(19)
`
`(12)
`
`Europaisches Patentamt
`
`European Patent Office
`
`Office europeen des brevets
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`
`(11)
`
`EP 1 403 714 A2
`
`EUROPEAN PATENT APPLICATION
`
`(43) Date of publication:
`31.03.2004 Bulletin 2004/14
`
`(21) Application number: 03256068.2
`
`(22) Date of filing: 26.09.2003
`
`(51) lnt Cl.7: G03F 7/20
`
`(84) Designated Contracting States:
`AT BE BG CH CY CZ DE DK EE ES Fl FR GB GR
`HU IE IT Ll LU MC NL PT RO SE Sl SK TR
`Designated Extension States:
`AL LT LV MK
`
`(30) Priority: 30.09.2002 EP 02256794
`
`(71) Applicant: ASML Netherlands B.V.
`5503 LA Veldhoven (NL)
`
`(72) Inventors:
`• van der Laan, Hans
`5501 CL Veldhoven (NL)
`
`• Baselmans, Johannes Jacobus Matheus
`5688 GG Oirschot (NL)
`• van Dijsseldonk, Antonius Johannes Josephus
`5527 BH Hapert (NL)
`• Leenders, Martinus Hendrikus Antonius
`3039 ER Rotterdam (N L)
`• Moors, Johannes Hubertus Josephina
`5709 MT Helmond (NL)
`
`(74) Representative: Leeming, John Gerard
`J.A. Kemp & Co.,
`14 South Square,
`Gray's Inn
`London WC1 R 5JJ (GB)
`
`(54)
`
`Lithographic apparatus and a measurement system
`
`(57)
`
`A lithographic projection apparatus comprising:
`
`characterized in that:
`
`a radiation system for providing a projection beam
`of radiation;
`for supporting patterning
`a support structure
`means, the patterning means serving to pattern the
`projection beam according to a desired pattern;
`a substrate table for holding a substrate;
`a projection system for projecting the patterned
`beam onto a target portion of the substrate, and
`a measurement system for measuring wave front
`aberrations of the projection system,
`
`Fig. 3
`
`said measurement system comprises: a diffractive
`element and structure for increasing the pupil filling
`of the radiation in the pupil of the projection system,
`both movable into the projection beam between the
`radiation system and the projection system; and a
`sensor module for sensing radiation that has tra(cid:173)
`versed the projection system for measuring wave
`front aberrations of the projection system.
`
`,...
`c. w
`
`19
`
`Printed by Jouve, 75001 PARIS (FR)
`
`Nikon Exhibit 1006 Page 1
`
`

`

`EP 1 403 714 A2
`
`2
`
`Description
`
`[0001] The present invention relates to the measure(cid:173)
`ment of wave front aberrations in a lithographic projec-
`tion apparatus, more particularly for a lithographic pro-
`jection apparatus comprising:
`
`a radiation system for providing a projection beam
`of radiation;
`for supporting patterning
`a support structure
`means, the patterning means serving to pattern the
`projection beam according to a desired pattern;
`a substrate table for holding a substrate;
`a projection system for projecting the patterned
`beam onto a target portion of the substrate.
`
`5
`
`10
`
`15
`
`[0002] The term "patterning means" as here em(cid:173)
`ployed should be broadly interpreted as referring to
`means that can be used to endow an incoming radiation
`beam with a patterned cross-section, corresponding to
`a pattern that is to be created in a target portion of the
`substrate; the term "light valve" can also be used in this
`context. Generally, the said pattern will correspond to a
`particular functional layer in a device being created in
`the target portion, such as an integrated circuit or other 25
`device (see below). Examples of such patterning means
`include:
`
`20
`
`electric field, or by employing piezoelectric actua(cid:173)
`tion means. Once again, the mirrors are matrix-ad(cid:173)
`dressable, such that addressed mirrors will reflect
`an incoming radiation beam in a different direction
`to unaddressed mirrors; in this manner, the reflect(cid:173)
`ed beam is patterned according to the addressing
`pattern of the matrix-addressable mirrors. The re(cid:173)
`quired matrix addressing can be performed using
`suitable electronic means. In both of the situations
`described hereabove, the patterning means can
`comprise one or more programmable mirror arrays.
