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
1
Background

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

2
Charge Deposition by a Radiation
Particle – Drift and Diffusion





Radiation particles - protons, neutrons, alpha particles and heavy
ions
Reverse biased p-n junctions are most sensitive to particle strikes
Radiation
Charge is collected at the
Particle
drain node through drift
and diffusion
VDD
G
Results in a voltage glitch
S
D
at the drain node
_ n+
n+
Depletion
+
System state may change
Region
_+ _+ E
if this voltage glitch is
_
+ E VDD - Vjn
_
captured by at least one
+
_
memory element
_ +
+
_
 This is called SEU

May cause system failure
+
p-substrate
B
3
Charge Deposited by a
Radiation Particle


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
4
Other Charge Collection
Mechanisms

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
Alpha-particle Source-drain Penetration
(ALPEN)

A radiation particle penetrates through both
source and drain diffusions
5
Modeling a Radiation Particle
Strike

A radiation particle strike is modeled by a current
pulse as
Q
t / t b
t / t
iseu (t ) 
t a t b
(e
a
e
)
where: ta is 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 simulation
should be performed
6
Single Event Upsets

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 gate
results in a Single Event
Transient (SET)
 If a memory latches wrong
value -> SEU
A particle strike in a memory
element may directly lead to an SEU event
7
Radiation Hardening
Approaches

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
8
System Level Radiation
Hardening Approaches

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% !!!!
9
Circuit Level Hardening

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
10
Diode Clamping based
Hardening Approach

Approach A - PN Junction Diode based SEU
Clamping Circuits
V (out)
Radiation
Strike
1V
in
out
G
0V
D2
1.4V
GP
Shadow Gate
0.8
0.6
0.4
0.2
0
D1
V (outP)
outP
-0.4V
time
Higher VT
device
0.8
0.6
0.4
0.2
0
-0.4
time
11
Our Radiation Hardening
Approach

Approach B - Diode-connected Device based SEU
V (out)
Clamping Circuits
Radiation
Strike
1V
in
out
G
0V
D2
Ids
1.4V
GP
time
D1
V (outP)
outP
-0.4V

0.8
0.6
0.4
0.2
0
Higher VT
slightly
device better
Performance of approach A is
than B but with a higher area penalty than B.
Therefore, we selected approach B
0.8
0.6
0.4
0.2
0
-0.4
time
12
Protection Performance Example


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
13
Our Split-output Approach

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

14
Gate Level Hardening Approach

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
in
out2
out2
out1
INV1
INV2
Radiation Particle
inp &
inn
out1n
inn
INV1
VDD - VTN
out1n
out1p
|VTP|
out2
INV2
Static Leakage Paths
15
Our Gate Level Hardening
Approach
Low VT transistors
inp
inp
out1p
inp &
inn
out1p
VDD - VTN
X
out2
out2
out1n
out1p
out1n
|VTP|
X
out2
inn
out1n
inn
Radiation Tolerant
Inverter
Leakage currents are
lower by ~100X
Modified Inverter
16
Radiation Tolerant Inverter
inp
M2
X
X
X
Radiation Particle
Strike
M8
out1p
M4
X
Radiation Particle
Strike
inp &
inn
M6
out2
out1n
out1p
X M5
out2
M3
out1n
inn
M1
X
M7
The voltage at
out2 isstrike
unaffected
A radiation particle
at any node of the
first inverter (radiation tolerant inverter) does
not affect the voltage at out2
17
Radiation Tolerant Inverter



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
inn
out1n
INV1
18
Block Level Radiation Hardening


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)
0
For all
1
inputs
P1 = 0.5
P0 = 0.5 1

1
P1 = 0.25
0 P0 = 0.75
3
2
0→
P11= 0.5
P0 = 0.5
Radiation
Particle
1
Gate
PLM
PSen
1
0.5
0.5
2
0.75
0.25
3
0
1
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
19
Block Level Radiation Hardening



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 



G
P
 Sen
All _ hardened _ G *
G
Sen
All _ gates _ G
P
100
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.)
20
Critical Charge (Qcri)


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
in
Operating frequency of
circuit determines Qcri
out1n
Qcri is the amount of
charge which results in a
voltage glitch of pulse
width T
out1p


CLK
out2
t1
T + t1
2T + t1
21
Experimental Results

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
22
Experimental Results

Average results over several benchmark circuits mapped
for area and delay optimality
Avg. Results
Area Mapped
Delay
Mapped


Coverage
% Area Ovh
% Delay Ovh
90%
62.4
28.9
100%
97.7
44.3
90%
58.15
27.9
100%
96.5
47.6
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
23
Comparison Our Hardening
Approach

Our approach is suitable for radiation
environments with high energy particles
90% Coverage
Zhou et al.
Our Approach
Area Ovh.
90%
58%
Delay Ovh.
8%
28%
Critical Charge
~150fC
>520fC
24
SRAM Hardening


Decrease recovery
time
Slow down feedback
path


Insert resistors in the
feedback paths
Resistor

Polysilicon
Gated

Increases write delay

25
Conclusions

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
26
THANK YOU
27