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Three types of planar structure microspring electro-thermal actuators with insulating beam constraints
Abstract
A new concept of using an electrically insulating beam as a constraint is proposed to construct planar spring-like electro-thermal actuators with large displacements. On the basis of this concept, three types of microspring actuators with multi-chevron structures and constraint beams are introduced. The constraint beams in one type (the spring) of these devices are horizontally positioned to restrict the expansion of the active arms in the x-direction, and to produce a displacement in the y-direction only. In the other two types of actuators (the deflector and the contractor), the constraint beams are positioned parallel to the active arms. When the constraint beams are on the inner side of the active arms, the actuator produces an outward deflection in the y-direction. When they are on the outside of the active arms, the actuator produces an inward contraction. Finite-element analysis was used to model the performances. The simulation shows that the displacements of these microspring actuators are all proportional to the number of the chevron sections in series, thus achieving superior displacements to alternative actuators. The displacement of a spring actuator strongly depends on the beam angle, and decreases with increasing the beam angle, the deflector is insensitive to the beam angle, while the displacement of a contractor actuator increases with the beam angle.
Supplementary resource (1)
... A rising temperature will increase the electrical resistance of the device due to the positive temperature coefficient of the resistivity of metals. The average temperature change of the thermal microactuator ΔT ave can be extracted from the change of the resistance by the relation [22] ...
... agreement with theoretical analysis in the literature [12], [22]. For both D1 and D2, the displacement of the tip reached a maximum at a power of 60 mW, specifically at values of ∼16 and 20 μm, respectively; beyond this, the displacement decreased with any further increase in power due to thermal degradation, as discussed later. ...
... The maximum temperature is near the middle of the hot arm. A previous analysis based on the thermal conduction model indicated that the maximum temperature increase of the hot arm ΔT max would be expected to be about 1.5 times the calculated average temperature increase ΔT ave [21], [22]. ...
In this paper, the thermal degradation of laterally operating thermal actuators made from electroplated nickel has been studied. The actuators investigated delivered a maximum displacement of ca. 20 mum at an average temperature of ~ 450degC , which is much lower than that of typical silicon-based microactuators. However, the magnitude of the displacement strongly depended on the frequency and voltage amplitude of the pulse signal applied. Back bending was observed at maximum temperatures as low as 240degC. Both forward and backward displacements increase as the applied power was increased up to a value of 60 mW; further increases led to reductions in the magnitudes of both displacements. Scanning electron microscopy clearly showed that the nickel beams began to deform and change their shape at this critical power level. Compressive stress is responsible for nickel pileup, while tensile stresses, generated upon removing the current, are responsible for necking at the hottest section of the hot arm of the device. Energy dispersive X-ray diffraction analysis also revealed the severe oxidation of Ni structure induced by Joule heating. The combination of plastic deformation and oxidation was responsible for the observed thermal degradation. Results indicate that nickel thermal microactuators should be operated below 200degC to avoid thermal degradation.
... Que et al. [10] have reported a simple bent beam actuator and chevron actuators, and from their experimental results, it was concluded that the chevron actuators produce more in-plane motion and force at less driving voltage. Luo et al. [11] proposed three different types of electrothermal actuators. Out of the three designs, bent beam actuator was employed in the present work. ...
... The heat loss due to convection and radiation contribute meager amount of heat loss in microdevices [17,18]. Therefore, the thermal conduction through solid anchor is only considered for analysis, the temperature distribution along the bent beam in S-direction shown in Fig. 2 and it is expressed by using classic heat transfer equation [11] given in Eq. (3). ...
A microelectromechanical system-based digital-to-analog converter (MEMS DAC) using a bent beam electrothermal compliant actuator as a single mechanical bit is proposed in this work. Two types of MEMS DAC namely asymmetric and symmetric structures with multi-bent beam actuators in a chevron arrangement are designed, modeled and simulated in MEMS CAD tool CoventorWare. The proposed DACs generate the mechanical displacement at the output from the digital input based on the principle of weighted stiffness which is analogous to an electronic weighted resistor DAC. The DACs are built using a standard SOIMUMPs process, and its coupled electrothermomechanical analysis is carried out to illustrate their performance. The device operates on 5 V which is well suited with conventional CMOS logic and in turn reduces the power consumption of the device. The simulation result shows that symmetric and asymmetric MEMS DACs produce a nonlinear error of 0.5 and 3.5% from its ideal straight line. The nonlinearity error is found to be less in symmetric design due to equal width of bent beam.
... A single mechanical binary digit of a MEMS DAC is formed by single Chevron type electro-thermal compliant actuator. The chevron type thermal actuator reported in [6], of length 200µm (L), anchored at an angle of 2.6 0 (ω) is used (Fig 2) for realizing MEMS DAC. The necessary stiffness of the each chevron beam is attained by choosing a suitable width to covert the given binary code. ...
... The working principle of the proposed MEMS based DAC uses the similar principle discussed in [1][2][3][4][5], by using this principle, MEMS based DAC is designed by incorporating chevron type electro-thermal actuators. The deflection of a chevron type thermal actuator depends on the length and the angle of the beams [6], the better deflection is possible when a lesser angle is used. The binary '1' condition ensues when the ETC biased with 5V voltage and it produces the maximum deflection and the binary, '0' ensues when they are not biased. ...
This paper describes the design and numerical modeling of chevron micro electro thermal compliant (ETC) actuator based Digital to Analogue Converter (DAC). The proposed DAC generate the mechanical displacement at the output from digital electrical input based on the principle of weighted stiffness. The device is made of 4 - horizontal chevron type ETC actuators of different stiffness. The DAC is built using a SOIMUMP’S process steps using MEMS CAD tool COVENTORWARE and its coupled electro-thermo-mechanical analysis is carried out to illustrate the performance. The device operates on 5V which is well suited with conventional CMOS logic and in turn reduces the power consumption of the device.
... This serial connection reduces its stiffness in half, but the analytic relationship for the bent-beam actuator is still valid. There is a similar actuator reported as a one-ring spring actuator [18], where this lateral bar is thermally isolated and operates as a mechanical constraint. In this paper, the lateral bar is used to simply connect two bent-beam type actuators, because perfect thermal insulation of the lateral bar is hard to implement with MEMS fabrication technologies. ...
... The stiffness of the oscillating plate is the summation of the stiffness of the eight folded springs [18] and can be expressed as: (5) where μ is the Poisson's ratio of silicon, and the other design parameters are indicated in Fig. 3. The value for each design parameter is listed in Table 1. ...
