Method of controlling wavelength of laser beam and laser apparatus

There is provided a method of controlling the wavelength of a laser beam. The method includes measuring an absolute wavelength of the laser beam; calculating a difference between a reference wavelength and the absolute wavelength of the laser beam; and adjusting the reference wavelength of the laser beam based on the difference between the reference wavelength and the absolute wavelength of the laser beam, at an interval shorter than an interval for which the absolute wavelength of the laser beam is measured.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No. 2013-066813, filed Mar. 27, 2013, which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method of controlling the wavelength of a laser beam, and a laser apparatus.

BACKGROUND ART

The miniaturization and increased levels of integration of semiconductor integrated circuits have led to a demand for increases in the resolutions of semiconductor exposure devices (hereinafter referred to as “exposure device”). Accordingly, advances are being made in the reduction in the wavelengths of light emitted from exposure light sources. Gas laser apparatuses are being used as exposure light sources instead of conventional mercury lamps. At present, a KrF excimer laser apparatus that emits ultraviolet light at a wavelength of 248 nm and an ArF excimer laser apparatus that emits ultraviolet light at a wavelength of 193 nm are being used as gas laser apparatuses for exposure.

As a current exposure technology, immersion exposure has been put to practical use, for reducing the apparent wavelength of an exposure light source by filling the space between the projection lens of an exposure device and a wafer with a liquid, and changing the refractive index of the space. In the case where immersion exposure is carried out using an ArF excimer laser apparatus as the exposure light source, the wafer is irradiated with ultraviolet light at a wavelength of 134 nm in the liquid. This technology is referred to as ArF immersion exposure or ArF immersion lithography.

The spontaneous oscillation spectral linewidth of a KrF or ArF excimer laser apparatus is as wide as 350 to 400 pm, and therefore a laser beam (ultraviolet light) which is reduced and projected on the wafer by the projection lens in the exposure device exhibits chromatic aberration. As a result, the resolution is dropped. It is therefore necessary to narrow the spectral bandwidth of the laser beam emitted from the gas laser apparatus until the chromatic aberration reaches a level that can be ignored. The spectral bandwidth may be referred to as a spectral width. Accordingly, the spectral width has been narrowed by providing a line narrowing module (LNM) having line narrowing elements in the laser resonator of the gas laser apparatus. Here, the line narrowing elements may be an etalon, a grating, and so forth. A laser apparatus having a narrowed spectral width in this manner is referred to as a line narrowing laser apparatus.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

A method of controlling a wavelength of a laser beam includes measuring an absolute wavelength of the laser beam; calculating a difference between a reference wavelength and the absolute wavelength of the laser beam; and adjusting the reference wavelength of the laser beam based on the difference between the reference wavelength and the absolute wavelength of the laser beam, at an interval shorter than an interval for which the absolute wavelength of the laser beam is measured.

A laser apparatus includes a laser resonator configured to output a laser beam; a spectroscope configured to measure a relative wavelength of the laser beam with respect to a reference wavelength of the laser beam; an absolute wavelength detector configured to measure an absolute wavelength of the laser beam; and a controller configured to calculate a difference between the reference wavelength and the absolute wavelength of the laser beam, and to adjust the reference wavelength of the laser beam based on the difference, at an interval shorter than an interval for which the absolute wavelength of the laser beam is measured.

DESCRIPTION OF EMBODIMENTS

Table of Contents

1. An exemplary laser apparatus

1.1 An example of configuration of the exemplary laser apparatus

1.2 An example of operation of the exemplary laser apparatus

2. An exemplary method of controlling the wavelength of a laser

beam according to related art

2.1 Flowcharts according to related art

2.2 Problems with related art

3. Embodiments of the method of controlling the wavelength of a

laser beam

Hereinafter, selected embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments to be described below are merely illustrative in nature and do not limit the scope of the present disclosure. Further, the configuration(s) and operation(s) described in each embodiment are not all essential in implementing the present disclosure. Note that like elements are referenced by like reference numerals and characters, and duplicate descriptions thereof will be omitted herein.

1. An Exemplary Laser Apparatus

1.1 An Example of Configuration of the Exemplary Laser Apparatus

FIG. 1is a drawing illustrating an exemplary laser apparatus. InFIG. 1, the straight lines without arrows indicate electrical connection between components. Meanwhile, the straight lines with arrows indicate the travelling direction of a laser beam.

The exemplary laser apparatus illustrated inFIG. 1may be a line narrowing laser apparatus100including a system for controlling the absolute wavelength of a laser beam. The line narrowing laser apparatus100including the system for controlling the absolute wavelength may be used with an exposure device91. The line narrowing laser apparatus100may be an excimer laser apparatus. The excimer laser apparatus may be an argon fluoride (ArF) excimer laser apparatus, or a krypton fluoride (KrF) excimer laser apparatus. The line narrowing laser apparatus100may be a solid-state laser apparatus including a wavelength-variable ultraviolet laser. The wavelength-variable ultraviolet solid-state laser apparatus may be a solid-state laser apparatus formed by combining a Ti-sapphire laser with a nonlinear crystal.

The line narrowing laser apparatus100may include a laser chamber10, an output coupling mirror21, a first beam splitter22, a laser exit shutter31, a shutter driver32, a line narrowing module (LNM)50, a wavelength detection system90, a wavelength controller61, a laser controller62, and a first driver63. The output coupling mirror21and the line narrowing module50may constitute a laser resonator configured to output a laser beam in the line narrowing laser apparatus100.

The line narrowing laser apparatus100may be a single-stage line narrowing ultraviolet laser. The line narrowing laser apparatus100may be mounted, as a master oscillator (MO) used with a power amplifier (PA) that amplifies a laser beam. The line narrowing laser apparatus100may be mounted in the double-stage system including a power oscillator (PO).

The laser chamber10may be provided on the optical path of the laser resonator. The laser chamber10may include a first window11, a second window12, a pair of electrodes13, and a power supply14. The laser chamber10may contain laser medium. When the line narrowing laser apparatus100is an ArF excimer laser apparatus, the laser medium may be mixed gas containing argon (Ar) gas, fluorine (F2) gas, and neon (Ne) gas. Meanwhile, when the line narrowing laser apparatus100is a KrF excimer laser apparatus, the laser medium may be mixed gas containing krypton (Kr) gas, fluorine (F2) gas, and neon (Ne) gas. The first window11and the second window12may be provided to allow a laser beam to pass therethrough. The pair of electrodes13may be provided to face the direction perpendicular to the plane ofFIG. 1, in the laser chamber10. The longitudinal direction of the pair of electrodes13may match the direction of the optical path of the laser resonator. The pair of electrodes13may be connected to the power supply14. The power supply14may apply a voltage to the pair of electrodes13such that the laser medium discharges electricity between the pair of electrodes13and the laser beam emitted from the laser medium is amplified due to the stimulated emission.

The output coupling mirror21may be coated with a film that allows part of the laser beam to be reflected, and the remaining laser beam to transmit therethrough.

The first beam splitter22may be provided on the optical path of the laser beam outputted from the output coupling mirror21. The first beam splitter22may be provided to transmit part of the laser beam outputted from the output coupling mirror21to the laser exit shutter31. The first beam splitter22may be provided to reflect the remaining laser beam to the wavelength detection system90.

The laser exit shutter31may be provided to allow the laser beam having passed through the first beam splitter22to pass to the exposure device91, or be provided to block the laser beam having passed through the first beam splitter22. The laser exit shutter31may be connected to the shutter driver32.

The shutter driver32may be provided to control the opening and closing of the laser exit shutter31according to a command from the laser controller62. The shutter driver32may be connected to the laser exit shutter31and the laser controller62.

The line narrowing module50may be configured to narrow the wavelength width (spectral width) of a laser beam. The line narrowing module50may include a plurality of prisms51, a grating52, and one or more rotation stages53. The number of the plurality of prisms51may be two. The plurality of prisms51may be provided to function as a beam expander. The grating52may be arranged in a Littrow configuration such that the angle of diffraction of the laser beam matches the angle of incidence of the laser beam. The number of the one or more rotation stages53may be two. At least one of the plurality of prisms51may be set on either the one or more rotation stages53. The angle of incidence of the laser beam on the grating52may be changed by rotating at least one of the plurality of prisms51on the one or more rotation stages53.

The wavelength detection system90may include a second beam splitter23, a third beam splitter24, a first reflecting mirror25, an optical sensor40, a spectroscope70, and an absolute wavelength detector80.

The second beam splitter23may be provided on the optical path of the laser beam reflected from the first beam splitter22. The second beam splitter23may be provided to transmit part of the laser beam having been reflected from the first beam splitter22to the third beam splitter24. The second beam splitter23may be provided to reflect the remaining laser beam having been reflected from the first beam splitter22to the optical sensor40.

