Writing Quality - Page 28

Atomic Force Microscopy

Despite the fact that the industry checks the writing quality of stampers with the use of CD/DVD analyzers, there are alternative ways with the use of Atomic Force Microscopy! This kind of technology is mostly used not for written media, but mostly during the development of dye layer or for testing the stamper quality.

Pit fidelity is a determining factor for the playability of a disc and the error rate of the disc. The final CDs produced for sale are not free of defects. Defects in the pit surface may be caused by mastering errors, stamper defects, and manufacturing (molding) defects. An elaborate error correction scheme with data redundancy is utilized to allow the compact disc to offer robust performance in the presence of some errors. Atomic Force Microscopy (AFM) is a tool for manufacturers to identify defects and their causes on both the stamper and disc surfaces.

Optical disc manufacturers continue to push for faster cycle times to increase capacity while maintaining disc quality. This creates the need for improved methods to analyze the quality of the disc and stamper surfaces. AFMs are ideally suited to the characterization of nanometer sized pit and bump structures in CD and DVD manufacturing.

AFM provides quantitative, three dimensional imaging of the disc or stamper surface within minutes. Similar quantitative information is possible using SEM or TEM, but these techniques are destructive, time consuming, measure in only two dimensions, and provide a limited field of view. Another major advantage of AFM over other techniques is that once an image is captured, cross sections can be obtained in seconds to provide pit depth, pit width, pit side-wall angle, and track pitch anywhere in the data set - and without physically damaging the disc.

For pit characterization with AFM, the discs may be examined after molding and before metallization. Representative three-dimensional AFM images of CD and DVD stamped discs (replicas) are shown below:

Both images were obtained at the same lateral magnification; the increased pit density and reduced track pitch for DVD are readily apparent. The types of measurements which AFM can provide for CD and DVD characterization are listed in the below table:

CD and DVD measurements readily accessible by AFM. Note that the position at which the width is measured will have to be determined depending on the ability of the tip to measure sidewall angles accurately.

The corresponding images below show changes in the CD pit structure which accompany both the observable (by eye) staining and BLER measurements.

In Figure a, the pit structure is uniform and replicates the stamper accurately.

In Figure b, the pits are locally deformed with polycarbonate piled up or smeared toward the perimeter of the disk. This deformation is manifested as visible staining and can lead to high block error rates depending on degree.

In Figure c, the polymer surface is severely distorted and some of the pits are barely recognizable.

In the lands (surfaces surrounding pits), the polymer appears to have stuck to the stamper during disc ejection leading to severe deformation. Note that the molding conditions used here to demonstrate severe staining are atypical; area [C] in particular represents surface deformations which are rarely seen in actual disc production.

- Manufacturers

- Digital Instruments (http://www.di.com and http://www.veeco.com)
- Advanced Surface Microscopy (http://www.asmicro.com/)