Microbeam analysis of lateral inhomogeneity in depth penetration of Pd in porous silicon

Porous silicon (PS) is widely utilized in gas sensors. Palladium is a good choice to sensitize the surface of PS to hydrogen. Ideally for highest sensitivity of the sensor, all the pores of PS should be completely covered with palladium. Rutherford backscattering spectroscopy (RBS) technique is routinely used to determine depth profile of elements in thin layers. By using microbeam analysis as a complementary technique, we clearly observed the lateral image of penetrated Pd in depth. In this work, we used electrochemically anodized-silicon wafer doped with Pd by the electroless process. In our analysis, we intentionally characterized both the area of PS where Electric Field is applied during silicon etching (EF area) and the Field Free region (FF area) of the sample. Microbeam analysis show that EF area of the sample is covered with thicker palladium layer, but the pores are rarely filled in this area, while in the FF area of the sample, there is a thin palladium film on the silicon surface, but the pores are partially filled. We used a computational simulation method based on columnar pore structure and linear variation of Pd depth profile in pores and determined the lateral variation of Pd penetration in pores in both parts of the sample. These simulations showed that there are 180 ± 80 nm thick Pd layer with 300 ± 200 nm penetration in pores in EF region and 80 ± 40 nm thin Pd layer with 800 ± 200 nm penetrate in pores in FF region.