Robert G. Azevedo

A SiC MEMS Resonant Strain Sensor for Harsh Environment Applications

In this paper, we present a silicon carbide MEMS resonant strain sensor for harsh environment applications. The sensor is a balanced-mass double-ended tuning fork (BDETF) fabricated from 3C-SiC deposited on a silicon substrate. The SiC was etched in a plasma etch chamber using a silicon oxide mask, achieving a selectivity of 5:1 and etch rate of 2500 Aring/min. The device resonates at atmospheric pressure and operates from room temperature to above 300degC. The device was also subjected to 10 000 g shock (out-of-plane) without damage or shift in resonant frequency. The BDETF exhibits a strain sensitivity of 66 Hz/muepsiv and achieves a strain resolution of 0.11 muepsiv in a bandwidth from 10 to 20 kHz, comparable to state-of-the-art silicon sensors

Silicon carbide microsystems for harsh environments

Introduction to Harsh MEMs.- Silicon Carbide Processing.- Silicon Carbide Electronics.- Silicon Carbide MEMS Devices.- Silicon Carbide MEMs Device Packaging.- System Integration.

Silicon carbide resonant tuning fork for microsensing applications in high-temperature and high G-shock environments

We present the fabrication and testing of a silicon carbide balanced mass double-ended tuning fork that survives harsh environments without compromising the device strain sensitivity and resolution bandwidth. The device features a material stack that survives corrosive environments and enables high-temperature operation. To perform high-temperature testing, a specialized setup was constructed that allows the tuning fork to be characterized using traditional silicon electronics. The tuning fork has been operated at 600°C in the presence of dry steam for short durations. This tuning fork has also been tested to 64,000 G using a hard-launch, soft-catch shock implemented with a light gas gun. However, the device still has a strain sensitivity of 66 Hz/µe and strain resolution of 0.045 µe in a 10-kHz bandwidth. As such, this balanced-mass double-ended tuning fork can be used to create a variety of different sensors including strain gauges, accelerometers, gyroscopes, and pressure transducers. Given the adaptable fabrication process flow, this device could be useful to microelectromechanical systems (MEMS) designers creating sensors for a variety of different applications.

Silicon carbide coated MEMS strain sensor for harsh environment applications

We present poly-SiC coating and subsequent operation of a Si-based double-ended tuning fork (DETF) resonant strain sensor fabricated in the Bosch commercial foundry process. The coating is applied post release and, hence, has minimal impact on the front end of the microfabrication process. The deposition thickness of nanometer-thin SiC coating was optimized to provide enhanced corrosion resistance to silicon MEMS without compromising the electrical and mechanical performance of the original device. The coated DETF achieves a strain resolution of 0.2 mue in a 10 Hz to 20 kHz bandwidth, which is comparable to the uncoated device. The coated DETF is locally heated with an IR lamp and is shown to operate up to 190 degC in air with a temperature sensitivity of -7.6 Hz/degC. The devices are also dipped in KOH at 80 degC for 5 minutes without etching the structures, confirming the poly-SiC coating provides a sufficient chemical barrier to the underlying silicon. The results demonstrate that SiC-coated poly-Si devices are an effective bridge between poly-Si and full poly-SiC films for applications requiring a high level of corrosion resistance and moderate operating temperatures (up to 200 degC) without compromising the performance characteristics of the original poly-Si device.

Passive Substrate Temperature Compensation of Doubly Anchored Double-Ended Tuning Forks

While microdevices have shown a number of key advantages over similar macrosize sensors including size, cost, and sensitivity, a key challenge has centered on reducing drift and error due to changes in temperature. This paper proposes a novel substrate temperature compensation mechanism for microelectromechanical systems double-ended tuning forks (DETFs). The device layer and substrate layer are purposefully made from differing materials. This mismatch induces thermal strains that cancel changes in the frequency due to a shift in the modulus of elasticity. Two polycrystalline silicon carbide DETFs of different physical dimensions are fabricated on single-crystalline silicon substrates. The devices are tested between 5°C and 320°C and exhibit temperature compensation as predicted by an analytical model. The DETFs exhibit peak temperature compensation near room temperature at 34°C and 38°C, respectively. Over a commercial temperature range from 0°C to 70°C, the devices display temperature sensitivities of 1.5 Hz/°C (7.4 ppm/°C) and 0.3 Hz/°C (1.7 ppm/°C), which is up to 17× better than a similar epitaxial silicon device. This work is broadly applicable to tuning-fork-based sensing systems such as strain gauges, pressure sensors, accelerometers, and gyroscopes.

