Mechanical Specifications for Silicon Wafers

Silicon wafers, especially those used in VLSI manufacturing, must adhere to stringent electrical, mechanical, chemical, and surface specifications. Below are the most common mechanical specifications for silicon wafers:

1) Crystal Growth and Doping Process

- specifies the wafer's single crystal growing process, i.e., whether CZ or FZ

- specifies the wafer's doping process, e.g., ion implantation, etc.

2) Wafer Diameter

- specifies the diameter of the wafer (excluding flats)

- the diameter specification is given with high precision to facilitate automated wafer handling; example: 150 +/- 0.2 mm.

- it is usually measured using a ring template as per ASTM Std F-613

3) Wafer Thickness

- specifies the thickness of the wafer

- wafer thickness is measured at the center of the wafer with a wafer thickness gauge

- wafer thickness is also an important parameter since a very thin wafer is prone to warpage or breakage while a very thick wafer may not allow processing in all wafer fab equipment

4) Total Thickness Variation (TTV)

- specifies the difference between the maximum and minimum values of wafer thickness

- excessive thickness variations can result in automated processing problems, especially with respect to mechanical handling and photolithographic processes

- TTV is measured using a non-contact thickness gauge

5) Bow

- specifies the concavity of the wafer due to sawing or the deformation of the wafer centerline due to thermal processing

- bow is a bulk property (and not just a surface property) expressed in microns, and can be measured per ASTM Std F-534-84

6) Warp

- specifies the difference between the minimum and maximum distance of the wafer center line from a reference plane

- warp, which is also a bulk property of the wafer, takes into account both concave and convex deviations of the wafer

- warp can be measured using ASTM Std F-657-80

7) Flat Dimensions - specifies the precise lengths and orientations of the wafer flats

8) Edge Contour

- specifies the rounding of wafer edges;

- wafer edges are contoured to reduce chipping and film build-up

- edge contour is measured by comparison with a template

9) Laser Markings - specifies any markings made by laser on the surface of the wafer

10) Other Mechanical Attributes

- specifies the presence of attributes like chip-outs and indents on the wafer

- chip-outs are regions of missing silicon that do not go all the way through the slice while indents are regions of missing silicon that go all the way through the slice

Primary Reference: Silicon Processing for the Vlsi Era: Process Technology

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