JPH01290276A - Lighting device - Google Patents

Abstract

PURPOSE:To nip the development of speckles in the bud, by giving optical path differences which are larger than coherent distances of laser beams to respective divided laser beams. CONSTITUTION:An excimer laser oscillator is manufactured in a stable resonator type which has a dispersing element in a resonator. A laser beam 1 which is emitted by the above oscillator enters into a beam dividing apparatus 2 and four pieces of laser beams having each intensity that is almost 25% of original one are emitted. Being a raw material for the beam dividing apparatus 2, a silica plate makes its thickness sufficiently thick so that an optical path difference of each beam becomes exceedingly larger than a coherent distance lambda<2>/ lambda and then, these beams become incoherent each other. As this approach makes respective beams divided in this way incoherent each other and allows a number of them to be lined up in the lateral direction, no speckles develop in this device.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a lighting device, and relates to a lighting device suitable for, for example, an exposure device for manufacturing integrated circuits using an excimer laser.

[Prior Art] In recent years, excimer lasers have attracted attention as light sources for exposure apparatuses used in the manufacture of integrated circuits.

This excimer laser is currently a powerful short-wavelength light source, and is suitable for achieving the high linewidth accuracy that is required with the recent increase in the degree of integration of integrated circuits.

As this type of exposure light source, excimer laser light sources that have been put into practical use are broadly classified into stable resonator type and injection lock type.

The one shown in FIG. 2 is called a stable resonator type. In the figure, a discharge tube 12a that causes stimulated emission
Two resonator mirrors lla and llb are arranged at both ends of the resonator, forming a resonator. As the light travels back and forth between the two resonator mirrors 11a and llb, the amplitude of the stimulated emitted light is strengthened, and the laser beam 15 is emitted.

A drawback of this stable resonator type laser light source is that the emitted laser beam 15 has a low spatial and temporal coherence range. In other words, low temporal coherency means that the half-width of the spectrum is wide (△λ~0.
4 nm). Therefore, it is necessary to correct the achromatic chromatic aberration of the projection lens, and it is difficult to make a practical lens in this wavelength range.

Moreover, the one shown in FIG. 3 is called an injection lock type. This is a two-stage configuration consisting of an oscillator stage (upper side in Figure 3) and an amplifier stage (upper side in Figure 3), and in the oscillator stage, two resonator mirrors 11a and 11b are used. The arrangement is similar to the stable resonator type described above. However, this type of oscillator stage is equipped with a dispersion element 13 such as an etalon or a diffraction grating for selecting a wavelength in a predetermined region. Furthermore, apertures 1 are provided at both ends of the discharge tube 12a of the oscillator stage to narrow down the laser beam.
6 is placed. By providing these dispersion probes 13 and apertures 16, the half width of the spectrum of the oscillated laser beam is narrow (Δλ~O, OO1 nm).
This improves monochromaticity. The laser beam oscillated from this oscillator stage is reflected by a mirror 17 and enters the amplifier stage.

On the other hand, mirrors 14a and 14b for unstable resonators are disposed at both ends of the discharge tube 12b in the amplifier stage, with their convex and concave surfaces facing each other, and the incident laser beam is directed to these mirrors for unstable resonators. It is amplified and emitted by 14a and 14b.

The laser beam 15 emitted from this injection-locked laser light source is characterized by high monochromaticity and high temporal coherence. Therefore, the projection lens does not need to be achromatic, and can be manufactured using only a single glass material (quartz), which has the advantage of being easy to design and manufacture. However, since this laser beam is amplified by an unstable resonator, its spatial coherence is extremely high. Therefore, speckled exposure unevenness (hereinafter referred to as speckle) due to interference occurs in the exposed area.

Conventionally, the method for removing speckles has been to lower spatial coherence and reduce speckles by placing a vibrating mirror in the optical path of the illumination system and vibrating the beam.
It has been proposed to average the
No. 6317).

