Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-07-31
2004-08-10
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000
Reexamination Certificate
active
06773854
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method of producing a perforated mask having a desired pattern of perforations for the projection of particle radiation onto a projection area. The preferred field of application of the invention is the production of masks for electron or ion-beam lithography in the fabrication of integrated circuits, in particular, for the lithographic formation of very fine structures with dimensions below 130 nm.
To produce integrated circuits, various layers of selected semiconductor, conductor, and insulating materials are applied one after another to a wafer-like substrate, referred to as a wafer, and are each processed selectively in predefined areas to form fine structures in the layers. Lithographic processes configure the pattern of the respective structure, in that, radiation having an effect in the desired manner on the respective material to be treated is projected onto the relevant layer through a mask. The projection mask is constructed in accordance with the desired pattern so that only the areas to be treated are irradiated and the remaining areas are shielded from the radiation. The irradiated layer is normally a film of a “resist” material, whose irradiated areas are washed away after a development operation to expose the layer lying underneath selectively in accordance with the desired pattern and, therefore, to make it receptive to specific selective processing such as etching, semiconductor doping, vapor deposition.
The lithographic process that is most common at present is optical lithography (photolithography), which operates with optical irradiation. Projection masks for optical lithography include a carrier material that is transparent to the radiation used (in the case of transmission masks) or is reflective like a mirror (in the case of reflection masks) and bears a shielding layer in those portions of its area that are to let through or reflect no radiation to the object being exposed. The lower limit for the size of the structures that can be defined by optical projection is directly proportional to the wavelength of the light used and inversely proportional to the numerical aperture of the imaging system. With increasing miniaturization of integrated circuits, the development of optical lithography processes is, therefore, moving to shorter and shorter wavelengths as far as the ultraviolet (UV) range and even beyond that.
Although radiation in the UV range still permits projection techniques by optical lenses, it permits resolutions only down to a fineness of the order of magnitude of about 100 nm. To permit even smaller structure sizes, special projection techniques and masks for electromagnetic radiation in the X-ray range have been developed or proposed. Because, in the case of X-rays, no optical lenses can be used for the reduced imaging of a mask, the mask has to be formed as a “1:1 mask” on the same scale as the structures to be provided. In addition, because flooded exposure with an appropriately broad parallel X-ray beam cannot apply the necessary energy density to the irradiated area, the 1:1 mask, together with the wafer to be exposed, has to be subjected to a scanning movement relative to the source of a narrowly collimated X-ray beam, for example, relative to a synchrotron ring. X-ray lithography, therefore, requires a complicated and expensive apparatus and, because of the time-consuming mechanical scanning technique, is barely suitable for the mass production of integrated circuits.
In view of such a problem, more recent developments are aimed at implementing lithography for very small structures (for example below 130 nm) by particle radiation instead of by electromagnetic radiation. The particles to be used in such a case are electrons or ions that, because of their electric charge, can be accelerated and focused by electric or magnetic lenses so that reduced imaging of the mask pattern onto the projection area is also possible. With such particle radiation, much smaller structures can be formed on a projection area than by conventional optical radiation or by X-rays because the equivalent wavelength of electrons or ions is many times smaller than the wavelength of the shortest-wave electromagnetic radiation. Although no optical radiation is used in particle beam lithography, the expression “exposure” is commonly also used here for the selective irradiation procedure.
There are materials that are sufficiently sensitive to exposure with particles such as ions or electrons to form a useable resist for particle radiation lithography. On the other hand, there are no materials that are sufficiently transparent to ions to be able to serve as a transmitting carrier material for a projection mask. Although there are materials that are transparent to electrons, high transmission losses occur when they are used. Masks for ion projection lithography (IPL), therefore, have to be perforated masks, that is to say, to be of a diaphragm of a material that is opaque to the particles used and is perforated in accordance with the desired projection pattern. The use of such perforated masks is also desirable for electron projection lithography (EPL) to avoid the aforementioned transmission losses.
The narrower the mask openings formed by the perforation, the thinner must the diaphragm be so that the ratio of depth to width of the openings remains small. Lithography of small structures, therefore, requires very thin diaphragms. Furthermore, it is desirable to give the diaphragm as large an area as possible so that a sufficient number of pattern components-can be accommodated on it to cover an entire wafer by full area exposure and so that no time-consuming scanning or blockwise exposure of the wafer is necessary.
The required low thickness and the desired large area of the diaphragm, and also the presence of the perforation, leads to the mask having a relatively low stiffness with respect to mechanical stresses in the directions of its main plane. This means that longitudinal and shear forces in the directions of the main plane lead to distortions of the perforation pattern. Because the requirements on the accuracy of placement of the mask openings become higher and higher as the size of the exposure structures decreases, calculation of all the mechanical distortions that occur is necessary. To be able to calculate the distortions exactly in advance and compensate for them, both the stresses that act and the actual stiffness of the perforated diaphragm must be made available in all areas.
The mechanical stresses depend on parameters of the production process and also on external influences, for example, on the mounting, on thermal effects, ion implantation and so on, and may be predicted quantitatively or determined empirically. The stiffness of the diaphragm, on the other hand, is not only a function of the material and of the thickness but also depends on the shape of the perforation pattern, that is to say, on the form, the size, and the density of the mask openings, and can, therefore, be very different from place to place within the diaphragm. If the actual stiffness of the diaphragm and its local fluctuations have been determined, this information, together with the information about the mechanical stresses, can be used to calculate the distortion that occurs, by the method of “finite elements” (FE method).
The FE method is a model calculation in which the overall area to be investigated is broken down into a finite number of adjacent polygonal “cells”, and the relevant elasticity values for each cell are determined numerically, namely, the modules of elasticity, the shear modulus, and the Poisson's ratio in the plane considered. The values determined for each cell are linked with those of the adjacent cells and with the mechanical stress that acts to determine the relative displacement of the corners of the cells vectorially. The vector array so obtained describes the distortion of the overall area. Suitable FE methods are in the prior art and, therefore, do not need to be descr
Ehrmann Albrecht
Haugeneder Ernst
Kamm Frank-Michael
Petraschenko Alexander
Schunck Stefan
Huff Mark F.
Infineon - Technologies AG
Locher Ralph E.
Mohamedulla Saleha
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