Ultra high numerical aperture (NA) enables extension of ArF lithography for the 45 technology node and beyond. The resulting changes in design rules drives feature sizes on the mask into the sub-wavelength regime. As 2-beam imaging techniques (off-axis illumination and alternating phase shift mask) are required for strong resolution enhancement in low-k1 lithography, traditional scalar and paraxial approximations used for optical image modeling are no longer valid in the ultra high NA regime. Vector and thick-mask based models are required to account for topographic effects and large angles of incident light at the reticle plane in ultra-high NA systems. Although vector-based imaging theory is well understood, experimental validation is required to ensure the appropriate topographical and optical parameters are being used. To address these issues, finite-difference time-domain rigorous electromagnetic simulation are compared to experimental measurements of the polarization dependent diffraction efficiencies on advanced optical reticles. Based on these results, the impact of mask induced polarization to vectorial imaging latitude is assessed. The impact of polarization purity, mask absorber profile, and Fresnel effects through the pellicle on process window and OPC are also discussed.
We have designed and constructed a microstepper for 157 nm immersion lithography. The lens, designed and fabricated at Newport, provides a numerical aperture of 1.3 and a field size of 60 μm with immersion liquids of index n=1.38. Because of a lack of system interferometer, final alignment has been ongoing in the field using actuators incorporated into the lens design. Lithography down to 250 nm has been demonstrated but lens alignment has proved difficult. We are currently implementing an image monitoring system to provide real-time feedback on lens performance and to allow expedited alignment.
Polarization dependent diffraction efficiencies in transmission through gratings on specially designed masks with pitch comparable to the wavelength were measured using an angle-resolved scatterometry apparatus with a 193 nm excimer source. Four masks - two binary, one alternating and one attenuated phase shift mask - were included in the experimental measurements. The validity of models used in present commercially available simulation packages and additional polarization effects were evaluated against the experimental scattering efficiencies.
We have measured the intrinsic scattering of water with an eye toward its potential impact on immersion lithography. Quantitative measurements of the elastic Rayleigh scatter agree well with theory and show a loss of 0.001 cm-1. Qualitative measurements of the inelastic Raman scattering show a strong peak at 206 nm, consistent with the O-H stretch present in water. Both are expected to contribute flare of < 10-6 of the incident intensity. We have also examined the possibility for bubbles in the immersion liquid, and in particular those which form near the resist surface. We have measured scattering from single bubbles and estimate that bubbles as small as 5 μm should be detectable in this fashion. In addition, we have measured the potential for bubbles due to laser induced resist outgassing by direct imaging. In 2500 resist images (~235 mm2 of surface), we have seen only one bubble candidate which, due to its persistence in the water, we do not believe represents a true outgassing-induced bubble. Finally, using a technique borrowed from biology, rapid cryofixation/freeze fracture, we have examined nanobubbles which form spontaneously on hydrophobic surfaces and found that degassing the water prevents their formation.
The photo-induced degradation of 157-nm anti-reflective (AR) coatings, and the role of water vapor in the ambient, have been studied with in-situ spectroscopic ellipsometry, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy. Using ellipsometric techniques, we find that MgF2 thin films develop a surface roughness layer under laser irradiation at an incident dose of ~0.1 MJ/cm2. These thin film changes occur well before any changes in 157-nm transmission are observed. The findings are confirmed by ex-situ post-irradiation AFM measurements. LaF3 does not exhibit this effect. Addition of ppm-levels of moisture suppresses surface roughness formation, suggesting that the surface roughness growth may be a precursor to the transmission degradation of full AR stacks that had been observed earlier.
A fabrication process has been developed which prevents solvent intermixing between layers of diazonaphthoquinone/novolac (DNQ/novolac) based resist. The process enables three-dimensional structures to be batch fabricated stereolithographically using integrated circuit-compatible resist, coating, and exposure techniques, followed by a single development step. To prevent solvent intermixing, a combination of solvent tailoring and surface treatment is employed. The photoresist is first constituted into a weaker, less polar solvent. Before coating a new layer, the surface is exposed to ozone, thus increasing the hydrophilicity of the surface and providing a less soluble barrier layer. This enables the formation of a stack of successively photoimaged layers of the same material, which are then developed in a single step. A new interlayer dose modulation technique to optimize the development process in positive tone resists such as DNQ/novolac is also described.
