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| A class of failure analysis techniques has evolved over
the last several years that uses a scanning laser beam to activate electrical
characteristics of the test device. Optical Beam Induced Current (OBIC) was the first of
the class. It was later followed by Light Induced Voltage Alterations (LIVA), Thermally
Induced Voltage Alterations (TIVA), and Seebeck Effect Imaging (SEI)1 |
The use of this class of techniques for failure
analysis is primarily in fault isolation. TIVA, for example, locates ohmic shorts due to
the high resistance change with thermal heating that occurs at most shorts. SEI locates
opens due to the electrical imbalance of the thermal electric effects at the open site. |
| A significant feature of this class of techniques is
their ability to work from both the topside and backside of an integrated circuit (IC).
Backside analysis is critical for "flip-chip" packages and devices with multiple
metal layers that obscure visibility from the topside. A
basic system for performing the fault isolation techniques uses a laser scanning
microscope (LSM) to sequentially scan a focused laser spot over the IC. Scanning can be
performed from the front or backside, with near infrared (IR) lasers used for backside
probing.
Some device preparation for backside scanning is also necessary. In
addition to unzipping the package, thinning and polishing of the substrate is generally
required. Use of an anti-reflection coating improves image quality and allows more of the
laser beam to penetrate into the substrate2.
As the focussed laser beam passes over an IC, it causes changes in
the device’s electrical characteristics through two effects, generation of
photocarriers and heating. If the laser wavelength is chosen to be below the semiconductor
bandgap3 then only heating occurs. Shorter wavelengths,
above the bandgap, will produce both photocarriers and heating. |
However, photocarrier effects are orders of magnitude
stronger and generally dominate any thermal signals. Both photocarrier generation and
heating can cause changes in the circuit resistance (photoconductive and
thermal-conductive effects) and cause currents to flow (photovoltaic and Seebeck effects).
For the photovoltaic and Seebeck effects, no external bias is needed
to observe a signal. For the photoconductive and thermal conductive effects, application
of a constant current to the IC causes small voltage alterations to occur across the
device in response to the optically induced changes in the circuit resistance.
The main application of these techniques, to date, has been fault
isolation. In some cases, faults are located by comparison of an image from a
"good" sample to the image of a failed device. Comparison is not generally
required for TIVA and SEI, which only produce signals at the fault, associated drivers and
drivelines. The types of faults detected are determined by the detailed physics of the
laser interaction.
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The
table below summarizes the capabilities of each technique
| Technique |
Physics |
Failure
Analysis Applications |
| OBIC |
Photovoltaic effect |
Location of junction defects |
| LIVA |
Photoconductive effect |
Location of open junctions and substrate damage |
| SEI |
Seebeck (Thermal voltaic or thermal couple)
effect |
Location of opens |
| TIVA |
Thermal-conductive effect |
Location of shorts, vias with incorrect
resistance |
|
TIVA image of Level 5 short |
TIVA image of FET in drive circuit of short |
| 1For further
details on these techniques see Tony Wilson "Theory and Practice of Scanning Optical
Microscopy", Academic Press, London (1984); E.I. Cole, Jr., J.M. Soden, J.L. Rife,
D.L. Barton, and C.L. Henderson, "Novel Failure Analysis Techniques Using Photon
Probing with a Scanning Optical Microscope", IRPS, 388-98 (1994); E.I. Cole, Jr., P.
Tangyunyong, and D.L. Barton, "Backside Localization of Open and Shorted IC
Interconnections", IRPS, 129-136 (1998); E.I. Cole, Jr., P. Tangyunyong, D.A. Benson
and D.L. Barton, TIVA and SEI Developments for Enhanced Front and Backside Interconnection
Failure Analysis", ESREF (1999). 2 For further details see P. Perdu, R. Desplats, and F. Beaudin,
"Comparative Study of Sample Preparation Techniques for Backside Analysis",
ISTFA, 161-72 (2000) and B.V. Davis, "Antireflection Coatings for Semiconductor
Failure Analysis", ISTFA, 155-60. (2000).
3 For topside probing, a dark coating can be
used to allow surface heating without photocarrier generation. |
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