Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
2002-09-03
2004-06-01
Siek, Vuthe (Department: 2825)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
C257S207000, C716S030000, C716S030000
Reexamination Certificate
active
06745380
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to producing fine structures on substrates in lithographic methods for producing structures of microelectronic components by using projection exposure. The invention also relates to a computer program for executing a method for optimizing and a method for producing a layout for a mask.
The wavelengths used in projection exposure are nowadays greater than the smallest dimensions of the structures to be produced on the substrate. Therefore, diffraction effects play a major part in imaging the structures. Diffraction effects flatten the intensity gradient at the edge of a structure because of light being scattered during the exposure of closely adjacent structures.
The unsharpness (i.e. fuzziness) of edges that are produced thereby and the resulting size distortion depend in principle on the relative proximity of the structures on the substrate, the optical wavelength, and the numerical aperture of the exposure device. Thus, given the same exposure (that is to say the same wavelength and numerical aperture), two structures that actually have the same dimensions exhibit different dimensions following exposure, depending on how closely these structures lie to other parts of the exposed layout. The phase and coherence of the light also has an influence. This change in the size relationships as a function of the relative proximity of the structures is called the “proximity effect”.
In order to reduce the proximity effect, it is known to carry out an optical proximity correction (OPC) of the structure before creating the mask. An OPC is conducted to ensure that the finished structures on the substrate actually have the sizes envisaged in the layout. The sizes are not obvious because, during the production of a microelectronic component, size changes may occur (for example as a result of the proximity effect mentioned). These are compensated by the OPC.
In that case, the OPC can be conducted by using a simulation program (simulation-based OPC) or a rule-based software system (rule-based OPC).
In order to improve the sharpness of the image in lithography, U.S. Pat. No. 5,242,770 issued to Chen et al. additionally discloses the practice of applying thin lines between the structures on the mask. These lines are so thin that they are not themselves imaged on the substrate by exposure. These scatter bars, as they are known, influence the intensity gradients at the edges of the structures, so that differences between closely packed parts of the structure and further spaced parts of the structures are compensated for. In this case, the scatter bars are disposed at predeterminable intervals parallel to parts of the structure. When placing the scatter bars, an intended ideal distance from the main structures is maintained when compatible with the layout.
The size reduction in semiconductor production has in the meantime progressed so far that structure sizes are smaller than the wavelength of the light used in the lithographic step. The smaller the structure sizes, the more important it is to modify the layouts by using OPC before mask production to account for diffraction effects during the optical imaging.
When the two methods are combined (OPC and scatter bars), however, the following difficulty arises because of their mutual effect: simulation-based OPC has to be carried out while accounting for the lithographic influence of the scatter bars, for which reason the latter has to be generated in the original layout before the correction. Because the dimensions of the main structures are changed by the OPC, deviations in the distances between main structures and scatter bars from the desired ideal distance occur simultaneously. If the scatter bars move too closely to the main structures, then their optical influence becomes so high that the exposure and fabrication tolerances lead to uncontrollable results; on the other hand, if they move too far away, then their positive influence on the process window is lost.
These problems have hitherto been solved by the distance of the scatter bars from the main structures in the original layout (before the correction) having been chosen to be so large that, with the maximum permissible broadening of the main structures as a result of OPC, a necessary minimum distance of the scatter bars from the corrected main structures was guaranteed. As a result, an undesirably large distance from the main structures was produced for the majority of these scatter bars. This forfeits some of the positive influence on the process window.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for optimizing and a method for producing a layout for a mask, preferably for use in semiconductor production, and a computer program therefor, that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that reduce the problems in the prior art.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of optimizing a layout for a mask for use in semiconductor production. The layout has main structures and scatter bars. The method includes the following steps. The first step is (a) defining one or more of the main structures. The next step is (b) defining one or more of the scatter bars in accordance with predetermined rules. The next step is (c) carrying out an OPC on the main structures while accounting for the scatter bars. The next step is (d) comparing the corrected main structures with the main structures before step (c). The next step is (e) repeating steps (b) to (e) based on the corrected main structures, depending on the comparison in step (d). In the following text, the above-described method will be designated “Iteration A”.
The scatter bars can consequently be iteratively matched optimally to the main structures corrected by using OPC by using the method of the invention.
In particular, the comparison in step (d) can be a comparison of the layers of the main structures, and the definition of the scatter bars in step (b) can be carried out on the basis of the position of the main structures and, when step (b) is repeated, on the basis of a mean value along the edges of the main structures before and after the preceding OPC. As a result, it is possible to avoid, for example, a situation where the scatter bars running along the edges of the corrected main structures are divided into excessively small portions. For instance, in the case of only slight fluctuations in the width of a corrected structure, it is expedient to calculate a mean value relative to which the position of a scatter bar can then be defined.
In general, the iteration should be terminated when the maximum or mean distance of the main structures between two successive iteration steps lies below a predefinable threshold value.
In one refinement of the invention, steps (b) to (e) are repeated if the largest change in a parameter as a result of OPC lies above a predetermined threshold value. By suitably defining the threshold value, the level of optimization of the method to the requirements in semiconductor production can be adapted individually. In this case, the parameter is advantageously a distance between two main structures. An alternative or additional termination criterion can be defined by the OPC grid, that is to say the iteration is terminated as soon as the variation between two successive iteration steps lies below the resolution of the OPC grid.
In another refinement, steps (b) to (e) are repeated only when the main structures do not remain unchanged by the OPC. In this refinement, a higher number of iterations is generally necessary, but this results in a higher level of optimization.
Depending on the refinement, when step (b) is repeated, the position and/or the dimensions of one or more of the scatter bars may be changed. Likewise, during the OPC, the position and/or the dimensions of one or more of the main structures may be changed.
In summary, the iteratively generated scatter bars
Bodendorf Christof Tilmann
Thiele Jörg
Do Thuan
Greenberg Laurence A.
Infineon - Technologies AG
Mayback Gregory L.
Siek Vuthe
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