Kevin M. Walsh

The viability of anisotropic conductive film as a flip chip interconnect technology for MEMS devices

The use of anisotropic conductive film (ACF) and stud bumping to form interconnects between die and substrates is one variation of current flip chip technologies with potential applications to MEMS devices. The key concerns associated with ACF are its long-term mechanical stability and consistent electrical performance. During this study a process methodology was developed for using ACF interconnections on MEMS devices by investigating key electrical parameters, and a practical example was investigated by packaging a pressure sensor using ACF flip chip techniques. Results of process development and contact resistance measurements using glass substrates and the anisotropic conductive film Sony FP1526 are discussed and analyzed. All required processing steps, such as stud bumping, application of ACF and thermocompression bonding of the die and substrate, were carried out using a digital wire bonder and a bench-top flip chip system. FP1526 yielded an average contact resistance of 10.23 mΩ, a stray capacitance measurement of 10 femtofarad and less than 0.1% change in contact resistance after being encapsulated in parylene C and being immersed in DI water for 72 h. Finally, the performance of an ACF-packaged MEMS piezoresistive pressure sensor was compared to that of a conventional wire-bonded device and showed a significant improvement in temperature stability (<0.34% deviation in offset voltage) with essentially no change in sensitivity.

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.

The effects of DRIE operational parameters on vertically aligned micropillar arrays

Vertically aligned silicon micropillar arrays have been created by deep reactive ion etching (DRIE) and used for a number of microfabricated devices including microfluidic devices, micropreconcentrators and photovoltaic cells. This paper delineates an experimental design performed on the Bosch process of DRIE of micropillar arrays. The arrays are fabricated with direct-write optical lithography without photomask, and the effects of DRIE process parameters, including etch cycle time, passivation cycle time, platen power and coil power on profile angle, scallop depth and scallop peak-to-peak distance are studied by statistical design of experiments. Scanning electron microscope images are used for measuring the resultant profile angles and characterizing the scalloping effect on the pillar sidewalls. The experimental results indicate the effects of the determining factors, etch cycle time, passivation cycle time and platen power, on the micropillar profile angles and scallop depths. An optimized DRIE process recipe for creating nearly 90° and smooth surface (invisible scalloping) has been obtained as a result of the statistical design of experiments.

Modeling the effect of oxygen on vapor phase diamond deposition inside micro-trenches

Abstract For the same feed gas composition, radical species concentrations vary with depth inside micro-trenches during diamond deposition. However, the feed gas compositions are typically used for predicting diamond deposition vs. non-diamond deposition. So, any type of modeling for predicting diamond deposition quality and uniformity inside trenches and onto three-dimensional objects is not possible. In this regard, a simple modeling approach utilizing a recently constructed C–H–O ternary diagram, based on radical species composition by Eaton and Sunkara [Eaton and Sunkara, Diam. Relat. Mater., 9 (2000) 1320], demonstrated the diamond deposition process inside trenches. Specifically, the effect of oxygen in the feed gases on the depths at which diamond can be deposited is modeled under both Knudsen and Fickian diffusion conditions and compared with experimental observations. The results suggested that certain feed gas phase compositions (C/O ratio) help increase the aspect ratio (>2.0) at which diamond deposition can be achieved.

Maskless Grayscale Lithography Using a Positive-Tone Photodefinable Polyimide for MEMS Applications

The novel use of a positive-tone photosensitive polyimide for the rapid production of grayscale features using a maskless lithography system is demonstrated. The removal rate of the polyimide, HD-8820, is characterized as a function of exposure dose. A broad contrast curve is found that is suitable for grayscale lithography. Three-dimensional polyimide structures up to 22 μm thick are demonstrated, and the surface roughness after the final cure is Ra = 4.4 nm, which is suitable for many microelectromechanical systems (MEMS) applications, including many optical applications. Tensile testing of 63 polyimide samples shows excellent mechanical properties for four different polyimide thicknesses produced with grayscale lithography. The modulus of elasticity is found to be 1.92 GPa, the yield strength to be 103 MPa, the fracture strength to be 133 MPa, and the percent elongation to be 51%. The test results show that the mechanical properties are consistent and do not change due to a partial exposure to UV light. The entire fabrication sequence, from computer-aided design file to cured structure, can be performed in less than 4 h, making this a fast low-cost method of producing polymer MEMS devices with excellent mechanical properties.

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.

The study of radiation effects in emerging micro and nano electro mechanical systems (M and NEMs)

The potential of micro and nano electromechanical systems (M and NEMS) has expanded due to advances in materials and fabrication processes. A wide variety of materials are now being pursued and deployed for M and NEMS including silicon carbide (SiC), III–V materials, thinfilm piezoelectric and ferroelectric, electro-optical and 2D atomic crystals such as graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2). The miniaturization, Semiconductor Science and Technology Semicond. Sci. Technol. 32 (2017) 013005 (14pp) doi:10.1088/1361-6641/32/1/013005 15 Author to whom any correspondence should be addressed. 0268-1242/17/013005+14

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

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Development of Ultra-Miniaturized Piezoresistive Pressure Sensors for Biomedical Applications

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.

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

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.