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    公开号:NL2012846A
    申请号:NL2012846
    申请日:2014-05-20
    公开日:2014-12-15
    发明作者:Olga Sytina；Erik Buurman；Ramon Hofstra；Niek Kleemans
    申请人:Asml Netherlands Bv；
    IPC主号:
    专利说明:

    LASER RADIATION SYSTEM, PLASMA RADIATION SYSTEM FOR LITHOGRAPHIC APPARATUS AND ASSOCIATED METHODS
    Field
    [0001] The present invention relates to a laser radiation system, and to a plasma radiation system such as that used with a lithographic apparatus. In particular the invention relates to a laser radiation system comprising a laser for providing a laser beam to a fuel source in a laser produced plasma (LPP) source of, for example, an EUV lithographic apparatus.
    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 stmctures to be manufactured.
    [0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
    (1)
    [0005] 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.
    [0006] 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.
    [0007] 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.
    [0008] The laser used to excite the fuel may be a gas laser (such as a C02 laser), which uses a gas as a gain medium, which is excited by an excitation electric field (such as a radio frequency (RF) electric field). The excitation electric field may be pulsed during an exposure period, such that during each excitation electric field “on” pulse, the gas laser outputs a number of laser pulses.
    [0009] The amount of EUV radiation produced in an LPP source is dependent on (amongst other factors) the output power of the gas laser exciting the fuel. It has been observed that the average power of the laser radiation produced by the gas laser during each excitation electric field “on” pulse may be insufficient or at least unsatisfactory when compared to the peak level of the laser radiation produced during this time.
    SUMMARY
    [0010] It is desirable, in an LPP source, to increase the average power output of radiation being output from a gas laser during an on pulse of the excitation electric field without necessarily increasing the peak power output.
    [0011] The invention in a first aspect provides for a laser radiation system comprising: a gas laser comprising a gas gain medium; and a power supply for supplying power for excitation of said gas gain medium; wherein the power supply is operable to supply power to the gas gain medium at an intermediate amplitude level which is greater than zero but below a lasing threshold, said lasing threshold being a minimal amplitude level required for laser emission.
    [0012] The invention in a further aspect provides for a method of operating a gas laser radiation source comprising: supplying power to a gas gain medium of the gas laser radiation source at an intermediate amplitude level which is greater than zero but below a lasing threshold, said lasing threshold being a minimal amplitude level required for laser emission.
    [0013] The invention in a further aspect provides for a method of generating output radiation from a plasma, comprising: supplying power to a gas gain medium of a gas laser at a lasing amplitude level which is above a lasing threshold required for laser emission, so such that said gas laser emits radiation; using said emitted radiation to excite a fuel at a plasma generation site in order to generate a plasma; and supplying power to the gas gain medium at an intermediate amplitude level which is greater than zero but below the lasing threshold, during at least some of the time when no output radiation is being generated.
    [0014] Further features and advantages of the invention, as well as the structure 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
    [0015] 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:
    [0016] Figure f depicts schematically a lithographic apparatus having reflective projection optics;
    [0017] Figure 2 is a more detailed view of the apparatus of Figure f, illustrating a laser radiation system and EUV radiation system in accordance with embodiments of the invention; and
    [0018] Figure 3 shows an alternative configuration for the LPP radiation source in the apparatus of Figures f and 2;
    [0019] Figure 4 is a signal diagram showing a typical drive power signal for a laser;
    [0020] Figure 5 is a signal diagram showing a typical drive power signal for a laser during a single miniburst and the resultant output signal;
    [0021] Figure 6 is a signal diagram showing a drive power signal for a laser during a single miniburst and the resultant output signal, operating in accordance with a first embodiment of the invention; and
    [0022] Figure 7 is a signal diagram showing a drive power signal for a laser during a single miniburst and the resultant output signal, operating in accordance with a second embodiment of the invention.
    [0023] 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
    [0024] Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises:
    [0025] an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation).
    [0026] 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;
    [0027] 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
    [0028] 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.
