Development of AFM tensile test technique for evaluating mechanical properties of sub-micron thick DLC films

This paper describes mechanical properties of submicron thick diamond-like carbon (DLC) films used for surface modification in MEMS devices. A new compact tensile tester operating under an atomic force microscope (AFM) is developed to measure Young's modulus, Poisson's ratio and fracture strength of single crystal silicon (SCS) and DLC coated SCS (DLC/SCS) specimens. DLC films with a thickness ranging from 0.11 /spl mu/m to 0.58 /spl mu/m are deposited on 19-/spl mu/m-thick SCS substrate by plasma-enhanced chemical vapor deposition using a hot cathode penning ionization gauge discharge. Young's moduli of the DLC films deposited at bias voltages of -100 V and -300 V are found to be constant at 102 GPa and 121 GPa, respectively, regardless of film thickness. Poisson's ratio of DLC film is also independent of film thickness, whereas fracture strength of DLC/SCS specimens is inversely proportional to thickness. Raman spectroscopy analyses are performed to examine the effect of hydrogen content in DLC films on elastic properties. Raman spectra reveal that a reduction in hydrogen content in the films leads to better elastic properties. Finally, the proposed evaluation techniques are shown to be applicable to sub-micron thick DLC films by finite element analyses.     

Wafer-level mechanical characterization of silicon nitride MEMS

The mechanical and physical properties of silicon nitride thin films have been characterized, particularly for their application in load-bearing MEMS applications. Both stoichiometric (high-stress) and silicon-rich (low-stress) films deposited by LPCVD have been studied. Young's modulus, E, has been determined using conventional lateral resonators and by bulge testing of membranes, and tensile strength has been determined using a specially designed microtensile specimen. All microdevices have been fabricated using standard micromachining. We have also measured the thermal expansion coefficient of stoichiometric silicon nitride. Our best estimate of E is 325/spl plusmn/30 GPa for stoichiometric and 295/spl plusmn/30 GPa for silicon-rich silicon nitride. The average tensile strength for the stoichiometric material is 6.4/spl plusmn/0.6 GPa, while that for the silicon-rich material is 5.5/spl plusmn/0.8 GPa; the burst strength of membranes of the stoichiometric material is 7.1/spl plusmn/0.2 GPa.     

Measurements of material properties using differential capacitive strain sensors

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

Mechanical property characterization of LPCVD silicon nitride thin films at cryogenic temperatures

T-shape, LPCVD silicon nitride cantilevers are fabricated to determine Young's modulus and fracture strength of silicon nitride thin films at room and cryogenic temperatures. A helium-cooled measurement setup is developed and installed inside a focused-ion-beam (FIB) system. A lead-zirconate-titanate (PZT) translator powered by a function generator and a dc voltage is utilized as an actuator, and a silicon diode is used as a temperature sensor in this setup. Resonant frequencies of identical cantilevers with different "milling masses" are measured to obtain thickness and Young's modulus of the silicon nitride thin films, while a bending test is performed to obtain fracture strength. From the experiment, the average Young's modulus of low-pressure chemical-vapor deposition (LPCVD) silicon nitride thin films varies from 260.5 GPa at room temperature (298 K) to 266.6 GPa at 30 K, and the average fracture strength ranges from 6.9 GPa at room temperature to 7.9 GPa at 30 K. The measurement setup and technique presented here can be used to characterize the mechanical properties of different MEMS materials at cryogenic temperatures.     

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.     

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

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

The critical role of environment in fatigue damage accumulation in deep-reactive ion-etched single-crystal silicon structural films

The importance of service environment to the fatigue resistance of n/sup +/-type, 10 /spl mu/m thick, deep-reactive ion-etched (DRIE) silicon structural films used in microelectromechanical systems (MEMS) was characterized by testing of electrostatically actuated resonators (natural frequency, f/sub 0/, /spl sim/40 kHz) in controlled atmospheres. Stress-life (S-N) fatigue tests conducted in 30/spl deg/C, 50% relative humidity (R.H.) air demonstrated the fatigue susceptibility of silicon films. Further characterization of the films in medium vacuum and 25% R.H. air at various stress amplitudes revealed that the rates of fatigue damage accumulation (measured via resonant frequency changes) are strongly sensitive to both stress amplitude and, more importantly, humidity. Scanning electron microscopy of high-cycle fatigue fracture surfaces (cycles to failure, N/sub f/>1/spl times/10/sup 9/) revealed clear failure origins that were not observed in short-life (N/sub f/<1/spl times/10/sup 4/) specimens. Reaction-layer and microcracking mechanisms for fatigue of silicon films are discussed in light of this empirical evidence for the critical role of service environment during damage accumulation under cyclic loading conditions.     

