Patent Number: 
Section: description

FIG. 1 shows diagrammatically an embodiment of a step-and-scan lithographic projection apparatus 1 in which an EUV radiation source according to the invention may be used and with which the method according to the invention may be performed. The apparatus comprises an illumination system for illuminating a mask MA and a mirror projection system for imaging a mask pattern, present in the mask, on a substrate W, for example, a semiconductor substrate which is provided with an EUV radiation-sensitive photoresist WR. The illumination system 10 shown in the left-hand part of FIG. 1 is designed in known manner in such a way that the illumination beam IB supplied by the system at the area of the ask MA has a cross-section in the form of an annular segment or a rectangle, and has a uniform intensity. The illumination system comprises, for example, three mirrors 11, 12 and 13 which are maximally reflecting for EUV radiation at, for example, a wavelength of the order of 13 nm because they have a multilayer structure of, for example, silicon layers alternating with molybdenum layers. The mask MA is arranged in a mask holder MH which forms part of a mask table MT. By means of this table, the mask can be moved in the scanning direction SD and possibly in a second direction perpendicular to the plane of the drawing, such that all areas of the mask pattern can be introduced under the illumination spot formed by the illumination beam IB. The mask table and the mask holder are shown only diagrammatically and may be constructed in different ways. The substrate W to be illuminated is arranged in a substrate holder WH which is supported by a substrate table WT, also referred to as stage. This table can move the substrate in the scanning direction SD but also in a direction perpendicular to the plane of the drawing. The substrate table is supported, for example, by a table bearing ST. For further details of a step-and-scan apparatus, reference is made by way of example to PCT patent application WO 97/33204 (PHQ 96004). For imaging the mask pattern on the substrate with a reduction of, for example, 4xc3x97, a mirror projection system 20 comprising, for example, four mirrors 21, 22, 23 and 24 is arranged between the mask and the substrate. For the sake of simplicity, the mirrors are shown as plane mirrors but actually these mirrors, as well as those of the illumination system 10, are concave and convex mirrors and the mirror projection system 20 is designed in such a way that the desired sharp image is realized at a reduction of, for example 4xc3x97. The design of the mirror projection system does not form part of the present patent application. Analogously as the mirrors of the illumination system, each mirror 21, 22, 23 and 24 is provided with a multilayer structure of first layers having a first refractive index, alternating with second layers having a second refractive index. Instead of four mirrors, the mirror projection system may alternatively comprise a different number of mirrors, for example, three, five or six. Generally, the accuracy of the image will be greater as the number of mirrors is larger, but there will be more radiation loss. Thus, a compromise will have to be found between the quality of the image and the radiation intensity on the substrate, which intensity also determines the velocity at which the substrates are illuminated and can be passed through the apparatus. Mirror projection systems having four, five or six mirrors for lithographic apparatuses are known per se. For example, a six-mirror system is described in EP-A 0 779 528. Since EBV radiation is absorbed by air, the space in which this radiation propagates must be a highly vacuum-exhausted space. Minimally, both the illumination system, from the radiation source to the mask, and the projection system, from the mask to the substrate, must be arranged in a vacuum-tight space, which is denoted by means of the envelope 16 in FIG. 1. Instead of being accommodated in the same envelope, the illumination system and the projection system may be alternatively accommodated in separate envelopes. The mask MA and the substrate W may be juxtaposed, as shown in FIG. 2, instead of opposite each other. In this Figure, the components corresponding to those in FIG. 1 have the same reference numerals or symbols. The separate mirrors of the illumination system are not shown in FIG. 2 but form part of the block 10 representing the illumination system, with which the illumination beam is given the desired shape and the uniform intensity. FIG. 2 is a plan view of a mask with a mask pattern C and a plan view of a substrate W with substrate fields, with an image of the mask pattern C being formed on each field. The mask and the substrate comprise two or more alignment marks M1 and M2, and P1 and P2, respectively, each, which are used for aligning the mask pattern with respect to the substrate or with respect to each substrate field separately before the mask pattern is projected. For checking the movements of the mask and the substrate, the lithographic projection apparatus comprises very accurate measuring systems, preferably