Creep behavior of undoped and La–Nb codoped PZT based micro-piezoactuators for micro-optical modulator applications

Time-dependent deformation (creep) behaviors of piezoelectric microactuators have been investigated. Position (or gap height) drift of microbridged actuator beam and its displacement amplitude change with time could be attributed to the anelastic behavior of the driving unit itself based on lead zirconate titanate (PZT) in the actuator, and as well as to the mechanical stress states established in the microactuator beam after the surface release of the micromachined actuator structure. From creep analyses of a simple microcantilever and microbridge beam structures, it was turned out that the overall creep behavior in microbridged piezoactuator structure was mainly dependent on the actuator beam initial deflection configuration governed both by residual stress of the actuating part Pt/PZT/Pt stack and the stress gradient through the silicon nitride (SiNx) microbridge beam.     

Integrated magnetic sensing of electrostatically actuated thin-film microbridges

The movement of electrostatically actuated microbridges is measured by sensing the field of a permanent magnet, deposited and patterned on top of the microbridge, with a spin valve magnetic sensor fabricated beside the bridge at the level of the substrate. The spin valve sensor is sensitive to the position of the magnet and thus to the position of the bridge. The thin-film microbridges are fabricated using thin-film technology and surface micromachining at low temperatures (/spl les/100/spl deg/C) on glass substrates. The bridges are electrostatically actuated by applying a voltage between the bridges and a gate counter electrode placed beneath them. The deflection of the bridge is at the same time characterized optically by focusing a laser on the structure and monitoring the position of the reflected beam with a photodetector. A comparison of the bridge position sensing using the optical and magnetic methods is made. The absolute movement of the structures is measured with a precision close to 0.1 /spl Aring/ using the integrated magnetic sensor. The deflection of the electrostatically actuated structures is studied as a function of the applied gate voltage and length of the bridges. The experimental results show qualitative agreement with an electromechanical model.     

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.     

Characterization of Polycrystalline Silicon-Germanium Film Deposition for Modularly Integrated MEMS Applications

The deposition of in situ boron-doped polycrystalline silicon-germanium (poly-SiGe) films in a conventional low-pressure chemical-vapor deposition reactor has been characterized using the design of experiments method. The dependencies of deposition rate, resistivity, average residual stress, strain gradient, and wet etch rate in hydrogen peroxide solution are presented. Structural layer requirements for general microelectromechanical system applications can be met within the process temperature constraint imposed by complementary metal-oxide-semiconductor (CMOS) electronics. However, residual stress and strain gradient requirements for inertial sensor applications will be difficult to meet with a single homogeneous layer of poly-SiGe that is about 2 mum thick. By correlating stress depth profile measurements with cross-sectional transmission electron microscopy images, we conclude that the large strain gradient is due to highly compressive stress in the lower (initially deposited) region of the film. For films deposited at very low temperature (near the range of amorphous film deposition), in situ boron doping enhances film crystallinity and reduces the strain gradient     

A new technique for producing large-area as-deposited zero-stressLPCVD polysilicon films: the MultiPoly process

Polysilicon films deposited by low-pressure chemical vapor deposition (LPCVD) exhibit tensile or compressive residual stresses, depending on the deposition temperature. Polysilicon films composed of alternating tensile and compressive layers can display any overall stress value between those of the individual layers, including a state of zero overall residual stress, depending on the relative thickness of each layer. The residual stress gradient can be similarly controlled by the layer thicknesses and distribution. This has been demonstrated with a ten-layer near-zero stress (<10 MPa), near-zero stress gradient (⩽0.2 MPa/μm) polysilicon film, containing flat cantilever beams whose length-thickness ratios exceed 150. Using multilayer deposition to control the stresses and stress gradients of polysilicon films is termed the MultiPoly process     

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.     

Determination of residual stress in deep submicron thick films by the critical buckling technique of annular thin plates

Critical buckling of annular plates has been proposed to measure residual stress in thin films with submicron thickness and succeeds in measuring stress ranging from decades of MPa to hundreds of MPa. In this paper, critical buckling of annular plates was further researched in order to enhance their sensitivity, and improved structures were proposed to measure ultra small residual stress in scale of several MPa. Specimens of improved annular structures with thickness down to 200 nm were prepared on silicon-on-insulator wafer, and released in hydrofluoric HF solution to get the critical etching length under an on line monitoring system. The critical etching length was inputted into the ANSYS software to predict the residual stress by the eigenvalue buckling analysis. Small compressive stress of 3.07 MPa within 100 nm film was resolved successfully with this improved annular structure, but the micro rotating structures fabricated in the same die cannot generate a distinguishable deformation.     

