Apparatus and method for controlling multi-focal imaging

Abstract

An apparatus and method for rapidly achieving target peak wavelength separation in a system that generates more than one wavelength (color) of laser radiation in which one or more actuators control the wavelength in response to a supplied waveform. A feature of the waveform is determined using a model reference control system.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application 63 / 395,368, filed on August 5, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to laser systems that generate light, such as excimer lasers, and systems and methods for controlling the central wavelength of such lasers. Background of the Invention

[0005] The photolithographic apparatus applies the desired pattern to a substrate such as a wafer of semiconductor material, typically to a target portion of the substrate. A patterning device, which may be a mask or reticle, may be used to generate the circuit pattern to be formed on a single layer of the wafer. Transfer of the pattern is typically accomplished by imaging a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain adjacent target portions that are patterned in sequence.

[0006] A lithographic apparatus comprises a so-called stepper, in which each target portion is irradiated by exposing the entire pattern onto the target portion at once, and a so-called scanner, in which each target portion is irradiated by scanning the pattern by a radiation beam in a given direction (the "scanning" direction) while simultaneously scanning the substrate parallel or antiparallel to that direction. The pattern can also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0007] The light source used to illuminate the pattern and project it onto the substrate can be any of a variety of configurations. Deep ultraviolet (DUV) excimer lasers commonly used in lithography systems include krypton fluoride (KrF) lasers with a wavelength of 248nm and argon fluoride (ArF) lasers with a wavelength of 193nm.

[0008] A lithographic apparatus can operate at a single wavelength, which can be referred to as a monochromatic mode. However, for certain applications, it is desirable to have the ability to change the wavelength, i.e., operate in a multicolor mode to control the depth of focus (DoF). For example, in the fabrication of 3D NAND memories, structures similar to NAND gates are stacked on top of each other, extending fabrication in a third dimension orthogonal to the xy plane of the 2D substrate. The transition from 2D to 3D NAND architectures requires significant changes in the fabrication process.

[0009] These considerations lead to the need for a larger DoF. Lithography DoF is given by the relationship DoF = ±m2λ / (NA) 2Determine, where λ is the wavelength of the illuminating light, NA is the numerical aperture, and m2 is an actual factor that depends on the resist process. Due to the larger DoF requirements in 3D NAND lithography, sometimes more than one exposure pass is performed on the wafer each time using different laser wavelengths.

[0010] Multifocus imaging (MFI) uses multiple focus levels (e.g., via multiple wavelengths) to effectively increase the DoF for a given NA of an objective lens. The technique can be specifically tuned to provide the amount of wavelength separation (peak separation) required for a specific DoF requirement. This enables the imaging NA and therefore the exposure latitude (process window) to be increased, while the DoF can be optimized by MFI based on production layer requirements.

[0011] Furthermore, the material that makes up the lenses that focus the laser radiation is dispersive, so different wavelengths focus at different depths. This is another reason why it is desirable to have the ability to change wavelength.

[0012] To achieve MFI, an element in an optical train moves back and forth between two angular positions, wherein (1) a source generates light having a first wavelength when the element is in one of the positions, and (2) the source generates light having a second wavelength when the element is in the other of the positions. The element moves under the control of a command voltage generated by an electro-actuated element (EAE), such as a piezoelectric transducer (PZT), a stepper motor, a valve, a pressure control device, an electromagnet, a solenoid, another type of piezoelectric device, a linear motor, a hydraulic actuator, a voice coil, and / or any other type of device capable of generating power under the command of a control signal.

[0013] In the single-color mode, two actuators, the stepper motor and the PZT, work in conjunction with each other to stabilize the central wavelength. In operation, the stepper motor has a limited resolution, and therefore, the PZT is used as the master actuator. However, in the two-color mode, the wavelength stability is based on the central or peak wavelength, the average of two alternating spectra, and in this mode, the PZT is tasked with producing a waveform that generates alternating wavelengths.

[0014] As a specific example, in an application where DUV light at two different wavelengths is generated, the reference wavelength has two set points during exposure, a first set point at the first wavelength and a second set point at the second wavelength. The reference wavelength will then be modulated between these two set points. A predetermined stabilization time is required for each wavelength target change.

[0015] DUV light sources include systems for controlling the wavelength of the DUV light. Typically, these wavelength control systems include a feed-forward compensator to facilitate wavelength stability. The feed-forward compensator compensates for command changes in the wavelength target, i.e., wavelength change events. When such events occur, a stabilization time must be allowed for the system to stably adapt to the new wavelength.

[0016] Typically, the MFI algorithm assumes that the laser will only operate in MFI mode at (or substantially close to) a specific repetition rate (e.g., 6kHz), and therefore the base waveform of the PZT dither is calibrated and optimized to achieve performance at that single operating point. This base waveform is then modified pulse by pulse using an Iterative Learning Control (ILC) algorithm to (reasonably) compensate for drift and operation outside of the expected operating point.

[0017] The desired peak separation performance is obtained by using an MFI algorithm, which depends on accurate knowledge of the PZT calibration results. However, there is uncertainty (non-linearity) in the performance of the PZT at the repetition rate of the PZT resonance frequency or a harmonic of this frequency. Therefore, the calibration gain of the PZT voltage is unreliable at these frequencies. The sequence of using accurate calibration results in the existing MFI control algorithm results in a slow transient response and large overshoot when the laser is fired at a repetition rate of about (2*PZT resonance) Hz. In fact, the peak separation can take about a hundred pulses to converge to the desired value, and the change in peak separation is used as a measure of system stability. This behavior essentially excludes the use of repetition rates at or near the PZT resonance and its harmonics.

[0018] It is against this background that the need for the subject matter disclosed herein arises. Summary of the invention

[0019] A simplified overview of one or more embodiments is presented below to provide a basic understanding of the embodiments. The content of the present invention is not a broad overview of all contemplated embodiments. It is not intended to identify any element of the embodiments as a key or important element, nor is it intended to describe the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a concise form as a prelude to a more detailed description presented later.

[0020] According to one aspect of the embodiment, a laser system is disclosed, which includes: a source of laser radiation, which is excited in one or more pulse trains, each pulse train consisting of multiple pulses; a wavelength controller, which is arranged to receive pulses and control the main wavelength of some of the pulses towards a first value, and control the main wavelength of other pulses of the pulses towards a second value that differs from the first value by a target main wavelength separation amount, the wavelength controller including at least one actuator, which operates in response to a control signal to act on the wavelength control of the pulses; and a model reference adaptive control system, which is suitable for generating a control signal based at least in part on the measured main wavelength separation amount so that the wavelength controller achieves the target main wavelength separation amount.

[0021] The source of laser radiation may be an excimer laser. The actuator may include a piezoelectric transducer. The wavelength controller may be a line narrowing module. The wavelength controller may be a line narrowing module comprising an optical element mechanically coupled to at least one actuator. At least one actuator may include a piezoelectric transducer.

[0022] According to another aspect of the embodiment, a multi-focus imaging lithography system is disclosed, which generates first wavelength pulses of deep ultraviolet radiation having a first main wavelength and second wavelength pulses of deep ultraviolet radiation having a second main wavelength that differs from the first main wavelength by a main separation amount. The multi-focus imaging lithography system includes: a wavelength controller, which is arranged to receive input pulses of deep ultraviolet radiation, and in response to a control signal, controls the main wavelength of a first subset of pulses to obtain first wavelength pulses, and controls the main wavelength of a second subset of input pulses to obtain second wavelength pulses; and a model reference adaptive control system, which is suitable for generating a control signal based at least in part on the measured wavelength main separation of the first wavelength pulse and the second wavelength pulse, so that the wavelength controller achieves and maintains a target main separation amount.

[0023] The source of laser radiation may be an excimer laser. The wavelength controller may include an electrically actuatable component. The wavelength controller may be a line narrowing module. The line narrowing module may include an electrically actuatable component. The electrically actuatable component may include a piezoelectric transducer.

[0024] According to another aspect of the embodiment, a system for controlling the wavelength of laser radiation emitted in one or more pulse trains is disclosed, each pulse train consisting of a plurality of pulses, the system comprising a wavelength controller arranged to receive the pulses and control the dominant wavelength of some pulses toward a first value, and to control the dominant wavelength of other pulses toward a second value that differs from the first value by a target dominant wavelength separation amount, the wavelength controller comprising at least one actuator that operates in response to a control signal to act on the wavelength control of the pulses; and a model reference adaptive control system suitable for generating a control signal based at least in part on a measured dominant separation amount so that the wavelength controller achieves the target dominant separation amount.

