Sacrificial wafer bonding for planarization after very deep etching

A new technique is presented that provides planarization after a very deep etching step in silicon. This offers the possibility for resist spinning and layer patterning as well as realization of bridges or cantilevers across deep holes or grooves. The sacrificial wafer bonding technique contains a wafer bond step followed by an etch back. Results of polymer bonding followed by dry etching and anodic bonding combined with KOH etching are discussed. The polymer bonding has been applied in a strain based membrane pressure sensor to pattern the strain gauges and to provide electrical connections across a deep corrugation in a thin silicon nitride membrane by metal bridges     

MEMS-based humidity sensor with integrated temperature compensation mechanism

This study applies conventional micro-electro-mechanical systems (MEMS) techniques to develop a novel low-cost humidity sensor comprising a silicon substrate, a freestanding cantilever and an integrated resistive thermal sensor. The cantilever has a composite structure comprising a thin layer of platinum (Pt) deposited on a silicon nitride layer and then covered with a polyimide sensing layer. The cantilever deflected in the upward direction as water molecules absorbed by the polyimide sensing layer. The humidity sensor chip caused a measurable change in the resistance of the platinum layer. By compensating the change in the measured resistance by the ambient temperature, the absolute value of the relative humidity can be directly derived. The experimental results show that the sensor has a time-response of 0.9 s when exposed to a sudden humidity change of 65%RH to 95%RH. The sensitivity of the sensors decreases as the temperature increases. Furthermore, the sensor with the longest Pt resistor has the greatest sensitivity. In additions, the temperature-calibrated resistance signal generated by the sensor varies linearly with the ambient humidity.     

Insect-model based microrobot with elastic hinges

This paper describes the new concept of making a microrobot based upon insects and its implementation using rigid plates and elastic hinges. One unique feature of an insect's structure is its deformable external skeleton, to which wings and legs are attached, forming a closed-loop mechanism. Frictionless deformation of elastic elements, instead of conventional rotational joints, allows for efficient actuation in the microscale. We propose three-dimensional micromechanisms based on an external skeleton and have fabricated several examples using polysilicon plates and polyimide hinges. By folding the polysilicon plates along the polyimide hinges just like paper, 3D structures can be constructed on silicon wafers. These structures can be easily actuated by electrostatic force. Closed-loop flapping mechanisms have been fabricated and electrostatically actuated to demonstrate the motion of the external skeleton system. Resonant vibration was observed in a frequency range of around 10 kHz. Actuation using electrostatic induction is also considered     

Thermal isolation of high-temperature superconducting thin filmsusing silicon wafer bonding and micromachining

Using a new micromachining technology, thermally isolated thin films of high-temperature superconductor have been microfabricated. The intended application for these structures is in infrared bolometers. A silicon wafer bonding process produces a low thermal mass island of single-crystal silicon on a silicon nitride membrane which provides thermal isolation. The silicon can act as a seed for the epitaxial growth of YBa₂Cu₃O₇ on a yttria-stabilized zirconia buffer layer. This paper describes the overall concept of the thermally isolated device, and demonstrates that the micromachined structure can be fabricated with high-quality superconducting films     

MEMS微型NO2气体传感器的研究

采用MEMS技术制作了硅基微型NO2气体传感器,选用高分子金属酞菁聚合物酞菁铜作为敏感膜,从半导体理论出发解释了酞菁铜的敏感机理。阐述了该传感器的结构与工艺流程,并测试了传感器的气敏特性、温度特性、响应时间和恢复时间等敏感特性。实验结果验证了酞菁铜对NO2气体的敏感性,该传感器可以检测到10^-6量级的NO2气体,且响应时间快。     

A low-power micromachined MOSFET gas sensor

This paper reports on the design, fabrication, and characterization of the first low-power consumption MOSFET gas sensor, The novel MOSFET array gas sensor has been fabricated using anisotropic bulk silicon micromachining. A heating resistor, a diode used as temperature sensor, and four MOSFETs are located in a silicon island suspended by a dielectric membrane. The membrane has a low thermal conductivity coefficient and, therefore, thermally isolates the electronic components from the chip frame. This low thermal mass device allows the reduction of the power consumption to a value of 90 mW for an array of four MOSFETs at an operating temperature of 170°C. Three of the MOSFETs have their gate covered with thin catalytic metals and are used as gas sensors. The fourth one has a standard gate covered with nitride and could act as a reference. The sensor was tested under different gaseous atmospheres and has shown good gas sensitivities to hydrogen and ammonia. The low-power MOSFET array gas sensor presented is suitable for applications in portable gas sensor instruments, electronic noses, and automobiles     

