`(19) FRENCH REPUBLIC
`
`NATIONAL INSTITUTE
`
`FOR 'NDUSTR'AL PROPERTY
`PARIS
`
`
`(11) Publication No.:
`(To be used only when
`ordering copies)
`
`2 554 302
`
`(21) National Registration No.:
`
`84 16445
`
`(51) int. or“: H 05 H 1/46.
`
`
`
` (12> PATENT APPLICATION A1
`
`
`
`(22) Filing date: 26th October 1984.
`
`15‘ November 1983
`(30) Priority: DD,
`N0. WP H 05 H/256 179.
`
`(71) Applicant(s): Company named: VEB
`CARL ZE/SS JENA, Company under
`German law. —— DD.
`
`(72) lnventor(s): Walther Gartner, Wolfgang
`Retschke and Klaus GUnther.
`
`(43) Date of publication of application:
`B.O.P.l. - "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.
`
`_l‘>,
`
`
`unu-‘“I
`gyms
`
`
`
`D
`
`Printed copies available for sale from the lMPRlMERlE NATIONALE (French National Press) ~ 75732 PARlS CEDEX 15
`
`FR2554302—A1
`
`ASML 1004
`
`

`

`1
`
`2554302
`
`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
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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
`
`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 the produced 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
`
`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
`
`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
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`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
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`operate by pulses, above a power level which is type—dependent and which is achieved
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`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 forthe 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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`2
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`2554302
`
`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
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`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
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`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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`(SPlE Vol. 174 [1979]. p.28...36, “Coherent illumination improves step—and~repeat printing on
`
`wafers” [Un éc/airage coherent amé/iore I’impression “gradual/e et re’pe’tée” sur les galettes’j, 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
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`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
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`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
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`contact é forte resolution 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
`
`be achieved to a degree as justified by its technical use.
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`3
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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 illuminationof photosensitive areas and which, as a result, ensures a high productivity
`
`in photolithographicinstallations 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
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`are no dead solid angles clue 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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`4
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`2554302
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`consists in placing the optical means which ensure the tocussing 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 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
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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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`1O
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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
`
`mirror or an ellipsoidal mirror.
`
`To obtain high power densities and to increase the lifespan, it is advantageous to provide the
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`15
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`chamber with an external cooling system.
`
`Various other characteristics of the invention further emerge from the following detailed
`
`description.
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`Embodiments of the subject of the invention are shown. by way of non—limiting examples, in the
`
`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
`
`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
`
`ellipsoidal reflector.
`
`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
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`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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`

`

`5
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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
`chamber 1. The coherent radiation 11 from the laser 9, which is a stationary 002 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
`
`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
`
`to high temperatures under the influence of the radiation 11. The radiation 15 from the plasma
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`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 COZ 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 COZ 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
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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
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`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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`incte'ases, the temperature falls and the spectral distribution approaches Planck’s function. 'As
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`presisure 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
`
`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
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`gas-itight chamber containing the discharge medium 19I1The coherent radiation 20 from a
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`pulsed C02 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, and in the Z direction, 26.
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`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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`6
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`2554302
`
`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. lt 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 (C02) 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.
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`46 serves as a source of secondary radiation for the optical system situated downstream and
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`10
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`starting at the condenser lenses 47, 48,
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`

`

`7
`
`
`Claims
`
`2554302
`
`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).
`
`10
`
`15
`
`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
`
`20
`
`chamber (19).
`
`25
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`30
`
`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 chambers
`
`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.
`
`The radiation source according to claim 1, characterised in that the inner wall of the
`
`

`

`8
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`2554302
`
`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).
`
`

`

`FR f3 55% 302
`
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`FR2554302-A?
`
`

