1.   
Introduction

A pressure sensor is a device that measures the pressure of gases or liquids.
A pressure sensor usually acts as a transducer;
it generates a signal as a function of the pressure imposed.

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Pressure sensors are not just used for
pressure measurement only but they are also used for control and monitoring in
many applications. Pressure sensors are also called pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers and manometers.

Pressure sensors
can differ in technology, design, performance and cost. As estimated, there are
possibly 50 different technologies and at least 300 companies fabricating pressure
sensors worldwide 3. The types of pressure sensors include Piezoresistive
strain gauge, Capacitive, Electromagnetic, Piezoelectric, Optical and Potentiometric. Pressure sensors can
also be used to indirectly measure other variables such as fluid/gas flow,
speed, water level, and altitude.

 

       Fig. 1.
Piezoelectric pressure sensor                                                       Fig.
2. Digital pressure sensor

Fig. 3. Capacitive pressure sensor

2.   
Analysis of
the Problem

High temperature pressure
sensors have been widely applied in modern industry. These applications include
automotive, aerospace, and energy industries. In most cases, piezoeresistive
and capacitive pressure sensors are used nowadays in high temperature implementations.
For example, in the aerospace industry, the pressure sensors are required to be
placed directly nearby the heat source and in such conditions, the sensors
operate at temperature up to 400 °C in gas ambiance, e.g. oxygen, nitrogen
oxide, sulfur oxide and many others 3.

In the automotive industry,
high temperature sensors are usually placed near the engine to record the
temperature, pressure and vibrations; These recordings are very beneficial so
that improvements to the efficiency and reliability of the engine system could
be made later on 1. Moreover, some pressure sensors are placed near the wheel
to monitor the tire pressure, these sensors are also exposed to high
temperature which would reach approximately 300°C.

These factors listed above
represent typical harsh environments in which sensors must operate efficiently and
without the risks of being damaged. Therefore, the packaging materials and
basic components including substrates, metallization materials, electrical
connections (such as wire bonds), die-attach and hermetic sealing must be able
to withstand an environment that is in the region of 400°C. Silicon pressure sensors based on piezoresistive and
capacitive sensing schemes have been already developed 1,3. However, silicon
structures suffer from severe mechanical performance degradation above 400°C and, thus, are inadequate for building reliable
high-temperature sensors.

 

3.      State of the Art

As discussed above, the Si structures suffer from degradation at high
temperatures, so therefore, researchers and scientists have proposed during
these past few years many technologies for high temperature sensors. These
innovations include single crystal 3C-SiC capacitive pressure sensors that are
able to support applications in the region of 400°C.
 Additionally, another state of the
art innovation centered around SOI pressure sensor using a Cu-Sn wafer level
bonding. Furthermore, there is a
possibility to increase the temperature resistance of pressure sensors by
implementing a thermostable electrode using SOI wafer. These three solution
will therefore be discussed in the section below.

3.1 High-temperature pressure sensor using single crystal
3C-SiC

Silicon carbide (SiC) is an inviting material
for high-temperature applications due to the fact that it is represents a
mechanical robust element, also due to its chemical inertness, and electrical
stability at elevated temperatures 5. Usually, 3C-SiC high temprerature
pressure sensors are developped using capacitive pressure sensing technology. These
devices can actually withstand high temperature applications. Besides, SiC is chosen mainly because it is relatively easy and non-expensive to fabricate
inside facilities. So therefore, the pressure sensor will be widely available
in the market with a very reasonable price.

Three distinct CVD
(chemical vapor deposition) processes used to produce SiC thin films. First we
have the atmospheric pressure CVD (APCVD) using a reactor. This reactor
utilizes RF-induction to heat silicon wafers. Then we have the production of polycrystalline SiC
(poly-SiC) films which is performed by low pressure CVD (LPCVD) using a conventional
horizontal tube furnace. Amorphous SiC films are deposited by conventional plasma enhanced CVD (PECVD).

