Model-based RET using interference maps, algorithms for random contacts at 65nm
09/01/2004
Wafer-exposure imaging conditions and interference maps are employed by a novel model-based approach for optimum placement of resolution enhancement features on subwavelength photomasks. The technology, called IML, applies multiple methods for deriving interference maps, which are used to define reticle patterns for various resolution enhancement techniques (RET). As part of photomask software, IML can generate reticle designs for random contact patterns typical of logic designs expected at the next-generation 65nm process node.
Imaging of contact and via layers continues to be one of the major challenges facing 65nm node lithography. Initial trials using ASML's CPL technology for printing contact arrays through pitch indicate its potential to extend contact imaging to a k1 near 0.30 [1]. CPL also has several favorable inherent characteristics: It uses symmetrical phase edges and does not require solving the phase assignment problem (which can lead to irresolvable phase conflicts); it is a single-reticle, single-exposure technique; and it can be applied to multiple layers within a device, both clearfield and darkfield.
The bottom line: CPL is an attractive, cost-effective solution to low-k1 imaging. However, real semiconductor circuit designs are much more complex than regular arrays of contact holes. A method for defining the CPL reticle design for full-chip circuit patterns is required for this technique to be feasible in volume manufacturing.
To address this challenge, IML technology has been developed as a model-based technique for defining optimum reticle patterns based on the imaging conditions used during wafer exposure. The technique provides a model-based approach for placing all types of resolution enhancement features, including scattering bars, antiscattering bars, nonprinting assist features, and phase-shifted and nonphase-shifted elements on photomasks. Figure 1 shows an IML interference map for an isolated contact, based on 193nm scanner simulations using an ASML AT:1150 tool with optics settings of 0.75NA and ASML's Quasar illumination option. This interference map can be applied to any reticle type, including binary (COG), attenuated PSM, alternating PSM, and CPL.
In this work, IML is used to generate CPL reticle designs for random contact patterns typical of future logic devices at the 65nm node. We examine the critical issues related to using CPL with IML technology, including controlling sidelobe printing, contact patterns with odd symmetry, and reticle manufacturing constraints. CPL reticle designs were also created with automated algorithms for contact pattern decomposition using MaskTools' MaskWeaver RET software.
Shrinking contact holes with CPL
The use of off-axis illumination (OAI) with CPL technology has been used successfully to improve the resolution for dense pitch features, including contact arrays [2]. Past wafer results using a CPL test reticle have shown the capability to get a through-pitch solution for 100nm contact holes with an overlapping process window of 0.30µm depth-of-focus (DOF) at 8% exposure latitude [3].
We have also demonstrated a process window of 0.26µm DOF at 8% exposure latitude at a k1 (half-pitch) of 0.300 by printing 80nm contact holes at 155nm pitch using an ASML PAS/1100 system. Using a high-NA 193nm lithography system, the ASML AT:1250, it was possible to image 65nm contacts at a pitch of 127nm, giving a k1 (half-pitch) of 0.280.
To image randomly placed contacts at all the pitches required for 65nm-node devices, a specific method for determining the reticle pattern is required. Historically, reticle designs were created by applying a set of rules to define the assist features through pitch, whether for binary or phase-shifting masks. To satisfactorily extend the printing resolution for randomly placed contact designs, it is necessary to go beyond the current antiscattering bar and assist feature methods currently employed in OPC software.
For this work, the objective was to explore a method of creating CPL reticle designs for full-chip production type patterns, which contain effectively, randomly placed contacts over a wide range of pitches — from dense to isolated features. To do this, it is necessary to better understand how all the printing and nonprinting features on a reticle interact to produce a desired target pattern. Also needed is the ability to apply this behavior to define a reticle pattern for any random placements of contacts by using a modeling algorithm, based on IML, in the RET software.
