Re: [NOISE] Nasty-Quibble-Punks
At 04:06 PM 3/31/96 -0800, Mike Duvos wrote:
"Perry E. Metzger" <perry@piermont.com> writes:
Uh huh.
Incidently, EEPROMs don't work by simply charging a capacitor or something silly like that. No insulator is perfect, no dielectric is perfect, and charge would eventually leak away were that the case. However, if it were, it would be fairly easy to determine the state of a cell without having to get particularly close to it. Beyond that, there is this insane notion you seem to have that a charged object will lose its charge if the "insulator" is "stripped off" -- I wasn't under the impression a vacuum, for instance, was a particularly good charge carrier.
Uh huh.
Turns out Perry is wrong about this. I believe that UV EPROMs and probably EEPROMs do indeed work by storing charge on a buried, totally-isolated capacitor. 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. (It's odd. That's why it's called "tunnelling.") The charge, surprisingly enough, is stable for years, in fact decades, and probably (statistically) centuries at room temperature. The reason the charge stays around so long is that the insulator, silicon dioxide, is extremely good. It has to be. If the capacitor were, say, 1 picofarad, and the resistance was 10E18 ohms (a billion gigohms) the resulting time constant would be 1E6 seconds, or about 12 days. Since EPROMs obviously hold data far longer than this (well over 100 times longer, or else our computers wouldn't work!), and since the capacitance is probably not nearly 1 pf, that tells you that the effective resistance is far above 1E20 ohms. 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. 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. Ion-beam milling would also remove SiO2, but as the name implies that's applying a current to the system. Fortunately, all this is moot: Since the floating gate is inherently part of a transistor, it isn't necessary to expose it to detect its charge state: Just activate the transistor in-circuit BTW, some PLD's have a so-called "security bit" which (when set) is designed to prevent reading of the state of the rest of the programmed bits. Years ago it occurred to me that if you knew where this particular bit was stored, you could expose this bit location alone to a UV source through a tiny mask to discharge it. Finding that location wouldn't be all that hard: Just expose the chip with a series of exposures, moving a linear mask slightly, and eventually the security bit will erase. Note the location of the mask, and rotate the mask 90 degrees and repeat the process. At that point, you've located the bit (this may require a few iterations), so you expose the target part through a tiny pinhole (Edmund Scientific sells them in many different sizes, exposing only that security bit location. Jim "Mr. Bell talks about one field" Bell (Let's see, I covered solid, liquid, and vapor phase chemistry, a bit of particle physics (ion-beam milling), semiconductor physics, minor optics, electronics, some trivial math, and maybe even some detective work!) jimbell@pacifier.com
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. $
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