We seek to harness microelectromechanical systems (MEMS) technologies to build biomimetic devices for low-power,
high-performance, robust sensors and actuators on micro-autonomous robot platforms. Hair is used abundantly in nature
for a variety of functions including balance and inertial sensing, flow sensing and aerodynamic (air foil) control, tactile
and touch sensing, insulation and temperature control, particle filtering, and gas/chemical sensing. Biological hairs,
which are typically characterized by large surface/volume ratios and mechanical amplification of movement, can be
distributed in large numbers over large areas providing unprecedented sensitivity, redundancy, and stability (robustness).
Local neural transduction allows for space- and power-efficient signal processing. Moreover by varying the hair structure
and transduction mechanism, the basic hair form can be used for a wide diversity of functions. In this paper, by
exploiting a novel wafer-level, bubble-free liquid encapsulation technology, we make arrays of micro-hydraulic cells
capable of electrostatic actuation and hydraulic amplification, which enables high force/high deflection actuation and
extremely sensitive detection (sensing) at low power. By attachment of cilia (hair) to the micro-hydraulic cell, air flow
sensors with excellent sensitivity (< few cm/s) and dynamic range (> 10 m/s) have been built. A second-generation
design has significantly reduced the sensor response time while maintaining sensitivity of about 2 cm/s and dynamic
range of more than 15 m/s. These sensors can be used for dynamic flight control of flying robots or for situational
awareness in surveillance applications. The core biomimetic technologies developed are applicable to a broad range of
sensors and actuators.
To power distributed wireless sensor networks on bridges, traditional power cables or battery replacement are
excessively expensive or infeasible. This project develops two power harvesting technologies. First, a novel parametric
frequency-increased generator (PFIG) is developed. The fabricated PFIG harvests the non-periodic and unprecedentedly
low frequency (DC to 30 Hz) and low acceleration (0.55-9.8 m/s<sup>2</sup>) mechanical energy available on bridges with an
average power > 2 μW. Prototype power conversion and storage electronics were designed and the harvester system was
used to charge a capacitor from arbitrary bridge-like vibrations. Second, an RF scavenger operating at medium and
shortwave frequencies has been designed and tested. Power scavenging at MHz frequencies allows for lower antenna
directivities, reducing sensitivity to antenna positioning. Furthermore, ambient RF signals at these frequencies have
higher power levels away from cities and residential areas compared to the UHF and SHF bands utilized for cellular
communication systems. An RF power scavenger operating at 1 MHz along with power management and storage
circuitry has been demonstrated. It powers a LED at a distance of 10 km from AM radio stations.
Bridges are an important societal resource used to carry vehicular traffic within a transportation network. As such, the
economic impact of the failure of a bridge is high; the recent failure of the I-35W Bridge in Minnesota (2007) serves as a
poignant example. Structural health monitoring (SHM) systems can be adopted to detect and quantify structural
degradation and damage in an affordable and real-time manner. This paper presents a detailed overview of a multi-tiered
architecture for the design of a low power wireless monitoring system for large and complex infrastructure systems. The
monitoring system architecture employs two wireless sensor nodes, each with unique functional features and varying
power demand. At the lowest tier of the system architecture is the ultra-low power Phoenix wireless sensor node whose
design has been optimized to draw minimal power during standby. These ultra low-power nodes are configured to
communicate their measurements to a more functionally-rich wireless sensor node residing on the second-tier of the
monitoring system architecture. While the Narada wireless sensor node offers more memory, greater processing power
and longer communication ranges, it also consumes more power during operation. Radio frequency (RF) and mechanical vibration power harvesting is integrated with the wireless sensor nodes to allow them to operate freely for long periods of time (e.g., years). Elements of the proposed two-tiered monitoring system architecture are validated upon an operational long-span suspension bridge.
An overview of wafer-level packaging technologies developed at the University of Michigan is presented. Two
sets of packaging technologies are discussed: (i) a low temperature wafer-level packaging processes for
vacuum/hermeticity sealing, and (ii) an environmentally resistant packaging (ERP) technology for thermal and
mechanical control as well as vacuum packaging.
The low temperature wafer-level encapsulation processes are implemented using solder bond rings which are
first patterned on a cap wafer and then mated with a device wafer in order to encircle and encapsulate the device at
temperatures ranging from 200 to 390 °C. Vacuum levels below 10 mTorr were achieved with yields in an optimized
process of better than 90%. Pressures were monitored for more than 4 years yielding important information on
reliability and process control.
The ERP adopts an environment isolation platform in the packaging substrate. The isolation platform is
designed to provide low power oven-control, vibration isolation and shock protection. It involves batch flip-chip
assembly of a MEMS device onto the isolation platform wafer. The MEMS device and isolation structure are
encapsulated at the wafer-level by another substrate with vertical feedthroughs for vacuum/hermetic sealing and
electrical signal connections. This technology was developed for high performance gyroscopes, but can be applied to
any type of MEMS device.