The third beam splitter24may be provided on the optical path of the laser beam having transmitted through the second beam splitter23. The third beam splitter24may be provided to transmit part of the laser beam having transmitted through the second beam splitter23to the spectroscope70. The third beam splitter24may be provided to reflect the remaining laser beam having transmitted through the second beam splitter23to the first reflecting mirror25.

The first reflecting mirror25may be provided to reflect the laser beam having been reflected from the third beam splitter24, to the absolute wavelength detector80.

The optical sensor40may be provided to detect the laser beam having been reflected from the second beam splitter23. The output terminal of the optical sensor40may be connected to the wavelength controller61.

The spectroscope70may be provided to receive the laser beam having transmitted through the third beam splitter24. The spectroscope70may be connected to the wavelength controller61. The spectroscope70may be configured to measure a relative wavelength of the laser beam with respect to the reference wavelength of the laser beam which serves as the standard for the control of the wavelength of a laser beam. The spectroscope70may be capable of measuring the wavelength of a laser beam per pulse.

The spectroscope70may be an etalon spectrometer. In this case, the spectroscope70may include a first diffuser element71, a monitor etalon72, a first condenser lens73, and an image sensor74. The monitor etalon72may be, for example, an air-gap etalon. The image sensor74may be a line sensor such as a one-dimensional CCD, or a photodiode array. In the spectroscope70, the first diffuser element71, the monitor etalon72, the first condenser lens73and the image sensor74may be arranged in the order as described.

The first diffuser element71may be provided to diffuse the laser beam having transmitted through the third beam splitter24.

The monitor etalon72may be provided to receive the laser beam having been diffused by the first diffuser element71. The monitor etalon72may be provided to interfere with the laser beam having been diffused by the first diffuser element71.

The first condenser lens73may be provided to focus the laser beam having transmitted through the monitor etalon72on the image sensor74.

The image sensor74may be provided on the focal plane of the first condenser lens73. The focal plane of the first condenser lens73may have an interference pattern of the laser beam having transmitted through the monitor etalon72. The image sensor74may detect the interference pattern of the laser beam having transmitted through the monitor etalon72. The square of the radius of the interference pattern generated on the focal plane of the first condenser lens73may be proportional to the wavelength of the laser beam. Wavelength A of the laser beam may be represented by an equation λ=λc+αr2. Here, r represents the radius of the detected interference pattern of the laser beam; λc represents the center wavelength of the detected interference pattern of the laser beam at which the intensity is maximized (reference wavelength of the laser beam); and a represents a proportional constant. The spectral profile of the laser beam may be detected based on the interference pattern of the laser beam that is detected by the image sensor74. The center wavelength and the spectral linewidth of the laser beam may be detected, based on the interference pattern of the laser beam that is detected by the image sensor74. The center wavelength and the width of the spectral line of the laser beam may be detected by an information processor (not shown), or be calculated by the wavelength controller61.

The spectroscope70may include a plurality of etalon spectroscopes having different free spectral ranges.

The spectroscope70may include a grating and an image sensor. The grating may be provided to diffract the laser beam having transmitted through the third beam splitter24. The spectral profile of the laser beam having been diffracted by the grating may be detected by the image sensor. The center wavelength and the width of the spectral line of the laser beam may be detected by the image sensor.

The absolute wavelength detector80may be provided to receive the laser beam having been reflected from the first reflecting mirror25. The absolute wavelength detector80may be configured to measure the absolute wavelength of the laser beam. The absolute wavelength detector80may be capable of measuring the absolute wavelength of the laser beam more accurately than the spectroscope70. By using the absolute wavelength detector80, the wavelength of the laser beam measured by the spectroscope70may be controlled.

The absolute wavelength detector80may include a uniaxial stage81, a second reflecting mirror82, a second driver83, a laser Galvatron™84, an optical system85, an optogalvanic signal detecting circuit86, a galvatron power supply87, a first damper88and a second damper89.

The uniaxial stage81may be provided to be able to move the second reflecting mirror82provided on the uniaxial stage81. The uniaxial stage81may be connected to the second driver83.

When the measurement of the absolute wavelength of the laser beam is not performed, the second reflecting mirror82may be positioned such that the laser beam reflected from the first reflecting mirror25is not incident on the optical system85and the laser galvatron84, but is reflected to the first damper88. In contrast, when the measurement of the absolute wavelength of the laser beam is performed, the second reflecting mirror82may be positioned such that the laser beam reflected from the first reflecting mirror25is incident on the optical system85and the laser galvatron84.

The second driver83may be provided to control the location of the second reflecting mirror82provided on the uniaxial stage81, according to a command from the wavelength controller61. The second driver83may be connected to the uniaxial stage81and the wavelength controller61.

The laser galvatron84may be provided to detect the absolute wavelength of the laser beam by using the optogalvanic effect. The laser galvatron84may include a hollow anode and a hollow cathode. The hollow anode and the hollow cathode in the laser galvatron84may be provided to be supplied with a voltage. When the line narrowing laser apparatus100is an ArF excimer laser apparatus, the material for the hollow cathode of the laser galvatron84may contain platinum (Pt). Meanwhile, when the line narrowing laser apparatus100is a KrF excimer laser apparatus, the material for the hollow cathode of the laser galvatron84may contain ferrum (Fe).

FIG. 2is a drawing schematically illustrating the configuration and the operation of the laser galvatron of the exemplary laser apparatus.

The laser galvatron84illustrated inFIG. 2may include a hollow anode84aand a hollow cathode84b. The optogalvanic signal detecting circuit86and the galvatron power supply87may be connected to the hollow anode84aand the hollow cathode84b. The galvatron power supply87may apply a voltage between the hollow anode84aand the hollow cathode84b. Then, an electric discharge occurs between the hollow anode84aand the hollow cathode84b, and the (metal) material of the hollow cathode84bis spattered, and therefore vapor of the atoms of the material of the cathode84bmay be generated. The vapor of the atoms of the material may be irradiated with a laser beam. The vapor of the atoms of the material of the cathode84bmay be irradiated with a laser beam having a wavelength for which the atoms of the material of the cathode84bare resonantly ionized. As a result, the atoms of the material of the cathode84bare resonantly ionized, and therefore ions of the material may be generated. The ions of the material of the cathode84b, which are generated by the laser beam, may increase the current flowing between the hollow anode84aand the hollow cathode84b(optogalvanic effect). The wavelength (absolute wavelength) for which the atoms of the material of the cathode84bare resonantly ionized may be determined based on the kind of the atoms of the material of the cathode84b. As illustrated by A, B, C, D, and E inFIG. 2, the current flowing between the hollow anode84aand the hollow cathode84bin the laser galvatron84may be detected, changing the wavelength of the laser beam. As illustrated by C inFIG. 2, the wavelength of the laser beam when the current flowing between the hollow anode84aand the hollow cathode84bin the laser galvatron84is maximum may be the wavelength for which the atoms of the material of the cathode84bare resonantly ionized. The absolute wavelength of the laser beam as illustrated by A, B, C, D, and E inFIG. 2may be obtained based on the difference from the wavelength for which the atoms of the material of the cathode84bare resonantly ionized as illustrated by C inFIG. 2.

The optical system85shown inFIG. 1may be provided to transmit the laser beam having been reflected from the first reflecting mirror25, to the laser galvatron84. The optical system85may include a second diffuser element85a, a second condenser lens85b, a pinhole85cand a lens85d.

The second diffuser element85amay be provided to diffuse the laser beam having been reflected from the first reflecting mirror25.

The second condenser lens85bmay be provided to focus the laser beam having been diffused by the second diffuser element85aon the pinhole85c.

The pinhole85cmay be provided at the focal position of the second condenser lens85b.

The lens85dmay be provided to allow the laser beam having passed through the pinhole85cto pass through the space in the hollow anode84aand the space in the hollow cathode84bin the laser galvatron84.

The optogalvanic signal detecting circuit86may be provided to detect the current flowing between the hollow anode84aand the hollow cathode84bin the laser galvatron84. The optogalvanic signal detecting circuit86may be provided to send the detected current value to the wavelength controller61. The optogalvanic signal detecting circuit86may be connected to the hollow anode84aand the hollow cathode84bin the laser galvatron84, and to the wavelength controller61.

The galvatron power supply87may be provided to generate an electric discharge in the laser galvatron84by applying a voltage between the hollow anode84aand the hollow cathode84bin the laser galvatron84, according to a command from the wavelength controller61. The galvatron power supply87may be connected to the hollow anode84aand the hollow cathode84bin the laser galvatron84, and to the wavelength controller61.

