From: mpd@netcom.com (Mike Duvos)
Date: Tue, 2 Apr 1996 00:20:43 +0800
To: cypherpunks@toad.com
Subject: Re: [NOISE] Nasty-Quibble-Punks
In-Reply-To: <m0u3bwV-0008zJC@pacifier.com>
Message-ID: <199604010642.WAA27208@netcom20.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


jim bell <jimbell@pacifier.com> writes:

 > Turns out Perry is wrong about this.

Shhhh.  Never say the "w-word" in front of Perry. :)

 > I believe that UV EPROMs and probably EEPROMs do indeed
 > work by storing charge on a buried, totally-isolated
 > capacitor.

That is correct.  A very thin layer of dielectric material is
placed on top of a MOS gate.  This "floating gate" can be charged
by applying enough voltage for electrons to tunnel through the
dielectric and charge the gate, which switches the transistor. In
the EPROM, the stored electrons can be given enough energy from
exposure to ultraviolet light to tunnel back out which erases the
device.

Unfortunately, as the geometry shrinks, a longer and longer
exposure to the light is required for erasure, which becomes
annoyingly long for sub-micron technologies.

 > The capacitor is charged with a system called
 > "Fowler-Nordheim tunneling," which involves placing a
 > relatively high voltage on a nearby electrode and causing
 > the thin interface to temporarily conduct.

Almost.  The EEPROM is an advance over the EPROM which permits
the device to be erased electrically.  Fowler-Nordheim Tunneling
is an effect whereby low energy electrons can sneak through the
dielectric in the presence of a very high electric field.  In the
EEPROM, this is used to discharge the floating gates in place of
the UV exposure.  Programming is still done by applying a voltage
high enough to tunnel through the dielectric as in the EPROM.

 > The charge, surprisingly enough, is stable for years, in
 > fact decades, and probably (statistically) centuries at room
 > temperature.

I've never done any calculations, but the charge stays around
"long enough."

The major drawback is that the dielectric is very thin, and
degrades after after hundreds of thousands or millions of write
cycles to the point where the floating gate can no longer retain
a charge.  Therefore such devices are limited in the number of
write cycles they can undergo before wearing out.

 > UVEPROMS are erased, naturally enough, by exposing them to
 > UV light, which is usually produced by a mercury vapor
 > lamp.  This UV causes enough electrons to be excited into
 > upper electron shells in the insulator to temporarily turn
 > it into a slight conductor, and the charge dissipates.  I
 > think EEPROMs are erased by, more or less, reversing the
 > voltage on the charging electrode.

It's more of a case of the trapped electrons absorbing a high
energy photon and getting enough energy to tunnel through the
dielectric, but you have the general idea.  EEPROM erasure is as
described above.

 > As for keeping the charge when that insulator is stripped
 > off, that would be a problem.  It isn't that a vacuum isn't
 > a good enough insulator; it is, but it would be hard to
 > imagine a technique to strip off an SiO2 insulator that
 > doesn't also allow a substantial amount of charge to flow.
 > You could strip it off with HF (hydrofluoric acid) but
 > that's electrically conductive.  Even a gas-phase process
 > would probably result in enough conductive products to
 > discharge the capacitor.

The charge is minute, the dielectric is thin, and damage to the
dielectric would leak the charge.  I'm not sure what a secondary
electron spectrum from a beam that penetrated the dielectric
would disclose about the charge on the gate, but I would tend to
think the dielectric would interfere with tunneling or atomic
force instruments trying to take such measurements.  Again, this
is Tim's area of expertise, and he can probably give you the gory
details on why the state of such devices is difficult to image.

--
     Mike Duvos         $    PGP 2.6 Public Key available     $
     mpd@netcom.com     $    via Finger.                      $