The long-term deterioration of large-scale infrastructure systems is a critical national problem that if left unchecked,
could lead to catastrophes similar in magnitude to the collapse of the I-35W Bridge. Fortunately, the past decade has
witnessed the emergence of a variety of sensing technologies from many engineering disciplines including from the
civil, mechanical and electrical engineering fields. This paper provides a detailed overview of an emerging set of sensor
technologies that can be effectively used for health management of large-scale infrastructure systems. In particular, the
novel sensing technologies are integrated to offer a comprehensive monitoring system that fundamentally addresses the
limitations associated with current monitoring systems (for example, indirect damage sensing, cost, data inundation and
lack of decision making tools). Self-sensing materials are proposed for distributed, direct sensing of specific damage
events common to civil structures such as cracking and corrosion. Data from self-sensing materials, as well as from
more traditional sensors, are collected using ultra low-power wireless sensors powered by a variety of power harvesting
devices fabricated using microelectromechanical systems (MEMS). Data collected by the wireless sensors is then
seamlessly streamed across the internet and integrated with a database upon which finite element models can be
autonomously updated. Life-cycle and damage detection analyses using sensor and processed data are streamed into a
decision toolbox which will aid infrastructure owners in their decision making.
Although MEMS technologies and device structures have made significant progress in the past three decades and have found widespread application in many areas, including Micro-Opto-Electro-Mechanical Systems (MOEMS), packaging and assembly techniques suitable for many of these emerging applications have not kept pace. Packaging is one of the most costly parts of microsystem manufacturing, and it is also often the first to fail or negatively influence the system response. This paper addresses the packaging and assembly challenges of microsystems and MEMS for different applications. Hermetic and vacuum micropackaging, wafer-level packaging and bonding, and miniature sealed interconnection and feedthrough technologies will be reviewed. Results from long-term accelerated testing, and from in-situ tests, especially in biological hosts, will also be discussed. Issues and challenges facing packaging of MOEMS will be discussed.
High aspect ratio beam/trench arrays are etched into silicon substrates using a Surface Technology Systems (STS) deep reactive ion etch (RIE) tool. Process input parameters are varied using high/low values for etch cycle time, passivation cycle time, RF power, and SF<SUB>6</SUB> flow rate. The silicon etch process is characterized using photo-resist masked trench arrays varied from 1.5micrometers through 6micrometers in both width and spacing. A design of experiments (DOE) approach is used to model the following measured outputs: 1) trench depth (R<SUP>2</SUP>=0.985), 2) lateral trench etch (R<SUP>2</SUP>=0.852), 3) trench sidewall angle (R<SUP>2</SUP>=0.815), and 4) aspect ratio dependent etch (R<SUP>2</SUP>=0.942), where R<SUP>2</SUP> represents the correlation between actual and model predicted values. The presented characterization models are employed to form beams as small as 300nm wide beams etched to a depth >15micrometers with near vertical sidewalls using standard photolithography equipment. In addition, the provided models are exploited to produce a dual re-entrant/tapered beam etch release process. Released silicon beams are demonstrated over 1200micrometers long and 30micrometers thick with a base width of 300nm.
This paper reviews recent developments in micromachining technologies for the fabrication of microsensors, microactuators, and integrated microsystems, and discusses the requirements that micromachining technologies have to satisfy for many present and emerging applications. First, the paper discusses the challenges that micromachining technologies have to overcome and features that they have to provide for many future applications. Micromachining technologies have to be simple so that high yield and low cost can be achieved in manufacturing , they have to be capable of producing microstructures with a variety of shapes and sizes in all three dimensions, many of them have to be compatible with integration with electronics, they have to be capable of providing packaging and encapsulation at the wafer level for many devices that require operation in hermetic and/or vacuum environments, and finally they have to be capable of supporting a mixed set of materials, technologies and devices. Significant progress has been made in all of these areas during the past few years and several groups have developed new techniques that satisfy some or all of these requirements. The paper also reviews the most recent advances in the three mainstream technologies of bulk silicon micromachining, surface micromachining, and electroplating techniques. As microsystems become more complex, these three mainstream technologies will be increasingly used and combined to build complex systems at low cost.
It has previously been shown that bulk silicon micromachining using a dissolved wafer process with an impurity based etch stop can be used to fabricate structures with variable thickness, high (thickness-to- width) aspect ratios, and overhanging features, as well as structures with multiple stacked levels. This technology has been further extended in the development of silicon micromachined thermal profilers (SMTPs). The SMTP consists of a polysilicon-gold thermocouple located at the tip of a probe shank that overhangs the edge of the substrate. The probe shank is suspended by flexible beams, and can be electrostatically excited into motion by comb drives. A polysilicon heater is built into the base of the shank to provide a thermal bias. As the thermocouple is scanned across a sample, the varying proximity and temperature of features on the sample surface can be mapped as a function of position, providing high resolution topographic and thermographic information. The same principle can also be used in photothermal absorption spectroscopy, microelectronic metrology, and microflow measurements. Advanced versions of the SMTP suspend the thermocouple on a dielectric diaphragm for improved thermal isolation, and replace it with a thermopile for a larger readout signal. Devices have been fabricated using an IC-compatible, 8 mask, single-sided process that has general applicability beyond the SMTP. Preliminary data from test scans is presented.
This paper reviews the field of silicon-based integrated microsensors. These devices are implemented using fabrication technologies developed for integrated circuits, and lend themselves easily to low cost and high volume production. They are typically much smaller, more precise and stable, have less drift, and are much more reliable than their discrete counterparts. The addition of on-chip integrated circuits to the silicon sensors enables the processing and amplification of low level signals recorded by the sensors, and allows the device to be more easily interfaced and integrated into an electronic measurement and control system. The fabrication of silicon integrated microsensors is reviewed, and a number of examples are discussed. These examples include a multichannel integrated silicon microprobe for the recording of neural signals form the brain cells, and a multi-element infrared thermal detector based on silicon-gold thermopiles.