With the introduction of the NXE:3400B scanner, ASML has brought EUV to High-Volume Manufacturing for sub10nm node lithography. And work has already been started on a successor high-NA system with NA=0.55. For both these systems, node resolution will go down faster than NA increases, resulting in decreasing k1-factors and tightening of aberration requirements. A crucial component for measuring and controlling aberrations in-situ is a diffuser to fill the full pupil of the projection optics appropriately.
This paper presents several new diffuser concepts, both reflective as well as transmissive, with their respective key performance metrics for both NA=0.33 and NA=0.55 EUV projection optics. These concepts can be used for measuring wavefront quality from dedicated fiducial plates, or for measuring directly from the imaging reticle. The latter would enable a combination of reticle alignment with lens aberration control without throughput penalty.
It will be shown that with these diffuser concepts, we have a solution for in-situ aberration control for 5nm nodes and below.
The development of industrial infrastructure for EUV lithography requires a wide array of optics beyond the mask and the scanner optics, which include optics for critical instruments such as exposure testing and actinic inspection. This paper will detail recent results in the production of a variety of high-precision multilayer coatings achieved to support this development. It is critical that the optical designs factor in the capabilities of the achievable multilayer gradients and the associated achievable precision, including impact to surface distortion from the added figure error of the multilayer coating, which adds additional requirements of a specific shape to the period distribution. For example, two different coatings may achieve a ±0.2% variation in multilayer period, but have considerably different added figure error.
Part I of the paper will focus on radially-symmetric spherical and aspherical optics. Typical azimuthal uniformity (variation at a fixed radius) achieved is less than ±0.005nm total variation, including measurement precision, on concave optics up to 200mm diameter. For highly curved convex optics (radius of curvature less than 50mm), precision is more challenging and the total variation increases to ±0.01nm total variation for optics 10-30mm in diameter. Total added figure error achieved has been as low as 0.05nm.
Part II of the paper will focus on multilayer designs graded in two directions, rather than radially, in order to accommodate the increased complexity of elliptical, toroidal and hyperbolic surfaces. In most cases, the symmetry of the required multilayer gradient does not match the symmetry of the optical surface, and this interaction must be countered via the process design. Achieving such results requires additional flexibility in the design of the deposition equipment, and will be discussed with several examples in the paper, such as the use of variable velocity of an inline substrate carrier in conjunction with a shaped target aperture to produce ±0.03nm total variation on an off-axis elliptical surface.
A critical component of high-performance EUV lithography source optics is the reflecting multilayer coating. The ideal multilayer will have both high reflectance and high stability to thermal load. Additionally the capping layers must provide resistance to degradations from exposure to an EUV source, and also be compatible with, or enhance, the systems used for cleaning an exposed multilayer coating. We will report on the results of development of C and B4C stabilized Mo/Si multilayers used to increase the as-deposited peak reflectivity (Rp) as well as decreasing the loss of peak reflectivity (Rp) as a function of annealing temperature. Previous results demonstrate that these layers prevent loss of Rp for temperatures up to 600º C. Results on the use of reactively-sputtered oxide capping layers such as SiO2 and ZrO2 will be presented as well, along with results of exposure testing. The deposition is performed in a dual processchamber inline magnetron system, using reactive sputtering for the production of capping layers. The reflectometer and exposure apparatus at the NIST Physics Laboratory is used for evaluation of the performance. Exposure results on the resistance to oxidation in the presence of water vapor will be presented and discussed.
The NIST Extreme Ultraviolet (EUV) Reflectometry Facility was designed in the 1990s to accommodate the largest multilayer optics envisioned at that time. However, with increasing power requirements for an EUV scanner, source collection optics have grown larger and more steeply curved than the original design would allow. To accommodate these changes, the mechanical and operational parameters of the facility have been upgraded. To access the entire surface of a larger optic, an auxiliary off-axis rotation stage has been installed allowing an increase in maximum optic size from 350 mm to 450 mm. Likewise, to deal with the deeper sags and steeper slopes of these optics, we have had to significantly expand our data analysis capabilities. In order to make these measurements, the incident radiation is reflected out of the vertical plane, allowing for measurements of effectively unpolarized radiation, an advantage for EUV lithography optics such as source collectors.
