JPH08334803A - Uv laser beam source - Google Patents

Abstract

PURPOSE: To provide a UV laser beam source which is capable of generating UV rays having sufficient outputs and low coherence as the light source of an exposure machine stably for a long time, is small in size and facilitates maintenance. CONSTITUTION: This UV laser beam source is composed of 10 pieces × 10 pieces, total 100 pieces of laser elements. The respective laser elements have laser beam generating sections 100 which generate long-wavelength light of visible light or IR light and wavelength conversion sections 14 which convert the generated laser beams to UV rays, These laser beam generating sections 100 have semiconductor lasers 11, optical fibers 12 and solid-state lasers 13. These wavelength conversion sections 14 are constituted to include nonlinear crystals which convert the wavelengths of incident light rays and outputs these rays.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source for an exposure machine used in a semiconductor manufacturing process, and more particularly to an ultraviolet laser light source capable of generating an ultraviolet laser beam.

[0002]

2. Description of the Related Art With the progress of information equipment, it is required to improve the function and storage capacity of a semiconductor integrated circuit, and for that purpose, it is necessary to increase the degree of integration. In order to increase the degree of integration, it is sufficient to make each circuit pattern small,
The minimum pattern size is determined by the performance of the exposure machine used in the manufacturing process.

The exposure machine optically projects and transfers the circuit pattern formed on the mask onto a semiconductor wafer. The minimum pattern size R on the wafer at that time is given by the following formula by the wavelength λ of the light used for projection by the exposure machine and the numerical aperture NA of the projection lens.

R = Kλ / NA where K is a constant determined by the illumination optical system and process, and usually takes a value of about 0.5 to 0.8.

Efforts to improve the resolution, that is, to reduce the minimum pattern size R, are made in the direction of decreasing the constant K, in the direction of increasing the numerical aperture NA, and in the direction of decreasing the wavelength λ of the exposure light. Is being made towards.

The methods for reducing the constant K are collectively called super-resolution in a broad sense. Up to now, improvement of illumination optical system, modified illumination, phase shift mask method, etc. have been proposed and studied. However, there were some problems such as the conditions that can be applied. On the other hand, the larger the numerical aperture NA, the smaller the minimum pattern dimension R can be made, but at the same time, the depth of focus also becomes small, so there is a limit to making it large. Usually, about 0.5 to 0.6 is suitable.

Therefore, the simplest and most effective way to reduce the minimum pattern size R is to reduce the wavelength λ of the light used for exposure, which generates light of a short wavelength.
It is to provide a light source for an exposure machine. The present invention has been made in response to this demand.

Here, in order to make a light source for an exposure machine, there are some conditions to be provided in addition to the realization of a shorter wavelength. Hereinafter, these conditions will be described.

First, a light output of several watts is required.
This is necessary to keep the time required for exposing and transferring the integrated circuit pattern short.

Secondly, in the case of ultraviolet light having a wavelength of 300 nm or less, the material usable as the lens of the exposure device is limited, and it becomes difficult to correct chromatic aberration. Therefore, it is required to set the line width of the emission spectrum to 1 pm or less. .

Thirdly, since the temporal coherence (coherence) increases with the narrow line width, when irradiating light with a narrow line width as it is, an unnecessary interference pattern called speckle occurs. Therefore, in order to eliminate it, the light source needs to reduce its spatial coherence.

Next, a description will be given of a typical light source for an exposure machine which has been used conventionally, and a problem when each light source satisfies the above conditions and tries to generate ultraviolet light. To do.

(1) Mercury lamp Among the emission lines of the mercury lamp, g line (wavelength 436 nm)
And i-line (wavelength 365 nm) have been used. The minimum pattern size obtained at this time (hereinafter referred to as the minimum size) is about 500 nm and about 350 nm, respectively. The spectral line width of these light sources was wider than that of the laser described later, and thus the temporal coherence was low. Even if the line width is wide, it was possible to correct the chromatic aberration of the lens at these wavelengths, so there was no problem in the prior art.
The spatial coherence of mercury lamps is also lower than that of lasers, and speckle generation is not a problem because of the low coherence of these two.

However, the above-mentioned mercury emission line has a long wavelength, and it has become difficult to meet the newly required minimum dimension. The method of using the shorter wavelength ultraviolet emission line of the mercury emission line was also used in some cases, but its spectral line width is wide and it is difficult to use an achromatic lens in the ultraviolet region, so it is difficult to use in the ultraviolet region. It is said that.

(2) KrF excimer laser The KrF excimer laser emits light of 248 nm.
Therefore, the minimum dimension is also around 250 nm. Since it is difficult to manufacture an achromatic lens at this wavelength, it is necessary to narrow the spectral line width of the light source laser to 1 pm or less.

However, with the narrowing of the band, the temporal coherence is increased and the occurrence of speckles becomes a problem. Therefore, for example, "Excimer laser stepper" (Kazuo Ushida, Optics, Vol. 23, No. 10, p602, 19)
In the example described in (October 1994), generation of speckles is suppressed by adding an optical system for reducing spatial coherence.

Although KrF excimer lasers for exposure machines have been developed and used, excimer lasers are expensive and large in size as compared with mercury lamps, and use toxic fluorine gas. Further, there is a problem that maintenance is required such as replacement of the optical system and fluorine gas, and the cost is high.

Since the light generated is pulsed light,
There is a problem that the peak power becomes larger than that of continuous light, and the laser and the optical components inside the exposure device are easily damaged by light.

(3) ArF Excimer Laser The ArF excimer laser emits light of 193 nm.
The minimum practical size at this time is about 190 nm. Currently, a laser for an exposure machine is under development, but this laser has the same disadvantages as the KrF excimer laser.
That is, expensive, large-scale, toxic fluorine gas is used, and further maintenance such as replacement of the optical system and fluorine gas is required, resulting in high cost.

