1. Field of the Invention
The present invention relates to a gas laser device, and in particular to a high output excimer laser device for exposure.
2. Description of the Related Art
A demand for improving the resolution of exposure devices for semiconductor substrates has arisen along with the recent advancement of refinement and integration of semiconductor integrated circuits. In order to meet this demand, studies have been made to shorten the wavelength of a laser beam emitted from an exposure light source. One type of typically used semiconductor exposure light source is gas laser devices which emit light with a shorter wavelength than that of conventional mercury lamps. As gas laser devices for exposure, KrF excimer laser devices emitting ultraviolet light having a wavelength of 248 nm and ArF excimer laser devices emitting ultraviolet light having a wavelength of 193 nm are typically used.
In recent years, there has been a trend of increased demand for excimer laser devices for exposure having still higher output as the wavelength of exposure light is decreased. A main reason for the trend lies in the fact that the resist used for exposure with ArF excimer lasers has lower sensitivity than the resist used for exposure with KrF excimer lasers. A demand for increased throughput is another reason.
One type of device outputting high output laser light is an excimer laser device performing both oscillation and amplification in a single laser chamber. Such an excimer laser device will be referred to as a “single-chamber excimer laser device”.
On the other hand, amplification may be performed by a two-stage laser system as shown in
The two-stage laser system is composed of an oscillation stage laser 10 for outputting narrow-band laser light, and an amplification stage laser 20 for amplifying the narrow-band laser beam (referred to as “seed light”).
Practical two-stage laser systems used are classified into two types, MOPO and MOPA systems, according to the difference in means for amplifying the laser light. A MOPO laser system includes an amplification stage laser having a resonator. The term “MOPO” is an abbreviation for master oscillator/power oscillator and is also referred to as “injection lock (IL) system”. On the other hand, a MOPA laser system includes an amplifier having no resonator. The term “MOPA” is an abbreviation for master oscillator/power amplifier. Particulars of the two-stage system will be described later.
Increase of the output of an excimer laser device can be realized, for example, by increasing the output energy per pulse if the oscillatory frequency is fixed. However, the increase of output energy per pulse will incur problems as described below.
A resonator is typically comprised of an output-side mirror for emitting laser light and a rear-side mirror having a high reflectance. The output-side mirror has a PR film (partial reflection mirror coating) with a reflectance of several tens of percent attached to one side, and an AR film (anti-reflection coating) attached to the other side. A rear-side mirror in a single laser chamber has a HR film (high reflection mirror coating) attached thereto, and a rear-side mirror in an amplification stage laser has a PR film with a high reflectance of about 80˜90% attached thereto. Laser light output from the amplification stage laser and reaching the output-side mirror is output only partially. Therefore, the laser energy inside the resonator is several times higher than the energy output to the outside.
In the case of the MOPO system, for example, a laser beam output from the oscillation stage laser exhibits an energy density of several mJ/cm2. However, since the laser energy is amplified in the amplification stage laser, the laser beam inside the resonator exhibits a high energy density of several tens of mJ/cm2 or higher. As a result, a laser beam with a high energy density passes through a chamber window of the amplification stage laser, and hence the amount of laser light absorbed by the surface and the inside of the window is increased, causing the window to generate heat. If this heat generation causes thermal stress in the window, a window formed from CaF2, for example, will be deteriorated. When the deterioration of the window progresses to a certain degree, the window cannot be used as an optical element any more, coming to the end of the window's lifetime.
As seen from
In contrast, when output energy per pulse is 15 mJ under a novel condition, the average energy density applied to the window is 42.3 mJ/cm2, and the peak energy density 114.2 mJ/cm2. The window comes to the end of its lifetime after one Bpls. The laser beam width is 0.33 cm.
Also as for the output-side mirror, the mirror has a lifetime of 24.1 Bpls or more under the conventional condition, whereas, under the novel condition, the lifetime of the mirror is only one Bpls.
According to the comparison results described above, when the output energy is increased by about 25% compared to that of the conventional condition when the laser beam width is fixed, the lifetime of the window is decreased to one fourteenth of that of the conventional condition. It is believed that this is because, since the output laser beam width is fixed, the peak energy density or the average energy density applied to the window is increased as the output energy is increased, and the deterioration of the window rapidly progresses when the density exceeds a threshold, resulting in drastic decrease of the lifetime of the window.
As described above, the output energy is increased in the amplification stage laser of the MOPO laser system, and the lifetime of the window provided in the amplification stage laser chamber or of the output-side mirror is drastically decreased once the peak energy density or average energy density in the resonator exceeds a predetermined threshold. In principle, this also applies to the laser oscillator or laser amplifier of an excimer laser device with a single laser chamber.
