Magnetic microactuators based on polymer magnets

Integrated permanent magnet microactuators have been fabricated using micromachined polymer magnets. The hard magnetic material utilized is a polymer composite, consisting of magnetically hard ceramic ferrite powder imbedded in a commercial epoxy resin to a volume loading of 80%. The magnets have the form of thin disks approximately 4 mm in diameter and 90 μm in thickness. These disks have been magnetized in the thickness direction, and even in this geometrically unfavorable direction showed typical permanent magnet behavior with an intrinsic coercivity H_ci of 4000 Oe (320 kA/m) and a residual induction B_r of 600 Gauss (60 mT). Cantilever beam-type magnetic actuators carrying a screen-printed disk magnet on their free ends have been fabricated on an epoxy board. A planar coil on the opposite side of the substrate is used to drive the beams vertically. The actuators exhibit hard magnetic behavior allowing both attraction and repulsion by reversing the current direction. Static and dynamic testing of the magnetic actuators have been performed. The experimental data are compared with theoretical results obtained from both finite element simulations and analytical models. Good agreement is obtained between simulation and experiment     

Microbridge testing on symmetrical trilayer films

In this paper, we extended the microbridge testing method to characterize the mechanical properties of symmetrical trilayer thin films. Theoretically, we analyzed the deformation of a trilayer microbridge sample with a deformable boundary condition and derived load-deflection formulas in closed-form. The slope of a load-deflection curve under small deformation gives the relationship between the bending stiffness and the residual force of a trilayer microbridge. Taking this relationship, we were able to assess simultaneously the Young's modulus of two kinds of materials composing the symmetrical trilayer film and the thickness-averaged residual stress of the film. Experimentally, we fabricated symmetrical trilayer microbridge samples of SiO/sub 2//Si/sub 3/N/sub 4//SiO/sub 2/ on 4-inch p-type (100) silicon wafers and conducted the microbridge tests with a load and displacement sensing nanoindenter system equipped with a microwedge indenter. The experimental results verified the proposed microbridge testing method. The thickness-averaged residual stress of the 1.1-/spl mu/m trilayer thin films was determined to be 8.8 MPa, while the Young's modulus of the 0.3-/spl mu/m silicon oxide layers and the Young's modulus of the 0.5-/spl mu/m silicon nitride layer were evaluated to be 31 GPa and 294 GPa, respectively.     

Determination of Young's modulus and residual stress of electroless nickel using test structures fabricated in a new surface micromachining process

This paper reportsin situ measurement of Young's modulus and residual stress of electroless nickel films through the use of microfabricated nickel test structures, including electrostatic microactuators and passive devices. Th test structures are fabricated in a new surface micromachining process, termed “nickel surface micromachining”, using electroless plated nickel as the structural layer and polysilicon as the sacrificial layer. Subsequent to fabrication, lateral resonant-type electrostatic microactuators of different geometries are resonated by electrical excitation. Using the measured resonant frequencies and knowledge of the device geometry, the Young's modulus of the film is determined. The passive electroless nickel microstructures deform upon completion of the fabrication process due to residual stress in the film. Measurement of this deformation in conjunction with an appropriate mechanical model is used to determine the residual stress in the films. This work was suported by ARPA under Contract No. FQ8761-9301675. This paper was not presented at the International Conference on High Aspect-Ratio Microstructure Technology HARMST '95 in July 1995.     

Conductive Blended Polymer MEMS Microresonators

This paper presents an all-polymer microelectromechanical system technology in which a crosslinker is used to modify the electromechanical properties. The structural material of these microelectromechanical systems (MEMS) structures is a poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate/polymethyl methacrylate (PEDOT/PSS/PMMA) blended conductive polymer. Microbridge resonators are fabricated using surface micromachining on glass substrates. The electromechanical properties of the polymer microbridges are studied using electrostatic actuation and optical and electrical detection. The resonance frequency of the polymer bridges occurs in the MHz range, with quality factors of the order of 100 when measured in vacuum. Addition of a silane-based crosslinker increases the Young's modulus of the polymer structural material which is reflected in higher resonance frequency, higher pull-in voltage, better long-term stability of the electrical conductivity, and in a decrease in the quality factor of the resonator. The mechanical properties of the polymer resonators are strongly affected by the residual stress because of the low Young's modulus, and by the measurement frequency and the measurement temperature due to the viscoelastic properties of the polymer structural material     

