NL2022477A

NL2022477A - Radiation source, lithographic system and Method

Info

Publication number: NL2022477A
Application number: NL2022477A
Authority: NL
Inventors: Everts Frank; Patrick Elisabeth Maria Op 't Root Wilhelmus; Philip Godfried Herman
Original assignee: Asml Netherlands Bv
Priority date: 2018-02-27
Filing date: 2019-01-30
Publication date: 2019-09-03
Also published as: WO2019166173A1

Abstract

A radiation source configured to provide a radiation beam, the radiation source being controlled by a controller, the controller comprising a first feedback algorithm configured to process a signal indicative of the radiation beam and control the radiation source in dependence on the received signal, wherein the controller is in communication with a processor, the processor being configured to receive an operating parameter of the radiation source, generate a second feedback algorithm based on the received operating parameter, and cause the controller to operate in accordance with the second feedback algorithm.

Description

FIELD [0001] The present invention relates to a lithographic apparatus and a device manufacturing method.

BACKGROUND [0002] A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the 1C, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.

[0003] A lithographic system comprises a radiation source and a lithographic apparatus. The lithographic system is configured to apply a desired radiation dose to target portions of the substrate. Any differences between a desired radiation dose and an actual radiation may be referred to as a radiation dose error. Radiation dose errors may be understood as being a culmination of inaccuracies associated with the radiation source and the lithographic apparatus. For example, an inaccurately calibrated radiation source may contribute to radiation dose errors on the substrate. As another example, changes in the radiation source and/or the lithographic apparatus over time (e.g., from wear and tear) may not be suitably accounted for over the lifetime of the lithographic system, resulting in radiation dose errors.

[0004] It is desirable to provide, for example, a radiation source and/or a lithographic system that obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.

SUMMARY [0005] According to an aspect of the invention, there is provided a radiation source configured to provide a radiation beam, the radiation source being controlled by a controller, the controller comprising a first feedback algorithm configured to process a signal indicative of the radiation beam and control the radiation source in dependence on the received signal, wherein the controller is in communication with a processor, the processor being configured to receive an operating parameter of the radiation source, generate a second feedback algorithm based on the received operating parameter, and, cause the controller to operate in accordance with the second feedback algorithm.

[0006] The second feedback algorithm causes a different control compared to the first feedback algorithm. That is, the processor acts to reconfigure the controller such that the controller controls the radiation source in a different manner compared to when the controller operates in accordance with the first feedback algorithm. Advantageously improved control is provided of the radiation source and the ability to adapt to changes in the radiation source performance over its lifetime.

[0007] The generation of the second feedback algorithm may depend upon a selected balance between a performance of the radiation source and a stability of the controller.

[0008] The generation of the second feedback algorithm may include determining one of a controller response and a radiation source response.

[0009] The processor may be configured to monitor the operating parameter.

[0010] The monitoring may be continuous.

[0011] The controller may be a proportional-integral-derivative (PID) controller.

[0012] The generation of the second feedback algorithm may include changing one of a proportional term, an integral term and a derivative term of the PID controller.

[0013] The operating parameter may be one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, an error of the radiation source, a gain of the radiation source, a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, and a variation of a loop gain of the controller.

[0014] According to a second aspect of the invention there is provided a lithographic system comprising a radiation source configured to provide a radiation beam, and a lithographic apparatus, the lithographic apparatus comprising an illumination system for conditioning the radiation beam, a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section, a substrate table for holding a substrate, and a projection system for projecting the patterned radiation beam onto a target portion of the substrate, wherein the lithographic system further comprises a controller comprising a first feedback algorithm configured to process a signal indicative of a radiative power incident on the substrate and control the radiation source in dependence on the received signal, wherein the controller is in communication with a processor, the processor being configured to receive an operating parameter of the lithographic system, generate a second feedback algorithm based on the received operating parameter, and cause the controller to operate in accordance with the second feedback algorithm.

[0015] The generation of the second feedback algorithm may depend upon a selected balance between a performance of the lithographic system and a stability of the controller.

[0016] The generation of the second feedback algorithm may include determining one of a controller response, a radiation source response and a lithographic apparatus response.

[0017] The processor may be configured to monitor the operating parameter.

[0018] The monitoring may be continuous.

[0019] The controller may be a PID controller.

[0020] The generation of the second feedback algorithm may include changing one of a proportional term, an integral term and a derivative term of the PID controller.

[0021] The operating parameter may be one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, an error of the radiation source, a gain of the radiation source, a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, a variation of a loop gain of the controller, a number of pulses of the radiation beam that are incident on the target portion of the substrate, tin illumination setting of the illumination system, and a measured value of a radiation dose error on the substrate.

[0022] According to a third aspect of the invention there is provided a method of configuring a controller of a radiation source, the radiation source being configured to generate a radiation beam, the controller comprising a first feedback algorithm configured to process a signal indicative of the radiation beam and control the radiation source in dependence on the received signal, the method comprising receiving an operating parameter of the radiation source, generating a second feedback algorithm based on the received operating parameter, and causing the controller to operate in accordance with the second feedback algorithm.

[0023] The generation of the second feedback algorithm may depend upon a selected balance between a performance of the radiation source and a stability of the controller.

[0024] The generation of the second feedback algorithm may include determining one of a controller response and a radiation source response.

[0025] The method may further comprise monitoring the operating parameter.