Accordingly, it is envisageable to increase the cross-sectional area of a beam applied to the optical elements by enlarging the discharge electrode width for the purpose of decreasing the peak energy density.
As shown in
However, the enlargement of the discharge electrode width will induce problems as described below.
If oscillation is repeated after simply enlarging the discharge electrode width, subsequent oscillation is performed before a discharge product generated in the gain region during the previous oscillation is sufficiently shifted. This will cause arc discharge 4 to occur on the downstream side of the discharge electrodes 24 and 25, deteriorating the stability of the output energy.
In order to prevent this arc discharge, it is envisageable to increase the flow rate of laser gas in the gain region by increasing the rotation speed N of a fan incorporated in the laser chamber.
However, when the flow rate of the laser gas is made proportional to the discharge electrode width T in order to prevent the arc discharge, the fan rotation speed N is proportional to the discharge electrode width T since the fan rotation speed N is proportional to the flow rate of the laser gas. That is, N∝T.
In general, a consumption current I of the fan is proportional to the cube of a rotation speed N of the fan. Therefore, the consumption current of the fan is proportional to the cube of the discharge electrode width T. That is, I∝T3. Thus, the consumption current of the fan is rapidly increased in proportion to the cube of the discharge electrode width. Currently, the consumption current of the fan is approximately its upper limit, and it is difficult to increase the consumption current any further. Consequently, it should be avoided to increase the rotation speed of the fan by enlarging the discharge electrode width.
The present invention has been made in view of the problems described above. It is an object of the present invention to provide an excimer laser device capable of suppressing deterioration of optical elements provided in a laser chamber even if the output energy per pulse is increased.
In order to achieve the object above, a first aspect of the invention provides an excimer laser device having laser beam width enlarging means for enlarging a width of a laser beam applied to an optical element provided in a laser chamber so that an energy density of the laser beam is reduced within such a range that a laser output of no less than a desired level is obtained.
The first aspect of the invention will be described with reference to
As shown in
On the other hand, as the tilt angle θ of the discharge electrode axis 32 is increased, the distance for which the laser beam is allowed to travel in the gain region is decreased. If the tilt angle θ is too large, it is expected that the laser beam reflected back and forth within the resonator cannot be utilized effectively in the gain region.
According to experimental results, when a gain G0 and an injected light amount are high, the output energy can be held substantially constant until the tilt angle becomes a diagonal angle θ1 shown in
This means that, as shown in
According to the first aspect of the invention, therefore, on the basis of the findings shown in
According to a second aspect of the invention based on the first aspect of the invention, the excimer laser device is of a single-chamber type.
The second aspect of the invention will be described with reference to
As shown in
In the single-chamber excimer laser device as well, the gain region width W1 can be enlarged by tilting the discharge electrode axis 32 with respect to the resonator optical axis 30. In this case, the tilt angle θ is set based on the findings shown in
According to a third aspect of the invention based on the first aspect of the invention, the excimer laser device is used for an amplification stage laser of a two-stage laser device comprised of an oscillation stage laser and the amplification stage laser.
The third aspect of the invention will be described with reference to
The laser chamber 23 shown in
In this configuration as well, the gain region width W1 can be enlarged by tilting the discharge electrode axis 32 with respect to the resonator optical axis 30. In this case, the tilt angle θ is set on the basis of the findings shown in
According to a fourth aspect of the invention based on the second or third aspect of the invention, the excimer laser device includes a resonator comprised of a rear-side mirror and an output-side mirror both of which are of a flat type, a laser chamber arranged inside the resonator, and a pair or discharge electrodes facing each other inside the laser chamber, in which the laser beam width enlarging means tilts a resonator axis formed by arranging the rear-side mirror and the output-side mirror of the resonator parallel to each other and an axis of the discharge electrode axes extending parallel to the longitudinal direction, in a plane parallel to an electrode width direction of the discharge electrodes.
The fourth aspect of the invention will be described with reference to
In the fourth aspect of the invention, as shown in
According to
Since the gain region width is enlarged by L sin θ, the width of the laser beam oscillated (amplified) in the resonator is also enlarged vertically in the drawing.
According to a fifth aspect of the invention based on the third aspect of the invention, the excimer laser device includes a resonator comprised of a rear-side mirror and an output-side mirror both of which are of a flat type, a laser chamber arranged inside the resonator, and a pair of discharge electrodes facing each other inside the laser chamber, in which the laser beam width enlarging means injects seed light generated by the oscillation stage laser into the amplification stage laser chamber, while tilting the seed light with respect to a resonator optical axis formed by arranging the rear-side mirror and the output-side mirror of the resonator parallel to each other, in a plane parallel to an electrode width direction of the discharge electrodes.
