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.     

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.     

Fracture Toughness, Fracture Strength, and Stress Corrosion Cracking of Silicon Dioxide Thin Films

The fracture toughness, fracture strength, and stress corrosion cracking behavior of thin-film amorphous silicon dioxide $(hbox{SiO}_{2})$ deposited on silicon wafers via plasma-enhanced chemical vapor deposition have been measured using specimens with length scales comparable to micromachined devices. Clamped–clamped microtensile specimens were fabricated using standard micromachining techniques. These devices exploit residual tensile stresses in the film to create stress intensity factors at precrack tips and stress concentrations at notches, in order to measure fracture toughness and fracture strength, respectively. The fracture toughness of thin-film $hbox{SiO}_{2}$ was $0.77 pm 0.15 hbox{MPa}cdot hbox{m}^{1/2}$, and the fracture strength was $0.81 pm 0.06 hbox{GPa}$. Stress corrosion cracking (slow crack growth) was also measured in the $hbox{SiO}_{2}$ devices with sharp precracks subjected to residual tensile stresses. These data are used to predict lifetimes for a $hbox{SiO}_{2}$-based microdevice. $hfill$[2007-0304]      

Mechanical characterization of polysilicon through on-chip tensile tests

Two new types of on-chip tests have been designed in order to evaluate the elastic Young modulus and the fracture strength of polysilicon used in microelectromechanical systems (MEMS). The former is a pure tension test, while the latter is a single-edge-notched tension test. The actuation in both tests is obtained by means of an ad hoc designed layout of parallel plates capacitors applying sufficiently high forces to reach significant strains in the tensile specimens and complete failure of the notched specimens. The pure tension tests on 20 specimens showed a low dispersion and gave a Young modulus for the polysilicon of 143 GPa. A total of 92 notched specimens were tested up to failure. The experimental results, supported by finite-element simulations, gave a value of the maximum stress for the notched specimens in the range 4144-4568 MPa.     

Micro-Raman measurement of bending stresses in micromachined silicon flexures

Micron-scale characterization of mechanical stresses is essential for the successful design and operation of many micromachined devices. Here we report the use of Raman spectroscopy to measure the bending stresses in deep reactive-ion etched silicon flexures with a stress resolution of /spl sim/10 MPa and spatial resolution of /spl sim/1 /spl mu/m. The accuracy of the technique, as assessed by comparison to analytical and finite-element models of the deformation, is conservatively estimated to be 25 MPa. Implications for the use of this technique in microsystems design are discussed.     

Characterization of selective polysilicon deposition for MEMS resonator tuning

Variations in micromachining processes cause submicron differences in the size of MEMS devices, which leads to frequency scatter in resonators. A new method of compensating for fabrication process variations is to add material to MEMS structures by the selective deposition of polysilicon. It is performed by electrically heating the MEMS in a 25/spl deg/C silane environment to activate the local decomposition of the gas. On a (1.0/spl times/1.5/spl times/100) /spl mu/m/sup 3/, clamped-clamped, polysilicon beam, at a power dissipation of 2.38 mW (peak temperature of 699/spl deg/C), a new layer of polysilicon (up to 1 /spl mu/m thick) was deposited in 10 min. The deposition rate was three times faster than conventional LPCVD rates for polysilicon. When selective polysilicon deposition (SPD) was applied to the frequency tuning of specially-designed, comb-drive resonators, a correlation was found between the change in resonant frequency and the length of the newly deposited material (the hotspot) on the resonator's suspension beams. A second correlation linked the length of the hotspot to the magnitude of the power fluctuation during the deposition trial. The mechanisms for changing resonant frequency by the SPD process include increasing mass and stiffness and altering residual stress. The effects of localized heating are presented. The experiments and simulations in this work yield guidelines for tuning resonators to a target frequency.     

Thermoelastic damping in fine-grained polysilicon flexural beam resonators

The design and fabrication of polysilicon flexural beam resonators with very high mechanical quality factors (Q) is essential for many MEMS applications. Based on an extension of the well-established theory of thermoelastic damping in homogeneous beams, we present closed-form expressions to estimate an upper bound on the attainable quality factors of polycrystalline beam resonators with thickness (h) much larger than the average grain size (d). Associated with each of these length scales is an independent damping mechanism; we refer to them as Zener and intracrystalline thermoelastic damping, respectively. For representative polysilicon beam resonators (h = 2 /spl mu/m; d = 0.1 /spl mu/m) at 300 K, the predicted critical frequencies for these two mechanisms are /spl sim/7 MHz and /spl sim/14 GHz, respectively. The model is consistent with data from the literature in the sense that the measured values approach, but do not exceed, the calculated thermoelastic limit. From the viewpoint of the maximum attainable Q, our model suggests that single-crystal silicon, rather than fine-grained polysilicon, is the material of choice for the fabrication of flexural beam resonators for applications in the gigahertz frequency range.     

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     

Fluidic packaging of microengine and microrocket devices for high-pressure and high-temperature operation

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20-600/spl deg/C and pressures in the range 100-300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600/spl deg/C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested.     

