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 mask 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, 4x, 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 4x. 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 EUV 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 IF 1 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 water droplets or xenon clusters 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 capillary tube 31, one end of which is connected to, for example, a water inlet, for example a water tank 32, and the other end projects into a vacuum space 33. Water is transported at a high pressure through this tube to the vacuum space, which is denoted by arrow 35. The space 33 is connected to a pump 34, for example, a turbo pump having a power of, for example, 1000 dm3/sec, with which the space 33 can be pumped to a vacuum of 10xe2x88x924 mbar. The capillary tube 31 is caused to vibrate, for example, by means of a piezoelectric driver 37. At a given vibration frequency, for example 0.3 MHz, the tube supplies a continuous flow of individual water droplets 39. The droplet formation is based on the principle which is comparable with that used in various ink jet printers for forming ink droplets. The water droplets may not only be formed by means of piezoelectric pulses but also by thermal pulses or by ultrasonic means. At a water flow 35 of, for example, 5.4 ml/hour, the water droplets have a diameter of the order of 20 xcexcm. 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 space 33. 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 is aligned with the centerline of the tube 31 along which the water droplets pass. An absorbing element 55 is arranged behind a second window 44 in the wall of the vacuum space 33, which absorbing element ensures that radiation of the laser beam exiting from the space 33 cannot enter the apparatus of which the radiation source unit forms part. The beam 42 is each time substantially focused on a water droplet which is instantaneously present at the position 46. The radiation spot 45 has a diameter of, for example, 30 xcexcm so that a water droplet is completely irradiated. The driver 37 is synchronized with the laser driver via an electronic circuit 48 comprising a delay element, so that a laser pulse is generated at the instant when a water droplet arrives at the position 46. Due to the laser energy supplied to the water droplet, a plasma 47 in which oxygen ions are present is produced at the location of this droplet. As a result of the extremely high energy density, for example, of the order of 1021 W/m3 of the laser beam at the location of the droplet, this plasma reaches a temperature which corresponds to an energy of the order of 30 eV. At this high temperature, the dominant ionization state of oxygen O is VI. Then, EUV radiation is generated at wavelengths around 11.6 nm and 13 nm. For further details about the way and circumstances in which EUV radiation is formed, reference is made to the article xe2x80x9cLaser produced oxygen plasmasxe2x80x9d in Proceedings of the 2nd Int. Symp. on Heat and Mass Transfer under Plasma conditions, 1999. One or more mirrors 49 for collecting, concentrating and directing the generated EUV radiation may be arranged in the space 33. 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 EUV generation process, not all water droplets are converted into a plasma. On their way through the source space, the droplets give off water vapor. Since the vapor pressure of water at room temperature is approximately 23 mbar, the water vapor and the water droplets which have not been converted must be removed from the source space 33 so as to comply with the vacuum requirement in connection with a free passage of the generated EUV radiation. As described in the article mentioned above, excess water droplets may be drained by introducing them into a second vacuum space 50 via a narrow aperture 51. In the space 50, a relatively low vacuum of, for example, 0.5 mbar is maintained by means of a further vacuum pump 53 having a power of, for example, 70 dm3/sec. By means of this special embodiment of the principle of differential pumping, i.e. separate pumping of two communicating spaces to different degrees of vacuum, a relatively satisfactory vacuum, of the order of 2xc3x9710xe2x88x924 mbar, can in principle be maintained in the source space 33. Moreover, the pumps may be chosen to be such that the remaining water droplets are drained as water vapor. Consequently, the radiation source can be operated continuously and at a constant, rest, pressure level. This pressure level is determined by the vapor pressure of the water droplets moving from the tube to the space 50. However, this vapor pressure is 23 mbar at room temperature so that, without further measures, still so many water molecules are present in the space 33 that EUV radiation coming from the plasma 47 may be absorbed in this space and the intensity of this radiation is reduced. The wall of the vacuum space 33 must be further provided with at least one aperture through which the generated EUV radiation can leave the space 33 so as to enable it to enter the space in which the mirrors of the illumination system, 10, 12 and 14 in FIG. 3, are present. This aperture may be located at the position where the window 43 is situated in FIG. 3. It has been found that, due to the presence of such an aperture, the problem not mentioned in the above-mentioned article xe2x80x9cLaser produced oxygen plasmaxe2x80x9d may occur that water emerging from this aperture may drip on the mirrors of the illumination system and on those of the projection system and may attack these mirrors, thus reducing their reflection. This is an important problem 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 velocity at which substrates can be illuminated. This problem can be eliminated, or at least sufficiently reduced, by making use of a radiation source unit in accordance with the concept of the invention. FIG. 4 shows a cross-section of an embodiment of this radiation source unit. 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. 4, the reference numeral 61 denotes the wall of a source space 60 which has the shape of, for example, a cylinder and in which the water flow 35 is introduced through the tube 31 and in which the water droplets (not shown) propagate. This wall is provided with, for example, narrow apertures 63, 64 having a diameter of, for example, 2.5 mm, via which the pulsed laser beam 41 can enter (63) the space 60, or can leave (64) this space. The generated EUV radiation can leave the source space 60, for example, via these apertures 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 34 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. According to the invention, not only a flow of water droplets but also 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 flow of individual water droplets leaving the tube 31. 