Platters are typically made using an aluminium orglass and ceramic substrate. In disk manufacturing, a thin coating is deposited on both sides of the substrate, mostly by a vacuum deposition process called magnetron sputtering. The coating has a complex layered structure consisting of various metallic (mostly non-magnetic) alloys[clarification needed] as underlayers optimized control of crystallographic orientation and grain size of the actual magnetic media layer on top of them, i.e. the film storing the bits of information. On top of it a protective carbon-based overcoat is deposited in the same sputtering process. In post-processing a nanometer thin polymeric lubricant layer gets deposited on top of the sputtered structure by dipping the disk into a solvent solution, after which the disk is buffed by various processes[clarification needed] to eliminate small defects and verified by a special sensor on a flying head for absence of any remaining impurities or other defects (where the size of the bit given above roughly sets the scale for what constitutes a significant defect size). In the hard disk drive the hard drive heads fly and move radially over the surface of the spinning platters to read or write the data. Extreme smoothness, durability, and perfection of finish are required properties of a hard disk platter.
In 2005-2006 a major shift in technology of hard disk drives and of magnetic disks/media began. Originally, in-plane magnetized materials have been used to store the bits but perpendicular magnetization is now taking over. (see perpendicular recording).
The reason for this transition is the need to continue the trend of increasing storage densities, with perpendicularly oriented media offering a more stable solution for a decreasing bit size. Orienting the magnetization perpendicular to the disk surface has major implications for the disk's deposited structure and the choice of magnetic materials, as well as for some of the other components of the hard disk drive (head, electronic channel, etc.).