Observation of carrier densities in silicon devices by infrared emission

Previous publications have shown that the absorption coefficient of silicon in the near infrared depends on the carrier concentration of the material considered. Consequently, determination of the absorption coefficient enables the carrier concentration to be deduced. Unfortunately the absorption is generally small for the doping levels used in integrated circuits, and therefore normal absorption methods require thick samples for accurate measurements. For measuring material with low absorption, an emissivity method is better. Such a technique is presented which enables carrier concentrations in standard integrated circuit diffusions to be evaluated with a spatial resolution approaching 20 pm. The samples are heated to approximately 373 K so that there is a net emission of thermal radiation, the amount emitted depending on the carrier concentration. A review of the main factors involved in the emission is presented, together with experimental verification on standard bipolar diffusions. The basic technique is applicable in most cases where observation of carrier distribution is required. 1 Introduction Most previous measurements of infrared emission from silicon devices have been made to determine the thermal distribution within them. To ensure a surface of constant emissivity, various coatings and paints are often used (Peterman and Workman 1967). Local hot spots can then be identified. These may be caused by defects in the bond between the chip and header, current hogging or secondary breakdown (Hamiter 1967, Schafft and French 1966, Agatsuma 1966). At shorter wavelengths, light emission and carrier recom-bination emission (1-1.2 pm) have been used to study micro-plasmas in reverse-biased junctions and current crowding in large-area transistors (Sunshine 1971, 1974). Although the presence of free carriers affects the infrared absorption properties of silicon, for the doped layers normally occurring in silicon devices the absorption is only a few per cent. Absorption techniques are therefore mainly used to detect non-uniformities in the silicon crystal and to determine carrier densities in relatively heavily doped diffusions (Tong 1973).