Surface micromachining by sacrificial aluminium etching

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Abstract
Sacrificial aluminium etching enables micromechanical structures integrated with circuitry to be fabricated using standard IC processes followed by simple post-processing. In this paper, the etching characteristics of CMOS aluminium in four etch solutions are reported. The solutions are (A) a commercially available aluminium etchant, (B) Krumm etch, (C) diluted hydrochloric acid, and (D) diluted hydrochloric acid with hydrogen peroxide. The etching of narrow channels is studied as a function of time and temperature. Initially, the etching process is reaction-rate controlled and then crosses over to a diffusion-controlled regime with reduced etch rate. Underetching distances larger than are readily achieved with etchants `A', `B', and `D'. The commercially available aluminium etchant has a low initial underetch rate of at but offers best control. The initial etch rate of hydrochloric acid with hydrogen peroxide is at . However, irregular etch fronts are obtained. Reliable protection of aluminium pads against etchants `A', `B', and `D' is guaranteed by Shipley's photoresist S1828 spun at 3000 rpm and hardbaked at .

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