Radiation system for controlling a burst of radiation pulses

Abstract

A radiation system for controlling a burst of radiation pulses includes an optical element, a controller, an actuator, and a sensor. The optical element is configured to interact with a radiation pulse to control a characteristic of the radiation pulse, the characteristic of the radiation pulse depending on a configuration of the optical element. The controller is operable to generate a control signal. The actuator is configured to receive the control signal from the controller and control the configuration of the optical element according to the control signal. The sensor is operable to determine the characteristic of the pulse that has interacted with the optical element. The control signal for a given pulse in a given burst depends on the determined characteristic from a corresponding pulse in a previous burst.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application No. 62 / 933,140, ​​filed on November 8, 2019, and entitled “RADIATION SYSTEM FOR CONTROLLING BURSTS OF PULSES OF RADIATION,” which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present disclosure relates to systems and methods for generating bursts of radiation pulses, such as for use in a lithographic apparatus. Specifically, for example, the present disclosure may relate to systems and methods for generating bursts of radiation pulses that switch between different radiation wavelengths within the burst. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (often also referred to as a "design layout" or "design") of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] As semiconductor manufacturing processes continue to advance, the size of circuit elements continues to decrease, while the number of functional elements (such as transistors) per device has steadily increased over the decades, following a trend commonly referred to as "Moore's Law." To keep up with Moore's Law, the semiconductor industry is pursuing technologies that can create smaller and smaller features. In order to project a pattern on a substrate, a lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature, which is patterned into the resist on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm and 13.5nm. Compared to lithography apparatuses using radiation with a wavelength of, for example, 193nm, lithography apparatuses using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4nm to 20nm (for example, 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Typically, excimer lasers are designed to operate with specific gas mixtures; therefore, changing the wavelength can be complex. In particular, changing the center wavelength from one discharge to the next (“pulse to pulse”) is challenging. However, there may be situations where the ability to change wavelength is desirable. For example, in 3D NAND hierarchies of memory (i.e., memories with structures similar to NAND (“NOT-AND”) gates stacked on top of each other). The transition from 2D to 3D NAND architecture requires significant changes in the manufacturing process. In 3D NAND manufacturing, the challenges are primarily driven by etching and deposition processes at extreme aspect ratios (i.e., the ratio of the hole diameter to its depth). Creating complex 3D structures with extremely high aspect ratio (HAR) features is complex and requires extremely high precision, and ultimately, process uniformity and repeatability are required to achieve scaling. Furthermore, as the height of the multilayer stack increases, the difficulty of achieving consistent etching and deposition at the top and bottom of the stack (e.g., a memory array) also increases.

[0007] These considerations lead to the need for a larger depth of focus. For single wavelength light, the depth of focus (DOF) is given by the relationship DOF = ±m2λ / (NA) 2 Determine, where λ is the wavelength of the illuminating light, NA is the numerical aperture, and m2 is a practical factor that depends on the resist process. Due to the greater depth of focus requirements in 3D NAND lithography, sometimes more than one exposure is performed on the wafer, each time using a different laser wavelength.

[0008] Furthermore, the material that makes up the lens that focuses the laser radiation is dispersive, so different wavelengths focus at different depths in the resist. This is another reason why having the ability to change wavelength is desirable.

[0009] Radiation systems, such as deep ultraviolet (DUV) radiation systems, include systems for controlling the wavelength of the generated radiation. These wavelength control systems can include feedback and feedforward compensators to improve wavelength stability. Characteristically, it is desirable that the target or reference wavelength, i.e., the wavelength commanded by the wavelength control system, does not change rapidly during laser operation. Therefore, the compensator can be primarily used to suppress transient interference. In applications where two different wavelengths of DUV light are generated, the reference wavelength can have two set points during exposure: a first set point at the first wavelength and a second set point at the second wavelength. The reference wavelength can be modulated between these two set points.

[0010] It may be desirable to provide a system and method for generating radiation pulses having controlled characteristics, such as wavelength, that at least partially addresses one or more problems associated with known systems and methods, whether identified herein or elsewhere. Summary of the Invention

[0011] According to a first aspect, there is provided a radiation system for controlling a burst of radiation pulses, comprising: an optical element configured to interact with a radiation pulse to control a property of the radiation pulse, the property of the radiation pulse being dependent on a configuration of the optical element; a controller operable to generate a control signal; an actuator configured to receive the control signal from the controller and to control the configuration of the optical element in dependence on the control signal; and a sensor operable to determine the property of a pulse that has interacted with the optical element; wherein the control signal for a given pulse in a given burst is dependent on the determined property of a corresponding pulse from at least one previous burst.

[0012] As now discussed, the radiation system according to the first aspect is advantageous. The radiation system contains a feedback loop that uses the determined property (e.g. wavelength) of a pulse that has interacted with the optical element from at least one previous burst. In particular, the control signal for a given pulse in a given burst can be given by the control signal that depends on the control signal for one or more corresponding pulses from a previous burst plus a feedback correction. The feedback correction can depend on (e.g. be proportional to) the difference between the determined property of the corresponding pulse from the previous burst and a nominal or target property of the corresponding pulse from the previous burst(s). Advantageously, this allows the radiation system to take into account and at least partially correct for transient effects caused by different control signals being used during each burst of pulses and between bursts of consecutive pairs of pulses. Such transient effects can be significant, particularly if the dynamic response of the actuator and optical element system is under-damped.

[0013] The control signal for a given pulse in a given burst can depend on the determined property of a group of pulses from at least one previous burst.

[0014] The group of pulses can comprise a plurality of pulses. The group of pulses from at least one previous burst, for example, can be a corresponding group of pulses from at least one previous burst. It will be appreciated that for each pulse in a burst, there can be a pulse in each previous burst that corresponds exactly to that pulse. For example, the 10th pulse in a given burst can correspond exactly to the 10th pulse in each of the other bursts. For a given pulse in a given burst, the corresponding group of pulses from a previous burst can comprise the pulse from the previous burst that corresponds exactly to the given pulse, in addition to one or more surrounding pulses from the previous burst. For example, for the 10th pulse in a given burst, the corresponding group of pulses from a previous burst can comprise the 9th, 10th and 11th pulses from the previous burst.

[0015] The control signal for a given pulse in a given burst can be given by the control signal for the corresponding pulse in at least one previous burst plus a correction term. The correction term can depend on the determined characteristics of the corresponding pulse from the at least one previous burst. Thus, the feedforward control signal for a given burst can include a feedback correction term based on the previous burst(s).

[0016] The correction term may be proportional to a difference between the determined characteristic of a corresponding pulse from at least one previous burst and a nominal value of the characteristic.

[0017] For example, the determined characteristic may be the wavelength of the radiation. The correction term for a given pulse in a given burst may be proportional to the difference between the measured wavelength and the nominal or target wavelength for the corresponding pulse in the previous burst. For embodiments in which the control signal for a given pulse in a given burst depends on the determined characteristics of corresponding pulses from more than one previous burst, the correction term for the pulse may, for example, be a weighted sum of the differences between the measured wavelength and the nominal or target wavelength for the corresponding pulse in each previous burst.

