This PDF file contains the editorial “Hot-Lot Equivalent of Publishing—The Immersion Special Section” for JM3 Vol. 3 Issue 01

This PDF file contains the editorial “Immersion Lithography” for JM3 Vol. 3 Issue 01

When imaging at high incident angles to the resist, both the reflected light from the resist surface and the image forming process of the transmitted light into the resist are polarization dependent. The transmitted TM component and the reflected TE and TM components tend to induce stray light as a function of the incident angle. In this work, these components are analyzed and evaluated quantitatively. The polarization-dependent stray light (PDS) and the system stray light found in regular optical imaging systems are incorporated into a simulation program for diffracted intensity distribution to construct exposure-defocus (E-D) trees and windows to evaluate the exposure latitude and depth of focus (DOF) of typical circuit elements. The DOF of line-space pairs and contact holes with and without PDS are compared in the cases of two- and three-beam interference, first- and second-order beams, binary intensity masks, and phase shifting masks. Even though the 193-nm immersion system is shown to be better than 157- and 193-nm dry systems, using polarized illumination can improve DOF, particularly in low k₁ situations, and in other situations when the stray-light-free image contrast is low.

Depth of focus (DOF) is the gating factor of resolution and an indicator of the feasibility of a given technology node. However, its meaning is ambiguous. Sometimes it is used to indicate the longitudinal range of a good image governed by diffraction. Other times it is used as a budget for mechanical and optical tolerances. Another source of confusion is that the longitudinal range of a good diffracted image is usually evaluated for an air medium, ignoring the wavelength compression effect of the photoresist, as well as the fact that at least two media have to be within the longitudinal range of the good image. An immersion system with a high-index coupling medium makes the situation even more complicated. We classify the types of DOF into diffraction DOF, required DOF, and available DOF, then show the mutual dependence of these parameters and their significance in subwavelength imaging. The impact and DOF-resolution tradeoff, with and without a high-index coupling medium, is also explored.

The premise behind immersion lithography is to improve resolution by increasing the index of refraction in the space between the final projection lens of an exposure system and the device wafer by inserting a high-index liquid in place of the low-index air that currently fills the gap. We present a preliminary analysis of the fluid flow characteristics of a liquid between the lens and the wafer. The objectives of this feasibility study are to identify liquid candidates that meet the fluid mechanical requirements and to verify modeling tools for immersion lithography. The filling process was analyzed to simplify the problem and identify important fluid properties and system parameters. Two-dimensional computational fluid dynamics (CFD) models of the fluid between the lens and the wafer are developed and used to investigate a passive technique for filling this gap, in which a liquid is dispensed onto the wafer as a puddle, and then the wafer and liquid move under the lens. Numerical simulations include a parametric study of the key dimensionless groups influencing the filling process, and an investigation of the effects of the fluid/wafer and fluid/lens contact angles and wafer direction. The model results are compared with experimental measurements.

With immersion in water (n = 1.44), the highest spatial frequency available with ArF-based (193-nm) lithography tools with a numerical aperture (NA) = n×sin θ of 1.3 (1.44×0.9) corresponds to a half-pitch of 37 nm. This suggests that the 45-nm half-pitch node should be accessible. A detailed vector simulation study is reported for two approaches to printing for this node. Both imaging interferometric lithography (IIL, with a single mask and multiple exposures incorporating pupil plane filters) and dipole illumination (with two masks separating the x and y oriented small features) are shown to be capable of printing arbitrary structures under these conditions. There is a substantial loss of contrast for TM polarization at this NA that demands that different polarizations be used to capture the high spatial frequencies in the x and y directions. Both dipole and IIL schemes offer this capability; IIL provides more robust imaging results.

Historically, the application of immersion optics to microlitho-graphy has not been seriously pursued because of the alternative technologies available. As the challenges of shorter wavelength become increasingly difficult, immersion imaging becomes more feasible. We present results from research into 193-nm excimer laser immersion lithography at extreme propagation angles. This is being carried out in a fluid that is most compatible in a manufacturable process, namely water. By designing a system around the optical properties of water, we are able to image with wavelengths down to 193 nm. Measured absorption is below 0.50 cm^-1 at 185 nm and below 0.05 cm^-1 at 193 nm. Furthermore, through the development of oblique angle imaging, numerical apertures approaching 1.0 in air and 1.44 in water are feasible. The refractive index of water at 193 nm allows for exploration of the following: k₁ values near 0.25 leading to half-pitch resolution approaching 35 nm at a 193-nm wavelength; polarization effects at oblique angles (extreme NA); immersion and photoresist interactions with polarization; immersion fluid composition, temperature, flow, and micro-bubble influence on optical properties (index, absorption, aberration, birefringence); mechanical requirements for imaging, scanning, and wafer transport in a water media; and synthesizing conventional projection imaging via interferometric imaging.

