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7-4 MEMS: Design and Fabrication

doped n -type epilayer over the lower-doped n-type epilayers for use as the sensing element is due to the
associated low resistivity and lesser sensitivity to temperature across the range of operating temperature.
However, this preference leads to a trade-off in lower strain sensitivity due to reduction in the GF.
In order to fabricate a SiC diaphragm transducer, a controllable etching process for thinning and form-
ing of deep cavities in the backside is required. For diaphragm fabrication, the desirable etch rates should be
in the range of 1µm/min. It has been shown [Okojie, PhD Thesis, 1996] that by using ECE in dilute
Hydrofluoric acid (HF), cavities with depth greater than 100µm in both p- and n-type 6H-SiC can be fab-
ricated. In many cases using dilute HF during the ECE process, a porous layer is formed; this layer has a tex-
ture that depends on the concentration of HF in water and the voltage or current density applied.At low HF
concentration, the anodization produces a soft, highly porous SiC layer, whereas anodization at higher HF
concentrations produces a hard layer of low porosity. Because these porous layers have large crystal surface
areas, enhanced thermal oxidation occurs within them relative to the bulk. Subsequent dipping in buffered
hydrofluoric (BHF) acid removes the now-oxidized porous material to reveal the etched cavity.
In n-type SiC, ECE is possible only when the positive anode voltage (with the SiC as the anode elec-
trode) is sufficiently high to cause the semiconductor space charge layer at the SiC/electrolyte interface to
break down. Under this condition, electrons are injected from the SiC/HF interface into the bulk of the
semiconductor, resulting in dissolution reactions. The voltage required for the breakdown depends on the
doping level of the semiconductor and on the concentration of the electrolyte.
Two ECE experiments were conducted with n-type SiC: (a) anodization in a 0.625% HF in a two-electrode
cell (SiC-anode with platinum counterelectrode) and subsequent thermal oxidation to remove the porous
layer, and (b) anodization in a 0.625% HF in a three-electrode configuration (SiC-anode, platinum-
counterelectrode, and reference standard calomel electrode) and oxidation to remove the porous layer.
Both experiments demonstrated rapid etching of n-type SiC. In the first experiment, the average etch rate
was 0.3µm/min, and in the second, an etch rate of 0.8µm/min was obtained. At an average current den-
2
sity (J 100mA/cm ) used in both experiments, higher etch-rates should have been obtained if etch-
av
ing was the only process that occurred. The actual etch rates obtained from the experiments suggested
that other processes were occurring (e.g. water decomposition), which would use up part of the anodiza-
tion current. Gaseous products released during the anodization process acted as masks to decrease the
reaction rate, thereby reducing the etch rate.
Materials must possess certain properties to be used as a mask in electrochemical etching. In a dilute
HF, the mask should be inert in the concentrations used, and no electrolytic reaction should occur dur-
ing anodization that might interfere with the etching process. Two masking materials were investigated.
Platinum, which is very resistant to HF, generated gas bubbles at the high anodic potentials used, result-
ing in nonuniform etching. Silicon nitride did not form bubbles because it is an insulator. However,
silicon nitride is not completely resistant to HF.
Because the process of deep etching to form a diaphragm takes a long time (200 minutes to etch a 200-µm
deep cavity at an average of 1 µm/min), a certain minimum thickness of nitride is required to survive the
dilute HF electrolyte during the anodization process; thus, the complete stripping of the nitride mask and
consequent undesired etching of the entire SiC surface can be prevented. The investigations briefly
described led to the following important experimental conclusions [Okojie, PhD Thesis, 1996], that at a
fixed voltage of 12V:

1. Etching in 2.5% HF occurs at higher rates than in 0.625% HF.
2. Etching in 2.5% HF produced deep etch pits and/or hillocks on the surface of SiC, whereas no etch
pits or hillocks were observed on surfaces etched in 0.625% HF. The pits are attributed to the
enhanced electrochemical etching around the micropipes in the SiC substrate.
3. The surface of the etched region was smoother and the depth more uniform when etched in
0.625% HF.

In an attempt to further optimize the etching process for forming n-type SiC diaphragms, an addi-
tional series of experiments was conducted with different etching parameters and concentrations of HF.

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