Wireless Intraocular Pressure Sensing Using Microfabricated Minimally Invasive Flexible-Coiled LC Sensor Implant

This paper presents an implant-based wireless pressure sensing paradigm for long-range continuous intraocular pressure (IOP) monitoring of glaucoma patients. An implantable parylene-based pressure sensor has been developed, featuring an electrical LC-tank resonant circuit for passive wireless sensing without power consumption on the implanted site. The sensor is microfabricated with the use of parylene C (poly-chlorop- xylylene) to create a flexible coil substrate that can be folded for smaller physical form factor so as to achieve minimally invasive implantation, while stretched back without damage for enhanced inductive sensor–reader coil coupling so as to achieve strong sensing signal. A data-processed external readout method has also been developed to support pressure measurements. By incorporating the LC sensor and the readout method, wireless pressure sensing with 1-mmHg resolution in longer than 2-cm distance is successfully demonstrated. Other than extensive on-bench characterization, device testing through six-month chronic in vivo and acute ex vivo animal studies has verified the feasibility and efficacy of the sensor implant in the surgical aspect, including robust fixation and long-term biocompatibility in the intraocular environment. With meeting specifications of practical wireless pressure sensing and further reader development, this sensing methodology is promising for continuous, convenient, direct, and faithful IOP monitoring

ULTRA-SMALL SCALE MECHANICAL PROPERTIES MEASUREMENT

In order for the microelectromechanical systems (MEMS) industry to continue to grow and advance, it is critical that methods are developed to determine the mechanical reliability of MEMS devices. This is particularly so for advanced devices with contacting, moving components, for which component strength is a key factor in determining reliability. The etching processes used to produce MEMS devices leave residual surface features that typically limit device strength and, consequently, device lifetime and reliability. In order to optimize MEMS device reliability, it is therefore necessary to understand and characterize the effects these etching processes have on MEMS-scale device strengths. At the micro and nano scales, however, conventional strength testing methods cannot be used, and a standardized test method for MEMS-scale strength measurement has yet to be established. The micro-scale theta specimen, shaped like the Greek-letter theta, acts as a tensile test specimen when loaded in compression by generating a uniform tensile stress in the central web of the specimen. Utilizing the theta specimen for strength measurements allows for simple micro-scale strength testing and assessment of etching effects, while removing the difficulties associated with gripping and loading specimens as well as minimizing potential misalignment effects. Micro-scale silicon theta samples were fabricated using techniques relevant to MEMS processing. Processing-structure relationships were determined with microscopy techniques measuring sample dimensional variations, etch quality, and surface roughness. Structure-properties relationships were determined using three techniques. Samples were tested by instrumented indentation testing (IIT) and finite element analysis determined sample strength. Sample set strength data were examined via Weibull statistics. Fractographic analysis determined initial fracture locations and fracture propagation behavior. Key scientific findings included: (1) directly relating the processing-induced etching quality of fabricated samples to sample strength, and (2) critical flaw size calculations from sample strength measurements that were consistent with sample surface roughness. Technical contributions included development of the micro-scale theta specimen fabrication methodology, super-resolution dimensional measurements, and extension of IIT to strength measurements. The micro-scale theta specimen and corresponding testing methodology have enabled successful determination of processing-structure-mechanical properties relationships for three processing approaches. This is vital to the determination of properties-performance relationships in MEMS devices

