Irregular Wet Etching of Cr Mask

Irregular Wet Etching of Cr Mask

Material Name: Chromium
Record No.: 54
Primary Chemical Element in Material: Cr
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: Both negative and positive photoresists used in through wafer deep etching tend to crack at cryogenic temperatures. Silicon oxide is a commonly used hard mask for cryogenic etching and has been used heavily in this research. Metal masks were also used in this research but were found to be not quite ideal, although they have a much higher selectivity. We used a chromium (Cr) thin film metal hard mask because of its higher selectivity to silicon in DRIE. Also, growing 0.004 mm of oxide by oxidation can take up to 5 days, while metal film sputtering takes only a few hours. We performed wet etching of a Cr film and the results were not uniform, with irregular Cr pieces found in many places. These irregularly shaped metal masks led to nonuniform etching of Si around the feature boundaries, as seen around the hole top and the island in Fig. 1.
Reference: Ankita Verma, Joshah Jennings, Ryan D. Johnson, Marc H. Weber, and Kelvin G. Lynn, Fabrication of 3D charged particle trap using through-silicon vias etched by deep reactive ion etching, J. Vac. Sci. Technol. B 31, 032001 (2013); doi: 10.1116/1.4799662.

Figure 1: Irregular wet etching of Cr mask.