Measurements of material properties using differential capacitive strain sensors

This paper describes a laterally deflecting micromachined device that offers high sensitivity and wide dynamic range to electronically monitor the thermal expansion coefficient, tensile and compressive residual strain and Young's modulus of microstructural materials, as well as the temperature dependence of these properties. The device uses sidewall capacitance between interdigitated tines to sense displacement caused by the release of residual stress in a bent-beam suspension. Electrostatic force is used to obtain load-deflection profiles. The suspensions and tines are arranged such that output is a differential readout, immune to common mode parasitic capacitance. Analytical and numerical modeling results are presented and the device concept is verified by three different fabrication approaches using polysilicon and nickel as structural materials. Measured values of residual strain, thermal expansion and Young's modulus are very consistent with measurements taken by other approaches and those reported previously. For example, the residual strain in certain electrodeposited Ni structures was tracked from 68.5 microstrain at 23/spl deg/C to -420 microstrain at 130/spl deg/C, providing an expansion coefficient of 8.2 ppm/K; the best fit Young's modulus provided by the device was 115 GPa.     

Polymeric magnetic microactuator with efficient permalloy loop design

In the study, two types of polyimide (PI) magnetic microactuator with different geometries were designed, fabricated and tested. Mathematic model is created by ANSYS software to design the two microactuators, respectively. The magnetic field generated by the planar coil with permalloy circuit loop design was simulated by ANSYS software. The simulation result shows that the magnetic force can be up to 34.69 mN. Fabrication of the electromagnetic microactuator combines with 10 m thick Ni/Fe (80/20) permalloy circuit loop deposition, 10 m thick permalloy plate with 800 × 800 m in area on PI diaphragm, high aspect ratio electroplating of copper planar coil with 10 m in thickness, bulk micromachining, and excimer laser ablation. They were fabricated by a novel concept avoiding the etching selectivity and residual stress problems during wafer etching. First, PI diaphragm with micro-electroplated Ni/Fe permalloy was released by anisotropic wet etching, followed by mask projection of 248 nm excimer laser ablation. The new concept of excimer laser ablation process demonstrates that microstructure can be post-ablated after bulk micromachining. By applying a current through planar coil, the PI diaphragm will be actuated. ANSYS software was used to predict the deflection angle of the two types of microactuators. The preliminary theoretical simulation shows a good agreement with experimental result.The authors would like to thank Dr. Tung-Chuan Wu and Mr. M.-C. Chou at MIRL of ITRI in Taiwan for their guidances, and National Science Council (NSC) for their financial supports to the project (granted number: NSC92-2622-E110-009-CC3 and NSC92-2212-E110-029).     

Characterization of Young's modulus and residual stress gradient of MetalMUMPs electroplated nickel film

Metal multi-user MEMS processes (MetalMUMPs) offered by MEMSCAP provide a 20 μm thick electroplated nickel film suitable for constructing micro RF tunable capacitors, RF inductors, relays, switches, etc. Currently the Young's modulus and the residual stress gradient of the MetalMUMPs nickel film have not been characterized. In this paper the resonance method is used to characterize the Young's modulus of the MetalMUMPs nickel film. The characterization results show that the nickel film has a Young's modulus of 155–164 GPa with an average of 159 GPa. A stress gradient induced free beam mechanism is proposed in this paper to characterize the residual stress gradient in the MetalMUMPs nickel film. Characterization results show that the residual stress in the electroplated nickel film has a gradient across the film thickness of −5.49 MPa/μm to −4.30 MPa/μm with the average of −4.72 MPa/μm. The residual stress change from the bottom surface to the top surface of the nickel film is −97.7 MPa. The Young's modulus and residual stress gradient of the MetalMUMPs nickel film obtained in this paper provide MetalMUMPs users an important reference for designing, optimizing and analyzing suspended nickel structures. The stress gradient induced free beam mechanism proposed in this paper provides a method of characterizing negative residual stress gradient in thin films without using trenches or through-wafer holes.     

