AU602113B2 - Purification process for dendritic web silicon crystal growth system and dendritic web silicon crystals made thereby - Google Patents

Description

%=4I Le1 P/00/011 Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: 8th September, 1987 Related Art: [Th s d ment c7 1 -ii t S ti o n 4 1) n i (I I k I It I; TO BE COMPLETED BY APPLICANT Name of Applicant: WESTINGHOUSE ELECTRIC CORPORATION i; 1 i: i i:j Address of Applicant: Actual Inventor: 1310 Beulah Road, Churchill. Pittsburgh PA UNITED STATES OF AMERICA.

15235 Edgar Leonard Kochka Paul Anthony Piotrowski William Clyde Higginbotham Address for Service: -HALFORD MAXWELL--- pelTR. N lx Pr<th Floor, ppa.oATW t-RO 49-51 York, Street, -SYDNEY. N. 2.0.00...

Complete Specification for the invention entitled: "PURIFICATION PROCESS FOR DENDRITIC WEB SILICON CRYSTAL GROWTH SYSTEM AND DENDRITIC WEB SILICON CRYSTALS MADE THEREBY" The following statement is a full description of this invention, including the best method of performing it known to p~M*xc us: SNote: The description is to be typed in double spacing, pica type face, in an area not exceeding 250 mm in depth and 160 mm in width, on tough white paper of good quality and it is to be inserted inside this form.

14599/78-L Printed by C. J. THOMPSON, Commonwealth Government Printer, Canberra la The present invention relates to a method or process for growing silicon dendritic web crystals having improved purity and thus improved efficiency relative to prior systems.

Silicon dendritic web crystals are long, thin ribbons of single crystalline material of high structural quality which are grown in the (111) orientation. The o,0 current impetus for developing silicon dendritic web is its 0 oo. application to the production of low-cost, highly efficiently solar cells for direct conversion of sunlight to electrical energy. The thin ribbon form of the crystal requires little additional processing prior to device fabrication, in contrast to wafer substrates from the more traditional Czochralski crystal which must be sliced, S 15 lapped' and polished prior to use, a costly process even though large volume economies are practiced. Additionally, the rectangular shape of the silicon ribbon leads to efficient packing of individual cells into large modules 0 and arrays of solar cells.

20 In order to effectively grow dendritic web I •4 6 crystals for practical application, it is necessary that such crystals and the system from which they are grown have a sufficient level of purity. For example, if too much oxygen is permitted to enter the growth chamber, silicon i dioxide can form in the gas phase above the melt or be present on the melt surface and affect crystal growth.

.3t 2 Oxygen also attacks the growing crystal, forming a dark blue and/or gold-colored oxide film that must be removed before the crystal can be processed into solar cells. In the past, therefore, dendritic web crystals have been grown in evacuated and purged gas chambers, into which argon gas has been introduced to minimize the effects of oxygen on the melt and the growing crystal.

However, argon gas alone has not solved all problems .relating to impurities in the growth system. For example, dark deposits tend to collect on the lids and shields of the system setting up adverse convection flows and affecting crystal growth. Dendritic web crystals are typically grown from a quartz crucible, which tends to react with the molten silicon when heated, producing gaseous silicon oxide, which condenses on the growing dendritic web as a light brown powdery oxide coating which must be removed. Adherent oxides, that is, those forming directly on the surface of the dendritic web are also commonly experienced, and must be removed with a wet chemical treatment. The growth system also tends to harbor moisture, which reacts with the internal molybdenum parts, freeing molybdenum, which can be transported through the system, adhere to the surface of the web and diffuse into the crystalline structure of the web. Molybdenum can be tolerated in only extreme minute concentrations before the web loses its value as a photoconductor.

While it is possible to measure directly the concentrations of the above impurities contained in a dendritic web crystal, this procedure is difficult, costly, and time-consuming. A more useful indication of crystal purity is the relative photovoltaic efficiency of a given photo cell manufactured from one crystal compared to cells manufactured from other crystals. In general, the purity of a crystal is directly related to the efficiency of a photovoltaic cell produced from that crystal. The greater the crystal purity, the greater the possible cell efficiency. In order to increase cell efficiency, it would therefore be useful to devise a means of producing crystals having greater purity than those grown previously. It would also be useful to develop a method for maintaining a higher degree of purity within the crystal growth system than is currently possible, in order to lessen the adverse conditions associated with impure systems.