Photo-induced contamination rates on 157-nm optical surfaces have been studied in controlled experiments with contaminants containing fluorocarbon, sulfur and iodine. The compounds investigated represent species generated in controlled outgassing studies of common construction materials and photoresists used in 157 nm steppers. No photocontamination was measured for highly fluorinated alkanes and ethers on an anti-reflective coating, at levels exceeding 10 ppm. Photocontamination with sulfur based compounds was similar to the behavior observed with hydrocarbon based derivatives. Sulfur containing residues, even from oxidized precursors, are fully cleanable in oxygen, with cleaning rates scaling proportionally with the level of oxygen. In contrast, at elevated levels of oxygen, non-volatile iodate complexes can form from iodine based contaminants. Sulfonium salts should therefore be considered over iodonium species in photoacid generators in 157 nm photoresists. In addition to studying these new classes of compounds, cleaning rates of hydrocarbon residues in trace levels of water were also studied.
An angle-resolved scattering detection system has been designed and implemented for use at 157 nm. This tool will enable the optimization of polishing and thin-film deposition, whith an eye towards minimizing small-angle scatter in projection lithography tools. In this test-bed, scattered rays can be collected to 4° from the directional ray of the specularly transmitted beam (corresponding to spatial wavelengths of surface roughness below 2 μm) over a dynamic range of 7 orders of magnitude, and to 0.5° with a dynamic range of 5 orders of magnitude. The angular scattering distributions in CaF2 samples and antireflective coatings are compared. From these results, the impact of scattering on image performance in exposure tools at 157 nm is estimated.
Photodeposition rates for ten hydrocarbon species have been measured on CaF2 substrates under 157-nm irradiation in the presence of ppm scale levels of oxygen. The species are representative of hydrocarbon based compounds observed in outgassing studies of common build materials used in 157-nm based lithographic systems. Photodeposition rates have also been measured for a subset of the hydrocarbon species on a MgF2 thin film, six anti-reflective dielectric stacks, and fluorine doped fused silica for comparison with the results on CaF2 substrates. Two contamination processes are observed. One is the formation of an equilibrium layer on the surfaces. The other is a quasi-permanent contamination which is most pronounced at elevated levels of contaminant.
Contamination rates of CaF2 substrates in the presence of trace levels of toluene vapor and oxygen under 157-nm irradiation have been studied to determine conditions which prevent contamination films from depositing on optical elements in lithographic projection systems. A 2 - 3 monolayer thick deposit, causing a 1 - 2% transmission drop per surface, can readily form over a range of contaminant levels in the sub-ppm range and typical background oxygen levels. In addition, stable partial surface coverage can be supported with either lower concentrations of contaminant or conversely much higher levels of oxygen. Contamination rates are also higher at lower fluences, and thus contamination effects are expected to impact the projection optics more severely than beam delivery and illumination components. Finally, a permanent degradation in transmission of coated optics has been observed on anti-reflective coatings exposed to sub-ppm levels of toluene. Taken together, the results suggest that even with hydrocarbon based contaminants, where oxygen can be introduced into the beam-line in trace levels (i.e. hundreds of ppb) without significantly degrading transmission, toluene contaminant levels will have to be maintained in the ppb range or below.
A UV-lamp-based cleaning station, serving as a load-lock to a VUV spectrometer, has been used to evaluate the cleaning of hydrocarbon residues on 157-nm reticles. UV lamp based cleaning is found to be an effective tool to remove both nanometer scale layers of physisorbed and significantly more resilient highly conjugated 'graphitized' layers on the mask substrate. Slight changes in reflectance and surface roughness are observed on the chromium absorber indicating some degree of photo-oxidation is occurring during lamp cleaning.