Fracture Properties of Silicon Carbide Thin Films by Bulge Test of Long Rectangular Membrane

This paper reports the mechanical properties and fracture behavior of silicon carbide (3C-SiC) thin films grown on silicon substrates. Using bulge testing combined with a refined load-deflection model of long rectangular membranes, which takes into account the bending stiffness and prestress of the membrane material, the Young's modulus, prestress, and fracture strength for the 3C-SiC thin films with thicknesses of 0.40 and 1.42 mum were extracted. The stress distribution in the membranes under a load was calculated analytically. The prestresses for the two films were 322 plusmn 47 and 201 plusmn 34 MPa, respectively. The thinner 3C-SiC film with a strong (111) orientation has a plane-gstrain moduli of 415 plusmn 61 GPa, whereas the thicker film with a mixture of both (111) and (110) orientations exhibited a plane-strain moduli of 329 plusmn 49 GPa. The corresponding fracture strengths for the two kinds of SiC films were 6.49 plusmn 0.88 and 3.16 plusmn 0.38 GPa, respectively. The reference stresses were computed by integrating the local stress of the membrane at the fracture over edge, surface, and volume of the specimens and were fitted with Weibull distribution function. For the 0.40-mum-thick membranes, the surface integration has a better agreement between the data and the model, implying that the surface flaws are the dominant fracture origin. For the 1.42-mum-thick membranes, the surface integration presented only a slightly better fitting quality than the other two, and therefore, it is difficult to rule out unambiguously the effects of the volume and edge flaws. [2007-0191].     

Mechanical properties of thin films from the load deflection oflong clamped plates

A plane-strain load-deflection model for long plates clamped to a rigid support is developed. The analytical model describes the nonlinear deflection of plates with compressive or tensile residual stress and finite flexural rigidity under uniform load. It allows for the extraction of the residual stress and plane-strain modulus of single-layered thin films. Properties of compressively and weakly prestressed materials are extracted with an accuracy achieved previously only with tensile samples. Two approximations of the exact model are derived. The first reduces the plates to membranes by neglecting their flexural rigidity. Considerable errors result from this simplification. The second approximation provides an exact expression for the linear plate response. Using the model, mechanical properties were extracted from two plasma-enhanced chemical-vapor deposition (PECVD) silicon nitride films with weakly tensile and compressive prestress, respectively. Measured residual stresses are 1.3±3.8 and -63±12.4 MPa, respectively. Corresponding plane-strain moduli are 134.4±3.9 and 142±2.6 GPa, respectively     

Fabrication and measured performance of a first-generation microthermoelectric cooler

The measured performance of a column-type microthermoelectric cooler, fabricated using vapor-deposited thermoelectric films and patterned using photolithography processes, is reported. The columns, made of p-type Sb/sub 2/Te/sub 3/ and n-type Bi/sub 2/Te/sub 3/ with an average thickness of 4.5 /spl mu/m, are connected using Cr/Au/Ti/Pt layers at the hot junctions, and Cr/Au layers at the cold junctions. The measured Seebeck coefficient and electrical resistivity of the thermoelectric films, which were deposited with a substrate temperature of 130/spl deg/C, are -74 /spl mu/V/K and 3.6/spl times/10/sup -5/ /spl Omega/-m (n-type, power factor of 0.15 mW/K/sup 2/-m), and 97 /spl mu/V/K and 3.1/spl times/10/sup -5/ /spl Omega/-m (p-type, power factor of 0.30 mW/K/sup 2/-m). The cooling performance of devices with 60 thermoelectric pairs and a column width of 40 /spl mu/m is evaluated under a minimal cooling load (thermobuoyant surface convection and surface radiation). The average cooling achieved is about 1 K. Fabrication challenges include the reduction of the column width, implementation of higher substrate temperatures for optimum thermoelectric properties, and improvements of the top connector patterning and deposition.     

MEMS中Ni-W合金薄膜力学性能的研究

研究了柠檬酸胺-1-羟基乙烷二膦酸（HEDP）镀液体系中Ni-W的力学性能。通过紫外曝光的光刻、电铸和注塑（UV—LIGA）技术制备出微拉伸试样和单轴微拉伸测试系统进行拉伸试验。结果表明：在Ni-W薄膜试样尺寸为5μm×50μm×100μm条件下,其杨氏模量约为100．4GPa,抗拉强度为1．96GPa,应变约为3．6％。     

Arrays of monocrystalline silicon micromirrors fabricated using CMOS compatible transfer bonding

In this paper, we present CMOS compatible fabrication of monocrystalline silicon micromirror arrays using membrane transfer bonding. To fabricate the micromirrors, a thin monocrystalline silicon device layer is transferred from a standard silicon-on-insulator (SOI) wafer to a target wafer (e.g., a CMOS wafer) using low-temperature adhesive wafer bonding. In this way, very flat, uniform and low-stress micromirror membranes made of monocrystalline silicon can be directly fabricated on top of CMOS circuits. The mirror fabrication does not contain any bond alignment between the wafers, thus, the mirror dimensions and alignment accuracies are only limited by the photolithographic steps. Micromirror arrays with 4/spl times/4 pixels and a pitch size of 16 /spl mu/m/spl times/16 /spl mu/m have been fabricated. The monocrystalline silicon micromirrors are 0.34 /spl mu/m thick and have feature sizes as small as 0.6 /spl mu/m. The distance between the addressing electrodes and the mirror membranes is 0.8 /spl mu/m. Torsional micromirror arrays are used as spatial light modulators, and have potential applications in projection display systems, pattern generators for maskless lithography systems, optical spectroscopy, and optical communication systems. In principle, the membrane transfer bonding technique can be applied for integration of CMOS circuits with any type of transducer that consists of membranes and that benefits from the use of high temperature annealed or monocrystalline materials. These types of devices include thermal infrared detectors, RF-MEMS devices, tuneable vertical cavity surface emitting lasers (VCSEL) and other optical transducers.     