[0018] The correction term may be proportional to the gain.For any pulse in the burst that is not the first pulse, the gain may be equal to the first gain, and for any pulse in the burst that is the first pulse, the gain may be equal to the second gain.

[0019] Advantageously, in this way, different gains can be applied to transients that occur within each burst and to transients that occur due to gaps between consecutive bursts.

[0020] The characteristic may be the wavelength of the radiation pulse.

[0021] The controller may be configured such that the control signal oscillates.

[0022] With this arrangement, the radiation system can generate bursts of radiation pulses whereby a characteristic (eg wavelength) of the pulses oscillates. Advantageously, using multiple (eg two) different wavelengths may be advantageous for use in a lithography system to increase depth of focus.

[0023] The actuator may include a piezoelectric element configured to rotate the optical element to control the angle of incidence of the radiation pulse on the optical element.

[0024] The optical element may comprise a grating configured to reflect the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to the output of the radiation system.

[0025] The optical element may comprise a prism configured to refract the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to the output of the radiation system.

[0026] The control signal for a given pulse in the first burst may be determined based on a dynamic model of the optical element and the actuator.

[0027] According to a second aspect, a lithography system is provided, comprising: a radiation system for controlling bursts of radiation pulses, the radiation system comprising: an optical element configured to interact with the radiation pulses to control characteristics of the radiation pulses, the characteristics of the radiation pulses depending on the configuration of the optical element; a controller operable to generate a control signal; an actuator configured to receive the control signal from the controller and to control the configuration of the optical element according to the control signal; and a sensor operable to determine characteristics of the pulses that have interacted with the optical element; wherein the control signal for a given pulse in a given burst depends on the determined characteristics of a corresponding pulse from at least one previous burst; and a lithography device configured to receive the radiation pulses from the radiation system, pattern the radiation pulses, and project the patterned radiation pulses onto a target.

[0028] According to a third aspect, there is provided a method of controlling bursts of radiation pulses, comprising: interacting with the radiation pulses using an optical element to control characteristics of the radiation pulses; determining characteristics of the pulses that have interacted with the optical element; generating a control signal; and controlling the configuration of the optical element based on the control signal; wherein the control signal for a given pulse in a given burst depends on the determined characteristics of a corresponding pulse from at least one previous burst.

[0029] The method according to the third aspect can be implemented using a radiation system according to the first aspect. The method uses a feedback loop that uses a determined characteristic (e.g. wavelength) of a pulse from a previous burst that interacts with an optical element. In particular, the control signal for a given pulse in a given burst can be given by the control signal for the corresponding pulse from the previous burst plus a feedback correction. The feedback correction can depend on (e.g. be proportional to) the difference between the determined characteristic of the corresponding pulse from the previous burst and the nominal or target characteristic of the corresponding pulse from the previous burst. Advantageously, this allows the method to take into account, and at least partially correct for, transient effects produced by different control signals that are used during the burst of each pulse and between bursts of consecutive pairs of pulses. Such transient effects may be significant, particularly if the dynamic response of the actuator and optical element system is underdamped.

[0030] The control signal for a given pulse in a given burst may depend on a determined characteristic of a group of pulses from at least one previous burst.

[0031] The control signal for a given pulse in a given burst may be given by the control signal for the corresponding pulse in the previous burst plus a correction term, the correction term being dependent on the determined characteristic of the corresponding pulse from at least one previous burst.

[0032] In this way, the feedforward control signal for a given burst may contain a feedback correction term based on the previous burst.

[0033] The correction term may be proportional to a difference between the determined characteristic of a corresponding pulse from at least one previous burst and a nominal value of the characteristic.

[0034] For example, the determined characteristic may be the wavelength of the radiation.The correction term for a given pulse in a given burst may be proportional to the difference between the measured wavelength and the nominal or target wavelength of the corresponding pulse in the previous burst.

[0035] The correction term may be proportional to the gain.For any pulse in the burst that is not the first pulse, the gain may be equal to the first gain, and for any pulse in the burst that is the first pulse, the gain may be equal to the second gain.

[0036] Advantageously, in this way, different gains can be applied to transients that occur within each burst and transients that occur due to gaps between consecutive bursts.

[0037] The characteristic may be the wavelength of at least one pulse of radiation.

[0038] The generated control signal may oscillate.

[0039] With this arrangement, the radiation system can generate bursts of radiation pulses whereby the pulse characteristics (eg wavelength) oscillate. Advantageously, using multiple (eg two) different wavelengths may be advantageous for use in a lithography system to increase the depth of focus.

[0040] The control signal for a given pulse in the first burst of a squadron can be determined based on the dynamic models of the optical element and the actuator. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Various versions of the systems and methods will now be described, by way of example only, with reference to the accompanying schematic diagrams, in which:

[0042] Figure 1 An overview of the photolithography apparatus is schematically depicted;

[0043] Figure 2 A diagram schematically depicts the overall concept of a lithography system according to one aspect of the disclosed technical solution;

[0044] Figure 3A diagram schematically depicts the overall concept of a lighting system according to one aspect of the disclosed technical solution;

[0045] Figure 4 is a schematic diagram of a radiation system for controlling bursts of radiation pulses;

[0046] Figure 5 Yes Figure 4 Schematic diagram of control of the wavelength of pulses within the radiation system shown;

[0047] Figure 6 Shows the control Figure 4 A flow chart of a method for radiating bursts of radiation pulses within a radiation system is shown;

[0048] Figure 7A Shown are an open-loop control mechanism (squares) and a more suitable adaptive control approach (circles) for the wavelength of each pulse, whereby the operating wavelength is varied between pulses (also known as a multi-focus imaging approach);

[0049] Figure 7B shows the control voltage applied to the actuator for each pulse in an open loop control regime and in an adaptive control regime more suitable for operation whereby the operating wavelength is varied between pulses (also known as a multi-focus imaging regime); and

[0050] Figure 8 Shown for Figure 6 The control voltage applied to the actuator as a function of pulse number (upper graph) and the wavelength for each pulse (lower graph) for the control scheme shown, with two graphs showing spanning 15 bursts of radiation pulses. DETAILED DESCRIPTION

[0051] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5-100 nm).

[0052] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam to correspond to the pattern to be created in a target portion of a substrate. The term "light valve" may also be used herein. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0053] Figure 1A lithographic apparatus LA is schematically shown. The lithographic apparatus LA comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0054] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in a cross-section in the plane of the patterning device MA.

[0055] The term "projection system" PS as used herein should be broadly interpreted as including various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, depending on the exposure radiation used and / or other factors (e.g., the use of an immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0056] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0057] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or preparatory steps for subsequent exposure of a substrate W may be performed on a substrate W positioned on one of the substrate supports WT while a pattern on another substrate W is exposed using another substrate W on another substrate support WT.

