Micromechanical Tensile Testing

A method is described where tensile tests can be performed in situ on micromachined structures. The testing equipment consists of a testing unit mounted on a micromanipulator in a Scanning Electron Microscope (SEM). The fracture loads of micromachined beam structures made from thick and thin film polysilicon as well as from electrodeposited nickel and nickeliron alloy were measured, and the fracture strengths then calculated via measurements of the test structures’ initial cross-sectional areas. The statistical scatter of the polysilicon fracture strength values were evaluated by Weibull statistics. The mean fracture strength and the Weibull modulus, a measure of the scatter, were obtained

Young's Modulus, Yield Strength and Fracture Strength of Microelements Determined by Tensile Testing

Some mechanical properties of thin film microelements, e.g. fracture strength, depend on the manufacturing process, the load application as well as on size and shape of the microelements. Hence, the test structures that are used to determine mechanical properties should have dimensions of the same order of magnitude as an application structure, i.e. microelements must be used to accurately characterise MEMS. The fabrication of test structures must be realised in the same process as an intended application in order to give accurate results. Microelements are easily viewed in an SEM, but to be handled and tested in situ a micromanipulator was developed. Test structures were designed as released beams fixed to the substrate at one end, with a ring at the other. A high-precision testing unit was mounted on the micromanipulator next to the test structures. In the SEM, the testing unit was manoeuvred to grip the ring of the test structure beam and a tensile test of the beam was then executed. From the test data Young's modulus and fracture strength of polysilicon and single crystalline silicon were evaluated. Relative measurement of test structures with different beam lengths enabled Young's modulus to be evaluated with an accuracy of ±5%. Young's modulus was determined to 172±7 GPa for polysilicon and 142±9 GPa for single crystalline silicon in the <100> direction. The fracture surfaces were examined and compared. Young's modulus, yield strength and fracture strength of microelements made from electroplated nickel and nickel-iron alloy were also measured. Young's modulus was evaluated to 231±12 GPa for nickel and 155±8 GPa for nickel-iron alloy composed of 72 at% nickel and 28 at% iron.

Residual stress in sputtered gold films on quartz measured by the cantilever beam deflection technique

With resonator applications in mind, the residual stress in sputtered gold electrodes on quartz has been investigated with respect to various deposition rates (2, 10, and 50 A/s), pressures (1.0 and 3.0 10(-3) mbar), deposition temperatures (80 degrees C and room temperature (RT)), film thicknesses (approx. 400 to 800 A), and substrate smoothnesses (lapped and polished), using the cantilever beam deflection method. Samples were monitored for 4 weeks at room temperature followed by 13 weeks of annealing at 85 degrees C. The initial stress (ranging from -180 to -60 MPa) was compressive for all samples but turned tensile (a few megaPascals) in some of the samples after annealing. A significant decrease in initial compressive stress appeared with samples coated at an elevated temperature. From samples prepared at lower pressure and differing only in film thickness and substrate roughness, an increased compressive stress was found in thicker films and on rougher surfaces. The stress relaxation has been fitted to an exponential expression, and an attempt to relate the stress to a frequency shift (typically a few parts per million for ordinary, 100-mum thick AT blanks) has been made. With the help of transmission electron microscopy (TEM) the film morphology was investigated and related to the deposition parameters and aging. Judging from the increase in compressive stress and grain refinement with increased deposition rate and decreased pressure, the atomic peening mechanism is the most likely reason for the induced stress. Rutherford backscattering spectrometry (RBS) was employed to rule out the inclusion of argon (below or around 0.5%) as an explanation. From the vague, but clearly discernible, trend toward faster RT stress relaxation with higher initial stress, together with the finer film morphology, the relief mechanism is believed to be stress-promoted grain boundary diffusion.

MR patterns of rotator cuff and labral lesions: comparison between low-field and high-field images

Eighteen people (10 men, 8 women) were examined on a 0.3-T (low-field) imager with T1-weighted axial and coronal images and either axial or coronal T2-weighted images. Thirty-two people (22 men, 10 women) were examined on a 1.5-T (high-field) imager with axial T1-weighted images and coronal, sagittal dual-echo images. Rotator cuff lesions were diagnosed by the following findings: abnormal signal in the tendon, irregularity or discontinuity at the musculo-tendinous junction, or muscle abnormality. Glenoid labrum tears were considered if one or more of the following criteria were present: labral deformity of unusual size, focal defect, amputation or displacement with fluid, or capsular stripping. Comparison between the high-field and low-field MR images in the evaluation of rotator cuff lesions suggests that the high-field imager is better than the low-field imager in the differentiation of tendinitis from tears, in the confirmation of bursitis and in detection of subscapularis lesions. A higher contrast-to-noise ratio (CNR), less time consumption, and more section planes (especially, sagittal sections) appear to be the main reasons for this. As far as labral lesions are concerned, the performances of the high-field imagers and low-field imagers were almost equal. The artifact of greater chemical shift with a high-field imager would more or less, we assume, degrade the better CNR achievable in the diagnosis of labral lesions.