Robust Microelectromechanical Systems Research Articles

Yield-aware multi-objective optimization of a MEMS accelerometer system using QMC-based methodologies

This paper proposes a novel yield-aware optimization methodology that can be used for mixed-domain synthesis of robust micro-electro-mechanical systems (MEMS). The robust Pareto front optimization of a MEMS accelerometer system, which includes a capacitive MEMS sensor and an analog read-out circuitry, is realized by co-optimization of the mixed-domain system where the sensor performances are evaluated using highly accurate analytical models and the circuit level simulations are carried out by an electrical simulator. Two different approaches for yield-aware optimization have been implemented in the synthesis loop. The Quasi Monte Carlo (QMC) technique has been used to embed the variation effects into the optimization loop. The results for both two- and three-dimensional yield-aware optimization are quite promising for robust MEMS accelerometer synthesis.

In-Situ Contact Surface Characterization in a MEMS Ohmic Switch under Low Current Switching

To develop robust microelectromechanical systems (MEMS) switching technology for low voltage direct current (DC) applications (1–12 V) there is a requirement for the investigation of wear caused by hot switching (contact operated while carrying a current load). Previous investigation of contact wear in the ohmic MEMS switch has been limited to either the completion of the contact switching cycles, where the device is destructively opened, or by low switching rates, making lifetime testing impractical. A novel MEMS testing platform is described that is capable of both resolving microscale changes on the contact surface between switching events and sustained high frequency switch cycling, enabling practical lifetime testing. The platform is used to investigate early surface changes in a thin-film Au contact pair on a cycle-by-cycle basis. The contact is closed at forces representative of a practical MEMS contact (<1 mN). The apparatus reveals the microscopic surface change between individual switching events. Hot switched contact wear is dominated by the molten metal bridge (MMB) phenomenon, linked to a characteristic voltage transient at contact opening and the gradual process of contact material transfer; however, during hot switching delamination phenomena are also observed, and associated with a step change in contact voltage and a greater level of surface damage.

Investigation of Al/CuO multilayered thermite ignition

The ignition of the Al/CuO multilayered material is studied experimentally to explore the effects of the heating surface area, layering, and film thickness on the ignition characteristics and reaction performances. After the description of the micro-initiator devices and ignition conditions, we show that the heating surface area must be properly calibrated to optimize the nanothermite ignition performances. We demonstrated experimentally that a heating surface area of 0.25 mm2 is sufficient to ignite a multilayered thermite film of 1.6 mm wide by a few cm long, with a success rate of 100%. A new analytical and phenomenological ignition model based on atomic diffusion across layers and thermal exchange is also proposed. This model considers that CuO first decomposes into Cu2O, and then the oxygen diffuses across the Cu2O and Al2O3 layers before reaching the Al layer, where it reacts to form Al2O3. The theoretical results in terms of ignition response times confirm the experimental observation. The increase of the heating surface area leads to an increase of the ignition response time and ignition power threshold (go/no go condition). We also provide evidence that, for any heating surface area, the ignition time rapidly decreases when the electrical power density increases until an asymptotic value. This time point is referred to as the minimum response ignition time, which is a characteristic of the multilayered thermite itself. At the stoichiometric ratio (Al thickness is half of the CuO thickness), the minimum ignition response time can be easily tuned from 59 μs to 418 ms by tuning the heating surface area. The minimum ignition response time increases when the bilayer thickness increases. This work not only provides a set of micro-initiator design rules to obtain the best ignition conditions and reaction performances but also details a reliable and robust MicroElectroMechanical Systems process to fabricate igniters and brings new understanding of phenomena governing the ignition process of Al/CuO multilayers.

Metallic Glass Hemispherical Shell Resonators

By utilizing bulk metallic glasses' (BMGs) unique combination of amorphous structure, material properties, and fabrication opportunities, ultrasmooth and symmetric 3-D metallic glass resonators that are complimentary metal oxide semiconductor (CMOS) post-processing compatible are fabricated. Surface roughness to size ratio fabrication precision in the order of 100 parts per billion is demonstrated with a 3-mm diameter Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 BMG hemispherical shell with a thickness variation <;100 nm and a surface roughness of <;1 nm Ra. The resonator exhibits a resonant frequency of 13.9440 kHz ± 0.1 Hz with 0.035% frequency mismatch between degenerate N = 2 wineglass modes with a quality factor of 6200. This performance was obtained in the asmolded state without any device tuning or trimming. Another resonator with N = 2 resonant modes at 9.393 and 9.401 kHz, and quality factors of 7800 and 6500 was mounted into an integrated electrode system. Electrical readout by capacitive sensing in both time and frequency domains showed a resonance shift to 9.461 and 9.483 kHz, respectively. The quality factor was reduced to 5400 and 5300, respectively. This investigation demonstrates that BMG resonators may serve as a basis for robust microelectromechanical systems resonator devices with increased performance and low-cost fabrication techniques that exploits the atomic structure, unique softening behavior, strength, formability, and toughness of metallic glasses.

