Christopher G. Levey

An untethered, electrostatic, globally controllable MEMS micro-robot

We present an untethered, electrostatic, MEMS micro-robot, with dimensions of 60 /spl mu/m by 250 /spl mu/m by 10 /spl mu/m. The device consists of a curved, cantilevered steering arm, mounted on an untethered scratch drive actuator (USDA). These two components are fabricated monolithically from the same sheet of conductive polysilicon, and receive a common power and control signal through a capacitive coupling with an underlying electrical grid. All locations on the grid receive the same power and control signal, so that the devices can be operated without knowledge of their position on the substrate. Individual control of the component actuators provides two distinct motion gaits (forward motion and turning), which together allow full coverage of a planar workspace. These MEMS micro-robots demonstrate turning error of less than 3.7/spl deg//mm during forward motion, turn with radii as small as 176 /spl mu/m, and achieve speeds of over 200 /spl mu/m/sec with an average step size as small as 12 nm. They have been shown to operate open-loop for distances exceeding 35 cm without failure, and can be controlled through teleoperation to navigate complex paths. The devices were fabricated through a multiuser surface micromachining process, and were postprocessed to add a patterned layer of tensile chromium, which curls the steering arms upward. After sacrificial release, the devices were transferred with a vacuum microprobe to the electrical grid for testing. This grid consists of a silicon substrate coated with 13-/spl mu/m microfabricated electrodes, arranged in an interdigitated fashion with 2-/spl mu/m spaces. The electrodes are insulated by a layer of electron-beam-evaporated zirconium dioxide, so that devices placed on top of the electrodes will experience an electrostatic force in response to an applied voltage. Control waveforms are broadcast to the device through the capacitive power coupling, and are decoded by the electromechanical response of the device body. Hysteresis in the system allows on-board storage of n=2 bits of state information in response to these electrical signals. The presence of on-board state information within the device itself allows each of the two device subsystems (USDA and steering arm) to be individually addressed and controlled. We describe this communication and control strategy and show necessary and sufficient conditions for voltage-selective actuation of all 2/sup n/ system states, both for our devices (n=2), and for the more general case (where n is larger.).

Planar Microassembly by Parallel Actuation of MEMS Microrobots

We present designs, theory, and results of fabrication and testing for a novel parallel microrobotic assembly scheme using stress-engineered MEMS microrobots. The robots are 240-280 mum times 60 mum times 7-20 mum in size and can be controlled to dock compliantly together, forming planar structures several times this size. The devices are classified into species based on the design of their steering arm actuators, and the species are further classified as independent if they can be maneuvered independently using a single global control signal. In this paper, we show that microrobot species are independent if the two transition voltages of their steering arms, i.e., the voltages at which the arms are raised or lowered, form a unique pair. We present control algorithms that can be applied to groups of independent microrobot species to direct their motion from arbitrary nondead-lock configurations to desired planar microassemblies. We present designs and fabrication for four independent microrobot species, each with a unique transition voltage. The fabricated microrobots are used to demonstrate directed assembly of five types of planar structures from two classes of initial conditions. We demonstrate an average docking accuracy of 5 mum and use self-aligning compliant interaction between the microrobots to further align and stabilize the intermediate assemblies. The final assemblies match their target shapes on average 96%, by area.

Integrating Magnetics for On-Chip Power: A Perspective

Integration of efficient power converters requires technology for efficient, high-power on-chip inductors and transformers. Increases in switching frequency, facilitated by advances in circuit designs and silicon or wide-bandgap semiconductors, can enable miniaturization, but only if the magnetics technology works well at the higher frequencies. Technologies, geometries, and scaling of air-core and magnetic-core inductors and transformers are examined, and their potential for integration is discussed. Air-core inductors can use simpler fabrication, and increasing frequency can always be used to decrease their size, but magnetic cores can decrease the required thickness without requiring as high a frequency.

Power delivery and locomotion of untethered microactuators

The ability for a device to locomote freely on a surface requires the ability to deliver power in a way that does not restrain the devices motion. This paper presents a MEMS actuator that operates free of any physically restraining tethers. We show how a capacitive coupling can be used to deliver power to untethered MEMS devices, independently of the position and orientation of those devices. Then, we provide a simple mechanical release process for detaching these MEMS devices from the fabrication substrate once chemical processing is complete. To produce these untethered microactuators in a batch-compatible manner while leveraging existing MEMS infrastructure, we have devised a novel postprocessing sequence for a standard MEMS multiproject wafer process. Through the use of this sequence, we show how to add, post hoc , a layer of dielectric between two previously deposited polysilicon films. We have demonstrated the effectiveness of these techniques through the successful fabrication and operation of untethered scratch drive actuators. Locomotion of these actuators is controlled by frequency modulation, and the devices achieve maximum speeds of over 1.5 mm/s.

