谐振式硅微机械加速度计研究进展

谐振式硅微机械加速度计作为一种新型的对加速度进行测量的传感器,是通过检测加速度施加前后谐振敏感元件谐振频率变化实现对加速度检测的。该传感器具有频率信号输出、稳定性好、灵敏度高、精度高等优点,己成为微传感器的重要发展方向之一。汇总了该传感器的国内外相关研究现状,并对加工工艺进行了总结,讨论了谐振式硅微加速度计设计中的关键技术因素,并给出了未来的发展方向。     

Wafer-level mechanical characterization of silicon nitride MEMS

The mechanical and physical properties of silicon nitride thin films have been characterized, particularly for their application in load-bearing MEMS applications. Both stoichiometric (high-stress) and silicon-rich (low-stress) films deposited by LPCVD have been studied. Young's modulus, E, has been determined using conventional lateral resonators and by bulge testing of membranes, and tensile strength has been determined using a specially designed microtensile specimen. All microdevices have been fabricated using standard micromachining. We have also measured the thermal expansion coefficient of stoichiometric silicon nitride. Our best estimate of E is 325/spl plusmn/30 GPa for stoichiometric and 295/spl plusmn/30 GPa for silicon-rich silicon nitride. The average tensile strength for the stoichiometric material is 6.4/spl plusmn/0.6 GPa, while that for the silicon-rich material is 5.5/spl plusmn/0.8 GPa; the burst strength of membranes of the stoichiometric material is 7.1/spl plusmn/0.2 GPa.     

Fibre-optical MEMS switches based on bulk silicon micromachining

 Fibre-optical micro-electro-mechanical systems (MEMS) switches for optical communication systems require high-precision mechanical subassemblies due to the sensitivity of single-mode fibre coupling against misalignments. The fibre diameter of 125 μm also demands for actuators with at least ±62.5-μm travel range. Bulk micromachining based on wet anisotropic etching of crystalline silicon allows fabricating actuators and alignment structures with the required accuracy. Two concepts for lensless moving-fibre switches with thermo-mechanical and electrostatic Si-micromachined actuators with large displacements are demonstrated. Received: 14 January 2002/Accepted: 1 February 2002 This paper was presented at the Workshop “Optical MEMS and Integrated Optics” in June 2001.     

采用MEMS技术制造硅磁敏三极管

阐述了采用MEMS技术与反外延技术相结合在硅片表面制造具有矩形板状立体结构的硅磁敏三极管的设计原理、结构和工艺。结果表明 ,设计的硅磁敏三极管制造技术不但能与IC工艺相兼容 ,而且便于集成化 ,将有广泛的应用领域。     

利用MEMS技术研制硅脉象传感器

利用MEMS技术在N型<100>晶向的单晶硅衬底上设计并制作了以纳米硅/单晶硅异质结为源极和漏极的P-MOSFETs脉象传感器.在方形硅膜上制作四个以纳米硅/单晶硅异质结为源极和漏极P-MOSFETs,并将P-MOSFETs的沟道电阻设计成惠斯通电桥结构,从而实现对微小脉象信号的准确检测.实验结果表明,该硅脉象传感器在恒压源-3.0 V供电条件下,灵敏度为1.623 mV/kPa,准确度为2.029%F·S.     

Porous polycrystalline silicon: a new material for MEMS

A new technique for the fabrication of thin patterned layers of porous polycrystalline silicon (polysilicon) and surface micromachined structures is presented. First, a multilayer structure of polysilicon between two layers of low-stress silicon nitride is prepared on a wafer of silicon. Electrochemical anodization with an external cathode takes place in an RF solution. A window in the outer nitride layer provides contact between the polysilicon and the HF solution; the polysilicon layer contacts the substrate through openings in the lower silicon nitride layer (remote from the upper windows). Porous polysilicon growth in the lateral direction is found at rates as high as 15 μm min^-1 in 12M (25%, wgt) HF to be controlled by surface-reaction kinetics. A change in morphology occurs when either the anodic potential is raised or the HF concentration is decreased, causing the polysilicon to be electropolished. The etch front advances proportionally to the square root of time as expected for a mass-transport-controlled process. Similar behavior is observed in HF anodic reactions of single-crystal silicon. Dissolution of the polysilicon layer is confirmed using profilometry and scanning electron microscopy. Enclosed cavities (chambers surrounded by porous plugs) are formed by alternating between pore formation and uniform dissolution. Porous polysilicon also forms over a broad-area layer of polycrystalline silicon that has been deposited without overcoating the silicon wafer with a thin film of silicon nitride. The resulting porous layer may be useful for gas-absorption purposes in ultrasonic sensors     

Cross-axis sensitivity reduction of a silicon MEMS piezoresistive accelerometer

This paper presents realization of a MEMS piezoresistive single axis accelerometer using dual doped TMAH solution. The silicon micromachined structure consists of a heavy proofmass supported by four thin flexures and sandwiched between top and bottom glass plates. Boron diffused piezoresistors located near fixed points of the flexure are used for sensing the developed stress due to applied acceleration. Based on the initial results an improved design has also been considered to achieve reduced cross-axis sensitivity and nonlinearity. The fabricated sensor tested upto 13 g acceleration shows average sensitivity of 0.556 mV/g along normal to the proofmass plane. The measured cross-axis sensitivity was 3.272 μV/g for X-axis and 3.442 μV/g for Y-axis which is less than 1% of Z-axis sensitivity. Comparing two designs there was an improvement of 63% sensitivity along Y-axis for the design with flexures placed along the proofmass edges.     