The fifth aspect of the invention will be described with reference to
According to
Subsequently, in the same manner, the laser beam is repeatedly reflected at a fixed reflection angle θ within the resonator, while the laser beam is shifted vertically on the sheet surface by M tan θ every time it is reflected. This means that the laser beam width is enlarged vertically on the sheet surface.
According to a sixth aspect of the invention based on the fifth aspect of the invention, the laser beam width enlarging means further includes means for causing the injected seed light to pass through substantially an entire gain region between the discharge electrodes.
In the sixth aspect of the invention as shown in
Specifically, in order to shift the optical axis 35 of the injected seed light to the lower side in the drawing by Gm, the position of the laser light guide mirror 34 to reflect the optical axis of the seed light is changed from a position K0 to a position K1. The change of the reflection position from K0 to K1 shifts the optical axis 35 of the seed light to the lower side in the drawing by Gm.
According to a seventh aspect of the invention based on the third aspect of the invention, the laser beam width enlarging means includes means for injecting seed light generated by the oscillation stage laser into the amplification stage laser chamber while tilting the seed light with respect to an axis of the discharge electrode axes extending parallel to the longitudinal direction, in a plane parallel to an electrode width direction of the discharge electrodes; means for causing the injected seed light to pass through a substantially entire gain region between the discharge electrodes; and means for arranging one of the rear-side mirror and the output-side mirror orthogonal to an axis parallel to the longitudinal direction of the discharge electrodes, while arranging the other mirror such that laser light reflected thereby passes through the gain region.
According to an eighth aspect of the invention based on the seventh aspect of the invention, the means for arranging the other mirror such that laser light reflected thereby passes through the gain region is means for tilting the mirror with respect to the other mirror around an axis extending in a direction orthogonal to both of the optical axis of the discharge electrodes and the electrode width direction of the discharge electrodes.
The seventh and eighth aspects of the invention will be described with reference to
As shown in
According to a ninth aspect of the invention based on the third aspect of the invention, the laser beam width enlarging means is means for arranging one of the rear-side mirror and the output-side mirror orthogonal to an axis extending along the longitudinal direction of the discharge electrodes, while arranging the other mirror such that laser light reflected thereby travels away from a gain region between the discharge electrodes.
According to the tenth aspect of the invention based on the ninth aspect of the invention, the means for arranging the mirror such that laser light reflected thereby travels away from the gain region between the discharge electrodes is means for tilting the mirror with respect to the other mirror around an axis orthogonal to both of the axis of the longitudinal direction of the discharge electrodes and the axis of an electrode width direction of the discharge electrodes.
The ninth and tenth aspects of the invention will be described with reference to
As shown in
According to an eleventh aspect of the invention based on the third aspect of the invention, the laser beam width enlarging means is means for injecting seed light generated by the oscillation stage laser into the laser chamber of the amplification stage laser such that the laser beam is spread in an electrode width direction of the discharge electrodes.
In the eleventh aspect of the invention as shown in
A laser beam deviated to the upper side of the drawing reaches the output-side mirror 22 at the tilt angle θ and then is reflected thereby. On the other hand, a laser beam deviated to the lower side of the drawing reaches the output-side mirror 22 at the tilt angle θ and is then reflected thereby. Accordingly, the width of the laser beams repeatedly reflected back and forth is gradually enlarged in the vertical direction in the drawing.
According to a twelfth aspect of the invention based on the third aspect of the invention, a beam expander is provided between the laser chamber of the amplification stage laser and the output-side mirror.
In the twelfth aspect of the invention as shown in
The laser beam width enlarging means according to any of the first to eleventh aspects of the invention is able to enlarge the laser beam width. Therefore, even if the laser output per pulse is higher than that in the prior art, the energy density applied to the optical elements provided in the laser chamber can be reduced, and hence deterioration of the windows can be suppressed.
In the case of the sixth aspect of the invention, since the seed light is allowed to pass through most of the gain region, deterioration of the windows can be suppressed and the discharge energy can be utilized effectively.
In the case of the seventh and eighth aspects of the invention, deterioration of the windows can be suppressed and the laser beam reflected back and forth within the resonator can be prevented from deviating from the gain region. Therefore, the discharge energy can be utilized effectively.
According to the twelfth aspect of the invention, deterioration of the windows can be suppressed and deterioration of the output-side mirror can be suppressed at the same time.
Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
First, the basic configuration and operation of an excimer laser device according to the present invention will be generally described using a two-stage laser system. The present invention is however applicable also to an excimer laser device having a single laser chamber, as described later in the description of the embodiments.
