10-05-2014, 02:59 PM
Fabrication and Characteristics of Si Piezoresistive Micropressure Sensors for Tactile Imaging Devices
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ABSTRACT
This paper describes the characteristics of a Si piezoresistive micropressure sensor fabricated
by using a three-electrode electrochemical etch-stop to reduce the pressure sensitivity variation,
as well as its application to both force and load distributions for instruments or tactile imaging
devices. The Si microdiaphragm thickness is precisely controlled by using a natural etch-stop in
aqueous tetramethyl ammonium hydroxide (TMAH) : isopropyl alcohol (IPA) : pyrazine solutions.
Using the electrochemical etch-stop technique, we have made 801 microdiaphragms on a 5-inch
Si wafer with 20-μm-thick n-epi Si grown epitaxially on p-type substrates. The average thickness
of the 801 microdiaphragms with sizes of 1.43 × 1.43 mm2 has been measured to be 20.03 μm
with a standard deviation of ±0.26μm. The etch-stopped microdiaphragm surface is extremely flat
without any noticeable taper or nonuniformity. The pressure sensitivity of the fabricated devices
is 1.72 mV/(V.Kgf.cm−2 ), and its variation is less than ±2.3 %. This result indicates that the
electrochemical etch-stop technique in TMAH : IPA : pyrazine solutions is useful for the manufacture
of the uniformly thick microdiaphragms needed for producing micro-electro-mechanical-systems
(MEMS) on a wafer scale.
INTRODUCTION
Rapid improvements over the last years in Si inte-
grated circuits and micromachining technologies have al-
lowed the development of various sensing instruments. A
reliable, low-cost, and high-resolution tactile sensor that
can be used in unfamiliar and harsh environments is par-
ticularly needed in the area of automation process and
industrial robot control sensors [1]. Recently, methods of
using tactile imaging devices to measure not only force
but also force distribution have been based on solid-state
pressure sensors utilizing an array of either piezoresistive
or capacitive cells [2,3]. When making these devices, it
is very important to develop a pressure sensor cell where
the variation of the pressure sensitivity is extremely small
over a large area.
EXPERIMENTS
1. Diaphragm Formation
The starting materials consisted of <100>-oriented 5-
inch Si wafers that had 20-μm-thick n-type Si grown epi-
taxially on p-type substrates. Both surfaces were covered
with a 4000- ̊
A-thick thermal oxide layer to protect them
from the etchant. On the p-type substrate surface, 801
diaphragm patterns were formed by SiO2 etching using
a photolithographic technique. In the n-type Si layer, we
implanted boron to form a contact, and the anodic con-
tact area was also opened up to apply the anodic voltage.
The optimum anisotropic etch condition is TMAH (20
wt.%) : IPA (8.5 vol.%) : pyrazine (0.5 g/100 ml) solu-
tions [10]. Under this condition, the etch rate is higher
than that in TMAH : IPA solutions, and the surface qual-
ity is excellent [11]. The three-electrode electrochemical
etch-stop was controlled using an EG & G 362 potentio-
stat. An Ag/AgCl-type electrode, whose working tem-
perature extends up to 100 ◦ C.
Pressure Sensor Fabrication
Si piezoresistive micropressure sensors were fabricated
using a standard Si IC process, combined with an elec-
trochemical etch-stop technique. A cross-section of each
fabrication step is shown in Fig. 2. The starting ma-
terial is an n/p epitaxial Si wafer, in which the 20-μm-
thick n-type epitaxial layer corresponds to the diaphragm
thickness of the pressure sensors. First, p-type piezore-
sistors were formed on the surface of the n-type epitaxial
layer by boron-ion implantation, φ = 3.5 × 1014 cm−2 ,
E = 100 keV, followed by annealing at 1140 ◦ C for 2 hr.
Finally, p-type piezoresistors with a 3 × 1018 cm−3 sur-
face impurity concentration were formed. Second, the Al
electrode used as the working electrode during the elec-
trochemical etch-stop was deposited on the surface of the
Si substrate. Then, the diaphragms were formed using
the electrochemical etch-stop method mentioned above.
Third, the contact holes were opened to the SiO2 layer of
the piezoresistor’s end terminals.
RESULTS AND DISCUSSION
For mass production, identical mechanical properties
are required in all devices. Hence, the reproducibility of
a uniform thickness in Si microdiaphragms is an impor-
tant fabrication feature. In order to evaluate the repro-
ducibility of the uniform thickness in Si microdiaphragms
across a wafer, we fabricated 801 microdiaphragms on a
5-inch Si wafer having a 20-μm n-type Si epilayer grown
on a p-type substrate, as shown in Fig. 4. After the elec-
trochemical etch-stop process in TMAH : IPA : pyrazine
solutions at a reverse bias of 0.8 V, the thickness of the
microdiaphragms was evaluated by using scanning elec-
tron microscopy (SEM).
CONCLUSION
In this work, high resolution Si piezoresistive pressure
sensors were fabricated using a three-electrode electro-
chemical etch-stop technique during the formation of Si
microdiaphragms, and the pressure sensors were evalu-
ated. The developed electrochemical etch-stop was use-
ful for the formation of microdiaphragms with flat and
uniform thicknesses on a wafer. The most significant
factor affecting the pressure sensitivity of Si diaphragm
pressure sensors is the diaphragm thickness variation. In
particular, the variation in pressure sensitivity could be
reduced to within a standard deviation of ±2.3 % from
wafer to wafer. Consequently, we demonstrated that the
electrochemical etch-stop is an easy-to-handle, yet pow-
erful, fabrication tool. Since almost any wafer can be
etched, the process can be automated if the current is
used as an automatic end-point detector, and there is no
danger of overetching the microdiaphragms.
The results of microdiaphragm formation by using an
electrochemical etch-stop presented in this work suggest
a very promising technique for the development of high-
resolution mechanical sensors with no variation in the
pressure sensitivity. Furthermore, this technique seems
to be especially attractive for applications to microsen-
sors and microactuators using microdiaphragms.