Sensors and Materials, Vol. 11, No. 8 (1999) 479-492
MYUTokyo
S &M 0390

Piezoresistive Accelerometer Fabricated by
Selective Formation and Etching of Porous Silicon
Jun-Hwan Sim, Chan-Seob Cho 1, Jin-Sup Kim 2,
Jung-Hee Lee 3 and Jong-Hyun Lee 3
Department of Electronics & Communication Engineering, Korea Maritime University,
Pusan 606-791, Korea
1 Department of Electronic & Electrical Engineering, Sangju National Polytech. University,
Sangju 742-170, Korea
2 Department of Electronic Engineering, lnje University, Kimhae 621-749, Korea
3
School of Electronic & Electrical Engineering, Kyungpook National University,
Taegu 702-701, Korea(Received March 16, 1998; accepted October 5, 1999)

Key words:

silicon micromachining, porous silicon etching, eight-beam structure, piezoresistive
silicon accelerometer

This is a report on the selective formation and etching of porous silicon, and thefabrication of a piezoresistive silicon accelerometer with eight beams on a selectively
diffused (111)-oriented nfn+/n substrate using a porous silicon micromachining technique.
This technique has many of the advantages of both bulk and surface micromachining
without their disadvantages, and is capable of precisely constructing a microstructure bydefining the n+-diffusion region and patterning the n-epitaxial layer, thereby, avoiding any
lateral etching or undercutting. Through the use of an eight-beam structure, the mechanicalstrength of the accelerometer could be greatly improved due to a smaller shear stress. The
lower sensitivity of the sensor was easily adjusted by combining the four output signals of
the four half-bridges. The effectiveness of the sensor was confirmed through an experi­
ment in which the accelerometer was characterized.

1.

Introduction

Silicon accelerometers have some attractive features such as small size, low cost andmass-production capabilities with standard IC technologies.< 1-2l Recently, the developmentof many kinds of silicon accelerometers based on piezoresistive, capacitive, and other

479

480

· Sensors and Materials, Vol. 11, No. 8 (1999)

sensing principles, have been reported_C3-6l These sensors are mainly fabricated using
either bulk or surface micromachining techniques_o.si
Bulk micromachining is normally conducted using isotropic chemical etching solu­
tions. To achieve a high-resolution etch and precise dimensional controllability, some·
additional special techniques such as deep etching, two-sided (front-back) etching, or
anodic bonding are often adopted, which result in a low yield and high cost of the device
fabrication. Surface micromachining usually utilizes deposited polycrystalline silicon as astructural or sacrificial layer, which is subsequently patterned and etched away to construct
the desired structure. However, polycrystalline silicon, which is widely used in surface
micromachining, has poor mechanical properties compared to those of single-crystalline
silicon and consequently, the film quality is closely related to the specific deposition
conditions.
Alternative surface micromachining techniques using a porous silicon etching tech­nique have been suggested, since these would not suffer from the problems mentioned
above. Porous silicon is formed by the electrochemical dissolution of monocrystalline
silicon in hydrofluoric (HF) acid. Since the porous silicon formation is strongly dependent
on both the substrate type and the doping leve1,<9l several methods for the selective
formation of a porous silicon layer and the subsequent etching of the layer have been
investigated.<1 0-12l However, the method described by Tu<10l requires several repetitious
implantations with proton and nitrogen, and it is also difficult to define the side of the
microstructure. Other methods using the n/p substrate, developed by Eijkel et at.< 11 l andLang et al., 112i were shown to have difficulties in controlling the air gap between the
microstructure and the· substrate and with the definition of the exact geometry of the
microstructure, due to undesirable undercutting caused by the isotropic anodic reaction.
A bridge-type accelerometer, which can support the mass in eight directions, has notbeen introduced up to now because of the difficulty in the precise shaping of the micro­
structure using a conventional isotropic silicon etching process and due to the lower
sensitivity of the structure than that of a two- or four-beam bridge-type accelerometer.
However, an eight-beam structure offers a lower cross-axis sensitivity to lateral accelera­tion parallel to the device surface than a two- or four-beam structure.
In this study, we present a highly selective, self-stopping<13l porous silicon surface
micromachining technique which is more effective for controlling the air gap height, thebeam thickness, and the shape of the micro structure due to the selective formation of n+porous silicon in a selectively diffused (111 )-oriented nfn+/n substrate. We will also
describe the detailed fabrication process and the experimental results of the production ofa piezoresistive silicon accelerometer with eight beams, using porous silicon
micromachining.