`

`3534302
`1
`La présente invention est relative é une source
`
`d9 rayonnement your appareils d‘optique, notamment pour
`
`systémes fie reproduction par photolithographie. Elle s‘ap-
`
`plique de préférence dans les cas 9% i1 faut une puissance
`
`de raycnnement supérieure é celle des lampes é vapaur dg
`mercuze sous pression, par exemplg dans les installatiOnS
`
`fie yhotolithagxaphie, pour l‘éclairement d‘una couche
`
`fie vernis photo sur une plaque d3 aemi~conducteur.
`
`On cannait actuallament de nombreux systémes fie
`
`sources de rayonnement qui sont utilisés &ans des appareils
`
`scientifiques et fiont les propriétés ont été largement
`afiaptées aux conditions inhérentes an domaine d‘utilisation.
`
`69$ propriétés sant relatives 3 1a répartitian spectrale
`
`6e l‘émissicn at a la &ensité de rayonnement susceptible
`
`d‘étre obtenue ainsi qu‘a la réyartition spatiale et
`
`angulaire du xayonnement profiuit. Le$ exigences ralatives
`
`é fies puissancas de rayonnement dépassant 1a puissance
`
`de rayonmement fipectrale d‘un corys moi: au~dessus du
`
`point 615: finsion fies some. soliées ne pauvent étée satis-
`faitas qua par du plasma. L33 piasmas s‘obtiannent par
`chauffage d‘un miliéu actif, da pxéférence par passage
`d'un courant élactriqua ou par action fie champ$ électro~
`magnétiques de haute fréquence. Les densités de rayonnement
`spectrales susceptiblas d‘étre atteintes sent limitées
`vers le haut par la vaieur maximale de la puissance élec-
`txique, pouvant étre miae an 363 par unité de volume, a
`
`laquelle les matériaux constituant les élactrcées et les
`parois peuvent résistar'thermiquement. Dans le cas §u
`
`chauffage é haute fréquence,
`:1 n‘y a plus de limitation
`due a la Charge das électrodes, mais le psobléme qui se
`pOSe alors est celui de la cancentration gpatiale de
`
`l‘énergxe fie haute fréquence.
`
`'
`
`Si l'on renance é um fonctionnement stationnaire
`
`de la source ée rayonnement, on pent obtenir, pendant un
`
`temps csurt, une augmentaticn, d‘un oxdra'ée gran§eur assez
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`2
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`2554302
`
`fie la puiSsance misa en jeu, du iaii que la
`important,
`transfermation an rayonnement de la puissance fournie
`
`s‘effectue beaucaup plus rapiéement qua sa transmission
`
`aux parois at, s‘il y en a, aux électxades fie la cavité
`
`de décharge. Ceyenfiant, méme avec ce made fie fonctionnémant,
`
`é cété fies charges mécaniques dues aux onées fie choc
`
`qui, cagenéant, n'ont une action suffisante que fians
`
`des Gas défavorables, la vaporisation at l‘érosian des
`
`matériaux qui formant les parois et les électrcdes cons-
`
`tituent,
`
`lorsque la source de rayannement éoit avoir une
`
`certaine durée de via, an obstacle é ia productien de
`
`flux fie rayonnement intense.
`
`I1 y a lieu de remarquer
`
`a ca sufiet que, dans le cas de sources ayant an fonction-
`
`nement stationnaire comma éans 1e cas de sources ayant
`
`§5
`
`un fonctionnfiment par impulsions, au~de$$u$ é'un niveau
`
`de puissance gal dépend an type aficpté et qui, dans les
`
`applications techniqnes, est gratiquement atteint partaut,
`
`taute augmentation suyplémentaire fie 1a puissance de
`
`rayonnement s'obtient aux déyens de la diminution de
`la durée de vie.
`
`Cagendant, ces sources d9 rayonnemeat de courte
`éurée sont inutilisahles pour beaucoup d‘applications, car
`
`elles augmenteat d'una maniére inaémissible les fixais
`
`d’entxetien deg appaxeils auxquels eiles sonfi incorgarées,
`
`éu fait qua la remplacement d‘une lamps entraine généw
`
`ralement un réglage compliqué et de lengues opérations
`
`é‘aéaptatian an systéme optique de transmission an flux
`
`&e rayonnement spécifique de la lampe an question. On
`pent, extra certaines lifiites, augmenter 1a puissance de
`rayonnemeni tout en conservant la charge tofiale fie l‘éner-
`gie électrique investie dan5 1e rayonnement, pour la
`
`longuaux é‘onde voulue at la largeur d‘étalement préférée.