Special Issue on Thermodynamics, Flow, and Heat Transfer in MEMS

Microelectromechanical systems (MEMS) equipment and products have gradually penetrated into modern life due to recent advances in fabrication processes. Accompanied with the fast-growing power consumption and increasing integration, requirement for miniaturization, compactness, and high performance have become more and more remarkable, resulting in problems such as higher heat loads and higher thermal resistance. Thermal management has emerged as one of the critical bottlenecks for further development of diverse, robust, and ultracompact MEMS. Thermodynamics, fluid flow, and heat transfer are important physical processes in advanced MEMS and affiliated parts. A better understanding of the mechanisms of thermodynamics, fluid mechanics, and heat transfer in MEMS is desired for advanced MEMS thermal management.Extensive efforts have been directed toward advanced cooling schemes for removing high heat fluxes, e.g., direct and indirect liquid cooling, jet impingement, liquid-vapor phase change cooling, etc. MEMS thermal management demands high heat transfer coefficient, low pressure drop penalty, and low load-driven temperature nonuniformity. Besides, junction temperature rise and hot spots should be carefully manipulated. The main problems for single-phase cooling are temperature nonuniformity and relatively low heat transfer coefficient, while two-phase cooling is challenged by flow instabilities and possible associated large pressure drop penalty. Therefore, enhancement techniques might be incorporated in present cooling approaches for further heat transfer augmentation and flow stabilization. In an effort to synthesize recent development in MEMS thermal management and to improve our fundamental understanding of the involved scientific issues related to thermodynamics, fluid mechanics, and heat transfer, invitations were sent to a group of researchers, who have been actively involved in the research and development of MEMS thermal management, to submit their articles for the special issue of Journal of Electronic Packaging (JEP). After careful reviews of all the submissions following the standard reviewing process for regular JEP papers, 11 articles were finally accepted for this special issue.These articles address a variety of topics, including enhancement of single-phase cooling (e.g., enhancement by offset strip fins, pin-fins, grooves and obstacles, and dimples and protrusions), analytic and numerical models from microchannel heat sink level to system level (e.g., resistance-capacity network modeling of multicomponent systems, and porous media modeling of two-phase cooling with nonuniform power distribution), development of three-dimensional multicavity microprocessor chip stacks, a three-stage design approach for microchannel systems of multicore processors, on-chip power generation using ultrathin thermoelectric generators, etc. Recent advances and suggestions in integrated cooling systems were presented for three-dimensional integrated circuit. Besides, advances of wafer level packaging were reviewed.Although a wide range of topics are touched upon, it is not possible to cover the entire field of MEMS thermal management in a single journal issue. Some other important topics on thermodynamics, flow and heat transfer in MEMS, such as junction temperature rise, evaporative instability, two-phase cooling enhancement and thermodynamics of advanced electronic packaging, are not considered in this issue. However, no attempt was taken to make it a comprehensive review either. Instead, it is hoped that this special issue presents the readers with a snapshot of the recent trends and developments in thermodynamics, flow and heat transfer in MEMS. Most likely, these papers will generate and stimulate more interests and discussions, and consequently lead to more innovations and technology advances in MEMS thermal management and to promote the development of the next generation cooling schemes for MEMS.Finally, all the contributors of this special issue are gratefully acknowledged for their support and cooperation. I really appreciate the JEP Editor Professor Bahgat Sammakia for giving me the opportunity to edit a special issue on very timely topics. I also thank the assistant Editor Dr. Gary Miller for help to manage the process of the special issue. The present volume is the first special issue on thermodynamics, flow, and heat transfer in MEMS, and it is hoped that it forms the beginning of a series that will periodically stimulate researchers to publish the highlights and original articles reporting new advances in MEMS.

Topology optimization of MEMS considering etching uncertainties using the level‐set method

SUMMARYBecause of the specific fabrication process of microsystems, the geometric uncertainties are relatively large compared with the size of the structures. However, the performance of microstructures is very sensitive to these uncertainties especially when slender structural members are included in the design. This paper presents a level‐set‐based topology optimization method for robust microelectromechanical systems. The presented method focuses on the effects of geometric uncertainties on the performance of microelectromechanical systems in the topology optimization. The geometric uncertainties arising from the etching process are treated as bounded‐but‐unknown uncertainties. This approach can be used to obtain designs with optimal performance for the worst‐case. The etching uncertainties can be conveniently included in topology optimization using the level‐set method. The signed distance property of the level‐set function allows easy construction of under‐etched and over‐etched structures. Therefore, models with variable etching‐induced uncertainties can be easily constructed. The effectiveness of the proposed approach is demonstrated using benchmark problems including compliance minimization and compliant‐mechanism design problems. Although the reinitialization scheme used to maintain the signed‐distance property can cause problems in the optimization process, this effect is limited using an approximate Heaviside function with a relatively large bandwidth. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Environmentally Robust MEMS Vibratory Gyroscopes for Automotive Applications

