Page 34 - MEMS Mechanical Sensors

2.3 Fabrication Techniques 23

order of 1,000:1 [11]. An illustration of the use of etch stop using boron doping is
shown in Figure 2.10.
The crucial difference between these etchants is in the etch rates of the masking
and other materials that are deposited on the substrate. Suitable masks for KOH are
silicon nitride or silicon carbide, which etch at negligible rates. Silicon dioxide, on
the other hand, is not an ideal mask due to an etch rate that is typically 1/200 of the
etch rate of {100} silicon. This may suffice in some circumstances, but for removing
large amounts of silicon, the thickness of the oxide mask required is impractical.
Another important consideration is that KOH is corrosive and therefore will dam-
age metals such as aluminum. Refractory metals, such as gold and titanium, how-
ever, are not attacked. Silicon dioxide can be used as a mask when etching with
TMAH, since the etch rate is negligible. This is a clear advantage. Another advan-
tage is that it is possible to reduce the etch rate of aluminum to an acceptable level by
the addition of silicon, polysilicic acid [12], (NH ) CO ,or(NH )HPO to the
4 2 2 4 4
etchant to lower the pH [13]. The drawback to this is that hillocks and rough sur-
faces are produced. These can be alleviated to some extent by the addition of an oxi-
dizer such as ammonium peroxydisulfate [14]. Both oxide and nitride can be used as
a mask for etching in EDP and, in addition, many metals are not attacked by EDP.
One exception is aluminum, although the etch rate of aluminum for some formula-
tions of the etchant can be reduced to useful proportions [15]. It is however
extremely hazardous, very corrosive, carcinogenic, and has to be used in a reflux
condenser. The surface roughness of the etched surface is also dependent on the
etchant used. For a 30%wt KOH solution at 70°C, the mean surface roughness of
the {100} plane is of the order of a few nanometers after etching ∼200 µm. The
smoothest surfaces obtained with TMAH are at concentrations above 20%wt
where the mean surface roughness is of the order of 100 nm. Unfortunately, at these
concentrations the pH is too high to make effective use of the methods used
to reduce the aluminum etch rate mentioned above. A typical formulation for
EDP is 750 ml ethylenediamine, 120g pyrochatechol, and 100 ml water used at
115°C. With this formulation surfaces comparable to KOH etched surfaces can be
obtained. A comparison highlighting the main differences between these etchants
can be found in Table 2.5.

Boron diffussion
through patterned
oxide

Deposit and pattern
oxide and nitride on
back of wafer

Etch silicon in
anisotropic
etchant

Figure 2.10 Boron etch stop technique. In this illustration the technique is used to create
freestanding structures such as cantilever beams.