阅读说明：本技术 控制系统和方法 (Control system and method ) 是由 H·P·戈德弗里德 F·埃弗茨 W·P·E·M·奥普特·鲁特 于 2019-02-21 设计创作，主要内容包括：一种用于控制激光器的控制系统,包括：用于感测物理值的传感器,物理值指示由激光器发出的激光束的特性；开关；第一控制器和第二控制器。每个控制器被配置为：从传感器接收又一传感器值；基于接收到的又一传感器值调整接收到的设定点值以给出输出值；并且使激光器根据该输出值进行操作。开关被配置为在控制器之间切换,使得输出值以循环的方式从每个控制器被提供。(A control system for controlling a laser, comprising: a sensor for sensing a physical value indicative of a characteristic of a laser beam emitted by the laser; a switch; a first controller and a second controller. Each controller is configured to: receiving a further sensor value from the sensor; adjusting the received set point value based on the received further sensor value to give an output value; and causes the laser to operate in accordance with the output value. The switches are configured to switch between the controllers such that the output values are provided from each controller in a cyclical manner.)

1. A control system for controlling a laser, the control system comprising:

a sensor for sensing a physical value indicative of a characteristic of a laser beam emitted by the laser;

a switch;

a first controller and a second controller, wherein each controller is configured to:

receiving a set point value;

receiving a further sensor value from the sensor;

adjusting the received set point value based on the received further sensor value to give an output value;

causing the laser to operate in accordance with the output value;

wherein:

the output value from the first controller is different from the output value from the second controller;

the switch is configured to switch between the controllers such that an output value is provided from each controller in a cyclic manner;

the controllers are configured to communicate with each other; and is

At least one adjustment to the set point value by the second controller is determined at least in part by at least one adjustment to the set point value by the first controller.

2. The control system of claim 1, wherein the laser is a pulsed laser and the switch is configured to switch between the controllers on a pulse-to-pulse basis.

3. A control system according to any preceding claim, wherein the laser is configured to provide more than one type of output.

4. The control system of claim 3, wherein each controller is configured to control a respective type of output from the laser.

5. A control system according to any preceding claim, wherein each controller comprises a limiter configured to adjust the output value according to a predetermined limit.

6. The control system of claim 5, wherein the limiters of each controller are configured to communicate with each other.

7. A control system according to any preceding claim, wherein each controller comprises a tuning unit configured to adjust the set point value to give the output value based on the received further sensor value.

8. A control system according to any preceding claim, wherein the output value is configured to control the energy dose of the laser.

9. The control system of any one of claims 1 to 8, wherein the output value is configured to control the wavelength emitted by the laser.

10. A radiation source comprising a laser and a control system according to any preceding claim, wherein the control system is configured to control the laser of the radiation source.

11. A lithographic system comprising a radiation source according to claim 10 and a lithographic apparatus, the lithographic apparatus comprising:

an illumination system for conditioning a radiation beam emitted by the radiation source;

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

a substrate table for holding a substrate; and the number of the first and second groups,

a projection system for projecting the patterned beam of radiation onto a target portion of the substrate.

12. A method for controlling a parameter of a laser using a first controller and a second controller, the method comprising:

cycling between a first controller and a second controller, wherein each controller performs the steps of:

a) receiving a set point value for the parameter;

b) receiving a further sensor value from the sensor;

c) based on the received further sensor value, adjusting the received set point value to give an output value;

d) causing the laser to operate in accordance with the output value;

wherein the controllers are in communication with each other, and wherein at least one adjustment to the set point value by the second controller is determined at least in part by at least one adjustment to the set point value by the first controller.

13. The method of claim 12, wherein the laser is a pulsed laser and the cycling between the controllers is on a pulse-to-pulse basis.

14. The method of any of claims 12 to 13, wherein each controller further controls at least one of:

a respective type of output from the laser; and

the output value according to a predetermined limit.

15. The method of any of claims 12 to 14, wherein the output value is used to control at least one of:

the energy dose of the laser; and

the wavelength emitted by the laser.

