(19) FRENCH REPUBLIC
`
`NATIONAL INSTITUTE
`FOR INDUSTRIAL PROPERTY
`
`PARIS
`
`(11) Publication No.:
`(To be used only when
`ordering copies)
`
`(21) National Registration No.:
`
`2 554 302
`
`84 16445
`
`(51) Int. Cl4: H 05 H 1/46.
`
`(12)
`
`(22) Filing date: 26th October 1984.
`
`(30) Priority: DD, 1st November 1983
`No. WP H 05 H/256 179.
`
`PATENT APPLICATION
`(71) Applicant(s): Company named: VEB
`CARL ZEISS JENA, Company under
`German law. — DD.
`
`A1
`
`(72) Inventor(s): Walther Gärtner, Wolfgang
`Retschke and Klaus Günther.
`
`(43) Date of publication of application:
`B.O.P.I. - "Patents" No. 18 of 3rd May 1985.
`
`(73) Proprietor(s):
`
`(60) References to other related national
`documents:
`
`(74) Agent(s): Cabinet Madeuf, industrial
`property consultants.
`
`(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 chamber 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
`via exit aperture 5.
`
`D
`
` Printed copies available for sale from the IMPRIMERIE NATIONALE (French National Press) – 75732 PARIS CEDEX 15
`
`FR 2 554 302 –A1
`
`ASML 1504
`
`

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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
`photolithographic reproduction systems. It is preferably applied in cases where a radiated power
`is required which is greater than that from pressurised mercury vapour lamps, such as in
`photolithographic appliances for illuminating a photoresist layer on a semiconductor wafer.
`
`Currently, numerous radiation source systems are known which are used in scientific devices and
`of which the properties have been widely adapted to the conditions in the field of use. These
`properties relate to the spectral distribution of the emission and to the obtainable radiation
`density, as well as to the spatial and angular distribution of the produced radiation. Requirements
`relating to spectral radiated powers which exceed the spectral radiated power of a black body
`above the melting point of solid bodies can only be satisfied through plasma. Plasmas are
`obtained by heating an active medium, preferably by passing an electric current through it or by
`the action of high-frequency electromagnetic fields. The achievable spectral radiation densities
`are upwardly limited by the maximum value of the harnessable electrical power per volume unit
`which can be thermally withstood by the constitutent materials of the electrodes and walls. In the
`case of high-frequency heating, limitation due to electrode loading no longer occurs, but the
`problem of the spatial concentration of the high-frequency energy does arise.
`
`If the stationary operation of the radiation source is dispensed with, an increase, by a fairly large
`order of magnitude, in the power harnessed can be obtained for a short time, since the
`conversion of the fed-in power into radiation proceeds significantly faster than its transmission to
`the walls and, if there are any, to the electrodes of the discharge cavity. However, even with this
`mode of operation, alongside mechanical stresses due to the shock waves which, however, have
`sufficient action only in unfavourable cases, the evaporation and erosion of the materials which
`form the walls and electrodes contistute, when the radiation source must have a certain lifespan,
`an impediment to the production of intense radiant flux. In this regard, it should be noted that in
`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
`obtained at the expense of a reduction in the lifespan.
`
`However, these short-lived radiation sources cannot be used for many applications because they
`unreasonably increase the maintenance costs for the devices into which they are incorporated,
`since changing a lamp generally entails complicated adjustment and long adaptation operations
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`of the optical transmission system to the specific radiant flux of the lamp in question. Within
`certain limits, it is possible to increase the radiated power whilst retaining the overall charge of
`the electrical energy invested in the radiation, for the desired wavelength and the preferred
`spread width. This can be achieved by giving the active medium an optimal composition and by
`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
`incompatibility, at working temperature, between various active media and the consituent
`materials of the electrodes and the walls, such that, taking into account the withstand time of
`these materials, discharge conditions which are far from optimal frequently have to be selected.
`In the case of non-stationary operation, further limitations result from the fact that the radiation
`source simultaneously has to fulfil the functions of an electrical heavy-duty switch and of a
`converter of electrical energy into radiation. In this case too, the scope for optimising the radiation
`production is restricted, because the safety of ignition and switching is linked to certain plasma
`states.