[0025] The actuator may comprise a piezoelectric transducer. The wavelength controller may be a line narrowing module. The wavelength controller may be a line narrowing module comprising an optical element mechanically coupled to the actuator. The actuator may comprise a piezoelectric transducer.

[0026] Each pulse train may include a plurality of pulses fired at a repetition rate, and the model reference adaptive control system may be adapted to generate a control signal based at least in part on the measured primary wavelength separation to enable the wavelength controller to achieve a target primary separation even when the repetition rate is within a critical range where operation of the electrically actuatable component would otherwise be unstable. The critical range may be + / - 10% of a resonant frequency of the electrically actuatable component or a harmonic of the resonant frequency of the electrically actuatable component. The electrically actuatable component may include a piezoelectric transducer.

[0027] According to another aspect of the embodiment, a method for controlling a multi-focus imaging lithography system to generate first wavelength radiation pulses having a first dominant wavelength and second wavelength radiation pulses having a second dominant wavelength having a dominant separation amount different from the first dominant wavelength is disclosed, the method comprising generating input pulses of laser radiation; using a wavelength controller to control the dominant wavelength of a first subset of the input pulses in response to a control signal to obtain first wavelength pulses, and controlling the dominant wavelength of a second subset of the input pulses to obtain second wavelength pulses; comparing the dominant wavelength separation of the first wavelength pulses and the second wavelength pulses with the dominant wavelength separation obtained from a reference model controlled by a reference signal to obtain an error signal; and modifying one or more parameters of the control signal based at least in part on the error signal so that the response of the wavelength controller to the control signal tracks the response of the reference model to the reference signal.

[0028] Generating the input pulses of laser radiation may be performed using an excimer laser.Using a wavelength controller may include using a line narrowing module.

[0029] Other features and exemplary aspects of the embodiments, as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. It should be noted that the scope of all possible embodiments is not limited to the specific embodiments described herein. Such specific embodiments are presented herein for illustrative purposes only. Based on the teachings contained herein, (one or more) persons skilled in the relevant art will understand other embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate embodiments and, together with the description, further serve to explain the principles of the embodiments and to enable one skilled in the relevant art(s) to make and use the embodiments.

[0031] Figure 1A , Figure 1B and Figure 1C A schematic diagram of a lithographic apparatus according to an embodiment is provided.

[0032] Figure 2A is a schematic diagram of a light source device according to an embodiment.

[0033] Figure 2Bis a schematic diagram of a spectral feature actuation system according to an embodiment.

[0034] Figure 2C is a schematic cross-sectional view of a line narrowing module according to an embodiment.

[0035] FIG. 3A to FIG. 3C is a schematic diagram of a laser pulse train consisting of laser pulses, illustrating some of the basic principles behind the operation of one aspect of the embodiments.

[0036] Figure 4 is a conceptual diagram of a lithography system according to an aspect of an embodiment.

[0037] Figure 5 is a graph of a unimodal spectrum of laser radiation according to an aspect of an embodiment.

[0038] Figure 6 is a graph of a bimodal spectrum of laser radiation according to an aspect of an embodiment.

[0039] Figure 7 An example of an average optical spectrum at a wafer is shown according to one aspect of the embodiment.

[0040] Figure 8 is a schematic diagram of a model reference adaptive control system for controlling a LNM of an MFI according to an aspect of an embodiment.

[0041] Fig. 9 is a flow chart of an operation process of a model reference adaptive control system for controlling a LNM of an MFI according to an aspect of an embodiment.

[0042] Fig.10 is a functional block diagram of a computer control system according to one aspect of the embodiment.

[0043] Features and exemplary aspects of the embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the accompanying drawings. In the accompanying drawings, the same reference numerals generally indicate the same, functionally similar, and / or structurally similar elements. Unless otherwise indicated, the accompanying drawings should not be interpreted as drawings to scale. DETAILED DESCRIPTION

[0044] The described embodiments and the description of "one embodiment", "an embodiment", "an exemplary embodiment", "an example embodiment", etc., indicate that the described embodiments may include certain features, structures, or characteristics, but not every embodiment must include the certain features, structures, or characteristics. In addition, such phrases do not necessarily refer to the same embodiment. In addition, when a specific feature, structure, or characteristic is described in conjunction with an embodiment, it should be understood that within the knowledge of those skilled in the art, such feature, structure, or characteristic may be implemented in other embodiments, whether or not explicitly described.

[0045] For ease of description, spatially relative terms may be used herein to describe the relationship of one element or feature to another element or feature shown in the figures. Spatially relative terms are intended to encompass different orientations of the device in use or operation, in addition to the orientation depicted in the figures. The device may be oriented in other ways (rotated 90 degrees or in other orientations), and the spatially relative descriptors used herein may be interpreted accordingly.

[0046] As used herein, the term "about" or "substantially" or "approximately" indicates a value of a given quantity that may vary based on a particular technology. Based on a particular technology, the term "about" or "substantially" or "approximately" may indicate a value of a given quantity that varies, for example, within a range of 1% to 15% of a value (e.g., ±1%, ±2%, ±5%, ±10%, or ±15% of a value).

[0047] Where the context permits, embodiments of the present disclosure may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present disclosure 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 a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), etc. In addition, firmware, software, routines, and / or instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and such actions are actually caused by a computing device, processor, controller, or other device that implements firmware, software, routines, instructions, etc.

[0048] However, before describing these embodiments in more detail, it is helpful to present an example environment in which embodiments of the present disclosure may be implemented.

[0049] refer to Figure 1A, the lithography system 100 includes an optical (or light) source 105 that provides a light beam 160 to a lithography exposure device 169, which processes a wafer 170 received by a wafer holder or stage 171. The light beam 160 is a pulsed light beam that includes pulses of light separated from each other in time. The lithography exposure device 169 includes a projection optical system 175 and a metrology system 172 through which the light beam 160 passes before reaching the wafer 170. The metrology system 172 may include, for example, a camera or other device capable of capturing an image of the wafer 170 and / or the light beam 160 at the wafer 170, or an optical detector capable of capturing data describing the characteristics of the light beam 160, such as the intensity of the light beam 160 at the wafer 170 in the xy plane. The lithography exposure device 169 may be a liquid immersion system or a dry system. The lithography system 100 may also include a control system 150 to control the light source 105 and / or the lithography exposure device 169.

[0050] For example, microelectronic features are formed on wafer 170 by exposing a layer of radiation-sensitive photoresist material on wafer 170 with beam 160. Also refer to Figure 1B , the projection optical system 175 includes a slit 176, a mask 174 and a projection objective, and the projection objective includes a lens 177. The light beam 160 enters the optical system 175 and impinges on the slit 176, and at least some of the light beams 160 pass through the slit 176. Figure 1A to Figure 1B In the example of , slit 176 is rectangular and shapes beam 160 into an elongated rectangular shaped beam. A pattern is formed on mask 174 and determines which portions of the shaped beam are transmitted by mask 174 and which portions are blocked by mask 174. The design of the pattern is determined by the specific microelectronic circuit design to be formed on wafer 170.

[0051] The shaped beam interacts with the mask 174. The portion of the shaped beam transmitted by the mask 174 passes through the projection lens 177 (and may be focused by the projection lens) and exposes the wafer 170. The portion of the shaped beam transmitted by the mask 174 forms an aerial image in the xy plane in the wafer 170. The aerial image is an intensity pattern formed by the light reaching the wafer 170 after interacting with the mask 174.

[0052] The system 100 is capable of forming multiple aerial images during a single exposure pass, wherein each of the aerial images is located at a spatially different location along the z-axis in the wafer 170. Figure 1C , which shows a cross-sectional view of a wafer 170 in the yz plane, the projection optical system 175 forms two aerial images 173a, 173b at different planes along the z-axis during a single exposure pass. As discussed in more detail below, each of the aerial images 173a, 173b is formed by light having a different dominant wavelength.

[0053] The position of the aerial image along the z-axis depends on the characteristics of the optical system 175 (including the projection lens 177 and the mask 174) and the wavelength of the light beam 160. The focal position of the lens 177 depends on the wavelength of the light incident on the lens 177. Therefore, changing or otherwise controlling the wavelength of the light beam 160 allows the position of the aerial image to be controlled. By providing pulses of light having different dominant wavelengths during a single exposure pass, multiple (two or more) aerial images (each at a different position along the z-axis) can be formed during a single exposure pass without moving the optical system 175 (or any component of the optical system 175) and the wafer 170 relative to each other along the z-axis.

