Abstract
In this paper, we report magnetic neutral loop discharge (NLD) plasma etching of fused silica (FS) and borosilicate glass (BSG), demonstrating high aspect ratio deep etch (100 μm) with vertical walls (<;3° deviation from vertical). This paper for the first time presents the systematic study of FS and BSG deep etching in NLD plasma. Four different masking materials have been explored including metal, amorphous silicon, bonded silicon, and photoresist. Etch parameters were optimized to eliminate unwanted artifacts, such as micro-masking, trenching, and faceting, while retaining a high aspect ratio (up to 7:1 for FS and 8:1 for BSG). In addition, a method for sidewall roughness mitigation based on postfabrication annealing was developed, showing the sidewall roughness reduction from the average roughness (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) 900 to 85 nm. Further advances in deep plasma etching processes may enable the use of FS and BSG in the fabrication of precision inertial MEMS, micro-fluidic, and micro-optical devices.
Highlights
F USED SILICA is the desired material for MEMS and micro-optical devices for its outstanding temperature stability, high electrical resistance, low optical loss, and low internal thermo-elastic loss [1]
In contrast to classical ICP and Reactive Ion Etching (RIE) systems, in this work we studied Magnetic Neutral Loop Discharge (NLD) plasma (ULVAC neutral loop discharge (NLD) 570) system for Fused Silica (FS) and Borosilicate Glass (BSG) etching
We studied the NLD plasma etching with three new approaches to achieve high aspect ratio features: (1) mask selection for NLD based deep etching, (2) optimum NLD plasma conditions to obtain defect-free etching, and (3) post-processing step to mitigate sidewall roughness
Summary
F USED SILICA is the desired material for MEMS and micro-optical devices for its outstanding temperature stability, high electrical resistance, low optical loss, and low internal thermo-elastic loss [1]. The glass dry etching suffers from orders of magnitude lower aspect ratio, limited mask selectivity, slower etch rate, and high surface roughness [3]. Etch depth of about 100 μm and aspect ratio of 3:1 were demonstrated on fused silica using a silicon mask in RIE plasma [8]. We studied the NLD plasma etching with three new approaches to achieve high aspect ratio features: (1) mask selection for NLD based deep etching, (2) optimum NLD plasma conditions to obtain defect-free etching, and (3) post-processing step to mitigate sidewall roughness. Four masking materials: electroplated Ni, KMPR photoresist, bonded silicon, and deposited amorphous silicon were explored Etch parameters, such as etching/inert gas, antenna power, bias power, chamber pressure, and chamber temperature were optimized. The paper concludes with an example of how the optimized etch process can be used for fabrication of MEMS micro-wineglass resonators
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