Investigations of oxygen precipitates in Czochralski silicon wafers by using infrared tomography

Abstract

Currently, oxygen atoms are intentionally introduced in Si crystals during Czochralski (CZ) pulling process. They usually come from a controlled out-diffusion from the crucible, the expected role of these impurities being to generate silicon oxide clusters or micro-precipitates. Afterwards, these small aggregates are able to getter residual metallic impurities which are introduced into the wafer during the integrated circuit fabrication process. This operation is called “internal gettering” and its successive steps, from “embryo” nucleation to precipitate condensation, require an adapted control adjustment. Convenient thermal cycles reduce the amount of metallic doping in the vicinity of the ICs, thus improving specifications such as diode leakage current or minority carrier lifetime or premature oxide breakdown. This is a key problem in VLSI technology as well as in CCDs. It is also worth noting that the device process itself (annealing) participates in the final texture of precipitates and in the dimension of the “precipitate free zone” (or denuded zone) underlying the wafer surface.Experimental means for observing the individual precipitates and their spatial distribution are not so diversified. X-rays, EBIC or chemical etching are used to obtain images of these defects. The latter technique is the most widely used but not so precise; three-dimensional exploration requires delicate preparation such as controlled bevel etching. It is also known that not all species of precipitates can be revealed by etching. Recently, a new technique called “laser scanning tomography” (LST) has been proposed. It consists of scanning the bulk of the wafer with a thin laser probe beam along a virtual plane; the scattered light image is recorded with a camera and computer reconstructed. LST is non-destructive, very sensitive to all kinds and sizes of microprecipitates and has three-dimensional capability; it does not need any special and time-consuming preparation or vacuum conditions. Internal structures such as depleted zone (DZ) or striations or layered structures are easily detected and measured. Both qualitative (profiles, features) and quantitative (densities, size, dimensions, classification) information is obtained from LST images. LST has previously been used for III–V compound defect analysis. In this communication we report the present state of the art of the investigations on CZ silicon.