Editing Soft error (section)

=== 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>