FR2597513A1 - PROCESS FOR REMOVING DENTRITIC SILICON TISSUE FILM - Google Patents

Abstract

A.PROCEDE FOR REMOVING A FILM OF DENTRITIC SILICON TISSUE, B.PROCEDE CHARACTERIZED IN THAT IT INCLUDES THE HEATING OF THE SAID FABRIC AT A TEMPERATURE OF 800 TO 1200C IN THE PRESENCE OF OXYGEN THANKS TO A COATING OF DIOXIDE FORMED OF SILICON ON THE FABRIC BELOW THE SAID FILM; AND THE REMOVAL OF SAID COATING, C. THE INVENTION RELATES TO A METHOD FOR REMOVING A FILM FROM DENTRITIC SILICON TABLES.

Description

"Process for removing a film from dentritic silicon tissue"

  This invention relates to a method for removing a film from dendritic silicon tissue.

  Dendritic Silicon Fabric is a ribbon-shaped flat material made of silicon and used primarily as an undercoat for solar cells. To obtain growth of the dendritic silicon fabric, a seed of a single silicon crystal in an argon atmosphere is reduced until it just comes into contact with polycrystalline silicon.

  melted and then it is slowly pushed from the molten bath.

  As the seed grows from the melt, a dendritic tissue of a single crystal of silicon

  grows from the base of the germ.

  Unfortunately, as the tissue grows, a film forms on its surface that is believed to be primarily various silicon oxides, but may also contain impurities from the atmosphere surrounding the tissue. This contaminating film must be removed from the tissue before it can be processed for use as a substrate in a solar cell. To remove this film, the fabric is dipped in concentrated hydrofluoric acid and rubbed with cotton. This treatment must not only be done by hand but also frequently

  the breaking of the very fine tissue of silicon.

  According to the present invention, a method of removing the film formed on a dentritic silicon fabric during the growth thereof by heating said tissue at a temperature of from 800 to 12000 C in the presence of oxygen, whereby a coating of silicon dioxide is formed on said fabric underneath said film; and removing said coating. It has been found that the film formed on silicon dendritic tissue as it grows can be easily and cleanly removed by heating the fabric in a temperature range of from 800 to 1200 C in the presence of oxygen and in particular

  Oxygen saturated with water vapor (wet oxygen).

  This forms a coating of silicon dioxide on the fabric underneath the film. When the fabric is then immersed in a solution of hydrofluoric acid, the silicon dioxide coating dissolves and the

  contaminating film is easily detached from the tissue.

  Unlike the prior process for removing the film, the preferred method of this invention does not require hand cleaning with cotton - and can be done with an automatic machine. Surprisingly we have found that the resulting fabric surface is smooth (specular), even and flat, and not bumpy, irregular

  or rough as one would have expected.

  The process of this invention is applicable to both dendritic pure silicon tissue and dendritic doped silicon tissue. Typical dopants include, for example, boron, phosphorus, gallium, aluminum, arsenic and antimony. The silicon dendritic photograph is typically 100 to 200 microns thick, one

  2.5 to 7.5 cm and maybe about any length.

  The exact nature of the film that forms on silicon as it develops is not known, but it is believed to be a mixture of various silicon oxides and contaminants that are in the atmosphere of the silicon. oven. In addition, soils arising from the use of doping substances may also give rise to

  film on the surface of the fabric. For example, boron stain is often formed on the fabric when the concentration of the boron load exceeds about 1020 atoms / cm.

  Although the film is only about 1-8 meters thick, it must be removed before the fabric can be treated again as a substrate for

solar cell.

  Although temperatures below 8000C can act, they are very slow. At temperatures above 1200 C, impurities in the film

  can penetrate the silicon.

