5 - MEMS devices

Tunable capacitors based on MEMS-technology uses interdigitated structures both for tuning and for the tunable capacitor itself. The effective area of the capacitor (right side) is varying in response of the distance changes caused by the varying electrostatic field inside the tuning unit (left side).

The inductors are based on the combination of metal pad on Si substrate (ci, Al or Au) and wirebonding (middle). Bulk-micromachined inductors have air isolation between the convolutions (left and right).

The basis of this switch is a cantilever beam. Electrostatic actuation of the beam causes a dimpled gold stripe located at teh beam's end to contact two ends of a gap in an RF line deposited on the switch substrate. The cantilever beam is parallel to the surface at a height of 2 mm. When the switch is actuated and the cantilever beam is pulled down, two springs are revealed. One spring, near the RF bar, insures that therer is sufficient contacting force for low contact resistance. The other spring, near the anchor of the beam, guarantees that the beam returns to its original position when actuating voltage is removed.

MEMS based switches are widely used in RF (Radio Frequency) applications. Applying the Pull-in voltage to the RF-line the capacitor closes without adding too much noise to the RF-signals. Top figure: shunt switch with electrostatic actuator Bottom figure: series switch with electrostatic actuator

Other actuation mechanisms:

• Electrostatic: positive and/or negative charges, set by applied voltages between certain structural members elicit Coulomb forces which produce motion.
• Piezoelectric: applied voltages on structures induce fields which change their dimensions, with the physical dimensional change used to communicate motion.
• Thermal: a current forced through an element causes it to heat up and expand with the physical dimensional change used to communicate motion.
• Magnetic: magnet-induced or current-induced magnetic forces produce motion.
• Bi-Metallic (shape-memory alloy): materials that upon experiencing deformation at a lower temperature can return to their original undeformed shape when heated. During this process the physical dimensional change is used to communicate motion.
  

Recent advances in millimeterwave systems and antennas allow integration of planar antennas with active and passive circuits on the same chip. Micromechanical system allows the fast actuation to reconfigure antenna structures with low power comsumption by microactuators. One end of the antenna arm is held by a rotational hinge locked on the substrate, which allows the arm to rotate. The arms are pulled or pushed by support bars connected to the comb drive actuators with movable rotation hinges on both ends. The movable rotation hinges translate the lateral movement of the actuators to circular movement of the antenna arms.

Microrelay structures are asked to replace standard "reeds" or electro-mechanical relays while satisfying the same specifications. Relatively low actuation power is required as well as large open airgaps and very low on-state contact resistance. The electromagnetic microrelay consists of two different chips: the electromagnet chip and the cantilever keeper chip. The cantilever design comprises two distinct parts, two beams for the elasticity of the system, and a large plate for closing the magnetic circuit between the two electromagnet poles. Both chips were assembled by gluing with a conductive paste ensuring an open rest airgap of 20 µm. On actuation, the cantilever is attracted against the electromagnet poles, thus closing the relay contact. The thermal microrelay consists of a U-shaped bimaterial arm. On actuation, the cantilever is bended against the relay contact closing them.

Thermal microrelay

Electromagnetic microrelay