Showing papers by "Xinxin Li published in 2006"

High-mode resonant piezoresistive cantilever sensors for tens-femtogram resoluble mass sensing in air

Abstract: According to the demand for an ultrasensitive mass sensor for bio/chemical molecular detection, resonant cantilever sensors are developed for detection in an air environment. Both a piezoresistive bridge and a metal coil are integrated in the cantilever for signal sensing and Lorentz-force resonance excitation, respectively. Compared with conventional first flexure mode resonance, measurement results for the second mode resonance show an improved mass-sensing resolution from 0.17 pg to 0.06 pg due to the higher quality factor. For further improving the resolution, an optimized electromagnetic excitation method specifically for the second resonant mode is proposed and developed. The optimized method provides a two-point excitation that matches the second mode shape function of the cantilever deflection and excites the second mode more efficiently. Compared to a cantilever with a conventional excitation method, the optimally excited cantilever improves the quality factor from 307 to 857. Based on the experimental results for the optimally excited second mode resonant sensor, 29 × 10−15 g resolution for in-air mass sensing is achieved. The developed second mode resonant cantilever sensor with piezoresistive sensing element integrated on-chip is promising for use in high-performance portable biological/chemical sensing applications.

A single-sided micromachined piezoresistive SiO2 cantilever sensor for ultra-sensitive detection of gaseous chemicals

Abstract: This paper presents a novel SiO2 microcantilever sensor for high-sensitive gaseous chemical detection. A thin single-crystalline silicon piezoresistor is integrated in the SiO2 cantilever for low-noise and high-resoluble signal readout. The paper relates the MEMS formation, modeling and detecting experiments of the sensor. The micro-fabrication of the cantilever sensor is processed at a single side (front side) of the silicon wafer, combined with functionalization of a self-assembled monolayer (SAM) on the sensing cantilever for specific molecular capturing. The model of the piezoresistive SiO2 cantilever sensor is given by using both analysis and simulation, resulting in good agreement with the measuremental data. With the cantilever modified by the specific SAM, the detecting performance of the sensor is experimentally obtained. Attributed to the high sensitivity and the low electric noise of the piezoresistive SiO2 cantilever, ammonia gas of 0.1 ppm level concentration and dimethyl methylphosphonate vapor of 10 ppb concentration have been well detected. The integrated cantilevers are promising for inexpensive, highly resoluble and portable chemical/biological detection.

Silicon dioxide microcantilever with piezoresistive element integrated for portable ultraresoluble gaseous detection

Abstract: Surface-stress sensing microcantilevers with on-chip signal readout are demanded for on-the-spot ultrasensitive biological/chemical detection. For enlarging the bending of the cantilever under surface stress induced by specific reaction, a novel SiO2 cantilever is developed which features much lower Young’s modulus than conventional Si or SiNx cantilevers. Thin single-crystalline-silicon piezoresistors are integrated with the SiO2 cantilevers for electric readout. For improving resolution, the piezoresistors are fully encapsulated by SiO2. Thus, the piezoresistors with SiO2 isolation show much lower leakage-related noise than those with p-n junction isolation. Following the description of microfabrication process, this letter gives the sensing model and discusses the thermal mechanism of the piezoresistive SiO2 cantilever. With a specific self-assembled monolayer functionalized on the cantilever surface, on-chip detection for the vapor of trinitrotoluene is performed with a resolution of about 20ppt (part...

Single-wafer-processed nano-positioning XY-stages with trench-sidewall micromachining technology

Abstract: For operation and manipulation with nanometric positioning precision, a single crystalline silicon micro XY-stage is developed by using double-sided bulk-micromachining technology. Front-side deep reactive ion etching combined with backside anisotropic etching constructs the high-aspect-ratio comb-driven XY-stage in a single standard silicon wafer (i.e., no silicon on insulator wafer is used). For integrating several electrostatic actuators in one silicon chip, different actuators are electrically isolated from each other using a trench-sidewall insulating technique. SiO2-refilled trench bars are formed on vertical trench sidewalls to isolate adjacent comb-drive elements. Combined with the reverse-biased p–n junction along the boron-diffused trench sidewall for comb driving, individual actuators can be operated independently. The developed XY-stage of 1600 × 1600 µm2 is suspended by four sets of folded-beam and bending-flexure composite springs. To maximize the moving distance, a two-segment comb finger with a gently curved transition is used for both improving the actuation efficiency and avoiding side instability of the stage. The experimental results verify the stage design including the gentle transition of a two-segment comb-drive scheme. Under 23 V driving voltage, a 10 µm moving stroke is measured in each of the four directions. Compared with a conventional comb structure, the two-segment comb fingers contribute 70% improvement in actuating amplitude. The positioning precision of the stage is evaluated with a nano-mechanical indenting experiment. A scanning probe microscopy probe with an electrical-heated nano tip is put in contact with the surface of a polymethyl methacrylate film that is coated on the stage surface. Along with the movement of the stage, pulsed heating on the nano tip produces serial nano-pitches. With the nano-indenting experiment, better than 18 nm positioning precision is obtained for the XY-stage.