Interference mapping
In IML technology, a full-pitch range of deep-subwavelength reticle patterns can be defined based on any illumination condition, including highly coherent on-axis (e.g. partial coherence <0.4) and strong off-axis illuminations (e.g., Quasar, double-dipole, and single-dipole illumination). Reticle features not printing on the wafer (subresolution and/or nonprinting features) can be added to the target pattern that will enhance the aerial image of the intended target pattern, resulting in higher-resolution imaging and larger process latitudes.
This method can be used for binary (electric field amplitudes of 0 and +1), attenuated PSM (electric field amplitudes of 0, +1, and -0.06), and phase-shift reticle patterns (electric field amplitudes of 0, +1, and -1). To demonstrate this approach, IML was used to define a reticle pattern for a darkfield CPL reticle that contained contact array patterns through pitch. However, this same method can be equally well applied to design a brightfield reticle for printing a full-pitch range of dark features.
The concept behind IML involves mapping the interaction between each point in the field with all the target geometry that is within the optical range of influence around that point [4]. The background (the field area) is divided up into an array of points or pixels. At each point in the field, it is determined what effect each possible reticle phase and transmission would have on the imaging of the six contacts. For a CPL reticle, the three possible reticle transmissions are 100% nonphase-shifted light, 100% phase-shifted light, and 0% transmission chrome (+1, -1, or 0).
Any contact that is near a point being analyzed will be strongly affected by the reticle transmission at that point and contacts farther away will be affected to a lesser degree. The interference value for a specific reticle transmission at a point in the field is the sum of all the interactions between that point and the target contact patterns. This interference value is calculated for each of the three possible reticle transmissions and the strongest positive effect value is assigned to that point. Repeating this analysis for every point in the field area derives the interference map for that arrangement of contacts.
Application of IML
Several factors will influence the interference map, including the particular arrangement of contacts, contact pitch, contact size and shape, exposure wavelength, NA, illumination condition, and characteristics of the imaging lens. For every point within the optical range of the contact, this mapping indicates whether 100% nonphase-shifted light, 100% phase-shifted light, or 0% transmission will have the greatest positive impact on imaging the isolated contact and what the magnitude of that effect will be.
 Figure 1. An interference map generated for an isolated contact imaged on an ASML AT:1150 ArF scanner using 0.75NA and Quasar illumination option. |
Assuming that the contact opening is defined as zero phase, the yellow
ed areas on the interference map, as shown in Fig. 1, are locations where zero phase light (in phase with the contact) will have the strongest positive effect on contact imaging. The blue areas are where π phase light (out of phase with the contact) will have the strongest positive effect on contact imaging, and black areas are where neither zero phase light nor π phase light will have a positive effect on the contact imaging.
 Figure 2. Possible MaskWeaver reticle designs based on the interference map shown. |
Using the interference map, it is now possible to define a CPL reticle pattern that places zero phase, π phase, and chrome at the locations where imaging of the desired isolated contact will be enhanced. Figure 2 shows examples of the reticle patterns that can be derived for various types of photomasks (CPL, att-PSM, etc.) based on the interference map. The amount of area surrounding the target contact used to create the patterned area will determine the extent to which imaging is improved. Obviously, the area closest to the contact will have the greatest impact, with the magnitude of the effect decreasing as the distance increases.
These designs do not always consist of what traditionally have been subresolution assist features. IML enables designs to go beyond conventional concepts of scattering bars and antiscattering bars to novel concepts employing features on reticles that are actually not subresolution. In this example, the "assisting features" can be larger than the target contact, but they are nonprinting. These nonprinting assist features (NPFs) can be either phase-shifted or nonphase-shifted, and that, while large enough to print, are defined to take advantage of destructive interference to make them nonprintable. In this way, sidelobe printing is suppressed, allowing for a large margin of over-exposure before any sidelobes begin to print. The interference map provides a method of identifying the desired placement locations and the shapes for traditional scattering bars and antiscatter bars, as well as nonprinting assist features, in a mask design.
Generating CPL reticle designs
A reticle pattern defined by IML technology uses algorithms in MaskWeaver software, which first generates an interference map and then applies a threshold to that map (to separate zero phase areas from π phase areas). The software then converts the resulting contours into reticle geometries, which are restricted to manufacturable shapes based on reticle manufacturing constraints. The table shows reticle requirements and constraints.
