Richard Mlcak

Micro hydraulic transducer technology for actuation and power generation

The paper introduces a novel transducer technology, called the solid-state micro-hydraulic transducer, currently under development at MIT. The new technology is enabled through integration of micromachining technology, piezoelectrics, and microhydraulic concepts. These micro-hydraulic transducers are capable of bi-directional electromechanical energy conversion, i.e., they can operate as both an actuator that supplies high mechanical force in response to electrical input and an energy generator that transduces electrical energy from mechanical energy in the environment. These transducers are capable of transducing energy at very high specific power output in the order of 1 kW/kg, and thus, they have the potential to enable many novel applications. The concept, the design, and the potential applications of the transducers are presented. Present efforts towards the development of these transducers, and the challenges involved therein, are also discussed.

Inorganic sensors utilizing MEMS and microelectronic technologies

Abstract The most significant recent advances in achieving sensor sensitivity and selectivity has been tied to the integration of thin film sensors with micromachined and/or microelectronic technologies. Arrays of micro-hotplates, microcantilever beams and metal oxide semiconductor devices show great promise as chemical sensor platforms exhibiting enhanced selectivity and markedly reduced power dissipation. Microcantilever beams, presently under development, demonstrate exceptional sensitivity for thermal, optical and magnetic stimuli.

Photoassisted electrochemical micromachining of silicon in HF electrolytes

Abstract We have demonstrated the ability to fabricate stress-free micromechanical cantilever beams by selective etching of silicon p—n structures in HF solutions utilizing a photoassisted electrochemical process. A particular novelty of this technique is that n or p regions of a p—n structure may be selectively etched at controlled rates by appropriate choice of cell bias, p—n junction bias, and illumination intensity. p-Si is selectively etched by either one of two ways. Illumination of the p—n junction serves to bias the p-layer anodically relative to the n-substrate, resulting in etch rates of up to 0.6 μm/min. Alternatively, p-Si etch rates up to 10 μm/min are attained without illumination by short circuiting the p—n junction and anodically biasing the n-Si substrate. n-Si, on the other hand, is selectively etched at rates up to 10 μm/min by illuminating and reverse biasing the p—n junction, driving the p-layer cathodic. At etch rates below approximately 1 μm/min, porous silicon layers form, which can be subsequently removed with chemical etchants. These processes are characterized by high-resolution etch-stops with smooth surfaces, rendering them potentially attractive for micromachining purposes. The effects of key variables, including doping type, cell bias, p—n junction bias, and illumination intensity, on etch rate, selectivity, and surface finish are discussed.

Design, fabrication, and testing of a piezoelectrically driven high flow rate micro-pump

Towards the development of novel class of miniature transducers with very high specific power, a high frequency and high flow rate hydraulic micro-pump with passive check valves was fabricated and tested. The micro-pump features a small piezoelectric (PZT-5H/PZN-PT) cylinder integrated into micromachined silicon and pyrex chips. The piezoelectric cylinder bonded to a thick circular disk serves as the drive element in the pump chamber, and two axisymmetric silicon membranes with hollow annular bosses serve as system check valves. Using silicone oil as the working fluid, the performance of the micro-pump was tested by varying voltages from 0 to 1600 V and frequencies from 1 kHz to 12 kHz. The micro-pump with PZT-5H element achieved a high flow rate of 2.5 ml/min at 1200 V and 4.5 kHz.

