NL2010306A - Radiation source for an euv optical apparatus and method of generating euv radiation. - Google Patents

Description

RADIATION SOURCE FOR AN EUV OPTICAL APPARATUS AND METHOD OF

GENERATING EUV RADIATION

Field

[0001] The present invention relates to a radiation source for an EUV optical apparatus (such as a lithographic apparatus) and a method of generating EUV radiation. Background

[0002] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0003] Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of kl.

[0004] In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0005] EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

[0006] When using a plasma source, contamination particles are created as a byproduct of the plasma generation. Generally, such contamination particles are undesired because they adhere for example to reflective surfaces of the lithographic apparatus. Build up of contamination particles on reflective surfaces of the lithographic apparatus (directly or via other non-reflecting surfaces) reduces the reflectivity of those surfaces, and consequently may reduce the achievable throughput of the lithographic apparatus.

SUMMARY

[0007] It is desirable to reduce the accumulation of contamination particles on reflective surfaces of the lithographic apparatus.

[0008] The invention in a first aspect provides radiation source configured to generate extreme ultraviolet radiation, the radiation source comprising: a plasma generator configured to excite a fuel so as to form a plasma at a plasma generation site; an inlet configured to allow gas to enter the radiation source and an outlet configured to allow gas to exit the radiation source; therefore providing for a gas flow within the radiation source; a contamination trap configured to trap debris particles that are generated with the formation of the plasma; and a shaker device for shaking said contamination trap.

[0009] The invention in a further aspect provides for a method of generating extreme ultraviolet radiation, the method comprising: exciting a fuel, thereby forming a plasma at a plasma formation site; establishing a gas flow within the radiation source to an outlet; providing a contamination trap configured to trap dehris particles that are generated with the formation of the plasma; and shaking said contamination trap.

[0010] Further features and advantages of the invention, as well as the stmcture and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0011] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts schematically a lithographic apparatus having reflective projection optics;

Figure 2a is a more detailed view of the apparatus of Figure 1;

Figure 2b shows an alternative source arrangement usable in the apparatus of Figure 2a;

Figure 3 schematically depicts a source of the lithographic apparatus provided with a contamination trap according to an embodiment of the invention;

Figures 4 and 5 show the contamination trap of Figure 3 in more detail; and

Figure 6 is a plot of fuel particle radius (m) on the x-axis, against gas flow velocity (m/s) on the y-axis.

[0012] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0013] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises: an illumination system ( illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g. a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0014] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0015] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0016] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0017] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0018] The projection system, like the illumination system, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0019] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0020] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines 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 exposure.

[0021] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source module SO may be part of an EUV radiation system including a laser, not shown in Figure 1, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0022] Tn such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0023] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0024] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0025] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. Tn scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0026] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0027] Figure 2a shows the apparatus 100 in more detail, including the source module SO, the illumination system IL, and the projection system PS. The source module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma may be formed at a plasma formation site 2 by a laser produced LPP plasma source. The function of source module SO is to deliver EUV radiation beam 20 from the plasma such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source module is arranged such that the intermediate focus IF is located at or near an aperture 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma.

[0028] From the aperture 221 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called “fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the wafer stage or substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated on substrate table WT and masked table MT perform synchronized movements 266, 268 to scan the pattern on patterning device MA through the slit of illumination.

[0029] Each system IL and PS is arranged within its own vacuum or nearvacuum environment, defined by enclosing structures similar to enclosing structure 220. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown in the Figures. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in Figure 2a.

[0030] Considering source module SO in more detail, laser energy source comprising laser 223 is arranged to deposit laser energy 224 into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li), creating the highly ionized plasma with electron temperatures of several 10's of eV. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector CO and focused on the aperture 221. The plasma and the aperture 221 are located at first focal point (plasma formation site 2) and second focal point of collector CO, respectively.

[0031] Although the collector CO shown in Figure 2a is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another. The grazing incidence collector may be suited for use in a DPP source.

