Gary K. Fedder

Endoscopic optical coherence tomography based on a microelectromechanical mirror

An endoscopic optical coherence tomography (OCT) system based on a microelectromechanical mirror to facilitate lateral light scanning is described. The front-view OCT scope, adapted to the instrument channel of a commercial endoscopic sheath, allows real-time cross-sectional imaging of living biological tissue via direct endoscopic visual guidance. The transverse and axial resolutions of the OCT scope are roughly 20 and 10.2mum, respectively. Cross-sectional images of 500x1000 pixels covering an area of 2.9 mmx2.8 mm can be acquired at ~5 frames/s and with nearly 100-dB dynamic range. Applications in thickness measurement and bladder tissue imaging are demonstrated.

Laminated high-aspect-ratio microstructures in a conventional CMOS process

Abstract Electrostatically actuated microstructures with high-aspect-ratio laminated-beam suspensions have been fabricated using a 0.8 μm three-metal CMOS process followed by a sequence of three maskless dry-etching steps. Laminated structures are etched of the CMOS silicon oxide, silicon nitride, and aluminum layers. The key to the process is the use of the CMOS metallization as an etch-resistant mask to define the microstructures. A minimum beam width of 1.2 μm, gap of 1.2 μm, and maximum beam thickness of 4.8 μm are obtained. These structural features will scale in size as the CMOS technology improves. The laminated material has an effective Youngs modulus of 61 GPa, an effective residual stress of 69 MPa, and a residual strain gradient of 2 × 10 −4 μm −1 . Multi-conductor electrostatic micromechanisms, such as self-actuating springs, x−y microstages, and nested comb-drive lateral resonators, are successfully produced. A self-actuating spring is a lateral electrostatic microactuator without a stator that is insensitive to out-of-plane curl. A spring 107 μm wide by 109 μm long excited by an 11 V a.c. signal has a measured resonance amplitude of 1 μm at 14.9 kHz. Finite-element simulation using the extracted value of Youngs modulus predicts the resonance frequencies of the springs to within 7% of the measured values.

Technologies for Cofabricating MEMS and Electronics

Microfabrication technologies initially developed for integrated electronics have been successfully applied to batch-fabricate a wide variety of micromechanical structures for sensing, actuating, or signal-processing functions such as filters. By appropriately combining the deposition, etching, and lithography steps for microelectromechanical devices with those needed for microelectronic devices, it is possible to fabricate an integrated microsystem in a single process sequence. This paper reviews the strategies for cofabrication, with an emphasis on modular approaches that do not mix the two process sequences. The integrated processes are discussed using examples of physical sensors (infrared imagers and inertial sensors), chemical and biochemical sensors, electrostatic and thermal actuators for displays and optical switching, and nonvolatile memories. By adding new functionality to integrated electronics, the use of microelectromechanical systems is opening new applications in sensing and actuating, as well as enhancing the performance of analog and digital integrated circuits.

Single-chip computers with microelectromechanical systems-based magnetic memory (invited)

This article describes an approach for implementing a complete computer system (CPU, RAM, I/O, and nonvolatile mass memory) on a single integrated-circuit substrate (a chip)—hence, the name “single-chip computer.” The approach presented combines advances in the field of microelectromechanical systems (MEMS) and micromagnetics with traditional low-cost very-large-scale integrated circuit style parallel lithographic manufacturing. The primary barrier to the creation of a computer on a chip is the incorporation of a high-capacity [many gigabytes (GB)] re-writable nonvolatile memory (in today’s terminology, a disk drive) into an integrated circuit (IC) manufacturing process. This article presents the following design example: a MEMS-based magnetic memory that can store over 2 GB of data in 2 cm2 of die area and whose fabrication is compatible with a standard IC manufacturing process.

A two-axis electrothermal micromirror for endoscopic optical coherence tomography

This paper reports a 1-mm/sup 2/, two-axis, single-crystalline-silicon (SCS)-based aluminum-coated scanning micromirror with large rotation angle (up to 40/spl deg/), which can be used in an endoscopic optical coherence tomography imaging system. The micromirror is fabricated using a deep reactive ion etch post-CMOS micromachining process. The static response, frequency response, resonance frequency shift, and thermal imaging of the device are presented. A 4/spl times/4 pixel display using this two-dimensional micromirror device has been demonstrated.