Rotational rheometers are used to measure paste properties, but the test would take too long to be useful for quality control (QC) on the job site. In this paper, a new type of rheometer is proposed based on a one degree of freedom (DOF) micro-electro-mechanical systems (MEMS)-based motion stage. Preliminary data will be presented to show the capability of the system to measure the viscoelastic properties of a paste. The parallel plate geometry rheometer consists of two plates, which move relative to each other to apply a strain to the material to be tested. From the stress measured and the strain applied, the rheological characteristics of the material can be calculated. The new device consists of an electrothermal actuator and a motion plate. For the rheological measurements, the device is designed to generate the shear stress up to 60 Pa and maintain its stiffness to less than 44 N/m. With these features, the device uses a square plate of 1.5 mm x 1.5 mm to provide enough area for a few micro-liter level volumes. The motion of the square plate is monitored by a capacitive sensor at the end of the oscillating plate which has a resolution of 1.06 μm. When a reference cementitious paste, Standard Reference Material (SRM)-2492, is placed between the oscillating plate of the presented motion stage and a fixed plate, the reduction in the displacement of the oscillating plate is monitored showing that the presented motion stage is reasonably designed to detect the response of the reference cementitious paste.
... Microactuators can be designed based on different methods. Electro-thermal, electro-static, pneumatic, piezoelectric and shape memory alloys (SMA) are different actuation mechanisms utilized to achieve this goal [15][16][17][18]. Smart actuators such as piezoelectric and SMA are used more than other types in recent studies because of their functional characteristics, such as ease of activation, high-frequency response, low-cost fabrication, bio-compatibility and high actuation energy density. ...
... In the late 1990s, Kohl et al fabricated a linear microactuator from thin film SMA, then used the same microactuator to manufacture a microgripper [16,33]. After this, the use of [17,34]. Given that the stress and strain applied to the SMA microactuators have direct impacts on their performance, they can be appropriately designed and optimized to increase their efficiency. ...
Microactuators are essential elements of MEMS and are widely used in these devices. Microgrippers, micropositioners, microfixtures, micropumps and microvalves are well-known applications of microstructures. In this paper, the design optimization of shape memory alloy microactuators is discussed. Four different configurations of microactuator with variable geometrical parameters, generating different levels of displacement and force, are designed and analysed. In order to determine the optimum values of parameters for each microactuator, statistical design of experiments (DOE) is used. For this purpose, the Souza et al constitutive model (1988 Eur. J. Mech. A 17 789-806) is adapted for use in finite element analysis software. Mechanical properties of the SMA are identified by performing experimental tests on Ti-49.8%Ni. Finally, the specific energy of each microactuator is determined using the calibrated model and regression analysis. Moreover, the characteristic curve of each microactuator is obtained and with this virtual tool one can choose a microactuator with the desired force and displacement. The methodology discussed in this paper can be used as a reference to design appropriate microactuators for different MEMS applications producing various ranges of displacement and force.
... For metal, the change in resistivity due to the piezoresistivity is much smaller than that caused by the temperature change. In the case of the small magnitude of external force in a cell manipulation, which has relatively small mechanical strains, the small resistivity change caused by the piezoresistive effect could be negligible [16][17][18][19]. It would be approximated that the resistance variation in metal is resulted solely from the temperature change; therefore, the self-sensing method using the electrical resistance as a control parameter can be used in dynamic situations when the temperature and the deformation of the actuator are changed at the same time. ...
The self-sensing technology of microactuators utilizes a smart material to concurrently actuate and sense in a closed-loop control system. This work aimed to develop a position feedback-control system of nickel electrothermal microactuators using a resistivity self-sensing technique. The system utilizes the change in heating/sensing elements’ resistance, due to the Joule heat, as the control parameter. Using this technique, the heating/sensing elements would concurrently sense and actuate in a closed loop control making the structures of microactuators simple. From a series of experiments, the proposed self-sensing feedback control system was successfully demonstrated. The tip’s displacement error was smaller than 3 µm out of the displacement span of 60 µm. In addition, the system was less sensitive to the abrupt temperature change in surroundings as it was able to displace the microactuator’s tip back to the desired position within 5 s, which was much faster than a feed-forward control system.
... Therefore, the microsprings in MEMS devices have many applications. Microsprings can be used in MEMS actuators [1][2][3][4][5], MEMS sensors [6][7][8], in the micro-2D stage (micro-X-Y stage) [9][10][11][12], and for energy harvesting [13,14]. Improving the performance of microsprings is a key research topic. ...
Even when made by brittle materials, awl-shaped serpentine microsprings (ASSMs) were found to have a nonlinear displacement–force relationship similar to springs made by ductile material. It is found that the nonlinear displacement–force relationship is due to the geometry and dimensions of the ASSMs. The geometric effect of the nonlinear force-displacement relationship of ASSMs for in-plane motion was investigated. A theoretical solution was derived to analyze this nonlinearity. By successfully fabricating and measuring an ASSM, the theoretical results agreed well with the experimental results. The results indicated that ASSMs have a nonlinear force-displacement relationship, which is similar to that of hardening springs. The taper angle has a significant effect on the nonlinear displacement of ASSMs. When the taper angle was small, no obvious effect appeared on the nonlinearity of the microsprings with different numbers of turns. When the beam length increased, the critical force for nonlinear deflection decreased.
... Thermomechanical actuators (TMAs) are considered to be an attractive choice for precision applications due to their high stiffness and force output in comparison to their electro-static counterparts as well as the capability to reach nanometer-level resolution with ranges of over several micrometers [1]. There have been prior attempts to achieve both expansion and contraction mechanisms for chevron-beam based TMA's [2]. However, they generally required separate designs where the location of constraint beams had to be altered for each mode. ...
... The boundary conditions requires that: first, the temperature of the anchors remains constant as initial temperature 0 , so that 1 (0) = 0 and 2 (2 + ) = 0 ; second because of symmetry, the temperature of the beams and the shuttle at = and = + should be the same, which implies 1 ( ) = ( ) and 2 ( + ) = ( + ) ; third, the rates of heat conduction at = and = + are also the same, which yields = , and by combining eqn. 2 ). The tip displacement of the Z-shaped beam is written as in (17). Detailed derivations can be found in our preliminary work [8]. ...
This paper presents a novel analytical model that provides a unique guidance on the design comparison and estimation of V- and Z-shaped electrothermal microactuators. A common analytical model for both V- and Z-shaped actuators is developed for the first time in this work using the proposed topological structure layout. The developed model is verified with finite-element (FE) simulation results using commercialized software ANSYS and with reported experimental results. Finally by virtue of the benchmark and the system model, systematic performance comparison with varying parameters of V- and Z-shaped actuators are conducted to provide the essential insights into the design analysis and selection of V- and Z- shaped eletrothermal microactuators.
... The device is an optical chip which consists of an array of micro mirrors capable of generating high resolution digital images for the improved performance of projection systems.Microelectrothermal actuators in general, are particularly promising for applications requiring a large specific work. They are capable of actuating either in-the-planeLuo et al. (2005) or out-ofthe-planeSingh et al. (2005) of the substrate. Actuation is usually achieved by thermal expansion of the materials caused by Joule heating of the substrate materials followed by ambient cooling. ...
