CN110849875A

CN110849875A - Method for analyzing microstructure of cast polycrystalline silicon

Info

Publication number: CN110849875A
Application number: CN201910960476.2A
Authority: CN
Inventors: 谢宇; 张发云; 饶森林
Original assignee: Xinyu University
Current assignee: Xinyu University
Priority date: 2019-10-10
Filing date: 2019-10-10
Publication date: 2020-02-28

Abstract

The invention discloses a method for analyzing a microstructure of cast polycrystalline silicon, which comprises the following steps of firstly, carrying out chemical polishing and Secco etching treatment on the cast polycrystalline silicon; step two, observing and photographing by using a large-scale metallographic microscope after the step one; and step three, analyzing after the step two. The invention has the beneficial effects that: the microstructure of commercial polysilicon and self-made polysilicon blocks is researched by chemical polishing and Secco etching methods, and after polishing, the defects of crystal boundaries, subgrain grains, half-crystal boundaries, small triangles and the like are clearly observed; after Secco etching, dislocation defects are prominent, and dense dislocation, concentrated dislocation, dislocation arrangement and the like are mainly existed; after polishing and Secco etching, dislocation defects in various forms are observed and found, the formation mechanism of the polycrystalline silicon crystal defects is further known through analyzing the microstructure of the polycrystalline silicon, and a certain basis can be laid for the formulation and optimization of the subsequent polycrystalline silicon ingot casting process.

Description

Method for analyzing microstructure of cast polycrystalline silicon

Technical Field

The invention relates to a method for analyzing a microstructure of cast polycrystalline silicon, and belongs to the technical field of structural analysis.

Background

With the continuous development of the photovoltaic industry, the polycrystalline silicon has a wide market prospect, and many enterprises at home and abroad produce the polycrystalline silicon. At present, cast polycrystalline silicon is the most main solar cell material, and compared with czochralski silicon, the preparation process of the cast polycrystalline silicon is relatively simple, but the cast polycrystalline silicon has more crystal defects and impurities, and the photoelectric property of the silicon material is seriously influenced. Therefore, it is highly desirable to understand the mechanism of polysilicon crystal defect formation and to improve the quality of cast polysilicon.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a method for analyzing the microstructure of the cast polysilicon, and the method is used for analyzing the microstructure of the polysilicon material, so that the formation mechanism of the polysilicon crystal defects is further understood, and certain theoretical guidance can be provided for reasonably formulating the production process of polysilicon ingots.

The invention is realized by the following scheme: a method of analyzing a cast polysilicon microstructure, comprising the steps of,

step one, polishing and etching the cast polysilicon;

step two, observing and photographing are carried out after the step one;

and step three, analyzing after the step two.

A method of analyzing a cast polysilicon microstructure, comprising the steps of,

step one, performing chemical polishing and Secco etching treatment on cast polycrystalline silicon to expose corrosion pits on the surface of a sample on internal defects of the crystal of the cast polycrystalline silicon;

step two, observing and photographing by using a large-scale metallographic microscope after the step one;

and step three, analyzing after the step two.

The chemical polishing solution adopted by the polishing is a mixed solution of hydrofluoric acid and nitric acid.

And mixing the hydrofluoric acid and the nitric acid according to the volume ratio of 1: 3.

The concentration of the hydrofluoric acid is 49%, and the concentration of the nitric acid is 68%.

The etching liquid adopted by the Secco etching treatment is a mixed liquid of hydrofluoric acid, potassium dichromate and glacial acetic acid.

And mixing the hydrofluoric acid, the potassium dichromate and the glacial acetic acid according to the volume ratio of 50:25: 1.

The concentration of the potassium dichromate is 0.15 mol/L.

Before the Secco etching treatment, the surface of the cast polycrystalline silicon needs to be cleaned, and organic pollution such as grease on the surface is removed.

And removing the mechanical damage layer on the surface of the silicon wafer before the Secco etching treatment.

The invention has the beneficial effects that: the microstructure of commercial polysilicon and self-made polysilicon blocks is researched by chemical polishing and Secco etching methods, and after polishing, the defects of crystal boundaries, subgrain grains, half-crystal boundaries, small triangles and the like are clearly observed; after Secco etching, dislocation defects are prominent, and dense dislocation, concentrated dislocation, dislocation arrangement and the like are mainly existed; after polishing and Secco etching, dislocation defects in various forms are observed and found, the formation mechanism of the polycrystalline silicon crystal defects is further known through analyzing the microstructure of the polycrystalline silicon, and a certain basis can be laid for the formulation and optimization of the subsequent polycrystalline silicon ingot casting process.