The first damper88may be provided to absorb the laser beam having been reflected from the second reflecting mirror82.

The second damper89may be provided to absorb the laser beam having passed through the laser galvatron84.

The absolute wavelength detector80may include a gas cell to absorb the laser beam and an optical sensor, instead of the laser galvatron84and the second damper89. When the line narrowing laser apparatus100is an ArF excimer laser apparatus, the gas cell to absorb the laser beam may be a Pt lamp. In this case, the wavelength of the laser beam is changed to obtain the minimal value of the detection signal of the optical sensor, so that the absolute wavelength of the laser beam may be detected.

The wavelength controller61may be connected to the optical sensor40included in the wavelength detection system90, the laser controller62, and the first driver63. The wavelength controller61may be configured to calculate the difference between the reference wavelength and the absolute wavelength of the laser beam. The wavelength controller61may be configured to adjust the reference wavelength of the laser beam, based on the difference between the reference wavelength and the absolute wavelength of the laser beam, at an interval shorter than the interval for which the absolute wavelength of the laser beam is measured.

The laser controller62may be connected to the wavelength controller61, the power supply14of the laser chamber10, the shutter driver32, and the exposure device controller92provided in the exposure device91.

The first driver63may be provided to control the rotation of the one or more rotation stages53included in the line narrowing module50, according to a command from the wavelength controller61. The first driver63may be connected to the wavelength controller61.

The line narrowing laser apparatus100illustrated inFIG. 1includes the shutter driver32, the wavelength controller61, the laser controller62, the first driver63, and the second driver83. Here, at least two of the shutter driver32, the wavelength controller61, the laser controller62, the first driver63and the second driver83may be integrated with each other.

FIG. 3is a drawing schematically illustrating the configuration of a controller of the exemplary laser apparatus.

Each of the above-described controllers may be constituted by general-purpose control equipment such as a computer and a programmable controller. For example, each of the controllers may be constituted as follows.

The controller may be constituted of a processor1000, and a storage memory1005, a user interface1010, a parallel I/O controller1020, a serial I/O controller1030, and an AD/DA converter1040which are connected to the processor1000. The processor1000may be constituted of a CPU1001, and a memory1002, a timer1003and a GPU1004which are connected to the CPU1001.

The processor1000may read out a program stored in the storage memory1005. In addition, the processor1000may execute the read program, read out data from the storage memory1005according to the execution of the program, and store the data in the storage memory1005.

The parallel I/O controller1020may be connected to a device that allows communication via a parallel I/O port. The parallel I/O controller1020may control the communication by digital signals via the parallel I/O port, which is executed by the processor1000in the course of the execution of the program.

The serial I/O controller1030may be connected to a device that allows communication via the serial I/O port. The serial I/O controller1030may control the communication by digital signals via the serial I/O port, which is executed by the processor in the course of the execution of the program.

The AD/DA converter1040may be connected to a device that allows communication via an analog port. The AD/DA converter1040may control the communication by analog signals via the analog port, which is executed by the processor1000in the course of the execution of the program.

By using the user interface1010, the operator may allow the processor1000to display the execution process of the program and to halt or interrupt the execution of the program.

The CPU1001of the processor1000may perform arithmetic processing according to the program. The memory1002may temporarily store the program in the course of the execution of the program by the CPU1001. Also, the memory1002may temporarily store the data in the course of the arithmetic processing. The timer1003may measure the time and the elapsed time, and output the time and the elapsed time to the CPU1001according to the execution of the program. When image data is inputted to the processor1000, the GPU1004may process the image data according to the execution of the program, and output the result of the process to the CPU1001.

The device connected to the parallel I/O controller1020, which allows communication via the parallel I/O port, may be the optical sensor40, the image sensor74, and other controllers.

The device connected to the serial I/O controller1030, which allows communication via the serial I/O port, may be other controllers.

The device connected to the AD/DA converter1040, which allows communication via the analog port, may be the optical sensor40, the image sensor74, the optogalvanic signal detecting circuit86, and so forth.

1.2 an Example of Operation of the Exemplary Laser Apparatus

The wavelength controller61may measure or calibrate the absolute wavelength of the laser beam at a predetermined cycle.

When measuring or calibrating the absolute wavelength of the laser beam, the wavelength controller61may inform the exposure device91of the measurement or calibration of the absolute wavelength of the laser beam by causing the laser controller62and the shutter driver32to close the laser exit shutter31. When the wavelength controller61measures or calibrates the absolute wavelength of the laser beam, the laser beam may not be inputted to the exposure device91.

When measuring or calibrating the absolute wavelength of the laser beam, the wavelength controller61may cause the laser controller62to oscillate the laser beam on a predetermined appropriate oscillating condition. The predetermined oscillating condition may be, for example, a repetition frequency of 100 Hz, and a power supply voltage of 15 kV.

The wavelength controller61may send a signal to turn the power supply of the laser galvatron84on, to the galvatron power supply87, and therefore generate an electric discharge between the hollow anode84aand the hollow cathode84bin the laser galvatron84.

The wavelength controller61may send a signal to the second driver83to allow the laser beam reflected from the first reflecting mirror25to enter the laser galvatron84.

The wavelength controller61may send to the first driver63a signal to scan the wavelength of the laser beam at a predetermined gradient, within a predetermined wavelength range. The predetermined wavelength range may be, for example, from 193.430 nm or more to 193.440 nm or less. The predetermined gradient to scan the wavelength may be, for example, 0.0001 nm/sec.

The wavelength controller61may read the data of the interference pattern of the laser beam generated by the monitor etalon72included in the spectroscope70, per pulse of the laser beam oscillating during the wavelength scanning. The wavelength controller61may calculate wavelength λ (the detection wavelength of the spectroscope70) of the laser beam based on the data of the interference pattern of the laser beam per pulse of the laser beam. The spectroscope70may measure relative wavelength drift δλ=λ−λc with respect to reference wavelength λc of the laser beam, and calculate Δ=δλ+λc, based on the reference wavelength λc of the laser beam and the relative wavelength drift δλ. When the spectroscope70is an etalon spectroscope, δλ may be proportional to the square of the radius of the interference pattern of the laser beam that is generated by the monitor etalon72.

The wavelength controller61may read the value detected by the optogalvanic signal detecting circuit86(intensity I of the optogalvanic signal) per pulse of the laser beam oscillating during the wavelength scanning.

The wavelength controller61may obtain detection wavelength λs of the spectroscope70at the peak of the intensity I of the optogalvanic signal, based on the relationship between the value of the detection wavelength λ of the spectroscope70and the value of the intensity I of the optogalvanic signal. The absolute wavelength of the laser beam at the peak of the intensity I of the optogalvanic signal may be wavelength λabs for which the atoms of the material of the cathode84bin the laser galvatron84are resonantly ionized. When the line narrowing laser apparatus100is an ArF excimer laser, the wavelength λabs for which the atoms of the material (Pt) of the cathode84bof the laser galvatron84are resonantly ionized may be 193.4369 nm. Meanwhile, when the line narrowing laser apparatus100is a KrF excimer laser, the wavelength λabs for which the atoms of the material (Fe) of the cathode84bof the laser galvatron84are resonantly ionized may be 248.327 nm.

The wavelength controller61may obtain difference Δλabs=λabs−λs between the detection wavelength λs of the spectroscope70at the peak of the intensity I of the optogalvanic signal and the wavelength λabs for which the atoms of the material of the cathode84bof the laser galvatron84are resonantly ionized. The wavelength λabs for which the atoms of the material of the cathode84bof the laser galvatron84are resonantly ionized may be the absolute wavelength of the laser beam at the peak of the intensity I of the optogalvanic signal.

When calibrating the absolute wavelength of the laser beam, the wavelength controller61may cause the first driver63to rotate at least one of the plurality of prisms51of the line narrowing module50, so as to add Δλabs to the reference wavelength λc of the laser beam set for the spectroscope70.

When controlling the wavelength of the laser beam, the wavelength controller61may cause the first driver63to rotate at least one of the plurality of prisms51of the line narrowing module50, so as to add predetermined wavelength difference δλ to the reference wavelength λc of the laser beam set for the spectroscope70. The wavelength controller61may adjust the reference wavelength λc of the laser beam, based on the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam, at an interval shorter than the interval for which the absolute wavelength of the laser beam is measured.

After completion of the measurement or calibration of the absolute wavelength of the laser beam, the wavelength controller61may cause the second driver83to move the second reflecting mirror82to prevent the laser beam from entering the second diffuser element85a. The wavelength controller61may stop sending the signal to scan the wavelength of the laser beam. The wavelength controller61may turn the galvatron power supply87off.

