Patent Number: 
Section: description

FIG. 1 shows a radiation source indicated by reference number 1, and a filter which is generally indicated by reference number 2. The processing organ that is used in the apparatus for, for example, extreme ultraviolet lithography, is not shown. This processing organ is located at the side of the filter 2 facing away from the radiation source 1. The filter 2 comprises a number of plates 3 positioned in a radial direction from the radiation source 1. It is possible to position said plates in a honeycomb construction, or as a plurality of concentric cones as shown in FIG. 3. FIGS. 1 and 2 show that in the direction of radiation from the source 1, the plates are positioned such as to be evenly distributed next to one another. The proximal end 4 of the filter 2 is at a distance X from the radiation source 1, which distance is selected depending on the pressure and the type of buffer gas in which the radiation source 1, the processing organ (not shown), and also the filter 2, are placed. If the apparatus is used for extreme ultraviolet lithography, the buffer gas is preferably krypton having a pressure of 0.5 Torr, and the value of X may be 5 cm. The length of the plates of the filter is indicated by L. The value of L is selected depending on the pressure of the buffer gas and the form of the filter 2. The value of L, that is to say the length of the filter, is at least 1 cm. In FIG. 1, this value is approximately 10 cm. The thickness of the plates 3 may be, for example, 0.1 mm, and the spacing between the plates at the side nearest the radiation source 1, may be approximately 1 mm. This may result in an optical transparency of the filter 2, which is determined by the formula       d          d      +              d        f              xc3x97  100  ⁢      xe2x80x83    ⁢  % in which d=the distance between two plates of the filter at the proximal side of the filter; and df=the thickness of a plate of the filter. The effectiveness of the filter can be promoted if the surface of the plates 3 is slightly roughened. When the apparatus is used for extreme ultraviolet lithography, radiation is used having a wavelength of 13.5 nanometers. Various inert gasses may be used as buffer gas, such as helium and krypton which, compared with other gasses have the lowest absorption coefficient at this wavelength. Krypton is better able to meet the requirements of the present application because the atomic mass of krypton is more compatible with that of the atomic- and microparticles emitted by the radiation source, which augments the inhibition of said undesirable particles. The krypton gas used is maintained at a pressure of at least several mTorr. It should be noted that taken over a distance of 20 cm at a pressure of 0.5 Torr, the optical transparency of krypton for the desired radiation is approximately 90%. The filter used in the apparatus is comprised of copper plates (other materials are also possible) which have a length of 7 cm and are positioned at 2 cm from the radiation source. At a plate thickness of 0.2 mm and with the plates being spaced at approximately 0.8 mm at the side of the radiation source, the filter will have a geometrical transparency of approximately 80%. The effectiveness of the filter was measured at room temperature and at a temperature of approximately xe2x88x9290xc2x0 C. At both these temperatures the effectiveness of the filter was shown to be very high, almost 100. It will be clear to the person skilled in the art that the various dimensions of the filter forming part of the apparatus according to the invention, as well as the distance from the filter to the radiation source, has to be determined in practice on the basis of the above-mentioned inter-relating ratios. It is therefore possible to apply diverse variations to the above description, without departing from the idea of the invention as specified in the appended claims. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.