`NATIONAL INSTITUTE
`FOR 'NDUSTR'AL PROPERTY
`PARIS
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`(11) l=T":.ible?llis<(3:da°tri‘;)vIV:el’1\lo.:
`°“'°”"“°°"'°s’
`(21) National Registration No.:
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`2 554 302
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`34 15445
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`(51) int. or‘: H 05 H 1/46.
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`(12)
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`PATENT APPLICATION
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`A1
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`(22) Filing date: 26th October 1984.
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`1“ November 1983
`(30) Priority: DD,
`No. WP H 05 H/256 179.
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`(71) App|icant(s): Company named: VEB
`CARL ZE/SS JENA, Company under
`German law. ~— DD.
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`(72) lnventor(s): Walther Gartner, Wolfgang
`Retschke and Klaus Gunther.
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`(43) Date of publication of application:
`B.O.P.l. ~ "Patents" No. 18 of 3'“_ May 1985.
`
`(73) Proprietor(s):
`
`(60) References to other related national
`documents:
`
`(74) Agent(s): Cabinet Madeuf, industrial
`property consultants.
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`(54) Radiation source for optical devices, notably for photolithographic reproduction systems.
`
`(57) Radiation source for optical devices, notably
`for photolithographic reproduction systems,
`characterised in that a gas—tight chamber 1 filled
`with a discharge medium 2 comprises at least one
`entry aperture 3 and 4 which allows laser
`radiation to pass and at least one exit aperture 5
`which allows plasma radiation to pass and in that
`the production and maintenance of a radiation-
`emitting plasma in the discharge medium are
`ensured, in a known manner, by at least one laser
`situated outside the cham ber 1, whereby optical
`means ensuring the focussing of the laser
`radiation in the discharge medium are mounted at
`an entry aperture, such that the plasma is situated
`at a certain distance from the wall of the chamber
`1 and that the plasma radiation exits the chamber
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`via exit aperture 5.
`
`D
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`Printed copies available for sale from the IMPRIMERIE NATIONALE (French National Press) — 75732 PARIS CEDEX 15
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`ASML 1304
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`FR2554302—A1
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`1
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`2554302
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`The present invention relates to a radiation source for optical devices, in particular for
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`photolithographic reproduction systems. It is preferably applied in cases where a radiated power .
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`is required which is greater than that from pressurised mercury vapour lamps, such as in
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`photolithographic appliances for illuminating a photoresist layer on a semiconductor wafer.
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`Currently, numerous radiation source systems are known which are used in scientific devices and
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`of which the properties have been widely adapted to the conditions in the field of use. These
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`properties relate to the spectral distribution of the emission and to the obtainable radiation
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`density, as well as to the spatial and angular distribution of theproduced radiation. Requirements
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`relating to spectral radiated powers which exceed the spectral radiated power of a black body
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`above the melting point of solid bodies can only be satisfied through plasma. Plasmas are
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`obtained by heating an active medium, preferably by passing an electric current through it or by
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`the action of high-frequency electromagnetic fields. The achievable spectral radiation densities
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`are upwardly limited by the maximum value of the harnessable electrical power per volume unit
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`which can be thermally withstood by the constitutent materials of the electrodes and walls. in the
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`case of high-frequency heating, limitation due to electrode loading no longer occurs, but the
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`problem of the spatial concentration of the high-frequency energy does arise.
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`if the stationary operation of the radiation source is dispensed with, an increase, by a fairly large
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`order of magnitude, in the power harnessed can be obtained for a short time, since the
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`conversion of the fed-in power into radiation proceeds significantly faster than its transmission to
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`the walls and, if there are any, to the electrodes of the discharge cavity. However, even with this 7
`mode of operation, alongside mechanical stresses due to the shock waves which, however, have
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`sufficient action only in unfavourable cases, the evaporation and erosion of the materials which
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`form the walls and electrodes contistute, when the radiation source must have a certain lifespan,
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`an impediment to the production of intense radiant flux. In this regard, it should be noted that in
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`the case of sources which operate in a stationary manner and in the case of sources which
`operate by pulses, above a power level which is type—dependent and which is achieved
`practically universally in the technical applications, any further increase in the radiated power is
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`obtained at the expense of a reduction in the lifespan.