[0026] The monitoring may be continuous.

[0027] The controller may be a PID controller and the generation of the second feedback algorithm may include changing one of a proportional term, an integral term and a derivative term of the PID controller.

[0028] The operating parameter may be one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, an error of the radiation source, a gain of the radiation source, a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, and a variation of a loop gain of the controller.

[0029] According to a fourth aspect of the invention there is provided a method of configuring a controller of a lithographic system, the lithographic system comprising a radiation source configured to provide a radiation beam, and a lithographic apparatus, the lithographic apparatus comprising an illumination system for conditioning the radiation beam, a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section, a substrate table for holding a substrate, and a projection system for projecting the patterned radiation beam onto a target portion of the substrate, wherein the lithographic system further comprises a controller, the controller comprising a first feedback algorithm configured to process a signal indicative of a radiative power incident on the substrate and control the radiation source in dependence on the received signal, the method comprising receiving an operating parameter of the lithographic system, generating a second feedback algorithm based on the received operating parameter, and causing the controller to operate in accordance with the second feedback algorithm. [0030] The generation of the second feedback algorithm may depend upon a selected balance between a performance of the radiation source and a stability of the controller.

[0031] The generation of the second feedback algorithm may include determining one of a controller response, a radiation source response and a lithographic apparatus response.

[0032] The method may further comprise monitoring the operating parameter.

[0033] The monitoring may be continuous.

[0034] The controller may be a PID controller and the generation of the second feedback algorithm may include changing one of a proportional term, an integral term and a derivative term of the PID controller.

[0035] According to a fifth aspect of the invention there is provided a computer program comprising computer readable instructions configured to cause a computer to carry out a method according to any one of the methods described above.

[0036] According to a sixth aspect of the invention there is provided a computer readable medium carrying a computer program according to the fifth aspect of the invention.

[0037] According to a seventh aspect of the invention there is provided a computer apparatus for a radiation source comprising a memory storing processor readable instructions, and a processor arranged to read and execute instructions stored in said memory, wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of the methods described above.

BRIEF DESCRIPTION OF THE DRAWINGS [0038] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 schematically depicts a lithographic system comprising a radiation source, a controller and a processor according to a described example;

Figure 2 shows a flow diagram of a known method of controlling a lithographic system;

Figure 3 shows a flow diagram of a method of configuring a controller of a lithographic system according to another described example;

Figure 4 shows a How diagram of an algorithm used by a processor in communication with a controller of a radiation source according to yet another described example; and,

Figure 5, consisting of Figure 5A and Figure 5B, shows two graphs which compare a radiation source being controlled by a controller operating in accordance with a first feedback algorithm and the same radiation source being controlled by the same controller operating in accordance with a second feedback algorithm according to a further described example.

DETAILED DESCRIPTION [0039] Although specific reference may 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 may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, 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. Tire 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) or a metrology or 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.

[0040] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0041] The term “patterning device” used herein should be broadly interpreted as referring to a 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. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0042] A patterning device may be transmissive or reflective. Examples of patterning device 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 phaseshift, 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; in this manner, the reflected beam is patterned.

[0043] The support structure holds the patterning device. It holds the patterning device in a way depending 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 can use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.

[0044] The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

[0045] The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.

[0046] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). 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.

[0047] The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0048] Figure 1 schematically depicts a lithographic system comprising a radiation source SO, a controller 32 and a processor 39 according to an embodiment of the invention. The lithographic system comprises:

an illumination system (illuminator) IL to condition a beam PB of radiation (e.g. UV radiation).

a support structure (e.g. a support structure) MT to support a patterning device (e.g. a mask) MA and connected to first positioning device PM to accurately position the patterning device with respect to item PL;

a substrate table (e.g., a wafer table) WT for holding or supporting a substrate (e.g., a resist coated wafer) W and connected to second positioning device PW for accurately positioning the substrate with respect to item PL; and a projection system (e.g., a refractive projection lens) PL configured to image a pattern imparted to the radiation beam PB by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0049] As here depicted, the lithographic system is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the lithographic system may be of a reflective type (e.g., employing a reflective mask or programmable mirror array of a type as referred to above).

[0050] The illuminator IL receives a beam of radiation from the radiation source SO. The radiation source SO and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising for example suitable directing mirrors and/or a beam expander. In other cases the radiation source SO may be integral part of the apparatus, for example when the radiation source SO is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0051] The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the 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 IL can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator IL provides a conditioned beam of radiation PB, having a desired uniformity and intensity distribution in its cross section.

[0052] The radiation beam PB is incident on the patterning device (e.g., mask) MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g., an interferometric device), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA with respect to the path of the beam PB, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning device PM and PW. However, in the case of a stepper (as opposed to a scanner) the support structure MT may be connected to a short stroke actuator only, or may be fixed. Patterning device MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0053] The depicted lithographic system can be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam PB is projected onto a target portion C in one go (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. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam PB 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 MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the support structure 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 beam PB 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. Tliis 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.