    [0029] 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.
    [0030] 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.
    [0031] 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.
    [0032] 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.
    [0033] 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.
    [0034] As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).
    [0035] 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.
    [0036] 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.
    [0037] In 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.
    [0038] 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.
    [0039] 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 PS1 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.
    [0040] The depicted apparatus could be used in at least one of the following modes:
    [0041] 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.
    [0042] 2. In 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.
    [0043] 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.
    [0044] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
    [0045] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectmm. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using C02 laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.
    [0046] The radiation system 42 embodies the function of source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector mirror 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.
    [0047] As part of an LPP radiation source, a laser radiation system 61 (described in more detail below) is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector mirror 50. Also, the radiation system includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a predetermined plasma position 73.
    [0048] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a 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 includes the wanted EUV radiation which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector 50 onto the intermediate focus point IF.
    [0049] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56 onto a patterning device (e.g., reticle or mask) positioned on a support (e.g., reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.
    [0050] 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 55, 56, 57. 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 of 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 collector module 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated in Figure 3. 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 2, 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.
    [0051] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR radiation present from the laser beam 63. The non-EUV wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non-EUV radiation. As schematically depicted in Figure 2, a transmissive SPF may be applied upstream of virtual source point IF. Alternatively or in addition to such a filter, filtering functions can be integrated into other optics. For example a diffractive filter can be integrated in collector 50 and/or mirrors 53, 54 etc., by provision of a grating structure tuned to divert the longer, IR radiation away from the virtual source point IF. Filters for IR, DUV and other unwanted wavelengths may thus be provided at one or more locations along the paths of beams 55, 56, 57, within source module (radiation system 42), the illumination system IF and/or projection system PS.
    [0052] Figure 3 shows an alternative LPP source arrangement which may be used in place of that illustrated in Figure 2. 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. Figure 3 shows the main laser 30 emitting a main pulse beam 31 delivered to a plasma generation site 32 via at least one optical element (such as a lens or folding mirror) 33. The EUV radiation 34 is collected by a grazing incidence collector 35 such as those used in discharge produced plasma (DPP) sources. Also shown is a debris trap 36, which may comprise one or more stationary foil traps and/or a rotating foil trap, and a pre pulse laser 37 operable to emit a pre pulse laser beam 38.
    [0053] To deliver the fuel, which for example is liquid tin, a droplet generator or target material supply 71 is arranged within the source chamber 47, to fire a stream of droplets towards the plasma formation position 73. In operation, laser beam 63 may be delivered in a synchronism with the operation of target material supply 71, to deliver impulses of radiation to turn each fuel droplet into a plasma. The frequency of delivery of droplets may be several kilohertz, or even several tens or hundreds of kilohertz. In practice, laser beam 63 may be delivered by a laser radiation system 61 in at least two pulses: a pre pulse PP 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 (or deforms it into a flattened “pancake” shape), and then a main pulse MP of' laser energy is delivered to the cloud at the desired location, to generate the plasma. In a typical example, the diameter of the plasma is about 0.2-0.5 mm. A trap 72 is provided on the opposite side of the enclosing structure 47, to capture fuel that is not, for whatever reason, turned into plasma.
    [0054] Referring to laser radiation system 61 in more detail, the laser in the illustrated example is of the ΜΟΡΑ (Master Oscillator Power Amplifier) type, although it may be any RF excited gas (usually C02) laser, for example a no master oscillator (ΝΟΜΟ) laser. The laser radiation system 61 includes a “master” laser or “seed” laser, labeled MO in the diagram, followed by a power amplifier system PA, for firing a main pulse of laser energy towards an expanded droplet cloud, and a pre pulse laser for firing a pre pulse of laser energy towards a droplet. A beam delivery system 65 is provided to deliver the laser energy 63 into the source chamber 47. In practice, the pre-pulse element of the laser energy may be delivered by a separate laser. Laser radiation system 61, target material supply 71 and other components can be controlled by a control module 20. Control module 20 may perform many control functions, and have many sensor inputs and control outputs for various elements of the system. Sensors may be located in and around the elements of radiation system 42, and optionally elsewhere in the lithographic apparatus. In one embodiment of the present invention, the main pulse and the pre pulse are derived from a same laser. In another embodiment of the present invention, the main pulse and the pre-pulse are derived from different lasers which are independent from each other.