Magnetic MEMS reconfigurable frequency-selective surfaces

A reconfigurable frequency-selective electromagnetic filter implemented by integrating hard magnetic materials with microelectromechanical systems (MEMS) provides a new variation of reconfigurable frequency-selective surfaces (FSS). By incorporating magnetically actuated dipole elements that are capable of being tilted away from the supporting surface, we can tune the FSSs operating frequency without having to physically alter the dimensions of the dipole elements. The 25/spl times/25 array of microactuators used in this work each consist of a 896/spl times/168/spl times/30 /spl mu/m/sup 3/ ferromagnetic plate made of 40Co-60Ni, layered with a 1-/spl mu/m-thick conductor (Au), attached to a pair of 400/spl times/10/spl times/1 /spl mu/m/sup 3/ polysilicon torsion beams, suspended just above the supporting substrate. The high remanent magnetization of the ferromagnetic material allows for relatively small magnetic fields (/spl sim/2.1 kA/m) to induce significant angular deflections (/spl sim/45/spl deg/). This innovative reconfigurable FSS design has successfully demonstrated electromagnetic-signal diplexing and tuning its resonant frequency over a bandwidth of 2.7 GHz at a frequency of 85 GHz.     

Vertical comb array microactuators

A vertical actuator fabricated using a trench-refilled-with-polysilicon (TRiPs) process technology and employing an array of vertical oriented comb electrodes is presented. This actuator structure provides a linear drive to deflection characteristic and a large throw capability which are key features in many sensors, actuators and micromechanisms. The actuation principle and relevant theory is developed, including FastCap simulations for theoretical verification. Design simplifications have been suggested that enable one to use parallel plate analytical expressions which match simulation results with /spl sim/5.6% error. Several actuators were designed and fabricated using the 7-mask TRiPs technology with calculated drive voltages as low as 45 V producing 10 /spl mu/m of deflection. The actuators employed a mechanical structure that was 18 /spl mu/m tall using a polysilicon layer 1.5 /spl mu/m thick and occupying a total area of 750 /spl mu/m by 750 /spl mu/m. The actuators were successfully tested electrostatically and several microns of deflection were observed.     

Polymer-based cantilevers with integrated electrodes

An innovative release method of polymer cantilevers with embedded integrated metal electrodes is presented. The fabrication is based on the lithographic patterning of the electrode layout on a wafer surface, covered by two layers of SU-8 polymer: a 10-/spl mu/m-thick photo-structured layer for the cantilever, and a 200-/spl mu/m-thick layer for the chip body. The releasing method is based on dry etching of a 2-/spl mu/m-thick sacrificial polysilicon layer. Devices with complex electrode layout embedded in free-standing 500-/spl mu/m-long and 100-/spl mu/m-wide SU-8 cantilever were fabricated and tested. We have optimized major fabrication steps such as the optimization of the SU-8 chip geometry for reduced residual stress and for enhanced underetching, and by defining multiple metal layers [titanium (Ti), aluminum (Al), bismuth (Bi)] for improved adhesion between metallic electrodes and polymer. The process was validated for a miniature 2/spl times/2 /spl mu/m/sup 2/ Hall-sensor integrated at the apex of a polymer microcantilever for scanning magnetic field sensing. The cantilever has a spring constant of /spl cong/1 N/m and a resonance frequency of /spl cong/17 kHz. Galvanometric characterization of the Hall sensor showed an input/output resistance of 200/spl Omega/, a device sensitivity of 0.05 V/AT and a minimum detectable magnetic flux density of 9 /spl mu/T/Hz/sup 1/2/ at frequencies above 1 kHz at room temperature. Quantitative magnetic field measurements of a microcoil were performed. The generic method allows for a stable integration of electrodes into polymers MEMS and it can readily be used for other types of microsensors where conducting metal electrodes are integrated in cantilevers for advanced scanning probe sensing applications.     

Mechanical characterization and design of flexible silicon microstructures

A variety of different silicon structures has been fabricated and characterized mechanically to optimize the design of silicon ribbon cables used in neural probes and multichip packaging structures. Boron-doped 3-/spl mu/m-thick silicon beams were tested in three modes: bending in plane, twisting (along beam axis), and pushing. Various cable configurations were investigated (straight beams, curved beams, meandered beams, etc.) as well the effects of length, width, cable termination, and the presence of reinforcing spans between multistranded cables. The results along with finite element modeling indicated that many simple modifications could be made to increase the strength and flexibility of silicon ribbon cables. One structure, a meandered beam 200-/spl mu/m wide and 2-mm long could be twisted up to 712/spl deg/. It also was seen that structures having multiple 20-/spl mu/m-wide beams were generally more robust than those with a single 500-/spl mu/m-wide beam. Finally, a method for easy determination of the bending fracture strain is analyzed and verified. It was seen that the silicon structures tested broke after a strain slightly above 2%.     

