An Optical Method For Surface Curvature Testing

Abstract

Introduction and principles of the measurement method Inspection and measurement of surface quality play increasingly an important role in most machining and polishing processes. A typical example is the manufacturing of magnetic disks. The quality of a disk file essentially depends on the surface flatness of the substrate material. For many measurement aspects,large area topography variations are of less interest than high local changes of slope and curvature of the surface to be tested. Mathematically, the surface curvature is expressed as the second derivative of the profile function of the substrate, while the first derivative is known as the slope. Rapid local variations of the slope, that means high curvature values, cause high vertical accelerations of the magnetic head flying over the disk surface in fractions of a micrometer flight-height. Such irregularities of the substrate in the azimuthal disk direction would lead to uncontrolled fluctuations of the air gap between disk and head causing an attenuation of the write /read signal, to head vibrations, or even to a direct contact of the head with the disk (head crash). In the radial direction, the high -speed radial positioning of the head by voice coil driven motors also may cause a head crash at high local changes of the disk slope. Limits of the tolerable head accelerations, found by experience and theoretically by calculations, are listed in manufacturing specifications. For a fast, large area disk quality inspection and evaluation, a compact and highly sensitive measuring method has been developed. A testing tool based on this method displays the test area superimposed with a clear fringe pattern on a TV screen. The fringe pattern represents the surface curvature. From this, both components of the disk curvature, the azimuthal as well as the radial component, can be measured. Coherent optical interference and Moire techniques are the basic principles of the method providing the fringe pattern of the surface area under test. Each fringe interconnects locations of equal surface slopes. Consecutive, iso -slope fringes differ by a constant angular slope increment. The lateral spacing between adjacent iso -slope fringes -or generally the density of fringes -is proportional to the second derivative of the profile function of the disk and thus related to its radius of curvature. Therefore, the map of fringes indicates the areas of low and high head accelerations.The measuring method As shown in Fig. 1, the surface area to be measured is illuminated by two collimated coherent laser beams (wavelength X) at a small angle 0 between their propagation directions. Both beams are reflected at the disk surface. They generate an interference grid pattern in the space where they overlap. At a distance R from the disk (typically 500 mm)/ the interference grid is projected onto an analyzer grid with the same grid constant g = X /(2sin0 /2)