[0032] To deliver the fuel, which for example is liquid tin, a droplet generator 226 is arranged within the enclosure 220, arranged to fire a high frequency stream 228 of droplets towards the desired location 2 of the plasma. In operation, laser energy 224 is delivered in a synchronism with the operation of droplet generator 226, to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 224 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 224 is delivered to the cloud at the desired location 2, to generate the plasma. A trap 230 is provided on the opposite side of the enclosing structure 220, to capture fuel that is not, for whatever reason, turned into plasma.

[0033] In an alternative configuration (not illustrated) the EUV radiation may be generated by causing a partially ionized plasma of an electrical discharge to collapse onto an optical axis (e.g. via the pinch effect). This source may be referred to as a discharge produced plasma (DPP) source. Partial pressures of for example 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be used to generate the EUV radiation emitting plasma.

[0034] Figure 2b shows an alternative LPP source arrangement which may be used in place of that illustrated in Figure 2a. A main difference is that the main pulse laser beam is directed onto the fuel droplet from the direction of the intermediate focus point IF, such that the collected EUV radiation is that which is emitted generally in the direction from which the main laser pulse was received.

[0035] Figure 2b shows the main laser beam delivery system 130 emitting a main pulse beam 131 delivered to a plasma formation position 132. At least one optical element of the beam delivery system, in this case a folding mirror 133 is located on the optical axis between plasma position 132 and the intermediate focus. (The term “folding” here refers to folding of the beam, not folding of the mirror.) The EUV radiation 134 emitted by a plasma at position 132, or at least the major portion that is not directed back along the optical axis O into the folding mirror 133 is collected by a grazing incidence collector 135. This type of collector is known already, but is generally used in discharge produced plasma (DPP) sources, not LPP sources. Also shown is a contamination trap 136. A pre-pulse laser 137 is provided to deliver a pre-pulse laser beam 138 to fuel droplets. In this example, the pre-pulse energy is delivered to the side of the fuel droplet that faces away from the intermediate focus point IF. It should be understood that the elements shown in this schematic diagram are not to scale. Furthermore, other alternative arrangements of the pre-pulse and main-pulse laser also also envisaged herein.

[0036] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 20, 21, 26. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source module, the X axis coincides broadly with the direction of fuel stream 228, while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 2a. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2a, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0037] Numerous additional components critical to operation of the source module and the lithographic apparatus as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector3and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus.

[0038] When using a laser produced plasma (LPP) source or discharge produced plasma (DPP) source, contamination may be produced in the form of debris such as fast ions and/or neutral particles (for example Sn (tin)). Such debris may build up on the reflective surface(s) of the collector 3, causing the collector to lose reflectivity and thereby reducing the efficiency of the collector. Contamination by debris may also cause other reflective components of the lithographic apparatus (for example mirrors 22, 24, 28, 30 or patterning device MA) to lose reflectivity over time. The throughput of the lithographic apparatus is dependent upon the intensity of EUV radiation which is incident on a substrate being exposed. Any reduction of reflectivity which arises due to the build up of debris on the collector or other reflective surfaces of the lithographic apparatus may reduce the throughput of the lithographic apparatus.

[0039] Figure 3 shows the source module SO of Figure 2a according to an embodiment of the invention. It should be noted that while the source module of Figure 2a is shown for illustration, the concepts disclosed herein are equally applicable to a source module such as that shown in Figure 2b, or to other types of plasma source modules (including LPP and DPP source modules). The skilled person will have no difficulty applying the teachings disclosed herein to any of the other source modules referred to in this text, or to other suitable plasma source modules.

[0040] For ease of illustration, in Figure 3 the source module SO is oriented vertically rather than at an angle. However, the source module SO may be oriented at any angle, although it will tend to be orientated with the collector towards the bottom end, as illustrated in both Figure 2a and Figure 6. The arrow labeled Fg shows the direction in which gravity may act if orientated in a particular example orientation. The wall 30 of the source module SO is shown, rather than merely showing a schematic box (as was done in Figure 2a).

