Oil-Filled Isolated High Pressure Sensor for High Temperature Application
ABSTRACT In order to solve pressure measurement problems in the fields of aerospace, petroleum and chemical industry, mobile and military industry, a oil-filled isolated piezoresistive high pressure sensor has been developed with the range of 0~100 MPa, and was able to work reliably under high temperature of above 200 C. Based on MEMS (Micro Electro-Mechanical System) and SIMOX (Separation by Implantation of Oxygen) technology, the piezoresistive sensor chip has been developed. By high temperature packaging process, the oil-filled isolated high pressure sensor was fabricated with the sensor chip and corrugated diaphragm. The experimental results showed that the oil-filled isolated high pressure sensor had good performances under high temperature of 200 C, such as linearity error of 0.07%FS, repeatability error of 0.04%FS, hysteresis error of 0.03%FS.
Oil-filled isolated high pressure sensor for high temperature application
Zhuangde Jiang1, 2, Libo Zhao1, Yulong Zhao1, Yuanhao Liu1, Philip D. Prewett2, Kyle Jiang2
1 State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University
No. 28 Xianning West Road, Xi’an, 710049, P. R. China
2 School of Mechanical Engineering, University of Birmingham
Edgbaston, Birmingham, B15 2TT, UK
Tel.: + 86  8266-8616 Fax: + 86  8266-8612 E-mail: firstname.lastname@example.org
In order to solve pressure measurement problems in the fields of aerospace, petroleum
and chemical industry, mobile and military industry, a oil-filled isolated piezoresistive high
pressure sensor has been developed with the range of 0~100 MPa, and was able to work
reliably under high temperature of above 200 C. Based on MEMS (Micro Electro-
Mechanical System) and SIMOX (Separation by Implantation of Oxygen) technology, the
piezoresistive sensor chip has been developed. By high temperature packaging process, the
oil-filled isolated high pressure sensor was fabricated with the sensor chip and corrugated
diaphragm. The experimental results showed that the oil-filled isolated high pressure sensor
had good performances under high temperature of 200 C, such as linearity error of 0.07%FS,
repeatability error of 0.04%FS, hysteresis error of 0.03%FS.
Keywords: Oil-filled, high temperature, high pressure sensor, chip, corrugated diaphragm
The pressure sensor based on piezoresistive theory plays an important role in the whole
pressure sensor world. Along with modern industry’s development, there is a wide need for
high temperature and high pressure sensor which is able to work stably under the temperature
of above 200 C in many fields, such as aerospace, petroleum and chemical industry, mobile
and military industry. But the conventional piezoresistive pressure sensor is just able to work
reliably under temperature up to 85 C, and its performances would be deteriorated because
the leakage current through the p-n junction increased as the temperature raised beyond
Some kinds of high temperature pressure sensor chips based on piezoresistive principle
such as SOI (Silicon on Insulator), SOS (Silicon on Sapphire), and sputtering film have been
developed. Based on SOS and sputtering film technology, the high temperature pressure
sensors’ elastic elements are fabricated by metallic materials, which long-term stability will
deteriorate because of creep characteristics of metallic materials. And based on SOI
technology [1, 2], the high temperature pressure sensor’s elastic and sensitive elements are all
silicon material, and its long-term stability is very good because of excellent mechanical
properties of silicon material. But, if the SOI pressure sensor’s sensitive chip contacts with the
measured fluid directly, this sensor can just be used to measure the pressure of fluids which
are compatible with stainless steel (1Cr18Ni9Ti and 316L, etc.), silicon, glass, Au or Al wires,
and other packaging materials, such as epoxy adhesive. In order to expand the pressure
sensor’s application scope, an oil-filled isolated high temperature and high pressure sensor
has been developed and produced in batch, and the sensor based on MEMS (Micro Electro-
Mechanical System) and SOI technology has good performances and is able to work reliably
under 200 C.
