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Sputtering is the process in which an energetic bombarding particle, usually
an ion accelerated by an electric field, is incident on a material and causes
the ejection of atoms, ions, and/or molecules from a surface. It has many
applications including thin-film deposition, etching and analytical
techniques. It also plays an important role in spacecraft propulsion, since
sputtering can damage the thruster and cause re-deposition on other
spacecraft surfaces. We employ the cavity ring-down spectroscopy (CRDS)
technique to help study sputtering characteristics. |
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CRDS is a highly
sensitive path-enhanced laser absorption method that is widely used for
trace-species measurement in flames, plasmas and atmosphere. It is capable
of doing quantitative measurements of ultra low density particles
non-intrusively in real time. The basic idea of CRDS is introduce an absorbing
species (i.e. the sputtered atoms) into a high finesse optical cavity formed
by high-reflectivity mirrors. The laser is tuned around a specific spectral
transition line of the absorber. A photo detector is placed behind the cavity
to measure the laser decay single inside the cavity (ring-down signal). Under
appropriate conditions, the ring-down signal decays single exponentially
versus time. By comparing the laser decay constant inside the cavity with
and without the absorber, the absorption and path-integrated number density
can be measured. |
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In our laboratory,
a broadly tunable optical parametric oscillator (OPO) laser system (doubled
idler) is used as the light source to probe the required optical transitions.
It provides a very large wavelength range (200 nm to 1200 nm) that allows us
to do measurement for many different materials. The optical cavity is formed
using a pair of high-reflectivity mirrors and integrated with a sputtering
facility consisting of an ion beam and sputtering target within a vacuum
chamber. The ring-down signals are collected behind the output mirror with a
fast photomultiplier tube and passed to a custom data acquisition code which
fits the signals with exponentials to extract the ring-down (1/e) times. A
given measurement requires recording of the absorbance spectrum and is
achieved by scanning the laser across the given absorption feature(s) and
determining the ring-down time at each laser wavelength step. |
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Real time in situ
monitoring of the concentration of sputtered particles would provide a powerful
tool for process control in ion beam etch systems, including end point monitoring.
An end-point detection is demonstrated by monitoring the time dependence of
Manganese concentration for a multi-layer target comprised of alternating
Manganese layers in near real time.
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| References: |
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- V. Surla, P.J. Wilbur, J.D. Williams, M. Johnson, A.P. Yalin,
"Sputter Erosion Measurements of Titanium and Molybdenum by Cavity
Ring-Down Spectroscopy", Review of Scientific Instruments
75 9, pp. 3025-3030 (2004)
- A.P. Yalin, V. Surla, "Velocity Measurements by Cavity Ring-Down
Spectroscopy", Optics Letters 30 3219 (2005)
- A.P. Yalin, V. Surla, M. Butweiller, J.D. Williams, "Detection of
Sputtered Metals using Cavity Ring-Down Spectroscopy", Applied
Optics 44 30, pp. 6496-6505 (2005)
- V. Surla, A.P. Yalin, "Differential Sputter Yield Measurements using
Cavity Ring-Down Spectroscopy", Applied Optics 46
19, pp. 3987-3994 (2007)
- L. Tao, N. Yamamoto, A.P. Yalin, "Cavity Ring-Down Spectroscopy
Sensor for Ion Beam Etch Monitoring and End-Point Detection of Multilayer
Structures", Review of Scientific Instruments 79
115107 (2008)
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