[0041] As shown in Figure 3, the collector CO includes an opening 31 through which the laser beam (not shown) passes. The laser beam is focused onto the plasma formation site 2, where it is incident upon a fuel droplet generated by the droplet generator 15 and thereby generates a plasma which emits EUV radiation.

[0042] The plasma emits the EUV radiation over a range of directions. However, due to the collector CO having a finite radius and having an opening 31, not all of the EUV radiation is collected by the collector. An example of a suitable collector CO radius is 320 mm, although collectors having other radii may be used. An inner edge 32a and an outer edge 32b of the collector CO together define a cone of EUV radiation 33 which is directed from the collector to an intermediate focus (the intermediate focus is beyond the edge of Figure 3 but is shown in Figure 2a). The EUV radiation cone 33 has an inner boundary which is represented by dotted lines 33a and an outer boundary which is represented by dotted lines 33b.

[0043] A gas (such as for example, Hydrogen, Argon or Nitrogen) may be provided from one or more inlets (for example through opening 31 and at the circumference 32b of the collector 3, although different inlets can be used as an alternative to this, or in addition) which introduce gas into the source module SO in order to allow a gas flow (illustrated here by arrows 39a and 39b) to be established from the inlet to an outlet (not shown, but may be in the region of the arrow 39b, outside of the light path). This gas flow may assist in drawing debris out of the source module SO via the outlet, and thereby reduce contamination of the collector CO and other reflective surfaces of the lithographic apparatus.

[0044] One or more contamination traps 34 may be provided in locations which are outside of the EUV radiation cone 33 formed by the collector 3. For example, a contamination trap 34 may be provided outside of the outer boundary 33b of the EUV radiation cone 33. A cut-away perspective view of the contamination trap 34 is illustrated in Figure 4. In addition, an outlet and associated contamination trap (which may comprise foils) may be provided within the inner boundary 33a of the EUV radiation cone 33, although this is not illustrated here. WO 2010/028899 describes such contamination traps in greater detail and is hereby incorporated by reference.

[0045] The contamination trap 34 provided outside of the outer boundary 33b of the EUV radiation cone 33 may comprise a plurality of foils 34a-h which extend from the wall 30 of the source SO. The foils 34a-h may be frustoconical, and are represented in perspective in three dimensions in Figure 4. The foils 34a-h may be arranged such that they do not extend into the EUV radiation cone 33, thereby preventing the foils reducing the amount of EUV radiation which is available for use by the lithographic apparatus.

The foils 34 reduce the likelihood of debris particles scattering or splashing back from the wall 30 of the source and being incident upon the collector 3.

[0046] The foils 34a-h may include any of the features or configurations described in WO 2010/028899. The contamination trap 34 may comprise tapering blocks, or some other suitable structure. The spacing between adjacent foils 34a-h may for example be less than 40 mm, less than 20 mm, less than 10mm, or less than 5mm.

[0047] The droplet generator 15 is shown in Figure 3 at an angle of approximately 70 degrees relative to the optical axis O of the source SO. Flowever, the droplet generator 15 may be provided at any suitable angle. For example, the droplet generator 15 may be oriented perpendicular to the optical axis O. Where this is the case, one or more of the foils 34a-h may be removed in order to accommodate the droplet generator (which would be moved to the right in Figure 3). Alternatively, a gap may be provided in one or more of the foils 34a-h. In general, it may be possible to provide a gap in one or more foils (i.e. such that the foil does not extend around a full circumference) in order to accommodate a component of the source SO. In such circumstances, the foil which is provided with the gap may still be described as being frustoconical.

[0048] Figure 5 is a side view of the contamination trap 34 of Figures 3 and 4. In Figure 5 the angle Θ of one of the foils 34a relative to a normal from the wall 30 of the source is labeled (for clarity of illustration the angles of the other foils are not labeled). As illustrated, the angle of each of the foils 34a-h is different, such that each foil points at the same location 2a. The plasma formation site 2a may be provided at this location. However, where the foils are so arranged, the plasma formation site will typically be one where the debris particles emitted from the plasma will be incident at a glancing angle on the foils 34a-h, such as locations 2b-2d (or at any location in between).