`More information on mirror arrays as here referred
`to can be gleaned, for example, from United States
`Patents US 5,296,891 and US 5,523,193, and PCT
`patent applications WO 98/38597 and WO
`98/33096, which are incorporated herein by refer(cid:173)
`ence. In the case of a programmable mirror array,
`the said support structure may be embodied as a
`frame or table, for example, which may be fixed or
`movable as required.
`A programmable LCD array. An example of such a
`construction is given in United States Patent US
`5,229,872, which is incorporated herein by refer(cid:173)
`ence. As above, the support structure in this case
`may be embodied as a frame or table, for example,
`which may be fixed or movable as required.
`
`A mask. The concept of a mask is well known in
`lithography, and it includes mask types such as bi(cid:173)
`nary, alternating phase-shift, and attenuated
`phase-shift, as well as various hybrid mask types.
`Placement of such a mask in the radiation beam
`causes selective transmission (in the case of a
`transmissive mask) or reflection (in the case of a
`reflective mask) of the radiation impinging on the
`mask, according to the pattern on the mask. In the
`case of a mask, the support structure will generally
`be a mask table, which ensures that the mask can
`be held at a desired position in the incoming radia(cid:173)
`tion beam, and that it can be moved relative to the
`beam if so desired.
`A programmable mirror array. One example of such
`a device is a matrix-addressable surface having a
`viscoelastic control layer and a reflective surface.
`The basic principle behind such an apparatus is that
`(for example) addressed areas of the reflective sur(cid:173)
`face reflect incident light as diffracted light, whereas
`unaddressed areas reflect incident light as undif(cid:173)
`fracted light. Using an appropriate filter, the said un(cid:173)
`diffracted light can be filtered out of the reflected
`beam, leaving only the diffracted light behind; in this
`manner, the beam becomes patterned according to
`the addressing pattern of the matrix-addressable
`surface. An alternative embodiment of a program(cid:173)
`mable mirror array employs a matrix arrangement
`of tiny mirrors, each of which can be individually tilt(cid:173)
`ed about an axis by applying a suitable localized
`
`For purposes of simplicity, the rest of this text may, at
`certain locations, specifically direct itself to examples in-
`30 volving a mask and mask table; however, the general
`principles discussed in such instances should be seen
`in the broader context of the patterning means as here(cid:173)
`above set forth.
`[0003] Lithographic projection apparatus can be
`35 used, for example, in the manufacture of integrated cir(cid:173)
`cuits (ICs). In such a case, the patterning means may
`generate a circuit pattern corresponding to an individual
`layer of the IC, and this pattern can be imaged onto a
`target portion (e.g. comprising one or more dies) on a
`40 substrate (silicon wafer) that has been coated with a lay(cid:173)
`er of radiation-sensitive material (resist). In general, a
`single wafer will contain a whole network of adjacent tar(cid:173)
`get portions that are successively irradiated via the pro(cid:173)
`jection system, one at a time. In current apparatus, em-
`45 ploying patterning by a mask on a mask table, a distinc(cid:173)
`tion can be made between two different types of ma(cid:173)
`chine. In one type of lithographic projection apparatus,
`each target portion is irradiated by exposing the entire
`mask pattern onto the target portion in one go; such an
`50 apparatus is commonly referred to as a wafer stepper.
`In an alternative apparatus
`commonly referred to as
`a step-and-scan apparatus
`each target portion is ir-
`radiated by progressively scanning the mask pattern un(cid:173)
`der the projection beam in a given reference direction
`(the "scanning" direction) while synchronously scanning
`the substrate table parallel or anti-parallel to this direc(cid:173)
`tion; since, in general, the projection system will have a
`magnification factor M (generally < 1 ), the speed V at
`
`55
`
`2
`
`Nikon Exhibit 1006 Page 2
`
`

`

`3
`
`EP 1 403 714 A2
`
`4
`
`which the substrate table is scanned will be a factor M
`times that at which the mask table is scanned. More in(cid:173)
`formation with regard to lithographic devices as here de(cid:173)
`scribed can be gleaned,
`for example,
`from US
`6,046, 792, incorporated herein by reference.
