HF溶液阳极氧化形成多孔硅层的机理

The anodic current-potential characteristics of four types of silicon in HF aqueous solutions have been studied. According to the surface condition of the samples anodized at different potentials, there are generally three regions on a current-potential curve: porous silicon formation in the region where current changes exponentially with potential, electropolishing of silicon in the constant current region, and a transition region in between. A diagram is presented to indicate  the anodic conditions for porous silicon formation and electropolishing for the silicon samples studied. Porous silicon formation is considered to be the result of the competition reactions between silicon oxide formation with H20 and direct dissolution of silicon by HF. Polishing of silicon occurs through dissolution of the oxide film when it completely covers the  silicon surface.  

Porous silicon can be formed by anodic polarization of  silicon in hydrofluoric acid. It was first reported in 1958 by  Turner in his study of the electropolishing of silicon in hydrofluoric acid solutions (1). Recently, porous silicon has  found applications in isolation technology for fabrication  of integrated circuits, particularly in silicon-on-insulator  (SOI) .

Table I shows the silicon samples used in this study. The  back side of the silicon was coated with a layer of aluminum to provide a low-resistance electrical contact. Except  for the exposed surface, the silicon specimen and a copper  strip for electrical connection were sealed with black wax.  The exposed surface area was 0.38 cm 2.  

The solutions were prepared by adding deionized water  to 49 weight percent (w/o) HF to the desired concentrations. The experiments were conducted at room temperature, about 22~ Except for n-type samples, which were  tested in the dark, no screening was used for any experiments, since ambient lighting was found to have z~o affect  on the results. Before each test, the silicon electrodes were  degreased with methanol and washed in deionized water.  The electrode potential was controlled with an EG&G  PAR (Model 273) potentiostat and the current was plotted  with an X-T recorder.  

Fig1

Figure 1 shows a typical plot of current vs. potential  characteristics of p§ silicon in 1% HF solution. Anodic current increases exponentially with the electrode potential at  low potentials. As the potential is increased, the current  exhibits a peak and then remains at a relatively constant  value. The current increases slightly again with further increasing potential.  

Since porous silicon is formed during the i-V curve  measurement and it increases in thickness as the measurement continues, the electrode surface condition is different at each potential. Several i-V curves were measured  successively on the same sample to determine whether the  porous silicon layer and its thickness influence the i-V  characteristics. The result is shown in Fig. '3. With the exception of the first i-V curve, successive curves are virtually the same. Figure 4 shows the effect of potential sweep  rate on the i-V curve for p+ sample in 1% HF solution. The  i-V curves are similar for different sweep rates in a range  from 2 mV/s to 100 mV/s.