Performance of nonplanar silicon diaphragms under large deflections

The successful realization of many high-performance microactuators, including many microvalves and micropumps, depends critically on the development of diaphragms which are capable of large displacements and free from fatigue. Thin nonplanar silicon diaphragms are promising candidates for such applications since they can be batch fabricated using techniques and materials that are compatible with the other portions of these devices. This paper reports the detailed simulation, fabrication, and characterization of such diaphragms, which are corrugated-bossed structures that unfold, accordion-like, to produce high boss deflections. Boron-doped diaphragms 1 mm on a side, 3 μm in thickness, and containing five 10-μm-deep corrugations produce boss deflections of more than 30 μm at 760 mmHg, in close agreement with simulations. The maximum deflection measured at diaphragm fracture is 38 μm under a 1050 mmHg differential pressure. The effects on load-deflection performance due to changes in diaphragm internal stress (residual stress), corrugation profile, and diaphragm thickness are also explored     

Theoretical and experimental studies on the parylene diaphragms for microdevices

In this paper, parylene diaphragms with planar and corrugated shapes are fabricated and tested. Although, recently in MEMS, parylene has been used for a diaphragm material due to its excellent property, no theoretical and analytical studies of the parylene diaphragm have been performed. Therefore, the characterization of the parylene diaphragms is studied through both experiments and simulations. Thermal and load-deflection analyses for diaphragms are carried out to find the effect of residual stress on the behavoir of the diaphragm. The measured deflections are compared with both analytical calculations and FEM simulations that considers the influence of the thermal stress. Those results show excellent agreement. In order to find an optimum shape of the corrugated diaphragm, the influence of the corrugation number and the corrugation depth on the mechanical sensitivity of the corrugated diaphragm is studied parametrically.The authors would like to thanks to Dr. Sang Sik Yang and coworkers in the Microsystems Lab., School of Electronics Engineering, Ajou University for the help with device processing. This research was sponsored by Institute for Applied Science and Technology of Sogang University.     

Design and fabrication of silicon condenser microphone usingcorrugated diaphragm technique

A novel silicon condenser microphone with a corrugated diaphragm has been proposed, designed, fabricated and tested. The microphone is fabricated on a single wafer by use of silicon anisotropic etching and sacrificial layer etching techniques, so that no bonding techniques are required. The introduction of corrugations has greatly increased the mechanical sensitivities of the microphone diaphragms due to the reduction of the initial stress in the thin films, For the purpose of further decreasing the thin film stress, composite diaphragms consisting of multilayer (polySi/Si_xN_y/polySi) materials have been fabricated, reducing the initial stress to a much lower level of about 70 MPa in tension. Three types of corrugation placements and several corrugation depths in a diaphragm area of 1 mm² have been designed and fabricated. Microphones with flat frequency response between 100 Hz and 8~16 kHz and open-circuit sensitivities as high as 8.1~14.2 mV/Pa under the bias voltages of 10~25 V have been fabricated in a reproducible way. The experimental results proved that the corrugation technique is promising for silicon condenser microphone     

Front-to-backside alignment using resist-patterned etch control andone etching step

A processing technique that aligns features on the front side of a wafer to those on its backside has been developed for bulk micromachining. A 30 μm-square and 1.6 μm-thick diaphragm serves as an alignment pattern. At the same time that the alignment diaphragm is made, much thicker, large-area diaphragms can be partially etched using `mesh' masking patterns in these areas. The mesh-masking technique exploits the etch-rate differences between (100) and (111) planes to control the depths reached by etch pits in selected areas. The large partially etched diaphragms (2 to 3 mm², roughly 100 μm thick) are sufficiently robust to survive subsequent IC-processing steps in a silicon-foundry environment. The thin alignment diaphragm can be processed through these steps because of its very small area. The partially etched diaphragms can be reduced to useful thicknesses in a final etch step after the circuits have been fabricated     

Silicon condenser microphones with corrugated silicon oxide/nitride electret membranes

Corrugated electret membranes are used in a micromachined silicon microphone. The membranes consist of a permanently corona-charged double layer of silicon dioxide and silicon nitride, known to have excellent charge-storing properties. This electret can replace the external bias needed for condenser microphones. The well-known LOCOS technique—also combined with dry etching—is used for the first time to fabricate membranes with corrugation depths of several microns. The membrane thickness amounts to 600 nm.

The interferometrically measured center deflection is up to 40 nm/Pa for diaphragms with four corrugations of up to 3.3 μm depth and a size of A_M=1 mm². This high mechanical sensitivity limits the dynamic range to sound pressures below 50 Pa. The obtained mechanical sensitivities are in excellent agreement with the theory.