In addition, it is difficult to narrow the oscillation line width of the laser to 1 pm or less in order to reduce the chromatic aberration of the exposure device, as compared with the KrF excimer laser.

Further, since the pulsed light of short wavelength has higher energy than the KrF laser,
There is a drawback in that the damage to the laser and the optical components of the exposure device is more severe than that of the KrF laser.

(4) Light Source by Harmonic Generation of Semiconductor Laser Excited Solid-state Laser As a method of generating ultraviolet light, long-wavelength light (visible light, infrared light) is converted into ultraviolet light by utilizing the second-order nonlinear optical effect. There is a way to convert it to light. For example, "Longitudinal
ly diode-pumped continuo
s-wave3.5-W green laser
(LY Liu, M. Oka, W. Wiechman.
n and S. Kubota, Optics Let
ters, Vol. 19 (1994), p. 189) "
A laser light source for wavelength-converting light from a solid-state laser excited by a semiconductor laser has been developed. This conventional example discloses a method of converting 1064 nm light emitted from an Nd: YAG laser and converting the wavelength using a non-linear crystal to generate 266 nm which is a fourth harmonic.

Such a conventional semiconductor laser pumped solid-state laser light source is advantageous in that it is compact, easier to maintain than an excimer laser, high in power efficiency, and easy to control light output. Have as. Another advantage is that continuous light can be generated in addition to pulsed light. Furthermore, even when the oscillation line width is reduced, it can be performed at the stage of long wavelength before wavelength conversion, and its control is easier than that of an excimer laser that requires direct control of ultraviolet light. , Has the advantage.

Despite such advantages, the above-mentioned conventional technique has not yet been applied to an exposure machine, and laser development has been carried out at a laboratory level. One of the reasons why it has not been used as a light source for an exposure machine is that the nonlinear crystal is damaged when the output power is increased and the life of the apparatus is shortened.

Further, for the reason described below, there is a drawback that the spatial coherence becomes higher than that in the case of the excimer laser and speckles are generated.

Next, the relationship between speckle generation and coherence will be described in more detail.

The removal of unnecessary interference patterns such as speckles can be achieved by reducing the temporal coherence of light or the spatial coherence. Lowering the temporal coherence means mixing light of various frequencies. On the other hand, lowering the spatial coherence means mixing light with different propagation locations and propagation directions.

However, the ultraviolet light used in the exposure machine is
There is a demand for the oscillation line width to be 1 pm or less, which results in increased temporal coherence.
Further, the light generated by the laser is composed of a finite number of transverse modes, and the small number of transverse modes means that the spatial coherence is high.

Conventionally, in an exposing machine using a KrF excimer laser, a beam is divided into a plurality of beams by using an oscillating reflecting mirror, and spatial coherence is lowered. The excimer laser originally oscillated in the transverse mode of several hundreds, and the spatial coherence was relatively low as a laser, so there was no problem in the above method.

However, when the wavelength of a solid-state laser is converted by a non-linear crystal, it is necessary to squeeze the beam strongly in the non-linear crystal, so that the laser usually oscillates in one transverse mode. This means that the spatial coherence is set to the highest state, and it has been difficult to reduce the spatial coherence in such a case.

[0031]

DISCLOSURE OF THE INVENTION The present invention relates to the above-mentioned problems of the prior art, for example, the problems that occur when an excimer laser is used as an ultraviolet light source of an exposure machine.
Non-linear optics for wavelength conversion, which is expected when using a solid-state laser excited by a semiconductor laser as an ultraviolet light source of an exposure machine, as well as problems such as large size of equipment, use of toxic fluorine gas, maintenance difficulty and cost. This was done in consideration of problems such as crystal damage and speckle generation due to an increase in spatial coherence.

An object of the present invention is to provide an ultraviolet laser light source capable of stably generating ultraviolet light having a sufficient output and a low coherence as a light source of an exposure machine for a long time, and being small in size and easy to maintain. To provide.

[0033]

The above object is to provide a laser light generator for generating light within a wavelength range from infrared to visible, and wavelength conversion of the generated light into ultraviolet light using a nonlinear optical crystal. This is achieved by an ultraviolet laser light source characterized in that a plurality of laser elements having a wavelength conversion optical system are bundled in parallel.

More specifically, the laser generating portion of each laser element is provided with, for example, a semiconductor laser, or a semiconductor laser and a solid-state laser excited by the light generated by the semiconductor laser.

[0035]

In the present invention, in each laser element,
For example, infrared light or visible light from a laser light generator including a semiconductor laser is wavelength-converted by a nonlinear optical crystal provided in a wavelength conversion optical system to generate ultraviolet light.

By bundling a plurality of laser elements having such a structure, the light outputs can be added together and finally the output from the entire light source device can be made high. Furthermore, by outputting light from laser elements independent of each other, temporal and spatial coherence can be reduced.

Furthermore, since the target optical output is obtained by bundling a plurality of laser elements, the output from each laser element can be made smaller than the target optical output. Therefore, it is possible to reduce the burden on the nonlinear optical crystal of the wavelength conversion unit, and to suppress the deterioration to the minimum.
Therefore, it can be operated stably for a long time,
The life of the device can be extended.

[0038]

EXAMPLE An example of an ultraviolet laser light source to which the present invention is applied will be described with reference to FIG.

As shown in FIG. 1, the ultraviolet laser light source of this embodiment is composed of 10 × 10 laser elements, for a total of 100 laser elements. Each laser element is a laser light generator 100 that generates long wavelength light of visible light or infrared light.
And a wavelength conversion unit 14 that converts the generated laser light into ultraviolet light. In this embodiment, the laser light generator 10
An example using a solid-state laser excited by a semiconductor laser will be described as 0.

