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.     

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     

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.     

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.     

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.     

Specimen size effect on tensile strength of surface-micromachinedpolycrystalline silicon thin films

A new tensile tester using an electrostatic-force grip was developed to evaluate the tensile strength and the reliability of thin-film materials. The tester was constructed in a scanning electron microscope (SEM) chamber for in situ observation and was applied for tensile testing of polycrystalline silicon (poly-Si) thin films with dimensions of 30-300 μm long, 2-5 μm wide, and 2 μm thick. It was found that the mean tensile strengths of nondoped and P-doped poly-Si are 2.0-2.8 and 2.0-2.7 GPa, respectively, depending on the length of the specimens, irrespective of the specimen width. Statistical analysis of these size effects on the tensile strength predicted that the location of the fracture origin was on the edge of the specimen, which was Identified by the SEM observation of the fracture surface of the thin films     

Mechanical Properties of Epitaxial 3C Silicon Carbide Thin Films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the     

Fracture of polycrystalline 3C-SiC films in microelectromechanical systems

The fracture of polycrystalline SiC films is investigated using a micrometer-sized fracture tester fabricated by micromachining techniques. A series of SiC cantilever beams varying in length are carried by a moving shuttle tethered to the substrate, and are bent in plane until fracture. The fracture strain of SiC films is calculated from the deflection of bending beams using nonlinear beam theory and determined to be 3.3%/spl plusmn/0.2%, which corresponds to a fracture stress of 23.4/spl plusmn/1.4 GPa. These values are significantly higher than those for polycrystalline silicon. In addition, the crack propagation in the polycrystalline SiC films is observed to be transgranular.     

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.     

Fracture strength of silicon carbide microspecimens

Polycrystalline silicon carbide tensile microspecimens 3.1 mm long were produced by deep reactive ion etching of wafers on the order of 150 /spl mu/m thick. The gage sections, which were nominally 200 /spl mu/m wide, were either straight, slightly curved, or contained double notches in order to vary the size of the highly stressed region. The fracture stresses of 190 specimens from three process runs were measured in a novel test setup. The average local fracture strengths for the last run were: straight 0.38/spl plusmn/0.13 GPa, curved 0.47/spl plusmn/0.15 GPa, notched 0.78/spl plusmn/0.28 GPa. The corresponding Weibull characteristic strengths were, 0.42 GPa, 0.53GPa, and 0.88 GPa with respective moduli 3.3, 3.4, and 3.1. These results show a clear increase in the strength of the material as the size of the highly stressed region decreases. Fractographic analyzes showed failures initiating from the bottoms of side grooves left by the etching process. The grains of the material were quite heterogeneous, varying from a few microns in size to columnar grains through the entire specimen thickness. The curved specimens were used as the base for predicting the probability of failure of the other two shapes. While the Weibull approach was quite accurate for the straight shape, it over-predicted the strengths of the notched specimens. Given the microstructure of the material relative to the size of the specimen, a continuum analysis is questionable.     

Effect of post-release sidewall morphology on the fracture and fatigue properties of polycrystalline silicon structural films

Surface properties can markedly affect the mechanical behavior of structural thin films used in microelectromechanical systems (MEMS) applications. This study highlights the striking difference in the sidewall surface morphology of n⁺-type polysilicon films from two popular MEMS processes and its effect on fracture and fatigue properties. The sidewall surface roughness was measured using atomic force microscopy, whereas silicon oxide thickness and grain size were measured using (energy-filtered) transmission electron microscopy. These measurements show that the oxide layers are not always thin native oxides, as often assumed; moreover, the roughness of the silicon/silicon oxide interface is significantly influenced by the oxidation mechanism. Thick silicon oxides (20 ± 5 nm) found in PolyMUMPs™ films are caused by galvanic corrosion from the presence of gold on the chip, whereas in SUMMiT V™ films a much thinner (3.5 ± 1.0 nm) native oxide was observed. The thicker oxide layers, in combination with differences in sidewall roughness (14 ± 5 nm for PolyMUMPs™ and 10 ± 2 nm for SUMMiT V™), can have a significant effect on the reliability of polysilicon structures subjecting to bending loads; this is shown by measurements of the fracture strength (3.8 ± 0.3 GPa for PolyMUMPs™ and 4.8 ± 0.2 GPa for SUMMiT V™) and differences in the stress-lifetime cyclic fatigue behavior.     

Characterization of Deformation Behaviors and Elastic Moduli of Multilayered Films in Piezoelectric Inkjet Head

A bulge testing system was developed to mechanically characterize the deformation behaviors and elastic moduli of multilayered films, mainly composed of polycrystalline silicon (polysilicon) and lead zirconate titanate (PZT), used in a multilayer actuator of a piezoelectric inkjet head. In the tests, commercial inkjet heads including a few tens of multilayer actuators were directly pressurized by air, and the corresponding deflections were measured via full-field optical measurement techniques. An analytic solution derived from a thin-plate theory and finite-element analysis were used to describe pressure-deflection behaviors of films, and the results were compared with the experimental data to evaluate the elastic modulus of individual film. The results showed that the elastic moduli of polysilicon and PZT films are ~110 and ~49 GPa, respectively. These values were consistent with the nanoindentation results. For polysilicon films, about 30% reduction in elastic modulus, compared with that calculated from single-crystal elastic constants, was observed, and this was most likely attributed to the presence of microdefects like voids and microcracks at grain boundaries between columnar grains.     

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.     