[0054] exist Figure 1B 170. In the example of FIG. 170, light passing through mask 174 is focused to a focal plane by projection lens 177. The focal plane of projection lens 177 is between projection lens 177 and wafer stage 171, wherein the position of the focal plane along the z-axis depends on the properties of optical system 175 and the wavelength of light beam 160. Aerial images 173a, 173b are formed by light having different wavelengths, and thus aerial images 173a, 173b are located at different positions in wafer 170. Aerial images 173a, 173b are separated from each other by a separation distance 179 along the z-axis. Separation distance 179 depends on the difference between the wavelength of light forming aerial image 173a and the wavelength of light forming aerial image 173b.

[0055] Separation distance 179 is due to the ability to control the dominant wavelength in the pulses passing through mask 174 during an exposure pass. Additionally, during the same exposure pass, both aerial images 173a and 173b are present on wafer 170. In other words, system 100 does not need to form aerial image 173a during a first exposure pass and does not need to form aerial image 173b during a subsequent second exposure pass.

[0056] The light in the first aerial image 173a interacts with the wafer at a depth 178a, and the light in the second aerial image 173b interacts with the wafer at a depth 178b. These interactions can form electronic features or other physical properties, such as openings or holes, on the wafer 170. Because the aerial images 173a and 173b are formed at locations displaced along the z-axis, forming the aerial images 173b and 173a can be used as part of a process for making three-dimensional features on the wafer 170. For example, the aerial image 173a can be used to form a peripheral region, and the aerial image 173b can be used to form a channel, groove, or recess at a different location along the z-axis. Therefore, the techniques discussed herein can be used to form three-dimensional semiconductor components, such as three-dimensional NAND flash memory components.

[0057] Before discussing additional details related to forming multiple aerial images during a single exposure pass, FIG. 2A to FIG. 2C , FIG. 3A to FIG. 3C and Figure 4 Example implementations of light source 105 and lithography system 100 are described.

[0058] refer to Figure 2A , a block diagram of a lithography system 200 is shown. The system 200 is a block diagram of the system 100 ( Figure 1A ). For example, in the lithography system 200, the light source 205 is used as the light source 105 ( Figure 1A ). Light source 205 generates a pulsed light beam 260, which is provided to lithography exposure device 169. Light source 205 can be, for example, an excimer light source that outputs a pulsed light beam 260 (which can be a laser beam). When pulsed light beam 260 enters lithography exposure device 169, it is guided through projection optical system 175 and projected onto wafer 170. In this way, one or more microelectronic features are patterned onto the photoresist on wafer 170, and then the one or more microelectronic features are developed and cleaned before subsequent process steps, and the process is repeated. Lithography system 200 also includes a control system 250, which is used in Figure 2A In the example of FIG. 250 , the control system is connected to components of the light source 205 and the photolithography exposure device 169 to control various operations of the system 200. The control system 250 is Figure 1A An example of an implementation of the control system 150 is provided.

[0059] exist Figure 2A , the light source 205 is a two-stage laser system including a master oscillator (MO) 212 that provides a seed beam 224 to a power amplifier (PA) 230. The MO 212 and the PA 230 can be considered as a subsystem of the light source 205 or a system that is part of the light source 205. The power amplifier 230 receives the seed beam 224 from the master oscillator 212 and amplifies the seed beam 224 to generate a beam 260 for use in the lithography exposure apparatus 169. For example, the master oscillator 212 can emit a pulsed seed beam with a seed pulse energy of about 1 millijoule (mJ) per pulse, and these seed pulses can be amplified by the power amplifier 230 to about 10 mJ to 15 mJ.

[0060] The master oscillator 212 includes a discharge chamber 240 having two elongated electrodes 217, a gain medium 219 which is a gas mixture, and a fan for circulating the gas between the electrodes 217. A resonator is formed between a line narrowing module (LNM) 216 on one side of the discharge chamber 240 and an output coupler 218 on a second side of the discharge chamber 240. The LNM 216 may include a diffractive optical device (such as a grating) that finely tunes the spectral output of the discharge chamber 240. Figure 2B and Figure 2C Additional details regarding LNM 216 are provided.

[0061] Figure 2B 2 is a block diagram of an example implementation of the spectral feature selection module 258. The spectral feature selection module 258 is coupled to the light propagating in the light source 205. In some implementations (e.g., Figure 2B As shown), the spectral feature selection module 258 receives light from the chamber 214 of the master oscillator 212 to achieve fine tuning of the spectral features (such as wavelength and bandwidth) within the master oscillator 212.

[0062] The spectral signature selection module 258 may include a control module, such as a spectral signature control module 254, which includes an electronic device in the form of any combination of firmware and software. The control module 254 is connected to one or more actuation systems, such as spectral signature actuation systems 255_1 to 255_n. Each of the actuation systems 255_1 to 255_n may include one or more actuators connected to corresponding optical features 256_1 to 256_n of the optical system 257. The optical features 256_1 to 256_n are configured to adjust specific characteristics of the generated light beam 260, thereby adjusting the spectral characteristics of the light beam 260. The control module 254 receives a control signal from the control system 250, which includes a specific command to operate or control one or more of the actuation systems 255_1 to 255_n. The actuation systems 255_1 to 255_n can be selected and designed to work together, ie, in tandem, or the actuation systems 255_1 to 255_n can be configured to work individually. In addition, each actuation system 255_1 to 255_n can be optimized to respond to a particular class of disturbances.

[0063] Each actuator of the actuation system 255_1 to 255_n may be an EAE for moving or controlling a corresponding optical feature 256_1 to 256_n of the optical system 257. The actuator receives energy from the control module 254 and converts the energy into some movement applied to the optical features 256_1 to 256_n of the optical system 257.

[0064] Each optical feature 256_1 to 256_n is optically coupled to a light beam 260 generated by the light source 105. The optical system 257 may be implemented as a LNM 216c, such as Figure 2CAs shown in . The line narrowing module includes a dispersive optical element (such as a reflective grating 291) and a refractive optical element (such as prisms 292, 293, 294 and 295) as optical features 256_1 to 256_n. One or more of the prisms 292, 293, 294 and 295 can be rotatable. An example of such a line narrowing module can be found in U.S. Patent No. 8,144,739 (the '739 patent), entitled "System Method and Apparatus for Selecting and Controlling Light Source Bandwidth", issued on March 27, 2012. In the '739 patent, a line narrowing module is described that includes a beam expander (including one or more prisms 292, 293, 294 and 295) and a dispersive element such as a grating 291.

[0065] All patent applications, patents, and printed publications cited herein are incorporated by reference in their entirety, except for any definitions, subject matter disclaimers, or disclaimers, unless the incorporated material is inconsistent with the explicit disclosure herein, in which case the language of the disclosure controls.

[0066] The corresponding actuation systems for the optical features, such as one or more of the prisms 292, 293, 294 and 295, are respectively provided by EAE 292a, 293a, 294a and 295a. Figure 2C 291 and 292. A reflector may also be present and rotated to change the angle of incidence of the light beam on the grating 291 and, therefore, change the dominant wavelength of the emitted light. The common element is the presence of an EAE that is caused to move under the command of a voltage command signal. Thus, typically, a line narrowing module includes one or more optical elements that rotate to change the dominant wavelength of the light leaving the module. These EAEs must be able to move the optical element very quickly between two positions, typically two angular positions, a process known as dithering.

[0067] Back to Figure 2A , the master oscillator 212 also includes a line center analysis module 220 that receives the output beam from the output coupler 218 and a beam coupling optical system 222 that modifies the size or shape of the output beam as needed to form a seed beam 224. The line center analysis module 220 is a measurement system that can be used to measure or monitor the wavelength of the seed beam 224. The line center analysis module 220 can be placed at other locations in the light source 205, or can be placed at the output end of the light source 205.

[0068] The power amplifier 230 includes a beam coupling optical system 232 that receives a seed beam 224 from the master oscillator 212 and directs the seed beam 224 through a discharge chamber 240 and to a beam steering optical element 248. The beam steering optical element 248 modifies or changes the direction of the seed beam 224 so that the seed beam is sent back to the discharge chamber 240. The discharge chamber 240 includes a pair of elongated electrodes 241, a gain medium that is a gas mixture, and a fan for circulating the gas mixture between the electrodes 241.

