Garth Robins

Measuring optical image aberrations with pattern and probe based targets

A theoretical foundation is given for the ability of programmed-probe based aberration targets to measure individual Zernike terms. The optimum targets are inverse Fourier transforms of the Zernikes and this allows the main features of the family of targets to be predicted in advance. Simulation of discretized versions shows an impressive 27%–36% increase, per 0.01 λ of rms aberration, in the intensity at the center of the target relative to the clear field intensity. The cross contamination by other targets is about 1/6 as large and it is thus possible to measure spherical aberration independent of focus.

Validation of the aberration pattern-matching OPC strategy

This paper validates the pattern matching methodology for locating and quantifying worst-case aberration-distortion of patterns through the comparison of theoretically predicted and simulated images. The matching process identifies those pattern element that will be most affected by the aberrations specific to the given lens. Once the highly impacted layout structures are identified, the region is extracted and simulated with and without aberrations using SPLAT to observe the induced pattern distortions. It is shown that even for good quality lenses, the resulting line- edge and line-end perturbations of PSM layouts for residual amounts of aberrations can exceed half those of optical proximity effects and may be quantified as additional input into the OPC process. The resulting feature edge shift is large and linear with aberration level for odd aberrations and much smaller for even aberrations whose electric fields add in quadrature. The effects of aberrations on binary marks are about half as large as the effects on phase-shift and phase-edge masks. The goal of the system is to allow measurements of aberrations across the field and among tools to be utilized int eh design process to inform designers of problematic features and to apply appropriate compensation on the mask.

Are pattern and probe aberration monitors ready for prime time

The first experimental results for interferometric pattern and probe-based aberration monitors designed for use at 193nm wavelength have been obtained using the Zeiss Aerial Image Measurement System (AIMSFab 193TM). Designs developed earlier are being tested on phase-shifting masks in collaboration with Photronics Inc. for use as precision instruments to measure aberrations. Comparison of the results with SEM measurements of the mask and simulations help to characterize second-order effects due to mask topography, high-NA electric-field vector addition, and mask fabrication tolerances in projection printing of advanced process monitors on special phase-shifting test reticles. For this study the aberration targets have been factored into their basic elements, such as probes, rings, lines, and rings surrounding probes. Through-focus studies of well-formed 120nm probes showed peak intensities for actual mask dimensions that were below ideal mask values by a factor of 0.70, 0.49, 0.26, and 0.29 for 0°, 90°, 180°, and 270°, respectively. Measurements for lines and outer rings were consistent with probes and showed intensities of 0.86 and 0.61 of those expected for ideal 0° and 180° 125nm lines in wafer dimensions. The focus sensitivity of the composite mask was clearly larger than that of typical features. However, to leverage the full sensitivity from interference with the probe, the probe must be resized as a function of its phase depth due to electromagnetic effects and the probes must be protected by the use of larger 2D feature biases. Operation at a partial coherence factor of 0.15 or below is recommended to preserve the contribution of the second ring and balance out unwanted proximity effects.

Interferometric-probe aberration monitor performance in the production environment

The performance of pattern and probe-based aberration monitors in the production environment, designed to measure individual Zernike aberration terms in 248nm wavelength high-NA (0.80) exposure tools is investigated via printed resist images. The results demonstrate the measurement operation of these monitors compared to their performance as designed through simulation, tightening the measurement accuracy of the focus monitor to 17nm or better than 1/10 of the Rayleigh depth of focus. The data shows a characteristic 50nm variation in focus across the field of the exposure tool. A comprehensive electric-field vector addition model of target operation is presented and shows how the center of the defocus target suffers from a lack of orthogonality to the normal proximity effect spillover. The target designed to detect coma aberration was investigated in-depth, but it continues to print in an unexpected manner, likely due to the electromagnetic performance of the mask and high-NA vector imaging effects. Finally, the target designed to measure spherical aberration was examined, but no noticeable spherical aberration signature/response was detected.

Experimental assessment of pattern and-probe aberration monitors

First experimental evidence of the high sensitivity of interferometric-probe based aberration targets on phase-shifting masks is presented. Measurements were made on an AIMS tool modified for NA = 0.2 with 150 μm imaging and 300 μm illumination pinholes to match an inadvertent 4× oversizing of the layout dimensions. Calibration of the actual NA (= 0.18) was accomplished through known phase-edge distances and comparison of images of isolated probes and large features with aerial image simulation. Even though only two-ring versions of the targets were measured the peak of the 90 deg. central probe in the defocus target increased linearly with focus at a rate of 47% of the clear field per Rayleigh unit (RU) of defocus when measured over a ±1/2 RU interval about best focus. The focal position can be measured to within 1/40 RU and the prediction of best focus on an absolute basis agrees with that determined by the Strehl ratio to within 1/35 of a Rayleigh focal length. The two-ring spherical and higher-order spherical targets showed decent orthogonality to focus with changes in their central peak intensities of only 0.47 and 0.37 of that of the defocus target even when viewed at an NA 10% smaller than their design.

