Advances in experimental technique for quantitative two dimensional dopant profiling by scanning capacitance microscopy

Several advances have been made toward the achievement of quantitative two-dimensional dopant and carrier profiling. To improve the dielectric and charge properties of the oxide-silicon interface, a method of low temperature heat treatment has been developed which produces an insulating layer with consistent quality and reproducibility. After a standard polishing procedure is applied to cross-sectional samples, the samples are heated to 300°C for 30 min under ultraviolet illumination. This additional surface treatment dramatically improves dielectric layer uniformity, scanning capacitance microscopy (SCM) signal to noise ratio, and C-V curve flat band offset. Examples of the improvement in the surface quality and comparisons of converted SCM data with secondary ion mass spectrometry (SIMS) data are shown. A SCM tip study has also been performed that indicates significant tip depletion problems can occur. It is shown that doped silicon tips are often depleted by the applied SCM bias voltage causing errors in the SCM measured profile. Worn metal coated and silicided silicon tips also can cause similar problems. When these effects are tested for and eliminated, excellent agreement can be achieved between quantitative SCM profiles and SIMS data over a five-decade range of dopant density using a proper physical model. The impact of the tip size and shape on SCM spatial accuracy is simulated. A flat tip model gives a good agreement with experimental data. It is found that the dc offset used to compensate the C-V curve flat band shift has a consistently opposite sign on p- and n-type substrates. This corresponds to a positive surface on p-type silicon and to a negative surface on n-type silicon. Rectification of the large capacitance probing voltage is considered as a mechanism responsible for the apparent flat band shift of (0.4-1) V measured on the samples after heating under UV irradiation. To explain the larger flat band shift of (1-5) V, tip induced charging of water-related traps is proposed and discussed.