[0069] The output beam 260 is directed through a bandwidth analysis module 262, where various parameters of the beam 260 (such as bandwidth or wavelength) can be measured. The output beam 260 can also be directed through a beam preparation system 263. The beam preparation system 263 can include, for example, a pulse stretcher, where each of the pulses of the output beam 260 is stretched in time, such as in an optical delay unit, to adjust the performance properties of the beam impinging on the lithographic exposure device 169. The beam preparation system 263 can also include other components capable of acting on the beam 260, such as, for example, reflective and / or refractive optical elements (such as, for example, lenses and mirrors), filters, and optical apertures (including automatic shutters).

[0070] The photolithography system 200 also includes a control system 250. Figure 2A In the illustrated implementation, the control system 250 is connected to various components of the light source 205. For example, the control system 250 can control when the light source 205 emits a light pulse or a light pulse train including one or more light pulses by sending one or more trigger signals to the light source 205. The control system 250 is also connected to the photolithography exposure device 169. Therefore, the control system 250 can also control various aspects of the photolithography exposure device 169. For example, the control system 250 can control the exposure of the wafer 170, and thus can be used to control how electronic features are printed on the wafer 170. In some implementations, the control system 250 can control the position of the slit 176 in the xy plane ( Figure 1B ) to control the scanning of the wafer 170. In addition, the control system 250 can exchange data with the metrology system 172 and / or the optical system 175.

[0071] The photolithography exposure apparatus 169 may also include, for example, temperature control equipment (such as air conditioning equipment and / or heating equipment) and / or power supplies for various electrical components. The control system 250 may also control these components. In some implementations, the control system 250 is implemented to include multiple sub-control systems, wherein at least one sub-control system (photolithography controller) is dedicated to controlling various aspects of the photolithography exposure apparatus 169. In these implementations, the control system 250 may be used to control various aspects of the photolithography exposure apparatus 169 instead of using a photolithography controller, or as a supplement to using a photolithography controller.

[0072] Control system 250 includes electronic processor 251, electronic storage device 252 and I / O interface 253. Electronic processor 251 includes one or more processors suitable for executing computer programs, such as general or special purpose microprocessors, and any one or more processors of any type of digital computer. Typically, the electronic processor receives instructions and data from read-only memory, random access memory, or both. Electronic processor 251 can be any type of electronic processor.

[0073] Electronic storage 252 may be a volatile memory or a non-volatile memory such as RAM. In some implementations, electronic storage 252 includes non-volatile and volatile portions or components. Electronic storage 252 may store data and information used in the operation of control system 250, components of control system 250, and / or systems controlled by control system 250. The information may be stored, for example, in a lookup table or database. For example, electronic storage 252 may store data indicating the values ​​of various properties of light beam 260 under different operating conditions and performance scenarios.

[0074] In addition, electronic storage device 252 may store various recipes or processing programs 259 that specify parameters of light beam 260 during use. For example, electronic storage device 252 may store a recipe that indicates the wavelength of each pulse in light beam 260 for a particular exposure pass. The recipe may indicate different wavelengths for different exposure passes. The wavelength control techniques discussed below may be applied on a pulse-by-pulse basis. In other words, the wavelength content of each individual pulse during an exposure pass may be controlled to facilitate forming an aerial image at a desired location along the z-axis.

[0075] Electronic storage device 252 may also store instructions, perhaps as a computer program, that when executed cause processor 251 to communicate with components in control system 250 , optical system 205 , and / or lithographic exposure apparatus 169 .

[0076] The I / O interface 253 is any type of electronic interface that allows the control system 250 to receive and / or provide data and signals to an operator, the optical system 205, the lithographic exposure apparatus 169, any component or system within the optical system 205 and / or the lithographic exposure apparatus 169, and / or an automated process running on another electronic device. For example, the I / O interface 253 may include one or more of a visual display, a keyboard, and a communication interface.

[0077] Light beam 260 (and light beam 160) is a pulsed light beam and may include one or more pulse trains separated from each other in time. Each pulse train may include one or more light pulses. In some implementations, the pulse train includes hundreds of pulses, such as 100 to 400 pulses. FIG. 3A to FIG. 3C An overview of the generation of pulses and pulse trains in the light source 205 is provided. Figure 3A shows the amplitude of the wafer exposure signal 300 as a function of time, Figure 3B shows the amplitude of the gate signal 315 as a function of time, and Figure 3C The amplitude of the trigger signal 330 is shown as a function of time.

[0078] The control system 250 may be configured to send a wafer exposure signal 300 to the light source 205 to control the light source 205 to generate the light beam 260. Figure 3A In the example shown, the wafer exposure signal 300 has a high value 305 (eg, logic 1) during a time period 307 when the light source 205 generates the light pulse train. Otherwise, when the wafer 170 is not exposed, the wafer exposure signal 300 has a low value 310 (eg, logic 0).

[0079] refer to Figure 3B , the control system 250 also controls the duration and frequency of the pulse train by sending a gate signal 315 to the light source 205. The gate signal 315 has a high value 320 (e.g., a logical 1) during the pulse train and a low value 325 (e.g., a logical 0) during the time between consecutive pulse trains. In the example shown, the duration that the gate signal 315 has a high value is also the duration of the pulse train 316. The pulse trains are separated in time by an inter-pulse train time interval. During the inter-pulse train time interval, the lithography exposure device 169 can position the next die on the wafer 170 for exposure.

[0080] refer to Figure 3C, the control system 250 also controls the repetition rate of the pulses within each pulse train using the trigger signal 330. The trigger signal 330 includes a trigger 340, which is provided to the light source 205 to cause the light source 205 to generate light pulses. Each time a pulse is to be generated, the control system 250 can send the trigger 340 to the source 205. Therefore, the repetition rate of the pulses (the time between two consecutive pulses) or other timing of the pulses generated by the light source 205 can be set by the trigger signal 330.

[0081] As described above, when the gain medium 219 is pumped by applying a voltage to the electrode 217, the gain medium 219 emits light. When the voltage is applied to the electrode 217 in a pulsed form, the light emitted from the medium 219 is also pulsed. Therefore, the repetition rate of the pulsed light beam 260 is determined by the rate at which the voltage is applied to the electrode 217, and each application of the voltage produces a light pulse. The light pulse propagates through the gain medium 219 and exits the chamber 214 through the output coupler 218. Therefore, a series of pulses are generated by periodically and repeatedly applying a voltage to the electrode 217. For example, the trigger signal 330 can be used to control the voltage to the electrode 217 and the repetition rate of the pulses, and for most applications, the repetition rate can range between about 500 Hz and 6,000 Hz. In some implementations, the repetition rate can be greater than 6,000 Hz, and can be, for example, 12,000 Hz or higher.

[0082] Signals from the control system 250 may also be used to control the electrodes 217, 241 within the master oscillator 212 and the power amplifier 230, respectively, to control the respective pulse energies of the master oscillator 212 and the power amplifier 230, and thus control the energy of the light beam 260. There may be a delay between the signal provided to the electrode 217 and the signal provided to the electrode 241. The amount of delay may affect the properties of the light beam 260, such as the amount of coherence in the pulsed light beam 260.

[0083] Pulsed beam 260 may have an average output power in the range of tens of watts, for example, from about 50 W to about 130 W. The irradiance (i.e., average power per unit area) of beam 260 at the output end may be about 60 W / cm 2 Up to 80W / cm 2 within the range.

[0084] Also refer to Figure 4 , the wafer 170 is irradiated by the light beam 260. The photolithography exposure device 169 includes an optical system 175 ( Figure 1A and Figure 1B ).exist Figure 4 In the example of FIG. 1 , the optical system 175 ( Figure 4Other parts (not shown) include an illuminator system 429, which includes an objective lens arrangement 432. The objective lens arrangement 432 includes a projection lens 177 ( Figure 1B ) and transfers the image from the mask 174 to the photoresist on the wafer 170. The illuminator system 429 adjusts the angular range of the beam 260 impinging on the mask 174. The illuminator system 429 can also homogenize (make uniform) the intensity distribution of the beam 260 in the xy plane passing through the mask 174.

