Lawrence Berkeley National Laboratory

Nanostructure-initiator mass spectrometry (NIMS) is a matrix-free desorption/ionization technique with high sensitivity for small molecules. Surface preparation has relied on hydrofluoric acid (HF) electrochemical etching which is undesirable given the significant safety controls required in this specialized process. In this study, we examine a conventional and widely used process for producing black silicon based on sulfur hexafluoride/oxygen (SF6/O2) inductively coupled plasma (ICP) etching at cryogenic temperatures and we find it to be suitable for NIMS. A systematic study varying parameters in the plasma etching process was performed to understand the relationship of black silicon morphology and its sensitivity as a NIMS substrate. The results suggest that a combination of higher silicon temperature and oxygen flow rate gives rise to the formation of black silicon with fine pillar structures, whose aspect ratio are ∼ 8.7 and depth are <1 μm resulting in higher NIMS sensitivity which is attributed to surface restructuring caused by their low melting point upon laser irradiation. Interestingly, we find selectivity of these black silicon substrates to different analytes depending on the etching parameters. Though, the sensitivity of the dry etching process is lower than the traditional "wet" electrochemical etching process, it is suitable for many applications and is prepared using conventional equipment without the use of HF.