The most compliant corrugated diaphragms result in a microphone sensitivity of 2.9 mV/Pa, an equivalent noise level of 39 dB(A) and a total harmonic distortion below 1.7% at 28 Pa (123 dB SPL). The corrugation depth in the sensors has been only 1.3 μm. All sensors cover the whole audio and low ultrasonic range. The temperature coefficient is between 0.05 and 0.1 dB/K.     

Design and fabrication of high sensitive microphone diaphragm using deep corrugation technique

In this paper, single deeply corrugated diaphragm (SDCD) with various corrugation depths and initial stresses are studied extensively for applications to micromachinined high-sensitivity devices. Both theoretical analysis and finite-element-model (FEM) simulation results show that significant improvement in sensitivity can be achieved using the diaphragm with larger corrugation depth. The measured mechanical sensitivity of the SDCD structure shows reasonable agreement with the theoretical prediction. The SDCD structures with various corrugation depths have been applied to the realization of high-sensitivity microphones. The measurements show that deep corrugation technique is promising in its applications to high-sensitivity devices.     

A simulation program for the sensitivity and linearity ofpiezoresistive pressure sensors

A simulation program is developed which is capable of calculating the output responses of piezoresistive pressure sensors as a function of pressure and temperature. Analytical models based on small and large deflection theories have been applied to predict the sensitivity and linearity of pressure sensors. Surface-micromachined diaphragms with square or circular shapes, fabricated by a low pressure chemical vapor deposition sealing process, are designed and tested to verify the program. They are made of polysilicon and have a standard width (diameter) of 100 μm and thickness from 1.5 to 2.2 μm. Various parameters of the piezoresistive sensing resistors, including length, orientation, and dopant concentration, have been derived and constructed on top of the diaphragms. For a 100-μm-wide 2-μm-thick square-shape pressure sensor, calculated and experimental results show that int sensitivity of 0.24 mV/V/(Ibf/in²) is achieved. Experimentally, non a maximum linearity error of ±0.1% full-scale span) is found out on a 100-μm-wide 2.2-μm-thick square-shape pressure sensor. Both sensitivity and linearity are characterized by the diaphragm thickness and the length of the sensing resistors     

Electrostatic micro torsion mirrors for an optical switch matrix

We have developed a new type of compact optical switch using silicon micromachining technique. Torsion mirrors (300 μm×600 μm) supported by thin polysilicon beams (16 μm wide, 320 μm long, and 0.4 μm thick) are arranged in a 2×2 matrix (total size 3 mm×5 mm, t 0.3 mm). The mirrors are independently attracted by electrostatic force of applied bias voltage to redirect the incident light in a free space. Using collimated beam fibers for optical coupling, we obtained small insertion loss (⩽-7.66 dB), considering the length of a light path (⩾10 mm), a large switching contrast (⩾60 dB), and small crosstalk (⩽-60 dB). The fabrication yield was higher than 80% thanks to the newly developed releasing technique that used a silicon oxide diaphragm as an etch-stop layer and as a mechanical support in the process. Holding voltage (⩽50 V) was lower than the voltage to attract the mirror (100~150 V) because of the hysteresis of angle-voltage characteristic of electrostatic operation     

压力传感器封装中波纹膜片的结构优化

对压力传感器隔离封装中的关键部件——波纹膜片,采用有限元分析的方法,分别得到了2,4,6个波纹的膜片的弹性特性,并根据封装要求选择了6波纹的膜片。通过比较不同波数的膜片对压力传感器输出信号的影响,验证了膜片结构优化的正确性。最后,使用6波纹的膜片进行封装,得到了性能稳定的压力传感器,并测试得到了其主要的静态参数。     

Fabrication, experiment of a microactuator using magnetic fluid for micropump application

This paper presents the characteristics of the first microactuator using magnetic fluid (MF). MF actuators with flat and corrugated diaphragms are fabricated by the micromachining technique and their deflections are measured. The MF actuator consists of an inductor, a fluid chamber filled with magnetic fluid, and a parylene diaphragm. In the presence of the magnetic field, magnetic fluid easily deforms its shape and generates some pressure. The relation between the MF pressure and magnetic flux density is obtained by the comparison of the deflection for the applied air pressure with that for the MF pressure. At 110 Gauss, the deflection of MF actuator with corrugated parylene diaphragm is more than 200 μm and the generated MF pressure is 2.8 kPa. The successful operation of the first MF actuator shows the high potential for the application to the micromachined devices.     

