In July 2004, the third FPA-5800 FS1 157-nm full-field scanner, developed by Canon Inc., was shipped to Selete. The scanner has an exposure field of 22 x 26 mm with a five-times reduction ratio. The numerical aperture is 0.80, which is the highest among all 157-nm scanners. We are now investigating tool performance, illumination uniformity, and imaging performance under various exposure conditions. In this paper, we will report our findings, focusing on the applicability of 157-nm dry lithography for the half-pitch 65-nm-node. We have obtained a resolution limit of 55-nm line-and-space (L&S) patterns with an alternating phase shifting mask. This corresponds to a 0.28 k1 factor. The depth of focus for these patterns at lens-center was 0.30 μm. For an attenuating PSM (Att-PSM) with annular illumination, the resolution limit was 65-nm L&S, which corresponds to 0.33 k1. The line width uniformity of 65-nm L&S with an Att-PSM was 15.0 nm. The dense-hole resolution was 80 nm. However, lens flare had a considerable effect on resist profiles, viewed as a difference between dark and bright field patterns. In addition, with the application of gate etching processes, the fabrication of a 40-nm line/120-nm pitch gate pattern was achieved. Improvement in the line width roughness (8.3 nm) is needed and should be attainable.
Centerline phase-shifting mask (CL-PSM), which has narrow chromium lines at the boundaries of a μ-phase shifter, is promising as a resolution enhancement technology for random-pitch line patterns. We compared the performance of the CL-PSM in fabricating sub-45 nm lines with that of the chrome-less phase-shifting mask (CLM) in 157-nm lithography. The simulation results showed the CL-PSM is superior to the CLM in resolution and depth of focus (DOF), especially in small pitch patterns. We optimized the layouts of CL-PSM and the CLM to 40-nm-wide, 140-nm-pitch line patterns through the simulation. In exposure experiments with optimized masks, the CL-PSM resolved 40-nm-wide line patterns with a minimum pitch of 110 nm, while the resolvable minimum pitch was 130 nm for the CLM. The DOFs for 40-nm-wide, 140-nm-pitch lines were 200 and 80 nm with CL-PSM and CLM, respectively. Furthermore, we estimated the resolution limit of CL-PSM in hyper-NA 193-nm lithography, and showed a pitch of 100 nm would be achieved with a 1.4 NA optics.
The FPA-5800FS1 157-nm scanner installed at Selete has demonstrated a minimum resolution of 55 nm for line-and-space (L/S) patterns with a numerical aperture (NA) of 0.8. The scanner has been used for 65-nm-node device fabrication and will be used for 45-nm-node device development. The approximately 20% shorter wavelength in 157-nm lithography has several advantages compared to 193-nm immersion lithography. For example, assuming the same k1 value, 157-nm lithography, which has a 20% smaller NA, has a 25% larger depth of focus and better resolution in two-dimensional patterns, for which polarized illumination is not effective. This 157- nm immersion lithography has the potential to be used for 32-nm-node device fabrication with a k1 of 0.3 in combination with a high-refractive-index immersion fluid. To demonstrate the process feasibility of 157-nm immersion lithography, we developed a two-beam interferometric stepper with a high-quality F2 laser and used it and a commercial perfluoroether as an immersion fluid to print 60-nm L/S patterns with a steep cross-sectional profile. Development of an immersion fluid with a high refractive index and low optical absorption is critical issue for making 157-nm immersion lithography practical. We have identified several fluorinated polymers with high diffractive indices and will continue searching for suitable 157-nm immersion fluids.
Aberration metrology is critical to the manufacture of quality lithography lenses in order to meet strict optical requirements. Additionally, it is becoming increasingly important to be able to measure and monitor lens performance in an IC production environment on a regular basis. The lithographer needs to understand the influence of aberrations on imaging and any changes that may occur in the aberration performance of the lens between assembly and application, and over the course of using an exposure tool. This paper will present a new method for the detection of lens aberrations that may be employed during standard lithography operation. The approach allows for the detection of specific aberration types and trends, as well as levels of aberration, though visual inspection of high resolution images of resist patterns and fitting of the aberrated wavefront. The approach consists of a test target made up of a 180-degree phase pattern array in a “phase wheel” configuration. The circular phase regions in the phase wheel are arranged so that their response to lens aberration is interrelated and the regions respond uniquely to specific aberrations, depending on their location within the target. This test method offers an advantage because of the sensitivity to particular aberration types, the unique response of multiple zones of the test target to aberrations, and the ease with which aberrations can be distinguished. The method of lens aberration detection is based on the identification of the deviations that occur between the images printed with the phase wheel target and images that would be produced in the absence of aberration. This is carried out through the use of lithography simulation, where simulated images can be produced without aberration and with various levels of lens aberration. Comparisons of printed resist images to simulated resist images are made while the values of the coefficients for the primary Zernike aberrations are varied.