[0049] The angle at which debris particles are incident upon a foil will depend upon the location of the plasma formation site 2a-d with respect to the foils 34a-h. Figure 5 illustrates this by showing some possible debris particle trajectories as dotted lines 35. In all cases, there is a line of sight from a side of each foil 34a-h to the plasma formation site 2a-d, and debris particles emitted from the plasma will be incident on a given foil 34a-h with a shallow angle. The shallow angle may for example be less than 45 degrees, may for example be less than 30 degrees, and may for example be less than 20 degrees. The shallow angle may be between 0 and 20 degrees, and may be between 0 and 10 degrees. The angle between the angle of incidence of the debris particle and the angle of each foil may be approximately constant.

[0050] When the plasma formation site is at location 2a, the foils 34a-h may be described as pointing towards the plasma formation site 2a. When the plasma formation site is at one of the other locations 2b-d (or some location in between 2a and 2d), then the foils may be described as pointing towards the vicinity of the plasma formation site.

[0051] A contamination trap which corresponds to that shown in Figures 3 to 5 may be provided at some location other than on the walls of the source SO.

[0052] An issue with this type of contamination trap 34 is that accumulation of tin on the foils leads to droplet formation, with large droplets forming (>10μηΤ8 up to mm’s). These large droplets will (for reasons explained below) fall from the foils onto the collector under the influence of gravity, rather than being swept out by the protective gas flow in the source. Removing the droplets from the collector requires shutdown of the system.

[0053] In order to address this, a shaker device 38 (for example an ultrasonic shaker device, such as those used in ultrasonic mixing applications) is provided. This shaker device 38, in combination with the gas protection flow 39a, 39b, can be used for more efficient tin-debris transport. The shaker device 38 may be attached to the contamination trap 34, or to one or more of the foils thereof. Multiple shaker devices may be provided. Similarly, one or more shaker devices may be attached to the contamination trap 136 (or other contamination trap) of the source module of Figure 2b.

[0054] The shaker device 38 shakes the contamination trap 34. The effect of this is to prevent the creation of, and/or to break-up, tin droplets that coalesce on the contamination trap foils 34a-34h, such that any droplets on the foils 34a-34h are kept to smaller than 1 pm in diameter, depending on gas flow conditions. Furthermore, the shaking action causes the tin droplets to be transported to the foil edges. In this way, small droplets that fall from the foils will not be dominated by the gravitational force Fg but by the neutral drag force resultant from the gas flow 39a, 39b, such that the gas flow 39a, 39b flushes the droplet away from the collector and into the gas exhaust.

[0055] In one embodiment the shaker 38 may provide a reciprocal force in a direction as indicated by the arrow on the device, that is along the longitudinal direction of the contamination trap 34. When the shaker device 38 is attached to a single foil, the force may be applied substantially perpendicular to the foil. The shaker may also provide a reciprocal force in two (or even three) dimensions.

[0056] Preferably, the foil edges should be made to be fuel (e.g. tin) repellent to prevent (re-)formation of large droplets and to aid an efficient pick-up of the droplet by means of the drag force. The shaker can be operated on various different timescales: continuous, during dark periods of the EUV source, during wafer swaps etc.

[0057] To achieve the desired effect, the balance of gas flow velocity and maximum droplet size needs to be considered such that the drag force prevails on the droplet, over the gravitational force Fg. The gravitational force is:

where m, r and p is the droplet mass, radius and density respectively.

[0058] The neutral drag force Fd exists in several representations, such as, for example:

where rp is the particle radius, nnis the neutral density, mnis the neutral mass, υ,ι, is the thermal velocity, υη is the neutral velocity (i.e. flow velocity) and υρ is the particle velocity; and

where η is the gas viscosity, dp is the particle diameter, u is the particle velocity V is the gas velocity and and Cc is a correction factor known as the Cunningham correction factor.

