SERESSA 2015
Non-volatile Memories and Their Space Applications A. S. Gerardin, M. Bagatin, A. Paccagnella
DEI - University of Padova Reliability and Radiation Effects on Advanced CMOS Technologies Group Outline
Introduction Overview . Storage mechanisms . Array organization and peripheral circuitry . Commercial and space market Details and radiation effects . Charge-based Cells • Floating Gate devices • Radiation effects on FG cells • Radiation effects on peripheral circuits . Phase change memories Conclusions
S. Gerardin, SERESSA 2015 2 Non-volatile Memories
A memory which retains information when powered off Key metrics . Speed, density, power (cost) . Retention: amount of time a memory is able to retain information (e.g., 10 years) . Endurance: maximum number of program/erase (read) cycles which can be performed on a memory (e.g., 105 cycles) Erase . An erase operation may be needed before program
S. Gerardin, SERESSA 2015 3 Non-volatile Storage Concepts
What physical quantity can be used to store information in a non-volatile way? . Charge • use a potential well to store charge • floating gate, nanocrystals, charge trap (SONOS, TANOS, NROM) . Phase • use materials which may be switched from amorphous to crystalline and vice versa • phase-change memories (PCM), calchogenide RAM (C-RAM) . Dielectric Polarization • use materials that retain their polarization (ferroelectric) • Ferroelectric RAM (FeRAM) . Magnetization • use materials that retain their magnetization (ferromagnetic) • Magnetic RAM (MRAM) . Other concepts: • Nanotube RAM (NRAM), Resistive RAM (RRAM), Conductive- bridging RAM (CBRAM), Millipede
S. Gerardin, SERESSA 2015 4 Array Architecture
Depending on the type of cell, different architectures are possible/needed: . Selection device for each cell • PCM, FeRAM, MRAM, … . Flash arrays for charge-based cells • NOR: cells in parallel • NAND: strings of memory cells (in series), two selection devices for each string of 16/32 cells . Crosspoint (no selection devices) • Memory elements at the intersection between bitlines and wordlines, no selection devices! • Ideal, best density, but parasitics…
S. Gerardin, SERESSA 2015 5 Peripheral Circuitry
Row and Column Decoders: block/page/cell Cell selection Array Microcontroller/State machine: executes complex program and erase Decoders algorithms Charge pumps: provide Charge high voltages/currents Buffers Pumps needed in some memories (e.g., floating gates) C Buffers: always, but may have different sizes
S. Gerardin, SERESSA 2015 6 Radiation Effects
Mix of elements determines radiation sensitivity . Storage mechanism . Architecture . Peripheral circuits elements
Rules of thumb . Cells may be hard, but periphery may be soft . Charge based cells more sensitive . When high voltages are needed for program/erase (or even read) operations, radiation sensitivity gets worse
S. Gerardin, SERESSA 2015 7 NAND FLASH Scaling Trends
S. Gerardin, SERESSA 2015 8 Mainstream Market
30 100
25 10
20 1 15 0.1 10
Total(B$)Sales 0.01 5 (Eb) Volume Total
0 0.001 1985 1990 1995 2000 2005 2010
Year
B$ Eb
S. Gerardin, SERESSA 2015 9 Mainstream Market Large (ever-increasing) capacity . now 128 Gbit at 16 nm, larger than SDRAM, and pushing Moore’s law, ahead of microprocessors; 3-D chips (V-NAND) Non-volatile’s are a growing share of the semiconductor memory market (Digital cameras, MP3 Players, …) So far dominated by Flash Floating Gate (FG) technology How long will survive FG/Charge-based beyond 20nm? Radiation Sensitivity . Low (100 krad or less, SEFI, SEU, SEL) because of cells (FG) and peripheral circuitry (all) Possible replacements (short/medium term?) . Phase change memories (good for radiation) for the NOR architecture S. Gerardin, SERESSA 2015 10 Rad-hard Memories Small capacity . Few Mbits, typically <64 Mbit Technologies . Charge Trap/SONOS devices . Magnetic RAM (MRAM) . Phase Change/Chalcogenide RAM (C-RAM) . Ferroelectric (FeRAM) Radiation Hardness . ~ Mrad . Robust cells (intrinsically or because of large feature size) . Rad-hard peripheral circuitry Future . Nanotube-based memories? …
S. Gerardin, SERESSA 2015 11 Space Applications
For space storage applications, flash NVMs feature: . the highest available storage density substantial savings in mass and volume occupancy . when in idle state, they may be kept unbiased without losing data energy saving . In case of device functional interrupts, power cycling can be used to restore the correct functionality without any data loss Yet, NVMs suffer, in comparison with SDRAM, of: . moderate data transfer rate . program/erase only at page/block level . limited endurance (105 cycles)
