Design of Mems-based Gas Sensor Micro Heat Plate

L X He

1, 2, *

F Wang

, G Q Niu

, H M Gong

, Z T Yang

, W He

and J M Cao

College of Optoelectronic Engineering of Shenzhen University, Shenzhen, 518060,

China

Department of Electrical and Electronic Engineering of Southern University of

Science and Technology, Shenzhen, 518055, China

Corresponding author and e-mail: L X He ,372331992@qq.com

Abstract. In this academic paper, a mems-based device: micro gas sensor micro heat plate

(MHP) was designed and produced. Unlike the traditional MHP, of which the heating

electrode and test electrode is with non-coplanar design, the MHP designed in this paper has

coplanar heating electrode and test electrode with higher heating efficiency. Platinum was

used as heating electrode material in the design. By taking advantage of platinum's property

of relatively stable resistance temperature coefficient, as well as by controlling the heating

voltage of MHP, the temperature of the gas sensor was accurately regulated. The design also

utilized Comsol, the finite element simulation software, to analyze and optimize the heating

performance and heat distribution of MHP.

1. Introduction

Micro-electromechanical Systems (MEMS) developed with the progress of semiconductor integrated

circuit micro machining technology and ultraprecise machining technology [1-3]. It can integrate

micro sensors, micro actuators, signal processing and control circuits, and even interface,

communication and power supply into a whole, characterized by such features as small volume, light

weight and practicality of mass production, etc [4-6].

Since 1962, metal oxide semiconductor (MOS) materials have been increasingly applied into gas

detection. Nevertheless, since a certain temperature is required for MOS materials to react with gas,

the power dissipation of sensor is excessive, consequently restraining its further development. In

contrast with traditional gas sensor, MOS gas sensor on MEMS technology is equipped with a host of

advantages on such aspects as consistency and microminiaturization, which is easier to achieve

integration and low power consumption [7-11]. Meanwhile, MOS gas sensor is compatible with the

existing silicon-based processing technology, which is the orientation of development for future gas

sensor. Within this type of sensor, the MHPs made via MEMS technology can provide heat for MOS

air-sensitive thin-film materials. And its thermal performance can exert influences on the displaying

of sensor's overall performance [12-13]. Consequently, it is of great necessity to carry out in-depth

analysis and research on MHP so that the structure of the sensor can be optimized [14-16].

Within the traditional MHP gas sensor, the heating electrodes and the test electrodes are not on

the same plane. With such traditional design, the manufacturing process is complex, and the heat

transfer is not very good, and a parasitic electric field is likely to take form among the heating layer,

610

He, L., Wang, F., Niu, G., Gong, H., Yang, Z., He, W. and Cao, J.

Design of Mems-Based Gas Sensor Micro Heat Plate.

In Proceedings of the International Workshop on Materials, Chemistry and Engineering (IWMCE 2018), pages 610-615

ISBN: 978-989-758-346-9

insulating layer and testing layer, exerting certain influences on the signal to be tested. In this new

design, the heating electrode and the testing electrode are placed on the same plane [16-21].

Compared with traditional devices, the production process has been streamlined, the heating process

and heat transfer is optimized, and the parasitic electric field among the "three-layer" structure is

avoided.

Platinum was used for both heating electrode and test electrode in this design. The resistance of

platinum is equipped with such merits as large range of measurement, good stability and anti-

oxidation property. Within a certain range of temperature, the resistance-temperature characteristic

curve is linear, and the temperature of platinum electrode can be effectively calculated based on the

resistance value of platinum. By gradually increasing the voltage at both ends of the heating electrode

of MHP and by measuring the resistance value of heating electrode during the heating and heat

generation process, the relation curve for voltage and resistance of the heating electrode can be

achieved. The relation curve for voltage and resistance can be further calculated by referring to the

linearity between resistance and temperature of the platinum resistance. Eventually, regulation of

working temperature can be realized by changing the working voltage of platinum electrode.

Simulation analysis can be carried out on various structures' temperature distribution in the

physical field with Multiphysics module in COMSOL, the finite element simulation software [22-23].

In this academic paper, a new-type MHP with four pins was designed, and by using finite element

analysis method, the temperature distribution of the new-type MHP was simulated; meanwhile, the

structure of MHP was optimized via simulation.

2. The design and fabrication of MHP

The structure of MHP mainly designed in this academic paper is shown in Figure 1 and the plane

dimension of the device is 6mm*6mm. With a thickness of approximately 500 µm, the base of MHP

consists of three layers: they are SiO

, Si and SiO

respectively from top to bottom, with the

thickness of Si being 500 µm and the thickness of SiO

being 2 µm. The upper SiO

layer is the

electrical insulating layer as well as the thermal insulating layer, and the bottom SiO

layer is

equipped with the function to prevent heat loss.

Figure 1. The structure of the electrode on the microplate; (a) overall structure of the electrodes, (b)

heating electrode and (c) test electrodes.

The fabrication process of MHP is shown in Figure 2, in which the SiO2 base in the upper layer is

processed via spreading photoresist and platinum with a thickness of 200 nm is deposited via electron

beam evaporation. The heating electrode and test electrode on the same plane was achieved after

getting rid of redundant photoresist and platinum. By proceeding with spreading photoresist, and

with plasma sputtering the heating electrode cladded by 200-nm-thick SiO2, the insulating property

between heating electrode and air-sensitive material is thus guaranteed.