Laser-produced plasma (LPP) sources for extreme ultraviolet lithography (EUVL) systems utilize CO2 lasers operating
with wavelength 10.6μm. Since multilayer-coated optics have high reflectivity for this infrared radiation (IR), a
significant and detrimental amount of IR is passed through the EUVL system. One method to remove the IR from the
system is to utilize a binary diffraction grating. When this grating is applied directly to the surface of the primary
collector optic of the source, the majority of the IR is diverted outside the radius of the exit aperture at the intermediate
focus (IF). This paper will report details on the performance of a full size (410mm diameter) Demonstration Collector
utilizing IR rejection (IRR) technology with the capability to produce over 125X suppression of IR, equaling the
performance of a IR spectral filter. Additional details will be reported on the technology development and use of a
glassy smoothing layer to enable high EUV performance, a weighted average multilayer reflectance of 50.9% for
unpolarized EUV radiation.
The most promising wavelength for the next generation EUV lithography in terms of maximizing throughput of an
optical system was found to be 6.63nm, where highest peak reflectivity is expected at this wavelength using
La(La2O3)/B4C structures. The optical throughput at 6.63nm is expected to be ~6 times lower than at 13.5nm due to the
higher resolution of multilayers at the smaller wavelengths.
La/B4C and La2O3/B4C multilayer structures were coated at RIT by using magnetron sputtering deposition technology.
EUV reflectivity of the multilayers was tested at CXRO and NewSUBARU. The round robin measurements
demonstrated a maximum deviation of 1.9% in the peak reflectivity and 0.0063nm in the peak position. The big
difference in the peak value can be explained by presence of the higher harmonics in the probe beam at NewSUBARU
which affected the accuracy of the measurements. The maximum peak reflectivity of 48.9% was measured from La/B4C
multilayer at 6.68nm. Maximum reflectivity of La2O3/B4C structure at this wavelength was 39.2% while reflectivity at
6.63nm was measured to be 42.68%. The measured band width of the reflectivity curves was about 20% lower than for
ideal structures. La2O3/B4C structure demonstrated a larger level of the imperfections resulting in much lower
EUV reflectivity of one of the La/B4C multilayers deposited in December 2000 was measured at NewSUBARU in
January 2011 and the results were compared with the measurements performed in January 2001 at CXRO. The
maximum reflectivity dropped from 42.6% to 37.6%. Reduction of the reflectivity band width from 0.044nm to 0.04nm
was also observed.
To perform actinic inspection of patterned EUV reticles with diffraction-limited resolution at 13.5 nm wavelength aspheric optical surfaces with surface figure errors and roughnesses well below 1 nm had to be developed.
The 3D surface topologies of prototype optical components were characterized over spatial periods ranging from the clear apertures down to 25 nanometers over 6 orders of magnitude by using a portfolio of instruments.
3D topography maps were Fourier analyzed and averaged Power Spectral Densities (PSDs) computed over the entire spatial frequency range. A good fit to the PSD was achieved with a linear function on a log-log scale. RMS values were computed over several spatial period ranges.
All optical surfaces were coated with high-reflectivity coatings to maximize optical throughput at 13.5 nm for the average angle-of-incidence of each optic. The spectral reflectivity of the HR coatings, consisting of Molybdenum-Silicon bi-layers (40 periods) were measured using synchrotron instruments at the NIST/DARPA EUV Reflectometry Facility and the Center for X-Ray Optics at Lawrence Berkeley National Laboratory. Total variations (PV) of peak-position within the clear-apertures ranged from 0.005 nm to 0.020 nm, with the one exception being a highly-curved convex surface yielding a PV variation of 0.040 nm. Peak reflectivity variation was typically 0.2% to 1% PV over the clear aperture, with some of the variation being instrument precision. One optic was coated with Ruthenium only, approximately 16nm thick, with less than ±0.1 nm variation in thickness. Detailed information on the spectral reflectivity for the coatings is discussed.
