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3. Bonding

3. Bonding.

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3. Bonding

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  1. 3.Bonding • Bonding Processes are widely used in micromachining applications to allow similar or dissimilar and/or components to be permanently coupled mechanically (and sometimes electrically). There are a variety of processes available, ranging from those that form covalent bonds between the substrates to those using intermediate adhesive layers as silple as commertial glues. IN many cases, hermetic sealing is desired, and this requirement is becoming more important as bonded-on cavities are being used for packaging micromachined devices that would be compromised by conventional plastic injection molded packing( very commonly used with low-cost integrated circuits). • Bonding Techniques 1)Anodic Bonding 2)Fusion Bonding 3)Eutectic Bonding 4)Adhesive Bonding 5)HF Bonding

  2. Bonding Techniques require • Low temperature bonding • Bio/Chemically compatible bonding • Wafer-to-wafer bonding • Chip-to-wafer bonding • Precise (or Self )alignment between wafers/chips • Void-free intermediate layer • Universal bonding for various materials • High bonding strength • Cheap and easy processing

  3. 3.1 Anodic Bonding

  4. 3.2 The Fusion Bonhding

  5. 4. Sensors • Mechanical Sensors • The basic mechanical sensing mechanisms( e.g. for force, displacement, strain, etc ) • Micromachined mechanical sensors • Read-out techniques Static methods:

  6. 1.1 Sensing Mechanisms • The following table summarizes the commonly used mechanical sensing mechanisms in micromachined devices. • Important considerations when choosing such a mechanism include the need for local circuitry, whether or not the transduction mechanism is DC-responding, temperature coefficients, long-term drift, overall system complexity, and others.

  7. 1.1.1 Resistive and piezoresistive Strain Sensors Strain sensors are an integral part of many micromachined devices, serving to measure strain or, indirectly, displacement of structures. A strain gauge is a conductor or semiconductor that is fabricated on or bonded directly to the surface to be measured. Changes in gauge dimensions result in proportional changes in resistance in the sensor. This is partly due to stretching (changes in dimensions) and partly due to the piezoresistive effect, discovered by Lord Kelvin in 1856. As might be expected, the sensitivity of gauges can be quite different, depending on their design. Strain gauges of all types can be very linear over considerable ranges of strain, making them attractive in a variety of applications. In general, the sensitivity is expressed by the gauge factor (dimensionless), R relative resistance change strain R R GF = = = L R L (here longitudinal strain, 1, is used). One can use partial derivatives to derive a general expression for the gauge factor in terms of the physical parameters of the strain gauge.

  8. Metal strain gauges may be made from thin wires or metal films (thin-film strain gauges ) that may be directly fabricate on top of microstructures. • Thin-film metal strain gauges are easier to fabicate (photolithographically) and allow for more complex shapes. • They are generally built on flexible plastic substrates (Sometimes self-adhesive) and can be glued onto a surface.

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