    [0055] To operate, the laser radiation system 61 requires a power supply such as a radio frequency power supply RF, which is used to excite the gas gain medium of the laser radiation system 61. The radio frequency power supply RF may be comprised within the C02 laser, within the power source, elsewhere within the lithography apparatus, or remote from the lithography apparatus (including outside of the clean room, provided the cables are sufficiently long). The radio frequency power supply RF may be dedicated to the power amplifier system PA, with a separate supply or supplies provided for the other elements of the laser radiation system 61. Alternatively, the same radio frequency power supply RF may power other elements of the laser radiation system 61 in addition to the power amplifier system PA. Radio frequency power supply RF may be controlled by controller 20 or by other means.
    [0056] Control module 20 in this example may control the location of the plasma 73 (the source of the EUV radiation), by controlling the injection of the fuel, and also for example the timing of energizing pulses from laser.
    [0057] In addition to monitoring the position of the plasma, sensors at the illumination system and sensors at the reticle level monitor the intensity of the EUV radiation, and provide feedback to control module 20. Intensity may be controlled for example by adjusting the energy of the laser pulses.
    [0058] In projection lithography it is desirable to keep an effective exposure dose within a tolerance during imaging. Dose is the term for the amount of energy (per unit area) that the photoresist is subjected to upon exposure. For optical lithography it is a function of the light intensity and the exposure time. Dose control of EUV energy/power can be achieved by providing the EUV energy in minibursts of pulses, with the number of pulses comprised within a miniburst being varied accordingly, whereby each pulse has a fixed EUV energy. It will be appreciated that this fixed energy represents the average energy supplied, and there will be some energy level fluctuation between pulses. “Fixed” therefore should be taken to mean that there is no intentional variation in pulse-to-pulse energy. Each EUV pulse is emitted as a result of a corresponding C02 laser pulse directed at the plasma formation position 73. In turn, the C02 laser pulses are emitted by the laser radiation system 61 when the gas gain medium (laser gas mix) is excited by a radio frequency (RF) power supply RF. The RF power supply may excite the gas by application of an electric field using a pair of electrodes comprised within the C02 laser. During a miniburst, a drive signal enables RF generators thereby triggering the C02 laser to fire. Between minibursts within one exposure, as well as between exposures, the RF power is disabled, and the C02 laser is turned off.
    [0059] Figure 4 is a signal diagram which illustrates this mode of operation. It shows the RF drive signal 400 (i.e., the RF voltage signal applied to the power supply electrodes) over time. Also shown are the corresponding C02 laser output pulses 410; which in turn correspond to EUV radiation output from the source. During an exposure period 430, it can be seen that the RF drive signal 400 is pulsed between two states (“on” and “off’), wherein “on” corresponds to a lasing amplitude level sufficient for laser radiation to be generated (at a desired power level), and “off’ is zero. Each time period during which the RF drive signal 400 is switched on defines a miniburst 420. Between each exposure period 430 is a dark time 440 during which the C02 laser is switched off. The typical duration of a miniburst 420 is about 1ms.
    [0060] In a typical example, energizing pulses of laser radiation are delivered at a rate of 40-50 kHz, corresponding to a period of 20-25 ps, and in bursts lasting anything from, say, 20 ms to 20 seconds; each burst defining an exposure period 430. The duration of an individual laser pulse (for example 410, 530) can vary between from around 0.1 -0.2ps, up to 1 ps depending on chosen laser embodiment. These values are provided purely for example and it is envisaged that higher frequency lasers, for example 80-100kHz or more, may be used in future.