SOI RF-MEMS Based Variable Attenuator for Millimeter-Wave Applications

The most-attractive feature of microelectromechanical systems (MEMS) technology is that it enables the integration of a whole system on a single chip, leading to positive effects on the performance, reliability and cost. MEMS has made it possible to design IC-compatible radio frequency (RF) devices for wireless and satellite communication systems. Recently, with the advent of 5G, there is a huge market pull towards millimeter-wave devices. Variable attenuators are widely employed for adjusting signal levels in high frequency equipment. RF circuits such as automatic gain control amplifiers, broadband vector modulators, full duplex wireless systems, and radar systems are some of the primary applications of variable attenuators. This thesis describes the development of a millimeter-wave RF MEMS-based variable attenuator implemented by monolithically integrating Coplanar Waveguide (CPW) based hybrid couplers with lateral MEMS varactors on a Silicon–on–Insulator (SOI) substrate. The MEMS varactor features a Chevron type electrothermal actuator that controls the lateral movement of a thick plate, allowing precise change in the capacitive loading on a CPW line leading to a change in isolation between input and output. Electrothermal actuators have been employed in the design instead of electrostatic ones because they can generate relatively larger in-line deflection and force within a small footprint. They also provide the advantage of easy integration with other electrical micro-systems on the same chip, since their fabrication process is compatible with general IC fabrication processes. The development of an efficient and reliable actuator has played an important role in the performance of the proposed design of MEMS variable attenuator. A Thermoreflectance (TR) imaging system is used to acquire the surface temperature profiles of the electrothermal actuator employed in the design, so as to study the temperature distribution, displacement and failure analysis of the Chevron actuator. The 60 GHz variable attenuator was developed using a custom fabrication process on an SOI substrate with a device footprint of 3.8 mm x 3.1 mm. The fabrication process has a high yield due to the high-aspect-ratio single-crystal-silicon structures, which are free from warping, pre-deformation and sticking during the wet etching process. The SOI wafer used has a high resistivity (HR) silicon (Si) handle layer that provides an excellent substrate material for RF communication devices at microwave and millimeter wave frequencies. This low-cost fabrication process provides the flexibility to extend this module and implement more complex RF signal conditioning functions. It is thus an appealing candidate for realizing a wide range of reconfigurable RF devices. The measured RF performance of the 60 GHz variable attenuator shows that the device exhibits attenuation levels (|S21|) ranging from 10 dB to 25 dB over a bandwidth of 4 GHz and a return loss of better than 20 dB. The thesis also presents the design and implementation of a MEMS-based impedance tuner on a Silicon-On-Insulator (SOI) substrate. The tuner is comprised of four varactors monolithically integrated with CPW lines. Chevron actuators control the lateral motion of capacitive thick plates used as contactless lateral MEMS varactors, achieving a capacitance range of 0.19 pF to 0.8 pF. The improvement of the Smith chart coverage is achieved by proper choice of the electrical lengths of the CPW lines and precise control of the lateral motion of the capacitive plates. The measured results demonstrate good impedance matching coverage, with an insertion loss of 2.9 dB. The devices presented in this thesis provide repeatable and reliable operation due to their robust, thick-silicon structures. Therefore, they exhibit relatively low residual stress and are free from stiction and micro-welding problems

Development of a three-axis MEMS accelerometer

While originally developed to deploy air bags for the automotive industry, Microelectromechanical Systems (MEMS) based accelerometers have found their way into everything from video game controllers to cells phones. As prices drop and capabilities improve, it is expected that the use of accelerometers will further expand in the coming years. Accelerometers currently have the second highest MEMS sales volume, trailing only pressure sensors [1]. In this work several single and three-axis accelerometers are designed, fabricated, and tested under a variety of conditions. The designed accelerometers are all based off of the piezoresistive effect, where the value of a resistor changes with applied mechanical stress [2]. When accelerated, the inertia of a suspended proof mass causes stress on piezoresistors placed on support arms. The corresponding changes in these resistor values are then converted to an output voltage using a Wheatstone bridge. To sense acceleration independently in all three axes, structures with three distinct modes of vibration and three sets of Wheatstone bridges are used. Devices were fabricated at the Semiconductor and Microsystems Fabrication Laboratory (SMFL), located at RIT. A modified version of the RIT bulk MEMS process was used, consisting of 65 steps, 7 photolithography masks, bulk silicon diaphragm etch, and top hole release etch [3]. Unfortunately the finished chips show poor aluminum step coverage into contact vias and over polysilicon lines. This results in open circuits throughout the chip, prohibiting proper operation. Process corrections have been identified, and with proper fabrication the designs are still expected to yield working devices. Since the finished accelerometers were not functional, several commercial accelerometers have been tested to characterize sensitivity, linearity, cross-axis sensitivity, frequency response, and device lifetime