`[0004]
`In a manufacturing process using a lithograph-
`ic projection apparatus, a pattern (e.g. in a mask) is im(cid:173)
`aged onto a substrate that is at least partially covered
`by a layer of radiation-sensitive material (resist). Prior
`to this imaging step, the substrate may undergo various
`procedures, such as priming, resist coating and a soft
`bake. After exposure, the substrate may be subjected
`to other procedures, such as a post-exposure bake
`(PEB), development, a hard bake and measurement/in(cid:173)
`spection of the imaged features. This array of proce(cid:173)
`dures is used as a basis to pattern an individual layer of
`a device, e.g. an IC. Such a patterned layer may then
`undergo various processes such as etching, ion-implan(cid:173)
`tation (doping), metallization, oxidation, chemo-me(cid:173)
`chanical polishing, etc., all intended to finish off an indi(cid:173)
`vidual layer. If several layers are required, then the
`whole procedure, or a variant thereof, will have to be
`repeated for each new layer. Eventually, an array of de(cid:173)
`vices will be present on the substrate (wafer). These de(cid:173)
`vices are then separated from one another by a tech-
`nique such as dicing or sawing, whence the individual
`devices can be mounted on a carrier, connected to pins,
`etc. Further information regarding such processes can
`be obtained, for example, from the book "Microchip Fab(cid:173)
`rication: A Practical Guide to Semiconductor Process-
`ing", Third Edition, by Peter vanZant, McGraw Hill Pub(cid:173)
`lishing Co., 1997, ISBN 0-07-067250-4, incorporated
`herein by reference.
`[0005] For the sake of simplicity, the projection sys-
`tem may hereinafter be referred to as the "lens"; how-
`ever, this term should be broadly interpreted as encom(cid:173)
`passing various types of projection system, including re(cid:173)
`fractive optics, reflective optics, and catadioptric sys(cid:173)
`tems, for example. The radiation system may also in(cid:173)
`clude components operating according to any of these
`design types for directing, shaping or controlling the pro(cid:173)
`jection beam of radiation, and such components may
`also be referred to below, collectively or singularly, as a
`"lens". Further, the lithographic apparatus may be of a
`type having two or more substrate tables (and/or two or
`more mask tables). In such "multiple stage" devices the
`additional tables may be used in parallel, or preparatory
`steps may be carried out on one or more tables while
`one or more other tables are being used for exposures.
`Dual stage lithographic apparatus are described, for ex(cid:173)
`ample, in US 5,969,441 and WO 98/40791, incorporat(cid:173)
`ed herein by reference.
`[0006] There is a desire to integrate an ever-increas-
`ing number of electronic components in an I C. To realize
`this it is necessary to decrease the size of the compo(cid:173)
`nents and therefore to increase the resolution of the pro(cid:173)
`jection system, so that increasingly smaller details, or
`line widths, can be projected on a target portion of the
`
`5
`
`10
`
`20
`
`25
`
`substrate. For the projection system this means that the
`projection system and the lens elements used in the pro(cid:173)
`jection system must comply with very stringent quality
`requirements. Despite the great care taken during the
`manufacturing of lens elements and the projection sys(cid:173)
`tem, they both may still suffer from wave front aberra(cid:173)
`tions, such as, for example, displacement, defocus,
`astigmatism, coma and spherical aberration across an
`image field projected with the projection system on to a
`target portion of the substrate. Such aberrations are im(cid:173)
`portant sources of variations of the imaged line widths
`occurring across the image field. It is important that the
`imaged line widths at different points within the image
`field are constant. If the line width variation is large, the
`15 substrate on which the image field is projected may be
`rejected during a quality inspection of the substrate. Us(cid:173)
`ing techniques such as phase-shifting masks, or off-axis
`illumination, the influence of wave front aberrations on
`the imaged line widths may further increase.
`[0007] During manufacturing of a lens element it is ad(cid:173)
`vantageous to measure the wave front aberrations of
`the lens element and to use the measured results to tune
`the aberrations in this element or even to reject this el-
`ement if the quality is not sufficient. When the lens ele(cid:173)
`ments are put together to form the projection system it
`may again be necessary to measure the wave front ab-
`errations of the projection system. These measure(cid:173)
`ments may be used to adjust the position of certain lens
`elements in the projection system in order to minimize
`wave front aberrations of the total projection system.
`[0008] After the projection system has been built into
`a lithographic projection apparatus, the wave front ab(cid:173)
`errations may be measured again. Moreover, since
`wave front aberrations are variable in time in a projection
`system, for instance, due to deterioration of the lens ma(cid:173)
`terial or lens heating effects (local heating of the lens
`material), it may be necessary to measure the aberra(cid:173)
`tions at certain instants in time during operation of the
`apparatus and to adjust certain moveable lens elements
`accordingly to minimize wave front aberrations. The
`short time scale, on which lens-heating effects may oc-
`cur, may require measuring the wave front aberrations
`frequently.