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`However, these short-lived radiation sources cannot be used for many applications because they
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`unreasonably increase the maintenance costs for the devices into which they are incorporated,
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`since changing a lamp generally entails complicated adjustment and long adaptation operations
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`2554302
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`of the optical transmission system to the specific radiant flux of the lamp in question. Within
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`certain limits, it is possible to increase the radiated power whilst retaining the overall charge of
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`the electrical energy invested in the radiation, for the desired wavelength and the preferred
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`spread width. This can be achieved by giving the active medium an optimal composition and by
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`creating optimal pressure and temperature conditions for the plasma during the production of the
`radiation. However, consideration should be given to the limitations which arise from the existing
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`‘incompatibility, at working temperature, between various active media and the consituent
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`materials of the electrodes and the walls, such that, taking into account the withstand time of
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`these materials, discharge conditions which are far from optimal frequently have to be selected.
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`In the case of non-stationary operation, further limitations result from the fact that the radiation
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`source simultaneously has to fulfil the functions of an electrical heavy~duty switch and of a
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`converter of electrical energy into radiation. In this case too, the scope for optimising the radiation
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`production is restricted, because the safety of ignition and switching is linked to certain plasma
`states.
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`In the case of the stationary operation and in the case of pulsed operation, there are, in electrode
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`radiation devices, dead solid angles in which the radiation cannot be used, although the insertion
`of suitable optical components, such as ellipsoidal reflectors and/or light-conducting fibres
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`theoretically make it possible to also use the areas formed by these angles and, as a result, to
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`provide the maximum amount of radiation energy to the optical system. To illuminate optical
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`systems used in photolithography microinstallations, lasers are also used as radiation sources
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`(SPIE Vol. 174 [1979], p.28...36, “Coherent illumination improves step—and~repeat printing on
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`wafers” [Un éc/airage cohérent ame’//ore /’impression ”gradue/Ie et répétée” sur/es galettes’7, by
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`Michel Lacombat et al.) The main limitations of these light sources result from their high spatial
`"coherence and the structural distortions which result therefrom, their high monochromy and the
`effects of the resulting standing waves in photosensitive materials. Furthermore, generally, lasers
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`with high radiated power or favourable efficiency are generally not present in advantageous
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`spectral areas. The use of “excimer” lasers which emit the necessary energy in the desired
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`wavelength region (UV region) are limited to contact—lithographic methods (SPIE Vol. 334 [1982],
`p.259...262, “Ultrafast high resolution contact lithography using excimer laser" [“Lithographie par
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`contact é forte résolution ultrarapide au moyen de laser eXcimer”], by K. Jain et al.), because the
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`partial spatial coherence necessary for the illumination of projection-lithography systems cannot
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`be achieved to a degree as justified by its technical use.
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`2554302
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`The aim of the invention is to achieve a highly powerful radiation source which has a long
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`lifespan and which makes it possible to include a substantial area of solid angles and precise
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`and fast illumination of photosensitive areas and which, as a result, ensures a high productivity
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`in photolithographic installations. Therefore the invention is intended to make it possible to
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`achieve a radiation source for optical devices, in particular for photolithographic reproduction
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`systems, which uses plasma radiation. By a spatial separation between the plasma and the wall
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`or other installations associated with a cavity and without use of electrodes mounted in the
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`cavity nor high-frequency fields for spatial concentration of the energy, it must make it possible
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`to obtain a long lifespan and high power density. Furthermore, there is a reduction of stresses
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`on the cavity though shock waves when the radiation source is in pulsed operation, and there
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`are no dead solid angles due to electrodes or other installations in the cavity. The radiation
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`source according to the invention is intended to possess a wide scope for optimisation of the
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`radiation production in the desired wavelength region, because the active media and pressure
`and temperature conditions must be selected regardless of the compatibility with the materials
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`which the electrodes are made of. With regard to the laser radiation, the radiation source has
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`the advantage that, especially in the case of photolithographic reproduction systems, it has a
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`significant partial spatial cohere.nce and that its spectral structure is such that the effects of
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`standing waves in the photosensitive material are attenuated.
