GB2190368A

GB2190368A - Method of removing film from dendritic web silicon

Info

Publication number: GB2190368A
Application number: GB08706918A
Authority: GB
Inventors: Prosenjit Rai-Choudhury; James Bernard Mcnally; Daniel Leo Meier
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1986-04-16
Filing date: 1987-03-24
Publication date: 1987-11-18
Also published as: GB2190368B; FR2597513A1; JPS6325300A; GB8706918D0; DE3712443A1; FR2597513B1

Description

1 GB2190368A 1

SPECIFICATION

Method of removing film from dendritic web silicon This invention relates to a method of removing film from a dendritic web silicon. 5 Dendritic web silicon is a flat ribbon-like material made of silicon, used primarily as a substrate for solar cells. To grow dendritic web silicon, a seed of single crystal in an argon atmosphere is lowered until it just contacts a melt of polycrystalline silicon, and is then slowly pulled from the melt. As the seed rises from the melt, a dendritic web of single crystal grows from the base of the seed. 10 Unfortunately, as the web grows, a film forms on its surface which is believed to be primarily various oxides of silicon, but which may also contains impurities from the atmosphere that surrounds the web. This contaminating film must be removed from the web before it can be processed for use as a substrate in a solar cell. To remove the film, the web is dipped into concentrated hydrofluoric acid, then rubbed with cotton. Not only must this procedure be done 15 by hand, but it frequently breaks the very thin silicon web.

According to the present invention a method of removing film formed on a web of dendritic silicon during growth thereof comprises a method of removing film formed on a web of dendritic silicon during growth thereof which comprises heating said web to from 800 to 1200'C in the presence of oxygen, whereby a coating of silicon dioxide forms on said web beneath said film; 20 and removing said coating.

It has been discovered that film formed on dendritic web silicon as it is grown can be easily and cleanly removed by heating the web in the range of from 800 to 1200'C in the presence of oxygen, particularly oxygen saturated with water vapor (---wet- oxygen). This forms a coating of silicon dioxide on the web beneath the film. When the web is then immersed into a solution of 25 hydrofluoric acid, the silicon dioxide coating is dissolved, and the contaminating film is easily dislodged from the web. Unlike the prior process for removing the film, the preferred process of this invention does not require hand cleaning with cotton and can be done by automatic machinery. Surprisingly, we have found that the resulting surface of the web is smooth (specu Jar), uniform, and flat, and is not bumpy, irregular, or rough, as might have been expected. 30 The ' method of this invention is applicable to both pure and doped dendritic web silicon.

Typical dopants include, for example, boron, phosphorus, gallium, aluminium, arsenic, and anti mony. Dendritic web silicon is typically from 100 to 200 microns in thickness, from 1 to 3 inches wide, and can be of almost any length. The exact nature of the film that forms on the silicon as it grows is not known, but it is believed to be a mixture of various silicon oxides and 35 contaminants that are in the furnace atmosphere. In addition, stains from the use of dopants can also result in a film on the surface of the web. For example, a boron stain is often formed on the web when the concentration of the boron dopant exceeds about 1020 boron atoms/em.

While the film is only a few hundred angstroms in thickness, it must be removed before the web can be further processed as a substrate for a solar cell. 40 While tempertures lower than 800'C will work, they are very slow. At temperatures above 1200'C, impurities in the film may penetrate into the silicon. Preferably, the web is maintained within the 800-1200'C temperature range for 15 minutes to an hour. If the web is heated for less time, the silcon dioxide layer may be insufficiently thick to fully remove the contaminating film, and heating for a longer time is usually unnecessary. The oxygen used in the process of 45 this invention may be either dry oxygen or wet oxygen. Dry oxygen is oxygen gas or a mixture of oxygen gas and an inert gas such as nitrogen; pure oxygen is preferred as it is simpler to work with. Wet oxygen is oxygen gas saturated with water vapor. Wet oxygen can be obtaining by bubbling oxygen gas through water heated to about 95'C, which saturates the oxygen gas with water vapor. Wet oxygen is preferred as it results in a much thicker silicon dioxide layer 50 and therefore faciliates removal of the contaminating film from the web. The thickness of the silicon dioxide layer is typically from 0.1 to 1 micron, although other thicknesses may also be suitable. After exposure to oxygen, it has been found desirable to cool the web and then immerse it in dilute or concentrated hydrofluoric acid at about room temperature. Concentrated hydrofluoric acid, which is about a 37% by weight solution in water, is preferred as it is 55 commercially available and is faster. Typically, from 2 to 6 minutes is required to etch away the silicon dioxide layer and thereby separate the contaminating film from the silicon web. The silicon web may then washed with water after which it is ready for further processing. Since the contaminating film forms on both sides of the silicon web, both sides are treated.

The invention will now be illustrated with reference to the following Example: 60 EXAMPLE

Pieces of dendritic web silicon about 2 in. long, about 3cm. wide, and about 120 microns thick, doped with 3 x 1015 atoMS/CM3 of boron, were used in these experiments. Before treat ment, the pieces had unacceptable films on both surfaces. Pieces were placed in dry oxygen at 65 2 GB2190368A 2 a flow rate of 350 standard cubic centimeters per minute, at temperatures of 700, 800, 900, and 1000'C for one hour. The following table gives the results:

Thickness of Silicon Dioxide Layer 5 Temperature (OC) (Angstroms) 700 Not measured 800 309 9 00 578 10 1000 599 All the treated pieces were placed in an aqueous solution of 37% hydrofluoric acid at room 15 temperature for 5 minutes. In all cases, both surfaces of the pieces became cleaner than a control piece that was immersed directly in the hydrofluoric acid solution without the dry oxygen treatment. However, these surfaces were not clean enough to continue processing into a solar cell without further treatment.

In similar experiments, oxygen was bubbled through water at WC and the wet oxygen was 20 passed over similar pieces of dendritic web silicon at various temperatures for an hour. The following table gives the temperatures and the approximate thicknesses of the silicon dioxide layers:

Thickness of 25 Dioxide Layer Temperature (OC) (Angstroms) 800 1000 900 3200 30 1000 4300 1100 6800 1200 9000 35 The pieces were again immersed in a 37% aqueous solution of hydrofluoric acid for 5 minutes at room temperature. In this case, all the pieces became clean enough to continue processing into solar cells without further treatment. The surfaces of the web were bright and smooth.

40

Claims

1. A method of removing film formed on a web of dendritic silicon during growth thereof which comprises heating said web to from 800 to 120WC in the presence of oxygen, whereby a coating of silicon dioxide forms on said web beneath said film; and removing said coating.

2. A method according to claim 1, wherein the coating is removed by immersing the web 45 into a solution of hydrofluoric acid.

3. A method according to claim 2, wherein the hydrofluoric acid is a 37% aqueous solution and is applied for from 2 to 6 minutes.

4. A method according to claim 1, 2 or 3 wherein the silicon is doped with boron, phospho- rus, gallium, aluminium, arsenic, antimony, or mixtures thereof. 50

5. A method according to any of claims 1 to 4, wherein the oxygen is saturated with water vapor.

6. A method according to any of claims 1 to 5, wherein the heating is applied for from 15 minutes to one hour.

7. A method according to any of claims 1 to 6, wherein the web is from 1 to 3 inches wide 55 and from 100 to 200 microns thick.

8. A method according to any of claims 1 to 7, wherein the coating of silicon dioxide is from 0.05 to 1 micron thick.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987.

Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.