Zhuo Wang, Tzu-Yu Wu, Mark Hamm, Alexander Altshuler, Anderson Mach, Donald Gilbody, Bin Wu, Santosh Ganesan, James Chung, Mitsuhiro Ikuta, Jacob Brauer, Seiji Takeuchi, Tokuyuki Honda
As one of the smallest endoscopes that have been demonstrated, the spectrally encoded endoscope (SEE) shows potential
for the use in minimally invasive surgeries. While the original SEE is designed for side-view applications, the forwardview
(FV) scope is more desired by physicians for many clinical applications because it provides a more natural
navigation. Several FV SEEs have been designed in the past, which involve either multiple optical elements or one
optical element with multiple optically active surfaces. Here we report a complete FV SEE which comprises a rotating
illumination probe within a drive cable, a sheath and a window to cover the optics, a customized spectrometer, hardware
controllers for both motor control and synchronization, and a software suite to capture, process and store images and
videos. In this solution, the optical axis is straight and the dispersion element, i.e. the grating, is designed such that the
slightly focused light after the focusing element will be dispersed by the grating, covering forward view angles with high
diffraction efficiencies. As such, the illumination probe is fabricated with a diameter of only 275 μm. The twodimensional
video-rate image acquisition is realized by rotating the illumination optics at 30 Hz. In one finished design,
the scope diameter including the window assembly is 1.2 mm.
We present the basic concept of a fast mask optimization method that utilizes target-intensity back propagation. This method decomposes the target-intensity using a two-dimensional (2-D) transmission cross coefficient. After applying normal incidence approximation to the decomposed target intensity, the spectrum of the mask is optimized in the pupil plane. Since the optimization is performed in the pupil plane, one can use relatively small sampling points, leading to a fast optimization. By setting a high-contrast target intensity, we can obtain an approximation of a phase shifting mask, whereas a low-contrast target intensity approximates a binary intensity mask or attenuated phase shifting mask.
EUVL is the most promising candidate of 32 nm generations and beyond. In this paper, we present Canon's
development status of EUVL technologies. The system design of the EUV full field high volume manufacturing tool
(VS2) is under way. The specification of VS2 is presented in this paper. The fabrication of six-aspheric-mirror prototype
projection optics (PO1) of NA 0.3 has been started. The PO1 is fabricated to evaluate and improve our technologies
of polishing and measuring the figure of mirrors. We present some results of the figuring accuracy of the mirror. EUVL
will be required to resolve sub-twenty nm L&S patterns. We are studying off-axis illumination technologies and high-
NA technologies. The simulation results of the resolution capability and the DOF are presented.
For lithography of 45-nm half-pitch and beyond, the resist blur due to photoacid diffusion is a significant issue. On the other hand, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. We study the influence of the resist blur on resolution in hypernumerical aperture ArF immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist performance for some of the latest commercial resists and estimated their acid diffusion lengths as 8 nm to 9 nm in sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, which is directly related to the resist resolution and is controllable by photoacid generator (PAG) anion size, polymer resin size, and post-exposure bake (PEB) temperature. We confirmed that there is a trade-off between resist resolution and sensitivity. Our results indicate that the resist blur is still a concern in order to extend lithography for 45 nm and beyond; however, it will not likely be a showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve ultimate resolution in hyper-NA immersion lithography.