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`This aim is achieved, according to the invention, by the fact that a gas-tight chamber filled with a
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`discharge medium contains at least one entry aperture which allows laser radiation to pass and at
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`least one exit aperture which allows plasma radiation to pass, and that the production and
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`maintenance of a radiation—emitting plasma in the discharge medium are ensured, in a known
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`manner, by at least one laser situated outside the chamber, whereby optical means for focussing
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`the laser radiation in the discharge medium are mounted at an entry aperture, such that the
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`plasma is at a certain distance from the wall of the chamber and that the plasma radiation exits
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`the chamber via the exit aperture.
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`When the radiated power of a laser as supplied is not sufficient for a discharge in the discharge
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`medium, it is advantageous that the device includes, to ignite the discharge medium, outside the
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`chamber, at least one further pulse—operated laser which is directed by optical means to ensure
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`focussing of the same volume at an entry aperture.
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`An advantageous variant, with regard to changing of position of the radiation-emitting plasma,
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`2554302
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`consists in placing the optical means which ensure the focussing of the laser radiation outside
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`the chamber. It is then possible to advantageously arrange installations which make it possible
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`to adjust theioptical means which ensure the focussing of the laser radiation.
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`It is possible to advantageously simplify the realisation of the radiation source by placing optical
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`means which ensure the focussing of the laser radiation inside and/or on the surface of the
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`chamber. in these conditions, the inner wall of the chamber constitutes an optical means for
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`focussing the radiation coming from outside. To include as large an area of dead solid angles as
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`possible, it is advantageous to give the inner wall of the chamber a shape such that it
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`constitutes an optical means for ensuring the reflection of the radiation coming from the plasma.
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`It is therefore advantageous for the inner wall of the chamber to have the shape of a convex
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`mirror or an ellipsoidal mirror.
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`To obtain high power densities and to increase the lifespan, it is advantageous to provide the
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`chamber with an external cooling system.
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`Various other characteristics of the invention further emerge from the following detailed
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`description.
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`Embodiments of the subject of the invention are shown, by way of non-limiting examples, in the
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`attached drawings.
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`‘Fig. 1 schematically shows an embodiment of the radiation source according to the invention.
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`Fig. 2 shows an exemplary embodiment in which the inner wall of the chamber has a shape
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`such that it constitutes an optical element.
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`Fig. 3 and 4 show embodiments wherein the discharge chamber has the shape of an
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`ellipsoidal reflector.
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`Fig. 1 schematically shows an embodiment of the radiation source according to the invention in
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`which a gas—tight chamber 1 contains the discharge medium 2. The chamber 1 includes two
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`entry apertures 3 and 4 which allows laser radiation to pass and an exit aperture 5 which allows
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`plasma radiation to pass. The entry aperture 3 is sealed by thewindow 6 which allows infrared
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`2554302
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`to pass, and the entry aperture 4 is sealed by the lens 7 which allows ultraviolet to pass. The
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`exit aperture 5 is provided with a window 8. The device includes two lasers 9 and 10 outside the
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`chamber 1. The coherent radiation 11 from the laser 9, which is a stationary CO2 gas laser,
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`penetrates into the chamber 1 through the window 6 and is focussed by the concave mirror 12
`mounted on the wall of the chamber. The radiation 13 from the laser 10, which is a nitrogen
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`pulse laser, is focussed on the same point by the lens 7 which allows ultraviolet to pass and
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`produces an electrical discharge there, and as a result an absorbent plasma 14 which is heated
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`to high temperatures under the influence of the radiation 11. The radiation 15 from the plasma
`can be fed into the downstream optical system through the window 8.