[0054] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0055] The radiation source SO is controlled by the controller 32. The controller 32 is configured to operate in accordance with a first feedback algorithm that is generated by a processor 39 that is in communication with the controller 32. The processor 39 is configured to receive one or more operating parameters 40a-e from the lithographic system and generate a second feedback algorithm based on the received operating parameters 40a-e. In particular, operating parameter 40a is received from the controller 32 and may, for example, be a loop gain of the controller 32. Operating parameter 40b is received from the radiation source SO and may, for example, be representative for an efficiency of the radiation source SO. Operating parameter 40c is received from the illumination system IL and may, for example, be an illumination setting (e.g., dipole illumination or quadrupole illumination) of the illumination system IL. Operating parameter 40d is received from the projection system PL and may, for example, represent a transmission of the projection system PL. Operating parameter 40e is received from an energy sensor (not shown) located at the substrate table WT and may, for example, represent a measurement error associated with the energy sensor. The processor 39 is further configured to cause the controller 32 to operate in accordance with the second feedback algorithm. The controller 32 and the processor 39 are discussed in greater detail below with respect to Figure 3, Figure 4 and Figure 5.

[0056] Figure 2 shows a flow diagram of a known method of controlling a lithographic system 10. The lithographic system 10 comprises a controller 12, a radiation source 13 and a lithographic apparatus 14. The lithographic apparatus 14 comprises an illumination system 15, a projection system 16 and a substrate 17 located on a substrate table. The substrate table may comprise an energy sensor 18 configured to measure the radiation incident on the substrate 17. Alternatively, the illumination system 15 may comprise an energy sensor that is configured to measure the radiation and produce a signal that is indicative of the radiation incident on the substrate 17. The energy sensor 18 may, for example, comprise a photodetector such as a photodiode. A desired set-point value 11 of the radiative power incident on the substrate 17 is determined. The set-point value 11 may, for example, be selected based upon the lithographic exposure that is to take place using the lithographic system 10. For example, the set-point value 11 may be determined from a desired radiation dose profile incident on the substrate 17. The set-point value 11 may be corrected over time to compensate for a drift in an optical transmission of the lithographic apparatus. For example, the set-point value 11 may be increased (e.g., linearly) over time to compensate for a reduced optical transmission of the projection system 16. The set-point value 11 may be further corrected to compensate for other effects such as, for example, unwanted scattered radiation (i.e., “stray light”). For example, the set-point value 11 for pulses of radiation occurring during a beginning and/or an end of an exposure of a target portion of the substrate 17 may be greater than the set-point value 11 for other pulses of radiation. The set-point value 11 may be provided by an operator of the lithographic system 10. Alternatively, the set-point value 11 may be retrieved from a lookup table in which multiple set-points for different uses of the lithographic system 10 are stored. The energy sensor 18 senses the radiative power incident on the substrate 17 and generates a signal 19 indicative of the radiative power incident on the substrate 17. The set point value 11 and the signal 19 indicative of the radiative power incident on the substrate 17 are provided to the controller 12. The controller 12 comprises a feedback algorithm that is configured to process the received signal 19 and control the radiation source 13 in dependence on the received signal 19.

[0057] The feedback algorithm produces a single configuration of the controller 12. That is, the feedback algorithm aims to control the radiation source 13 in a way that balances a performance of the lithographic system 10 and a stability of the controller 12. For example, the feedback algorithm may be configured to increase a performance of the lithographic system 10 whilst maintaining a minimum stability of the controller 12. The performance of the lithographic system 10 corresponds to the average radiation dose error occurring at the substrate 17. The stability of the controller 12 corresponds to the prevalence of radiation dose outliers experienced at the substrate 17. A radiation dose outlier may be defined as a dose of radiation which is either too large or too small, resulting in a target portion of the substrate being incapable of performing its intended function. Alternatively, a radiation dose outlier may be defined as any radiation dose error that falls outside of a distribution (e.g., a Gaussian distribution) that is representative of a majority of radiation dose errors occurring for a lithographic system. A radiation dose error that falls outside of the normal distribution representing the majority of dose errors may be indicative of a non-linear reaction (i.e., an overreaction) of the controller 12. The stability of the controller 12 may determine an overreaction of the controller 12. For example, an instable controller 12 may overcompensate either side of a set point 11 when attempting to reach a set point, resulting in the signal 19 getting further and further away from the set point value 11. The stability of the controller 12 may represent a stability of a control loop of which the controller is a part of. The feedback algorithm is determined once for the lifetime of the lithographic system 10. The lithographic system 10 may change over the course of its lifetime. For example, an efficiency of the radiation source 13 may decrease over the lifetime of the lithographic system 10. The feedback algorithm does not account for the decrease in efficiency of the radiation source 13. The same feedback algorithm may be applied to multiple lithographic systems despite differences between those lithographic systems. For example, one projection system may have a larger transmission of the radiation beam compared to another projection system. The feedback algorithm does not account for the difference in transmission between the projection systems. The same feedback algorithm may be applied to multiple uses of the lithographic system despite differences between those uses of the lithographic system. For example, one use of the lithographic system 10 may involve a dipole illumination setting of the illumination system 15 whereas a different use of the lithographic system 10 may involve a quadrupole illumination setting of the illumination system 15. The feedback algorithm does not account for the different illumination settings of the lithographic system 10.