The laser light generator 100 has a semiconductor laser 11, an optical fiber 12, and a solid-state laser 13. In addition, the wavelength conversion unit 14 is configured to include a nonlinear crystal that converts a wavelength. In addition, the laser element,
Besides, the optical elements include a reflecting mirror, a lens, a wave plate, a polarizer and the like, but the detailed configuration thereof will be described in the following embodiments, and the description thereof will be omitted here.

The cross-sectional size of each laser element except the semiconductor laser 11 is about 5 mm × 5 mm. In addition,
In the present embodiment, a configuration in which 100 laser elements are combined is taken as an example, but the number of laser elements is not limited to this, and the present invention assumes about 2 to 1000 laser elements.

The semiconductor laser 11 is a solid-state laser 13.
Used to excite. The solid-state laser 13 excited by the light from the semiconductor laser 11 oscillates visible light or near-infrared light. In this embodiment, it is converted into short wavelength ultraviolet light by the nonlinear crystal of the wavelength conversion unit 14.

There is also a method (internal resonator method) in which a nonlinear crystal for converting the wavelength is inserted into the resonator structure of the solid-state laser, and the solid-state laser 13 and the wavelength converting section 14 are integrated. An ultraviolet laser light source using this structure will be described in detail in Examples described later.

According to the present embodiment, by bundling a plurality of laser elements having the above-mentioned structure, the light outputs can be added together to obtain a high output, and at the same time, the laser elements output light independently of each other. Temporal and spatial coherence can be reduced.

Furthermore, since it is not necessary to increase the output from each laser element by bundling a plurality of laser elements, the load on the nonlinear crystal of the wavelength conversion unit 14 can be reduced, and the deterioration thereof can be minimized. Can be suppressed, so
The life of the device can be extended.

The line width of the wavelength generated by the laser element having the structure of the present embodiment can be made sufficiently smaller than 1 pm, and the difference in wavelength between the laser elements is also caused by the solid laser medium, the laser resonator length, It can be adjusted to 1 pm or less by adjusting the configuration of the wavelength selection optical system.

Next, the semiconductor laser 11 will be described in more detail.

The semiconductor laser emits light by the transition of electrons inside the semiconductor. This electron is excited by a current injected from the outside. At present, the wavelength of emitted light is in the range of 600 nm to 1500 nm (1.5 μm), and it is known that a light emitting element having a light output of 10 W class alone can emit light in the vicinity of 800 nm.

However, the light emitted from such a high-power semiconductor laser has poor transverse mode properties, and the intensity distribution of the beam is not uniform. Therefore, the efficiency is low even if the wavelength is converted by the nonlinear crystal.

On the other hand, the above-mentioned semiconductor laser having a high light output usually has a sufficient condensing property to excite a solid-state laser. Further, there is an advantage over other laser light sources that the optical output is large with respect to the input electric power, it is compact, and the oscillation output and the oscillation wavelength can be finely adjusted by the inflowing current. Therefore, the semiconductor laser is most suitable for realizing the compactness of the light source.

The semiconductor laser is usually a box-shaped device having a size of several cm and is capable of outputting light of about 10 W, and moreover, light can be extracted by an optical fiber.

As the semiconductor laser 11 of this embodiment,
For example, a semiconductor laser having a high light output as described above is used.

Also, unlike the high-power semiconductor laser described above, a single transverse-mode semiconductor laser is also known, but the output is generally small. At present, the maximum is about 200 mW. However, high output ones are gradually being developed. Therefore, such a single transverse mode semiconductor laser may be used as the semiconductor laser 11 of this embodiment.

When this single transverse mode semiconductor laser is used as the semiconductor laser 11 of this embodiment, the light output from the semiconductor laser 11 can be directly wavelength-converted by a nonlinear crystal to obtain ultraviolet light. . Even in that case,
By using a plurality of lasers in parallel, which is a feature of this embodiment, the effects of reducing the optical damage of the nonlinear crystal and reducing the temporal and spatial coherence can be achieved as in the case of the present embodiment. .

Next, the solid-state laser 13 will be described in more detail.

The solid-state laser is a general term for lasers having a solid laser medium. The semiconductor laser is also one of the solid-state lasers, but usually the solid-state laser refers to a solid-state laser excited by light. Here too, we make that distinction.

In this embodiment, the excitation light is obtained from the semiconductor laser 11. The solid-state laser 13 is composed of a solid-state laser medium and optical components such as a reflecting mirror. Further, as will be described in Examples described later, a configuration in which a nonlinear crystal for wavelength conversion is incorporated may be used.

As the solid-state laser 13, for example, Nd
Doped Yttrium Aluminum Ga
A laser that uses rnet (Nd: YAG) as a laser medium and emits 1064 nm light is used. At this time, the semiconductor laser 11 as the excitation source is 808n
The one that generates a wavelength near m is used.

If the currently developed high-power semiconductor laser emitting blue-green light near 500 nm can be used, Ti-doped sapphire (Ti: Sapphire) can be used as the solid-state laser. The light generated by the solid-state laser has a good transverse mode distribution (the transverse mode is single), and it is possible to efficiently perform wavelength conversion to a shorter wavelength by using a nonlinear crystal.

Next, the configuration of the nonlinear optical crystal included in the wavelength converting section 14 will be described in more detail, including its wavelength converting action.

A crystal called a nonlinear optical crystal having a second-order nonlinear susceptibility, for example, β-BaB ₂ O ₄ (BBO).
And, LiB ₃ O ₅ (LBO) is a light frequency omega ₁ (wavelength lambda _1), to convert (second harmonic generation) to 2 [omega ₁ of the light (wavelength _{λ 2 = λ 1/2)} , the frequency From light of ω ₁ and frequency ω ₂ , light of frequency ω ₃ = ω ₁ + ω ₂ (wavelength λ ₃ is 1 / λ ₃ = 1 / λ ₁
+ 1 / λ ₂ ) can be generated (sum frequency generation).

