Photons - Page 1
Forum section: electronics / projects

author: MickF
author_rank: Newbie
timestamp: June 14, 2015, 09:57:09 am
content: Last week I was researching some info on watch dog timers and discovered an article about cosmic rays. The article said that as electronics become smaller, photons have the energy to flip a bit. How is it possible to protect against this problem, if indeed this is a problem?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2015, 11:54:30 am
content: The cosmic rays include particles / photons of different energy, and a few of them are very strong and can even upset rather large dimension semiconductors (even building the logic from 2N3055 size transistor will not be a definite solution). However these high energy particles are very rare.The more real problems are alpha decay from inside the chip and muons from cosmic rays. It's well known, that they can flip bits in DRAM cells. The solution is to use parity bits and better error correcting codes. In logic circuits its more difficult. With doing everything twice one could detect errors - with 3 times the logic one could use the majority to decide. This is actually done in some space applications, where radiation levels are higher. However logic chips are more resistant to radiation than memory. With critical calculations one might decide to run the same calculation twice, even on different computers to catch hardware bugs as well. It the results are different - it's up to a third.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2015, 12:09:31 pm
content: This has been a concern for many decades, not just recently. Cosmic rays contain a lot of energy, and when one rips through a semiconductor device, it rips up charge carriers along the way, which diffuse across junctions, into nearby conductors, and lodge in insulators. If a higher concentration of electrons or holes (one or the other) gets deposited in a charged well or node, it can change the voltage locally.Atoms struck in the process may also get dislodged, causing crystal defects, which result in faster charge recombination in minority carrier devices (indeed, ultrafast rectifier diodes are often electron irradiated to enhance this effect), increased leakage current (junctions and insulators), reduced breakdown voltage (insulators), etc.This goes for anything from soft x-rays to gammas and relativistic particles. (Alphas probably not so much, because they are easily stopped by matter, but betas are more penetrating and give off x-rays when stopped (bremsstrahlung). It's worth noting that alphas in contact with the die are a big concern, hence the demand for low-alpha lead, used with solder-bump flip chips.)The effect is worst for floating nodes -- DRAM, E(E)PROM, Flash. Less important for driven nodes (SRAM, sequential logic), but enough of anything will still cause a problem.It wasn't much of a concern back in the day, because terrestrial radiation levels are low, process scales were large (more node capacitance means a given particle has less effect) and memory stores were small (megs). Basic protection was generally provided by an additional parity bit of RAM per byte (so your 640kB system RAM would have an 18 bit bus to the memory controller), and the usual action taken on parity fail was NMI (non-maskable interrupt), usually set to direct the processor to a JMP -02 instruction, hard coded in the BIOS ROM (i.e., loop forever, interrupts disabled, complete hard lockup).Today, process scales are quite small and memory stores are huge (gigs just for system RAM, plus hundreds to thousands for Flash storage), so error detection would be appalling; error correction is a necessity.I don't know offhand how much (if any) error correction takes place in modern RAM. It could be they just get lucky...Modern NAND Flash is heavily error correcting, by design necessity: the process is quite unreliable (in semiconductor terms*), so additional hardware is used to map out bad locations, and to remap addresses from time to time (wear leveling, because the aggressive programming process itself tends to dislodge atoms).*When a semiconductor is tested, traditionally, the ones that fail are simply discarded, end of the line. That's fine when you get a thousand logic gates from a wafer, and only ten of them are bad (suggesting there were 10 defects on the wafer, either to begin with, or due to any step along the process -- logic chips are simple, so not many steps are required). It's still not too bad if you get, say, 500 MCUs off a wafer -- maybe a hundred are failed in some way (stuck bit in the RAM, or a timer skips some counts, etc.?), but you're still doing 80%. But that sucks if you're doing huge CPUs or RAM chips, where the die area is in the cm^2, and you only get a hundred chips or less from one wafer. Now a single bit error ruins several gigs worth of chip at a time, and the fail rate might be over 50%. Many early processors ran into the 90%, even >98% range, while the process was dialed in -- but they still moved real product, at obscene introductory prices, because that's business.Without error management methods, we certainly wouldn't have 2G+ thumb drives and SD cards for bucks each, and the highest capacity parts might not be possible at all.As for critical applications (aerospace, military, medical), some structural changes are used, but mostly design is key. The most important structural change is SOI (silicon on insulator), where the underlying insulating layer blocks the creation and spread of free charge carriers. (It's also significantly faster because of reduced capacitances, so most or all high speed CMOS circuitry is built on it, including CPUs and RAM, I think.) Beyond that, error correcting and detecting methods (codes and algorithms), and extreme steps like redundancy (e.g., running two or more processor cores in lockstep with identical inputs, and verifying that they remain matched), cover the rest.Tim

author: Dragon88
author_rank: Regular Contributor
timestamp: June 14, 2015, 02:56:22 pm
content: https://www.sparkfun.com/news/1670http://www.militaryaerospace.com/articles/2011/05/radiation-hardened-electronics.htmlhttp://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.136.9216&rep=rep1&type=pdf

author: Gyro
author_rank: Super Contributor
timestamp: June 14, 2015, 06:07:13 pm
content: I remember Intel having problems back in the late 70s / early 80s with higher than expected numbers of soft errors in their DRAMs.It turned out to be due to low level radiation (alpha presumably) produced by their ceramic dip packages.