S. Gerardin, SERESSA 2015 12 Space Applications/2
Flash NVMs are not ideal for the workspace memories: . They cannot grant fast random access to small data entries Instead, NVMs are the right choice for mass storage applications: . mass memories typically buffer data entities of many kbytes without any need for data modification . the frequency of write accesses to the same storage location is often rather low, less than 10 write accesses/day less than 3.6 ∙ 104 operations within 10 years (i.e., less than 60 % of the endurance window) . wear-out issues can be mitigated by more storage capacity. Further, wear leveling distributes the storage locations rather uniformly over the address space, with integrated bad block management
S. Gerardin, SERESSA 2015 13 Charge-based Cells
Control Gate Blocking Dielectric Charge-storage element Tunnel - - - - - Oxide
Source/Drain
Storage concept: Inject or remove charge between the control gate and the channel, for instance in a floating gate Charge storage element: floating polysilicon gate (FG), charge-trap layer, nanocrystals
S. Gerardin, SERESSA 2015 14 Charge-based Cells
CHARGE STORAGE ELEMENT
Interpoly dielectric Control Gate ControlControl GateGate
Tunnel Floating Gate Floating Gate oxide
SourceSource DrainDrain
S. Gerardin, SERESSA 2015 15 Charge-based Cells: Vth
Positive or no With negative charge in the charge in the storage element storage element The presence of Erased Programmed electrons/holes in the charge-storage element alters the device Vread Vth threshold voltage Read: sense the drain current at a proper gate bias and compare it
Drain Current [A] with a reference current Gate voltage [V]
+ - Q Vth Q Vth
S. Gerardin, SERESSA 2015 16 FG: History
History . 1967: first proposal of Floating Gate memory (Kahng & Sze) . 1971: Electrical Programmable Read Only Memory (EPROM) is invented, after investigations of faulty MOSFET where the gate connection had broken (Frohman) . 1978: Electrically Erasable ROM (E2PROM) is introduced (Perlegos) . 1985: First attempts to build a NOR Flash memory . Mid 1990s: NAND Flash enjoys commercial success . 2003: NAND Flash overcomes NOR in terms of volumes State-of-the-art . NAND: 16-nm 128 Gbit . NOR: 45-nm 8 Gbit multi-level Future . Replacements are needed because development of FG cells is rapidly approaching scaling limits, but some countermeasures are already in place: 3-D NAND memories!
S. Gerardin, SERESSA 2015 17 FG: Coupling
Floating Gate (FG) C potential is the key P Control gate . Coupling with the other other C cells PP terminals - - - - - C C FG device equations S D can be obtained by substituting gate Source CB Drain voltage with floating gate voltage in
Vth = - QFG/CPP standard MOSFET’s equations V = (V -V ) + V + … FG G G th D D Important differences G=CPP/(CPP+CD+…) . Drain turn-on
x are coupling coefficients . No saturation with VD occurs
S. Gerardin, SERESSA 2015 18 FG: Band Diagram Floating gate Source - Cell Control Tunnel - Oxide - structure gate - ~10 nm - Blocking Drain ONO
- - Ideal energy band diagram
Erased Programmed
S. Gerardin, SERESSA 2015 19 Injection/Removal Mechanisms Channel hot electrons (holes) . Carriers are heated in the channel and acquire enough energy to surmount the potential barrier . Efficiency is low, a lot of drain current for a just a few injected carriers Fowler-Nordheim tunneling . Quantum-mechanical tunneling. The carriers do not have the energy classically needed to overtake the potential barrier . Slower than CHE/CHH (UV exposure) High voltages are required . charge pumps are needed to generate high voltages for single supply operation
S. Gerardin, SERESSA 2015 20 Threshold Voltage Distributions
Especially with large “1” Read “0” arrays, the statistical voltage distributions of parameters are extremely important [log#] Cell Distribution Cell Distribution
Vth [V] Ideal distributions
S. Gerardin, SERESSA 2015 21 Threshold Voltage Distributions
Especially with large Read arrays, the statistical distributions of voltage parameters are “0” extremely important “1” Vth distributions are typically gaussian
[log#] Tightening program/erase algorithms are used Cell Distribution Cell Distribution Behavior of tail bits Vth [V] may be dominant Erase Program Distributions are not verify verify visible to the end- user, only digital Actual distributions values
S. Gerardin, SERESSA 2015 22 FG: Multiple Bits per Cell
More than one bit per cell can be stored, by modulating the amount of charge injected in the floating gate
