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IEEE P2427:Proposing the Essential Framework for Measuring Defect Coverage in
Analog Circuits
… not officially speaking for the IEEE P2427 working group
Jeff RearickSenior Fellow, AMD
Full Disclosure / Disclaimer
� This presentation reflects work done by an IEEE working
group, but it is not an official statement from that group.
The opinions expressed are merely those of this author
(however reasonable they may seem ☺).
2
Outline
� Background, Mission, Guiding Principles
� Six important decisions
� Framework for analog defect coverage analysis
� Conclusions
3
Outline
� Background, Mission, Guiding Principles
� Six important decisions
� Framework for analog defect coverage analysis
� Conclusions
4
Background: analog defect coverage
� Digital tests are graded using fault coverage metrics
� Digital fault coverage measurement is
well-defined (EDA tools, IEEE 1804)
(…yet!)
The IEEE P2427 Working Group is now underway to tackle this challenge and define the standard
� Analog tests are, well, complicated
� There have been many approaches used
in the last 3 decades; nothing standard
5
Mission: IEEE P2427
� Define the following processes:
• Given a circuit, produce the universe of defects• Given a test for that circuit, determine which
defects are detected
� Standardize the following information:
• Definition and communication of defect models• Definition and communication of detection criteria• Reporting of defect coverage
6
Guiding principles
� Keep it simple
� Keep it real
7
Outline
� Background, Mission, Guiding Principles
� Six important decisions
� Framework for analog defect coverage analysis
� Conclusions
8
Decisions that P2427 needs to make
Either this: … or this?
Defect-oriented Spec-based
Behavioral Circuit-level
Faults Defects
Single Multiple
Manufacturing Reliability
Catastrophic Variation
9
123456
Decision tree
Defect-oriented or Spec-based?
�Defect-oriented: requires a defect model applied
to a circuit netlist, then simulation to determine
detection; coverage is ratio of detected / total
�Spec-based: requires documentation and
simulation of performance-based measurements
for complex specifications; what is “coverage”?
1
Keep it simple 10
Behavioral or Circuit-level?2
�Behavioral: uses a high-level model which
mathematically/logically mimics circuit-level
behavior
�Circuit-level: requires a schematic or layout-
extracted representation of the circuit which
matches the physical design
Keep it real 11
Faults or Defects?3
�Faults: abstractions of defects into unwanted
significant behavioral changes from the
intended function of the circuitry
�Defects: unintended significant changes in a
Design-intent primitive circuit element
{R,L,C,M,Q,D} or connections between them
Keep it real 12
Single or Multiple?4
�Single: the “single fault assumption” has been
fundamental to digital fault coverage
measurement for decades
�Multiple: this would add complexity and impair
scalability, but does not seem to add much value
to the quantification of coverage: detecting single
defect (often) covers multiple defects
Keep it simple 13
http://central.oak.go.kr/journallist/journaldetail.do?article_seq=11532&tabname=abst&resource_seq=-1&keywords=null
Manufacturing or Reliability?5
�Manufacturing: “time-zero” flaws determine
yield (follow the money), and a test that can
detect time-zero defects can be re-used later
in the life cycle
�Reliability: latent defects are far less
common, far harder to predict, and (almost by
definition) impossible to detect at time zeroKeep it simple 14
reliabilitysolutions.co.uk
Catastrophic or Variation?6
� This is a tough question! ??
15
� Keep it simple
� Keep it realProcess variations
� Process variations are not simple, but they are real
• Hundreds of process variables• Some are correlated, some are not• Many circuit elements can vary at once• Analog designers spend lots of time in Monte Carlo• We can’t not consider some aspect of variations…
Catastrophic or Variation? Both!6
�Catastrophic: hard defects are simple to
enumerate, and they are representative of a
number of realistic manufacturing flaws
�Variation: the trick here will be to specify a
scalable, enumerable, yet interesting set of
variation defects which will indicate the
quality of a test (our fundamental mission)
Keep it real, find a way to keep it simple 16
Decisions that P2427 has made
Either this: … or this?