2.

Porous Silicon Micromachining

In this section, two different processes are presented for the fabrication of free-standing
microstructures. The first fabrication process is shown in Fig. 1. It begins with the
diffusion of a highly doped n+ layer over an n-type substrate and the subsequent growth ofa lightly doped n-epitaxial layer. Next, three-dimensional structures are patterned by dry

481

Sensors and Materials, Vol. 11, No. 8 (1999)

n-epi.
rt-n-sub.
( a) n/n+ /n substrate(b) Microstructure define

�n•��

n-sub.

n-sub.

(c) PSL formation( d) PSL etching

Fig. 1. Process sequence for fabricating silicon microstructures using a porous silicon etching
method.

etching or wet chemical etching through the epitaxial layer. These structures are then freed
from the substrate by forming, and subsequently dissolving porous silicon in portions of
the n+ layer.
Figure 2 shows a scanning electron microscopy (SEM) micrograph of a free-standing
cross with a 5 µm thickness, 5 µm width, and 200 µm length, fabricated by porous siliconetching on a blank diffused (111)-oriented n/n+/n substrate. A porous silicon layer wasformed on a 20-µm-thick n+-nonselectively diffused layer by anodization in a 20% aqueous
HF solution. Then a current density of 10 mA/cm2 was applied for 5 min at room
temperature. After the porous silicon layer was removed in a 5 wt% NaOH solution, acompletely undercut cross pattern with a uniform thickness and no cusp was observed.
Since the dopant concentration in the n+-diffused layer was much higher than that in the n­epitaxial layer and the substrate, the anodic reaction occurred only in the n+-diffused layer.
However, during the reaction, porous silicon etching spread laterally into the n+ region
beneath the edge of the n-epitaxial layer, which caused undesirable undercutting as the n+layer became diffused over the entire n-type substrate.
For application in sensors, a microstructure must have exactly defined areas without
any lateral undercut. This can be successfully achieved using a selectively diffused (111)­
oriented nfn+/n substrate, as shown in Fig. 3. Figure 4 shows SEM micrographs ofcantilevers fabricated by the selective formation and etching of porous silicon in a
selectively diffused nfn+/n silicon substrate. The widths are 30, 50, 70, and 100 µm, and thelengths and thicknesses are all 700 µm. Step 1 in Fig. 4 is the result of isotropic siliconetching through an n-epitaxial layer, which reveals the n+-diffused layer, and step 2 is theSensors and Materials, Vol. 11, No. 8 (1999)

482

Fig. 2. SEM micrograph of a 5-µm-thick, 5-µm-wide, and 200-µm-long free-standing cross fabricatedby porous silicon etching.

n-epi.
rt-diffn-sub.
(a)

(b )

(c)

(d)

Fig. 3. Process flow for free-standing microstructures fabricated using a selectively diffused nfn+/n
silicon substrate. (a) Formation of a selectively diffused nfn+/n multilayer, (b) definition of three­dimensional structures by reactive ion etching (RIE) or wet chemical etching, (c) selective conversionof an n+ layer to a porous silicon layer, and (d) removal of a porous silicon layer in a weak NaOH
solution to form free-standing structures.

