Julia Aebersold

Design and development of a MEMS capacitive bending strain sensor

The design, modeling, fabrication and testing of a MEMS-based capacitive bending strain sensor utilizing a comb drive is presented. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist with the diagnosis of spinal fusion. ABAQUS/CAE finite-element analysis (FEA) software was used to predict sensor actuation, capacitance output and avoid material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 ?m sidewalls with 25 ?m spacing between the adjacent fingers. The sensor was adhered to a steel beam and subjected to four-point bending to mechanically change the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers. At the unstrained state, the capacitive output was 7.56 pF and increased inversely to 17.04 pF at 1571 ?? of bending strain. The FEA and analytical models were comparable with the largest differential of 0.65 pF or 6.33% occurring at 1000 ??. Advantages of this design are a dice-free process without the use of expensive silicon-on-insulator (SOI) wafers.

Design, modeling, fabrication and testing of a MEMS capacitive bending strain sensor

Presented herein are the design, modelling, fabrication and testing of a MEMSbased capacitive bending strain sensor utilizing a comb drive. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist orthopaedic surgeons with the diagnosis of spinal fusion. ABAQUS/CAE version 6.5 finite element analysis (FEA) modelling software was used to predict sensor actuation, capacitance output and the avoidance of material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 µm sidewalls with 25 µm spacing between the adjacent fingers. For testing, the sensor was adhered to a steel beam, which was subjected to four-point bending. This mechanically changed the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers, beginning with an initial capacitance of 7.56 pF at the unstrained state and increasing inversely to 17.04 pF at 1571 µe of bending strain. The FEA and analytical models were comparable with experimental data. The largest differential of 0.65 pF or 6.33% occurred at 1000 µe.

Bistable out-of-plane stress-mismatched thermally actuated bilayer devices with large deflection

In this paper, we explore microfabricated bistable actuators released as thin films from a silicon wafer. The actuators are based on a serpentine design where two cantilevers are coupled at the tips by a thin-film bar. These devices are parameterized by two lengths: cantilever length and the length of the coupling bar. These two dimensions are systematically varied to study the effect of design parameters on bistability. The three-dimensional devices have extremely large deflection (hundreds of microns rather than tens of microns for most planar microactuators of similar size) and are thermally actuated out of the plane of the wafer by applying a bias across either the left or right side of the serpentine. The bistability of these devices is evaluated using electron and optical microscopy. Potential applications include non-volatile mechanical memory, optical shutters, and reconfigurable antenna elements.

Single element 3-terminal pressure sensors: A new approach to pressure sensing and its comparison to the half bridge sensors

We report the development of a novel 3-terminal single element piezoresistor for ultra-miniature pressure sensor applications and compare its performance to that of a traditional half Wheatstone bridge design. The pressure sensors reported here are 0.69-French in size (1F= 333µm) and are designed and batch-fabricated using SOI (silicon on insulator) and DRIE (deep reactive ion etching) technologies. One of the major applications of this device is for blood pressure monitoring using ultra-miniature 1F catheters. The combination of SOI and DRIE technologies results in uniform diaphragm thickness and complete elimination of the post-processing dicing step by micromachining “die separation streets” during the DRIE process. The novel 3-terminal single element design and half Wheatstone bridge sensors were optimized using finite element analysis (FEA). Performance characteristics of the half bridge and 3-terminal sensors, i.e. sensitivity, nonlinearity (NL%), temperature coefficient offset (TCO) and drift were measured and compared. It was determined that the 3-terminal pressure sensors (3-TPS) had greater sensitivity, better non-linearity and lower drift compared to half bridge design sensors. The 3-TPS devices were also less sensitive to alignment errors.

Development of Ultra-Miniaturized Piezoresistive Pressure Sensors for Biomedical Applications

Ultra miniaturized 0.69-French piezoresistive pressure transducers are designed and fabricated for biomedical applications. Silicon on insulator (SOI) and deep reactive ion etching (DRIE) technologies are used for the fabrication of the pressure sensors. A combination of SOI and DRIE technologies eliminates the dicing step and results in uniform diaphragm thickness. The dimensions of the final fabricated sensor die are 650 mum times 230 mum times 150 mum (length, width, thickness) with 2.5 mum thick diaphragms. Sensitivity of the sensors with half Wheatstone bridge configuration is determined to be 27-31 muV/V/mmHg.