In the following description of the embodiments, the term “the sheet surface” means a plane parallel to a width direction of discharge electrodes in the drawings. When a rear-side mirror 21 and an output-side mirror 22 are arranged parallel to each other, the direction of a resonator optical axis 30 defined by the mirrors 21 and 22 is defined as a lateral direction on the sheet surface, while a direction orthogonal to the resonator optical axis 30 on the sheet surface is defined as a vertical direction. When the mirrors 21 and 22 are not parallel to each other, the direction of an axis (discharge electrode axis) 32 parallel to the longitudinal direction of discharge electrodes 24 and 25 is defined as a lateral direction on the sheet surface, while a direction orthogonal to the discharge electrode axis 32 on the sheet surface is defined as a vertical direction. It should be noted that, in the present invention, a tilt angle is always an angle as small as a few mrad. The tilt angle is always an angle formed in a plane parallel to the discharge electrode width direction.
As shown in
The amplification stage laser 20 includes a Fabry-Perot etalon type resonator composed of a rear-side mirror 21 and an output-side mirror 22 both of which are of a flat type. A laser chamber 23 having laser gas sealed therein is arranged between the mirrors 21 and 22.
A pair of discharge electrodes 14 and 15 and another pair of discharge electrodes 24 and 25 are arranged in the respective laser chambers 13 and 23 of the oscillation stage laser 10 and the amplification stage laser 20. Windows 17, 17 and windows 27, 27 are provided on the laser optical axis of the discharge electrodes 14 and 15 and discharge electrodes 24 and 25 so as to be parallel with each other. The windows are formed from a material having permeability to laser oscillation light, such as CaF2. The windows 17, 17 and the windows 27, 27 are arranged at a Brewster angle to the laser light for decreasing the reflection loss.
The oscillation stage laser 10 includes a laser resonator comprised of a rear-side mirror in a narrowband module 11 and an output-side mirror 12. A laser chamber 13 having laser gas sealed therein is arranged between these mirrors. The narrowband module 11 is provided therein with a prism and a grating, for example, and the grating also functions as a mirror.
A laser light guide 18 is provided between the oscillation stage laser 10 and the amplification stage laser 20. The laser light guide 18 includes a plurality of laser light guide mirrors to guide seed light generated by the oscillation stage laser 10 to the amplification stage laser 20.
As shown in
In the case of the MOPO system, the laser beam has an average energy density of several mJ/cm2 in the oscillation stage laser 10, whereas the average energy density of the laser beam becomes several tens of mJ/cm2 in the amplification stage laser 20. The energy density in the laser beam is not uniform. In general, the energy density is distributed to be higher in a central part of the beam and lower in the skirts of the beam. Therefore, a peak energy density is typically several times higher than the average energy density.
When the MOPO system is a KrF excimer laser device, a laser gas composed of krypton (Kr) gas, fluorine (F2) gas, and a buffer gas such as helium (He) or neon (Ne) is sealed in the respective laser chambers 13 and 23 of the oscillation stage lasers 10 and the amplification stage laser 20. When the MOPO system is an ArF excimer laser device, a laser gas composed of argon (Ar) gas, fluorine (F2) gas, and a buffer gas such as helium (He) or neon (Ne) is sealed in the respective laser chambers 13 and 23 of the oscillation stage laser 10 and the amplification stage laser 20.
The terms used herein in relation to the laser beam and the gain will be described.
The spectrum of the laser light output from the output-side mirror has distribution such that a peak energy density is present at a central portion and the energy density is decreased from the central portion towards the skirts. The term “laser beam width” as used in the present invention is defined as a region (width) having an energy density of 5% or more of the peak energy density. The term “average energy density” is defined as an average value of the energy density distribution within the laser beam width.
A value indicating how much the laser beam is amplified while passing through a unit distance (mm) in the gain region is denoted by G0. That is, G0 denotes an amplification factor per unit distance of the gain region.
In the conventional excimer laser device as shown in
In contrast, in the excimer laser device according to the first embodiment as shown in
When a length in the longitudinal direction of the discharge electrodes 24 and 25 is denoted by L, and a gain region width as viewed from the side of the resonator optical axis 30 is denoted by W1, the gain region width W1 can be approximately represented by the equation W1=W0+L sin θ. In other words, the gain region width can be enlarged by L sin θ in comparison with the conventional configuration. Since the gain region width as viewed from the side of the resonator optical axis 30 can be enlarged, the oscillation width in the resonator can be enlarged.
As the tilt angle θ of the discharge electrode axis 32 is increased, the distance for which the laser beam is allowed to travel in the gain region is decreased. This means that, if the tilt angle θ becomes too large, the laser beam that is reflected back and forth within the resonator will possibly not be amplified effectively in the gain region.