Position control of parallel-plate microactuators for probe-based data storage

In this paper, we present the use of closed-loop voltage control to extend the travel range of a parallel-plate electrostatic microactuator beyond the pull-in limit. Controller design considers nonlinearities from both the parallel-plate actuator and the capacitive position sensor to ensure robust stability within the feedback loop. Desired transient response is achieved by a pre-filter added in front of the feedback loop to shape the input command. The microactuator is characterized by static and dynamic measurements, with a spring constant of 0.17 N/m, mechanical resonant frequency of 12.4 kHz, and effective damping ratio from 0.55 to 0.35 for gaps between 2.3 to 2.65 /spl mu/m. The minimum input-referred noise capacitance change is 0.5 aF//spl radic/Hz measured at a gap of 5.7 /spl mu/m, corresponding to a minimum input-referred noise displacement of 0.33 nm//spl radic/Hz. Measured closed-loop step response illustrates a maximum travel distance up to 60% of the initial gap, surpassing the static pull-in limit of one-third of the gap.

Emerging simulation approaches for micromachined devices

In this survey paper, we describe and contrast three different approaches for extending circuit simulation to include micromachined devices. The most commonly used method, that of using physical insight to develop parameterized macromodels, is presented first. The issues associated with fitting the parameters to simulation data while incorporating design attribute dependencies are considered. The numerical model order reduction approach to macromodeling is presented second, and some of the issues associated with fast solvers and model reduction are summarized. Lastly, we describe the recently developed circuit-based approach for simulating micromachined devices, and describe the design hierarchy and the use of a catalog of parts.

A CMOS-MEMS mirror with curled-hinge comb drives

A micromirror achieves up to /spl plusmn/4.7/spl deg/ angular displacement with 18 Vdc by a comb-drive design that uses vertical angled offset of the comb fingers. Structures are made from a combination of CMOS interconnect layers and a thick underlying silicon layer. Electrical isolation of the silicon fingers is realized with a slight silicon undercut etch, which disconnects sufficiently narrow pieces of silicon under the CMOS microstructures. The 1 mm by 1 mm micromirror is made of an approximately 40 /spl mu/m-thick single-crystal silicon plate coated with aluminum from the CMOS interconnect stack. The mirror has a peak-to-peak curling of 0.5 /spl mu/m. Fabrication starts with a conventional CMOS process followed by dry-etch micromachining steps. There is no need for wafer bonding and accurate front-to-backside alignment. Such capability has potential applications in biomedical imaging, optical switches, optical scanners, interferometric systems, and vibratory gyroscopes.

Fabrication, characterization, and analysis of a DRIE CMOS-MEMS gyroscope

A gyroscope with a measured noise floor of 0.02/spl deg//s/Hz/sup 1/2/ at 5 Hz is fabricated by post-CMOS micromachining that uses interconnect metal layers to mask the structural etch steps. The 1 /spl times/ 1 mm lateral-axis angular rate sensor employs in-plane vibration and out-of-plane Coriolis acceleration detection with on-chip CMOS circuitry. The resultant device incorporates a combination of 1.8-/spl mu/m-thick thin-film structures for springs with out-of-plane compliance and 60-/spl mu/m-thick bulk silicon structures defined by deep reactive-ion etching for the proof mass and springs with out-of-plane stiffness. The microstructure is flat and avoids excessive curling, which exists in prior thin-film CMOS-microelectromechanical systems gyroscopes. Complete etch removal of selective silicon regions provides electrical isolation of bulk silicon to obtain individually controllable comb fingers. Direct motion coupling is observed and analyzed.

A hierarchical circuit-level design methodology for microelectromechanical systems

A circuit-level methodology for hierarchical design and nodal simulation of microelectromechanical systems (MEMS) is presented. A layout-based schematic view is introduced as a geometrically intuitive MEMS representation that is transformed into a schematic suitable for behavioral simulation. As examples of the design methodology, suspended-MEMS inertial sensors, resonant actuators, and filters are designed using a small set of geometrically parameterized plate, beam, gap, and anchor elements. Effects of manufacturing variations are evaluated by simply changing parameter values of the elements.