The Berkeley MET5, funded by EUREKA, is the world’s highest-resolution EUV projection lithography tool. With a 0.5-numerical aperture (NA) Schwartzchild objective, the Berkeley MET5 is capable of delivering 8-nm resolution for dense line/space patterns. In order to achieve this resolution, optical aberrations must be accurately characterized and compensated, a task that is complicated by the difficulty in finding a bright, high quality reference wave, and nonlinear effects associated with high incident angles on interferometry targets. The Berkeley MET5 was designed with an in-situ lateral shearing interferometer (LSI) to provide real-time wavefront diagnostics alongside its imaging capabilities.
The geometry of the MET5 makes it a particularly difficult optical system to measure interferometrically. Unlike EUV production tools, the 2-bounce Schwartzchild design is non-telecentric at the image, with an image plane whose normal vector is tilted 1.12 degrees with respect to the optical axis. Shearing interferometers have shown good results measuring EUV wavefronts at low to medium NAs (0.1 - 0.33) with telecentric geometry. However, to accommodate the MET5 geometry, a generalized model of LSI was developed to inform the design and build of a lateral shearing interferometer capable of operating at high-NA and with a tilted image plane. This model predicts non-negligible systematic errors that must be compensated in the analysis.
Specialized pinhole arrays were patterned onto the mask to fill the pupil with spatially filtered light that is incoherently multiplexed from multiple apertures. Due to the relatively large amount of DC flare compared with the signal in the interferograms, illumination profiles were chosen to match the NA of the obscuration so that zero-order light coming through the mask absorber is blocked in the pupil, which results in a finite coherence function width. Because of this, the design of the arrays required balancing the efficiency of the pattern while maintaining enough separation between apertures to accommodate the coherence function width.
Analysis of the interferometric data shows a total RMS wavefront error of 0.6 nm after removal of systematic errors predicted by the LSI model. The bulk of this error lies in astigmatism and coma terms which can be corrected by field position and small adjustments to the alignment of the Schwartzchild optic respectively. The aberration signature of this wavefront is in good agreement with preliminary print data of aberration targets according to aerial image modeling of these features.
The interferometric capability of the Berkeley MET5 is an indispensable part of commissioning the tool, and will allow for the diagnosing and monitoring of tool performance as it begins user operations in the coming months.
We present a lateral shearing interferometer suitable for high-NA EUV wavefront metrology. In this interferometer, a geometric model is used to accurately characterize and predict systematic errors that come from performing interferometry at high NA. This interferometer is compatible with various optical geometries, including systems where the image plane is tilted with respect to the optical axis, as in the Berkeley MET5. Simulation results show that the systematic errors in tilted geometries can be reduced by aligning the shearing interferometer grating and detector parallel to the image plane. Subsequent residual errors can be removed by linear fitting.
Mask 3D effects are an area of active research in EUV mask technology. Mask-side numerical aperture, illumination, feature size and absorber thickness are key factors modulating mask 3D effects and affecting printability and process window. Variable mask-side NA and flexible illumination make the SHARP actinic EUV microscope a powerful instrument for the study of mask 3D effects. We show an application example, comparing mask 3D effects for a standard Tantalum Nitride absorber and a thinner, 40-nm Nickel absorber. Data is presented for 0.33 4xNA and anamorphic 0.55 4x/8xNA. The influence of different illumination settings on mask 3D effects is discussed.
In this paper we compare two non-interferometric wavefront sensors suitable for in-situ high-NA EUV optical testing. The first is the AIS sensor, which has been deployed in both inspection and exposure tools. AIS is a compact, optical test that directly measures a wavefront by probing various parts of the imaging optic pupil and measuring localized wavefront curvature. The second is an image-based technique that uses an iterative algorithm based on simulated annealing to reconstruct a wavefront based on matching aerial images through focus. In this technique, customized illumination is used to probe the pupil at specific points to optimize differences in aberration signatures.