`
`In the case of the stationary operation and in the case of pulsed operation, there are, in electrode
`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
`theoretically make it possible to also use the areas formed by these angles and, as a result, to
`provide the maximum amount of radiation energy to the optical system. To illuminate optical
`systems used in photolithography microinstallations, lasers are also used as radiation sources
`(SPIE Vol. 174 [1979], p.28...36, “Coherent illumination improves step-and-repeat printing on
`wafers” [Un éclairage cohérent améliore l’impression “graduelle et répétée” sur les galettes”], by
`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
`with high radiated power or favourable efficiency are generally not present in advantageous
`spectral areas. The use of “excimer” lasers which emit the necessary energy in the desired
`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
`contact é forte résolution ultrarapide au moyen de laser excimer”], by K. Jain et al.), because the
`partial spatial coherence necessary for the illumination of projection-lithography systems cannot
`be achieved to a degree as justified by its technical use.
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`The aim of the invention is to achieve a highly powerful radiation source which has a long
`lifespan and which makes it possible to include a substantial area of solid angles and precise
`and fast illumination of photosensitive areas and which, as a result, ensures a high productivity
`in photolithographic installations. Therefore the invention is intended to make it possible to
`achieve a radiation source for optical devices, in particular for photolithographic reproduction
`systems, which uses plasma radiation. By a spatial separation between the plasma and the wall
`or other installations associated with a cavity and without use of electrodes mounted in the
`cavity nor high-frequency fields for spatial concentration of the energy, it must make it possible
`to obtain a long lifespan and high power density. Furthermore, there is a reduction of stresses
`on the cavity though shock waves when the radiation source is in pulsed operation, and there
`are no dead solid angles due to electrodes or other installations in the cavity. The radiation
`source according to the invention is intended to possess a wide scope for optimisation of the
`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
`which the electrodes are made of. With regard to the laser radiation, the radiation source has
`the advantage that, especially in the case of photolithographic reproduction systems, it has a
`significant partial spatial coherence and that its spectral structure is such that the effects of
`standing waves in the photosensitive material are attenuated.
`
`This aim is achieved, according to the invention, by the fact that a gas-tight chamber filled with a
`discharge medium contains at least one entry aperture which allows laser radiation to pass and at
`least one exit aperture 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 known
`manner, by at least one laser situated outside the chamber, whereby optical 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 and that the plasma radiation exits
`the chamber via the exit aperture.
`
`When the radiated power of a laser as supplied is not sufficient for a discharge in the discharge
`medium, it is advantageous that the device includes, to ignite the discharge medium, outside the
`chamber, at least one further pulse-operated laser which is directed by optical means to ensure
`focussing of the same volume at an entry aperture.
`
`An advantageous variant, with regard to changing of position of the radiation-emitting plasma,
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`consists in placing the optical means which ensure the focussing of the laser radiation outside
`the chamber. It is then possible to advantageously arrange installations which make it possible
`to adjust the optical 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
`means which ensure the focussing of the laser radiation inside and/or on the surface of the
`chamber. In these conditions, the inner wall of the chamber constitutes an optical means for
`focussing the radiation coming from outside. To include as large an area of dead solid angles as
`possible, it is advantageous to give the inner wall of the chamber a shape such that it
`constitutes an optical means for ensuring the reflection of the radiation coming from the plasma.
`It is therefore advantageous for the inner wall of the chamber to have the shape of a convex
`mirror or an ellipsoidal mirror.
`
`To obtain high power densities and to increase the lifespan, it is advantageous to provide the
`chamber with an external cooling system.
`
`15
`
`Various other characteristics of the invention further emerge from the following detailed
`description.
`
`20
`
`Embodiments of the subject of the invention are shown, by way of non-limiting examples, in the
`attached drawings.
`
`Fig. 1 schematically shows an embodiment of the radiation source according to the invention.
`
`25
`
`Fig. 2 shows an exemplary embodiment in which the inner wall of the chamber has a shape
`such that it constitutes an optical element.
`
`Fig. 3 and 4 show embodiments wherein the discharge chamber has the shape of an
`ellipsoidal reflector.