[0058] The radiation source 13 may be a pulsed radiation source. The controller 12 may control the radiation source 13 by setting a voltage of the radiation source 13. The controller 12 may set the voltage of the radiation source 13 on a pulse-by-pulse basis. The feedback algorithm may act in part to compare the signal 19 that is indicative of the radiative power on the substrate 17 with the set-point value 11 to determine a radiative dose error. The feedback algorithm may then convert the error to a change of the voltage of the radiation source 13. The controller 12 may then apply the change of the voltage applied to the radiation source 13 in order to reduce the error. The conversion of a radiative dose energy error to a voltage performed by the feedback algorithm may not be accurate. For example, the radiation source 13 may change and/or deteriorate over time, causing the actual conversion of radiative dose error to voltage to differ from the feedback algorithm’s conversion of radiative dose error to voltage. This may in turn negatively affect the performance of the lithographic system 10. For example, the average radiation dose error may increase over the lifetime of the lithographic system 10 and/or the prevalence of radiation dose outliers may increase over the lifetime of the lithographic system 10.

[0059] Figure 3 shows a flow diagram of a method of configuring a controller of a lithographic system 30 according to an example described herein. The lithographic system 30 comprises a controller 32, a radiation source 33 and a lithographic apparatus 34. The lithographic apparatus 34 comprises an illumination system 35, a projection system 36 and a substrate 37 located on a substrate table. The substrate table may comprise an energy sensor 38 configured to measure the radiative power incident on the substrate 37. The energy sensor 38 may, for example, comprise a photodetector such as a photodiode. A desired set-point value 31 of the radiative power incident on the substrate 37 is determined. The set-point value 31 may, for example, be selected based upon the lithographic exposure that is to take place using the lithographic system 30. The energy sensor 38 generates a signal 41 indicative of the radiative power incident on the substrate 37. The set point value 31 and the signal 41 indicative of the radiative power incident on the substrate 37 are provided to the controller 32. The controller 32 comprises a first feedback algorithm configured to process the signal 41 indicative of a radiative power incident on the substrate 37 and control the radiation source 33 in dependence on the received signal. The controller 32 is in communication with a processor 39. The processor 39 is configured to receive one or more operating parameters 40a-e, as described with reference to Figure 1, of the lithographic system 30 and generate a second feedback algorithm based on the received operating parameters 40a-e. The processor 39 is configured to cause the controller 32 to operate in accordance with the second feedback algorithm.

[0060] The one or more operating parameters 40a-e are used by the processor 39 to update the configuration of the controller 32. The processor 39 may be configured to monitor the one or more operating parameters 40a-e. The monitoring may be continuous. The processor 39 is configured to process the received operating parameters 40a-e and determine whether or not a performance of the lithographic system 30 could be improved by generating a second feedback algorithm and causing the controller 32 to operate in accordance with the second feedback algorithm. Using the operating parameter 40a-e to determine a second feedback algorithm advantageously enables changes in the lithographic system 30 to be accounted for. For example, the radiation source 33 may become less efficient over its lifetime. The operating parameter 40b received by the processor 39 from the radiation source 33 may indicate an efficiency of the radiation source 33. The processor 39 may receive the operating parameter 40b indicating an efficiency of the radiation source 33 and determine a second feedback algorithm that accounts for the change in efficiency of the radiation source 33. The processor 39 then causes the controller 32 to operate in accordance with the second feedback algorithm. The second feedback algorithm may, for example, result in a reduced average radiation dose error for the lithographic system 30 (i.e., an improved performance of the lithographic system 30). For example, the second feedback algorithm may cause the controller 32 to operate such that the radiation source 33 operates at an increased voltage to account for the reduced efficiency of the radiation source 33.

[0061] The generation of the second feedback algorithm may depend upon a selected balance between a performance of the lithographic system 30 and a stability of the controller 32. An operator of the lithographic system 30 may select a balance between a performance of the lithographic system 10 and a stability of the controller 32. The processor 39 may store the selected balance in memory. Whenever the processor 39 generates a second feedback algorithm, the processor 39 may access the selected balance and generate the second feedback algorithm in accordance with the selected balance. A different balance may be selected, e.g., by an operator, between the performance of the lithographic system 10 and the stability of the controller 32 at any time. For example, a balance may be selected that minimises the average radiation dose error on the substrate 37 whilst accepting an increased risk of radiation dose outliers. Alternatively, a balance may be selected, e.g., by an operator, that allows a larger average radiation dose error (i.e., a decreased performance of the lithographic system) whilst reducing the number of radiation dose outliers (i.e., increasing a stability of the controller).

[0062] The one or more operating parameters 40a-e may, for example, be one or more of a power of the radiation beam 40b, a variation of the power of the radiation beam 40b, a spectrum of the radiation beam 40b, a radiation source noise 40b, an operating voltage of the radiation source 40b, an efficiency of the radiation source 40b, an error of the radiation source 40b, a gain of the radiation source 40b; a rate limiter of the radiation source 40b, a linearity of the radiation source 40b, a loop gain factor of the controller 40a, a variation of a loop gain of the controller 40a, a number of pulses of the radiation beam that are incident on the target portion of the substrate 40d, an illumination setting of the illumination system 40c, a transmission of the projection system 40e and a measured value of a radiation dose error on the substrate 40d.

[0063] The controller 32 may be a PID controller. The generation of the second feedback algorithm may include changing one of a proportional term, an integral term and a derivative term of the PID controller 32. A proportional-integral-derivative controller (PID controller or three term controller) comprises a control loop feedback mechanism used in applications requiring continuously modulated control. The PID controller may continuously calculate an error value as the difference between a desired setpoint and a measured process variable and apply a correction based on proportional, integral, and derivative terms.

