Magnetic Parallel Assembly Improves 3-D Processing
Ruth DeJule, Associate Editor -- Semiconductor International, 6/1/2000
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 1. The square flap remains flat until a magnetic field produces rotation about the three hinges (on left). (Source: University of Illinois) |
Typically, hinged microstructures are rotated using a DI water rinse to randomly actuate the hinged flap. As expected, the yield is low. Others integrate an independent actuator positioned near the flaps. However, this requires additional wafer real estate for the actuator and, more importantly, individual addressing of each hinge. With magnetic actuation of hinged microstructures, no additional wafer area is needed because a magnetic field is applied externally, actuating all flaps in parallel (Fig. 1). Once positioned, the magnetic field is removed. The degree of angular displacement of a hinged structure is determined by the volume of the magnetic material or by the stiffness of an auxiliary flexural loading spring. Varying the amount of magnetic material attached to the flaps, noted Dr. Liu, controls the speed at which the parts fold into position. His group has developed a technique that allows multiple pieces to effectively interlock, thus forming a stable 3-D assembly.
 2. A Fresnel microlens displays a pattern that has been etched into the silicon. (Source: University of Illinois) |
Based on standard silicon processing, the fabrication of magnetic actuation 3-D structures begins with the formation of polysilicon device structures on sacrificial layers of phosphosilicate glass (PSG), ~1.5 µm thick, deposited by low-pressure chemical vapor deposition (LPCVD). The micromachined flap is made of polysilicon ~1 µm thick. A magnetic material, Permalloy, is electroplated and integrated into the hinged microstructures. Other combinations of structural and sacrificial materials also are possible.
 3. An array of DC probes shows the effect of this 3-D MEMS process technology. (Source: University of Illinois) |
This new fabrication capability enables the realization of new MEMS devices and applications. Dr. Liu's team has demonstrated 3-D structures millimeters high in several microoptics and rf applications including Fresnel microlens (Fig. 2) and vertical tunable capacitors. Other demonstrations include vertical probes for neuron recording and DC probe arrays (Fig. 3). This unique fabrication process makes possible the development of modular building blocks for the construction of a new class of integrated microsensors – for example, future autonomous robots that have a high degree of sensory sophistication to effectively perceive and respond to their environment. •