However, according to the research of the inventor of the present application, in order to effectively eliminate speckles by vibrating the beam, it is necessary to synchronize with the laser oscillation pulse to uniform the intensity distribution of the fly-eye lens of the illumination system. It has become clear that it is necessary to swing the beam two-dimensionally the number of times corresponding to the arrangement of the lens elements of the lens element. Therefore,
In the above method, it is necessary to swing the beam two-dimensionally a considerable number of times using a vibrating mirror, etc., which makes it difficult to obtain the appropriate exposure amount in a short time. The problem is that it has to be done.

To compensate for this problem, an excimer laser light source shown in FIG. 4 has been developed. This type of laser light source has a dispersion element 13 such as an etalon, prism, or diffraction grating disposed in the resonator of the above-mentioned stable resonator type laser light source as a band narrowing means, and the spectrum of the emitted laser beam is The width is narrow (Δλ~0.003 nm). The emitted laser beam is characterized by improved temporal coherence due to the provision of the dispersion element 13, and lower spatial coherence than that of the injection locking type.

[Problems to be Solved by the Invention] Since a stable resonator type laser light source equipped with a dispersion element as described above has low spatial coherence, it was expected that speckles would not occur unlike an injection locking type. However, in reality, a small amount of speckle (interference fringes) occurs, which has been found to hinder the formation of fine patterns in integrated circuits.

The present invention has been made in view of the above points, and an object thereof is to provide a lighting device that does not cause speckles.

[Means for Solving the Problems] In order to achieve the above object, an illumination device of the present invention includes a laser light source provided with means for narrowing the spectral width of laser light to be output, and a laser light source that is output from the light source. The laser beam is divided into a plurality of laser beams arranged parallel to its traveling direction and at approximately equal intervals, and each of the divided laser beams is provided with an optical path larger than the coherence length of the laser beam. The apparatus is equipped with means for providing a difference, and means for converging the plurality of laser beams divided by the dividing means into an appropriate diameter to illuminate an object to be irradiated.

[Function] In the illumination device according to the present invention, since a laser light source equipped with means for narrowing the spectral width is used, a laser beam with poor monochromaticity can be obtained.

The laser beam emitted from this light source is divided by the dividing means into a plurality of laser beams arranged parallel to the width direction of the beam cross section and at equal intervals, and this is based on the following reason.

Spatial coherence of a laser is the coherence of light within the cross section of the laser beam, and increases as the number of modes in the laser decreases. Generally, the number of modes of a laser is expressed as a2/λL (where d: laser beam diameter (aperture diameter); L: resonator length or laser beam pulse length).

A typical example of a laser beam cross-sectional shape is 20mmX
For a beam cross section of 7mm, L-, 4m, λ=0
．． Assuming 25 μm, the number of modes in the longitudinal direction is approximately 400,
The number of modes in the lateral direction is approximately 50, which differs by nearly one order of magnitude between the longitudinal and lateral directions.

This indicates that speckles tend to occur in the transverse direction of the beam cross section. For example, the semiconductor sea urchin surface (
When projecting a mask onto the reticle surface, a lattice-like speckle pattern (interference pattern) is generated on the eight surfaces, which are arranged in the transverse direction of the beam cross section.

Therefore, in order to eliminate this speckle, it is sufficient to increase the number of modes in the lateral direction isometrically.

The method is to split the laser beam and make each of the split beams incoherent with each other.
Then, you can line up a large number of them in the short direction. The larger the number of arranged elements, the better, but in reality, it is appropriate that the number be equal to the number of modes in the longitudinal direction. For example, in the case of the above example, it is appropriate to arrange 400150=8 pieces.

In this case, there are two possible ways to make each divided laser beam incoherent.

One of them uses temporal coherence,
By employing splitting means that provides each split laser beam with an optical path difference larger than the coherence length of the laser beams, the beams can be made incoherent with each other.

The other method takes advantage of the fact that polarizations perpendicular to each other do not interfere with each other. It uses birefringent crystals (e.g. calcite, quartz, MgF, KDP, A
DP, etc.), a mutually incoherent two-split beam can be obtained. However, since this method can only apply two divisions, the present invention adopts a means that utilizes the above-mentioned temporal coherence.

