The Role of Microdefects in Silicon Starting Materials as Quality Reducing Factors in Semiconductor Devices

Transmission electron microscope studies have shown that commercial silicon materials have varying concentrations of microdefects which are not detectable by normal evaluation procedures. The size of these defects ranges from less than 100Å up to microns. The following types of defects have been found: small loop-shaped defects, large loop-shaped defects, inclusions of several kinds, and precipitates including those associated with crystallographic defects. The chemical composition of these particles, precipitates, and inclusions are generally uncertain; but some evidence exists that SiC, SiO2, and possibly Si3N4 are present. The widespread occurrence of microdefects in standard or premium quality silicon emphasizes the need for developing improved materials growth techniques and rapid procedures for detecting microdefects. The frequency of occurrence of the different varieties of microdefects differs with the type of material; however, crystals grown by the three principal techniques - float-zone, Czochralski, and crucibleless pulling - did not differ significantly in the degree of the overall microdefect problem. The effects of microdefects on devices depends strongly on the nature and size of the defect, whether it is conducting (metallic), non-conducting, or purely crystallographic. The size of the defect is important in determining the behavior of the device under electrical stress and during long time operation. Indirect effects, in particular precipitation of fast-diffusing heavy metal impurities on microdefects, are believed to be a significant cause of poor device performance. Qualitative relationships between microdefects and the quality of p-n junction devices have been established.