The object of the present invention is to produce crystals having greater purity and thus providing greater cell efficiencies in photovoltaic cells. Such dendritic web crystals are produced by introducing helium gas to the gas chamber as a pre-treatment step prior to growing dendritic web crystals from the system. This lessens the deposits typically found in the growth system, and lessens their effect on crystal growth. While the precise mechanism is not understood, it is believed that the helium in some way purges the growth system, driving off impurities that have collected on the walls of the gas chamber, susceptor, crucible and elsewhere within the system.

4 Additionally, when the pre-treatment helium is added as described above, deposition of the dark blue adherent oxides on the crystal surfaces is greatly reduced or eliminated, possibly resulting in the elimination of the wet chemical treatment necessary for crystals grown from prior processes.

This invention as described in the claims will become more apparently by reading the following description of the preferred embodiments and preferred methods of practicing the invention in conjunction with the accompanying drawings, in which: Figure 1 is a schematic diagram in partial cross section of a dendritic web crystal growth system which can So be used in accordance with the presently claimed invention.

Figure 2 is an elevation view of a susceptor lid commonly used for growing silicon dendritic web crystals.

Figure 3 is an elevation view of a gas chamber which can be used in accordance with the presently claimed invention.

I_ I_( Figure 1 shows a typical system, generally 1, used for dendritic web crystal growth. As shown, a susceptor 28 contains a crucible 30 containing molten polycrystalline silicon 31. A susceptor lid 15. is positioned over the crucible/susceptor system. As shown in Figure 2, the susceptor lid 15 contains a slot 16 through which dendritic web crystals 32 can be pulled. As shown, the dendritic web crystal 32 is bounded by dendrites which are growing in the molten silicon 31.

Also as shown in Figure 1, the dendritic web growth system contains a heating element 17. This heating element is typically a coiled induction heater, which is used to bring the silicon 31 contained in the crucible up to the melting point of silicon, 1412 0 C at atmospheric pressure. The entire dendritic web growth system is housed in a gas chamber 18 which preferably contains a gas inlet line 19, a gas outlet line 20, and a chimney 21. The gas inlet line 19 preferably includes a ringed portion 26 which extends around the circumference of the interior of the gas chamber 18, preferably near the bottom 27 of the gas chamber. The ringed portion 26 preferably has performations (not shown) which allow the gas to fill the chamber 18. The chamber 18 typically contains a side viewing port 22 and one or more top viewing ports 24 and an exit port 25 through the chimney 21 on the top portion 23 of the chamber 18. Viewing ports 24 on the top portion 23 of the chamber 18 may also be provided with auxiliary lines for additional gas flows. The chimney 21 includes a cap 21a with a valve 21b that is used to close the chimney 30 during evacuation and backfill of the gas chamber,18. The valve 21b is opened iring the initial purging of the chamber 18, and the cap 21a is totally removed for the crystal to be pulled through exit port In the present invention, the gas chamber 18 is preferably first evacuated by conventional means, for example, by closing all ports and valves and connecting the gas outlet line 20 to a vacuum source. After the gas chamber 18 is evacuated, argon gas is introduced through the inlet line 19 in order to effect a preliminary purge of the system. In a most preferred embodiment of the present invention, the argon gas includes up to 3% hydrogen gas.

The argon gas is then evacuated from the gas chamber 18 through the outlet line 20. This procedure is preferably done three times, and preferably includes a pre-heating step in which the heating element 17 is used to preheat the system to approximately 400 0 C following the addition of argon.

In one preferred procedure, the argon gas purge is accomplished by first verifying that the gas inlet valve 19a and all exhaust valves on the gas chamber are closed.

Then, the chamber is evacuated using a vacuum pump, until the pressure in the gas chamber reaches 20 milli-Torr or less. Then, the vacuum valve 20a is closed, the gas inlet valve 19a is opened, and then the gas chamber is backfilled with argon gas to a pressure of about 150 mm Hg (3 psi).

Preferably, the steps of opening the vacuum valve until 20 milli-Torr or less is achieved, closing said valve, and opening the gas inlet valve 19a to admit argon gas are repeated three times.