in the form of interferometer systems IF1 and IF2. The block denoted by reference numeral 2 in FIGS. 1 and 2 comprises an EUV radiation source unit in which EUV radiation is generated by irradiating a solid medium, for example, a metal with a high-intensity laser beam. FIG. 3 is a cross-section of an embodiment of such a radiation source unit. This unit comprises a transport device 30 in the form of, for example, a supply reel 31 and a take-up reel 32 for transporting a tape 33 of, for example, metal through a vacuum source space 34. This space is connected to a pump 35, for example, a turbo pump having a power of, for example, 1000 dm3/sec, with which the space 34 can be pumped to a vacuum of 10xe2x88x924 mbar. The radiation source unit further comprises a high-power laser 40, for example, an Nd-YAG laser which supplies laser pulses at a frequency of, for example, 10 Hz and at a pulse duration of, for example, 8 ns and with an energy content of 0.45 Joule. The optical frequency of the laser radiation may be doubled in known manner so that laser radiation with a wavelength of the order of 530 nm is obtained. An excimer laser, for example a Kr-F laser emitting at a wavelength of 248 nm, may be alternatively used as a laser source. The beam 41 emitted by the laser 40 enters, through a window 43, into the wall of the source space 34. This beam is focused by a lens system 42, illustrated by a single lens element, to a radiation spot 45 in a position 46 in a plane which substantially coincides with the laser-facing surface 36 of the tape 33. The pulsed beam 42 is each time substantially focused on a part of the tape which is instantaneously at the position 46. The radiation spot 45 has a diameter of, for example, 10 xcexcm. As a result of the extremely high energy density, for example, of the order of 1021 W/m3 of the laser beam at the bombarded area on the tape, this area partly explodes so that material, for example, metal particles are repelled from the tape. The repelled particles constitute a plasma, as is indicated in FIGS. 4a and 4b.  In these Figures, the reference numeral 36 denotes the bombarded surface of the tape 33 and the reference numeral 41 denotes the laser beam. The plasma is denoted by the reference numeral 47. This plasma reaches a temperature corresponding to an energy of the order of several tens of eV. Then, EUV radiation is generated at a wavelength in the range of several nm to several tens of rm. The wavelength of the generated radiation is dependent on the process parameters, such as the material of the tape 33. FIG. 4a illustrates the situation immediately after the laser beam has bombarded an area on the tape. At that instant, energy-rich ions 51 and atoms 52 are repelled from the plasma. A few moments later, hot pieces of metal 53, or clusters of metal particles, are evaporated, as is shown in FIG. 4b. For further particulars about the way and conditions in which EUV radiation is formed, reference is made to the above-mentioned article: xe2x80x9cCharacterization and control of laser plasma flux parameters for soft X-ray projection lithographyxe2x80x9d. According to the invention, the surface 36 of the tape 33 is provided with pits 37, in which the width of these pits is, for example approximately equal to the cross-section of the laser beam at the area of the tape, as is shown in FIG. 5. A cross-section in this Figure shows a small part of the tape 33 in which a pit is present. As is shown in FIG. 5, the tape may have a constant small thickness and the pit is formed by a local protuberance of the tape. The pit may alternatively have the shape of a local indentation in a thicker tape. The pits may be cylindrical or spherical. Due to this shape of the local surface of the tape which is irradiated, the plasma formed there is concentrated in a smaller volume, so that the plasma has a considerably higher density and temperature than when using a flat tape as a plasma-forming medium. Due to the higher density and temperature, the emitted EUV radiation has a considerably higher intensity than in known EUV metal plasma radiation sources. In addition to the high intensity gain, the use of the tape with pits as a plasma-forming medium provides another advantage which is not less important. Due to the pit structure, the ions 39, atoms 41 and the metal pieces 42 are also concentrated, i.e. the spatial angle at which these particles are repelled is reduced considerably. This provides the possibility to collect these particles within the source space by means of a particle collector, or receptacle 48, arranged within this space. The tape 36 may consist of various metals such as iron, tin or carbon. Instead of a metal, another solid material may be used as a medium. The requirements imposed on such a material are that it should form an EUV emitting plasma upon bombardment with a high power laser beam and can be brought to a shape which is suitable for transport through the source space. As regards the shape of the medium, there are various possibilities, as is illustrated in FIGS. 6, 7 and 8. FIG. 6 is an elevational view, in the direction of the laser beam, of the above-mentioned