[0085] In some implementations, an immersion medium may be supplied to cover the wafer 170. The immersion medium may be a liquid (such as water) for liquid immersion lithography. In other implementations where the lithography is a dry system, the immersion medium may be a gas, such as dry nitrogen, dry air, or clean air. In other implementations, the wafer 170 may be exposed to a pressure controlled environment (such as a vacuum or partial vacuum).

[0086] A plurality of N pulses of the beam 260 illuminate the same area of ​​the wafer 170. N may be any integer greater than 1. The number N of pulses of the beam 110 that illuminate the same area may be referred to as an exposure window or exposure pass process 400. The size of the window 400 may be controlled by the slit 176. For example, the slit 176 may include a plurality of blades that are movable such that the blades form an aperture that is open in one configuration and closed in another configuration. The size of the window 400 may also be controlled by arranging the blades of the slit 176 to form an aperture of a particular size.

[0087] The N pulses also determine the exposure dose of the exposure pass. The exposure dose is the amount of optical energy delivered to the wafer during the exposure pass. Therefore, the properties of the N pulses, such as the optical energy in each pulse, determine the exposure dose. In addition, as discussed in more detail below, the N pulses can also be used to determine the amount of light ( Figure 1C ). In particular, the recipe may specify that among the N pulses, a certain number of pulses have a first dominant wavelength forming the aerial image 173a, and a certain number of pulses have a second dominant wavelength forming the aerial image 173b. These pulses having wavelengths different from each other may be interspersed, for example, pulse to pulse or otherwise (i.e., alternating groups of pulses).

[0088] In addition, the slit 176 and / or mask 174 can be moved in the xy plane in the scanning direction so that only a portion of the wafer 170 is exposed at a given time or during a particular exposure scan (or exposure pass). The size of the area on the wafer 170 exposed by the beam 160 is determined by the distance of the blades in the non-scanning direction and the length (distance) of the scan in the scanning direction. In some implementations, the value of N is in units of ten, for example, each point on the wafer can receive light from 10 to 100 consecutive pulses during the scanning of the slit relative to that point. In other implementations, the value of N is greater than 100 pulses, for example, 100 to 500 pulses. The exposure field 479 of the wafer 170 is the physical area of ​​the wafer 170 that is exposed in one scan of the exposure slit or window within the lithographic exposure device 169.

[0089] The wafer stage 171, the mask 174 and the objective lens arrangement 432 are fixed to an associated actuation system, thereby forming a scanning arrangement. In the scanning arrangement, one or more of the mask 174, the objective lens arrangement 432 and the wafer 170 (via the stage 171) can move relative to each other in the xy plane. However, except for occasional relative operational motion between the wafer stage 171, the mask 174 and the objective lens arrangement 432, these elements do not move relative to each other along the z-axis during the exposure pass process.

[0090] Reference again Figure 2A Typically, tuning of the wavelength of beam 224, and therefore beam 260, occurs in LNM 216. A typical technique for line narrowing and tuning of a laser is to provide a window at the rear of the discharge chamber of the laser through which a portion of the laser beam enters LNM 216. There, a portion of the beam is expanded by a prismatic beam expander and directed to a grating that reflects a narrow, selected portion of the broader spectrum of the laser light back into the discharge chamber where it is amplified, as shown in FIG. Figure 2C The laser is typically tuned by changing the angle at which the beam strikes the grating 291 using an actuator, such as, for example, a piezoelectric actuator.

[0091] In some implementations, multiple prisms 292 to 295 can be used to adjust the final angle of incidence, and therefore adjust the selected wavelength. For example, prism 292 can have more control over the final angle of incidence than prism 293. That is, in some implementations, the controller 250 uses prisms 292, 293 in a dual-stage configuration, where prism 292 is used for large jumps and prism 293 is desaturated, which is used to make more subtle changes to the final angle of incidence. Controlling prisms 292, 293 is particularly important for MFI operations, which require more than just adjusting around a set point, but rather, in addition to accurately controlling the center point of the sinusoid (i.e., the center wavelength), it is also necessary to accurately track the sinusoid at the Nyquist frequency. There is a process for controlling the center wavelength of imaging operations (such as MFI operations).

[0092] The multi-focus imaging operation may include a dual-color mode. In dual-color mode, the wavelength target may alternate between two known set points within a pulse train (e.g., each pulse, pulse to pulse), and an electrically actuated component, which may be implemented as a piezoelectric transducer (PZT), may be used to track, i.e., adjust the wavelength toward a rapidly changing wavelength target. As described above, for some applications, it is beneficial to be able to generate one or more pulses having one wavelength, and then be able to switch to generating one or more pulses having a different wavelength.

[0093] In some implementations, the MFI operation provides an actuator that moves the motion of the control prism 293 during a pulse train. That is, these processes provide an intra-pulse train solution for addressing changes in the center wavelength. A dynamic model of the actuator can be used to calculate an optimal control waveform for actuating the actuator to minimize the difference between the actual wavelength and the wavelength target.

[0094] In some embodiments, a jitter waveform (or sequence) may be combined with an offset to move the actuator of prism 293. For example, the jitter waveform may be in the form of an application of noise for random quantization. The offset may be updated at the end of a burst (EOB) and / or at set pulse intervals. In some embodiments, the EOB update may move the actuator of prism 293 to zero an estimated center wavelength drift obtained by averaging the wavelength measurements for the entire burst. In some embodiments, the interval update may be based on an estimation process.

[0095] The optimal control waveform can be calculated using any of several methods. For example, dynamic programming can be used to calculate the optimal control waveform. This method is very suitable for processing complex models containing nonlinear dynamics. If an actuator model with strong nonlinear dynamics is adopted, dynamic programming can be used to generate the optimal control signal for a given wavelength target. However, dynamic programming does bring challenges, it requires a lot of computing resources, and these computing resources may not be implemented in real time. To overcome this, a data storage device such as a pre-filled lookup table or a pre-programmed field programmable gate array (FPGA) can be used, which contains the optimal control parameters for at least some of the different repetition rates at which the source can operate.

[0096] As another example, model inverse feedforward control can be used to determine the optimal control waveform. This method relies on a model of the actuator dynamics to build a digital filter that inverts the actuator dynamics. By passing the desired waveform of the desired actuator trajectory through this filter, the optimal control waveform can be generated in real time to achieve zero steady-state error tracking.

[0097] As another example, obtaining an optimal solution for two independent wavelengths in a stable manner is achieved using a learning algorithm to ensure error convergence over several iterations of learning. Embodiments of the systems and methods disclosed herein can potentially achieve two separated wavelengths that are 1000 femtoseconds (fm) apart with a separation error below 20 fm.

[0098] refer to Figure 5 , shows a spectrum 601A of a light pulse 600A. The light pulse 600A has a non-zero intensity within a wavelength band. The wavelength band may also be referred to as the bandwidth of the pulse 600A.

[0099] like Figure 5 The information shown is the instantaneous spectrum 601A (or emission spectrum) of pulse 600A. Spectrum 601A contains information about how the optical energy or power of the pulses of light beam 260 is distributed at different wavelengths (or frequencies). Spectrum 601A is depicted in the form of a graph in which the spectral intensity (not necessarily with absolute calibration) is plotted as a function of wavelength or optical frequency. Spectrum 601A may be referred to as the spectral shape or intensity spectrum of the pulses of light beam 260. Pulse 600A has a dominant wavelength 602A at Figure 5In the example of , the dominant wavelength is the peak intensity. Although the discussion of the pulses of beam 260 and the aerial image formed by the pulses of beam 260 refers to the dominant wavelength of the pulses, pulses include wavelengths other than the dominant wavelength and pulses have a finite bandwidth that can be characterized by a metric. For example, the full width of spectrum 601A at some fraction (X) of the maximum peak intensity of the spectral shape (referred to as FWXM) can be used to characterize the beam bandwidth. As another example, the spectral width containing some fraction (Y) of the integrated spectral intensity (referred to as EY) can be used to characterize the beam bandwidth.

[0100] Pulse 600A is shown as an example of a pulse that may be in beam 260 .

[0101] When pulse 600A is used to expose a portion of wafer 170, the light in the pulse forms an aerial image. The position of the aerial image in the z direction ( Figures 1A to 1C ) is determined by the value of dominant wavelength 602A. Various pulses in beam 260 may have different dominant wavelengths. For example, to generate two aerial images during a single exposure pass, some of the pulses of beam 260 have one dominant wavelength (a first dominant wavelength) while other pulses of beam 260 have another dominant wavelength (a second dominant wavelength). The first dominant wavelength and the second dominant wavelength are different wavelengths. The wavelength difference between the first dominant wavelength and the second dominant wavelength may be referred to as spectral separation. Although the wavelengths of various pulses in beam 260 may be different, the spectral shapes of the pulses may be the same.

