Reliability of ECC-Based Memory
Architectures with Online Self-Repair
Capabilities
Gian Mayuga1, Yuta Yamato1, Tomokazu Yoneda1, Yasuo Sato2, Michiko Inoue1
1Nara Institute of Science and Technology, Ikoma, Nara, Japan
2Kyushu Institute of Technology, Iizuka, Fukuoka, Japan

Outline
• Research Background
• Proposed ECC-Based Memory Architecture
• Proposed Online Repair Strategy
• Reliability Evaluation
• Results
• Conclusion
12/19/14 IEICE DC 2

Issues on Embedded Memory
• Memory takes up most area in large-scale SoC’s
• Post-manufacturing failures highly occur in memory
• Periodic field-level test and repair, and error correction
required to maintain reliabilitySource: Semico Research Corp http://www.semico.com/content/semico-systems-chip-%E2%80%93-braver-new-world
20
38 44 53 58 65 69
0
20
40
60
80
100
1999 2000 2005 2008* 2011* 2014* 2017*
PercentofArea
% area new logic % area reused logic % area memory
SoC Area Partitioning
12/19/14 IEICE DC 3

Memory Errors
and Mechanisms for Repair and Correction
• Conventionally, errors treated independently
– Hard error
• Repair
– Soft error
• Correction
• Recently, Combined Approach is used
– Hard error also Corrected
– Errors in memory word can be classified under:
• Uncorrectable error
• Correctable error
Repair
Hard Errors Soft Errors
Alpha Ray
Cosmic Ray
Row/Column Faults
Array Faults
Correction
…
Given m faulty bits in a word, if
correction capable up to m-bits,
then word can be corrected
Memory
word
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Random
Bit Faults

Issue on Memory Word Reliability (1/2)
• Memory word with Uncorrectable error
– Must be Repaired
• Memory word with Correctable error
– Corrected
– More vulnerable if word already has faulty bit
512/19/14 IEICE DC
Word with 2-bit Errors
For example, using Single-Error Correction
Spare Word
Word with 1-bit Error Corrected Word
✓
Word with 1-bit Error Word failure

Issue on Memory Word Reliability (2/2)
• Memory word with Uncorrectable error
– Must be repaired
• Memory word with Correctable error
– Can be corrected
– Can also be repaired
612/19/14 IEICE DC
Repair
Hard Errors Soft Errors
Alpha Ray
Cosmic Ray
Row/Column Faults
Array Faults
Correction
Random
Bit Faults

Combined Approach of Repair and Correction
[2] T.H. Wu, et al. A Memory Yield Improvement Scheme Combining Built- In Self-Repair and Error Correction Codes. ITC 2012.
[5] C.L. Su, et al. An Integrated ECC and Redundancy Repair Scheme for Memory Reliability Enhancement. DFT 2005.
[6] M. Nicolaidis, P. Papavramidou. Transparent BIST for ECC-Based Memory Repair, IOLTS 2013.
Reference
Target Error Time Test
Performed
Remark
ECC Repair
[2] Correctable Uncorrectable Manufacturing Enhances Yield
[5] Correctable Uncorrectable In-Field
Transparent
BIST w/ ECC
[6] Soft Hard In-Field
Repairs all
Hard errors
Proposed Correctable
Uncorrectable
and
Correctable
In-Field
Enhances
Reliability
12/19/14 IEICE DC 7
Comparison of Studies using Combined Approach

Proposed Online Repair Scheme
To enhance reliability, erroneous words are repaired with spare
words as long as possible
• Both Uncorrectable error and Correctable error
– Repair (Address Remapping)
• Uncorrectable error is repaired
• Correctable error is repaired if spare space is available
• Remapped word with Correctable error is cancelled if
needed
– Correction
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Test, Repair and Correction in Field
• Test (mBIST)
– Test performed to identify errors
• Repair (mBISR)
– Use Spare Space to remap address
of memory word
– Physical replacement performed in
manufacturing test
• Correction (ECC)
– Use ECC to correct errors
• Scrubbing
– Re-write data to eliminate soft errors
Processors
Cache Cache
Logic block
Memory
Logic block
Test and Repair
Sample SoC with Embedded Memory
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Correction

Proposed Architecture and Strategy
Overview
• Normal Mode
• ECC and Scrubbing Controller
• Test Mode
• BIST and Diagnosis CAM
• Remap CAM and Remap Controller
– Proposed Online Repair Scheme
» Remap CAM Strategy
» Cases for Remap CAM
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Proposed ECC-Based Memory Architecture