A Technology Overview of the PowerChip Development Program

The PowerChip research program is developing technologies to radically improve the size, integration, and performance of power electronics operating at up to grid-scale voltages (e.g., up to 200V) and low-to-moderate power levels (e.g., up to 50W) and demonstrating the technologies in a high-efficiency light-emitting diode driver, as an example application. This paper presents an overview of the program and of the progress toward meeting the program goals. Key program aspects and progress in advanced nitride power devices and device reliability, integrated high-frequency magnetics and magnetic materials, and high-frequency converter architectures are summarized.

Planning and control for microassembly of structures composed of stress-engineered MEMS microrobots

We present control strategies that implement planar microassembly using groups of stress-engineered MEMS microrobots (MicroStressBots) controlled through a single global control signal. The global control signal couples the motion of the devices, causing the system to be highly underactuated. In order for the robots to assemble into arbitrary planar shapes despite the high degree of underactuation, it is desirable that each robot be independently maneuverable (independently controllable). To achieve independent control, we fabricated robots that behave (move) differently from one another in response to the same global control signal. We harnessed this differentiation to develop assembly control strategies, where the assembly goal is a desired geometric shape that can be obtained by connecting the chassis of individual robots. We derived and experimentally tested assembly plans that command some of the robots to make progress toward the goal, while other robots are constrained to remain in small circular trajectories (closed-loop orbits) until it is their turn to move into the goal shape. Our control strategies were tested on systems of fabricated MicroStressBots. The robots are 240-280 μm × 60 μm × 7-20 μm in size and move simultaneously within a single operating environment. We demonstrated the feasibility of our control scheme by accurately assembling five different types of planar microstructures.

Power delivery and locomotion of untethered micro-actuators

This paper presents a micro-actuator that operates free of any physically restraining tethers. We show how capacitive coupling can be used to deliver power to MEMS devices, independently of the position and orientation of those devices. Then, we provide a simple mechanical release process for detaching MEMS devices from the fabrication substrate once chemical processing is complete. To produce these untethered micro-actuators in a batch-compatible manner while leveraging existing MEMS infrastructure, we have devised a novel post-processing sequence for the PolyMUMPS process. Through the use of this sequence, we show how to add, post hoc, a layer of dielectric between two previously-deposited polysilicon films. We have demonstrated the effectiveness of these techniques through the successful fabrication and operation of untethered scratch drive actuators. Locomotion of these actuators is controlled by frequency modulation, and the devices achieve speeds of over 1.5 mm/sec.

Fabrication of thin-film V-groove inductors using composite magnetic materials

A new fabrication process is described for high performance embedded or integrated inductors for power converters. The process includes etching V-grooves in a silicon substrate, depositing granular composite magnetic materials, and electroplating the copper conductors.

Microfabricated V-groove power inductors using multilayer Co-Zr-O thin films for very-high-frequency DC-DC converters

Microfabricated V-groove inductors targeted to operate above 10 MHz are fabricated and tested. Multilayer nano-granular Co-Zr-O/ZrO2 magnetic thin films are used as the core material of the inductors to improve the magnetic performance of the films deposited on the sidewalls of V-grooves and to control eddy-current loss in the core. Prototype V-groove inductors are fabricated based on optimization results for 7-V to 3.3-V, 1-A dc-dc buck converters. The inductors exhibit inductance of 3.4 nH from 10 MHz to 100 MHz, dc resistance of 3.83 mΩ, and quality factor up to 66. The prototype inductors are a promising candidate for high-power-density high-efficiency dc-dc converters. The measured inductor performance indicates that they could be used to make a 7-V to 3.3-V, 1-A converter that would exhibit power density of 2.5 W/mm2 and efficiency of 86% at 100 MHz; or power density of 0.36 W/mm2 and efficiency of 91% at 11 MHz.

Simultaneous Control of Multiple MEMS Microrobots

We present control algorithms that implement a novel planar microassembly scheme using groups of stress-engineered microrobots controlled through a single global control signal. The global control signal couples the motion of the devices, causing the system to be highly underactuated. Despite the high degree of underactuation, it is desirable that each robot be independently maneuverable. By exploiting differences in the designs and the resulting electromechanical interaction with the control signal, the behavior of the individual robots can be differentiated. We harness this differentiation by designing the control signal such that some devices remain confined in small circular orbits (limit cycles), while the non-orbiting robots perform work by making progress towards the goal. The control signal is designed to minimize the number of independent control voltage levels that are used for independent control, allowing us to maximize the number of simultaneously controllable devices.