handbook of modern sensors

10.5 Piezoresistive Sensors 347
bridge design. At the same time, it greatly simplifies the additional circuitry necessary
to accomplish calibration and temperature compensation. Nevertheless, the singleelement
strain gauge electrically is analogous to the bridge circuit. Its balance (offset)
does not depend on matched resistors, as it would be in a conventional bridge, but on
how well the transverse voltage taps are aligned.
A thin diaphragm with 1-mm 2 area size may be formed by using one of the commonly
used silicon etching solutions [e.g., hydrazine–water (N 2 H 4·H 2 O) anisotropic
etchant]. A SiO 2 or Si 3 N 4 layer serves as an etch mask and the protective layer on
the bottom side of the wafer. The etching time is about 1.7 µm/min at 90 ◦ C in reflux
solution. The final diaphragm thickness is achieved at about 30 µm.
Another method of diaphragm fabrication is based on the so-called silicon fusion
bonding (SFB), where single crystal silicon wafers can be reliably bonded with nearperfect
interfaces without the use of intermediate layers [9]. This technique allows the
making of very small sensors which find use in catheter-tip transducers for medical
in vivo measurements. The total chip area may be as much as eight times smaller
than that of the conventional silicon-diaphragm pressure sensor. The sensor consists
of two parts: the bottom and the top wafers (Fig. 10.6A). The bottom constraint wafer
(substrate) is first anisotropically etched with a square hole which has the desirable
dimensions of the diaphragm. The bottom wafer has a thickness about 0.5 mm and the
diaphragm has side dimensions of 250 µm, so the anisotropic etch forms a pyramidal
cavity with a depth of about 175 µm. The next step is SFB to a top wafer consisting of a
p-type substrate with an n-type epi layer. The thickness of the epi layer corresponds to
the desired final thickness of the diaphragm. Then, the bulk of the top wafer is removed
by a controlled-etch process, leaving a bonded-on single crystal layer of silicon which
forms the sensor’s diaphragm. Next, resistors are ion implanted and contact vias are
etched. In the final step, the constrain wafer is ground and polished back to the desired
thickness of the device—about 140 µm. Despite the fact that the dimensions of the
SFB chip are about half of those of the conventional chip, their pressure sensitivities
are identical. A comparison of conventional and SFB technology is shown in Fig.
(A)
(B)
Fig. 10.6. Silicon fusion bonding method of a silicon membrane fabrications: (A) productions
steps; (B) comparison of an SFB chip size with a conventionally fabricated diaphragm.