Phase shift mask
View Patent ↗Semitransparent and trenchlike, absorber-free structure elements are formed jointly on a photomask formed using phase mask technology. The trenchlike structure elements are formed as trench or mesa structure using CPL technology. In a layout, dense, but also if appropriate semi-isolated and isolated, but relatively thin pattern portions are selected to fabricate them on the photomask using CPL technology. By contrast, isolated, wider pattern portions are formed as semitransparent structure elements using halftone phase mask technology. The respective process windows are relatively large and are adapted to one another. The joint process window is enlarged. In the area of dynamic memory chips, structures in a memory cell array can be formed using CPL technology and the support regions using halftone phase mask technology. In logic circuits, thin conductor tracks using CPL technology and wider conductor tracks using halftone phase mask technology can be fabricated.
1. A phase shift mask for the projection of a pattern of structure elements formed on the mask onto a substrate, the phase shift mask comprising:
a first portion of the pattern including a plurality of first structure elements formed as elevated ridges made from a semitransparent, phase-shifting material, which is arranged on a planar surface of a transparent mask substrate; and
a second portion of the pattern being free of a phase shifting or semitransparent material, the second portion including a plurality of second structure elements formed by trenches with closely adjacent trench edges in the planar surface of the mask substrate with a depth, or formed by ridges with closely adjacent ridge edges within a surface etched with the depth into the surface of the mask substrate, such that a light beam transmitted through the mask substrate within the trenches or ridges experiences a phase shift of 180 +/−20 degrees with respect to a light beam transmitted through the mask substrate on the planar surface outside the trenches.
2. The phase shift mask as claimed in claim 1 , further comprising:
further structure elements formed by trenches, the further structure elements adjoining the first structure elements at least in part directly as RIM structures.
3. The phase shift mask as claimed in claim 1 , wherein the second structure elements formed by trenches are surrounded by a planar surface not covered by material.
4. The phase shift mask as claimed in claim 1 , wherein the second structure elements formed by adjacent trench or ridge walls have a width less than or equal to a predetermined limit value.
5. The phase shift mask as claimed in claim 4 , wherein the limit value relative to the wafer scale is 90 nanometers.
6. The phase shift mask as claimed in claim 4 , wherein the limit value relative to the wafer scale is 70 nanometers.
7. The phase shift mask as claimed in claim 4 , wherein the limit value relative to the wafer scale is 60 nanometers.
8. The phase shift mask as claimed in claim 1 , wherein the first structure elements formed by semitransparent, phase-shifting material have a width greater than a predetermined limit value.
9. The phase shift mask as claimed in claim 1 , wherein the pattern represents a circuit plane of a memory chip with a cell array;
the first portion has structure elements arranged in the support or peripheral region outside the cell array, and
the second portion has structure elements arranged within the cell array.
10. The phase shift mask as claimed in claim 1 , wherein the substrate is a semiconductor wafer.
11. A method for forming the pattern on the phase shift mask, comprising:
providing a mask blank with a mask substrate, a semitransparent, phase-shifting layer, an absorber layer, and a photosensitive resist;
exposing and developing the resist so that bright regions to be formed by the first portion and bright and dark regions to be formed by the second portion are opened in the resist, the second portion being free of a phase shifting or semitransparent material;
transferring the bright regions of the first portion for forming the first structure elements and the bright and dark regions of the second portion of the pattern into the absorber layer;
removing the resist and renewed application of a second resist;
exposing and developing the second resist for covering openings formed by the transferred bright regions of the first portion in the absorber layer and the semitransparent layer and for opening the dark regions to be formed on the mask, or the bright regions to be formed on the mask of the second portion of the pattern in the resist;
transferring the openings formed by the dark regions or by the bright regions of the second portion from the resist into the mask substrate as far as a depth representing a phase difference of 180 +/−20 of the light beams passing through compared with mask regions without an absorber which have not undergone mask substrate etching for the formation of the second structure elements, the second structure elements being formed as ridge (mesa) in the case of the transferred bright regions, or as trench in the case of the transferred dark regions with adjacent ridge or trench walls;
removing the further resist; and
removing the absorber layer to uncover the semitransparent, elevated ridges of the first structure elements.
12. The method as claimed in claim 10 , wherein the transferring the bright regions includes etching and the transferring the openings includes etching.
13. The method as claimed in claim 12 , wherein the etching is performed anisotropically.
14. The method as claimed in claim 13 , wherein in the first and second patterns with which a resist is respectively exposed onto the phase shift mask, none of the first structure elements directly adjoins one of the second structure elements.