As the design of microsystems (MST) or microelectromechanical systems (MEMS) matures and migrates from process centric to performance based design, MEMS designers would need a rational method for selecting an appropriate material that is not based on the ease of processing alone. While there is a growing number of thin film materials that can be used for MEMS devices, the selection of a particular material is rarely based on quantifiable criterion that relates directly to the optimum performance of the device. This article describes the need of materials selection for MEMS from a designer’s point of view. The performance indices and the selection of most appropriate material for MEMS is also described. From MEMS structures to Ashby’s material selection method is reviewed and presented in following sections. E↵ects of length scale is documented as described by followed by mechanical behavior and properties of materials is further documented. Influence of materials on the performance of microsystems is described in next section followed by common issues in microsystem designing. Further, comparison of common materials is described with the help of two years studies of MEMS structures. Ashby’s materials selection process is also described for thin films followed by summary of this article.
... where, is the coefficient of thermal expansion of silicon, W is the beam width, H is the beam height, and an average temperature rise over the beam can be described [26] as: (4) where, k is the thermal conductivity of silicon, is the resistivity of silicon, and V is the driving electrical voltage applied to the actuator. The load P can be expressed as a function of temperature and this relationship is described in equations (3) and (4). ...
This paper presents the design, fabrication and characterization of a single-layer out-of-plane electrothermal actuator based on MEMS (Micro-Electro-Mechanical System). The proposed electrothermal actuator is designed to generate motions along the out-of-plane or normal to a wafer by a Joule heating when the current flows through the actuator. This out-of-plane electrothermal actuator is based on a single layer of a SOI (Silicon on Insulator) wafer and two notches near the middle of the actuator beams. Due to these notches, the thermal expansion of the beams in the actuator generates an eccentric loading, which converts into the bending of the beams. This bending of the beam finally generates the out-of-plane motion at the middle of the beam. This behavior is described by the prepared analytic equations and compared by the results from FEM (Finite element Model) analysis. With fabricated samples, a 30 μm displacement is measured along out-of-plane at 5 V driving voltage. The 1st mode of the resonant frequency for the out-of-plane motion is expected to occur at 74.9 kHz from FEA. The proposed actuator is based on the standard SOI-MUMPs (SOI-Multi User Manufacturing Process), so it has good integration capability with other system employing same fabrication techniques. To test its integration capability, a MEMS XYZ stage is fabricated by embedding the proposed out-of-plane electrothermal actuator onto an existing MEMS XY stage. The range of motion of the fabricated XYZ stage is measured about 35 μm x 35 μm x 30 μm along X, Y and Z axes without any changes on its fabrication process.
... This increases the electrical resistance of the device due to the positive temperature coefficient of the metal resistivity. The average temperature of the thermal microactuator, T ave , can be extracted from the change of the resistance by the following equation [18]; ...
Metal based thermal microactuators normally have lower operation temperatures than those of Si-based ones; hence they have great potential for applications. However, metal-based thermal actuators easily suffer from degradation such as plastic deformation. In this study, planar thermal actuators were made by a single mask process using electroplated nickel as the active material, and their thermal degradation has been studied. Electrical tests show that the Ni-based thermal actuators deliver a maximum displacement of ~20 m at an average temperature of ~420 °C, much lower than that of Si-based microactuators. However, the displacement strongly depends on the frequency and peak voltage of the pulse applied. Back bending was clearly observed at a maximum temperature as low as 240 °C. Both forward and backward displacements increase with increasing the temperature up to ~450 °C, and then decreases with power. Scanning electron microscopy observation clearly showed that Ni structure deforms and reflows at power above 50mW. The compressive stress is believed to be responsible for Ni piling-up (creep), while the tensile stress upon removing the pulse current is responsible for necking at the hottest section of the device. Energy dispersive X-ray diffraction analysis revealed severe oxidation of the Ni-structure induced by Joule-heating of the current.
... An electro-thermal actuator with planar structure may provide an out-of-plane motion [24] or an in-plane motion [25]. Furthermore, in accordance with the structure of the device, the electro-thermal actuator may be a thermal double-layer microactuator (e.g. the bimetallic structure) or a mechanical thermal expansion microactuator. ...
... The deflection of a heatuator is given by [86,87] ...
Diamond and diamond-like carbon (DLC) thin films possess a number of unique and attractive material properties that are unattainable from Si and other materials. These include high values of Young's modulus, hardness, tensile strength and high thermal conductivity, low thermal expansion coefficient combined with low coefficients of friction and good wear resistance. As a consequence, they are finding increasing applications in micro-electro-mechanical systems (MEMS). This paper reviews these distinctive material properties from an engineering design point of view and highlights the applications of diamond and DLC materials in various MEMS devices. Applications of diamond and DLC films in MEMS are in two categories: surface coatings and structural materials. Thin diamond and DLC layers have been used as coatings mainly to improve the wear and friction of micro-components and to reduce stiction between microstructures and their substrates. The high values of the elastic modulus of diamond and DLC have been exploited in the design of high frequency resonators and comb-drives for communication and sensing applications. Chemically modified surfaces and structures of diamond and DLC films have both been utilized as sensor materials for sensing traces of gases, to detect bio-molecules for biological research and disease diagnosis.
... Piezoelectric [1], magneto-static [2], electro-static [3,4] and electro-thermal [5][6][7][8] are popular drive mechanisms implemented in micro-actuators. Piezoelectric material is typically not process-compatible with silicon. ...
Reported presently are two designs to improve the performance of a “chevron” electro-thermal in-plane actuator. One incorporates beams with uniform cross-sections but nonuniform lengths and tilt angles to accommodate the thermally induced expansion of the “shuttle”; the other incorporates beams with non-uniform cross-sections to achieve a wider spread of the high temperature “expansion” regions of the beams. With the product of the actuation force and displacement defined as a figure-of-merit, it is verified using finite-element simulation that the incorporation of non-uniform lengths and tilt angles, and non-uniform beam cross-sections leads to respective improvement of 10 and 65% in the figure-of-merit. The effectiveness of these designs was also tested by micro-fabricating actuators occupying fixed device areas.
... While MEMS consist of micrometer-scale components, they have the capability of achieving nanometer displacement resolution and femto- Newton load resolution. In this context, both electrostatic [5] and thermal actuators [6]–[9] were demonstrated as possible actuators. In particular, MEMS actuators have been successfully used in on-chip testing of mechanical properties of MEMS materials [10], [11]. ...
As the design of microsystems (MST) or microelectromechanical systems (MEMS) matures and migrates from process centric to performance-based design, MEMS designers would need a rational method for selecting an appropriate material that is not based on the ease of processing alone. While there is a growing number of thin film materials that can be used for MEMS devices, the selection of a particular material is rarely based on quantifiable criterion that relates directly to the optimum performance of the device. This article describes the need of materials selection for MEMS from a designer׳s point of view. The performance indices and the selection of most appropriate material for MEMS is also described. From MEMS structures to Ashby׳s material selection method is reviewed and presented in following sections. Effects of length scale is documented as described by followed by mechanical behavior and properties of materials is further documented. Influence of materials on the performance of micro- systems is described in next section followed by common issues in microsystem designing. Further, comparison of common materials is described with the help of two years studies of MEMS structures. Ashby׳s materials selection process is also described for thin films followed by summary of this article.