    [0061] Because the electrical discharge in the C02 laser is switched off for relatively long periods of time between exposures, and even between minibursts, there is a delay in the generation of C02 pulses, and consequently EUV pulses, after the drive signal is switched on. This is illustrated in Figure 5, which shows the RF drive signal 400 and the corresponding C02 laser output pulses 410 during a single miniburst. It can be seen that generation of the C02 laser output pulses 410 only begins after a substantial delay 520. Also, as a result of thermal transients and gain build up in the laser gas, output C02/EUV energy is lower for the first few C02 laser pulses 530. As a consequence of these effects, the average pulse energy 540 is lower than maximal during the miniburst.
    [0062] The delay time between RF drive signal and actual C02/EUV pulses is determined by (i) the intrinsic properties of RF generators (how fast RF power gets to the required level thus providing gain build up in laser gas); and more fundamentally by (ii) the time needed to create discharge/plasma in the C02 tubes. It is known that this delay time can vary between 150ps and 200+ps long. For a 40kHz repetition rate, and assuming that a miniburst comprises about 40 C02 laser output pulses, a 200ps delay would mean a loss of about 20% average pulse energy (8 pulses out of 40). In fact this loss may be a little greater when the thermal transient effects are taken into account.
    [0063] One possibility for reducing this delay time would be to keep the C02 laser switched on continuously, and preventing C02/EUV light entering the scanner between exposures, either by providing a C02 beam dump or an EUV shutter. However, at high powers there is risk of damage to the apparatus using such a method.
    [0064] Figure 6 illustrates an improved proposal for reducing this delay time. In this proposal the RF drive signal 400 is maintained at an intermediate amplitude level (labeled “low” on Figure 6) during dark time, instead of being switched off completely. As a consequence, an intermediate RF power level is applied to the discharge gas in the C02 tubes during this dark time, such that the discharge does not quench completely. The dark time during which an RF drive signal is applied to the discharge gas at this intermediate amplitude level, may comprise the time between each exposure, and optionally, also the time between each miniburst.
    [0065] As a result of this methodology, the delay 520 between onset of RF drive signal 400 and generation of C02 pulses 410 is minimized. In addition, the effect of transients in C02/EUV energy is also reduced insofar as that there are fewer lower power pulses 530, and/or their power levels are not as low compared to when the RF signal has been switched off completely. The increase in average EUV pulse energy, and therefore EUV power, may be as much as 28% while maintaining the same dose control overhead: 20% of which being attributable to fewer missing pulses (an additional 6-8 pulses per miniburst) and 8% being attributable to the reduction of transient effects. Dose control overhead per miniburst is defined as the available pulses between the EUV Gate before lowering of the RF gate (RF drive signal).
    [0066] The low RF level applied during dark time should be set at a level such that no laser output power is generated during dark time. It is known that gas discharge lasers, such as C02 lasers, only begin generating output power after gain in the laser gas exceeds losses, which happens when an RF drive signal power reaches or exceeds a required lasing threshold. The lasing threshold being a minimal power amplitude level required for laser emission. This lasing threshold is shown on Figure 6 by the line It. Provided that the RF drive signal power remains below this lasing threshold, the C02 laser output power will be zero. In this way, there should be no additional infra-red heat load between exposures. Above this lasing threshold It the output laser power varies proportionally with the RF signal power. Preferably, the level of low RF power should be adjusted for each power amplifier in the C02 laser.
    [0067] The principle of maintaining a reduced power in the discharge gas is supported by experimental data on the variation of dark lime between minibursls and the number of missing pulses. It can be shown that with a higher duty cycle (shorter dark time between minibursts), fewer pulses are missing at the beginning of the miniburst.
    [0068] Figure 7 illustrates a refinement to the RF drive signal 400, compared to that of Figure 6. When the RF drive signal is lowered (as may be the case when the desired dose per miniburst is reached), there may be a contribution resultant from after-pulses. To limit the effect of these after-pulses on the dose algorithm, the RF drive signal 400 may take the form of that shown in Figure 7. In this embodiment, the RF drive signal is switched off completely for a short time period immediately after a miniburst, and then raised to the intermediate amplitude level already described prior to the beginning of the next miniburst.