`
`30
`
`Fig. 1 schematically shows an embodiment of the radiation source according to the invention in
`which a gas-tight chamber 1 contains the discharge medium 2. The chamber 1 includes two
`entry apertures 3 and 4 which allows laser radiation to pass and an exit aperture 5 which allows
`plasma radiation to pass. The entry aperture 3 is sealed by the window 6 which allows infrared
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`to pass, and the entry aperture 4 is sealed by the lens 7 which allows ultraviolet to pass. The
`exit aperture 5 is provided with a window 8. The device includes two lasers 9 and 10 outside the
`chamber 1. The coherent radiation 11 from the laser 9, which is a stationary CO2 gas laser,
`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
`pulse laser, is focussed on the same point by the lens 7 which allows ultraviolet to pass and
`produces an electrical discharge there, and as a result an absorbent plasma 14 which is heated
`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.
`
`If the radiation source is meant to be pulse-operated, the continuous laser 9 is replaced by a
`pulsed CO2 carbon dioxide laser. As a rule, it is possible to dispense with the pulsed laser 10,
`because the field strength of the pulsed CO2 carbon dioxide laser is in many cases sufficient to
`bring about the discharge. With such a device, it is possible to obtain, near-ellipsoidal plasmas
`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
`the temperature can be varied within a vast range by altering the pressure. As the pressure
`increases, the temperature falls and the spectral distribution approaches Planck’s function. As
`pressure decreases, the temperature increases, and the emission becomes linear.
`Temperatures far in excess of 20000 K can be reached by using, as active medium, helium
`which in conventional pulsed light sources, operating electrically, can no longer be used
`practically due to the heavy wear and tear on the electrodes. In these conditions, the density of
`radiation and its spectral distribution can be altered in a much wider range than in the case of
`conventional radiation sources.
`
`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
`gas-tight chamber containing the discharge medium 19. The coherent radiation 20 from a
`pulsed CO2 carbon dioxide laser 21 is focussed by the lens 22 which lets infrared pass and
`penetrates the chamber via the window 23 which allows infrared to pass. The pulsed laser 21 is
`mounted displaceably in the X direction, 24, and in the Y direction, 25, and the lens for infrared
`22 can be displaced in the X direction, 24, and in the Y direction, 25, and in the Z direction, 26.
`Accordingly, the position of the focal point, which corresponds to the position of the plasma 27,
`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
`optical system placed downstream.
`
`The gas-tight chamber is surrounded by a container 30. The free space 31 which they demarcate
`is traversed by a refrigerating means 32 which enters, via the tube 33, and exits via the tube 34
`and evacuates the heat produced by the pulsed laser radiation 21 and plasma radiation 27. It is
`possible to dispense with the quartz window 18 if the condenser lens 29 is installed instead.
`
`Figs. 3 and 4 show embodiments wherein the discharge chambers 35 and 36 are constituted by
`ellipsoidal reflectors. The radiation 37 from the carbon dioxide (CO2) laser 38 is focussed by the
`focussing elements, a concave mirror 39 or a lens 40 which allows infrared to pass, onto focal
`points 41 and 42 of the ellipsoid formed by the reflecting layers of the ellipsoidal mirror 43 and 44.
`The light emitted by the plasma producing the radiation is concentrated by the ellipsoidal mirror
`onto the second focal point 45 or 46 of the ellipsoid. The plasma formed at these focal points 45,
`46 serves as a source of secondary radiation for the optical system situated downstream and
`starting at the condenser lenses 47, 48.
`
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`Claims
`
`2554302
`
`1.
`
`2.
`
`3.
`
`4.
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`5.
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`A radiation source for optical devices, in particular for photolithographic reproduction
`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
`least one exit 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
`known manner, by at least one laser situated outside the chamber (1), whereby optical
`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
`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-
`operated laser (10) which is directed by optical means (7) to focus it on the same volume
`after passing in an entry aperture (4).
`
`The radiation source according to one of claims 1 or 2, characterised in that the optical
`means (22) which ensure the focussing of the laser radiation (21) are situated outside the
`chamber (19).
`
`The radiation source according to claim 3, characterised in that the installation
`includes devices for adjusting the optical means which ensure the focussing of the
`laser radiation.
`
`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.
`
`30
`
`6.
`
`The radiation source according to claim 5, characterised in that the inner wall of the
`chamber has a shape such that it constitutes an optical means for focussing the
`laser radiation coming from outside.
`
`7.
`
`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
`emitted by the plasma.
`
`The radiation source according to claim 7, characterised in that the inner wall of the
`chamber partially has the shape of a concave mirror or an ellipsoidal mirror (43, 44).
