Editing Soft error (section)

== Causes of soft errors ==

=== Alpha particles from package decay ===
Soft errors became widely known with the introduction of [[dynamic RAM]] in the 1970s. In these early devices, ceramic chip packaging materials contained small amounts of [[radioactive]] contaminants. Very low decay rates are needed to avoid excess soft errors, and chip companies have occasionally suffered problems with contamination ever since. It is extremely hard to maintain the material purity needed. Controlling alpha particle emission rates for critical packaging materials to less than a level of 0.001 counts per hour per cm<sup>2</sup> (cph/cm<sup>2</sup>) is required for reliable performance of most circuits. For comparison, the count rate of a typical shoe's sole is between 0.1 and 10 cph/cm<sup>2</sup>.

Package radioactive decay usually causes a soft error by [[alpha particle]] emission.  The positive charged alpha particle travels through the semiconductor and disturbs the distribution of electrons there.  If the disturbance is large enough, a [[Digital data|digital]] [[signal (information theory)|signal]] can change from a 0 to a 1 or vice versa.  In [[combinational logic]], this effect is transient, perhaps lasting a fraction of a nanosecond, and this has led to the challenge of soft errors in combinational logic mostly going unnoticed.  In sequential logic such as [[Latch (electronic)|latches]] and [[Random Access Memory|RAM]], even this transient upset can become stored for an indefinite time, to be read out later.  Thus, designers are usually much more aware of the problem in storage circuits.

A 2011 [[Black Hat Briefings|Black Hat]] paper discusses the real-life security implications of such bit-flips in the Internet's [[Domain Name System]]. The paper found up to 3,434 incorrect requests per day due to bit-flip changes for various common domains. Many of these bit-flips would probably be attributable to hardware problems, but some could be attributed to alpha particles.<ref>{{cite web |url=https://media.blackhat.com/bh-us-11/Dinaburg/BH_US_11_Dinaburg_Bitsquatting_WP.pdf |title=Bitsquatting - DNS Hijacking without Exploitation |author=Artem Dinaburg |date=July 2011 |access-date=2011-12-26  |archive-date=2018-06-11  |archive-url=https://web.archive.org/web/20180611050923/https://media.blackhat.com/bh-us-11/Dinaburg/BH_US_11_Dinaburg_Bitsquatting_WP.pdf |url-status=dead }}</ref> These bit-flip errors may be taken advantage of by malicious actors in the form of [[bitsquatting]].

[[Isaac Asimov]] received a letter congratulating him on an accidental prediction of alpha-particle RAM errors in a 1950s novel.<ref>[[Gold (Asimov)|Gold]] (1995): "This letter is to inform you and congratulate you on another remarkable scientific prediction of the future; namely your foreseeing of the dynamic random-access memory (DRAM) logic upset problem caused by alpha particle emission, first observed in 1977, but written about by you in Caves of Steel in 1957." [Note: Actually, 1952.] ... "These failures are caused by trace amounts of radioactive elements present in the packaging material used to encapsulate the silicon devices ... in your book, Caves of Steel, published in the 1950s, you use an alpha particle emitter to 'murder' one of the robots in the story, by destroying ('randomizing') its positronic brain. This is, of course, as good a way of describing a logic upset as any I've heard ... our millions of dollars of research, culminating in several international awards for the most important scientific contribution in the field of reliability of semiconductor devices in 1978 and 1979, was predicted in substantially accurate form twenty years [Note: twenty-five years, actually] before the events took place</ref>