The wavelength controller61may cause the laser controller62to stop the laser oscillation.

The wavelength controller61may cause the laser controller62and the shutter driver32to inform the exposure device controller92of the completion of the measurement or calibration of the absolute wavelength of the laser beam and to open the laser exit shutter31.

2. An Exemplary Method of Controlling the Wavelength of a Laser Beam According to Related Art

2.1 Flowcharts According to Related Art

FIG. 4is a flowchart illustrating a process performed by the wavelength controller according to related art.

The wavelength controller61may start the process when the line narrowing laser apparatus100is set up, when the line narrowing laser apparatus100is stopped for a long time, just after the spectroscope70is replaced, or just after the absolute wavelength detector80is replaced.

In step S401, the wavelength controller61may read initial parameters regarding the process performed by the wavelength controller61. The initial parameters may include initial reference wavelength λc0 of the laser beam set for the spectroscope70, and cycle K1 for which the absolute wavelength of the laser beam is measured (hereinafter “absolute wavelength measurement cycle K1”). The absolute wavelength measurement cycle K1 may be, for example, within a range from about one day or more to about ten days or less.

In step S402, the wavelength controller61may execute a subroutine for measuring the absolute wavelength of the laser beam (hereinafter “absolute wavelength measurement subroutine”). Contents of the absolute wavelength measurement subroutine will be described later. In the absolute wavelength measurement subroutine, the wavelength controller61may calculate the difference Δλabs between the initial reference wavelength λc0 and the absolute wavelength of the laser beam.

In step S403, the wavelength controller61may calibrate the initial reference wavelength λc0 of the laser beam by the difference Δλabs between the initial reference wavelength λc0 and the absolute wavelength of the laser beam. The reference wavelength λc of the laser beam set for the spectroscope70may be calculated according to an equation λc=λc0+Δλabs.

In step S404, the wavelength controller61may reset a timer that measures the period of time for which the absolute wavelength of the laser beam is controlled, and start measuring time T1 after the previous measurement of the absolute wavelength of the laser beam.

In step S405, the wavelength controller61may execute a subroutine for controlling the wavelength λ of the laser beam (hereinafter “wavelength control subroutine”). Contents of the wavelength control subroutine will be described later. In the wavelength control subroutine, the wavelength controller61may add the predetermined wavelength difference δλ to the reference wavelength λc of the laser beam (λc+δλ) to control the wavelength λ(=λc+δλ) of the laser beam. The wavelength control subroutine may be executed during the exposure by the exposure device91.

In step S406, the wavelength controller61may determine whether or not the time T1 after the previous measurement of the absolute wavelength of the laser beam is equal to or longer than the absolute wavelength measurement cycle K1. When T1 is shorter than K1 (T1<K1), the wavelength controller61may move the step back to the step S405and repeat the wavelength control subroutine to continue to control the wavelength λ of the laser beam. On the other hand, when T1 is equal to or longer than K1 (T1≧K1), the wavelength controller61may stop executing the wavelength control subroutine, and move the step to step S407.

In the step S407, the wavelength controller61may cause the exposure device controller92of the exposure device91to determine whether or not to allow to execute the absolute wavelength measurement subroutine. When receiving a signal from the exposure device91, which indicates that the absolute wavelength measurement subroutine can be executed, the wavelength controller61may move the step to step S408. On the other hand, when receiving a signal from the exposure device91, which indicates that the absolute wavelength measurement subroutine cannot be executed, the wavelength controller61may move the step back to the step S405.

In the step S408, the wavelength controller61may execute the absolute wavelength measurement subroutine. In the absolute wavelength measurement subroutine, the wavelength controller61may calculate the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam.

In step S409, the wavelength controller61may calibrate the reference wavelength λc of the laser beam by adding the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam, to the reference wavelength λc of the laser beam.

The timing at which the absolute wavelength of the laser beam is measured in the step S408may match the timing at which the reference wavelength λc of the laser beam is calibrated in the step S409.

In step S410, the wavelength controller61may determine whether or not to stop controlling the wavelength of the laser beam. When stopping the control of the wavelength of the laser beam, the wavelength controller61may stop the operation for controlling the wavelength of the laser beam. On the other hand, when not stopping the control of the wavelength of the laser beam, the wavelength controller61may move the step back to the step S404, and continue to control the wavelength of the laser beam.

FIG. 5is a flowchart illustrating a process of the absolute wavelength measurement subroutine included in the process executed by the wavelength controller according to related art.

In step S501, the wavelength controller61may cause the laser controller62to output a signal to start measuring the absolute wavelength of the laser beam, to the exposure device controller92.

In step S502, the wavelength controller61may set conditions of the laser oscillation when the absolute wavelength of the laser beam is measured. The conditions of the laser oscillation when the absolute wavelength of the laser beam is measured may include, for example, charging voltage HV (kV), repetition frequency f (Hz), wavelength change gradient v (pm/sec), a range of wavelength scanning λr (nm), and the number of measurements during the wavelength scanning. The charging voltage HV may be within a range from 13 kV or more to 20 kV or less. The repetition frequency f may be within a range from 100 Hz or more to 1000 Hz or less. The wavelength change gradient v may be within a range from 0.01 pm/sec or more to 0.1 pm/sec or less. The range of wavelength scanning λr may be within a range from 193.430 nm or more to 193.440 nm or less. The number of measurements during the wavelength scanning may be within a range from 100 or more to 1000 or less. The wavelength controller61may send data of the setting values for the charging voltage and the repetition frequency, to the laser controller62.

In step S503, the wavelength controller61may turn the galvatron power supply87on. The wavelength controller61may cause the second driver83to move the second reflecting mirror82such that the laser beam reflected from the first reflecting mirror25enters the laser galvatron84.

In step S504, the wavelength controller61may cause the laser controller62and the shutter driver32to close the laser exit shutter31.

In step S505, the wavelength controller61may cause the laser controller62to perform laser oscillation at the charging voltage and the repetition frequency set in the step S502.

In step S506, the wavelength controller61may execute a subroutine for calculating the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam, that is, the wavelength difference Δλabs between the spectroscope70and the absolute wavelength detector80(hereinafter “wavelength difference measurement subroutine”). The wavelength difference measurement subroutine will be described later.

In step S507, the wavelength controller61may cause the laser controller62to stop the laser oscillation.

In step S508, the wavelength controller61may turn the galvatron power supply87off. The wavelength controller61may cause the second driver83to move the second reflecting mirror82to block the laser beam reflected from the first reflecting mirror25, that is, to prevent the laser beam from entering the laser galvatron84.

In step S509, the wavelength controller61may cause the laser controller62and the shutter driver32to close the laser exit shutter31.

In step S510, the wavelength controller61may cause the laser controller62to output a signal indicating that the measurement of the absolute wavelength of the laser beam has been completed, to the exposure device controller92.

FIG. 6is a flowchart illustrating a process of the wavelength difference measurement subroutine included in the process executed by the wavelength controller according to related art.

In step S601, the wavelength controller61may read initial parameters for the process of the wavelength difference measurement subroutine. The initial parameters may include the number m0 of measurement points having to be obtained for the detection wavelength λ of the spectroscope70and the intensity I of the optogalvanic signal, and the ideal value λabs of the absolute wavelength of the laser beam.

The number m0 of measurement points having to be obtained may be, for example, within a range from 100 or more to 1000 or less. When the line narrowing laser apparatus100is an ArF excimer laser, the ideal value λabs of the absolute wavelength of the laser beam may be 193.4369 nm, which is the wavelength for which the atoms of Pt are resonantly ionized. Meanwhile, when the line narrowing laser apparatus100is a KrF excimer laser, the ideal value λabs of the absolute wavelength of the laser beam may be 248.327 nm, which is the wavelength for which the atoms of Fe are resonantly ionized. The number m of measurement points to be obtained for the detection wavelength λ of the spectroscope70and the intensity I of the optogalvanic signal may be set to zero by the wavelength controller61.

In step S602, the wavelength controller61may determine whether or not a signal from the optical sensor40has been detected. The signal from the optical sensor40may indicate that the laser oscillation has been performed. When determining that the signal from the optical sensor40has been detected, the wavelength controller61may move the step to step S603. On the other hand, when determining that any signal from the optical sensor40has not been detected, the wavelength controller61may repeat the step S602until a signal from the optical sensor40is detected.

In the step S603, the wavelength controller61may add one to the number m of the measurement points. The wavelength controller61may add one to the number m of the measurement points every time obtaining a measurement point for the detection wavelength λ of the spectroscope70and the intensity I of the optogalvanic signal.

In step S604, the wavelength controller61may cause the optogalvanic signal detecting circuit86to obtain the intensity I(m) of the optogalvanic signal.