[0102] The light source 205 can dither or switch the dominant wavelength between the first dominant wavelength and the second dominant wavelength on a pulse train to pulse train, pulse to pulse, or even intra-pulse basis. For the pulse to pulse case, each pulse has a different dominant wavelength than the pulse immediately preceding or following it in time. In these implementations, assuming that all pulses in the light beam 260 have the same intensity, distributing the first dominant wavelength and the second dominant wavelength in this manner results in two aerial images with the same intensity at different locations in the z direction.

[0103] In some implementations, a portion (e.g., 33%) of the pulses have a first dominant wavelength and the remainder (67% in this example) have a second dominant wavelength. Here and elsewhere, "first" and "second" are used merely as distinguishing labels, not as a temporal order, unless the context dictates otherwise. In these implementations, assuming that all pulses in the beam 260 have the same intensity, two aerial images with different intensities are formed. The intensity of the aerial image formed by the pulses having the first dominant wavelength is approximately half the intensity of the aerial image formed by the signal having the second dominant wavelength. In this way, the dose provided to a specific location in the wafer 170 along the z-axis can be controlled by controlling the portion of the N pulses having the first dominant wavelength and the portion of the N pulses having the second dominant wavelength.

[0104] For an exposure pass, the portion of the pulse having a particular dominant wavelength may be specified in a recipe file 259 stored in electronic storage device 252 (see Figure 2A ). The recipe file 259 specifies the ratios of various dominant wavelengths for an exposure pass. The recipe file 259 may also specify ratios for other exposure passes so that different ratios may be used for other exposure passes and the aerial image may be adjusted or controlled on a field-by-field basis.

[0105] refer to Figure 6 , shows spectrum 601B of pulse 600B. Pulse 600B is another example of a pulse of light beam 260. Spectrum 601B of pulse 600B has a different shape from spectrum 601A. In particular, spectrum 601B has two peaks corresponding to two main wavelengths 602B_1 and 602B_2 of pulse 600B. Pulse 600B is a portion of light beam 260. When pulse 600B is used to expose a portion of wafer 120, the light in the pulse forms two aerial images at different positions along the z-axis on the wafer. The position of the aerial image is determined by the wavelengths of main wavelengths 602B_1 and 602B_2. Therefore, one objective control of the control system according to the embodiment is to direct the main wavelengths toward the corresponding target values, that is, to converge each main wavelength to its target value, and thus to make the separation amount reach the target amount.

[0106] Figure 5 and Figure 6 The pulses shown can be formed by any hardware capable of forming such pulses. For example, a pulse similar to Figure 2CThe line narrowing module of LNM 216C is used to form a pulse train of pulses, such as pulse 600A. As described above, the wavelength of light diffracted by grating 291 depends on the angle of the light incident on the grating. A mechanism for changing the angle of incidence of light interacting with grating 291 can be used with such a line narrowing module to create a pulse train having N pulses for an exposure pass process, wherein the dominant wavelength of at least one of the N pulses is different from the dominant wavelength of another of the N pulses. For example, one of prisms 292, 293, 294, and 295 can be rotated to change the angle of light incident on grating 291 on a pulse-by-pulse basis. In some implementations, the line narrowing module includes a mirror located in the path of beam 260 that is movable to change the angle of light incident on grating 291. For example, an example of such an implementation is discussed in U.S. Patent No. 6,192,064, issued on February 20, 2001, entitled "NarrowBand Laser with Fine Wavelength Control."

[0107] Reference again Figure 4 , during a single exposure pass, a group of light pulses passes through the mask 174 toward the wafer 170. As described above, during an exposure pass, N light pulses may be provided to the wafer 170. The N light pulses may be consecutive light pulses in the beam 260. The exposed portion of the wafer 170 sees an average of the spectrum of each of the N pulses during the exposure pass. Thus, if a portion of the N pulses have a first dominant wavelength and the remainder of the N pulses have a second dominant wavelength, the average spectrum at the wafer 170 will be a spectrum that includes a peak at the first dominant wavelength and a peak at the second dominant wavelength. Similarly, if all or some of the individual pulses in the N pulses have multiple dominant wavelengths, these dominant wavelengths may form peaks in the average spectrum. Figure 7 An example of an average spectrum 701 at the wafer 170 is shown. The average spectrum 701 includes a first dominant wavelength 702_1 and a second dominant wavelength 702_2. Figure 7 In the example of FIG. 7 , a first dominant wavelength 702_1 and a second dominant wavelength 702_2 are separated by spectral separation 703. Spectral separation 703 makes the first dominant wavelength 702_1 and the second dominant wavelength 702_2 different, and the average spectrum 701 includes a spectral region with little or no intensity between the wavelengths 702_1 and 702_2.

[0108] As mentioned above, the technical challenge of trying to base the control signal on feedforward control and standard mathematical models is that the behavior of the PZT in the LNM lacks predictability at and near the PZT resonant frequency and harmonics (integer multiples) of that resonant frequency. The PZT resonant frequency may be, for example, about 2100 Hz. This means that the PZT behavior is unpredictable at or near that repetition rate, and at or near a repetition rate of 4200 Hz, and so on. The practical effect of this is that the user is restricted from avoiding these repetition rates n*f r The frequency at or near ±Δf, where n is a positive integer, f r is the resonant frequency of the PZT, and ±Δf is the range of repetition rates around the resonance or harmonics where the PZT behavior is unpredictable, typically within 10% of the resonant frequency of the PZT or a harmonic of the resonant frequency of the PZT. Otherwise, the peak separation may not stabilize until late in the pulse train. In this article, the term "critical range" refers to repetition rates within 10% of the resonant frequency of the PZT or a harmonic of the resonant frequency of the PZT.

[0109] According to one aspect of the embodiment, model reference adaptive control (MRAC) is used to resolve the uncertainty in the PZT parameters to achieve a constant value even during laser excitation, e.g. at 2*f r Around Hz and including 2*f r The desired peak separation can be achieved quickly even in the range of Hz. The unknown performance of the PZT near resonance is treated as a parameter uncertainty in the PZT reference model. The control system does not rely on accurate PZT calibration results because it is able to adjust the control parameters in response to the resonance uncertainty. Such a control system also allows real-time feedback control that can better manage any external disturbances. The use of such a control system makes it possible for the peak separation to achieve its desired value early in the pulse train, for example by the third pulse. In effect, this removes the limitation of operating at a repetition rate related to the PZT resonant frequency. In particular, this is achieved at 2*f r The FETs provide sufficiently reliable performance to allow operation at this repetition rate in MFI mode.

[0110] like Figure 8 As shown, the MRAC system 1000 is configured to control the operation of a controlled system (e.g., an actuator) 1010. The output of the controlled system 1010 is supplied as a feedback signal to an adaptive controller 1040. However, since the behavior of the controlled system 1010 lacks predictability during operation within a certain repetition rate range, according to one aspect of the embodiment, the depicted embodiment of the MRAC system 1000 includes a reference model 1020, a parameter adaptation module 1030, and an adaptive controller 1040.

[0111] A reference input is applied to the reference model 1020 and the adaptive controller 1020. The adaptive controller 1020 generates a control law signal u(t) based on the reference input. The reference model 1020 generates a reference output in response to the reference input. The controlled system 1010 (e.g., a line narrowing module having one or more actuators) generates an output in response to the signal u(t) and the feedback signal. The output y(t) is provided to the parameter adaptation module 1030. The parameter adaptation module 1030 determines the difference between the reference output and the output y(t) as a tracking error and provides the adapted operating parameters to the adaptive controller 1020. The adaptive controller 1020 formulates the control law u(t) based on the adapted operating parameters. The parameter adaptation module 1030 automatically adjusts the controller parameters so that the behavior of the output y(t) of the closed-loop controlled system 1010 closely follows the behavior of the reference model 1020. In other words, as the control parameters are adjusted, the tracking error converges so that the behavior of the controlled system 1010 tracks the behavior of the reference model 1020.

