Towards the Sensors Implementation and Integration Using the CMOS MEMS Platform
National Tsing Hua University, Taiwan*Weileun Fang, Chih-Ming Sun

Abstract -This study presents a novel double-side CMOS post-process to monolithically integrate various capacitance type CMOS MEMS sensors on asingle chip. As a result, the monolithic integration of (1) accelerometers in three different axes, (2) pressure sensors, and (3) Pt temperature sensor, aredemonstrated. The measurement results of the pressure sensors show the sensitivity is 2.7mV/kPa. The 3-axes accelerometer has the sensitivity of
0.54mV/G in X-axis, 0.32mV/G in Y-axis, and 0.19mV/G in Z-axis. The sensitivity of temperature senor is 1.74mV/oC. In summary, this study
demonstrates a SoC approach for sensors integration based on standard CMOS process.
Keyword: CMOS post-process, pressure sensor, accelerometer, sensors integration.
difference on both sides of the diaphragm will lead to the deformation of

1. INTRODUCTION

the membrane, so as to change the gap and the capacitance between theThe CMOS MEMS process has the advantage of monolithic integration

sensing electrodes.

Fig.1c shows the thermal-resist type temperatureof the IC and micro mechanical components. In addition, the maturesensor. In this study, the Pt film will be deposited and patterned as theCMOS fabrication processes are available in many IC foundries. Thus, thethermal-resist type temperature sensor at the post-CMOS process. Thus,

CMOS-based micro fabrication technology provides a promising approach

the temperature change will be detected by the variation of the Pt

to implement MEMS sensors. Presently, various CMOS-based MEMS

resistance.

sensors have been reported, for instance, the inertial sensors [1], chemicalgas sensors [2], microphones [3], and pressure sensors [4].

3. CMOS MEMS DOUBLE-SIDE POST PROCESSING

Although there are many available micro sensors implemented using theFig.2 shows the process flow to integrate sensors. The chip was firstly

CMOS MEMS processes [5], it is difficult to monolithically integrate thesefabricated using the TSMC 0.35m 2P4M CMOS process (Fig.2a).

CMOS MEMS sensors since they have different post-processes. It wouldFigs.2b-g illustrate the presented low-temperature CMOS post-process. In

be useful to establish a generic post-process platform to simultaneouslyFig.2b, the 1500 A platinum layer was deposited onto the standard CMOS

implement various CMOS MEMS sensors, so as to further realize thelayer by E-gun, and then the shape of Pt thermal sensor was patterned bymonolithic integration of sensors.

lift-off. In Fig.2c, the backside silicon substrate was etched by DRIE to2. CONCEPT OF SENSORS DESIGN

expose the dielectric and metal layers. In Fig.2d, the liquid spin-on-glasswas dispensed to protect Pt layer during the following metal wet etching.

Fig.1 shows the design of the monolithic CMOS-MEMS chip containing
inertia, pressure, and temperature sensors. Fig.1a shows the 3-axis
accelerometer consisting of one proof-mass, two supporting frames, threesets of springs and gap-closing sensing electrodes. These three sets ofsprings are designed only flexible in one axis. A novel out-of-plane (Z-axis)
serpentine spring is designed, thus its out-of-plane stiffness is two orders
smaller than its in-plane stiffness. The coupling of sensing signals between
Z-axis and the in-plane axes is significantly suppressed. Fig.1b shows thedesign of capacitive pressure sensor. The pressure sensor consists ofdeformable sensing electrodes embedded in a diaphragm and stationary

After that, the metal and tungsten-via layers were etched by the H2SO4 andH2O2 solutions from both sides of the substrate to partially release theMEMS structures and also to define the out-of-plane sensing gaps forsensors, as in Fig.2e. In Fig.2f, the RIE anisotropic etching was thenemployed to remove the passivation and dielectric films from thefront-side of substrate. The top metal layer (M-4) acted as the etching mask
to define the planar shape of sensors. Thus, the movable components of the3-axis accelerometer were fully released from substrate. In Fig.2g, thesubstrate was further sealed by Pyrex 7740 glass at backside for pressure
sensor.

sensing electrodes embedded in a suspended structure. The pressure
Z-folder spring

Z-spring
Sensing circuit
Proof mass

Y-axisZ-axis

X-spring

X-axisM-4

X-axis sensing element
Y-axis sensing element
X-sensing electrodeM-1

Z-sensing electrode(b)

Y-sensing electrode Y-spring

Diaphragm and deformable electrode(c)

Pt-thermal couple

2.3 PmEtching hole
Stationary electrode

Single connect

Fig.1 Design concept of the integrated chip, (a) tri-axis accelerometer, (b) capacitive pressure sensor, and (c) Pt temperature sensor.

