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Displaced-Beam Modulated Thermoreflectance Optical Inspection System

IP.com Disclosure Number: IPCOM000113386D
Original Publication Date: 1994-Aug-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 93K

Publishing Venue

IBM

Related People

Edelstein, DC: AUTHOR [+2]

Abstract

Fig. 1. Illustration of displaced-beam modulated thermoreflectance principle. Pump (P) and probe (PR) laser beams are focussed onto and scanned (arrow) across a sample metal wire (W). Simulated data traces (above diagram) show amplitude and phase of probe beam synchronous component as beam pair is scanned across a crack (CR) and void (V) in the wire.

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Displaced-Beam Modulated Thermoreflectance Optical Inspection System

Fig. 1.  Illustration of displaced-beam modulated thermoreflectance
principle.  Pump (P) and probe (PR) laser beams are focussed onto and
scanned (arrow) across a sample metal wire (W).  Simulated data
traces
(above diagram) show amplitude and phase of probe beam synchronous
component as beam pair is scanned across a crack (CR) and void (V) in
the
wire.

      A non-contact scanning optical inspection system is described
which can detect microscopic cracks or discontinuities in metal
interconnects on chip or packaging structures.  A modulated focused
laser pump beam generates thermal waves in a given interconnect, and
temperature changes are detected by probe beam thermoreflectance at a
location trailing the pump beam in the scan direction.  The thermal
wave transmission between the two beams is reduced or interrupted
when the beams straddle a notch or break in the interconnect.  This
system provides a rapid, noncontact, nondestructive method for
inspecting large arrays of parallel x- or y-running interconnects.

      The system is illustrated in Fig. 1.  Here a pump laser beam
(P) and probe beam (PR) are focussed and incident on the sample (S)
as shown.  The pump laser is 100%-modulated by an acousto-optic
modulator at a high rate such as 1 MHz, with average power high
enough to cause a temperature rise of a few degrees Celsius when
incident on the metal.  The probe laser is displaced by one or more
beamwidths from the pump laser along the length of the interconnect
wire (W) to be scanned, and the scanning direction (arrow) is also
along this length.  Differential, phase-sensitive detection is used
to pass the part of the probe reflectance which is modulated in phase
with the pump laser.

      The beams may be shaped (such as with a cylindrical lens) to
offer higher spatial resolution in the scanning direction while
covering the full width of the wire.  If the two beams are now
scanned across a major defect such as a crack (CR) or void (V) in the
wire, there will be a marked change in the amplitude and phase of the
detected signal relative to that over non-flawed regions.  Thi...