Technical Field

The present invention relates to a control system and method for controlling a radiation source and has particular, but not exclusive, application to a radiation source for a lithographic apparatus.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. For example, lithographic apparatus can be used to manufacture Integrated Circuits (ICs). In that case, a patterning device (alternatively referred to as a mask or a reticle) can be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or more dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). Typically, 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.

The development of memory cells for computer memories has led to the creation of "3D" stacked memories that are arranged in multiple floors rather than as a single floor. The use of 3D memory means that the resist becomes significantly thicker. For example, the thickness of a single layer memory (also referred to as a "2D" memory) may be about 200nm, whereas a 3D memory may be several microns thick. Such thick resists may require exposure to high doses throughout the thickness of the resist. However, increased resist thickness may lead to problems of poor focus throughout the thickness of the resist. In addition, such high topography can lead to significant wafer bow, leading to further focusing problems.

Current solutions attempt to optimize focus by targeting a location near the middle of the thick resist. For 3D, "curved" wafers, focusing can also be achieved with large adjustments to the wafer stage height and tilt. However, this may lead to local stresses in the wafer and problems with alignment of the wafer stage.

It is desirable to obviate or mitigate one or more problems in the prior art, whether identified herein or elsewhere.

Disclosure of Invention

According to a first aspect of the present invention there is provided a control system for controlling a laser, the control system comprising: a sensor for sensing a physical value indicative of a characteristic of a laser beam emitted by the laser; a switch; a first controller and a second controller. Each controller is configured to: receiving a set point value; receiving a further sensor value from the sensor; adjusting the received set point value based on the received further sensor value to give an output value; and causing the laser to operate in accordance with the output value. The output value from the first controller is different from the output value from the second controller, and the switch is configured to switch between the controllers such that the output value is provided from each controller in a round-robin fashion. The controllers are configured to communicate with each other, and at least one adjustment to the set point value by the second controller is determined at least in part by at least one adjustment to the set point value by the first controller.

In this way, improved control of the radiation source is advantageously provided. In addition, by enabling communication between the controllers of the control system, adjustments to the operation of the laser caused by the control system may be set so as to minimize detrimental stress on the laser. Communication between the controllers may be accomplished in various ways known to the skilled person.

The laser may be a pulsed laser and the switch may be configured to switch between the controllers on a pulse-to-pulse basis. Alternatively, the switches may switch between the controllers according to different switching schemes, depending on requirements.

The laser may be configured to provide more than one type of output, e.g., laser beams of different wavelengths. Where the laser is configured to provide more than one type of output, each controller of the control system may be configured to control the respective type of output, e.g. each controller controls a laser beam having a respective wavelength. In this way, lasers providing more than one type of output (such as laser beams of different wavelengths) can be more accurately controlled with a single control system.

Advantageously, each controller may comprise a limiter configured to adjust the output value according to a predetermined limit. In this way, the limits can be set so as to avoid continuous adjustment that places excessive stress on the laser. Further, the limiters of each controller may be configured to communicate with each other. For example, the limiters may be configured to communicate instances of conditional resets performed by each limiter.

Each controller may comprise a tuning unit configured to adjust the set point value based on the received further sensor value to give an output value. The tuning unit may include a PID module, a PIID module, a PII module, a PDD module, or other architecture depending on the requirements.

The output value may be configured to control the energy dose of the laser. Alternatively or additionally, the output value may be configured to control the wavelength emitted by the laser.

According to a second aspect of the present invention, there is provided a radiation source comprising a control system according to the first aspect, wherein the control system is configured to control a laser of the radiation source.

According to a third aspect of the invention, there is provided a lithographic system comprising a radiation source according to the second aspect and a lithographic apparatus comprising: an illumination system for conditioning a radiation beam emitted by a radiation source, a support structure for supporting a patterning device, a patterning device for imparting 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.

According to a fourth aspect of the present invention there is provided a method for controlling a parameter of a laser using a first controller and a second controller, the method comprising: cycling between a first controller and a second controller, wherein each controller performs the steps of: a) receiving a set point value for a parameter; b) receiving a further sensor value from the sensor; c) adjusting the received set point value based on the received further sensor value to give an output value; d) causing the laser to operate in accordance with the output value; wherein the controllers are in communication with each other, and wherein at least one adjustment to the set point value by the second controller is determined at least in part by at least one adjustment to the set point value by the first controller.