Rely on tight statistical control of Vth distribution More complex (and slower) program algorithms Increased density, lower cost but poorer performance and reliabilty
“10”“00” “01” “11” [log#] L0 L1 L2 L3 Cell Distribution
Vth
S. Gerardin, SERESSA 2015 23 Array Architecture
Electrically Programmable ROM (EPROM) . Erasable through UV exposure Electrically Erasable and Programmable ROM (E2PROM ) . Erasable at the single cell level . Requires a selection device for each cell density penalty Flash arrays . Erasable in blocks . No/few selection devices . Higher density . Combine the best of EPROM (density) and EEPROM (usability)
S. Gerardin, SERESSA 2015 24 NOR
Bit Lines (BL) NOR . Cells in parallel, bit size ~ 10 F2 (F=Feature size) Performance . Fast random access ~100 ns
Word Lines Lines (WL) Word . Word program ~ 5s . Block (Mb) erase ~200 ms Applications . Read-mostly memory
Source Source lines . Code storage
S. Gerardin, SERESSA 2015 25 NOR Operation Program ~5V . Channel hot electrons . WL high & BL high & SL GND . High programming HC current low parallelism
~9V 0V
S. Gerardin, SERESSA 2015 26 NOR Operation Program FLOAT FLOAT FLOAT . Channel hot electrons . WL high & BL high & SL GND ~-9V . High programming current 0V low parallelism Erase ~-9V . FN tunneling ~-9V . Low WL & high SL & float BL 0V . Sequence of pulses, erase ~-9V verify reliability margin
S. Gerardin, SERESSA 2015 27 NOR Operation Program . Channel hot electrons . WL high & BL high & SL GND . High programming current low parallelism Erase . FN tunneling ~5V . Low WL & high SL & float BL 0V . Sequence of pulses, erase verify reliability margin Read . WL on & sense BL . ECC may be present in MLC
S. Gerardin, SERESSA 2015 28 NAND Bit Lines BL NAND . Cells arranged in strings (16/32), bit size ~ 4F2 . The smallest feature size!
Drain . Source Selection Line (SSL), and
Selection Drain Selection Line (DSL) devices for each 16-32 FG string Performance . Series arrangement, - speed, + density and parallelism . High throughput ~ 40MB/s . Page (kB) program ~0.2 ms
Word Lines WL Word . Block (MB) erase ~2ms . Compulsory use of ECC Applications
Source . Used for data storage Selection
S. Gerardin, SERESSA 2015 29 NAND Operation
FLOAT 0V 0V Program . FN tunneling ~3V . Selected WL very high & ~10V other WL’s high & BL GND
~18V
~10V
~3V
S. Gerardin, SERESSA 2015 30 NAND Operation
FLOAT Program . FN tunneling FLOAT . Selected WL very high & ~0V other WL’s high & BL GND Erase ~0V . FN tunneling with reversed polarity
~0V
FLOAT Body ~20V
S. Gerardin, SERESSA 2015 31 NAND Operation
Program . FN tunneling ~4.5V . Selected WL very high & other WL’s high & BL GND ~4.5V Erase . FN tunneling with reversed ~0V polarity Read . Unselected WL’s biased at Vread both erased and ~4.5V programmed cells are on at Vread ~4.5V . Selected WL biased at 0V only erased cells are on . ECC needed
S. Gerardin, SERESSA 2015 32 Reliability of FG’s
Complex and critical . Intrinsic phenomena occurring in all cells in a uniform way . Single-bit failures occurring just in a few bits out of a large number of cells, due to • extrinsic defects (particles) • unfortunate configurations (alignment) of intrinsic point defects Endurance . Limited by generation of traps and charge trapping during high- voltage operations (10 MV/cm) in the tunnel oxide . Typical = 105 cycles Retention . Limited by Stress induced leakage current . Typical = 10 years
S. Gerardin, SERESSA 2015 33 FG: Radiation Effects Until a few years ago… . Only effects in the peripheral circuitry were of concern . Charge pumps weakest component …but nowadays . Effects in the peripheral circuitry are still very important but FG array sensitivity is an issue as well TID SEEs on: . Cells . Periphery: • Page buffer • Microcontroller
S. Gerardin, SERESSA 2015 34 FG: from Vth Shifts to Errors
When the threshold voltage shift is large enough to bring the cell beyond the read voltage, an error occurs
Intermittent errors may take place when Vth close to read voltage
Read voltage “0” “1” Error Vth,CL
No Error Cell Distribution
S. Gerardin, SERESSA 2015 35 FG: TID
Intrinsic distribution Uniform shifts of the V distributions 1 th "1" "0" 0.1 Movement towards 0.01 the intrinsic 0.001 distribution, i.e., the one the cells would 10-4 have with no net 10-5 Fresh charge in the FG 10-6 10krad(Si) Percentage of cells Percentage of 100krad(Si) discharge of the FG 10-7 012345678910 Typical doses for Vth [V] errors ~ nx10 krad(Si) G. Cellere, et al., TNS 2004 Dosimeters?