The extendibility of 2D-TCC technique to an isolated line of 45 nm width is investigated in this paper. The 2D-TCC
technique optimizes mask patterns placing assist pattern automatically. For 45 nm line patterns, the assist pattern width
generally becomes much smaller than the exposure wavelength of 193 nm. Thus, the impact of the topography of a mask
is examined using an electro-magnetic field (EMF) simulation. This simulation indicates that unwanted assist pattern
printings are brought about by assist patterns with a smaller size than expected by the Kirchhoff's approximation. The
difference, however, can be easily solved by giving a bias to the main pattern in the optimized mask. The main pattern
bias decreases DOF very little. Furthermore, DOF simulated with a thick mask model is roughly the same as that
simulated with a thin mask model. Therefore the topography of the optimized mask does not have an influence on the
assist pattern position of the optimized mask. From these results, we have confirmed that the 2D-TCC technique can be
extended to the optimization of 45 nm line patterns. As one of the notable features, the optimized aperiodic assist pattern
greatly reduces MEEF compared with the conventional periodic assist pattern. To verify the feasibility of the 2D-TCC
technique for 45 nm line, we performed experiment with an optimized mask. Experimental results showed that DOF
increased with the number of assist pattern as simulation indicated. In addition, a defect whose length was twice that of
the assist pattern did not have an influence on CD. From these results we have confirmed that the 2D-TCC technique can
enhance the resolution of 45 nm line and has practical feasibility.
The wavefront measurements have been performed with the EUV Wavefront Metrology System (EWMS) for the first
time using a prototype projection optic as a test optic. The wavefronts of the test optic was measured at the five positions
in the exposure field with the Digital Talbot Interferometer (DTI). The RMS magnitude of the wavefront errors ranged
from 0.71 λ (9.58 nm) to 1.67 λ (22.75 nm). The results obtained with the DTI were compared to those with the Cross
Grating Lateral Shearing Interferometer (CGLSI). As a result of a repeatability assessment, it was found that the EWMS
can stably measure the wavefronts of the test optic. Additionally, unwrapping of the phase map was found to be related
to the precision of the measurement.
In this paper, a new resolution enhancement technique named 2D-TCC technique is proposed. This method can
enhance resolution of line patterns as well as that of contact hole patterns by the use of an approximate aerial image.
The aerial image, which is obtained by 2D-TCC calculation, expresses the degree of coherence at the image plane of a
projection optic considering mask transmission at the object plane. OPC of desired patterns and placement of assist
patterns can be simultaneously performed according to an approximate aerial image called a 2D-TCC map. Fast
calculation due to truncation of a series in calculating an aerial image is another advantage. Results of mask
optimization for various line patterns and the validity of the 2D-TCC technique by simulations and experiments are
reported.
A newly developed sub-resolution assist feature (SRAF) placement technique with two-dimensional
transmission cross coefficient (2D-TCC) is described in this paper. In SRAF placement with 2D-TCC, Hopkins'
aerial image equation with four-dimensional TCC is decomposed into the sum of Fourier transforms of diffracted
light weighted by 2D-TCC, introducing an approximated aerial image so as to place SRAFs into a given reticle
layout. SRAFs are placed at peak positions of the approximated aerial image for enhanced resolution. Since the
approximated aerial image can handle the full optical model, SRAFs can be automatically optimized to the given
optical condition to generate the optimized reticle. The validity of this technique was confirmed by experiment
using a Canon FPA6000-ES6a, 248 nm with a numerical aperture (NA) of 0.86. A binary reticle optimized by this
technique with mild off-axis illumination was used in the experiment. Both isolated and dense 100 nm contacts (k1
= 0.35) were simultaneously resolved with the aid of this technique.
ArF water immersion exposure systems with a numerical aperture (NA) of over 1.3 are currently being developed and
are expected to be used for the node up to 45-nm half-pitch. Although there are multiple candidates for the next
generation node, we here focus on ArF immersion lithography using high-index materials. The refractive index of highindex
fluids is typically about 1.64 and is larger than that of fused silica (~1.56). In this situation, the NA is limited by
the refractive index of silica and is at most 1.45. An exposure system with 1.45 NA is not suitable for 32-nm hp node,
but may be used for 37-nm hp node. In spite of this limitation, the system has the advantage of slight alterations from
the current system using water as immersion fluid. On the other hand, high-index lens material is effective to increase
the NA of projection optics further. At present, LuAG, whose refractive index is 2.14, is most promising as high-index
lens material. The combination of high-index fluid and high-index lens material can enhance the NA up to about 1.55
and the exposure system would be available for the 32-nm half-pitch node.
