Coupled effects of gold electroplating and electrochemical discharge machining processes on the performance improvement of a capacitive accelerometer

Coupled effects of an electroplated gold layer and damper holes drilled by Electro Chemical Discharge Machining (ECDM) process on the performance improvement of a quad beam capacitive accelerometer is presented in this paper. A simple quad beam-proof mass configuration with its beams located symmetrically at the centre of all the edges of the proof mass and connected to an outer supporting frame is considered in the present study. For a fixed damping ratio, prime-axis sensitivity of the sensor is increased by the damper holes whereas an electroplated gold layer improves the prime-axis sensitivity, cross-axis sensitivity, and Brownian Noise Equivalent Acceleration (BNEA). Moreover, the increased weight of the proof mass due to an electroplated gold layer further reduces the damping of the device which in turn helps to increase the prime-axis sensitivity more. A new figure of merit called Performance Factor (PF), defined as the ratio of the product of the prime-axis sensitivity and resonant frequency to the cross-axis sensitivity at a fixed damping ratio of 0.7 is used as a quantitative index to evaluate the performance improvement caused by the coupled effects of gold electroplating and ECDM processes. Simulation results show that for a device with damper holes of 8 μm diameter and electroplated gold layer of dimensions 3,000 μm × 3,000 μm × 20 μm, the prime-axis sensitivity is increased by more than 500 times, rotational cross-axis sensitivity and BNEA are reduced by around 10 and 30%, respectively and the PF is improved by around 482 times at a fixed damping ratio of 0.7.

     

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare^™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 10¹⁸ atoms/cm³ were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively.     

A high-sensitivity silicon accelerometer with a folded-electrode structure

A high-sensitivity capacitive silicon accelerometer with a new device structure is presented in this paper. The structure uses a fixed rigid electrode suspended between a proof mass and a stiff moving electrode to provide differential capacitance measurement and force rebalancing. High sensitivity is achieved by forming a thick silicon proof mass and a narrow uniform air gap over a large area. The mechanical noise floor is reduced by incorporating damping holes in the electrodes. The accelerometer structure is all silicon and is fabricated on a single silicon wafer. The measured sensitivity for a device with 2.6 mm /spl times/ 1 mm proof mass and 1.4 /spl mu/m air gap is /spl ap/11 pF/g per electrode. The calculated mechanical noise floor for the same device is 0.18 /spl mu/g//spl radic/Hz at atmosphere.     

An all-silicon single-wafer micro-g accelerometer with a combinedsurface and bulk micromachining process

This paper reports an all-silicon fully symmetrical z-axis micro-g accelerometer that is fabricated on a single-silicon wafer using a combined surface and bulk fabrication process. The microaccelerometer has high device sensitivity, low noise, and low/controllable damping that are the key factors for attaining μg and sub-μg resolution in capacitive accelerometers. The microfabrication process produces a large proof mass by using the whole wafer thickness and a large sense capacitance by utilizing a thin sacrificial layer. The sense/feedback electrodes are formed by a deposited 2-3 μm polysilicon film with embedded 25-35 μm-thick vertical stiffeners. These electrodes, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proof mass becomes possible. The polysilicon electrodes are patterned to create damping holes. The microaccelerometers are batch-fabricated, packaged, and tested successfully. A device with a 2-mm×1-mm proof mass and a full bridge support has a measured sensitivity of 2 pF/g. The measured sensitivity of a 1-mm×1-mm accelerometer with a cantilever support is 19.4 pF/g. The calculated noise floor of these devices at atmosphere are 0.23 μg/√Hz and 0.16 μg/√Hz, respectively     

硅微电容式二维加速度传感器的加工与测试

设计了一种基于体硅加工技术的单敏感质量元差分电容式二维加速度传感器,并采用硅-玻璃静电键合、ICP工艺释放等技术完成了二维加速度传感器的加工。测试结果表明:该二维加速度传感器两个检测方向上的灵敏度基本一致,线性度较好,交叉干扰较小。X、Y方向的灵敏度分别为58.3 mV/gn、55.6 mV/gn;线性相关系数分别为0.9968、0.9961,交叉灵敏度分别为6.17%、7.82%。     

