Michael W. Judy

Electrostatic-comb drive of lateral polysilicon resonators

This paper mvestlgates the elcctrostatlc dnve and sense of polynhcon resonators parallel to the substrate, using an mterdlgtated capacitor (electrostatic comb) Three expenmental methods are used nucroscoplc observation mth contmuous or stroboscopic iummatlon, capacltlve sensing using an amphtude-modulation technique and SEM observation The mtnnslc quality factor of the phosphorus-doped low-pressure chemlcal-vapordeposited (LPCVD) polyslhcon resonators is 49 000 + 2000, whereas at atmosphenc pressure, Q < 100 The finger gap IS found to have a more pronounced effect on comb charactenstlcs than finger width or length, as expected from simple theory

Self-adjusting microstructures (SAMS)

Composite LPCVD polysilicon/silicon nitride flexures have been fabricated on the sidewalls of previously patterned polysilicon mesas by anisotropic reactive-ion etching. Cantilever beams 450 nm thick (150 nm of silicon nitride and 300 nm of polysilicon) and 2.5 mu m wide (the mesa height) were fabricated. Upon release from the sidewall, the cantilever deflects laterally away from the mesa due to a large built-in bending moment arising from the compressive residual stress in the polysilicon layer and the tensile residual stress in the silicon nitride layer. End deflections of about 20 mu m are observed for 70 mu m-long cantilevers. This self-adjusting microstructure (SAMS) makes use of residual stresses in thin films to reduce intercomponent clearances or to apply preloads in micromechanical systems. The authors present a design theory for SAMS, describe the fabrication process in detail, and discuss the results of initial experiments.<<ETX>>

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

A modular process for integrating thick polysilicon MEMS devices with sub-micron CMOS

A new MEMS process module, called Mod MEMS, has been developed to monolithically integrate thick (5-10um), multilayer polysilicon MEMS structures with sub-micron CMOS. This process is particularly useful for advanced inertial MEMS products such as automotive airbag accelerometers where reduced cost and increased functionality is required, or low cost, high performance gyroscopes where thick polysilicon (>6um) and CMOS integration is required to increase poly mass and stiffness, and reduce electrical parasitics in order to optimize angular rate sensing. In this paper we will describe the new modular process flow, development of the critical unit process steps, integration of the module with a foundry sub-micron CMOS process, and provide test data on several inertial designs fabricated with this process.

Accurate Assessment of Packaging Stress Effects on MEMS Sensors by Measurement and Sensor–Package Interaction Simulations

In this paper, packaging-induced stress effects are assessed for microelectromechanical systems (MEMS) sensors. A packaged MEMS sensor may experience output signal shift (offset) due to the thermomechanical stresses induced by the plastic packaging assembly processes and external loads applied during subsequent use in the field. Modeling and simulation to minimize the stress-induced offset shift are essential for high-precision accelerometers, gyroscopes, and many other MEMS devices. Improvement of plastic package modeling accuracy is accomplished by correlating finite-element analysis package models using measured material properties and package warpage. Using a refined reduced-order MEMS sensor and package interaction model, device offset is simulated, optimized, and compared with data collected from a unique three-axis accelerometer, which uses a single mass for all three axes sensing. As a result, this accelerometer has achieved very low offset in all axes over device operation temperature range of to . Device offset performance was improved by at least five times after the MEMS design optimization as compared with the one prior to the optimization.

Dual-axis optical mirror positioning using a nonlinear closed-loop controller

This paper describes tilt angle control of a dual-axis optical mirror from the perspective of a fully integrated solution. Economical integration of the entire control system and high voltage drivers on-chip forces the use of simplified controllers that are realizable using standard small circuit blocks. Despite these constraints, the controller must guarantee fast 10 millisecond performance in the face of cross-axis coupling, electrostatic nonlinearity, nonlinear damping, higher order modes, and process variation. Following a brief description of mirror dynamics, a control solution conducive to integration is presented with experimental proof of concept using microprocessor control to emulate on-chip circuits.

Polysilicon hollow beam lateral resonators

The first microfabrication of hollow polysilicon beams is reported. Arrays of lateral resonators are designed, processed, and tested with resonant frequencies from 8 kHz to 0.5 MHz. The quality factor as a function of pressure of the hollow beam resonators is compared with solid beam resonators, and values as high as 34000 are obtained in vacuum. The hollow beam resonators are verified with theory and compared to resonators with solid cross sections.<<ETX>>

A very low cost, 3-axis, MEMS accelerometer for consumer applications

We report here on a novel 3-axis MEMS accelerometer intended to meet the requirements of a high volume, very low cost sensor in consumer gaming and hand-held applications. In order to balance the performance and cost requirements of such applications, this device achieves 3-axis sensing with a single proof-mass. This accomplishment required significant process, device, and test design novelty. Because of the extreme cost constraints in these markets, this design reexamines the core system trade-offs between single-chip integrated and 2-chip MEMS solutions. A 2-chip solution was chosen in this case. Minimizing package costs lead to the use of chip stacking in an over-molded plastic package. Finally, a sophisticated method of probing the sensor-only die was developed, which allowed the further development of a “no-shake” sensitivity trim algorithm which does not require mechanical stimulation of the device. Based on the design methodologies described here, a very low cost sensor with superior performance was achieved. The acceleration sensitivity achieved is 300mV/g with a noise limited resolution of 150 µg/√Hz. The offset voltage temperature coefficient is less than 0.3mV/degC.

Single-chip 1×84 MEMS mirror array for optical telecommunication applications

The inherent ability of silicon micromachining to provide a multitude of precision aligned optical components on a single die naturally facilitates optical communication trends towards installing larger cross-connects and transmitting many channels on individual fibers. A micromachined mirror array has been designed and fabricated using the Analog Devices, Inc. (ADI) Optical iMEMS (R) process. The single-chip mirror die consists of 84 mirrors arranged in a linear array with an average pitch of 95 μm. Each mirror is equipped with a pair of polysilicon actuation electrodes located beneath the mirror. These two electrodes allow each mirror to be independently rotated around the axis parallel to the long dimension of the array using off-chip voltage commands. An operating mirror tilt of +/- 2 degrees is achieved with less than 130 volts of actuation. The design objectives including high mirror fill factor, optimal air damping, low mirror-to-mirror cross talk, acceptable voltage levels, and robustness posed significant challenges. This paper will describe how these challenges were overcome using an interdigitated mirror layout. The mirrors were successfully fabricated with good yield and characterized through both customer and ADI testing.

Three-terminal test structure to measure stiction force using I-V data

Stiction is a major failure mechanism during the operation of accelerometers and hence it is important to know the stiction force that the structures encounter during use. We explore the possibility of devising an electrical technique for the direct measurement of in use stiction force. We have designed and fabricated three terminal test structures to measure both vertical and horizontal in use stiction. The measurement is not visual and is based on I-V data with the possibility of automation in the future. The structure consists of cantilever beams of different lengths each with an actuating pad and a detection pad. We measure the pull in voltage applied to the actuating pad, VPI , required to bring the cantilever beam in contact with the detection pad and the pull out voltage, VPO, at which the contact is broken. Using the Finite Element tool, ANSYS, a coupled electromechanical model is developed to determine the stiction force from the pull-in and pull-out voltages. We discuss the measurements in terms of the advantages and the shortcomings. We also discuss the sensitivity of the model to various material and geometric parameters and to the accuracy of the measurement.