A simulation package has been developed for predicting the influence of immersion, i.e., the presence of a uniform liquid layer between the last objective lens and the photoresist, on optical projection lithography. This technology has engendered considerable interest in the microlithography community during the past year, as it enables the real part of the index of refraction in the image space, and thus the numerical aperture of the projection system, to be greater than unity. The simulation program described here involves a Maxwell vector solution approach, including polarization effects and arbitrary thin film multilayers. We examine here the improvement in process window afforded by immersion under a variety of conditions, including λ = 193 nm and 157 nm, annular illumination, and the use of alternating phase shift mask technology. Immersion allows printing of dense lines and spaces as small as 45 nm with acceptable process window. We also examine the effect of variations in liquid index on the process window and conclude that the index of the liquid must be known to and maintained within a few parts per million. This has important implications for the temperature control required in future liquid immersion projection systems.

The scattering of bubbles in water by 193-nm light is characterized analytically with Mie scattering theory. The angular-resolved spectra with bubble sizes 100 nm, 1 µm, and 10 µm are calculated. For large bubbles, the forward scattering becomes very strong and therefore introduces a pattern-dependent flare. The normalized cross section with variant bubble size is also calculated. For bubble sizes smaller than the incident wavelength, the cross section decreases steeply and is explained by Rayleigh scattering. A contour plot of the normalized cross section versus the bubble size and the variance of refractive index is also calculated. This plot is explained with the Born approximation and is used to characterize the temperature control of the water during exposure. Finally, a statistical model is suggested to predict the image degradation caused by the phase information loss after the scattering. A bubble density 0.03/µm³ with 1-µm bubble size causes a loss of exposure latitude from 28 to 22%.

We present accurate measurements of several important optical constants of high-purity water for wavelengths near 193.39 nm at 21.50°C: the absolute refractive index n = 1.436 62(2), the dispersion dn/dλ = -2.109(17)×10^-3 nm^-1, and the thermo-optic coefficient dn/dT = -1.00(4)×10^-4 (°C)^-1. We determine these values by two independent approaches, the minimum deviation prism method and an interferometric technique, and find that they give mutually consistent results.

More than 50 fluorocarbon liquids are measured for transparency over the wavelength range 150 to 200 nm for the purpose of identifying a suitably transparent fluid for use in 157-nm liquid immersion lithography. Purification methods such as degasification, distillation, silica gel drying, and supercritical fluid fractionation are investigated to determine the impact of residual contaminants on absorbance. The purification processes are monitored by gas chromatography-mass spectrometry and Fourier-tranform infrared spectroscopy (for organics), ¹⁹F-nuclear magnetic resonance spectroscopy (for molecular structure), gel permeation chromatography (for molecular weight), Karl Fisher analysis (for water), and for residual dissolved oxygen. We find that in most cases, the absorbance is dominated by dissolved oxygen and water. Once the contaminant levels are reduced, the most transparent perfluoroether (PFE) measured is perfluoro-1,2-bis(2-methoxyethoxy)ethylene glycol (perfluorotriglyme) at 0.52 cm^-1, the most transparent perfluoroalkane (PFA) measured is perfluorohexane at 1.1 cm^-1, the most transparent hydrofluoroether (HFE) was 1-(1H-tetrafluoro)ethoxy-2-(1-trifluoromethyl)tetrafluoroethoxy-2-trifluoromethyl-1,1,2-trifluoroethane at 2.6 cm^-1, and the lowest projected absorption coefficient for a hydrofluoroalkane (HFA) is decafluoro-2H,3H-pentane at <2 cm^-1. Our chemical analysis shows that some impurities still remain in these materials, and further reductions in absorption are likely. Even so, our current absorption values should allow lens-to-wafer working distances (assuming 95% transmission) of 428, 203, 83, and 111 µm, respectively, for the four classes of fluids. The identification of these four classes (PFEs, PFAs, HFEs, and HFAs) of fluids for potential use as 157-nm immersion fluids, each with their own ranges of viscosity, vapor pressure, refractive index, dn/dT, synthetic routes, and cost, should allow for flexibility in performing tradeoff analyses for various 157-nm immersion lithography engineering designs and cost of ownership estimates.