[0058] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure characteristics of the projection system PS or characteristics of the radiation beam B. The measurement stage can accommodate multiple sensors. The cleaning devices can be arranged to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of the system for providing immersion liquid. The measurement stage can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0059] In operation, a radiation beam B is incident on a patterning device, for example a mask MA held on a mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the radiation beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner PM and possibly another position sensor ( Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, the substrate alignment marks P1, P2 may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe lane alignment marks.

[0060] For clarity, a Cartesian coordinate system is used. A Cartesian coordinate system has three axes, the x-axis, the y-axis, and the z-axis. Each of these three axes is orthogonal to the other two. Rotation about the x-axis is called Rx-rotation. Rotation about the y-axis is called Ry-rotation. Rotation about the z-axis is called Rz-rotation. The z-axis can usually coincide with the optical axis of the lithographic apparatus (e.g., in Figure 1 The x-axis and y-axis may define a plane perpendicular to the optical axis (e.g. Figure 1The Cartesian coordinate system does not limit the present invention and is provided for illustration only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used for illustration. The Cartesian coordinate system may be oriented differently, for example, with the z-axis having an element along the horizontal plane.

[0061] Figure 2 A lithography system 100 is schematically shown and includes an illumination system 105. As described more fully below, the illumination system 105 includes a light source that generates a pulsed light beam 110 and directs it to a lithography exposure apparatus or scanner 115, which patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer stage 125, which is constructed to hold the wafer 120 and is connected to a positioner that is configured to precisely position the wafer 120 according to certain parameters.

[0062] Photolithography system 100 uses a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, for example, 248 nanometers (nm) or 193 nm. The minimum size of a microelectronic feature that can be patterned on wafer 120 depends on the wavelength of light beam 110, with lower wavelengths allowing for smaller minimum feature sizes. When the wavelength of light beam 110 is 248 nm or 193 nm, the minimum size of the microelectronic feature can be, for example, 50 nm or less. The bandwidth of light beam 110 can be the actual instantaneous bandwidth of its spectrum (or emission spectrum), which contains information about how the optical energy of light beam 110 is distributed across different wavelengths. Scanner 115 includes an optical device having, for example, one or more focusing lenses, a mask, and an objective lens arrangement. The mask can be moved in one or more directions, for example, along the optical axis of light beam 110 or in a plane perpendicular to the optical axis. The objective lens arrangement includes a projection lens and enables the transfer of an image from the mask to the photoresist on wafer 120. The illumination system 105 adjusts the angular range of the light beam 110 irradiated onto the mask. The illumination system 105 also homogenizes (uniforms) the intensity distribution of the light beam 110 on the mask.

[0063] Scanner 115 may include, among other features, a photolithography controller 130, air conditioning, and power supplies for various electrical components. Photolithography controller 130 controls how each layer is printed on wafer 120. Photolithography controller 130 includes a memory that stores information, such as a process recipe. The process or recipe determines the length of the exposure on wafer 120, based on, for example, the mask used and other factors that affect the exposure. During photolithography, multiple pulses of light beam 110 illuminate the same area of ​​wafer 120 to create an exposure dose.

[0064] The lithography system 100 also preferably includes a control system 135. In general, the control system 135 includes one or more digital electronic circuits, computer hardware, firmware, and software. The control system 135 also includes memory, which can be read-only memory and / or random access memory. Memory devices suitable for containing computer program instructions and data include all forms of non-volatile memory, including, for example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks.

[0065] The control system 135 can also include one or more input devices (e.g., keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (e.g., speakers or displays). The control system 135 also includes one or more programmable processors, and one or more computer program products tangibly embodying computer programs for execution by the one or more programmable processors. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Typically, a processor will receive instructions and data from memory. Any of the foregoing can be supplemented by, or incorporated in, special purpose logic circuitry. The control system 135 can be centralized or partially or entirely distributed throughout the lithography system 100.

[0066] Reference is made to Figure 3 The exemplary illumination system 105 is a pulsed laser source that produces a pulsed laser beam as the beam 110. Figure 3 A gas discharge laser system is shown, for example, in block diagram form, in accordance with embodiments of certain aspects of the disclosed technology. The gas discharge laser system can include, for example, a solid state or gas discharge seed laser system 140, an amplification stage, such as a power ring amplifier ("PRA") stage 145, relay optics 150, and a laser system output subsystem 160. The seed system 140 can include, for example, a master oscillator ("MO") chamber 165.

[0067] The seed laser system 140 may further include a master oscillator output coupler ("MOOC") 175, which may include a partially reflecting mirror formed by a reflective grating (not shown) in a line narrowing module ("LNM") 170, in which the seed laser 140 oscillates to form a seed laser output pulse, i.e., a master oscillator ("MO"). The seed laser system may further include a line core analysis module ("LAM") 180. The LAM 180 may include an etalon spectrometer for fine wavelength measurement and a grating spectrometer for coarser resolution. The MO wavefront engineering box ("WEB") 185 may be used to redirect the output of the MO seed laser system 140 to the amplifier stage 145, and may include beam expansion using, for example, a multi-prism beam expander (not shown) and coherence destruction, for example, in the form of an optical delay path (not shown).

[0068] The amplification stage 145 may include, for example, a PRA lasing chamber 200, which may also be an oscillator, formed, for example, by seed beam injection and output coupling optics (not shown), which may be incorporated into the PRA WEB 210 and may be redirected back through the gain medium in the chamber 200 by a beam reverser 220. The PRA WEB 210 may include partially reflecting input / output couplers (not shown) and maximum reflective mirrors for the nominal operating wavelength (e.g., about 193 nm for an ArF system) and one or more prisms.

[0069] A bandwidth analysis module ("BAM") 230 at the output of the amplifier stage 145 can receive the pulsed output laser beam from the amplifier stage and pick a portion of the beam for metrology purposes, such as to measure the output bandwidth and pulse energy. The pulsed laser output beam then passes through an optical pulse stretcher ("OPuS") 240 and an output combined automatic shutter metering module ("CASMM") 250, which can also be the location of a pulse energy meter. One purpose of the OPuS 240 can be, for example, to convert a single output laser pulse into a train of pulses. The secondary pulses generated from the original single output pulse can be delayed relative to each other. By distributing the original laser pulse energy into a series of secondary pulses, the effective pulse length of the laser can be extended and the peak pulse intensity can be reduced. The OPuS 240 can therefore receive the laser beam from the PRA WEB 210 via the BAM 230 and direct the output of the OPuS 240 to the CASMM 250. Other suitable arrangements can be used in other embodiments.

[0070] The PRA laser emission chamber 200 and MO 165 are configured as a chamber in which a discharge between electrodes can induce a laser emission gas discharge in the laser emission gas to create an inversion population of high energy molecules (including, for example, Ar, Kr and / or Xe) to produce relatively broadband radiation, which can be line-narrowed to a relatively very narrow bandwidth and a center wavelength selected in a line-narrowing module (“LNM”) 170, as is known in the art.