Microfabrication of submicron nozzles in silicon nitride

A novel microfabrication process is described for obtaining nanometer apertures in highly cusped nozzle-like structures fabricated in silicon nitride, having apex angles of up to a few degrees. The process is based on a sacrificial etch technology using single-crystal silicon as the mold and silicon nitride as the material for the nozzle. The nitride coating on the apex of the pyramid shaped mold is selectively etched off using a polymer layer as the etch mask, which leaves the tip of the silicon mold protruding from the masked nitride, thus defining the aperture of the nozzles. The silicon mold is then removed in an alkaline etchant, which leaves the freestanding nozzles. The process is applicable to fabrication of similar structures in a variety of other materials such as silicon dioxide, boron-doped silicon, polysilicon, and refractory and noble metals. The main requirement is the preferential etchability of the mold with respect to material for the nozzles     

Fabrication and flow-sensor application of flexible thermal MEMS device based on Cu on polyimide substrate

A Cu on polyimide (COP) substrate was proposed as a MEMS material, and the fabrication process for a flexible thermal MEMS sensor was developed. The COP substrate application to MEMS devices has the advantage that typical MEMS structures fabricated in a SOI wafer in the past—such as a diaphragm, a beam, a heater formed on a diaphragm—can also be easily produced in the COP substrate in the flexible fashion. These structures can be used as the sensing element in various physical sensors, such as flow, acceleration, and shear stress sensors. A flexible thermal MEMS sensor was produced by using a lift-off process and sacrificial etching of a copper layer on the COP substrate. A metal film working as a flow sensing element was formed on a thin polyimide membrane produced by the sacrificial etching. The fabricated flexible thermal MEMS sensor was used as a flow sensor, and its characteristics were evaluated. The obtained sensor output versus the flow rate curve closely matched the approximate curve derived using King’s law. The rising and falling response times obtained were 0.50 and 0.67 s, respectively.

     

Microcantilever-based weather station for temperature,humidity and flow rate measurement

The current study develops a new process for the fabrication of Pt resistor temperature detectors (RTDs), cantilevers covered with a water-absorbent polyimide layer for humidity measurement and the bending-up of cantilevers to determine the flow rate. Pt RTDs are fabricated on the silicon substrate. The temperature measurement is based on the linear resistance variations when temperature changes. The polyimide layer is spun on the cantilever to form a humidity sensing layer. A variation in humidity causes moisture-dependent bending of the micro-cantilever, which changes the measured resistance of the resistor on the micro-cantilever. The same type of micro-cantilever, without spinning on polyimide, is used to form an anemometer. It is found that the cantilever bends slightly upward as a result of the released residual stress induced in the beam during the fabrication. When wind passes over the cantilever beam, a small deformation occurs. Variations in the flow rate can therefore be determined by measuring the changes in resistance caused by the beam deflection, using a LCR meter.     

The design and operation of a MEMS differential scanning nanocalorimeter for high-speed heat capacity measurements of ultrathin films

A MEMS sensor has been developed for use as a calorimetric cell in an ultra-sensitive, thin-film, differential scanning calorimetric technique. The sensor contains a freestanding, thin (30 nm to 1000 nm), low-stress silicon nitride membrane with lateral dimensions of a few millimeters. This membrane, along with a thin (50 nm) metallization layer, forms a calorimetric cell with an exceptionally small addenda. This small addenda creates a very sensitive calorimetric cell, able to make heat capacity measurements of nanometer-thick metal and polymer films. The sensor fabrication and various design considerations are discussed in detail. The calorimetric technique and examples of applications are described.     

