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Weir Dose Uniformity Mapping and the ASML DoseMapper1
See Also: Weir PW Brochure
Applications: Process comparison, Process window setup, OPC or Phase Shift feature design evaluation, Process Control tuning , yield enhancement, reticle process window derivation, reticle qualification, OPC characterization, dose mapping, improved process window selection.
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Weir PW can be used to model and calculate the uniformity of Dose, Focus, Feature Profiles, Films, Depth-of-Focus, Exposure Latitude and other process variables across the wafer and exposure field. However the mention of Dose Uniformity Mapping sometimes leads the Scanner Engineer to ask just how these functions are similar to the DoseMapper product offered by ASML2.
This report will discuss the similarities and differences of the two products. The discussion will also review the ways in which ASML DoseMapper users can benefit from Weir PW.
Keywords: Dose Response, algorithm, Batch, automation, trend chart, html, OPC, Optical Proximity Correction, Phase, FEM, Focus-Exposure Matrix, duty cycle, period, IsoFocal Dose, IsoFocal Bias, IsoFocal Feature Size, Isolated-Dense, IsoDense, bias, Bossung, Best Focus, Best Dose, IFD, DoF, Depth-of-Focus, process characterization, process response, Process Window, Reticle design, process control
DoseMapper is an advanced exposure-dose control system that is part of ASML's Ultra-k1 TM control portfolio. The software itself resides on a Unix based server called the LithoServer TM . The DoseMapper provides a real-time graphic front end, or GUI, that allows the user to adjust dose-map configurations and exposure sensitivity to explore the benefits to Critical Dimension (CD) uniformity across the exposure field and wafer. However the fundamental application of the product is in Advanced Process Control (APC) implementations for real-time control of production.
Weir PW's Dose Uniformity Mapping functions as part of a broader scope of analysis tools. Although each Engineer's implementation of these functions can be incorporated into an automated Weir Macro for both focus and dose production mapping, Weir PW cannot directly control exposure sequences on any scanner or stepper.
Weir PW is an analysis workbench for the Engineer. It is capable of bringing in any form of log, metrology and reticle data to characterize, match and calculate corrections and setup for the process. As part of it's functionality Weir PW can calculate base precision for process. metrology and exposure tool sub-assemblies searching for the greatest contributions to systematic process perturbations. The product provides full-profile as well as full-field analysis of process windows, and spatial distributions.
Weir PW puts much more control in the hands of the user through the tools of data culling, multiple models and mouse interactive graphics and data than DoseMapper. Because of this, when the Weir PW algorithms are automated through Weir DMA, it can be used as an improved monitor and dose-model builder for the DoseMapper application.
Weir PW cannot replace the DoseMapper but it can help you to improve control and production monitoring. You don't need to have the ASML DoseMapper to use these Weir PW functions. The DoseMapper is not a fix for many tuning problems on your exposure tool/ process interface that arise from daily production. However with Weir PW you can directly measure the improvements obtained from DoseMapper and anticipate and correct process and tool drift components that would otherwise cause the DoseMapper corrections to drift out of control.
Dose Sensitivity Enhancement
The success of the DoseMapper is dependent upon the derivation of a valid Dose Response sensitivity curve for the photoresist. Exposure dose response to a photoresist is masked by many process factors. Subtle changes in the manufacture of the reticle feature convolute with localized lens and slit elements to perturb the simple feature response to dose curve significantly from site-to-site. Determining feature response for a full reticle must at least remove the localized defocus errors of a lens wavefront. Besides varying from lens to lens, the wavefront can vary it's signature with scan direction, time and temperature.
Weir PW can remove these perturbations to obtain an improve dose sensitivity characteristic as needed for the DoseMapper's conversion of CD size to localized dose. The advanced, multi-level methods of "poor" metrology data culling embedded in Weir PW can also have a strong impact on the proper derivation of the process's sensitivity to dose.
Improved Slit and Scan Mapping
DoseMapper can implement a parabolic gray filter correction to across-slit symmetric variations in the exposure field dose control. An analysis with Weir PW can quickly show that these variations are most often not symmetric nor are they centered in the slit. If the deviation is small then the DoseMapper will work correctly. However production systems are never rock-solid and the slit response can change with scan direction, reticle bowing, reticle mount degradation, illumination source aging and many other factors. Lens contamination from photoresist gassing occurs slowly but surely in production and can only be monitored with Weir PW and Weir DM.
Scan corrections in the DoseMapper can be corrected using a 4th order polynomial. The DoseMapper ignores the higher order variations than can result from slit-scan-speed variations at the end and start of scan as well as bearing and scan stage degradation that is incipient with hard use.
Improved Wafer Mapping of Dose
Substrate corrections of dose variation are modeled in DoseMapper using a fixed element radial symmetric model and some data culling that the user has no control over. Weir PW models can be tuned by the user to cull elements automatically or interactively with the mouse and include various terms using the Weir PW Adaptive Modeling automation or manually.
DoseMapper can derive and partially compensate for the stable systematic dose variations seen in production arising from full-wafer radial variations and slit and scan variation across the exposure field. With the use of multiple levels of APC feedback loops between the track, stepper and offline metrology, these corrections can be updated.
The DoseMapper cannot be tuned to remove the greatest contributors to dose variation as embodied in the reticle itself. These errors derive from either manufacturing or design response of Reticle Enhanced Techniques (RET) that include phase shift and Optical Proximity correction structures to the unique signature of the individual exposure tool.
DoseMapper cannot correct or monitor basic process and biases such as feature orientation (H-V) Bias, proximity, phase, polarization, IsoDense, IsoFocal, Etch and Develop loading effects.
Weir PW will derive the Dose Response with greater accuracy through improved data culling and the removal of errors introduced by spatial and lens localized aberration. Weir PW can monitor the higher order effects of bias, proximity, IsoFocal bias etc and trend their influence on the process segment
Regular calling of a Weir DMA macro by an APC tool can automate this extensive monitor and sensitivity calculation to improve the overall performance of your exposure tool whether it is using the DoseMapper or any other control system.
TEA Systems does not support nor is it supported by ASML. We welcome commentary or criticism of these analyses and can provide significant proof through publications of the successes involving the Weir PW method.
TEA Systems offers products to model films, photomasks, wafers, feature profiles, process and lens data for characterization and setup of semiconductor design, simulators, tools and the process.
TEA Systems, a privately held corporation since 1988, specializes in advanced, intelligent modeling of the semiconductor process and toolset. Products from TEA allow the user to decouple process, tool and random perturbations for enhanced process setup & control.
TEA Systems products include:
Weir PSFM: Full-wafer/field/scan analysis tool for FOCUS derived from proprietary defocus sensitive features.
Weir PW: Reticle/Full-wafer/field/scan analysis for any metrology with advanced process window capabilities for both wafer and photomask control.
LithoWorks PEB: A tool to link and correlate profile, film and critical element control to thermal reactions such as PEB and ChillPlate
Weir DMA: Macro Automation interface for Weir PSFM and Weir PW for external program calling, automated data gathering or one-button analysis of commonly used sequences. Includes data trending and web interface.
2. H. van der Laan et. al, "Etch, reticle and track CD fingerprint corrections with local dose compensation", SPIE (2005) Vol 5755-13.
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TEA Systems does not endorse or recommend ASML products.
This publication should not be taken as an endorsement by ASML of TEA Systems or any Weir component.
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