`
`The radiation source according to claim 1, characterised in that the chamber is equipped
`with an external cooling system (31, 32, 33, 34).
`
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`9.
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`

`
`FR 2 554 302
`
`l, James ll/lcGill, of Murgltroyd & Company, Scotland House, 165-169 Scotland Street,
`
`Glasgow G5 8PL, hereby declare that I am the translator of the document attached and
`
`certify that the following is a true translation to the best of my knowledge and belief.
`
`.
`
`é
`
`..........
`
`.................................. .. Dated this 15th of December 20%-4
`
`9
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`

`
`népusuous FRANCAISE
`INST”-UT NATIONAL
`DE LA PROPHET: INDUSTRIELLE
`PARIS
`
`2 554 302
`
`@ N"de publication:
`la n'utiliser que pour les
`commandas de reproduction)
`® N“ d'enregistrement national :
`
`@ Int cu‘ : H 05 H 1/46.
`
`DEMANDE DE BREVET D'INVENT|Oi\l
`
`Date de dépiit : 26 octobre 1984.
`
`Priorité : DD,
`n° WP H 05 W256 179.
`
`1°’ novémbre
`
`@ Demandeurlsl : Entreprise dite : VEB CARL ZEI
`JENA, Entreprise de droit allemand — DD.
`'
`
`@ lnventeurlsl : Walter Gartner, Wolfgang Retschke et
`Klaus Gunther.
`
`Date de la miss a disposition du public de la
`demande : BOPl « Brevets is n“ 18 du 3 mai 1985.
`
`Références é d'autres documents nationaux appa-
`rentés:
`
`@ 1'rtu|aire(sl :
`
`@ Source de rayonnement pour appareils d'optique, notamment pour systémes de reproduction par photolithographie.
`
`Mandataire(s): Cabinet Madeuf, Conseils en propriété
`industrielle.
`—
`'
`
`Source de rayonnement pour appareils d'optique, notam-
`ment pour systémes de reproduction photolithographique, ca-
`ractérisée en ce qu'une enceinte 1 étanche aux gaz remplie
`par un milieu de décharge 2 comporte au moins une ouverture
`d’entrée 3 et 4 laissant passerun rayonnement laser et au
`moins une ouverture de sortie 5 Iaissant passer un rayonne-
`ment de plasma et en ce que la production et |’entretien d’un
`plasma émettant un rayonnement dans le milieu de décharge
`sont assurés. d'une maniére connue, par au moins on laser
`situé a l'extérieur de l'enceinte 1, des moyens optiques assu-
`rant
`la focalisation du rayonnement
`laser dans le milieu de
`décharge étant montés au niveau d’une ouverture d'entrée, de
`sorte que Ie plasma se trouve a une certaine distance de la
`paroi’de |'enceinte 1 et que le rayonnement du plasma sort de
`l'enceinte par l'ouverture de sortie 5.
`
`Vents des fascicules 3 FIMPRIMERIE NATIONALE. 27. rue de la Convention — 75732 PARIS CEDEX 15
`
`10
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`FR2554302-A1
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`
`1
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`2554302
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`La présente invention est relative a une source
`
`de rayonnement pour appareils d'optique, notamment pour
`systémes de reproduction par photolithographie. Elle s'ap—
`plique de préférence dans les cas ou il faut une puissance
`de rayonnement supérieure a celle des lampes a vapeur de
`mercure sous pression, par exemple dans les installations
`
`de photolithographie, pour l'éclairement d'une couche
`
`de vernis photo sur une plaque de semi—conducteur.r
`
`On connait actuellement de nombreux systemes de'
`
`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 1'émission et a la densité de rayonnement susceptible
`d'étre obtenue ainsi qu'a la répartition spatiale et
`
`angulaire du rayonnement produit. Les exigences relatives
`
`a des puissances de rayonnement dépassant la puissance
`
`de rayonnement spectrale d'un corps noir au-dessus 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
`
`vers le haut par la valeur maximale de la puissance é1ec-
`trique, pouvant étre mise en jeu par unité de volume, a
`
`laquelle les matériaux constituant les électrodes et les
`
`parois peuvent résister thermiquement. Dans le cas du
`
`chauffage a haute fréquence, il n'y a plus de limitation
`
`due a la charge des électrodes, mais le probléme qui se
`
`pose alors est celui de la concentration spatiale de
`l'énergie de haute fréquence.
