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We investigate the MEMS resonant cantilevers for high-performance thermogravimetric analysis (TGA) of chemical decomposition, featuring high accuracy and minimized thermal lag. Each resonant cantilever is integrated with a microheater for sample heating near the free end, which is thermally isolated from the resonance excitation and readout elements at the fixed end. Combining finite element modeling and experiments, we demonstrate that the sample loading region can stabilize within ~11.2 milliseconds in response to a step heating of 500 °C, suggesting a very fast thermal response of the MEMS resonant cantilevers of more than 104 °C/s. Benefiting from such a fast thermal response, we perform high-performance TG measurements on basic copper carbonate (Cu2(OH)2CO3) and calcium oxalate monohydrate (CaC2O4·H2O). The measured weight losses better agree with the theoretical values with 5–10 times smaller thermal lags at the same heating rate, compared with those measured by using conventional TGA. The MEMS resonant cantilevers hold promise for highly accurate and efficient TG characterization of materials in various fields. [ABSTRACT FROM AUTHOR]

Micro-cantilever piezoresistive sensors are optimally designed and fabricated in a double-layer silicon-on-insulator (SOI) wafer. The sensor geometry is optimized by placing the sensing piezoresistor at the cantilever root region to increase effective piezoreisistive sensing area. According to finite-element simulation results, high sensitivity can be obtained by design the cantilever into a wide and short shape. In order to use single-crystalling silicon to fabricate both the cantilever and the piezoresistor for high sensitivity, double-layer SOI wafer, which has two active layers and two insulating layers, is proposed to fabricate the self-sensing micro-cantilever sensor. The piezoresistor is made of the top active-layer single-crystalline silicon. Without p-n junction isolation, such a piezoresistor can be free from leakage-current relative noise that helps to achieve fine sensing resolution. The bottom active-layer is used to form the cantilever, with well controlled cantilever thickness and high fabrication yield. With the top surface of the micro-cantilever is modified with the functionalized self-assembled monolayer, detection of trace-concentration Trinitrotoluene (TNT) vapor is experimentally carried out, with reproducible sensing response to 7.6 ppb TNT. [ABSTRACT FROM AUTHOR]

A resonant cantilever sensor is, for the first time, dressed in a water-proof ‘diving suit’ for real-time bio/chemical detection in liquid. The μ-‘diving suit’ technology can effectively avoid not only unsustainable resonance due to heavy liquid-damping, but also inevitable nonspecific adsorption on the cantilever body. Such a novel technology ensures long-time high-Q resonance of the cantilever in solution environment for real-time trace-concentration bio/chemical detection and analysis. After the formation of the integrated resonant micro-cantilever, a patterned photoresist and hydrophobic parylene thin-film are sequentially formed on top of the cantilever as sacrificial layer and water-proof coat, respectively. After sacrificial-layer release, an air gap is formed between the parylene coat and the cantilever to protect the resonant cantilever from heavy liquid damping effect. Only a small sensing-pool area, located at the cantilever free-end and locally coated with specific sensing-material, is exposed to the liquid analyte for gravimetric detection. The specifically adsorbed analyte mass can be real-time detected by recording the frequency-shift signal. In order to secure vibration movement of the cantilever and, simultaneously, reject liquid leakage from the sensing-pool region, a hydrophobic parylene made narrow slit structure is designed surrounding the sensing-pool. The anti-leakage effect of the narrow slit and damping limited resonance Q-factor are modelled and optimally designed. Integrated with electro-thermal resonance excitation and piezoresistive frequency readout, the cantilever is embedded in a micro-fluidic chip to form a lab-chip micro-system for liquid-phase bio/chemical detection. Experimental results show the Q-factor of 23 in water and longer than 20 hours liquid-phase continuous working time. Loaded with two kinds of sensing-materials at the sensing-pools, two types of sensing chips successfully show real-time liquid-phase detection to ppb-level organophosphorous pesticide of acephate and E.coli DH5α in PBS, respectively. The proposed method fundamentally solves the long-standing problem of being unable to operate a resonant micro-sensor in liquid well. [ABSTRACT FROM AUTHOR]

