SEMI PV39 - Test Method for In-Line Measurement of Cracks in PV Silicon Wafers by Dark Field Infrared Imaging
Silicon (Si) wafers for PV applications cut from a Si ingot or Si brick by multiple-wire sawing contain artifacts characteristic for this cutting process, so called saw marks.
The saw marks consist of topographic features, such as grooves, steps (see Related Information 2), on or in the Si wafer surface and that extend along the wire direction.
Saw marks may significantly impact the quality of wafers. They interfere with printing the contact fingers on solar cells. Extreme saw mark dimensions may interrupt the contact fingers or create too wide fingers.
Saw marks are frequently specified for Si wafers for solar cells with respect to their maximum peak-to-valley within a finite distance, or window.
Standardized test methods providing reproducible values for saw marks are required to specify this aspect of wafer quality.
Process and quality control during manufacturing of wafers requires continuous monitoring of saw marks with a noncontact method that supports high throughput.
This Test Method determines maximum peak-to-valley of saw marks of multi or single crystal Si wafers that typically run across the entire wafer surface and along the wire direction.
It describes an in-line, noncontacting and nondestructive method that determines the height changes of steps and grooves of clean, dry as-cut silicon wafers supported by two belts that move the test specimen through the measurement equipment.
This Test Method covers square and pseudo-square PV Si wafers, with a nominal edge length ≥125 mm and a nominal thickness ≥100 µm. It applies to both single-crystalline and multicrystalline Si wafers.
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 a light sectioning technique (see Related Information 1) where patterns of line segments or spots of light are projected onto a wafer surface and the saw marks are oriented perpendicular to the direction of wafer transport.
Other measurement techniques may also provide similar information about saw marks of a wafer 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
SEMI PV22 — Specification for Silicon Wafers for Use as Photovoltaic Solar Cells