S. Gerardin, SERESSA 2015 36 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 37 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 38 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 39 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 40 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 41 TID: FG cells
Device irradiated during read cycles: Vth distributions and errors build-up as a function of accumulated dose 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 42 TID: FG cells No errors are observed in erased cells because the
neutral distribution is below Vref in these devices 1000000 100000 Errors at the output pins 10000 1000 800000 100 600000
number of cells 10
1 400000 Programmed ("0") Vref # bit# biterrorserrors Erased ("1") 200000 Vth distributions 0 M. Bagatin, et al., TNS 2009 0 100 200 300 400 Dose [krad] S. Gerardin, SERESSA 2015 43 FG: TID Basic Mechanisms (3) Three basic Ionizing - - mechanisms: radiation 1. Charge injection in - - - - the FG + (2) 2. Charge trapping in the tunnel oxide 3. Internal Photoemission Effects do not depend much on (1) scaling (remember + + oxides can hardly be Control Floating Silicon scaled because of gate gate bulk reliability)
S. Gerardin, SERESSA 2015 44 Flash: Charge Pumps Radiation Effects
Degradation of program charge pump TID, shift in the in a NAND Flash memory threshold voltage . Failure dose usually < 100 krad Single Event Gate Rupture, rupture of the gate oxide One of the most sensitive building blocks
M. Bagatin, et al., TNS 2009
S. Gerardin, SERESSA 2015 45 Total Dose Effects
Use cases 1. Mostly off: memory off most of the time and powered at regular intervals (about 2 krad), during which operating and retention tests are carried out 2. Low duty: memory powered in the selected state (ready to operate) and exercised at regular intervals (about 2 krad), during which operating and retention tests are carried out 3. High duty: memory continuously operated through a sequence of E/R/P/R operations with no dead time. Every 10 E/R/P/R cycles a retention test is carried out.
S. Gerardin, SERESSA 2015 46 Total Dose Fails in SLC memories
140 Failure criteria
120 Erase . Retention: 1% of bit Retention 1% corrupted 100 Program . Erase: failure to 80 erase a block 60 . Program: failure to
40 program a page Failure Dose Failure [krad(Si)
20 In SLC . Retention failures 0 SLC Mostly Off SLC Low Duty SLC High Duty independent of operating conditions Failure levels for 34-nm SLC NAND Flash . Erase and program manufactured by Micron fails occur later when S. Gerardin, et al., TNS 2010 memory is mostly off
S. Gerardin, SERESSA 2015 47 Total Dose Fails in MLC memories
80 Retention errors 70 . Occur at a higher 60 rate than in SLC and Erase 50 earlier than P/E fails Retention 1% 40 Program Program/Erase
30 . Failure levels lower than SLC 20 Failure Dose [krad(Si)] Dose Failure . Contrary to SLC, 10 mostly-off and low 0 MLC Mostly Off MLC Low Duty MLC High Duty duty are almost equivalent conditions for this type of Failure levels for 25-nm MLC NAND Flash manufactured by Micron failures S. Gerardin, et al., TNS 2010 . High duty most severe condition
S. Gerardin, SERESSA 2015 48 FG: Heavy ions
A secondary peak Read voltage appears in the 105 distributions right 104 after exposure to Fresh heavy ions 103 After rad . Distance between 100 Errors the two peaks is average Vth
Number cells of 10 . Height of the peak 1 (number of cells in 45678 the secondary V [v] distribution) related th to the number of struck FGs G. Cellere, et al., TNS 2001
S. Gerardin, SERESSA 2015 49 FG: Heavy ions
A transition region Read voltage appears in the 105 distributions 104 between the primary Fresh and secondary 103 After rad peaks 100 Errors Such region is correlated to
Number cells of 10 energetic delta 1 electrons emitted by 45678the ion hitting V [v] th outside the FG cell area G. Cellere, et al., TNS 2001
S. Gerardin, SERESSA 2015 50 FG & Heavy Ions: LET and Electric Field (1)
Charge loss linearly dependent on 3.5 . electric field
2.5 . LET Not dependent on [V] th 1.5 charge yield! V
Strong function of 0.5 scaling: the smaller the FG, the more -0.5 -1 1 3 5 7 severe the effect Tunnel oxide electric field [a.u.]
G. Cellere, et al., TNS 2004
S. Gerardin, SERESSA 2015 51 FG & Heavy Ions: LET and Electric Field (2)
Charge loss linearly dependent on 4 . electric field 3 . LET Small FG Not dependent on [V] th 2 charge yield! V Strong function of 1 scaling: the smaller Large FG the FG, the more 0 0050 50 100 severe the effect LET [MeV·cm2/mg]
G. Cellere, et al., TNS 2004
S. Gerardin, SERESSA 2015 52 FG & Heavy Ions: Basic Mechanisms
Charge loss linearly dependent on . electric field . LET Not dependent on - - - - - charge yield! Strong function of scaling: the smaller the FG, the more severe the effect Heavy ion strikes can Transient leakage path due to discharge the FG high density of electron/hole pairs . Transient conductive created by radiation path . Transient carrier flux
S. Gerardin, SERESSA 2015 53 Angular effects of heavy ions
What happens when changing the irradiation angle? In real world (e.g., space) the ion flux is ~isotropic
After 1217-MeV Xe irradiation 45°60°15°75°30°0°
At 75° more than 20 FGs corrupted by a single ion!