[0071] Typically, tuning occurs in the LNM. A conventional technique for line narrowing and tuning a laser is to provide a window on the back of the laser's discharge chamber, through which a portion of the laser beam enters the LNM. There, a portion of the beam is expanded using a prismatic beam expander and directed to an optical element, such as a grating, which reflects a narrow, selected portion of the laser's broader spectrum back into the discharge chamber, where it is amplified. The laser is typically tuned by using an actuator (e.g., a piezoelectric actuator) to change the angle at which the beam strikes the grating. Alternatively, a transmissive optical element, such as a prism, can be used to transmit a narrow, selected portion of the laser's broader spectrum back into the discharge chamber, where it is amplified. The laser can be tuned by using an actuator (e.g., a piezoelectric actuator) to change the angle at which the beam strikes the prism.

[0072] As mentioned above, for some applications, it is beneficial to be able to generate a burst of one or more pulses at one wavelength and then switch to generating a burst of one or more pulses at a different wavelength. However, switching wavelengths between pulses is challenging. One reason is that the settling time, i.e., the amount of time it takes for the system to settle after a wavelength change, is typically longer than the inter-pulse interval. According to one aspect, the transient settling period caused by changing the reference wavelength is shortened by pre-positioning the actuator between bursts to achieve the upcoming new target wavelength between bursts.

[0073] According to another aspect, a dynamic model of the actuator is used to calculate an optimal control waveform, which is used to actuate the actuator to minimize the difference between the actual wavelength and the wavelength target.

[0074] The optimal control waveform can be calculated using any of several methods. For example, dynamic programming can be used to calculate the optimal control waveform. This method is well suited to processing complex models that contain nonlinear dynamics. If an actuator model with strong nonlinear dynamics is used, dynamic programming can be used to generate the optimal control signal for a given wavelength target. However, dynamic programming does have the challenge of requiring a large amount of computing resources, which may not be available in real time. To overcome this challenge, a data storage device such as a pre-populated lookup table or a pre-programmed field programmable gate array (FPGA) can be used, which contains the optimal control parameters for at least some different repetition rates at which the source can operate.

[0075] As another example, model inversion feedforward control can be used to determine the optimal control waveform. This method relies on a model of the actuator dynamics to construct a digital filter that inverts the actuator dynamics. By passing the desired waveform of the desired actuator trajectory through the filter, the optimal control waveform can be generated in real time to achieve zero steady-state error tracking.

[0076] As another example, achieving an optimal solution for two separated wavelengths in a stable manner is achieved using a learning algorithm to ensure error convergence during multiple iterative learning processes.

[0077] Embodiments of the systems and methods disclosed herein can potentially achieve two separate wavelengths separated by 1000 fm with a separation error below 20 fm.

[0078] According to another aspect, the optimal control waveform can be fed to the actuator at a very high rate by using an FPGA.

[0079] Figure 4 is a schematic diagram of a radiation system 300 for controlling bursts of radiation pulses. The radiation system 300 may be formed Figure 3 At least a portion of the lighting system 105 shown and described above. The radiation system 300 may be combined Figure 3 Any features of the illustrated illumination system 105. The radiation system 300 includes: an optical element 310; a controller 320; an actuator 330; and a sensor 340.

[0080] The optical element 310 is configured to interact with the radiation pulse (schematically indicated by the dashed arrow) to control the characteristics of the radiation pulse (e.g., the wavelength of the radiation pulse). The characteristics of the radiation pulse may depend on the configuration of the optical element 310. The optical element 310 may include the above-referenced Figure 31 and 2. The optical element 310 may include one or more portions of a line narrowing module ("LNM") 170 as described herein. The optical element 310 may include a reflection grating configured to reflect the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to the output of the radiation system 300. Additionally or alternatively, the optical element 310 may include a prism configured to refract the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to the output of the radiation system 300.

[0081] The sensor 340 is operable to determine a characteristic (eg, wavelength) of a pulse interacting with the optical element 310. For example, the sensor 340 may include, or may be formed of Figure 3 A portion of a Linear Analysis Module ("LAM") 180 of the type shown and described above. Sensors 340 may include etalon spectrometers for fine wavelength measurement and grating spectrometers for coarser resolution, but it will be appreciated that other sensors may be used.

[0082] The optical element 310 and the sensor 340 both form a portion 350 of the radiation system 300 that is operable to physically interact with the radiation pulses (as schematically indicated by the dashed arrows). Thus, the portion 350 of the radiation system 300 provides a physical interface between the other portions of the radiation system 300 and the radiation pulses.

[0083] Controller 320 is configured to generate a control signal 325. In turn, actuator 330 is configured to receive control signal 325 from controller 320 and control the configuration of optical element 310 according to control signal 325. Actuator 330 may include a piezoelectric element configured to rotate optical element 310 to control the angle of incidence of the radiation pulse on optical element 310.

[0084] In use, a burst of multiple radiation pulses is generated. For example, Figure 3 The illumination system 105 of the form shown and described above generates a burst of radiation pulses. Each radiation pulse interacts with the optical element 310, so that the optical element 310 affects the characteristics (e.g., wavelength) of each pulse. The characteristics of each pulse (which have been determined by the optical element 310) are then measured by the sensor 340, which also interacts with each radiation pulse.

[0085] The controller 320 is configured such that the control signal 325 for a given pulse in a given burst depends on the determined properties of at least one corresponding pulse from at least one previous burst. That is, for a given pulse in a given burst, a portion of the control signal 325 that affects the position of the optical element 310 (via the actuator 330) depends on the determined properties of a corresponding pulse from at least one previous burst, as discussed further below. The control signal 325 can comprise a feedforward signal for each pulse in each burst.

[0086] As described further below, the control signal 325 is updated for each burst of pulses. However, within any given burst of pulses, the properties of the pulses (e.g. wavelength) can be controlled by an open loop scheme.

[0087] Reference is now made to Figure 5 The control of the wavelength of the pulses within the radiation system 300 is discussed.

[0088] A first input into the control of the radiation system 300 can comprise a plurality of voltages V(k), V(k) being a feedforward signal of a voltage to be applied to the actuator 330 for the kth pulse. That is, V(k) can represent a voltage to be applied to the actuator 330 when the kth pulse in a burst is incident on the optical element 310 to control the position of the optical element 310. Optionally, these voltages V(k) can be converted into a time dependent waveform V(t) using an analog to digital converter 410. The waveform V(t) can be applied by the actuator 330 to control the optical element 310. It will be appreciated that this waveform V(t) can be reduced to a plurality of voltages V(k) when sampled by the pulses that interact with the optical element 310, and any suitable interpolation between the pulses can be used to generate the waveform V(t).

[0089] The control signal 325 generated by the controller 320 can be considered to comprise a plurality of voltages V(k). Additionally or alternatively, the control signal 325 generated by the controller 320 can be considered to comprise a waveform V(t).

[0090] In turn, the application of the time dependent waveform V(t) to the actuator 330 results in a time dependent output wavelength λ(t), which can be output by a subsystem 420 of the radiation system 300. The response of the subsystem 420 determines the time dependent output wavelength λ(t) based on the time dependent waveform V(t). The response of the subsystem 420 can be considered to depend on an analog low pass filter, the response of the actuator 330 to the input voltage V(t) (which can or can not be, for example, generally linear), and / or any optical gain that arises from the pulses interacting with the optical element 310.

