Page 258 - A Practical Guide from Design Planning to Manufacturing

230 Chapter Seven

supply, it is left to the keeper gate to pull it up the rest of the way. Because
the keeper is sized to be weak, this could take a full clock cycle or more.
Hold noise is caused when the data input switches just after the latch
closes. Because the latch is not completely closed, the storage node Q is
disturbed from its ideal value; once again it is left to the keeper gate to
restore the proper value. The amount of setup and hold noise that can be
tolerated must be considered when performing maxdelay and mindelay
timing checks.
In the late 1970s, transistors had been scaled down to small enough
sizes to be affected by another source of noise, radiation. Ceramics and
lead solder in chip packaging often contain some radioactive isotopes.
When they decay, these atoms release alpha particles, which can pene-
trate the silicon chip in the package. The level of radiation is trivial, but
the tiny transistors of modern semiconductors act as extremely sensitive
detectors. As alpha particles travel through the silicon, they collide with
electrons in the valence band, knocking them loose from their silicon
atoms. Tens of thousands of electrons and holes can be created in this
fashion and if all the electrons are collected by one piece of diffusion, they
can be enough to switch the voltage on that node. Memory bits are the most
susceptible because they have the least capacitance and therefore require
the smallest amount of added charge to flip. These errors are called soft
errors because although the current data being held is lost, the memory
cell is not permanently damaged.
Alpha particles are charged, which prevents them from traveling very
far through any solid material. Any emissions from outside the computer
or even outside the processor package are extremely unlikely to cause an
error. Awareness of the problems caused by radioactive isotopes has led
to them being almost completely eliminated from semiconductor pack-
aging, but a more difficult problem is eliminating sunlight.
Sunlight contains high-energy neutrons, which can also cause soft errors.
Because neutrons are uncharged, they pass easily through solid material.
When passing through a semiconductor, they can create free charges in
the same manner as alpha particles (Fig. 7-31). Trying to shield proces-
sors is simply not practical. A 1-ft thick concrete shield might only reduce
the soft error rate (SER) from neutrons by 25 percent. Miles of atmosphere
do block out many neutrons, reducing soft error rates at lower altitudes.
Computers in Denver, which has an altitude of 5000 ft, have SER four
2
times those found at sea level. Notebook computers being used in air-
planes would suffer even more soft errors.
A common way soft errors are reduced is by increasing the capacitance
of sequential storage nodes. With each new fabrication generation, smaller
dimensions tend to reduce the capacitance on all nodes. This allows higher

2
Ziegler et al., “IBM Experiments in Soft Fails.”