Cellere et al, TNS 2007
S. Gerardin, SERESSA 2015 54 Heavy-ion Induced Vth Tails
S. Gerardin, et al., TNS 2010 Heavy ions 1000000 collectively Pre‐rad produce: 100000 Post‐rad . Secondary Peak: the number of Secondary 10000 cells is in good Peak agreement with 1000 the number of Transition strikes going 100 Region through FGs Density distribution [a.u] distribution Density . Transition Region: 10 origin not yet Vth [a.u.] completely clear 65-nm Micron NOR Flash memories irradiated with 3∙107 cm-2 Silicon ions (E=121 MeV, LET=9.8 MeV∙cm2∙mg-1)
S. Gerardin, SERESSA 2015 55 Geant4 Application
65-nm NOR devices were modeled in 3D M3 Heavy-ions and neutrons strikes M2 were simulated Simulations M1(BL) provide energy deposition in WL the floating Substrate, gate and in the Silicon Floating gates tunnel oxide of particles crossing cells
S. Gerardin, SERESSA 2015 56 Simulations
Experimental Vth shifts Simulated energy deposition in the FG 1000000 1000000
100000 100000
10000 10000
1000 1000
100 100 Density distribution [a.u] distribution Density Density distribution [a.u] distribution Density 10 10 0.2 2.2 4.2 02040 LET [MeV·cm2·mg-1] Vth [V] 65-nm NOR irradiated with Ni The experimental data and the energy deposition can
be experimentally related through a Vth(LET)
S. Gerardin, SERESSA 2015 57 Vth vs LET
The experimental 3.5 Vth vs LET can 0.6739 3 y = 0.2988x R² = 0.9959 be fitted with a 2.5 simple relation,
[V] 2 either a power y = 0.0874x + 0.3962 th
V 1.5 R² = 0.9988 law or linear 1 relation 0.5 65nm Linear relation 0 with non-zero 0 10203040intercept is non- LET [MeV∙cm2∙mg-1] Experimental data on 65-nm NOR Flash physical memories irradiated with ions having different LETs
S. Gerardin, SERESSA 2015 58 Secondary peak
Energy deposition in Energy deposition in Floating Gate Tunnel Oxide 1000000 100000000 Experimental 10000000 Experimental 100000 Simulations 1000000 Simulations 100000 10000 10000 1000 1000 100 100 10 Density distribution [a.u] distribution Density Density distribution [a.u] distribution Density 1 10 0.2 2.2 4.2 0.2 2.2 4.2 65-nm NOR Vth [V] Vth [V] irradiated with Ni Agreement is much better when considering energy deposition in the FG as opposed to the tunnel oxide Width of the secondary peak related mostly to variability in energy deposition
S. Gerardin, SERESSA 2015 59 Small Events
In addition to the large energy depositions caused Small events by ions going Large event through FG (large events)… there are several small depositions by energetic electrons going through FG (small events), we can think of them in terms of total dose
S. Gerardin, SERESSA 2015 60 Vth vs TID
0.5 The experimental 0.45 Vth vs TID can be 0.4 y = 0.0154x fitted with a simple 0.35 linear relation 0.3
[V] The extracted
th 0.25
V conversion
0.2 0.15 coefficient is 15.4 65nm 0.1 mV/krad 0.05 If we weigh the small 0 events with this 0 10203040number… TID [krad]
Experimental data on 65-nm NOR Flash memories irradiated with x rays
S. Gerardin, SERESSA 2015 61 Transition Regions
Ni Si E=212.8 MeV LET=28.4 MeVcm2mg-1 E=121 MeV LET=9.8 MeVcm2mg-1 1000000 1000000 Experimental Experimental Sim. Large Events 100000 Sim. Large Events 100000 Sim. Small Events Sim. Small Events 10000 10000
1000 1000
100 100 Density distribution [a.u] distribution Density Density distribution [a.u] distribution Density 10 10 0.2 2.2 4.2 0.2 1.2 2.2
Vth [V] Vth [V] 65-nm NOR
A good agreement is obtained with different ions
S. Gerardin, SERESSA 2015 62 FG & Heavy Ions: Annealing
Even though, the oxides surrounding 26 the FG are quite thin (8.5 nm in NAND 4 24 FG) charge x 10 22 trapping can take 20 place
FG errors 18 Removal of trapped charge may cause 16 0.1 1 10 100 1000 some errors to Time after irradiation [hours] disappear both after Silver E=266 MeV, TID and heavy-ion LET=57 MeV cm2/mg exposures M. Bagatin, et al., TNS
S. Gerardin, SERESSA 2015 63 FG & Heavy Ions: Permanent Leakage Paths
In addition to prompt discharge, permanent damage can occur Cells hits by high-LET - - - - - heavy-ions and then reprogrammed, experience a charge loss over time Caused by permanent defects in the tunnel Permanent leakage path due to oxide leading to defects created by radiation Radiation-Induced Leakage Current (compare with SILC)
S. Gerardin, SERESSA 2015 64 Radiation Effects on VTH distributions
Large tails after irradiation 99.99% Number of bits in tail 93.4% does not depend on 2x107ions/cm2 30.7% ion LET (it depends on fluence) 4.8%
VTH strongly 0.67% depends on ion LET 0.09% I
0.012% Fresh Cumulative probability Ag Ni 0.002%
• These cells (hit) only 345678 are considered in the VTH (V) next experiments
S. Gerardin, SERESSA 2015 65 Data retention in hit FG cells
• Hit devices only 99.99% were re- 93.4% After 20 programmed days 30.7% 4.8% • After only 30min 0.67% a clear tail 0.09% appears… 0.012% Cumulative probability • …which 0.002% increases more 345678 and more with After 30 After time VTH (V) min program
After 20 days, VTH~4V!!!
