circuits are DRAMs, PROMs, EPROMs, flash
EEPROMs, and ROMs.
Dynamic random access memory devices (DRAMs)
store information dynamically, that is, as a charge on a
capacitor. These designs feature one field-effect tran-
sistor (FET) to assess information for both reading and
writing and a thin-film capacitor for information
storage. Most nonvolatile cells rely on trapped charge
stored on the floating gate of the FET. These units can
be rewritten many times, the limit being determined by
programming stress-induced degradation of the dielec-
tric. Erasure of the charge from the floating grid is
accomplished by tunneling or by exposure to ultraviolet
light.
DRAMs are volatile, the average memory is about
10 years. Programmable memories can be programmed
at least once and some can be programmed a million
times. A few nonvolatile memories are programmable
just once. These have an array of diodes or transistors
with fuses or antifuses in series with each semicon-
ductor cross point.
Electrically programmable read only memory
devices (EPROMs) are usually used to describe cells
that are electronically written and UV erased. EEPROM
is probably the most common technology used. Static
random access memory devices (SRAMs) are some-
times connected to EEPROM for storage when power is
removed. Flash EPROMs require bulk erasure and
therefore cannot be written over by the consumer.
Read only memory (ROM) is the only form of semi-
conductor storage that is permanently nonvolatile. Even
with no power source present, information is retained in
a ROM without any information loss.
Optical storage devices, the CD and DVD , are
popular for long term archiving for the following
reasons:- Disk medium is highly standardized.
- Disk medium is multimedia (sound, data, still
images, moving images). - Disk medium format has a commercial life expec-
tancy of many decades. - Disk medium is an efficient and evolving medium.
- Disk medium has good chemical and mechanical
resistance. - Disk medium has good resistance to harsh environ-
mental conditions. - Disk medium has contactless reading—i.e., nonde-
structive. - Disk medium is cost effective.
- Disk medium is an unrecordable system in the ROM
version that prevents erasing or overwriting.
Archived CDs must be chemically stable, have good
resistance against scratching, breaking, etc., and must
be tolerant to extreme conditions of temperature,
humidity, and electromagnetic fields. Some companies,
such as DIGIPRESS, produce a stable CD. Rather than
using a polycarbonate substrate, the CENTURY-DISC
ARK from DIGIPRESS has a desalcanized etched
tempered glass substrat, which is covered with titanium
nitride—a very resistant metal. They can reach a life-
time of over 200 years—not forever maybe, but a great
deal better than our present mediums can.Acknowledgments
Many thanks to Jean Roche from MessageRepeaters.com for her inputs and editing of this chapter.
Additional Reading
Storage Technology Assessment: Final Report, Technical Report RE-0016. NML National Media Lab, St. Paul, MN,
(612) 740-3670.