[0064] The processor 39 may be used with a radiation source 33 independent of the lithographic apparatus 34. For example, the processor 39 may only receive an operating parameter from the controller 32 and/or the radiation source 33, e.g. operating parameters 40a-b.

[0065] Figure 4 shows a flow diagram of an example algorithm that may be used by a processor in communication with a controller of a radiation source according to an embodiment of the invention. Operating parameters 42 of the radiation source are measured and/or otherwise determined. In the example of Figure 4, the operating parameters 42 consist of a radiation dose error on the substrate 44, a radiation source noise 46, a loop gain function of the controller 48 (e.g., minimum and maximum values of a loop gain function of the controller), a voltage setting of the radiation source 50, an efficiency of the radiation source 5'2 and a signal indicative of the radiative power at the substrate

57. A larger or smaller number of operating parameters 42 may be used. Other operating parameters 42 may be used. Operating parameters 46-57 are provided to a model 54. The model receives a signal indicative of the radiative power at the substrate 57 and uses the operating parameter 57 to determine how the controller would respond to the signal when operating in accordance with the first feedback algorithm. That is, the model 54 uses the operating parameter 57 to calculate a radiation source response 51, a controller response 53 and a loop sensitivity 55 to the operating parameter 57 resulting from the first feedback algorithm. In the example process of Figure 4, the generation of the second feedback algorithm includes determining the controller response 53 and the radiation source response 51. The controller response 53 may be determined from a controller transfer function. The controller transfer function may correspond to a sensitivity function of the controller (i.e., a closed loop output disturbance suppression of the controller in a frequency domain). For example, if a component of noise with a given frequency has a sensitivity function amplitude that is greater than one, then this indicates that the noise component will be amplified by the controller, whereas an amplitude below one indicates that the controller will suppress that noise component. The radiation source response may be determined from the radiation source transfer function. The radiation source transfer function indicates a response of the radiation source relative to a set-point value provided by the controller. For example, if the radiation source transfer function has an output of one then this indicates that a calibration conversion between the set-point value and the power of the radiation produced by the radiation source is accurate.

The model 54 uses the operating parameters 46-52 to determine how the radiation source response 51, the controller response 53 and the loop sensitivity 55 will affect the radiation source and the radiation produced by the radiation source. That is, the model 54 uses the operating parameters 46-52 to predict via a processor 56 a voltage setting of the radiation source 58, a phase of the radiation beam pulses 60 and an amplitude of the radiation beam pulses 62. The processor then compares the predicted values

58, 60, 62 to limit values, that are e.g., set by an operator, of the radiation source via a series of decisions 70a-d. These limit values are determined by a selected balance between a performance of the radiation source and a stability of the controller. A first decision 70a determines whether or not the predicted voltage setting 58 of the radiation source is greater than a maximum threshold value of the voltage setting of the radiation source. A second decision 70b determines whether or not the predicted voltage setting 58 of the radiation source is less than a minimum threshold value of the voltage setting of the radiation source. Outputs for the first decision 70a and the second decision 70b may be determined simultaneously. Alternatively the processor may only determine an output of the first decision 70a or an output of the second decision 70b. For example, the processor may, given the knowledge of the voltage setting of the radiation source 50 and the limit values, e.g., set by a user, determine whether an output of the first decision 70a should be determined or whether an output of the second decision 70b should be determined. A third decision 70c determines whether or not the predicted phase of the radiation beam pulses 60 is outside a desired phase margin. A fourth decision 70d determines whether or not the predicted amplitude of the radiation beam pulses 62 is outside a desired amplitude margin. The stability of the controller may be determined, at least in part, by the maximum threshold value of the voltage setting of the radiation source, the minimum threshold value of the voltage setting of the radiation source, the desired phase margin of the radiation pulses and the desired amplitude margin of the radiation pulses.

[0066] The next step in the example processing of Figure 4 is represented by an OR logic gate 72 followed by a true or false decision 74. If any of the answers to the decisions 70a-d are “Yes” then the controller would have gone beyond the limit values (i.e., the selected balance between performance and stability) by operating in accordance with the first feedback algorithm. That is, the first feedback algorithm would have reduced a stability of the controller. If any of the answers to the decisions 70ad are “Yes” then the output of the OR gate 72 will be “1” and thus the subsequent decision 74 will output “Yes”. In this case, a decision 76 determines whether the stability of the controller can be improved without reducing the performance of the lithographic system below a selected minimum performance. In order to make this decision 76, the processor may generate a second feedback algorithm and re-run the model 54 using the second feedback algorithm instead of the first feedback algorithm. Alternatively, the processor may determine whether the stability of the controller can be improved without reducing the performance of the lithographic system below a selected minimum performance without re-running the model 54. For example, at decision 76 the processor may compare a difference (i.e., an available margin) between the predicted values and the threshold values associated with decisions 70a-70d to a proposed modification to the first algorithm and determine whether the proposed modification to the first algorithm (i.e., the second algorithm) would result in a change that is well-within the available margin. If the answer at decision 76 is “No”, then the processor does not cause the controller to operate in accordance with the second feedback algorithm and the process ends at block 80. That is, the processor does not re-configure the controller because the processor cannot do so without increasing an average radiation dose error (i.e., decreasing performance) beyond a, e.g., operator-defined, limit value.