[0091] The time-dependent output wavelength λ(t) is then sampled at a repetition rate by radiation pulses 430 that may be incident on the optical element 310 to produce discrete output wavelengths λ(k), where the output wavelength λ(k) is the wavelength of the kth pulse.

[0092] In some embodiments of the illumination system 300, the illumination system 300 can be configured such that the output pulsed beam comprises a train of pulses in which the wavelength of the pulses alternates between two different set-point wavelengths. That is, odd-numbered pulses have a first set-point wavelength and even-numbered pulses have a second set-point wavelength. To achieve this, the controller 320 can be configured such that the control signal 325 oscillates, for example, at a frequency equal to half the repetition rate of the radiation pulses. For example, the time-dependent waveform V(t) applied by the actuator 330 to control the optical element 310 can be periodic and can oscillate at a frequency equal to half the repetition rate of the radiation pulses (or an integer multiple thereof). With this arrangement, the radiation system can generate bursts of radiation pulses, whereby the wavelength of the pulses oscillates. Advantageously, as described above, using multiple (e.g., two) different wavelengths can be advantageous for use in a lithography system to increase the depth of focus.

[0093] Different portions of the radiation system 300 may operate at different frequencies or at different repetition rates.

[0094] For example, the sensor 340 can be operated to determine the characteristics of each pulse of radiation. Thus, the sensor 340 can be operated to acquire data and output it as an output signal 345 to the controller 320 at a frequency given by the repetition rate of the radiation system 300. The repetition rate of the radiation system 300 can be, for example, on the order of 6 kHz.

[0095] In order to make the wavelength of the pulses oscillate between pulses, the actuator 330 may also be operated at a frequency that depends on the repetition rate of the radiation system 300. For example, the actuator may be moved once between each pair of consecutive pulses to achieve an oscillating output wavelength. With this arrangement, the time-dependent waveform V(t) (see Figure 5 ) can typically be in the form of a square wave or a sine wave. Alternatively, the actuator 330 can be operated at a frequency higher than the repetition rate of the radiation system 300. For example, the actuator 330 can be operated at 10 or 15 times the repetition rate of the radiation system 300. With this arrangement, the actuator 330 can be moved multiple times (e.g., 10 or 15 times) between each pair of consecutive pulses to achieve an oscillating output wavelength. With this arrangement, the time-dependent waveform V(t) (see Figure 5) can be generally sinusoidal. By operating actuator 330 at a higher frequency than the repetition rate of radiation system 300, V(t) can excite λ(t) at a higher frequency than the gain, and thus can increase the (pulse-to-pulse) spacing seen by λ(k). In general, the gain ratio λ(t) / V(t) depends on the spectrum of waveform V(t). Furthermore, in general, for a given power of waveform V(t), the power of λ(t) will be higher if V(t) has a higher frequency.

[0096] In some embodiments, the control signal 325 for a given pulse in a given burst depends on the determined characteristics of the corresponding pulse from the previous burst, as now referred to. Figure 6 described. Figure 6 A flow chart of a method for controlling bursts of radiation pulses is shown.

[0097] As a first step S10, the burst counter i is set to 1. The generation of pulses of the first burst is then started. In a second step S20, the pulse counter k of the pulses within the current burst is set to 1. The generation of the first pulse in the current burst of pulses is then started.

[0098] At step S30, a determination is made as to whether the current burst (burst i) has ended. For example, in some embodiments, a new burst may begin when a certain amount of time has passed since the last pulse, which is also referred to as a "burst timeout." For such embodiments, step S30 may include determining whether a certain amount of time has passed since the last pulse. Alternatively, by determining whether a pulse counter k for the pulses in the current burst is greater than the total number of pulses N in each burst. burst (For example, for each burst with a certain number N burst If it is determined in step S30 that the current burst has ended, then in step S40, the burst counter i is incremented by 1, and the method returns to step S20 to prepare to start generating pulses in the next burst.

[0099] If it is determined at step S30 that the current burst of pulses has not ended, then generation of the current burst has not ended, so the method proceeds to generating the next pulse in the burst (steps S50 to S120, as now discussed).

[0100] In step S50, a previous feedforward signal FF is obtained. The previous FF signal will be used to generate the k-th pulse in the i-th burst. The previous feedforward signal FF can be retrieved, for example, from a lookup table or some other memory within the controller 320. During the generation of the first pulse in the first burst, the previous feedforward signal FF can be set to a default value. Subsequently, the previous feedforward signal FF is defined as follows. During the generation of the first pulse in the burst (i.e., when k=0), the previous feedforward signal FF is the signal FF(N burst , i-1), that is, the Nth burst in the (i-1)th burst burst During the generation of a pulse that is not the first pulse in a burst (i.e., when k≠0), the previous feedforward signal FF is the signal FF(k-1,i), i.e., the feedforward signal of the (k-1)th pulse in the i-th burst.

[0101] In step S60, during the generation of the k-th pulse in the i-th burst, the optical element 310 is activated using the previous feedforward signal FF. For example, when the k-th pulse in the burst is incident on the optical element 310, the voltage V(k) applied to the actuator 330 depends on the previous feedforward signal FF. The voltage V(k) applied to the actuator 330 may, for example, be proportional to (e.g., equal to) the previous feedforward signal FF. As described above with reference to Figure 5 As discussed, the voltage V(k) may first be converted into a time-dependent voltage waveform V(t) that is applied to the actuator 330 .

[0102] In step S70, the wavelength error W of the kth pulse in the i-th burst is determined. err (k, i) (i.e., the wavelength error of the currently generated pulse). The wavelength error W err (k, i) is determined based on the wavelength λ(k, i) determined by the sensor 340 and the target or nominal wavelength. Specifically, the wavelength error W err (k,i) is determined as the difference between the wavelength λ(k,i) determined by the sensor 340 and the target or nominal wavelength.

[0103] Next, the method proceeds to step S80.

[0104] At step S80, if the current pulse being generated is the first pulse in a burst (i.e., k=1), and the current burst is not the first burst being generated (i.e., i>1), the method proceeds to step S90, in which the last pulse in the current burst (Nth pulse) is set according to the following equation: burst pulses) of the feedforward signal:

[0105] FF(N burst , i)=FF(N burst, i-1)=+g2·W crr (1, i) (1)

[0106] Among them FF(N burst , i) is the last pulse in the current burst (Nth burst pulses) of the feedforward signal, FF(N burst , i-1) is the last pulse in the previous burst (Nth burst pulses), g2 is the gain parameter, and W err (1, i) is the wavelength error determined during the generation of the current (ie, first) pulse of the current burst of pulses.

