1

1
Physical Debug

• Manufacturing test, electrical debug, and
  physical debug
• Scanning Electron Microscopy (SEM)
• Electron-Beam Probing (E-Beam)
• Picosecond Imaging Circuit Analysis (PICA)
• Infrared Navigation with Fiducials
• Laser Voltage Probing (LVP)
• Focused Ion Beam (FIB) Editing
• Spare Hardware for FIB ECOs
• Micromechanical Probing

2
References

• Design for Physical Debug for Silicon
  Microsurgery and Probing of Flip-Chip Packaged
  Integrated Circuits
• Livengood Medeiros, ITC 99, p.877-882
• Novel Optical Probing Technique for Flip-Chip
  Packaged Microprocessors
• Paniccia, Eiles, et. al., ITC 98, pp. 740-747
• Picosecond Imaging Circuit Analysis
• Tsang et. al., IBM Journal of RD
• www.research.ibm.com/journal/rd/444/tsang.html
• Electron Beam Testing Technology
• Thong, 1993, Plenum Press

3
Manufacturing Test vs. Debug

• Test screens for manufacturing defects
• Not all die are faulty (some known good die
  available)
• Different die fail in different ways due to
  different defects
• Debug searches for design validation escapes
  which affect all die
• Functional design errors which escaped validation
• Unexpectedly slow critical path missed by timing
  analysis
• Other behaviors not correctly modeled by
  simulators
• Goals of debug diagnosis work-around
• Root-cause the failure to fix in next stepping
• Peel the onion to debug multiple problems in
  one stepping

4
Electrical Debug

• Controllability observability of 1s and 0s
• Use DFT (design-for-test) hooks
• Scan
• Feature disable modes
• Fault diagnosis
• Identify set of all modeled faults
• Use fault simulation to find which tests detect
  which faults
• Given pass / fail status of a specimen for each
  test, intersect fault lists to diagnose the fault

T2
T1
Fault List
5
Physical Debug

• Why physical debug?
• Need to control or observe signal with no test
  point or scan cell
• Need to observe analog behavior (delays, slopes)
• Need to modify layout to fix a timing or
  functional bug
• Use DFD (design-for-debug) hooks
• Probe points
• FIB cut and connection points
• Working with transistors wires, not modeled
  faults

6
Scanning Electron Microscopy

• Aim electron beam at object to be photographed
• Secondary electrons reflected back from surface
• Electrons collected analyzed with spectrometer
• Synchronized raster produces image
• Magnification 100x - 100,000x
• Different materials have different absorption
  behavior
• 3-D resolution due to reflection angles and
  blockages

7
SEM for Defect Diagnosis
8
SEM Voltage Contrast

• Place e-beam over a chip connected to a tester
• Metal with negative bias reflects more electrons
  and appears brighter
• Metal with positive bias reflects fewer electrons
  and appears darker

9
Fault Diagnosis with SEM Diff

• Diff known good image and bad image to find
  errors

10
E-Beam Probing

• Focus electron beam on a single conductor instead
  of capturing a whole image
• Run a test in a loop and take multiple samples to
  average out noise and construct a voltage
  waveform
• Probe top metal layers directly
• Aim beam between wires or through DFD probe holes
  to access lower metals
• Possible to probe through insulators, but always
  target conductors

11
E-beam as Invasive Probe

• Probing is normally non-invasive due to low
  electron count in beam relative to carrier count
  in circuit
• Increase beam power to charge up a MOS gate for
  primitive controllability

12
The Packaging RevolutionFrom Wire-Bond to
Flip-Chip

• Wire bond packages
• Pads on die perimeter
• Pads connected to package by wires
• Die mounted face-up in package
• Can see and probe top layer of metal
• Access to lower layers between top wires and
  through holes
• Flip-chip packages
• Pads across entire die surface
• Pads connected to package by solder balls
• Die mounted face-down in package
• Wires buried under silicon - too thick to e-beam
  through
• Could drill holes through silicon to expose wires
• Non-destructive probing technique is preferable

13
Electrons are OutInfrared Photons Are In

• Carriers emit photons as they flow across P-N
  junctions
• Materials and dopant concentrations are chosen in
  LEDs to make the photon wavelength in the visible
  spectrum
• Normal MOSFETs emit infrared photons when they
  switch (no steady-state current flow)
• Silicon is transparent to infrared light
• IR photons pass through substrate, out the
  backside of the die
• Focus an IR camera on the die to collect photons
• Probability of emission is 1 in a million, so
  test needs to be run through millions of loops to
  collect enough photons

14
Picosecond Imaging Circuit Analysis (PICA)
15
Infrared Navigation with Fiducials

• How do I know what Im looking at?
• Frontside access Use top metal geography to get
  oriented
• Backside access No way to tell whats under
  flat silicon
• Place DFD fiducial beacons across the chip
• Under IR camera, look like bright lights
  surrounded by darkness
• Align a CAD overlay to locate hidden layout
  features

16
Laser Voltage Probing (LVP)

• Probing version of IR microscopy
• Analogous to e-beam
• Reflected optical power is proportional to
  voltage across reverse-biased P-N junction
• Place DFD probe-able diodes at gate inputs

17
Focused Ion Beam (FIB) Editing

• Used for debug workarounds engineering samples
• Not reliable enough for production
• Physically modify a circuit
• Break existing wires
• Deposit tungsten to create new wires
• Destroy repair to access buried wires
• Cannibalization
• Success rate throughput time metrics
• DFD features to make FIB access easier
• Deterministic opportunistic

18
Drilling Holes with FIB
19
Spare Hardware

• Spare combinational and/or sequential cells
• Need to choose types density requirements
• Spare wires
• Need to choose lengths, metal layers, and
  accessibility
• Placement policy decisions
• Add concurrently with functional design to
  guarantee availability?
• Add in leftover space after design to minimize
  overhead?
• How much spare hardware should be added?
• Leave floating or tie to power rail?
• DFD features for FIB accessibility, or ECO only?

20
Example Complex FIB Edit
21
SEM Photos of Edit
22
Micromechanical Probing

• Touch 1-um probe to wire or contact point
• Used to probe top metal directly when geometries
  were bigger
• Now, probe FIB-deposited metal pads
• Typically used for power clock, not signals