A novel direct identification of double-strand DNA is proposed by using real-time enzyme-digestion in a resonant-cantilever embedded microfluidic chip. The new gene-level detection method is expected to replace the conventional DNA-hybridization based gene-detection that suffers from not only nonspecific adsorption induced false-positives but also complicated single-strand DNA preparation and hybridization. Since a detected DNA chain features a unique cutting site for a certain restriction-enzyme, the accurately cut-off mass (representing the length of the digested segment) can be online recorded by the frequency-shift signal of the resonant micro-cantilever sensor. This enzyme-digestion technique is confirmed by experimental identification of the stx2 gene of E. coli O157:H7. The direct-PCR sample is directly analyzed by using our lab-made cantilever-embedded microfluidic-chip. The 3776 bp DNA is immobilized via biotin–streptavidin binding and the added mass is recorded by a frequency-decrease of 15.9 kHz within 10 min. Then, with EcoRV-enzyme digestion at the site of 2635 bp, the cut-off mass is real-time detected by a frequency-increase of 10.2 kHz within 6 min. The detected frequency-shift ratio of 15.9/10.2 = 64.2% is consistent with the length ratio between the cut-off fragment and the whole DNA chain (2635/3776 = 69.8%). Hence, the simple and accurate double-strand detection method is verified experimentally. [ABSTRACT FROM AUTHOR]

This paper reports a novel top-down/bottom-up combined resonant microcantilever chemical sensor, where the nanosensing material of a functionalized mesoporous thin film (MTF) is directly self-assembled on the sensing region of the integrated microcantilever. By using the batch-producible nano-on-micro construction technique, a large number of such sensors can be batch fabricated with uniform performance and low cost. More importantly, the sensing molecule terminals can be simultaneously constructed at the pore inner surface when the MTF is directly grown on the cantilever. With -\NH2-group-functionalized MTF directly grown onto the surface of the cantilever free end, the micro-/nanocombined gravimetric sensor has experimentally exhibited quick response and highly sensitive detection of \CO2 gas. [ABSTRACT FROM PUBLISHER]

Abstract: In this paper, an integrated probe card is proposed and developed for wafer-level IC testing. Based on micromachining technology, totally about 26,000 cantilever-tip probes can be formed simultaneously in one 4-in. silicon wafer, with the minimum pitch of 35μm for adjacent probing tips. The probe card is designed with a novel composite structure that combines both single-crystalline silicon and electroplated metals. In the composite structure, a novel bypass through-silicon-via with a low aspect ratio can be high-yield fabricated for transferring the testing signals from the probing sided (at the wafer bottom side) to the I/O interface (at the front side). The probe card makes full use of the advantages of the single-crystal silicon and the electroplated nickel and copper. Bulk micromachined silicon cantilevers behave uniform probing height and a good elastic deformation property, while the electroplated nickel probing tips promise high hardness and satisfactory electric contact performance with the dies-under-test (DUT). Measurements show that the fabricated cantilever is able to withstand a contact force of 80mN by a tip displacement of 20μm. The measured contact resistances on metal pads (Al, Cu, and Au) are all below 1Ω, whereas the maximum current leakage is 64pA for 3.3V voltage across two adjacent tips. After a probing reliability test of 100,000 cycles, the cantilever-tip shows no sign of any performance degradation. [Copyright &y& Elsevier]

This research investigates the air drag damping effect of the micromachined cantilevers in different resonance modes on the quality factor, which are operated in ambient air. Based on a simplified dish-string model for air drag force acting on the resonant cantilever, the air drag damping properties of the cantilevers vibrating in different modes are analyzed with theoretic vibration mechanics, which is complemented and further confirmed with finite-element simulation. Four kinds of integrated cantilevers, which resonate in the first flexural mode, the second flexural mode, the first torsional mode, and the second torsional mode, respectively, are designed and fabricated by using micromachining techniques. Finally, biomolecular sensing experiments are carried out to verify the theoretical results obtained before. From both the modeling and experimental results, it can be seen that damping characteristics of the torsional cantilever resonators are generally better than that of the flexural ones, and quality factor of the cantilever resonator in a higher-frequency mode is always superior to that in a lower-frequency one. Among the four kinds of microcantilever resonators operated in our experiments, the one operated in the second flexural modes exhibits the highest Q factor and the best biomass sensing performance. [ABSTRACT FROM AUTHOR]