According to experiment results, if the gain G0 and the injected light amount are both high, the output energy can be held substantially constant until the tilt angle becomes a diagonal angle θ1 of the discharge electrodes 24 and 25 shown in
The horizontal axis represents the tilt angle θ of the discharge electrode 32, and the vertical axis represents the laser beam width B (arbitrary) or the laser output P (arbitrary). As seen from the configuration shown in
As seen from
Although a desired laser output cannot be obtained if the tilt angle θ of the discharge electrodes is too large, the laser beam width can be enlarged while maintaining the output energy of the laser light substantially constant as long as the tilt angle θ is smaller than the diagonal angle θ1. Consequently, the laser beam illuminated area of the optical elements arranged in the resonator can be enlarged.
According to the first embodiment as described above, the laser beam width is enlarged such that the energy density of the laser beam applied to the optical elements provided in the laser chamber is decreased within such a range that no less than desired output energy of the laser light is obtained. This makes it possible to suppress deterioration of the optical elements provided in the laser chamber even if the output energy per pulse is increased more than the prior art without changing the discharge electrode width.
The description so far does not take into account the magnitude of the gain G0 or the magnitude of the injected light amount. The following description will show simulation results calculated by additionally using the gain G0 and the injected light amount as parameters.
As seen from
In contrast, the discharge electrode axis 32′ of the first embodiment is tilted at a tilt angle θ with respect to the resonator optical axis 30. When the length in the longitudinal direction of the discharge electrodes is denoted by L, the gain region width W1 as viewed from the direction of the resonator optical axis 30 is approximately represented by the equation W1=W0+L sin θ. Thus, the gain width as viewed from the direction of the resonator optical axis 30 is enlarged by L sin θ. In principle, the gain width is greater than the laser beam width.
According to
According to
According to
According to
Accordingly, the width B of the laser beam applied to the window can be enlarged 33% compared to the prior art even if the laser light output energy is increased 33% compared to the prior art. It is therefore possible, in calculation, to decrease the energy density of the laser light applied to the window to an equivalent level or lower compared to the prior art.
As described above, the simulation results also reveal that the laser beam width can be enlarged by the enlargement of the gain width. A specific value of the tilt angle θcan be obtained by simulation, and such value can be used as a guideline for performing experiments.
The description of the first embodiment has not specifically mentioned the resonator forming the resonator optical axis 30.
A second embodiment of the present invention assumes the amplification stage laser 20 of the two-stage laser system 1 shown in
In the second embodiment, the axis in the longitudinal direction of the discharge electrodes is tilted with respect to a resonator optical axis formed by arranging a rear-side mirror 21 and an output-side mirror 22 parallel to each other.
In
As shown in
In contrast, according to the second embodiment as shown in
According to
Since the gain region width is enlarged by L sin θ, the width of a laser beam oscillated (amplified) in the resonator is also enlarged in the vertical direction on the sheet surface orthogonal to the resonator optical axis 30. The enlarged laser beam width is denoted by B1. The laser beam width B1 is smaller than the enlarged gain region width W1. That is, W1>B1.
According to the second embodiment as described above, the laser beam width is also enlarged along with the enlargement of the gain region width. Therefore, the energy density of a laser beam applied to windows 27, 27 provided in the laser chamber 23 can be reduced.
According to
In
As is obvious from
Although the description of the second embodiment has been so far made using the amplification stage laser of the two-stage laser system, the invention of the second embodiment is also applicable to a single laser chamber.
Although in
According to the second embodiment as described above, either in the amplification stage of the two-stage laser system or in the single-chamber excimer laser device, the laser beam width can be set to reduce the energy density of a laser beam applied to the windows within such a range that a laser output of no less than a desired level is obtained.
Accordingly, even if the output energy per pulse is increased more than the prior art, the laser beam width can be enlarged to reduce the energy density of the laser beam applied to the optical elements.
This makes it possible to suppress deterioration of the optical element provided in the laser chamber even if the output energy per pulse is increased more than the prior art.
A third embodiment of the present invention is applicable to a MOPO system using seed light.
A basic principle and simulation results of the third embodiment will be described.
A rear-side mirror 21 and an output-side mirror 22 are arranged in an amplification stage laser 20 parallel to each other, forming a resonator optical axis 30. A discharge electrode axis 32 is parallel to the resonator optical axis 30.