Optimization studies of thermal bimorph cantilevers, electrostatic torsion actuators and variable capacitors

In this dissertation, theoretical analyses and optimization studies are given for three kinds of MEMS devices: thermal bimorph cantilevers, electrostatic torsion actuators, and variable capacitors. Calculation, simulation, and experimental data are used to confirm the device behavior and demonstrate the application of the design approaches. For thermal bimorph cantilevers, an analytical model is presented which allows theoretical analysis and quantitative optimization of the performance based on material properties and device dimensions. Bimorph cantilevers are divided into two categories for deflection optimization: either the total thickness is constant, or the cantilever has one constant and one variable layer thickness. The optimum equations are then derived for each case and can be used as design rules. The results show that substantial improvements are possible over existing design approaches. Other parameters like static temperature distribution, power consumption, and dynamic behavior are also discussed, as are design tradeoffs such as feature size, application constraints, fabrication feasibility, and cost. The electrostatic torsion actuator studies are conducted for two device types: round and rectangular. The first case describes an analytical study of the pull-in effect in round, double-gimbaled, electrostatic torsion actuators with buried, variable length electrodes, designed for optical cross-connect applications. It is found that the fractional tilt at pull-in for the inner round plate in this system depends only on the ratio of the length of the buried electrode to the radius of the plate. The fractional tilt at pull-in for the outer support ring depends only on the ratio of the length of the buried electrode to the outer radius of the ring and the ratio of the ring\u27s inner and outer radii. Expressions for the pull-in voltage are determined in both cases. General relationships are also derived relating the applied voltage to the resulting tilt angle, both normalized by their pull-in values. Calculated results are verified by comparison with finite element MEMCAD simulations, with fractional difference smaller than 4% for torsion mode dominant systems. For the second case, a fast, angle based design approach for rectangular electrostatic torsion actuators based on several simple equations is developed. This approach is significantly more straightforward than the usual full calculation or simulation methods. The main results of the simplified approach are verified by comparing them with analytical calculations and MEMCAD simulations with fractional difference smaller than 3% for torsion mode dominant actuators. Also, good agreement is found by comparison with the measured behavior of a micro-fabricated full-plate device. In the last topic, ultra-thin silicon wafers, SU-8 bonding and deep reactive ion etching technology have been combined for the fabrication of folded spring, dual electrostatic drive, vertical plate variable capacitor devices with displacement limiting bumpers. Due to the presence of the bumpers, the variable capacitor with parallel plate drive electrodes has two tuning voltage regimes: first a parabolic region that achieves roughly a 290% tuning range, then a linear region that achieves an additional 310%, making the total tuning range about 600%. The variable capacitor with comb drive electrodes has a parabolic region that achieves roughly a 205% tuning range, then a linear region that achieves an additional 37%, making its total tuning range about 242%. The variable capacitors have Q factors around 100 owing to the use of silicon electrodes other than lower resistivity metal

Design fabrication and calibration of MEMS actuators for in-situ materials testing

Many MEMS devices utilize thin metallic films as mechanical structures. The elastic and plastic properties of these thin films (thickness \u3c 1μm) are significantly different from those of the bulk material. At these scales the volume fraction of material defects such as: grain boundaries, dislocations and interstitials become quite significant and become a chief contributor the physical and mechanical material properties of the thin films. Aluminum (Al), Copper (Cu), Nickel (Ni) and Gold (Au) are popular thin film materials used in MEMS/NEMS. Various studies have been conducted in recent years to study the mechanical properties of freestanding thin films in situ in TEM to study their failure mechanisms. Some of these studies utilize MEMS devices as actuators. These actuators are often co-fabricated with the specimen being tested therefore limiting the type of specimen that could be tested. Also these MEMS actuators are almost never traceably calibrated and their response is calculated. This thesis describes the design and fabrication process of a MEMS actuator for materials testing in-situ in TEM. The actuator is fabricated independent of the specimen. A setup was designed to calibrate these devices with a method that can be traced back to NIST standards. It has been shown that the calibrated response of these MEMS actuators is different from its calculated response and the use of un-calibrated devices for materials testing can lead to misleading results

