Page 217 - An Introduction to Microelectromechanical Systems Engineering

196 MEM Structures and Systems in RF Applications

the length of overlap and thus the capacitance between the fingers. The capacitance
scales linearly with the number of fingers and the finger thickness and is inversely
proportional to the gap. Thus, the use of DRIE for high-aspect-ratio trenches results
in a large capacitance per unit area. Fabricating a massive number of fingers in par-
allel, typically in several separate blocks, gives the desired capacitance. As with the
surface-micromachined capacitors, resistance in the springs dominates the Q if
other sources of loss are minimized.
Several generations of interdigitated-finger variable capacitors have been
designed and fabricated at the Rockwell Science Center of Thousand Oaks, Califor-
nia [9–11]. The earliest was fabricated using a single mask to DRIE though the top
layer of silicon on a SOI wafer, stopping on a 2-µm-thick buried layer of silicon
dioxide. Hydrofluoric acid removes the buried oxide in the regions with moving
−3
20
parts, followed by supercritical drying. Even heavy doping (>10 cm ) of the sili-
con springs and fingers does not yield as low a sufficiently low resistance, so their
resistance is lowered by sputtering on a thin, unpatterned layer of aluminum. This
coats the tops and sidewalls of the silicon. The top of the underlying silicon substrate
is also coated, but gaps are formed due to the undercut of the oxide, avoiding short-
ing. Even with the thick buried oxide, the underlying low-resistivity silicon handle
substrate acts as the lossy plate of a capacitor, giving both low Q and a large para-
sitic capacitance [see Figure 7.1(a)]. Changing the underlying silicon substrate to
high-resistivity material improves the Q and lowers the parasitic capacitance by a
factor of ten.
Further improvement in Q is achieved by using a glass substrate, which requires
major changes in the process flow [see Figure 7.3(b)]. These capacitors are fabricated
by bonding the thin-silicon side of a SOI wafer to the glass wafer with 20 µmof
epoxy. The Young’s moduli of epoxies are normally orders of magnitude lower than
those of silicon and glass, providing some stress isolation between the two substrates.
The thick side of the SOI wafer is removed by grinding and polishing off most of the
silicon, completing the silicon removal by etching in TMAH, and finally etching off
the buried oxide to reveal the 20 µm-thick layer of silicon. Next, a 2-µm layer of alu-
minum is deposited for a low series resistance and patterned using standard lithogra-
phy and plasma etching. The photoresist and aluminum act as a mask in a DRIE step
to etch straight down through the silicon to the buried layer of epoxy. The epoxy is
etched in an oxygen plasma, which undercuts the silicon to free the moving parts. An
additional 0.25 µm of aluminum is sputtered to coat the sidewalls for an even lower
series resistance. Fabricated capacitors have fingers that are 2 µm wide, 20 µm thick,
a gap of 2 µm, and an initial overlap of a few micrometers. A nominal capacitance of
2 pF requires 1,200 sets of interdigitated fingers. The measured value for Q is 61 at 1
GHz, the self-resonance frequency is 5 GHz, and the tuning range is 4.55:1 at 5.2V
from a finger motion of 23 µm.
A characteristic common to all mechanical devices with masses and springs,
whether surface or bulk micromachined, or even macro- or microscale, is that the
mass will move unintentionally when the device is subject to an external accelera-
tion, such as vibration (this sensitivity results in noise that is casually referred to as
microphonics). The displacement and corresponding capacitance change in vari-
able capacitors depends on the acceleration, the mass, and the spring constant in
the direction of motion. The allowed capacitance change is application-dependent.