In this paper, we propose MEMS based Electro-thermal actuator that can achieve minimum step size less of 100 nm; Actuator consisted of slider and griper. In the slider structure used of the two electro-thermal chevrons that led to linear forward motion of the slider. The gripper is consisting of clutch and chevrons. The clutch used to connect the slider to the substrate, and the chevrons used to release the clutches. With apply DC voltage of 0.5 Volts; a force is produced by the slider that is 8 mN and a stroke is equal to 2.2 μm. Large stroke is achieved by repeating the step sizes sequentially. Also an important property in this work is the normally latched or zero-power latching scheme.
This work investigates a novel awl-shaped serpentine microspring for a suspension structure, with a lower spring constant under the same unit layout area in out-of-plane motion. Using Castigliano’s theorem, the spring constant of the microspring was theoretically derived and simulations were performed using COMSOL Multiphysics to verify the theoretical results. The proposed awl-shaped serpentine microspring was successfully fabricated using silicon-based micromachining. Experiments were conducted to compare the theoretical and numerical results, which were in close agreement. In addition, a parameter of spring constant to layout area ratio (K/A) is defined to be used as the index for comparing spring constants under the same unit area. Accordingly, the awl-shaped serpentine microspring has a lower K/A value than the traditional serpentine microspring with the same total effective length and folds. With a greater taper angle, more folds, a smaller beam width, and lower beam thickness, the awl-shaped serpentine microspring has a smaller K/A value. Using the proposed mathematical model, the spring constants of microsprings of various sizes and geometric structures can be calculated in out-of-plane motion before the microstructure is fabricated. Thus, it saves time when designing a microspring with a proper spring constant.
This paper presents the design, fabrication and testing of a Micro Electro Mechanical Systems (MEMS) based positioning stage which is capable of generating translational motions along X, Y and Z axes, respectively. For this purpose, two existing 1 Degree of Freedom (DOF) in-plane positioning stages and one 1 DOF out-of-plane actuator are merged together for 3 DOF motions. For successful integration between three independent systems, a platform in the chosen 1 DOF in-plane stage is utilized to embed another system. For a successful combination between three independent systems, two nested structures are adapted as a serial kinematic mechanism. With the nested structures, one 1 DOF in-plane positioning stage is embedded into the other 1 DOF in-plane stage for in-plane translational motions along the X and the Y axes. And then, one 1 DOF out-of-plane actuator is embedded for the translational motion along the Z axis. The proposed 3 DOF system has demonstrated an ability to generate at least 20 μm along X, Y and Z axes, respectively. The cross talk between the three axes is also measured and is less than 4 percent of the intended displacement.
A new design of three degrees-of-freedom (DOF) translational motion stage (XYZ stage) is presented in this paper. This XYZ stage is based on MEMS and designed by combining three existing one-DOF motion stages through a nested structure. By utilizing the previously developed stages, this approach can reduce the effort for the design and analysis steps and ensure reasonable reliability. For successful implementation, electrical connection to the engaged stages, electrical isolation among them, and additional floating frames are introduced for the chosen one-DOF motion stages. With these features, the presented XYZ stage is successfully fabricated and demonstrates the range of motion of 53.98, 49.15, and 22.91 µm along X, Y, and Z axes, respectively. The coupled motion errors among the engaged stages can be reduced to be less than 1 µm with the proposed compensation method.
Recently, microgrippers are finding more importance in the field
of tissue engineering and microassembly in semiconductor electronics.
Large force and, hence, large displacement, low power, and low temperature
are the essential features to be considered during the design of the
microgrippers. The electrical and mechanical behaviors of electrothermally
actuated silicon microgrippers are presented. The effect of increasing
the flexure length and the cold arm area to improve the displacement is
discussed. These microgrippers are normally of the open type, in which
the arms have an initial open gap of 20 μm, and they move away from
each other with the applied voltage. The displacement of each arm is
observed to be 24 μm for the applied voltage of 10 V. The response
time of the device is less than 5 ms, and the maximum power dissipation
is 110 mW. The displacement of the microgrippers can be increased with
increased flexure length and cold arm length with short extended arms.
This structure also shows lower stress
This work presents an SU-8 surface micromachining process using amorphous silicon as a sacrificial material, which also incorporates two metal layers for electrical excitation. SU-8 is a photo-patternable polymer that is used as a structural layer for MEMS and microfluidic applications due to its mechanical properties, biocompatibility and low cost. Amorphous silicon is used as a sacrificial layer in MEMS applications because it can be deposited in large thicknesses, and can be released in a dry method using XeF2, which alleviates release-based stiction problems related to MEMS applications. In this work, an SU-8 MEMS process was developed using α-Si as a sacrificial layer. Two conductive metal electrodes were integrated in this process to allow out-of-plane electrostatic actuation for applications like MEMS switches and variable capacitors. In order to facilitate more flexibility for MEMS designers, the process can fabricate dimples that can be conductive or nonconductive. Additionally, this SU-8 process can fabricate SU-8 MEMS structures of a single layer of two different thicknesses. Process parameters were optimized for two sets of thicknesses: thin (5–10 µm) and thick (130 µm). The process was tested fabricating MEMS switches, capacitors and thermal actuators.
Realizing out-of-plane actuation in micro-electro-mechanical systems (MEMS) is still a challenging task. In this paper, the design, fabrication methods and experimental results for a MEMS-based out-of-plane motion stage are presented based on bulk micromachining technologies. This stage is electrothermally actuated for out-of-plane motion by incorporating beams with step features. The fabricated motion stage has demonstrated displacements of 85 µm with 0.4 µm (mA)−1 rates and generated up to 11.8 mN forces with stiffness of 138.8 N m−1. These properties obtained from the presented stage are comparable to those for in-plane motion stages, therefore making this out-of-plane stage useful when used in combination with in-plane motion stages.
The present study is aimed to develop an optimization approach which is employed to design an electro-thermal microactuator based on numerical simulation of the device performance. The microactuator basically adopts a bimorph structure which consists of a P-type silicon layer and an aluminum layer. A finite-element multi-physics code (ANSYS 10.0) is used to develop the direct problem solver for predicting the stress/stain, electric, and thermal fields associated with the varying material layers thicknesses during the iterative optimization process. The simplified conjugate-gradient method (SCGM) is used as the optimization method. Regarded as a test case, the modifications of a conventional design, which suffered from high temperature, excessive power consumption, and oxidization of aluminum layer, are attempted. In this study, firstly the direct problem solver is used to predict the performance of various design models so as to select the best one with highest performance. Parametric study of the best design model is then performed, and based on the solutions yielded from the parametric study, the subtle influence of thicknesses of the P-type silicon and the aluminum layers (tS and tAL) on the performance of the microactuator is observed. Finally, the developed optimization approach is applied to predict the optimal combination of the geometrical parameters. The optimal thicknesses of the material layers used in the microactuator are successfully determined, and the optimization process leads to a great improvement in the performance of the microactuator. Results also show that the approach is robust and the obtained optimal combination of the geometrical parameters is independent of the initial guess for the particular case.