[0067] If the answer to the decision 76 is “Yes” then the processor causes the controller to act in accordance with the second feedback algorithm 78. In this example the second feedback algorithm 78 will cause the controller to act less aggressively when controlling the radiation source of the lithographic system compared to the first feedback algorithm. For example, the controller may act less aggressively by increasing the average radiation dose error whilst simultaneously reducing the prevalence of radiation dose outliers. The second feedback algorithm 78 replaces the first feedback algorithm in the model 54 and is used for future modelling by the processor.

[0068] Alternatively, if all of the answers to the series of decisions 70a-d are “No” then the controller would not have gone beyond the limits set by the selected balance between performance and stability by operating in accordance with the first feedback algorithm. That is, the first feedback algorithm may have increased a performance of the radiation source, but there may be room for further improvement. If all of the answers to the series of decisions 70a-d are “No” then the output of the OR gate will be “0” and thus the subsequent decision 74 will output “No”. In this case, a decision 82 determines whether the performance of the lithographic system can be increased without reducing the stability of the controller below a selected minimum stability. In order to make this decision 82, the processor generates a second feedback algorithm and re-runs the model 54 using the second feedback algorithm instead of the first feedback algorithm. If the answer at decision 82 is “No” then the processor does not cause the controller to operate in accordance with the second feedback algorithm and the process ends at block 80. That is, the processor does not re-configure the controller because the processor cannot do so without increasing the prevalence of radiation dose outliers beyond a, e.g., operator-defined, limit value. If the answer at decision 82 is “Yes” then the processor causes the controller to operate in accordance with the second feedback algorithm 84. In this case, the second feedback algorithm 84 will cause the controller to act more aggressively when controlling the radiation source of the lithographic system compared to the first feedback algorithm. The second feedback algorithm 84 replaces the first feedback algorithm in the model 54 and is used for future modelling by the processor.

[0069] An input of the processor algorithm may be one or more of the following: a desired radiation dose incident on a target portion of the substrate, a minimum stability of the controller, a limitation of the controller, a frequency with which to generate a second feedback algorithm, a maximum limit on the amount of data to be used by the processor when determining one of a controller transfer function, a radiation source transfer function and a lithographic apparatus transfer function, a rate limiter of the radiation source (for example, this may be used to determine a nonlinear behaviour of the radiation source), a maximum and/or minimum voltage setting of the radiation source (for example, this may be used to determine a non-linear behaviour of the radiation source), a number of pulses of the radiation beam that tire incident on the target portion of the substrate, a repetition rate of the radiation source, etc.. The number of pulses of the radiation beam that are incident on the target portion of the substrate may, for example, be between about 25 and about 100. [0070] The controller 32 may be a PID controller. The generation of the second feedback algorithm 78, 84 may include changing one of a proportional term, an integral term and a derivative term of the PID controller 32. An input of the processor algorithm may, for example, be an integral gain of the PID controller, a proportional gain of the PID controller, a derivative gain of the PID controller, a ratio between an actual loop gain and a calibrated loop gain (i.e., the loop gain factor of the controller), etc.. The integral gain of the PID controller and/or the proportional gain of the PID controller may, for example, be between about 0.1 and about 10. The derivative gain of the PID controller may, for example, be about 1. The loop gain factor of the controller may, for example, be between about 0.25 and about 2. A loop gain factor of 1 means that the controller is perfectly calibrated. A loop gain factor that is larger than 1 means that a stability of the controller is less than a desired stability, resulting in an increased number of radiation dose outliers. A loop gain factor that is less than 1 means that the controller will be slow to respond to the signal indicative of the radiation beam, resulting in an increase in the average dose error on the substrate (i.e., a reduced performance of the radiation source).

[0071] An operating parameter 42 of the lithographic system that is provided to the processor may, for example, be one or more of the following: a noise of the radiation source such as a pulse-topulse noise (for example, this may be used to predict a radiation dose error), a radiation source noise spectrum (for example, this may be used to determine a sensitivity of the controller), a variation in a conversion of the radiation source voltage setting to an energy of the radiation beam (for example, this may be used to ensure controller stability criteria are being met), an efficiency of the radiation source (for example, this may be used to determine the non-linear behaviour of the radiation source and/or a maximum and/or minimum voltage setting of the radiation source, e.g., a clipping response), a change in radiative transmission of the lithographic apparatus, one or more measured radiation dose errors (e.g., maximum does error, minimum dose error, mean dose error and/or standard deviation of dose error), a setting of the illumination system (e.g., a dipole illumination setting or a quadrupole illumination setting) and its impact on the radiation dose received by a target portion of the substrate, and changes of a rate limiter of the radiation source over time. A pulse-to-pulse noise of the radiation source may, for example, be between about 5% and about 10%. A minimum voltage setting of the radiation source may, for example, be about 0.1 kV. A maximum voltage setting of the radiation source may, for example, be about 30 kV.

[0072] Using an operating parameter 42 to generate the second feedback algorithm 78, 84 may advantageously provide improved performance for individual lithographic systems. That is, differences between different lithographic systems may be accounted for by the processor when generating the second feedback algorithm 78, 84 such that the controller is tuned to the characteristics and uses of an individual lithographic system. This is turn may reduce an average radiation dose error of the lithographic system.