聚酰亚胺隔热悬挂结构设计与制作

加热器在原子钟、气体传感器、生物传感器等器件中有着广泛的应用,降低加热功耗对于这些器件的实用化具有重要意义。设计并制作了一种集成了加热器和温度传感器的聚酰亚胺隔热悬挂结构,用于承载待加热的芯片,并利用集成制作在聚酰亚胺膜上的2组Pt线圈分别对芯片进行加热和温度检测。实验证明:聚酰亚胺隔热悬挂结构由热传导引起的功耗仅为没悬挂结构时的0.95%,大大降低了由热传导引起的功耗,即便在没有真空封装的情况下,在芯片工作在100℃时,功耗也可降低到36.7%;同时该结构也具有良好的机械性能,承载体积为14 mm×7 mm×1 mm的硅基芯片时,安全系数可达到56.58,具有很好的应用前景。     

Mechanical property characterization of LPCVD silicon nitride thin films at cryogenic temperatures

T-shape, LPCVD silicon nitride cantilevers are fabricated to determine Young's modulus and fracture strength of silicon nitride thin films at room and cryogenic temperatures. A helium-cooled measurement setup is developed and installed inside a focused-ion-beam (FIB) system. A lead-zirconate-titanate (PZT) translator powered by a function generator and a dc voltage is utilized as an actuator, and a silicon diode is used as a temperature sensor in this setup. Resonant frequencies of identical cantilevers with different "milling masses" are measured to obtain thickness and Young's modulus of the silicon nitride thin films, while a bending test is performed to obtain fracture strength. From the experiment, the average Young's modulus of low-pressure chemical-vapor deposition (LPCVD) silicon nitride thin films varies from 260.5 GPa at room temperature (298 K) to 266.6 GPa at 30 K, and the average fracture strength ranges from 6.9 GPa at room temperature to 7.9 GPa at 30 K. The measurement setup and technique presented here can be used to characterize the mechanical properties of different MEMS materials at cryogenic temperatures.     

Development of a flexible three-axial tactile sensor array for a robotic finger

In this paper, we propose a flexible three-axial tactile sensor array for measuring both normal and shear loads. The sensor array has 16 tactile sensor units based on piezoresistive strain gauge. It is constructed on a Kapton polyimide film using advanced polymer micromachining technologies. Thin metal strain gauges are embedded in polyimide to measure normal and shear loads, which are tested by applying forces from 0 to 1?N. The developed sensor unit had a hysteresis error of about 9% and repeatability error of about 1.31%. The sensor showed a good resulting image when pressed by a circle-shaped object with 10?N loads. The proposed flexible three-axial tactile sensor array can be applied in a curved or compliant surface that requires slip detection and flexibility, such as a robotic finger.     

Electromagnetically force balanced polymer accelerometer

An acceleration sensor from polymer has been developed which balances a proof mass by magnetic forces. The sensor is fabricated from a polyimide membrane with conductor paths from gold patterned by photolithography and etching, a frame manufactured by ultrasonic hot embossing, and permanent magnets fixed to the frame. Except the conductor path and permanent magnets, all components are made of polymers on a planar substrate, and then the frame is kinked forming the desired three-dimensional structure. In a first try, a sensitivity of 0.46 V/(m/s²) was achieved, and cross axis sensitivity error was less than 3 %.     

微机械热电堆红外探测器的设计

阐述了微机械热电堆的设计原理、所用材料及主要结构,并研究了3×3阵列的微机械制造工艺,该热电堆结构支撑结构为氮化硅—氧化硅—氮化硅的复合介质膜。热电偶材料采用多晶硅和铝,热电偶采用并列排布的结构,对冷端覆盖了绝热层,用以提高热电堆的探测率。由于该制作工艺与标准IC工艺兼容,使得硅基热电堆红外探测器得到了越来越广泛的应用。     

Flexible hot-film anemometer arrays on curved structures for active flow control on airplane wings

A set of flexible MEMS sensor arrays for flow measurements in boundary layers is presented. The sensor principle of these anemometers is based on convective heat transfer from a hot-film into the fluid. Each sensor consists of a nickel sensing element between copper supply tracks. The functional layers are attached either on a ready-made polyimide foil or on a spin-on polyimide layer. These variants are designed to meet the requirements of measurements in different environments. Spin-on technology enables the use of very thin polyimide layers, ideally suited for measurements in transient flows. It is a unique characteristic of the presented arrays that their total thickness can be scaled from 7 to 52 μm. This is essential, because the sensor thickness has to be adapted to the varying thickness of the boundary layers in different aerodynamic tests. With these sensors we meet the special requirements of a wide range of fluid mechanic experiments but in particular those of future active flow control on airplane wings. For less critical flow conditions with much thicker boundary layers, thicker sensors might be sufficient and cheaper, so that sensors fabricated on ready-made foils are perfect for these applications. Since the presented sensors are flexible, they can be attached on curved aerodynamic structures without any geometric mismatches. The entire development, starting from theoretical investigations, is described. Further, the micro-fabrication is discussed, including photolithography, sputtering and wet-etching. In particular the wet-etching of the sensing element is found to be critical for the functional characteristics.     