A dual-axis MEMS capacitive inertial sensor with high-density proof mass

This paper reports a novel dual-axis microelectromechanical systems (MEMS) capacitive inertial sensor that utilizes multi-layered electroplated gold. All the MEMS structures are made by gold electroplating that is used as a post complementary metal-oxide semiconductor (CMOS) process. Due to the high density of gold, the Brownian noise on the proof mass becomes lower than those made of other materials such as silicon in the same size. The single gold proof mass works as a dual-axis sensing electrode by utilizing both out-of-plane (Z axis) and in-plane (X axis) motions; the proof mass has been designed to be 660 μm × 660 μm in area with the thickness of 12 μm, and the actual Brownian noise in the proof mass has been measured to be 1.2 \({\upmu}{\text{G/}}\sqrt {\text{Hz}}\) (in Z axis) and 0.29 \({\upmu}{\text{G/}}\sqrt {\text{Hz}}\) (in X axis) at room temperature, where 1 G = 9.8 m/s². The miniaturized dual-axis MEMS accelerometer can be implemented in integrated CMOS-MEMS accelerometers to detect a broad range of acceleration with sub-1G resolution on a single sensor chip.     

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 %.     

Design and simulations of a new biaxial silicon resonant micro-accelerometer

This paper presents a new biaxial silicon resonant micro-accelerometer. The device basically consists of a single proof mass, four pairs of decoupled beams, four lever mechanisms and two pairs of resonators, which provides 2-D in-plane acceleration measurement with the decoupled two pairs of resonators. Structure optimization is implemented by taking advantage of the finite element analyses. From the simulation results we can see that the effective frequencies of two acceleration sensitive modes are 1010.18 and 1010.13 Hz respectively, and the undesired modes and effective modes are isolated apparently. Additionally, high linear relationship between the input acceleration and the resonant frequency shifts of resonators are demonstrated by the input–output characteristic simulation. Moreover, simulation results reveal the scale factor for the x-axis is 180.03 Hz/g, and the scale factor for y-axis is 180.75 Hz/g, while the cross-axis sensitivity for x-axis is 0.046 Hz/g, and the cross-axis sensitivity for y-axis is 0.027 Hz/g. The high sensitivity and low cross-axis sensitivity are thus adequately confirmed. By the way, the simulation of temperature dependent characteristics demonstrate that the differential scheme can effectively suppress the influence of temperature variation, and the thermal analysis shows that the device can bear the thermal stress induced by temperature change. All these simulations above can verify the feasibility of the structure design.     

BESOI-based integrated optical silicon accelerometer

The design, simulation, fabrication and characterization of a new integrated optical accelerometer is presented in this paper. The reduction of fabrication, packaging and thermomechanical stresses are considered by keeping the weak mechanical parts free of stresses. The mechanical sensor consists on a quad beam structure with one single mass. In addition, there are two waveguides on the frame of the chip self-aligned to one on the mass of the accelerometer. Four lateral beams increase the mechanical sensitivity and allow the flat displacement of the optical waveguides on the mass. The working principle is based on the variation of the output light intensity versus the acceleration due to the misalignment of the waveguides. The devices have been optimized by the finite-element method to obtain a mechanical sensitivity of 1 /spl mu/m/g. The fabrication technology is based on BESOI wafers combining bulk an surface micromachining. Moreover, machined glass wafers with cavities are bonded to the silicon wafer for packaging and damping control. Special packaging considerations as dicing, polishing and alignment are also presented. Optical measurements at 633 nm shown an optical sensitivity of 2.3 dB/g for negative and 1.7 dB/g for positive acceleration. This difference in the sensitivity has been demonstrated as a consequence of the passivation layer located over the core of the waveguides.     

Fluid Effects in Vibrating Micromachined Structures

Squeeze film damping and hydrodynamic lift for a micromechanical perforated proof mass are calculated and measured. This paper has resulted in closed-form expressions that can be used to design accelerometers, tuning-fork gyroscopes (TFGs), and other micromechanical devices. The fluid damping and lift are determined using finite-element analyses of the normalized and linearized governing equations where the boundary condition of the pressure relief holes is derived using pipe flow analysis. The rarefaction of gas is incorporated in the governing equations based on slip flow condition. As a further check, a one-dimensional (1-D) network model is developed to account for the boundary condition of the holes on a tilted proof mass. Both closed-form and numerical solutions are compared against experimental data over a range of pressures.hfillhbox[1221]     

