The invention describes the fabrication and use of lithographically produced resist structures used as mold material for imprint lithography for fabricating micro and nanostructures.
THE STATE OF THE ART
Structures dimensioned on the nanometer scale may be economically fabricated in large numbers by imprint lithography. (S. Y. Chou et al. Vac. Sci. Technol. B 15(6) (1997), 2897; U.S. Pat. No. 5,772,905). In this context, cost-intensive and time consuming electron beam lithography is used only for fabricating the mold. A large number of imprints may then be produced with the fabricated molds by thermal embossing of layers of polymeric materials on different substrates. The properties of the electron beam resists hitherto available do not, however, permit their direct use as molds. Following its development process, the structural layout imprinted directly on the resist layers must be transferred to more stable materials by several partial steps, such as plasma etching and galvanizing (Semi-Conductor Lithography, Principles, Practices, and Materials; W. M. Moreau, Plenium Press, New York, 1988). The structure transfer of nano-scaled resist structures to more stable inorganic materials poses a number of problems. The plasma etching resistance of conventional hyper-sensitive electron beam resists is insufficient, and their galvanic copying process places special demands on the profile of the structure, and on the thermal stability and solubility of the resist structures (Introduction to Microlithography; L. E. Thompson et al., ACS Professional Reference Book, American Chemical Society, Washington D.C., 1994). This may usually be realized by combinations of different polymers or by using special auxiliary layers in multiple-layer systems and necessitates further partial steps and, therefore, a loss of precision and resolution of the structures.
It is an object of the present invention to overcome the disadvantages inherent in conventional processes of mold fabrication.
In accordance with the invention, the object is accomplished by use of a negative system the lithographically fabricated structures of which satisfy the demands placed on a mold for embossing thin polymeric layers.
The details of the method in accordance with the invention are as follows. In the context of the invention, mixtures of photo-reactive epoxy resins are preferably used for fabricating a master structure. In general, they are multi-functional aromatic, hetero-aromatic, aliphatic and cyclo-aliphatic epoxy resins and include photo-initiators. For micro-structuring, the masks are preferably made by optical lithography, and for nano-structuring they are made by electron beam lithography. The sensitivity of the used materials against ultraviolet light and high-energy radiation makes it possible to increase their structural stability by subjecting their surface to ultraviolet irradiation and by thermal after-treatment of the developed structures. This results in complete hardening, and the stability attained by the structures renders them suitable as embossing tools. The high degree of cross-linking prevents thermal flow up to the thermal decomposition temperature of the polymers at about 200° C. The embossing temperatures of many polymers lie substantially below this temperature. It is particularly advantageous if the layer to be embossed also consists of a curable material based on a photo-reactive epoxy resin. The embossing process may be carried out in a manner similar to the one used in connection with thermoplastic polymers. In the absence of UV radiation, cross-linking is substantially prevented at temperatures up to 180° C. Because of the low glass temperature and molar mass of such systems, the preferred embossing temperatures lie in the range of 60-140° C., preferably 80-120° C., at an embossing pressure in the range of 40-100 bar, preferably 60-80 bar. Structural profiles of excellent thermal and mechanical stability are attained, separate from the embossing tool, by subsequent UV surface irradiation and thermal after-processing at up to 150° C. The embossed structures may thus also be used as molds. The surface UV irradiation may be carried out by a high pressure mercury lamp.
The preferred dosage is in the range of 100-1000 mJ/cm2. Thermal processing may take place at a temperature up to 200° C., without any thermal decomposition of the cross-linked polymer being noticeable. Instead of photo-reactive epoxy systems, it is, of course, also possible to emboss other thin-layered polymers. Examples of such polymers are thermoplastic polymers having a glass temperature of <150° C., and curable pre-polymers. The thermal stability of most mold materials lies at about 200° C.
The principle of the structure transfer is shown schematically in the figure. The master structure fabricated in a resist by electron beam lithography is embossed (1) under pressure and temperature, into a coated material. This results in a negative image of the mold (2) used. Where the polymer layer to be embossed consists of photo-reactive epoxy resins, the embossed image may also be used as a mold (3) which then results in the original structural layout of the initial mold (4). In order to increase the thermal and mechanical stability, following each embossing step, UV surface irradiation and thermal after-processing are performed to yield the required stability.
Without claiming completeness, the following examples will describe the invention in greater detail.