`[0009] Previously there has been proposed a meas(cid:173)
`urement system for measuring wave front aberrations
`of a projection system using the principle known as a
`"shearing interferometer". According to this proposal dif(cid:173)
`ferent portions of the projection beam from a particular
`location at the level of the patterning means, travel along
`50 different paths through the projection lens. This can be
`achieved by a diffractive element located in the projec(cid:173)
`tion beam between the radiation system and the projec(cid:173)
`tion system. The diffractive element, such as a grating,
`also known as the object grating, diffracts the radiation
`55 and spreads it out such that it passes through the pro(cid:173)
`jection system along a plurality of different paths. Light
`that has traversed the projection system then impinges
`on a further diffractive element, such as a pinhole or a
`
`30
`
`35
`
`40
`
`45
`
`3
`
`Nikon Exhibit 1006 Page 3
`
`

`

`5
`
`EP 1 403 714 A2
`
`6
`
`element and a structure for increasing the pupil fill(cid:173)
`ing of the radiation in the pupil of the projection sys(cid:173)
`tem, both movable into the projection beam be(cid:173)
`tween the radiation system and the projection sys(cid:173)
`tem; and a sensor module for sensing radiation that
`has traversed the projection system for measuring
`wave front aberrations of the projection system.
`
`5
`
`grating, known as the image grating. This further diffrac(cid:173)
`tive element acts as the "shearing mechanism" which
`combines radiation from multiple paths through the lens
`to get interference, for example interference of different
`diffracted orders from different paths through the lens.
`For example the zero order from one path may be made
`to interfere with the first order from another path. This
`results in a diffraction pattern, which can be detected by
`a sensor to reveal information on the wave front aber(cid:173)
`ration at a particular location in the image field.
`[001 0] However, there is a problem, particularly for
`some types of radiation, in spreading the radiation such
`that it fills the entire pupil of the projection lens (filling
`the pupil of the projection lens corresponds to incoher(cid:173)
`ent light, i.e. the light entering the projection lens having
`no particular angular bias). If the radiation does not ad(cid:173)
`equately fill the pupil of the projection lens, then the ab(cid:173)
`erration of the lens is not necessarily accurately meas(cid:173)
`ured, because it is only sampled for particular paths of
`radiation through the lens. If there is not a sufficient de-
`gree of pupil filing, then higher order aberrations cannot
`be measured at all.
`[0011] A further problem is as follows. Previously, it
`has also been proposed to measure defocus by project-
`ing images of two alignment marks, one of which is tel-
`ecentric and the other of which is non-telecentric. The
`distance between the images of the marks is known for
`the correct distance between the reticle and substrate.
`However, because one beam is non-telecentric, if the
`substrate is at the wrong height (i.e. not at the best focus
`position) then the distance between the marks will be
`different. In fact the amount of lateral shift of the non(cid:173)
`telecentric mark is directly proportional to the amount of
`defocus. For radiation such as DUV, the method of gen(cid:173)
`erating the mark that produces the non-telecentric beam
`is to attach wedges, prisms or similar structures onto the
`reticle mask. However there is a problem that this can(cid:173)
`not be done for a reflective EUV mask.
`[0012]
`It is an object of the present invention to pro(cid:173)
`vide a measurement system for measuring the wave
`front aberrations in a lithographic projection apparatus,
`which alleviates, at least partially, any of the above prob(cid:173)
`lems. According to the present invention there is provid(cid:173)
`ed a lithographic projection apparatus comprising:
`
`a radiation system for providing a projection beam
`of radiation;
`for supporting patterning
`a support structure
`means, the patterning means serving to pattern the
`projection beam according to a desired pattern;
`a substrate table for holding a substrate;
`a projection system for projecting the patterned
`beam onto a target portion of the substrate, and
`a measurement system,
`
`characterized in that:
`
`said measurement system comprises: a diffractive
`
`[0013] This is advantageous because increasing the
`10 pupil filling can permit higher order aberrations of the
`projection system to be measured, and can improve the
`overall measurement of aberration.
`[0014] Preferably, the structure for increasing the pu(cid:173)
`pil filling comprises a structure for diffusing the radiation.