We describe an assessment of the pressure rise that may be induced by the short-duration but high-power pulses associated with the immersion lithography exposure process. A conservative model provides an upper bound on the pressure rise related to the expansion of the fluid near the wafer due to rapid heating. This rapid heating process is simulated as a constant heat flux from the substrate. The resulting temperature rise causes a change in pressure that propagates into the fluid at the speed of sound. The net change in the mass of the fluid contained within the pressure wavefront must be zero. This continuity requirement allows an estimate of the pressure rise and its penetration depth into the gap. For the nominal conditions associated with 193-nm immersion lithography, the model predicts that the pressure near the wafer surface may rise by as much as 7.3 kPa during the laser pulse. At the end of the laser pulse, this pressure rise will extend nominally 75 µm into the gap. Following the laser pulse, this pressure rise will rapidly decay as the pressure wavefront continues to propagate across the gap and eventually out of the under-lens region.

Advanced dioptric projection lenses from Carl Zeiss are used in some of the world's most advanced deep ultraviolet projection lithography systems. These lenses provide a resolution of better than 100 nm across the entire field of view with a level of aberration control that maximizes critical dimension uniformity and lithographic process latitude. These dioptric projection lenses are currently being used for critical layer device patterning for a wide array of complex logic, memory, and application specific integrated circuits. Zeiss' involvement in the development of ultraviolet lenses goes back to the year 1902, more than 100 years ago, when von Rohr calculated the first monochromatic ultraviolet microobjectives for ultra-high-resolution microphotography using a line-narrowed source. The modern dioptric projection lenses for lithography are influenced by the collective experience in the field of microscopy, and the more recent experience with early step-and-repeat lenses. We discuss some of the foundations of modern dioptric designs in the context of this history, demonstrating that rapid synthesis of designs is possible using combinations of monochromatic microscope objectives and early step-and-repeat lenses from the 1970s. The problems associated with ultrahigh numerical aperture objectives are discussed. Specifically, it is demonstrated that aspheres can be used effectively to reduce the volume of full field projection lenses, making the mechanical implementation of a 0.90 NA lens feasible in production. Several contemporary dioptric projection lens designs are reviewed in detail. The extension of these designs to numerical apertures greater than 1.0 using immersion techniques is demonstrated. These immersion lenses give the potential for 40-nm resolution.

Immersion lithography has an advantage in the numerical aperture of optics by a factor of refractive index n of the liquid filled into the space between the bottom lens and wafer. In case of 193-nm exposure tools, water (n = 1.44) has been found as the best liquid. It is shown, by using imaging simulations, that ArF (193-nm) immersion lithography (NA = 1.05 to 1.23) has almost equivalent performance to F2 (157-nm) dry (NA = 0.85 to 0.93) lithography. Issues in the ArF immersion exposure tools are discussed with fluid-dynamic and thermal simulations results. In the fundamental issues, there seems to be no showstoppers so far, however, there exist several challenges to realize viable exposure tools.

Liquid immersion has been used for more than 100 years to increase the numeric aperture (NA) and resolution in optical microscopy. We explore the benefits and limitations of immersion technology in lithography. Immersion optical lithography has the potential to extend the resolution below 40 nm. The theory of immersion is decribed. Simulations show that a 193-nm immersion system at NA = 0.95 can double the depth of focus as compared to a dry system. Also, an immersion 193-nm system at NA = 1.05 has slightly more depth of focus than a 157-nm dry system at NA = 0.85. However, the exposure latitude at 193 nm is decreased due to the impact of polarization in imaging. Design schemes are presented to realize an immersion step and scan system. Two configuration approaches are proposed and explored. A localized shower type solution may be preferred over a bath type solution, because the impact on the step and scan platform design is significantly less. However, scanning over the wafer edge becomes the main design challenge with a shower solution. Studies are presented that look at the interaction of immersion fluids with the lens and the photoresist. Water seems to be a likely candidate, as it does not impact productivity of the step and scan system; however, focus and aberration levels need to be carefully controlled. For 157 nm, per-fluor-polyether (PFPE) materials are currently being studied, but their characteristics may limit the productivity of the exposure system. Further research on fluid candidates for 157-nm immersion is required.

