In situ X-ray crystallography powder diffraction studies on beta silicon carbide (3C-SiC) in the temperature range 25–800 °C at the maximum peak (111) are reported. At 25 °C, it was found that the lattice parameter is 4.596 Å, and coefficient thermal expansion (CTE) is 2.4 ×10−6/°C. The coefficient of thermal expansion along a-direction was established to follow a second order polynomial relationship with temperature (α11=−1.423×10−12T2+4.973×10−9T+2.269×10−6). CASTEP codes were utilized to calculate the phonon frequency of 3C-SiC at various pressures using density function theory. Using the Gruneisen formalism, the computational coefficient of thermal expansion was found to be 2.2 ×10−6/°C. The novelty of this work lies in the adoption of two-step thermal expansion determination for 3C-SiC using both experimental and computational techniques.

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Thermal Expansion of 3C-SiC Obtained from In-Situ X-ray Diffraction at High Temperature and First-Principal Calculations

Femtosecond laser ablation has become one of the important structural processing methods for the third-generation semiconductor material, silicon carbide (SiC), and it is gradually being employed in the manufacture of microelectromechanical systems and microelectronic devices. Experimental study has been performed on infrared single and multiple pulses (1035 nm) femtosecond laser ablation of SiC at various processing parameters. Diameters of laser ablation spots on 4H-SiC were measured to estimate the absorption threshold for material modification and structural transformation, which were 2.35 J/cm2 and 4.97 J/cm2, respectively. In the multiple-pulse scribing ablation for microgrooves, the ablation threshold dropped to 0.70 J/cm2 due to the accumulation effect when the effective pulse number reached 720. The calculated average of the thermally stimulated ablation depth of 4H-SiC is 22.4 nm, which gradually decreased with the raising of the effective pulse number. For obtaining square trenches with precise and controllable depths and a smooth bottom in 4H-SiC, the effects of processing parameters on the material removal rate and surface roughness are discussed. The ablation rate per pulse is almost constant, even if the effective pulse number varies. The reduction of laser spot overlapping ratio in x direction has a greater weakening effect on the material removal rate than that in y direction. The precise amount of material removal can still be controlled, while modulating the surface roughness of the ablated features by changing the hatch rotation angle. This research will help to achieve controllable, accurate, and high-quality machining results in SiC ablation, using infrared femtosecond laser.

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Experimental Investigation on Ablation of 4H-SiC by Infrared Femtosecond Laser

The widespread application of bulk silicon carbide in pressure sensing has been largely limited by the slow etching rate of traditional micromachining processes. This paper proposes a piezoresistive bulk 4H-SiC pressure sensor with diaphragm which has controllable thickness realized by femtosecond laser. A modified stress model was established by finite element analysis to fit the SiC circular diaphragm prepared by laser micromachining. The test results proved that the fabricated sensor with diaphragm thickness of <inline-formula> <tex-math notation="LaTeX">$60 \mu \text{m}$ </tex-math></inline-formula> had sensitivity of 1.42 mV<inline-formula> <tex-math notation="LaTeX">$/\text{V}/$ </tex-math></inline-formula>MPa under the applied pressure of up to 5 MPa at room temperature. Small hysterisis error of 0.17 %/FSO and nonlinearity of 0.20 %<inline-formula> <tex-math notation="LaTeX">$/$ </tex-math></inline-formula>FSO was achieved. The sensor was able to work in a broad temperature range with the temperature coefficient of sensitivity of −0.23% FSO/°C at −50 °C and −0.10% FSO/°C at 300 °C. The dynamic response results exhibited that the resonance frequency of the sensor was 60.11 kHz and the rise time was <inline-formula> <tex-math notation="LaTeX">$18 \mu \text{s}$ </tex-math></inline-formula>. The SiC pressure sensor also had the noise-limited resolution of 0.42 kPa and the dynamic range of 44.9 dB. The research demonstrates the prospect of employing the femtosecond laser technology to prepare bulk SiC pressure sensors for extreme temperature environment.

Piezoresistive 4H-SiC Pressure Sensor With Diaphragm Realized by Femtosecond Laser

A safety and arming device with a condition feedback function has been designed in this article to improve the intelligence and safety of ignition devices. The device achieves active control and recoverability by virtue of four groups of bistable mechanisms which consist of two electrothermal actuators to drive a semi-circular barrier and a pawl. According to a specific operation sequence, the barrier is engaged by the pawl at the safety or the arming position. The four groups of bistable mechanisms are connected in parallel, and the device detects the contact resistance generated by the engagement of the barrier and pawl by the voltage division of an external resistor to determine the parallel number of the mechanism and give feedback on the device’s condition. The pawl as a safety lock can restrain the in-plane deformation of the barrier in the safety condition to improve the safety function of the device. An igniter (a NiCr bridge foil covered with different thicknesses of Al/CuO films) and boron/potassium nitrate (B/KNO3, BPN) are assembled on both sides of the S&A device to verify the safety of the barrier. The test results show that the S&A device with a safety lock can realize the safety and arming functions when the thickness of the Al/CuO film is set to 80 μm and 100 μm.