Wavelength conversion in these nonlinear optical crystals is
There is a limit to the short wavelength side of the converted wavelength. One factor that determines the limit is the limit of the transmittance of the nonlinear optical crystal, and the other factor is the limit of phase matching that matches the phase velocities of two types of light before and after conversion due to the birefringence of the crystal.

From the viewpoint of transmittance, BBO is limited to around 190 nm. Although the LBO is transparent up to around 155 nm in transmittance, it cannot generate ultraviolet light in the second harmonic generation, and can reach up to 187 nm near by using the sum frequency generation of different wavelengths.

As a non-linear optical crystal, KBe ₂
BO ₃ F ₂ (KBBF) and Sr ₂ Be ₂ B ₂ O ₇ (SBB
It is also possible to use O). By using these, there is a possibility that shorter wavelength ultraviolet light can be generated.

The conversion efficiency of wavelength conversion by the non-linear optical effect is proportional to the intensity (power per unit cross section) of the fundamental wave that is the basis. That is, the stronger the light, the more efficiently it is converted. In order to utilize this property, in this embodiment, for example, any one of the following three methods or a combination thereof is used.

1. The laser beam incident on the nonlinear optical crystal is narrowed down by a condenser lens or a reflecting mirror.
In order to do this, it is necessary that the transverse mode of the underlying solid-state laser is a single mode.

2. A nonlinear crystal is placed in the resonator, and the efficiency of wavelength conversion is improved by using the enhancement of the light intensity by multiple reflection of light in the resonator. As a specific example of this method, an internal resonator (i.
In addition to the so-called intra-cavity method, another resonator is provided in addition to the resonator of the solid-state laser, and an external crystal is inserted in the resonator.
Resonant Cavity method.

3. By pulsing the laser oscillation and concentrating the energy in a short time, the instantaneous light intensity is increased. In this method, the nonlinear crystal is not placed in the resonator, and the nonlinear crystal is arranged alone and light is allowed to pass through in one direction.

The structure corresponding to each of the above methods will be described in more detail in the embodiments described later.

In addition, optical damage to the nonlinear crystal is another point to be noted in the structure of this embodiment.
That is, the non-linear crystal has a problem that it is damaged by excessive light intensity and the conversion efficiency is reduced. On the other hand, since it is necessary to increase the intensity of light in order to increase the conversion efficiency in the nonlinear crystal, this becomes a dilemma in the design of the device.

At present, there is almost no optical damage up to a certain light intensity. In the case of an actual UV-generating laser, the output of UV light is 1 when the continuous-wave UV light (266 nm) is generated by BBO.
The damage is small at 00 mW, but the damage becomes significant when it exceeds 1 W.

In this embodiment, the ultraviolet laser light source as a whole produces a light output of several watts, but from several to several tens.
A plurality of zero laser elements share the light output, and the output per laser element is kept low. With this configuration, optical damage is unlikely to occur in the plurality of nonlinear crystals included in this embodiment, and stable operation for a long period of time is possible.

In addition, in this embodiment, in consideration of application to an exposure device, each laser element has a line width of an individual oscillation wavelength of 1 pm or less, and a wavelength difference between a plurality of laser elements. Two conditions are required that the difference be 1 pm or less.

In the present embodiment, for example, the wavelength is controlled so as to satisfy the above two conditions as follows.

Each laser element is adjusted to oscillate in only one longitudinal mode among several longitudinal modes (corresponding to the oscillation wavelength). For that purpose, the resonator length is adjusted, and an optical element having wavelength selectivity is inserted when necessary.

The oscillation line width of one longitudinal mode is typically 0.01 pm or less. Therefore, oscillation in one longitudinal mode (Single frequency operation)
The oscillation line width of each laser element becomes less than the required 1 pm.

Further, the natural wavelengths of the longitudinal modes are periodically present, and the wavelength interval Δλ is such that the length of one round trip of the laser resonator is 2 L, the refractive index of the substance inside the resonator is n, and the wavelength for oscillation is Let λ be given by the following equation.

Δλ = λ2 / (2L · n) As a typical example of this embodiment, L = 10 cm, N
d: 1064 nm of the YAG fundamental wave is used and n is N
When 1.7 is used as the average of d: YAG and the nonlinear crystal, Δλ is 2.9 pm, and Δλ = 0.6 pm is obtained at the fifth harmonic of 213 nm.

Normally, when one longitudinal mode is oscillated, the longitudinal mode of the wavelength having the largest amplification factor of the laser medium oscillates. This wavelength is determined by the laser medium, and the oscillated wavelengths of the plurality of laser elements are aligned near the wavelength peculiar to the laser medium. More specifically, with respect to the wavelength of the solid-state laser medium having the highest amplification factor, the maximum of each laser element is 1/2 of the longitudinal mode interval (± 0.6 pm / 2 = ± in the above example).
Only 0.3 pm).

When the cavity length L is smaller than that described here, the mode interval Δλ becomes large. Therefore, the oscillation wavelength can be adjusted by adjusting the cavity length L of each laser element or the characteristics of the wavelength selection element. You can arrange.

Further, in this embodiment, since light is generated from a plurality of laser elements, the spatial coherence is originally low. Further, the coherence is lower than that of a conventional laser in which a single beam is divided into a plurality of beams. This is because a plurality of laser elements generate very close wavelengths (wavelength difference is 1 pm or less) as described above, but the difference in frequency is usually about 1 GHz.

The oscillation frequency of each laser naturally deviates from several hundred MHz to about 1 GHz or more without any hand. By the way, in the vicinity of the wavelength of 200 nm, the difference of ± 0.3 pm is ± 2.2 GHz in terms of frequency difference.