=== Cosmic rays creating energetic neutrons and protons ===
Once the electronics industry had determined how to control package contaminants, it became clear that other causes were also at work. [[James F. Ziegler]] led a program of work at [[IBM]] which culminated in the publication of a number of papers (Ziegler and Lanford, 1979) demonstrating that [[cosmic ray]]s also could cause soft errors. Indeed, in modern devices, cosmic rays may be the predominant cause. Although the primary particle of the cosmic ray does not generally reach the Earth's surface, it creates a [[Air shower (physics)|shower]] of energetic secondary particles. At the Earth's surface approximately 95% of the particles capable of causing soft errors are energetic neutrons with the remainder composed of protons and pions.<ref name="Ziegler1996">
{{cite journal |last1=Ziegler |first1=J. F. |title=Terrestrial cosmic rays |journal = [[IBM Journal of Research and Development]] |volume=40 |issue=1 |pages=19–39 |date=January 1996 |doi=10.1147/rd.401.0019 | issn = 0018-8646 }}</ref>
IBM estimated in 1996 that one error per month per 256&nbsp;[[MiB]] of RAM was expected for a desktop computer.<ref name="cosmicRayAlert" />
This flux of energetic neutrons is typically referred to as "cosmic rays" in the soft error literature. Neutrons are uncharged and cannot disturb a circuit on their own, but undergo [[neutron capture]] by the nucleus of an atom in a chip. This process may result in the production of charged secondaries, such as alpha particles and oxygen nuclei, which can then cause soft errors.

Cosmic ray flux depends on altitude. For the common reference location of 40.7°&nbsp;N, 74°&nbsp;W at sea level ([[New York City]], NY, USA), the flux is approximately 14 neutrons/cm<sup>2</sup>/hour. Burying a system in a cave reduces the rate of cosmic-ray-induced soft errors to a negligible level. In the lower levels of the atmosphere, the flux increases by a factor of about 2.2 for every 1000&nbsp;m (1.3 for every 1000&nbsp;ft) increase in altitude above sea level. Computers operated on top of mountains experience an order of magnitude higher rate of soft errors compared to sea level. The rate of upsets in [[aircraft]] may be more than 300 times the sea level upset rate. This is in contrast to package decay-induced soft errors, which do not change with location.<ref name="GordonGoldhagen2004">{{cite journal |last1=Gordon |first1=M. S. |last2=Goldhagen |first2=P. |last3=Rodbell |first3=K. P. |last4=Zabel |first4=T. H. |last5=Tang |first5=H. H. K. |last6=Clem |first6=J. M. |last7=Bailey |first7=P. |title=Measurement of the flux and energy spectrum of cosmic-ray induced neutrons on the ground |journal=IEEE Transactions on Nuclear Science |volume=51 |issue=6 |date=2004 |pages=3427–3434 |issn=0018-9499 |doi=10.1109/TNS.2004.839134 |bibcode=2004ITNS...51.3427G|s2cid=9573484 }}</ref>
As [[Moore's law|chip density increases]], [[Intel]] expects the errors caused by cosmic rays to increase and become a limiting factor in design.<ref name="cosmicRayAlert">{{cite magazine |last=Simonite |first=Tom |date=March 2008 |title=Should every computer chip have a cosmic ray detector? |url=https://www.newscientist.com/blog/technology/2008/03/do-we-need-cosmic-ray-alerts-for.html |magazine=[[New Scientist]] |archive-url=https://web.archive.org/web/20111202020146/https://www.newscientist.com/blog/technology/2008/03/do-we-need-cosmic-ray-alerts-for.html |archive-date=2 December 2011 |access-date=26 November 2019}}</ref>

The average rate of cosmic-ray soft errors is ''inversely'' proportional to sunspot activity. That is, the average number of cosmic-ray soft errors decreases during the active portion of the [[sunspot cycle]] and increases during the quiet portion. This counter-intuitive result occurs for two reasons. The Sun does not generally produce cosmic ray particles with energy above 1&nbsp;GeV that are capable of penetrating to the Earth's upper atmosphere and creating particle showers, so the changes in the solar flux do not directly influence the number of errors. Further, the increase in the solar flux during an active sun period does have the effect of reshaping the Earth's magnetic field providing some additional shielding against higher energy cosmic rays, resulting in a decrease in the number of particles creating showers. The effect is fairly small in any case resulting in a ±7% modulation of the energetic neutron flux in New York City. Other locations are similarly affected.{{citation needed|date=December 2015}}

