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Winter Semester 2010

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  1. Emerging Systems Course No. 4: Expanding Bio-Inspiration: Towards Reliable MuxTree Memory Arrays – Part 1 – ”Politehnica” University of Timisoara Winter Semester 2010

  2. Outline • Chapter 1: Bio-Inspired Reliability(With a plea for bio-inspiration and a comparison between artificial Embryonics cells and the stem cells from biology) • Chapter 2: A Bird’s Eye View Over Faults (Includes fault tolerance motivation, causes of unexpected, soft errors and a description of the physical phenomena involved) • Chapter 3: Embryonics and SEUs (Particularities of the project, datapath model in memory structures, and reliability analysis)

  3. Chapter 1: Bio-Inspired Reliability (1) 1.1. Introduction • Performance VS Fault Tolerance ↓ Performance AND Fault Tolerance • Hot issues in bio-inspiration: • mimicking nature, also mimic results? • Intersection point: where? • exporting biological features toward computer engineering: possible?

  4. Chapter 1: Bio-Inspired Reliability (2) 1.2. A Plea for Bio-Inspiration • Existing: intrinsic robustness of living beings • Sustaining many minor wounds/illnesses: self-diagnosing • Quick healing: growing/replacing damaged tissues • Perfected in time • Wanted: robust computing systems • Self-testing and self-repair mechanisms through redundancies • Immunotronics: bio-inspired immune system for silicon devices

  5. Chapter 1: Bio-Inspired Reliability (3) 1.2. A Plea for Bio-Inspiration • HW redundancy • Spares take over faulty resources * • Process supervised centrally • Overall reliability limited by centralized unit • Bio-inspired redundancy (Embryonics) • Spares take over faulty resources * • Process distributed • Multiple level redundancy

  6. Chapter 1: Bio-Inspired Reliability (4) 1.3. Embryonics and Stem Cells • Human brain • Exponent of epigenetic systems • Complexity given by 1010 cells and 1014 interconnections • All from a single, original cell: the zygote • Stem cells • Recently discovered • Division potential apparently unlimited • Offspring: identical or able to specialize • Anyone potentially becoming a fetus

  7. Chapter 1: Bio-Inspired Reliability (5) 1.3. Embryonics and Stem Cells Similarities: • Cells made by molecules. No limits on cell number or cellular dimensions • Indefinite cellular division. Process not material, but informational • Specialization via gene expression: information governs over functionality

  8. Presentation Outline • Chapter 1: Bio-Inspired Reliability(With a plea for bio-inspiration and a comparison between artificial Embryonics cells and the stem cells from biology) • Chapter 2: A Bird’s Eye View Over Faults (Includes fault tolerance motivation, causes of unexpected, soft errors and a description of the physical phenomena involved) • Chapter 3: Embryonics and SEUs (Particularities of the project, datapath model in memory structures, and reliability analysis)

  9. Chapter 2: A Birds-Eye View Over Faults (1) 2.1. Bio-Inspired Storage • Genetic program stored into cyclic memory structure – macro-cell • Large genomes → large macro-cells → increased error likelihood • Two-level self-repair mechanism refers to system functionality only • Data integrity extremely important, functionality governed by genome • No data integrity protection implemented

  10. Chapter 2: A Birds-Eye View Over Faults (4) 2.2. Fault Taxonomy Over Time • Permanent faults: affect normal device operation constantly, over an indefinite period of time; solid or hard fails • Non-permanent faults: random occurrence, effect over indefinite but finite periods of time; • Intermittent: non-environmental causes (parameter variations, timing, loose connections, etc); difficult debugging process • Transient: environmental causes (atmospheric parameters, supply characteristics, cosmic rays); very difficult/impossible to model ↓ • Soft fails, soft errors or single event upsets (SEUs): transient type

  11. Chapter 2: A Birds-Eye View Over Faults (6) 2.3. Single Event Upsets: An Analysis • Affect normal execution process • Mostly caused byelectronic noise: ionization electron-hole pairs • Particles capable of generating, energetic nucleons and nuclear fragments: • α–particles • radioactive isotopes • other particles grouped under the more general term of cosmic rays

  12. Chapter 2: A Birds-Eye View Over Faults (7) 2.3. Single Event Upsets: An Analysis • Radiation affecting devices long known • 1978, Ziegler (IBM): if α–particles affect semiconductor devices, then other particles from the outer space (cosmic rays) also might • 1979, May and Woods (Intel): radioactive isotopes affect memories • Confirmation under device irradiating • Studied since 1980, hot field today

  13. Chapter 2: A Birds-Eye View Over Faults (8) 2.3. Single Event Upsets: An Analysis • Protective techniques used in modern devices • Focus on memories, prone to soft fails: • Techniques well understood, relatively easy to implement, not expensive • Memory area in systems proportionally significant • Combinational logic much less susceptible to soft errors • SER not constant, dependant on technological advancement