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`If theradiation source is meant to be pu|se—operated, the continuous laser 9 is replaced by a
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`pulsed CO2 carbon dioxide laser. As a rule, it is possible to dispense with the pulsed laser 10,
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`because the field strength of the pulsed CO2 carbon dioxide laser is in many cases sufficient to
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`bring about the discharge. With such a device, it is possible to obtain, near—ellipsoida| plasmas
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`from 4 mm to 5 mm in diameter up to a temperature of 16000 K, for example in an argon or
`xenon atmosphere as active medium with a working pressure of 106 Pa. The optical depth and
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`the temperature can be varied within a vast range by altering the pressure. As the pressure
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`increases, the temperature falls and the spectral distribution approaches Planck’s function. As
`pressure decreases, the temperature increases, and the emission becomes linear.
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`Temperatures far in excess of 20000 K can be reached by using, as active medium, helium
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`which in conventional pulsed light sources, operating electrically, can no longer be used
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`practically due to the heavy wear and tear on the electrodes. In these conditions, the density of
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`radiation and its spectral distribution can be altered in a much wider range than in the case of
`conventional radiation sources.
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`Figure 2 shows an embodiment in which the inner wall of the chamber constitutes, by its shape,
`an optical element. A casing 16, the concave mirror 17 and the quartz window 18 constitute the
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`gas-tight chamber containing the discharge medium 19. The coherent radiation 20 from a
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`pulsed CO2 carbon dioxide laser 21 is focussed by the lens 22 which lets infrared pass and
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`penetrates the chamber via the window 23 which allows infrared to pass. The pulsed laser 21 is
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`mounted displaceably in the X direction, 24, and in the Y direction, 25, and the lens for infrared
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`22 can be displaced in the X direction, 24, and in the Y direction, 25, andin the Z direction, 26.
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`Accordingly, the position of the focal point, which corresponds to the position of the plasma 27,
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`may be adjusted relative to the optical axis 28. The plasma radiation 27 is sent directly, and by
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`2554302
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`means of the concave mirror 17, through the quartz window 18 to the condenser lens 29 of the
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`optical system placed downstream.
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`The gas—tight chamber is surrounded by a container 30. The free space 31 which they demarcate
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`is traversed by a refrigerating means 32 which enters, via the tube 33, and exits via the tube 34 ~
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`and evacuates the heat produced by the pulsed laser radiation 21 and plasma radiation 27. It is
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`possible to dispense with the quartz window 18 if the condenser lens 29 is installed instead.
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`Figs. 3 and 4 show embodiments wherein the discharge chambers 35 and 36 are constituted by
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`ellipsoidal reflectors. The radiation 37 from the carbon dioxide (CO2) laser 38 is focussedby the
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`focussing elements, a concave mirror 39 or a lens 40 which allows infrared to pass, onto focal
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`‘ points 41 and 42 of the ellipsoid formed by the reflecting layers of the ellipsoidal mirror 43 and 44.
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`The light emitted by the plasma producing the radiation is concentrated by the ellipsoidal mirror
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`onto the second focal point 45 or 46 of the ellipsoid. The plasma formed at these focal points 45,
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`46 serves as a source of secondary radiation for the optical system situated downstream and
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`starting at the condenser lenses 47, 48.
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`'
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`
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`2554302
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`Claims
`
`A radiation source for optical devices, in particular for photolithographic reproduction
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`systems, characterised in that a gas—tight chamber (1) filled with a discharge medium (2)
`
`contains at least one entry aperture (3 and 4) which allows laser radiation to pass and at
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`least oneexit aperture (5) which allows plasma radiation to pass, and that the production
`
`and maintenance of a radiation—emitting plasma in the discharge medium are ensured, in a
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`known manner, by at least one laser situated outside the chamber (1), whereby optical
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`means for focussing the laser radiation in the discharge medium are mounted at an entry
`
`aperture, such that the plasma is at a certain distance from the wall of the chamber (1) and
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`that the plasma radiation exits the chamber via the exit aperture (5).
