thin film of an atom layer having a predetermined thickness on the substrate; and depositing an upper silicon thin film on the crystalline silicon thin film functioning as a seed layer.

[2] The method of claim 1, wherein said depositing the crystalline silicon thin film of atom layer comprises:

(a) supplying silicon precursors into the inside of a reaction tube having a substrate placed therein to make the silicon precursors react with the substrate while maintaining the inside of the reaction tube at a predetermined pressure and a temperature;

(b) pumping and purging to remove silicon precursors not having reacted and reaction byproducts remaining inside the reaction tube;

(c) supplying reductive plasma into the inside of the reaction tube to form solid state silicon by reducing the silicon precursors deposited on the substrate; and

(d) pumping and purging to remove various ions not having reacted and reaction byproducts remaining inside the reaction tube.

[3] The method of claim 2, wherein the steps (a), (b), (c) and (d) are repeated to form the crystalline silicon thin film having the predetermined thickness on the substrate.

[4] The method of claim 2 or 3, wherein the silicon precursors are a halogen compound.

[5] The method of claim 4, wherein the silicon precursors each are any one of SiCl 4 , SiF4 , SiH2 Cl2 , SiHCl3 , SiH3 Cl,

SiI 4 , Si 2 Cl 6 , and Si 2 F 6.

[6] The method of claim 5, wherein a pressure inside the reaction tube ranges from 10 mTorr to 10 Torr to cause the silicon precursors to react.

[7] The method of claim 5, wherein the silicon precursors are supplied into the inside of the reaction tube during a time from 1 second to 10 minutes.

[8] The method of claim 2 or 3, wherein

H or D is used as a gas to form the reductive plasma.

2 2 ^{& r}

[9] The method of claim 8, wherein one of capacitively-coupled plasma(CCP), inductively-coupled plasma(ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma is used as a source for the reductive plasma.

[10] The method of claim 9, wherein when the ICP is used as the source for the reductive plasma, a plasma power is maintained between 100 watts and 20 kirowatts, a pressure is maintained between 5 mTorr and 5 Torr, and a flow rate of a gas for generating the reductive plasma is maintained between 100 Seem and 50 Slpm.

[11] The method of claim 2 or 3, wherein an inert gas or H is used for said purging.

[12] The method of claim 2 or 3, wherein a temperature of the substrate is maintained between 300 and 500⁰C.

[13] The method of claim 12, wherein the substrate is heated using one of a resistance heating technique, an induction heating technique, and a ramp heating technique.

[14] The method of claim 2 or 3, wherein a thickness of the crystalline silicon thin film of atom layer ranges from lnm to lOOnm.

[15] The method of claim 1 , wherein the upper silicon thin film is formed using one of a PECVD method, a LPCVD method, an APCVD method, and a sputtering method.

[16] The method of claim 15, wherein when the upper silicon thin film is deposited using the PECVD method, silicon precursors, H , and inert gases are simultaneously supplied into the inside of the reaction tube, plasma is applied to the inside of the reaction tube, and a temperature of the substrate is maintained between 200 and 500⁰C.

[17] The method of claim 16, wherein the silicon precursors each are any one of SiH , Si H , SiCl , SiF , SiH Cl , ^{r J} 4 2 6 4 4 2 2

SiHCl 3 , SiH 3 Cl, SiI 4 , Si 2 Cl 6 and Si 2 F 6.

[18] The method of claim 16, wherein any one of capacitively-coupled plasma(CCP), inductively-coupled plasma(ICP), microwave plasma, electron cyclotron (ECR) plasma, and helicon plasma is used as a source for generating the plasma.

[19] The method of claim 15, wherein when the upper silicon thin film is deposited using the LPCVD method, silicon precursors, H , and inert gases are simultaneously supplied into the inside of the reaction tube, and a temperature of the substrate is maintained between 300 and 500⁰C. [20] The method of claim 19, wherein the silicon precursors each are any one of SiH , Si H , SiCl , SiF , SiH Cl ,

4 2 6 4 4 2 2

SiHCl 3 , SiH 3 Cl, SiI 4 , Si 2 Cl 6 and Si 2 F 6.