Although high-index immersion lithography is attractive since it is effective in raising resolution, such new materials
should be examined if these materials can be used for high precision projection optics. Here, we have investigated
optical characteristics of high-index materials in order to realize high-index immersion systems.
The resist blur due to photoacid diffusion is a significant issue for 45-nm half-pitch node and beyond.
Furthermore, it has been generally recognized that there is a trade-off between resist resolution and sensitivity. In
this paper, we study the influence of the resist blur on resolution and sensitivity in hyper-numerical aperture ArF
immersion lithography by utilizing a two-beam interferometric exposure tool. We evaluated the current photoresist
performance for some of the latest commercial resists, and estimated their acid diffusion lengths as 8 to 9 nm in
sigma assuming Gaussian blur kernel. In addition, we found that the acid diffusion length, that is, the resist
resolution was controllable by PAG anion size, polymer resin size, and PEB temperature. We also found that there
was the trade-off between resist resolution and sensitivity. Our results indicated that the resist blur is still a concern
in order to extend ArF lithography for 45-nm half-pitch node and beyond, however, it will not likely be a
showstopper. We consider that total optimization of resists and exposure tools is important in order to achieve
ultimate resolution in hyper-NA immersion lithography.
The chemical amplification provides high sensitivity of resists for deep-uv and extreme-uv (EUV) lithography. On the other hand, the chemical amplification involves photoacid diffusion that causes contrast degradation of the latent image or, in other words, resist blur. We study the influence of the resist blur in high-numerical aperture ArF immersion lithography by using an interferometric exposure tool. The contrast ratio between the resist latent image and the original aerial image was measured for half pitches from 45 to 80 nm. Acid diffusion length for a high-resolution ArF resist was determined as 11 nm in sigma (26 nm in full width at half maximum) assuming a Gaussian blur kernel. The results revealed that the influence of the resist blur is a significant issue for the 45-nm half-pitch node. The reduction of acid diffusion length is highly desirable. Given the tradeoff between the resist resolution and sensitivity, increasing illumination intensity in exposure tools can be an effective means to overcome the challenge of the resist blur. We also demonstrate resist imaging of 30-nm line-and-space pattern with a high-index fluid. While our focus is on ArF immersion lithography, our findings are also relevant to EUV lithography.
As the resolution of optical lithography is being pushed for 45-nm half-pitch node, there is a growing concern about the printing capability of chemically amplified resists. The chemical amplification involves photoacid diffusion that causes contrast degradation of latent image or, in other words, resist blur. In this paper, we study the influence of the resist blur in high-NA ArF immersion lithography by using an interferometric exposure tool. Contrast ratio between the resist latent image and the original aerial image was measured for half pitch from 50 nm to 80 nm. Acid diffusion length for a high-resolution ArF resist was determined as 11 nm in sigma (26 nm in full width at half maximum) assuming Gaussian blur kernel. The results revealed that the influence of resist blur is in fact a significant issue for the 45-nm half pitch node. We consider that reduction of acid diffusion length is highly desirable. Given the tradeoff between the resist resolution and sensitivity, increasing illumination intensity in the exposure tools can be an effective means to overcome the challenge of the resist blur. We also demonstrate resist imaging of 30-nm line-and-space pattern with high-index immersion fluid. The reduction of acid diffusion will be even more important if the ArF immersion is to be extended beyond 45-nm half-pitch node with high-index fluids. While the focus of the paper is on high-NA ArF immersion lithography, our findings are also relevant to EUV lithography.
