raob@munnari.oz (Richard Oxbrow) (05/16/89)
The is a very late article following up the discussion on
non-volatile RAMs/memory . It has been reproduced from an
article in an Australian newspaper about a FRAM cell ,
currently under developement by Ramtron Australia .
Basically the FRAM cell is very similar to a RAM cell does,
but retains its "memory" when the power is turned .
Longish article follows
Stolen from "The Australian" page 57 , (a newspaper from Australia)
Tuesday May 2 , 1989
Ferroelectric RAM (FRAM)
[/* the first 6 paragraphs have been omitted */]
The "ferro" part of the word is misleading.Apparently the effect
was thought to be related in some way to the ferromagnetic properties
of iron compounds.
It was instead a characteristic of the small electrically-
asymmetical elements, or dipoles, within some crystals. The dipoles
become polarised spontaneously under the influence of an externally-
applied electric field and remain polarised after the field is
removed.
Reversal of the field causes spontaneous polarisation in
the opposite direction.
This means that ferroelectric material has two stable
polarisation states, and can be modelled as a "bistable capacitor"
with two distinct polarisation voltage thresholds.
No electrical field or current is required for the material to
remain polarised in either state, hence a true nonvolatile
ferroelectric digital memory capacitor can be built for storing
1s and 0s.
Ramtron's development is based on a complex thin-film ceramic
comprising primarily ferroelectric PZT (lead-zirconate-titanate).
This is related to material used in ceramic filters
and the Perovskite ceramics used in warm superconducting.
This is also compatible with standard semiconductor
fabrication techniques.
The ferroelectronic process uses the thin film of ceramic
PZT compound sandwiched between two metal electrodes to form
nonvolatile digital memory capacitors that lie directly above the
existing semi-conductor circuitry.
The PZT file is extremely rugged and remains ferroelectric
from below -80 C to above 350 C. This is well beyond the operating
temperature range of existing silicon circuits.
This result is also highly dense, since the features and
electrical properties of the underlying circuit are preserved and no
additional area is required.
In this way Ramtron's technology integrates ferroelectric
materials with conventional semiconductor devices without
altering their underlying characteristics, and builds non volatile
components which offer performance, density and cost advantages over
existing semiconductor products.
The technology is an add-on and complementary technology to
existing semiconductor production techniques, including silicon-
based CMOS and bipolar, as well as gallium arsenide processes.
A Cross section of FRAM Device ..
TE :: Top electrode
PZT TF :: PZT thin Filem
BE :: Bottom electrode
P :: Poly
So :: Source
Dr :: Drain
_________________ ____________
__________|____ Metal __|_______|___Metal _|_____
____\_ _/___ _____\ /
____|_TE__\____/_____\____/_____|\ /
|_PZT TF__\____/_____\____/_____|_\__/____
-------|_____BE__\____/_____\____/_______________|____
\____/ \____/
Glass
__________________ _____ _________________
\____/__P__\______/
SiO _____ _____
____2_____________/ So \___/ Dr \__________________
\ Well\_____/ \_____/ /
\_________________________/
Si substrate
----------------------------------------------------
______________________________________________________________________________
Richard Oxbrow ACSnet : raob@mullian.mu.oz
SITEE - (Elec Eng) uunet : uunet!munnari!mullian!raob
University of Melbourne internet : raob@mullian.mu.oz.au
Australia raob%mullian.oz.au@uunet.uu.nn
==============================================================================mo@prisma (05/18/89)
The Ramtron people doing the ferrorams are here in Colorado Springs, almost in eyeshot of my office. I have some data sheets for their first chips. They look like they will be quite interesting gadgets when they get the dense parts going. Essentially a DRAM looking device which can be "frozen" upon command. -Mike