Piezoelectric microphone with on-chip CMOS circuits

An IC-processed piezoelectric microphone with on-chip, large-scale integrated (LSI) CMOS circuits has been designed, fabricated, and tested in a joint, interactive process between a commercial CMOS foundry and a university micromachining facility. The 2500×2500×3.5 μm ³ microphone has a piezoelectric ZnO layer on a supporting low-pressure chemical-vapor-deposited (LPCVD), silicon-rich, silicon nitride layer. The optimum residual-stress-compensation scheme for maximizing microphone sensitivity produces a slightly buckled microphone diaphragm. A model for the sensitivity dependence of device operation to residual stress is confirmed by applying external strain. The packaged microphone has a resonant frequency of 18 kHz, a quality factor Q≈40, and an unamplified sensitivity of 0.92 mV/Pa. Differential amplifiers provide 49 dB gain with 13 μV A-weighted noise at the input     

Micromachined thermal shear-stress sensor for underwater applications

This paper reports the development of micromachined thermal shear-stress sensors for underwater applications. The thermal shear-stress sensor is a polysilicon resistor sitting atop a vacuum-insulated nitride diaphragm. Special challenges for underwater measurements, such as the waterproof coating and minimization of pressure crosstalk, have been addressed. More rigid diaphragms than the aerial sensors are implemented to increase the operating range and reduce pressure crosstalk, with the cost of larger power consumption and lower sensitivity. Sensors with different diaphragm dimensions and resistor lengths have been fabricated and tested. Nearly zero pressure sensitivity has been achieved by either reducing the diaphragm width or adjusting the sensing element length. The effects of overheat ratio and operating mode on the sensor's pressure crosstalk have been discussed. Parylene C is chosen as the waterproof material for the underwater shear-stress sensors. The primary failure mode is identified as the corrosion of the soldering pads.     

Modelling of silicon condenser microphones

Several models concerning the sensitivity of capacitive pressure sensors have been presented in the past. Modelling of condenser microphones, which can be considered to be a special type of capacitive pressure sensor, usually requires a more complicated analysis of the sensitivity, because they have a strong electric field in the air gap. It is found that the mechanical sensitivity of condenser microphones with a circular diaphragm, either with a large initial tension or without any initial tension, increases with increasing bias voltage (and the corresponding static deflection), whereas the mechanical sensitivity of other capacitive pressure sensors does not depend on the static deflection. It is also found that the mechanical sensitivity increases with increasing input capacitance of a preamplifier. In addition, the open-circuit electrical sensitivity and, consequently, the total sensitivity too, also increases with increasing bias voltage (or static deflection). However, the maximum allowable sound pressure at which the diaphragm collapses, an effect that has to be taken into account, decreases with increasing static deflection in most cases, ulthnately resulting in an optimum value for the bias voltage. The model for microphones with a circular highly tensioned diaphragm has been verified successfully for two microphone types.     

Analysis of partly corrugated rectangular diaphragms using theRayleigh-Ritz method

In this paper, the Rayleigh Ritz method is applied to static analyses of partly corrugated four-edges-clamped rectangular diaphragms. We use coupled variational equations (derived from the principle of virtual displacement), and describe some practical considerations needed to analyze multiregional diaphragms by the method. We approximate a sinusoidal corrugation by an equivalent flat single layer, and derive equivalent in-plane and bending stiffnesses using the theory of engineering mechanics. We demonstrate the effectiveness and usefulness of the present method through examples (in one example, we compare the calculated deflection with that obtained by a finite-element program). We examine a partly corrugated rectangular plate with residual stress, and investigate the effects of corrugation parameter variations. The analysis technique presented in this paper is very convenient, efficient, and reliable for analyzing seemingly difficult microelectromechanical system devices built on partly corrugated thin diaphragms with various residual stresses     

Dynamical modeling and experimental validation of a micro-speaker with corrugated diaphragm for mobile phones

As the design trend of modern cellular phones evolves to be miniaturization and versatile sound in quality, the electro-mechanical components including the micro-speaker are essential toward size reduction and broad frequency range of sound. To reduce size, a diaphragm type micro-speaker is commonly employed in industry, while to broaden the sound frequency range corrugations on the diaphragm are adopted. The corrugations are generally capable of leading to fairly flat response over a broad range since diaphragm stiffness is decreased in axial direction. To confirm the aforementioned capability, the modeling on a corrugated diaphragm is performed first using finite element method (FEM) to obtain the associated dynamic equations in terms of modal coordinates; then the equations are next combined with the magneto-electrical model of the voice coil motor (VCM) that is attached to the bottom side of the diaphragm. Finally, the acoustic effects of the air inertia on the diaphragm and vents of the outer case are modeled using basic acoustic theories. Assembling all derived system equations and solving them, the frequency response of the micro-speaker in sound pressure level (SPL) can be simulated. It shows that the diaphragm corrugation in fact helps flatten the SPL response of the micro-speaker (especially in high frequency range) to lessen sound distortion. Furthermore, the corrugation angle approximately below 45° is favored over other angles or flatter SPL response. Experiments are also conducted to verify the theoretical findings.     