    [0069] Although specific reference may be made in this text to the provision and operation of an EUV radiation source in a lithographic apparatus, it should be understood that the EUV radiation apparatus described herein may have other applications in EUV optical apparatus. Further in the case of a lithographic apparatus, this may have other applications besides 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.
    [0070] 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.
    [0071] 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A laser radiation system comprising: a laser comprising a gas gain medium; and a power supply for supplying power for excitation of said gas gain medium; wherein the power supply is operable to supply power to the gas gain medium at an intermediate amplitude level which is greater than zero but below a lasing threshold, said lasing threshold being a minimal amplitude level required for laser emission.
    2. A laser radiation system as claimed in clause 1 wherein said intermediate amplitude level is sufficient to maintain discharge in the gas gain medium without lasing.
    3. A laser radiation system as claimed in clause 1 or 2 wherein said power supply is a radio frequency power supply.
    4. A laser radiation system as claimed in clause 1, 2 or 3 wherein said laser is a carbon dioxide laser, said gas gain medium comprising carbon dioxide.
    5. A laser radiation system as claimed in any preceding clause wherein said laser is of the Master Oscillator Power Amplifier type.
    6. A laser radiation system as claimed in any of clauses 1 to 4 wherein said laser is of the no Master Oscillator type.
    7. A laser radiation system as claimed in any preceding clause wherein said power supply comprises a pair of electrodes being operable to supply power to the gas gain medium by generating an electric field within the gas gain medium.
    8. A laser radiation system as claimed in any preceding clause wherein said power supply is operable to supply power to the gas gain medium at one of at least two amplitude levels: a lasing amplitude level which is above the lasing threshold; and said intermediate amplitude level.
    9. A plasma radiation system comprising: a laser radiation system as claimed in any one of clauses 1 to 8 being operable to emit a beam of radiation; and a fuel source operable to supply fuel to a plasma generation site where said fuel will be contacted by said beam of radiation to form a plasma which emits output radiation; wherein the power supply is operable to: supply power to the gas gain medium at said intermediate amplitude level during at least some of the time when no output radiation is to be generated.
    10. A plasma radiation system according to clause 9 wherein the power supply is operable to supply power to the gas gain medium at said lasing amplitude level so as to cause said laser to emit said beam of radiation for forming said plasma, during periods when output radiation is to be generated.
    11. The plasma radiation system as claimed in clause 9 or 10 being operable to emit said output radiation during exposure periods, wherein said power supply is operable to supply power to the gas gain medium at said intermediate amplitude level during at least some of the time between exposure periods.
    12. The plasma radiation system as claimed in clause 11 being operable such that, during each of said exposure periods, said power supply supplies power to the gas gain medium in pulses such that the supplied power is pulsed between said lasing amplitude level and said intermediate amplitude level.
    13. The plasma radiation system as claimed in clause 12 wherein the laser radiation system is operable such that the laser emits a plurality of pulses of radiation of substantially fixed amplitude during each pulse of power supplied to the gas gain medium at said lasing amplitude level.
    14. The plasma radiation system as claimed in clause 13 wherein the number of pulses of radiation emitted by said laser during each pulse of power supplied to the gas gain medium at said lasing amplitude level is variable so as to control the output dose level of said output radiation.
    15. The plasma radiation system as claimed in any of clauses 11 to 14 wherein the laser radiation system is operable such that no power is applied to the gas gain medium during a period immediately following each exposure period, the power supply being operable to supply power to the gas gain medium at said intermediate amplitude level immediately following this period, and prior to the next exposure period.
    16. The plasma radiation system as claimed in any of clauses 10 to 15 wherein said output radiation has a wavelength of 20nm or less.
    17. A method of operating a gas laser radiation source comprising: supplying power to a gas gain medium of the gas laser radiation source at an intermediate amplitude level which is greater than zero but below a lasing threshold, said lasing threshold being a minimal amplitude level required for laser emission.