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For any algorithm employing IML, the most significant challenges to automatically generating a CPL reticle design are:
- defining the contour threshold at different values depending on the proximity to other geometry;
- converting irregular continuous contour structures into shapes that can be manufactured by the maskwriting tools; and
- developing a model-based OPC method that is capable of applying the proper edge corrections (i.e., biases) to an IML pattern consisting of chrome, zero phase, and π phase geometries.
Using a model-based algorithm that satisfied these requirements, patterns were automatically generated using the RET software. The patterns were compared to the interference map to verify the proper placement of zero and assist features, and also compared to the corresponding reticle patterns that were manually drawn based on the interference map.
 Figure 3. Determination of antiscattering bar placements using IML Technology for both isolated and semi-isolated contacts. |
Next, the algorithm was applied to a random contact pattern to test the ability to place zero and π phase antiscattering bars in areas where the interference map had different thresholds. The example in Fig. 3 shows how the threshold for the desired contour (which is then converted to the reticle geometry) will be different in the area surrounding an isolated contact, compared to the area surrounding the contacts that have nearby neighbors.
 Figure 4. Example of a random contact pattern containing isolated, dense, semi-dense, and random contacts, treated with an IML algorithm within MaskWeaver. |
Figure 4 shows an example of a final CPL reticle design where zero phase and π phase assist features have been placed corresponding to an interference map. Because IML is based on deriving the interference pattern that results from the target pattern and the illumination conditions used to expose that target pattern, there is no phase assignment or coloring required for this technique. Zero phase and π phase reticle features are explicitly defined by the interference map and, unlike alternating phase-shifting masks, these phase assignments cannot be arbitrarily switched. As a result, phase conflicts cannot exist and patterns with odd symmetry present no difficulty, as shown in Fig. 4. Because IML is based on modeling the interference pattern, it is able to properly place assist features through pitch even if the pitch varies continuously and can range from very dense through the forbidden pitch to semi-isolated to completely isolated contacts.
Conclusion
Model-based IML technology can be applied to the placement of resolution assist features in a range of reticle types — binary (COG), att-PSM, high-transmission att-PSM, and CPL photomasks. This technique was demonstrated by creating a CPL reticle pattern for a random contact design that represented a typical 65nm-node logic device. Using the interference map, it was shown that zero phase and π phase assist features could be placed, and this placement could be determined for dense, semi-dense, isolated, and randomly placed contacts.
The IML algorithm in MaskWeaver was able to extract the proximity-dependent contour necessary to overcome the difference in the interference map threshold values between isolated, semi-dense, and dense contacts. The complex mapping contours were then converted to relatively simple CPL mask patterns required for manufacturing. This demonstrated that contact hole mask patterns for random contact designs can be optimized with a well defined algorithm and that full-chip automation is possible using this RET software.
References
- D. Van Den Broeke, et al., "Near 0.3 k1 Full Pitch Range Contact Hole Patterning Using CPL," Proceedings of BACUS 2003, SPIE, Vol. 5256, pp. 297–308.
- D. Van Den Broeke, et al., "Complex 2D Pattern Lithography at Lambda/4 Resolution Using Chromeless Phase Lithography (CPL)," Optical Microlithography XV, Proceedings of SPIE, Vol. 4691, pp.196–214, 2002.
- V. Wiaux, et al., "Through Pitch Low-k1 Contact Hole Imaging with CPL Technology," Photomask and Next-Generation Lithography Mask Technology XI, Proceedings of SPIE, Vol. 5446-109, to be published, 2004.
- R. Socha, et al., "Contact Hole Reticle Optimization By Using Interference Mapping," Optical Microlithog. XVII, Proc. of SPIE, Vol. 5377-20, to be pub., 2004.
Contact Douglas J. Van Den Broeke at ASML MaskTools, ph 408/855-0502; fax 408/855-5040, e-mail [email protected].