High Resolution Measurements of Two-dimensional Dopant Diffusion in Silicon

We report detailed experimental results on the electrochemical selective etching of doped Si. By using transmission electron microscopy analyses and spreading resistance measurements we investigated the dependence of the etching selectivity on the different parameters of the electrochemical cell, i.e., bias voltage and chemical solution. In B-doped samples immersed in buffered HF, the increase of bias voltage from 0.5 to 1 V produces a slight improvement of the etching selectivity and a B concentration as low as 1 x 10(17) cm(-3) can be successfully delineated at 1 V. A further improvement is achieved by using HF:HNO(3):CH(3)COOH or HF:HCl chemical mixtures for which the delineation sensitivity approaches the value of 1 x 10(16) cm(-3). In buffered HF As-doped regions can be delineated to a concentration of 2 x 10(17) cm(-3), independently of the bias voltage, in the range 2-4 V. These results were used to measure the 2D doping diffusion profiles in silicon wafers patterned with polycrystalline Si strips and implanted with As or B, by using different tilt and twist angles. The high resolution of the electrochemical delineation allowed us to evaluate very accurately the effects of the implant angles on the lateral doping distribution.