Automotive applications are known to impose quite harsh environmental conditions such as vibration, shock, temperature, and thermal cycling on inertial sensors. Micromachined gyroscopes are known to be especially challenging to develop and commercialize due to high sensitivity of their dynamic response to fabrication and environmental variations. Meeting performance specifications in the demanding automotive environment with low-cost and high-yield devices requires a very robust microelectromechanical systems (MEMS) sensing element. This paper reviews the design trend in structural implementations that provides inherent robustness against structural and environmental parameter variations at the sensing element level. The fundamental approach is based on obtaining a gain and phase stable region in the frequency response of the sense-mode dynamical system in order to achieve overall system robustness. Operating in the stable sense frequency region provides improved bias stability, temperature stability, and immunity to environmental and fabrication variations.

Fabrication of implantable microshunt using a novel channel sealing technique

This letter reports the development of a novel mi- croshunt by combining biocompatible materials with well- established, robust microelectromechanical systems MEMS fabrication processes. The key features of our mi- croshunt fabrication process are summarized as follows: 1 Control over the thickness of channels' enclosure, 2 chan- nels' height scalable by multiple thick resist patterning, 3 structural material deposition by electroplating, and 4 an easy fabrication process suitable for mass production. These features were realized using electroplating for the gold structural material, photoresist sacrificial material, and e-beam evaporation of adhesion layers. The combination of these techniques allows precise control on the microchan- nels width, depth, and length. A novel channel sealing tech- nique that is extendable to more complex geometries is also presented. The developed method involves using a thin gold seed layer deposited via evaporation prior to electroplating the last structural layer, the cover of the channels. This method enhanced the electroplating process, yielding more control of the final thickness of the cover layer compared to methods consisting of overgrowing the channels until they seal. © 2008 Society of Photo-Optical Instrumentation Engineers.

Demonstration of Alternative Fabrication Techniques for Robust MEMS Device

This work describes efforts in the fabrication and testing of robust microelectromechanical systems (MEMS). Robustness is typically achieved by investigating non-silicon substrates and materials for MEMS fabrication. Some of the traditional MEMS fabrication techniques are applicable to robust MEMS, while other techniques are drawn from other technology areas, such as electronic packaging. The fabrication technologies appropriate for robust MEMS are illustrated through laminated polymer membrane based pressure sensor arrays. Each array uses a stainless steel substrate, a laminated polymer film as a suspended movable plate, and a fixed, surface micromachined back electrode of electroplated nickel. Over an applied pressure range from 0 to 34 kPa, the net capacitance change was approximately 0.14 pF. An important attribute of this design is that only the steel substrate and the pressure sensor inlet is exposed to the flow; i.e., the sensor is self-packaged.

Design of microelectromechanical systems for variability via chance-constrained optimization

In this paper we describe a robust design method for microelectromechanical systems (MEMS) subject to inherent geometric and material uncertainties. We propose to formulate the robust MEMS design with variability problem as a specific optimization problem, namely chance-constrained posynomial programming, in which the statistical variations of both the process parameters and design variables can be easily incorporated. Using such constrained optimization approach, automated robust design can be obtained with very suboptimal design cost, and parametric yield of each specification can be guaranteed. Finally, the method is illustrated by considering the example of a simple crab-leg resonator.

Growth and doping of SiC-thin films on low-stress, amorphous Si3N4/Si substrates for robust microelectromechanical systems applications

The N2-doped 3C-SiC thin films have been grown by low-pressure, chemical vapor deposition (LPCVD) on amorphous Si3N4/p-Si (111) substrates using the single, organosilane-precursor trimethylsilane [(CH3)3SiH]. The effects of N2 flow rate and growth temperature on the electrical properties of SiC films were investigated by Hall-effect measurements. The electron-carrier concentration is between 1017-1018/cm3. The lowest resistivities at 400 K and 300 K are 1.12 × 10-2 and 1.18 × 10-1 cm, respectively. The corresponding sheet resistances are 75.02 Ω/□ and 790.36 Ω/□. The SiC film structure was studied by x-ray diffraction. The 3C-SiC films oriented in the 〈111〉 direction with a 2θ peak at 35.5° and line widths between 0.18-0.25° were obtained. The SiC/Si3N4 interface is very smooth and free of voids. The fabrication of microelectromechanical (MEMS) structures incorporating the SiC films is discussed.