In
the last few years, semiconductor circuit features
have shrunk to sub-100 nanometer (nm) dimensions,
while the size of the thin silicon wafers that these
circuits are constructed on has grown from 200 millimeters (mm) to 300 mm (about
12 inches). The payoff is a higher yield of finished devices from fewer wafers.
The
tough part, however, is to make wafers substantially
larger while simultaneously meeting higher quality
control specifications. The optics and materials
for "printing" nanoscale circuit lines require that
the wafers used are perfectly flat and of uniform
thickness. To help the semiconductor industry meet
its 2010 quality control roadmap goals, researchers
at the National Institute of Standards and Technology
(NIST) recently developed a new instrument that accurately
measures differences in thickness across a 300 mm
wafer with an excellent repeatability of 5 nm. The
researchers hope the tool, with further refinements,
will allow them to establish a new calibration service
for "master wafers" used in the industry to measure
wafer thickness.
The NIST researchers will describe the instrument,
the Improved Infrared Interferometer, or IR3 for
short, at a technical conference* in late July. Like
all interferometers, the IR3 uses intersecting waves
of light to create interference patterns, which in
turn can be used as a ruler to measure nanoscale
dimensions. While most interferometers use red laser
light, the IR3 uses infrared laser light. And unlike
visible light, these much longer wavelengths pass
right through a silicon wafer. This means that IR3
can illuminate the top and bottom on a 300 mm wafer
and produce a detailed spatial map of differences
in thickness in one pass. Conventional tools require
spinning the wafer and measuring at multiple locations.
The
NIST researchers make precision measurements of
the wafer's index of refraction--the amount that
light is "bent" as it passes through the silicon--as
a critical step in correctly interpreting the interference
patterns. Increased precision in the refractive index
measurement will be necessary before "absolute" measurements
of thickness rather than relative differences will
be possible with the new instrument.
*Q. Wang, U. Griesmann and R. Polvani. Interferometric
thickness calibration of 300 mm silicon wafers. ASPE
Summer Topical Meeting on Precision Interferometric
Metrology (July 20-22, 2005).
Contact: Gail Porter
gail.porter@nist.gov
301-975-3392
National Institute of Standards and Technology
(NIST)
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