Photo-assisted electrochemical machining of micromechanical structures

Stress-free micromechanical cantilever beams have been fabricated by selective etching of silicon p-n structures in HF solutions utilizing a photo-assisted electrochemical process. With this technique, n or p regions can be selectively etched at controlled rates by appropriate choice of cell bias, p-n junction bias, and illumination intensity. p-Si can be selectively etched by utilizing illumination of the p-n junction to anodically bias the p-layer relative to the n-substrate. Etch rates of up to 5 mu m/min resulted in the formation of porous layers readily removed with chemical Si etching solutions. n-Si can be selectively etched by illuminating and applying a reverse bias across the p-n junction, driving the p-layer cathodic. Etch rates up to 10 mu m/min and high resolution etch stops with smooth surfaces were obtained. The effects of key variables, including doping type, cell bias, p-n junction bias, and illumination intensity, on etch rate, selectivity, and surface finish are discussed.<<ETX>>

Advanced Sensor Technology Based on Oxide Thin Film—MEMS Integration

Many of the most significant advances in achieving improved sensor performance have been tied to the integration of thin film oxides with micromachined and microelectronic technologies (MEMS). Arrays of microcantilever beams or microhotplates show great promise as chemical sensor platforms exhibiting enhanced selectivity and markedly reduced power dissipation. Microcantilever beams demonstrate exceptional sensitivity to chemical, thermal and other physical stimuli. Microhotplates, provide the opportunity to discriminate between chemical species by control of reaction rates. We review recent key developments in the micromachining of suitable structures, the means for incorporating active oxide films and the performance of individual and arrays of sensor devices.

Photo-assisted silicon micromachining : opportunities for chemical sensing

Abstract Micromachining methods allow for the fabrication of small, three-dimensional structures including membranes, micro-valves, and pumps, channels, etc., in materials such as silicon. When integrated with microcircuitry this provides for the opportunity of creating unique miniature ‘smart’ chemical sensing devices. We present a novel method of photo-assisted electrochemical silicon micromachining which allows for the fabrication of stress-free submicrometer thick beams, membranes, and high aspect ratio three-dimensional structures, whose complex surface topologies were previously inaccessible by conventional processing routes. We discuss a number of gas and chemical sensor structures which benefit from MEMS technology and would benefit further from the processing versatility provided by our new micromachining fabrication method.

MEMS Microresonators for High Temperature Sensor Applications

Microelectromechanical Systems (MEMS) are being extensively investigated as a means of miniaturizing piezoelectric sensors thereby offering higher sensitivity, reduced power consumption, and ability to form compact multi-sensor arrays. Such devices typically employ one or more silicon micromechanical elements (e.g. membranes, cantilever beams and tethered proof masses) driven electromechanically by a polycrystalline piezoelectric film. The use of polycrystalline materials results in inherently less stable and irreproducible device characteristics. For elevated operating temperatures, more robust and refractory materials are also required. In this paper, we describe a MEMS microresonator array capable of operating to temperatures exceeding 600°C enabled by the integration of epitaxially grown piezoelectric AlN films onto single crystal SiC tethered plates. The operation of the microresonators as sensors is illustrated by examining their response to temperature, pressure and chemical analytes.

A High-Frequency, High-Stiffness Piezoelectric Micro-Actuator for Hydraulic Applications

A piezoelectric micro-actuator capable of high stiffness actuation in micro-hydraulic systems was fabricated and experimentally tested to frequencies in excess of 100 kHz. The actuator was fabricated from a bonded stack of micromachined silicon-on-insulator (SOI) and borosilicate glass layers. Actuation was provided by 1mm sized piezoelectric cylinders, which were integrated within a tethered piston structure and electrically and mechanically attached using a thin-film AuSn eutectic bond. Die-level anodic bonding techniques were developed to assemble the supporting structural silicon and glass layers. The microfabrication, device assembly, experimental testing procedures, and actuator performance are discussed in this paper. Issues such as piezoelectric material preparation, requisite dimensional tolerancing, micromachining of the silicon tethered structures, and integration of multiple piezoelectric elements within the micro-actuator structure are detailed.

MEMS-based gravimetric sensors for explosives detection

A compact, rapid system for trace detection of explosives using MEMS-bases resonant structures is described. Various software algorithms are used to eliminate problems in previous implementations of array-based detectors. Results for detection of trace levels of explosives and other threat agents are given.