`15 This is advantageous because the structure for diffusing
`the radiation reduces the coherence of the radiation and
`improves the filling of the pupil of the projection system.
`[0015] According to one preferred embodiment, a sin-
`gle member has the function of both the diffractive ele(cid:173)
`ment and the structure for diffusing the radiation. This
`has the advantage that there is no extra overhead in in-
`cluding the diffusing structure in the measurement sys(cid:173)
`tem. The provision of the two functions may be com(cid:173)
`bined in and/or on a single surface of the member. For
`example, the surface may be provided with a radiation
`diffracting structure embodied as a grating and a sur-
`face-structure for diffusing by diffracting or scattering ra(cid:173)
`diation for improving filling of the pupil of the projection
`system.
`[0016] The diffractive element may comprise a reflec(cid:173)
`tive grating in which the reflective portions comprise a
`structure for diffusing the radiation. The structure for dif(cid:173)
`fusing the radiation may comprise an array of reflective
`portions randomly staggered in height. This structure
`35 advantageously acts as a random phase diffuser and
`can substantially fill the pupil of the projection system.
`Preferably each reflective portion comprises a multi lay(cid:173)
`er structure, which advantageously can function as are(cid:173)
`flector for EUV radiation.
`[0017] Alternatively, the structure for diffusing the ra(cid:173)
`diation may comprise sub-resolution absorptive fea(cid:173)
`tures, such as a random array of absorptive dots, which
`can advantageously diffuse the radiation by random am(cid:173)
`plitude modulation.
`[0018] According to another embodiment, the diffrac(cid:173)
`tive element comprises a transmissive grating, and the
`measurement system further comprises a mirror for di(cid:173)
`recting the projection beam to illuminate the grating from
`behind, wherein the structure for diffusing the radiation
`50 comprises imperfections in the mirror. Preferably the
`mirror is curved to provide a focusing effect. This has
`the advantage of increasing the intensity of the radiation
`sensed by the sensor module thereby improving the sig(cid:173)
`nal to noise ratio of the interferogram and reducing the
`time required to collect the data for measuring the ab(cid:173)
`erration.
`[0019] According to a further embodiment the diffrac(cid:173)
`tive element comprises a transmissive grating, and the
`
`20
`
`25
`
`30
`
`40
`
`45
`
`55
`
`4
`
`Nikon Exhibit 1006 Page 4
`
`

`

`7
`
`EP 1 403 714 A2
`
`8
`
`measurement system further comprises a mirror for di(cid:173)
`recting the projection beam to illuminate the grating from
`behind, wherein the mirror is curved to provide a focuss(cid:173)
`ing effect and comprises the structure for increasing the
`pupil filling of the radiation in the pupil of the projection
`system.
`[0020] Another aspect of the invention provides a lith(cid:173)
`ographic projection apparatus comprising:
`
`a radiation system for providing a projection beam
`of radiation;
`a support structure
`for supporting patterning
`means, the patterning means serving to pattern the
`projection beam according to a desired pattern;
`a substrate table for holding a substrate;
`a projection system for projecting the patterned
`beam onto a target portion of the substrate, and
`a measurement system for measuring defocus of
`the apparatus,
`
`characterized in that:
`
`said measurement system comprises a transmis-
`sive grating and a mirror for directing the projection
`beam to illuminate the grating from behind, wherein,
`in use, the mirror is tilted at an angle relative to the
`plane of the grating to provide a tilted illumination
`beam.
`
`measurement may be used to adjust certain lens
`elements to minimize the wave front aberrations.
`
`5
`
`[0022] Although specific reference may be made in
`this text to the use of the apparatus according to the
`invention in the manufacture of ICs, it should be explic(cid:173)
`itly understood that such an apparatus has many other
`possible applications. For example, it may be employed
`in the manufacture of integrated optical systems, guid-
`10 ance and detection patterns for magnetic domain mem(cid:173)
`ories, liquid-crystal display panels, thin-film magnetic
`heads, etc. The skilled artisan will appreciate that, in the
`context of such alternative applications, any use of the
`terms "reticle", "wafer" or "die" in this text should be con-
`15 sidered as being replaced by the more general terms
`"mask", "substrate" and "target portion", respectively.
`[0023]
`In the present document, the terms "radiation"
`and "beam" are used to encompass all types of electro(cid:173)
`magnetic radiation, including ultraviolet radiation (e.g.
