Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

My CD-ROM discs sometimes contain milky areas when viewed from the readout side. When I call this to the attention of their manufacturer, I am told that the effect is only cosmetic. Sometimes I cannot read such discs. Is it possible that my quality problems are related to this unusual appearance?

Yes, it is very possible that your problems are related to the milky appearance that is called cloudiness. These clouds are also known as stains, ghosts, or sticking. Clouds in optical discs are caused by high concentrations of extremely small defects that cannot be seen individually by the human eye, even with high magnification. Only sophisticated tools such as scanning electron or atomic force microscopes will clearly define these defects.

The presence of one or a few very small defects is not visually observable. Increase the number of defects to millions, and they collectively diffract light to create visible cloudiness in optical discs. An analogy is clouds in the sky that consist of millions of water vapor molecules. One or a few molecules are not visible, but millions of them are collectively observable as clouds in the sky.

Similar effects result in visible clouds in optical discs. Defects regularly occur during the mastering and molding process. Random, isolated defects are easily corrected in the C1 circuitry. Dense defects may generate cloudiness and can cause severe interchange problems.

One source of clouds are the multiple separations required during mastering and molding. The father is first electroplated onto a metallized glass master. After separation, cleaning, and surface passivation, nickel mothers are electroplated onto the father. After separation and surface passivation, nickel stampers are electroplated onto the mother. Stampers are then separated and mounted in the mold cavity, liquid plastic is injected, and the hot disc is separated from the stamper after partial cooling. Any relative motion or sticking during separation will deform millions of pits, resulting in clouds.

These multiple separations are sensitive to pit shape. High aspect ratio pits having steep walls can result in sticking and pit deformation during separation. The resulting defects can be smooth if the hot replica is still plastic, and then may be most severe near the warmer center of the disc. Deformation upon separation can also result in tearing if the replica, stamper, or mother is rigid at the time of separation, such as near the outer rim of the disc. Either type of deformation produces clouds of dense defects that can result in data errors.

Another source of clouds is the electroplating process itself. Impurities in the bath, incorrect current densities, wrong temperatures, poor circulation, and variations in other process parameters can produce patterns of microscopic defects on fathers, mothers, or stampers. These defects are then faithfully reproduced onto every replica.

Defects responsible for clouds can be associated with the pits or lands of replicas, or can be found with CD-R pregrooves or associated land areas. Even if data errors do not directly result from clouds, the defects can interfere with radial tracking or focus servos. Such servo problems are unpredictable, and quality tests may indicate that discs are acceptable while their performance in other drives having different servo designs may be unacceptable. DVD discs are even more susceptible to such defects because of their smaller geometries.

Although minor cloudiness may not generate interchange problems, it is difficult to utilize only visual inspection to differentiate minor cloudiness from major effects that can cause interchange failures. Replicators should establish and maintain mastering and molding processes that do not produce clouds, stains, or ghosts. Cloudiness can and always should be avoided to minimize interchange failures.