Passivation
M-4
M-3
M-2
M-1

Sensing circuit

TSMC 0.35Pm 2P4M
Pressure sensor(b)

Tri-axis accelerometer(b)

DRIE holeFig.4 The fabricated chip, (a) after backside DRIE Si etching, (b) bonding
with Pyrex glass, and (c) wire bonding and packaged on ceramic housing.

(a)Protect SiO2(d)GPowersupplyChip

Function
Generator

Shaker

3.5
3.0
2.5
2.0
1.5
1.0
0.5

Spectrum

Wet etching hole

(e)

(c)

Glass

1500A Pt

Temperature sensor

(c)

(a)

'Vout (mV)

(a)

01

2

345

Acceleration (G)Wet etching holePowersupplyChip

Function
Generator

Chamber(g)'Vout (mV)

(b)
(f)

X-axisY-axisZ-axisSpectrum60
50403020
100'V vs relative pressure

05101520Pressure (KPa)(c)Si

SiO2AlViaPt

Passivation

V_inChipHot plane
Fig.2 Fabrication process steps for Sensors integration on a single chip

4. RESULTS

140120100
8060
40
200'V vs Temperature'Vout (mV)

GlassMeter

20 40 60 80 100 120 140 160

Fig.3 shows a typical fabricated CMOS-MEMS sensors integrated

Temperature (0C)

device (2.5mm × 2.5mm), contains tri-axis accelerometer, pressure andtemperature sensor. The device after sealing with glass and wire bonding isshown in Fig.4. Fig.5 shows the experiment setup and the related
measurement results. The sensitivity of the tri-axis accelerometer in Fig.5aFig.5 Measurement setups and typical characterization results of thedifferent sensors, (a) tri-axis accelerometer, (b) pressure sensor, and (c)temperature sensor.

5. CONCLUSION

is 0.54mV/G (X-axis), 0.32mV/G (Y-axis), and 0.19mV/G (Z-axis),
respectively. Fig.5b-c shows the characterization of pressure andtemperature sensors. The sensitivities of pressure and temperature sensors
are 2.7mV/KPa and 1.74mV/ °C, respectively. Other detail specificationsof sensors are summarized in Table1. The measurement results
demonstrate the feasibility of the presented CMOS-MEMS device.

the capacitive-type tri-axis accelerometer, the capacitive-type pressuresensor, and the thermal-resist type Pt temperature sensor, on a single chip.
In addition, the signal processing ICs are also integrated in the same chip.
Table 1 Specifications of the presented integration sensor.

(a)

(b)
YZ
X
(c)

This study successfully demonstrates novel monolithic different sensors
using the CMOS MEMS processes. This CMOS-MEMS device consists ofPt thermal couple

G-sensor

P-sensor

T-sensor

Sensitivity

0.54 / 0.32 /0.19
X/Y/Z (mV/G)

2.7 mV/KPa

1.74 mV/°C

Non-linearity
Sensing range< 5%
0.8~5 G

4.58 %
0~20 KPa

4.29 %
25~150 °C

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pp.188-195.
[2] M. Graf, R. Jurischka, D. Barrettino, and A. Hierlemann, J. Micromech.

Pressure sensor

Microeng, 15, 2005, pp. 190-200.
[3] J. Neumann, and K. Gabriel, Sensors and Actuators A, 95, Issue 2-3,2002, pp. 175-182.

Fig.3 The SEM micrographs of, (a) the full chip, (b) tri-axis accelerometer,(c) Pt temperature sensor, and (d) pressure sensor.

[4] O. Paul, and H. Baltes, Sensors and Actuators A, 46/47, no.1-3, 1995,
pp. 143-146.
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