Cubic millimeter power inductor fabricated in batch-type wafer technology

A hybrid technology for the realization of three-dimensional (3-D) miniaturized power inductors is presented. Our devices consist of planar Cu coils on polyimide substrates, and mm-size ferrite magnetic cores, obtained by three-dimensional micro-patterning of ferrite wafers using powder blasting. The coils are realized using an in-house developed high-resolution polyimide spinning and Cu electroplating process. Winding widths down to 5 /spl mu/m have been obtained and total device volumes are ranging between 1.5 and 10 mm/sup 3/. Inductive and resistive properties are characterized as a function of frequency; inductance values in the 100 /spl mu/H range have been obtained.     

Materials Selection and Design of Microelectrothermal Bimaterial Actuators

A common form of MEMS actuator is a thermally actuated bimaterial, which is easy to fabricate by surface micromachining and permits out of plane actuation, which is otherwise difficult to achieve. This paper presents an analytical framework for the design of such microelectrothermal bimaterial actuators. Mechanics relationships for a cantilever bimaterial strip subjected to a uniform temperature were applied to obtain expressions for performance metrics for the actuator, i.e., maximum work/volume, blocked (force) moment, and free-end (displacement) slope. Results from finite-element analysis and closed form relations agree well to within 1%. The optimal performance for a given pair of materials and the corresponding thickness ratio were determined. Contours of equal performance corresponding to commonly used substrates (e.g., Si, SiO₂) were plotted in the domain of governing material properties (thermal expansion coefficient and Young's modulus) to identify candidate materials for further development. These results and the accompanying methodology provide a rational basis for comparing the suitability of "standard" materials for microelectrothermal actuators, as well as identifying materials that might be suitable for further research     

Two-axis single-crystal silicon micromirror arrays

This work presents the design, fabrication, and testing of a two-axis 320 pixel micromirror array. The mirror platform is constructed entirely of single-crystal silicon (SCS) minimizing residual and thermal stresses. The 14-/spl mu/m-thick rectangular (750/spl times/800 /spl mu/m/sup 2/) silicon platform is coated with a 0.1-/spl mu/m-thick metallic (Au) reflector. The mirrors are actuated electrostatically with shaped parallel plate electrodes with 86 /spl mu/m gaps. Large area 320-mirror arrays with fabrication yields of 90% per array have been fabricated using a combination of bulk micromachining of SOI wafers, anodic bonding, deep reactive ion etching, and surface micromachining. Several type of micromirror devices have been fabricated with rectangular and triangular electrodes. Triangular electrode devices displayed stable operation within a (/spl plusmn/5/spl deg/, /spl plusmn/5/spl deg/) (mechanical) angular range with voltage drives as low as 60 V.     

Uncooled multimirror broad-band infrared microbolometers

A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earth's Radiant Energy System) instrument to measure the radiation flux at the Earth's surface and the radiant energy now within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 /spl mu/m, a wide-band channel of 0.3 to 100 /spl mu/m and a window channel from 8 to 12 /spl mu/m each housing a 1.5 mm x 1.5 mm microbolometers or alternatively 400 /spl mu/m x 400 mm microbolometers in a 1 /spl times/ 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 /spl mu/m wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si/sub 3/N/sub 4/ bridge layer. The temperature coefficient of resistance was measured to be -2.8%/K. The voltage responsivities were over 10/sup 3/ V/W, detectivities above 10/sup 8/ cm Hz/sup 1/2//W, noise equivalent power less than 4 /spl times/ 10/sup -10/W/Hz/sup 1/2/ and thermal time constant less than 15 ms.     

A finite element analysis of an axisymmetrically loaded orthotropic shell of revolution

The objective of this study was to develop a finite element matrix method of analysis for symmetrically loaded orthotropic shells of revolution using closed form elasticity solutions for the element. A computer program for structural analysis was developed based on this method.

The program was used to analyze orthotropic cylindrical shells with edge loads, orthotropic spherical shells with edge loads, and pressurized ellipsoidal shells.

For the ellipsoidal shells, the ratio of the major to minor axis (a/b) varied from 0.2 to 1.8. The orthotropic materials used had ratios of Young's modulus in the meridional direction to Young's modulus in a direction tangent to a parallel circle (E₁/E₂) that ranged from 0.2 to 1.8.

