SEMI M20 - Practice for Establishing a Wafer Coordinate System
This Standard was technically approved by the Silicon Wafer Global Technical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on November 11, 2014. Available at www.semiviews.org and www.semi.org in February 2015; originally published in 1991; previously published November 2010.
Processing systems now employed in advanced device manufacturing use aligning mechanisms to position the wafer rotationally and in x-y prior to processing. Many of these scan the wafer periphery and determine the geometric center of the wafer surface. This is most often seen on stepping aligners, to minimize the effects of wafer-to-wafer diameter variation in mixed aligner type fabs. Similar center-referencing subsystems are found on many characterization systems. The wafer coordinate system provides a method for referencing any other coordinate system, such as a site, pattern, or mapping array, to the physical geometry of the wafer surface.
If the points of the array lie on the front surface of the wafer, only the x and y (or r and θ) coordinates are relevant. It has become increasingly important in semiconductor material and device manufacturing to describe, in unambiguous terms, the position of a point on a wafer that automatic processing, test, or characterization equipment can recognize and locate. For example, characterization equipment needs to report the precise locations of defects and anomalies discovered in wafers before or after processing in order to relate the presence or absence of such defects and anomalies to device yield variations. The wafer coordinate system can be used to establish the coordinates of each point of interest, and, through transformation to the yield analysis coordinate system, relate them to the die yield map.
In response to these needs, this Practice defines a wafer coordinate system to facilitate the precise locating and reporting of points on the wafer surface. If the point or points lie above or below the surface, the z-coordinate must also be used. Because the zero point on the z-axis is application specific, this practice treats the x-y-z (or r-q -z) system separately from the surface coordinate system.
This Practice covers procedures for defining a wafer coordinate system for locating uniquely any point on a wafer surface using the wafer center as the origin and either Cartesian (x-y) or polar (r-θ) coordinates.
For unpatterned wafers, this wafer coordinate system can be used directly or in conjunction with a rectangular or polar overlay array.
This wafer coordinate system can also be used to locate the origins or other reference points of other coordinate systems used to define or report position data of site, die, or map arrays on the front or back surface of a patterned or unpatterned wafer. In this way, the array coordinate system may be referenced to the physical geometry of the wafer. Selected modes of application of the wafer coordinate system are given for information only in Related Information 1.
This Practice also covers procedures for defining a three-dimensional x-y-z (or r-q -z) coordinate system for the wafer.
Referenced SEMI Standards
SEMI E5 — SEMI Equipment Communications Standard 2 Message Content (SECS-II)
SEMI M1 — Specification for Polished Single Crystal Silicon Wafers
SEMI M12 — Specifications for Serial Alphanumeric Marking of the Front Surface of Wafers
SEMI M13 — Specification for Alphanumeric Marking of Silicon Wafers
SEMI M17 — Guide for a Universal Wafer Grid
SEMI M59 — Terminology for Silicon Technology