AFM probes fabricated with masked-maskless combined anisotropic etching and p + surface doping

Abstract: The paper presents a newly developed high-yield micro-fabrication technology for single-crystalline silicon atomic force microscope (AFM) probes. Both the tips and the cantilevers are simultaneously formed by a masked?maskless combined anisotropic etching process. Compared to a conventional tip-to-cantilever sequential fabrication scheme, this tip-and-cantilever simultaneous formation can effectively increase fabrication yield by avoiding the tips damaged during the following processed photolithographic steps for defining the cantilevers. By heavy boron doping at the surface, the conductive AFM probe provides an electrical path to the electric ground of the AFM that helps to eliminate the electrostatic accumulation of charges and, therefore, eliminate undesirable electrostatic forces between the probes and the samples. A fabrication yield as high as 90% has been obtained for the AFM probes for 4 inch wafers. The tips after oxidation-sharpening treatment generally have a radius of 10?30 nm. The cantilever spring constant can be well controlled in the range of 0.025?40 N m?1. High-quality sample scanning results with the formed AFM probes are obtained with a slightly better resolution than that from commercial probes without surface conductive treatment.

A silicon cantilever probe card with tip-to-pad electric feed-through and automatic isolation of the metal coating

Abstract: A new microfabrication technology for a silicon cantilever array probe card is developed, with technical requirements from Sino IC Technology Co. Ltd for dies under test of 8 inch wafers. A probe card consisting of 96 silicon probes for simultaneously testing two dies is designed and fabricated with bulk micromachining technologies. According to the requirements, a probing force of 25 mN is designed for over-drive displacement no lower than 20 µm. By using a double-sided metal overlapping scheme, electric feed-through is realized from the probe tips at the bottom to the automatic testing equipment interface on the topside. A recessed concave step is designed and formed to automatically isolate the adjacent probes when a conductive metal layer is sputtered on the surface of the cantilever tips. Characterization of the probe card shows that the resistance at the feed-through via holes is generally lower than 0.8 Ω. In addition, the parasitic capacitance between adjacent probes is as low as 0.02–0.03 pF. The spring constant of the cantilevers is measured as 1126.8 N m−1, which agrees well with the design.

Nanofabrication based on MEMS technology

Abstract: In this paper, a novel nanofabrication method that develops from the traditional microelectromechanical system (MEMS) technology of anisotropic etching, deep reaction ion etching, and sacrificial layer process has been reviewed based on our work. With such a technology, nano tips, nano wires, nano beams even nano devices can be fabricated in a batch process. Beams with thickness of only 12 nm, a nano tip with a heater on the beam, and a nano wire whose width and thickness is only 50 nm are demonstrated. The scale effect of the Young's modulus of silicon has been observed and the nano-electronic-mechanical data storage has been presented

A Microgyroscope With Piezoresistance for Both High-Performance Coriolis-Effect Detection and Seesaw-Like Vibration Control

Abstract: A novel piezoresistive scheme is proposed to solve the problem of piezoresistive gyros in terms of low-sensitivity and high-temperature drift. Based on the piezoresistive scheme, a micromachined vibratory gyroscope is developed. By using axially stressed piezoresistive tiny-beams, piezoresistive detection to Coriolis-acceleration can be realized with both high sensitivity and high resonant frequency, i.e., high gyro operation frequency. Thanks to the high piezoresistive sensitivity, precise frequency matching between the driving and detecting modes is unnecessary. Another four-terminal transverse piezoresistive element is used to monitor and stabilize the amplitude of the seesaw-like torsional vibration through a feedback loop. With the same feedback loop, temperature drift of the piezoresistive angular-rate sensing signal can be on-chip compensated, as the transverse piezoresistance tracks the temperature drift of the angular-rate sensing piezoresistors. The gyro is designed and fabricated by bulk micromachining. Measurement results verify the proposed piezoresistive gyro scheme and show noise-limited angular-rate resolution of 0.33deg/s for plusmn300deg/s range. Considering the mature fabrication technology and simple signal-readout for piezoresistive sensors, the piezoresistive microgyros are promising for low-cost applications