Design and optimization of parylene nanomechanical cantilevers with integrated piezoresistors for surface-stress based biochemical sensing

The sensitivity of surface-stress based cantilever sensor can be significantly increased by using polymer as the cantilever material. In our previous research, parylene nanomechanical cantilevers with integrated poly-crystal silicon piezoresistors have already been developed for biochemical applications. However, parylene cantilevers exhibit different behaviors compared with their more rigid counterparts. In this paper, a new analytical model has been developed and verified using finite element simulations. The design optimization has also been discussed based on the new analytical model, resulting in several useful design guidelines that are unique to parylene or more generally to polymer cantilevers with integrated poly-crystal or single-crystal silicon piezoresistors for surface stress sensing.     

Sealing of micromachined cavities using chemical vapor depositionmethods: characterization and optimization

This paper presents results of a systematic investigation to characterize the sealing of micromachined cavities using chemical vapor deposition (CVD) methods. We have designed and fabricated a large number and variety of surface-micromachined test structures with different etch-channel dimensions. Each cavity is then subjected to a number of sequential CVD deposition steps with incremental thickness until the cavity is successfully sealed. At etch deposition interval, the sealing status of every test structure is experimentally obtained and the percentage of structures that are sealed is recorded. Four CVD sealing materials have been incorporated in our studies: LPCVD silicon nitride, LPCVD polycrystalline silicon (polysilicon), LPCVD phosphosilicate glass (PSG), and PECVD silicon nitride. The minimum CVD deposition thickness that is required to successfully seal a microstructure is obtained for the first time. For a typical Type-1 test structure that has eight etch channels-each 10 μm long, 4 μm wide, and 0.42 μm tall-the minimum required thickness (normalized with respect to the height of etch channels) is 0.67 for LPCVD silicon nitride, 0.62 for LPCVD polysilicon, 4.5 for LPCVD PSG, and 5.2 for PECVD nitride. LPCVD silicon nitride and polysilicon are the most efficient sealing materials. Sealing results with respect to etch-channel dimensions (length and width) are evaluated (within the range of current design). When LPCVD silicon nitride is used as the sealing material, test structures with the longest (38 μm) and widest (16 μm) etch channels exhibit the highest probability of sealing. Cavities with a reduced number of etch channels seal more easily. For LPCVD PSG sealing, on the other hand, the sealing performance improves with decreasing width but is not affected by length of etch channels     

A microcantilever-based gas flow sensor for flow rate and direction detection

Utilizing conventional micro-electro-mechanical systems techniques, this study fabricates and characterizes a novel micro gas flow sensor comprising four silicon nitride/silicon wafer cantilever beams arranged in a cross-form configuration. The residual stresses induced within the beams during their fabrication cause the tip of each beam to curve slightly in the upward direction. However, as air travels over the surface of the sensor, the upstream cantilevers are deflected in the downward direction, while the downstream cantilevers are deflected in the upward direction. The velocity of the air flow is then determined by measuring the corresponding change in resistance of the piezoresistors patterned on the upper surface of each cantilever beam. It is shown that by measuring the change in resistance of all four cantilever beams, the proposed sensor can detect not only the velocity of the air flow, but also its direction.     

Fabrication flaws and reliability in MEMS thin film polycrystalline flow sensor

A micro hot wire anemometer sensor has been constructed. The process consists in depositing a thin doped polycrystalline silicon layer on silicon substrate, using a micro-machined technique. This paper discusses the reliability and the fabrication flaws of this sensor. The different steps of fabrication are oxidation, deposit, photolithography, chemical attack, ionic implantation and annealing. An additional step, allowing the release of the suspended structures, is added. With each technological process step, a certain number of problems can be met. Each of these problems can potentially give rise to a defect of the final structure. Various tests are carried out on the final structure to make a first approach of the micro flow sensor flaws.