It is to be understood that the terms 'first' and 'second' are used without time limitation. In particular, the second controller may control the laser before the first controller and vice versa.

According to a fifth aspect of the invention, there is provided a computer program comprising computer readable instructions configured to cause a computer to perform the method according to the fourth aspect.

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

According to a seventh aspect of the present invention, there is provided a computer arrangement for a radiation source, the computer arrangement comprising: a memory storing processor readable instructions; and a processor arranged to read and execute instructions stored in the memory, wherein the processor-readable instructions comprise instructions arranged to control a computer to perform a method according to the fourth aspect.

Drawings

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:

FIG. 1 depicts a lithographic apparatus arranged according to examples described herein;

FIG. 2 depicts an example of a control system for controlling a radiation source;

figures 3a to 3c depict example configurations for a tuning unit of a control system for controlling a radiation source;

FIG. 4 depicts a flow chart of an example of a method for controlling a radiation source; and

fig. 5 depicts a flow chart of another example of a method for controlling a radiation source.

Detailed Description

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. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) 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.

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 126nm) 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.

The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section, such as to create a pattern in a target portion of the substrate. 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.

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

The support structure holds the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support may use mechanical clamping, vacuum, or other clamping techniques, such as electrostatic clamping under vacuum conditions. The support structure may be, for example, a frame or a table, 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".

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 liquid 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".

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".

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.

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.

FIG. 1 schematically depicts a lithographic apparatus according to a particular implementation of the invention. The device includes:

an illumination system (illuminator) IL configured to condition a radiation beam (e.g. UV radiation or EUV radiation) PB;

a control system 100 for controlling the radiation beam PB;

a support structure (e.g. a support structure) MT to support a patterning device (e.g. a mask) MA and connected to a 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 a substrate (e.g. a resist-coated wafer) W and connected to a 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 B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

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

The illuminator IL receives a radiation beam from a radiation source 214 SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer source 214. 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 source may be an integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjusting component 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 can be adjusted. IN addition, the illuminator IL generally includes various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation PB, having a desired uniformity and intensity distribution in its cross-section.

The radiation beam PB is incident on the patterning device (e.g., mask) MA, which is held on the support structure MT. After traversing patterning device MA, beam PB passes through projection system PL, which focuses the beam onto a target portion C of substrate W. With the aid of the second positioner 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. Likewise, the first positioning device PM and another position sensor (which is not explicitly depicted in fig. 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 devices 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 M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus 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 at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. 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, while a pattern imparted to the beam PB is projected onto a target portion C, so as to hold the programmable patterning device, and the substrate table WT is moved or scanned. In this mode, a pulsed radiation source 214 is typically employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

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

The radiation source SO is controlled by the control system 100. For example, the control system 100 may determine a voltage value to be supplied to the radiation source SO. The control system 100 may adjust the voltage value provided to the control system 100 according to a feedback loop. An example implementation of the control system 100 is described in more detail below with reference to fig. 2 and 3.

Fig. 2 shows an example implementation of a control system 200 for controlling the radiation source 214. For example, the control system 200 may be used to control a radiation source 214 of an illumination system of a lithographic apparatus. For example, the control system 200 and the radiation source 214 may correspond to the control system 100 and the radiation source SO, respectively, of FIG. 1. The control system 200 may control various parameters of the radiation source 214, such as a dose of the radiation beam, a wavelength of the radiation beam, or another parameter associated with the radiation source 214 or the illumination system. The radiation source 214 may be a pulsed radiation source 214, i.e., the radiation source 214 may be configured to output a radiation beam in discrete intervals through an intermediate cycle that does not produce a radiation beam. Each individual beam of radiation may be referred to as a "pulse" of radiation. The radiation source 214 may be a laser.

The following description generally relates to the use of control system 200 for a lithography system. However, it is to be understood that the control system 200 need not be used in association with a lithography system and may be used to control any suitable radiation source for any of a variety of applications.