  14. Chapter 2: A Birds-Eye View Over Faults (9) 2.3.1. Radioactive Isotopes • Due to proximity contamination in semiconductor facilities • Famous “HERA” problem, 1987: • IBM produced LSI memories hit by anomalous behavior • Chips produce in US and Europe, but only US-produced batches affected • Package responsibility ruled out

  15. Chapter 2: A Birds-Eye View Over Faults (10) 2.3.1. Radioactive Isotopes • Finally, traces of Po210 discovered • Product of the uranium decay chain • Strangely, other expected particles totally missing • After months of searching, the culprit found as a bottle of nitric acid, used in the process • Intel also hit by problems: 2107 series, 16Kb DRAM memories • Cause: radioactive contamination • Reason: factory built downstream, close to an old uranium mine

  16. Chapter 2: A Birds-Eye View Over Faults (11) 2.3.2. Cosmic Ray Influence • Strong electromagnetic perturbations induced by any ionizing particle hitting an electronic device • Disturbances translate into electron–hole pairs • If local fields sufficiently strong, pairs cannot recombine; instead, find a way out to the nearest appropriate contact • Charge collection provoke soft errors

  17. Chapter 2: A Birds-Eye View Over Faults (12) 2.3.2. Cosmic Ray Influence • Particles entering atmosphere collide: elastic and inelastic collisions • Secondary nuclear fragments generated in avalanche-type phenomena: nucleons, pions, light ions (2H, 3H, 3He and α–particles), and heavy residual nuclei (O, C, Mg) • Disturbances created by cosmic ray collision with semiconductor nuclei • High-energy particle flux born, reaching the surface

  18. Chapter 2: A Birds-Eye View Over Faults (13) 2.3.2. Cosmic Ray Influence • Final flux distribution affected by: • Altitude: Due to atmospheric filtering effect, the lower the altitude, the smaller particle rate • Geomagnetic region (GMR): shielding of Earth‘s magnetic field; cosmic rays penetration smallest around equator, largest at poles • 11-year solar cycle: strongly affects particle flux; increased sun activity leads to lower rate of particles, because shielding effect of magnetic field around the earth also strengthened

  19. Chapter 2: A Birds-Eye View Over Faults (14) 2.3.2. Cosmic Ray Influence • Attempts on flux measurement as early as 1980s (IBM). At least 3 categories of cosmic rays • Primary cosmic rays: lurking particles in the outer space, eventually may hit the planet; product of intense reactions

  20. Chapter 2: A Birds-Eye View Over Faults (15) 2.3.2. Cosmic Ray Influence • Sun also major source of primary cosmic rays • Measured flux of primary cosmic rays about 105/m2·s • Secondary cosmic rays: born by collisions when primary cosmic rays enter atmosphere; also known as “cascade particles”

  21. Chapter 2: A Birds-Eye View Over Faults (16) 2.3.2. Cosmic Ray Influence • Terrestrial cosmic rays: particles actually reaching sea-level • max. 1% originate from primary cosmic rays • the rest cascade-generated particles, from the 3rd to 7th generations • extremes in sea-level particle flux lag solar cycle by 1-2 years • Final flux made of hadrons (collide due to atmospheric density), leptons and photons

  22. Chapter 2: A Birds-Eye View Over Faults (18) 2.3.2. Cosmic Ray Influence • Flux measurement : 360/m2·s • Most particles decay spontaneously or reach thermal energies (absorbed by the atmosphere) • Peak of cascading density at an altitude of about 15 km, usually referred to as the Pfotzer point • Altitude used by many commercial aircraft • This is where the fail rate of electronic devices is about 100 times worse than at terrestrial altitudes

  23. Chapter 2: A Birds-Eye View Over Faults (19) 2.3.2. Cosmic Ray Influence • After protons hit atmosphere most resulting fragments decay or absorbed • Proton flux largely affected by interactions with atmospheric electrons • Muons dominate the medium/ high energy particle spectrum • Energetic levels of most concern for SER between 200 and 3000MeV

  24. Chapter 2: A Birds-Eye View Over Faults (21) 2.3.3. Modelling Cosmic Ray Influence • Significant effort towards particle measurements • Vast effort in providing computational models • CREME96 (Cosmic Ray Effects on Micro Electronics Code): numerical models for ionizing radiations (near-Earth orbits) and effects in spacecraft • NUSPA (NUclear SPAllation): initial focus on sea-level interactions (where protons and neutrons play the most significant role); extended model interactions at high altitudes, including pion interactions

  25. Chapter 2: A Birds-Eye View Over Faults (22) 2.3.4. Introduction to Particle Physics • All known particles made of 2 types of building bricks: leptons and quarks • Leptons: do not interact by strong nuclear force • electrons, muons, tau particles, and their associated neutrinos • believed to be point-like particles • Hadrons: interact by strong nuclear force • Mesons: made by 2 quarks (pions) • Baryons: made by 3 quarks (protons and neutrons)