[0107] It can be understood that, as shown in equation (1), the last pulse in the current burst (Nth burst The feedforward signal of the Nth pulse is set equal to the last pulse in the previous burst (Nth pulse). burst It should be understood that FF(N burst , i) is the last pulse in the current burst (Nth burst pulses), which will actually be used later (see steps S50 and S60) to generate the first pulse in the (i+1)th burst. Similarly, FF(N burst , i-1) is the last pulse in the previous burst (Nth burst The correction term depends on the wavelength error W determined during the generation of the current (i.e. first) pulse of the burst of current pulses. err (1, i).

[0108] Note that Figure 6 As shown in the flowchart, once W has been measured err (k, i), then in step S90 the feedforward signal FF (N burst , i) (which will be actually used, see steps S50 and S60, and subsequently used to generate the first pulse in the (i+1)th burst). However, in general, the feedforward signal FF (N burst , i) can be set at any time according to equation (1), which is set at W err After (k, i) has been measured, and before the next burst begins to be generated. For example, in some embodiments, N may not be known until the generation of the current burst has ended. burst (For example, if the next burst begins when a certain amount of time has passed since the last pulse.) For these embodiments, the feed-forward signal FF (Nburst , i) can be set immediately before or after step S40.

[0109] Once the last pulse (Nth pulse) in the current burst has been set in step S90 burst pulses), the method proceeds to step S120.

[0110] At step S80 , if the current pulse being generated is not the first pulse in a burst (ie k≠1) or the current burst is the first burst being generated (ie i=1), the method proceeds to step S100 .

[0111] At step S100, if the current pulse being generated is not the first pulse in the burst (i.e., k>1), the method proceeds to step S110; otherwise, the method proceeds to step S120. Note that the path directly from step S100 to step S120 will only be used during the generation of the first pulse in the first burst.

[0112] In step S110, the feedforward signal of the next burst, i.e., the (k-1)th pulse in the (i+1)th burst, is set according to the following equation:

[0113] FF(k-1,i+1)=FF(k-1,i)-g1·W crr (k,i) (2)

[0114] Where FF(k-1, i+1) is the feedforward signal of the k-1th pulse in the next burst, FF(k-1, i) is the feedforward signal of the k-1th pulse in the current burst, g1 is the gain parameter and W err (k, i) is the wavelength error determined during the generation of the current (ie, kth) pulse of the current burst.

[0115] It will be appreciated that in this manner, the feedforward signals for all pulses in the next burst can be set equal to the feedforward signals for the corresponding pulses in the current burst plus a correction term. The correction term depends on the wavelength of the corresponding pulse determined from the current burst. Here, as given by equation (2), the feedforward correction term for the feedforward signal for the (k-1)th pulse is proportional to the wavelength error determined during the generation of the current (i.e., kth) pulse (recall that the feedforward signal for the (k-1)th pulse will actually be used to generate the kth pulse).

[0116] At step S120 , the pulse counter k is incremented by 1, and the method returns to step S30 to prepare to start generating the next pulse.

[0117] Thus, for each pulse in all bursts except the first burst, the feedforward signal (corresponding to the signal 325 output by the controller 320) is set equal to the feedforward control signal for the corresponding pulse in the previous burst plus a correction term. The correction term is proportional to the difference between the determined wavelength and the nominal value of the characteristic for the corresponding pulse from the previous burst.

[0118] As will be discussed further below (with reference to Figure 8 ), the method for controlling a burst of radiation pulses is generally of the type described above with reference to Figure 6 , which can allow the voltage waveform (defined by the feedforward control signal) to converge towards a steady state waveform over a certain number of bursts of pulses. Thus, it can be that once this steady state is reached, the method can simply use these steady state values during the generation of subsequent pulses, rather than calculating a correction term based on the wavelength error in the previous pulse. For example, at the control step S30 (see Figure 6 ), the method can determine whether a given number of bursts of pulses has been generated (the number generally being large enough so that the voltage waveform should have converged to the steady state waveform, e.g. the given number of groups of pulses can be 20. For example, at or near the step S30, the method can determine whether i < 20. If yes (i.e. i < 20), the method can proceed as described above using steps S40-S120. However, if no (i.e. i > 20), the method can implement a simpler set of steps, e.g. only steps S40 and S50 (selecting and using the relevant stored feedforward signal to generate the pulse), after which the method can proceed directly to step S120 (preparing for the generation of the next pulse). Optionally, the measurement of the wavelength error can be performed at step S70, and this information can be stored for later analysis.

[0119] In the above described example (as schematically shown in Figure 6 ), the feedforward signal for each pulse in all bursts except the first burst is set equal to the feedforward control signal for the corresponding pulse in the previous burst plus a correction term. In the described example, the correction term (see equations (1) and (2)) is proportional to the difference between the determined wavelength and the nominal wavelength for the corresponding pulse from the previous burst. Thus, the described example can be considered as a single burst review scenario, in which the correction is applied based on the wavelength error for the corresponding pulse in a single previous burst. However, in alternative embodiments, the control signal 325 can depend on several or all previous bursts. This can be referred to as an n-burst review scenario.

[0120] Furthermore, in the above described example (as schematically shown in Figure 6), only the feedforward signal and wavelength error of a single corresponding pulse in the previous burst are used to define the feedforward signal for a given pulse in a given burst. However, in alternative embodiments, more than one corresponding pulse in the previous burst(s) may be used. For example, in addition to the pulse from the previous burst(s) that exactly corresponds to the current pulse, one or more surrounding pulses may also be used. In general, the correction term may depend on the wavelength error for a group of corresponding pulses in the previous burst(s), where the group of corresponding pulses may include a plurality of corresponding pulses.

[0121] Typically, the wavelength error W can be used err All measurement histories of (k, i) are used to determine the feedforward signal. For example, in general, equations (1) and (2) can be replaced by:

[0122] FF(k,i)=f(FF(1,2,...,N bursi ; 1, 2, ..., i-1), W crr (1, 2, ..., N burst ; 1, 2, ..., i-1)(3)

[0123] where f is the value of the previous burst FF (1, 2, ..., N burst ; 1, 2, ..., i-1) and the wavelength error W for the previous burst pulse crr (1, 2, ..., N burst ; any function of the feedforward signal of 1, 2, ..., i-1).

[0124] The inventors of the present invention have recognized that in radiation generation methods where the wavelength target changes within a burst (e.g., each pulse), a new burst transient state appears in the system. This is because, as explained above, actuator 330 and optical element 310 must be placed in an oscillating state to change the wavelength of each pulse, but during the pulse interval between successive bursts of radiation, this physical system of actuator 330 and optical element 310 returns to rest. In short, the voltage waveforms used by actuator 330 to (a) quickly initiate oscillation and (b) maintain oscillation are different.

[0125] This is Figure 7A and 7B As shown in Figure 7A and 7B An open-loop control approach (represented by squares) is compared to a more adaptive control approach (represented by circles) whereby the operating wavelength is varied between pulses (also known as a multi-focus imaging approach). Figure 7A and 7B The wavelength for each pulse and the control voltage applied to the actuator 330 for each pulse are shown, respectively.

