Comparison of techniques for detecting metal contamination in silicon wafers

Highlights: • The contaminant diffusivity determine the best method for contamination monitoring. • For fast diffusers TXRF and minority carrier lifetime give comparable results. • DLTS is best to detect slow diffuser contaminants in the silicon volume. • For palladium minority carrier lifetime is the most sensitive approach. In this work we present the results of experiments aimed at comparing the performances of various techniques for the detection of metal contamination in the silicon technology. Techniques for the measurement of surface contamination such as Total Reflection X-Ray Fluorescence (TXRF) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are compared with techniques for the measurement of contamination in the silicon volume, specifically the Deep Level Transient Spectroscopy and techniques for the measurement of carrier lifetime. Carrier lifetime measurements were obtained by photocurrent measurements and by Surface Photovoltage measurements. In a first experiment, Synchrotron-Radiation TXRF (SR-TXRF) was used as the reference technique to assess the sensitivity of a commercial TXRF instrument and of carrier lifetime measurements to detect iron contamination in wafers cleaned by ordinary cleaning processes. Samples intentionally contaminated by spinning with various elements were used for another experiment comparing TXRF and ToF-SIMS measurements of surface contamination. Then, a few case studies are discussed, specifically tungsten contamination by sputtering in ion-implanted samples and palladium contamination due to contact with a contaminated chuck. In all these experiments, advantages and disadvantages of the different techniques are discussed. The results of this study clearly show that it is not possible to define a unique recipe that can be applied in all cases. The maximum tolerated contaminant concentration per unit area depends on the contaminant diffusivity, and is much lower for slow diffusers. The contaminant diffusivity and solid solubility in silicon determine the in-depth distribution of the contaminant, and hence the most effective approach.