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 helium flow 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 water droplets are now encapsulated in a tubular and viscous flow of helium which has a sufficient suction power. As a result, water vapor from the droplets remains enclosed within the helium column and is taken along by the helium flow and transported to the vacuum pump 75. This also applies to water droplets which have not been converted into plasma. The tube 70 ensures that the helium flow is a laminar flow so that the helium gas and the elements of the water present therein cannot flow back. Due to the interaction of the flow of water droplets 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 helium so that an argon flow has a better suction power than helium. However, argon is more absorbing than helium. In the choice of the gas, a compromise must be made between the minimal absorption and the maximal suction power. By suitable choice of the diameter of the tube 70 and the tube 71, if any, and of the pump speed, it can be ensured that the quantity of rare gas which may leak through the apertures 63, 64 into the space 65 is sufficiently small to maintain a helium pressure of at most 0.1 mbar in the space by means of a suitable pump for this space. FIG. 5 is a cross-section of a part of a second embodiment of the radiation source unit according to the invention. This embodiment differs from that in FIG. 4, inter alia, in that the source space 60 has a smaller diameter, for example 5 mm, and consists of three parts. The lower part is surrounded by a wall 81. The upper part is closed by the wall 82 of the tube 88 surrounding the inlet tube 31 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 medium present in this ambience and removes it. 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 63 between the tube 31 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 water vapor and water droplets, if any. Due to the jet pump configuration of the source space, diffusion of water vapor 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. 4. 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 water droplets pass. Then there is a greater risk that the beam radiation does not impinge upon a desired water droplet, 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. When irradiating a water droplet and forming a plasma, matter particles, such as highly energetic ions and radicals, may be released, while the number of these particles increases with an increasing density of the laser energy. These particles may also get into the high-vacuum space 65 and reach the mirrors of the illumination system and of the projection system, where they attack the mirror coatings and reduce their reflection. These possible problems are mitigated by the embodiment shown in FIG. 6. 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 both the water vapor of the water droplets and the water droplets which have not been converted into a plasma as well as the energy-rich ions and radicals from a plasma remain enclosed and are drained to the pump 75. The jet pump configuration ensures that the gas curtain moves downwards at a great velocity and prevents rare gas particles 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. Moreover, the energy density of the laser radiation in a droplet is considerably smaller in the embodiment of FIG. 5, so that the number of energy-rich ions and radicals repelled by the plasma is smaller. The embodiment of FIG. 6 thus combines the advantages of the embodiment of FIG. 5 with those of the embodiment of FIG. 4. For the theoretical background and details about ejector pumps, reference is made to the article xe2x80x9cExit Flow Properties of Annular Jet-Diffuser Ejectorsxe2x80x9d in Journal of the Chinese Society of Mechanical Engineers, Vol. 18, No. 2, pp. 1113-125, 1997. The fact that water droplets are used as a medium for forming the plasma in the embodiment described hereinbefore does not mean that the invention is limited thereto. As has been described in the article xe2x80x9cDebris elimination in a droplet target laser plasma soft X-ray sourcexe2x80x9d in Rev., Sci. Instruments 66 (10), October 1995, pp. 4916-4920, ethanol droplets may be alternatively used as a medium for forming a plasma emitting EUV radiation. Similar problems as with water droplets occur, which problems can be solved by using the invention. Water and ethanol are only two examples of possible liquid media which, if irradiated with a high-power pulsed laser, form an EUV radiation emitting plasma and can be used in a laser-generated plasma EUV radiation source. Generally, the invention can be used in all EUV radiation sources in which a liquid medium is converted by a high-power pulsed laser into an EUV radiation-emitting plasma and in which the problems occur that the medium raises the (vapor) pressure in the source space and the plasma formed repels contaminating particles which may penetrate the high-vacuum space and reduce the reflection of the mirrors which are present in this space. Gaseous media instead of liquid media may be alternatively used in an EUV radiation source. For several years, theories have been developed about and experiments have been carried out on the interaction between high laser and xenon clusters for creating a plasma which emits EUV radiation to a sufficient extent, as was recently reported from different research centers at the OSA conference on applications of high field and short-wavelength sources VIII (1999). However, xenon gas absorbs EUV radiation to a great extent so that, without further measures, the EUV radiation output of a xenon plasma source would be too small to operate projection lithography with such a source. By enclosing the xenon gas and draining it with a flow or a curtain of rare gas such as helium according to the invention, it can be achieved that absorption of the generated EUV radiation is reduced considerably. The embodiments shown in FIGS. 4, 5 and 6 may be used, in which xenon clusters are supplied via a tube which is similar to the tube 31 in the Figures, which tube is then not caused to vibrate. As regards their physical state, xenon clusters occupy a position between molecules and a solid material. Such clusters can be injected into a source space by means of a pulsed valve having an aperture diameter of, for example, 2 mm. If such a cluster in this space is excited by laser pulses of, for example, a Kr-F excimer laser, which laser pulses have a very short pulse duration of, for example, 0.35 p/sec and a power of, for example, 20 mjoule, then there will be a strong ionization of the cluster and it will emit EUV radiation at a wavelength of the order of 11 nm. The strong absorption of the EUV radiation formed can be prevented and the collection and drainage of energy-rich ions can be ensured by making use of the present invention and the various embodiments described. The invention may also be used in an EUV radiation source unit in which a tape or wire of a metal is used as a medium. The problem when using a metal medium is that, when it is impinged upon by a laser beam to obtain the desired plasma, the metal is locally caused to explode so that metal particles are released. If these particles get outside the source space, they will have a destructive effect on the optical components of the apparatus incorporating the radiation source unit. The present invention and its various embodiments provide an eminent possibility of preventing this. 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 an X-ray microscope or an X-ray analysis apparatus, hence the use in the claims of the term extremely short-wave radiation which is understood to be EUV radiation and X-ray radiation.