SEMI PV71 - Test Method for In-Line, Noncontact Measurement of Thickness and Thickness Variation of Silicon Wafers for Photovoltaic (PV) Applications Using Laser Triangulation Sensors
This Standard was technically approved by the Photovoltaic – Materials Global Technical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on December 4, 2015. Available at www.semiviews.org and www.semi.org in January 2016.
Wafer thickness and its variation across a wafer are important parameters for solar cell manufacturing. Excessive thickness variations within a lot from wafer to wafer or within a wafer may negatively impact process yield and solar cell efficiency.
Both parameters are part of the specification for solar cell wafers (see SEMI PV22), which define a thickness range as well as an upper limit for the total thickness variation (TTV).
In addition, careful process and quality control of the wafer thickness and its variation during wafer and solar cell manufacturing requires continuous monitoring of thickness by the supplier of wafers for PV applications as well as by the user of such wafers.
Therefore a standardized test method providing reproducible data for thickness and its variation is required to establish agreement between business partners regarding the specification of wafers.
The present Document defines a noncontact, high throughput in-line method for measuring wafer thickness and its total variation using laser triangulation sensors.
This Test Method covers the in-line, noncontact, nondestructive measurement of the thickness and the TTV of clean, dry silicon (Si) wafers supported on two belts that move the test specimen through the measurement equipment.
The test method is applicable to square or pseudo-square multi- as well as single-crystalline Si wafers with edge length ≥125 mm and with thickness ≥100 µm.
The Test Method is based on simultaneously measuring the wafer top and bottom surface position with an aligned pair of laser triangulation sensors when the wafer passes through the gap formed between the laser triangulation sensor pair.
The Test Method does not cover measurement of surface flatness, warp, bow or sori of wafers.
The Test Method is intended for in-line high throughput measurements. Therefore it is mandatory to operate system under tight statistical process control (SPC), for example ISO 11462, in order to obtain reliable, repeatable and reproducible measurement data.
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
SEMI PV40 — Test Method for In-Line Measurement of Saw Marks on PV Silicon Wafers by a Light Sectioning Technique Using Multiple Line Segments
SEMI PV41 — Test Method for In-Line, Noncontact Measurement of Thickness and Thickness Variation of Silicon Wafers for PV Applications Using Capacitive Probes