MEMS on silicon for integrated optic metrology and communication systems

 Integrated optic micro-electromechanical systems (IO-MEMS) based on silicon-, Al₂O₃-SiO₂ and TiO₂-SiO₂ waveguides, doped with Ti, Cr, and Er on silicon substrates allow to generate complex metrology and optical communication systems. They exhibit low loss across a wide spectral range, occupy small space, and exhibit high functionality at low production cost. The waveguides are deposited by CVD and patterned by anisotropic plasma etching. The micro-electromechanical structures are formed by standard micro-machining processes and anodic bonding of silicon-glass. An integrated optical pressure sensor using the interferometer principle, a gas monitor for the near and middle infrared using elevated silicon single mode waveguides, an electrostatic-tuned Ti:/Cr:-sapphire laser, and a UV-VIS-NIR-spectrometer including integrated broad banded light sources represent metrology systems. Optical communication systems are described like a waveguide grating based wavelength demultiplexer, an optical transceiver using self aligned detector and emitter as well as a tapered fibre coupler, and an integrated optical amplifier. Received: 10 July 2001/Accepted: 15 August 2001 This work was funded by Deutsche Forschungsgemeinschaft, the Ministry of Research and Technology and the City of Hamburg. This paper was presented at the Workshop “Optical MEMS and Integrated Optics” in June 2001.     

Performance of thin film silicon MEMS on flexible plastic substrates

The fabrication and characterization of thin film silicon MEMS microbridges on flexible polyethylene terephthalate substrates are described. Surface micromachining using an aluminum sacrificial layer and a maximum processing temperature of 110 °C was used for device fabrication. These microbridges are electrostatically actuated and their deflection at resonance and at low frequencies is measured optically. Quasi-DC deflection with a quadratic dependence of the actuation voltage is observed, and resonance frequencies up to 2 MHz and quality factors of around 500 are measured in vacuum. Bending measurements are performed by subjecting these devices to tensile and compressive strain. The low frequency response (bridge deflection as a function of the applied voltage) was measured in air before bending and after every bending step. Under tensile strain, 16.6% of the devices survive the maximum bending with a radius of curvature of 1 cm, equivalent to a tensile strain 1.25%. In contrast, for compressive strain, 50% of the devices survive the bending corresponding to a radius of curvature of −0.5 cm, equivalent to a compressive strain of −2.5%. Thin film silicon microresonators on flexible plastic substrates can withstand more compressive strain than tensile.     

Localized silicon fusion and eutectic bonding for MEMS fabricationand packaging

Silicon fusion and eutectic bonding processes based on the technique of localized heating have been successfully demonstrated. Phosphorus-doped polysilicon and gold films are applied separately in the silicon-to-glass fusion bonding and silicon-to-gold eutectic bonding experiments. These films are patterned as line-shape resistive heaters with widths of 5 or 7 μm for the purpose of heating and bonding. In the experiments, silicon-to-glass fusion bonding and silicon to gold eutectic bonding are successfully achieved at temperatures above 1000°C and 800°C, respectively, by applying 1-MPa contact pressure. Both bonding processes can achieve bonding strength comparable to the fracture toughness of bulk silicon in less than 5 min. Without using global heating furnaces, localized bonding process is conducted in the common environment of room temperature and atmospheric pressure. Although these processes are accomplished within a confined bonding region and under high temperature, the substrate temperature remains low. This new class of bonding scheme has potential applications for microelectromechanical systems fabrication and packaging that require low-temperature processing at the wafer level, excellent bonding strength, and hermetic sealing characteristics     

A distributed MEMS phase shifter on a low-resistivity silicon substrate

Distributed MEMS phase shifters using CMOS-grade low-resistivity silicon have been successfully developed. Kapton films were utilized as dielectric layers to reduce RF signal attenuation in the lossy silicon substrate. The scattering parameters were evaluated from DC to 26 GHz. The phase shifting reaches 43° and insertion losses are less than 1.4 dB. The manufacturing process is simple and compatible with CMOS and post CMOS processes.     