Defect-oriented Spec-based
Behavioral Circuit-level
Faults Defects
Single Multiple
Manufacturing Reliability
Catastrophic Variation
17
123456
[Provisionally, of course]
Outline
� Background, Mission, Guiding Principles
� Six important decisions
� Framework for analog defect coverage analysis
� Conclusions
18
Framework for analog defect coverage
19
1. Defect Models and Universe
3. Defect Coverage Reporting
2. Detection Criteria1. Circuit-level enumeration
of catastrophic and variation-based time-zero defects
2. Circuit-level simulation comparing responses of defect-free and defective circuits using measurable parameters
3. Summary of total, detected, and untestable defects
Proposed analog defect models
� Catastrophic (hard defects: change netlist topology)
20Photos: https://www.semiwiki.com/forum/content/3677-catching-ic-manufacturing-defects-slack-based-transition-delay-testing.html
�Extra interconnect material (i.e. shorts)�Unintended connection between …
�… terminals of an element (schematic) �… adjacent nodes (layout)
� Variation (soft defects: change element/model values)
• This is still under discussion in the Working Group (next slides)
�Missing interconnect material (i.e. opens)�Unintended disconnection of …
�… an element terminal from node (schematic)�… fanout stem and/or branch (layout)
Visualizing variation defects: simplistic R
21
Acceptable
process
variationDEFECTDEFECT
Principal
parameter
value (R)
Nominal
design-
intent
value (R0)
like
lih
oo
d
R0
Catastrophic
open defect
value
Ropen
R0
Catastrophic
short defect
value
RshortR0
High
variation
defect
value
r
R0+r
Low
variation
defect
value
r
R0-r
Implicit failImplicit fail
Variation defect models: options
1. Out-of-bounds variation of a principal parameter (R, L, C, W/L,
Ion, Ron, Vt, B, …) of a single circuit element {R,L,C,Q,M,D}
• Is this a realistic defect model?
2. Out-of-bounds variations of a principal parameter of all
circuit elements (i.e. a global parameter shift, not multiple defects)
• How would we handle correlation between parameters?
3. In- or Out-of-bounds variations of multiple different principal
parameters of many different circuit elements
• Doesn’t scale: can’t define a finite defect universe
• Others? P2427 WG work in progress22
Illustrating the P2427 framework (741 op-amp)
23
Untestable shorts 3
Testable defects 141 (?) [1]
Notes:[1] Proof of untestability is necessary for determining when test generation
is “done”: in digital ATPG terms: “fault coverage” vs. “test coverage”
Element Count Defects
Bipolar transistors 20 120
Resistors 11 22
Capacitors 1 2
Total defects 144
Nodes 24
TBD [2]
Inputs 2
Outputs 1
InOuts 2
Supplies 2
[2] Node-related defects have some open questions:a) Inject too many shorts [(n)(n-1)/2] in schematic?b) Inject too few opens [one per terminal per node] in schematic?c) What should we do with Primary Inputs / Primary Outputs?
Analog defect coverage flow
24
Defect list
Derive list of n defects
Responses different?
circuitnetlist*
Simulate defect-free circuit (once)
Simulate defectivecircuit (n times)
Test stimulus
More in list?
* Netlist could be schematic or layout-extracted; in fact, both should be employed
Detectedlist
Y
Y
Done: report Detected/n
N
Detection criteria
Outline
� Background, Mission, Guiding Principles
� Six important decisions
� Framework for analog defect coverage analysis
� Conclusions
25
Conclusions
� IEEE P2427 will enable uniform assessment of analog
tests by quantifying the coverage of analog defects
� This is crucial for automotive quality
� This framework is a long-overdue step toward standardized
analog defect coverage measurement
� The P2427 Working Group has made substantial progress
toward our mission, but still has tough decisions to make:
� Defining and enumerating variation defects� Pre-identifying untestable defects� Documenting detection criteria, defect models & values
� Join us if you’ve got some good ideas!26
Framework for analog defect coverage
27Work in progress, but we’ve got a great crew!
1. Defect Models and Universe
3. Defect Coverage Reporting
2. Detection Criteria