This means that the interference fringes due to the light emitted from a plurality of lasers blink at a frequency of about 1 GHz. On the time scale of exposure of a semiconductor wafer, the interference fringes created between each laser element are averaged out. This effectively means that the light emitted from each laser does not interfere.

According to this embodiment, since the spatial coherence is lowered in this way, it is more advantageous in speckle reduction than in the case of the conventional single-beam solid-state laser.

Another embodiment to which the present invention is applied will be described with reference to FIGS. Here, FIG. 2 is an overall view of the ultraviolet laser light source of this embodiment, and FIG. 3 is a configuration diagram of each laser element.

As shown in FIG. 2, the ultraviolet laser light source of this embodiment is composed of a total of 100 laser elements, 10 in length and 10 in width. Each laser element includes a semiconductor laser 21 that generates excitation light and an optical fiber 2 that transmits the excitation light.
2 and an internal cavity type solid-state laser including a nonlinear crystal 2
And 3.

The solid-state laser 23 emits 213 nm continuous ultraviolet light in the right direction, and as shown in FIG. 3, Nd: YAG3, which is a laser medium, is contained in the resonator.
4 and four nonlinear crystals 35, 36, 3 for converting the wavelength
7, 39, reflecting mirrors 33, 40, and a wave plate 38.

In this embodiment, the ultraviolet light output per laser element is expected to be about 100 mW (0.1 W), at which time the entire light source of this embodiment has an output of about 10 W. Further, the solid-state laser 23 of each laser element is
The cross-sectional dimension is about 3 mm square, and a bundle of 100 in total makes up a light source of about 50 mm square. Each laser element is cooled by a cooling mechanism (not shown). As the cooling mechanism, for example, a mechanism in which each laser element is embedded in a block made of copper and this copper block is cooled by a cooling device can be used.

Since the optical fiber 22 is flexible and the length thereof can be several cm to several m, the arrangement method can be freely changed. In particular, the size of the semiconductor laser 21 is several cm cubic (reduced in FIG. 2).
On the other hand, since the cross-sectional dimension of the solid-state laser 23 is a few mm square, the optical fiber 22 is arranged by making the most of its flexibility.

The solid-state laser 23 of this embodiment generates the fifth harmonic of the inputted light. For example, the fourth harmonic generated from the conventionally reported laser for generating the fourth harmonic is further added. It is converted to generate a fifth harmonic wave.

As the semiconductor laser 21, the oscillation wavelength is 808 nm, and the output at the exit of the optical fiber 22 is 1
Use the one of about 0W. Excitation light (wavelength 808 nm) from the semiconductor laser 31 passes through the optical fiber 32,
It is guided to the laser resonators (33 to 40) of the solid-state laser 23, and passes through the reflecting mirror 33, which is the laser medium Nd:
Excite the YAG rod 34.

The cross-sectional size of the Nd: YAG rod 34 is 3
The length should be 10m or less (a circular cross section is acceptable).
Set to about m. The reflecting mirror 33 has a high transmittance for the excitation light having a wavelength of 808 nm and a high reflectance for the fundamental wave of the solid-state laser having a wavelength of 1064 nm. The reflector 33 does not have to be an individual part, but the Nd: YAG rod 3
Alternatively, a reflective film may be attached to the left end surface of No. 4 for substitution.

Wavelength 1064 emitted from laser medium 34
The fundamental wave (frequency ω) of nm is generated by the nonlinear crystals 35, 36,
37, the wave plate 38, and the nonlinear crystal 39, and reciprocates in the laser resonator formed between the reflecting mirror 33 and the other reflecting mirror 40. The cross-sectional size of each nonlinear crystal is about 3 mm square and the length is about 10 mm.

When the light of the fundamental wave travels back and forth in the laser resonator, the fundamental wave absorbs energy due to reflection and scattering at the end faces of each nonlinear crystal, internal absorption, and conversion of energy into higher harmonics. Although lost, it undergoes intensity amplification as it passes through the Nd: YAG rod 34 of the laser medium. As a result, the intensity of the fundamental wave in the resonator increases, reaching approximately tens to hundreds of watts.

When the fundamental wave passes from the left to the right through the nonlinear crystal 35, a double wave having a wavelength of 532 nm (frequency 2ω) is generated (second harmonic generation, ω + ω = 2ω). Non-linear crystal 3
LBO is used as 5. The fundamental wave loses some of its energy by conversion, but its absolute intensity is still high and can maintain high intensity. Here, so that so-called type I phase matching is established,
Determine the cutting direction of the LBO end face. In Type I phase matching, a fundamental wave having a vertical polarization direction generates a second harmonic wave having a horizontal polarization direction.

The generated second harmonic advances to the right along with the fundamental wave, and advances to the next nonlinear crystal (LBO) 36. Therefore,
The sum frequency generation (ω + 2ω = 3ω) of the second harmonic wave and the fundamental wave is performed, and the third harmonic wave (wavelength 355 nm) is generated. Then 2
The intensity of both the fundamental wave and the harmonic wave is slightly reduced, but the fundamental wave still maintains high intensity. Here, the LBO end face is cut so that so-called type II phase matching is performed. In type II phase matching, a fundamental wave in the vertical direction and a second harmonic wave in the horizontal direction generate a third harmonic wave having a vertical component. There is also a horizontal component, but it is irrelevant here.

Further, the following non-linear crystal (LBO) 37 generates a sum frequency of ω + 3ω = 4ω, and generates a fourth harmonic wave of 266 nm. Here, type I phase matching is performed and the fourth harmonic has a horizontal polarization direction.

Next, the wave plate 38 makes the polarization direction of the fourth harmonic wave vertical without changing the polarization direction (perpendicular) of the fundamental wave. The vertically polarized fourth harmonic wave enters a nonlinear crystal (BBO) 39.

