1 MAPLD C192Degalahal SESEE: A Soft Error Simulation & Estimation Engine V Degalahal 1, S M Çetiner 2, F Alim 2, N Vijaykrishnan 1, K Ünlü 2, M J Irwin 1 1 Emerging and Mobile Computing Center(EMC^2) 2 Radiation Science and Engineering Center (RSEC) Pennsylvania State University MAPLD 2004

Outline Introduction Soft Errors: Physics  Scattering  Absorption Monte Carlo Method: Overview Tool Outline  Neutron Transport Simulation  Modeling Neutron-Si interactions  Ion transport simulation  Circuits simulation Case Study

Introduction Soft errors or transient errors are circuit errors caused due to excess charge carriers induced primarily by external radiations These errors cause an upset event but the circuit itself is not damaged. Same as SEU (single event upset) SEU for space-born errors

Soft Errors For a soft error to occur at a specific node in a circuit, the Q collected at that particular node should be more then Q critical As CMOS device sizes decrease, the charge stored at each node decreases (due to lower nodal capacitance and lower supply voltages). This potentially leads to a much higher rate of soft errors

Soft Errors Soft Errors can cause problems in 3 different ways  Affects memory elements like caches and memory  Affects the data path if the error propagates to the pipeline registers.  Change the character of a SRAM-Based FPGA circuit(Firm Error)

B S D p substrate G n+ n channel Soft Errors + - A particle strike Current 3.6 eV for one electron hole pair

Interaction Mechanisms Scattering  Elastic scattering  Potential scattering  Resonance scattering  Interference scattering  Inelastic scattering Absorption

Elastic Scattering Potential Scattering  Neutron scatters elastically off of the nuclear potential without penetrating the nuclear surface Resonance and Interference Scattering  The neutron is first absorbed by the target nucleus creating a compound nucleus  Creation of compound nucleus is then followed by the reemission of neutron  The target nucleus returns to its ground state.

Energy Transfer through Inelastic Scattering The incident neutron is first absorbed by the nucleus forming a compound nucleus The nucleus subsequently decays by reemitting a neutron Unlike elastic scattering, the final nucleus is left at an excited state

Energy Transfer through Inelastic Scattering Such reactions occur only for relatively high energies. Kinetic energy is not conserved When the incident energy of the neutron exceeds 280 MeV, secondary pions can also be produced The identity of the incoming particle is lost, and the creation of secondary particles is associated with energy exchanges of the order of MeV or larger

Energy Transfer through Absorption Incoming particle is captured by the nucleus. The absorption might be followed by a subsequent gamma emission depending upon the state of the compound nucleus The absorption process of our interest is the 10B fission capture:

Monte Carlo Method Utilizes Stochastic Technique  based on the use of random numbers and probability statistics to investigate problems Obtains answers by simulating individual particles and recording some aspects of their average behavior Does not solve an explicit equation

Monte Carlo Method Does not need averaging approximations required in space, energy and time Well suited to solving complicated three- dimensional, time-dependent problems The use of the Monte Carlo method as a radiation transport research tool was started at Los Alamos National Laboratory during 1940s We will be using two MC based codes: MCNP and TRIM in our tool to simplify the complexity

Terrestrial Cosmic Ray Spectrum SESEE:Flow Diagram Neutron Si interaction Ion transport Q collected Circuit: Netlists & Layouts Q critical SER

Terrestrial Cosmic Ray Spectrum SESEE:Flow Diagram Neutron Si interaction Ion transport Q collected Circuit: Netlists & Layouts Q critical SER

NEUTRON TRANSPORT A general-purpose, continuous-energy, generalized-geometry, time-dependent, coupled Monte Carlo N–Particle (MCNP) transport code  Used for neutron, photon, electron, or coupled neutron/photon/electron transport, including the capability to calculate eigenvalues for critical systems

