Optical Physics Company
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Nanometrology Application
National Institute of Standards and Technology (NIST) plays a critical and unique role in the evolution of nanotechnology into nanoscale products and addresses both metrology and manufacturing-related issues.
OPC is
working with the NIST Quantum Electrical
Metrology Division which maintains the US Farad standard. The
Farad standard is based on the NIST calculable
capacitor, a unique high accuracy
instrument that directly links the capacitance unit to the
mechanical unit of length. The NIST calculable capacitor is the
primary standard in the
Three-axis Interferometer
Optical Physics Company (OPC)is
working with the National Institute of Standards and Technology
(NIST) on a three axis interferometer to be used in the NIST
calculable capacitor application.
The required linear displacement resolution for
the three-axes interferometer is 0.1 nm with an angular resolution
of 0.1 arc-second.
OPC has assembled the interferometer (shown below). OPC
personnel are
currently in the process of finalizing the alignment of this device
and characterizing its performance.
The basic layout for the 3 axis
interferometer is a tip-tilt interferometer shown below.
Its operating principles are explained as
follows: The interferometer is configured as a Michelson
interferometer where the interference takes place between the return
signal from the target mirrors and a fixed reference mirror. The
target mirror’s reflective surfaces are arranged such that one can
measure the return from both mirrors simultaneously. A small tilt
in the reference mirror produces the interference fringes for each
of the target mirrors, and a focal plane array is used to measure
the interference patterns. By measuring the spatial frequency and
phase of the interference pattern, one can calculate the change in
tip-tilt and piston between the target mirrors and a reference beam
splitter. The expected image on the focal plane is
shown below. On the left side of the image the two target mirrors
are misaligned and there is a relative tip-tilt between the
mirrors. This results in a change of the fringe patterns between
the two mirrors which allows for the adjustment of the relative tip
tilt of the two mirrors until they are parallel to each other as
shown on the right.
Once the tip tilt angles between the two
mirrors are aligned, the relative phase of the two fringe patterns
provides a measurement of the distance between the two mirrors.
The fringe pattern will change by 1 cycle when the mirror moves by ½
of laser wavelength. By measuring the phase of the fringe
pattern accurately one can get an accurate measurement of the
distance. An iodine stabilized HeNe laser is used to
provide the precision wavelength reference for accurate piston
measurements. Single wavelength measurement of the piston is
periodic with a period of λo/2,
where λo is the
measurement wavelength. In order to resolve this ambiguity, a
second measurement wavelength is used. Since the wavelength of the
auxiliary laser needs to be known precisely, the assembled
instrument includes a reference interferometer to calibrate the
auxiliary laser wavelength. A configuration of the final
instrument is shown below.