Contamination of optical elements during photoresist exposure is a serious issue in optical lithography. The outgassing of photoresist has been identified as a problem at 248nm and 193nm in production because the organic films that can be formed on an exposure lens can cause transmission loss and sever image distortion. At these exposure energies, the excitation of the photo acid generator, formation of acid, and cleavage of the protecting group are highly selective processes. At 157nm, the exposure energy is much higher (7.9 eV compared to 6.4 eV at 193nm) and it is known from laser ablation experiments that direct laser cleavage of sigma bonds occurs. The fragments formed during this irradiation can be considered as effective laser deposition precursors even in the mid ppb level. In this study, methods to quantify photoresist outgassing at 157 nm are discussed. Three criteria have been set up at International SEMATECH to protect lens contamination and to determine the severity of photoresist outgassing. First, we measured film thickness loss as a function of exposure dose for a variety of materials. In a second test we studied the molecular composition of the outgassing fragments with an exposure chamber coupled to a gas chromatograph and a mass spectrometer detector. Our third method was a deposition test of outgassing vapors on a CaF2 proof plate followed by analysis using VUV and X-ray photoelectron spectroscopies (XPS). With this technique we found deposits for many different resists. Our main focus is on F- and Si- containing resists. Both material classes form deposits especially if these atoms are bound to the polymer side chains. Whereas the F-containing films can be cleaned off under 157nm irradiation, cleaning of Si-containing films mainly produces SiO2. Our cleaning studies of plasma deposited F-containing organic films on SiO2 did not indicate damage of this surface by the possible formation of HF. Despite that we strongly recommend engineering measures to overcome contamination by resist, such as optimizing the purge flow between the final lens element and wafer surface or utilization of a lens pellicle.
A 157nm interference lithography system which is capable of patterning features at sub-100-nm pitch has been implemented. Initial results demonstrate approximately 50 nm line and space patterns exposed in a commercial deep-UV photoresists. Little line edge roughness is observed, indicating that the intrinsic properties of the resist may meet CD-control requirements to at least 50 nm. In addition, this system may be used to measure the spatial coherence of the 157-nm F2 laser source. Preliminary estimates show that the coherence length is approximately 40 micrometers .
In-situ laser cleaning is shown to be an effective tool for removal of organic contaminants on CaF2 windows. To study laser cleaning in a controlled fashion, CaF2 substrates were pre-contaminated with 5 to 10 nm of poly(methyl methacrylate), poly(4-hydroxy styrene), poly(norbornene), and poly((beta) -pinene) thin films. Irradiation of all the polymer films showed similar trends. Initially, a high rate of material removal occurs, which depends on the chemistry of the polymer. During this period, the material also undergoes significant bond rearrangement, forming a more tightly bound highly conjugated network. Removal of this residual 'graphitized' film is significantly more difficult, but can be accelerated by the presence of modest levels of oxygen. For oxygen concentrations between 10-1000 ppm, the measured removal rate is approximately 3 nm/(kJ/cm2) ppm oxygen. No effect on removal rate was observed as pulse energy or purge gas flow rate was varied over ranges expected to be used in practical systems.
Photolithography using 157-nm pulsed F2 lasers has emerged as the leading candidate technology for the 0.1 micrometer lithography node for the post-193-nm generation. The extension of operating wavelength to the VUV range presents new challenges for thin film metrology tools, such as ellipsometers and spectrophotometers, most of which have not yet shown robust performance at high accuracy at wavelengths below 193 nm. Knowledge of material optical properties near 157 nm is essential for several areas of microlithography, such as (1) optimization of resist and bottom antireflectance coating (BARC) and lithographic performance modeling; (2) development of thin dielectric layers for lens coatings, including antireflectance, beamsplitter and high reflectance designs; and (3) development of resolution enhancement techniques, such as attenuating phase shifting masks. In this work we review our experience with VUV spectrophotometers, as well as techniques for obtaining stable reflection and transmission measurements necessary for deriving optical constants of thin films. In particular, we find that reliably accurate reflection data can be obtained only using absolute reflectance methods. Extraction of optical constants is performed utilizing global optimization methods with a commercially available software package. Kramers-Kronig- consistent dispersion relations are used to describe the material dielectric constants. We will present real and imaginary refractive index values of various thin films, as determined from reflection/transmission data into the deep UV wavelengths to as low as 140 nm. A separate study designed to understand scatter losses of materials at 157 nm will also be described. We have constructed a 157-nm laser-based scatterometer for obtaining bidirectional reflection distribution function (BRDF) measurements. By correlating scatter signals with total transmission losses, we are able to separate absorption from scatter effects.