For the structures and orthotropic materials studied, it was found that the edge effect, as signified by the meridional moment, was affected by the Young's moduli ratio E₁/E₂, the radius of curvature R₂ in the plane containing a normal to the shell surface and a tangent to a parallel circle, and Poisson's ratio v₂, the latter being more prominent for large E₁/E₂ values. The range of the E₁/E₂ ratio caused the meridional edge moment to double, increasing as the E₁/E₂ ratios increased from 0.2 to 1.8, for pressurized ellipsoidal shells. The meridional edge moment more than doubled as the ellipsoidal axes ratio, a/b, ranged from 0.2 to 1.8.     

A MEMS-Based Evaluation of the Mechanical Properties of Metallic Thin Films

Characterizing the mechanical properties of metal thin films is critical for the design and fabrication of metal microelectromechanical systems and integrated circuit devices. This paper focuses on wafer-level determination of the mechanical behavior of sputtered aluminum and nickel thin films, using a variety of measurement techniques. Elastic moduli have been determined in devices fabricated with standard micromachining techniques using bulge testing of square diaphragms and lateral resonator structures. We find a Young's modulus of ~70 GPa for Al and ~200 GPa for Ni, in agreement with data for the bulk metals. Using pressurize/depressurize cycles, the load-deflection curves of the membranes have also been determined, and in conjunction with finite element simulations, were used to determine the yield strength and fracture strength of these films. Residual stresses in the films have also been investigated using wafer curvature, bulge testing, and X-ray diffraction. The merits of each measurement technique are discussed.     

Self-assembly of three dimensional micro mechanisms using thermal shrinkage of polyimide

This paper describes three-dimensional microstructures fabricated using a simple self-assembly process involving the thermal shrinkage of polyimide. The proposed method enables hinged structures to be automatically rotated out of the wafer plane and to remain bent without the need to use any interlocking mechanisms. The hinged structures were fabricated using surface micromachining techniques involving heating in a furnace. An increase in the bending angle due to the shrinkage of polyimide was observed with increasing heating temperature, heating time, and length of the polyimide hinge. Of these three parameters, the heating time was found to be the most suitable for precise control of the bending angle. Furthermore, microcubes were fabricated by this method and the self-assembly process was successfully visualized using a CCD camera.     

高g_n值MEMS加速度传感器封装的研究

对一种新型双悬臂梁高gn 值MEMS加速度传感器进行有限元模拟。采用双悬臂梁传感芯片的一种实际封装结构 ,进行频域分析和时域分析 ,讨论封合传感器芯片和封装基体的封合材料对其输出信号的影响。频域分析表明 ,封合材料的杨氏模量对封装后加速度传感器整体的振动模态有一定影响 ,封合胶的杨氏模量很小时 ,会致使加速度传感器的信号失真 ,模拟表明可选用杨氏模量足够高的环氧树脂类作高gn 值传感器的封合材料。时域分析静态模拟表明 ,封合材料的杨氏模量 ,对最大等效应力和沿加载垂直方向的正应力最大最小值基本无影响。时域分析动态模拟表明 ,随着封合材料杨氏模量的提高 ,动态模拟输出的悬臂梁末端节点位移的波形和其经数字滤波后输出的信号变好 ,封合材料的杨氏模量不影响输出信号的频率和均值 ,在加速度脉冲幅值输入信号变化时 ,悬臂梁末端位移平均值输出信号与输入有良好的线性关系。     

Characterizations of VO2-based uncooled microbolometer linear array

Vanadium dioxide (VO₂) thin films are materials for uncooled microbolometer due to their high temperature coefficient of resistance (TCR) at room temperature. This paper describes the design and fabrication of eight-element uncooled microbolometer linear array using the films and micromachining technology. The characteristics of the array is investigated in the spectral region of 8–12 μm. The fabricated detectors exhibit responsivity of over 10 kV/W, detectivity of approximate 1.94×10⁸ cm Hz^1/2/W, and thermal time constant of 11 ms, at 300 K and at a frequency of 30 Hz. Furthermore, the uncorrected response uniformity of the linear array bolometers is less than 20%.     