`20 with a wavelength of 365, 248, 193, 157 or 126 nm) and
`EUV (extreme ultra-violet radiation, e.g. having a wave(cid:173)
`length in the range 5-20 nm), as well as particle beams,
`such as ion beams or electron beams.
`[0024] Embodiments of the invention will now be de(cid:173)
`scribed, by way of example only, with reference to the
`accompanying schematic drawings in which:
`
`25
`
`Figure 1 depicts a lithographic projection apparatus
`according to an embodiment of the invention;
`Figure 2 depicts an embodiment of a wave front ab(cid:173)
`erration measuring system incorporated in the lith(cid:173)
`ographic projection apparatus of figure 1;
`Figure 3 is a detailed cross-section of a portion of
`an object grating for use in one embodiment of the
`present invention;
`Figure 4 is a cross-section of an object grating in a
`reticle module for use in another embodiment of the
`present invention;
`Figure 5 is a cross-section of an object grating in a
`reticle module for use in a further embodiment of
`the present invention; and
`Figure 6 is a cross-section of an object grating in a
`reticle module with a tilted mirror for use in another
`embodiment of the present invention.
`
`[0025]
`In the Figures, corresponding reference sym(cid:173)
`bols indicate corresponding parts.
`
`[0026] Figure 1 schematically depicts a lithographic
`projection apparatus according to a particular embodi(cid:173)
`ment of the invention. The apparatus comprises:
`
`·a radiation system Ex, IL, for supplying a projection
`beam PB of radiation (e.g. EUV radiation), which in
`this particular case also comprises a radiation
`source LA;
`
`[0021] The invention also relates to a measurement
`system for measuring wave front aberrations of a pro(cid:173)
`jection system, said measurement system comprising:
`
`a radiation system for providing a projection beam
`of radiation; and
`a projection system holder for holding a projection
`system in the projection beam such that the projec(cid:173)
`tion system is illuminated by the projection beam;
`
`30
`
`35
`
`characterized in that said measurement system
`further comprises:
`
`40
`
`a diffractive element and a structure for increasing
`the pupil filling of the radiation in the pupil of the
`projection system, both movable into the projection
`beam between the radiation system and the projec-
`tion system; and a sensor module for sensing radi(cid:173)
`ation that has traversed the projection system for
`measuring wave front aberrations of the projection
`system.
`With this system it is possible during the manufac(cid:173)
`turing of lens elements to measure more accurately
`the wave front aberrations of a single lens element
`and consequently to choose this element or even
`to reject it if the quality is not sufficient. It is also
`possible to measure more accurately the wave front
`aberrations of a projection system wherein different
`lens elements are put together. The results of such
`
`45
`
`50
`
`55
`
`5
`
`Nikon Exhibit 1006 Page 5
`
`

`

`9
`
`EP 1 403 714 A2
`
`10
`
`a long-stroke module (course positioning) and a short(cid:173)
`stroke module (fine positioning), which are not explicitly
`depicted in Figure 1. However, in the case of a wafer
`stepper (as opposed to a step-and-scan apparatus) the
`mask table MT may just be connected to a short stroke
`actuator, or may be fixed.
`[0031] The depicted apparatus can be used in two dif(cid:173)
`ferent modes:
`
`1. In step mode, the mask table MT is kept essen(cid:173)
`tially stationary, and an entire mask image is pro(cid:173)
`jected in one go (i.e. a single "flash") onto a target
`portion C. The substrate table WT is then shifted in
`the x and/or y directions so that a different target
`portion C can be irradiated by the beam PB;
`2. In scan mode, essentially the same scenario ap(cid:173)
`plies, except that a given target portion C is not ex(cid:173)
`posed in a single "flash". Instead, the mask table
`MT is movable in a given direction (the so-called
`"scan direction", e.g. the y direction) with a speed
`v, so that the projection beam PB is caused to scan
`over a mask image; concurrently, the substrate ta(cid:173)
`ble WT is simultaneously moved in the same or op(cid:173)
`posite direction at a speed V = Mv, in which M is the
`magnification of the lens PL (typically, M = 1/4 or
`1/5). In this manner, a relatively large target portion
`C can be exposed, without having to compromise
`on resolution.