TiNi (shape memory) films silicon for MEMS applications

TiNi shape memory alloy in thin film form is an excellent candidate for MEMS microactuation. Using RF sputter deposition, thin films of TiNi (51.7 at% Ti-48.3 at% Ni) have been formed on silicon substrates and produced shape memory behavior at approximately 60°C. Films were amorphous when deposited and were subsequently annealed at 515°C for 30 min. to crystallize the shape memory microstructure. Excellent adherence was achieved onto silicon, SiO₂ and poly-silicon surfaces. Microfabrication was used to create TiNi diaphragms, which exhibited useful shape memory microactuation and other desirable mechanical properties. The diaphragms recovered greater than 2% strain when heated through the phase transformation temperature, providing a maximum work density of at least 5×10⁶ J/m ³. This work density is higher than that of any other type of microactuation     

MEMS单晶硅加热雾化芯片设计与制作

为实现高粘度液体的雾化,提出一种微机电系统(MEMS)单晶硅快速加热雾化芯片。对单晶硅加热芯片结构进行了设计,芯片为10 mm×10 mm的方形,布置了168个边长为500μm的方形雾化孔。通过ANSYS有限元软件进行了电—热耦合仿真,以评估其温度分布均匀性。衬底采用5×10~(-3)Ω·cm的4 in(l in=2.54 cm)N型(100)硅片,基于各向异性湿法腐蚀工艺完成了微孔阵列和芯片的制造。测试结果表明:室温下芯片电阻约为0.6Ω,且电阻值与温度呈正相关;芯片温度分布均匀,最低温与最高温相差约12.7%;施加3.7 V电压时,芯片在4 s内可升温至300℃,能够实现对甘油的快速雾化。该芯片结构和制作工艺简单,易于实现批量制造。     

Oxidized bridges technology for suspended MEMS fabrication using standard silicon wafer

Microsystem Technologies - This paper presents a new method for electrically isolating released single crystal silicon MEMS structures. The technology employees double-side processing deep reactive...     

Fabrication and characterization of PECVD silicon nitride for RF MEMS applications

The structural, optical and electrical properties of plasma enhanced chemical vapor deposited silicon nitride layers are investigated, which have been used as a dielectric layer during RF MEMS fabrication. During growth, the gas ratio (SiH₄/NH₃) is varied between 0.33 and 0.5 and pressure is varied between 400 and 700 mTorr while deposition time is kept constant. The results in the films show differing properties. The thicknesses of the resultant films are between 150 to 220 nm with different gas flow ratios and pressures whereas the deposition time was kept constant. A Bruggeman effective medium approximation is utilized to model the refractive index of the films. Reflectance measurements were carried out in the range of 210–250 nm. The refractive indexes of the films varied between 1.79 and 2.03, with a dielectric constant varying from 6.66 to 7.22. Capacitance voltage measurements yield a fixed dielectric charge value in the low ?10¹² cm^?2 while a breakdown voltage of 915 V μm^?1 is achieved for films grown at the lowest gas ratio and pressure. The quality of Si/Si_xN_y interface is also considered.     

A silicon test chip for the thermomechanical analysis of MEMS packagings

This paper reports on a method for the investigation of mechanical stress on MEMS sensor and actuator structures due to packaging processes. A silicon test chip is developed and manufactured to validate the simulation results. Finite element analysis (FEA) is used to optimize the geometric parameters and to find a stress sensitive sensor geometry. A diaphragm structure is used as mechanical amplifier for bulk induced stresses during the packaging process. Piezo resistive solid state resistors are doped into the surface of the chip to measure the stress in the diaphragms and at the contact pads being most significant locations for analysis. A high precision ohmmeter was used to measure the resistance prior and past the packaging process. The captured data allows for computation of the resulting stress loads in magnitude. Therefore, a stress evaluation of different packaging technologies is conducted and the impact of the packaging process on reliability can be estimated immediately.     

Friction and dynamically dissipated energy dependence on temperature in polycrystalline silicon MEMS devices

Microsystem Technologies - In this paper, we report on the influence of capillary condensation on the sliding friction of sidewall surfaces in polycrystalline silicon micro-electromechanical...     

Quantifying adsorbed water monolayers on silicon MEMS resonators exposed to humid environments

This study investigated the influence of temperature and humidity on the adsorbed water layer on micron-scale monocrystalline silicon (Si) films in air, using a Si-MEMS kHz-frequency resonator. Both temperature and relative humidity induced a reversible change in resonant frequency, attributed to the temperature-dependent properties of Si and to a change in adsorbed water layer. The excellent precision in resonant frequency measurement (0.02 Hz, or 0.5 ppm) allowed precise calculation of the changes in adsorbed water layer thickness over the specimen surface. The increase in water thickness with relative humidity was a function of temperature and could not be described with simple multimolecular adsorption theories such as the BET theory. A likely explanation is the presence of hydrocarbon contaminants on the Si surface. Guidelines are provided to accurately measure the influence of temperature and relative humidity on the adsorbed water layer thickness on micron-scale Si surfaces, using this technique.