Comparison of Three MEMS Thermal Conductivity Sensors

Tu Xiang Zheng

MEMS sensors have greatly matured and proliferated in use amongst many disciplines. There has been great interest in MEMS thermal conductivity sensors due to the benefits of miniaturization: low cost, small device footprint, low power consumption, greater sensitivity, integration with on-chip circuitry, etc. which can be used to analyze a whole range of gas mixtures, provided that there are only two gases present and that the two gases have significantly different thermal conductivities. 

A single silicon wafer MEMS thermal conduction sensor has developed by the present author. The sensor consists of a heat transfer cavity with a flat bottom and an arbitrary plane shape, which is created in a silicon substrate. A heated resistor with a temperature dependence resistance is deposed on a thin film bridge, which is the top of the cavity. A heat sink is the flat bottom of the cavity and parallel to the bridge completely. The heat transfer from the heated resistor to the heat sink is modulated by the change of the thermal conductivity of the gas or gas mixture filled in the cavity. This change can be measured to determine the composition concentration of the gas mixture or the pressure of the air in a vacuum system.

In order to know the advantages of the present MEMS thermal conductivity sensors a comparison of several MEMS thermal conductivity sensors is made. The others MEMS thermal conductivity sensors were developed by Arndt et al’s and Hsiai et al’s.

 

1. Comparison and contrast between the Arndt et al.’ sensors and the present sensors are shown in Fig.1. Several significant differences between them can be found from the comparison and contrast. These significant differences include the follows:

(1) The Arndt et al.’ sensors are not single silicon wafer MEMS thermal conduction sensors. Actually it is constructed by three wafer including one base plate and two porous cover plates. In contrast, the present sensors are single wafer MEMS thermal conduction sensors that are constructed from single silicon wafers.

(2) The heat transfer cavity of the Arndt et al.’ sensors can not be arbitrary shape because it is formed by KOH etching that limits the shape of the cavity to be bounded by the (111) crystal planes of the silicon wafer. Compared with the Arndt et al.’ sensors, the heat transfer cavity of the present sensors can be arbitrary shape because it is by anodization in HF solution which is not governed by the crystal structure of the silicon wafer.

(3) The side wall of the heat transfer cavity of the Arndt et al.’ sensors can not be curved due to limitation of the crystal structure of the silicon wafer. On the contrary, the side wall of the heat transfer cavity of the present sensors can be curved due to its formation without limitation of the crystal structure of the silicon wafer.

(4) The Arndt et al.’ sensors have two heat transfer cavities with one on the top of the base plate and the other on the bottom of the base plate. Unlike the Arndt et al.’ sensors, the present sensors only have one heat transfer cavity.

(5) The two heat transfer cavities of the Arndt et al.’ sensors with each is formed by bonding a silicon base plate and a porous plate together. Contrasted with the Arndt et al.’ sensors, the heat transfer cavity of the present sensors are formed inside the silicon wafer without using an additional wafer.

(6) The Arndt et al.’ sensors have two adhesive layers that are used for bonding three different plates together. Conversely the present sensors have no adhesive layers because it is a single wafer structure without wafer bonding.

In summery, almost no similarities between the Arndt et al.’ sensors and the present sensors have been found. There is impossible for the Arndt et al.’ sensors to teach the present sensors.

2. Comparison and contrast between the Hsiai et al.’ sensors and the present sensors are shown in Fig.2. It can be found that the Hsiai et al.’ sensors and the present sensors are related to different types of sensors according to the following reasons:

The Hsiai et al.’ sensors are fluid shear stress sensors that are based on heat transfer by fluid convection. But the present sensors are gas thermal conduction sensors that are based on heat transfer by gas conduction.

The Hsiai et al.’ sensors do not need a heat transfer cavity because fluid convection occurs over the diaphragm. However the present sensors do need a heat transfer cavity because gas conduction occurs inside the cavity.

The cavity of the Hsiai et al.’ sensors have no bottom used for heat sink because they have no heat transfer function. Nevertheless the heat transfer cavities of the present sensors have a bottom so that the heat generated by the heater on the top of the cavity can be transferred to the bottom through the gas filled in the cavity.

In view above mentioned three major differences between the Hsiai et al.’ sensors and the present sensors, it should be very clear that the present thermal conduction sensors are not able to learn from the Hsiai et al.’ fluid shear stress sensors.

3. The present sensors are fabricated using porous silicon micromachining technique instead of KOH etching micromachining technique that is used by Arndt et al. and Hsiai et al.

(1) KOH etching of silicon is an anisotropic etching. The main characteristics of this micromachining technique can be listed as:

Etch rates of crystallographic planes are: (100) > (110) > (111)

(111) family of crystallographic planes are the “stop” planes

producing standard anisotropic etching structures: V-grooves and pyramidal cavities

From these characteristics it can conclude that the cavities of the Arndt et al.’ sensors and the Hsiai et al.’s sensors are pyramidal cavities with (111) sidewall planes, because they all use KOH etching to create their cavities. It also can be conclude that the heat transfer cavity with curved sidewalls provided by the present sensors can not be created using KOH etching technique.

(2) In addition to KOH micromachining technique, a porous silicon based micromachining technique is more and more widely used for MEMS devices fabrication. Porous silicon layers can be fabricated by partially electrochemical dissolution of silicon wafers with etching masks covered on the surface of the silicon wafers. Then the porous layers are etched away to form microstructures including cavities. The shape of the cavities is determined by the etching masks without limitation based on the silicon crystallographic structure. Like the present sensors the cavities of the sensors can have arbitrary shape and curved sidewalls.

4. Conclusion

Comparing and contrasting between the Arndt et al.’s sensors and the present sensors and between the Hsiai et al.’s sensors and the present sensors have been made respectively. Six significant differences between the Arndt et al.’s sensors and the present sensors have been found. It can be seen that it is impossible for the present inventor to learn from the Arndt et al.’s sensors for designing and fabricating the present sensors. Three major differences have been found between the Hsiai et al.’s sensors and the present sensors. Actually, the Hsiai et al.’s sensors and the present sensors belong to different sensor types. It is not able for the present inventor to learn how to design the present sensors from the Hsiai et al.’s sensors. Moreover the present sensors use porous silicon based micromachining technique for fabricating the present sensors, which is different from KOH micromachining technique used by the Arndt et al.’s sensors and the Hsiai et al.’s sensors. Hwy the heat transfer cavity with arbitrary shape and curved sidewalls provided by the present sensors can not been created in the Arndt et al.’s sensors and the Hsiai et al.’s sensors, because they all use KOH micromachining technique instead of porous silicon based micromachining technique.