A new technique for measuring the mechanical properties of thinfilms

Accurate measurement of mechanical properties is very difficult for films that are only a few microns thick. Previously, these properties have been determined by indirect methods such as cantilever beam and diaphragm bulge tests. This paper presents a new technique to measure the Young's modulus of thin films in a direct manner consistent with its definition. Strain is measured by a laser-based technique that enables direct and accurate recording of strain on a thin-film specimen. Load is recorded with a 1-lb load cell, and an air bearing is used to eliminate friction in the loading system. The specimen is phosphorus-doped polysilicon that has a gage cross section of 3.5 μm thick by 600 μm wide. All 29 uniaxial tensile tests show brittle behavior, and the average values of Young's modulus and fracture strength are measured to be 170±6.7 GPa and 1.21±0.16 GPa, respectively. One fatigue test is also reported in this paper     

Silicon-on-insulator microfluidic device with monolithic sensor integration for /spl mu/TAS applications

A novel concept for the integration of liquid phase charge sensors into microfluidic devices based on silicon-on-insulator (SOI) technology is reported. Utilizing standard silicon processing we fabricated basic microfluidic cross geometries comprising of 5-10-mm-long and 55-/spl mu/m-wide channels of 3 /spl mu/m depth by wet sacrificial etching of the buried oxide of an SOI substrate. To demonstrate the feasibility of fluid manipulation along the channel we performed electroosmotic pumping of a dye-labeled buffer solution. At selected positions along the channel we patterned the 205-nm thin top silicon layer into freely suspended, 10-/spl mu/m wide bars bridging the channel. We demonstrate how these monolithically integrated bars work as thin-film resistors that sensitively probe changes of the surface potential via the field effect. In this way, a combination of electrokinetic manipulation and separation of charged analytes together with an on-chip electronic detection can provide a new basis for the label-free analysis of, for example, biomolecular species as envisaged in the concept of micrototal analysis systems (/spl mu/TAS) or Lab-on-Chip (LOC).     

Two-axis single-crystal silicon micromirror arrays

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

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.     

Microfabrication of palladium-silver alloy membranes for hydrogen separation

In this paper, a process for the microfabrication of a wafer-scale palladium-silver alloy membrane (Pd-Ag) is presented. Pd-Ag alloy films containing 23 wt% Ag were prepared by co-sputtering from pure Pd and Ag targets. The films were deposited on the unetched side of a <110>-oriented silicon wafer in which deep grooves were etched in a concentrated KOH solution, leaving silicon membranes with a thickness of ca. 50 /spl mu/m. After alloy deposition, the silicon membranes were removed by etching, leaving Pd-Ag membranes. Anodic bonding of thick glass plates (containing powder blasted flow channels) to both sides of the silicon substrate was used to package the membranes and create a robust module. The hydrogen permeability of the Pd-Ag membranes was determined to be typically 0.5 mol H/sub 2//m/sup 2//spl middot/s with a minimal selectivity of 550 for H/sub 2/ with respect to He. The mechanical strength of the membrane was found to be adequate, pressures of up to 4 bars at room temperature did not break the membrane. The results indicate that the membranes are suitable for application in hydrogen purification or in dehydrogenation reactors. The presented fabrication method allows the development of a module for industrial applications that consists of a stack of a large number of glass/membrane plates.     

Magnetic induction machines integrated into bulk-micromachined silicon

This paper presents the design, fabrication, and characterization of laminated, magnetic induction machines intended for high-speed, high-temperature, high-power-density, silicon-based microengine power generation systems. Innovative fabrication techniques were used to embed electroplated materials (Cu, Ni/sub 80/Fe/sub 20/, Co/sub 65/Fe/sub 18/Ni/sub 17/) within bulk-micromachined and fusion-bonded silicon to form the machine structures. The induction machines were characterized in motoring mode using tethered rotors, and exhibited a maximum measured torque of 2.5 /spl mu/N/spl middot/m.     

Piezoelectric Al/sub 0.3/Ga/sub 0.7/As longitudinal mode bar resonators

This paper reports the modeling, fabrication, and experimental characterization of piezoelectric longitudinal mode bar resonators based on thin film single crystal Al/sub 0.3/Ga/sub 0.7/ As. Fabricated resonators with lengths ranging from 1000 /spl mu/m to 100 /spl mu/m have been characterized for operation in their first five odd longitudinal modes. Resonance frequencies range from 2.5 to 75 MHz, with quality factors up to 25 390 at 21.8 MHz in vacuum. Power handling capacity as high as -2.6 dBm is demonstrated at 18.8 MHz. Motional resistance and temperature stability of the resonators are also evaluated.     

Magnetic composite electroplating for depositing micromagnets

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