Metal-Assisted Etching of High-Aspect-Ratio Structures for Solar Cell Applications: Controlling the Porosity of Au Thin Films

Fabrication of high aspect ratio structures in silicon independent of crystal orientation using metal assisted etching

“…Etching a thinner Si segment can potentially reduce this diameter further. However, as demonstrated in previous work [6], the major barrier to forming very high aspect ratio grooves using MacEtch is the formation of breaks in the Au layer, causing a bridge of unetched Si across the width of the groove. This limits the lengths of the spiral sections capable of being removed from the wafer with current processing capability.…”

“…Using the methods of [6], it was demonstrated that the MacEtch process could be conducted effectively on a Si wafer patterned with a spiral structure (where the spiral grooves are 30 μm-wide). Figure 2 shows a spiral which was patterned with 20 nm Au and subjected to an accelerated dewetting process at 90 °C.…”

“…This can be attributed to the fact that as the pore density increases, there will be a corresponding decrease in the robustness of the films, with breaks in the Au layer becoming more likely. The process by which a high density of pores leads to Au film breakage and Si bridge formation has been discussed elsewhere [6].…”

“…Using a metal (commonly Ag or Au) as catalyst in an etch solution containing both an oxidant and acid component, the Si can be etched to create high aspect ratio grooves, pores or nanowires in Si. MacEtch can be used to etch through the entire thickness of a relatively thick (650 μm) Si wafers [6], which has application for processes such as the formation of through-silicon vias [7] and MEMS resonators [8] as well as for the production of SLIVER solar cells [9].…”

“…In our previous study [6] we demonstrated that dewetting the Au catalyst layer allows for the formation of a robust layer which also has the requisite porosity for the solution to access the etch front. The same technique has been applied to this work, with Au catalyst layers of 20 nm and 30 nm thickness.…”