Sensors and Materials, Vol. 11, No. 8 (1999)

483

(a)

(b)

Fig. 4. SEM micro graphs of cantilevers fabricated by selective formation and the etching of porous
silicon. (a) Free-standing cantilevers with various widths and (b) close-up of a connecting part
between a cantilever and the substrate.
edge between the n+-diffusion region and the n-substrate. Since the n+-region was
selectively diffused on the n-silicon substrate, the porous silicon was formed only in the n+­region through an anodic reaction, as shown in Fig. 4(b), while the lightly doped epitaxial
silicon and substrate provided etch stops. The undesirable undercutting phenomenon of
the microstructure shown in Fig. 2 was, therefore, not observed in the nfn+/n structure witha buried n+ layer. Consequently, we were able to precisely control the dimensions of the
microstructures by defining the n+-diffusion region and patterning the n-epitaxial layer.
For this reason, porous silicon micromachining was used in the fabrication of the micro­
structure of an accelerometer, as discussed below.

Sensors and Materials, Vol. 11, No. 8 (1999)

484

3.

Fabrication of Piezoresistive Accelerometer

3.l Experimental

The detailed procedure for the fabrication of a silicon accelerometer is shown in Fig. 5.
(111)-Oriented, 5-10 Qcm, n-type silicon wafers were used as starting materials. A 0.8µm-thick silicon dioxide masking layer was thermally grown and patterned. Then a 20µm-thick n +-region, which served as a sacrificial layer and provided an air gap, wasselectively formed by deep phosphorous diffusion. The resultant doping density of the n+­buried layer, confirmed by a spreading resistance measurement, was above 1 x l 0 17/cm3,
which was high enough to be anodizable in a HF solution. A low-doped n-type epitaxial
layer with an As doping density of 5 xl 0 15/cm3, which is used as a movable microstructure,
was then deposited on the substrate (Fig. 5(a)).
Another thin surface oxide was thermally grown on a low-doped n-epitaxial layer. The
piezoresistors were formed by boron ion implantation with a dose of 2.7 x 10 14 cm-2 at40
keV through a thin surface oxide which was followed by annealing at 1,070'C for 70 min
in N2 atmosphere (Fig. 5(b)). The measured junction depth and sheet resistance of thepiezoresistors were about I µm and 240 Q/0, respectively. After 1,000-A-thick passiva­tion oxide and 1,500-A-thick stress-relief nitride were both deposited, the back of the wafer
was heavily implanted with phosphorus to ensure a uniform current density during
anodization (Fig. 5(c)).
A 1,000-A-thick Ni-Cr glue layer and 6,000-A-thick Au layer were sequentiallydeposited by e-beam evaporation and then patterned by wet etching as the masking layer
for anodization. In order to provide a pass for anodization, the uncovered parts of thenitride and oxide layers were removed by plasma and wet chemical etching, respectively,
and the n-epitaxial silicon layer was etched to a depth of 8 µm using reactive ion etching(RIE). A 1-µm-thick aluminum back ohmic contact was deposited by e-beam evaporation(Fig. 5(d)).
The anodic reaction was performed in a 10% aqueous HF solution for 25 min at room
temperature by applying a constant current density of 15 mA/cm2 to the wafer. The porous
silicon layer was then etched away in a 5 wt% NaOH solution. After the Au/Ni-Cr masking
layer was removed, contacts for the piezoresistors were made by etching the nitride andoxide through contact windows. A new Au (6,000A)/Ni-Cr (1,000A) layer was depositedand patterned to form both the resistor pads and the mass paddle. The wafers were thenannealed at 330'C in nitrogen ambient for 30 min to reduce the contact resistance of thepiezoresistors and to provide good adhesion to the surface of the silicon paddle (Fig. 5(e), (f)).
Finally, the solder mass was loaded on the mass paddle by dispensing a Pb/Sn/Ag(Pb:Sn:Ag=35:63:2) solder pastec 14i and the die was then heated using a reflow soldering
process to form a solder ball on top of the Au/Ni-Cr mass paddle (Fig. 5(g)). The
completed accelerometer consisted of a loaded mass at the center of the structure, eightbeams supporting the mass, eight piezoresistors located on the beams, and eight fixedresistors outside the beams. The radius of the mass paddle (the region suspended by theeight beams in full symmetry) was 500 µm. The length, width, and thickness of the beams
were 350 µm, 100 µm, and 5 µm, respectively. The air gap between the microstructure andsubstrate was 20 µm and the overall size of the chip was 3.7 mm x 3.7 mm.