`
`5
`
`10
`
`15
`
`20
`
`25
`
`. a first object table (mask table) MT provided with
`a mask holder for holding a mask MA (e.g. a reticle),
`and connected to first positioning means for accu(cid:173)
`rately positioning the mask with respect to item PL;
`. a second object table (substrate table) WT provid(cid:173)
`ed with a substrate holder for holding a substrate W
`(e.g. a resist-coated silicon wafer), and connected
`to second positioning means for accurately posi(cid:173)
`tioning the substrate with respect to item PL;
`a projection system ("lens") PL (e.g. mirror group)
`for imaging an irradiated portion of the mask MA on(cid:173)
`to a target portion C (e.g. comprising one or more
`dies) of the substrate W.
`
`[0027] As here depicted, the apparatus is of a reflec(cid:173)
`tive type (e.g. has a reflective mask). However, in gen(cid:173)
`eral, it may also be of a transmissive type, for example
`(e.g. with a transmissive mask). Alternatively, the appa(cid:173)
`ratus may employ another kind of patterning means,
`such as a programmable mirror array of a type as re-
`ferred to above.
`[0028] The source LA (e.g. a laser-produced or dis(cid:173)
`charge plasma source) produces a beam of radiation.
`This beam is fed into an illumination system (illuminator)
`IL, either directly or after having traversed conditioning
`means, such as a beam expander Ex, for example. The
`illuminator IL may comprise adjusting means AM for set-
`ting the outer and/or inner radial extent (commonly re(cid:173)
`ferred to as cr-outer and cr-inner, respectively) of the in(cid:173)
`tensity distribution in the beam. In addition, it will gen-
`erally comprise various other components, such as an
`integrator IN and a condenser CO. In this way, the beam
`PB impinging on the mask MA has a desired uniformity
`and intensity distribution in its cross-section.
`[0029]
`It should be noted with regard to Figure 1 that
`the source LA may be within the housing of the litho(cid:173)
`graphic projection apparatus (as is often the case when
`the source LA is a mercury lamp, for example), but that
`it may also be remote from the lithographic projection
`apparatus, the radiation beam which it produces being
`led into the apparatus (e.g. with the aid of suitable di(cid:173)
`recting mirrors); this latter scenario is often the case
`when the source LA is an excimer laser. The current in(cid:173)
`vention and Claims encompass both of these scenarios.
`[0030] The beam PB subsequently intercepts the
`mask MA, which is held on a mask table MT. Having
`been selectively reflected by the mask MA, the beam
`PB passes through the lens PL, which focuses the beam
`PB onto a target portion C of the substrate W. With the
`aid ofthe second positioning means (and interferometric
`measuring means IF), the substrate table WT can be
`moved accurately, e.g. so as to position different target
`portions C in the path of the beam PB. Similarly, the first
`positioning means can be used to accurately position
`the mask MA with respect to the path of the beam PB,
`e.g. after mechanical retrieval of the mask MA from a
`mask library, or during a scan. In general, movement of
`the object tables MT, WT will be realized with the aid of
`
`30
`
`35
`
`40
`
`[0032] Figure 2 shows a wave front aberration meas(cid:173)
`urement system incorporated in the lithographic projec(cid:173)
`tion apparatus of Figure 1. Only a portion of the litho(cid:173)
`graphic projection apparatus is shown in Fig.2, and this
`portion includes the projection system PL. The meas-
`urement system comprises a grating module 3 and a
`sensor module 5. The grating module 3 may be put on
`the mask table MT occasionally, or may form a part of
`said table, and comprises an object grating 7. The sen(cid:173)
`sor module 5 may be put on the substrate table WT oc(cid:173)
`casionally or may form a part of said substrate table WT,
`and comprises an image grating 9, which is a transmis-
`sion grating, a detector 11 and a luminescent layer 13.
`The image grating 9 corresponds to the object grating
`7, but scaled by the magnification M of the projection
`45 system PL mentioned above. The detector 11 can be a
`ceo chip and the luminescent layer 13 converts inci(cid:173)
`dent radiation from the projection beam, such as EUV
`radiation, to radiation to which the detector 11 is more
`sensitive, such as visible light. Depending on the partic(cid:173)
`ular radiation and sensitivity of the detector 11, the lu(cid:173)
`minescent layer 13 is optional and can be omitted.
`[0033] Figure 3 shows an enlarged view of the grating
`module 3. The object grating 7 in this embodiment is a
`reflection grating consists of non-reflective regions 15
`'alternating with reflective regions 17. The grating can
`be a 1 D grating or a 20 grating, or indeed more than
`one differently orientated 1 D grating