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Single Event Upsets (SEUs) – Soft Errors

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Single Event Upsets (SEUs) – Soft Errors

By:

Rajesh Garg

Sunil P. Khatri

Department of Electrical and Computer Engineering,

Texas A&M University, College Station, TX

- pn junction behavior
- Electric field
- Depletion region

- Energy band diagram of Si
- Energy transferred to Si may excite an electron from valence band to conduction band
- e-h pairs can be generated

- Radiation particles - protons, neutrons, alpha particles and heavy ions
- Reverse biased p-n junctions are most sensitive to particle strikes

- Charge is collected at the drain nodethrough drift and diffusion
- Results in a voltage glitch at the drain node
- System state may change if this voltage glitch is captured by at least one memory element
- This is called SEU
- May cause system failure

Radiation Particle

VDD

G

S

D

_

+

n+

n+

Depletion Region

+

_

+

_

E

_

+

_

E

+

VDD - Vjn

_

+

_

+

_

+

p-substrate

B

- Linear Energy Transfer (LET) is a common measure of the energy transferred by a radiation particle when it strikes a material
- Relationship between Q,LET and t

Charge of 1 electron

Therefore the charge deposited by a unit LET (for a track length of 1µm)

So the charge deposited by a radiation strike (in terms of LET and track length) is

- Bipolar Effect
- Parasitic bipolar transistor exists in MOSFETs
- For example, n-p-n (S–B–D) in an NMOS transistor

- Holes accumulation in an NMOS transistor may turn on this bipolar transistor

- Parasitic bipolar transistor exists in MOSFETs
- Alpha-particle Source-drain Penetration (ALPEN)
- A radiation particle penetrates through both source and drain diffusions

- A radiation particle strike is modeled by a current pulse as
where: tais the collection time constant

tb is the ion track establishment constant

- The radiation induced current always flows from n-diffusion to p-diffusion
- For an accurate analysis, device level simulationshould be performed

- Single Event Upsets (SEUs) or Soft Errors
- Troublesome for both memories and combinational logic
- Becoming increasingly problematic even for terrestrial designs

- A particle strike at the output of a combinational gateresults in a Single Event Transient (SET)
- If a memory latches wrongvalue -> SEU

- A particle strike in a memory element may directly lead to an SEU event

- Can be classified into three categories
- Device level
- Circuit level
- System level

- Device level – Fault avoidance
- SOI devices are inherently less susceptible to radiation strikes
- Low collection volumes

- Still needs other hardening techniques to achieve SEU tolerance
- Bipolar effect significantly increases the amount of charge collected at the drain node

- SOI devices are inherently less susceptible to radiation strikes

- Fault detection and fault correction approaches
- SEU events are detected using built in current sensors (BICS) (Gill et al.)
- Error correction codes (Gambles et al.)
- Triple modulo redundancy based approaches (Neumann et. al)
- Classical way of radiation hardening
- Area and power overheads are ~200% !!!!

- Fault avoidance approach
- Gate sizing is done to improve the radiation tolerance of a design (Zhou et al.)
- Radiation tolerance improves
- Higher drive capability
- Higher node capacitance

- Area, delay and power overheads can be large
- Selectively harden critical gates

- Approach A - PN Junction Diode based SEU Clamping Circuits

V (out)

Radiation Strike

0.8

1V

0.6

0.4

in

out

0.2

G

0

time

0V

D2

D1

1.4V

V (outP)

0.8

outP

GP

0.6

0.4

Shadow Gate

-0.4V

0.2

Higher VT device

0

time

-0.4

- Approach B - Diode-connected Device based SEU Clamping Circuits

V (out)

Radiation Strike

0.8

1V

0.6

0.4

in

out

0.2

G

0

time

0V

D2

D1

Ids

1.4V

V (outP)

0.8

outP

GP

0.6

0.4

-0.4V

0.2

Higher VT device

0

- Performance of approach A is slightly better than B but with a higher area penalty than B. Therefore, we selected approach B

time

-0.4

- Circuit simulation is performed in SPICE
- 65nm BPTM model card is used
- VDD = 1V
- VTN= | VTP| = 0.22V