  26. Chapter 2: A Birds-Eye View Over Faults (23) 2.3.4. Introduction to Particle Physics • At sea-level approx. 94% of hadron total flux made by neutrons, 2% protons • Neutrons (uncharged particles) usually go through electrical circuits with no interactions • Unless combined into a nucleus, a free neutron follows a β-decay process (10.3 min half-life)

  27. Chapter 2: A Birds-Eye View Over Faults (24) 2.3.4. Introduction to Particle Physics • Only 1 neutron out of 4·104 hit a silicon nucleus • Hit very likely to produce soft fail • About 105 neutrons/cm2·year at sea level above 20MeV • Neutron flux exponentially influenced by altitude

  28. Chapter 2: A Birds-Eye View Over Faults (24) 2.3.4. Introduction to Particle Physics • Pions (π-mesons): unstable particles, 135MeV mass, 2% of total hadron flux at sea-level • neutral pion (its anti-particle being itself), with a lifetime of 10-16s • positive pion (its anti-particle being the negative pion), longer lifetimes of about 26ns (26·10-9s) • SER contribution expected negligible due to small numbers compared to other nucleons

  29. Chapter 2: A Birds-Eye View Over Faults (25) 2.3.4. Introduction to Particle Physics • Particles released during pion decay: electron, positron, gamma ray, muon, and neutrino • Charged pions can interact with matter • Low-energy positive pions repelled by the nucleus

  30. Chapter 2: A Birds-Eye View Over Faults (27) 2.3.4. Introduction to Particle Physics • Energetic, positive pion may lose kinetic energy to a nucleus • Pion capture, most contributive to SER • Pion-capture: entire pion mass is transformed into nucleonic energy, provoking a nuclear fision

  31. Chapter 2: A Birds-Eye View Over Faults (28) 2.3.4. Introduction to Particle Physics • Every negative pion capture within the active volume of an electronic circuit leads to a soft fail • Pion capture into silicon about 8.5/cm3·year at sea-level • For intermediate energies (between 100 and 250 MeV) pion contributions to SER of modern chips (16-Mb DRAMs) about 5 times greater that SER caused by protons • Pions may have significant impact at aircraft altitudes • Studies of pion-induced soft errors still in progress

  32. Chapter 2: A Birds-Eye View Over Faults (29) 2.3.4. Introduction to Particle Physics • Muons (μ-leptons):lifetime of 2.2 us • Produced in the upper atmosphere by pion decay • Two effects leading to soft fails: • electrostatic muon scattering from nuclei • muon capture (about 510/cm3·year): orbiting a Si nucleus; most of the initial mass-energy, the neutrino

  33. Chapter 2: A Birds-Eye View Over Faults (30) 2.3.5. Ion-Induced SEUs • Discrete satellite components highly resistant to radiations; modern satellites sensitive • Energetic heavy ions (100MeV Fe) at least partly responsible for reported fails • Croley et al.: roughly two-thirds of the fails due to ions with Z ≥ 6 (C, O, Fe, N, Ne, Mg, Si, S, Ar, Ca) • At satellite altitudes heavy energetic ions responsible for at least 45% (roughly equal proportions with the proton-induced) of SEUs

  34. Chapter 2: A Birds-Eye View Over Faults (31) 2.3.6. Neutron-Induced SEUs • Pathway of electron–hole pairs due to the energetic impact of a high-energy neutron striking a p-n junction • Minimum charge resulting in a soft error called critical charge (QCRIT) • Two types of neutron interactions: • Elastic scattering (target nucleus not excited, until QCRIT becomes smaller that 50fC minor role in SER) • Inelastic scattering

  35. Chapter 2: A Birds-Eye View Over Faults (32) 2.3.6. Neutron-Induced SEUs • Inelastic scattering nucleon + target → X1 + X2 + …+ Xn + residual nucleus • much higher exchange energies (order of MeV or larger) • identity of incoming particle lost • pions may also be produced • Example of inelastic scattering:

  36. Chapter 2: A Birds-Eye View Over Faults (32) 2.3.7. α-Induced SEUs • Natural decay processes (of U, Th, Ac and Np) generate helium ions 4He or α–particles • α–particles large mass and charge, most upsetting • α–decay may release energetic α–particles

  37. Chapter 2: A Birds-Eye View Over Faults (33) 2.3.7. α-Induced SEUs • Also due to impurities in semiconductor chip materials and packaging • Technological progress, careful supervision of materials quality: α–particle SER momentum lost • α–particles also emitted by at most 3% of captured muons

  38. Chapter 2: A Birds-Eye View Over Faults (33) 2.3.8. Proton-Induced SEUs • Particle distribution change with altitude • Inelastic scattering example: • Uncertainties concerning proton influences over silicon