3.1.1       3C-SiC architecture
and operating principle

In this section, everything related to the
fabrication process, materials and components used to deliver high temperature
pressure sensors will be discussed.

Fig. 4. SiC pressure sensor

Fig. 4 demonstrates the final
configuration of the pressure sensor. In this figure, it is shown that when
applying an external pressure, the SiC diaphragm deflects over a sealed cavity,
on a silicon substrate. This bend or deflection, has a unique effect, it
increases the capacitance value between diaphragm and substrate. And the
relationship between pressure and capacitance increases linearly when the SiC
diaphragm touches the silicon substrate at touch point pressure (PT)
5. And Fig. 5, clearly shows the capacitance vs pressure.

Fig. 5. Pressure sensor characteristic response

By modifying the diaphragm thickness and radius,
one can achieve different cavity depth and touch point pressure that will lead
to a change in the capacitance value. This can be proven by using the formula
below:

PT =  + 0.488() + 4?()                                                                             (1)

Where, r:
diaphragm radius, t: diaphragm thickness, g: cavity depth, D: diaphragm
flexural rigidityand ?: residual stress.

Fig. 6. Sensor fabrication. (a) Recession
formation. (b) 3C-SiC growth and PSG. (c) wafer bonding. (d) diaphragm
formation. (e) contact metallization.

Fig. 6 shows the fabrication process for the
discussed capacitive pressure sensor. A 4 inch N-type silicon wafer is etched by a
reactive ion etch (RIE) process to form a 2µm recess, then there
is a deposition of 2500 Å
phosphorus silicate glass (PSG) as an
insulation layer, Fig. 6 (a) 5. Next, a 0.5µm single-crystal 3C-SiC is grown on the surface of another 4 inch N-type silicon
wafer using the APCVD technique which was briefly discussed earlier. The SiC surface is
then polished by a chemical mechanical polishing (CMP) to deal with the surface
roughness and defects. According to Fig. 6 (b), 2500 Å of PSG are then placed on top of the SiC
surface. This PSG layer serves as a coating layer and is also essential in the
wafer bonding step. The wafers are then bonded together at a
pressure of approximately
48Kpa, as shown in Fig. 6 (c). A high-temperature
annealing step at 1000°C for 2 h is then performed to enhance the bonding quality 5. Moreover,
the silicon substrate which is on the top layer is removed to form a 0.5µm-thick SiC diaphragm according to Fig. 6 (d). Thus, due
to the differential pressure, the diaphragm bends towards the substrate. Finally,
according to Fig. 6 (e), a 5000 Å of nickel
layer is introduced on the top and bottom sides of the
wafer using 100 Å of titanium that acts as a fixing agent in this case. Eventually, gold wire bonding is
implemented and high-temperature silver epoxy is applied to establish
electrical contacts to the top and bottom electrodes of the sensor 5.

To test the temperature resitivity of this
sensor, we attach it to a ceramic substrate by high temperature epoxy with gold
wire connections between all parts of the circuit.

      Fig. 7. Capacitance vs External Pressure

The 3C-SiC capacitive pressure sensor have
demonstrated sensing capabilities
up to 400ºC according to Fig. 7, suitable for various
high-temperature applications 5.

3.2 .
High Temperature Pressure Sensor using Thermostable Electrode

The sensor
discussed in this section, is based on the piezoresistive pressure sensor. Based on SOI (silicon on insulator) wafers, piezoresistive pressure sensors are
more preferred nowadays due to their simple structures compared
with the capacitive pressure sensors.

