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. $