Design of Mems-Based Gas Sensor Micro Heat Plate

611

Figure 2. Fabrication process of the MHP for gas sensor based on MEMS.

3. The test of MHP's property

Within a certain range of temperature, the resistance value of platinum metal electrode is linearly

correlated with the temperature value. By increasing the temperature after placing MHP into precise

temperature-controlled box and by measuring the resistance value of platinum heating electrode

during the temperature rise, the relation between the resistance of platinum heating electrode and

temperature can be achieved. As illustrated in Figure 3(a), by measuring the resistance value of

heating electrode with a width between 40 µm and 50 µm when being heated at 30°C -190°C , the

curve demonstrating how electrode resistance value is varied with the change of temperature can be

achieved. Based on Figure 3(a), it is also observed that the resistance value of platinum heating

electrode is not linearly correlated with temperature. Furthermore, the experiment also showed that

the electrode was lower than its initial value instead of returning to its initial value after being cooled

from the heating process.

It is found that the structure of platinum metal film achieved via electron beam evaporation

coating is not stable and the resistivity is much higher than normal resistivity. It is necessary to

process the platinum metal film with high-temperature annealing so that the compactness of its metal

film structure can be restored and its resistivity can be reduced.

In the design, MHP with 50-µm -thick heating electrode was put in tube furnace and heated at

500°C for three hours before it was cooled. After three times of annealing, the resistance value of the

heating electrode before and after annealing was measured at 1025Ω, 561Ω, 556Ω and 552Ω

respectively. According to the resistance value change of platinum heating electrode, it is known that

the change of resistance value was the greatest after first-time annealing, while the change of

resistance value after the second-time and third-time annealing was less than 10 Ω, indicating that the

resistance value of the heating electrode three hours after the 500°C annealing has already been

stabilized.

The annealed MHP was once again put into precise temperature-controlled box so as to measure

how the resistance value of platinum heating electrode changed with temperature, and Figure 3

shows how the heater strips with a width of 40 µm and 50 µm change when the temperature varies

between 30°C and 200°C . By fitting the data from the test in Origin, the linear equation about how

the resistance value of MHP with corresponding sizes varies with the changes of temperature can be

achieved.

In the test, the annealed MHP was fixed by probe station, the probe was pricked into the pins at

both ends of the heating electrode, and stable voltage was applied to both ends of the heating

electrode via the probe station to make the platinum electrode generate heat; after the resistance of

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612

platinum electrode became stabilized, the resistance value was recorded. The changes of platinum

electrode's resistance value when the voltage rose from 0V to 70V was recorded and shown in Figure

3(b). By making use of the measured linear relation between platinum electrode's resistance value

and temperature to replace the resistance value on the ordinate in Figure 3. with temperature value,

the relation about how platinum electrode's temperature changes with voltage was obtained and was

illustrated in Figure 3(c, d).

(a) (b)

Figure 3. The test of MHP's property.

4. Simulation verification

An analysis was carried out on the heating performance of the designed MHP via the finite element

simulation software: COMSOL Multiphysics, and a contrast was made with the test result, so that the

optimization of the structure of the initially-designed finished product as well as the improvement of

the performance was further achieved.

In the first place, a simulated geometric model was constructed in Comsol. Then, corresponding

materials were selected for different sections of the geometric model and the material property was

also set up. Afterward, the physical field and condition required by simulation were set up and the

whole model was finalized in a gridding state. Finally, the simulation software was operated whereby

the heat distribution of the constructed model was obtained.

The module with 50-µm -thick heating electrode was chosen and with the heating voltage of 60V,

the thickness of the silicon layer at its base was gradually reduced so at to carry out the simulation.

When the constant heating voltage was achieved, the changes of heating temperature in accordance

with the change of silica base thickness are shown in Figure 4. It is thereby known that when the

heating voltage remains constant, the thinner the silica base is, the higher the value of the MPH at

stable heating temperature will be.

Design of Mems-Based Gas Sensor Micro Heat Plate

613

(a) Silicon layer thickness of 500um (b) Silicon layer thickness of 5um

Figure 4. Simulation verification.

5. Conclusions

During the preparation process of MHP, the resistance value of metal film electrode obtained by

evaporation coating is not stable and highly susceptible to temperature, and thus needs high-

temperature annealing. The resistance value of the annealed metal electrode tends to stabilize, and the

resistance value before heating and after cooling is no longer changes.

The non-optimized devices in this design have relatively higher heat loss, and this is primarily due

to that in the initial design, no consideration is given to the fact that the actual resistance value will

increase by a large extent in contrast with the theoretical value after the molding of platinum via

evaporation coating. Consequently, it is necessary to carry out a simplified design on the area and

length of the platinum electrode, making sure that heating electrode has relatively small resistance

and reduces the heating power of the device. Meanwhile, in consideration of conduction dissipation

of heat and via simulation, the silicon surrounding the metal electrode is processed by tunneling, so

that the heat loss of the device can be effectively improved.

In this design, such procedures as design, production, test and simulation were completed for a

new type of MHP for gas sensor. Via the heating and Adding voltage experiment, the relation

between the 's working temperature and working voltage was clarified. Thereby, during the actual

operation of the, the control over the working temperature of the micro sensor can be realized via

changing the heating voltage.

Acknowledgment

This research was supported by Department of Electrical and Electronic Engineering of Southern

University of Science and Technology.

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