[0126] like Figure 7B As shown, open loop control applies a control voltage that alternates between the same two set point voltages V1 and V2. Figure 7A As can be seen in FIG, at least at the beginning of each burst of radiation (here, the current burst starts at pulse number 2400), under this control regime the wavelength tends to overshoot and oscillate transiently about the nominal or target set point values ​​λ1, λ2.

[0127] like Figure 7B As shown, the adaptive control mode applies an alternating control voltage, but wherein the amplitude of the oscillation increases over the initial period. Figure 7A As can be seen in FIG, under this control regime, the wavelength tends to converge on the nominal or target set point values ​​λ1, λ2 significantly faster than under open loop control regime.

[0128] All feedforward signals for each pulse in all bursts except the first (corresponding to signal 325 output by controller 320) are set equal to the feedforward control signal for the corresponding pulse in the previous burst plus a correction term. Note that although the correction term is proportional to the difference between the determined wavelength and the nominal value of the characteristic of the corresponding pulse from the previous burst, different gains (g2) are used for the first pulse in the burst and for all other pulses (g1). Advantageously, in this way, different gains can be applied to transients occurring within each burst, as well as to transients occurring due to gaps between consecutive bursts.

[0129] Radiation system 300 is advantageous because it includes a feedback loop that uses determined characteristics (e.g., wavelength) of pulses that have interacted with optical element 310. In particular, a control signal 325 for a given pulse in a given burst can be given by the control signal 325 for the corresponding pulse from the previous burst plus a feedback correction. The feedback correction can depend on (e.g., be proportional to) the difference between the determined characteristics of the corresponding pulse from the previous burst and the nominal or target characteristics of the corresponding pulse from the previous burst. Advantageously, this allows radiation system 300 to account for and at least partially correct for transient effects caused by different control signals used during each burst of pulses and between bursts of consecutive pairs of pulses. Such transient effects can be significant, particularly if the dynamic response of the actuator and optical element system is underdamped.

[0130] Although the general shape of the transient behavior may be known and, for example, modeled, in practice it is difficult to model this behavior accurately enough to keep the wavelength error within acceptable limits.

[0131] Figure 8The control voltage applied to the actuator 330 as a function of the number of pulses (top graph) and the wavelength for each pulse (bottom graph) is shown. The above reference Figure 6 The control scheme described shows these results and shows two plots across a burst of 15 radiation pulses.

[0132] It can be seen that the initial control scheme for the first burst of radiation pulses generally has the form described above with reference to Figure 7A and 7B The adaptive control mechanism described above. For example, the voltage waveform of the initial burst of pulses can be determined by a suitable model. Although generally having the correct "fitted" form, it can be appreciated from Figure 8 that there is still a significant wavelength error.

[0133] However, using the control scheme described above with reference to Figure 6 it can be seen that over several bursts of pulses, the voltage waveform converges towards the steady state waveform and in doing so, the wavelength accuracy is improved both in the transient and steady state.

[0134] The feedforward signal for the first burst can be determined based on a dynamic model of the optical element 310 and the actuator 330. For example, to address the constraint on the actuator 330 voltage variation, a quadratic programming with constraints can be used to help find the best feedforward signal within the feasible operating region. Quadratic programming (QP) is a technique for finding the best solution to a given quadratic cost function with mathematical constraints. In the technique described here, the aim is to find a feedforward control that satisfies the actuator 330 constraints while minimizing the error between the actuator position and the desired control waveform.

[0135] Alternatively, the initial condition can be that the feedforward signal for the first burst of pulses is set to zero (although such an embodiment can take longer to converge to a steady state).

[0136] Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications. Possible other applications include 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.

[0137] Although specific reference can be made in this text to embodiments of the application in the context of lithography apparatus, embodiments of the application can be used in other apparatus. Embodiments of the application can form part of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus are often referred to as lithographic tools. Such lithographic tools can use vacuum conditions or environmental (non-vacuum) conditions.

[0138] Embodiments of the application can be implemented in hardware, firmware, software, or any combination thereof, depending on the context and preferences of one of ordinary skill in the art. Embodiments of the application can also be implemented as instructions stored on a machine-readable medium, which can be read and executed by one or more processors. A machine-readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from processing of the firmware, software, routines, instructions, etc. by computing devices, processors, controllers, or other devices, and that the actions are not to be interpreted as being performed by the firmware, software, routines, instructions, etc.

[0139] Implementations of the present disclosure can be further described using the following clauses:

[0140] 1. A radiation system for controlling a burst of radiation pulses, comprising:

[0141] an optical element configured to interact with a radiation pulse to control a property of the radiation pulse, the property of the radiation pulse being dependent on a configuration of the optical element;

[0142] a controller operable to generate a control signal;

[0143] an actuator configured to receive the control signal from the controller and to control the configuration of the optical element in accordance with the control signal; and

[0144] a sensor operable to determine the property of a pulse that has interacted with the optical element;

[0145] wherein the control signal for a given pulse in a given burst is dependent on the determined property of a corresponding pulse from at least one previous burst.

[0146] 2. The radiation system of clause 1, wherein the control signal for a given pulse in a given burst is dependent on the determined property of a group of pulses from at least one previous burst.

[0147] 3. The radiation system of clause 1, wherein the control signal for a given pulse in a given burst is given by the control signal for a corresponding pulse in at least one previous burst plus a correction term that is dependent on the determined property of the corresponding pulse from at least one previous burst.

[0148] 4. The radiation system of clause 3, wherein the correction term is proportional to a difference between the determined property and a nominal value of the property for a corresponding pulse from at least one previous burst.

[0149] 5. The radiation system of clause 4, wherein the correction term is proportional to a gain, the gain being equal to the first gain for any pulse in the burst that is not the first pulse, and the gain being equal to the second gain for any pulse in the burst that is the first pulse.

[0150] 6. The radiation system of clause 1, wherein the property is a wavelength of the radiation pulse.

[0151] 7. The radiation system of clause 1, wherein the controller is configured such that the control signal oscillates.

[0152] 8. The radiation system of clause 7, wherein the control signal oscillation provides pulses having wavelengths that alternate between two different setpoint wavelengths within the burst of pulses.

[0153] 9. The radiation system of clause 1, wherein the actuator comprises a piezoelectric element configured to rotate the optical element to control an angle of incidence of the radiation pulses on the optical element.

[0154] 10. The radiation system of clause 1, wherein the optical element comprises a grating configured to reflect the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of the radiation is transmitted to an output of the radiation system.

[0155] 11. The radiation system of clause 1, wherein the optical element comprises a prism configured to refract the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of the radiation is transmitted to an output of the radiation system.

[0156] 12. The radiation system of clause 1, wherein the control signal for a given pulse in the first burst is determined based on a dynamic model of the optical element and the actuator.

[0157] 13. The radiation system of clause 1, wherein the controller is configured such that the control signal provides pulses having wavelengths that alternate between two different setpoint wavelengths within the burst of pulses.