Detailed Description

The present invention is further illustrated below, but the scope of the invention is not limited to the disclosure.

In the following description, for purposes of clarity, not all features of an actual implementation are described, well-known functions or constructions are not described in detail since they would obscure the invention with unnecessary detail, it being understood that in the development of any actual embodiment, numerous implementation details must be set forth in order to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, changing from one implementation to another, and it being recognized that such development effort might be complex and time consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art.

and step three, analyzing after the step two.

Example (b): analyzing the microstructures of a commercial polycrystalline silicon wafer and a self-made polycrystalline silicon ingot, firstly, chemically polishing and Secco etching silicon blocks at a horizontal part and a vertical part, which are taken out of the commercial polycrystalline silicon wafer and the self-made polycrystalline silicon ingot, under the same conditions to expose corrosion pits on the surface of a sample on the internal defects of the crystal, then observing and photographing by using a large-scale metallographic microscope, and finally analyzing. Considerations before dislocation corrosion: (1) cleaning the surface of a silicon wafer, and removing organic pollution such as grease on the surface; (2) the mechanically damaged layer on the surface of the silicon wafer is removed because it may introduce dislocations.

The conclusion is as follows:

(1) after polishing a commercial polycrystalline silicon wafer, surface defects such as crystal boundary, subgrain boundary, semi-crystal boundary and the like can be observed under an optical microscope, but linear defects such as dislocation and the like cannot be observed at the moment because an etch pit does not exist or is too fine; after Secco etching, dislocation is prominent, and dense dislocation, concentrated dislocation, dislocation arrangement and the like are mainly existed.

(2) After polishing and Secco etching, the self-made polycrystalline silicon block found defects similar to those of commercial polycrystalline silicon wafers, which indicated that the polycrystalline silicon ingot could be prepared using experimental equipment, and the grains of the self-made polycrystalline silicon were found to be smaller than those of the commercial polycrystalline silicon, which may be caused by the smaller crucible of the laboratory ingot furnace.

The reason for the formation of dislocations: the heat dissipation from the edge to the center, bottom to upper part of the ingot by casting is uneven when the ingot is cooled, which causes thermal stress to be generated in the ingot; in addition, the thermal expansion coefficients of crystalline silicon and quartz crucibles are different, and thermal stress is also generated during cooling. A large number of dislocations in the grains are caused by thermal stress, and thus the efficiency of the solar cell is seriously affected.

In conclusion, in the initial stage of crystal growth, the growth rate and the temperature gradient are as small as possible, so that the grown crystal has the minimum defect density; during slicing operation, dislocation can be introduced into a damaged layer on the surface of the silicon wafer by a machine, and the introduced dislocation can also extend into the silicon wafer in the subsequent thermal processing process; the dislocations introduced by the thermal stress may be caused by non-uniform temperature distribution at the center and edge of the wafer during the thermal processing of the wafer.

Although the invention has been described and illustrated in some detail, it should be understood that various modifications may be made to the described embodiments or equivalents may be substituted, as will be apparent to those skilled in the art, without departing from the spirit of the invention.

Claims

1. A method for analyzing a microstructure of cast polycrystalline silicon, comprising: which comprises the following steps of,

step one, polishing and etching the cast polysilicon;

step two, observing and photographing are carried out after the step one;

and step three, analyzing after the step two.

2. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: which comprises the following steps of,

and step three, analyzing after the step two.

3. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: the chemical polishing solution adopted by the polishing is a mixed solution of hydrofluoric acid and nitric acid.

4. A method of analyzing a cast polysilicon microstructure as recited in claim 3, wherein: and mixing the hydrofluoric acid and the nitric acid according to the volume ratio of 1: 3.

5. A method of analyzing a cast polysilicon microstructure as recited in claim 3, wherein: the concentration of the hydrofluoric acid is 49%, and the concentration of the nitric acid is 68%.

6. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: the etching liquid adopted by the Secco etching treatment is a mixed liquid of hydrofluoric acid, potassium dichromate and glacial acetic acid.

7. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: and mixing the hydrofluoric acid, the potassium dichromate and the glacial acetic acid according to the volume ratio of 50:25: 1.

8. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: the concentration of the potassium dichromate is 0.15 mol/L.

9. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: before the Secco etching treatment, the surface of the cast polycrystalline silicon needs to be cleaned, and organic pollution such as grease on the surface is removed.

10. A method of analyzing a cast polysilicon microstructure as recited in claim 1, wherein: and removing the mechanical damage layer on the surface of the silicon wafer before the Secco etching treatment.