A High Gauge Factor Capacitive Strain Sensor and its Telemetry Application in Biomechanics

A highly sensitive strain sensing system has been developed using a capacitive MEMS bending strain sensor for telemetry application in biomechanics such as spinal fusion monitoring. This telemetry sensor system is capable of detection with a linear gauge factor as high as 249 in frequency domain. The task is accomplished by converting the capacitive strain to frequency using a low power capacitance-frequency converter circuit that modulates the 125 kHz magnetic carrier source from the interrogating reader. The reader demodulates the 125 kHz signal and recovers the strain information from the sensor. Experimentally, various situation tests were performed with loads on a material test system (MTS) machine up to 1000 micro- strains to simulate corpectomy model on a stainless rod. Strain measurements were proved reliable within 10 cm range.

Microfabrication Process and Characteristic Testing of a MEMS-Based Preconcentrator

Due to terroristic activities it has become of significant importance to accurately detect vapor or trace particles of explosive materials. This task is difficult due to the limited number of particles available to be gathered for analysis. In order to combat this problem a preconcentrator has been developed to increase collection efficiency and the signal to noise ratio. Described is the fabrication process and characteristic testing prior to use with a portable ion mobility spectrometer (IMS).

Utilization of Direct Write Lithography to Develop Ultra High Aspect Ratio (>100:1) DRIE Silicon Pillars

Aspect ratios for deep reactive ion etching (DRIE) typically do not exceed 100. The discussed technique uses direct write lithography utilizing a Heidelberg DWL 66FS laser pattern generator followed by DRIE and hydrofluoric acid etching to create a nano pillar array on a silicon wafer. Prior to the direct write process, chrome was patterned on the wafer to act as an etch stop once the positive photoresist was consumed via DRIE. The sample was then oxygen-plasma etched to remove any remaining C4F8 residuals, as confirmed by Energy Dispersive X-ray spectroscopy. Remaining oxides were removed with dilute hydrofluoric acid and the wafer was rinsed with de-ionized water. The resulting structure was a nano pillar array with aspect ratios of 129.9:1, where pillar lengths were 50 μm with widths of 380 nm.

Efforts to Increase Utilization & Reduce the Subsidy for a Mid-Size Cleanroom Facility

The annual operating budget of the University of Louisvilles 10,000 ft2 100/1000 class cleanroom or the Micro/Nano Technology Center is heavily subsidized to cover salaried expenses by the engineering school of the University of Louisville (Speed School of Engineering). Expenses of the cleanroom include liquid nitrogen, gowning supplies, chemicals, compressed gas cylinders, sputtering targets, glassware, maintenance, repairs, etc. It is expected that these expenses are paid via cleanroom fees, while the salaried positions are subsidized. Yet, aging of the facility has increased expenses and the MNTC has been challenged to cover its operational expenses via collected cleanroom fees. Additionally, budget cuts from the state of Kentucky have given the impetus to reduce the subsidy from the engineering school and increase cleanroom revenue by means of increasing internal users, external users and service center work.

The UofL MNTC and the KY NanoNET - Two Initiatives to Promote Nano-Science in the State of Kentucky

The University of Louisville constructed its first cleanroom in 1997, a modestly-sized 1,500 sq. ft. class 1,000 facility. Since then, the “micro/nano/MEMS” revolution has exploded nationally and U of L has positioned itself to be the leader of this effort for the state of Kentucky. With the success of existing faculty and the hiring of new faculty over the last 10 years in the diverse disciplines that utilize micro/nanotechnology, U of L outgrew its original cleanroom. In 2001, plans were initiated for the design of a new greatly-expanded, multi-user cleanroom, large enough to support the emergent research activities at U of L and throughout the state and region. In 2006, construction was completed on a new 120,000 sq. ft.,