The SHARP high-numerical aperture actinic reticle review project is a synchrotron-based, extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP emulates the illumination and imaging conditions of current EUV lithography scanners and those several generations into the future. An anamorphic imaging optic with increased mask-side numerical aperture (NA) in the horizontal and increased demagnification in the vertical direction has been proposed to overcome limitations of current multilayer coatings and extend EUV lithography beyond 0.33 NA. Zoneplate lenses with an anamorphic 4×/8× NA of 0.55 are fabricated and installed in the SHARP microscope to emulate anamorphic imaging. SHARP’s Fourier synthesis illuminator with a range of angles exceeding the collected solid angle of the newly designed elliptical zoneplates can produce arbitrary angular source spectra matched to anamorphic imaging. A target with anamorphic dense features down to 50-nm critical dimension is fabricated using 40 nm of nickel as the absorber. In a demonstration experiment, anamorphic imaging at 0.55 4×/8× NA and 6 deg central ray angle (CRA) is compared with conventional imaging at 0.5 4× NA and 8 deg CRA. A significant contrast loss in horizontal features is observed in the conventional images. The anamorphic images show the same image quality in the horizontal and vertical directions.
The SHARP High numerical aperture Actinic Reticle review Project is a synchrotron-based, extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP emulates the illumination and imaging conditions of current EUV lithography scanners and several generations into the future. An anamorphic imaging optic with increased mask side-NA in the horizontal and increased demagnification in the vertical direction has been proposed to overcome limitations of current multilayer coatings and extend EUV lithography beyond 0.33 NA.<sup>1</sup> Zoneplate lenses with an anamorphic 4x/8x NA of 0.55 are fabricated and installed in the SHARP microscope to emulate anamorphic imaging. SHARP’s Fourier synthesis illuminator with a range of angles exceeding the collected solid angle of the newly designed elliptical zoneplates can produce arbitrary angular source spectra, matched to anamorphic imaging. A target with anamorphic dense features down to 50-nm critical dimension is fabricated using 40-nm of nickel as the absorber. In a demonstration experiment anamorphic imaging at 0.55 4x/8xNA and 6° central ray angle is compared to conventional imaging at 0.5 4xNA and 8° central ray angle. A significant contrast loss in horizontal features is observed in the conventional images. The anamorphic images show the same image quality in the horizontal and vertical directions.
Characterizing and removing optical aberrations remains a key part of achieving ultimate resolution in EUV exposure tools. Common-path approaches such as lateral shearing interferometry (LSI) have had success at moderate numerical apertures (NA ≈ 0:3); however, these techniques run into several obstacles when applied at higher NA (NA > 0:4). Chief among these obstacles are systematic aberrations due to high incident angles on the diffraction grating and non-planar Talbot surfaces that create regions of low fringe contrast across the pupil. In this paper, we present strategies for addressing these obstacles to extend LSI to high numerical apertures. These strategies will be employed in the installation and alignment of the 0.5-NA SEMATECH Berkeley Microfield Exposure Tool (MET5).
In this paper, we present an experimental verification of Zernike phase contrast enhanced EUV multilayer (ML) blank defect detection using the SHARP EUV microscope. A programmed defect as small as 0.35 nm in height is detected at focus with signal to noise ratio (SNR) up to 8. Also, a direct comparison of the through-focus image behavior between bright field and Zernike phase contrast for ML defects ranging from 40 nm to 75 nm in width on the substrate is presented. Results show the advantages of using the Zernike phase contrast method even for defects with both phase and absorption components including a native defect. The impact of pupil apodization combined with Zernike phase contrast is also demonstrated, showing improved SNR is due to the stronger reduction of roughness dependent noise than defect signal, confirming our previous simulation results. Finally we directly compare Zernike phase contrast, dark field and bright field microscopes.
The authors are expanding the capabilities of the SHARP microscope by implementing complementary imaging modes. SHARP (the SEMATECH High-NA Actinic Reticle Review Project) is an actinic, synchrotron-based microscope dedicated to extreme ultraviolet photomask research. SHARP’s programmable Fourier synthesis illuminator and its use of Fresnel zoneplate lenses as imaging optics provide a versatile framework, facilitating the implementation of diverse modes beyond conventional imaging. In addition to SHARP’s set of standard zoneplates, we have created more than 100 zoneplates for complementary imaging modes, all designed to extract additional information from photomasks, to improve navigation, and to enhance defect detection. More than 50 new zoneplates are installed in the tool; the remaining lenses are currently in production. We discuss the design and fabrication of zoneplates for complementary imaging modes and present image data, obtained using Zernike phase contrast and different implementations of differential interference contrast (DIC). First results show that Zernike phase contrast can significantly increase the signal from phase defects in SHARP image data, thus improving the sensitivity of the microscope. DIC is effective on a variety of features, including phase defects and intensity speckle from substrate and multilayer roughness. The additional imaging modes are now available to users of the SHARP microscope.