S. Gerardin, SERESSA 2015 66 Model vs. Experimental
Model: multi-trap 10-18 assisted tunneling 10-19 current through 10-20 tunnel oxide 10-21 -22
Experimental data (A) (A) 10 GG II obtained from 10-23 0.04m2 device 10-24 irradiated with I 10-25 Experimental -26 Simulated previously shown 10 Bars variance 0.8 1 1.2 1.4 1.6 1.8 FOX (MV/cm) (spread) of •With 20 defects in the tunnel experimental data oxide, good agreement on: Lines – Mean value calculations – Extreme values (spread)
S. Gerardin, SERESSA 2015 67 NAND Flash: Dynamic Errors and SEFIs
Typical errors during read operation 106 105 SEFI under exposure to Dynamic errors heavy ions 120 Static errors 100 . Static errors, linked 80 to FG cells 60 . Dynamic errors,
Total errors Total 40 linked to the 20 peripheral circuitry 0 . Single Event 0 105 2x105 3x105 Functional -2 Fluence [ion cm ] Interruptions, due to strikes in the M. Bagatin, et al., TNS 2008 microcontroller memory
S. Gerardin, SERESSA 2015 68 NAND Flash: Read Protocol
In the NAND architecture a large page FG Array buffer (PB) is present A page is transferred from (to) the FG array into FG Array the PB before (after) being read (written) at the pins
S. Gerardin, SERESSA 2015 69 NAND Flash: Read Protocol
In the NAND architecture a 1 P 0 large page a 1 FG Array 0 buffer (PB) is g .. e 0 present 1 A page is Row Decoder transferred from (to) the FG array into FG Array the PB before (after) being read (written) at the pins
S. Gerardin, SERESSA 2015 70 NAND Flash: Read Protocol
In the NAND architecture a 1 Page P 0 large page a 1 0 buffer (PB) is Buffer g .. (PB) e 0 present 1 A page is transferred from (to) the FG array into the PB before (after) being read (written) at the pins
S. Gerardin, SERESSA 2015 71 NAND Flash: Read Protocol
In the NAND architecture a 1 B large page buffer Y P 0 Page T (PB) is present a 1 0 E Buffer g .. A page is (PB) e 0 transferred from 1 (to) the FG array into the PB before (after) being read (written) at the pins While in the PB, data are sensitive to radiation Dynamic Errors
S. Gerardin, SERESSA 2015 72 Flash: Single Event Functional Interruption
Error Description Solutions Type Strikes in the Block- The device’s ready Reset power only. microcontroller may erase signal never exits the busy state. lead to inconsistent SEFI states Partial Block-erase suspends Repeat the erase erase at the first address. process. . Read fails on whole Few blocks are erased. SEFI pages/blocks Write Write operation Reset power only. . Program/Erase failure SEFI completion’s status suspends Functionality can Read Sensing circuitry Repeat the read usually be restored SEFI suspends due to process or cycle charge built-up. Next power. through: read operation doesn’t clear errors. . Repeat operation Irregular Read operation locks Reset power only. . Reset SEFIs into endless loop. Write operation stops. . Power-cycle D.N. Nguyen, et al., REDW2002
S. Gerardin, SERESSA 2015 73 Future Memories Phase change memories . Phase Change Materials (e.g., chalcogenides) can be electrically switched between the amorphous and crystalline state, through Joule heating . These two states feature one or two orders of magnitude of difference in resistivity, which can be sensed with a proper scheme Nanotube RAM (NRAM) . Cell: single walled nanotube suspended over an electrode . “0” = suspended “1” = in contact (van der Waals forces) . Great potential for radiation hardness Resistive RAM (RRAM) . Based on materials whose resistivity can be changed electrically, with a breakdown-like path . Good scalability, cross-point arrays Conductive-bridging RAM (CBRAM, PMC) . Relocation of ions inside an electrolyte . Presence or absence of a nanowire between two electrodes Millepede . Use pits in a polymer material created with a MEMS-probe . Great scalability