The control system 200 includes a sensor 202, a switch 204, and a plurality of controllers 206a, 206 b. Fig. 2 shows a first controller 206a and a second controller 206b (collectively referred to as controllers 206), and for ease of description with reference to fig. 2, the operation of the control system 200 will be described with reference to the two controllers 206a, 206 b. However, the skilled person will appreciate that any suitable number of controllers 206 may be provided. Each controller 206a, 206b includes similar or equivalent components that are identified in the figures by the same reference numerals. Components of the first controller 206a are denoted by the reference numeral followed by 'a' and components of the second controller 206b are denoted by the same reference numeral followed by 'b', and these components may also be collectively referred to by reference numerals without 'a' or 'b'.

Each controller 206 is arranged to control the radiation source 214 to provide radiation having different characteristics. For example, the radiation source 214 may be configured to output radiation at different wavelengths, each wavelength directed to a different depth (or height or "layer") within the resist. Each controller may be associated with a particular wavelength and configured to control the radiation source 214 when the radiation source 214 is outputting radiation of that wavelength.

In general, and as described in more detail below, within each channel of the control system 200 (e.g., each channel controls a single pulse of the radiation source 214), one of the controllers 206a, 206b receives a respective set point value 208a, 208b and a respective further sensor value 210a, 210b from the sensor 202. The controller 206 is configured to adjust (or "tune") the received respective set point values 208a, 208b based on the received respective further sensor values 210a, 210b to generate respective output values 212a, 212b (collectively referred to as output values 212) and to output the respective output values 212a, 212b to cause the radiation source 214 to operate in accordance with the respective output values 212a, 212 b. The output value 212a received from the first controller 206a is different from the output value 212b received from the second controller 206 b. The switch 204 is configured to switch between the controllers 206a, 206b such that the output values 212a, 212b are provided to the radiation source 214 in a cyclic manner.

In the example of fig. 2, the control system 200 controls the dose of the radiation beam. The dose is the energy per unit area that the photoresist is subjected to when exposed by the lithography system. For optical lithography, the dose is equal to the light intensity of the radiation beam multiplied by the exposure time. It is to be understood that when a continuous wave light source is used, the exposure time is the period of time during which radiation from the light source exposes the resist. However, in the case of a pulsed light source, the total exposure time may be the sum of a plurality of individual pulses of radiation used to expose the resist. In other words, in the case of a pulsed light source, it may be necessary to sum the dose of each pulse of a series of individual pulses to obtain the total dose. The set point value 208 indicates a desired dose. In case the radiation source 214 is a pulsed radiation source 214, the set point values 208 indicate the desired dose per single pulse. The desired dose per single pulse is set so as to obtain a desired dose distribution (in other words, a desired total dose) over the series of pulses. In some embodiments, the number of individual pulses used to administer the total dose may be in the range of 20 to 80 pulses. It is to be appreciated that the dose distribution may be 'flat' or 'non-flat', i.e. the set point value 208 may be the same for successive pulses or may be different for successive pulses, depending on requirements. In particular, a non-flat dose profile may be required, for example, to account for disturbances within the system and/or stray light reaching the wafer stage.

The switch 204 is operable to supply the set point values 208a, 208b to one of the controllers 206a, 206b of the control system 200 depending on the current cycle (e.g., depending on which wavelength of radiation the radiation source 214 is to output). The control system 200 may be maintained in a given cycle for multiple channels of the control system (e.g., multiple consecutive pulses of the laser 214). For example, in a first cycle, the first controller 206a may execute multiple pulses of the laser 214 before the switch switches to the second controller 206b to execute multiple pulses in the next cycle.

The values of the set point values 208a, 208b may depend on the pattern to be applied to the resist to be exposed, or on any other variable as will be appreciated by those skilled in the art.

A further sensor value 210a, 210b is indicative of the value of the parameter being controlled (in the case of the presently described example, the pulse energy of the radiation beam), which is measured by the sensor 202 by the previous pulse emitted by the radiation source 214. The first further sensor value 210a indicates a measurement value for a previous pulse controlled by the first controller 206a, and the second further sensor value 210b indicates a measurement value for a previous pulse controlled by the second controller 206 b. Typically, each controller 206 includes equivalent components.