In the configuration described above, seed light is injected at a tilt angle θ with respect to the resonator optical axis 30 and reaches the output-side mirror 22 (this pass is referred as the “first pass”). When a distance between the rear-side mirror 21 and the output-side mirror 22 is denoted by M, the injected seed light will be shifted to the upper side in the drawing by M tan θ while the seed light travels from the rear-side mirror 21 to reach the output-side mirror 22. The laser beam reflected by the output-side mirror 22 at a reflection angle θ reaches the rear-side mirror 21. The laser beam is further shifted to the upper side in the drawing by M tan θ while traveling to reach the rear-side mirror 21. The laser beam reflected by the rear-side mirror 21 at the reflection angle θ reaches the output-side mirror 22 (this pass is referred to as the “second pass”). From then on, the reflection in the resonator is repeated similarly at a fixed reflection angle θ, and the laser beam is shifted to the upper side in the drawing by M tan θ at every reflection.
In
Part of the laser beam reaching the output-side mirror 22 is emitted through the output-side mirror 22 in the direction indicated by the arrow E as first-pass output energy P1. An image of the first-pass laser beam is indicated by a region G1.
Most of the laser beam reaching the output-side mirror 22 is reflected at the reflection angle θ, again passes through the gain region G, and reaches the rear-side mirror 21 after being amplified. The laser beam reaching the rear-side mirror 21 is reflected at the reflection angle θ, again passes through the gain region G, and reaches the output-side mirror 22 after being amplified (second pass). The second-pass laser beam is shifted to the upper side in the drawing by 3M tan θ with respect to the injected laser beam.
Part of the laser beam reaching the output-side mirror 22 is emitted through the output-side mirror 22 in the direction indicated by the arrow E as second-pass output energy P2. An image of the second-pass laser beam is indicated by a region G2.
Most of the laser beam reaching the output-side mirror 22 is reflected at the reflection angle θ, again passes through the gain region G, and reaches the rear-side mirror 21 after being amplified. The laser beam reaching the rear-side mirror 21 is reflected at the tilt angle θ, again passes through the gain region G, and reaches the output-side mirror 22 after being amplified (third pass). The third-pass laser beam is shifted to the upper side in the drawing by 5M tan θ with respect to the injected laser beam.
Part of the laser beam reaching the output-side mirror 22 is emitted through the output-side mirror 22 in the direction indicated by the arrow E as third-pass output energy P3. An image of the third-pass laser beam is indicated by a region G3. Subsequently, a similar event is repeated.
As shown in
The lower part of
The gain Gp represents an increasing rate at which the laser beam is amplified while passing through the gain region G, and is determined depending upon how the laser beam passes through the gain region. As the gain Gp is greater, the amplification factor becomes higher and the output of the laser light becomes higher. The gain Gp becomes smaller as the laser beam is further away from the gain region G (or as the passage passing through the gain region G is shorter). Therefore, the simulation was performed on the assumption that the position where the gain Gp becomes 0.35 or higher is the position where the peak energy density becomes 5% or higher. This means that the range (width) in which gain Gp 0.35 is higher corresponds to the laser beam width.
As shown in
It is assumed here that the energy loss per unit length is fixed. Therefore, an absorption length La in each pass is the same as the resonator length.
In
As shown in
The horizontal axis in
The total gain Gs has a same shape as the spectrum of the output laser. According to
According to
In the configuration of
Seed light generated by an oscillation stage laser (not shown) is guided by a laser light guide mirror 34 in a laser light guide 18, and injected into an amplification stage laser chamber 23 in parallel with the discharge electrode axis 32. This means that the optical axis 35 of the injected seed light is parallel to the resonator optical axis 30.
In contrast, in the case of the third embodiment, as shown in
According to
According to the third embodiment as described above, the laser beam width can be enlarged by injecting the seed light generated by the oscillation stage laser into the amplification stage laser 20 while making an angle with respect to the resonator optical axis 30.
Consequently, the energy density of the laser beam applied to the windows of the amplification stage laser chamber can be decreased even if the output energy per pulse is increased more than the prior art. This makes it possible to suppress deterioration of the windows provided in the amplification stage laser chamber.
Although the laser light guide mirror 35 is rotated anti-clockwise in
In the case of the third embodiment, laser light output from the output-side mirror 22 is tilted at the tilt angle θ with respect to the resonator optical axis 30, the tilt angle can be corrected while the laser light travels to reach the exit of the excimer laser device.
A fourth embodiment of the present invention is applicable to a MOPO system using seed light.
In
In the fourth embodiment as shown in
Specifically, in order to shift the optical axis 35 of the injected seed light to the lower side in the drawing by Gm, the position where the optical axis of the seed light is reflected by the laser light guide mirror 34 is changed from the position K0 to the position K1. The change of the reflection position from K0 to K1 shifts the optical axis 35 of the injected seed light to the lower side in the drawing by Gm. The optical axis of the injected seed light thus changed is denoted by 35′.
