Silicon 45° micromirrors fabricated by etching in alkaline solutions with organic additives

Krzysztof P Rola,Irena Zubel,Adrian Zakrzewski,Konrad Ptasiński

doi:10.1007/s00542-013-1859-z

Krzysztof P Rola, Irena Zubel + Show 2 more

Open Access

https://doi.org/10.1007/s00542-013-1859-z

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Abstract

The fabrication of 45° micromirrors by silicon anisotropic etching in potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) solutions containing organic additives is investigated in this paper. The reflective planes are formed by {110} sidewall planes inclined at 45° towards the Si (100) wafer. Isopropyl alcohol and Triton X-100 surfactant are used as additives, because they are supposed to provide the etch rate ratio R(100)/R(110) > 1, which is necessary for {110} sidewalls development. The fabricated spatial microstructures with 45° sidewalls are examined in terms of the {110} surface roughness and the quality of the {110} sidewall profile. The KOH solution saturated with the alcohol gives the striped {110} surface, though the stripes almost disappear after addition of Triton surfactant to KOH and TMAH etchants. The 45° sidewall profiles fully defined by {110} planes are obtained in KOH as well as TMAH solutions containing additives. The measurements of micromirrors’ reflectivity indicate that replacement of the alcohol by Triton surfactant in the KOH solution reduces the optical power loss caused by the reflection. The achieved reflectivity is comparable with the one obtained by etching in the TMAH solution with surfactant.

Highlights

Wet chemical anisotropic etching of silicon is widely used for bulk micromachining in MEMS and MOEMSThe {110} planes can develop in the sidewalls on condition that the edges of the etching mask are aligned perpendicularly to h100i directions and the etch rate ratio R(100)/R(110) is higher than one
The fabrication of 45° micromirrors by silicon anisotropic etching in potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) solutions containing organic additives is investigated in this paper
The surface morphology of (110) surface displays a striped pattern after etching in the KOH solution saturated with isopropanol

Summary

Introduction

The {110} planes can develop in the sidewalls on condition that the edges of the etching mask are aligned perpendicularly to h100i directions and the etch rate ratio R(100)/R(110) is higher than one. The tetramethylammonium hydroxide (TMAH) solution containing Triton X-100 nonionic surfactant was proposed for silicon etching by Resnik et al (2005). This etching mixture allowed one to obtain high etch rate ratio R(100)/ R(110) and relatively smooth {110} sidewalls. Another nonionic surfactant NCW-1002 was applied for TMAH etching of {110} micromirrors (Xu et al 2010, 2011; Yagyu et al 2010). Triton X-100 surfactant is suggested as an additive to KOH solution in the 45° micromirrors

Experimental details

Results and discussion

Summary