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No Slide Title - Ohio University


									 ECE 617 - Fault Testable Design
 Dr. Janusz Starzyk

 School of EECS
 Ohio University
 Athens, OH, 45701

Partially based on Prof. Vishwani D. Agrawal lecture VLSI Testing
and book by S. Mourad, Y. Zorian, "Principles of Testing Electronic Systems”
IC Testing Machine
3360-P VLSI Test System
Outline                        0.18u VLSI silicon neurons
  Reliability and testing
  Design Process
  Verification & testing
  Faults and their detection
  Fault coverage
  Types of tests
  Test applications
  Design for Test
  Test economics
Reliability and Testing
   Reliability of electronics systems is no longer
   limited to military, aerospace or banking
   Used by almost everyone in the workplace
   Applied to smaller and smaller devices
   Have continually new failure modes
   Reliability depending on being error free
   Failures in both software and hardware
   Here we concentrate on hardware
Test Objective
 The goal over time is to reduce the cost of
 manufacturing the product by reducing the per-part
 recurring costs:

 - reduction of silicon cost by increasing volume and
 yield, and by die size reduction (process shrinks or
 more efficient layout)

 - reduction of packaging cost by increasing volume,
 shifting to lower cost packages if possible (e.g., from
 ceramic to plastic), or reduction in package pin count
Test Objective
 - reduction in cost of test by:
     - reducing the vector data size
     - reducing the tester sequencing complexity
     - reducing the cost of the tester
     - reducing test time
     - simplifying the test program
A System on a Chip

               Interface Block   Controller
     RAM         (RT Level )     (algorithm)

                       FPGA          UDL
Verification and Testing
   Testing a circuit prior to fabrication is known as
   design verification
   Verification is certainly done at various stages of the
   design process
   Most viable design verification is through simulation
   Testing is identifying that the fabricated circuit is free
   from errors
   Need to specify what errors testing is looking for
DFT Cycle

                    Behavioral              Technology
                      DFT                    Mapping

                  RTL Description
                                            P arameter
                      Logic                 Extraction
                       DFT                                    Libraries
                  Gate Description

                                              P roduct
                   Test P attern
                                           Test Application

            low       Fault         high    Good P roduct
Test Programming
Types of
Types of
Faults and their Detection
   Physical failures are manifested as electrical
   failures and are interpreted as faults on the
   logic level
   Several physical defects may be mapped into
   few fault types
   The main fault type is Stuck-at Fault
   A fault is detected by a test pattern
   Test pattern is an input combination that
   confirms the presence of the fault
Possible Defects
            R            Z
            L                    B

                                     A             Z
   A   R1
                                               A       Z
       R2          B
                   A         Z

                   (a)                   (b)
   Two technologies, two physical defects map into the
   same stuck-at zero fault
   Notation used - A SA0, A@0, or A/0
Detecting Stuck-at Faults
     A                         Fill in the blanks in
     B                         faulty response A/0 and A/1

 Inputs      FF                      Faulty Response
   AB     Response       A/0   B/0     Z/0      A/1    B/1   Z/1
   00         0                 0       0               0     1
   01         0                 0       0               0     1
   10         0                 0       0               1     1
   11         1                 0       0               1     1
Detecting Stuck-at Faults

 Inputs      FF                  Faulty Response
  AB      Response   A/0   B/0     Z/0     A/1   B/1   Z/1
   00         0       0     0       0       0     0     1
   01         0       0     0       0       1     0     1
   10         0       0     0       0       0     1     1
   11         1       0     0       0       1     1     1
Detecting Stuck-at Faults

  Inputs   Fault Free               Faulty Responses
  AB       Response         A/0   B/0 Z/0 A/1 B/1      Z/1
  00       0                0     0     0     0    0   1
  01       0                0     0     0     1    0   1
  10       0                0     0     0     0    1   1
  11       1                0     0     0     1    1   1
Sequential Circuit
                     A                  Q

                S         2

Inputs     FF                   Faulty Response
 SR      Response   A/0   S/0     R/0    A/1      S/1   R/1
 01         0       0     0       X         0     0     1
 00         0       1     0       X         1     0     1
 10         1       1     0       1         0     1     1
 11         0       0     0       1         1     1     1
Types of
    Types of Tests
The exhaustive test used to detect the faults on a
2-input AND gate is not practical for circuits with
20 or more primary inputs
Pseudo-exhaustive: exhaustive for components in
the circuits
   segmentation or partitioning
A random test is also viable to detect faults, but
pseudo-exhaustive tests are more realistic for
Stuck-at Faults
Deterministic or fault oriented tests
    Functional Testing

Exhaustive & pseudo-exhaustive testing :

Partial dependence circuits:
-a circuit in which primary outputs (PO)
       depend on all the primary inputs (PI)
- each output tested using 2ni inputs
       (ni < n shows inputs affecting PO)
  Functional Testing