SiC Materials and Processing Technology

This chapter contains a broad review of SiC materials and processing technology necessary to create SiC electronics, micromechanical transducers, and packaging. Details on deposition and etching methods are covered. The material properties of various forms of SiC (single crystalline, polycrystalline, and amorphous) along with their use for creating the various components of harsh environment microsystems will also be discussed. Current status and future research are highlighted with regards to both materials and processing technologies.

Influence of Sensor Substrate Geometry on the Sensitivity of MEMS Micro-Extensometers

In order to determine the influence of the silicon chip substrate on measurement fidelity in a silicon MEMS micro-extensometer, finite element modeling of strain transfer efficiency from a steel beam through a bond layer and silicon chip is investigated over a range of chip and steel beam geometries under both axial and pure bending load conditions. The finite element model results are verified against experimental data. An analytical model that incorporates both influence of the bonded substrate on the effective load and shear-lag phenomenon in the bond is developed and is shown to compare favorably to the finite element model over a wide range of chip and beam geometries. Based on these results, a partially-trenched silicon chip is also investigated as an alternate means of locally enhancing the strain transfer to the micro-extensometer without compromising the ability of the substrate to act as part of the encapsulation of moving elements of the micro-extensometer from the environment. The partially-trenched substrate in bending is experimentally shown to generate strains that are 118% of the strain applied to the substrate—a 23% percent improvement over the equivalent unpatterned substrate geometry.Copyright

Low temperature ion beam sputter deposition of amorphous silicon carbide for wafer-level vacuum sealing

This paper presents a novel low temperature, wafer-level vacuum sealing method that uses line-of-sight deposition of amorphous SiC with ion beam sputter deposition. The ion beam sputter deposition system allows substrate tilting for off-normal deposition and operates with a pressure of approximately 3 times 10-6 torr during deposition. The amorphous SiC films have demonstrated compressive intrinsic stresses for growth rates between 0.06 - 0.13 nm/min test scaffold structures were fabricated by etching holes and trenches into bare Si wafers. The topography of sealing films deposited on the test scaffold structures shows that the film growth is directional with no visible down-hole deposition. The termination of the seal and the chemical resistance of the sealing films have been confirmed with a hot KOH immersion experiment.

Silicon-to-steel bonding using rapid thermal annealing

This paper presents the rapid, low-temperature bonding between silicon and steel using the rapid thermal annealing process. Three different thin-film adhesion layer systems including silver, gold, and nickel were utilized as the intermediate bonding material to assist the eutectic Pb/Sn bonding between silicon and steel. The bonding temperature was set at 220/spl deg/C for 20 s, with a 20-s ramp-up time. Five experiments were conducted to determine the strength of the bond, including static tensile and compressive four-point bend tests, axial extension tests, tensile bending fatigue tests, and corrosion resistance tests. The test results have shown that the gold adhesion layer is the most robust, demonstrating minimal creep during fatigue tests, no delamination during the tensile or compressive four-point bend tests, and acceptable strength during the axial extension tests. Additionally, all adhesion layers have withstood four months of submersion in various high-temperature solutions and lubricants without failure. Simulations of the axial stresses and strains that developed during the four-point bend and axial extension tests were performed and showed that the presence of the silicon die provides a local reinforcement of the bond as observed in the experimental tests.

Temperature-insensitive silicon carbide resonant micro-extensometers

This work presents thin-film polycrystalline silicon carbide resonant micro-extensometers fabricated on a single-crystalline silicon substrate that are temperature-insensitive near room temperature. The slight difference in thermal expansion coefficient between the device layer and the substrate induces a slight tensile strain near room temperature, which counteracts material softening of the resonator. An analytical model that accounts for these thermal effects predicts that this system will exhibit a turnover temperature near room temperature. A temperature sensitivity of 0.7 Hz/°C (3.6 ppm/°C) from 17 to 65 °C is measured experimentally, without compromising the strain-sensing capability of the resonator. This is nearly a 10-fold improvement over an equivalent epitaxial silicon resonant miro-extensometer (−30 ppm/°C) [1].