Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-09-28
2003-09-16
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C430S312000, C430S320000, C430S394000, C430S396000
Reexamination Certificate
active
06620559
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates in general to photomasks for lithographic exposure and structuring methods with the aim of producing microscopic material structures such as photoresist structures and, on this basis, of producing microscopic material structures such as magnetic memory element configurations or the like. In particular, the invention relates in this case to phase masks and their utilization in these methods and the production of dimensionally critical material structures.
For the production of the memory level (XMR level) of magnetic memory components such as magnetic RAM data memories (MRAM), as described for example in S. Mengel: Technologieanalyse Magnetismus Band 2, XMR-Technologien [Technological analysis, magnetism volume 2, XMR technologies], published by VDI-Technologiezentrum Physikalische Technologien, 1997, it is a general object to produce the individual magnetic memory elements in increasing densities, in order to increase the storage capacity of the entire memory component. Hitherto, such structures have been produced either electron-optically by means of a direct writing method or by means of conventional optical lithography, corresponding to the conventional methods for producing microelectronic circuits.
In photolithographic processes, the structures are projected optically onto light-sensitive layers such as photoresist layers on a substrate in a conventional manner by photomasks. Because of the diffraction effects, the resolution power of such a projection system is limited and mask structures having dimensions below the inverse value of this resolution capacity, the dimensionally critical structures as they are known, become blurred or projected unsharply. In order to be able to produce magnetic memory elements with high density, it is previously necessary for a photoresist layer in the form of a matrix-like configuration of resist clusters or dots or of cutouts or depressions in a resist layer to be structured. Using conventional lithography, however, it is very difficult to reach below the resolution limit which is normally given, wherein half the distance between two resist dots is given by k
1&lgr;
/NA with (k
1
≈0.38, &lgr; the carrier wavelength of the illumination, NA the numerical aperture). At the least, such dense resist dots cannot be produced with a non-negligible process margin in the conventional optical way. Furthermore, the conventional optical projection is very sensitive to fluctuations in the mask dimensions which, for example, can be described by the “mask error enhancement factor” (MEF).
The above problems constitute limiting factors for the cost-effective and competitive fabrication of MRAM memory components with critical dimensions below 100 nm with conventional lithography and mask technique.
These difficulties may be overcome with phase masks, as they are known. In phase masks, the destructive interference effect of two closely adjacent and coherent light beams with phases shifted by 180° is utilized.
The various types of phase masks are described, for example, in the book “Technologie hochintegrierter Schaltungen” [The technology of highly integrated circuits] by Widmann, Mader, and Friedrich, 2nd edition, Springer-Verlag, pages 135ff. An extensive overview of phase mask technology is given in the publications “Improving Resolution in Photolithography with a Phase-Shifting Mask” by M. D. Levenson et al. in IEEE Trans. Electron. Devices 29 (1982), 1828ff. and “Wavefront Engineering for Photolithography” by M. D. Levenson in Physics Today, July 1993, p. 28ff.
In the case of MRAM memory components, it is additionally advantageous to produce the individual magnetic memory elements as elliptically shaped structures of high density, since they thus impart a preferred direction to the magnetic storage medium. This has been shown in the publications “Giant magnetoresistance by melt-spun CU—CO alloys” by J. Wecker et al. in Appl. Phys. Lett. 62 (1993), pp. 1985-1987, and “GMR angle detector with an artificial antiferromagnetic subsystem (AAF)” in J. Magn. Mat. 165 (1997) p. 524, using magneto-resistance elements.
Conventional optical lithography is also problematic under this last-named aspect. This is the case because using the conventional binary chromium photomasks, defined production of elliptical structures, that is to say both depressions (holes) in resist layers and resist dots, can generally not be carried out. In the case of dense structures, the necessary reserve on the mask structures would, under certain circumstances, be too great for practical mask production.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for lithographic structuring of a material layer, in particular a photoresist layer, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows microscopic, in particular elliptical material structures to be produced. In particular, it is an object of the present invention to specify a suitable photomask and its use for implementing such a method. Furthermore, it is an object of the present invention to provide a method on this basis of producing a magnetic memory component such as an MRAM memory component with the aid of the structuring method.
With the foregoing and other objects in view there is provided, in accordance with the invention, a photomask for lithographic structuring methods, comprising:
a configuration of at least partially transparent strip-like areas arranged immediately adjacent and parallel to one another; and
wherein adjacent areas are formed such that rays of light passing therethrough have a phase difference of 180° relative to one another.
With the above objects in view there is also provided, in accordance with the invention, a lithographic structuring method which comprises providing a material layer, in particular a photoresist layer, and exposing the material layer through the above-summarized photomask.
Finally, there is also provided a method of producing a matrix-like configuration of magnetic memory elements, which comprises:
producing a matrix-like arrangement of microscopic photoresist structures in accordance with the above method; and
subsequently producing the matrix-like configuration of magnetic memory elements with the aid of the arrangement of microscopic photoresist structures.
A significant idea of the invention consists in using at least one photomask, which is formed as a phase mask, in the exposures of a material layer to be structured, in particular a photoresist layer. Such a photomask has an arrangement of at least partially transparent strip-like areas, which are arranged immediately adjacent and parallel to one another, adjacent areas being formed in such a way that rays of light passing through them have a phase difference of 180° from one another.
Using such a photomask, a first exposure can be carried out, narrow strip-like sections remaining unexposed on the surface of the material layer to be exposed, because of the destructive interference of the rays of light passing through adjacent areas, said sections being defined by the boundaries between the adjacent areas.
A second exposure can then be carried out, wherein, for example, a second corresponding photomask is used, the alignment of the strip-like arrangement of the photomasks being rotated through an angle, in particular 90°, between the exposures. At the crossing points of the strip-like arrangements, hole-like or cluster-like resist structures are formed, so that the result produced is a matrix-like arrangement of such resist structures.
The novel method has the advantage that hole-like or cluster-like material structures such as resist structures can be produced, their extent, at least in one direction, falling below critical dimensions. For example, the case may occur wherein structures are required which have to be subcritical only in one direction but in the other direction have an extent above the critical dimensions
Czech Günther
Fülber Carsten
Henke Wolfgang
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
McPherson John A.
Sagar K
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