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     

Poisson's Ratio of Low-Temperature PECVD Silicon Nitride Thin Films

In this paper, the Poisson's ratio of low-temperature plasma-enhanced chemical vapor deposited silicon nitride thin films has been determined by a modified double-membrane bulge test. This test method utilizes a square membrane and a large-aspect-ratio rectangular membrane that is fabricated alongside from the same thin film. The Poisson's ratio is determined from the ratio of the bulge deflections of the two membranes under an applied pressure. The method is suitable for determining of either stress-free thin films or those containing low tensile residual stresses. Poisson's ratio values of 0.23 0.02 and 0.25 0.01 were measured for films that were deposited at 125 and 205 , respectively.     

Surface modification of a-Si<Subscript>0.60</Subscript>C<Subscript>0.40</Subscript>:H films by Al-assisted photochemical etching: humidity sensing application

The paper investigates the formation of thin porous amorphous silicon carbide (PASiC) by Al-assisted photochemical etching using HF/AgNO₃ solution under UV illumination at λ = 254 nm. Different etching times varying from 2 to 10 min have been used on thin a-Si_0.60C_0.40:H films, which are elaborated by co-sputtering DC magnetron using a single crystal Si target and who deposited onto 86 of hot pressed polycrystalline 6H-SiC stripes of 12.5 mm³. Because of the high electrical resistivity of the thin a-Si_0.60C_0.40:H film higher than 2 MΩ cm, and in order to facilitate the chemical etching, a thin metallic film of high purity aluminum (Al) has been deposited under vacuum, follow-up of a thin palladium deposited under a grid to reduce attacked surface and reinforced solution etching. The etched surface was characterized by scanning electron microscopy, infrared spectroscopy, spectrophotometer UV, and photoluminescence. Results show that the morphology of etched a-Si_0.60C_0.40:H surface evaluates with etching time and presents a spongy and macroporous layers. Where, the diameter of pore size increases with the increasing etching time. A humidity sensors were fabricated through evaporating coplanar interdigital gold electrodes on PASiC and the humidity sensing properties were tested, it show, that the measured resistance Au-PASiC structure, depends highly on the applied bias voltage. Finally, the sensing performances are attributed to the unique surface structure, morphology of the pore and its size, that provide an effective pathway for vapor transportation and enlarged the sensing area of Au-PASiC.     

阵列化锰铜高压传感器的研制

通过传感器的结构设计、敏感材料和封装材料的研制以及采用新的传感器制备工艺,制作了一种新型的薄膜式锰铜传感器。采用熔融石英材料作为绝缘基板。在绝缘基板上沉积锰铜敏感薄膜。并在敏感薄膜的上面沉积SiO2封装层薄膜。根据"后置"式传感器由阻抗匹配原则,计算出铝靶板中的最高压力为51.68GPa,SiO2封装材料中的压力为35.396GPa。     

Micromechanical elements for detection of molecules and molecular design

Photolithographic preparation of thin films and stacks of them were combined with anisotropic silicon etching and free standing film technology in order to realize three dimensional micro components for studies in detection and optimization of biomolecules. A polymer based SFM sensor was developed and tested in the measurement of thin film roughness and in the detection of holes in molecular films as well as in the detection of single DNA molecules. This novolever shows surprisingly high mechanical stability and provides high resolution SFM images of sensible molecules. Experimental arrangements of miniaturized chemical parallel processing for combinatorial and evolutionary synthesis strategies including silicon micro compartment arrays with free standing optical membranes and thin film filters have been proposed and the manufacturing of micro compartment arrays is described.We thank M. Sossna, B. Rau, H. Porwol, I. Menzel, F. Jahn and W. Schubert for technical assistance. The Inst. of Semiconductor Physics Frankfurt/O is acknowledged for CVD-deposition of membrane films. The work on SFM-sensors was supported by the Max-Planck-Gesellschaft. The work on micro compartment arrays was supported by the BMBF (No 0310713)     

Toward Flexible Thermoelectric Flow Sensors: A New Technological Approach

A new technological approach on thin flexible sensors is presented. As proof of concept, a thermoelectric flow sensor on a 10-mum-thick polyimide foil has been realized. The advantages of silicon as a thermoelectric material and the stability of low-pressure chemical vapor deposition (LPCVD)-silicon nitride as a protective coating are combined with the flexibility of polymer substrates. The thermoelectric flow sensor is fabricated on a standard silicon wafer for handling purposes. Only the functional layers that are embedded in 600 nm of LPCVD-silicon nitride are transferred onto a 10-mum-thick polyimide. The bulk silicon has been removed using deep reactive ion etching. Samples have been fabricated and tested, proving the potential of this new technological concept. The first characterization results show that the sensor layout has to be adapted to the properties of the polymer substrate.     

Piezoelectric micromotors for microrobots

The authors have begun research into piezoelectric ultrasonic motors using ferroelectric thin films. The authors have fabricated the stator components of these millimeter diameter motors on silicon wafers. Ultrasonic motors consist of two pieces: a stator and a rotor. The stator includes a piezoelectric film in which bending is induced in the form of a traveling wave. A small glass lens placed upon the stator becomes the spinning rotor. Piezoelectric micromotors overcome the problems currently associated with electrostatic micromotors such as low torque, friction, and the need for high voltage excitation. More importantly, they may offer a much simpler mechanism for coupling power out. Using thin films of lead zirconate titanate on silicon nitride membranes, various types of actuator structures can be fabricated. By combined new robot control systems with piezoelectric motors and micromechanics, the authors propose creating micromechanical systems that are small, cheap and completely autonomous