Shigeru Hirukawa

Double-patterning requirements for optical lithography and prospects for optical extension without double patterning

Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are being considered. This paper focuses on the requirements of the most complex forms of DP, pitch-splitting (where line density is doubled through two exposures) and spacer processes (where a deposition process is used to achieve the final pattern). Budgets for critical dimension uniformity and overlay are presented along with tool and process requirements to achieve these budgets. Experimental results showing 45-nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are presented to highlight some of the challenges. Finally, alternatives to DP are presented.

Automatic laser-scanning focus detection method using printed focus pattern

An innovative method of determining best focus with an optical exposure tool has been developed. The method uses wedge shaped marks in photoresist that can be measured automatically. The results show that best focus can be measured with a repeatability of 20 nm. The automatic focus measurement system can be used to characterize lens astigmatism, field curvature and tilt. Data shows good correlation with conventional methods using SEM linewidth measurement.

Double patterning requirements for optical lithography and prospects for optical extension without double patterning

Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are being considered. This paper focuses on the requirements of the most complex forms of DP, pitch splitting, where line density is doubled through two exposures, and sidewall processes, where a deposition process is used to achieve the final pattern. Budgets for CD uniformity and overlay are presented along with tool and process requirements to achieve these budgets. Experimental results showing 45 nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are presented to highlight some of the challenges. Finally, alternatives to double patterning are presented.

Development of polarized-light illuminator and its impact

Nikon has developed an illuminator with special options for RET (Resolution Enhancement Technique). For one of the solutions of RET, Nikon has pursued the development of a loss-less polarized illumination system. When the polarization direction is the same as the direction of the printed pattern, this technique improves image contrast and extends the process margin. We have simulated the impact of the RET with polarized illumination, in the case of dipole illumination and phase-shift masks, and we have estimated the dominant parameters for high performance polarized illumination. In addition, we have constructed a polarized-light illuminator and installed it in an ArF full-field scanner. We have measured and optimized the degree and distribution of polarization at the wafer plane with a special tool, and we have investigated image performance with polarized dipole illumination. Results show that the new polarized-light illuminator has extended the process margin, especially with respect to dose latitude. The results of the image simulations and experiments will be reported.

Analysis of imaging performance degradation

The wavefront aberration that is generally represented with Zernike polynomials is an approximate representation. The reasons are that, in actual optics, the wavefront has such frequency component that cannot be represented with Zernike polynomials. Moreover, it can never be scalar or monochromatic. Instead, it must be vector and polychromatic. Higher frequency component beyond Zernike representation could cause a local flare that will be observed in the surrounding area of nominally bright patterns. Vector aspect of light leads to imaging degradation combined with birefringence of the material. Even with a narrowed spectral bandwidth of excimer lasers, chromatic aberration could be a factor that impacts imaging performance. Lateral, rather than axial, chromatic aberration can be critical because it is influential to CD uniformity across the field. This paper describes the factors that deteriorate imaging performance based on Nikon’s optics, and finally concludes hat our optics is well balanced among these factors.

Characterization of spatial coherence uniformity in exposure tools

A novel technique to characterize variations of the spatial (partial) coherence (sigma) across the image field in modern steppers and scanners has been developed and experimentally tested. It is based on the high sensitivity of the length L of macroscopically large diamond-shaped marks printed in photoresist to (sigma) variation. Variations in the (sigma) value across the image field lead to variations in the length of marks printed at different image field locations. The mark lengths are measured rapidly with high accuracy by a built-in optical system and then converted into (sigma) values using the calibration dependence L((sigma) ) measured in the same exposure tool. Simulation and experimental studies show that the level of projection lens aberrations in modern Nikon tools have practically no effect on (sigma) measurements obtained with this technique. Our results demonstrate that in the conventional illumination scheme, (sigma) distribution can be measured with an accuracy of 2.5%. The main advantage of the presented method is that (sigma) variation over the image field is characterized by the exposure tool itself, avoiding expensive and time-consuming SEM measurements. Moreover, since the measurement procedure is based on the wedge-shaped marks and laser scanning system currently used in Nikon tools for automated focus detection, implementation of the technique does not require any hardware or software modification.

Impact of Zernike cross-term on linewidth control

Recent introduction of phase measurement interferometer (PMI), to measure wavefront aberrations brought about rapid reduction of residual aberrations in stepper and scanner projection lenses. Zernike sensitivity method (ZSM) is useful to capture and to understand the lens aberrations impact on the imaging performances, and to guide the improvements in patterning performance of the projection tools. In this paper we present ZSM for CD-Focus curve capable of precisely predicting CD at any focus position. We found that cross-term interactions of several Zernike combinations impact ZSM for CD-Focus. We present an example of V-H difference for which the cross-term interactions dominate. Aerial image simulation results presented here are illustrated by corresponding to experimental results.

New projection optical system for beyond 150-nm patterning with KrF and ArF sources

Two types of new optical system for 150 nm lithography are studied. One is the system with KrF source and high numerical aperture (NA), the other is the system with ArF source. By aerial image simulation, the adequate NA of each projection lens is searched, and the value was 0.68 for KrF source and 0.60 for ArF source. Then the projection lens is fabricated and evaluated. The results are almost same as those of simulation.

Innovative image formation: coherency controlled imaging

Two types of innovative imaging systems are studied. One is the system with non-incoherent effective source and the other is the system with non-coherent pupil function. When the optics are configured in such a way that a certain pattern would be illuminated by a group of waves diffracted by the pattern itself or an exactly the same pattern, the effective source can be made no longer incoherent. Then the coherence of the effective source can be a new parameter to improve image quality. The simulation shows that the contrast of L/S pattern imaging under the off-axis illumination, for instance, can be boosted up to 99.5% with this system, while to only 90.6% with conventional optics. A new pupil filter is proposed; the area of which is divided into some annular zones to be made mutually incoherent. Then the phase difference becomes less crucial and large DOF can be achieved. The simulation shows that this filter is effective for the exposure of contact hole patterns.

Aberration optimizing system using Zernike sensitivity method

We introduce a projection lens adjustment procedure that is customer application oriented. This technique is based on the simulated imaging performance using Zernike sensitivity, the measurement results of wavefront aberration and wavefront change by lens element position change. This system finds the optimum combination of lens position where the amount of specific imaging performance error is in tolerance. In this paper, the idea of optimization and some optimization results are shown.