One experiment measured the soft error rate at the sea level to be 5,950&nbsp;[[failures in time]] (FIT = failures per billion hours) per DRAM chip.  When the same test setup was moved to an underground vault, shielded by over {{Convert|50|feet|m}} of rock that effectively eliminated all cosmic rays, zero soft errors were recorded.<ref>{{cite web|author-last=Dell|author-first=Timothy J.|date=1997|title=A White Paper on the Benefits of Chipkill-Correct ECC for PC Server Main Memory|url=https://asset-pdf.scinapse.io/prod/48011110/48011110.pdf|access-date=2021-11-03|website=ece.umd.edu|page=13}}</ref>  In this test, all other causes of soft errors are too small to be measured, compared to the error rate caused by cosmic rays.
 
Energetic neutrons produced by cosmic rays may lose most of their kinetic energy and reach thermal equilibrium with their surroundings as they are scattered by materials. The resulting neutrons are simply referred to as [[thermal neutrons]] and have an average kinetic energy of about 25 millielectron-volts at 25&nbsp;°C. Thermal neutrons are also produced by environmental radiation sources, including the decay of naturally occurring radioactive elements such as [[uranium]] and [[thorium]]. The thermal neutron flux from sources other than cosmic-ray showers may still be noticeable in an underground location and an important contributor to soft errors for some circuits.

=== Thermal neutrons ===
Neutrons that have lost kinetic energy until they are in thermal equilibrium with their surroundings are an important cause of soft errors for some circuits. At low energies many [[neutron capture]] reactions become much more probable and result in fission of certain materials creating charged secondaries as fission byproducts. For some circuits the capture of a [[thermal neutron]] by the nucleus of the <sup>10</sup>B [[isotopes of boron|isotope of boron]] is particularly important. This nuclear reaction is an efficient producer of an [[alpha particle]], [[lithium|<sup>7</sup>Li]] nucleus and [[gamma ray]]. Either of the charged particles (alpha or <sup>7</sup>Li) may cause a soft error if produced in very close proximity, approximately 5&nbsp;[[μm]], to a critical circuit node. The capture cross section for <sup>11</sup>B is 6 [[orders of magnitude]] smaller and does not contribute to soft errors.<ref name="BaumannHossain1995">{{cite book |last1=Baumann |first1=R. |title=33rd IEEE International Reliability Physics Symposium |last2=Hossain |first2=T. |last3=Murata |first3=S. |last4=Kitagawa |first4=H. |chapter=Boron compounds as a dominant source of alpha particles in semiconductor devices |date=1995 |pages=297–302 |doi=10.1109/RELPHY.1995.513695 |isbn=978-0-7803-2031-4|s2cid=110078856 }}</ref>

[[Boron]] has been used in [[Borophosphosilicate glass|BPSG]], the insulator in the interconnection layers of integrated circuits, particularly in the lowest one. The inclusion of boron lowers the melt temperature of the glass providing better [[reflow soldering|reflow]] and planarization characteristics. In this application the glass is formulated with a boron content of 4% to 5% by weight. Naturally occurring boron is 20% <sup>10</sup>B with the remainder the <sup>11</sup>B isotope. Soft errors are caused by the high level of <sup>10</sup>B in this critical lower layer of some older integrated circuit processes. Boron-11, used at low concentrations as a p-type dopant, does not contribute to soft errors. Integrated circuit manufacturers eliminated borated dielectrics by the time individual circuit components decreased in size to 150&nbsp;nm, largely due to this problem.

In critical designs, depleted boron{{mdashb}}consisting almost entirely of boron-11{{mdashb}}is used, to avoid this effect and therefore to reduce the soft error rate. Boron-11 is a by-product of the [[nuclear power|nuclear industry]].