应力对硅悬空薄膜的机械灵敏度的影响

运用四周固支薄板小挠度理论,分析了存在较大残余应力的浓硼重掺杂硅悬空薄膜的力学特性.给出了小挠度下,在外界载荷作用下总应力表达式和薄膜机械灵敏度表达式,并分析了残余应力对二者的影响.研究表明,对存在较大残余应力的硅薄膜,总应力和机械灵敏度主要由残余应力决定,其中机械灵敏度较忽略残余应力时的值减小三个数量级.     

Experimental evaluation of sensitivity and non-linearity in polysilicon piezoresistive pressure sensors with different diaphragm sizes

MEMS-based piezoresistive pressure sensors are widely popular due to advantages such as small size, low cost, simple fabrication, and DC output. In this work, the design simulation, fabrication process, and characterization of four pressure sensors with square diaphragms of edge-length 1,060, 1,280, 1,480, and 1,690 µm are reported. Several design principles such as appropriate boundary condition, piezoresistor placement, and fracture stress are considered in the design phase. The sensors have novel shaped polysilicon piezoresistors and equal diaphragm thickness of 50 µm. The sensors are fabricated simultaneously by putting the different designs on the same mask set so that the best design can be determined after characterization. The uncompensated and unamplified output response of the different sensors are reported at three temperatures (?5, 25 and 55 °C). Out of the four sensors with different diaphragm sizes, the sensor with a diaphragm edge length of 1,280 μm is found to have optimum characteristics. For the diaphragm with edge-length of 1,280 µm, in the pressure range of 0–30 Bar, sensitivity of 3.35–3.73 mV/Bar, non-linearity of <0.3 %, and hysteresis of <0.1 % are obtained. The different sensors can be used in the specified pressure range for suitable applications.     

Electrothermally activated paraffin microactuators

A new family of electrothermally activated microactuators that can provide both large displacements and forces, are simple to fabricate, and are easily integrated with a large variety of microelectronic and microfluidic components are presented. The actuators use the high volumetric expansion of a sealed, surface micromachined patch of paraffin heated near its melting point to deform a sealing diaphragm. Two types of actuators have been fabricated using a simple three mask fabrication process. The first device structure consists of a 9 μm thick circularly patterned paraffin layer ranging in diameter from 400 to 800 μm all covered with a 4-μm-thick metallized p-xylylene sealing diaphragm. All fabricated devices produced a 2.7-μm-peak center deflection, consistent with a simple first order theory. The second actuator structure uses a constrained volume reservoir that magnifies the diaphragm deflection producing consistently 3.2 μm center diaphragm deflection with a 3-μm-thick paraffin actuation layer. Microactuators were constructed on both glass and silicon substrates. The actuators fabricated on glass substrates used between 50-200 mW of electrical power with response times ranging between 30-50 ms. The response time for silicon devices was much faster (3-5 ms) at the expense of a larger electrical power (500-2000 mW)     

A surface-micromachined resonant-beam pressure-sensing structure

The first study on an entirely surface-micromachined resonant-beam pressure sensor is presented. Using a fully surface-micromachined process, an encapsulated beam resonant pressure-sensor structure with a pressure-sensitive diaphragm of 100×150×2 μm has been fabricated. The resonating beam is fully enclosed inside the reference vacuum cavity formed beneath the diaphragm. The new design enables high pressure sensitivity and a miniature chip size, essential for sensors such as catheter-mounted intravascular blood pressure sensors. The pressure sensitivity is measured at 3.2%/bar with a beam resonance frequency of about 700 kHz     

Analysis,modeling and experimental validation of temperature-changing effect on mechanical properties of pneumatic artificial muscle

This paper focuses on quantify how temperature affects the mechanical properties of pneumatic artificial muscle (PAM). The influence of temperature variations on PAM’s structure size and internal friction was analyzed firstly, based on the thermal characteristics of rubber diaphragm and fiber mesh, then a novel model considering the influence of temperature variation is proposed, without any fitting parameter or any parameter with uncertain physical meanings. In order to verify the proposed model, temperature-rising experiments using heating device and continuous operation were both carried out separately. Through the comparison of characteristic curves under different temperatures, PAM’s characteristics were proven to be affected by temperature variations. The results show that inside temperature changing of PAM is why there are drifting and time-varying phenomenon of their mechanical characteristics in long-term operation, and the proposed model can predict the variations caused by temperature changing well. Meanwhile, the accuracy of the new model is higher than Chou model and lower than Andrikopoulos model; it is understandable because our new model has no fitting parameters, but the advantage of new model is that it takes temperature as independent variables and, therefore, can be used at different temperatures directly without any calibration.