  In a preferred manner the fabric is maintained in the temperature range of 800-1200 C for 15 minutes to 1 hour. If the fabric is heated for a shorter time, the silicon dioxide layer may be insufficiently thick to completely remove the contaminating film, and heating for a longer time is usually unnecessary. The oxygen used in the process of the 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 because it is simpler to use. Wet oxygen is gaseous oxygen saturated with water vapor. The moist oxygen can be obtained by bubbling gaseous oxygen in water heated to about 95, which saturates the oxygen gas with water vapor. Moist oxygen is preferred because it gives a much thicker layer of silicon dioxide and

  therefore, facilitates the departure of the contaminating film from the tissue. The thickness of the silicon dioxide layer is typically 0.1 to 1 micron, although other thicknesses may also be appropriate. After exposure to oxygen, it has been found preferable to cool the tissue and then immerse in dilute or concentrated hydrofluoric acid at a temperature of about room temperature. Concentrated hydrofluoric acid, which is an approximately 37% by weight solution in water, is the preferred one because it is commercially available and has a faster action.

  Typically a time of 2 to 6 minutes is required to remove the silicon dioxide layer and thereby separate the contaminating film from the silicon fabric. The silicon fabric can then be washed with water and after which it is ready for further processing. Since the contaminating film is formed on both sides of the silicon fabric, both sides are processed.

  The invention will now be illustrated with reference to the following example. EXAMPLE

  Pieces of dentritic silicon fabric about 5 cm long, about 3 cm wide and about 120 microns thick, loaded with 3 x 1015 boron atoms / cm 3 were used in these experiments.

  Before treatment, the parts have an unacceptable film on each of the surfaces. The pieces are placed in dry oxygen at a flow rate of 350 cm / min and at temperatures of 700, 800, 900 and 10000 ° C for one hour. The following table provides the results.

  Thickness of Silicon Dioxide Layer (in 1-10m) Temperature (C)

700 800

900 1000

not measured

309 578 599

  All the treated parts were placed in a 37% aqueous solution of hydrofluoric acid at room temperature for 5 minutes. In either case, both sides of the pieces become cleaner than a control piece which has been immersed directly in the hydrofluoric acid solution without the dry oxygen treatment. However these surfaces were not clean enough to continue manufacturing in one cell

  solar without additional treatment.

  In similar experiments, oxygen was bubbled through 95 water and wet oxygen passed through similar pieces of dendritic silicon cloth at various temperatures.

during one hour.

  The following table gives the temperatures and the approximate thickness of the silicon dioxide layers. Renewal (C)

900 1000 1100 1200

  Thickness of the layer of dioxide (1-10m) (1 slm)

1000 3200 4300 6800

  The pieces were again immersed in 37% hydrofluoric acid solution for minutes at room temperature. In this case all the parts became clean enough to allow the continuation of the manufacture in solar cells without further processing. The faces of the fabric were shiny

and smooth.

FRENCH REPUBLIC

NATIONAL INSTITUTE

BE THE INDUSTRIAL PROPERTY

PARIS:: M.

2,597,514

  This issue did not result in any publication I

Claims (6)

R E V E N D I C A T I 0 N S   1) Process for removing the film formed on a dendritic silicon fabric during the growth thereof, characterized in that it comprises heating said fabric at a temperature of 800 to 1200 ° C in the presence of oxygen by means of which a coating of silicon dioxide is formed on the said fabric below the   said film; and removing said coating.   2) The method of claim 1, wherein the coating is removed by immersing the   tissue in a solution of hydrofluoric acid.   3) Process according to claim 2, wherein the hydrofluoric acid is an aqueous solution   at 37% and is applied for 2 to 6 minutes.   4) Process according to any one of   Claims 1, 2, 3 wherein the silicon is charged   with boron, phosphorus, gallium, aluminum,   arsenic, antimony or mixtures thereof.    The process of any one of claims 1, 2, 3, 4 wherein the oxygen is saturated water vapor.   Method according to any one of claims 1, 2, 3, 4, 5, in which recourse is made   heating for 15 minutes to one hour.   7) Process according to any one of   Claims 1, 2, 3, 4, 5, 6, wherein the fabric has   2.5 to 7.5 cm wide and 100 to 200 microns thick.   8. A process according to any one of claims 1, 2, 3, 4, 5, 6, 7, wherein the silicon dioxide coating is 0.05 to 1 micron thick.