In micro-electro-mechanical systems (MEMS) it is difficult to obtain a large range of motion with a small coupled error. This limitation was overcome by designing and fabricating a nested structure as a serial kinematic mechanism (SKM). In this paper, a MEMS-based XY stage is reported for multifinger manipulation application. The SKM MEMS XY stage is implemented by embedding a single degree-of-freedom (DOF) stage into another single DOF stage. The proposed MEMS XY stage is fabricated by deep reactive ion etching (DRIE) from both sides of a silicon-on-insulator (SOI) wafer. This SKM MEMS stage has the capability to generate more than 50 µm displacements along each X- and Y-axes. This nested structure also suppressed the coupled motion error to 0.6% of the original actuation displacement. For the demonstration on the micro-particle manipulation, a 15 µm sized polypropylene particle is manipulated and rotated by operating two individual fingers attached to proposed MEMS stages.
This paper describes a novel micro xy-stage, driven by double-hot arm horizontal thermal micro-actuators integrated with a piezoresistive sensor (PS) for low-voltage operation and precise control. This micro xy-stage structure is linked with chevron beams and optimized to amplify the displacement generated by the micro-actuators that provide a pull force to the movable platform. The PS employed for in situ displacement detection and feedback control is fabricated at the base of a cold arm, which minimizes the influence of temperature change induced by electro-thermal heating. The micro xy-stage structure is defined through the use of a simple micromachining process, released by backside wet etching with a special tool. For an input power of approximately 44 mW, each chevron actuator provides about 16 µm and the total displacement of the platform is close to 32 µm. The sensitivity of the PS is better than 1 mV µm−1, obtained from the amplified voltage output of the Wheatstone bridge circuit. The potential applications of the proposed micro xy-stage lie in micro- or nano-manipulation, as well as the positioning of ultra-small objects in nanotechnology.
Static and dynamic electro-mechanical performance of a microactuator is a key factor in the functioning of an integrated microsystem composed of moving components such as optical shutters/switches, micropumps, microgrippers, and microvalves. Therefore, the development of such systems primarily focuses on the overall design and parameter optimization of an actuator as the major driving element with respect to the desired performance parameters, e.g., displacement, force, dimensional constraints, material, actuation principle, and method of fabrication. This study presents results on the static and dynamic electro-mechanical performance analysis of an in-plane electro-thermally driven linear microactuator. Each microactuator, having a width of 2220 mm and made of 25 mm thick nickel foil, consisted of a pair of cascaded structures. Connecting several actuation units in a series formed each cascaded structure. Several microactuators with a different number of actuation units were fabricated using the laser micromachining technology. The static performance of these microactuators was evaluated with respect to the maximum linear output displacements, actual resistance, applied current, and consumed electric power. The maximum displacements varied approximately from 3 to 44 mm, respectively, depending on the number of actuation units. The dynamic performance was studied as a response function on constant applied current with respect to the output displacements. In addition, the response time was evaluated for different applied currents and for actuators with 2, 4, and 6 actuation units. The microactuators’ performance results are promising for applications in MEMS/MOEMS, microfluidic, and microrobotic devices.
A new nanoscale tension testing platform with on-chip actuation and the unique capability for nanoscale mechanical characterization of highly deformable and strong nanostructures is presented. The specimen force and extension measurements are based on optical imaging, supported by digital image correlation, which allows the resolution of 20 nm specimen extensions and force measurements better than 30 nN, without the use of high-resolution electron microscopy. The breakthrough of this nanomechanical testing platform is the ability to study the mechanical behavior of nanostructures subjected to a wide range of forces (30 nN–300 µN) and displacements (20 nm–100 µm), which are significantly beyond the limits of typical surface micromachined MEMS with on-chip actuators, such as comb-drives and thermal actuators. The utility of this device in experimental nanomechanics is demonstrated by investigating the mechanical behavior of electrospun polyacrylonitrile nanofibers with diameters of 200–700 nm subjected to strains as high as 200%. The mechanical property measurements were compared to and agreed well with off-chip measurements by an independent testing method, which validates the capability of this on-chip testing platform to characterize strong and highly ductile nanomaterials.
To reduce power consumption and operation temperature for micro-thermal actuators, metal-based micro-mechanical locks with multi-locking positions were analyzed and fabricated. The micro-locks consist of two or three U-shaped thermal actuators. The devices were made by a single mask process using electroplated Ni as the active material. Tests showed that the metal based thermal actuators deliver a maximum displacement of ~20µm at a much lower temperature than that of Si-based actuators. However Ni-actuators showed a severe back bending, which increases with increasing applied power. The temperature to initiate the back bending is as low as ~240°C. Back bending increases the distance between the two actuators, and leads to locking function failure. For practical application, Ni-based thermal actuators must be operated below 200°C.
The present study is aimed to develop an electro-thermal microactuator. The microactuator adopts a deformable plate with a double-layer structure consisting of a silicon layer and an aluminum layer. As the microactuator is heated by an implanted electric heater, a displacement at the tip of the deformable plate is generated due to the different thermal expansion between these two layers. A serpentine area of N-type silicon is formed by ion implantation in the silicon layer and used as the electric heater. The structure of the microactuator is proposed, modeled, and fabricated in this study. Finite-element analysis is first performed to predict the performance of the microactuator with a multiphysics modeling package (ANSYS 10.0), and then the dimensions of the device are determined. The designed electro-thermal microactuator is fabricated by the semiconductor manufacturing technologies. Experiments on the displacement of the microactuator have been carried out by a microscopic image system to evaluate the performance of the design as well as to demonstrate validity of the modeling.
In this paper, we present the fabrication process of a shape memory alloy (SMA) thin film in both monolithic and hybrid configurations. This provides an effective actuation part for a gripper made of SU-8 thick photoresist. We also extensively describe and discuss the assembly of the SMA thin film with the SU-8 mechanism. Measurements show that the SU-8 gripper is able to achieve an opening action of 500 μm in amplitude at a frequency of 1 Hz. Finite element model simulations indicate that a force of 50 mN, corresponding to 400 μm of opening amplitude, should be produced by the SMA actuator. Although the assembly of the TiNi SMA thin film with the SU-8 mechanism is demonstrated, the bond reliability needs further development in order to improve the thermal behavior of the interface. In this paper, we show that SU-8 is well suited as a structural material for microelectromechanical systems (MEMS) applications. An attractive feature in the MEMS design is that the SMA generated force is well matched with the elastic properties of SU-8. From the application point of view, a SMA-actuated SU-8 high-aspect-ratio microgripper can serve as a secure means to transport microelectronics device, because it provides good grasping and safe insulation. This is also a preliminary result for the future development of biogrippers.
