Conoscope: an apparatus for determining crystal orientation of SAW wafers

Conoscopic interference is a commonly used optical technique in polarizing microscopy with applications in areas such as material science, mineralogy and biological studies. The principle is based on the optical birefringence exhibited by various anisotropic crystals. When linearly polarized light is incident on such a crystal at an arbitrary angle, it effectively decomposes into two mutually orthogonal components. These components then travel through the crystal at different phase velocities. Upon exit from the crystal and the recombination of the two components, the transmitted light is elliptically polarized due to the cumulated phase retardation difference: The elliptical polarization can be analyzed by a polarizer. Using a cone of incident light, the resulted families of interference fringes, which are characteristic of the crystal type, its crystal orientation, and sample thickness, can be examined. The conoscope is a low-cost, convenient optical apparatus that exploits this phenomenon. It can be applied to both uncut and diced SAW wafers over a region as small as the dimension of the wafer thickness. In its simplest form, it can be used for verification of SAW substrates from crystal vendors, and for gross identification of crystal types and cut angles of an unknown wafer or a diced chip. With somewhat more elaborate setup, the cut angle of a SAW wafer can be quantitatively determined with good accuracy which approaches what substantially more expensive X-ray machines offer. For example, it will be shown that the cut angle relative to the surface normal of the polished side of the ST family of quartz wafers can easily be determined to an accuracy of 5 minutes. This paper discusses the principle, operation, measurement accuracy, and limitations of a conoscope that has been constructed for use in a production environment. Results from commonly used substrates, including various Rayleigh wave and leaky wave cuts of quartz, lithium niobate and lithium tantalate will be presented.<>