After the final argon gas purge is evacuated, the vacuum line valve 20a is closed, and the gas chamber 18 is backfilled with helium gas by introducing helium through the gas inlet line 19, by opening the inlet valve 19a, keeping the chimney 21 closed, until the chamber 18 is filled with helium gas at atmospheric pressure. The valve 21b is opened, and the helium gas flow is adjusted. The flow rate of helium gas is not critical, but preferably is sufficient to maintain a slight positive pressure while the helium gas is filling the gas chamber 18. After the chamber 18 is filled with helium, helium should continue to flow into the chamber, maintaining a slight positive pressure, with any excess helium being vented to the atmosphere through the valve 21b. It is preferable that the helium used in the backfilling procedure be ultra-pure, meaning having a purity of at least 99.999% helium.

After the gas chamber 18 has been backfilled with helium, the system is preferably heated using the heating element 17 to a temperature of approximately 1300-1350OC, preferably using the established heating rate, or "ramp" rate, for the particular system. The helium gas is continually fed into the gas chamber, using a slight positive pressure, during this heating step.

Following the heating step, the argon gas under which the dendritic web crystal will be grown is introduced to the gas chamber. This is done by turning on the argon gas supply, and introducing the argon gas through the inlet gas line, while maintaining the flow of helium to the system. In a most preferred embodiment of the present invention, the argon gas includes up to 3% hydrogen gas.

Because argon is heavier than helium, the argon gas will displace the helium, which will leave the system through the open valve 21b on the chimney 21. After the argon gas has substantially displaced the helium gas, the helium gas supply may be turned off. At this point, the helium purification process is completed, and the system should continue to be heated to approximately 1412 0 C, to establish appropriate conditions for web growth. Once such temperature conditions are established, generally about 2 hours after turning off the helium gas supply, web growth may proceed in the conventional manner.

Table 1 shows the improved cell efficiency made possible using the helium purification process of the present invention. As shown in Table 1, in nine out of ten cases in which helium pre-treatment was used, the average efficiency of solar cells produced from such crystals was superior to cells produced from crystals not grown using the helium pre-treatment step.

c TABLE 1 CELL EFFICIENCY DATA Furnace Pre- Furnace Crystal Processing Treatment Run No. No. Batch No.

No. of Average* Cells Efficiency He He He He None None He He He He He He None 7-289 1 5251-49W Cells with aluminum BSR Baseline Cells 7-289 1 5301-01E 14.4 13.9 12.8 14.3 13.8 13.4 15.2 14.6 14.4 14.9 13.9 14.5 13.8 Baseline Cells *Calculated according to standard test measurements known in the art.

Al Lhuuyl- hha iv Liin as beenl dC ibed detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except s- it may be limited by the claims. For example, alt hMgh the present invention has been used primarily or dendritic web crystals, it is expected that t e skilled in the art would appreciate the invention' advantages in other crystal growth processes in whic igher crystal and/or growth system purity is a '3r f 1 1 T 7-, U~T~Tt~ff+~--Ftm-n-FFI-

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Claims (8)

1. A method of growing a dendritic web silicon crystal from a crucible containing molten silicon disposed in a heated susceptor which is housed in a gas chamber characterized by the steps of: evacuating said chamber; back filling said evacuated chamber with helium to produce a slight positive pressure therein; raising the temperature within the chamber to about 1300 0 C; introducing a second gas into said chamber; and allowing said second gas to displace said helium in said chamber prior to commencing the growth of the dendritic web silicon crystal.

2. The method of claim 1 characterized by the steps hel cry acc< DAT on 0 (0 4 4 WES' Pat PETI of: evacuating the chamber; back filling the chamber with the second gas and evacuating the chamber a plurality of times prior to back filling said evacuated chamber with helium.

3. The method of claims 1 or 2 characterized in that the second gas is argon.

4. The method of claims 1 or 2 characterized in that the second gas is argon containing about 3% hydrogen.

The method of claims 1 or 2 characterized in that the helium is 99.999% pure.

6. The method of claim 3 characterized in that the helium is 99.999% pure. 4 4 4 Q 1 0'. -9-

7. The method of claim 4 characterized in that the helium is 99.999% pure.

8. A method of growing a dendritic web silicon crystal as hereinbefore described with reference to the accompanying drawings. DATED this 28th day of June, 1990. oo 0 '0 0 o 0 r 0 u ft f WESTINGHOUSE ELECTRIC CORPORATION Patent Attorneys for the Applicant: PETER MAXWELL ASSOCIATES. r i L ftI4 I ft 0 t ft t f i r j 1