Surface micromachining by sacrificial aluminium etching

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 .

A repeatable and scalable fabrication method for sharp, hollow silicon microneedles

“…where A is the proportionality constant that accounts for molecular weight and density of the oxide layer; this equation takes the same form presented by Liu [46] and Westberg [47].…”

Integrated MEMS: Opportunities & Challenges

“…The first, Figure 20a uses the gate poly as the mechanical material and the second, Figure 20b, (Fedder, 1996) a combination of oxide and metal used in standard processes. An alternative approach is to use one of the aluminium layers as a sacrificial layer, and protecting the remaining aluminium layers with oxide (Westberg, 1996). The resulting structure is shown in Figure 21.…”

Water‐Soluble Sacrificial Layers for Surface Micromachining

“…More generally, we found that the presence of 0.1 equivalents of Ca 2 + in solution relative to the sum of all the monovalent cations was sufficient to maintain the insolubility of the PAA-Ca 2 + films. This approach did not, however, prove successful in very alkaline solutions, because the calcium ions precipitate in the form of Ca(OH) 2 , and could not be maintained at a sufficiently high concentration to maintain the ionic cross-linking of PAA. As a result, alkaline developers with pH values of 12-13 (such as the 351 developer used for Shipley positive photoresists) are currently not compatible with the PAA films.…”

“…Although some non-silicate-based materials (e.g., titanium and aluminum) can be used as sacrificial layers with a HF etch, [2] the poor selectivity of this etch limits its usefulness with fragile materials, and its toxicity makes it inconvenient or hazardous for inexperienced users. HF-free etching solutions for aluminum are available, based on mixtures of acids and oxidants (i.e., concentrated phosphoric and nitric acids, hydrogen peroxide, and acetic acid), [2] but are also incompatible with some fragile materials. Porous silicon is also used for the fabrication of microsystems, and is coupled with a final dissolution in alkaline environment (KOH).…”