[0112] In this example, the controlled system 1010 is an actuator that is adjusted to control the peak separation of two wavelengths generated by the laser. The parameter adaptation module 1030 determines the peak separation difference between the reference output and the output y(t) as a tracking error and provides the adapted operating parameters to the adaptive controller 1040. As described above, the peak separation is ideally stabilized to a stable value quickly at the beginning of the pulse train. Also as described above, when the laser is operated at a repetition rate at or near the resonance of the actuator (e.g., a PZT actuator) and a harmonic of the resonance, it is difficult to achieve such a rapid onset of stability. However, using the described system, stable peak separation can be achieved quickly after the start of the pulse train even at repetition rates at or near these resonant frequencies and their harmonics.

[0113] Fig. 9 is a flow chart describing an adaptive model reference adaptive control system according to one aspect of an embodiment. In step S10, a reference model is formulated. In step S20, a reference signal is applied to the reference model and the adaptive controller. At step S30, the adaptive controller generates a control signal based on the reference signal. In practice, it is advantageous to make the parameters of the operating control signal as close to the expected value as possible. In step S40, the controlled actuator is driven using the control signal. In step S50, the error or difference between the output of the reference model and the output of the controlled actuator is determined. In step S60, the operating control parameter is adjusted to reduce the error or difference signal between the output of the reference model and the output of the controlled actuator. In this way, the error converges and the behavior of the controlled actuator conforms to the behavior of the reference model.

[0114] like Fig.10As shown, various embodiments and components therein can be implemented, for example, using one or more well-known computer systems, such as, for example, the example embodiments, systems and / or devices shown in the figures or otherwise discussed. Computer system 1200 can be any well-known computer capable of performing the functions described herein.

[0115] Computer system 1200 includes one or more processors (also referred to as central processing units or CPUs), such as processor 1210. Processor 1210 is connected to a communication infrastructure or bus 1220.

[0116] One or more processors 1210 may each be a graphics processing unit (GPU). In one embodiment, a GPU is a processor of electronic circuits specifically designed to process mathematically intensive applications. A GPU may have a parallel structure that is efficient for parallel processing of large blocks of data, such as mathematically intensive data commonly used in computer graphics applications, images, videos, etc.

[0117] The computer system 1200 also includes user input / output devices 1230 , such as a monitor, keyboard, pointing device, etc., which communicate with the communication infrastructure 1220 through a user input / output interface 1240 .

[0118] The computer system 1200 also includes a main memory or primary storage 1250, such as a random access memory (RAM). The main memory 1250 may include one or more cache levels. The main memory 1250 has stored therein control logic (ie, computer software) and / or data.

[0119] The computer system 1200 may also include one or more secondary storage devices or memories 1260. The secondary storage 1260 may include, for example, a hard disk drive 1280 and / or a removable storage device or drive 1290. The removable storage drive 1290 may be a floppy disk drive, a tape drive, an optical drive, an optical storage device, a tape backup device, and / or any other storage device / drive.

[0120] Removable storage drive 1290 may interact with removable storage unit 1300. Removable storage unit 1300 includes a computer usable or readable storage device having computer software (control logic) and / or data stored thereon. Removable storage unit 1300 may be a floppy disk, a magnetic tape, a compact disk, a DVD, an optical storage disk, and / or any other computer data storage device. Removable storage drive 1290 reads from and / or writes to removable storage unit 1300 in a well-known manner.

[0121] According to an example embodiment, secondary memory 1260 may include other components, tools, or other methods for allowing computer system 1200 to access computer programs and / or other instructions and / or data. Such components, tools, or other methods may include, for example, a removable storage unit 1310. Examples of removable storage unit 1310 may include a program cartridge and a cartridge interface (such as found in video game devices), a removable memory chip (such as an EPROM or PROM) and an associated socket, a memory stick and USB port, a memory card and an associated memory card slot, and / or any other removable storage unit and associated interface.

[0122] The computer system 1200 may also include a communication or network interface 1320. The communication interface 1320 enables the computer system 1200 to communicate and interact with any combination of remote devices, remote networks, remote entities, etc. (individually and collectively referred to as reference numeral 1330). For example, the communication interface 1320 may allow the computer system 1200 to communicate with the remote device 1330 via a communication path 1340, which may be wired and / or wireless and may include any combination of a LAN, a WAN, the Internet, etc. Control logic and / or data may be transmitted to and from the computer system 1200 via the communication path 1340.

[0123] In an embodiment, a non-transitory tangible apparatus or article of manufacture including a non-transitory, tangible computer usable or readable medium having control logic (software) stored thereon is also referred to herein as a computer program product or program storage device. This includes, but is not limited to, computer system 1200, main memory 1008, secondary memory 1010, and removable storage units 1018 and 1022, and tangible articles of manufacture embodying any combination of the foregoing. When executed by one or more data processing devices (such as computer system 1200), such control logic causes these data processing devices to operate as described herein.

[0124] Based on the teachings contained in this disclosure, it will be clear to those skilled in the relevant art how to use the Fig.10 The embodiments of the present disclosure may be made and used with data processing devices, computer systems, and / or computer architectures other than those shown. In particular, the embodiments may operate with software, hardware, and / or operating system implementations other than those described herein.

[0125] Although specific reference has been made above to the use of embodiments in the context of optical lithography, it will be appreciated that embodiments may be used in other applications, such as imprint lithography, and that embodiments are not limited to optical lithography where the context permits.

[0126] It should be understood that the phraseology or terminology herein is for the purpose of description rather than limitation, so that the phraseology or terminology of this specification will be interpreted by persons skilled in the relevant art(s) based on the teachings herein.

[0127] It should be understood that the Detailed Description section, rather than the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments contemplated by the inventors, and thus, are not intended to limit the embodiments and the appended claims in any way.

[0128] Embodiments have been described above with the aid of functional building blocks, which illustrate the implementation of specified functions and their relationships. For ease of description, the boundaries of these functional building blocks are arbitrarily defined herein. Alternative boundaries may be defined as long as the specified functions and their relationships are properly performed.

[0129] The above description of the specific embodiments will fully reveal the general nature of the embodiments, so that others can easily modify and / or adjust various applications of such specific embodiments without departing from the general concept of the embodiments by applying knowledge within the technical scope of the art without undue experimentation. Therefore, based on the teachings and guidance set forth herein, these adjustments and modifications are intended to be within the equivalent meaning and scope of the disclosed embodiments.

[0130] The breadth and scope of an embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0131] The embodiments may be further described using the following terms.

[0132] 1. A laser system, comprising:

[0133] a source of laser radiation, the laser radiation being excited in one or more pulse trains, each pulse train consisting of a plurality of pulses;

[0134] a wavelength controller arranged to receive the pulses and to control the dominant wavelength of some of the pulses towards a first value and to control the dominant wavelength of other of the pulses towards a second value that differs from the first value by a target dominant wavelength separation amount, the wavelength controller comprising at least one actuator operative in response to a control signal to effect wavelength control of the pulses; and

[0135] A model reference adaptive control system is adapted to generate the control signal based at least in part on the measured dominant wavelength separation to cause the wavelength controller to achieve the target dominant wavelength separation.

[0136] 2. The laser system of clause 1, wherein the source of the laser radiation is an excimer laser.

[0137] 3. The laser system of clause 1, wherein the actuator comprises a piezoelectric transducer.

[0138] 4. The laser system of clause 1, wherein the wavelength controller is a line narrowing module.

[0139] 5. The laser system of clause 1, wherein the wavelength controller is a line narrowing module comprising an optical element mechanically coupled to the at least one actuator.

[0140] 6. The laser system of clause 5, wherein the at least one actuator comprises a piezoelectric transducer.

[0141] 7. A multi-focus imaging lithography system, the multi-focus imaging lithography system generating a first wavelength pulse of deep ultraviolet radiation having a first main wavelength and a second wavelength pulse of deep ultraviolet radiation having a second main wavelength different from the first main wavelength by a main separation amount, the multi-focus imaging lithography system comprising:

[0142] a wavelength controller arranged to receive input pulses of deep ultraviolet radiation and, in response to a control signal, control a dominant wavelength of a first subset of said pulses to obtain pulses of said first wavelength and to control a dominant wavelength of a second subset of said input pulses to obtain pulses of a second wavelength; and

[0143] A model reference adaptive control system is adapted to generate the control signal based at least in part on the measured primary wavelength separation of the first wavelength pulse and the second wavelength pulse to cause the wavelength controller to achieve and maintain the target primary separation.