S. Gerardin, SERESSA 2015 74 Phase Change Memories
Memory Cells . Based on the reversible phase transition of a chalcogenide material
Ge2Sb2Te5 (GST): amorphous XTL RESET SET
S. Gerardin, SERESSA 2015 75 Phase Change Memories
Phase Change Memories . Periphery • Many of the same issues as in standard CMOS circuits, but it may be hardened by design/process . Memory Cells • Believed to be hard and totally immune to radiation effects, because storage mechanism is not based on charge • As we will see that’s not entirely true
S. Gerardin, SERESSA 2015 76 Neutron Results 45-nm PCM
10 Devices in 9 rad] 8 retention mode pre 7 (i.e. off) 1CK rad/ 6
5 0CK No significant [post 4 ALL0 increase in bit 3 rate
errors caused by 2 Meas. noise neutrons, error 1
0 Bit regardless of 01234567 pattern Irradiated site
Irradiated with neutrons @ ISIS
S. Gerardin, SERESSA 2015 77 Heavy Ions along the Bit line 45-nm PCM
10 Highest used 9 rad]
LET(Si) = 59.2 8 -1 2 pre MeV∙mg ∙cm 7 rad/ 6 High fluence = 5 7 2
[post 3∙10 ions/cm 4 rate
3 No significant 2 Meas. increase in bit error 1 noise
Bit errors 0 0 20406080 Irradiation angle along BL [°]
Irradiated with 267-MeV iodine @ SIRAD
S. Gerardin, SERESSA 2015 78 Heavy Ions along the Word line (1)
45-nm PCM Highest used LET(Si) = 59.2 -1 2 60 MeV∙mg ∙cm rad]
High fluence 50 7 2 pre 3∙10 ions/cm 40 rad/ Large increase 30 [post
at high
rate 20 incidence
angles > 40°
error 10
Bit Meas. Errors only in 0 noise 0 20406080cells set to ‘1’ Irradiation angle along WL [°] prior to the Cells set to ‘1’. exposure Irradiated with 267-MeV iodine (crystalline) @ SIRAD
S. Gerardin, SERESSA 2015 79 Heavy Ions along the Word line (2) 45-nm PCM
60 Threshold LET rad] 50 ≥ than 40 -1 2 pre
MeV∙mg cm 40 rad/ Consistent with 30 neutron results: [post
20 no effect rate expected
error 10
Meas. Bit 0 noise 0 10203040506070 LET [MeV cm2/mg]
Cells set to ‘1’. Irradiated @ SIRAD
S. Gerardin, SERESSA 2015 80 Current Distributions
Errors only in cells set to ‘1’ prior to 1.0E+00 the exposure 1.0E‐01 '0' (crystalline) 1.0E‐02 '1' 1.0E‐03 About one order of 1.0E‐04 magnitude 1.0E‐05 distribution between the lower 1.0E‐06 Cell edge of the ‘1’ 1.0E‐07 1.0E‐08 current distribution 1.0E‐09 and the upper 0.1 1 10 100 edge of the ‘0’ Current [a.u.] distribution
Representative pre-rad current distribution
S. Gerardin, SERESSA 2015 81 Cell Design Cell is asymmetrical In particular, the Crystalline GST heater section has storage element one litho-driven dimension and one sub-litho dimension Heavy‐ion The interface between damage track the heater and the Heater chalcogenide material is rectangular, with the long side oriented Word line along the word line Damage at the Cartoon of a PCM cell (not to heater/GST interface scale)
S. Gerardin, SERESSA 2015 82 Latent Track
Energetic heavy ions can create latent tracks of damage in several materials, including insulators and semiconductors Thermal spike model: due to electron-phonon coupling, a heavy ion heats the material in a small cylinder around its trajectory, causing the melting temperature to be locally reached
No data exist for Ge2Sb2Te5, and no general rules are available to predict latent track formation
Due to the quick cooling, the GST material may be amorphized after the heavy-ion strike!
S. Gerardin, SERESSA 2015 83 Conclusions
Non-volatile memories have been dominating the commercial market They have become more and more appealing for space applications, thanks to their non-volatility and high density, not matched by rad-hard components Yet, these devices are sensitive to both TID and SEEs, in both the cell arrays and peripheral circuitry They are intensively studied, in order to evaluate the more resilient devices and, correspondingly, the best conditions for their use in different radiation environments New unexpected effects may appear due to nano-size!