`
`Si l'on renonce a un fonctionnement stationnaire
`
`de la source de rayonnement, on peut obtenir, pendant un
`
`temps court, une augmentation, d'un ordre de grandeur assez
`
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`2554302
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`important, de la puissance mise en jeu, du fait que la
`
`transformation en rayonnement de la puissance fournie
`
`s'effectue beaucoup plus rapidement que sa transmission
`aux parois et, s'il y en a, aux électrodes de la cavité
`
`de décharge. Cependant, méme avec ce mode de fonctionnement,
`
`é c6té des charges mécaniques dues aux ondes de choc
`
`qui, cependant, n'ont une action suffisante que dans
`
`des cas défavorables, la vaporisation et l'érosion des
`
`matériaux qui forment les parois et les électrodes cons-
`
`tituent, lorsque la source de rayonnement doit avoir une
`
`certaine durée de vie, un obstacle a la production de
`
`I1 y a lieu de remarquer
`flux de rayonnement intense-
`a ce sujet que, dans le cas de sources ayant un fonction—
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`nement stationnaire comme dans le cas de sources ayant
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`un fonctionnement par impulsions, au-dessus d'un niveau
`
`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.
`7
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`Cependant, ces sources de rayonnement de courte
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`durée sont inutilisables pour beaucoup d'app1ications, car
`
`elles augmentent d'une maniére inadmissible les frais
`
`d'entretien des appareils auxquels elles sont incorporées,
`du fait que le remplacement d'une lampe entraine géné-
`
`ralement un réglage compliqué et de longues opérations
`
`d'adaptation du systéme optique de transmission au flux
`
`de rayonnement spécifique de la lampe en question. On
`peut, entre certaines limites, augmenter la puissance de
`rayonnement tout en conservant la charge totale de l'éner-
`
`gie électrique investie dans le rayonnement, pour la
`
`longueur d'onde voulue et la largeur d'étalement préférée.
`
`on peut y parvenir en donnant au milieu actif‘une compo-
`
`sition optimale et en réalisant des conditions de pres-
`
`sion et de température optimales pour le plasma lors de
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`la production du rayonnement. I1 y a lieu cependant dep
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`tenir compte de limitations qui.découlent de 1'incompa—
`
`tibilité existant, a la température de fonctionnement,
`
`entre différents milieux actifs et les matériaux qui'
`
`constituent les électrodes et les parois, de sorte que,
`
`compte tenu de la durée de résistance de ces matériaux,
`
`les conditions de décharge doivent étre choisies souvent
`
`de telle maniére qu'elles s'écartent sensiblement des
`
`valeurs optimales. D'autres limitations résultent, dans
`
`le cas d'un fonctionnement non stationnaire, du fait
`
`que la source de rayonnement doit remplir en méme temps
`
`les fonctions de commutateur électrique a grande puissance
`
`et de transformateur d'énergie électrique en rayonnement.
`Dans ce cas également,
`le jeu pour l'optimisation de la
`production d'un rayonnement efficace se trouve limité,
`
`3
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`car la sécurité de l'allumage et de la commutation est
`
`liée a certains états du plasma.
`Dans le cas du fonctionnement stationnaire comme
`
`dans le cas du fonctionnement par impulsions, il y a,
`
`dans les appareils de rayonnement a électrodes, des angles
`
`solides morts dans lesquels le rayonnement ne peut pas
`
`étre utilisé bien que l'insertion d'éléments_optiques
`
`convenables, comme, par exemple, des réflecteurs ellip-
`
`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
`
`systéme optique le maximum d'énergie de rayonnement. Pour
`
`1'éc1airement des systémes optiques utilisés dans les
`
`micro-installations de photolithographie, on utilise
`
`également, comme sources de rayonnement, des lasers (SPIE
`
`Vol. 174 (1979) p. 28 ... 36 "Un éclairage cohérent
`
`améliore 1‘impression "graduelle et répétée" sur les
`galettes" par Michel Lacombat et autres). Les principales
`limitations de ces sources lumineuses résultent de leur
`
`35
`
`grande cohérence spatiale et des distorsions de structure
`
`13
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`4
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`2554302
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`qui en résultent, de leur forte monochromie et des effets
`
`d'ondes stationnaires qui en résultent dans les matériels
`
`sensibles a
`
`la lumiére. De plus, en général, dans les
`
`zones du spectre avantageuses, il n‘y a pas de laser
`ayant une Qrande puissance de rayonnement ou un rendement
`d'efficacité favorable. L'utilisation de 1aSerS"excimer“,
`
`qui émettent l'énergie nécessaire dans le domaine de
`longueurs d'ondes voulu (domaine ultraviolet),
`se limite
`
`a des procédés de lithographie par contact
`
`(SPIE Vol. 334
`
`(1982) p 259 ... 262 “Lithographie par contact 3 forte
`
`résolution ultrarapide au moyen de laser excimer" par
`
`K. Jain et autres), car la cohérence partielle spatiale
`
`nécessaire a l'éclairement des systemes de lithographie
`
`par projection ne peut pas étre réalisée a un degré tel
`que son utilisation technique se justifie.
