The actual data layer comprises glass into
which slots are etched. The reflection layer made of metal ensures that the
disc is readable in any BD or DVD drive.
Production of a glass disc
The production process has more to do with
the production of chips than with the normal burning of a disc. Therefore it is
also carried out in a dust-free room.
1.
Put on photoresistor
A layer of lacquer is applied on the glass
disc.
2.
Allow to harden
The lacquer serves as a data layer and must
first be hardened.
3.
Laser writes data
A laser burns bits into the lacquer. It is
removed in the exposed areas.
4.
Carry out dry etching
The data structure is transferred with the
help of lacquer mask using ion bombardment.
5.
Apply metal sheet
Metal alloy is added by cathode sputtering.
6.
Stick disc
The disc is sealed with a second glass
layer using UV glue.
Memory vault: flash protection
Sandisk offers a 100-year guarantee on the
flash device preserved with corrosion protection. In purely theoretical terms,
it could last for 1,300 years.
Data storage is moving away from discs and
magnetic plates in the direction of solid-state flash in the form of USB
sticks, memory cards and SSDs. These data storage media do not have a good
reputation since flash cells wear off after a certain number of read/write
operations. However, this does not matter in the case of long-term archiving,
when data will be written only a couple of times and then be read much later.
Sandisk has launched a flash memory product for this application called Memory
Vault. In the beginning, it allows the in-built controller to overwrite and
delete data. The Vault switches to a read-only mode after a certain number of
write/delete cycles. Sandisk says nothing about when exactly this happens but
the user will notice it due to the increasingly fast blinking of the in-built
LED light. This mechanism prevents exhaustion of the flash cells, but they are
still exposed to natural decay. The decay rate depends strongly on the storage
temperature.
Sandisk
has launched the Memory Vault in two versions: It costs $49.99 for 8GB and
$89.99 for 16GB.
Flash lasts longest when stored in cool places
A flash cell is principally a transistor
with an additional component: the floating gate which stores electrons. The
voltage created ensures that the electrons drift from the channel through the
isolating oxide gate into the floating gate or out again in case of a deletion
operation. The quantity of electrons stored in this way determines the bit
value of the flash cell. Whether or not electrons tunnel through the oxide gate
is determined by the energy supply. Besides current, even heat can trigger
electrons tunneling even though the flash memory is inactive. Flash lasts very
long at room temperatures before the effect becomes apparent.
In order to test the durability of flash
cells in the Memory Vault, Sandisk has increased the temperature tolerance to
125°C and measured the point at which the flash memory loses its data. It goes
relatively smoothly at this temperature: after almost 350 hours, data security
could no longer be guaranteed for the Vault. The lifespan of 105 years was
established when scaled down at a still-uncomfortable storage temperature of
40°C since the proportion between temperature and lifetime is exponential and
not linear.
Officially, Sandisk advertises this product
with a 100 year lifespan but if one converts the test result into storage at
room temperature (20°C), the Vault should conserve its data for up to 1,300
years. Other external factors play a role in case of these calculations, since
even corrosion affects the circuitry of the Vault. Sandisk has coated the
memory chips with a special additional protection layer. The measures are
reflected in the price: an 8 GBVault costs approx. $51 and the 16GB variant
costs $93 - durability doesn't come very cheaply.
The flash cell as data storage
If voltage is created at the control gate,
electrons drift from the channel in the floating gate and are stored. If you
apply a voltage with reverse polarity, they flow out again.
The
flash cell as data storage
Durability of flash
A test according to the JESD22-A117B
industrial standard determines when flash memory breaks down. It is based on
the fact that the lifespan of a cell decreases exponentially depending on the
temperature.
Durability
of flash