NEUTRON TRANSPORT MCNP  Uses a continuous energy scheme, rather than energy groups  Neutron energy range: MeV to 150 MeV  photon and electron energy range from 1 keV to 1 GeV  Has generalized 3-D geometry capabilities with elaborate plotter capabilities  Has elaborate tally capabilities  “Ptrac” gives elaborate details of the neutron transport location, collision, absorption for each neutron

MCNP We get the different collisions and there positions in the silicon  Can be aborption  Or Inelastic scattering creating charged particles like Carbon, Al or Mg etc  Currently each resultant reaction is assumed to be of the equal probability, we plan to augment the tool through the use of the more detailed codes like MECC

MCNP The reactions modeled are The sample cell contained both P-type (containing Boron) and N-type regions (containing Phosphorus)

MCNP Model Side View of the model Top View of the model Side View of the model B: p well P: n well

Neutron Transport: Results The MCNP input is the terrestrial cosmic ray PDF The MCNP output is from the ‘ptrac’ card  The Ptrac result is parsed and converted into a form that TRIM can accept as input For 300 million neutrons injected,  There are 1100 inelastic scattering event  A 10 absorption events

Terrestrial Cosmic Ray Spectrum SESEE:Flow Diagram Neutron Si interaction Ion transport Q collected Circuit: Netlists & Layouts Q critical SER

Ion Transport Neutron collision or absorption may be followed by charged particle creation Charged particles create random electron hole pairs and get collected at junction The process can be modeled by Monte Carlo We use TRIM to simulate the transport of charged particles TRIM calculates the ion tracks and the resultant ionization

Ion Transport On plotting the ionization and the particle penetration depth (Bragg’s Curve) Resultant ionization is assumed to give a point charge The results are stored as a table

Typical Results using TRIM The typical reaction byproducts 4 He, 12 C, p and n create the ionization of the range of 2.5E4e-h/um, 4E5 e-h/um, 3.7E3 e-h/um, 0 These correspond to a Charge of 3.5e-13C, 1.8e- 13C, 2.321e-13C

Terrestrial Cosmic Ray Spectrum SESEE:Flow Diagram Neutron Si interaction Ion transport Q collected Circuit: Netlists & Layouts Q critical SER

Terrestrial Cosmic Ray Spectrum SESEE:Flow Diagram Neutron Si interaction Ion transport Q collected Circuit: Netlists & Layouts Q critical SER

Circuits At circuit level Q critical is the most common metric to evaluate the robustness Evaluated using circuit simulators such as Hspice. Transient Pulse modeled as current pulse with a sharp rise and slow decay The measured Q critical is stored in the file along with the node name as a table The input to the tool are the GDSII files with the different node names and the tabulated Q critical values

Circuits 0

GDSII  GDSII is a binary file format which is classified as a "data interchange format", used for transferring mask-design data between the IC designer and the fabrication facility  GDSII gives the spatial information unlike spice netlists which account for only electrical properties  Effective in evaluating the schemes such as interleaving and their effect on soft errors.  Hence, the spice netlists accounts for the process, device and circuit while GDSII files of the circuits account for topology

Circuits Charge collection and Funneling depth are given by the TRIM simulation which give the Q collected The Q collected is weighed based on the depth D and the distance from the circuit node. The GDSII files are parsed using the xrlcad2.0 library package An failure is assumed of the ratio of the weighed Q collected / Q critical lesser than 1

Circuits Charge Sharing  Past studies have shown the charge sharing to be additive or linear for related nodes  For non related nodes and inverse square rule is used to explain the charge sharing.  Such an approximation is used to reduce the computation time as the alternate of simulating the charge transport in silicon is very time tedious and requires extensive device level simulations.

Case Study A Custom designed SRAM is chosen  The Qcritcal for the two nodes was calculated by simulating in Hspice, and found as 15 and 330fC  So for a typical inelastic scattering, the Soft Error will observed from only those particles that generate a secondary 4 He particles.

Further work Effect of charge depth has to be modeled Make use of Monte Carlo based nuclear internal cascade codes for precise modeling of inelastic scatterings