S. Gerardin, SERESSA 2015 84 Terrestrial Effects: Neutrons
Atmospheric neutrons . Not charged, they do not directly ionize materials . They can give rise to secondary charged byproducts (= heavy ions) by interacting with chip materials . Wide range of energies . The generated charge particles have a wide distribution of LETs
S. Gerardin, SERESSA 2015 85 Neutron Induced Vth Tails
100000000 10000000 Atmospheric-like neutrons produce: 1000000 [a.u]
Pre‐rad . Secondary charged 100000 Post‐rad byproducts 10000 . Linear Vth tails in distribution 1000 log-lin scale 100 Density 10 1
Vth [a.u.]
65-nm NOR Flash memories irradiated with 3.6∙1010 cm-2 neutrons with atmospheric-like spectrum
S. Gerardin, SERESSA 2015 86 Neutrons
10000 Geant4 used to assess neutron Experimental byproducts, 1000 heavy-ion and x- Sim. Large Events ray data for Sim. Small Events conversion 100 coefficients Small events fit the 10 tail at lower Vth
Density distribution [a.u] distribution Density Large events fit the 1 tail at higher Vth 0.25 0.75 1.25 1.75 Good agreement V [V] despite limitations th (energies, angles Tail on L3 for 65-nm NOR Flash memories of heavy ions used irradiated with 3.6∙1010 cm-2 neutrons with for conversion) atmospheric-like spectrum
S. Gerardin, SERESSA 2015 87 Neutron FG error cross section NAND MLC samples 1.E10‐13‐13 Vendor A ‐14 Non-zero error rate (pre-rad errors) even 1.E10‐14 Vendor B
] without radiation, due to read and program 2 1.E‐15‐15 10 ref Vendor A disturb, erratic tunneling, stress induced 1.E‐16‐16 ref Vendor B [cm 10 leakage current, etc.
1.E‐17‐17 10 1.E‐18‐18 Bit 10 1.E10‐19‐19 1.E10‐20‐20 20 30 40 50 60 70 80 90 100 Node [nm] The number of errors due to neutrons is significantly larger than pre-rad errors during our accelerated tests exponentially increases as the cell feature size is reduced The FG error bit averaged on all levelsfor 25-nm samples is more than one order of magnitude larger than for 50-nm samples
S. Gerardin, SERESSA 2015 88 Neutron FG error cross section MLC samples SLC samples 1.E10‐13‐13 101.E‐13 Vendor A Vendor A ‐14 1.E‐14 1.E10‐14 Vendor B 10 ]
] Vendor B 2 2 1.E‐15‐15 ref Vendor A 1.E‐15 10 10 ref Vendor A 1.E‐16‐16 ref Vendor B 1.E‐16 [cm [cm 10 10 ‐17 1.E‐17 1.E‐17 ‐17 10 10 No errors ‐18 1.E‐18 Bit 1.E‐18 Bit 10 10 1.E10‐19‐19 101.E‐19 1.E10‐20‐20 101.E‐20 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 Node [nm] Node [nm] 34-nm SLC (vendor A) is one of the first generations of single-bit cells to be sensitive to neutrons Errors are all ‘0’ to ‘1’ flips (programmed to erased) The neutron cross section of 34-nm SLC is more than 3 orders of magnitude smaller with respect to 25-nm MLC samples
S. Gerardin, SERESSA 2015 89 Model: Neutron Scaling Trends
Through the modeling of 101E‐12 threshold LET, neutron Vendor A 101E‐13 Vendor B cross section is modeled ] 1E‐14 2 10 Model 101E‐15 and fitted to our
[cm Observability Limit 1E‐16
10 experimental data 101E‐17 101E‐18 A turnaround is expected 101E‐19 between 10 and 20 nm Neutron 101E‐2020 101E‐21 . For one or few more 101E‐22 generations we expect the 0 255075100125150 neutron to increase Feature size [nm] . Afterwards a significant decrease should occur, due to M. Bagatin, et al., TNS 2014 the fact that the FG becomes smaller and smaller than the heavy-ion track
S. Gerardin, SERESSA 2015 90 Alpha FG error cross section
For MLC samples the 110 E‐10‐10 110 E‐11‐11 MLC alpha error is about 3 1 E‐12‐12 SLC
] 10 orders of magnitude larger 2 110 E‐13‐13 1 E‐14‐14 with respect to the neutron [cm 10
1 E‐15‐15 error 10 No errors 1 E‐16‐16 Bit 10 From 50-nm MLC devices 110 E‐17‐17 110 E‐18‐18 to 25-nm ones, the error 110 E‐19‐19 sensitivity increases by 10 20 30 40 50 60 70 Feature size [nm] almost 2 orders of magnitude - MLC and SLC samples by vendor A The most scaled SLC 241 irradiated with Am alpha particles samples we tested (34 nm) - Empty symbols indicate the observability limit of events after a fluence of 3·107 cm-2 are not sensitive to alpha particles yet
S. Gerardin, SERESSA 2015 91