不同检测电容结构对MEMS电容传感器性能的影响分析

改进传感器检测电容几何结构能有效改善传感器的性能。本文对梳齿电极结构、栅形电极结构及梳栅电极结构检测电容的性能特点进行分析比较,重点分析了振子质量、空气阻尼、系统阻尼系数比以及灵敏度等特性,得出在相同的外轮廓尺寸、支撑梁、振子厚度以及振子到衬底的距离的条件下,栅形结构传感器的振子质量最大,空气阻尼最小,适合制作高分辨率的传感器;在大气下,梳齿结构灵敏度增加的同时空气阻尼力也会增加,且振子质量较小,适合制作高灵敏度,低分辨率传感器结构;梳栅结构的特点居于两者之间,适合制作需要兼顾分辨率和灵敏度的传感器。通过实例计算,证明了该结果。     

A piezoelectric micro-cantilever bio-sensor using the mass-micro-balancing technique with self-excitation

A biosensor was developed for using in a Lab-On-a-Chip (LOC). The sensor detects the change in the resonance frequency of a micro-cantilever with a piezoelectric film. This is the mass micro-balancing technique, which has been successfully used for detecting bio-materials in the quartz crystal microbalance (QCM). The PZT film, a piezoelectric film, is designed to act as both sensor and actuator. The geometry of the micro cantilever is optimized to maximize the sensitivity and minimize the environmental effects such as viscous damping and added mass effect in liquid. The fabricated sensor is composed of a 100 μm long, 30 μm wide, and 5 μm thick cantilever with a 2.5 μm thick piezoelectric (PZT) layer on it. The ratio of thickness to length of the micro cantilever is very high compared to others in micro cantilever-based studies. This high aspect ratio is the key to maximize the sensitivity and minimize the environmental effects. The fabricated micro sensor was tested by detecting the mussel gluing protein, the insulin-anti insulin binding protein and the poly T-sequence DNA.     

铅黏弹性阻尼墙力学性能分析研究

为提高黏弹性阻尼墙耗能能力,改善其耗能方式,本文提出一种复合型铅黏弹性阻尼墙,介绍了铅黏弹性阻尼墙的构造与原理.对4种不同硬度天然橡胶进行材性试验及本构参数拟合,设计24组不同参数铅黏弹性阻尼墙,采用ABAQUS有限元软件进行模拟分析,研究了不同黏弹性材料、黏弹性层面积、铅芯直径、铅芯布置方式、复合黏弹性层厚度和单层薄钢板与黏弹性层厚度比对阻尼墙滞回性能及力学性能的影响,并给出了设计建议值.     

Rapid replication of polymeric and metallic high aspect ratio microstructures using PDMS and LIGA technology

This paper present a method of rapid replication of polymeric high aspect ratio microstructures (HARMs) and a method of rapid reproduction of metallic micromold inserts for HARMs using polydimethylsiloxane (PDMS) casting and standard LIGA processes. A high aspect ratio (HAR) metallic micromold insert, featuring a variety of test microstructures made of electroplated nickel with 15:1 height-to-width ratio for 300 μm microstructures, was fabricated by the standard LIGA process using deep X-ray lithography (DXRL). A 10:1 mixture of pre-polymer PDMS and a curing agent were cast onto the HAR metallic micromold insert, cured and peeled off to create reverse images of the HAR metallic micromold insert in PDMS. In addition to the replication of polymeric HARMs, replicated PDMS HARMS were coated with a metallic sacrificial layer and electroplated in nickel to reproduce another metallic micromold insert. This method can be used to rapidly and massively reproduce HAR metallic micromold inserts in low cost mass production manner without further using DXRL.     

Design, fabrication and analysis of micromachined high sensitivity and 0% cross-axis sensitivity capacitive accelerometers

This paper presents the design and fabrication of three MEMS based capacitive accelerometers. The first design illustrates the achievement of an accelerometer with 0% cross-axis sensitivity and has been fabricated using PolyMUMPs, a multi-user surface-micromachining process. A unidirectional parallel plate configuration is utilized in this design to illustrate the achievement of 0% cross-axis sensitivity and an acceptable performance range. In addition, a method is introduced to improve the sensitivity of a capacitive sensor employing a transverse configuration based on the relationship of initial gaps setup in comb-finger arrangements. A design based on this technique and the PolyMUMPs fabrication process is illustrated which demonstrates a sensitivity value of 4.07 fF/μm, with a nonlinearity of 2.05% for a ±3 μm sensor operating range. The last design based on this method and the SOIMUMPs fabrication process exhibits a sensitivity of 3.45 pF/μm for ±1 μm operating range of the sensor.     