[0158] 14. A lithography system, comprising:

[0159] a radiation system for controlling a burst of radiation pulses, the radiation system comprising:

[0160] an optical element configured to interact with the pulses of radiation to control a property of the pulses of radiation, the property of the pulses of radiation being dependent on a configuration of the optical element;

[0161] a controller operable to generate a control signal;

[0162] an actuator configured to receive the control signal from the controller and to control the configuration of the optical element in dependence on the control signal; and

[0163] a sensor operable to determine a property of a pulse that has interacted with the optical element;

[0164] wherein the control signal for a given pulse in a given burst is dependent on the determined property of a corresponding pulse from at least one previous burst; and

[0165] a lithographic apparatus configured to receive the pulses of radiation from the radiation system, to pattern the pulses of radiation, and to project the patterned pulses of radiation onto a target.

[0166] 15. The lithographic system of clause 14, wherein the controller is configured such that the control signal oscillates to provide pulses having wavelengths that alternate between two different setpoint wavelengths within a burst of pulses.

[0167] 16. The lithographic system of clause 15, wherein the lithographic apparatus is configured to receive pulses having wavelengths that alternate between two different setpoint wavelengths.

[0168] 17. A method of controlling a burst of pulses of radiation, comprising:

[0169] interacting with the pulses of radiation using an optical element to control a property of the pulses of radiation;

[0170] determining a property of a pulse that has interacted with the optical element;

[0171] generating a control signal; and

[0172] controlling a configuration of the optical element in dependence on the control signal;

[0173] wherein the control signal for a given pulse in a given burst is dependent on the determined property of at least one corresponding pulse from at least one previous burst.

[0174] 18. The method of clause 17, wherein the control signal for a given pulse in a given burst is dependent on the determined property of a group of pulses from at least one previous burst.

[0175] 19. A method according to clause 17, wherein the control signal for a given pulse in a given burst is given by the control signal for the corresponding pulse in the previous burst plus a correction term, the correction term being dependent on the determined characteristic of the corresponding pulse from at least one previous burst.

[0176] 20. The method of clause 19, wherein the correction term is proportional to the difference between the determined characteristic of the corresponding pulse from at least one previous burst and a nominal value of the characteristic.

[0177] 21. A method according to clause 19, wherein the correction term is proportional to a gain, the gain being equal to a first gain for any pulse in the burst that is not the first pulse, and the gain being equal to a second gain for any pulse in the burst that is the first pulse.

[0178] 22. The method of clause 17, wherein the characteristic is the wavelength of at least one pulse of radiation.

[0179] 23. The method of clause 17, wherein the generated control signal oscillates.

[0180] 24. The method of clause 17, wherein the control signal for a given pulse in the first burst is determined based on a dynamic model of the optical element and the actuator.

[0181] 25. The method of clause 17, wherein the generated control signal oscillates to provide pulses having a wavelength that alternates between two different set point wavelengths.

[0182] While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A radiation system for controlling bursts of radiation pulses, comprising: an optical element configured to interact with the radiation pulses to control a wavelength of the radiation pulses, the wavelength of the radiation pulses being dependent on a configuration of the optical element; a controller operable to generate a control signal; an actuator configured to receive the control signal from the controller and control the configuration of the optical element according to the control signal; as well as a sensor operable to determine the wavelength of a pulse that has interacted with the optical element; wherein said control signal for a given pulse in a given burst depends on said determined wavelength of a corresponding pulse from at least one previous burst, Wherein the control signal oscillates to provide pulses having wavelengths alternating between two different set point wavelengths within the burst of pulses. 2 . The radiation system of claim 1 , wherein the control signal for the given pulse in the given burst depends on the determined wavelength of a group of pulses from at least one previous burst.

3. A radiation system according to claim 1, wherein the control signal for a given pulse in a given burst is given by the control signal for the corresponding pulse in at least one previous burst plus a correction term, the correction term being dependent on the determined wavelength of the corresponding pulse from at least one previous burst.

4. The radiation system of claim 3, wherein the correction term is proportional to the difference between the determined wavelength of the corresponding pulse from at least one previous burst and a nominal value of the wavelength.

5. The radiation system of claim 4 , wherein the correction term is proportional to a gain, the gain being equal to a first gain for any pulse in the burst that is not the first pulse, and the gain being equal to a second gain for any pulse in the burst that is the first pulse.

6. The radiation system of claim 1, wherein the actuator comprises a piezoelectric element configured to rotate the optical element to control an angle of incidence of the radiation pulse on the optical element.

7. The radiation system of claim 1, wherein the optical element comprises a grating configured to reflect the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to an output of the radiation system.

8. The radiation system of claim 1, wherein the optical element comprises a prism configured to refract the radiation pulses in a wavelength-dependent manner such that a selected wavelength band of radiation is transmitted to an output of the radiation system.

9. The radiation system of claim 1, wherein the control signal for a given pulse in a first burst is determined based on a dynamic model of the optical element and the actuator.

10. A photolithography system comprising: A radiation system for controlling bursts of radiation pulses, the radiation system comprising: an optical element configured to interact with the radiation pulses to control a wavelength of the radiation pulses, the wavelength of the radiation pulses being dependent on a configuration of the optical element; a controller operable to generate a control signal; an actuator configured to receive the control signal from the controller and control the configuration of the optical element according to the control signal; and a sensor operable to determine the wavelength of a pulse that has interacted with the optical element; wherein said control signal for a given pulse in a given burst depends on said determined wavelength of a corresponding pulse from at least one previous burst; and a lithographic apparatus configured to receive radiation pulses from the radiation system, pattern the radiation pulses, and project the patterned radiation pulses onto a target; wherein the controller is configured to cause the control signal to oscillate to provide pulses having wavelengths alternating between two different set point wavelengths within the burst of pulses.

11. The lithographic system of claim 10, wherein the lithographic apparatus is configured to receive the pulses having a wavelength that alternates between the two different setpoint wavelengths.

12. A method of controlling a burst of radiation pulses, comprising: using an optical element to interact with the radiation pulse to control the wavelength of the radiation pulse; determining the wavelength of the pulse that has interacted with the optical element; generating a control signal; as well as controlling the configuration of the optical element according to the control signal; wherein said control signal for a given pulse in a given burst depends on said determined wavelength of a corresponding pulse from at least one previous burst, The control signal is generated to oscillate to provide pulses having a wavelength that alternates between two different set point wavelengths.

13. The method of claim 12, wherein the control signal for the given pulse in the given burst depends on the determined wavelength of a group of pulses from at least one previous burst.

14. The method of claim 12 , wherein the control signal for a given pulse in a given burst is given by the control signal for the corresponding pulse in a previous burst plus a correction term, the correction term being dependent on the determined wavelength of the corresponding pulse from at least one previous burst.

15. The method of claim 14, wherein the correction term is proportional to the difference between the determined wavelength of the corresponding pulse from at least one previous burst and a nominal value of the wavelength.

16. A method according to claim 14, wherein the correction term is proportional to a gain, the gain is equal to a first gain for any pulse in the burst that is not the first pulse, and the gain is equal to a second gain for any pulse in the burst that is the first pulse.

17. The method of claim 12, wherein the control signal for a given pulse in a first burst is determined based on a dynamic model of the optical element and actuator.

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