Disk film optical measurement system

Abstract

A method and apparatus for inspecting a reflective surface, or material on such surface, such as lubricant and planarizing layers on a magnetic media storage disk. A beam of controlled polarization impinges obliquely at a spot in the plane of the disk. A collector such as an integrating sphere is spaced away from the disk and has an opening oriented to catch the oblique specular reflectance from the surface. Preferably the opening is substantially larger than the beam, so that disk run out does not send the beam astray or defeat its measurement, and the oblique beam is aimed at an angle lying between the Brewster angle of the lubricant and that of the adjacent layer. A temperature-controlled laser diode with constant-current driver provides a beam that is free of wavelength hops and amplitude changes, making beam aiming repeatable and allowing point-by-point comparisons of the detected reflectance. The disk spins at high speeds on an encoded shaft, and the assembly is radially stepped to allow inspection of the disk at radial and angular positions while a microprocessor synchronized with the spinning disk collects light data to compile a map of surface reflectance. The polarization may be controlled to enhance image quality for different coatings and improve signal discrimination. Simple logic applied to point-by-point inequalities identifies the nature of physical changes responsible for observed effects. Polarization contrast and time difference contrast images allow visualization of defects and changes in substantially transparent layers well under a quarter wave thickness.