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From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: sci.electronics
Subject: Re: OTP (really?) EPROMS
Summary: packaging
Keywords: EPROMS
Message-ID: <15357@vlsisj.VLSI.COM>
Date: 13 Oct 89 05:11:27 GMT
References: <841@dms.UUCP>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Distribution: usa
Organization: VLSI Technology Inc., San Jose, CA
Lines: 47

In article <841@dms.UUCP> albaugh@dms.UUCP (Mike Albaugh) writes:
>	Suppose for the sake of argument that somebody had access to
>a fair number of "One Time Programmable" EPROMS that were already
>programmed. Is there a way (like, f'rinstance X-rays) to erase them
>anyway? 

You'd better believe they test the whole thing with wafer probing.  No
semiconductor company worth its salt would ship partially-tested chips.
Of course, before they're packaged the company can simply zap 'em in the
tanning booth.

In theory, the chips should also be tested after packaging (there is some
loss - wires break, etc.).  I think they could be erased after testing with
X-rays.  I quote my semiconductor materials textbook ("Device Electronics
for Integrated Circuits", Richard S. Muller & Theodore I. Kamins, p. 452):

	The buried polysilicon layer in a FAMOS device retains the charge
	on the gate for any practical time period unless some source of
	external energy is able to liberate the electrons held there.  A
	suitable energy source to excite them is photon irradiation which
	can give the electrons sufficient energy to pass over the barrier
	at the interface between the polysilicon layer and the oxide.

As long as the plastic package is transparent to X-rays, you should be able
to do it.

So why do people buy these things in the first place?  My guess is that the 
package simply costs less and so they're cheaper for people who aren't going
to change the programming very often.  Of course, there is a fair market for 
ROMs; OTP EPROMs are a pretty easy way of making ROMs without the masking 
expense or difficulty of changing the code.

We at VLSI have stopped making larger ROMs (1Mbits is as large as we go) 
because no one is willing to commit to freezing code enough to make them in 
volume.  They all worry about patches, bug fixes, etc.  Think about it - 128 
Kbytes is a lot of code!

>	Any Suggestions? I'll take email and post a summary if I get
>a usable suggestion.

E-mail doesn't seem to work well for me; sorry.  A lot of stuff I send just
bounces three days later.
-- 
		David Chapman

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