Page 150 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

130 SILICON MICROMACHINING: BULK

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Figure 5.13 Basic arrangement of a four-electrode electrochemical cell for silicon etch-stop. Volt-
ages are indicated relative to the reference electrode RE

The conventional junction etch-stop permits the formation of n-type structures. If the
doping types of the silicon in Figures 5.12 and 5.13 are reversed such that the bias
is applied to a p-type layer, and the n-type is exposed to solution, the diode is now
forward-biased by the applied potential. If the forward-diode current density is low
enough, it is possible that the n-type silicon will not passivate, even if the potential
applied to the p-type layer is greater than the passivation potential for silicon. If this
is the case, the n-type silicon will etch until it reaches the p-type layer, thus permit-
ting the formation of a p-type membrane. In practice, this process can be very difficult
to implement because it requires a very tight control of the potential applied to the
p-type layer and it is very sensitive to the diode-forward characteristics. To circum-
vent this weakness of the conventional electrochemical etch-stop technique, an alter-
native dopant-selective technique that uses pulsed anodising voltages applied to silicon
samples immersed in etching solutions was developed (Wang et al. 1992). This alternative