[0073] The controller may be reconfigured by the second feedback algorithm 78, 84 such that the prevalence of radiation dose outliers of a lithographic system is reduced. For example, if a controller is configured to act too aggressively w'hen the radiation source is operating under a corner case condition, i.e. when one or more settings of the radiation source are at extreme values, then the prevalence of dose outliers will be relatively large. The second feedback algorithm 78, 84 could reconfigure the controller during operation at a corner case condition in order to reduce the prevalence of radiation dose outliers. This may be of particular use when the lithographic process involves exposing a sensitive substrate for which even a small prevalence of radiation dose outliers would have a significant negative effect on the yield of the lithographic process.

[0074] Figure 5, consisting of Figure 5A and Figure 5B, shows two graphs which compare a radiation source being controlled by a controller operating in accordance with a first feedback algorithm and the same radiation source being controlled by the same controller operating in accordance with a second feedback algorithm according to an embodiment of the invention. Figure 5A is a graph of a radiation dose error at the substrate as a function of the loop gain of the controller. When operating in accordance with the first feedback algorithm 90 the controller becomes unstable at a loop gain of 1.5. Figure 5B is a graph of a loop gain distribution as a function of the loop gain of the controller. The prevalence of radiation dose outliers is proportional to the area under the loop gain distribution graph above a loop gain value at which the controller becomes unstable. When the controller is operating in accordance with the first feedback algorithm 90 the prevalence of radiation dose outliers is proportional to the area under the loop gain distribution graph above a loop gain value of 1.5.

[0075] The processor receives an operating parameter. In the example of Figure 5, the operating parameter is a variation of a loop gain of the controller. Other operating parameters may be provided to the processor. Multiple operating parameters may be provided to the processor. After a period of time a variation of the loop gain improves (i.e., the loop gain stability improves). With reference to Figure 5B, this results in a narrowed loop gain distribution 92. The processor receives the variation of the loop gain of the controller and determines that there has been an improvement in the loop gain variation. The processor uses this information to generate a second feedback algorithm (e.g., via the process exemplified in Figure 4). The second feedback algorithm depends upon a selected balance between a performance of the radiation source and a stability of the controller. In the example of Figure 5, the balance is selected, e.g., by an operator or user, such that the performance of the radiation source is increased as much as possible whilst maintaining a minimum stability of the controller. The processor generates the second feedback algorithm in accordance with the selected balance between performance and stability. Because there has been an improvement in the loop gain variation of the controller, the second feedback algorithm causes the controller to act more aggressively than the first feedback algorithm. With reference to Figure 5A, when the controller acts in accordance with the second feedback algorithm 92 the radiation dose error is reduced compared to the first feedback algorithm 90. When operating in accordance with the second feedback algorithm 92, the controller becomes unstable at a loop gain of 1.4 compared to 1.5 when operating in accordance with the first feedback algorithm 90. However, with reference to Figure 5B, the area under the second feedback algorithm distribution 92 after the loop gain value at which the controller becomes unstable (i.e., 1.4 onwards) is about the same as the area under the first feedback algorithm 90 above a loop gain value at which the controller becomes unstable (i.e., 1.5 onwards). As discussed above, the number of dose error outliers is proportional to the area under the loop gain distribution graph above a loop gain at which the controller becomes unstable. When the controller is operating in accordance with the second feedback algorithm the number of radiation dose outliers experienced by the lithographic system is the same as when the controller acts in accordance with the first feedback algorithm. However, the average radiation dose error has reduced when operating in accordance with the second feedback algorithm (see Figure 5A). That is, when switching from the first feedback algorithm 90 to the second feedback algorithm 92 a stability of the controller has been maintained whilst a performance of the radiation source has improved.

[0076] The processor may be configured to monitor the operating parameter. The monitoring may be continuous. Alternatively the monitoring may be periodic at and desired frequency. As another alternative, the monitoring may be event driven, e.g., the processor may receive the operating parameter once per target portion of a substrate, or once per substrate, or one per lot of substrates in a lithographic process, etc.. As yet another alternative, an operator may select when the processor receives the operating parameter. That is, an operator may cause the processor to receive the operating parameter and determine whether or not to generate a second feedback algorithm when the operator feels that a different controller configuration is required, e.g., when a new illumination setting is selected and/or when a different pattern is to be projected onto the substrate.

[0077] Whilst the invention has been discussed above in relation to a UV radiation source and a transmissive UV lithographic apparatus, the invention described herein may be used in conjunction with an EUV radiation source and/or a reflective EUV lithographic apparatus.

[0078] Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may 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 may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may 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 computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc..

[0079] 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 description is not intended to limit the invention. 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 radiation source configured to provide a radiation beam, the radiation source being controlled by a controller, the controller comprising a first feedback algorithm configured to process a signal indicative of the radiation beam and control the radiation source in dependence on the received signal, wherein the controller is in communication with a processor, the processor being configured to:

receive an operating parameter of the radiation source;

generate a second feedback algorithm based on the received operating parameter; and cause the controller to operate in accordance with the second feedback algorithm.

2. The radiation source of clause 1, wherein the generation of the second feedback algorithm depends upon a selected balance between a performance of the radiation source and a stability of the controller.

3. The radiation source of clause 1 or clause 2, wherein the generation of the second feedback algorithm includes determining one of a controller response and a radiation source response.

