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Lilliputian techniques for measuring the mechanical response of microscale specimens are being developed to characterize the performance and reliability of microelectromechanical systems (MEMS) and other small-scale entities. The challenges associated with the preparation, handling, and testing of small volumes of material have spawned a variety of techniques; this review focuses on uniaxial testing. Results from these experiments provide valuable insight into size-scale effects on the elastic, brittle, and ductile behavior of micron-sized structures. Fundamental elastic interactions show no size effect; in-plane moduli can be predicted from anisotropic elastic constants if crystallographic texture is properly considered. Intrinsic fracture toughness is also size independent, although the fracture strength of brittle MEMS materials is extremely dependent on flaw size and distribution. By contrast, size effects on the strength of ductile materials suggest that the operation of intrinsic dislocation processes in greatly reduced or confined volumes alters their generation, multiplication, interaction, and motion.

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Mechanical design of MEMS requires the ability to predict the strength of load-carrying components with stress concentrations. The majority of these microdevices are made of brittle materials such as polysilicon, which exhibit higher fracture strengths when smaller volumes or areas are involved. A review of the literature shows that the fracture strength of polysilicon increases as tensile specimens get smaller. Very limited results show that fracture strengths at stress concentrations are larger. This paper examines the capability of Weibull statistics to predict such localized strengths and proposes a methodology for design. Fracture loads were measured for three shapes of polysilicon tensile specimens - with uniform cross-section, with a central hole, and with symmetric double notches. All specimens were 3.5 mu m thick with gross widths of either 20 or 50 mu m. A total of 226 measurements were made to generate statistically significant information. Local stresses were computed at the stress concentrations, and the fracture strengths there were approximately 90% larger than would be predicted if there were no size effect (2600 MPa versus 1400 MPa). Predictions based on mean values are inadequate, but Weibull statistics are quite successful. One can predict the fracture strength of the four shapes with stress concentrations to within +or-10% from the fracture strengths of the smooth uniaxial specimens. The specimens and test methods are described and the Weibull approach is reviewed and summarized. The CARES/Life probabilistic reliability program developed by NASA and a finite element analysis of the stress concentrations are required for complete analysis. Incorporating all this into a design methodology shows that one can take "baseline" material properties from uniaxial tensile tests and predict the overall strength of complicated components. This is commensurate with traditional mechanical design, but with the addition of Weibull statistics

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Silicon dioxide thin film is a common component in electronic devices and in MEMS, but its mechanical properties have rarely been studied. Techniques have been adapted and developed to conduct tensile tests on 1.0 μm thick silicon dioxide specimens that are 100, 150, and 200 μm wide and either 1 or 2 mm long. One end of the specimen remains fastened to the substrate, and the other is glued to a silicon carbide fiber attached to a 30 g load cell mounted on a piezoelectric translation stage. Strain is measured by digital imaging of two gold lines applied to the gage section of the transparent specimen. Twenty-five tests yield a Young's modulus of 60.1±3.4 GPa and a fracture strength of 364±57 MPa.

ABSTRACT--Tensile specimens of polysilicon are deposited on a silicon wafer; one end remains affixed to the wafer and the other end has a relatively large paddle that can be gripped by an electrostatic probe. The overall length of the specimen is less than 2 mm, but the smooth tensile portion can be as small as 1.5 x 2 Ixm in cross section and 50 Ixm long. The specimen is pulled by a computer-controlled translation stage. Force is recorded with a 100-g load cell, whereas displacement is recorded with a capacitance-based transducer. Strain can be measured directly on wider specimens with laser-based interferometry from two small gold markers deposited on the smooth portion of the specimen. The strength of this linear and brittle material is measured with relative ease. Young's modulus measurement is more difficult; it can be determined from either the stress-strain curve, the record of force versus displacement or the comparison of the records of two specimens of different sizes. Specimens of different sizes--thicknesses of 1.5 or 3.5 Ixm, widths from 2 to 50 Ixm and lengths from 50 to 500 txmnwere tested. The average tensile strength of this polysilicon is 1.45 • 0.19 GPa (210 + 28 ksi) for the 27 specimens that could be broken with electrostatic gripping. The average Young's modulus from force displacement records of 43 specimens is 162 4-14 GPa (23.5 -4-2.0 • 103 ksi). This single value is misleading because the modulus values tend to increase with decreasing specimen width; that is not the case for the strength. The three methods for determining the modulus agree in general, although the scatter can be large.KEY WORDS--Microelectromechanical systems, Young's modulus, strength, polysilicon, tensile tests Microelectromechanical systems (MEMS) are generally on the order of millimeters or less in size, with features in the micrometer range. Several different materials are used in these tiny devices, but polysilicon is currently the most widely used MEMS material. As its name implies, polysilicon is simply polycrystalline silicon, which is deposited by chemical vapor deposition and can only be produced in layers a few microns thick. Microelectronics fabrication techniques are employed to pattern it into microdevices such as accelerometers and pressure transducers. Figure 1 is a scanning electron microscopy (SEM) photograph of a typical polysilicon MEMS product, which illustrates several important features of MEMS. First, the microdevice is planar, with a complicated shape permitted by the photolithographic processes of the microelectronics industry. Second, it is thin; the active components are 2 txm thick. Third, the supporting arms are long and thin, which makes them amenable to simple analysis.Obviously, one must know the mechanical properties of MEMS materials for intelligent design and life prediction. Furthermore, these should be measured on specimens that are roughly the same size as the microdevices themselves and are manufactured in the same manner. This requires the testing of specimens that have cross-se...