Page 144 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

124 SILICON MICROMACHINING: BULK

dw bd
w, 'b+V2 d w b d
H—M •H H K- K--H / ---Hh*-HK--

(a) (d)

Figure 5.7 Four process steps to make and release the cantilever

4. The wafer is then etched in BOE to remove all the SiO 2 and is then cleaned and
oxidised to grow a fresh layer of SiO 2 that is 1 urn thick. The wafer is spin-coated
with a layer of positive photoresist and patterned with mask 2. After the unprotected
oxide is etched away in BOE, the resist is removed in acetone. The wafer is then
etched in NaOH at 55 °C until the bulk silicon is completely under-etched in the
areas that are directly underneath the beam. Figure 5.7(c, d) shows the evolution of
the silicon cantilevers etched in this way at different stages of the final etching in
NaOH.

5.3 ETCH-STOP TECHNIQUES

Many different chemical etchants for silicon are known. The properties that make some of
these etchants indispensable to micromachining of three-dimensional structures are selec-
tivity and directionality. As etching processes in polar solvents are fundamentally charge
transport phenomena, it is not surprising that the etch-rate may be dopant-type-dependent,
dopant-concentration-dependent, and bias-dependent. Etch processes can be made selec-
tive by the use of dopants - heavily doped regions etch more slowly - or even halted elec-
trochemically when observing the sudden rise in current through an etched n-p junction.
A region where wet (or dry) etching tends to slow down (or halt) is called an etch-stop.
There are several ways in which an etch-stop region can be created. In the following
subsections, two such methods by which etch-stops are created are discussed. These
methods are:

• doping-selective etching (DSE)

• bias-dependent etching BSE

5.3.1 Doping-Selective Etching (DSE)

Silicon membranes are generally fabricated using the etch-stop phenomenon of a thin,
heavily boron-doped layer, which can be epitaxially grown or formed by the diffusion
or implantation of boron into a lightly doped substrate. This stopping effect is a general
property of basic etching solutions such as KOH, NaOH, ethylenediamine pyrocatechol