485

Sensors and Materials, Vol. 11, No. 8 (1999)
n ·- ee1

C

J

n'

D

sub.

(a)

Boron Implant

I

C

I

n - ee1.

n+

D

C

n
n+

t

F

r

eel.

n+

sub.

(d)

IEJE:11
PSL

D

rJ
I

sub.

j

Implaol ""'"

Buffer oxide

Phosphorus
implant

3

�

- Au
Ni-Cr

Al

3

(e)
��v�r

PR

__ Nitride

sub.

(c)
IElBI
D

]

sub.

(b)
D

t

I

j

I····· ••••••••••£•••••••••••••• •••• I

7

_ Au
Ni-Cr

,::c,

n sub.

(f)7

Mass(Pb/Sn/Ag),QOO§/fn sub.

(g)

J

Fig. 5. Fabrication process of the silicon accelerometer.

486

Sensors and Materials, Vol. 11, No. 8 (1999)

3.2 Results and discussion
Surface and cross-sectional SEM views of the fabricated microstructure of the acceler­ometer are shown in Fig. 6. Since the anodic reaction did not proceed to the lightly dopedn-type substrate, the reaction stopped automatically after the complete conversion of then+-region to porous silicon. The resultant microstructure shown in Fig. 6(b) was thus welldefined and uniform with no cusp or any evidence of the undercutting phenomenon. This
indicates that the beam thickness and the air gap height of the microstructure can be
precisely adjusted by controlling the thicknesses of the n-epitaxial layer and n+-diffusedlayer, respectively.

(a)

(b)

Fig. 6. Surface and cross-sectional SEM views of the 8-beam microstructure: (a) x 50 and (b) x 500.

487

Sensors and Materials, Vol. 11, No. 8 (1999)

It is known that beams are mainly formed in the <110> direction on a (100) substratesince piezoresistive coefficients are maximum in the <110> direction. Thus, in the case ofa (100) plane, p-type resistors must be carefully placed along a <110> direction. On theother hand, the positioning of p-type resistors on a (111) plane is not critical because
piezoresistive coefficients for longitudinal as well as for transverse stresses are the circularsymmetry of all the crystal directions in this plane.c 15J
Figure 7 shows a (100) and a (111) silicon wafers that have various cleavage planes
cutting a crystal.C 16l When we break or cleave a silicon wafer, it will cleave along the <110>direction between (111) planes, separating or breaking (110) bonds. In the case of the(100) silicon slice there are two cleavage planes 90 degrees apart, whereas in the case of the(111) slice there are three cleavage planes 120 degrees apart. Figure 8 shows theaccelerometers with 4 beams on a (100) substrate and 8 beams on a (111) substrate.
Usually, stress is concentrated at the edges of beams when force is applied to an accelerom­eter. In the case of the (100) plane, the edges of the beams are parallel to the <110>cleavage direction and thus the beams can be easily broken at the edges. However, in thecase of the (111) plane, we can see that the edges of the beams are not parallel to the <110>cleavage direction since the beams can be placed in all the crystal directions; hence, thebeams will not fracture easily. Consequently, microstructures formed on a (111) substrate
are mechanically stronger than those fabricated on a (100) substrate because the fracture
strength can be stronger according to the geometry of the beams on the wafer.

(a)

I

(110)

(b)

I

(101)
Fig. 7. The (100) and (111) silicon wafers that have various cleavage planes cutting a crystal. (a)Natural cleavage of (100) substrate and (b) natural cleavage of (111) substrate.

488

Sensors and Materials, Vol. 11, No. 8 (1999)
beam edge(a)---,-(110)

I
(110)

(b)

(101)
Fig. 8. The accelerometers with 4 beams on a (100) substrate and 8 beams on a (111) substrate. (a)4-beam accelerometer on a (100) substrate and (b) 8-beam accelerometer on a (111) substrate.

