Shigeki Nakao

Effect of temperature on fracture toughness in a single-crystal-silicon film and transition in its fracture mode

A change in the fracture mode of a micrometer-sized single-crystal-silicon (SCS) film was observed at a temperature slightly higher than room temperature (RT). Silicon films with a notch on one side were tested under tensile stress at temperatures ranging from RT to 500 °C. The mean fracture toughness was 1.28 MPa at RT and the value remained similar up to 60 °C. However, it drastically increased to nearly twice this value above 70 °C. Differences in the fracture mode and dislocation activity were found by scanning and transmission electron microscopy (SEM and TEM). These results show that the fracture mode changed even at a low temperature near 70 °C due to the activation of dislocations at the notch tip.

Effect of crystal orientation on fracture strength and fracture toughness of single crystal silicon

This paper presents the dependence of fracture toughness, fracture strength, and fracture behavior, such as crack propagation, on the crystal orientation of single-crystal silicon. We conducted on-chip tensile testing to measure fracture strength and fracture toughness of single-crystal silicon films with [100] and [110] surface in the <100> and <110> loading direction. The loading direction had a significant effect on fracture toughness, which was 2.17 MPa/spl radic/m in the <100> direction and 1.27 MPa/spl radic/m in <110>. However, the fracture stress varies with both loading direction and surface orientation. We observed a fracture specimen on which a [111] cleavage plane eventually appeared on any crystal types of the specimen.

A method for measuring the fracture toughness of micrometer-sized single crystal silicon by tensile test

We developed a method of measuring the fracture toughness,-which is a material constant in the macroscopic domain, of single crystal silicon on a micrometer-scale. We notched the thin film specimen on a single edge and then conducted uniaxial tensile test to failure. The average value of measured fracture toughness of specimens on the (100) plane was 1.58 MPa/spl middot/m/sup 1/2/ with scatter. This is slightly higher than, but comparable to, the value for bulk silicon. Scanning electron microscope (SEM) observation of the failed specimens revealed that the fracture developed mainly along the [110] cleavage plane.

Tensile test of single crystal silicon film at elevated temperatures

A tensile-testing system that allows tests at elevated temperature was developed. Using this system, we evaluated the mechanical properties of micro-scale single-crystal silicon film at 573 K. The silicon test specimens had a surface orientation of (100), and a tensile direction of <110>, The average measured Youngs modulus and fracture stress were 144 GPa and 5.19 GPa, respectively, at 573 K. At room temperature, Youngs modulus and fracture stress were 137 GPa and 4.89 GPa. These similar values show that the mechanical properties of single-crystal silicon film hardly change at 573 K compared to room temperature.

Effects of Environmental Condition on the Strength of Submicron-Thick Single Crystal Silicon Film

We developed a quasi-static tensile test system that controls environmental conditions, such as pressure, temperature, and surrounding gasses. Using this system, we evaluated the fracture properties of micron- and submicron-thick single-crystal-silicon film under several conditions. The strength of silicon measured in vacuum or helium was slightly higher than that in laboratory air. We measured the fracture toughness at different temperatures ranging from room temperature (RT) to 500degC and found a brittle-to-ductile transition at 70degC for micron-sized silicon film. The fracture toughness drastically increased at the transition temperature and saturated at a level of 2.5 MParadicm, which is twice the value at RT. On the other hand, submicron-thick silicon was less brittle: its fracture toughness was already 2.7 MParadicm at RT.

Temperature effects on fracture behavior of notched silicon film specimen

We performed tensile tests on single-crystal-silicon film at temperatures ranging from RT (room temperature) to 600/spl deg/C. We made notched specimens by using a focused ion beam (FIB) process. The fracture toughness did not change from room temperature to 60/spl deg/C. However, between 60/spl deg/C and 80/spl deg/C, it rapidly increased to almost double that at room temperature, and it saturated at temperatures higher than 80/spl deg/C. Some specimens tested at 150/spl deg/C and 300/spl deg/C showed the nonlinear relationships between stress and strain. Shapes of the fractures that occurred from 80/spl deg/C to 600/spl deg/C were clearly different from the fracture shape at 60/spl deg/C. These results suggest that dislocation motion occurred even at low temperatures near 80/spl deg/C.