Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 122

Table 4.4 Characteristics of Different Anisotropic Wet Etchants

Parameters KOH NaOH TMAH EDP

Boron concentration (cm e 3 ) 10 20 10 20 10 20 10 19

> > 3 > > 3

Etch rate ratio Si/Si þþ

> 20 500 10 40 100 10

e e

All etchants are selective to silicon nitride and silicon dioxide. Thus, these two materials can be used as
masks for anisotropic etching processes.
Controlled etch stop is an important technique for precise fabrication with anisotropic wet etching.
Different methods to slow down or eliminate the etch rate are:

Using selectivity of etchants, coating silicon surfaces with a protective layer such as nitride or
oxide;

Using orientation dependency of etch rates; and

Using controlled hole generation.

The first method is often used for selective etching with a layer of silicon dioxide and silicon nitride
as a mask. By combining multiple silicon/nitride layers, structures with different depths can be
realized. Since the etch rate of the {111}-plane is two orders of magnitude slower than those of {110}-
and {100}-planes, the etch front stops at the {111}-plane. This unique property can be used to fabricate
microchannels with well-defined shape.
According to (4.5) , electrons are essential for a successful wet-etching process. Etching away one
silicon atom requires four electrons. Holes are generated when electrons are released. The holes attract
more hydroxide ions to the substrate surface and speed up the etching process. There are two ways of
controlling the availability of holes: highly boron-doped p-silicon and electrochemical etching with
ap

n junction.
Silicon can be doped by a solid or gaseous boron source where silicon dioxide or silicon nitride
may work as a diffusion barrier. The depth of the doped layer depends on the diffusion process and is
limited by a maximum value on the order of 15

e

m m. Table 4.4 compares the etch rate reduction of

different etchants in highly boron-doped silicon.
Etch rates can also be controlled electrochemically. If the silicon surface is biased with
a positive potential relative to a platinum electrode, hydroxide ions are attracted to the substrate
surface and speed up the etching process ( Fig. 4.3 (a)). There are two potential values critical for
the electrochemical etch process: the open circuit potential (OCP) and the passivation potential
(PP). Open circuit potential (OCP) is the potential resulting in a zero current. At this potential, no
electron supply exists and the etching process works, as in the case without the circuit. OCP is on
the order of 1.56 V. Decreasing the potential from OCP increases the current. The current reaches
its maximum value and decreases again because of oxide formation, which prevents further
etching. The potential at which oxide formation is reached is called passivation potential. PP is on
the order of 1 V.
The above-mentioned electrochemical characteristics can be used for controlling etch stop with
ap

n junction is reverse biased, most of the
voltage drops at this junction. Thus, p-silicon is allowed to float at OCP and is etched away
( Fig. 4.3 (b)). Etching away p-silicon destroys the p

n junction as described in Fig. 4.3 (a). Because the p

e e

n junction. The voltage across the two electrodes

e