In step S605, the wavelength controller61may cause the image sensor74to obtain the detection wavelength λ (m) of the spectroscope70.

In step S606, the wavelength controller61may determine whether or not the number m indicating the number of measurement points having been obtained is m0 or more that is the number of the measurement points having to be obtained. When the number m of the measurement points is m0 or more that is the number of the measurement points having to be obtained, the wavelength controller61may move the step to step S607. On the other hand, when the number m of the measurement points is less than m0, the wavelength controller61may move the step back to the step S602, and continue to obtain the measurement points for the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal.

In the step S607, the wavelength controller61may obtain an approximate curve indicating the relationship between the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal, based on the obtained measurement points for the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal. In order to obtain the approximate curve, the wavelength controller61may apply the least squares method to the obtained measurement points for the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal.

FIG. 7is a drawing illustrating calculation of an approximate curve in the wavelength difference measurement subroutine.

The graph shown inFIG. 7has the horizontal axis representing the detection wavelength λ (m) of the spectroscope70, and the vertical axis representing the intensity I (m) of the optogalvanic signal. The measurement points obtained for the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal may be plotted in the graph as illustrated inFIG. 7. By applying the least squares method to the measurement points plotted in the graph, the wavelength controller61may obtain the approximate curve indicating the relationship between the detection wavelength λ (m) of the spectroscope70and the intensity I (m) of the optogalvanic signal. The approximate curve may be a curve having the peak of the intensity I(m) of the optogalvanic signal with respect to the detection wavelength λ (m) of the spectroscope70.

In step S608, the wavelength controller61may detect the peak of the approximate curve indicating the relationship between the detection wavelength λ (m) of the spectroscope70and the intensity I(m) of the optogalvanic signal, and calculate the detection wavelength λs of the spectroscope70corresponding to the peak of the approximate curve.

In step S609, the wavelength controller61may calculate the difference Δλabs between the detection wavelength λs of the spectroscope70corresponding to the peak of the approximate curve and the ideal value λabs of the absolute wavelength of the laser beam according to an equation Δλabs=λabs−λs.

FIG. 8is a flowchart illustrating the wavelength control subroutine in the process executed by the wavelength controller according to related art.

In step S801, the wavelength controller61may cause the laser controller62to read target wavelength λt sent from the exposure device controller92.

In step S802, the wavelength controller61may determine whether or not a signal from the optical sensor40has been detected. The signal from the optical sensor40may represent that the laser beam has oscillated. When determining that the signal from the optical sensor40has been detected, the wavelength controller61may move the step to step S803. On the other hand, when determining that any signal from the optical sensor40has not been detected, the wavelength controller61may repeat the step S802until a signal from the optical sensor40is detected.

In step S803, the wavelength controller61may cause the image sensor74to detect relative wavelength δλ with respect to the reference wavelength λc of the laser beam set for the spectroscope70. When the spectroscope70is an etalon spectroscope, the relative wavelength δλ may be calculated according to an equation δλ=α·r2, where r may represent the diameter of the interference pattern generated by the etalon spectroscope, and a may represent a proportional constant.

In step S804, the wavelength controller61may calculate wavelength λ of the laser beam based on the reference wavelength λc of the laser beam set for the spectroscope70and the relative wavelength δλ detected by the spectroscope70, according to an equation λ=λc+δλ.

In step S805, the wavelength controller61may calculate the difference Δλt between the calculated wavelength λ of the laser beam and the target wavelength λt, according to an equation Δλt=λ−λt.

In step S806, the wavelength controller61may cause the first driver63to rotate at least one of the plurality of prisms51of the line narrowing module50, so as to change the wavelength λ of the laser beam detected by the spectroscope70by the value of the calculated Δλt.

2.2 Problems with Related Art

FIG. 9is a drawing illustrating problems with calibration of the absolute wavelength of the laser beam according to related art.

In the spectroscope70calculating the wavelength λ of the laser beam based on the reference wavelength λc and the detected relative wavelength δλ of the laser beam, adrift in the reference wavelength λc may occur during a predetermined period of time. The drift in the reference wavelength λc of the laser beam may be about 100 fm for ten days. The drift in the reference wavelength λc of the laser beam may occur due to a change in the pressure or temperature in the spectroscope70, and fluctuations in the property of the etalon over time. For example, as illustrated inFIG. 9, even if the reference wavelength λc of the laser beam is initially calibrated to the ideal wavelength λabs of the laser beam, the wavelength λ of the laser beam may fluctuate over time due to the drift in the reference wavelength λc of the laser beam.

As illustrated inFIG. 9, after controlling the wavelength of the laser beam by means of the spectroscope70until the time T1 after the previous measurement of the absolute wavelength of the laser beam reaches the absolute wavelength measurement cycle K1, the wavelength controller61may measure the absolute wavelength of the laser beam and calibrate the reference wavelength λc of the laser beam. Alternately, the wavelength controller61may calibrate the reference wavelength λc of the laser beam by adding the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam to the reference wavelength λc of the laser beam.

When the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam is added to the reference wavelength λc of the laser beam, the wavelength λ of the laser beam is changed by Δλabs. When the change Δλabs in the wavelength λ of the laser beam is not so small for the exposure device91, it may affect the adjustment of the wavelength λ of the laser beam used in the exposure device91.

3. Embodiments of the Method of Controlling the Wavelength of a Laser Beam

FIG. 10is a drawing illustrating the method of controlling the wavelength of a laser beam according to Embodiment 1.

As illustrated inFIG. 10, the time interval until the first measurement, the interval between the first and second measurements, . . . the interval between the previous and n-th measurements of the absolute wavelength of the laser beam may be calculated as ΔT(1), ΔT(2), . . . , ΔT (n), respectively. The reference wavelength λc of the laser beam set for the spectroscope70may be initially calibrated to the ideal value λabs of the wavelength of the laser beam, and the differences between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements may be calculated as Δλabs(1), Δλabs(2), . . . , Δλabs(n), respectively.

The reference wavelength λc of the laser beam may be adjusted based on the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam, at a time interval shorter than time interval ΔT(n) for which the absolute wavelength of the laser beam is measured. The reference wavelength λc of the laser beam may be adjusted after the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval for which the absolute wavelength of the laser beam is measured, that is, may be adjusted for ΔT(2), . . . , ΔT (n).

The reference wavelength λc of the laser beam may be adjusted with a gradient for the time interval, which is substantially the same as the drift gradient of the reference wavelength λc of the laser beam for the time interval for which the absolute wavelength of the laser beam is measured. The gradient for the time interval to adjust the reference wavelength λc of the laser beam may be referred to as a control gradient.

The reference wavelength λc of the laser beam may be adjusted with the control gradient smaller than the drift gradient of the reference wavelength λc of the laser beam.

The amount of the variation in the drift of the reference wavelength λc of the laser beam may be substantially the same as the difference (Δλabs(1), Δλabs(2), . . . , Δλabs(n)) between the reference wavelength λc and the absolute wavelength of the laser beam.

Since the reference wavelength λc of the laser beam is adjusted with the control gradient equal to or smaller than the drift gradient of the reference wavelength λc of the laser beam, it may be possible to reduce the variation in Δλabs(n) of the reference wavelength λc of the laser beam. Since the variation in Δλabs(n) of the reference wavelength λc of the laser beam is reduced, it may be possible to reduce the variation in Δλabs(n) of the wavelength λ of the laser beam. Accordingly, it may be possible to reduce the effect on the adjustment of the wavelength λ of the laser beam used in the exposure device91.

FIG. 11is a flowchart illustrating the method of controlling the wavelength of a laser beam according to Embodiment 1.

In step S1101, the wavelength controller61may read initial parameters regarding a process performed by the wavelength controller61. Similarly to the step S401inFIG. 4, the initial parameters may include the initial reference wavelength λc0 of the laser beam set for the spectroscope70, and the absolute wavelength measurement cycle K1. The initial parameters may additionally include a period K2 for which the reference wavelength is controlled (“hereinafter a reference wavelength control period K2), an initial value n of the number of times at which the absolute wavelength is measured, and an initial value of the drift gradient G(0). The reference wavelength control period K2 may be within a range from the absolute wavelength measurement cycle K1/1000 or more to the absolute wavelength measurement cycle K1/100 or less. The initial value n of the number of times at which the absolute wavelength is measured may be set to zero. The initial value of the drift gradient G(0) may be set to zero.

Step S1102may be the same as the step S402ofFIG. 4.

Step S1103may be the same as the step S403ofFIG. 4.

Step S1104may be the same as the step S404ofFIG. 4.

