Wet chemical anisotropic and selective etching for silicon bulk micromachining

Wet chemical anisotropic etching allows for the precise three-dimensional structuring of large and miniature silicon structures in an IC compatible way. The three main properties that make this technique so widely applicable are the dependence of the etch rate on crystal orientation, dopant concentration and on the applied electrical potential in the case of electrochemical etching.

The crystallographic anisotropy gives the possibility for a very precise lateral and vertical machining of micromechanical devices by proper alignment of structural contours (masks) with either fast or slow etching crystal planes. The dependence on the dopant concentration and on electrical potential allows for the incorporation of well defined etch stop layers by either using a high boron concentration with NB > 5×1019 cm-3, or else exploiting the potential drop across a pn-junction. SiO2 and/or Si3N4 layers can be used as masking layers without problems for creating 3D-structures were very deep patterns are required.

The corresponding doping of etch stop layers is carried out in our Institute by conventional ion implantation, writing focused ion beam (FIB) implantation and subsequent annealing or drive in diffusion.

The main etch parameters for the developed anisotropic etch process are:

Etch solution KOH/H2O-solution of concentration c = 30 %
Etch temperature T = (80±1) °C
Etch samples 2", 3", 4" Si wafers
One-side processed wafers containing device structures can be etched

Etch rate R (µm/h) Anisotropy Selectivity Surface roughness r (µm)
R<100> = 69,8
R<111> = 1,4
RSiO2 = 0,15
RSi3N4 = 0
R<100>/R<111> = 49,8
R<100>/R<110> = 0,5-2
R<100>/RSiO2 = 465
R<100>p/R<100>p+ = 10...103
R<100>n/R<100>p+ = 103
ra = 0,033
rp = 0,11
rv = 0,12
rt = 0,24