`Le but de l'invention est la réalisation d‘une
`
`source de rayonnement de grande puissance qui ait une
`longue durée de vie et permette 1'inclusion d'une zone
`
`importante d'angles solides et un éclairement précis et
`
`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-
`
`ment pour systémes de reproduction photolithographiques,
`
`qui utilise le rayonnement d'un plasma. Par une séparation
`spatiale entre le plasma et la paroi ou d'autres instal-
`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
`
`et une densité de puissance élevée. De plus, il y a dimi-
`
`nution des charges imposées a la cavité par les ondes
`
`de choc en cas de fonctionnement par impulsions de la
`source de rayonnement et i1 n‘y a pas de zones d'angles
`solides morts dues a des électrodes ou a d'autres instal-
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`5
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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 le choix des
`milieux actifs et des conditions de pression et de tempe-
`rature doit se faire indépendamment de la compatibilité
`avec les matériaux constituant les électrodes. En ce
`qui concerne le rayonnement laser, la source de rayon-
`nement présente l'avantage que, notamment dans le cas
`des systemes de reproduction photolithographiques, elle
`présente une cohésion partielle spatiale notable et que
`sa structure spectrale est telle que les effets d'ondes
`
`stationnaires dans le matériel photosensible sont atténués.
`
`Ce but est atteint, suivant l'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—
`
`ture de sortie laissant passer un rayonnement de plasma
`et que la production et le maintien d'un plasma‘émettant
`
`un rayonnement dans le milieu de décharge sont assurés,
`d'une maniére connue, par un laser au moins situé a
`l'extérieur de l'enceinte, des moyens optiques assurant
`la focalisation du rayonnement laser dans le milieu de
`décharge étant montés au niveau d'une ouverture d'entrée,
`de sorte que le plasma se trouve a une certaine distance
`
`de la paroi de l'enceinte et que le rayonnement de
`
`plasma sort de l'enceinte par l'ouverture de sortie.
`
`Lorsque la puissance de rayonnement d'un laser
`
`telle qu'el1e est fournie n'est pas suffisante pour une
`décharge dans le milieu de décharge, il est avantageux
`que l'appareil comporte, pour l'a11umage du milieu de
`
`décharge, a l'extérieur de l'enceinte, au moins un autre
`
`laser fonctionnant par impulsions qui est dirigé par des
`
`moyens optiques pour assurer la focalisation, au niveau
`
`35 d'une ouverture d'entrée, du méme volume.
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`15
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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 l'enceinte.
`
`On peut alors disposer avantageusement des installations
`
`permettant le réglage des moyens optiques assurant la
`
`focalisation du rayonnement laser.
`
`On peut simplifier avantageusement la réalisation
`de la source de rayonnement en-placant les moyens optiques
`assurant la focalisation du rayonnement laser a l'intérieur
`et/ou a la surface de l'enceinte. Dans ces conditions,
`la paroi intérieure de l'enceinte constitue un moyen
`optique assurant la focalisation du rayonnement provenant
`de l'extérieur. Pour inclure une zone d'angles solides
`
`morts aussi grande que possible, il est avantageux de
`donner 3 la paroi intérieure de l'enceinte une forme telle
`
`qu'elle constitue un moyen optique assurant la réflexion
`
`du rayonnement provenant du plasma. 11 est alors avantageux
`que la paroi intérieure de l'enceinte ait la forme d'un
`
`miroir convexe ou d'un miroir ellipsoidal.
`
`Il est avantageux, pour obtenir de fortes densités
`
`de puissance et pour augmenter la d