Bonding pressure sensors

Most mechanical sensors have to operate in a stable, well controlled environment, thus requiring perfectly sealed cavities. One of the most important achievements of the anodic bonding technique is the fabrication of such hermetically sealed cavities. Anodic bonding can be performed in different atmospheres and at different pressures, which can both be preserved within a sealed cavity. The creation of reliable reference chambers for pressure sensors or for good control of squeeze-film damping [46] are just some of the opportunities offered by this technique. Nevertheless, this technique also faces some challenges, such as the feedthrough of the conductors to and from the sealed cavity. Several solutions have been proposed ... 

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

Fig. 5.1.15 Anodic-bonded silicon sensor with sensor for absolute pressure (left) and relative pressure (right) on the same chip...

Fig. 5.1.16 Piezoresistive pressure sensor made by silicon fusion-bonding (SFB) to obtain small chip size and anodic bonding to form a sealed vacuum cavity...

Fig. 5.1.17 Absolute pressure sensor for good media compatibility with pressure inlet port on the bottom of the silicon diaphragm and an on-chip vacuum reference volume, made possible by triplestack anodic bonding...

A. Cozma, Development of wafer bonding techniques with application to self-testable silicon pressure sensors, unpublished, Ph.D. Thesis, K. Univ. Leuven, Belgium, 1998. 

Wafer bonding or cavity sealing is present on many MEMS products. Wafer-level packaging was first used in pressure sensors (Fig. 7.1.13a) to create an absolute... 

The pressure-sensing element is protected by a three-layer protection structure comprising a SiN passivation film with superior moisture resistance, a parylene film for protecting the bonding wire, and gel for protecting the whole element. This structure makes it possible to achieve the creation of a surface-detection intake pressure sensor with high contamination immunity. 

The actual pressure sensor is offered in a package made of resin or metal. When the pressure device is directly bonded to the package, the sensor characteristics are affected by the difference in physical properties such as thermal expansion coefficient and Young s modulus between silicon and the package material. The glass base is used for lessening the effect of the difference. We have learnt, however, that the thermal expansion coefficient of the glass base itself also has a profound effect on the temperature characteristics of the sensor. 

Two approaches were investigated to provide selective encapsulation for surface-micromachined pressure sensor devices [9]. The first method uses a glass-frit-bonded bulk micromachined silicon cap wafer, and the second method uses a patterned polymeric gel-like material in the assembly area to create this dam. 

The second approach for selective encapsulation presented here uses a bulk-mi-cromachined bonded wafer with the surface micromachined pressure sensor. 

ABSTRACT Dif sion of acetone at the interface of a bonded pressure sensitive adhesive tape was measured using single frequency capachance measurements (SFCM) and a novel interdigitated electrode sensor design. The relative concentration of acetone at the bondline as a function of distance from the edge of the specimen and exposure time was correlated to adhesion loss measured by the 90° peel test. 

Our research builds on this limited body of work concerning diffusion at the interface of adhesive joints. We have measured the interfacial diffusion process of acetone into a bonded pressure sensitive adhesive tape bj employing single frequency capacitance measurements and a novel interdigitated electrode sensor design. 

Anodic bonding is the primary method for packaging silicon microstructures, for example packaging of pressure sensors, accelerometers and solar cell panels, because of... 

Glass-Polymer Bonding, Figure 1 Experimental setup for leak tests. The bonded chip Is mounted on top of an Inverted light microscope for optical leak detection. The pressure is generated with a syringe pump at the chip inlet side and measured at the outlet side with a MEMS gage pressure sensor... 

T. A Knecht, Bonding techniques for solid state pressure sensors, the 4th Int. Conf on Solid-state Sensors and Actuators, (1987-6), 95-98... 

---