JPH02230776A - Manufacture of solar battery - Google Patents

Abstract

PURPOSE:To uniformly eliminate a high concentration doped layer on an N<+> layer surface with excellent reproducibility by a method wherein, after an N- type layer is formed on a P-type silicon substrate surface by impurity diffusion of group V element, a thermal oxide film is formed and eliminated by etching. CONSTITUTION:After a P<+> layer 2 is formed on a P-type single crystal silicon substrate, an N<+> layer 3 is formed by thermal diffusion of phosphorus. A wafer 1 treated in the above manner is subjected to thermal oxidation in an oxygen atmosphere at 850 deg.C or less, thereby forming a thermal oxide film of several 10's-120Angstrom in thickness on the whole surface of the wafer 1. The water 1 coated with the thermal oxide film 4 is dipped in etchant whose main component is hydrofluoric acid, and the thermal oxide film is once completely eliminated. At this time, the vicinity of the surfaces of the P<+> layer 2 and the N<+> layer 3 is also eliminated a little, and a P<+> layer 2-1 and an N<+> layer 3-1 are formed. After that, by performing anew thermal oxidation, the passivation film of a thermal oxide film 5 is formed and a solar battery cell is manufactured.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for manufacturing solar cells, and is particularly useful when used for manufacturing space silicon solar cells used as power sources for artificial satellites. It is something.

(Prior Art) In solar cells that require high output and high reliability, such as space silicon solar cells, various methods are being researched and developed to further increase the output (high efficiency). Among them, it is important to improve the quality of the light-receiving surface, which directly controls the output characteristics of the solar cell, and the main ones are as follows.

(1) Improving the quality of the light-receiving surface (n+ layer) surface area (2)
Oxidation passivation of the light-receiving surface (n+ layer) (1) above
The improvement of the quality of the n+ layer surface area is described, for example, in J. As reported by Lindmeyer et al. (Conf.
Rec. IEEE Photo. Spec. Co
nf, 9 thp88 1972) The vicinity of the surface of the n+ layer is heavily doped with impurities, causing significant damage to the crystal, and as a result, it is possible that the effective lifetime of minority carriers generated near the surface is extremely short. This is an attempt to remove the above-mentioned highly doped layer in order to increase the output power of the cell.

The above (2) oxidation passivation of the n+ layer is a technology that has been mainly studied in recent years for terrestrial cells.
+ This is an attempt to reduce the recombination rate of minority carriers on the surface of the layer. For example, in dry oxygen
It has been reported that forming a thermal oxide film for about 100 to 200 years on the surface at 00 to 850°C is effective in reducing the recombination rate of the minority carriers (for example, K. Morita et al. .J. Appl. Ph
ys. Vol 26, No. 5, May 1987
pp. See L547-9).

(Problems to be Solved by the Invention) Although the above-mentioned means have been individually studied in the past, they have not yet fully met the requirements for increasing the output of silicon solar cells for use in space.

Regarding (1) above, the thickness of the highly doped layer is extremely thin, on the order of several tens to 100 λ, and
For example, it is not easy to remove it uniformly and with good reproducibility using a method such as chemical etching.

This becomes especially difficult for wafers with a diameter of 4 inches or more, which are commonly used in recent solar cell production processes. Additionally, if an electrochemical method such as anodic oxidation, which allows for precise control of the thickness of the n+ layer to be removed, is introduced, a new wafer cleaning process will be required after removal, resulting in equipment investment and process savings. The increased complexity increases the price of solar cells.

Regarding (2) above, during thermal oxidation, a new solid phase diffusion of doping impurities from the n+ layer into the silicon substrate progresses, resulting in an increase in the thickness of the n+ layer and a deepening of the junction. In space solar cells, deep junctions are undesirable because they lead to increased damage due to radiation in space. Therefore, it is necessary to find thermal oxidation conditions that do not promote solid phase diffusion from the n+ layer.

(Means for solving all problems) After forming an n layer on the surface of a p-type silicon substrate by diffusing group V element impurities, a thermal oxide film of 120X or less is formed on the surface in an oxygen atmosphere at 850°C or less. After removing the surface of the n-layer together with this thermal oxide film by etching, a thermal oxide film was further formed for 100 to 200 years on the etched surface in an oxygen atmosphere at 850° C. or lower.

(Function) By forming a thermal oxide film of 120X or less on the surface of the n+ layer in advance and etching it, the highly doped layer on the surface of the n+ layer can be removed uniformly and with good reproducibility. Furthermore, as mentioned above, the highly doped layer near the surface of the n+ layer is a region where the lifetime of minority carriers is extremely short, and at the same time it is a region where the surface recombination rate is high. Oxide film passivation is much more effective in improving the performance of solar cells.

(Embodiment) FIGS. 1(a) to 1(i) are schematic cross-sectional views of each step of an embodiment of the present invention.

First, a p-type silicon single crystal substrate (hereinafter referred to as a wafer) 1 having a predetermined thickness as shown in FIG. , by performing thermal diffusion of boron at 900 to 1000°C,
As shown in FIG. 1(b), a p layer 2 having a thickness of about 1 pm is formed on the surface of the wafer 1.

