Key Engineering Materials Vols. 645-646

Abstract: The variable stiffness micro-springs’ load and deformation have nonlinear relationship, thus they can be used in special occasions. MEMS processing technology can manufacture any complex structures in a plane, using this feature, a variable stiffness design idea for the planar micro-spring is proposed. That is, using one type of structure named contact pairs to achieve stiffness change during the micro-spring’s stretching process. Using contact pairs, two types of variable stiffness springs are designed: stiffness increase spring and stiffness convexity spring. Because of the influence of processing precision, the contact pairs’ sizes of the first type of variable stiffness spring are different each other, the machining error makes the test results and simulation results various. In order to avoid the above problem of above spring, this paper designs another two variable stiffness micro-springs, test results indicate that the improved variable stiffness springs can realize the variable stiffness’ tendency.

Abstract: The hot-filament vacuum gauge is a traditional gauge for the vacuum measurement below 10^-1Pa. It consists of the cathode, grid and anode. The cathode is used to emit the electron to collide with a gaseous molecular to form a pair of ion and electron. The number of these ions is proportional to the gaseous molecule density and the gas pressure. Unlike the traditional gauge with the large dimension, this paper develops a CMOS-compatible hot-filament vacuum gauge. With it, the vacuum gauge and its control circuit can be fabricated monolithically on a chip which will decrease the noise and be easy to use. Besides that, the optimal heating current for the cathode is also considered in this paper. The electro-thermal modeling using COMSOL software is introduced to calculate the heating current for cathode. The results show that for 200μm×5μm×0.2μm tungsten resistor, 200mA current is needed to heat the tungsten resistor to 1800°C to emit the electron.

Abstract: Accelerometer in MEMS always is made by capacitive or piezoresistive, whose dynamic response is not good, the operating frequency is narrow, and the cross-axis sensitivity is low. A new type of piezoelectric micro-accelerometer is designed, and its structure is “x” type. The sensing unit is piezoelectric PZT films, which is achieved by sol-gel method. The accelerometer is a triaxial accelerometer. The theoretical and simulation analysis is used to achieve the charge sensitivity and response frequency, and also get the optimal structural parameters. A new circuit connection is proposed to improve the sensitivity and avoid the cross-axis sensitivity. The design achieves the z-axis sensitivity with more than 40 pC/g, x, y-axis sensitivity with more than 8pC/g, and the response frequency is about 3000Hz.

Abstract: In this paper, a catalyst-free and non-oxidant digestion method based on UVC and thermal coupled mechanism was developed for the digestion of total phosphorus (TP). Two kinds of digestion devices, “hamburg-type” and “screw-type”, were designed and compared to get higher digestion efficiency. Both of the two devices contain a quartz tube, UV light tubes, a flexible heating unit, UV light reflecting layer, and a temperature meter, etc. UVC light with the wavelength of 254nm is used to produce hydroxyl radicals to degrade TP into phosphate in water. Compared with the “hamburg-type” digestion device, the “screw-type” device exhibited a higher digestion rate. At the condition of 1 hour and 80°C, the digestion rate of 1mg/L (by weight of P) water sample can come to 96% by using the “screw-type” device.

Abstract: An optimization method for fabricating 3D microneedle arrays with larger cone angles through backside exposure is demonstrated in this paper. A photo mask was designed to fabricate SU-8 microneedle based on diffraction of UV light. A circular hole diffraction was simulated with Matlab to obtain light intensity distribution. The simulation results show that the cone angles and surface profile can be adjusted by changing the thickness of substrate and exposure dose. Based on the simulation results, the microneedles with heights of 265 μm to 380 μm and cone angles in the range of 5.1° to 15.6° were fabricated by the backside exposure technology through one time UV lithography. Compared with previous approaches, the fabrication process in this paper takes advantages of simple, low cost and mass production.

Abstract: With the development of MEMS, more attention has been paid to the uniformity between the performance of the products in practical processing and the expected performance. A case study is conducted here with fixed-fixed beam and comb-drive resonator as the pattern to explore how geometry size deviation and geometry shape error influence the devices. It is discovered that the length, inclining profile and undercutting of the anchor is the key factor which influences the vibration mode of the fixed beam, and the geometry size and corrugating profile mainly affect the comb-drive resonator.

Abstract: A novel type of variable resistor micro system based on electro-explosion is described and its performance test results are presented. The variable resistor consists of four metallic leads in parallel with different width and length thus different resistances and four one-time shot MEMS solid on-off switches. By killing off the metallic lines, the variable resistor varies its resistance. Switching is accomplished by a voltage pulse. The resistor can be driven by external signal and its predicted lifetime is very long. Besides, the resistor has no movable parts, therefore the resistor can be used for long term store systems and harsh environments.

Abstract: In the space environments, the size and weight of composite materials will produce small changes. In order to measure the small changes accurately, a micro-deformation measurement device has been designed. The carbon fiber tube was made from cyanate ester resin or epoxy resin was tested in vacuum and hygrothermal environments. The deformation in the vacuum test is -1.7 to 3.8 microns, in the hygrothermal test, the deformation is -1.3 to -6.4 microns. The same as theoretical analysis, the environmental stability of cyanate ester resin better than epoxy resin.

Abstract: A fourth-order cascade low-pass transconductance capacitor filter is designed, and the coefficients of the filter are calculated. For the implementation of large-time constants the current shunt technique and the current cancellation technique are employed in the operational transconductance amplifiers. The whole filter is simulated in Hspice, the the cutoff frequency is 150.2Hz and the THD of the filter is 65dB, which meet the technical requirements of the inertial applications.

Abstract: This paper presents a novel and effective characterization method for giant piezoresistive properties of silicon nanowires by using the reference structures. This contrast detection approach investigates the influences of quantum size effect and surface defects effect on piezoresistive coefficients of silicon nanowires by direct comparison of the resistivity change ratio of silicon wires with nanoscale-to-microscale width under the same applied stress conditions. The characterization experiments based on four-point bending tensile test demonstrate that piezoresistive coefficient of small nanowidth silicon nanowire can be significantly increased to about five times higher levels than that of bulk silicon under the same impurity concentration, which indicates that the silicon nanowire can have giant piezoresistive effect. On the other hand, to solve the problem on nanowires pick-up, we proposed a nanowire piezoresistive detection approach, whose validity is confirmed by the dynamic LDV resonance test. Meanwhile, to investigate the influence of undercut arising from the wet chemical release process of the suspended silicon nanowire, a three-dimensional finite element simulation is also carried out for the fundamental resonant frequency using ANSYS software. The numerical and experimental results show that our piezoresistive detection is accurate and effective and the undercut should be carefully considered in the design of the high frequency resonator and mixer. The findings of this paper provide some useful references for the piezoresistive effect measurement and the piezoresistive pick-up in nanoelectromechanical system.