Present Status of the Surface Photovoltage Method (SPV) for Measuring Minority Carrier Diffusion Length and Related Parameters

Abstract

We present an overview of the physics and practical issues related to surface photovoltage measurement of the minority carrier diffusion length and its application to monitoring recombination center defects in silicon. A tutorial description is given of the role of pertinent processes like injection, recombination and trapping. The evolution of the SPV method is presented followed by a description of most recent refinements addressing the measurement of long diffusion lengths in silicon wafers with emphasis on accuracy, measuring speed, tool-to-tool reproducibility and, practically the most important question of monitoring, iron concentration.

Surface photovoltage phenomena: theory, experiment, and applications

“…More recently, Faifer et al [538] have examined the relation between the spot and electrode size in scanning systems and concluded that the latter should be bigger than the former by at least one diffusion length on either side in order to reduce errors due to lateral diffusion effects. This point was further investigated by Lagowski et al [530]. They concluded that when the electrode and spot size are essentially the same, as in Fig.…”

“…Sopori et al [537] noted relatively early on that the spot size must be large enough to avoid lateral diffusion effects. They predicted that lateral diffusion would be negligible and the true value of L obtained only if the spot size is at least 30 times as wide as L. In view of the results of, e.g., Lile [427] and Lagowski et al [12,529], this quantitative criterion seems to be much too harsh. Nevertheless, the qualitative conclusion is correct.…”

“…This is because the feedback scheme required to maintain a constant SPV value, which slows the measurement down considerably, is absent in the linear SPV technique. The linear SPV approach has therefore become the method of choice in Si-wafer monitoring applications [12,529]. A speed advantage is always very important in an industrial environment as it increases the throughput of the measurement.…”

“…According to Phillips's theory, this is because the thickness of the overlayer(s) is negligible with respect to the bulk diffusion length. It should be noted, however, that when measuring oxidized samples, special care should be taken to correct re¯ectivity changes due to interference, and also a possible loss of signal due to oxide-passivation of the SCR [529]. Commercial SPV equipment has recently been installed with corona-induced oxide charging capabilities to overcome the latter obstacle.…”

“…In the 1960s, diffusion lengths in Si wafers were of the order of tens of microns and the condition L ( l was easily satis®ed. In the 1990s, high-purity Si wafers typically feature diffusion lengths of at least many hundreds of microns and in some cases over a mm [529,549]. These values are of the order of, or exceed, typical wafer thicknesses.…”