Finite element models have been developed and refined to simulate the mechanical distortions associated with mask blank fabrication, pattern transfer, and exposure clamping. By modeling the substrate with layers associated with the mask fabrication process and then by prestressing specified layers, the resulting out-of-plane and in-plane distortions of the mask blank have been determined. Etching procedures were subsequently simulated to assess the pattern transfer distortions associated with both dark and bright field masks. Investigations included substrate materials which have acceptable optical transmission for wavelengths below 180 nm. Additional mechanical distortions associated with clamping the reticle into the exposure mount have also been considered.
If optical lithography is to be extended into the 157 nm regime, controlling mask-related distortions will be a necessity. Thermomechanical distortions during exposure could be a major source of pattern placement error, especially if alternative materials such as CaF2 or MgF2 are employed. Full 3D finite element heat transfer and structural models have been developed to simulate the response of the reticle during both full-field and scanning exposure systems. Transient and periodic steady-state temperature distributions have been determined for typical exposure duty cycles. Corresponding in-plane and out-of- plane thermal distortions have been identified for both fused silica and calcium fluoride substrates. Under equivalent exposure conditions, the distortions in the CaF2 are significantly higher.
We update previously reported results on the absorption of optical materials and coatings for use in 157 nm based optical projection system. New results include the transmissions spectrum of a modified from of fused silica with suitable initial transmission for use as a mask substrate. We also report on a more systematic study of the effects of surface contaminants on optical components at 157 nm. We have modified our vacuum spectrometer to allow in- situ cleaning to enable a closer examination of purging requirements and cleaning procedures.
We have measured the transparencies of a number of a candidate resist materials for 157 nm, with an emphasis on determining which chemical platforms would allow resist to be used at maximum thicknesses while meeting requirements for optical density. Assuming an ideal resist optical density of 0.4, our findings show that all existing commercially available resists would need to be < 90 nm thick, whereas specialized hydrocarbon resists could be made approximately 120 nm thick, and new resists based on hydrofluorocarbons, siloxanes, and/or silsesquioxanes could be engineered to be used in thicknesses approaching 200 nm. We also assess the tradeoff between these thicknesses and what current information exists regarding defects as a function of resist thickness.
One of the main obstacles encountered in designing low noise, high efficiency, heterodyne receivers and local oscillator sources at submillimeter wavelengths is the quality and cost of waveguide structures. At wavelengths shorter than 400 micrometers, rectangular waveguide structures, feed-horns, and backshorts become extremely difficult to fabricate using standard machining techniques. We have used a new laser milling technique to fabricate high quality, THz waveguide components and feedhorns. Once metallized, the structures have the properties of standard waveguide components. Unlike waveguide components made using silicon wet-etching techniques, laser-etched components can have almost any cross section, from rectangular to circular. Under computer control, the entire waveguide structure (including the corrugated feedhorn a submillimeter-wave mixer or multiplier can be fabricated to micrometer tolerances in a few hours. Laser etching permits the direct scaling of successful waveguide multiplier and mixer designs to THz frequencies. Since the entire process is computer controlled, the cost of fabricating submillimeter waveguide components is significantly reduced. With this new laser etching process, the construction of high performance waveguide array receivers at THz frequencies becomes tractable. In this paper we will describe the laser etching technique and discuss how it can be used to construct THz imaging arrays. We will also describe the construction of a prototype 810 GHz mixer which utilizes these new construction techniques.
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