High
temperature piezoresistive pressure sensor is based on piezoresistive effect. The
silicon diaphragm will bend out of shape when a pressure is applied to the
diaphragm. Therefore, this pressure will cause resistance change. The
relationship between stress and resistance change is given by:

 =   = ?l*?l + ?t*?t                                                                                                                    (2)

where R: resistance, ?: resistivity, ?l:
longitudinal stress, ?t: transverse
stress, ?l: longitudinal piezoresistive coefficient, ?t: transverse piezoresistive coefficient. is

Moreover, it is based on SOI wafer which has
many advantages such as high temperature resitiance and low cost. The operation of the SOI pressure sensor counts greatly on the thermal
stability of ohmic contact electrode 3. However, conventional ohmic contact electrodes such as
Al, Cr/Au, Ti/Au,
Ti/Pt/Au might show some bad signs at elevated temperatures due to the fact that
some of these metals may spread into silicon substrate 3.

To clear up
this issue, scientists have recently proposed TiSi2/Ti/TiN/Pt/Au
multilayer electrode. So eventually, the high temperature pressure sensor was
developped using this multilayer electrode.

3.2.1.   
Electrode
characteristics

The electrode
discussed in this section is formed using different materials put on top of
each others by lift-off process. These layers
include Ti, TiN, Pt and Au.

Fig. 8. Multilayer electrode

 

 

 

3.2.2. Sensor
fabrication

Acoording to Fig. 9.(a), a 4-inch SOI wafer is
used to fabricate the piezoresistive pressure sensor. 400µm is the thickness of the wafer
itself. The second step is to pattern the device layer about 0.34µm (which is
the top layer of the wafer) according to Fig. 9.(b) 3. Moreover, as indicated
in Fig.9.(c), the device layer is doped so as to form piezoresistives. The
passive layers SiO2/Si3N4 are deposited by
LPCVD 3.

According to Fig.9.(e), TiSi2-Si
ohmic contact is fabricated using self-aligned silicide process.
The Ti/TiN/Pt/Au multilayer films are sputtered and patterned using lift-off to form
electrodes and metal interconnects as seen in Fig.9.(f) 3. So basically, these comopnents
introduced in this step (Ti, TiN, Pt, Au) represent the multilayer electrode
which was discussed earlier. To protect the wafer from etching, it is required
to coat the top side with ProTEK B3. As indicated in Fig.9.(g) SiO2/Si3N4 layers on the bottom side of
the wafer are then removed using reactive ion etching. Furthermore, tetramethylammonium hydroxide (TMAH)
solution is applied to fabricate silicon diaphragm and cavity (which is the
curvature seen in Fig. 9.(g)). Finally, The SOI wafer is anodically bonded to a glass wafer in an EVG wafer bonder,
this can be seen in Fig.9.(h). The strain point of the glass wafer is 669ºC and the thermal miss-match between the silicon wafer and
glass wafer is small 3.

Fig. 9. Sensor fabrication process. (a) 4” SOI
wafer. (b) device layer of SOI wafer patterned. (c) device layer doped to form
piezoresistors. (d) SiO2/Si3N4 deposited by LPCVD. (e) TiSi2-Si ohmic contact added. (f) Ti, TiN, Pt and Au films are sputtered and
patterned using lift-off. (g) wafer etched in TMAH solution. (h) SOI-glass
bonding.

According to measurements made on this specific
sensor, the pressure range
is 30-150kPa and the temperature range is from 25 ºC
to 500ºC. According to Fig.
10, the nonlinearity error is quite negligeable (0.1%FS) and the sensitivity is
also quite small (0.24mV/kPa). And in Fig. 11, we can see the pressure sensor
packaged and ready for usage. This figure also shows, the gold wiring, Kovar base,
pins of the pressure sensor and the pressure sensor chip itself.

                                                                

          Fig. 10.
Test Results                                                                                  Fig.
11. Pressure sensor

 

3.3. High Temperature Pressure Sensor
using Cu-Sn wafer level packaging

A 4-inch double-side polished SOI wafer is used
to fabricate the piezoresistive pressure sensor and in between one can notice the
presence of buried oxide. Firstly, the top
layer of the wafer is patterned and heavily doped to form pressure sensing resistors
(Fig. 12b,c). After that, SiO2/Si3N4 layers are placed by low pressure
chemical vapor deposition (LPCVD). TiSi2-Si ohmic contact is fabricated using selfaligned
silicide process (Fig. 12e). To form electrodes and metal interconnects, the Ti/TiN/Pt/Au
multilayer films are then introduced using lift-off process 4. Moreover, ProTEK
B3 primer coating is used on the top side of the wafer to protect the metals.
Furthermore, according to Fig. 12g, the pressure sensor diaphragm and cavity
are achieved using a special solution that dissolves the layers on the bottom
side of the wafer.

