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From: buyno@voder.UUCP (Matthew Buynoski)
Newsgroups: sci.physics,sci.space,comp.lsi
Subject: Re: Radiation effects on semiconductors
Message-ID: <2968@voder.UUCP>
Date: Thu, 6-Aug-87 20:13:25 EDT
Article-I.D.: voder.2968
Posted: Thu Aug  6 20:13:25 1987
Date-Received: Sat, 8-Aug-87 14:54:09 EDT
References: <1356@ssc-vax.UUCP> <107@rdlvax.UUCP> <169@bernina.UUCP>
Organization: National Semiconductor, Santa Clara
Lines: 52
Keywords: radiation semiconductors
Summary: rad hard and semiconductors hole electron pairs
Xref: mnetor sci.physics:1985 sci.space:2380 comp.lsi:198

In article <169@bernina.UUCP>, peter@ethz.UUCP (Peter Beadle) writes:
> In article <107@rdlvax.UUCP> kopaz@rdlvax.UUCP (John Anthony 'Echo' Kopaz) 
> 
> 1. What are the primary causes of radiation (in the largest sense of
> the word) induced circuit failure.
> 
> 2. What proportion of the effects are permenant and what proportion are
> transitory.
> 
> 3. What makes a radiation hard circuit "hard". We have already heard
> about some quantitive measures of "hard", what I would like to know is
> what I have to do to design a "hard" circuit.
> 
> 4. What effects do process parameters have on radiation sensitivity.
> 
> 5. I hear a lot about alpha particle hits on memories. What does an
> alpha particle do as it passes through a 1Mbit D-ram.

	Answer to 1 (as I know it). The chief causes are: hole-electron pair
generation in the oxide, and outright smushing of the semiconductor lattice.
To the former: energetic particles in the oxide layers over the semiconductor
create hole-electron pairs in the oxide. The electrons so generated are mobile
enough to enter the semiconductor, but the holes are not. This leads to a build-up of positive holes along the Si-SiO2 interface. That in turn affects the
electrical properties of the semiconductor near-surface region. Typical effects
are the inversion of p-regions (bipolar bases, e.g.), shifts of threshold voltage in MOSFETs, lowered breakdown voltages due to field-induced junctions.
	The second effect is the outright damage to the lattice by the energeticparticles. These events tear up everything...all the diodes start to leak, oxides break down, etc.
	Usually the first mechanism occurs first.

	Answer to 2.  They are mostly all permanent. Alas. The alpha-particle
problem in memories is an example of transitory radiation "damage". In this case, many more hole-electron pairs are created in the holding capacitor of the
DRAM cell, which effectively messes up the stored signal. A single alpha can
create about 10e6 such pairs. The same alpha may well have also created a few
hole-electron pairs in the oxide above the cell, doing permanent-type damage
but at a slower rate (the oxide is thinner, and it is more difficult to create
pairs in it than silicon, since the bandgap is considerably larger in SiO2).
Actually, some of the effects are reversible, but with annealing at, say, 250 to300 degrees C. Not very practical in a working circuit.

	Answer to 3. Most of the things done to "rad hard" parts are done to 
reduce the surface state and fixed charge densities. Surface states are very
nice homes for the radiation-generated holes and tend to bind them right on
the interface, which is of course the absolute worst place for them. IF the
holes don't have as many places to "settle in", it is possible that some of
them may wander around and eventually find an electron to combine with..poof
goes a bit of your trouble. Reducing fixed surface charge helps in that it is
positive and if it is less, then it takes more holes to get you up to the point
where the total positive charge is too much. Or, it pushes back the day of
reckoning for a while.

	I think the answers to 4 and 5 are somewhere in the bulk of the answers
to 1,2, and 3, so I won't repeat them here.

	Hope this helps.