`
`The radiation source according to claim 1, characterised in that the ignition of the
`discharge medium is ensured outside the chamber (1) by at least one further pulse-
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`operated laser (10) which is directed by optical means (7) to focus it on the same volume
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`after passing in an entry aperture (4).
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`The radiation source according to one of claims 1 or 2, characterised in that the optical
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`means (22) which ensure the focussing of the laser radiation (21) are situated outside the
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`chamber (19).
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`The radiation source according to claim 3, characterised in that the installation
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`includes devices for adjusting the optical means which ensure the focussing of the
`laser radiation.
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`The radiation source according to one of claims 1 or 2, characterised in that optical
`
`means which ensure the focussing of the laser radiation are placed inside and/or on the
`wall of the chamber.
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`The radiation source according to claim 5, characterised in that the inner wall of the
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`chamber has a shape such that it constitutes an optical means for focussing the
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`laser radiation coming from outside.
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`The radiation source according to claim 1, characterised in that the inner wall of the
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`2554302
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`chamber has a shape such that it constitutes an optical means for reflecting the radiation
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`emitted by the plasma.
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`The radiation source according to claim 7, characterised in that the inner wall of the
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`chamber partially has the shape of a concave mirror or an ellipsoidal mirror (43, 44).
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`The radiation source according to claim 1, characterised in that the chamber is equipped
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`with an external cooling system (31, 32, 33, 34).
`
`
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`FR 2 554 302
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`I, James McGiH, of Murgitroyd & Company, Scotland House, 165-169 Scotland Street,
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`Glasgow G5 8PL, hereby declare that I am the translator of the document attached and
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`certify that the following is a true translation to the best of my knowledge and belief.
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`M25,/‘M (Wm
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`Dated this 15th of December 2014
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`............. ..
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`’.
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`
`
`
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`...........o
`REPUBLIOUE FRANCAISE
`
`msmur NATIONAL
`DE LA PnoPmé'ré INDUSTRIELLE
`
`PARIS
`
`_ @ N" de publication :
`lé rfutilisar que pour les
`commandos do reproduction)
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`2 554 302
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`® N’ d'enregistrement national :
`
`84 16445
`
`@ um cl‘ : H 05 H 1/46.
`
`@
`
`DEMANDE DE BREVET D'lNVEI\ITIOl\l
`
`A1
`
`@ Damandeurisl : Entreprise dite: VEB CARL ZEI
`J1.-‘NA, Entreprise de droit al/emand. -- DD.
`'
`
`@ lnventeurisi : Walter G'a'rtner, Wolfgang Retschke et
`Klaus Gunther.
`.
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`Titulairelsl :
`
`i @
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`Mandataire(s): Cabinet Madeuf. Consells en proprlété
`industrielle.
`-
`'
`
`
`
`@ Date de dépiit : 26 octobre 1984.
`’
`1°’ novembre
`
`@ Priorité: DD,
`n° WP H 05 H/256179.
`
`1 983,
`
`@ Date de la misaé disposition du public de la
`demanda : BOPl « Brevets» n° 18 du 3 mai 1985.
`
`Référances fa d'autres documents nationaux appa-
`rentés:
`'
`
`® Source de rayonnement pour appareils d’optique, notamment pour systémes do reproduction par photolithographie.
`
`@ Source de rayunnement pour appareils dbpfique, notam-
`ment pour systémes de reproduction photolithographique, ca-
`ractérisée en ce qu‘une enceinte 1 étanche aux gaz remplia
`par un milieu de décharge 2 comporta au moins une ouvertura
`d’entrée 3 et 4 laissant passer'un rayonnement laser at au
`moins une ouverture do sortie 5 Iaissant passer un rayonne~
`ment de plasma et en ce que la production et l'entretien cl'un
`plasma émettant un ravonnement dans le milieu de décharge
`sont assurés, d'une maniére connue. par au moins un laser
`situé a |’extérieur de l'enceinte_ 1. des moyens optiques assu-
`rant
`Ia focalisation du rayonnement
`laser dans la milieu de
`décharge étant montés au niveau d’une ouvarture d'entrée, de
`sorta que la plasma se trouve 3 une certalne distance de la
`parolde l'enceinte ‘l at qua la rayonnement du plasma sort do
`Venceinte par l'ouverture de sortie 5.