Illumination, mask, and tool effects on pattern and probe-based aberration monitors

The practicality and manufacturability of pattern and probe-based aberration monitors for characterizing optical lithography tools in light of tool and mask performance issues is investigated via simulation. The effects of the partial coherence of the illumination, the use of off-axis illumination, alignment optics obscuration, mask pixel size, and intensity imbalance effects on the ability of the pattern and probe aberration monitors to quantify residual aberrations at levels approaching 0.01(lambda) rms is assessed. Targets from nine rectangles to over 100 rectangles, in sizes up to seven rings were simulated with SPLAT under various illumination conditions. Results show that the targets respond best for nearly coherent illumination ((sigma) equals 0.1 to 0.2). Phase-compensation for reversal in sign of the mutual coherence function is shown to be feasible and will be essential for off-axis illumination. While the intensity imbalance for phase shifting masks can be significant, the affect on the aberration measurement is relatively small.

Operational model for pattern and probe based aberration monitors

A one-dimensional treatment of pattern and probe based aberration monitors is discussed, yielding significant insight into the characteristics of optimum targets for measuring Zernike aberrations. One key new result is that each target possesses a “double-width” thick ring, resulting from the stationary phase condition between the aberration and the wave front incident upon the pupil. The characteristic radius of the ring is given by R≈(0.186×n+y0)λ/NA. The use of simple on/off mask openings to synthesize targets proves to be beneficial, increasing sensitivity by a factor of 4/π, but it is also shown to increase cross contamination between aberrations on the order of 10%. A full algebraic formulation of the principle of operation including amplitude and phase effects is presented, demonstrating that effects of target phase variation outweigh the effect of amplitude variation by a factor of 2π/λ.

Interferometric-probe aberration monitors: aerial image and in-resist performance

Printed resist images of pattern and probe-based aberration monitors at 248 nm wavelength on an AIMS tool and a projection printer at several NAs will be presented. The results will demonstrate the measurement operation of these monitors compared to their performance as designed through simulation. Initial experiments indicate that the focus monitor has sufficient sensitivity to see systematic focus trends across the die. The focus monitor is estimated to measure focus to 25nm accuracy of 1/8 of the Rayleigh depth of focus, indicating a 2-8x improvement over determination of best focus via the point spread function. This work also shows that the optimum conditions for reading the targets is when the intensity of the probe is just at the exposure threshold of the resist. The target designed to detect coma abberation did not work as expected and this is likely due to the electromagnetic performance of the mask and high-NA vector imaging effects.

Simulation of image quality issues at low k1 for 100-nm lithography

Mask quality issues in pushing lithography to features below 0.5(lambda) /NA are identified and quantified through simulation of mask interactions and images. Guidelines summarize the results from detailed studies of aberrations, phase-shift mask image imbalance, 3D phase defects and EUV buried defects. Programmed-probe based aberration targets are extended to distinguish both even and odd lens aberrations and their mask tolerance requirements are assessed. Complex diffraction coefficients and results for cross-talk simulation are used to set guidelines for phase-shifting mask design. An antireflection coating (50 nm MoO3) is shown to reduce cross-talk between trenches. Type, location and size data are given for 3D phase-defects and the end regions of lines are shown to be more vulnerable to CD variation. Results for buried 3D Gaussian defects in EUV multilayers show a worst isolated defect size of half of the resolution and that 2nm high defects of any size can be tolerated.

Feature level test patterns for characterizing residual process effects

Layout test patterns are being pursued that are more sensitive than circuit patterns in detecting and quantifying residual processing effects. These patterns permit the rapid searching of layouts for the locations of worst-case process impacts, and may facilitate layout compensation at OPC speeds. These patterns have been taped-out along with snippets of circuits in preparation for experimental verification of the ability to link residual process effects to electronic design. The collection includes pattern-and-probe-based targets for measuring aberrations, illumination non-uniformity and etch-depth errors in phase-shifting masks, plasma etching with loading effects related to area and perimeter factors, and patterns for CMP orientation and feature proximity. The goal is to use these test patterns to develop maximum lateral impact functions for each individual process effect for use in fast-CAD techniques capable of inspecting large layouts.