Proposed ECC-Based Architecture
Normal Mode
Remap CAM enabled for remapping operation, and ECC/Scrubbing are in effect
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Remap CAM:
If memory address
stored, it is remapped
to spare address
Scrubbing
Controller:
Protection against
soft error
ECC:
Protection against
errors

Proposed ECC-Based Architecture
Test Mode
Diagnosis
CAM:
Error
information
saved in
Diagnosis
CAM, error
information
includes no.
of errors
Remap Controller:
Based on error
classification, faulty
words are remapped
BIST:
Performs test and
determines error
information
Remap CAM:
Where
remapping
information is
stored
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Test mode performed when memory is idle

Remap CAM Strategy
Spare Available? No. of Hard Errors Action Remark
Yes
1 Repair -
≥2 Repair -
Limited ~ None
1 - Correction
≥2 Repair
Cancel
word with 1
error
In this work, Single Error Correction ECC used
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Traditional Scheme
Proposed Scheme
No. of Hard Errors Action Remark
1 - Correction
≥2 Repair -

Uses Two Counters: RC2F and RC1F
Remap CAM
Faulty Address
RC2F
Bottom
Address
Top
Address
RC1F
Faulty Words
with 2 or more
errors
Faulty Words
with 1 error
12/19/14 IEICE DC 14
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
RC2F and
RC1F points
at next
address to
be written to

How Remap CAM works – Spare Available
(1/2)
Faulty Address
Bottom Address
Top Address
RC1F
Faulty Words
with 2 or more
errors
Faulty Words
with 1 error
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Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Write
Faulty
Address
with ≥2
errors
Faulty Address
RC2FRC2F points at next
address to be written to

How Remap CAM works – Spare Available
(2/2)
Faulty Address
RC2F
Bottom
Address
Top
Address
Faulty Words
with 2 or more
errors
Faulty Words
with 1 error
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Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Write
Faulty
Address
with 1
error
Faulty Address
RC1F
RC1F points at next
address to be written to

How Remap CAM works – FULL (1/2)
Faulty Address
Bottom Address
Top Address
Faulty Words
with 2 or more
errors
Faulty Words
with 1 error
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Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Write
Faulty
Address
with ≥2
errors RC2F
RC1F
Previous
remapping
cancelled

How Remap CAM works – FULL (2/2)
Faulty Address
Bottom
Address
Top
Address
RC2F
RC1F
Faulty Words
with 2 or more
errors
Faulty Words
with 1 error
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Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Spare Address
Faulty Address
Faulty Address
Faulty Address
Faulty Address
Write
Faulty
Address
with 1
error
Do nothing

time
Normal
Mode
Reliability of Proposed Architecture
0
scrubbing
1st Self-Test and
Repair period
ECC
Normal
Mode
scrubbing
ECC
2nd Self-Test and
Repair period
Normal
Mode
k-th Self-Test and
Repair period
scrubbing
ECC
Poisson
distribution for
hard error
λh=10-11 ,10-10
Poisson
distribution for
soft error
λs=10-7
Memory is reliable if there are no uncorrectable errors
that are not repaired/corrected at any given time
12/19/14 IEICE DC 19
Self-test: March-like test that repeatedly perform write and read in cells
Assume little chance of soft errors between read and the last write

Reliability Evaluation
• Evaluation done using R
• Conditions: λh<<λs since hard errors occur less frequently
• Scrubbing period: every 6 minutes
• Self-test period: every 10 days
• Reliabilities observed up until 50 years
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Remapping Scheme
• Proposed
– Proposed Method
• Traditional
– Repair only Uncorrectable word
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Reliability of proposed scheme vs
traditional scheme λh=10-11
Hard errors do not occur frequently and reliability is expected
to be near 1
Reliability
Years
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Reliability of proposed scheme vs
traditional scheme λh=10-10
When hard errors occur more often, proposed scheme may
use all the spare words, but still has better reliability than
traditional scheme
Reliability
Years
12/19/14 IEICE DC 23

Conclusion
• Given an ECC-based memory architecture, online repair
scheme that repair uncorrectable words and possibly
correctable word is proposed
• Novel memory reconfiguration using remap CAM is
proposed
• Reliability is evaluated under Poisson distributions of
hard errors and soft errors
• Reliability of proposed scheme is demonstrated to be
effective, and extends memory lifetime
12/19/14 IEICE DC 24