On the formation of black silicon in SF₆-O₂ plasma: The clear, oxidize, remove, and etch (CORE) sequence and black silicon on demand

Black silicon (BSi or silicon micro/nanograss) is a frequently encountered phenomenon in highly directional etching of silicon using mainstream plasma etch tools. The appearance of BSi in most studies is considered to be caused by micromasks unintentionally present on the silicon surface that locally prevent silicon from etching.

Particularly, under highly directional and selective plasma etch conditions, these chaotically arranged micromasks develop into tall grasslike structures that will absorb incoming light and make the etched silicon appear black. There are many different sources that might contribute to the formation of BSi.

Most of them can be prevented by proper pretreatment of the surface and careful control of the etch parameters. However, the masking related to the in situ plasma passivation (typically FC- or O-species) and insufficient ion etching of this layer causing residues at horizontal surfaces remains a resilient issue that is difficult to control or predict.

This study is built on a recently developed highly directional etch procedure called CORE (meaning Clear, Oxidize, Remove, and Etch) in which the usual FC inhibitor of the Bosch process is replaced by oxygen. Due to the self-limiting property of the oxidation step, the formation and controllability of BSi in the CORE sequence is different from how BSi presents itself in the FC-based sequences.

In this work, the effects of different process parameters on the creation of masks and formation of BSi are carefully investigated. The authors show that the time in the removal (R) step of the passivating oxide layer in tight combination with the undercut time in the isotropic etch (E) step are the most important parameters to consider.

By manipulating these two parameters and utilizing the self-limiting property of the oxidation (O) step, the CORE process can easily be modified to create either BSi-full or BSi-free surfaces independent of the aspect ratio of the etching features. The latter distinguishes the BSi formation clearly from other directional processes.

The proposed CORE process thus provides the authors a versatile tool for creating BSi anywhere anytime or—as we call it—“BSi on Demand.”