We describe the application of a linear birefringence measurement system at the optical lithographic wavelengths (157, 193, and 248 nm). Using this instrument, we obtain linear birefringence maps of a variety of calcium fluoride samples with different crystal orientations and at different DUV (deep ultraviolet) wavelengths. Comparing the birefringence results measured at DUV and visible wavelengths, we find that some calcium fluoride samples show a strong correlation in birefringence measured at different wavelengths while others do not. Significant differences in birefringence patterns are observed at 157 and 633 nm for certain calcium fluoride samples when the light beam propagates along the [111] crystal axis. We explain the discrepancy in the birefringence patterns observed at 157 and 633 nm based on the crystalline structural quality of calcium fluoride samples.

Optical lithography with 157-nm light is expected to bridge the gap between 193-nm technology and next-generation lithography. One important practical difficulty facing the implementation of 157-nm technology is gas absorption of 157-nm light. The exposure process for 193-nm technology is carried out in an air environment, but oxygen gas and water vapor severely attenuate 157-nm radiation. However, 157-nm exposure can be carried out in a nitrogen environment, which can be achieved by purging. A challenging aspect of the nitrogen purging process is the evacuation of the volume delineated by the pellicle frame, and the 800-µm-thick hard pellicle plate, which can fracture when subjected to an excessive pressure difference. A technique for pellicle purification via a cyclic purging and filling process is investigated. A theoretical analysis of the gas flow and pressure variation in the system is presented. The maximum stress induced in the hard pellicle during the process is predicted using finite element modeling. The minimum time for purification without causing excessive stress in the pellicle plate is estimated for a nominal set of conditions. Finally, a parametric analysis of important geometric variables including the size and number of purging holes as well as the filter resistance is presented.

Discharge-produced plasma (DPP) devices have been proposed as a light source for EUV lithography. A key challenge for DPP is achieving sufficient brightness to support the throughput requirements of exposure tools for high-volume manufacturing lithography. To simulate the environment of the EUV source and optimize the output of the source, an integrated model is being developed to describe the hydrodynamic and optical processes that occur in DPP devices. The model includes both plasma evolution and magnetohydrodynamic processes as well as detailed photon radiation transport. The total variation diminishing scheme in the Lax-Friedrich formulation for the description of magnetic compression and diffusion in a cylindrical geometry is used. Several models are being developed for opacity calculations: a collisional radiation equilibrium model, a self-consistent field model with Auger processes, and a nonstationary kinetic model. Radiation transport for both continuum and lines with detailed spectral profiles are taken into account. The developed models are integrated into the HEIGHTS-EUV computer simulation package. Preliminary results of a numerical simulation of xenon gas hydrodynamics and EUV radiation output are presented for various plasma conditions.

Reticle blanks for extreme ultraviolet lithography (EUVL) are fabricated by depositing reflective Mo/Si multilayer films on superpolished substrates. To obtain a reasonable cost of ownership for EUVL, the multilayer films must be nearly defect free, have excellent reflectance/thickness uniformity, and have a high EUV reflectance. Small particle contaminants on the substrate that can nucleate printable Mo/Si phase defects are a serious concern. We develop an ion-beam thin film planarization process for mitigating the effect of small substrate contaminants that relies on enhancing the smoothing capability of Mo/Si multilayer films; we observe that etching of the Si layers in between deposition steps can yield a significant improvement in smoothing. Using this process substrate particles as large as ~50 nm in diameter are smoothed to ~1 nm in height, rendering them harmless. We further develop this process so that it retains these particle-smoothing capabilities while also achieving a high EUV reflectance and excellent uniformity.

The employment of an insulating shield for the improvement of the current distribution on 3-in. wafer substrates is considered. Numerical analysis is used to evaluate the influence of shield shape and position on the deposition uniformity, and the simulation results are compared to experimental data for nickel deposition from a Ni sulfamate bath. The use of a shield is shown to be an effective and simple way to improve current distribution uniformity, reducing the measured disparity between the average current density and the current density at the substrate center from ~35% to less than about 10% for the cases studied.