An analytical model that can accurately predict the performance of a polysilicon thermal flexure actuator has been developed. This model is based on an electrothermal analysis of the actuator, incorporating conduction heat transfer. Heat radiation from the hot arm of the actuator to the cold arm is also estimated. Results indicate that heat radiation becomes significant only at high input power, and conduction heat losses to both the substrate and the anchor are mainly responsible for the operating temperature of the actuator under routine operations. Actuator deflection is computed based on elastic analysis of structures. To verify the validity of the model, polysilicon thermal flexure actuators have been fabricated and tested. Experimental results are in good agreement with theoretical predications except at high input power. An actuator with a 240 µm long, 2 µm thick, 3 µm wide hot arm and a 180 µm long, 12 µm wide cold arm deflected up to 12 µm for the actuator tip at an input voltage of 5 V while it could be expected to deflect up to 22 µm when a 210 µm long cold arm is used.
For Part I see L. Que, J.S. Park and Y.B. Gianchandani, ibid.,
vol.10, pp.247-54 (2001). This paper reports on the use of bent-beam
electrothermal actuators for the purpose of generating rotary and
long-throw rectilinear displacements. The rotary displacements are
achieved by orthogonally arranged pairs of cascaded actuators that are
used to rotate a gear. Devices were fabricated using electroplated Ni, p
<sup>++</sup> Si, and polysilicon as structural materials. Displacements
of 20-30 μm with loading forces >150 μN at actuation voltages
<12 V and power dissipation <300 mW could be achieved in the
orthogonally arranged actuator pairs. A design that occupies <1 mm
<sup>2</sup> area is presented. Long-throw rectilinear displacements
were achieved by inchworm mechanisms in which pairs of opposing
actuators grip and shift a central shank that is cantilevered on a
flexible suspension. A passive lock holds the displaced shank between
pushes and when the power is off. This arrangement permits large output
forces to be developed at large displacements, and requires zero standby
power. Several designs were fabricated using electroplated Ni as the
structural material. Forces >200 μN at displacements >100 μm
were measured
This paper describes electrothermal microactuators that generate
rectilinear displacements and forces by leveraging deformations caused
by localized thermal stresses. In one manifestation, an electric current
is passed through a V-shaped beam anchored at both ends, and thermal
expansion caused by joule heating pushes the apex outward. Analytical
and finite element models of device performance are presented along with
measured results of devices fabricated using electroplated Ni and
p<sup>++</sup> Si as structural materials. A maskless process extension
for incorporating thermal and electrical isolation is described. Nickel
devices with 410-μm-long, 6-μm-wide, and 3-μm-thick beams
demonstrate 10 μm static displacements at 79 mW input power; silicon
devices with 800-μm-long, 13.9-μm-wide, and 3.7-μm-thick beams
demonstrate 5 μm displacement at 180 mW input power. Cascaded silicon
devices using three beams of similar dimensions offer comparable
displacement with 50-60% savings in power consumption. The peak output
forces generated are estimated to be in the range from 1 to 10 mN for
the single beam devices and from 0.1 to 1 mN for the cascaded devices.
Measured bandwidths are ≈700 Hz for both. The typical drive voltages
used are ⩽12 V, permitting the use of standard electronic interfaces
that are generally inadequate for electrostatic actuators
The thickness uniformity within a specimen and the cross-sectional profiles of electroplated individual Ni-microstructures have been investigated as a function of the electroplating conditions. It was found that the uniformity and profiles of microstructures could be controlled by varying the process conditions. A uniform thickness distribution and microstructures with flat profiles could be obtained at optimal plating conditions of 8mA/cm 2 and 60°C. Above this optimal plating current density, the microstructure has a rabbit-ears profile and the thickness of a narrow microstructure is thicker than that of wide ones. While below this current density, the microstructure has a cap-like cross-sectional profile, and a narrow structure is thinner than wide ones. Increasing the plating temperature enhances the non-uniformity, whereas other process parameters have insignificant effects on it. The current crowding observed in patterned specimens is responsible for the rabbit-ears profile of individual microstructures, while a combination of the fluidic friction on the sidewall of the photoresist and the electrophoresis of the ions in the solution are believed to be responsible for the abnormal cap-like profile of individual microstructures and the thinning effect on narrow microstructures.
The in-plane motion of microelectrothermal actuator ("heatuator") has
been analysed for Si-based and metallic devices. It was found that the
lateral deflection of a heatuator made of a Ni-metal is about ~60%
larger than that of a Si-based actuator under the same power
consumption. Metals are much better for thermal actuators as they
provide a relatively large deflection and large force, for a low
operating temperature, and power consumption. Electroplated Ni films
were used to fabricate heatuators. The electrical and mechanical
properties of electroplated Ni thin films have been investigated as a
function of temperature and plating current density, and the process
conditions have been optimised to obtain stress-free films suitable for
MEMS applications. Lateral thermal actuators have been successfully
fabricated, and electrically tested. Microswitches and microtweezers
utilising the heatuator have also been fabricated and tested.
A number of in-plane spring-like micro-electro-thermal-actuators with
large displacements were proposed. The devices take the advantage of the
large difference in the thermal expansion coefficients between the
conductive arms and the insulator clamping beams. The constraint beams
in one type (the spring) of these devices are horizontally positioned to
restrict the expansion of the active arms in the x-direction, and to
produce a displacement in the y-direction only. In other two types of
actuators (the deflector and the contractor), the constraint beams are
positioned parallel to the active arms. When the constraint beams are on
the inside of the active arms, the actuator produces an outward
deflection in the y-direction. When they are on the outside of the
active arms, the actuator produces an inward contraction. Analytical
model and finite element analysis were used to simulate the
performances. It showed that at a constant temperature, analytical model
is sufficient to predict the displacement of these devices. The
displacements are all proportional to the temperature and the number of
the chevron sections. A two-mask process is under development to
fabricate these devices, using Si3N4 as the
insulator beams, and electroplated Ni as the conductive beams.
This article investigates rectangular two-beam microelectromechanical thermal actuators and provides a method for their optimization. The thermal actuators investigated consisted of two asymmetric parallel arms,one thin and one wide. Under an electric current load, the thin arm heats and expands more than the wide arm, thereby bending the entire structure. Simplified models of the heat transfer mechanisms are used to determine the temperature profile. From the thermal expressions for expansion of the arms, equations are derived to predict the deflection as well as the buckling loads. Measurements of the actuator deflection as a function of voltage are presented. Design guidelines are introduced for optimization of a thermal actuator.
Surface micromachined polycrystalline silicon thermal micro- actuators
provide large deflections while requiring low drive voltages and
occupying small device areas. The force provided by thermal actuators is
not currently known. Therefore, force testers have been designed and
fabricated. The force testers were used to measure the force of
individual actuators over a range of design parameters including flexure
length, hot arm width, arm separation,and actuator thickness. The force
testers have also been connected to arrays of 2 to 20 actuators coupled
together. Measurements of force tester deflection versus actuator drive
power have been taken. The data shows that individual actuators are
capable of generating greater than 4.8 micronewtons of force with less
than 15 milliwatts of power. The test also show that coupling the
actuators together successfully combines the force of individual
actuators. These measurements allow the creation of a general set of
design rules for efficient thermal actuators.