In this paper, we present a complete study on mask blank and patterned mask inspection utilizing the Zernike phase
contrast method. The Zernike phase contrast method provides in-focus inspection ability to study phase defects with
enhanced defect sensitivity. However, the 90 degree phase shift in the pupil will significantly reduce the amplitude
defect signal at focus. In order to detect both types of defects with a single scan, an optimized phase shift instead of 90
degree on the pupil plane is proposed to achieve an acceptable trade-off on their signal strengths. We can get a 70% of its
maximum signal strength at focus for both amplitude and phase defects with a 47 degree phase shift. For SNR, the tradeoff
between speckle noise and signal strength has to be considered. The SNR of phase and amplitude defects at focus can
both reach 11 with 13 degree phase shift and 50% apodization. Moreover, the simulation results on patterned mask
inspection of partially hidden phase defects with die-to-database inspection approach on the blank inspection tool show
that the improvement of the Zernike phase method is more limited. A 40% enhancement of peak signal strength can be
achieved with the Zernike phase contrast method when the defect is centered in the space, while the enhancement drops
to less than 10% when it is beneath the line.
The authors are expanding the capabilities of the SHARP microscope by implementing complementary imaging modes.
SHARP (the SEMATECH High-NA Actinic Reticle review Project) is an actinic, synchrotron-based microscope
dedicated to extreme ultraviolet (EUV) photomask research. SHARP’s programmable Fourier Synthesis Illuminator and
its use of Fresnel zoneplate lenses as imaging optics provide a versatile framework, facilitating the implementation of
diverse modes beyond conventional imaging. In addition to SHARP’s set of standard zoneplates, we have created more
than 100 zoneplates for complementary imaging modes, all designed to extract additional information from photomasks,
improve navigation and enhance defect detection. More than 50 new zoneplates are installed in the tool; the remaining
lenses are currently in production. In this paper we discuss the design and fabrication of zoneplates for complementary
imaging modes and present image data, obtained using Zernike Phase Contrast and different implementations of
Differential Interference Contrast.
In this paper, we address a new inspection method which provides in-focus inspection capability and higher defect sensitivity compared with conventional mask inspection methods. In the Zernike phase contrast microscope, an added phase shift to background wave combines with the phase of bump and pit defects to achieve higher contrast at focus. If we use a centralized apodization to half the lens radius to further reduce the intensity of the phase-shifted background wave, the signal strength can be improved up to 6-fold of its original value. Simulation results further show that this apodization for a typical EUV mask power spectral density results in the noise decreasing in absolute level similar to the clear field reference signal. Thus large improvements in signal to noise ratios are possible with the Zernike phase contrast microscope type systems for EUV mask inspection applications.
We present an update of the AIS wavefront sensor, a diagnostic sensor set for insertion in the upgraded 0.5 NA SEMATECH Albany and Berkeley METs. AIS works by using offset monopole illumination to probe localized regions of the test optic pupil. Variations in curvature manifest as focus shifts, which are measured using a photodiode- based grating-on- grating contrast monitor, and the wavefront aberrations are reconstructed using a least-squares approach. We present results from an optical prototype of AIS demonstrating an accuracy of better than λ/30 rms for Zernike polynomials Z<sub>4</sub> through Z<sub>10</sub>. We also discuss integration strategies and requirements as well as specifications on system alignment.
We present a novel approach for wavefront sensing based on scanning diffraction imaging suitable for high-NA optics inspection, where common metrology techniques show limitations. This approach employs ptychography, whereby a well-characterized object is scanned at the focus of the aberrated test optic, and the resulting scat tered light is captured on a CCD. Under the Fresnel approximation, the diffraction patterns are processed in an iterative algorithm to reconstruct the test optic aberrations. We discuss the applicability of this wavefront metrology, present numerical simulations that validate the reconstruction, and show first experimental results from an optical prototype.
We present a new form of optical testing for exposure tools based on measuring localized wavefront curvature. In this method, offset monopole illumination is used to probe localized regions of the test optic pupil. Variations in curvature manifest as focus shifts, which are measured using a photodiode-based grating-on-grating contrast monitor, and the wavefront aberrations are reconstructed using a least-squares approach. This technique is attractive as it is independent of the numerical aperture of the system and does not require a CCD or a separate interferometer branch.