In step S1105, the wavelength controller61may reset a timer that measures the period of time for which the reference wavelength λc of the laser beam set for the spectroscope70is controlled, and start measuring time T2 after the previous control of the reference wavelength λc of the laser beam.

Step S1106may be the same as the step S405ofFIG. 4.

In step S1107, the wavelength controller61may determine whether or not the time T2 after the previous control of the reference wavelength λc of the laser beam is equal to or longer than the reference wavelength control period K2. The reference wavelength control period K2 may be, for example, within a range from about one day or more to about ten days or less. When T2 is shorter than K2 (T2<K2), the wavelength controller61may move the step back to the step S1106, and repeat the wavelength control subroutine to continue to control the wavelength λ of the laser beam. On the other hand, when T2 is equal to or longer than K2 (T2≧K2), the wavelength controller61may stop executing the wavelength control subroutine, and move the step to step S1108.

In the step S1108, the wavelength controller61may control the reference wavelength λc of the laser beam. The wavelength controller61may add a value obtained by multiplying the drift gradient G(n) by the reference wavelength control period K2, to the reference wavelength λc of the laser beam.

Step S1109may be the same as the step S406ofFIG. 4.

Step S1110may be the same as the step S407ofFIG. 4.

In step S1111, the wavelength controller61may add one to the number of times n at which the absolute wavelength is measured.

Step S1112may be the same as the step S408ofFIG. 4. The wavelength controller61may calculate the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam.

In step S1113, the wavelength controller61may substitute the value of T1 for the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured, and also substitute Δλabs for the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam. ΔT(n) may be substantially constant with respect to n. Δλabs(n) may be substantially constant with respect to n.

In step S1114, the wavelength controller61may calculate the drift gradient G(n). The drift gradient G(n) may be defined by the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured, and the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam. The drift gradient G(n) may be defined by an expression Δλabs(n)/ΔT(n). G(n) may be substantially constant with respect to n.

Step S1115may be the same as the step S410ofFIG. 4.

According to Embodiment 1, since the step S1108and so forth are provided instead of the step S409ofFIG. 4, it may be possible to reduce the variation in the reference wavelength λc of the laser beam. Δλabs is not added to the reference wavelength λc of the laser beam, but a value being approximately the same as Δλabs×(K2/K1) is added to the reference wavelength λc of the laser beam. By this means, it may be possible to reduce the variation in the reference wavelength λc of the laser beam from Δλabs to Δλabs×(K2/K1).

FIG. 12is a drawing illustrating the method of controlling the wavelength of a laser beam according to Embodiment 2.

As illustrated inFIG. 12, the time interval until the first measurement, the interval between the first and second measurements, the interval between the previous and n-th measurements of the absolute wavelength of the laser beam may be calculated as ΔT(1), ΔT(2), . . . , ΔT(n), respectively. The reference wavelength λc of the laser beam set for the spectroscope70may be initially calibrated to the ideal value λabs of the wavelength of the laser beam, and the differences between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements may be calculated as Δλabs(1), Δλabs(2), . . . , Δλabs(n), respectively.

The reference wavelength λc of the laser beam may be adjusted based on the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam, at a time interval shorter than the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured. The reference wavelength λc of the laser beam may be adjusted after the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval for which the absolute wavelength of the laser beam is measured, that is, may be adjusted for ΔT(2), . . . , ΔT(n).

The reference wavelength λc of the laser beam may be adjusted with the control gradient greater than the drift gradient of the reference wavelength λc of the laser beam for the time interval for which the absolute wavelength of the laser beam is measured. The reference wavelength λc of the laser beam may be adjusted with the control gradient that is an integral multiple of the drift gradient of the reference wavelength λc of the laser beam.

The amount of the variation in the drift of the reference wavelength λc of the laser beam may be substantially the same as the difference (Δλabs(1), Δλabs(2), . . . , Δλabs(n)) between the reference wavelength λc and the absolute wavelength of the laser beam.

The reference wavelength λc of the laser beam may be adjusted with the control gradient smaller than the drift gradient of the reference wavelength λc of the laser beam that affects the adjustment of the wavelength of the laser beam in the exposure device91.

The reference wavelength λc of the laser beam is adjusted with the control gradient that is greater than the drift gradient of the reference wavelength λc of the laser beam but does not affect the adjustment of the wavelength of the laser beam in the exposure device91. Therefore, it may be possible to reduce the variation in Δλabs(n) of the reference wavelength λc of the laser beam. Since the variation in Δλabs(n) of the reference wavelength λc of the laser beam is reduced, it may be possible to reduce the variation in Δλabs(n) of the wavelength λ of the laser beam. Accordingly, it may be possible to reduce the effect on the adjustment of the wavelength λ of the laser beam used in the exposure device91.

The reference wavelength λc of the laser beam may be adjusted at a time interval shorter than the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured, depending on the control gradient. When the reference wavelength λc of the laser beam is adjusted with the control gradient that is an integral multiple of the drift gradient of the reference wavelength λc of the laser beam, the adjustment may be performed at a time interval obtained by dividing the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured by the integral multiple.

FIG. 13is a flowchart illustrating the method of controlling the wavelength of a laser beam according to Embodiment 2.

In step S1301, the wavelength controller61may read initial parameters regarding a process performed by the wavelength controller61. The initial parameters may include the same initial parameters as in the step S1101ofFIG. 11. The initial parameters may additionally include a ratio h of the control gradient to the drift gradient. The ratio h of the control gradient to the drift gradient may be a value within a range from 1 or more to 5 or less. When the ratio h of the control gradient to the drift gradient is 1, Embodiment 2 may be the same as Embodiment 1. The ratio h of the control gradient to the drift gradient may be an integer. The ratio h of the control gradient to the drift gradient may be a value that does not affect the adjustment of the wavelength of the laser beam in the exposure device91.

Step S1302may be the same as the step S1102ofFIG. 11.

Step S1303may be the same as the step S1103ofFIG. 11.

Step S1304may be the same as the step S1104ofFIG. 11.

Step S1305may be the same as the step S1105ofFIG. 11.

Step S1306may be the same as the step S1106ofFIG. 11.

Step S1307may be the same as the step S1107ofFIG. 11.

In step S1308, the wavelength controller61may control the reference wavelength λc of the laser beam set for the spectroscope70. The wavelength controller61may add a value obtained by multiplying the drift gradient G(n) by the ratio h of the control gradient to the drift gradient and the reference wavelength control period K2, to the reference wavelength λc of the laser beam.

In step S1309, the wavelength controller61may determine whether or not the time T1 after the previous measurement of the absolute wavelength of the laser beam is equal to or greater than a period obtained by dividing the absolute wavelength measurement cycle K1 by the ratio h of the control gradient to the drift gradient. When T1 is shorter than K1/h (T1<K1/h), the wavelength controller61may move the step back to the step S1305, and repeat the wavelength control subroutine to continue to control the wavelength λ of the laser beam. On the other hand, when T1 is equal to or longer than K1/h (T1≧K1/h), the wavelength controller61may stop executing the wavelength control subroutine, and move the step to step S1311.

Step S1310may be the same as the step S1110ofFIG. 11.

Step S1311may be the same as the step S1111ofFIG. 11.

Step S1312may be the same as the step S1112ofFIG. 11.

Step S1313may be the same as the step S1113ofFIG. 11.

Step S1314may be the same as the step S1114ofFIG. 11.

Step S1315may be the same as the step S1115ofFIG. 11.

According to Embodiment 2, the step1308and the step S1309are provided instead of the step S1108and the step1109ofFIG. 11. Therefore, it may be possible to reduce the variation in the reference wavelength λc of the laser beam. Δλabs is not added to the reference wavelength λc of the laser beam, but a value being approximately the same as Δλabs×(K2/K1)×h is added to the reference wavelength λc of the laser beam. By this means, it may be possible to reduce the variation in the reference wavelength λc of the laser beam from Δλabs to Δλabs×(K2/K1)×h. The value being approximately the same as Δλabs×(K2/K1)×h is added to the reference wavelength λc of the laser beam at the time interval K1/h, and therefore it may be possible to prevent the reference wavelength λc of the laser beam from being excessively adjusted.

FIG. 14is a drawing illustrating the method of controlling the wavelength of a laser beam according to Embodiment 3.

As illustrated inFIG. 14, the time interval until the first measurement, the interval between the first and second measurements, the interval between the previous and n-th measurements of the absolute wavelength of the laser beam may be calculated as ΔT(1), ΔT(2), . . . , ΔT(n), respectively. The reference wavelength λc of the laser beam set for the spectroscope70may be initially calibrated to the ideal value λabs of the wavelength of the laser beam, and the differences between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements may be calculated as Δλabs(1), Δλabs(2), . . . , Δλabs(n), respectively.

