SEMI PV52 - Test Method for In-Line Characterization of Photovoltaic Silicon Wafers Regarding Grain Size -

Abstract

Multicrystalline silicon (mc-Si) wafers consist of a multitude of crystallographically differently oriented grains. The term multicrystalline includes also so-called ‘mono-like’ Si wafers in the context of this Test Method.

The number and size of these grains vary significantly depending on the crystallization method and the wafer’s position in the ingot.

Grain boundaries may be regions of high recombination activity in a multicrystalline wafer. They also may getter unwanted impurities from the interior of the grains and diminish their detrimental impact on solar cell conversion efficiency.

An appropriate solar cell manufacturing process can reduce the detrimental effects of grain boundaries and internal grain defects.

An optimized grain size distribution for a specific solar cell manufacturing process is preferable.

A standardized test method for measuring the grain sizes and their distribution is required to establish wafer specifications regarding grain sizes.

This Test Method evaluates dimensional characteristics of cross-sections of grains of mc-Si as they appear on a wafer surface.

It employs an in-line, noncontacting and nondestructive method for characterizing clean, dry Si wafers that are supported by a mechanism that move the test specimen through the measurement equipment.

The surface condition of the wafers may be as cut or as etched.

The test method covers square and pseudo-square Si wafers for photovoltaic (PV) applications, with a nominal edge length ≥125 mm and a nominal thickness ≥100 µm.

The test method is intended for in-line high throughput measurements. Therefore it is mandatory to operate the measurement system under statistical process control (SPC) [e.g., ISO 11462] in order to obtain reliable, repeatable and reproducible measurement data.

The test method is based on recording and evaluating images of the wafer surface obtained by a digital camera under directional transmitted or reflected light illumination.

Two procedures for evaluating the grain size characteristics are defined. Both methods are based on obtaining a digital image of the wafer surface displaying the grain structure that is digitally processed. The first procedure straightforwardly evaluates the grain sizes; the second procedure follows a statistical approach used in metallurgy as described in ASTM E112. The constraints of the second procedure must be checked before applying it.

The test may also be used for off-line characterization of Si wafers provided the requirements of the test method are met.

Other measurement techniques may also provide similar information as compared to this Test Method, but they are not the subject of this Test Method.

Referenced SEMI Standards

SEMI E89 — Guide for Measurement System Analysis (MSA)
SEMI M59 — Terminology for Silicon Technology
SEMI MF1569 — Guide for Generation of Consensus Reference Materials for Semiconductor Technology

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