Then, a Ti adhesion layer (300Å) and Cu seed
layer (2000Å) are introduced on the bottom part of the wafer (Fig. 12h). After that, a special process is employed to define the pattern
for Cu sealing ring (Fig. 12i) 4.

Finally, a 6.5?m-thick Cu layer is electroplated on the seed layer (Fig. 12j). At last, the photoresist pattern
is thus removed (Fig. 12k).

Fig. 12. Sensor chip (top wafer)

 

Fig. 13 shows the
substrate wafer fabrication process. A 4-inch double-side polished silicon wafer is used to fabricate the substrate.

Fig. 13. Substrate wafer (bottom wafer)

As for the substrate
wafer, first of all, a Ti adhesion layer (300Å) and Cu seed layer (2000Å) are
layered on the wafer (Fig. 13m). Next, a photoresist film (~16?m) is added on the top side (Fig. 13n). Then, as seen in Fig. 13o and fig. 13p, Cu and Sn are added
simultaneously in order to avoid oxidization. After removing the photoresist
layer and also the Ti/Cu layers, the substrate wafer is acquired (Fig. 13p).

Then, the cap wafer and
substrate wafer are aligned and brought into a EVG wafer bonder.
The temperature-time profile for Cu-Sn TLP bonding process is shown in Fig. 14. 4

Fig. 14. Temperature-time profile for Cu-Sn
bonding process

 

Fig. 14 shows that
the two wafers are heated to 160ºC for 5min time to remove humidity. Then, an increase in temperature from 160ºC to 250ºC
(above the melting point of
Sn) for 5min to ensure sufficient infiltration. Next, the bonding process is carried out for 30min with a
temperature of 350ºC and 500N force. Thus, Cu-Sn compound will be formed. After
that, the temperature is reduced to 25ºC again.

Fig. 15. Packaged pressure sensor

Table 1: Pressure sensor performance

Temperature
( ºC)

25

300

Pressure
Range (kPa)

30-150

30-150

Supply
Voltage (V)

5

5

Full
scale output (mV)

68.5

42.1

Sensitivity
(mV/kPa)

0.46

0.28

Nonlinearity
error

0.1

0.11

 

3.4. Comparison,
Advantages/Disadvantages

Complicated wafer-level processes have been
extensively used in both pressure sensor device fabrication and in pressure sensor wafer level packaging. Wafer to wafer bonding has many
advantages and disadvantages. Basically all three technologies discussed
in  this section represent wafer to wafer
bonding.

Table 2: Advantages and Disadvantages of wafer to
wafer bonding used for all technologies discussed previously

Advantages

Disadvantages

process complexity lower

additional wafer needed

device area
can vary in a wide range

thinning of
the bonded stack required to reduce chip heigh

mechanically robust

to allow
flip-chip bonding vertical vias must be created

controlled
atmosphere and pressure inside the cavity

sealing can
impact device performance

 

Table 3: Comparisons

3C-SiC

Thermostable
electrode

Cu-Sn

Low cost

Low cost

Low cost

Relatively old technology

Relatively new technology

Relatively new technology

Silicon-Silicon (Direct) bonding

Anodic bonding

Transient liquid phase bonding

Capacitive pressure sensing technology

Piezoresistive sensing technology

Piezoresistive sensing technology

High temperature, High pressure resistant up to
350-400°C.

High temperature, High pressure resistant up to 400°C

High temperature, High pressure resistant up to 300°C

 

Direct bonding:

Process
temperature

x

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