<title>Characterization Of Optical Disks</title>

Abstract

AbstractOne of the key elements of an optical storage system is the recording medium. An important part to the successful development of media technology are the various techniques for characterization of the media. In this paper, several of the techniques are described. It is shown that useful information concerning the performance of the media can be derived without actual recording and playback. These techniques are obviously important when the recording technology is based on a non-erasable, write-once, read-only type of media.IntroductionThe commercial introduction of the optical videodisc player has stimulated a great aeal of interest in recording systems based on similar optical principles. One of the key elements of such systems is the recording medium (Ref. 1). Many different media have been investigated. Currently the most attractive approaches appear to center around the non­ erasable ablative machining of physical features in thin metallic or organic films. Since the actual recording of data on such an optical disk is by its nature a destructive pro­ cess, techniques for evaluating the performance of the medium, without physically recording on the medium, become extremely important. We describe in this paper several of the techniques we have developed, and some of the experiences we have acquired in using these techniques. These measurements are summarized in Table 1.Table 1. Characterization of Optical Discs.PropertiesMaterial ParametersSubstrate ThicknessSurface ProfileDefectsTypeLocalLocalGlobalLocalResolution10° mm10° mmDisc103 mmMeasurementsOptical constantsInterf erometryDesensitized interf erometryLaser scanningOptical propertiesThe optical properties of the recording material are of interest, because the primary experimentally measured quantities of reflection and transmission affect the signal char­ acteristics, and because the secondary derived quantities of optical constants and film thickness can be used to infer the composition, homogeneity, and uniformity of the deposit­ ed film. Figure 1 shows a schematic diagram of the measurement. Experimentally we measure three quantities, the air incident reflectivity R, the substrate incident reflectivity R1 , and the transmissivity T = T1 , us_ing a laser beam at normal incidence. Given the optical constants of the film material n = (n, k) and the thickness d, the theoretical calcu­ lation of the reflectivities R, R', and the transmissivity T is straight forward (Ref. 2). Thus, in principle, we have a system of three nonlinear relationship,R(n, k, d) = RexR1 (n, k, d) = R1ex (1)T(n, k, d) exfrom which we can solve for the in situ values of three unknowns n, k, and d.Figure 2 shows the experimental measurements for Te at the 633 nm and 442 nm wave­ lengths. Here the thicknesses d are the mass thicknesses measured on a quartz crystal thickness monitor, and d . are che optical thicknesses obtained from solving Eq. (1). The optical constants are ^given in Figure 3. By measuring R, R1 , and T, solving for n, k, and d, we can obtain in situ information about film thickness and deposition quality.