A mathematical model to predict shape evolution during through‐mask electrochemical micromachining (EMM) has been developed.
Boundary element method has been used to solve the Laplace equation for electric potential with appropriate boundary conditions
that describe the metal dissolution process under ohmic control. The influence of mask wall angle on shape of the evolving
cavity, current distribution within the cavity and etch factor have been determined. For mask wall angles less than 90°, the
etch factor increased due to the shadowing effect of the mask, whereas the etch factor decreased for mask wall angles greater
than 90°. The influence of mask wall angle has been found to diminish with increasing metal film thickness.
A novel electro-thermally and laterally driven microactuator made of polysilicon has been designed, fabricated, and tested. The operational principle is based on the asymmetrical thermal expansion of the microstructure with different lengths of two beams, but not based on the variable cross sections of the microstructure. A microgripper to demonstrate one possible application of the microactuator is fabricated and characterized. The input voltage of this design is less than 10 dc to produce 20 µm displacement with about 0.6 mJ heat dissipation, and the maximum temperature is less than 600 C. A gripping force up to 2.8 µN can be generated. Simulation results are compared with the experimental data and show good agreement. Some design parameters strongly influencing the performance of the microactuator are discussed also.
The manipulation of mechanical and biological microobjects is important for many applications in basic research and, possibly, for industrial processes. The end-effector is the key component for obtaining safe and effective micromanipulation. Among the different technologies which can be used for fabricating the end-effector, we intend to exploit LIGA. The design of the LIGA-fabricated microgripper is described in detail; the performance of two test microgrippers (fabricated by standard mechanical machining) are described and critically analysed.
Thermal actuator-based microtweezers with three different driving configurations have been designed, fabricated and characterized. Finite element analysis has been used to model the device performance. It was found that one configuration of microtweezer, based on two lateral bimorph thermal actuators, has a small displacement (tip opening of the tweezers) and a very limited operating power range. An alternative configuration consisting of two horizontal hot bars with separated beams as the arms can deliver a larger displacement with a much-extended operating power range. This structure can withstand a higher temperature due to the wider beams used, and has flexible arms for increased displacement. Microtweezers driven by a number of chevron structures in parallel have similar maximum displacements but at a cost of higher power consumption. The measured temperature of the devices confirms that the device with the chevron structure can deliver the largest displacement for a given working temperature, while the bimorph thermal actuator design has the highest operating temperature at the same power due to its thin hot arm, and is prone to structural failure.
The handling and assembling of microcomponents is usually considered as a bottleneck in the fabrication process of hybrid microsystems. This is especially true for very small components that require high positioning tolerances. In this paper we present gripping solutions for various applications concerning the handling and assembling of microcomponents as well as the major microgripper manufacturing technologies.
Polycrystalline-Si microactuators based on electrothermal principles exhibit many interesting features but their practical use is severely limited by permanent damage that may occur due to accidental overheating. Under these conditions, polycrystalline-Si structures will display irreversible structural changes ranging from slight geometrical deformations to complete damage. In this paper, an approach is presented to avoid permanent structural deformation of B-doped polycrystalline-Si based electrothermal actuators by overheating. The method allows us to distinguish reversible and irreversible actuation conditions and is demonstrated under environmental and vacuum conditions. It enables full utilization of the capabilities of B-doped polycrystalline-Si based electrothermal actuators with reproducible performance.
Force and power characteristics are experimentally measured for three types of toggling microthermal actuators: (i) standard two-arm thermal actuators, (ii) chevron or 'V' type actuators and (iii) chevron actuators with a toggle amplifier. Micro-Newton scale forces and micron scale displacements are experimentally measured using an off-chip probe. For each type of actuator, force versus deflection curves are measured to determine mechanical advantage and maximum ranges. For each type of actuator, work output versus input power curves are measured to determine actuator effectiveness. Different actuator designs are compared in terms of force, displacement, power and effectiveness.
A comprehensive thermal model for an electro-thermal-compliant (ETC) microactuator is presented in this paper. The model accounts for all modes of heat dissipation and the temperature dependence of thermophysical and heat transfer properties. The thermal modelling technique underlying the microactuator model is general and can be used for the virtual testing of any ETC device over a wide range of temperatures (300-1500 K). The influence of physical size and thermal boundary conditions at the anchors, where the device is connected to the substrate, on the behaviour of an ETC microactuator is studied by finite element simulations based on the comprehensive thermal model. Simulations show that the performance ratio of the microactuator increased by two orders of magnitude when the characteristic length of the device was increased by one order of magnitude from 0.22 to 2.2 mm. Restricting heat loss to the substrate via the device anchors increased the actuator stroke by 66% and its energy efficiency by 400%, on average, over the temperature range of 300-1500 K. An important observation made is that the size of the device and thermal boundary conditions at the device anchor primarily control the stroke, operating temperature and performance ratio of the microactuator for a given electrical conductivity.
This paper examines the time and frequency response characteristics of two-arm micromachined thermal actuators. Two types of thermal actuators are considered: hot/cold arm actuators and 'V' or 'chevron' shaped actuators. A heat transfer equation governing the temperature profile along the thermal actuators is derived. Equations for the thermal time constants and the frequency responses are presented. The thermal time constants and frequency responses are measured experimentally and compared to analytical predictions.
At the micro-scale, thermal actuation provides larger forces compared to the widely-used electrostatic actuation. In this paper, we highlight another advantage of thermal actuation, viz. the ease with which it can be utilized to achieve a novel embedded electro-thermal-compliant (ETC) actuation for MEMS. The principle of ETC actuation is based on the selective non-uniform Joule heating and the accompanying constrained thermal expansion. It is shown here that appropriate topology and shape of the structures give rise to many types of actuators and devices. Additionally, selective doping of silicon ETC devices is used to enhance the non-uniform heating and thus the deformation. A number of novel ETC building blocks and devices are described, and their analysis and design issues are discussed. The devices were microfabricated using MCNC’s MUMPs foundry process as well as a bulk-micromachining process called PennSOIL (Penn silicon-on-insulator layer). The designs are validated with the simulations and the experimental observations. The experimental measurements are quantitatively compared with the theoretical predictions for a novel ETC microactuator with selective doping.
Electro-thermal (E-T) actuators have been developed to complement the capabilities of electrostatic actuators. The thermal actuators presented here can be arrayed to generate high forces. Equally significant is that the single actuators and arrays of actuators operate at voltages and currents that are directly compatible with standard microelectronics. This paper demonstrates how combinations of two or more electro-thermal actuators can be applied to a variety of basic building-block micromechanical devices: array of ten lateral actuators; array of ten vertical actuators; vertically actuated two-axis tilting mirror; corner-cube retroreflector actuated by lateral array; grippers over the die edge with flip-over wiring; rotary stepper motor; flip-up optical grating on rotary stepper motor; linear stepper motor; and a linear stepper motor for assembly of hinged structures.