The reference wavelength λc of the laser beam may be adjusted based on the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam and the initial value of the drift gradient of the reference wavelength λc of the laser beam, at the time interval ΔT(n) shorter than the time interval for which the absolute wavelength of the laser beam is measured. The value of the drift gradient of the reference wavelength λc of the laser beam may correspond to the gradient of the net drift of the reference wavelength λc of the laser beam for the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured.

The reference wavelength λc of the laser beam may be adjusted with the initial value of the drift gradient (a predetermined gradient) of the reference wavelength λc of the laser beam, for the time interval ΔT(1) for which the absolute wavelength of the laser beam is measured until the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured.

After the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured, the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam may be corrected based on the initial value of the drift gradient of the reference wavelength λc of the laser beam. The correction values of the differences Δλabs(1), Δλabs(2), . . . , Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements, may be calculated as D(1), D(2), . . . , D(n), respectively. The correction values of the differences Δλabs(1), Δλabs(2), Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements, may depend on the initial value of the drift gradient of the reference wavelength λc of the laser beam.

After the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured, the reference wavelength λc of the laser beam may be adjusted based on the values G(1), G(2), . . . , G(n) of the drift gradient of the reference wavelength λc of the laser beam.

According to Embodiment 3 compared to Embodiment 1 or Embodiment 2, the reference wavelength λc of the laser beam may be adjusted as well until the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured.

According to Embodiment 3, compared to Embodiment 1 or Embodiment 2, it may be possible to reduce the differences Δλabs(1), Δλabs(2), . . . , Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements, respectively.

According to Embodiment 3, compared to Embodiment 1 or Embodiment 2, it may be possible to reduce the variation in Δλabs(n) of the reference wavelength λc of the laser beam. Since the variation in Δλabs(n) of the reference wavelength λc of the laser beam is reduced, it may be possible to reduce the variation in Δλabs(n) of the wavelength λ of the laser beam. Accordingly, it may be possible to reduce the effect on the adjustment of the wavelength λ of the laser beam used in the exposure device91.

FIG. 15is a flowchart illustrating the method of controlling the wavelength of a laser beam according to Embodiment 3.

In step S1501, the wavelength controller61may read initial parameters for a process performed by the wavelength controller61. The initial parameters may include the same parameters as in the step S1101ofFIG. 11. The initial parameters may additionally include the initial value of the time interval ΔT(0) for which the absolute wavelength of the laser beam is measured. In the step S1101ofFIG. 11, the initial value of the drift gradient G(0) may be set to zero. Here, in step S1501, the initial value of the drift gradient G(0) may be set to a value other than zero. Similarly, the initial value of the time interval ΔT(0) for which the absolute wavelength of the laser beam is measured be set to a value other than zero.

Step S1502may be the same as the step S1102ofFIG. 11.

Step S1503may be the same as the step S1103ofFIG. 11.

Step S1504may be the same as the step S1104ofFIG. 11.

Step S1505may be the same as the step S1105ofFIG. 11.

Step S1506may be the same as the step S1106ofFIG. 11.

Step S1507may be the same as the step S1107ofFIG. 11.

Step S1508may be the same as the step S1108ofFIG. 11.

Step S1509may be the same as the step S1109ofFIG. 11.

Step S1510may be the same as the step S1110ofFIG. 11.

In step S1511, the wavelength controller61may calculate amount of the correction of the drift D (n) by multiplying the drift gradient G(n) by the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured. The drift gradient G(n) and the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured will be defined later.

Step S1512may be the same as the step S1111ofFIG. 11.

Step S1513may be the same as the step S1112ofFIG. 11.

Step S1514may be the same as the step S1113ofFIG. 11.

In step S1515, the wavelength controller61may calculate the actual amount of the drift D by adding the amount of the correction of the drift D(n−1) that is a value obtained by taking into account that one has been added to n in the step S1512, to the λabs(n) obtained in the step S1514.

In step S1516, the waveform controller61may calculate drift gradient ΔG(n) for adjusting the reference wavelength λc of the laser beam based on the actual amount of the drift obtained in the step S1515, according to an equation ΔG(n)=D/ΔT(n).

Step S1517may be the same as the step S1115ofFIG. 11.

According to Embodiment 3, the initial value of the drift gradient G(0)=G0 may be used in the step S1501, instead of the initial value of the drift gradient G(0)=0 shown inFIG. 11. The reference wavelength λc of the laser beam may be adjusted as well until the absolute wavelength of the laser beam is first measured since the start of the measurement of the time interval ΔT(n) for which the absolute wavelength of the laser beam is measured. It may be possible to reduce the differences Δλabs(1), Δλabs(2), . . . , Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements, respectively.

FIG. 16is a drawing illustrating the method of controlling the wavelength of a laser beam according to Embodiment 4.

As illustrated inFIG. 16, the drift gradient G(n) may fluctuate over time. The wavelength controller61may calculate a prediction point of the drift gradient, from a plurality of drift gradient measurement points, based on the measured value of the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam. An approximate curve for the plurality of drift gradient measurement points may be calculated by the least squares method. The prediction point of the drift gradient may be calculated based on the approximate curve by the extrapolation method. The wavelength controller61may obtain not only the last measured value of the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam, but also the prediction point of the drift gradient after the measurement, from the plurality of drift gradient measurement points. It may be possible to improve the accuracy of the value of the drift gradient, and therefore to improve the control of the reference wavelength λc of the laser beam.

FIG. 17is a flowchart illustrating the method of controlling the wavelength of a laser beam according to Embodiment 4.

Step S1701may be the same as the step S1101ofFIG. 11.

Step S1702may be the same as the step S1102ofFIG. 11.

Step S1703may be the same as the step S1103ofFIG. 11.

In step S1704, the waveform controller61may reset the timer that measures the period of time since the start of the measurement of the time interval for which the absolute wavelength of the laser beam is measured, and start measuring time T3 since the start of the measurement of the time interval for which the absolute wavelength of the laser beam is measured.

Step S1705may be the same as the step S1104ofFIG. 11.

Step S1706may be the same as the step S1105ofFIG. 11.

Step S1707may be the same as the step S1106ofFIG. 11.

Step S1708may be the same as the step S1107ofFIG. 11.

Step S1709may be the same as the step S1108ofFIG. 11.

Step S1710may be the same as the step S1109ofFIG. 11.

Step S1711may be the same as the step S1110ofFIG. 11.

Step S1712may be the same as the step S1111ofFIG. 11.

Step S1713may be the same as the step S1112ofFIG. 11.

Step S1714may be the same as the step S1113ofFIG. 11.

Step S1715may be the same as the step S1114ofFIG. 11.

In step1716, the wavelength controller61may execute a subroutine for calculating a predicted value of the drift gradient G(n) (hereinafter “drift gradient predicting subroutine”), based on the plurality of values of the drift gradient G(n) calculated in the step S1715.

Step S1717may be the same as the step S1115ofFIG. 11.

According to Embodiment 4, the predicted value of the drift gradient G(n) obtained based on the plurality of values of the drift gradient G(n) may be used, instead of the drift gradient G(n) calculated in the step S1715. It may be possible to improve the accuracy of the value of the drift gradient, and therefore to improve the control of the reference wavelength λc of the laser beam.

FIG. 18is a flowchart illustrating the drift gradient predicting subroutine in the method of controlling the wavelength of a laser beam according to Embodiment 4.

In step S1801, the wavelength controller61may read the number of pieces of data L having to be used to predict the drift gradient G(n). Here, L may be an integer equal to or more than three.

In step S1802, the wavelength controller61may determine whether or not the number of pieces of data n of the drift gradient G(n) is equal to or greater than the number of pieces of data L having to be used to predict the drift gradient G(n). When the number of pieces of data n of the drift gradient G(n) is equal to or greater than the number of pieces of data L having to be used to predict the drift gradient G(n) (n≧L), the wavelength controller61may move the step to step S1803. On the other hand, when the number of pieces of data n of the drift gradient G(n) is smaller than the number of pieces of data L having to be used to predict the drift gradient G(n) (n<L), the wavelength controller61may end the drift gradient predicting subroutine.

In the step S1803, the wavelength controller61may read n pieces of data of the drift gradients G(1) G(n).

In step S1804, the wavelength controller61may obtain a function G(T) of the approximate curve of the n pieces of data of the drift gradients G (1) G (n) by the least squares method.

In step S1805, the wavelength controller61may calculate a predicted value of the drift gradient G (T3+K1) during the period of time since the start of the measurement of the time interval for which the absolute wavelength of the laser beam is measured, by using the function G(T) of the approximate curve.