Thin-Film AlN-on-Silicon Resonant Gyroscopes: Design, Fabrication, and Eigenmode Operation

Resonant MEMS gyroscopes have been rapidly adopted in various consumer, industrial, and automotive applications thanks to the significant improvements in their performance over the past decade. The current efforts in enhancing the performance of high-precision resonant gyroscopes are mainly focused on two seemingly contradictory metrics, larger bandwidth and lower noise level, to push the technology towards navigation applications. The key enabling factor for the realization of low-noise high-bandwidth resonant gyroscopes is the utilization of a strong electromechanical transducer at high frequencies. Thin-film piezoelectric-on-silicon technology provides a very efficient transduction mechanism suitable for implementation of bulk-mode resonant gyroscopes without the need for submicron capacitive gaps or large DC polarization voltages. More importantly, in-air operation of piezoelectric devices at moderate Q values allows for the cointegration of mode-matched gyroscopes and accelerometers on a common substrate for inertial measurement units. This work presents the design, fabrication, characterization, and method of mode matching of piezoelectric-on-silicon resonant gyroscopes. The degenerate in-plane flexural vibration mode shapes of the resonating structure are demonstrated to have a strong gyroscopic coupling as well as a large piezoelectric transduction coefficient. Eigenmode operation of resonant gyroscopes is introduced as the modal alignment technique for the piezoelectric devices independently of the transduction mechanism. Controlled displacement feedback is also employed as the frequency matching technique to accomplish complete mode matching of the piezoelectric gyroscopes.Ph.D

A 3-D micromechanical multi-loop magnetometer driven off-resonance by an on-chip resonator

This paper presents the principle and complete characterization of a single-chip unit formed by microelectromechanical system magnetometers to sense the 3-D magnetic field vector and a Tang resonator. The three sensors, nominally with the same resonance frequency, are operated 200-Hz off-resonance through an ac current whose reference frequency is provided by the resonator embedded in an oscillating circuit. The sensors gain is increased by adopting a current recirculation strategy using metal strips directly deposited on the structural polysilicon. At a driving value of 100 μArms flowing in series through the three devices, the magnetometers show a sub-185 nT/Hz Hz resolution with a selectable bandwidth up to 50 Hz. Over a ±5-mT full-scale range, the sensitivity curves show linearity errors lower than 0.2%, with high cross-axis rejection and immunity to external accelerations. Under temperature changes, the stability of the 200-Hz difference between the magnetometers and the resonator frequency is within 55 ppm/K. Offset is trimmed down to the microtesla range, with an overall measured Allan stability of about 100 nT at 20-s observation time. [2016-0030

Novel Applications of a Thermally Tunable Bistable Buckling Silicon-on-Insulator (SOI) Microfabricated Membrane

Buckled membranes are commonly used microelectromechanical systems (MEMS) structures. Recent work has demonstrated that the deflection and stiffness of these membranes can be tuned through localized joule heating. These devices were implemented into the design and fabrication of two novel device applications, a tunable pressure sensor and a steerable micromirror. A differential pressure across the membrane causes de reflection, up or down, which can be measured and related to a specific pressure. By tuning the stiffness of the membrane, its pressure response is varied providing a wider range of application for the pressure sensor. A 2.0mm by 2.0mm square membrane demonstrated a 60 percent decrease in pressure sensitivity from 1.433m/psi to 0.55m/psi. A steerable micromirror was realized by selectively heating a single quadrant of a buckled membrane, localized heating results in membrane de deflection constrained to that quadrant