Each controller 206a, 206b may include a respective feed-forward branch 216a, 216b and a respective tuning branch 218a, 218 b. The feed-forward branches 216a, 216b receive the respective setpoint values 208a, 208b and provide the setpoint values 208a, 208b to respective summing units 224a, 224 b. The tuning branches 218a, 218b receive the respective further sensor value 210a, 210b and further receive from the respective delay unit 219a, 219b the respective previous set-point value 208a ', 208 b' associated with the pulse to which the respective further sensor value 210a, 210b relates. The tuning branches 218a, 218b use the received further sensor value 210a, 210b and the previous set-point value 208a ', 208 b' to generate a respective tuning value 227a, 227b, which is accordingly tuned to be provided to a respective summing unit 224a, 224 b. The tuned values 227a, 227b and the set point values 208a, 208b are combined at respective summing units 224a, 224b to provide respective tuned set point values 229a, 229 b.

In the example of fig. 2, the tuning branches 218a, 218b of each controller 206a, 206b comprise a respective tuning unit 220a, 220b configured to modify the respective set-point value 208a, 208b based on the respective received further sensor value 210a, 210b according to a predetermined tuning setting. Each tuning unit 220a, 220b may include a PID controller (reference numerals 220, 220a, 220b are used herein for both the tuning unit and the PID controller). However, the skilled person will appreciate that other tuning methods may be used, such as model predictive control or predictive functional control, for example. Alternatively, iterative learning control may be used so that the control system may be self-tuned. Where tuning unit 220 is a PID controller, the PID controller may include any combination of proportional, integral, and/or derivative gain units, each of which may occur more than once. The proportional, integral and/or derivative gain units may have respective gains in the range from 0.1 to 10 (e.g. from 0.1 to 5, in particular from 0.5 to 5). The tuning elements 220 in each controller 206a, 206b may have the same configuration, or the tuning elements in different controllers 206 may have different configurations. For example, the PID controller tuning units may include the same or different gain units and may set the gains to the same or different values.

Each controller 206a, 206b is configured to output a respective output value 212a, 212 b. The output values 212a, 212b may be different from each other. The output values 212a, 212b provided by the respective controllers 206a, 206 may be used to cause the radiation source 214 to operate in accordance with the output values 212a, 212 b. When the switch 204 is switched between the controllers 206a, 206b, the output values 212a, 212b provided to the radiation source 214 change, thus causing the output of the radiation source 214 to change. For example, in the case of a substrate having a relatively very thick resist, it is beneficial to aim the resist to be exposed over its entire thickness. In particular, to expose the resist through its thickness, two (or more) focal planes may be selected, and the radiation source 214 may be controlled by the control system 200 to alternate between the selected focal planes. For example, the radiation source 214 may be configured to provide radiation at different wavelengths, each wavelength having a different plane of best focus. A single one of the controllers 206a, 206b may control the radiation source 214 for each focal plane, so that the properties of the pulses directed at the different focal planes may be controlled and adjusted individually.

Due to the operation of the switch 204, each controller 206a, 206b may apply an appropriate correction to each controller's respective output value 212a, 212b based on a further sensor value 210a, 210b from the previous pulse that the controller controlled. In this way, situations where one or each of the output values 212a, 212b deviates further and further from the set point values 208a, 208b may be avoided, since, for example, a correction that would have been applied to the output value 212a for controlling the dose at the first focal plane is erroneously applied to the other output value 212b for controlling the dose at the second focal plane.

In case of a delay from the respective delay unit 223a, 223b, the previous set-point values 208a ', 208 b' are supplied to a further summing unit 222a, 222b, which further summing unit 222a, 222b also receives a further sensor value 210a, 210 b. The further sensor value 210a, 210b is subtracted from the respective set point value 208a ', 208 b' to give a respective error value 225a, 225 b. The error values 225a, 225b are provided to respective PID controllers 220a, 220b, which perform a tuning function on the error values 225a, 225b and output tuning values 227a, 227b to respective second further summing units 224a, 224 b. The second further summing units 224a, 224b also receive the respective current set-point values 208a, 208b via the respective feed-forward branches 216a, 216 b. The tuning values 227a, 227b are then added to the respective set point values 208a, 208b by respective second further summing units 224a, 224b to produce respective tuned set point values 229a, 229 b. The skilled person will appreciate that the tuning values 227a, 227b may be positive or negative. Tuning enables the set point values 208a, 208b to be modified to account for any errors in the amount of pulse energy emitted in previous pulses (e.g., caused by noise received at the radiation source 214 or other interference sources) so that the total dose emitted by the radiation source 214 more closely corresponds to the desired total dose.

