Tensile testing of MEMS materials—recent progress

Tension tests, while standardized for common structural materials, are currently being developed and used for MEMS materials by a small number of researchers. This paper presents recent progress at Hopkins in four areas: Comparison of the tensile test method with different approaches; agreement is found with Young's modulus measurements from membrane tests and with fracture strengths from other tensile tests. Tension-tension fatigue; increased life with decreased applied stress is measured, yielding S-N plots similar to those of metals. Stress versus axial and lateral strain of thick-film silicon carbide; Young's modulus = 420 GPa, Poisson's ratio = 0.21, fracture strength = 0.8 GPa. Polysilicon stress-strain behavior at high temperatures; it deforms inelastically at temperatures above 750°C     

Influence of the frequency on fatigue of directly wafer-bonded silicon

Fatigue tests on directly wafer-bonded silicon samples were performed using pre-cracked Micro-Chevron samples applying cycling loading frequencies between 0.3 and 40 Hz. The experimental lifetime results were compared with a theoretical prediction using measured subcritical crack growth parameters under static loading conditions. The experimental investigations revealed that the number of cycles required to break the samples increased with frequency. In contrast, the corresponding time-to-failure values did not depend on frequency. Both the qualitative behavior and the quantitative life-time results agreed very well with a prediction based on a fracture mechanical model. Therefore, it could be concluded that fatigue behavior in the considered frequency range is solely controlled by stress corrosion in the bonded interface. Furthermore, the results demonstrate an available approach for life-time prediction of wafer-bonded micro-electro-mechanical systems components stressed by cycling loading.     

Fracture strength of polysilicon at stress concentrations

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 /spl mu/m thick with gross widths of either 20 or 50 /spl 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 /spl plusmn/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.     

Investigations of the influence of dicing techniques on the strength properties of thin silicon

Thin silicon offers a variety of new possibilities in microelectronical, solar and micromechanical industries, e.g. for 3D-integration (stacked dies), thin microelectromechanical packages or thin single crystalline solar cells. The wafers in this investigation were thinned back by grinding and subsequent spin etching steps for stress relief followed by separation into single test dies by sawing or etching. In order to characterize and optimize relevant process steps in terms of quality and fabrication yield, the mechanical properties were investigated considering the defect formation and strength. In this paper the influence of three different dicing technologies on the mechanical strength of thin silicon samples was investigated by 3-point bending tests. Sawing, Dicing-by-Thinning with sawn grooves and Dicing-by-Thinning with dry-etched trenches were used as dicing technologies. Analytical and numerical calculations were performed to calculate fracture stresses from fracture forces in 3-point bending tests taking into account the non-linear relationship of force and displacement during testing. Thus the fracture stress as a parameter of strength could be calculated for all tested samples. The results were statistically evaluated by the Weibull distribution based on the weakest link theory. This approach allows a more comprehensive understanding of the influence of the process on strength properties independently of geometric factors. Samples, being separated by “Dicing-by-Thinning”, have much higher strength than simply sawed samples. If trenches are fabricated by dry-etched process the strength can be increased tremendously.