Data is stored on the rotating platters used in hard drives, which are coated with carefully deposited layers of magnetic storage media. It is due to the rigid nature of these platters that the name hard disk drive originated.
It is common for multiple platters to be mounted onto a single spindle, upon which the data can be stored using both surfaces. The data upon a platter is accessed using a pair of read/write heads, which fly at a specified distance, just above the surface of the recording medium. If these platters are damaged in any way, this will severely increase the complexity of the data recovery, so it is important that they are free of any contaminants.
Common Substrate Material
It is essential that the material used to manufacture hard disk platters is stable and does not interact with the recording medium at the magnetic level. For a long time it was traditional to manufacture these platters from aluminium. However as technology has progressed, allowing higher recording density, the necessity for the read/write heads to fly at lower distances, has required smoother substrates to be used. Glass was initially used, but now many platters are made from a glass and ceramic composite.
These glass and ceramic materials also give an enhanced rigidity while using thinner platters, which allows for more platters to be held in a hard drive. Glass and ceramic substrates also expand less than aluminium at higher temperatures, allowing for higher engineering tolerances.
Layer Deposition & Polishing
Although the platter substrates are highly polished, they are still not smooth enough, so it is essential to deposit a hard Nickel Phosphate layer, which after polishing provides a perfectly smooth surface. The roughness as a result of this process is less than a tenth of a nanometre; which is approximately the size of an atom.
Many different complex layers are deposited upon this base layer, which increases the roughness to about four tenths of a nanometre. This is the maximum permissible roughness allowed for the reliable operation of read/write heads which fly above the platter at a distance of two nanometres.
Data Storage Layers & Protection
The traditional magnetic data storage layers were made from a composite of iron and nickel. Technology has since move to a storage layer which is a cobalt, nickel and iron alloy. This introduction of cobalt allows for a much greater control of the magnetic orientation, while also providing a much better signal to noise ratio.
The magnetic data storage layer is soft, so it is essential that a two nanometre layer of carbon overcoat is deposited. This is applied using either ion-beam or plasma-enhanced chemical vapour deposition techniques. A one nanometre layer of lubricant is then coated over the top. To remove any asperities (microscopic unevenness) it is important for the platters to undergo tape burnish and head burnish processes, which also removes any loose particles.
The final process is to test the platters by placing them in a glide test rig, which certifies that there are no asperities present on the platter surface, which ensures that the read/write heads will not crash into the disk surface.