[0059] The important thing to note from the above equations is that the gravitational force scales with r3 while the drag force scales with either r or r where r is the droplet radius.

[0060] Figure 6 is a plot of particle radius (m) on the x-axis, against flow velocity (m/s) on the y-axis. The shaded area represents radius/velocity combinations for which the gravitational force will prevail, and are therefore to be avoided. The unshaded area represents radius/velocity combinations for which the drag force will prevail, and for which the droplet will be carried out of the source chamber by the gas flow.

[0061] In practice, the gas flow velocity may be anywhere between 1.8 and 20 m/s. Taking 1.8 m/s as the worst case gas flow velocity, and then using a factor of 100 safety margin GFWc, results in a maximum particle radius Rpmax of 0.01 pm to remain outside of the shaded area of Figure 6. This is shown on the drawing. Consequently the shaker could, in one embodiment, be configured to prevent droplets larger than a threshold of 0.01 pm from forming. However, this threshold can be varied where a different gas flow velocity is known to be used, or a different safety margin is chosen, and in other embodiments, the threshold may be 10pm, or 1pm or 0.1pm.

[0062] The shaker device may be an ultrasonic shaker device of the type used in ultrasonic mixing of emulsions. These help the emulsifying process by reducing the droplet sizes of the liquids being mixed, thereby increasing area. The ultrasonic frequency and/or amplitude should be carefully chosen to produce the required droplet size, such that the droplets are forced to drop before they become too large. For example, the frequencies may be in the range of 10 kHz to 10 Mhz.

[0063] The contamination trap may be heated to a temperature which is sufficiently high such that the debris particles trapped by the contamination trap remain in a liquid state. Such heating may be provided by an induction heater.

[0064] A main advantage of this system is that it can work in-situ so there is no need to shutdown the source for cleaning. As a result, uptime is improved. Also, as well as preventing build-up of contamination particles on reflective surfaces, the concepts herein can also protect sensors/detectors comprised in the source which may also be susceptible to contamination·

[0065] While the concepts disclosed herein have been described specifically in combination with LPP sources, they are also applicable to other types of sources, such as DPP sources. Also the contamination trap may take other forms to that shown, such as comprising a plurality of tapering blocks instead of foils, or of foils in a different arrangement.

[0066] Although specific reference may be made in this text to the use of lithographic apparatus, it should be understood that the invention is more generally applicable to any EUV optical device requiring a plasma source, for example an aerial image measurement system or a reticle inspection system. Where lithographic apparatuses are described, they may have applications other than the manufacture of ICs, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0067] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0068] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A radiation source configured to generate extreme ultraviolet radiation, the radiation source comprising: a plasma generator configured to excite a fuel so as to form a plasma at a plasma generation site; an inlet configured to allow gas to enter the radiation source and an outlet configured to allow gas to exit the radiation source; therefore providing for a gas flow within the radiation source; a contamination trap configured to trap debris particles that are generated with the formation of the plasma; and a shaker device for shaking said contamination trap.

2. The radiation source of clause 1 wherein said shaker device is an ultrasonic shaker device.

3. The radiation source according to clause 1 or clause 2 wherein said shaker device is operable to prevent formation of fuel droplets on said contamination trap of a size where the action of gravity upon the fuel drop will prevail over the drag force upon the fuel drop provided by said gas flow.

4. The radiation source according to clause 3 wherein said shaker device is operable to prevent formation of fuel droplets on said contamination trap that are larger than lOpm.

5. The radiation source according to clause 3 wherein said shaker device is operable to prevent formation of fuel droplets on said contamination trap that are larger than 1 pm.

6. The radiation source according to clause 3 wherein said shaker device is operable to prevent formation of fuel droplets on said contamination trap that are larger than 0.1 pm.

7. The radiation source according to any preceding clause wherein said shaker device is operable substantially continuously during source operation.

8. The radiation source according to any of clause 1 to 6 wherein said shaker device is operable non-continuously, such as during dark periods of the source or during wafer swaps.