strip or tape 33 provided with the pits 37. When this type of tape is used, an extra provision may be made in the radiation source unit so as to ensure that a laser pulse each time impinges upon a pit. As is shown in FIG. 3, the transport device can be synchronized with the laser driver via an electronic circuit 50 comprising a delay element, so that a laser pulse is formed at the instant when a pit 37 arrives at the position 46. FIG. 7 is a perspective view of a medium in the form of a bent tape 55 which is transported through the source space in such a way that the concave side 56 faces the laser. This embodiment of the medium provides the advantage that each laser pulse automatically impinges upon a tape section of the desired concave shape. This also applies to the medium shown in FIG. 8. This medium has the shape of a concave wire 57 whose concave surface 58 must face the laser beam upon transport through the source space 34. The wall of this space is provided with one or more apertures (not shown) through which the generated EUV radiation can exit. One or more mirrors 49 for collecting, concentrating and directing the generated EUV radiation may be arranged in this space. Alternatively, such mirrors may be arranged outside the space so as yet to concentrate and direct the EUV radiation exiting from this space. The number of mirrors required is dependent on the percentage of the EUV radiation which must be collected and used and is emitted by the plasma in all directions. In the radiation source unit described, the problem may occur that the metal elements 51, 52 and 53 present in the source space 34 absorb EUV radiation which may reach other spaces in the apparatus via the apertures for passing EUV radiation. These particles may then damage the mirrors arranged in these spaces. This is an important problem, notably in lithographic projection apparatuses because a reduced reflection of the mirrors, whereby less radiation can reach the mask and notably the substrate, has a direct influence on an important performance parameter of such an apparatus, namely the rate at which substrates can be illuminated. This problem can be eliminated, or at least sufficiently reduced, by passing a flow of rare gas through the source space, parallel to the direction of movement of the medium 33. FIG. 9 shows a first embodiment of a radiation source unit in which this is realized. In this Figure, the elements corresponding to those in FIG. 3 are denoted by the same reference numerals. Furthermore, the components of the radiation source unit which are not important for the present invention are no longer shown in this Figure and subsequent Figures. In FIG. 9, the reference numeral 61 denotes the wall of a source space 60 which has the shape of, for example, a cylinder and through which the solid medium 33 is moved. This wall is provided with, for example, a narrow aperture 63 having a diameter of, for example, 2.5 mm, via which the pulsed laser beam 41 can enter the space 60. The generated EUV radiation can leave the source space 60, for example, via this aperture or other apertures (not shown) and enter a space 65. In this space, which is only shown diagrammatically and in which a high vacuum of, e.g., 10xe2x88x924 mbar is maintained by means of the vacuum pump denoted by the reference numeral 35 in FIG. 3, the EUV radiation is guided towards the mask via the mirrors of the illumination system. The space 65 may also be filled with a rare gas such as helium, or with hydrogen at a low pressure of, for example 10xe2x88x921 mbar. A flow 77, 78 of rare gas, for example helium, is introduced into the source space 60 so that the helium flow is parallel to the direction of movement of the tape 33. To this end, the source space has a tube 70 which communicates with a helium outlet, for example, in the form of a tank 73. This tube has a diameter of, for example, 5 mm. A vacuum pump 75 connected to the source space ensures that a continuous flow of helium is maintained and that the helium pressure in the source space will not exceed, for example, 10xe2x88x921 mbar. At this low helium pressure, the generated EUV radiation is not absorbed. The tape 33 is now embedded in a tubular and viscous flow of helium which has a sufficient suction power. As a result, the medium particles are enclosed within the helium column and are taken along by the helium flow and transported out of the source space. The tube 70 ensures that the helium flow is a laminar flow so that the helium gas and the medium particles present therein cannot flow back. Due to the interaction of the tape 33 with the helium flow, the desired flow profile of the helium flow may be disturbed. To prevent this, a second tube 71 connected to the helium tank 73 is preferably arranged in the source space 60, so that a second helium flow 78 is established coaxially with the first flow 77. The flow profile can be restored again by means of the second flow. Instead of helium, another rare gas may be used for draining water vapor and excess water droplets from the source space. An example of another gas is argon having larger molecules than those of helium so that