In this work, we use a high accuracy synchrotron-based reflectometer to experimentally determine the effects of angular bandwidth limitations on high NA EUV performance. We characterized mask blank and mask pattern diffraction performance as a function of illumination angle, scatter angle, and wavelength. A variety of pattern feature sizes ranging down to coded sizes of 11 nm (44 nm on the mask) are considered. A Rigorous Coupled-Wave Analysis (RCWA) model is calibrated against the experimental data to enable future model-based performance predictions. The model is optimized against the clearfield data and verified by predicting the mask pattern diffraction data. We thus have confirmed the degradation and asymmetry of diffraction orders at high AOI.
In this paper, we present an aerial image monitor suitable for use in high-NA EUV lithography tools, and discuss
an application in an in-situ image-based reconstruction of the optical system aberrations. The working principle of
the aerial image monitor relies on a scanning aperture that employs a binary, 2-dimensional uniformly redundant
array (URA), which simultaneously provides high flux throughput and high spatial frequency bandwidth. Aerial
images are captured through focus, and are fed into a computer algorithm that matches the measured images
to a computer model with a trial set of pupil aberrations. The aberrations are then modified until the modeled
images match the ones from the experiment. The Reduced Optical Coherent Sum (ROCS) decomposition for
partially coherent aerial image calculation greatly reduces the computation time of each iteration which makes
this method more computationally tractable.
EUV exposures at the SEMATECH Berkeley Microfield Exposure Tool have demonstrated patterning down to 15 nm
half pitch in a chemically amplified resist at a dose of 30 mJ/cm2. In addition, the sensitivity of two organic chemically
amplified EUV resists has been measured at 6.7 nm and 13.5 nm and the sensitivity at 6.7 nm is shown to be a factor of
6 lower than the sensitivity at 13.5 nm. The reduction of the sensitivity of each resist at 6.7 nm relative to the sensitivity
at 13.5 is shown to be correlated to a reduction of the mass attenuation coefficients of the elements involved with
Although Extreme ultraviolet lithography (EUVL) is now well into the commercialization phase, critical challenges
remain in the development of EUV resist materials. The major issue for the 22-nm half-pitch node remains
simultaneously meeting resolution, line-edge roughness (LER), and sensitivity requirements. Although several materials
have met the resolution requirements, LER and sensitivity remain a challenge. As we move beyond the 22-nm node,
however, even resolution remains a significant challenge. Chemically amplified resists have yet to demonstrate the
required resolution at any speed or LER for 16-nm half pitch and below. Going to non-chemically amplified resists,
however, 16-nm resolution has been achieved with a LER of 2 nm but a sensitivity of only 70 mJ/cm<sup>2</sup>.
Next generation EUV optical systems are moving to higher resolution optics to accommodate the smaller length
scales targeted by the semiconductor industry. As the numerical apertures (NA) of the optics become larger,
it becomes increasingly difficult to characterize aberrations, which broaden the point-spread function and thus
limit the ultimate resolution of an optical system. Lateral shearing interferometry (LSI) provides an attractive
alternative to conventional interferometric techniques such as point diffraction interferometry due to its experimental
simplicity, stability, relaxed coherence requirements, and its ability to scale to high numerical apertures.
In this paper we present an analytical solution to the LSI interferogram in various NA regimes. We demonstrate
that systematic aberrations present in high NA interferograms due to grating distortion of the diffracted order
angular spectrum are measurable and must be compensated for in the reconstruction algorithm.
This paper describes a method to arbitrarily shape and homogenize high-coherence extreme ultraviolet sources
using time-varying holographic optical elements and a scanning subsystem to mitigate speckle. In systems with
integration times longer than 100 ms, a speckle contrast below 1% can be achieved.
As commercialization of extreme ultraviolet lithography (EUVL) progresses, direct industry activities are being focused
on near term concerns. The question of long term extendibility of EUVL, however, remains crucial given the magnitude
of the investments yet required to make EUVL a reality. Extendibility questions are best addressed using advanced
research tools such as the SEMATECH Berkeley microfield exposure tool (MET) and actinic inspection tool (AIT).
Utilizing Lawrence Berkeley National Laboratory's Advanced Light Source facility as the light source, these tools
benefit from the unique properties of synchrotron light enabling research at nodes generations ahead of what is possible
with commercial tools.
The MET for example uses extremely bright undulator radiation to enable a lossless fully programmable coherence
illuminator. Using such a system, resolution enhancing illuminations achieving k1 factors of 0.25 can readily be attained.