FIG. 19is a drawing illustrating the method of controlling the wavelength of a laser beam according to Embodiment 5.

As illustrated inFIG. 19, the number of pulses until the first measurement, the number of pulses between the first and second measurements, the number of pulses between the previous and n-th measurements of the absolute wavelength of the laser beam may be calculated as ΔP(1), ΔP(2), . . . , ΔP(n), respectively. The reference wavelength λc of the laser beam may be initially calibrated to the ideal value λabs of the wavelength of the laser beam, and the differences between the reference wavelength λc set for the spectroscope70and the absolute wavelength of the laser beam at the first, second, . . . , n-th measurements may be calculated as Δλabs(1), Δλabs(2), Δλabs(n), respectively.

The reference wavelength λc of the laser beam may be adjusted based on the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam, at a time interval shorter than the number of pulses ΔP(n) for which the absolute wavelength of the laser beam is measured. The reference wavelength λc of the laser beam may be adjusted after the absolute wavelength of the laser beam is first measured since the start of the measurement of the number of pulses for which the absolute wavelength of the laser beam is measured, that is, may be adjusted for ΔP (2), . . . , ΔP(n).

The reference wavelength λc of the laser beam may be adjusted with the gradient for the number of pulses being substantially the same as the drift gradient of the reference wavelength λc of the laser beam for the number of pulses for which the absolute wavelength of the laser beam is measured. The gradient for the number of pulses to adjust the reference wavelength λc of the laser beam may be referred to as the control gradient.

The reference wavelength λc of the laser beam may be adjusted with the control gradient smaller than the drift gradient of the reference wavelength λc of the laser beam.

The amount of the variation in the drift of the reference wavelength λc of the laser beam may be substantially the same as the difference (Δλabs(1), Δλabs(2), . . . , Δλabs(n)) between the reference wavelength λc and the absolute wavelength of the laser beam.

Since the reference wavelength λc of the laser beam is adjusted with the control gradient equal to or smaller than the drift gradient of the reference wavelength λc of the laser beam, it may be possible to reduce the variation in Δλabs(n) of the reference wavelength λc of the laser beam. Since the variation in Δλabs(n) of the reference wavelength λc of the laser beam is reduced, it may be possible to reduce the variation in Δλabs(n) of the wavelength λ of the laser beam. Accordingly, it may be possible to reduce the effect on the adjustment of the wavelength λ of the laser beam used in the exposure device91.

FIG. 20is a flowchart illustrating the method of controlling the wavelength of a laser beam according to Embodiment 5.

In step S2001, the wavelength controller61may read initial parameters for a process performed by the wavelength controller61. Similarly to the step S401ofFIG. 4, the initial parameters may include the initial reference wavelength λc0 of the laser beam set for the spectroscope70. The initial parameters may additionally include cycle C1 for which the absolute wavelength of the laser beam is measured (the number of pulses for which the absolute wavelength is measured), the number of pulses C2 for which the reference wavelength is controlled, the initial value n of the number of times at which the absolute wavelength is measured, and the initial value of the drift gradient G(0). The number of pulses C2 for which the reference wavelength is controlled may be within a range from one-thousandth or more of the number of pulses C1 for which the absolute wavelength is measured to one-hundredth or less of the number of pulses C1 for which the absolute wavelength is measured. The initial value n of the number of times at which the absolute wavelength is measured may be set to zero. The initial value of the drift gradient G (0) may be set to zero.

Step S2002may be the same as the step S402ofFIG. 4.

Step S2003may be the same as the step S403ofFIG. 4.

In step S2004, the wavelength controller61may reset a pulse counter that measures the number of pulses for which the absolute wavelength of the laser beam is controlled, and start measuring the number of pulses P1 after the previous measurement of the absolute wavelength of the laser beam.

In step S2005, the wavelength controller61may reset a pulse counter that measures the period of time for which the reference wavelength λc of the laser beam is controlled, and start measuring the number of pulses P2 after the previous control of the reference wavelength λc of the laser beam.

Step S2006may be the same as the step S405ofFIG. 4.

In step S2007, the wavelength controller61may determine whether or not the number of pulses P2 after the previous control of the reference wavelength λc of the laser beam is equal to or greater than the number of pulses C2 for which the reference wavelength is controlled. When P2 is smaller than C2 (P2<C2), the wavelength controller61may move the step back to the step S2006, and repeat the wavelength subroutine to continue to control the wavelength λ of the laser beam. On the other hand, when P2 is equal to or greater than C2 (P2≧C2), the wavelength controller61may stop executing the wavelength control subroutine, and move the step to step S2008.

In the step S2008, the wavelength controller61may control the reference wavelength λc of the laser beam. The wavelength controller61may add a value obtained by multiplying the drift gradient G(n) defined later by the number of pulses C2 for which the reference wavelength is controlled, to the reference wavelength λc of the laser beam.

In step S2009, the wavelength controller61may determine whether or not the number of pulses P1 after the previous measurement of the absolute wavelength of the laser beam is equal to or greater than the number of pulses C1 for which the absolute wavelength is measured. When P1 is smaller than C1 (P1<C1), the wavelength controller61may move the step back to the step S2005, and repeat the wavelength control subroutine to continue to control the wavelength λ of the laser beam. On the other hand, when P1 is equal to or greater than C1 (P1≧C1), the wavelength controller61may stop executing the wavelength control subroutine, and move the step to step S2010.

The step S2010may be the same as the step S407ofFIG. 4.

In step S2011, the wavelength controller61may add one to the number of times n at which the absolute wavelength is measured.

Step S2012may be the same as the step S408ofFIG. 4. The wavelength controller61may calculate the difference Δλabs between the reference wavelength λc and the absolute wavelength of the laser beam.

In step S2013, the wavelength controller61may substitute the value of P1 for the number of pulses ΔP(n) for which the absolute wavelength of the laser beam is measured, and also substitute Δλabs for the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam. AP(n) may be substantially constant with respect to n. Δλabs(n) may be substantially constant with respect to n.

In step S2014, the wavelength controller61may calculate the drift gradient G(n). The drift gradient G(n) may be defined by the number of pulses ΔP(n) for which the absolute wavelength of the laser beam is measured, and the difference Δλabs(n) between the reference wavelength λc and the absolute wavelength of the laser beam. The drift gradient G(n) may be defined by an expression Δλabs(n)/AP(n). G(n) may be substantially constant with respect to n.

Step S2015may be the same as the step S410ofFIG. 4.

According to Embodiment 5, the step S2008and so forth are provided, instead of the step S409ofFIG. 4, and therefore it may be possible to reduce the variation in the reference wavelength λc of the laser beam. Δλabs is not added to the reference wavelength λc of the laser beam, but a value being approximately the same as Δλabs×(C2/C1) is added to the reference wavelength λc of the laser beam. By this means, it may be possible to reduce the variation in the reference wavelength λc of the laser beam from Δλabs to Δλabs×(C2/C1).

FIG. 21is a flowchart illustrating the wavelength control subroutine in the method of controlling the wavelength of a laser beam according to Embodiment 5.

Step S2101may be the same as the step S801ofFIG. 8.

Step S2102may be the same as the step S802ofFIG. 8.

When laser oscillation occurs in the step S2102, in step S2103, the wavelength controller61may add one to the number of pulses P1 after the previous measurement of the absolute wavelength of the laser beam, and also add one to the number of pulses P2 after the previous control of the reference wavelength λc of the laser beam.

Step S2104may be the same as the step S803ofFIG. 8.

Step S2105may be the same as the step S804ofFIG. 8.

Step S2106may be the same as the step S805ofFIG. 8.

Step S2107may be the same as the step S806ofFIG. 8.

The wavelength controller61may increment the number of pulses P1 after the previous measurement of the absolute wavelength of the laser beam and the number of pulses P2 after the previous control of the reference wavelength λc of the laser beam.

In the flowcharts illustrated inFIGS. 13, 15 and 17, the timer may be substituted for a pulse counter. K1, K2, and K3 may be substituted for C1 and C2.

The above-described embodiments and the modifications thereof are merely examples for implementing the present disclosure, and the present disclosure is not limited thereto. Making various modifications according to the specifications or the like falls within the scope of the present disclosure, and it is apparent from the above description that other various embodiments are possible within the scope of the present disclosure.

The terms used in this specification and the appended claims should be interpreted as “non-limiting.” For example, the terms “include” and “be included” should be interpreted as “including the stated elements but not limited to the stated elements.” The term “have” should be interpreted as “having the stated elements but not limited to the stated elements.” Further, the modifier “one (a/an)” should be interpreted as “at least one” or “one or more.”

REFERENCE SIGNS LIST