In the non-linear crystal 39, the sum frequency generation of ω + 4ω = 5ω is performed by the type II phase matching, and 5
A harmonic wave (wavelength 213 nm) is generated. Even after leaving BBO39, the fundamental wave is still high in intensity. The generated fifth harmonic wave is expected to have an output of about 100 mW.

At the right end of the solid-state laser 23 (laser resonator), the fundamental wave is reflected by the reflecting mirror 40 having wavelength selectivity to be folded back into the laser resonator.
The generated fifth harmonic wave is transmitted. The harmonics from the second harmonic to the fourth harmonic have weakened intensities, and thus may be transmitted or reflected. The fundamental wave reflected by the reflecting mirror 40 returns in the resonator in the reverse direction and is again amplified by the Nd: YAG rod 34.

In the present embodiment, the Nd: YAG rod 34, the nonlinear crystal 35, and the optical components in the laser resonator,
An antireflection film is applied to the end faces of 36, 37, 39 and the like. Further, the reflection may be prevented by bringing the optical components into close contact (adhesion or optical contact) without using the antireflection film. Further, similarly to the reflecting mirror 33, the reflecting mirror 40 may be configured as a substitute by not forming the reflecting mirror 40 as an individual component but by attaching a reflecting film to the end face of the nonlinear crystal 39 to form a reflecting surface.

Further, in the present embodiment, the harmonics and the fundamental wave other than the fifth harmonic wave are also output through the reflecting mirror 40 although the intensity is low. If they adversely affect the exposure, a filter is provided outside the laser, thereby excluding it.

When the laser element of this embodiment does not oscillate in a single longitudinal mode, a wavelength selecting element may be added.

According to this embodiment, an ultraviolet laser light source with a wavelength of 213 nm, a total output of about 10 W, a spectral line width of 1 pm or less, less damage to a nonlinear crystal, and low spatial coherence can be realized. .

Another embodiment to which the present invention is applied will be described with reference to FIG. Here, FIG. 4 is a diagram showing the configuration of each laser element in the present embodiment and the propagation path of the laser beam inside thereof.

The ultraviolet laser light source of this embodiment is constituted by bundling a plurality of laser elements in parallel as in the embodiments of FIGS. 2 and 3, and has an optical structure as shown in FIG. Have. That is, each laser element of this embodiment includes a small laser 41 that generates a long wavelength laser,
Lenses 42 and 44 provided on the non-linear crystals 43 and 45 for wavelength conversion, between the small laser 41 and the non-linear crystal 43, between the non-linear crystal 43 and the non-linear crystal 45, and on the emission side of the non-linear crystal 45, respectively. 46 and.

In this embodiment, as the small laser 41, a semiconductor laser 41 of a single transverse mode which emits a wavelength of 820 nm and has an output of about 150 mW is used, and the wavelength conversion method used is an external resonator method.

As the external resonator method, for example, "Gene
relation of 41 mWof blue ra
direction by frequency doub
ling of a GaAlAs diode la
ser (W. J. Kozlovsky, W. Lent
h, E. E. FIG. Latta, A .; Moser and G.
L. Bona, Applied Physics Le
tters, Vol. 56, p. 2291, (199
0)) ”as described in 856 nm to 428 nm.
There is one that converts the wavelength. The laser element of the present embodiment is a two-stage conversion from 820 nm to 410 nm, and further from 410 nm to 205 nm by using such an external cavity method.

In this embodiment, the light having a wavelength of 820 nm emitted from the semiconductor laser 41 is incident on the non-linear crystal 43 with its direction and convergence angle adjusted by the lens 42. Here, LBO is used as the nonlinear crystal 43.

The left and right end surfaces of the nonlinear crystal 43 are polished into inclined convex mirror surfaces as shown in FIG.
High reflectance for the fundamental wave of m, wavelength 410nm
A wavelength-selective reflective film having a high transmittance for the second harmonic wave is formed. The cross section of the nonlinear crystal 43 is 5
The length is about 15 mm and the length is about mm square.

The inside of the nonlinear crystal 43 forms a ring resonator structure. That is, the structure is a structure (ring resonator) that traps light in a triangular shape by using the total reflection of the left and right end surfaces and the flat surface-polished lower surface. A resonator made of an integrated crystal as described above is called a monolithic resonator.

The intensity of the 820 nm fundamental wave incident on the nonlinear crystal 43 is strengthened in the resonator structure, and the second harmonic generation occurs strongly at the horizontal beam position where the beam is most narrowed. A 410 nm harmonic light is generated horizontally (to the right).

Most of the generated second harmonic wave is emitted from the right end of the non-linear crystal 43, passes through the lens 44, is adjusted in angle and convergence angle, and is incident on the next non-linear crystal 45. on the other hand,
Most of the 800 nm fundamental wave is a nonlinear crystal 43.
The light emitted from the right end face is confined, and the intensity is low.

BBO is used as the nonlinear crystal 45. In addition, 410 nm is provided on the left and right ends of the nonlinear crystal 45.
A wavelength-selective reflective film having a high reflectance for the second harmonic wave and a high transmittance for the fourth harmonic wave of 205 nm is formed. Non-linear crystal 45
Is a monolithic resonator having the same configuration as the non-linear crystal 43.
A 205-nm fourth harmonic is generated from the harmonic, and the generated fourth harmonic is emitted from the right end face thereof. The size of the nonlinear crystal 45 is also
The same degree as that of the nonlinear crystal 43.

The generated fourth harmonic wave is incident on the lens 46, the direction and the divergence angle are adjusted, and is output as an ultraviolet laser output. The final output of 205 nm ultraviolet light is expected to be about 50 mW per laser element.