`
`On peat y parvenir en donnant au milieu actif_une compo—
`
`sition ogtimale at an réalisant fies conditiens as pres‘
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`sion at fie température optimales pour 19 plasma 10:8 fie
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`la production du rayonnement. 11 y a lieu cependant fie
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`tenir compte ée limitations qui.éécoulent de l’incompaa
`
`tibilité axistant, a la température de fonctionnement,
`
`entre aifférents milieux aciifs et les matériaux qui
`
`constituent les électredes et les parois, de sorta qua,
`
`compte tenu de la éurée de résistance de ces matériaux,
`
`las conditicns fie fiécharga éoivent étre chaisias souvent
`
`de telle maniéra qufelles s‘écartent sensiblement des
`valeurs optimales. D‘autres limitatians résultent, dans
`le Gas fi‘un fonctionnement non stationnaire, du fait
`
`que la sourca $9 rayonnement doit remplir en méme temps
`
`les fonctiong fie commutateur électrique é gxanfia puissance
`
`et de transformateur é‘énergie éleetrique en rayonnement.
`
`Sans 09 Gas éga§ement,
`
`la jeu pour l'ogtimisaticn de la
`
`production d‘un rayonnememt efficace 5e trcuve limité,
`
`car la sécurité de l’allumaga at de la commutation est
`
`liée é cert&ins états du plasma.
`Dans 15 Gas fin fonctiennement stationnaire comma
`
`Gang le cas du fonctionnement par impulsions,
`
`i1 y a,
`
`flans leg app&reils d8 rayonnement é électrodes, deg angles
`
`solides morts dans lesquels la rayennam&nt ne peut pas
`étre utilisé hian que l‘insertion d‘élémants optiques
`convenables, comma, par example,
`fies réflecteuxs ellipw
`soiéaux et/ou fies fibres conductrices fie la lumiére,
`
`permette
`
`théoriguement d’utiliser égalament leg zones
`
`farmées par Gas anglas at, 69 as fait; fie fournir au
`
`Systéme optique 1e maximum é‘énargie de rayonnement. Pour
`l‘éelairement des systémes optiques utilisés dans les
`
`micrc-installations fie yhotolithograghie, on utilisa
`
`également, camme saurces fie rayonnement,
`
`fies la$ers {SPEE
`
`V01. 174 {39?9) p. 28 ..‘ 36 “Sn éclaixage cchérent
`
`améliore l‘imprezsion “grafiuelle at répétée“ sur les
`
`galettes“ pai Michel Lacembat et antres}. Les principales
`limitations de ces sources lumineuses résultant fie ieur
`
`granéa cohérence spatiale at fies fiistorsions de structure
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`qui an résultent, da leur forta monochromie et des effets
`&’ondes stationnaireg qui en résultent fians lea matériels
`
`sensibles a
`
`la lumiére. De plus, en général, dans les
`
`zones du 5§ectre avantageuses,
`
`i1 n‘y a pas de laser
`
`ayant una granée puissance de rayonnement on an rendemgnt
`d‘affieacité favorable. L‘utilisatian de 3fi8€r§“excimer“,
`
`qui émettent l’énergie nécess&ire &ans le domaine fie
`longueurs é‘ondes voulu (domaine ultravislet§,
`se limits
`
`é des prccééés de lithographie par contact
`
`(SPIE Vol. 334
`
`10
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`15
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`(1982} p 259 ‘.. 262 ”Lithographie par cantact a forte
`
`résolutian ultrarapide an moyan da laser excimer“ p&r
`
`K. Jain et autres), car la cahérence partielle spatiale
`
`nécessaire é l'éclairement des systémes fie lithegraphie
`
`par projection ma pent gas étre réalisée a un fiegxé tel
`
`que son utilisatimn technique se justifie.
`Le but fie l‘invention est la réalisation a‘une
`
`source &a rayonnement de granae puissance qui ait une
`
`longue éurée de vie &t permette l’inclusion d‘une zone
`importante d'angles solidas at an éclairemant précis at
`
`rayida de zones photosensibles et qui, de ce iait, assure
`a des inatallations fie photolithographie une granfie
`
`proéuctivité. L'inventien doit donc permettre de réaliser
`une source fie rayonnement pour appareils fl'optique, metam-
`
`ment pour systémeg d9 reprofiuction phetolithographiques,
`
`qui utilise le rayannamant d'un ylasma. gar une séparation
