Kurt E. Petersen

Silicon as a mechanical material

Single-crystal silicon is being increasingly employed in a variety of new commercial products not because of its well-established electronic properties, but rather because of its excellent mechanical properties. In addition, recent trends in the engineering literature indicate a growing interest in the use of silicon as a mechanical material with the ultimate goal of developing a broad range of inexpensive, batch-fabricated, high-performance sensors and transducers which are easily interfaced with the rapidly proliferating microprocessor. This review describes the advantages of employing silicon as a mechanical material, the relevant mechanical characteristics of silicon, and the processing techniques which are specific to micromechanical structures. Finally, the potentials of this new technology are illustrated by numerous detailed examples from the literature. It is clear that silicon will continue to be aggressively exploited in a wide variety of mechanical applications complementary to its traditional role as an electronic material. Furthermore, these multidisciplinary uses of silicon will significantly alter the way we think about all types of miniature mechanical devices and components.

Bulk micromachining of silicon

Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.

Process for in-plane and out-of-plane single-crystal-silicon thermal microactuators

Abstract A process to manufacture single-crystal thermal actuators using silicon fusion bonding and electrochemical etch stop is presented. The process permits the simultaneous creation of in-plane and out-of-plane thermal actuators together with levers suitable for both directions of actuation. A final dry-release step is used, permitting the manufacture of MOS or bipolar devices in conjunction with actuators. Out-of-plane actuation of vertically levered devices has been demonstrated. The −3 dB response frequency of out-of-plane actuators is approximately 1000 Hz in air. Novel levered in-plane devices which achieve deflections of up to 200 μm have been fabricated. An estimate of the upper bound of thermal actuator efficiency is presented.

Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures

Abstract New developments in deep reactive ion etching (DRIE) technology, when combined with silicon fusion bonding (SFB), make it possible, for the first time, to span nearly the entire range of microstructure thicknesses between surface and bulk micromachining, using only single-crystal silicon. The combination of these two powerful micromachining tools forms a versatile new technology for the fabrication of micromechanical devices. The two techniques are described and a process technology is presented. Some of the experimental structures and devices that have been demonstrated using this new process technology are discussed.

Accelerometer with integral bidirectional shock protection and controllable viscous damping

A micromachined accelerometer includes integral bidirectional shock protection and controllable viscous damping. The accelerometer includes a frame in which a seismic mass is disposed and coupled to the frame by one or more cantilever beams. Upper and lower stops are provided around the periphery of the seismic mass and around the interior of the frame to limit the travel distance of the seismic mass. The accelerometer is fabricated, preferably from monocrystalline silicon, by defining an annular recess which extends into a first surface of a silicon substrate. Next, a layer is formed over the surface of the substrate but not in contact with the lower surface of the recessed region. An annular-shaped region of the substrate extending from the bottom surface of the substrate to the layer is then removed to define the seismic mass and frame. Finally, portions of the layer are removed to define the cantilever beams and integral bidirectional stops.

Ultra-stable, high-temperature pressure sensors using silicon fusion bonding

Abstract Piezoresistive silicon-based pressure sensors have been fabricated which are capable of high-precision operation at temperatures as high as 250°C. These devices are fabricated by a unique silicon fusion bonding process in which resistors from one wafer are bonded to the oxidized surface of a second wafer. The intermediate oxide layer electrically isolates the resistors from each other, thereby providing the high-temperature capability. Chip performance is excellent, especially linearity and the stability of offset voltage and pressure sensitivity.

Simulation of microfluidic pumping in a genomic DNA blood-processing cassette

Microfluidic cassettes that perform integrated biological sample preparation and DNA analysis require fluidic control and transport mechanisms built into the device. In this study, pneumatically actuated diaphragm pumps and valves were employed to achieve precise fluidic manipulation and enabled the execution of several sample-processing steps within a single cassette. However, the design of the microfluidic cassette to accomplish this multi-step fluidic protocol required a complex three-dimensional fluid path through valves, bends, various sized passageways and a porous filter for cell capture. In order to understand the fluidic behavior in such a device, measurements were taken of the pneumatic pressure delivered to the diaphragm pump as it pushed sample through the complicated fluidic pathway. Simultaneously monitored were the resulting volumetric flow rate, and the corresponding pre- and post-filter fluid pressures. The data enabled the construction of a model that simulated the fluidic action through the device using established fluid mechanics theory that closely matched flow rate and pressure data. The ability to simulate the behavior of diaphragm pumping and resulting fluidic movements in complex microfluidic devices provides a greater comprehension of this phenomenon and a useful tool in the application to future devices for biochemical analysis.

Modeling of thermal and mechanical stresses in silicon microstructures

Abstract Modeling of sensors and microstructures using the finite element method (FEM) is described. In this work two modeling techniques are presented which ar

Fabrication of SOI wafers with buried cavities using silicon fusion bonding and electrochemical etchback

Abstract This paper describes a new technique for batch fabrication of silicon-on-insulator (SOI) wafers for microelectromechanical systems (MEMS) applications by silicon wafer bonding techniques. The process permits the inclusion of buried cavities in the SOI wafers, providing a useful tool for sensor and actuator fabrication using the resulting wafers. A low-cost electrochemical etchback step is used to define accurately the thickness of the remaining single-crystal material even though the two wafers are bonded with an intermediate insulating oxide layer. The results presented include guidelines for backside contact definition which maximize the useful silicon area as a function of doping level. The final single-crystal silicon thickness is uniform to within 0.05 μm (standard deviation) and does not require any costly high-accuracy polishing steps.

Toward Next Generation Clinical Diagnostic Instruments: Scaling and New Processing Paradigms

Looking toward future clinical diagnostic instruments, there is little debate as to the features that need improvement over the current state-of-the-art. Increasing the speed and sensitivity of the assays, while reducing costs are clear goals. Recently, it has become possible to microminiaturize fluidic and sensing components using micromachining and precision injection molding. There has been a large amount of interest and effort in the area of miniaturization of such systems, yet not all of the properties of fluidics and sensing methods improve as they are drastically reduced in size. It is clear that implementing miniaturized diagnostic instruments is not a matter of simply “shrinking” their conventional counterparts, nor of automating existing manual procedures. What is required to harness the full potential of scaling technologies is the use of design methods that take into account scaling effects and the development of completely new processing approaches. Beginning with a general overview of the relevant scaling principles, sample preparation and detection approaches are addressed in this context.