`
`
`
`Vents des faacicules 3 VIMPRIMERIE NATIONALE. 27, rue de 18 Convention -— 75732 PARIS CEDEX 15
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`9 F
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`R2554302
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`2554302
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`La présente invention est relative a une source
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`de rayonnement pour appareils d'optique, notamment pour
`systémes de reproduction par photolithographie. Elle s'ap—
`plique de préférence dans les cas ot il faut une puissance
`de rayonnement supérieure a celle des lampes 5 vapeur de
`mercure sous pression, par exemple dans les installations
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`de photolithographie, pour l'éc1airement d'une couche
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`de vernis photo sur une plaque de semi—conducteur..
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`_
`On connait actuellement de nombreux systémes do‘
`sources de rayonnement qui sont utilisés dans des appareils
`scientifiques et dont les propriétés ont été largement
`adaptées aux conditions inhérentes au domaine d'utilisation.
`Ces propriétés sont relatives A la répartition spectrale
`de l'émission et a la densité do rayonnement susceptible
`d'étre obtenue ainsi qu'a la répartition spatiale et
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`angulaire du rayonnement produit. Les exigences relatives
`a des puissances de rayonnement dépassant la puissance
`de rayonnement spectrale d'un corps noir aurdessus du
`point de fusion des corps solides ne peuvent étre satis-
`faites que par du plasma. Les plasmas s'obtiennent par
`chauffage d'un milieu actif, de préférence par passage
`d'un courant électrique ou par action de champs électro—
`magnétiques de haute fréquence. Les densités de rayonnement
`spectrales susceptibles d'étre atteintes sont limitées
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`vers le haut par la valeur maximale de la puissance élec—
`trique, pouvant étre mise en jéu par unité de volume, A
`laquelle les matériaux constituant les électrodes et les
`parois peuvent résister thermiquement. Dans le cas du
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`chauffage a haute fréquence, il n'y a plus de limitation
`due a la charge des électrodes, mais le problems qui se
`pose alors est celui de la concentration spatiale de
`l'énergie fie haute fréquence.
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`Si 1'on renonce a un fonctionnement stationnaire
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`de la source de rayonnement, on peut obtenir, pendant un
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`temps court, une augmentation, d'un ordre de grandeur assez
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`important, de la puissance mise en jeu, du fait que la
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`transformation en rayonnement de la puissance fournie
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`s'effectue beaucoup plus rapidement que sa transmission
`aux parois et, s'i1 y en a, aux électrodes de la cavité
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`de décharge. Cependant,’méme avec ce mode de fonctionnement,
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`a cbté des charges mécaniques dues aux ondes de choc
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`qui, cependant, n'ont une action suffisante que dans
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`des cas défavorables, la vaporisation efi 1'érosion des'
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`matériaux qui forment les parois et les électrodes cons-
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`tituent, lorsque la source de rayonnement doit avoir une
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`certaine durée de vie, un obstacle a la production de
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`I1 y a lieu de remarquer
`flux de rayonnement intense.
`a ce sujet que, dans le cas de sources ayant un fonction~
`nement stationnaire comme dans le cas de sources ayant
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`un fonctionnement par impulsions, au-dessus»d'un niveau
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`de puissance qui dépend du type adopté et qui, dans les
`applications techniques, est pratiquement atteint partout,
`toute augmentation supplémentaire de la puissance de
`rayonnement s'obtient aux dépens de la diminution de
`la durée de vie.
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`Cefiendant, ces sources de rayonnement de courte
`durée sont inutilisables pour beaucoup d‘app1ications, car
`elles augmentent d'une maniére inadissible les frais
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`d'entretien des appareils auxquels elles sont incorporées,
`du fait que le remplacement d'une lampe entraine géné~
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`ralement un réglage compliqué et de longues opérations
`d'adaptation du systéme optique de transmission au flux
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`de rayonnement spécifique de la lumps an question. On
`peut, entre certaines lifiites, augmenter la puissance de
`rayonnemenu tout en conservant la charge totale de 1'éner-
`gie électxique investie dans le rayonnement, pour la
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`longueur d'onde voulue et la largeur d‘éta1ement préférée.
