Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-04-20
2004-02-17
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S022000, C430S030000
Reexamination Certificate
active
06692875
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a mask for optical projection systems and to a process for its production.
Optical projection systems are used, for example, in semiconductor fabrication for transferring image structures.
During the projection of image structures standing individually or standing partially individually on the mask, distortion of the image occurs, as compared with compact image structures that are distinguished by the fact that further structures are disposed beside an image structure. The distortion leads to a line width deviation in the separated structure, which can lead to the individually standing image structure not being capable of being projected at the same time as the compact image structure. Furthermore, the depth of focus of the individually standing image structure is very low.
One known solution to the problem consists in changing the mask, in which the line width on the mask is changed in such a way that the optical distortion of the projection is counteracted. This method is also referred to as optical proximity correction. The optical proximity correction has the disadvantage, however, that the layout correction of the mask is very complicated and in each case necessitates preliminary trials, which are included iteratively in the distortion correction of the mask. Furthermore, the method of proximity correction suffers from the drawback that the distortion correction of the masks can be carried out only with finite, incremental steps. However, the decisive drawback consists in the small process window that can be achieved for the individually standing or partially individually standing image structures. The process window is understood here to be a limited field in the two-dimensional space that is covered by the focal axis, that is to say a spatial position of the focal plane and the dose axis. The process window is limited in the direction of the focal axis by the depth of focus and in the direction of the dose axis by the exposure freedom.
One known photomask technique consists in the use of “embedded phase shifters”. These are masks that are specifically used to shift the phase of the light. One example of such a mask is given in U.S. Pat. No. 5,700,606. There, a phase shifting layer is applied to a semitransparent carrier material. An opaque layer is then disposed on the phase shifting layer. The “embedded phase shifter” technique leads to the imaging of undesired structures, which are produced by side bands (side lobes) being avoided.
A further known solution consists in the generation of what are known as sub-resolution structures, which are disposed in the vicinity of the individually standing or partially individually standing image structure. Sub-resolution structures are understood to be structures which, on account of their low geometric extent in at least one spatial dimension, are not transferred into a photosensitive layer. They are also referred to as lithographic dummy structures.
One drawback of the sub-resolution structure consists in the low structure size, which cannot be produced on the mask with the necessary accuracy and reproducibility. Furthermore, at present there are also unsolved problems for these structures in defect inspection and therefore also in defect repair.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a mask for optical projection systems, and a process for its production which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, with which a compact image structure can be transferred into a photolithographic recording medium at the same time as an isolated image structure, with the same exposure dose.
With the foregoing and other objects in view there is provided, in accordance with the invention, a mask for an optical projection system. The mask contains a transparent carrier material, an image structure disposed on the carrier material, and a dummy structure disposed on the carrier material. The dummy structure is spaced apart from the image structure and differs from the image structure in terms of transparency and phase rotation.
With respect to the process, the object set is achieved by a process for producing the mask for an optical projection system, with the steps of forming the image structure on the transparent carrier material, and forming the dummy structure on the carrier material. The dummy structure is spaced apart from all the image structures and differing from the image structure in terms of transparency and phase rotation.
The present invention provides for the dummy structure to be spaced apart from all the image structures and to differ from the image structure in terms of transparency and phase rotation. As a result, an enlarged process window for the projection of the image structures is achieved, which has an advantageous effect on the reproducibility and accuracy of the structures to be projected.
One advantageous embodiment of the invention provides for the dummy structure to be semitransparent. The use of a semitransparent and, if necessary, also phase shifting material, which in the most favorable case produces a phase rotation of 360°, makes it possible to apply the dummy structures which, in terms of their geometric dimensions, do not have to be smaller than the image structures to be projected. The semitransparency results in that the dummy structures are not transferred into the photolithographic recording medium. The advantage resides in the fact that only the same conditions are placed on the dummy structures, relating to minimum structure width, reproducibility and lithographic resolution during the production of the mask, as those placed on the actual image structures to be transferred. The semitransparent dummy structures achieve the situation where the process window for the relevant image structures is enlarged, so that the depth of focus of individual image structures is enlarged and the dose fluctuation sensitivity is reduced. The semitransparent dummy structures are therefore not geometrically below the resolution limit but in the sense of the photolithographic sensitivity of the recording medium. The dose that is transmitted through the semitransparent layer, the dose being lower as compared with the transparent carrier material, exposes the recording medium below its tolerance threshold.
A further advantageous embodiment of the invention provides for the smallest lateral extent of the dummy structure to be at least half as large as the smallest lateral extent of the image structure. In this case, the advantage resides in the relatively large dummy structures, which can be formed at the same order of magnitude as the image structures. As a result, the same conditions relating to minimum structure width, reproducibility and lithographic resolution during the production of the mask are placed on the dummy structures as those placed on the image structures actually to be transferred.
It is advantageous if the smallest lateral extent of the dummy structure is greater than
0.25
·
λ
NA
,
where &lgr; is a wavelength of a projecting light and NA is a numerical aperture of the projecting system.
A further advantageous embodiment of the invention provides for the smallest lateral extent of the dummy structure to be at least as large as the smallest lateral extent of the image structure. The fact that the dummy structure is semitransparent results in that it does not have to be significantly smaller, in terms of its geometric dimensions, but preferably can be at most exactly as small as the image structure to be transferred. As a result, the requirements on the mask production are significantly more relaxed, more reproducible and it is possible to inspect these structures more simply for defects and to carry out repair measures.
In a further advantageous embodiment of the mask according to the invention, the dummy structure is composed, in terms of its geometric dimensions and its transparency, in such a way that it is not
Fischer Werner
Gans Fritz
Pforr Rainer
Thiele Jörg
Greenberg Laurence A.
Huff Mark F.
Infineon - Technologies AG
Mayback Gregory L.
Sagar Kripa
LandOfFree
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