As a structure for matching the resonance wavelength of the two nonlinear crystal monolithic cavities with the oscillation wavelength of the semiconductor laser, for example, “Longitudue” is used.
internally diode-pumped conti
neutral-wave 3.5-W green la
ser (LY Liu, M. Oka, W. Wiech
mann and S. Kubota, Optics
Letters, Vol. 19, p. 189 (199
4)) ”, a known electric circuit for servo control may be provided.

The resonance line width of a monolithic cavity is
Usually, it is sufficiently smaller than 1 pm, but further, by selecting one having a lasing wavelength to be used from a plurality of semiconductor lasers and matching the dimensions of the nonlinear crystal among the laser elements, the laser elements are It is possible to reduce the difference in wavelength.

If the wavelength matching is still insufficient,
A wavelength selection element such as an etalon may be inserted, or a well-known method called injection lock in which seed light is incident on each semiconductor laser may be used to make the wavelengths of the laser elements match.

According to the ultraviolet laser light source of this embodiment, 1
An ultraviolet light output of about 5 W is expected for a total of 00 laser elements. As the semiconductor laser 41, 772
4th harmonic is 193 nm if you use the one that emits nm light
And has the same wavelength as the ArF excimer laser. In this case, it can be used as an alternative to the excimer laser. At that time, as a crystal for generating the fourth harmonic, KB
BF is used.

Another embodiment of the ultraviolet laser light source to which the present invention is applied will be described with reference to FIG.

The ultraviolet laser light source of this embodiment is the same as that shown in FIG.
Similar to the embodiment of FIG. 5, a plurality of laser elements are bundled in parallel and each laser element has an optical structure as shown in FIG.

That is, each laser element includes a semiconductor laser 51 that generates excitation light, a laser resonator that is excited by the excitation light to generate light of a fundamental wave, and that converts the fundamental wave into a double wave (solid-state laser). ) 501 and the 2
A wavelength converter 502 having an external resonator structure for converting a harmonic into a fourth harmonic, a laser resonator 501, and a wavelength converter 50.
2 and a condensing lens 57 provided between the two, and finally outputs 266 nm ultraviolet light.

The laser resonator 501 includes a lens 52,
Reflecting mirrors 53 and 56 and solid-state laser medium Nd: YAG
It has a rod 54 and a nonlinear crystal 55 that performs wavelength conversion. Further, the wavelength conversion unit 502 includes a nonlinear crystal 59 that performs wavelength conversion, and reflecting mirrors 58 and 60 arranged on both sides of the nonlinear crystal 59.

Wavelength from semiconductor laser 51 is 808 nm
The excitation light (output 3 W) is condensed by the lens 52 and enters the Nd: YAG rod 54 through the reflecting mirror 53.
Here, similarly to the embodiment of FIG. 3, it is also possible to use an optical fiber to introduce the light from the semiconductor laser to the reflecting mirror.

The reflecting mirror 53 forms a reflective film having a high transmittance for the excitation light of the wavelength 808 nm, a high reflectance for the light of the wavelength 1064 nm, and a high transmittance for the light of the wavelength 532 nm. The Nd: YAG rod 54 receives the excitation light from the semiconductor laser 51 and generates a fundamental wave of 1064 nm, and has a cross-sectional dimension of about 3 mm square and a length of about 10 mm.

The generated fundamental wave enters the nonlinear crystal 55 and is converted into a second harmonic wave there. The nonlinear crystal 55 is
Nonlinear crystal KTiOPO4 (KTP) 5 for second harmonic generation
5, including Nd: YAG rod 54
It has almost the same dimensions as.

The reflecting mirror 56 is provided with a reflecting film having a high reflectance for the fundamental wave having a wavelength of 1064 nm and a high transmittance for the second harmonic wave of 532 nm. In addition to the above configuration, the laser resonator 501 further includes a wavelength plate and a polarizing element (both not shown) for making the longitudinal mode single.

In this embodiment, a high-intensity fundamental wave of 1064 nm is confined in the laser resonator 501, the nonlinear crystal 55 generates light of a 532 nm doubled wave, and this doubled wave is reflected. It is emitted from the mirror 56.

Here, the reflecting mirror 53 of the present embodiment may be replaced by forming a reflecting film on the left end surface of the laser medium 54. Similarly, the reflecting mirror 56 is connected to the nonlinear crystal 5
Alternatively, the right end of 5 may be replaced with a reflecting surface.

The second-harmonic 532 nm light emitted from the laser resonator 501 is adjusted in its convergence angle by the lens 57 to form a resonator structure including the nonlinear crystal 59 (hereinafter referred to as an external resonator). Called). BBO is used as the nonlinear crystal 59. External resonator 502
The 532 nm second harmonic light incident on is converted into a 266 nm fourth harmonic light.

A well-known electric servo circuit may be provided so that the resonance wavelength of the external resonator 502 is tuned to the wavelength of light generated from the laser resonator 501. Further, instead of the reflecting mirrors 58 and 60 of this embodiment,
Both ends of the non-linear crystal 59 may be processed to form reflecting films, and the reflecting mirrors 58 and 60 may be omitted.

According to this embodiment, when the semiconductor laser 51 having an output of about 3 W is used, the optical output of 266 nm ultraviolet light finally output from each laser element is expected to be about 100 mW. .

Another embodiment of the ultraviolet laser light source to which the present invention is applied will be described with reference to FIG. This embodiment uses pulsed light instead of the continuous light used in the embodiments of FIGS.

The ultraviolet laser light source of this embodiment is also constructed by arranging a plurality of laser elements in parallel, as in the embodiment of FIG. 2, and each laser element is an optical element as shown in FIG. It has a structure.

That is, each laser element of this embodiment is
It has a pulse laser by the well-known Q-switch method, a nonlinear crystal 67 for conversion to a second harmonic, and a nonlinear crystal 68 for conversion to a fourth harmonic.