Next, after coating the side and back surfaces of the wafer 1 with wax or the like, the ten p-layers on the front surface are removed by etching, and as shown in FIG. be exposed.

Next, for example, using PH9, POCl 3, etc.
Thermal diffusion of phosphorus is carried out at about 800 to 900°C, and as shown in FIG.
m layers 3 are formed.

Next, the wafer 1 treated in this way is thermally oxidized in an oxygen atmosphere at 850°C or lower to form a thermal oxide film of several tens of λ to 120×8 degrees over the entire surface of the wafer 1, as shown in FIG. 1(e) K. form 4. As an example of specific conditions, thermal oxidation is performed at 800° C. for several seconds to 17 minutes in dry oxygen at 4 to 5 t/min.

Next, the wafer/% 1 covered with this thermal oxide film 4 is
For example, if the thermal oxide film is completely removed by immersion in an etchant whose main component is hydrofluoric acid, the result will be as shown in FIG. 1(f). In this etching process, the vicinity of the surfaces of the p+ layer 2 and n+ layer 3 are also slightly removed, resulting in slightly thin p+ layers 2-1 and n+48-1. after that,
Again, as shown in Figure 1(g), 100x~2
A thermal oxide film 5 having a thickness of about 1,000 yen is formed on the entire surface of the wafer 1.

An example of this specific condition is 4 to 5 tons of dry oxygen.
Thermal oxidation is carried out at 800° C. for 13-80 minutes in /mIn. The thermal oxide film 5 thus formed becomes a passive basis film.

Next, the front electrode 6, the back surface [pole 7,
When the antireflection film 8 is formed, the state shown in FIG. 1(h) is obtained,
By separating this into predetermined sizes, individual solar cells shown in FIG. 1(i) are completed.

Thermal oxidation conditions for forming a thermal oxide film for removing the highly doped layer on the surface of the n+ layer and the thermal oxidation conditions for forming a passivation film were obtained from the following study results.

FIG. 2 is a graph showing the dependence of the thermal oxide film thickness on the oxidation time when the n+ layer 8 formed by the above method was thermally oxidized in dry oxygen at 800°C. In order to form a thermal oxide film of approximately 100 to 200 A, which is necessary for oxidized passivation aimed at reducing the surface recombination rate, approximately 18
It can be seen that an oxidation time of ~32 minutes is required.

FIG. 8 similarly shows the heating time dependence of the junction depth (thickness of the n+ layer) when a wafer on which the n+ layer 8 was formed by the above method was heated in dry oxygen at 800°C. In order to reduce damage to the cell due to radiation in the space environment, the junction depth is set to K and O. It is found that it is desirable to set the heating time to about 100 μm or less, and for that purpose, the heating time must be set within about 30 minutes.

From the above two items, the time allowed for the formation of a thermal oxide film on the surface of the n+ layer 8 before removing it is 8
If it is 00°C, it will take less than 17 minutes. In other words, if the total oxidation time for forming the passive oxide film and the oxidation time for forming the first thermal oxide film exceeds 30 minutes, the junction depth becomes excessive. Note that if the temperature is 800°C or higher, the time will be even shorter.

It can be seen from FIG. 2 that the thickness of the thermal oxide film formed when the oxidation time is 17 minutes at 800° C. is about 120λ. When etching the thermal oxide film 4, 12
If OA can be removed, it can be said that the high definition doped layer has been removed sufficiently.

Furthermore, when a thermal oxide film is formed at a temperature of 850° C. or higher, the solid phase diffusion of phosphorus from the R+ layer is significant, making it difficult to find appropriate oxidation conditions.

(Effects of the Invention) According to the invention, it is possible to improve the quality of the light-receiving surface and improve the conversion efficiency of light into electric power without causing an increase in the cost of the solar cell. In addition, the reliability of the space solar cell is improved because the light-receiving surface can be oxidized passivated without reducing the radiation resistance characteristics of the cell. Furthermore, since the oxidation bashing is performed after removing the highly doped layer near the surface of the n+ layer, the panosybasic effect can be made even more pronounced.
Contributes to higher efficiency of space cells.

[Brief explanation of the drawing]

Figures 1<a) to (1) are cross-sectional views of essential parts of each process of an embodiment of the present invention, Figure 2 is a graph showing the dependence of the thickness of a thermal oxide film on oxidation time, and Figure 8 is a graph showing the dependence of junction depth on oxidation time.

Claims (1)

[Claims] 1. After forming a diffusion layer on the surface of a p-type silicon substrate by diffusing group V element impurities, a thermal oxide film of 120 Å or less is formed on the surface in an oxygen atmosphere at 850°C or less, and then thermally removed by etching. After removing the highly doped layer on the surface of the diffusion layer together with the oxide film, a thermal oxide film with a thickness of 100 to 200 Å is further formed on the surface of the diffusion layer in an oxygen atmosphere at 850° C. or lower. Production method.