2. Design and fabrication of sensor chip
2.1 Mask layout design
The layout of sensor chip with the range of 0~100 MPa was design to silicon cup
structure with “C” type and its side length of 3 mm × 3 mm, so the sensor chip can be
simplified to peripheral clamped side membrane structure. In order to improve the sensor
chip’s sensitivity, the four piezoresistors R1~R4 were designed with four-fold structure and
were arranged at the centers of the sides and along with the crystal orientation  or 
in the (100) crystal plane, as shown in Fig. 1. The effective length and width of every resistor
was 400 μm and 10 μm, respectively. In order to obtain fine linearity for the senor chip, the
silicon material should be in elastic deformation when the maximum pressure applied on the
sensor chip. It is known that elastic limit value of Si material is about 80 MPa, the
corresponding strain value is about 500 με . On the basis of this design principle and the
sensor’s range of 0~100 MPa, the height and side length of effective side membrane of the
sensor chip can be calculated and simulated. Figure 2 was the FEM (Finite Element Method)
simulation result by the ANSYS software when the pressure value of 100 MPa was applied on
the sensor chip.
According to the elastic deformation theory and Fig. 2, we know that the two
piezoresistors mainly endure maximum compressive stress, and the other two piezoresistors
mainly endure maximum tensile stress. Based on the piezoresistive theory, the variable
resistances ratios of four piezoresistors are expressed as following:
Where πl, πt are longitudinal and transverse piezoresistive coefficients, respectively, and
σl, σt are longitudinal and transverse stresses, respectively.
The four piezoresistors with the same initial resistance value can compose a Wheatstone
full bridge in a five-terminal version, as shown in Fig. 3, and the series resistor RZ is used to
compensate the zero offset voltage .
Fig. 1. Mask layout of sensor chip. Fig. 2. Simulation result by FEM. Fig. 3. Wheatstone full bridge.
2.2 Fabrication process
The main fabrication processes [5, 6] of the sensor chip wre shown in Fig. 4.
The starting material is p-type (100) double-sided polished single-crystal SOI silicon
material which had been fabricated with SIMOX technology. The SOI silicon material was
obtained with the thickness of top Si layer and buried SiO2 layer of 200 nm and 400 nm,
respectively. In order to satisfies the need of piezoresistivity, the top silicon layer’s thickness
was increased to about 1.5 μm by LPCVD (Low Pressure Chemical Vapor Deposition)
technique, as shown in Fig. 4 (a).
In Fig. 4 (b), the implantation and diffusion of boron was performed from a B2O3
constant source into the top silicon layer, and then annealing and activation process was
carried out in N2 for 30 minutes at 1100 C in order to eliminate some crystal lattice defects
and improve the conducting power of the top silicon layer. The top measuring circuit silicon
layer was obtained with the boron concentration 2×1020/cm3. And SiO2 layers were formed
onto the top silicon layer and the bottom of substrate with the oxidation process in the O2 for
30 minutes at 1100 C.
Next, four piezoresistors with four-fold structure were fabricated by RIE (Reactive Ion
Etch) technology, as shown in Fig. 4 (c). The single piezoresistor’s width size was 10 μm and
the total effective length was 400 μm. The sheet resistance of the four piezoresistors was
10Ω/□. The piezoresistors were fabricated to rilievo-type, in other words, the piezoresistors
were embossed from the substrate.
In order to match internal stresses and etch the cavity, the Si3N4 layers were fabricated
onto the top and bottom of SiO2 layer by LPCVD technology, as shown in Fig. 4 (d).
And then, the resistor holes and cavity window were etched by the RIE or ICP
(Inductively Coupled Plasma), as shown in Fig. 4 (e). Afterward, the cavity like cup structure
was etched by wet etching with KOH solution to form the sensitive membrane according to
pressure measurement range of the sensor, as shown in Fg. 4 (f).
In Fig. 4 (g), In order to make sensor chip work under temperature of over 200 C, the Ti-
Pt-Au multi metal inner lead layers were grown by sputtering technology with the thickness
of 50 nm, 50 nm and 500 nm, respectively, which were used to connect four piezoresistors to
form the Wheatstone Bridge. Then, thermal treatment for ohms connection and metallization
was carried out. In Fig. 4 (h), the wafer was diced to dies by the scribing technology. Figure 5
showed the fabricated piezoresistive sensor chip in the top view.
(a) Epitaxy silicon by
(b) Implanting and
diffusing boron ion
(c) piezoresistors etched
(d) LPCVD Si3N4 in
(e) Etching resistor holes
and cavity windows
p (100) Si
(f) Etching the cavity
with KOH solution
(g) Ti-Pt-Au inner leads
by sputtering technology
(h) Metallization and diced
Fig. 4. Mainly fabrication process.