Electrothermal responses of attached and suspended lineshape microstructures fabricated by surface micromachining are investigated. A three-dimensional electrothermal model has been established. This model is based on an axial, one-dimensional electrothermal analysis and a heat-conduction shape factor that represents heat transfer perpendicular to the axial direction. Experimental devices have been built by the surface-micromachining process and a 2 μm gap is constructed for suspended microstructures. Electrical power is passed through these microstructures to characterize the electrothermal responses. Bubble-formation experiments which use local electrical heating to generate micro thermal bubbles in a working liquid have been tested by using these microstructures and the experimental results are consistent with the theoretical model.
Structures of very high aspect ratio which are mechanically stiff in the substrate direction and flexible in the direction parallel to the substrate are studied. Such structures can be exploited to produce thermal flexure actuators which are capable of large motion produced by thermal expansion. The magnitude of the deflection depends strongly on geometry and material properties. Structures fabricated by laser micro-machining are characterized and compared to numerical simulations.
A passive micro strain gauge based on the strain magnification
technique has been designed, demonstrated, and characterized. This
strain gauge can be fabricated in situ along with active micro sensors
or actuators on the same chip for monitoring residual strain effects.
Both tensile or compressive strain could be easily observed under
optical microscopes and the resolution of strains as small as 0.001%
could be achieved. The residual strain study of RTA (rapid thermal
annealing) polysilicon is reported
Deep X-ray lithography and metal plating when coupled with a
sacrificial layer, SLIGA, lends itself to the fabrication of very high
aspect ratio metal structures which are mechanically stiff in the
substrate direction and can be very flexible in the direction parallel
to the substrate. These properties can be exploited by producing a
family of new flexure actuators which can produce very significant
motion via thermal expansion and magnetic forces. The magnitude of
thermal effects and magnetic forces are dependent on actuator geometry.
An understanding of each effect allows the design of an actuator which
is dominated by one or both effects. The end result is devices intended
for large motion actuators in microswitch and positioning applications.
They are also useful for material constant measurements of electroplated
metals
The design, fabrication, and characterization of surface
micromachined piezoelectric accelerometers are presented in this paper.
The thin-film accelerometers employ zinc oxide (ZnO) as the active
piezoelectric material, with different designs using either polysilicon
or ZnO bimorph substrates. Sensitivity analyses are presented for two
specific sensor designs. Guidelines for design optimization are derived
by combining expressions for device sensitivity and resonant frequency.
Two microfabrication techniques based on SiO<sub>2</sub> and Si
sacrificial etching are outlined. Techniques for residual stress
compensation in both fabrication processes are discussed. Accelerometers
based on both processes have been fabricated and characterized. A
sensitivity of 0.95 fC/g and resonant frequency of 3.3 kHz has been
realized for a simple cantilever accelerometer fabricated using the
sacrificial SiO<sub>2</sub> process. Sensors fabricated in the
sacrificial Si process with discrete proof masses have exhibited
sensitivities of 13.3 fC/g and 44.7 fC/g at resonant frequencies of 2.23
kHz and 1.02 kHz, respectively
A new type of MEMS microprobe was designed and fabricated which
can be used for a nest generation wafer probe card. A prototype MEMS
probe card consisting of an array of microprobes individually actuated
by bimorph heating to make contact with the test chip was also
fabricated. This probe card is called the CHIPP (Conformable, HIgh-Pin
count, Programmable) card and can be designed to contact up to 800 I/O
pads along the perimeter of a 1-cm<sup>2</sup> chip with a microprobe
repeat distance of approximately 50 μm. Microprobes for a prototype
CHIPP probe card have been fabricated with a variety of cantilever
structures including Al-SiO<sub>2</sub>, W-SiO<sub>2</sub> and Al-Si
bimorphs, and with the resistive heater placed either inside or on the
surface of the cantilever. Ohmic contacts between tips and bond pads
were tested with contact resistance as low as 250 mΩ. The
deflection efficiency varies from 5.23-9.6 μm/mW for cantilever
lengths from 300-500 μm. The maximum reversible deflection is in the
range of 280 μm. The measured resonant frequency is 8.16 kHz for a
50×500 μm device and 19.4 kHz for a 40×300 μm device.
Heat loss for devices operating in air was found to be substantially
higher than for vacuum operation with a heat loss ratio of about 2/1 for
a heater inside the structure, and 4.25/1 for a structure with the
heater as an outer layer of the cantilever
We examine a new class of sensitive and compact passive strain
sensors that utilize a pair of narrow bent beams with an apex at their
mid-points. The narrow beams amplify and transform deformations caused
by residual stress into opposing displacements of the apices, where
vernier scales are positioned to quantify the deformation. An analytical
method to correlate vernier readings to residual stress is outlined, and
its results are corroborated by finite-element modeling. It is shown
that tensile and compressive residual stress levels below 10 MPa,
corresponding to strains below 6×10<sup>-5</sup> can be measured
in a 1.5-μm-thick layer of polysilicon using a pair of beams that are
2 μm wide, 200 μm long, and bent 0.05 radians (2.86°) to the
long axis of the device. Experimental data is presented from bent-beam
strain sensors that were fabricated from boron-doped single crystal
silicon using the dissolved wafer process and from polycrystalline
silicon using surface micromachining. Measurements from these devices
agree well with those obtained by other methods
In order to extract macroscopic mechanical work out of
microelectromechanical systems, we have proposed the concept of
distributed micromotion systems (DMMS). The key idea of DMMS is to
coordinate simple motions of many microactuators in order to perform a
task. Design, fabrication, and operation of a type of DMMS, called a
ciliary motion system, are presented. A bimorph thermal actuator using
two types of polyimides with different thermal expansion coefficients
and a metallic microheater in between them was fabricated. The
cantilever-shaped actuator curled up from the substrate owing to the
residual stress in polyimides which built up during the cooling process
after they were cured at 350°C. It flattened and moved downward by
flowing current in the heater. The dimensions of the cantilever were 500
μm in length, 100 μm in width, and 6 μm in thickness. The tip
of the cantilever moved 150 μm in the direction vertical to the
substrate and 80 μm in the horizontal direction; these were the
maximum displacements obtained with 33 mW dissipated in the heater. The
cut-off frequency was 10 Hz. On a 1-cm-square substrate, 512 cantilevers
were fabricated to form an array. Two sets of cantilevers were placed
opposing to each other. We operated them in coordination to mimic the
motion and function of cilia and carried a small piece of a silicon
wafer (2.4 mg) at 27-500 μm/s with 4-mW input power to each actuator
Making submicron interelectrode gaps is the key to reducing the
driving voltage of a micro comb-drive electrostatic actuator. Two new
fabrication technologies, oxidation machining and a post-release
positioning method, are proposed to realize submicron gaps. Two types of
actuator (a resonant type and a nonresonant type) with submicron gaps
were successfully fabricated and their operational characteristics were
tested experimentally. The drive voltage was found to be lower than that
of existing actuators. The stability of comb-drive actuators is
discussed
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