For applications in medical electronic devices this soft error mechanism may be extremely important. Neutrons are produced during high-energy cancer radiation therapy using photon beam energies above 10&nbsp;MeV. These neutrons are moderated as they are scattered from the equipment and walls in the treatment room resulting in a thermal neutron flux that is about 40&nbsp;×&nbsp;10<sup>6</sup> higher than the normal environmental neutron flux. This high thermal neutron flux will generally result in a very high rate of soft errors and consequent circuit upset.<ref name="WilkinsonBounds2005">{{cite journal |last1=Wilkinson |first1=J. D. |last2=Bounds |first2=C. |last3=Brown |first3=T. |last4=Gerbi |first4=B. J. |last5=Peltier |first5=J. |title=Cancer-radiotherapy equipment as a cause of soft errors in electronic equipment |journal=IEEE Transactions on Device and Materials Reliability |volume=5 |issue=3 |date=2005 |pages=449–451 |issn=1530-4388 |doi=10.1109/TDMR.2005.858342|s2cid=20789261 }}</ref><ref name="Franco">Franco, L., Gómez, F., Iglesias, A., Pardo, J., Pazos, A., Pena, J., Zapata, M., SEUs on commercial SRAM induced by low energy neutrons produced at a clinical linac facility, RADECS Proceedings, September 2005</ref>

=== Other causes ===
Soft errors can also be caused by [[random noise]] or [[signal integrity]] problems, such as inductive or capacitive [[crosstalk]]. However, in general, these sources represent a small contribution to the overall soft error rate when compared to radiation effects.

Some tests conclude that the isolation of [[DRAM]] memory cells can be circumvented by unintended side effects of specially crafted accesses to adjacent cells.  Thus, accessing data stored in DRAM causes memory cells to leak their charges and interact electrically, as a result of high cells density in modern memory, altering the content of nearby memory rows that actually were not addressed in the original memory access.<ref name="kyungbae">{{cite book |author-first1=Kyungbae |author-last1=Park |author-first2=Sanghyeon |author-last2=Baeg |author-first3=ShiJie |author-last3=Wen |author-first4=Richard |author-last4=Wong |title=2014 IEEE International Integrated Reliability Workshop Final Report (IIRW) |chapter=Active-precharge hammering on a row induced failure in DDR3 SDRAMs under 3× nm technology |pages=82–85 |publisher=[[IEEE]] |date=October 2014 |doi=10.1109/IIRW.2014.7049516 |isbn=978-1-4799-7308-8|s2cid=14464953 }}</ref>  This effect is known as [[row hammer]], and it has also been used in some [[privilege escalation]] computer security [[Exploit (computer security)|exploits]].<ref>{{cite web |url=http://users.ece.cmu.edu/~yoonguk/papers/kim-isca14.pdf |title=Flipping Bits in Memory Without Accessing Them: An Experimental Study of DRAM Disturbance Errors |date=2014-06-24 |access-date=2015-03-10 |author-first1=Yoongu |author-last1=Kim |author-first2=Ross |author-last2=Daly |author-first3=Jeremie |author-last3=Kim |author-first4=Chris |author-last4=Fallin |author-first5=Ji Hye |author-last5=Lee |author-first6=Donghyuk |author-last6=Lee |author-first7=Chris |author-last7=Wilkerson |author-first8=Konrad |author-last8=Lai |author-first9=Onur |author-last9=Mutlu |publisher=[[IEEE]] |website=ece.cmu.edu}}</ref><ref>{{cite web |url=https://arstechnica.com/security/2015/03/cutting-edge-hack-gives-super-user-status-by-exploiting-dram-weakness/ |title=Cutting-edge hack gives super user status by exploiting DRAM weakness |date=2015-03-10 |access-date=2015-03-10 |author-first=Dan |author-last=Goodin |publisher=[[Ars Technica]]}}</ref>