An Axial-beam Piezoresistive Accelerometer for High-performance Crash Detection of Automotive Industry

Abstract: A high-performance piezoresistive accelerometer is developed for crash assessment of automobiles. For meeting the technical requirements, a novel sensor scheme is used that features both high sensitivity and high dynamic characteristics (including high resonant frequency and precise damping ratio). A three-beam seismic mass structure is proposed with the combination of a comb damper. Considering the demand in low-cost volume production, standard silicon wafers (instead of SOI wafers) are used here for micro-fabrication of the sensors. Double-sided deep RIE is processed for the micromechanical structure that is combined with the piezoresistive integration process. Test results show that the normalized sensitivity of the accelerometer is measured as 0.11 mV/g/5V within the full-range of 2000 g. The resonance frequency is 31 KHz, with the band of plusmn5% variation in sensitivity wider than 5 kHz 15000 g shocking test has been implemented to the packaged device, resulting in safe survival.

A Post-CMOS Concave-Suspending MEMS Process in Standard Silicon Wafers for High-Performance Solenoidal-DNA-Configured Micro-Transformers

Abstract: Solenoidal-DNA-Configured transformers are fabricated in standard silicon substrate by using a post-CMOS concave-suspending MEMS process. The concavely suspended structure of the transformer effectively depresses the substrate effects including eddy current and capacitive coupling between the coil and the substrate, thereby, achieving both high Q-factor and high resonant frequency for the self-inductors. With broad useable frequency band for high mutual reactive coupling coefficient, high available gain is measured in wide frequency band for high performance radio frequency ICs.

Competition between Host Aggregates and Isolated Guest Chromophores in Trapping Excitons in Polybenzazole Copolymers and Blends

Abstract: Host-guest systems have been prepared using the blends and copolymers consisting of the host molecule poly(p-phenylene benzobisoxazole) (PBO) with a higher bandgap of 2.93 eV and the guest molecule poly(2,5-thienylbenzobisoxazole) (PBOT) with a lower bandgap of 2.57 eV. These systems have been investigated using photoluminescence (PL) spectra and time-resolved PL decay dynamics. Both PBOT-PBO copolymers and PBOT/PBO blends with the PBOT compositions less than 20% demonstrate higher intensities and narrower bandwidths in solid-state emission compared to that of PBOT, as well as larger fractions of the shorter lifetime component in PL decay dynamics. A general scheme on intrachain and interchain exciton migration and trapping mechanism has been proposed to interpret the phenomena in both solutions and thin films. Particularly, a competition in trapping exciton between PBO aggregates and isolated PBOT chromophores has been revealed.

Trace TNT Vapor Detection with an SAM-functionalized Piezoresistive SiO2 Microcantilever

Abstract: Presented is an ultra-sensitive piezoresistive SiO2 microcantilever sensor for trace TNT detection. A thin single crystalline silicon piezoresistor is encapsulated in a SiO2 cantilever for on-chip electric readout. The cantilever can lead to a more sensitive characteristic compared with conventional silicon cantilever because of a much lower Young's modulus of SiO2. Moreover, fully covered by SiO2, the piezoresistor behaves a much lower current leakage relative noise comparing to those with p-n junction isolation. In addition to the performance improvement of the cantilever sensor, an optimized self-assembled monolayer (SAM) of 6-mercaptonicotinic acid (6-MNA) is immobilized on the cantilever as specifically sensitive coating to capture TNT molecules and, then, causes surface stress for piezoresistive detection. Experimental results shows that the sensor yields a rapid, reversible and reproducible response to TNT vapor, and can achieve a detection limit of about 20-30 ppt.