Exhaustive & pseudo-exhaustive testing
Example :

                                   Exhaustive test
                                   for each gate
      Functional Testing

Exhaustive & pseudo-exhaustive testing
Partitioning technique :
the circuit is partitioned into segments such that
each segment has small number of inputs
each segment is tested exhaustively
usually inputs & output of each segment are not
PIs or POs so we need to control segment
inputs using PIs and observe its outputs using
PO - this lead to sensitizing partitioning
      Functional Testing

Example : Consider the following circuit :
        Functional Testing

Example: the following shows 8
input vectors to test exhaustively h.
        Functional Testing

Add vectors 5 - 8 to test exhaustively g
and 9 -10 to test exhaustively y
     Functional Testing

Add missing combinations to vectors
4 and 9 to test exhaustively x
  Types of Testing
Verification testing, characterization testing
  Verifies correctness of design and correctness of
  test procedure
  May require correction of either or both
Manufacturing testing
  Factory testing of all manufactured chips for
  parametric and logic faults, and analog
  Burn-in or stress testing
Acceptance testing (incoming inspection)
  User (customer) tests purchased parts to ensure
  Verification Test
Very expensive
  Applied to selected parts
  Used prior to production or manufacturing test
May comprise:
  Scanning Electron Microscope tests
  Bright-Lite detection of defects
  Electron beam testing
  Artificial intelligence (expert system) methods
  Repeated functional tests
   Manufacturing Test
Determines whether manufactured chip meets
Must cover high % of modeled faults
Must minimize test time (to control cost)
No fault diagnosis
Test at rated speed or at maximum
speed guaranteed by supplier
  Burn-in or Stress Test
  Subject chips to high temperature and over-voltage
  supply, while running production tests
Catches infant mortality cases
  These are damaged or weak (low reliability) chips that will fail
  in the first few days of operation
  Burn-in causes bad devices
  to fail before they are
  shipped to customers
  Manufacturing Test Scenarios
Wafer sort or probe test
  Done before wafer is scribed and cut into chips
  Test devices are checked with specific patterns to
   • Gate threshold
   • Polysilicon field threshold
   • Poly sheet resistance, etc.
Packaged device tests
    Types of Tests
Parametric – measures electrical properties of pin
electronics – delay, voltages, currents, etc. – fast and
Functional – used to cover very high % of modeled
faults – test every transistor and wire in digital
circuits – long and expensive

     Functional Test
ATE and Manufacturing World – any vectors applied
to cover high % of faults during manufacturing test
Automatic Test-Pattern Generation World – testing
with verification vectors, which determine whether
hardware matches its specification – typically have
low fault coverage (< 70 %)
   Levels of testing

   • Boards put together
   • System-on-Chip (SoC)
  System in field             Mixed Signal VLSI Circuit
Cost – Rule of 10
  It costs 10 times more to test a device as we
  move to higher levels in the product
  manufacturing process
  Levels of testing
Other ways to define levels – these are
important to develop correct “fault models”
and “simulation models”
Focus: Chip level testing
  – gate level design
      Typical Test Program
1. Probe test (wafer sort)
   Catches gross defects
2. Contact electrical test
3. Functional & layout-related test
4. DC parametric test
5. AC parametric test
    Unacceptable voltage/current/delay at pin
    Unacceptable device operation limits
Rise/fall Time Tests
Set-up and Hold Time Tests
       Propagation Delay Tests
1. Apply standard output pin load (RC or RL)
2. Apply input pulse with specific rise/fall
3. Measure propagation delay from input to output
       Delay between 5 ns and 40 ns (ok)
       Delay outside range (fails)
 On Line Testing
  Embedded checkers – error detection
  Periodic diagnostic programs
  Watchdog checkers

Circuit Under       N         Output     N
     T est

            On- vs Off-Chip Testing

 High Bandwidth
                                                                High Bandwidth
                                               Low Bandwidth

                                   Logic                                  Logic

  Source/                          RAM                                    RAM

                                   An alog                                An alog

External test      Embedded test             External test     Embedded test

   Off chip test                                      On chip test
       Test Specifications & Plan
Test Specifications:
  Functional Characteristics
  Type of Device Under Test (DUT)
  Physical Constraints – package, pin numbers, etc.
  Environmental Characteristics – power supply,
  temperature, humidity, etc.
  Reliability – acceptance quality level (defects/million),
  failure rate, etc.
Test plan generated from specifications
  Type of test equipment to use
  Types of tests
  Fault coverage requirement
  Test Data Analysis

Uses of ATE test data:
  Reject bad DUTs
  Fabrication process information
  Design weakness information
Devices that did not fail are good only if tests
covered 100% of faults
Failure mode analysis (FMA):
  Diagnose reasons for device failure, and find
  design and process weaknesses
  Improve logic and layout design rules
          Cost of Testing
 Testers cost over
 $1 000 000

