Semiconductor manufacturing requires that wafers be exposed to a plasma for more than 13 of the manu

1
The effects of vacuum ultraviolet radiation on
the processing of electronic materialsJ. Leon
Shohet, Univ. of Wisconsin-Madison, DMR 0306582
THE ISSUE
OBJECTIVE
Semiconductor manufacturing requires that wafers
be exposed to a plasma for more than 1/3 of the
manufacturing stepsincluding etching,
deposition, ashing, etc. In many cases, plasma
processing is the only way in which these
manufacturing steps can be done. However, when
dielectric surfaces which are present in various
locations on the wafer are exposed to a plasma,
both charged particles and radiation strike the
dielectric. This often results in electric
charge building up on the surface of the
dielectric which can result in destruction of the
dielectric by electric currents tunneling through
or arcing across the dielectric. The figure
below shows the absolute VUV spectrum from an
oxygen plasma. Significant VUV radiation appears
above the bandgap of SiO2 where it can produce
electron-hole pairs and photoemission. This can
result in additional charge accumulating or
depletion of existing charge, depending on the
photon energy, the properties of the dielectric
and its thickness.
To mitigate the damage caused during plasma
exposure by depleting excess charge.
THE PROPOSED SOLUTION
When radiation in the ultraviolet to extreme
ultraviolet portion of the spectrum strikes the
dielectric layers, experimental evidence shows
that excess charge can be drained off by
photoconductive/photoinjection effects. However,
this radiation can also cause photoemission of
electrons that can work against the
photoconductive effects especially for positively
charged layers. We have shown that a positively
charged dielectric can be depleted/neutralized of
its charge by both photoinjective and/or
photoconductive effects, while a negatively
charged surface can be depleted/neutralized of
its charge by photoemissive and/or
photoconductive/photo-injective effects provided
that the photon energy is adjusted for the
dielectric properties and layer thickness. Thus,
by carefully tailoring the wavelength of
supplemental radiation to a particular
dielectric, it is possible to deplete the charge
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The effects of vacuum ultraviolet radiation on
the processing of electronic materialsJ. Leon
Shohet, Univ. of Wisconsin-Madison, DMR 0306582
EXPERIMENTS
Dielectric Extreme UV interaction
In our laboratory, we simulate plasma processing
exposure by placing silicon oxide dielectric
layers of thickness between 500-3000 Ångstroms to
plasmas generated by electron-cyclotron
resonance. Depending on the operating
parameters, the dielectric layer will be charged
positively or negatively. After plasma exposure
the charged dielectric layers are exposed to
synchrotron radiation at wavelengths between 100
and 2000 Ångstroms and both photoemission and
photoinjection/photocon-duction currents are
measured during synchrotron exposure as shown
below. These currents may charge or discharge the
dielectric. The amount and distribution of charge
before and after synchrotron exposure is measured
with a Kelvin probe.
Many processing plasmas produce measurable
amounts of Extreme Ultraviolet (EUV) radiation.
The figure below shows the surface potential map
measured using a Kelvin Probe on a 3000 Å SiO2
layer after synchrotron EUV exposure. Peak
potentials of the order of 28V are clearly seen
in the surface-potential map (shown at the right)
which is much higher than that formed by
photoemission during VUV exposure. It is
important to note that these high surface
potentials are formed outside of the EUV exposure
region, in contrast to VUV exposure where the
highest potentials occur at the VUV beam location.
Our measurements of the charging/discharging
current during VUV exposure show that (1)
dielectric space charge develops in the thick
oxides, thus decreasing the substrate current.
(2) As thickness decreases, this amount of charge
produces tunneling currents which cause the
current to increase, while (3) for the thinnest
oxides, the current is nearly constant due to
photoinjection from the substrate, because the
VUV photons pass completely through the oxide.
From these effects, we can determine the photon
energies needed for charge depletion as a
function of the dielectric and its thickness.
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The effects of vacuum ultraviolet radiation on
the processing of electronic materialsJ. Leon
Shohet, Univ. of Wisconsin-Madison, DMR 0306582
Education and outreach to underrepresented
minorities
Jean Calderon from the University of Puerto Rico
and Damien Kenney from Georgia Perimeter College
spent eight weeks during the summer of 2005
working in our laboratory and at the UW
Synchrotron (DMR-0084402) learning how to perform
plasma processing, expose test structures to
synchrotron radiation and to measure the charge
distribution.
Jean Calderon and Erik Hanley setting up VUV
exposures at the Synchrotron
Jean Calderon using a Kelvin Probe to measure
surface Potential
Erik Hanley and Damien Kenney exposing wafers to
plasma before VUV exposure