157-nm lithography processes together with optimization of mask feature size and illumination conditions and chromeless mask (CLM) of mesa-type were used to fabricate a 45-nm gate by combining a high numerical aperture (NA) lens with off-axis illumination (OAI) and using Si-containing resist. It was observed that the minimum pitch for forming a 45-nm line was 140-nm. It was also shown that quadrupole illumination was the optimum OAI condition and the optimum mask feature size for forming a 45-nm line of 200-nm pitch was between 50 nm to 55 nm. In these conditions the normalized image log-slope value was about 3.0. It was demonstrated that a 45-nm SRAM gate with a depth of focus of 150 nm could be fabricated by combining these resolution enhancement techniques with high NA lithography and Si-containing resist. Furthermore the 45-nm SRAM-gate pattern was successfully transferred with a bi-layer process. From these results it was proven that fabrication of 45-nm node device could be achieved by using CLM with high NA lithography.
Chromeless Phase Lithography is known as an effective resolution enhancement technique for isolated line patterns. We fabricated a chromeless phase lithography mask for 157-nm lithography, and evaluated the lithographic performance using a 0.90 numerical aperture 157-nm microstepper. To obtain the best resolution, illumination condition was optimized to conventional illumination with 0.7 partial coherence (σ) using lithography simulation. In the exposure experiment, 30-nm-wide isolated line, 30-nm-wide 140-nm-pitch line-and-space, and 30-nm-wide static random access memory (SRAM) gate patterns were resolved. Further lithography simulation results indicated that the resolution limit of 24-nm would be obtained by eliminating the image degradation factors such as the aberration, flare, and central obscuration.
The Rayleigh equation given by R equals k1 X (lambda) /NA is often used to predict the resolution (R) of optical lithography. Since the design rule is approaching half of wavelength, however, lithographic performance imperfectly follows the Rayleigh equation. In other words, the constant k1 does not represent the process difficulty expressed as contrast, exposure dose latitude, and mask error enhancement factor (MEEF). We propose a modified Rayleigh equation that considers the influence of image fluctuation on lithographic resolution, and confirmed that lithographic performance can be predicted more accurately by the modified equation than by the conventional equation. According to the modified equation, resolution will not be more enhanced than that predicted by the conventional Rayleigh equation, even if lens numerical aperture is increased.
To reduce mask error enhancement factor (MEEF), we have developed the new type half-tone phase shift mask (HTPSM) in which transparent regions are surrounded by opaque rims. We evaluated the imaging performance of contact hole patterns including the MEEF and the depth of focus (DOF). Using this new method, we obtained about 2.0 MEEF and 0.7-micrometers DOF for 180-nm isolated hole, which was much better than that in the conventional mask such as binary mask or HTPSM (the MEEF more than 3). The advantage of our method was that it was possible to attain both the MEEF reduction and the DOF enhancement by the optimization of mask hole size and rim width. Furthermore, we confirmed that this new method was effective not only for improving the exposure dose latitude but also for attenuating side-peak effect.
The influence of spherical aberration on imaging performance was evaluated by resist simulation for various resist thicknesses and other resist parameters. The best focus variation in terms of pattern size in L and S was not appropriate as a lithographic criterion because it varied not only with pattern size but also with resist characteristics and thickness. General rules for the best focus of L and S based on CD-defocus characteristics are proposed. The lithographic performance represented by the CD uniformity and the best focus variation of isolated patterns were predicted approximately from the result of aerial image calculation for ideal resist performance. In conclusion, the reduction in spherical aberration that leads to improved CD accuracy is not always achieved by the decrease in best focus shift.
The stage vibration effect on imaging performance, such as DOF and CD uniformity is evaluated experimentally and compared with simulation analysis. Various kinds of 0.25 - 0.18 micrometer patterns are investigated by using KrF excimer scanner with 0.6 NA and 0.75 partial coherency and two types of chemically amplified positive resists. In the case of a standard resist for 0.25 micrometer level patterning, the CD at the best focus changed and the DOF decreased rapidly with increasing moving standard deviation (MSD) in 0.18 micrometer level pattern formation. Allowable MSD value of L&S pattern was estimated to be around 25 nm. To improve the stage synchronous error margin, the application of a high resolution resist was effective on L&S and isolated space patterns (about 40 nm), but showed little improvement for isolated line and hole patterns. Therefore, totally allowable MSD value was still about 30 nm. In particular it was found that both isolated line and hole patterns were very sensitive to stage vibration effect. Strict stage control has to be required for 0.18 micrometer patterns even if the high resolution resist is used.
To enhance depth of focus (DOF) for isolated line patterns, we have developed a new assistant pattern method. In this method, opaque additional patterns are placed beside the attenuated phase-shift main pattern. DOF enhancement effects of this and conventional assistant pattern method were evaluated by means of a KrF excimer exposure tool with variable NA (0.45, 0.50 and 0.55). Using the new method with off-axis illumination, we obtained 1.5 micrometers DOF for 0.25 micrometers isolated line pattern, much wider than that achieved in the conventional method (0.9 micrometers ). Furthermore, we confirmed that this new method was effective not only for improving the exposure dose latitude but also for reducing the optical proximity effect.
In this study, the optical proximity effect (OPE) of positive and negative tone line patterns is compared under a variety of exposure conditions. 0.25 micrometers lines with various pitch sizes were printed by a KrF stepper, and the CD variation as a function of pitch was evaluated. We found that the OPE was suppressed significantly in negative patterns under various conditions. The effect of resolution enhancement techniques on the OPE is also investigated. We found that negative patterning with the combination of off- axis illumination and attenuated phase-shift masks not only improved DOF but also gave a small OPE.