Development of a geometrical evaluation apparatus for ultrahigh 100 GB optical disk masters

We report on the development of a prototype system for the automatic characterization of the pit shape in optical master disks (OMDS) and its relationship with the corresponding optical readout signal. The system consists of two basic components: an optical disk drive tester (ODDT) which uses a laser beam pickup to convert physical pits on the master disk into electrical signals and an integrated atomic force microscope (AFM). In this system, the ODDT scans the OMD and records the positions of errors. Using these recorded positions, the integrated tapping-mode AFM unit then automatically positions the AFM head to the corresponding locations on the OMD and initiates a scan which serves to record topographical information of pit shape in the error regions. In an initial trial, the system was used to automatically detect and record topographical information on randomly written error patterns ( 1 μ m in length) recorded on a 12 cm diam optical disk. For each identified region, 50 μ m square AFM scans were then automatically recorded. The system thus allows the direct observation of the correlation between errors detected in the optical readout signal and the topology of the corresponding pit on the nanometer scale. As high reliability of the AFM unit is a prerequisite for automatic operation, a carbon nanotube based probe was developed and proved to be much more reliable than conventional Si based cantilevers.