Tens Femtogram Resoluble Piezoresistive Cantilever Sensors with Optimized High-Mode Resonance Excitation

Abstract: Ultra-sensitive mass sensor is demanded for bio/chemical molecular detection. A piezoresistive resonant cantilever sensor with optimized is developed with an optimized electromagnetic excitation for high-mode flexure resonance. In our experiment, it has been proved that the quality factor of the resonant sensor can be improved by this optimized high-mode resonance exciting technique. Experimental results have shown that the optimized excitation for the resonant cantilever sensor can effectively improve the mass-sensing resolution to 29fg, thereby, improving the bio/chemical sensor performance.

Single-wafer-processed trench-sidewall integration and its application in a micro resonant detector for vacuum monitoring

Abstract: A trench-sidewall technology is developed for single-wafer-based integration of micro sensors and actuators with in-plane lateral configuration. Piezoresistive sensing and electrostatic actuating elements can be integrated together onto the deep-trench sidewalls of in-plane microelectromechanical structures by sidewall boron diffusion. Adjacent sidewall elements are electrically isolated by previously formed SiO2-refilled trenches. Electric transference from the sidewall to wafer surface is performed via the overlaps between the sidewall-diffusion and previously doped p+ at wafer surface. As a versatile micro-fabrication process, the technology promises to be widely used. As a typical application, a cantilever-shaped micro resonant vacuum detector was formed with both sidewall piezoresistive sensing and electrostatic resonance-driving elements integrated. In addition to satisfactory electric principles measured to verify the trench-sidewall technology, the resonant sensor was used to detect vacuum by meas...

Study of humidity properties of Zinc Oxide modified Porous Silicon

Abstract: In this paper, we discussed the humidity sensing behavior of Zinc Oxide modified porous silicon (ZnO/PS) composite structure. The porous silicon substrates were prepared by the electrochemical etching process first. Then, by sol-gel technique, it is possible to obtain a uniform Zinc Oxide films on the porous silicon substrates. The electrical conductivities of the porous silicon and Zinc Oxide modified porous silicon structures under different humidity levels were measured. Our study indicate that the modification of porous silicon by sol-gel Zinc Oxide increase the sensitivity and shorten the response time to the relative humidity, probably due to the increment of the specific surface area of the porous silicon. The other parameters, such as the concentration of zinc oxide precursors, which can affect the sensing performance, were also discussed. Therefore, the Zinc Oxide modified porous silicon composite structure studied is potential to develop the humidity sensor with high performance.

Fabrication of the Isolated Nano-beams in the Normal (111) Si Wafers with KOH Etching

Abstract: Described in this paper is a novel method to fabricate the isolated nano-beams in the normal (111) Si wafers The cantilever and the double clamped beams with nano thickness are anchored through the metal wires, which are isolated to the substrate electrically The thickness of the beams is determined by dry etching The beams are released by KOH etching As the bottom of the beams is (111) plane, which is etched very slowly in KOH etching, the etching is self-ended after the beams are released The gaps between the beams and the substrates are determined by RIE etching before releasing, due to the fact that the surfaces of the substrates are also (111) planes When the gaps between the beams and the substrates are large enough, the nano beams can be released without the special treatment The surface roughness of (111) wafer is also discussed in the paper

A Single-Wafer-Processed XY-Stage Fabricated with Trench-sidewall Doping and Refilled-Trench Isolating Technology

Abstract: For nano-metric positioning and manipulation, a single-crystalline-silicon XY-stage is fabricated by using a double-sided bulk-micromachining technology. For defining different electrostatic actuators in one ordinary wafer (instead of SOI wafer), a trench-sidewall electric isolation method is developed. Previously insulator-refilled trench-bars are used to cut and isolate the different comb-drive actuating elements on the structural trench-sidewalls. Combined with the reverse-biasd isolation of p-n junctions along the boron-diffused trench- sidewall for comb-driving, individual actuators can be operated independently. For maximizing the actuating stroke that is limited by the fabricated minimal comb-gap, a two-segment comb with a gentle-curve transition is designed for both improving actuation-amplitude and avoiding side-instability of the stage. Under 23 V actuating voltage, the moving stroke is about 10 mum in each of the four directions. Compared with conventional comb structure, the new comb design contributes 70% improvement in driving amplitude. Nano pitches on PMMA film are recorded by an electric-heated SPM probe. Coated with PMMA film, the stage movement is precisely controlled, resulting in controllable nano recording.