Mass-loading detection of multiple kinds of analyte with a single resonant microcantilever is proposed and experimentally validated. By exciting the cantilever in different resonance modes and adsorbing the different analytes at different locations of the cantilever, the specific mass of either kind of analyte can be independently detected. The proposed micromechanical resonant cantilever sensor is designed and fabricated using silicon micromachining techniques. Used as simulant adsorbates, Au and Cr thin films are selectively implemented on the cantilever to verify the bianalyte detecting function. The testing results are consistent with the theoretical analysis, with the detection error being an order of magnitude lower than the analyzed mass. [ABSTRACT FROM AUTHOR]

Torsion-mode resonance is built in an integrated cantilever sensor for ultraresoluble detection of specifically bio/chemical mass adsorption. The superior mass resolution of the torsion-mode cantilever to a conventional bending-mode one is verified by energy-dissipation analysis and Q-factor simulation. With integrated transverse piezoresistance for frequency-shift signal readout and Lorentz force for resonance excitation, the torsion-mode sensor is optimally designed for high sensitivity. The microfabricated torsion-mode sensor is measured with a high close-loop Q factor in air. By Allan-variance analysis for the measured frequency stability, 23 fg resolution is obtained for the torsion-mode sensor, which is much improved compared to the 313 fg for the conventional flexure-mode sensor. The torsional sensor is used to recognize biotin-avidin specific combination, resulting in 443 Hz frequency shift for 50 μM streptavidin solution. [ABSTRACT FROM AUTHOR]

Micromechanical resonant sensor offers many advantages for chemical detection, but it fails to maintain high quality factor ( Q -factor) when working directly in liquid because of the viscous damping. To solve the problem, a gas/liquid separated sensing method is introduced to detect volatile organic compounds (VOCs) in solution with a resonant cantilever gas sensor. With the help of a waterproof and breathable expanded polytetrafluoroethylene (ePTFE) film, the resonant sensor can be physically isolated from the analyte solution. Thus, the sensor can resonate in gas phase environment with a high Q -factor, meanwhile the interference from the solvent emission can be significantly suppressed. Loaded with the sensing-group functionalized mesoporous-silica nanoparticles (MSNs), the resonant cantilever can detect the target VOC molecules that permeate from the flowing solution sample at the other side of the film. Two typical kind of resonant microcantilever VOC sensors are tested to verify the proposed method, which are loaded with carboxyl (–COOH) and amino (–NH 2 ) sensing groups functionalized MSNs, respectively. The sensors exhibit highly sensitive ( mg/L level resolution) and reproducible detection ability to aniline and acetic-acid solution, respectively. This gas/liquid separated sensing technique is promising in various on-site chemical detection applications. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents a self-sensing microcantilever for bio/chemical detection in liquid. The developed silicon piezoresistance encapsulated SiO2 cantilever (Pr-Oxi-Lever) is with the electric interconnection wires insulated by a SU-8 coating layer for specifically detecting bio/chemical molecules in liquid environment. Functionalized with specific sensing terminals, siloxane-based molecule layer is directly modified on the cantilever SiO2 surface via reliable Si–O covalent bond. Instead of the previously used thiol self-assembly molecule layers that show long-term instability, the modified siloxane sensing layer secures long-term sensitivity stability and long working life. The sensors have been successfully used for detection of 2.5% aqueous tetramethylammonium hydroxide (TMAH) and avidin at 10−11 mol/mL trace level. An interesting phenomenon of opposite surface-stress sensing signals for different-sized target-molecules is experimentally observed. A simple model is proposed for preliminary explanation of this phenomenon. [ABSTRACT FROM AUTHOR]

Abstract: A dynamic collision model is proposed and built to analyze over-range protection of cantilever beam–mass structured micro-accelerometers. The model encloses angular momentum of the seismic mass, squeeze-film air damping, inertial force and supporting force (acting at the cantilever-root) during the collision between the seismic mass and the over-range stopping bumpers. Based on angular momentum balance equation we obtain the relationship among the supporting force F, the collision time duration t and the rotation center c where the bumpers are located. In the best case of F=0 which means no supporting force acts on the cantilever, we find the optimal bumper location of c. The accelerometers have been fabricated with the over-range protection design according to the dynamic collision model. Experimental test shows that 5g measure-range accelerometers can endure 9000g acceleration and there is no sign of performance degradation, which well verified the dynamic collision model. [Copyright &y& Elsevier]