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Design of an Intelligent MEMS Safety and Arming Device with a Condition Feedback Function

• A stress amplification structure employing free-standing SiC sensing elements was utilised to enhance 3C-SiC/Si pressure sensor sensitivity to 0.276 mV/V/kPa. • Analytical and numerical methods show that the stress in the structure is amplified by more than 750% compared to a traditional structure. • The fabricated sensor had a non-linearity of 2.2% and was highly repeatable with a low hysteresis of 0.91%. • The dual 3C-SiC thin film on the sensor provides a strong chemical protective capability due to its chemical inertness. SiC based pressure sensors show tremendous promise for harsh environment applications thanks to their excellent mechanical, electrical, thermal, and chemical properties. This paper presents the design, fabrication, and characterisation of a highly sensitive and robust 3C-SiC/Si pressure sensor. The sensor utilises a stress amplification structure consisting of four Si pillars built up from the 3C-SiC/Si membrane, supporting a series of released n-type 3C-SiC sensing elements. When pressure is applied to the diaphragm, the pillars act to locally concentrate and amplify strain in the 3C-SiC sensing elements, resulting in over 7 times higher stresses/strains in these sensing elements compared to a traditional structure. Additionally, the front side of the sensor is fully covered by a 3C-SiC thin film, which provides a strong chemical protective capability, allowing the sensor to operate in harsh chemically corrosive environments. The robust device utilises the full Wheatstone bridge to negate the effects of temperature. Experimental results show that the fabricated sensor is highly stable, repeatable, has a high sensitivity of 0.276 mV/V/kPa and a maximum non-linearity of 2.2% in the 0-100 kPa region. The results indicate that this smart-structure pressure sensor is promising for applications that require highly precise pressure sensing in aggressively corrosive environments. 

Highly Sensitive and Robust 3C-SiC/Si Pressure Sensor with Stress Amplification Structure

Making full use of the availability and excellent performance of silicon carbide (SiC) substrates, SiC-based MEMS piezoresistive pressure sensors have been extensively investigated. In this article, a femtosecond laser was adopted instead of dry plasma etching technology to realize mass fabrication of 128 bulk SiC piezoresistive pressure sensor diaphragms from half a 4H-SiC wafer. These diaphragms were completed in 2 h with an average thickness error of less than 7.5% and a surface roughness of 108 nm. The 4H-SiC sensor was characterized with a nonlinear error of 0.13% FS and a repeatability error of 1.49% FS in the pressure range of 0–5 MPa at room temperature. The tolerance to temperature was measured in high and low temperature of −50 °C–300 °C with the temperature coefficient of sensitivity (TCS) of −0.11% FS°C at −50 °C and −0.16% FS°C at 300 °C. In addition, resistance measurement of the sensor in strongly alkaline corrosive solution proved the robustness with an average daily drift of only 1.61%. This integrated solution that combines femtosecond laser technology and MEMS processing provides exciting opportunities for rapid and large-scale manufacturing of bulk SiC pressure sensors.

Development of Laser-Micromachined 4H-SiC MEMS Piezoresistive Pressure Sensors for Corrosive Environments

Considering the safety of ammunition, safety and arming (S&A) devices are usually designed in pyrotechnics to control energy transfer through a movable barrier mechanism. To achieve both intelligence and miniaturization, electrothermal actuators are used in MEMS S&A devices, which can drive the barrier to an arming position actively. However, only when the actuators’ energy input is continuous can the barrier be stably kept in the arming position to wait for ignition. Here, we propose the design and characterization of a double-layer electrothermal MEMS S&A Device with a bistable mechanism. The S&A device has a double-layer structure and four groups of bistable mechanisms. Each bistable mechanism consists of two V-shape electrothermal actuators to drive a semi-circular barrier and a pawl, respectively, and control their engagement according to a specific operation sequence. Then, the barrier can be kept in the safety or the arming position without energy input. To improve the device’s reliability, the four groups of bistable mechanisms are axisymmetrically placed in two layers to constitute a double-layer barrier structure. The test results show that the S&A device can use constant-voltage driving or the capacitor–discharge driving to drive the double-layer barrier to the safety or the arming position and keep it on the position passively by the bistable mechanism.

https://www.mdpi.com/2072-666X/13/7/1076/pdf?version=1657266902

Jiakai Liu

Papers

Development of Laser-Micromachined 4H-SiC MEMS Piezoresistive Pressure Sensors for Corrosive Environments

Design of a Double-Layer Electrothermal MEMS Safety and Arming Device with a Bistable Mechanism