Fig. 5. photo of sensor chip in top view.
3. Fabrication of oil-filled isolated pressure sensor
By high temperature oil-filled packaging process, an oil-filled isolated high temperature
and high pressure sensor was fabricated with the developed sensors chip and corrugated
diaphragm . The detailed illustration of the mechanical structure for the oil-filled isolated
pressure sensor was shown in Fig. 6, and the pressure sensor was shown in Fig. 7.
1: Kovar alloy pin; 2: Glass slurry; 3: Oil-filled hole; 4: Base; 5:Metal ring;
6: Conical pin; 7: Corrugated diaphragm; 8: Gold wires; 9: Sensor chip;
10: Pyrex glass; 11: Insulation gasket; 12: Printed circuit board;
13: Silicone oil; 14:High temperature cable.
Fig. 6. Mechanical schematic drawing of the sensor.
Fig. 7. Sensor photo.
The 316L stainless steel was selected as the sensor base 4’s material. The Gilded kovar
allyou pin 1 and others were sintered in the base 4 by the glass slurry 2 at 950 C. The Pyrex
glass 10 bonded with the sensor chip 9 was packaged on the base 4 by silicone adhesive of
Dow Corning® 3145. By gold wire bonder or ultrasonic hot-pressing welding machine, the
gold wires 8 with width of 40 μm were welded on one end of kovar alloy pins 1 from the pads
of the sensor chip 9. Then, the corrugated diaphragm 7 with sinusoidal structure was welded
with the metal ring 5 and base 4 together by laser welding or electron beam welding
technology. The corrugated diaphragm 7's structure and size parameters are important to
improve the sensor's performances . After the gas in the silicon oil 13 was removed by
several heating cycles, the silicon oil 13 of Dow Corning® 550 was filled under vaccum
pressure and high temperature. In order to reduce the effect of silicon oil 13 on the sensor's
performance and meet the requirements of high temperature application, the quantity of
silicon oil 13 should be more little and need be controlled accurately. After the silicon oil 13
was filled with the calculated quantity, the conical pin 6 was pressed into the oil-filled hole 3,
then was welded by laser welding technology with the base 4 to avoid the leakage of siliocn
oil 13. In order to compensate the zero offset voltage, zero drift coefficient and sensitivity
drift coefficient for the sensor, several metal film resistors were soldered on the polyethylene
printed circuit board (PCB) 12 with high temperature soldering, and the high temperature
calbe 14 was also soldered on the PCB 12.
When the sensor was used to measure the pressure of fluid, the pressure acted on the
corrugated diaphragm 7 directly, and was passed to the sensor chip 9 through silicon oil 13.
The sensor chip 9 was powered with constant voltage, and would output the electrical signal
proportional to the pressure.
In order to eliminate the residual stress effect, some heat treatment was carried out.
Firstly, the sensor was placed under the high temperature of 180 C for 4 h, then cooled down
to -40 C for 2 h. This process should be performed for four cycles. It was necessary to
control the temperature changed moderately in the heating-up and cool-down processes in
order to prevent some small cracks occur .
4. Calibration experiments
The static calibration  for the sensor was carried out with 5V constant voltage
powered under 200 C. The test data were obtained as shown in the Table 1. Figure 8 showed
the output data curves. The static performance index were calculated with the least square
method, such as linearity error of 0.081%FS, repeatability error of 0.03%FS, hysteresis error
of 0.03%FS, and the sensor’s accuracy was 0.114%FS.
Table 1. Test Data under 200℃
Fig. 8. Curve of output data under 200C.
(1) (2) (1) (2)
100 109.45 109.45 109.44 109.44
The high temperature piezoresistive pressure sensor chip based on SIMOX technique had
been developed. With the sensor chip, a high temperature pressure sensor with range of
0~25MPa was manufactured and calibrated, it was able to work successfully under
temperature of 200 C. From the calibration experiments results, the sensor’s static accuracy
was 0.114%FS and dynamic response frequency was about 694.4 kHz.
The authors would like to thank the financial supports by the National Natural Science
Foundation Key Project of China (Grant numbers: 50535030 and 50836004), and Youth
Foundation of School of Mechanical Engineering, Xi’an Jiaotong University.
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