Figure 9 shows a SEM image of the fabiicated accelerometer. A hemispherical solderball with a mass of about 2 mg was formed on the top of the Au/Ni-Cr mass paddle. The
method of mass loading suggested in this study offers another advantage that various
masses of shape or weight can be formed on the same silicon chip simply by changing theshape of the mass paddle or the dispensing rate of the solder. It is, therefore, possible tofabricate silicon accelerometers with a customized mass according to acceleration from
low to high range because the mass is loaded after the other device fabrication processes
are completed.
The output response of the fabricated devices has been measured by summing the four
outputs of the four half-bridges, which consisted of two piezoresistors located on the beams
of opposite sides and two fixed resistors outside the beams. Figure 10 shows a summing
circuit for signal conditioning of the fabricated accelerometer. A constant 5 V was applied
acnlss the bridge, and the voltage differences between each bridge were amplified at thefirst-stage differential amplifier with a gain of 200. The outputs of the four bridges (inputsof the summing circuit) were then added at the second stage summing circuit with a unity gain.
The characteristics of the fabricated devices were evaluated with a calibration systemusing a high-precision reference accelerometer (B&K 4366). The fabricated accelerometerand the reference accelerometer were both placed on the same vibration exciter and theoutput responses of the sensors were measured using a FFT spectrum analyzer.

489

Sensors and Materials, Vol. 11, No. 8 (1999)

Fig. 9. SEM image of the accelerometer with the loaded seismic mass.

,------------7

A

I
I
I

I
I
I

I
I
I

I
I
I

I

I

I

S1

I
I
I

S2

I
I
I

S3

S4

I
I

SensorBridgeI
I

I

D4,__L.JW--__J

Selection
SW

I

I

L ____________ _J

Summing Circuit

L _____ _J

BridgeAmp

Fig. 10. Block diagram of the electronic circuitry with a summing circuit for signal conditioning of
the accelerometer.

490

Sensors and Materials, Vol. 11, No. 8 (1999)

The measured result in Fig. 11 shows the output of the accelerometer sinusoidally
excited with 10 G at 50 Hz. No distortion was observed in the output signal of the device,similar to the reference sensor. Figure 12 shows a linear characteristic for the device in theacceleration range from 0 to 10 G. The output deviation of each bridge was ±4%, possibly
due to a misalignment between the two centers of the mass and the mass paddle, andvariations in the piezoresistors. The sensitivity measured through the summing circuit was
1.21 mV/G and the nonlinearity of the sensor was less than 2% in our measurement range.
When our sensor is compared with a two- or four-beam bridge-type accelerometer reported
by others, the sensitivity of our device is decreased due to the additional beams. For
example, the output voltage of one half-bridge of an eight-beam accelerometer is one­quarter of that of a two-beam sensor. If the four output signals of the four half-bridges aretotalled through the summing circuit in Fig. 10, the total output will be equal to that of thetwo-beam acceleromeler. Therefore, the lowering of the sensitivity due to the eight beams
can be simply compensated by summing the four output signals of the four half-bridges
through a summing circuit. Furthermore, the eight-beam structure provides a lower cross­axis sensitivity than two- or four-beam structures and is mechanically more stable due to alower shear stress. In general, the sensitivity of a typical piezoresistive accelerometer for
airbag applications is obtained between a few hundred µV/G arid several mV/G. There­
fore, we can estimate that the obtained sensitivity_ of this accelerometer (1.21 mV/G) issufficient for airbag applications.
The cross-axis sensitivity measured through the summing circuit was found to be lessthan 3.5% in all eight beam directions on the surface. It is believed that this was caused by
upward shifting of the center of mass along the z axis due to a one-side loaded mass. Whenthe vibrating magnitude was approximately 1 G, the excitation frequency varied from 10
Hz to 3.2 kHz, as shown in Fig. 13. The measured first resonant frequency was about 2.82
kHz, a 6% lower value than the designed frequency. It was found that an error in the loaded
mass is the major source of discrepancy between the estimated and the measured resonant
frequency.
(a) Reference Sensor

Fig. 11. Output signal to sine excitation with an amplitude of 10 G at 50 Hz.

