When the silicon substrate is prepatterned with an HF-resistant material as the Au mask, the pore formation is expected to be limited to the uncovered areas, as sketched in Figure a. Figure b,c shows the SEM cross-sectional tilted view of a macroporous silicon layer resulting from the etching of a Ti/Au prepatterned surface (WP1 geometry). An as-prepared HF/DMF mixture was used as electrolyte (pH = 4, conductivity = 2 mS/m). The etching current density was set at the constant value 91 mA/cm2
for 25 min. An etching voltage (Vetch
was necessary to sustain such current. A slow increment of Vetch
as a function of the etching time was observed in all experiments. The metal mask was detached by the substrate before the end of the etching. As shown in Figure c, the underetching of the masked area reduced the contact area between the metal layer and the silicon surface up to the detachment of the masking layer. The observed etching profile induces to suppose a catalytic role of the Ti/Au layer in the etching of the silicon in the close proximity of the metal layer, similar to what happened in the electroless metal-assisted chemical etching of silicon [10
Figure 1 Schematic representation of the process. (a) SEM cross-sectional tilted view of a macroporous layer grown on a Ti/Au prepatterned substrate (WP1 geometry) (b) and (c). The etching current was set at the constant value of 91 mA/cm2 for 25 min. A fresh (more ...)
Figure shows the SEM cross-sectional tilted view of Ti/Au prepatterned samples (geometry WP1 (a) and (b) and geometry WP2 (c)) etched used as an electrolyte of a 20-day-old HF/DMF mixture. As in the previous case the etching current density was set at the constant value 91 mA/cm2 for 25 min.
Figure 2 SEM cross-sectional tilted view of a macroporous layer grown on Ti/Au patterned silicon substrates. Geometry WP1 (a and b) and geometry WP2 (c). A 20-day-old HF/DMF mixture was used as electrolyte. The current density was set at the constant value of (more ...)
The different characteristics of the electrolyte (pH =6, conductivity = 30 mS/m) affected the etching parameters as well as the morphology of the resulting macroporous layer. A lower etching voltage, Vetch≈1.5 V, was necessary to sustain Jetch = 91 mA/cm2. Even in this case, a slow increase of the etching voltage as a function of the etching time was observed. A different etching profile was observed in close proximity of the masking layer. The metal layer was sufficiently adherent to the silicon substrate to sustain the etching process and the sample cutting for analysis.
Using as-prepared solutions as well as 20-day-old solutions, the pore distribution was affected by the presence of the Ti/Au mask. More specifically, the pores tend to grow in proximity of the metal edges. As a consequence, the pores grow with a random distribution in the direction parallel to the metal strips, and they grow with a regular spacing in the transverse direction. In the case of geometries with 2-μm pitches, a silicon wall in the middle of the unmasked area is formed. The depth and the high roughness of the top surface of this wall are due to the nucleation phase in which the initiation of the pore growth follows an almost-uniform etching of the surface. The dimension of this wall, as well as the pore diameter, depends on the applied current density. Figure shows the top and cross-sectional SEM views of a MPS layer with metal electrodes of WP1 geometry. A 20-day-old HF/DMF mixture was used as electrolyte. The etching current density was set at the constant value of 26 mA/cm2 for 25 min. Both walls, the one sustaining the metal layer and the one between the electrodes, are larger with respect to the samples shown in Figures and because of the smaller etching current. On the contrary, the thinner perpendicular wall separating the pores does not show a strong dependence on the applied etching current.
Figure 3 Comparison of experimental and simulation results for the WP1 geometry. Top (a) and cross-sectional tilted views (b) of the macroporous layer. The current density was set at the constant value of 26 mA/cm2. The frame (c) shows the COMSOL-simulated cross-section. (more ...)
Two-dimensional simulations of the interface silicon/mask/electrolyte were performed using COMSOL MultiphysicsⓇ (COMSOL AB, Stockholm, Sweden) to better understand the etching behavior of how the metal pattern geometry affects the macroporous layer morphology. To model the electrolyte/mask/silicon interface behavior, we considered a semiconductor/metal contact at the front side of a p-type wafer (1015 at./cm3) with a bias of -1 V applied on the conductor border at 30 μm from the semiconductor surface. Figure c shows the simulated cross-section of a metal pattern with geometry WP1. Holes current density (shading) and hole flux streamlines are shown. The flux streamlines follow the electric field lines. The simulation results show that, in the initial stage of the etching, the current density is higher at the metal electrode edges and converges with the field lines towards the metal strip corners. A direct consequence of the current density distribution is the experimentally observed growth of the pores in proximity of the mask edges. Even if during the nucleation phase pores start to grow in all the unmasked area, the pores on the corners of the mask are privileged by the higher current density that leads to a bigger growth rate. In case of smaller pitches, this behavior is reflected in the formation of the middle walls because the space between the metal strips is not sufficient to support the formation of another pore. In case of bigger spacing between the metal strips, the formation of the thicker middle wall is avoided. In fact, as clearly visible in Figure that shows the SEM cross-sectional tilted view (a and b) and the simulated cross-section for a Ti/Au prepatterned surface with geometry WP3, the pores on the middle go further the nucleation phase. The higher current at the mask edges in the initial stage of the etching is reflected in a bigger nucleation and pore depth.
Comparison of experimental and simulation results for the WP3 geometry. SEM cross-sectional views of the macroporous layer (a and b). COMSOL-simulated cross-section (c)