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CD-ROM Manufacturing

The term premastering is often used quite broadly, to refer to all of the steps which proceed the first stage in the manufacturing process proper. Strictly, data preparation, indexing, testing (also called simulation), and creating the image are actually performed before premastering. The encoding performed at this stage is usually referred to as ISO-level encoding. This disc image then undergoes what is known as the CD-level encoding, or final encoding. Basically, this involves taking the image data and merging it with other coding – which adds sector addresses, header, synchronisation, error correction and detection, mode and other required bytes – to produce a single ISO 9660 volume file. This is the premastered file that is used in the production process. The hardware and software used for premastering – usually referred to as ISO formatters – vary in capabilities and features. In the past, it was common for premastered image files to be transferred to mastering plants via some form of tape media. The advent of CD-R drives and media brought the entire premastering process within the scope of the home PC user and the premastered source that drives the manufacturing process is now often a CD-R disc.

Since glass substrate blanks are recycled, the first stage in the manufacturing process is to strip the old photoresist from the surface of the approximately 10in diameter, 6mm thick glass plate. This is followed by cleaning and final washing using de-ionised water. After drying, polishing and coating with a primer, a liquid layer of photoresist is spin coated evenly onto the glass. When this has been baked dry, the so-called glass master is ready for exposing by laser light.

The glass master is placed into a Laser Beam Recorder (LBR) that is connected to a computer with access to the premastered source. The CD-ROM image is read from the source and recorded to the photoresist on the glass master using a laser. The laser beam – blue or violet for CD-ROM mastering – is modulated by an electromagnetic current to expose the photoresist where pits should be while the glass master spins at exactly the correct linear velocity and is moved gradually and smoothly to maintain the correct track pitch and linear velocity. The photoresist dissolves where the laser hits, creating pits in the glass. The data is recorded from the centre of the glass master in a spiral track that goes towards the outside of the glass master.

After recording, the glass master is developed by spinning a sodium hydroxide solution over the glass master to remove the photoresist exposed by the laser, creating pits in the surface. These pits should extend right through the photoresist to the glass underneath to achieve good pit geometries as specified in the Red Book. The glass itself is unaffected by this process and acts merely as a carrier for the photoresist. The developed glass master is then placed into a vacuum chamber and a small amount of silver is deposited (only molecules thick) onto the surface of the photoresist. The developed glass master coated with silver is called the metalised glass master.

In a process known as electroforming, the metalised glass master has a layer of nickel grown onto its silver surface by immersion in a tank of nickel sulphamate solution. This sheet of nickel – referred to as the father – is subsequently removed from the silver. The father is a reverse image of the data and could be used to stamp discs. However, it is not. Instead, the father is returned to the electroforming tank where another layer of nickel is grown and subsequently removed to create a mother. The mother undergoes the same process to produce a stamper (sometimes referred to as a son). Several stampers can be grown from the same mother.

CD

The stampers are the forms used to mould the pit structure onto the CD’s polycarbonate layer, which is then coated with aluminium and the final acrylic layer. However, a stamper requires finishing – an important stage in the manufacturing process as it can impact the quality of the final discs – before any discs are actually replicated from it. Before being fitted to the press each stamper is carefully checked visually, the back polished, it is punched to the required outside diameter size, a centre hole is accurately punched and it is checked on a device known as a stamper player. Given the speed of modern CD-ROM drives, the accuracy of the positioning of the centre hole is particularly important. Similarly, stamper thickness must be uniform to avoid unbalance problems in the finished discs. Finished stampers are stored in protective plastic packages ready to be fitted to a moulding machine.

Injection moulding is a common industrial process to produce plastic products of all shapes. CD-ROM injection moulding machines fitted with the appropriate stampers, stamp or press the a CD’s molten polycarbonate material. The replicate (also known as substrate) is allowed to cool before it is moved for the application of an aluminium coating and finally, a layer of protective acrylic lacquer.

In 1990 a couple of problems with the pressed CD manufacturing process came to light. A CD is read by having laser light in the reader reflect back from the reflective layer in the CD. The first issue concerned whether the lacquer coating fully covered the edges of the CD. If it did not, then oxidation attacked the reflectivity layer and caused some data reading problems. The second issue was that some labelling inks used in the silk-screening process were chemically active even after UV curing, and this activity interfered with the reflective layer, causing readback problems. Both problems were quickly solved by changes to the CD manufacturing processes.

Filed Under: CD-ROM

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