A monolithic three-axis micro-g micromachined silicon capacitive accelerometer

A monolithic three-axis micro-g resolution silicon capacitive accelerometer system utilizing a combined surface and bulk micromachining technology is demonstrated. The accelerometer system consists of three individual single-axis accelerometers fabricated in a single substrate using a common fabrication process. All three devices have 475-/spl mu/m-thick silicon proof-mass, large area polysilicon sense/drive electrodes, and small sensing gap (<1.5 /spl mu/m) formed by a2004 sacrificial oxide layer. The fabricated accelerometer is 7/spl times/9 mm/sup 2/ in size, has 100 Hz bandwidth, >/spl sim/5 pF/g measured sensitivity and calculated sub-/spl mu/g//spl radic/Hz mechanical noise floor for all three axes. The total measured noise floor of the hybrid accelerometer assembled with a CMOS interface circuit is 1.60 /spl mu/g//spl radic/Hz (>1.5 kHz) and 1.08 /spl mu/g//spl radic/Hz (>600 Hz) for in-plane and out-of-plane devices, respectively.     

Electroless nickel films: properties and fabricated cavity structure

Properties of 6-/spl mu/m-thick films of electroless, phosphorus-containing nickel grown on evaporated Ni seed layers were measured as a function of bath conditions. The bath variables were temperature, pH, and hypophosphorus acid concentration. Residual mechanical stress at room temperature was measured by a bulge test method, as was elastic modulus. Growth rate, crystallinity, and phosphorus content were also measured. Stress was sensitive to all bath variables, and the range encountered was 4 MPa compressive to 255 MPa tensile. A proper set of nickel growth conditions combined with the use of a stiction-inhibiting parylene-D film led to the successful fabrication of a self-supported roof structure forming a large-area, sealable cavity.     

Encapsulated submillimeter piezoresistive accelerometers

While micromachined accelerometers are widely available and used in various applications, some biomedical applications require extremely small dimensions (相似文献   

Low-voltage, large-scan angle MEMS analog micromirror arrays with hidden vertical comb-drive actuators

This paper reports on novel polysilicon surface-micromachined one-dimensional (1-D) analog micromirror arrays fabricated using Sandia's ultraplanar multilevel MEMS technology-V (SUMMiT-V) process. Large continuous DC scan angle (23.6/spl deg/ optical) and low-operating voltage (6 V) have been achieved using vertical comb-drive actuators. The actuators and torsion springs are placed underneath the mirror (137/spl times/120 /spl mu/m/sup 2/) to achieve high fill-factor (91%). The measured resonant frequency of the mirror ranges from 3.4 to 8.1 kHz. The measured DC scanning characteristics and resonant frequencies agree well with theoretical values. The rise time is 120 /spl mu/s and the fall time is 380 /spl mu/s. The static scanning characteristics show good uniformity (相似文献   

Static and electrically actuated shaped MEMS mirrors

A novel digitally-actuated shaped micromirror for on-off optical switch applications is described. Reflective static spherical mirrors were designed and fabricated using conventional surface micromachining and the MultiPoly process, a technique for depositing multilayers of LPCVD polysilicon in order to control the overall stress and stress gradient. The resulting mirrors were measured to have radii of curvature of approximately 9 mm in agreement with design predictions. Based upon these static mirrors, an actuatable micromirror (diameter=500 /spl mu/m, static radius curvature=6.4 mm) was designed for snap action. This mirror was simulated using an electromechanical coupled-field model and fabricated using the MultiPoly process. Its performance was measured dynamically using an interferometer. A curved-to-flat digital actuation of the mirror was successfully achieved with a pull-in voltage of 38 V.     

VHF single crystal silicon capacitive elliptic bulk-mode disk resonators-part II: implementation and characterization

This work, the second of two parts, reports on the implementation and characterization of high-quality factor (Q) side-supported single crystal silicon (SCS) disk resonators. The resonators are fabricated on SOI substrates using a HARPSS-based fabrication process and are 3 to 18 /spl mu/m thick. They consist of a single crystal silicon resonant disk structure and trench-refilled polysilicon drive and sense electrodes. The fabricated resonators have self-aligned, ultra-narrow capacitive gaps in the order of 100 nm. Quality factors of up to 46 000 in 100 mTorr vacuum and 26000 at atmospheric pressure are exhibited by 18 /spl mu/m thick SCS disk resonators of 30 /spl mu/m in diameter, operating in their elliptical bulk-mode at /spl sim/150 MHz. Motional resistance as low as 43.3 k/spl Omega/ was measured for an 18-/spl mu/m-thick resonator with 160 nm capacitive gaps at 149.3 MHz. The measured electrostatic frequency tuning of a 3-/spl mu/m-thick device with 120 nm capacitive gaps shows a tuning slope of -2.6 ppm/V. The temperature coefficient of frequency for this resonator is also measured to be -26 ppm//spl deg/C in the temperature range from 20 to 150/spl deg/C. The measurement results coincide with the electromechanical modeling presented in Part I.     

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].