The tuned set point values 229a, 229b are supplied to converters 226a, 226b which convert the respective tuned set point values 229a, 229b to High Voltage (HV) signals 231a, 231b for output to the radiation source 214. In some implementations, HV signals 231a, 231b are first processed by respective limiters 228a, 228b before being passed to radiation source 214. The limiters 228a, 228b may perform rate limiting and/or clipping to ensure that the respective HV signals 231a, 213b are not set at a level that may damage the radiation source 214. In particular, the upper and lower limits of the HV signals 231a, 231b may be predetermined, and the limiters 228a, 228b may be configured to ensure that the HV signals 231a, 231b do not exceed the predetermined limits. For example, if the HV signals 231a, 231b exceed an upper limit, the limiter 228a, 228b may "clip" the HV signals 231a, 231b to output respective limited HV signals 231a, 231b below the upper limit. Additionally, the limiters 228a, 228b may monitor the rate of change of the HV signals 231a, 231b and operate to ensure that the rate of change of the HV signals 231a, 231b is controlled without causing undesirable stress to the radiation source 214. That is, if the rate of change of the HV signals 231a, 231b is above a predetermined threshold, the limiter 228a, 228b may cause the value of the respective HV signal 231a, 231b to be adjusted such that the rate of change of the HV signals 231a, 231b is reduced to within the predetermined threshold.

In some cases, the limiters 228a, 228b may cause the operation of the respective tuning units 220a, 220b (e.g., PID controllers) to reset. In some implementations, the reset may occur only under certain conditions. For example, where the limiters 228a, 228b have limited the HV signals 231a, 231b for a previous pulse or pulses, the limiters 228a, 228b may signal on respective reset lines 234a, 234b to reset the operation of the tuning units 220a, 220b to reduce the likelihood that the HV signals 231a, 231b need limiting for subsequent pulses. The limiter 228a of the controller 206a may communicate with the limiter 228b in the other controller 206b to notify the other controller 206b when such a "conditional reset" occurs. Of course, it will be appreciated that the controllers 206a, 206b may also communicate via other means in the event of a condition reset. For example, in some implementations, components of the controllers 206a, 206b other than the limiters 238a, 238b may provide communication between the controllers 206a, 206 b.

By way of additional example, in the presence of PID controller 220a, the following indication may be responded to: the HV signal 231a generated by the other controller 206b has been modified by the limiter 228b of the other controller 206b to reset the integral gain of the PID controller 220a of the controller 206 a. This may help to avoid problems associated with integral saturation, such as large variations in the correction value from pulse to pulse and unacceptable overshoot errors.

Once the output value 212a has been supplied to the radiation source 214, the switch 204 may switch from the first controller 206a to the second controller 206 b. The second controller 206b may operate in substantially the same manner as the first controller 206 a. The switch 204 is configured to switch between the controllers 206 such that each controller 206a, 206b operates in turn in a cyclic manner. In this way, the radiation source 214 may be controlled to alternate between different controlled parameter values. For example, the radiation source 214 may be controlled to alternate between different doses. If the radiation source 214 is a pulsed radiation source 214, the switch 204 may be configured to switch between the controllers 206a, 206b on a pulse-to-pulse basis. Alternatively, the switch 204 may be configured to switch between the controllers 206 at different frequencies. Depending on requirements, the switch 204 may be configured such that one of the controllers 206a, 206b provides more output values than the other of the controllers 206a, 206b for a given cycle. Alternatively, the switch 204 may be configured such that the controllers 206a, 206b each provide the same number of output values 212a, 212 b.

