The substrate material of which the
platters are made forms the base upon which the actual recording media is
deposited. The media layer is a very thin coating of magnetic material which is where the
actual data is stored; it is typically only a few millionths of an inch in thickness.
Older hard disks used oxide media. "Oxide" really means iron
oxide--rust. Of course no high-tech company wants to say they use rust in
their products, so they instead say something like "high-performance oxide media
layer". 
But in fact that's basically what oxide media is, particles of rust
attached to the surface of the platter substrate using a binding agent. You can actually
see this if you look at the surface of an older hard disk platter: it has the
characteristic light brown color. This type of media is similar to what is used in audio
cassette tape (which has a similar color.)
Oxide media is inexpensive to use, but also has several important shortcomings. The
first is that it is a soft material, and easily damaged from contact by a read/write head.
The second is that it is only useful for relatively low-density storage. It worked fine
for older hard disks with relatively low data density,
but as manufacturers sought to pack more and more data into the same space, oxide was not
up to the task: the oxide particles became too large for the small magnetic fields of
newer designs.
Today's hard disks use thin film media. As the name suggests, thin film media
consists of a very thin layer of magnetic material applied to the surface of the platters.
(While oxide media certainly isn't thick by any reasonable use of the word, it
was much thicker than this new media material; hence the name "thin film".)
Special manufacturing techniques are employed to deposit the media material on the
platters. One method is electroplating, which deposits the material on the platters
using a process similar to that used in electroplating jewelry. Another is sputtering,
which uses a vapor-deposition process borrowed from the manufacture
of semiconductors to deposit an extremely thin layer of magnetic material on the
surface. Sputtered platters have the advantage of a more uniform and flat surface than
plating. Due to the increased need for high quality on newer drives, sputtering is the
primary method used on new disk drives, despite its higher cost.
Compared to oxide media, thin film media is much more uniform and smooth. It also has
greatly superior magnetic properties, allowing it to hold much more data in the same
amount of space. Finally, it's a much harder and more durable material than oxide, and
therefore much less susceptible to damage.
|
A thin film 5.25" platter (above) next to an oxide
5.25" platter (below).
Thin film platters are actually reflective; taking photographs of them
is like trying to take a picture of a mirror! This is one reason why
companies always display internal hard disk pictures at an angle. |
After applying the magnetic media, the surface of each platter is usually covered with
a thin, protective, layer made of carbon. On top of this is added a super-thin lubricating
layer. These material are used to protect the disk from damage caused by accidental
contact from the heads or other foreign matter that might get into the drive.
IBM's researchers are now working on a fascinating, experimental new substance that may
replace thin film media in the years ahead. Rather than sputtering a metallic film onto
the surface, a chemical solution containing organic molecules and particles of iron and
platinum is applied to the platters. The solution is spread out and heated. When this is
done, the iron and platinum particles arrange themselves naturally into a grid of
crystals, with each crystal able to hold a magnetic charge. IBM is calling this structure
a "nanocrystal superlattice". This technology has the potential to increase the areal density capability of the recording media
of hard disks by as much as 10 or even 100 times! Of course it is years away, and will
need to be matched by advances in other areas of the hard disk (particularly read/write
head capabilities) but it is still pretty amazing and shows that magnetic storage still
has a long way to go before it runs out of room for improvement.
Next: Tracks and Sectors