Si基Cu/NiFe薄膜的生长及其粘附特性研究

微机械(MEMS)工艺和集成电路(IC)工艺中,在硅(Si)片上电铸高深宽比坡莫(NiFe)合金材料常出现脱落现象.提出了一种电铸NiFe合金材料的新方法,这种方法制作的合金薄膜厚度达200 μm时不脱落.此方法即对等离子刻蚀后的硅片溅射种子层铜(Cu),然后对种子层进行电镀,当其厚度达到约15 μm时,再进行NiFe合金的电铸.本文用扫描电镜、x射线衍射仪和剥离实验研究了薄膜粘附特性.研究结果表明当对种子层电镀后,随着Cu种子层厚度的增加,Cu/NiFe薄膜与基体的粘附强度增加,而薄膜的残余应力降低;同时Cu膜表面粗糙度增加,也增加了NiFe膜与Cu膜的粘附强度.     

薄膜热膨胀系数测试技术研究

热膨胀系数是薄膜的重要热学性能参数,也是薄膜热应力和残余应力计算分析过程中的关键数据.文章基于热诱导弯曲原理,分别采用单基片法和双基片法对氮化钛(TiN)和铝(Al)薄膜的热膨胀系数进行测试,并着重对双基片法的测试误差和适用性进行了分析.研究结果表明,薄膜在不同材质基底上弹性模量的差异是影响双基片法薄膜热膨胀系数测试精度的重要因素.当不同材质基片上薄膜弹性模量差异较小时,双基片法测得的热膨胀系数与单基片法所获结果基本一致;而当不同材质基片上薄膜弹性模量相差较大时,双基片法将不再适用.此外,文章结合薄膜的形貌、结构和残余应力表征测试,对TiN和Al薄膜热膨胀系数与其块体材料的差异进行了分析,结果显示残余压应力会导致薄膜热膨胀系数增大,而残余拉应力则具有相反的效果.     

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.     

Nonlinear response of composite steel-concrete bridges

This paper is concerned with the overload behavior of a composite beam-slab type highway bridge consisting of steel stringers supporting a cast-in-place reinforced concrete slab. A nonlinear finite element analysis is used to determine the complete state of stress and deformation at any level of overload. A layered model together with an incremental iterative procedure is employed to determine the response of the bridge in the post-elastic range. The results of this analysis are compared with available experimental results for the case of a simply supported bridge. The bridge response under overloads is investigated, and the effects of some major design parameters are studied. Among these parameters are: beam size, torsional constant, slab thickness, Poisson's ratio, yield stress of steel girders and the ratio of transverse to longitudinal stiffness of the slab. The results obtained from this computer analysis compare favorably with experimental results in the elastic and the post-elastic range.     

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.     

Calibration of residual stress difference of MetalMUMPs silicon nitride films

The metal multi-user MEMS processes (MetalMUMPs) provide one nickel film, two silicon nitride films and one polysilicon film for constructing various nickel MEMS devices. The two silicon nitride films are either bonded together as a bi-layered structure or they sandwich the polysilicon film to form a tri-layered structure to support nickel structures. The residual stress difference of the two silicon nitride films causes undesired deformations of suspended MetalMUMPs devices. In this paper, the residual stress difference of the two MetalMUMPs silicon nitride thin films is calibrated and the result is 169 MPa. The Young’s modulus of the MetalMUMPs nitride films is also measured, which is 209 GPa.     

Rapid thermal annealing of polysilicon thin films

In comparison with conventional heat treatment, high-temperature rapid thermal annealing (RTA) in a radio frequency (RF) induction-heated system can reduce or eliminate residual stresses in thin films in a few seconds. In this work, changes in the stress level due to the RTA of polycrystalline silicon thin films were studied as a function of annealing time and temperature. The corresponding variations in the microstructure and surface layer of the thin films were experimentally investigated by a variety of analytical tools. The results suggest that the residual stress evolution during annealing is dominated by two mechanisms: 1) microstructure variations of the polysilicon thin film and 2) effects of a surface layer formed during the heat treatment. The fact that the microstructure changes are more pronounced in samples after conventional heat treatment implies that the effects of the formed surface layer may dominate the final state of the residual stress in the thin film     

Process Temperature–Dependent Mechanical Properties of Polysilicon Measured Using a Novel Tensile Test Structure

A new test structure was developed to measure three major unknown mechanical parameters of deposited thin films, i.e., fracture strength, Young's modulus, and residual stress. The structure was designed to have plural specimens of a deposited thin film bridging the gap of the silicon substrate and enables the easy and efficient tensile testing of the film. It was used to measure those parameters of various polysilicon films. Polysilicon is commonly used as a structural material of microelectromechanical systems (MEMS) after being deposited at a temperature below 600 degC and annealed at a temperature around 1000 degC to remove the residual stress. On the other hand, polysilicon can be also deposited at a temperature higher than 600 degC. The three parameters of polysilicon films depend on process temperature and were evaluated using the new test structure. Concerning the strength, films deposited at 560 degC had the highest strength when annealed at 850 degC. Films deposited at 625 degC and annealed at 1050 degC were weaker than those deposited at 560 degC and annealed at 1050 degC. Young's modulus was found to behave in a similar way. The trend of the residual stress was the same as already reported, but its local evaluation was possible in combination with the tensile strength determination     

Microcavity interferometry for MEMS device characterization

We have developed a high resolution optical technique to measure the electromechanical properties of MEMS microstructures. The technique is applied to microbridges developed for capacitive switching in coplanar radio frequency (RF) waveguides. The thin metal ground plane on the substrate and the bottom of the bridge together form a microcavity for an optical beam. The wavelength of a cavity mode is a sensitive measure of the bridge position relative to the substrate. The technique is applied to the measurement of resonances and damping times of microbridges of varying lengths. It is also used to measure dc changes in bridge height of tenths of nanometers, driven ac displacements of less than a picometer, and bridge displacement noise of hundreds of femtometers per root Hertz. This extreme sensitivity exceeds previously demonstrated optical characterization methods.     