[0144] 8. A multi-focus imaging lithography system according to clause 7, wherein the source of the laser radiation is an excimer laser.

[0145] 9. The multi-focus imaging lithography system of clause 7, wherein the wavelength controller comprises an electrically actuatable component.

[0146] 10. The multi-focus imaging lithography system of clause 7, wherein the wavelength controller is a line narrowing module.

[0147] 11. The multi-focus imaging lithography system of clause 10, wherein the line narrowing module comprises an electrically actuatable component.

[0148] 12. The multi-focus imaging lithography system of clause 11, wherein the electrically actuatable component comprises a piezoelectric transducer.

[0149] 13. A system for controlling the wavelength of laser radiation excited in one or more pulse trains, each pulse train consisting of a plurality of pulses, the system comprising:

[0150] a wavelength controller arranged to receive the pulses and to control the dominant wavelength of some of the pulses towards a first value and to control the dominant wavelength of other of the pulses towards a second value that differs from the first value by a target dominant wavelength separation amount, the wavelength controller comprising at least one actuator operative in response to a control signal to effect wavelength control of the pulses; and

[0151] A model reference adaptive control system is adapted to generate a control signal based at least in part on the measured primary separation to cause the wavelength controller to achieve the target primary separation.

[0152] 14. The system of clause 13, wherein the actuator comprises a piezoelectric transducer.

[0153] 15. The system of clause 13, wherein the wavelength controller is a line narrowing module.

[0154] 16. The system of clause 13, wherein the wavelength controller is a line narrowing module comprising an optical element mechanically coupled to the actuator.

[0155] 17. The system of clause 16, wherein the actuator comprises a piezoelectric transducer.

[0156] 18. A system according to claim 13, wherein each pulse train comprises the plurality of pulses emitted at a repetition rate, and wherein the model reference adaptive control system is adapted to generate the control signal based at least in part on a measured primary wavelength separation so that the wavelength controller can achieve the target primary separation even when the repetition rate is within a critical range in which operation of the electrically actuatable component would otherwise be unstable.

[0157] 19. The system of clause 18, wherein the critical range is + / - 10% of a resonant frequency of the electrically-actuatable component or a harmonic of the resonant frequency of the electrically-actuatable component.

[0158] 20. The system of clause 18, wherein the electrically actuatable component comprises a piezoelectric transducer.

[0159] 21. A method of controlling a multi-focus imaging lithography system to generate a first wavelength radiation pulse having a first dominant wavelength and a second wavelength radiation pulse having a second dominant wavelength that differs from the first dominant wavelength by a dominant separation, the method comprising:

[0160] generating an input pulse of laser radiation;

[0161] using a wavelength controller to control a dominant wavelength of a first subset of the input pulses to obtain the first wavelength pulses and to control a dominant wavelength of a second subset of the input pulses to obtain the second wavelength pulses in response to a control signal;

[0162] comparing a dominant wavelength separation of the first wavelength pulse and the second wavelength pulse with a dominant wavelength separation obtained from a reference model controlled by a reference signal to obtain an error signal; and modifying one or more parameters of the control signal based at least in part on the error signal so that a response of the wavelength controller to the control signal tracks a response of the reference model to the reference signal.

[0163] 22. The method of clause 21, wherein generating the input pulses of laser radiation is performed using an excimer laser.

[0164] 23. The method of clause 22, wherein using a wavelength controller comprises using a line narrowing module.

[0165] The implementations described above and other implementations are within the scope of the following claims.

Claims

1. A laser system, comprising: a source of laser radiation, the laser radiation being excited in one or more pulse trains, each pulse train consisting of a plurality of pulses; a wavelength controller arranged to receive the pulses and to control the dominant wavelength of some of the pulses towards a first value and to control the dominant wavelength of other of the pulses towards a second value that differs from the first value by a target dominant wavelength separation amount, the wavelength controller comprising at least one actuator operative in response to a control signal to effect wavelength control of the pulses; and A model reference adaptive control system is adapted to generate the control signal based at least in part on the measured dominant wavelength separation to cause the wavelength controller to achieve the target dominant wavelength separation.

2. The laser system of claim 1, wherein the source of the laser radiation is an excimer laser.

3. The laser system of claim 1, wherein the actuator comprises a piezoelectric transducer.

4. The laser system of claim 1, wherein the wavelength controller is a line narrowing module.

5. The laser system of claim 1, wherein the wavelength controller is a line narrowing module comprising an optical element mechanically coupled to the at least one actuator.

6. The laser system of claim 5, wherein the at least one actuator comprises a piezoelectric transducer.

7. A multi-focus imaging lithography system, the multi-focus imaging lithography system generating a first wavelength pulse of deep ultraviolet radiation having a first main wavelength and a second wavelength pulse of deep ultraviolet radiation having a second main wavelength different from the first main wavelength by a main separation amount, the multi-focus imaging lithography system comprising: a wavelength controller arranged to receive input pulses of deep ultraviolet radiation and, in response to a control signal, control a dominant wavelength of a first subset of said pulses to obtain pulses of said first wavelength and control a dominant wavelength of a second subset of said input pulses to obtain pulses of said second wavelength; as well as A model reference adaptive control system is adapted to generate the control signal based at least in part on the measured primary wavelength separation of the first wavelength pulse and the second wavelength pulse to cause the wavelength controller to achieve and maintain a target primary separation.

8. The multi-focus imaging lithography system of claim 7, wherein the source of the laser radiation is an excimer laser.

9. The multi-focus imaging lithography system of claim 7, wherein the wavelength controller comprises an electrically actuatable component.

10. The multi-focus imaging lithography system of claim 7, wherein the wavelength controller is a line narrowing module.

11. The multi-focus imaging lithography system of claim 10, wherein the line narrowing module comprises an electrically actuatable component.

12. The multi-focus imaging lithography system of claim 11, wherein the electrically actuatable component comprises a piezoelectric transducer.

13. A system for controlling the wavelength of laser radiation excited in one or more pulse trains, each pulse train consisting of a plurality of pulses, the system comprising: a wavelength controller arranged to receive the pulses and to control the dominant wavelength of some of the pulses towards a first value and to control the dominant wavelength of other of the pulses towards a second value that differs from the first value by a target dominant wavelength separation amount, the wavelength controller comprising at least one actuator operative in response to a control signal to effect wavelength control of the pulses; and A model reference adaptive control system is adapted to generate the control signal based at least in part on the measured primary separation so as to cause the wavelength controller to achieve the target primary separation.

14. The system of claim 13, wherein the actuator comprises a piezoelectric transducer.

15. The system of claim 13, wherein the wavelength controller is a line narrowing module.

16. The system of claim 13, wherein the wavelength controller is a line narrowing module comprising an optical element mechanically coupled to the actuator.

17. The system of claim 16, wherein the actuator comprises a piezoelectric transducer.

18. The system of claim 13, wherein each pulse train comprises the plurality of pulses excited at a repetition rate, and wherein the model reference adaptive control system is adapted to generate the control signal based at least in part on the measured dominant wavelength separation so that the wavelength controller achieves the target dominant separation even when the repetition rate is within a critical range in which operation of the electrically actuatable component would otherwise be unstable.

19. The system of claim 18, wherein the critical range is + / - 10% of a resonant frequency of the electrically-actuatable component or a harmonic of the resonant frequency of the electrically-actuatable component.

20. The system of claim 18, wherein the electrically actuatable component comprises a piezoelectric transducer.

21. A method of controlling a multi-focus imaging lithography system to generate a first wavelength radiation pulse having a first dominant wavelength and a second wavelength radiation pulse having a second dominant wavelength that differs from the first dominant wavelength by a dominant separation, the method comprising: generating an input pulse of laser radiation; using a wavelength controller to control a dominant wavelength of a first subset of the input pulses to obtain the first wavelength pulses and to control a dominant wavelength of a second subset of the input pulses to obtain the second wavelength pulses in response to a control signal; comparing a dominant wavelength separation of the first wavelength pulse and the second wavelength pulse with a dominant wavelength separation obtained from a reference model controlled by a reference signal to obtain an error signal; as well as One or more parameters of the control signal are modified based at least in part on the error signal so that a response of the wavelength controller to the control signal tracks a response of the reference model to the reference signal.

22. The method of claim 21, wherein generating the input pulses of laser radiation is performed using an excimer laser.

23. The method of claim 22, wherein using a wavelength controller comprises using a line narrowing module.

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