VLSI Test System TS600
 Cost of Testing

Design for testability (DFT)
  Chip area overhead and yield reduction
  Performance overhead
Software processes of test
  Test generation and fault simulation
  Test programming and debugging
Manufacturing test
  Automatic test equipment (ATE) capital cost
  Test center operational cost
    Cost of Manufacturing Testing

Example test cost:
  0.5-1.0GHz, analog instruments,1024 digital
  pins: ATE purchase price
    = $4.272M
  Running cost (five-year linear depreciation)
    = Depreciation + Maintenance + Operation
    = $0.854M + $0.085M + $0.5M
    = $1.439M/year
  Test cost (24 hour ATE operation)
    = $1.439M/(365 x 24 x 3,600)
    = 4.5 cents/second

PCB for 16 channel pin
card for IC tester
    Test Economics

Time to Market

                                         Loss of

                                      Time to
                                                     Time in Months
                                      Market    T

The life cycle of a product is shorter than its design cycle
Time to market needs to be shorten
Testing is necessary for reliability and for improving yield
        VLSI Defects

                         Good chips
                        Faulty chips

                                        Smaller dies
                                        Wafer yield = 78/88 = 0.88

  Unclustered defects
Wafer yield = 12/22 = 0.55

                                       Clustered defects (VLSI)
                                       Wafer yield = 17/22 = 0.77
Yield and Defect Level

Defect Level
% DPM                          Y=50%   Y=90%
1    10000

0.1   1000

0.01 100

0.001 10
                .01     0.1    1   10 TT%
               99.99   99.9   99   90 C%

           Test transparency
Fault coverage
  Multi-site Testing

One ATE tests several (usually identical)
devices at the same time
Both probe and package test
DUT interface board has > 1 sockets
Usually tests 2 or 4 DUTS at a time
Usually test 32 or 64 memory chips at a time
Limits: # instruments available in ATE, type
of handling equipment available for package
       Example VLSI Test Systems

Advantest T3347B               40 MHZ testing speed.
Low-cost Parallel Testing of   Accommodates up to 512 I/O pins.
Four High-end MCU and          Simultaneous testing of up to four
Testing of Large ASIC          devices per station.
T6682 ATE Block Diagram
     T6682 ATE Specifications
Uses 0.35 mm VLSI chips in implementation
1024 pin channels
Speed: 250, 500, or 1000 MHz
Timing accuracy: +/- 200 ps
Drive voltage: -2.5 to 6 V
Clock/strobe accuracy: +/- 870 ps
Clock settling resolution: 31.25 ps
Pattern multiplexing: write 2 patterns in one
ATE cycle
Pin multiplexing: use 2 pins to control 1 DUT pin
T6682 Pattern Generation
Sequential pattern generator (SQPG):
  stores 16 M vectors of patterns to apply to DUT,
  vector width determined by # DUT pins
Algorithmic pattern generator (ALPG):
  32 independent address bits,
  36 data bits
Scan pattern generator (SCPG)
  supports JTAG boundary scan,
  greatly reduces test vector memory for full-scan testing
  T6682 Test Data Analysis
Uses of ATE test data:
  Reject bad DUTS
  Fabrication process information
  Design weakness information
Devices that did not fail are good only if tests
covered 100% of faults
Failure mode analysis (FMA)
  Diagnoses reasons for device failure
  Finds design and process weaknesses
  Allows improvement of logic & layout design rules
   T6682 Probe Card
Probe card – custom printed circuit board (PCB)
on which DUT is mounted in socket
  may contain custom measurement hardware
Probe needles
  come down and scratch the pads to stimulate/read pins
Membrane probe – for unpackaged wafers
  contacts printed on flexible membrane, pulled down
  onto wafer with compressed air
Intended for SOC test
  digital, analog, and memory test
  supports scan-based test
  can be upgraded with additional instruments
enVision Operating System
  maximum 64 M vectors memory storage
1 or 2 test heads per tester, maximum of 1024 digital pins,
1 GHz maximum test rate
Analog instruments:
  DSP-based synthesizers, digitizers, time measurement, power test,
  radio frequency source and measurement capability (up to 4.3 GHz)
         ADVANTEST Model T2000 ATE
Scalable Architecture

Microsoft Windows 2000

C++(Microsoft Visual Studio Professional)

OTPL(Open Architecture Test System
Programming Language)

Re-configurable Program Structure for test
data and algorithm

T2000 System Software Emulator

Wave Tool (Logic Analyzer, Oscilloscope).
        ADVANTEST T6577

Tests SoC/Mixed-Signal
Supports for a maximum of
1024 logic and/or I/O channels.
Performs parallel test of up to
32 devices
Supports baseband, DVD read
channel, and jitter test
At-speed test of high-speed
memory interfaces
Test rates of up to 667 Mbps
maximum of eight channels

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