Given the MET numerical aperture of 0.3, this translates to an ultimate resolution capability of 12 nm. Using such
methods, the SEMATECH Berkeley MET has demonstrated resolution in resist to 16-nm half pitch and below in an
imageable spin-on hard mask. At a half pitch of 16 nm, this material achieves a line-edge roughness of 2 nm with a
correlation length of 6 nm. These new results demonstrate that the observed stall in ultimate resolution progress in
chemically amplified resists is a materials issue rather than a tool limitation. With a resolution limit of 20-22 nm, the
CAR champion from 2008 remains as the highest performing CAR tested to date.
To enable continued advanced learning in EUV resists, SEMATECH has initiated a plan to implement a 0.5 NA
microfield tool at the Advanced Light Source synchrotron facility. This tool will be capable of printing down to 8-nm
Microfield exposure tools (METs) play a crucial role in the development of extreme ultraviolet (EUV) resists and masks.
One of these tools is the SEMATECH Berkeley 0.3 numerical aperture (NA) MET. Using conventional illumination this
tool is limited to approximately 22-nm half pitch resolution. However, resolution enhancement techniques have been
used to push the patterning capabilities of this tool to half pitches of 18 nm and below. This resolution was achieved in a
new imageable hardmask which also supports contact printing down to 22 nm with conventional illumination. Along
with resolution, line-edge roughness is another crucial hurdle facing EUV resists. Much of the resist LER, however, can
be attributed to the mask. We have shown that intenssionally aggressive mask cleaning on an older generation mask
causes correlated LER in photoresist to increase from 3.4 nm to 4.0 nm. We have also shown that new generation EUV
masks (100 pm of substrate roughness) can achieve correlated LER values of 1.1 nm, a 3× improvement over the
correlated LER of older generation EUV masks (230 pm of substrate roughness). Finally, a 0.5-NA MET has been
proposed that will address the needs of EUV development at the 16-nm node and beyond. The tool will support an
ultimate resolution of 8 nm half-pitch and generalized printing using conventional illumination down to 12 nm half pitch.
Interferometry, the long-standing method for optical characterization, is difficult to perform at EUV wavelengths
due to the lack of high power coherent EUV sources and difficult experimental setup. These problems are
exacerbated by systematic errors from geometrical effects as EUV tools move to higher numerical apertures (NA)
which require stricter tolerances for the optical elements involved in interferometry. In this paper we propose an
iterative, image-based, in-situ method for optical characterization that is independent of the operating wavelength
of light, illumination coherence, and NA of the system. In this method, a known pattern is imaged through focus,
and matched to an experimental model with a trial set of pupil aberrations. The aberrations are then changed
iteratively until the modeled images match the ones from the experiment. The Reduced Optical Coherent Sum
(ROCS) decomposition for partially coherent aerial image calculation greatly reduces the computation time of
each iteration which makes this method more computationally tractable.
Lateral shearing interferometry (LSI) provides a simple means for characterizing the aberrations in optical
systems at EUV wavelengths. In LSI, the test wavefront is incident on a low-frequency grating which causes
the resulting diffracted orders to interfere on the CCD. Due to its simple experimental setup and high photon
efficiency, LSI is an attractive alternative to point diffraction interferometry and other methods that require
spatially filtering the wavefront through small pinholes which notoriously suffer from low contrast fringes and
improper alignment. In order to demonstrate that LSI can be accurate and robust enough to meet industry
standards, analytic models are presented to study the effects of unwanted grating and detector tilt on the system
aberrations, and a method for identifying and correcting for these errors in alignment is proposed. The models are
subsequently verified by numerical simulation. Finally, an analysis is performed of how errors in the identification
and correction of grating and detector misalignment propagate to errors in fringe analysis.
Microfield exposure tools (METs) continue to play a dominant role in the development of extreme ultraviolet (EUV)
resists. One of these tools is the SEMATECH Berkeley 0.3-NA MET operating as a SEMATECH resist and mask test
center. Here we present an update summarizing the latest resist test and characterization results. The relatively small
numerical aperture and limited illumination settings expected from 1st generation EUV production tools make resist
resolution a critical issue even at the 32-nm node. In this presentation, sub 22 nm half pitch imaging results of EUV
resists are reported. We also present contact hole printing at the 30-nm level. Although resist development has
progressed relatively well in the areas of resolution and sensitivity, line-edge-roughness (LER) remains a significant
concern. Here we present a summary of recent LER performance results and consider the effect of system-level
contributors to the LER observed from the SEMATECH Berkeley microfield tool.