According to calculation, when the tilt angle of the seed light is 0.6 mrad, for example, Gm is 0.43 mm. In this case, therefore, the injection optical axis 35 may be shifted to the lower side in the drawing by 0.43 mm. The reflection position K1 of the laser light guide mirror 34 for shifting the injection optical axis 35 to the lower side in the drawing by 0.43 mm can be found by experiments.
According to the fourth embodiment as described above, the laser light can be caused to pass through the entire gain region G by shifting the optical axis 35 of the injected seed light to the lower side in the drawing by a predetermined distance. Therefore, the discharge energy of the entire gain region G can be utilized for amplification of the laser light.
This makes it possible to utilize the discharge energy more effectively than in the third embodiment, while obtaining the advantageous effects of the third embodiment.
A fifth embodiment of the present invention is applicable to a MOPO system using seed light.
In the case of
In the fifth embodiment, therefore, configuration is such that the entire gain region can be effectively utilized and the laser beam that is reflected back and forth within the resonator is prevented from moving away from the gain region G.
In the case of
What is different from
As shown in
If the tilt angle θ of the injection optical axis 35 is positive, the output-side mirror 22 is rotated anti-clockwise. If the tilt angle θ of the injection optical axis 35 is negative, the output-side mirror 22 is rotated clockwise.
When an injection angle of seed light applied to the output-side mirror 22 with respect to the discharge electrode axis 32 is denoted by θ (>0), a tilt angle of the output-side mirror 22 is denoted by θ2 (>0), a reflection angle of the laser beam reflected by the output-side mirror 22 with respect to the discharge electrode axis 32 is denoted by θ3 (>0), the equation θ3=θ−θ2 is established. This means that, in the fifth embodiment, the reflection angle θ3 at which the seed light is reflected is always smaller than the injection angle θ at which the seed light is input to the output-side mirror 22. Thus, according to the fifth embodiment, a reflected laser beam Z1 when the output-side mirror 22 is tilted is reflected at a lower angle than a reflected laser beam Z0 when the output-side mirror 22 is not tilted. Consequently, the shift amount of the reflected laser beam shifted to the upper side in the drawings can be suppressed.
However, if the tilt angle θ2 is made too large to make the tilt angle θ3 too small, the laser beam width formed by reflecting the laser beam back and forth in the resonator cannot be enlarged. Accordingly, it is required to preliminarily obtain an optimal tilt angle θ2 through experiments.
The experiment results revealed that the optimal tilt angle θ2 of the output-side mirror 22 is 0.04 mrad when the tilt angle of the seed light is 0.6 mrad.
When the tilt angle is set as described above, the reflection angle of the seed light reflected by the output-side mirror 22 is 0.56 mrad (first pass). The reflected laser light is reflected by the rear-side mirror 21 at a same angle as the incident angle, namely at a reflection angle of 0.56 mrad, and again is incident to the output-side mirror 22. The laser light incident to the output-side mirror 22 at a tilt angle of 0.56 mrad is reflected by the output-side mirror 22 at a tilt angle obtained by subtracting the tilt angle θ1 of the output-side mirror 22, namely at a tilt angle of 0.52 mrad (second pass). Subsequently, every time the laser is reflected by the output-side mirror 22, the reflection angle of the laser light is decreased by 0.04 mrad in the same manner. According to the decrease of the reflection angle, the shift amount of the laser beam shifted to the upper side in the drawing is also deceased every time the laser light is reflected back and forth.
As described with reference to
In contrast, according to the fifth embodiment of the present invention, the laser beam width is enlarged to 0.42 cm. The beam enlargement factor is 1.27. According to the present invention, therefore, the average energy density applied to the windows is 33.2 mJ/cm2, and the peak energy density is 89.7 mJ/cm2. These values are equivalent to the average energy density and the peak energy density according to the conventional condition shown in
Although the output-side mirror 22 is tilted in
According to the fifth embodiment as described above, the laser beam width can be enlarged and the laser light can be reflected back and forth within the gain region. Therefore, the discharge energy of the gain region can be utilized effectively.
Although the discharge electrode axis 32 is arranged laterally on the sheet surface in
In this case, the discharge electrode axis 32 should be tilted such that laser light reflected by the output-side mirror 22 is not deviated from the gain region G. That is, the discharge electrode axis 32 is rotated clockwise. This makes it possible to prevent the laser light reflected by the output-side mirror 22 from deviating from the gain region. Consequently, the discharge energy can be utilized even more effectively than in the case of
A sixth embodiment is applicable to a MOPO system and a single excimer laser device.