Influence of gold thin-film interlayers on anodic bonding of copper microstructures produced by LIGA

 Neither pure copper nor solid gold can be anodically bonded to glass. It is only the gold coating on the copper which allows a joint to be built up as a result of the copper ions diffusing into the gold layer, but not many of them being able to migrate into the glass. To encapsulate microstructures produced by the LIGA technique, anodic bonding of gold-coated copper to Corning 0211-type glass was studied. For demonstration purposes, a glass platelet made of Corning 0211 was anodically bonded to a LIGA linear actuator consisting of electroplated copper coated with 1 μm of gold. A better understanding about the decisive parameters in anodic bonding was obtained by varying the bonding temperature and the thickness of the gold layer. Glass can be bonded on to the entire surface of gold layers 0.5–1 μm thick at temperatures as low as 300 °C; however, when the systems cool to room temperature, stress-induced cracks arise in the glass. On the other hand, thicker gold layers of 2.5 to 10 μm thickness require higher bonding temperatures for the same period of heating, but prevent the occurrence of such cracks because of their higher ductility. Received: 11 December 1997/Accepted: 11 March 1998     

A predictive technique for evaluating structural vibration gain of damped suspensions in hard disk drives

The problem of resonant vibration of suspensions used in hard disk drives has been the subject of ongoing research. This study focuses on implementation of a finite element modeling methodology to predict the response for a suspension that includes an added damping component, either as a traditional constraint layer damper or as a design tool. High gains in suspensions result in off-track of the slider, which is the mechanism for reading and writing data. Analytical methodology uses the concept of modal strain energy to predict the modal damping of the structure at each resonant mode. Loss factors and strain energies of each material used in the structure quantify the amount of modal damping at a particular mode. Development of the methodology allows optimization studies of standard damping applications, such as a constraint layer damper. In addition, the methodology enables FEA as a design tool to develop innovative solutions for improved efficiency and higher performance.     

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.     

Lyapunov optimization of a damped system

Our aim is to optimize the damping of a linear vibrating system. The penalty function is the average total energy, which is equal to the trace of the corresponding Lyapunov solution. We prove the existence and the uniqueness of the global minimum, if the damping varies over the set of all possible positive definite matrices. The minimum is shown to be taken on the so-called modal critical damping, thus confirming a long existing conjecture. We also give some preliminary results concerning dampings which depend linearly on the viscosity parameters whereas the damper positions are kept fixed. We produce physical examples on which the minimum is taken on a negative viscosity or which have several local minima. Both phenomena seem to be a consequence of a bad choice of the damper positions.     

Fabrication of a MEMS capacitive accelerometer with symmetrical double-sided serpentine beam-mass structure

This paper presents a symmetrical double-sided serpentine beam-mass structure design with a convenient and precise process of manufacturing MEMS accelerometers. The symmetrical double-sided serpentine beam-mass structure is fabricated from a single double-device-layer SOI wafer, which has identical buried oxides and device layers on both sides of a thick handle layer. The fabrication process produced proof mass with though wafer thickness (860 μm) to enable formation of a larger proof mass. Two layers of single crystal silicon serpentine beams with highly controllable dimension suspend the proof mass from both sides. A sandwich differential capacitive accelerometer based on symmetrical double-sided serpentine beams-mass structure is fabricated by three layer silicon/silicon wafer direct bonding. The resonance frequency of the accelerometer is measured in open loop system by a network analyzer. The quality factor and the resonant frequency are 14 and 724 Hz, respectively. The differential capacitance sensitivity of the fabricated accelerometer is 15 pF/g. The sensitivity of the device with close loop interface circuit is 2 V/g, and the nonlinearity is 0.6 % over the range of 0–1 g. The measured input referred noise floor of accelerometer with interface circuit is 2 μg/√Hz (0–250 Hz).