Media Manufacturing

Media Manufacturing

An ideal storage disk will have no imperfections and store data for an indefinite amount of time.
The latter is theoretically achievable with today's magnetic materials assuming the environment within the HDA does not change - that there is no contamination or malfunction of components within the disk enclosure.
The former is not possible today but is often manageable.

The data is recorded onto the disk from signals emanating from the head transducer. A recorded region is called a bit cell. For an areal density of 1Gb/sq inch, the size of a bit cell is 1 billionth of a square inch. A 3.5-Inch - 1Gb/sq. inch disk can have in excess of 4 billion bit cells per surface.
In order to achieve these staggering numbers, strict manufacturing processes and advances in magnetic materials are required.

A disk is defined as a thin film medium. That is multiple thin film layers of various materials are deposited onto the disk through a sputtering process. The general structtire of a disk is shown below. The substrate on which the magnetic and other materials are deposited is always aluminum for 3.5-Inch HDD's (glass substrates are used for 2.5-Inch drives to improve shock characteristics).

Prior to material deposition which is performed in a clean room environment, the aluminum substrate is machined sized and ground to an acceptable finish. As can be seen in the diagram multiple layers of various materials are required to manufacture disks.

Each step of this process can introduce imperfections in the media. An imperfection the size of a bit cell will therefore prevent the cell from having the appropriate magnetic properties, thus a media defect is created. It is possible that the imperfection may encompass more than one bit cell and cause additional media defects.

Another source of media defects is through the HDD manufacturing process. Although extreme precautions are taken in the handling of disks, some microscopic scratches can occur as disks are mounted onto the motor spindle hubs and the head stack assemlies are merged onto the media.
In certain cases contamination during the drive build process can also cause particles to be deposited onto the media and cause latent defects.

Although defects are inevitable, HDD test processes can map them out prior to leaving the factory. However, some latent defects may not be detectable initially and may translate into inaccessible sectors in the field.

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