- Radiation strike at output of 2X INV
- Q = 24 fC
- ta= 145ps
- tb= 45ps

- Approach B is used

- Phase 1
- Gate level hardening

- Phase 2
- Block level hardening
- Selectively harden critical gates in a circuit
- To keep area and delay overheads low
- Reduce SER by 10X

in

- A radiation particle strike at a reverse biased p-n junction results in a current flow from n-type diffusion to p-type diffusion
- A gate constructed using only PMOS (NMOS) transistors cannot experience 1 to 0 (0 to 1) upset

Radiation Particle

inp

out1p

inp & inn

VDD - VTN

out2

out2

out1

out1n

out1p

|VTP|

INV1

INV2

out1n

Radiation Particle

inn

out2

INV2

INV1

Static Leakage Paths

inp & inn

VDD - VTN

out1n

out1p

|VTP|

out2

Low VT transistors

inp

inp

out1p

out1p

X

out2

out2

out1n

X

inn

out1n

Leakage currents are lower by ~100X

inn

Radiation Tolerant Inverter

Modified Inverter

Radiation Particle Strike

inp

X

Radiation Particle Strike

M8

M2

X

out1p

inp & inn

X

M4

X

M6

out1n

out2

out1p

X

M5

out2

M3

out1n

The voltage at out2 is unaffected

X

M7

M1

A radiation particle strike at any node of the first inverter (radiation tolerant inverter) does not affect the voltage at out2

inn

- Radiation particle strike at the outputs of INV1
- Implemented using 65nm PTM with VDD=1V
- Radiation strike: Q=150fC, ta=150ps & tb=38ps

inp

out1p

out2

out1n

inn

INV1

- 100% SEU tolerance can be achieved by hardening all gates in a circuit but this will be very costly
- Protect only sensitive gates in a circuit to achieve good SEU tolerance or coverage
- We obtain these sensitive gates using Logical Masking
- PLM (G) is the probability that the voltage glitch due to a radiation particle strike gets logically masked
- PSen(G) = 1 – PLM(G)
- If we want to protect only 2 gates then we should to protect Gates 1 and 3 to maximize SEU tolerance
- Gate 3 is the most sensitive

P1 = 0.25 P0 = 0.75

0

0

For all inputs P1 = 0.5 P0 = 0.5

1

1

1

3

2

1

→ 1

0

P1 = 0.5 P0 = 0.5

Radiation Particle

- Obtained PSen for all gates in a circuit using a fault simulator
- Sort these gates in decreasing order of their PSen
- Harden gates until the required coverage is achieved
- Coverage is a good estimate for SER reduction (Zhou et al.)

- Gates at the primary output of a circuit need to be hardened since PSen = 1 for these gates
- The dual outputs of the hardened gates at the primary outputs drive the dual inputs of an SEU tolerant flip-flip (such as the flip-flop proposed by Liu et al.)

- Minimum amount of charge which can result in an SEU event
- Our hardened gates can tolerate a large amount of charge dumped by a radiation particle
- Operating frequency of circuit determines Qcri

- Qcriis the amount of charge which results in a voltage glitch of pulse width T

CLK

in

out1n

out1p

out2

t1

T + t1

2T + t1

- We implemented a standard cell library L using a 65nm PTM model card with VDD = 1.0V
- Implemented both regular and hardened versions of all cell types

- Applied our approach to several ISCAS and MCNC benchmark circuits
- We implemented
- A tool in SIS to find the sensitive gates in a circuit
- An STA tool to evaluate the delay of a hardened circuit obtained using our approach
- Layouts were created for all gates in our library for both regular and hardened versions

- Average results over several benchmark circuits mapped for area and delay optimality

- Our SEU immune gates can tolerate high energy radiation particle strikes
- Critical charge is extremely high (>520fC) for all benchmark circuits
- Suitable for space and military application because of the presence of large number of high energy radiation particles

- Our approach is suitable for radiation environments with high energy particles

- Decrease recovery time
- Slow down feedback path
- Insert resistors in the feedback paths

- Resistor
- Polysilicon
- Gated
- Increases write delay

- SEUs are troublesome for both memories and combinational logic
- Becoming increasingly problematic even for terrestrial designs

- Applications demand reliable systems
- Need to efficiently design radiation hardening approaches for both combinational and sequential elements
- Also need efficient analysis techniques to estimate SER of complex circuits
- SEU susceptibility can be checked during design phase
- Reduce the number of design iterations