`
`spatiale entre la plasma et la paroi cu d'autres instalw
`latians associées a une cavité at sans utilisation
`
`d’électredes montéas dans la cavité ni 6e champs fie haute
`fréquance pour la concentfation séatiale fie l‘énexgia,
`
`39
`
`@116 $01: parméttre d‘ohtenir une longue aurée fie vie
`
`et une densité de puissance élevéa. De plug, 11 y a dimi-
`
`nution fies chaxges imposéas é la cavité gar les ondes
`de choc en Gas &9 functionnement par impulsions fie la
`
`source fie rayonnement at 11 n’y a pas da ZGDB$ d’angles
`solides morts dues a des élactrodes cu é d‘autres instal—
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`lations montées dans la cavité. La source de rayonnemant
`
`suivant l'invention doit comparter un,jeu large pour
`
`l'optimation da la production fin rayonnemant dans la
`
`éamaine de langueurg d'onde vaulu, car 1e choix des
`
`milieux actifs et des conditions fie pression at as tempé~
`rature doit se faire inéépenaamment §e la compatibilité
`avec les matériaux constituant lea électroées. En ce
`
`qui concerne 1e rayonnement laser, 1a source de rayon~
`nement présente l'avantage qua, notamment dans le cas
`
`des systémes 3e reprcéuction photolithogxaphiques, elle
`
`présente une cohésion partielle spatiale notable at Qua
`sa structure syectrale est tellg que les effiets d'ondes
`
`stationnaires dans la matérial photosensible sent atténués.
`Ce but est atteint, suivant 1’inventinngdu fait
`
`qu‘ane enceinte étanche aux gaz remplie par un milieu
`de déchazge comporte au moins une suverture é'antrée lais~
`sant passer un rayonnement laser at au moins une caverw
`
`tare fie scrtie laissant passer un rayonnement ae plaSma
`
`et que la production at Xe maintien d‘un plasma"émettant
`
`un rayonnément dans le milieu as déchaxge sant assurés,
`
`d‘une maniére connua, par un Eager au moins situé é
`
`fies moyens optiquea agsurant
`l'extérieur de l'enceinte,
`la facalisatiofi du raycnnament lase: dans la milieu de
`
`10
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`35
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`20
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`25
`
`décharge étant montés au niveau a‘une ouverture d’entrée,
`fie sorta que le ylasma se trouve % une certaine distance
`
`de la paroi de l‘enaeinte et que la rayonnemant fie
`plasma sart de l'enceinte par l‘ouverturfi fie sortie.
`
`Lorsque la puissance de rayannement d‘un laser
`
`telle qfi’elle est fournie n‘est pas suffisante your une
`
`§écharge dans le miliau de décharge, il est avantageux
`qua l‘appareil comporte, pour l'allumage du milieu de
`éécharge, & l'extérieur de l‘enceinte, au moins un autre
`
`laser fonctionnant par impuisions qui est dirigé par des
`mcyens optiques pour assure: la focalisation, au niveau
`
`d’une cuverture d‘entrée, du méme volume.
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`fine variante avantageuse,&n ce qui concerne la
`
`changement de position du plasma émettant 1e raycnnemant,
`
`congiste é placer les moyens optiques assurant 1a foca~
`
`lisation an rayonnement laser a l'extéxieur de l‘enceinte.
`
`On pent alors disposer avantageusement fies installations
`
`permettant la réglagg fies moyens optiques assurant la
`fecalisatian du rayannement laser.
`
`0n peat simplifier avantageusement la réalisation
`
`de la source de rayonnement en~placant lea moyens 0§tiques
`assurant la focalisation du rayonnement Ease: é l‘intérieur
`
`et/ou a la surface d8 l'enceinte. Dans ces canaitions,
`
`la paroi intérieurs fie l'encainte canstitue an moyen
`
`cptique assurant la focalisation fiu rayonnement provenant
`
`fie l'extérieur. Paar inclura une zone d‘angles solides
`
`morts aussi granée qua yossible, il est avantageux de
`
`dunner & 1a paxoi intérieura de l‘enceinte une forme tells
`
`qu'elle constitue an moyen optique assurant 1a réflexion
`
`an rayonnement provenant an plasma. Il est aiors avantagaux
`que la paroi intérieure de l’enceinte ait la form