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Background Material. Top Metal Layer (Metal 3 in RT54SX32S, Metal 4 in RT54SX72S). Barrier Metal Layer. Amorphous Silicon. Barrier Metal Layer. Second Metal Layer (Metal 2 in RT54SX32S, Metal 3 in RT54SX72S). What is an Antifuse?. Antifuses start life as capacitors

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Top Metal Layer

(Metal 3 in RT54SX32S, Metal 4 in RT54SX72S)

Barrier Metal Layer

Amorphous Silicon

Barrier Metal Layer

Second Metal Layer

(Metal 2 in RT54SX32S, Metal 3 in RT54SX72S)

What is an Antifuse?

  • Antifuses start life as capacitors
    • Layer of amorphous silicon between layers of titanium nitride barrier metal

Conducting Filament

Programming an Antifuse

  • Programming
    • High voltage causes the amorphous silicon to break down
      • As conduction begins to occur, localized heating causes the amorphous silicon and barrier metal to fuse, forming a conducting filament
      • Careful control of voltage and current during programming pulses ensures tight distribution of programmed antifuse resistance

About Programmed Antifuse Failures

  • 11 Confirmed failures. Other cases are suspected.
  • "Cluster of failures" - small number of users reporting a small percentage of failures on 0.25µm MEC SX-A/RTSX-S devices
  • Failures are occurring in a “dirty” or "out-of-spec" environment
  • Data strongly suggests early failures under "dirty" electrical environment
  • Experiments ongoing:
    • Determine reliability under “in-spec” conditions with respect to I/O and VCCA noise
    • Determine sensitivity to “out-of-spec” conditions, with respect to I/O and VCCA noise.

Usage History

  • More than 1.2 Million commercial 0.25 µm MEC SX-A units shipped
    • Four aerospace customers with confirmed failures report an approximate failure rate of 2%-7%
      • These are from a small sample of shipped units
    • Actel does not see an overall 2% functional failure return rate
    • Actel Failure Analysis group reports 58 units of SX-A products submitted for analysis in the last 2 years
      • One of these 58 units were traced to antifuse damage
      • Breakdown of other failure causes
        • Design Timing Fault
        • EOS of the non-antifuse nature (e.g., clock pin EOS damage, ref IAT)
        • ESD
actel published mec 0 25 m reliability data
Actel Published MEC 0.25µm Reliability Data
  • High Temperature Operating Life(HTOL):
    • 1 antifuse failure out of 805 devices
      • Equivalent 601,440 device hours
      • Failure occurred after 184 hrs (1st pull point)
      • The burnin system induced out of specification spikes of 9V on VCCA. This sequence has been since corrected.
  • Low Temperature Operating Life(LTOL):
    • 0 failures out of 334 devices
      • Equivalent 309,000 device hours
eos model reliability data
EOS Model Reliability Data
  • Clean Burn-in (no VCCA spikes observed >3V)
    • 0/77 failures for 168 hrs HTOL at 150°C
  • No I/Os switching
    • Special burn-in stress: antifuses are stressed with operating current levels using VPP
    • 0/100 failures at 168 hrs HTOL. Subsequent additional 168 hours in LTOL had zero failures.
  • Contractor X application
    • Contractor X found zero failures in their applications. Typical designs were 33MHz, one at 120MHz.
    • Contractor X measured I/O undershoot of –1.8V. But only 1 or 2 pins switching.
    • On the PCI bus with 33 Simultaneously Switching Outputs, care was taken to limit the undershoot to less than 500 mV.
    • The above results support the Simultaneously Switching Undershoot model
early failure under out of specification conditions
Early failure under out of specification conditions
  • Actel data and userdata indicates an early failure problem under out of specification conditions:
    • Actel experiments:
      • 10/246 units failed at 168 hours HTOL, LTOL “dirty” burn-in (out of spec -1.1V undershoots on I/Os)
        • First pull point for both HTOL and LTOL (first observation point) was at 168 hours
      • 0/207 fail at 500 hours cumulative HTOL, LTOL (same “dirty” burn-in)
      • 29 units held back due to burnin board capacity issues.
    • Single failure reported in Actel published reliability data was at first observation (184 hrs)
    • Extended burn-in
      • 0/8 RT54SX32S failures with 5000 hrs burn-in at 125°C
      • 0/8 RT54SX32S failures with 2000 hrs burn-in at 150°C
      • 0/22 RT54SX32S failures with 2000 hrs burn-in at 125°C
early failure under out of specification conditions1
Early failure under out of specification conditions
  • Sanitized Actel Customer data
    • Contractor A
      • 3/101 failed at first observation on ATE (unconfirmed failures)
      • Contractor A has no flight box failures. More than 3500 device hours on system. Some units >400hrs. (reported on 2/13/04)
    • Contractor B
      • Measured I/O undershoots exceed Actel maximum operating conditions.
      • 8 failures reported out of 100+ devices. Four parts confirmed antifuse failures. One failure could not be duplicated at Actel. One part was not returned to Actel. FA ongoing on other two units.
      • 1 failure was immediate. 6 failures less than 168 hrs. 1 failures at 0-400hrs (first observation)
      • No more failures seen. Contractor B has approx. 63,000 failure free hours (reported Dec 16, 2003). 18 parts >2000 hrs. 18 parts >1000 hrs. 18 parts >500 hrs.
    • Contractor C
      • Programmer with expired calibration, intermittent fan found. 1/5 device failure rate, of every 5 units programmed one failed
      • 10 failures reported out of 100+ devices. All in one design. All at first observation (< 1hr). Antifuse failure confirmed on 1 device.
      • No more failures seen after new programmer (replace bad fan). More than 100,000 device hrs (reported 2/13/04). Some parts more than 1year testing.
early failure under out of specification conditions2
Early Failure under out of specification conditions
  • Sanitized Actel Customer data (cont\'d)
    • Contractor D
      • Awaiting information relating to device operating conditions
      • 2/6 failed at device level testing at time zero. Remaining 4 units pass on board level.
    • Contractor E
      • Industry review of post-programming burn-in setup identified critical design deficiencies
        • no bypass capacitors
        • clock signal termination not employed
      • 1/59 failed from 96 hours in post-programming burn-in test. No failures seen since.
field failures
Field failures

Hundreds of units zero failures