Chemical Specifications for Silicon Wafers

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

1) Bulk Structural Defects

- a starting wafer must not have any structural defect as revealed by x-ray topography or any suitable inspection process

2) Surface Orientation

- specifies the orientation of the surface of the wafer as well as the tolerance for misorientation

- generally, <100> material is cut on orientation +/- 0.5 deg

- generally, <111> material is cut off orientation 3 +/- 0.5 deg

- wafer surface orientation is usually verified per ASTM Std F-26

3) Oxygen Content

- specifies the amount of oxygen within the silicon wafer

- oxygen content needs to be specified because it affects wafer strength, impurity gettering mechanisms, thermal donors, etc.

- oxygen content is measured using FTIR, as per ASTM F-121

- interstitial oxygen content specifications may be tailored to customer requirements

4) Carbon Content

- specifies the amount of carbon within the silicon wafer

- carbon content is measured using FTIR, as per ASTM F-123

- carbon content measurement can be difficult due to low levels of carbon in modern wafers (in the order of 1 ppma)

5) Swirl Defects

- swirl defects, which exhibit a spiral or swirl-like pattern, should not be present in wafers before processing

- swirl defects are the result of the agglomeration of point defects when the crystal cooled down

- swirl defects consist of two variants: 1) the A-swirls, which can be delineated as hillocks; and 2) the B-swirls, which can be delineated as small and shallow pits

- wafer crystals can now be grown without swirl defects

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

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