1. Field of the Invention
The invention relates to a diffractive optical element having a multiplicity of binary blazed diffraction structures. The diffractive optical element is particularly intended for use in microlithographic projection exposure apparatus.
2. Description of the Prior Art
Conventional blazed gratings have diffraction structures of triangular, in particular sawtoothed cross section which extend mutually parallel with a spacing equal to the grating constant g. One edge of the diffraction structures, the blaze edge, has an inclination with respect to the base surface of the grating such that the reflection or refraction law is satisfied for one diffraction order of the incident light, and the majority of the intensity of the diffracted light is therefore contained in the order favoured by the blaze edge. The traditional method of producing such blazed gratings consisted in scratching the diffraction structures in a master grating with the aid of diamonds and making corresponding copies of this master grating. This mechanical method is highly elaborate, on the one hand, and on the other hand it encounters limitations with very short wavelengths of the light for which the grating is intended to be used, since the structures to be produced are too small.
Efforts have therefore been made to employ the process technology used for the production of semiconductor components, in which a substrate is coated with photoresist, exposed, subsequently developed and etched, in order to produce the diffraction structures of blazed gratings. The approach firstly involved using successions of such process cycles to achieve diffraction structures which are supposed to approximate the blaze edge by a stepped edge. If four such steps are used, for example, then diffraction efficiencies of more than 80% can be achieved in the first order. With a further process cycle, eight stages are obtained by which a first-order diffraction efficiency of about 95% can be achieved. In general, 2n steps can be produced by using n process cycles. With increasing n, the stepped profile of the edge becomes closer and closer to the sawtooth profile of ideal blazed gratings in conventional, mechanically produced gratings, the diffraction efficiency of which is 100% in the first order according to scalar theory. The production of such a grating, however, is cost-intensive and error-prone because it is necessary to carry out the process cycle repeatedly.
Attempts have also been undertaken to simulate the blaze profile of the diffraction structures by using binary structures whose dimensions are smaller than the wavelength of the electromagnetic radiation for which the grating was defined. These attempts are based on the fact that light is no longer diffracted at the small substructures, but can only be scattered. This leaves only the zeroth diffraction order which picks up the effect of the substructures merely in the form of a local effective refractive index in phase gratings, or merely in the form of a local shade of grey in amplitude gratings.
A first example of such a binary blazed grating is described in the article by Joseph N. Mait et al. “Diffractive lens fabricated with binary features less than 60 nm”, Optics Letters, 15 Mar. 2000, pages 381 et seqq. The substructure used here is a multiplicity of lines, all of which extend parallel to the diffraction structure and whose spacing is less than the effective wavelength.
The article by Philippe Lalanne et al. “Design and fabrication of blazed binary diffractive elements with sampling periods smaller than the structural cut off”, J. Opt. Soc. Am. A, May 1999, pages 1143 et seqq. describes blazed diffractive elements of the type mentioned in the introduction, in which the diffraction structures are resolved into individual substructures consisting of rectangular or square pillars. Different “fill factors” can be achieved by varying the pillar width for a predetermined pillar spacing, and this corresponds to a local variation of the effective refractive index. As an alternative, the pillars may also be arranged at different spacings with a constant width.
A common feature of all these attempts to produce binary blazed diffractive optical elements is that the substructures are minutely configured and have a very high aspect ratio (structure height to structure width). They are therefore technologically highly elaborate and expensive to produce, and cannot be made with sufficient accuracy.