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Proceedings Paper

Megasonic enhanced KOH etching for {110} silicon bulk micromachining
Author(s): Dirk Nusse; Martin Hoffmann; Edgar Voges
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Paper Abstract

An optimized KOH micromachining process is presented, which offers perfectly flat sidewalls and a homogenous etching rate. We investigate the influence of ultrasound to the KOH wet etching process for different frequencies and operational modes. Especially small and deep grooves with a high aspect ratio cause difficulties in conventional etching because the time controlled etching process is often not reproducible. Rough bottoms of etched cavities are also found. The main reason for these problems is hydrogen, which is a byproduct of the chemical reaction. It settles down to the surface of the wafer until the hydrogen bubble is big enough to turn up into the solution. During that retention period, the silicon is micro-masked and the reaction in that area, consequentially, is interrupted for some time. To drive hydrogen out of small gaps is even more difficult, so that the etching rate decreases with the depth of the groove. Without any ultrasound, anisotropic KOH etching of deep trenches always leads to insufficient results, e.g. rough surfaces. Using common ultrasonic frequencies such as 35 kHz or 130 kHz reduces the effect of masking bubbles, but the risk of partial damage is much higher in comparison to a 1 MHz excitation frequency. Wafers etched with additional megasound show flat and homogeneous planes. Aspect ratios up to 60 have successfully been fabricated in {110} silicon.

Paper Details

Date Published: 25 October 2004
PDF: 8 pages
Proc. SPIE 5602, Optomechatronic Sensors, Actuators, and Control, (25 October 2004); doi: 10.1117/12.570220
Show Author Affiliations
Dirk Nusse, Univ. Dortmund (Germany)
Martin Hoffmann, SMI GmbH (Germany)
Edgar Voges, Univ. Dortmund (Germany)

Published in SPIE Proceedings Vol. 5602:
Optomechatronic Sensors, Actuators, and Control
Kee S. Moon, Editor(s)

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