Page 146 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

126 SILICON MICROMACHINING: BULK

in a lightly doped n- or p-type substrate. If the etch-stop concentration threshold lies in
between the substrate and diffusion concentrations, the p-substrate (or n -substrate for that
matter) is etched out from underneath the high boron diffusion. A silicon microstructure
with a geometry defined by the diffusion mask and a thickness close to the diffusion depth
is hence left freely suspended.
The main benefits of the high boron etch-stop are the independence of crystal orien-
tation, the smooth surface finish, and the possibilities it offers for fabricating released
structures with arbitrary lateral geometry in a single etch step. On the other hand, the
high levels of boron required are known to introduce considerable mechanical stress
into the material; this may even cause buckling or even fracture in a diaphragm or
other double-clamped structures. Moreover, the introduction of electrical components for
sensing purposes into these microstructures, such as the implantation of piezoresistors,
is inhibited by the excessive background doping. The latter consideration constitutes an
important limitation to the applicability of the high boron dose etch-stop. Consequently,
bias-dependent BSE, commonly referred to as an electrochemical etch-stop, is currently
the most widely used etch-stop technique.

5.3.2 Conventional Bias-Dependent BSE or Electrochemical
Etch-Stop

In electrochemical etching of silicon, a voltage is applied to the silicon wafer (anode), a
counter electrode (cathode) in the etching solution. The fundamental steps of the etching
mechanism are as follows:

1. Injection of holes into the semiconductor to raise it to a higher oxidation state Si +
–
2. Attachment of negatively charged hydroxyl groups, OH , to the positively charged Si
3. Reaction of the hydrated silicon with the complexing agent in the solution
4. Dissolution of the reaction products into the etchant solution
In bias-dependent etching, oxidation is promoted by a positive voltage applied to the
silicon wafer, causing an accumulation of holes at the Si-solution interface. Under these
conditions, oxidation at the surface proceeds rapidly while the oxide is readily dissolved
by the solution. Holes such as H + ions are transported to the cathode and released there
as hydrogen gas bubbles. Excess hole-electron pairs can, in addition, be created at the
silicon surface, for example, by optical excitation, thereby increasing the etch rate.
Figure 5.9 shows an electrochemical cell that is used to etch Si in a 5 percent hydro-
fluoric (HF) solution. The cathode plate used is made of platinum. In the etching situation
shown in Figure 5.9, holes are injected into the Si electrode and they tend to reside at
+
the Si surface where they oxidise Si at the surface to Si . The oxidised silicon interacts
with incoming OH~ that are produced by dissociation of water in the solution to form
the unstable Si(OH), which dissociates into SiO 2 and H 2 gas. The SiO 2 is then dissolved
by HF and removed from the silicon surface.
The current density-voltage characteristics for different silicon types and resistivities
are shown in Figure 5.10. It is apparent from Figure 5.10 that the current density is very
much dependent on the type and the resistivity (doping level) of Si. This dependence on
the type and resistivity is the property that is utilised in the electrochemical etch-stop
phenomenon.