- SEMI MF534 - Test Method for Bow of Silicon Wafers
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 January 2015; originally published by ASTM International as ASTM F534-77T; previously published July 2007.
The flatness of a wafer surface must be controlled to suit the requirements of fixtures and equipment used in microelectronic processing. Bow is a contributor to the lack of surface flatness of a wafer. Although moderate amounts of bow can be removed during processing by vacuum chucking or by clamping the wafer, excessive amounts of bow cannot be removed in this fashion.
In particular, photolithographic processes are adversely affected if the front surface of the wafer is not flat.
This Test Method is intended for use for materials acceptance and process control purposes. Estimates of the magnitude of bow on a representative sample from a given lot of wafers aid in determining whether or not wafers from that lot are acceptable for the intended processing steps. Measurements made on wafers following a particular processing step aids in determining the amount of bow introduced by that processing step.
If the median surface of a free, unclamped wafer has a curvature that is everywhere the same, bow is a measure of its concave or convex deformation, independent of any thickness variation that may be present. Positive values of bow denote a convex (mounded) median surface when the wafer is positioned with its front surface up. Conversely, negative values of bow denote a concave (dished) median surface when the wafer is positioned with its front surface up. Although bow may be caused by unequal stresses on the two exposed surfaces of the wafer, it cannot be determined from measurements on a single exposed surface.
Bow is only one of several geometrical properties that affect surface flatness. Procedures for determining other such properties are given in SEMI MF533, SEMI MF657, SEMI MF1390, SEMI MF1451, and SEMI MF1530.
When this Test Method was developed in the 1970s, noncontact bow and warp gages employing manual 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 MF1390).
Despite the fact that this test method is not commonly used in its present form, it embodies all the basic elements of wafer bow measurement and a simple analysis of data. Thus, it provides useful guidance in the fundamentals and application of differential noncontact wafer bow measurements.
This Test Method covers determination of the average amount of bow of nominally circular silicon wafers, polished or unpolished, in the free (nonclamped) condition.
This Test Method is intended primarily for use with wafers that meet the dimension and tolerance requirements of SEMI M1.
This Test Method can also be applied to circular wafers of other semiconducting materials, such as gallium arsenide, or electronic substrate materials, such as sapphire, that have a diameter of 25 mm or greater, a thickness of 0.18 mm or greater, and a ratio of diameter to thickness up to 250. Wafers to be tested may have one or more fiducial flats provided they are located in such a way that the wafer can be centered on the support pedestals (see ¶ 7.1.2) without falling off.
The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are for information only.
Referenced SEMI Standards
SEMI M1 — Specifications for Polished Single Crystal Silicon Wafers
SEMI M59 — Terminology for Silicon Technology
SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers
SEMI MF657 — Test Method for Measuring Warp and Total Thickness Variation on Silicon Wafers by Noncontact Scanning
SEMI MF1390 — Test Method for Measuring Warp on Silicon Wafers by Automated Noncontact Scanning
SEMI MF1451 — Test Method for Measuring Sori on Silicon Wafers by Automated Noncontact Scanning
SEMI MF1530 — Test Method for Measuring Flatness, Thickness, and Total Thickness Variation on Silicon Wafers by Automated Noncontact Scanning