In the sixth embodiment as shown in
Although the output-side mirror 22 is rotated clockwise to make the tilt angle θ in
Seed light injected in the direction of the discharge electrode axis 32 directly reaches the output-side mirror 22 (first pass). Since the output-side mirror 22 is tilted at the tilt angle θ, the seed light is reflected at a reflection angle θ that is the same as the tilt angle θ. When a distance between the rear-side mirror 21 and the output-side mirror 22 is denoted by M, the laser beam reflected by the output-side mirror 22 is shifted to the upper side in the drawing by M tan θ before reaching the rear-side mirror 21. The laser beam reflected by the rear-side mirror 21 is further shifted to the upper side in the drawing by M tan θ and reaches the output-side mirror 22 (second pass). Since the output-side mirror 22 is tilted at the tilt angle θ, the laser beam incident thereto at the reflection angle θ is reflected at a reflection angle 2θ. The reflected laser beam is shifted to the upper side in the drawing by M tan 2θ before reaching the rear-side mirror 21. The laser beam reflected by the rear-side mirror 21 is further shifted to the upper side in the drawing by M tan 2θ and reaches the output-side mirror 22 (third pass). Subsequently, in the same manner, the reflection angle is increased by θ every time the laser beam is reflected by the output-side mirror 22, while at the same time the shift amount is increased. In other words, the laser beam is shifted to the upper side in the drawing as the number of passes increased from the first to second pass, from the second to third pass, and so forth. This means that the laser beam width is enlarged to the upper side in the drawing according to the sixth embodiment as well.
According to the sixth embodiment as described above, the width of the laser beam output by the output-side mirror 22 can be enlarged. Accordingly, the energy density of the laser beam applied to the windows provided in the laser chamber in the resonator can be reduced, and thus deterioration of the windows can be suppressed.
A seventh embodiment is applicable to a MOPO system using seed light.
In the case of the seventh embodiment as shown in
According to
The subsequent mode of reflection of these two laser beams is entirely similar to the case of the third embodiment in which seed light injected into the laser chamber 23 at a tilt angle θ with respect to the resonator optical axis 30. In other words, the laser beam deviated to the upper side in the drawing has its laser beam width enlarged to the upper side in the drawing. Similarly, the laser beam deviated to the lower side in the drawing has its laser beam width enlarged to the lower side of the drawing.
According to the seventh embodiment as described above, the width of the laser beam reflected back and forth within the resonator can be enlarged. Therefore, the energy density of the laser beam applied to the windows provided in the laser chamber in the resonator can be reduced, and hence deterioration of the windows can be suppressed.
In the embodiments described so far, the rear-side mirror and the output-side mirror forming the resonator are both of a flat type. However, according to the present invention, the mirrors forming the resonator need not necessarily be of a flat type.
The resonator configurations as described above also enable enlargement of the laser beam width by repeatedly reflecting the laser beam back and forth within the resonator.
The eighth embodiment is applicable to both a MOPO system and a single chamber laser device.
The energy density applied to the output-side mirror 22 can be decreased further by combining the technique to enlarge the laser beam width with a well-known beam expander (BEX) technique. An embodiment employing such combination will be described as a ninth embodiment of the present invention.
As shown in
The beam expander 36 has wedge-shaped permeable optical elements 37, 37 arranged on a laser optical axis, and is able to enlarge laser light.
According to the ninth embodiment, the width of a laser beam applied to windows 27, 27 provided on the laser chamber 23 can be enlarged, and additionally the width of the laser beam applied to the output-side mirror 22 also can be enlarged by the beam expander 36.
This makes it possible to reduce the energy density of laser light applied to the output-side mirror 22, and thus deterioration of the output-side mirror 22 can be suppressed even if high output energy is output from the output-side mirror 22.
In the MOPO system described above, the seed light generated by the oscillation stage laser 10 is invariably injected from the rear face of the rear-side mirror 21. This method is referred to as the rear injection method. In the present invention, the method of injecting the seed light is not limited to the rear injection method, but other injection methods can be employed.
For example, the side injection method and the front injection method are suitable for injecting the seed light while tilting the same with respect to the resonator optical axis with the rear-side mirror 21 and the output-side mirror 22 arranged parallel to each other. In this case, the width of the laser beam in the resonator may be optimized by adjusting the reflection angle of the mirrors 21 and 22.
In the embodiments described above, the laser beam width is enlarged by enlarging the gain region width without changing the discharge electrode width. However, if it is convenient, the discharge electrode width may be enlarged to thereby enlarge the gain region width, so that the laser beam width is enlarged as a result.
Number | Date | Country | Kind |
---|---|---|---|
2006-216915 | Aug 2006 | JP | national |
This application is a divisional of U.S. application Ser. No. 11/882,938, filed Aug. 7, 2007, the entirety of which is incorporated herein by reference.
Number | Date | Country | |
---|---|---|---|
Parent | 11882938 | Aug 2007 | US |
Child | 13048159 | US |