Metal-Assisted Etching of p-Type Silicon Using Patterned Platinum and Silver Films in Single Layers and Bimetal Double Layers

Metal-assisted etching (metal-assisted chemical etching) is a promising wet process for silicon (Si) microfabrication. The etching behavior depends on the catalyst metal, but detailed differences are not fully understood. In this study, we compared platinum (Pt)- and silver (Ag)-assisted etching in the fabrication of microhole arrays using widely spaced patterned metal films in single and double layers on p-type Si, where remote etching is pronounced. The Pt single-layer film caused significant remote etching, represented by widened macropores formed beneath the catalysts and a continuous porous layer covering the entire substrate surface. The Ag single-layer film exhibited a lower degree of remote etching, yielding macropores of nearly constant diameter in depth and a relatively smooth substrate surface. In the double-layer systems, inserting Pt beneath Ag enhanced remote etching, whereas inserting Ag beneath Pt suppressed it and produced the deepest macropores. These morphological trends were explained by the differences in overpotentials of metals for hydrogen peroxide reduction and in cathodic polarization curves of Si with single- and double-layer metal films. These results deepen our understanding of the roles of the catalyst metal and suggest a potential strategy for controlling the degree of remote etching.