In a joint-development, Rohwedder and Osmic have designed and built a low-defect dual-ion beam reactive-sputtering tool. The tool has been specifically targeted for developing low-defect lithography mask photoblank coatings intended as DUV absorbers and phase-shifting films. The Osmic/Rohwedder collaboration will continue into NGL - the present tool also serves as an R&D platform for EUVL mask blanks. The deposition tool and robotic substrate handler have been integrated and delivered to Osmic in the 2nd quarter of 2003. In this paper, we present initial capability for production of thin-film lithography coatings, including spectrophotometric performance, defect levels and film uniformity. Future reports will share results from more in-depth process development and optimization.
In a joint-development, Rohwedder and Osmic have designed and built a low-defect dual-ion beam reactive-sputtering tool. The tool has been specifically targeted for developing low-defect lithography mask photoblank coatings intended as DUV absorbers and phase-shifting films. The Osmic/Rohwedder collaboration will continue into NGL -- the present tool also serves as an R&D platform for EUVL mask blanks. The deposition tool and robotic substrate handler have been integrated and delivered to Osmic in the 2nd quarter of 2003. In this paper, we present initial capability for production of thin-film lithography coatings, including spectrophotometric performance, defect levels and film uniformity. Future reports will share results from more in-depth process development and optimization.
We present the results of theoretical calculations pertaining to the control of the film thickness distribution in two dimensions. The calculations are relevant to magnetron sputter deposition in which two different deposition geometries are considered. One of which is for an 'in-line' system where the substrate passes along a linear path in front of each target, and the other for cylindrical deposition geometry where the substrates are mounted on a rotating drum. Results of various thickness gradients on flat as well as spherical and cylindrical substrates are shown. The thickness distribution in one dimension is controlled by the use of a contoured shield which appropriately intercepts sputtered material between the target and substrate. The film thickness in the other dimension is controlled by changing the velocity of the substrate through the deposition region. The shield contour and velocity profile required to achieve these gradients are also given.
Molybdenum/Silicon soft x-ray optical coatings for extreme ultraviolet lithography are being developed for both projection optics and masks, and have only recently been produced on a production scale via magnetron sputtering. A number of critical factors must be met for successful development of these coatings for implementation into commercial EUV Lithography. We report on our results for several factors with a state-of-the-art in-line sputtering system. All coatings in a lithography system must match to obtain maximum energy transmission. Hence, process repeatability and characterization of the reflectivity and central wavelength is essential. Run-to-run performance and substrate uniformity is shown to have achieved less than 1% deviation in soft x-ray central wavelength between any two points on any two substrates coated in separate batches; coating uniformity on a given wafer is on the order of 0.3% total deviation. Hard x-ray measurements of d-spacing and reflectivity were correlated to soft x-ray measurements; these correlations were used to improve process control. Furthermore, the coatings must be as defect-free as possible; elimination of aerosol-based particulate generation has allowed improvements by nearly four orders of magnitude. Finally, efforts to understand and control coating stress as a function of processing parameters and post-coating annealing schedules will be reviewed. Results of the effects of deposition method, ion bombardment and interactions between sputter power, sputter pressure and deposition rate are reviewed.
A three-aspherical mirror system for extreme ultraviolet lithography (EUVL) has been developed. The mirrors were fabricated using a computer-controlled optical surfacing (CCOS) process and a phase-shift interferometer. The figure error of the mirrors is 0.58 nm. To achieve a high reflectivity in the clear aperture, Mo/Si multilayer films with an optimized d-spacing were successfully deposited on the mirrors. These results show that we have nearly achieved the target specifications for EUVL mirrors.