Page 151 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

ETCH-STOP TECHNIQUES 131

technique is called selective etching by pulsed potential anodisation and is described in
the following section.

5.3.3 Selective Etching of n -Type Silicon by Pulsed Potential
Anodisation

The pulsed potential anodisation technique selectively etches n-type silicon (Wang et al.
1992). The difference in the dissolution time of anodic oxide formed on n- and p-type
silicon samples under identical conditions is used to create an etch selectivity. The mech-
anism responsible for this dissolution time difference is not fully understood at present.
However, it is believed to be due to a difference in oxidation rates caused by the limited
supply of holes in n-type samples (Wang et al. 1992). This technique is applicable in a
wide range of anodising voltages, etchant compositions, and temperatures. It differs from
the conventional p-n junction etch-stop in that the performance of the etch-stop does
not depend on the rectifying characteristics or quality of a diode. Using this technique,
p-type microstructures of both low and moderate doping can be fabricated. Hence, the
pulsed potential anodisation technique opens up the possibility for the creation of fragile
microstructures in p-type silicon.
The main problems with the conventional electrochemical etch-stop and the pulsed
potential anodisation technique are related to the etch holders required for contacting the
epitaxial layer (and the substrate for two, three, or four electrodes) and for protecting
the epitaxial side of the wafer from the etchant. Any leakage in these holders interferes
with the correct operation of the etch-stop. Moreover, mechanical stress introduced by
the holder is known to reduce substantially the production yield in many cases. There-
fore, development of a reliable wafer holder for anisotropic etching with electrochemical
etch-stop is not straightforward. The process of making contact with the wafer itself can
also be critical and difficult to implement. Therefore, single-step fabrication of released
structures with either the conventional electrochemical etch-stop or the pulsed poten-
tial anodisation techniques may be troublesome. An alternative etch-stop technique that
does not require any external electrodes (or connections to be made to the wafer) has
been recently developed. This new technique is what is referred to as the photovoltaic-
electrochemical etch-stop technique (PHET) (Peeters et al. 1994).

5.3.4 Photovoltaic Electrochemical Etch-Stop Technique (PHET)

The PHET approach is able to produce the majority of structures that can be produced
by either the high boron or the electrochemical etch-stop (Peeters et al. 1994). PHET
does not require the high impurity concentrations of the boron etch-stop and does not
require external electrodes or an etch holder as is required in conventional electrochem-
ical etch-stop or in pulsed anodisation technique. Free-standing p-type structures with
arbitrary lateral geometry can be formed in a single etch step. In principle, PHET is to
be seen as a two-electrode electrochemical etch-stop in which the potential and current
required for anodic growth of a passivating oxide is not applied externally but is generated
within the silicon itself. The potential essentially consists of two components, namely,
the photovoltage across an illuminated p-n junction and the 'Nernst' potential of an
n-Si/metal/etchant solution electrochemical cell.