Diffuser micropumps are commonly fabricated using the standard lithography techniques with silicon as the base material. The important components of this type of micropumps are flow-directing diffusers and a moving diaphragm. Different diffuser designs show various flow rates and pump efficiency. In this work a polymer is used as the base material instead of silicon. It is demonstrated that polymer-based micropumps can be conveniently fabricated using the laser machining technique. Moreover, because of the flexibility of polymer materials, there is great potential to improve the performance of the polymer micropumps. The fluid flow inside the diffuser polymer micropump is also calculated using computational fluid dynamics methods and the simulated results are compared with the experimental data.

_{Spin-coated films of nonphotosensitive engineering thermoplastics mixed with photosensitive agent diazonaphthoquinone (DNQ) can be clearly imaged with near-UV light. The selected engineering thermoplastics are commercially available poly(bisphenol A carbonate), polyarylate (U polymer^®) and polyetherimide (Ultem^®), and synthesized fluorinated polyimide, which have no specific functional groups. Development with a solution including ethanolamine dissolves the irradiated areas to give positive fine patterns. The two-component photosensitive systems shows good photosensitivity and resolution (line/space 10/10 µm) with about 10 to 15 µm in thickness. Gel-permiation chromatography (GPC) and ¹H-NMR measurements that can give information on the structure of components dissolved from the irradiated regions are carried out to make clear the imaging mechanism, which we call reaction development patterning (RDP). RDP-based photosensitive polymers showed high heat resistance up to their glass transition temperature (Tg) or above.}

We describe a simple and effective method, based on a combination of centrifugation and spin coating, developed for the fabrication by self-assembly of photonic crystal structures. The process produces compact 3-D arrays of polystyrene microspheres (opals) that are organized into crystalline lattices with single-crystal regions up to several hundred micrometers in size. These arrays are subsequently used as templates for infiltration with polyurethane. The resulting inverse opals are characterized by Fourier transform IR reflection spectroscopy and exhibit strong reflectivity for a band of wavelengths in excellent agreement with band structure calculations. This process offers the ability to rapidly form 3-D photonic crystals using inexpensive instrumentation, which makes it attractive for an array of applications. Process parameters, fabricated structures, and their experimental characterization are presented.

A new class of printing strategies is described for the manufacture of microstructures and nanostructures. This class collectively is referred to as molecular transfer lithography (MxL). The approach is based on the room-temperature fabrication of water-soluble polymer templates by spin casting a polyvinyl alcohol film-forming solution to replicate surface patterns. The templates are useful not only for pattern formation, but also for materials transfer printing, employing a low-cost, convenient, biocompatible chemical approach to high-resolution processing. Results are provided to demonstrate deep submicrometer feature sizes of holes, pillars and lines, 3-D patterns, materials transfer printing of metallic thin films, planarization of wafer topography, and water-soluble polymer templates for 100- and 200-mm wafer patterning. The alignment tooling is discussed and it is shown that MxL can be adapted for use on standard contact aligners with a replacement of the quartz photomask with a water-soluble polymer template to improve resolution without a change of equipment. A high-throughput alignment system for MxL is also discussed. The MxL class of pattern formation and materials transfer printing strategies is differentiated with respect to imprint lithography and soft lithography methods.

We propose a new approach to estimating a pointing error of the electron probe of a scanning electron microscope. The approach is formulated to estimate the pointing errors using a specimen image. Specimen images are numerically simulated by a mathematical model and identified with measured images of the specimen using a least-squares procedure to determine the pointing errors. The pointing errors are estimated by the proposed approach and used to design a controller for vibration isolation of a scanning electron microscope. Acceleration sensors are located at the root of the specimen chamber and are used to detect any environmental disturbance into the microscope. The designed controller is based on a transfer function from the sensor outputs to the pointing errors, and the transfer function is determined by sinusoidal excitation tests for the microscope. The controller is implemented as a digital filter on a PC and is used to move the electron probe to cancel the pointing errors using image-shifting coils. The pointing errors are successfully reduced by the controller in a lower frequency region that contains the first four natural frequencies.