Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 129

The last problem is in the release step. The sacrificial etching process and the subsequent rinsing
process are both wet processes. While drying, the liquid bridge between the freestanding structure and
the substrate causes it to collapse and stick to the surface. There are several solutions for this problem.
Dry etchant, such as vapor phase HF, can be used to avoid the liquid phase. However, gaseous HF also
attacks silicon nitride. Thus, silicon nitride should not be exposed to HF during the release process.
Solvents with lower surface tension, such as methanol, can be used for the rinsing process. Dimples
can be introduced in the design to avoid stiction. Alternative drying techniques, such as CO 2 critical
drying and freeze drying, can avoid the liquid phase. An organic film can work as sacrificial layer,
which only requires dry oxygen plasma for removal. Finally, surface modification with a self-
assembled monolayer (SAM) can minimize the surface energy and consequently stiction.
4.1.3.2 Fabrication of microchannels with silicon surface micromachining
A general surface micromachining process for microchannels starts with deposition of the sacrificial
layer ( Fig. 4.8 (a)). The channel material is then deposited over the structured sacrificial layer. After
opening etch accesses through the channel wall, the sacrificial layer is etched to hollow out the
channel. A subsequent deposition of channel material seals the etch access.
Lin et al. reported the fabrication of a microchannel in a microneedle using the above technique
[29] . PSG is deposited as the sacrificial layer over a ground silicon nitride layer. The structured
sacrificial layer is then encapsulated by LPCVD silicon nitride. Etch access is opened with RIE of the
nitride wall. After removing the sacrificial PSG, a second LPCVD process seals the empty channel
FIGURE 4.8
Surface-micromachined channels: (a) polysilicon channel; (b) oxide/nitride channel; and (c) metal channel.