In this work, we discuss several parameters influencing extreme ultraviolet (EUV) resist qualification test results. The witness sample resist qualifications test is implemented on several different instruments world wide. The original protocol for the test developed by the tool manufacturer requires that a 300 mm wafer be exposed to a dose to clear in one hour. Also required is that the carbon contamination rate on the witness sample be mass limited, i.e., that the rate be saturated with respect to the EUV or e-beam intensity. Simple arguments have been presented in the past to show that the thickness of the carbon growth should be inversely proportional to the pumping speed and proportional to the area exposed if dose and duration were held constant. The present experiments demonstrate the real-life limitations of these arguments and provide validation to our area scaling procedure to equate test results done with 200 mm wafers to results expected for 300 mm wafers. In the process of studying the dependence of carbon growth on pumping speed, we encountered the important effect of increased partial pressure on the degree of intensity saturation.
We report on optics contamination rates induced by exposure to broad-bandwidth, high-intensity EUV radiation peaked
near 8 nm in a new beamline at the NIST synchrotron. The peak intensity of 50 mW/mm2 allows extension of previous
investigations of contamination by in-band 13.5 nm radiation at intensities an order of magnitude lower. We report nonlinear
pressure and intensity scaling of the contamination rates which is consistent with the earlier lower-intensity
studies. The magnitude of the contamination rate per unit EUV dose, however, was found to be significantly lower for
the lower wavelength exposures. We also report an apparent dose-dependent correlation between the thicknesses as
measured by spectroscopic ellipsometry and XPS for the carbon deposits created using the higher doses available on the
new beamline. It is proposed that this is due to different sensitivities of the metrologies to variations in the density of the
deposited C induced by prolonged EUV irradiation.
We describe a null-field ellipsometric imaging system (NEIS) that provides for the real-time imaging of carbon
deposition profiles on extreme-ultraviolet (EUV) optics in a vacuum system. NEIS has been demonstrated at NIST on a
small chamber that is used for EUV optics lifetime testing. The system provides images of carbon deposition spots with
sub-nanometer resolution thickness measurements that maintain good agreement with those from ex-situ spectral ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). The system will be implemented on several synchrotron beamlines for real-time monitoring of carbon film growth on optics during EUV irradiation.
The goal of our ongoing optics-contamination program is to estimate the magnitude and scaling laws of the
contamination rates of optics exposed to extreme-ultraviolet (EUV) radiation in the presence of various contaminant
species expected in the EUV-lithography-tool environment by exposing samples to in-band 13.5 nm light from our
synchrotron in the presence of fixed partial pressures of admitted gases. We report contamination rate measurements on
TiO2-capped samples for species observed in separate resist-outgassing measurements (benzene, isobutene, toluene and
tert-butylbenzene) in the pressure range (10-8 to 10-5) Pa. We use two spatially-resolved surface probe techniques,
spectroscopic ellipsometry and X-ray photoelectron spectroscopy, to determine the thickness of deposited carbon. The
correlation and sensitivities of these techniques are discussed. The high sensitivity of ellipsometry shows that
contamination rates for some species have a pronounced non-linear intensity dependence and can be strongly influenced
by admixtures of water vapor, while the rates for other species are linear over the same intensity range and are less
affected by ambient water. Understanding scaling laws is critical when estimating optic lifetimes or cleaning cycles by
extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.
The primary, publicly reported cause of optic degradation in pre-production extreme-ultraviolet (EUV) lithography systems is carbon deposition. This results when volatile organics adsorb onto optic surfaces and then are cracked by EUV-induced reactions. Hence the deposition rate depends on the adsorption-desorption kinetics of the molecule-surface system as well as the basic photon-stimulated reaction rates, both of which may vary significantly for different organic species. The goal of our ongoing optics-contamination program is to estimate the contamination rate of species expected in the tool environment by exposing samples to in-band 13.5 nm light from our synchrotron in the presence of fixed partial pressures of admitted gases. Here we report preliminary results of contamination rates on TiO2-capped samples for species observed in resist-outgassing measurements (benzene, isobutene, toluene and tert-butylbenzene) in the pressure range (10-6 to 10-4) Pa which all display an unexpected logarithmic dependence on pressure. This scaling is in agreement with previous EUV exposures of other species at NIST as well as independent measurements of coverage performed at Rutgers University. These results are consistent with a molecular desorption energy that decreases with coverage due to molecular interactions (Temkin model). Use of the proper scaling law is critical when estimating optic lifetimes by extrapolating over the 3-to-6 orders of magnitude between accelerated-testing and tool-environment partial pressures.
The ability to predict the rate of reflectivity loss of capped multilayer mirrors (MLMs) under various conditions of
ambient vacuum composition, intensity, and previous dose is crucial to solving the mirror lifetime problem in an EUV
stepper. Previous measurements at NIST have shown that reflectivity loss of MLMs exposed under accelerated
conditions of dose and pressure can be a very complicated function of these variables. The present work continues this
effort and demonstrates that reflectivity loss does not scale linearly for accelerated exposure doses over the range of
0-350 J/mm2 either for partial pressures of MMA in the range 10-8-10-7 Torr or acetone in the range 10-7-10-6 Torr. We
suggest that this nonlinear scaling may be the result of a varying damage rate as the surface of the growing
contamination layer moves through the EUV standing wave created by exposure of any MLM to resonant radiation. To
further investigate the potential influence of these resonance effects, we report new measurements showing large
variations of the secondary electron yield as a function of thickness of carbon deposited on top of a MLM.
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