Performance of thin film silicon MEMS on flexible plastic substrates

The fabrication and characterization of thin film silicon MEMS microbridges on flexible polyethylene terephthalate substrates are described. Surface micromachining using an aluminum sacrificial layer and a maximum processing temperature of 110 °C was used for device fabrication. These microbridges are electrostatically actuated and their deflection at resonance and at low frequencies is measured optically. Quasi-DC deflection with a quadratic dependence of the actuation voltage is observed, and resonance frequencies up to 2 MHz and quality factors of around 500 are measured in vacuum. Bending measurements are performed by subjecting these devices to tensile and compressive strain. The low frequency response (bridge deflection as a function of the applied voltage) was measured in air before bending and after every bending step. Under tensile strain, 16.6% of the devices survive the maximum bending with a radius of curvature of 1 cm, equivalent to a tensile strain 1.25%. In contrast, for compressive strain, 50% of the devices survive the bending corresponding to a radius of curvature of −0.5 cm, equivalent to a compressive strain of −2.5%. Thin film silicon microresonators on flexible plastic substrates can withstand more compressive strain than tensile.     

Vibration analysis of new Galata Bridge—experimental and numerical results

The new Galata Bridge, which is one of the largest bascule type bridges in the world, was built during the period of 1985–1993 to link both sides of the Golden Horn in Istanbul, replacing old pantoon-type bridge built in 1912 at the same location. Soon after it has been taken into service, some very serious cracks and deformations were noticed on the flaps of bridge in 1998. In this study, the results of experimental stress and vibration studies on one of the flaps are given after the damages are repaired. Moreover, the first natural frequency, higher resonance frequencies and corresponding mode shapes have been obtained by using finite element method and results compared with experiment measurements. These studies unveiled that the first natural frequency of the flap structure is in the range of the hydraulic oil circuit natural frequency that causes resonancy during the expanding operation of flaps, especially when flap opening angle is within a certain range. As the second stage of the study, some design variations are applied to flap structure model and frequency responses are obtained for different design trials. Consequently, some design rules are proposed that should be considered during the design stage of bascule type bridges. On the other hand, results show that several natural frequencies of flaps, particularly during the opening period, are within the earthquake frequency ranges. This fact has been proposed as another important problem especially for this bridge as Istanbul is located near a major earthquake fault.     

Predicting part deformation based on deformation force data using Physics-informed Latent Variable Model

Part deformation prediction and control is a crucial issue for obtaining tight dimensional accuracy so as to ensure product quality with high performance, and deformation prediction is the fundamental of the deformation control. However, existing machining deformation prediction methods are based on the prediction or measurement of residual stress and suffering from two challenges: (i) the measurement accuracy of residual stress field is limited by physical principle and (ii) low prediction in accuracy. In order to address these issues, this paper presents a method for predicting part machining deformation based on deformation force using the proposed Physics-informed Latent Variable Model involved physics knowledge. Deformation force is introduced to represent the inner unbalanced residual stress state of the workpiece, and it is a much easier and more accurate signal compared with residual stress. Machining deformation is predicted by fusing the data-driven method and the prior knowledge of deformation mechanical relationship by taking advantage of the latent variable. The proposed method was verified both in simulation and actual machining environment, and accurate machining deformation prediction has been achieved. The proposed method can be readily extended to the prediction problems involved with difficult-to-measure physical quantities.     

Optical micro-paddle beam deflection measurement for electrostatic mechanical testing of nano-scale thin film application to MEMS

The authors describe their design for a paddle-like cantilever beam sample to relieve non-uniform stress distribution in beam-bending tests of the mechanical properties of thin film applications to MEMS. We added the sample to a custom-designed system equipped with an electrostatic panel and optical interferometer. The system overcomes problems associated with using nano-indentation for testing, and reduces errors tied to the amount of contact force required to bend the beam. Accurate paddle cantilever beam deflection was obtained using a four-step phase-shifting process with a Michelson interferometer. Film strain was determined using a simple force equilibrium equation. Residual stresses were measured at −41.3 MPa for 150 nm silver film, −3.2 MPa for 150 nm gold film, and −16.8 MPa for 150 nm copper film. We observed residual stresses for copper films at different thicknesses. The results indicate high tensile stress forms during the early deposition stage for thin copper film due to grain coalescence, and a decrease in stress with an increase in film thickness. In copper films with thicknesses greater than 153 nm, lattice relaxation associated with the surface mobility of metallic atoms changed residual stress from tension to compression.