    18. A method as claimed in clause 17 wherein said intermediate amplitude level is sufficient to maintain discharge in the gas gain medium without lasing.
    19. A method as claimed in clause 17 or 18 wherein said power supply is a radio frequency power supply.
    20. A method as claimed in clause 17, 18 or 19 wherein said gas laser is a carbon dioxide laser, said gas gain medium comprising carbon dioxide.
    21. A method as claimed in any of clauses 17 to 20 wherein said gas laser is of the Master Oscillator Power Amplifier type.
    22. A method as claimed in any of clauses 17 to 20 wherein said gas laser is of the no Master Oscillator type.
    23. A method as claimed in any of clauses 17 to 22 comprising the further step of supplying power to the gas gain medium at a lasing amplitude level which is above the lasing threshold.
    24. A method of generating output radiation from a plasma, comprising: supplying power to a gas gain medium of a laser at a lasing amplitude level which is above a lasing threshold required for laser emission, such that said laser emits beam of radiation; using said emitted beam of radiation to excite a fuel at a plasma generation site in order to generate a plasma; and supplying power to the gas gain medium at an intermediate amplitude level which is greater than zero but below the lasing threshold, during at least some of the time when no output radiation is being generated.
    25. A method as claimed in clause 24 wherein said intermediate amplitude level is sufficient to maintain discharge in the gas gain medium without lasing.
    26. A method as claimed in clause 24 or 25 wherein said applied power is a radio frequency power.
    27. A method as claimed in clause 24, 25 or 26 wherein said laser is a carbon dioxide laser, said gas gain medium comprising carbon dioxide.
    28. A method as claimed in any of clauses 24 to 27 wherein said laser is of the Master Oscillator Power Amplifier type.
    29. A method as claimed in any of clauses 24 to 27 wherein said laser is of the no Master Oscillator type.
    30. A method as claimed in any of clauses 24 to 29 comprising supplying power to the gas gain medium at said intermediate amplitude level during at least some of the time between exposure periods, said exposure periods being periods during which output radiation is emitted.
    31. A method as claimed in clause 30 comprising pulsing the power supplied to the gas gain medium between said lasing amplitude level and said intermediate amplitude level during each of said exposure periods.
    32. A method as claimed in clause 31 wherein the laser emits a pulse of radiation of substantially fixed amplitude for each pulse of power supplied to the gas gain medium at said lasing amplitude level.
    33. A method as claimed in clause 31 or 32 comprising the step of selecting the number of pulses of radiation emitted by said laser during each of said exposure periods so as to control the output dose level of said output radiation.
    34. A method as claimed in any of clauses 30 to 33 wherein no power is supplied to the gas gain medium during a period immediately following each exposure period; and power is supplied to the gas gain medium at said intermediate amplitude level immediately following this period, and prior to the next exposure period.
    35. A method as claimed in any of clauses 24 to 34 wherein said output radiation has a wavelength of 20nm or less.
    36. A lithographic apparatus comprising the plasma radiation system of any of clauses 9 to 16.
    37. A method for controlling a laser comprising a gas gain medium by providing power for excitation of said gas gain medium at: a lasing amplitude level which is above a lasing threshold to effectuate laser emission from the laser, said lasing threshold being a minimal amplitude level required for laser emission; and, an intermediate amplitude level which is greater than zero but below the lasing threshold during at least some time when no laser emission is required.
    权利要求:
    Claims (1)
    [1]
    A lithography device comprising: an illumination 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.
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    同族专利:
    公开号 | 公开日
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    法律状态:
    2015-06-10| WDAP| Patent application withdrawn|Effective date: 20150106 |
    优先权:
    申请号 | 申请日 | 专利标题
    US201361834196P| true| 2013-06-12|2013-06-12|
    US201361834196|2013-06-12|
    US201361875244P| true| 2013-09-09|2013-09-09|
    US201361875244|2013-09-09|
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