- SEMI MF533 - Test Method for Thickness and Thickness Variation of Silicon Wafers
This standard was technically approved by the global Silicon Wafer Committee. This edition was approved for publication by the global Audits & Reviews Subcommittee on January 20, 2010. Initially available at www.semi.org in February 2010. Originally published by ASTM International as ASTM F533-77T; previously published November 2009.
NOTICE: This Document was reapproved with minor editorial changes.
Wafer thickness and thickness variations must be controlled to suit the requirements of fixtures and equipment used in microelectronic processing. Estimates of these parameters, based on a representative sample from a given lot of wafers, aids in determining whether or not wafers from that lot are acceptable for the intended processing steps.
Wafers that are too thin may break during normal processing operations. Wafers that are too thick may cause mechanical jamming. Wafers with thickness outside the desired tolerance may not have appropriate thermal mass or electrical resistance for certain processing steps.
Excessive thickness variations may cause problems with mechanical handling of the wafers during processing. In addition, such variations may cause deviations from surface flatness that adversely affect photolithographic processes. The effect of thickness variations on photolithographic processes depends on the line width and registration requirements of individual circuit designs, as well as on the specific optical and mechanical design of the photolithographic processing equipment being used.
This Test Method is intended for use for materials acceptance and process control purposes. The test methods may be applied at any point during the processing of unpolished wafers into polished wafers or substrates.
When the test method was developed for flatted wafers in the 1970s, noncontact thickness gages employing manual wafer positioning, which are the basis of this Test Method, were in routine use. More recently, faster, automated instruments have replaced these manual gages for most common uses in the semiconductor industry. In these automatic systems, microprocessors or microcomputers are used to control wafer positioning, operate the instrument and to analyze the data (see SEMI MF1530).
Despite the fact that this Test Method is not commonly used in its present form, it embodies all the basic elements of the measurement method and a simple analysis of data. Thus, this Standard provides useful guidance in the fundamentals and application of differential non-contact wafer thickness measurements. It is also suitable for use with very large diameter wafers before the automatic systems become available.
This Test Method covers measurement of the thickness of silicon wafers, polished or unpolished, and estimation of the variation in thickness across the wafer for both flatted and notched wafers. For flatted wafers of 200 mm diameter or less, a five-point pattern offset from the bisector of the primary flat is used. For notched wafers of 200 mm diameter or more, either a symmetrical five-point or nine-point pattern is used.
This Test Method is intended primarily for use with wafers that meet the dimension and tolerance requirements of SEMI M1. However, it can be applied to circular silicon wafers or substrates of any diameter and thickness that can be handled without breaking.
This Test Method is suitable for both contact and contactless gaging equipment. Precision statements have been established for each in the case of the five-point method applied to flatted wafers.
The values stated in inch-pound units are to be regarded as the standard for measurements on wafers of 3 inch diameter or less while the values in metric units are to be regarded as the standard for measurements on wafers of 100 mm diameter or more.
Referenced SEMI Standards
SEMI M1 — Specification for Polished Single Crystal Silicon Wafers
SEMI M59 — Terminology for Silicon Technology
SEMI MF1530 — Test Method for Measuring Flatness, Thickness, and Thickness Variation on Silicon Wafers by Automated Noncontact Scanning