an argon flow has a better suction power than helium. However, argon absorbs more EUV radiation than helium. In the choice of the gas, a compromise must be made between the minimal absorption and the maximal suction power. FIG. 10 is a cross-section of a part of a second embodiment of the radiation source unit in which a rare gas flow is used. This embodiment differs from that in FIG. 9, inter alia, in that the source space 60 has a smaller diameter, for example 5 mm, and consists of three parts. A wall 81 surrounds the lower part. The upper part is closed by the wall 82 of the tube 88 surrounding the tape 33 for the supply of the rare gas. The central part 85 of the source space, at the area of the radiation path of the laser beam 41, communicates with the ambience. At the area of this central part, the walls 81 and 82 are slightly bent outwards so that a so-called ejector configuration, or geometry, is obtained. The combination of the vacuum pump 75 and its specific wall shape at the area of the central part of the source space operates as a so-called ejector pump or jet pump. Such a pump prevents helium or other particles from leaking to the ambience of the source space because it also sucks up possible particles present in this ambience and removes them. The open central part 85 of the source space 60 only needs to have such a height that the converging laser beam 41 can enter the source space in an unhindered way. Helium gas or another rare gas is supplied from a helium inlet 73 between the tape 33 and the wall 82. This helium gas is sucked downwards by the vacuum pump 75 in the form of a laminar flow and takes along the medium particles. Due to the jet pump configuration of the source space, migration of medium particles and loss of helium gas to the high-vacuum space 65 is prevented, and this to a stronger extent than is the case in the embodiment of FIG. 9. In this way, the helium gas pressure in the space 65 may be further reduced. To be able to operate as a jet pump, the straight part of the source space 60 must have a small diameter, for example, 5 mm. Then, the laser beam must be focused substantially on the position where the plasma-forming medium passes. Then there is a greater risk that the beam radiation does not impinge upon a desired pit in this medium, as compared with the case where the laser beam is focused at some distance from said position and hence this beam has a larger diameter at this position. Moreover, when focusing the laser beam on said position, the laser radiation has a large energy density at that position. This possible problem is mitigated by the embodiment shown in FIG. 11. This embodiment also comprises a jet pump. However, the inlet tube 90 now has an annular cross-section, with the width of the ring, for example 1 mm, being considerably smaller than its internal diameter which is, for example, 10 mm. The wall portions 92 and the upper parts of the wall 81 again constitute an ejector configuration. The gas curtain supplied through the tube 90 ensures that the medium particles remain entrapped and are drained. The jet pump configuration ensures that the gas curtain moves downwards at a great velocity and prevents rare gas from leaking to the high-vacuum space 65. Since the jet tube has an annular cross-section, the source space 60 may have a relatively large diameter so that the laser beam can be focused at some distance from the position where the water droplets pass so that the risk of missing a droplet will thus become smaller. The embodiment of FIG. 11 thus combines the advantages of the embodiment of FIG. 10 with those of the embodiment of FIG. 9. For the theoretical background and details about ejector pumps, reference is made to the article xe2x80x9cExit Flow Properties of Annular Jet-Diffluser Ejectorsxe2x80x9d in Journal of the Chinese Society of Mechanical Engineers, Vol. 18, No. 2, pp. 1113-125, 1997. EUV radiation sources may not only be used in lithographic projection apparatuses but also in EUV microscopes having a very high resolving power. The radiation path of the EUV radiation in such a microscope must be in a high vacuum. To prevent the vacuum from being attacked from the radiation source and from contaminating the optical components, the invention and its various described embodiments may be used to great advantage. It has been noted hereinbefore that EUV radiation is also known as soft X-ray radiation because its wavelength is close to that of real X-ray radiation having a wavelength of the order of 1 nm or less. It has also been noted that the wavelength of the radiation generated with the described radiation sources is dependent on, inter alia, the medium used. For generating X-ray radiation, similar radiation sources, with similar problems as for generating EUV radiation may therefore be used. For this reason, the present invention may also be used to great advantage in X-ray sources and this invention also relates to these sources and apparatuses such as X-ray microscopes or X-ray analysis apparatuses, hence the use in the claims of the term extremely short-wave radiation which is understood to be EUV radiation and X-ray radiation.