MEMS COMPONENTS – EXAMPLES
· Micromachined High Q Inductors
Bulk micromachining has reduced the parasitics associated with conventional on-chip planar inductors, which used to lower their quality factor (Q).
Figure shows an example of a bulk-micromachined inductor in which the substrate has been eliminated from underneath the spiral trace. Measured Qs range from 6 to 28 at frequencies from 6 to 18 GHz, with typical inductor values around 1nH.
Similarly, machining has been exploited to create solenoid-like inductors above the substrate. Figure 5 shows an example of such an approach. A quality factor of 25.1 at 8.4 GHz and an inductance of 2.3 nH were obtained.
· MEMS Varactors
MEMS-based varactors broadly are of two types — parallel plate and interdigitated capacitor. Some variants of the above types have also been demonstrated.
In the parallel (two or three) plate approach, the top plate is suspended a certain distance from the bottom plate by suspension springs, and this distance is made to vary in response to the electrostatic force between the plates induced by an applied voltage
In the interdigitated approach, the effective area of the capacitor is varied by changing the degree of engagement of the fingers of comblike plates. Comb drive type actuators make use of this approach.
· MEMS Switches
MEMS switches offer low insertion loss, high isolation and high linearity. Many switches, based on a number of actuation mechanisms and topologies, have been demonstrated. These include the Electrostatic, Piezoelectric, Thermal, Magnetic, Bi-Metallic (shape-memory alloy).
Structure RF MEMS (Shunt) Switches are shown in following figure.
· Cavity Resonators
The performance levels of macroscopic waveguide resonators may be approached using MEMS. As an example, micro machined cavity resonator for X-band applications has been demonstrated to have unloaded Q of 506 for a cavity with dimensions 16× 32× 0.465 mm. This was just 3.8 percent lower than the unloaded Q obtained from a rectangular cavity of identical dimensions.
· Micromechanical Resonators
Mechanical resonators are capable of exhibiting Q in the 10,000-to-25,000 range. Micromechanical resonators can also achieve this using vertical displacement resonator, in which a cantilever beam is set into a diving board-like vertical vibration in response to an electrostatic excitation, and the lateral displacement resonator, in which the motion is excited by exciting a comb-like structure. For higher frequencies, the Film Bulk Acoustic wave Resonator(FBAR) consisting of a layer of piezoelectric material disposed between can be used.
· Micromachined Gears
Figure shows electrostatic microengine output gear coupled to a double level gear train that drives rack and pinion slider. This is fabricated using Sandia Ultra-planar Multi-level MEMS Technology(SUMMiT).
· MEMS Rotary Motor
An electrostatic rotary motor has been fabricated using Multi-user MEMS process.
Packaging is very important for MEMS devices. Reliability, performance of a MEMS device largely depends on packaging and this is one of the factors which have affected the growth of MEMS technology. It should
• provide protection and be robust enough to withstand its operating environment.
• allow for environmental access and connections to physical domain.
• minimize EMI.
• dissipate generated heat.
• minimize stress from external loading.
• handle power from electrical connection leads without disruption.
Hermetic sealing is a universal requirement in packaging many MEMS devices, especially in micro fluidics and micro optical switches. Since MEMS device structures are typically very small, stresses and thermal effects, induced in the substrates during the packaging and interconnection steps, can adversely affect their mechanical performance. For devices like accelerometer which can be hermetically sealed, packaging is relatively much easier than the packaging for microfluidic sensors (e.g., pressure, flow, chemical, etc.). They need an interface for real world and hence, isolation of electrical interconnects from environment and protection of MEMS structure increases the complexity in these devices.
Types of MEMS packages used are
o Ceramic Packaging
· Hermetic when sealed
· High Young’s Modulus
· Flip Chip or Wirebonding
o Plastic Packaging
· Not Hermetic
o Metal Packaging
· Hermetic when sealed
· Easy to assemble
· Low Pin Count
Since the substrate on many ICs requires an electrical connection to bias it, sensor dies are usually mounted to a die attach pad in the package using a conductive bond. The die attach pad is typically joined to a metal lead frame with wire bonds providing the electrical connections to the lead frame fingers. Various bonding media include AuSi eutectic bonding, epoxy bonding (conductive or insulating depending on filler material) and glass usually loaded with silver. The package is subsequently formed by plastic moulding (as in the case of moulded plastic packages), sealed ceramic or metal caps (ceramic packages), or with a brazed metal cap to the base of a metal package.
Wire bonding (Ultrasonic and thermosonic) is the most common technique for electrically connecting the die. In flip-chip (FC) technology the chips are bonded face down to a substrate via bumps; materials include solder, gold, copper and nickel. On heating, the bump material melts and simultaneously forms all the electrical and mechanical connections between the chip and the substrate.
In order to protect MEMS devices, thin-film coatings of usually silicon dioxide or silicon nitride are deposited on the components using plasma enhanced chemical vapour deposition (PECVD); the process called passivation. It increases wear resistance and electrical insulation. Common encapsulants such as epoxies, silicones and polyurethanes are used to protect the sensor die against adverse influences from the environment (contaminants, mechanical vibration and shock).
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