the development of metrology for extreme-ultraviolet (EUV)
optics, the maintenance of national primary standards for radiometry in
the EUV and adjoining spectral regions, and the operation of national
user facilities for EUV science and applications.
INTENDED OUTCOME AND BACKGROUND
The intended outcomes of this program are: maintenance and continuous
improvement of the national primary measurement standards for extreme
ultraviolet radiation (EUV: wavelengths between 4 nm and 250 nm,
i.e., from soft x rays to vacuum ultraviolet), development of
techniques for fabricating and characterizing EUV optical systems, and
the development of a synchrotron-based, national primary standard for
source-based optical radiometry.
The Division has longstanding responsibility for the national
primary radiometric standards in the
EUV region of the spectrum. EUV radiation is an important
tool for determining the electronic structure of materials,
diagnosing plasmas, measuring dynamics of the upper atmosphere,
and probing the structure and dynamics of astrophysical objects.
One of the top candidates for next-generation semiconductor
manufacturing technology is an EUV micropatterning tool, since
operation at this short a wavelength (13 nm vs. 193 nm for present,
production ultraviolet lithography) enables diffraction-limited
imaging of features with smaller critical dimensions. We are
working actively with the semiconductor industry to develop
new metrology and testing capabilities as needs arise in their
effort to commercialize this next-generation lithography.
The Division’s key tool for EUV metrologyis the NIST Synchrotron
Ultraviolet Radiation Facility (SURF III). SURF III,
the successor to the world’s first dedicated source of synchrotron
radiation, is a low-energy (<400 MeV), high beam-current
(up to 1 A), perfectly circular electron storage ring. Its
operational characteristics are ideal for EUV metrology. It does
not produce the hard x-ray radiation of higher energy sources,
and it can be operated over a wide range of beam energies to
match the spectral response of systems of interest. As a calculable
source of radiation from the far infrared through EUV spectral
regions, SURF is also used as a primary standard for source-based
radiometry throughout the optical spectrum.
Accomplishments
Characterization of Damaged EUV Condenser Optics
| |
Figure 6. Measured EUV reflectivity over surface
of a condenser optic, and an SEM image of a
small region showing the striations associated
with the erosion of the multilayers. |
EUV lithography is a candidate process for producing the next
generation of semiconductor devices, with feature sizes that are
50 nm or less. One of the outstanding issues that must be
solved in order for EUV lithography to proceed to commercialization
is the durability of condenser optics, which collect and direct the
radiation from its source. EUV radiation is produced by
high-temperature plasmas generated by electrical discharges or
high-intensity lasers. These plasmas emit not only EUV radiation,
but also broadband radiation, electrons, and ions. This hostile
environment can damage the condenser optics and reduce its lifetime,
which must be months long for EUV lithography to be commercially viable.
In collaboration with SEMATECH we have analyzed one such
condenser element, which had operated for three months in
the Engineering Test Stand at Sandia National Laboratories.
This condenser had a Mo/Si multilayer reflective coating,
and it was used to collect the radiation from a laser-produced
Xe plasma. After exposure to the plasma, the optic was covered
by a visible array of erosion-associated striations. The origin
of these stripes was a mystery.
As shown in Fig. 6, the reflectivity of the optic was
measured at the NIST/DARPA EUV facility, while the surface
composition and structure were measured using the Electron
Physics Group’s SEM, AUGER, and AFM facilities. This combination
of tools allowed an understanding of the optic’s EUV reflectivity
in terms of the nanoscale compositional and structural properties
of the surface.
The measurements showed that the low-reflectance stripes were due
to periodic stripes of increased roughness, and that each stripe
corresponded to one additional Mo/Si bilayer of the multilayer
coating being removed.
Long-Term Endurance Testing
of EUV Projection Optics
The lithography tool that exposes the wafers in the patterning
process is called a stepper. It consists of a radiation source
and its condenser optics to illuminate the patterning mask,
projection optics to image the mask onto a resist-coated wafer,
and a mechanical device that aligns the wafer to the mask.
Although the system is operated under vacuum, there is still a
significant amount of water and hydrocarbons present. These
materials interact with the EUV and cause the performance of
the optics in the system to degrade. The effect is significant,
and it is a critical factor in determining the operational
lifetime of a stepper.
NIST recently commissioned a beamline on SURF III dedicated to
longterm exposure studies of EUV multilayer projection optics.
It contains a spherical, 75 mm diameter, Mo/Si multilayer
focusing mirror. This images the EUV beam onto a sample optic
with a spot size of approximately 0.6 mm × 0.8 mm
(FWHM). The sample chamber is separated from the mirror chamber
with a removable, 0.3 µm thick beryllium foil. It filters out
long-wavelength radiation and allows a differential vacuum between
the sample chamber and SURF. This permits exposure of the sample
optics to EUV radiation in the presence of larger partial pressures
of water and other species, to simulate the environments present in
EUV steppers.
Nanoscale Chemical Imaging
| |
Figure 7. Simulated reconstruction of a polymer sample,
as imaged by transmission electron microscopy.
Length of bar in lower right is 1 µm.
|
We are developing state-of-the-art measurement techniques and
algorithms to acquire three-dimensional, chemically resolved images
of nanoscale samples, using transmission electron microscopy.
We seek an enhanced, fundamental understanding of the probe-sample
interactions relevant to electron microscopy, including incoherent
and coherent scattering, electron energy loss spectroscopy, and
x-ray generation and propagation. In this joint project, we use
instrumentation to acquire data in the Chemical Science and
Technology Laboratory and investigate reconstruction algorithms
to form the images with the Information Technology Laboratory.
Earlier work led to reconstructions of integrated circuit interconnects
using an x-ray microscope, as well as to the detailed observation of
M x-ray absorption edges in tantalum and tungsten. More recently,
the possibility of performing reconstructions on polymer samples up to
8 µm in size using a scanning transmission electron
microscope has been demonstrated in simulation. The total size is
noteworthy because nearly all tomographic reconstructions use smaller
samples to avoid the multiple-scattering regime. Here it is shown that
multiple scattering can be accounted for mathematically, and
reconstructions may be performed, even in the presence of a few
scattering events. A simulated reconstruction appears in Fig. 7.
Absolute Radiometry at
SURF III
| |
Figure 8. Monochromator during alignment.
|
The SURF III facility recently became the source of the most
accurate ultraviolet lamp calibrations in the world.
The SURF upgrade in the mid-1990s was motivated by the need to
improve our source calibration capabilities in the ultraviolet
spectral region. To accomplish this, the magnet system had to be
improved, which in SURF’s case meant replacement of the coils, the
steel used in the back legs, the power supply, and the control system.
Recently these immense improvements were put to the test during a
Consultative Committee for Photometry and Radiometry (CCPR) key
comparison between several national measurement institutes,
piloted by the German Physikalisch-Technische Bundesanstalt (PTB).
This comparison focused on the calibration of specially prepared
deuterium lamps in the spectral range from 200 nm to 350 nm.
NIST was the sole laboratory to employ synchrotron radiation in the
calibration process, leading to the smallest uncertainties among all
participating laboratories. The uncertainty achieved at SURF was 0.5 %
(coverage factor k = 1) for the entire spectral range.
For this comparison, a detection system, consisting of a monochromator,
a diffusing device, and a photodetector, was calibrated against SURF,
utilizing the calculability of synchrotron radiation in conjunction
with our accurate monitoring of its operational parameters.
Figure 8 shows a monochromator during alignment. Once the
system was calibrated, it could be used to determine the spectral
irradiance of a lamp.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