The pulse laser is a semiconductor laser 61.
And a solid-state laser (laser resonator) 601. Laser resonator 601
Has a laser medium 64 and a modulator 65, and reflecting mirrors 63 and 66 arranged on both sides thereof.

The excitation light from the semiconductor laser 61 is guided to the laser resonator 601 through the optical fiber 62,
The Nd: YLF crystal 64 as a laser medium is excited through the reflecting mirror 63. In addition, instead of using the optical fiber 62, the light is condensed by the lens as in the embodiment of FIG.
The configuration may be such that it leads to the laser resonator 601.

The laser resonator 601 contains a modulator 64 based on the acousto-optic effect, and generates pulsed light having a wavelength of 1064 nm by the so-called Q-switch method. The pulse width of the generated pulsed light is about 10 ns,
The energy of one pulse is about 100 μJ, and the pulse repetition frequency is about 10 kHz. According to this configuration,
The average energy output is about 1W.

Since the pulsed light output from the laser resonator 601 has a large peak output, highly efficient wavelength conversion is performed without using a resonator structure for wavelength conversion.

In this embodiment, the fundamental wave of 1064 nm emitted from the laser resonator 601 is converted into a second harmonic wave of 532 nm by the first nonlinear crystal (KTP) 67 and is converted by the second nonlinear crystal (BBO) 68 into 4 Generates 266 nm ultraviolet light of the double wave. In addition, the BBO crystal causes
It is also possible to generate a 213 nm ultraviolet light by generating a harmonic sum frequency.

Here, in order to further increase the wavelength conversion efficiency, a condenser lens may be provided to collect the laser light and then pass the light through the nonlinear crystal.

According to the structure of the laser element of this embodiment,
UV light output (average output) of 266 nm finally obtained
Is expected to be about 100 mW. Therefore, a total output of about 10 W is expected for the entire ultraviolet laser light source obtained by bundling a total of 100 × 10 × 10 laser elements.

According to this embodiment, it is possible to realize an ultraviolet laser light source that can achieve advantages such as compactness, low spatial coherence, and ease of maintenance, and can generate pulsed light. can do.

Since pulsed light is used in this embodiment, the spectrum line width is wide as it is, and the line width of the output ultraviolet light is about 100 pm. Therefore, when it is used in an exposure machine, it is used in an exposure machine with an achromatic design.

However, it is also possible to reduce the spectral line width to 1 pm or less by a known method called injection lock.

[0146]

[Effect] According to the present invention, it is compact, easy to maintain, has high power utilization efficiency, and is easy to control the light output.
In addition, it is possible to provide an ultraviolet laser light source that generates ultraviolet light having a low spatial coherence with less damage to the nonlinear crystal.

[0147]

[Brief description of drawings]

FIG. 1 is a perspective view showing an overall configuration of an embodiment of an ultraviolet laser light source according to the present invention.

FIG. 2 is a perspective view showing the overall configuration of another embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an example of an optical structure of a laser element of the embodiment of FIG.

FIG. 4 is an explanatory diagram showing an example of an optical structure of a laser element according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an example of an optical structure of a laser element according to another embodiment of the present invention.

FIG. 6 is an explanatory diagram showing an example of an optical structure of a laser element according to another embodiment of the present invention.

[Explanation of symbols]

11 ... Semiconductor laser, 12 ... Optical fiber, 13 ... Solid-state laser, 14 ... Wavelength converter, 21 ... Semiconductor laser, 22 ... Optical fiber, 23 ... Solid-state laser, 33 ...
Reflecting mirror, 34 ... Laser medium, 35, 36, 37 ... Non-linear crystal, 38 ... Wave plate, 39 ... Non-linear crystal, 40 ... Reflecting mirror, 100 ... Laser light generating section.

Claims (9)

[Claims] 1. A laser having a laser light generator for generating light in a wavelength range from infrared to visible and a wavelength conversion optical system for converting the generated light into ultraviolet light by using a nonlinear optical crystal. An ultraviolet laser light source characterized by being configured by bundling multiple elements in parallel. 2. The ultraviolet laser light source according to claim 1, wherein the laser light generator includes a semiconductor laser. 3. The ultraviolet laser light source according to claim 2, wherein the laser light generator further comprises a solid-state laser excited by the light generated by the semiconductor laser. 4. The laser element according to claim 1, wherein the laser element is a semiconductor laser, a laser medium excited by the light generated by the semiconductor laser, and a fundamental wave excited by the laser medium is a fifth harmonic wave. And a laser resonator having a plurality of nonlinear optical crystals arranged in parallel, the ultraviolet laser light source. 5. The ultraviolet laser light source according to claim 1, wherein the wavelength conversion optical system has one or more nonlinear crystal monolithic resonators arranged in series. 6. The laser element according to claim 1, wherein the laser element is a semiconductor laser, a laser medium excited by the light generated by the semiconductor laser, and nonlinear optics for wavelength-converting the light excited by the laser medium. An ultraviolet laser comprising: a laser resonator having a crystal; and a resonator having a non-linear optical crystal for generating ultraviolet light by further wavelength-converting the wavelength-converted light in the laser resonator. light source. 7. The ultraviolet laser light source according to claim 1, wherein the laser light generator is a pulse laser that generates pulsed light. 8. The laser element according to claim 4, wherein a plurality of portions of each laser element excluding the semiconductor laser are bundled in parallel to form an emission end face of ultraviolet light. The ultraviolet laser light source further comprising an optical fiber for introducing light generated by the semiconductor laser into the laser resonator. 9. The ultraviolet laser light source according to claim 1, wherein the plurality of laser elements are arranged in parallel so that the emission end faces of the ultraviolet light of the respective laser elements form a matrix.