As imaging properties of ArF Immersion optics are evaluated in a hyper-NA region, the polarization of illumination systems and vectorial mask diffraction play an important role. We investigate the effectiveness of polarized illumination for practical patterns including the border of dense line-and-space (L/S) patterns, semi-dense L/S patterns, isolated lines, and contact holes. The results show that polarized illumination is effective in projecting many patterns except semi-dense L/S patterns and relatively large contact holes. Secondly, we examine how bias settings of alternating phase-shift masks (AltPSMs) are affected by vectorial mask diffraction, which depends on the polarization of incident light and feature size on the mask. Although a reduction ratio of 8x facilitates bias settings compared with that of 4x, it is necessary to take into account the effect of vectorial mask diffraction even in the case of 8x. Since polarized illumination also simplifies bias settings, the illumination is useful for 4x projection optics.
High-index fluids have recently attracted considerable attention because they are capable of extending the numerical aperture of projection optics beyond the refractive index of water (n=1.44). We study imaging properties of 1.50NA projection optics with an immersion fluid of n=1.64 and the preliminary requirements of fundamental optical characteristics of the fluid.
Immersion lithography systems with a 193 nm light source are being pursued in the industry. This paper presents the results of the study we have made on various aspects of the exposure system, and gives the status of exposure system development together with the challenges involved. If there are fluctuations in the flow rate of immersion fluid, i.e. ultrapure water, the positioning accuracy of the wafer stage may be affected. Similarly, temperature changes in the fluid can significantly influence imaging performance of the projection optics. We have developed an ultrapure water supply control system which allows direct connection to the ultrapure water line of the existing fabs and enables constant-temperature, constant-flow rate control of the water with high stability. The evaluation results of this system will be shown. Photoresist materials such as photo-acid generator, PAG, dissolved into the water are a cause of concern for lens contamination. The challenge for exposure tool suppliers in terms of contamination control is to specify the permissible dissolution amount. To this end, wet contamination tests are in progress, and the findings to date will be discussed in this paper. Two verification tools for immersion exposure are built: a two-beam interference exposure tool and a full-field alpha-site scanner. Using the alpha tool, the evaluation results of full wafer CD uniformity including edge dies will be presented. Also, defect analysis results will be shown, specifically the impact of air bubbles on patterning.
We present selected results of our feasibility study on ArF Immersion lithography from the viewpoint of the exposure-tool development. First, we show that utilizing finite bubble lifetime in degassed water can eliminate air bubbles that are generated by wafer scanning. Second, it is shown that thermal fluctuation of immersion liquid as well as vectorial diffraction effect from the mask is not significant in terms of imaging performance. Third, we demonstrate resist imaging of 60-nm and 45-nm line-and-space patterns in interferometric exposure experiments with an ArF laser at the power level of the actual exposure tools. Fourth, the increase of the depth of focus is confirmed using an alpha exposure tool of ArF immersion. All these results indicate that the ArF immersion lithography is promising for 65-nm half-pitch node and beyond.
KEYWORDS: Diffraction gratings, Holography, Polymerization, Diffusion, Polymers, Diffraction, Digital holography, Data storage, Holographic data storage systems, Digital recording
We investigated compositional volume grating formation in the Polaroid medium that utilizes the cationic-ring-opening photoinitiated polymerization process, and compared our conclusions with the current physical model describing polymer holographic recording. We identified the effects of diffusion and polymerization during illumination, as well as significant postexposure grating development. Holographic recording in this medium allows for final strong gratings with high recording sensitivity (S approximately 2 cm/mJ), that were not limited at the higher recording intensities (I less than or equal to 250 mW/cm2) corresponding to photon (exposure) limited recording. The results of the present analysis allow for more comprehensive physical description of grating formation in the photoinitiated CROP process, and evaluation of the polymer recording process in a nonvolatile holographic storage system.
The penetration depth of light into a processed surface is interferometrically ieasured by optically contacting it on a glass plate with a resolution of 3 urn. This principle is based on the ieasurement of the difference between the phase changes of light in reflections on the object and the glass plate. Also the depth is siiiultanecusly measured by an optical systeii with a phase-locked laser interferometer and an optical displacement sensor. 1 .
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