In some embodiments, the adjustment of the set point values 208a, 208b performed by one of the controllers 206a, 206b is completely independent of the adjustment of the set point values 208a, 208b performed by the other of the controllers 206a, 206 b. In other embodiments, each controller 206a, 206b is configured to receive a further value 232 and, in response to the received further value 232, adjust the setpoint value 208a, 208b based on an adjustment to the setpoint value 208a, 208b made by the other controller. For example, tuning settings (e.g., gain in the PID controller 220) may be adjusted based on information communicated from the other one of the controllers 206a, 206 b. For example, the limiters 228a, 228b may communicate with each other to indicate whether the HV signals 231a, 231b have been modified to remain within predetermined upper and lower limits of the HV signals 231a, 231 b. If the HV signals 231a, 231b generated by one of the controllers 206a, 206b has been modified by the limiter 228a, 228b, the tuning settings of the other one of the controllers 206a, 206b may be altered in response. By way of general example, the limiters 228a, 228b may adjust the control signals provided by one or more controllers 206a, 206b in the event that the adjustments made by each controller 206a, 206b are in opposite directions. For example, in the case where the first controller 206a has provided a negative control signal and the second controller 206b will provide a positive control signal next, one of the limiters 228a, 228b may limit the control signal provided by the second controller 206b so as not to overstress the radiation source.

From the foregoing, it will be appreciated that the example arrangement depicted in fig. 2 is merely exemplary, and that other arrangements are possible. For example, while the switch 204 is depicted as selecting which of the controllers 206a, 206b to send a set point value and a sensor value in each channel of the control system, it is to be understood that in other arrangements, each controller 206a, 206b may receive each set point value and sensor value in each channel of the control system, and the switch may select between the outputs of each controller 206a, 206 b.

In various implementations, the tuning units 220a, 220b may include various forms of tunable feedback control. For example, some installations may benefit from using a PID module, PI module, PIID module (with second stage integrator), PII module, PDD module, or other architecture depending on the characteristics of the system being regulated. For example, different approaches may have advantages depending on whether the laser source is a pulsed or continuous wave source, which may have different noise characteristics that need to be controlled. Some exemplary implementations of tuning units 320-1, 320-2, 320-3 (PID, PIID, PII, respectively, where I1 refers to a first integral and I2 refers to a second integral in the figures) are shown in fig. 3a to 3c, which show error value 225 as an input and tuning value 227 as an output. The skilled person will appreciate that these configurations are illustrated by way of example only, and that other configurations may also be possible.

Fig. 4 shows a flow chart of an embodiment of a method for controlling a parameter of a radiation source. The method may be performed by the control system described with reference to fig. 2 and 3. The control system includes a plurality of controllers. In step 402, a first controller receives a set point value. In step 404, the first controller receives a further sensor value from the sensor. In step 406, the controller adjusts the received set point value based on the received further sensor value to give an output value. The adjustment may also be based on previously received set point values (e.g., received set point values for previous pulses). The output value is output in step 408 such that the radiation source operates according to the output value. Then in step 410, the switch switches from the first controller to the second controller and the method is repeated.

Fig. 5 shows a flow chart of another embodiment of a method for controlling a parameter of a radiation source. The method is similar to the method described with reference to fig. 4 and may be performed by the control system described with reference to fig. 2 and 3. In step 502, a first controller receives a set point value. In step 504, the first controller receives a second sensor value from the sensor (e.g., the next reading from the sensor). Then, in step 506, the controller generates an adjustment value for the received setpoint value based on the received second sensor value. In step 508, a decision is made as to whether the adjustment value should be limited. If so, the method proceeds to step 510, where the adjustment value is limited. For example, as described above, where the controller includes a limiter, the limiter may limit the adjustment value to avoid detrimental stress on the laser. In step 512, the controller communicates the adjustment value that has been limited to other controller(s) within the control system. In step 514, the set point value is adjusted based on the adjustment value (which may or may not have been limited in the previous step). In step 516, the adjusted set point values are output to cause the radiation source to operate in accordance with the adjusted set point values. Then in step 518, the switch switches from the first controller to the second controller and the method is repeated. If it is determined in step 508 that no restriction is required, then processing transfers directly from step 508 to step 514.

The second controller may adjust the set point value based on the communicated adjustment to the set point value, which is sent from the first controller in step 512.

