Optical imaging of microroughness on polished silicon wafers

Abstract

The microscopic surface morphology of device quality polished silicon wafers has been imaged using laser scanning optical microscopy (SOM) in differential phase contrast (DPC) mode. The SOM-DPC technique has subnanometre vertical sensitivity and submicron lateral resolution. Surface texture was observed on all wafers examined. This was found to depend mainly on the originating wafer supplier and year of manufacture, rather than on any characteristic of the silicon material itself, and was assigned to silicon surface roughness. The amplitude of the directional 1–10 μm wide polishing features observed on many of the as-received wafers was estimated to be in the region of 1–2 nm. An additional isotropic submicron-scale roughness (incompletely resolved optically) was present in all cases. Atomic misorientation steps were also observed on specially-prepared wafers.