4. The radiation source of any preceding clause, wherein the processor is configured to monitor the operating parameter.

5. The radiation source of clause 4, wherein the monitoring is continuous.

6. The radiation source of any preceding clause, wherein the controller is a P1D controller.

7. The radiation source of clause 6, wherein the generation of the second feedback algorithm includes changing one of a proportional term, an integral term and a derivative term of the PID controller.

8. The radiation source of tiny preceding clause, wherein the operating parameter is one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, an error of the radiation source, a gain of the radiation source, a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, and a variation of a loop gain of the controller.

9. A lithographic system comprising;

a radiation source configured to provide a radiation beam; and a lithographic apparatus, the lithographic apparatus comprising:

an illumination system for conditioning the radiation beam;

a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section;

a substrate table for holding a substrate; and a projection system for projecting the patterned radiation beam onto a target portion of the substrate, wherein the lithographic system further comprises a controller comprising a first feedback algorithm configured to process a signal indicative of a radiative power incident on the substrate and control the radiation source in dependence on the received signal, wherein the controller is in communication with a processor, the processor being configured to;

receive an operating parameter of the lithographic system;

10. The lithographic system of clause 9, wherein the generation of the second feedback algorithm depends upon a selected balance between a performance of the lithographic system and a stability of the controller.

11. The lithographic system of clause 9 or clause 10, wherein the generation of the second feedback algorithm includes determining one of a controller response, a radiation source response and a lithographic apparatus response.

12. The radiation source of any of clauses 9 to 11, wherein the processor is configured to monitor the operating parameter.

13. The lithographic system of clause 12, wherein the monitoring is continuous.

14. The lithographic system of any of clauses 9 to 13, wherein the controller is a PÏD controller.

15. The lithographic system of clause 14, wherein the generation of the second feedback algorithm includes changing one of a proportional term, an integral term and a derivative term of the P1D controller.

16. The lithographic system of any clauses 9 to 15, wherein the operating parameter is one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, an error of the radiation source, a gain of the radiation source; a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, a variation of a loop gain of the controller, a number of pulses of the radiation beam that are incident on the target portion of the substrate, an illumination setting of the illumination system,; and a measured value of a radiation dose error on the substrate.

17. A method of configuring a controller of a radiation source, the radiation source being configured to generate a radiation beam, the controller comprising a first feedback algorithm configured to process a signal indicative of the radiation beam and control the radiation source in dependence on the received signal, the method comprising:

receiving an operating parameter of the radiation source;

generating a second feedback algorithm based on the received operating parameter; and causing the controller to operate in accordance with the second feedback algorithm.

18. The method of clause 17, wherein the generation of the second feedback algorithm depends upon a selected balance between a performance of the radiation source and a stability of the controller.

19. The method of clause 17 or clause 18, wherein the generation of the second feedback algorithm includes determining one of a controller response and a radiation source response.

20. The method of any of clauses 17 to 19, further comprising monitoring the operating parameter.

21. The method of clause 20, wherein the monitoring is continuous.

22. The method of any of clauses 17 to 21, wherein the controller is a P1D controller, and wherein the generation of the second feedback algorithm includes changing one of a proportional term, an integral term and a derivative term of the PID controller.

23. The method of any of clauses 17 to 22, wherein the operating parameter is one of a power of the radiation beam, a variation of the power of the radiation beam, a spectrum of the radiation beam, a radiation source noise, an operating voltage of the radiation source, an efficiency of the radiation source, tin error of the radiation source, a gain of the radiation source, a rate limiter of the radiation source, a linearity of the radiation source, a loop gain factor of the controller, and a variation of a loop gain of the controller.

24. A method of configuring a controller of a lithographic system, the lithographic system comprising:

an illumination system for conditioning the radiation beam;

a substrate table for holding a substrate; and a projection system for projecting the patterned radiation beam onto a target portion of the substrate, wherein the lithographic system further comprises a controller, the controller comprising a first feedback algorithm configured to process a signal indicative of a radiative power incident on the substrate and control the radiation source in dependence on the received signal, the method comprising:

receiving an operating parameter of the lithographic system;

generating a second feedback algorithm based on the received operating parameter; and causing the controller to operate in accordance with the second feedback algorithm,

25. The method of clause 24, wherein the generation of the second feedback algorithm depends upon a selected balance between a performance of the radiation source and a stability of the controller.

26. The method of clause 24 or clause 25, wherein the generation of the second feedback algorithm includes determining one of a controller response, a radiation source response and a lithographic apparatus response.

27. The method of any of clauses 24 to 26, further comprising monitoring the operating parameter.

28. The method of clause 27, wherein the monitoring is continuous.

29. The method of any of clauses 24 to 28, wherein the controller is a PID controller, and wherein the generation of the second feedback algorithm includes changing one of a proportional term, an integral term and a derivative term of the PID controller.

30. A computer program comprising computer readable instructions configured to cause a computer to carry out a method according to any one of clauses 17 to 29.

31. A computer readable medium carrying a computer program according to clause 30.

32. A computer apparatus for a radiation source comprising:

a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in said memory;

wherein said processor readable instructions comprise instructions arranged to control the computer to carry out a method according to any one of clauses 17 to 29.

Claims

CONCLUSION

A lithography apparatus comprising:

an illumination device adapted to provide a radiation beam;

a carrier constructed to support a patterning device, which patterning device is in

5 is capable of applying a pattern in a cross-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.

1/4

40b