Evidences for redox reaction driven charge transfer and mass transport in metal-assisted chemical etching of silicon.

Lingyu Kong,Parsian K Mohseni,Binayak Dasgupta,Minghui Hong,Yi Ren,Sing Yang Chiam,Xiuling Li,Wai Kin Chim

doi:10.1038/srep36582

Abstract

In this work, we investigate the transport processes governing the metal-assisted chemical etching (MacEtch) of silicon (Si). We show that in the oxidation of Si during the MacEtch process, the transport of the hole charges can be accomplished by the diffusion of metal ions. The oxidation of Si is subsequently governed by a redox reaction between the ions and Si. This represents a fundamentally different proposition in MacEtch whereby such transport is understood to occur through hole carrier conduction followed by hole injection into (or electron extraction from) Si. Consistent with the ion transport model introduced, we showed the possibility in the dynamic redistribution of the metal atoms that resulted in the formation of pores/cracks for catalyst thin films that are ≲30 nm thick. As such, the transport of the reagents and by-products are accomplished via these pores/cracks for the thin catalyst films. For thicker films, we show a saturation in the etch rate demonstrating a transport process that is dominated by diffusion via metal/Si boundaries. The new understanding in transport processes described in this work reconcile competing models in reagents/by-products transport, and also solution ions and thin film etching, which can form the foundation of future studies in the MacEtch process.

Highlights

Si (100) substrates used for metal-assisted chemical etching (MacEtch) are boron doped p-type Si, with resistivity in the range of 0.1 to 1 Ω-cm, and phosphorous doped n-type Si, with resistivity in the range of 1 to 10 Ω-cm, that were cut into square pieces, each with area of 1.5 × 1.5 cm[2]
For the carrier transport and redox mechanism that we have proposed in this work, they will have important consequences in the basic understanding and unification of MacEtch processes in general
When the catalyst film is thicker, the transport is accomplished via the boundaries of the metal/Si interface

Summary

Introduction

Si (100) substrates used for MacEtch are boron doped p-type Si, with resistivity in the range of 0.1 to 1 Ω-cm, and phosphorous doped n-type Si, with resistivity in the range of 1 to 10 Ω-cm, that were cut into square pieces, each with area of 1.5 × 1.5 cm[2]. Since the metal film thickness used in this work is significantly thinner than the height of the photoresist nano-dots, a subsequent photoresist lift-off process is not necessary. Cross-sectional SEM was performed for the structures etched with 20 nm and 40 nm Au thickness to ascertain the accuracy in using this approach (see Supplementary Fig. S16). MacEtch samples that underwent the depth profiling analysis were structures of circular dots with diameters ranging from 80 to 1000 μm. These samples were fabricated with a shadow mask and they were used to ensure that the etched area is bigger than the X-ray spot size of ~500 μm in diamet

Methods

Results

Conclusion