Sensors and Materials, Vol. 11, No. 8 (1999)

491

3.0-r.======:;----------,
2.5

D
o
b.
'1
•

: Bridge I: Bridge 2: Bridge 3: Bridge 4: Sum of four bridges

> 2.0
1.5

-s
a.. 1.0
0.5 -1---------

l.l l.l16-H�E=-<r-,�-=--�

0.0...f-l'o::;-.........,........,-.-,-.-,c--r--,c--r--,,-,-,-,-....-....-.-1

0 1 2 3 4 5 6 7 8 9 10 11 12

Acceleration [ G ]Fig. 12. Linearity of the accelerometer response for the acceleration range Oto 10 G and a voltage
amplification gain of 200.

-10 ···· ·············· ·····-······································ ·· ·····

-30

o.ot

· ·· ·· ········· ······· ·-·················· ·· · ······· ··· ··· ···· ··· ··············· ······ ·· ···· ·····
0.51(
I.Ok
2.(1(
2.5k
3. OI:
1.5k

Frequency [kHz]Fig. 13. Output response as a function of the excitation fre quency as measured at a constantexcitation of 1 G.

Sensors and Materials, Vol. 11, No. 8 (1999)

492

4.

Conclusions

We have shown that the porous silicon etching method has made it possible to precisely
define the dimensions of the beam and air gap in a microstructure by controlling the
thickness of the n-epitaxial and n+-diffusion layers on a mechanically strong (111)-oriented
nfn+/n structure. We are convinced that this micromachining technique provides a meansof fabricating monocrystalline silicon microstructures for micromechanical applications.
A bridge-type piezoresistive accelerometer with eight beams fabricated using selective
porous silicon micromachining and mass loading methods has also been presented. The
mechanical strength of an accelerometer can be greatly improved by adopting an eight­
beam structure. The lower sensitivity of this device, compared with two- or four-beambridge-type accelerometers, can be easily overcome by summing the four bridge output'
signals through a summing circuit.

Acknowledgments
The authors would like to express their thanks to Dong-Ki Kim, Dong-Kwon Kim and
Jae-Seung Lee for their help in developing and characterizing the accelerometer. This
work was supported by the Sensor Technology Research Center (STRC).

References
1 J.C.Greenwood: J. Phys. E; Sci. Instrum. 21 (1988) 1114.
2 J.M.Giachino: Sensors and Actuators 10 (1986) 239.
3 T. Suminto: Proc. 7th Int. Conf.Solid-State Sensors and Actuators (Transducers '91) (IEEE,San Francisco, CA, 1991) p. 104.
4 H. Seidel, H. Riedel, R. Kolbeck, G. Miick, W. Kupke and M.Koniger: Sensors and Actuators
A 21-23 (1990) 312.
5 W. Yun, R. T. Howe and P.R. Gray: Tech. Digest, IEEE Solid-State Sensors and Actuator
Workshop, (IEEE, Hilton Head Island, SC, 1992) p. 21.
6 M.E. Motamedi: IEEE Trans. Ultrason. Ferroelec. Freq. Contr. UFFC-34 (1987) 237.
7 E. Bassous: IEEE Trans.Electron Devices ED-25 (1978) 1178.
8 R. T. Howe: J.Vac. Sci.Technol.B 6 (1988) 1809.
9 M. J. J. Theunissen: J.Electrochem. Soc. 119 (1972) 351.
10 X. Z.Tu: J.Electrochem. Soc.135 (1988) 2105.
11 C. J. M. Eijkel, J. Branebjerg, M. Elwenspoek and F. C. M. van de Pol: Electron Dev. Lett. 11(1990) 588.
12 W. Lang, P. Steiner and H.Sandmaier: Sensors and Actuators A 51 (1995) 31.
13 R.P. Holmstrom and J. Y. Chi: Appl. Phys. Lett.42 (1983) 386.
14 N.Peteron: Proc.NEPCON/West '86, (Reed Exhibition Companies, Anaheim, CA, 1986) p.
619.
15 Y.Kanda: IEEE Trans.Electron Devices ED-29 (1982) 64.
16 G. E. McGuire: Semiconductor Materials and Process Technology Handbook (Noyes
Publications, New Jersey, 1988) Chap. 4.