9. The radiation source according to any preceding clause wherein said contamination trap is provided on a wall of the source, the contamination trap being configured to reduce an amount of debris particles that scatter or splash from the wall of the source onto a collector of the source.

10. The radiation source according to clause 9, wherein the contamination trap is provided outside an outer boundary of an EUV radiation cone formed by the collector.

11. The radiation source according to any preceding clause wherein the contamination trap comprises a foil trap formed from a plurality of foils, the foils being constructed and arranged to trap debris particles arising from the generation of plasma at the plasma formation site.

12. The radiation source according to clause 11 wherein said shaker is operable to facilitate transport of fuel droplets on said contamination trap to the outer periphery of said foils.

13. The radiation source according to clause 11 or 12 wherein said outer periphery of said foils are made of, or coated with, a material having properties which repel said fuel.

14. The radiation source according to clause 11, 12 or 13, wherein at least part of each foil has a line of sight to the plasma formation site.

15. The radiation source according to clause 14, wherein each foil is arranged at an angle of less than 45 degrees with respect to a trajectory which extends radially from the plasma formation site.

16. The radiation source according to clause 11, 12 or 13 wherein at least part of each foil points towards the plasma formation site, or towards a location which is in the vicinity of the plasma formation site.

17. The radiation source according to any of clause 11 to 16 wherein the shaker device is operable to provide a reciprocal force in a direction perpendicular to one of the foils.

18. The radiation source according to any of clause 1 to 16 wherein the shaker device is operable to provide a reciprocal force in a direction along the longitudinal direction of the contamination trap.

19. The radiation source according to any of clause 1 to 10, wherein the contamination trap comprises a plurality of tapering blocks, the tapering blocks being constructed and arranged to trap debris particles arising from the generation of plasma at the plasma formation site.

20. The radiation source according to any preceding clause wherein a plurality of shaker devices are provided.

21. The radiation source according to any preceding claim, further comprising a heater for heating the contamination trap to a temperature sufficient to melt debris that is trapped by the contamination trap.

22. The radiation source according to clause 21, where in the heater is an induction healer.

23. The radiation source according to any preceding clause wherein the shaker device is operable to shake the contamination trap with a frequency in the range of 10 kHz to 10 Mhz.

24. An EUV optical apparatus, comprising: a radiation source as claimed in any preceding clause configured to generate a beam of EUV radiation; and an illumination system configured to condition the beam of EUV radiation.

25. A method of generating extreme ultraviolet radiation, the method comprising: exciting a fuel, thereby forming a plasma at a plasma formation site; establishing a gas flow within the radiation source to an outlet; providing a contamination trap configured to trap debris particles that are generated with the formation of the plasma; and shaking said contamination trap.

26. The method according to clause 25 wherein said shaking prevents formation of fuel droplets on said contamination trap of a size where the action of gravity upon the fuel drop will prevail over the drag force upon the fuel drop provided by said gas flow.

27. The method according to clause 25 or 26 wherein said shaker device is operated substantially continuously during radiation generation.

28. The method according to clause 25 or 26 wherein said shaker device is operated non-continuously, such as during dark periods of the source or during wafer swaps.

29. The method according to any of clauses 25 to 28 wherein the contamination trap comprises a foil trap formed from a plurality of foils, the foils being constructed and arranged to trap debris particles arising from the generation of plasma at the plasma formation site.

30. The method according to clause 29 wherein said shaking facilitates transport of fuel droplets on said contamination trap to the outer periphery of said foils.

31. The method according to clause 29 or 30 wherein the shaking is provided in a direction perpendicular to one of the foils.

32. The method according to any of clauses 25 to 30 wherein the shaking is provided in a direction along the longitudinal direction of the contamination trap.

33. The method according to any of clauses 25 to 32, further comprising heating the contamination trap to a temperature sufficient to melt debris that is trapped by the contamination trap.

34. The method according to any of clauses 25 to 33 wherein the shaking is performed at a frequency in the range of 10 kHz to 10 Mhz.

Claims (1)

A lithography device comprising: an exposure device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.