Oct 27, 2015

Plastically deformed Si-crystal wafers for neutron-monochromator elements

Plastically deformed Si-crystal wafers were characterized by monochromatic neutron diffraction. During the cylindrically curved deformation, a resolution-limit Bragg peak changes into a box-type angular profile in accordance with the bulk curvature, associated with an enhancement in the angle-integrated intensity (Iθ). Stacking such wafers is efficient in amplifying Iθ further. We propose an application to neutron-focusing monochromator (or analyzer) crystals in order to design a quite compact spectrometer.

Keywords

Oct 10, 2015

A study on separating of a silicon wafer with moving laser beam by using thermal stress cleaving technique

This study describes the characteristics of separating a silicon wafer with a moving Nd:YAG laser beam by using a thermal stress cleaving technique. The applied laser energy produces a thermal stress that causes the wafer to split along the irradiation path. The wafer separation is similar to crack extension. In this study, the micro-groove was prepared at the leading edge of the silicon wafer to facilitate the fracture. In order to study the thermal effect in the separating process, the temperature at the laser spot was measured by using a two-color pyrometer with an optical fiber, and the mechanism of crack propagation was observed by using an acoustic emission (AE) sensor. The influence of the micro-groove length and depth was also examined. Thermal stress distribution was calculated using the finite-element method (FEM) by considering the temperature from the experimental result. The result indicates that the wafer separation occurred in two stages, fracture initiation and intermittent crack propagation. A higher temperature resulted in faster fracture initiation and higher repetition of the crack propagation signal. The wave mark on the cleaved surface was consistent with the AE signal. The influences of laser power, temperature and the groove parameters to the fracture initiation, crack propagation and cleaved surface features are explained based on the experimental results, while the thermal stress condition is clarified with FEM analysis.

Keywords

  • Thermal stress cleaving
  • Two-color pyrometer
  • Acoustic emission (AE)
  • Silicon wafer;
  • Cleaving temperature
  • Finite-element method (FEM)