High aspect ratio fabrication method using O2 RIE and electroplating

A polyimide-based process for the fabrication of vertical structures with high aspect ratio has been developed. O₂ reactive ion etching (O₂ RIE) has been employed in the polyimide processing. Achieved etching characteristics of the O₂ RIE system are: 4.0 m/min etching rate, 15 aspect ratio, 75 m etching depth. Polyimide has excellent chemical and thermal properties which makes it a good building material for micromachines. Polyimide could be also used as molds for electroplating. Electroplated copper structures were formed in the polyimide molds and metal gears were fabricated by these fabrication technologies. New possibilities for micromachining were opened by the use of O₂ RIE and electroplating.This work was supported by Japanese ministry of Education Science and Culture under a grant-in-Aid No. 03102001.     

Selection of high strength encapsulant for MEMS devices undergoing high-pressure packaging

Deflection behavior of several encapsulant materials under uniform pressure was studied to determine the best outer encapsulant for MEMS device. Encapsulation is needed to protect movable parts of MEMS devices during high-pressure transfer molded packaging process. The selected outer encapsulant material has to have surface deflection of less than 5 μm under 100 atm vertical loading. Deflection was simulated using Coventorware ver.2005 software and verified with calculation results obtained using shell bending theory. Screening design was used to construct a systematic approach for selecting the best encapsulant material and thickness under uniform pressure up to 100 atm. Materials considered for this study were SMC polyimide, liquid crystal polymer (LCP) carbon fiber and polyphenylene sulfide (PPS) high modulus carbon fiber. It was observed that PPS high modulus carbon fiber has deflection of less than 5 μm for all thickness and pressure variations. LCP carbon fiber is acceptable and SMC polyimide is unsuitable as high strength encapsulant. PPS high modulus carbon fiber is considered the best encapsulation material for MEMS under high-pressure packaging process due to its high strength. The generalized mathematical model and equations developed for predicting deflection of encapsulation under uniform loading could be used to determine the suitability of any candidate material and encapsulation design with similar domed shaped structure.     

Magnetic composite electroplating for depositing micromagnets

This paper reports a novel magnetic composite materials deposition technique called magnetic composite electroplating (MCE). Thin films and micromagnets arrays of a composite matrix consisting of magnetic particles and a ferromagnetic alloy have been fabricated based on this technique. In a typical MCE process, magnetic particles are electrochemically and mechanically embedded into electroplated ferromagnetic thin films to form a magnetic particle-alloy composite. The magnetic particle selected is a barium ferrite magnet (BaFe/sub 12/O/sub 19/) and the ferromagnetic matrix is a pulse-reverse electroplated CoNiP alloy. The particle embedded fraction (w.t. %) directly affects magnetic properties and is experimentally determined by its energy dispersive spectrum (EDS). Various factors including electrolyte particle concentration, applied current, electrolyte pH, and the presence of cationic surfactants affecting the particle embedded fraction are experimentally investigated. Arrays of BaFe/sub 12/O/sub 19/-CoNiP magnets with a variety of dimensions and features as small as 8/spl mu/m have been realized by MCE. Experimental analysis shows that the composite exhibits magnetic properties, such as a high coercivity (H/sub c/) of up to 1.75/spl times/10/sup 5/ A/m, particularly well suited for MEMS actuators.     

Determination of Young’s modulus and residual stress of electroless nickel using test structures fabricated in a new surface micromachining process

This paper reportsin situ measurement of Young’s modulus and residual stress of electroless nickel films through the use of microfabricated nickel test structures, including electrostatic microactuators and passive devices. Th test structures are fabricated in a new surface micromachining process, termed “nickel surface micromachining”, using electroless plated nickel as the structural layer and polysilicon as the sacrificial layer. Subsequent to fabrication, lateral resonant-type electrostatic microactuators of different geometries are resonated by electrical excitation. Using the measured resonant frequencies and knowledge of the device geometry, the Young’s modulus of the film is determined. The passive electroless nickel microstructures deform upon completion of the fabrication process due to residual stress in the film Measurement of this deformation in conjunction with an appropriate mechanical model is used to determine the residual stress in the films.