It will be apparent to the skilled person that the method steps described with reference to fig. 4 and 5 do not necessarily have to be performed in the order stated. For example, steps 402 and 404 may be performed in reverse order or simultaneously. This also applies to steps 502 and 504. Likewise, step 502 does not necessarily have to be performed before step 504. Likewise, step 512 need not be performed prior to step 514.

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); a magnetic disk storage medium; an optical storage medium; a flash memory device; 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 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.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. This description is not intended to limit the invention. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the terms set out below.

1. A control system for controlling a laser, the control system comprising:

a sensor for sensing a physical value indicative of a characteristic of a laser beam emitted by the laser;

a switch;

a first controller and a second controller, wherein each controller is configured to:

receiving a set point value;

receiving a further sensor value from the sensor;

adjusting the received set point value based on the received further sensor value to give an output value;

causing the laser to operate according to the output value;

wherein:

the output value from the first controller is different from the output value from the second controller;

the switch is configured to switch between the controllers such that the output value is provided from each controller in a cyclic manner;

the controllers are configured to communicate with each other; and is

The at least one adjustment to the set point value by the second controller is determined at least in part by the at least one adjustment to the set point value by the first controller.

2. The control system according to clause 1, wherein the laser is a pulsed laser and the switch is configured to switch between the controllers on a pulse-to-pulse basis.

3. The control system of any one of the preceding clauses wherein the laser is configured to provide more than one type of output.

4. The control system according to clause 3, wherein each controller is configured to control a respective type of output from the laser.

5. The control system according to any preceding clause, wherein each controller comprises a limiter configured to adjust the output value according to a predetermined limit.

6. The control system of clause 5, wherein the limiters of each controller are configured to communicate with each other.

7. The control system according to any preceding clause, wherein each controller comprises a tuning unit configured to adjust the set point value based on the received further sensor value to give the output value.

8. The control system according to any preceding clause, wherein the output value is configured to control an energy dose of the laser.

9. The control system of any of clauses 1 to 8, wherein the output value is configured to control the wavelength emitted by the laser.

10. A radiation source comprising a laser and a control system according to any of the preceding clauses, wherein the control system is configured to control the laser of the radiation source.

11. A lithographic system comprising a radiation source according to clause 10 and a lithographic apparatus, the lithographic apparatus comprising:

an illumination system for conditioning a radiation beam emitted by the radiation source;

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 the number of the first and second groups,

a projection system for projecting the patterned beam of radiation onto a target portion of the substrate.

12. A method for controlling a parameter of a laser using first and second controllers, the method comprising:

cycling between a first controller and a second controller, wherein each controller performs the steps of:

a) receiving a set point value for a parameter;

b) receiving a further sensor value from the sensor;

c) adjusting the received set point value based on the received further sensor value to give an output value;

d) causing the laser to operate according to the output value;

wherein the controllers are in communication with each other, and wherein the at least one adjustment to the set point value by the second controller is determined at least in part by the at least one adjustment to the set point value by the first controller.

13. The method according to clause 12, wherein the laser is a pulsed laser and the cycling between the controllers is on a pulse-to-pulse basis.

14. The method according to clause 12 or 13, wherein the laser provides more than one type of output.

15. The method of clause 14, wherein each controller further controls a respective type of output from the laser.

16. The method according to any of clauses 12 to 15, wherein each controller further adjusts the output value according to a predetermined limit.

17. The method according to any of clauses 12 to 16, wherein each controller comprises a PID controller, and the set point value is further adjusted to give an output value based on the received further sensor value.

18. The method according to any of clauses 12 to 17, wherein the output value is used to control the energy dose of the laser.

19. The method according to any of clauses 12 to 18, wherein the output value is used to control the wavelength emitted by the laser.

20. A computer program comprising computer readable instructions configured to cause a computer to perform a method according to any of clauses 12 to 19.

21. A computer readable medium carrying out a computer program according to clause 20.

22. A computer apparatus for a radiation source, the computer apparatus comprising:

a memory storing processor readable instructions; and

a processor arranged to read and execute instructions stored in the memory;

wherein the processor-readable instructions comprise: instructions arranged to control a computer to perform a method according to any of clauses 12 to 19.