`
`On peut y parvenir en donnant au milieu actif,une compo-
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`sition optimale et en réalisant des conditions de pres-
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`sion et de température optimales pour le plasma loxs de
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`I1 y a lieu cependant de_
`la production du rayonnement.
`tenir compte de limitations qui.décou1ent de l'incompa-
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`tibilité existant, a la température de fonctionnement,
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`entre différents milieux actifs et les matériaux qui'
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`constituent les électrodes et les parois, de sorte que,
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`compte tenu de la durée de résistance de ces matériaux,
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`les conditions de décharge doivent étre choisies souvent
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`de telle maniere qu'elles s'écartent sensiblement des
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`valeurs optimales. D'autres limitations résultent, dans
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`le cas d'un fonctionnement non stationnaire, du feit
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`que la source de rayonnement doit remplir en meme temps
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`les fonctions de commutateur électrique é grande puissance
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`et de transformateur d'énergie électrique en rayonnement.
`Dans ce cas également,
`le jeu pour 1'optimisation de la
`production d'un rafonnement efficace se trouve limité,
`’ car la sécurité de 1'allumage et de la commutation est
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`I
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`liée é certains états du plasma.
`Dans le cas du fonctionnement stationnaire comme
`
`dans le cas du fonctionnement par impulsions, il y a,
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`dans les appareils de rayonnement a électrodes, des angles
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`solides morts dans lesquels le rayonnement ne peut pas
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`étre utilisé bien que l'insertion d'éléments_optiques
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`convenables, comme, par exemple, des réflecteurs ellip-
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`soidaux et/ou des fibres conductrices de la lumiere,
`permette
`théoriquement d'utiliser également les zones
`formées par ces angles et, de ce fait, de fournir au
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`systeme optique le maximum d'énergie de rayonnement. Pour
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`l'éclairement des systémes optiques utilisés dans les
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`micro-installations de photolithographie, on utilise
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`également, comme sources de rayonnement, des lasers (SPIE
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`Vol. 174 (1979) p. 28 ... 36 "Un éclairage coherent
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`améliore l‘impression "graduelle et répétée" sur les
`galettes" par Michel Lacombat et autres). Les priucipales
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`30
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`limitations de ces sources lumineuses résultent de leur
`grande cohérence spatiale et des distorsions de structure
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`qui en résultent, de leur forte monochromie et des effets
`d'ondes stationnaires qui en résultent dans les matériels
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`sensibles a
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`la lumiére. De plus, en général, dans les
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`zones du spectre avantageuses, il n’y a pas de laser
`ayant une orande puissance de rayonnement ou un rendement
`d'efficacité favorable. L'utilisation de 1aS€rS"excimer",
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`qui émettent l'énergie nécessaire dans le domaineude
`longueurs d'ondes voulu (domaine ultraviolet),
`se limite
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`a des procédés de lithographie par contact
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`(SPIE Vol. 334
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`(1982) p 259 ... 262 "Lithographie par contact a forte
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`résolution ultrarapide au moyen de laser excimer" par
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`K. Jain et autres), car la cohérence partielle spatiale
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`nécessaire a l'éclairement des systémes de lithographie
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`par projection ne peut pas étre réalisée a un degré tel
`que son utilisation technique se justifie.
