The Large Optical Test and Integration Site (LOTIS) at the Lockheed Martin Space Systems Company in Sunnyvale,
CA is designed for the verification and testing of optical systems. The facility consists of a large, temperature
stabilized vacuum chamber that also functions as a class 10k cleanroom. Within this chamber and atop an advanced
vibration-isolation bench are the 6.5 meter diameter LOTIS Collimator and Scene Generator, LOTIS alignment and
support equipment. The optical payloads are also placed on the vibration bench in the chamber for testing. The Scene
Generator is attached to the Collimator forming the Scene Projection System (SPS) and this system is designed to
operate in both air and vacuum, providing test imagery in an adaptable suite of visible/near infrared (VNIR) and
midwave infrared (MWIR) point sources, and combined bandwidth visible-through-MWIR point sources, for testing
of large aperture optical payloads. The heart of the SPS is the LOTIS Collimator, a 6.5m f/15 telescope, which projects
scenes with wavefront errors <85 nm rms out to a ±0.75 mrad field of view (FOV). Using field lenses, performance
can be extended to a maximum field of view of ±3.2 mrad. The LOTIS Collimator incorporates an extensive integrated
wavefront sensing and control system to verify the performance of the system, and to optimize its actively controlled
primary mirror surface and overall alignment. Using these optical test assets allows both integrated component and
system level optical testing of electro-optical (EO) devices by providing realistic scene content. LOTIS is scheduled to
achieve initial operational capability in 2008.
The effects upon imaging due to varying the spatial coherence of the illumination in an optical system are studied. A rotating diffuser is located directly behind the object in an optical system and is trans-illuminated with spatially coherent monochromatic light. The statistical properties of the diffuser surface determine the scattering cone angle and the partial coherence effects in the image. A model is presented that can be used to determine the diffuser properties required to yield incoherent imaging. Two metrics are used to determine if an image is incoherent: the apparent transfer function and image contrast.
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.
Aerial image measurements, SEM measurements from printed photoresist images, and simulations are used 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. Challenging phase-shifting mask designs have been developed in collaboration with DuPont Photomask, Inc., Photronics Inc., ASMLithography, and Advanced MicroDevices for use as precision instruments to measure aberrations, system illumination, and the quality of the photomask itself. The results presented herein on pattern and probe-based aberration monitors show that, while the imaging of the probe portion of the target is dominated by errors in mask geometry and electromagnetic edge effects, high-NA effects play very little role in their image formation. The results also show that the full target suffers from mask edge electromagnetic effects and high-NA vector effects, implying that the ring patterns in the target emphasize those locations in the lens which are most susceptible to high-angle effects.
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.
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.
Printed resist images of pattern and probe-based aberration monitors at 248 nm wavelength on an AIMS tool and a projection printer at several NA's 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.
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.
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.
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.
The Lithography Analysis using Virtual Access (LAVA) web site at http://cuervo.eecs.berkeley.edu/Volcano/ has been enhanced with new optical and deposition applets, graphical infrastructure and linkage to parallel execution on networks of workstations. More than ten new graphical user interface applets have been designed to support education, illustrate novel concepts from research, and explore usage of parallel machines. These applets have been improved through feedback and classroom use. Over the last year LAVA provided industry and other academic communities 1,300 session and 700 rigorous simulations per month among the SPLAT, SAMPLE2D, SAMPLE3D, TEMPEST, STORM, and BEBS simulators.
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.