`Le but de l'invention est la realisation d'une
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`source de rayonnement de grande puissance qui ait une
`longue durée de vie et permette l'inc1usion d'une zone
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`importante d'ang1es solides et un éclairement précis et
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`rapide de zones photosensibles et qui, de ce fait, assure
`a des installations de photolithographie une grande
`productivité. L'invention doit donc permettre de réaliser
`une source de rayonnement pour appareils d'optique, notam-
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`ment pour systemes de reproduction photolithographiques,
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`qui utilise_le rayonnement d'uh plasma, Par une séparation
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`spatiale entre le plasma et la paroi ou d'autres insta1~
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`lations associées_a une cavité et sans utilisation
`d'électrodes montées dans la cavité ni de champs de haute
`fréquence pour la concentration spatiale de l'énergie,
`elle doit permettre d'obtenir une longue durée de vie
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`et une densité de puissance élevée. De plus, il y a dimi-
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`nution des charges imposées a la cavité par les ondes
`de choc en cas de fonctionnement par impulsions de la
`source de rayonnement et il n’y a pas de zoues d'angles
`solides morts dues a des électrodes ou a d'autres instal-
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`lations montées dans la cavité. La source de rayonnement
`suivant l'invention doit comporter un.jeu large pour
`1'optimation de la production du'rayonnement dans le
`domaine de longueurs d'onde voulu, car 1e choix des
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`milieux aetifs et des conditions de pression et de tempé~
`rature doit se faire indépendamment'de la compatibilité '
`avec les matériaux constituent les électrodes. En ce
`
`la source de rayon~
`qui concerne le rayonnement laser,
`nement présente 1'avantage que, notamment dans le cas
`des systémes de reproduction photolithographiques, elle
`présente une cohesion partielle spatiale notable et que
`ea structure spectrale est telle que les effets d'ondes
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`stationnaires dans le matériel photosensible sont atténués.
`
`Ce but est atteint, suivant 1'invention;du fait
`qu'une enceinte étanche aux gaz remplie par un milieu
`de décharge comporte au moins une ouverture d'entrée lais~
`sant passer un rayonnement laser et au moins une ouver—
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`ture de sortie laissant passer un rayonnement de plasma
`et que la production et le maintien d'un p1asma’émettant
`
`un rayonnement dans le milieu de décharge sont assurés,
`d'une maniere connue, par un laser au moins situé 3
`1'extérieur de l'enceinte, des moyens optiques assurant
`la foealisation du rayonnement laser dans le milieu.de
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`décharge étant montés au niveau d'une ouverture d'entrée,
`de sorte que le plasma se trouve 3 une certaine distance
`
`de la paroi de l'enceinte et que le rayonnement de
`plasma sort de l'enceinte par l'ouverture de sortie.
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`Lorsque la puissance de rayonnement d'un laser
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`telle qu'e11e est fournie n'est pas suffisante pour une
`décharge dans le milieu de décharge, il est avantageux
`que l'appareil comporte, pour l'allumage du milieu de
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`décharge, a l’extérieur de l'enceinte, au moins un autre
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`laser fonctionnant par impulsions qui est dirigé par des
`moyens optiques pour assurer la focalisation, au niveau
`d'une ouverture d'entrée, du meme volume.
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`Une variante avahtageuse,en ce qui concerne le
`changement.de position du plasma émettant le rayonnement,
`consiste a placer les moyens optiques assurant la foca-
`lisation du rayonnement laser a 1'extérieur de 1'enceinte;
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`On peut alors disposer avantageusement des installations
`permettant le réglage des moyens optiques assuraht la
`focalisation du rayonnement laser.‘
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`On peut simplifier avantageusement la réalisation
`de la source de rayonnement en-placant les moyens optiques
`assurant la focalisation du rayonnement laser a 1'intérieur‘
`et/ou A la surface de 1'enceinte. Dans ces conditions,
`la paroi intérieure de 1'enceinte'constitue un moyen
`optique assurant la focalisation an rayonnement provenant
`de 1'extérieur. Pour_inc1ure une zone d'angles solides
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`morts aussi grande que possible, il est avantageux de
`donner a la paroi intérieure de l’enceinte une forme telle
`qu'e11e constitue un moyen optique assurant la réflexion
`du rayonnement provenant du plasma. Il est alors avantageux
`que la paroi intérieure de 1'enceinte ait la forme d'un
`
`miroir convexe ou d'un miroir ellipsoidal.
`
`Il est avantageu