Note that if the number of modes in the transverse direction of the beam cross section is increased, it is conceivable that the transverse direction becomes longer than the longitudinal direction. For example, in the above-mentioned example of the cross-sectional shape of the laser beam, if the number of modes in the longitudinal direction and the transverse direction is made equal, for 20 mn1 in the longitudinal direction, 7×13 in the transverse direction
=56mm. Therefore, in the present invention, the divided beams are focused to an appropriate diameter by the shaping optical means to illuminate the object to be irradiated. This also makes the beam diffusivity equal.

For example, the beam divergence angle of an excimer laser is usually about 3 mrad in the longitudinal direction and about 1 mrad in the transverse direction.
If an optical system that reduces the beam size by /3 is adopted, the beam divergence angle in the lateral direction will be tripled to 3 mrad. Therefore, the beam divergence angles in both directions are approximately the same.

[Embodiments] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

In the following explanation, as an example of a laser beam, the width of the beam cross section in the transverse direction is approximately 7 mm, and the divergence angle is approximately 3 mrad.
This will be explained by taking an x1 mrad excimer laser as an example.

FIG. 1 is a block diagram showing an embodiment of the present invention.

In the figure, a laser beam 1 emitted from a stable resonator-type excimer laser oscillation device (not shown) having a dispersion element within the resonator is incident on a beam splitter 2 . This beam splitter 2 is a parallel plate with reflective films attached to both sides of a quartz plate, and its reflective surface a.

The reflectances of b, c, d, and e are 75%, 66°5%, respectively.
They are 50%, 0%, and 100%. That is, approximately 25% of the intensity of the laser beam 1 before entering the beam splitter 2.
Four beams are output. The thickness of the quartz plate forming this beam splitter 2 is such that the optical path difference between each beam is the interference distance λ2.
/Δλ is set to be sufficiently thick.

Therefore, these beams are mutually incoherent. Furthermore, the inclination of the beam splitter 2 is adjusted so that the optical axes of the respective beams are shifted from each other by about 14 mm.

The four beams output from this beam splitter 2 are composed of birefringent crystals (e.g. calcite, MgF, quartz, KDP, A
DP, etc.) 3 into two beams each with a different deflection. Therefore, in total, it will be divided into 8 wooden beams. In this case, the eight beams do not interfere with each other because the different polarized lights do not interfere with each other.

This birefringent crystal 3 has been processed to have an appropriate orientation and thickness depending on the type of crystal, and the distance between the centers of the two beams is approximately 7 mm1li.

Therefore, among the eight beams, the centers of adjacent beams are spaced apart from each other by about 7 mm1l. The width of each beam in the transverse direction is approximately 7 mm, so the total width is approximately 20 mm x 56 mm.
The beam will be approximately -mm in diameter.

On the other hand, the divergence angle of this beam is approximately 3 mrad x 1 mrad
However, this beam is passed through the cylindrical lenses 4a, 4
The beam is focused into a beam of approximately 20 mm x 20 mm by a beam shaping optical system according to b. Then, the spread angle is approximately 3mrad x 3m
It becomes rad.

In the above embodiment, a beam splitting means is described in which the beam splitter 2 and the birefringent crystal 3 are combined for the purpose of obtaining a large number of split beams, but the beam splitter 2 may be used alone. Further, the present invention can be similarly applied even when the cross-sectional shape of the laser beam l is close to a square, and the beams divided by the beam splitter 2 may be partially overlapped.

Additionally, it is also possible to split the beam using the birefringent crystal 3 alone without using the beam splitter 2. However, when using an excimer laser as in this embodiment, the number of beams split by the birefringent crystal 3 is only two, which is not suitable for obtaining the desired effect. Therefore, when using the birefringent crystal 3, it is always necessary to combine it with the beam splitter 2. However, in the case of other types of laser beams, it is possible to obtain the minimum necessary effect using only a birefringent crystal. In this case, the two divided beams do not need to have an optical path difference greater than the coherence length.

[Effects of the Invention] As explained above, according to the illumination device according to the present invention, a laser beam is divided, and an optical path difference larger than the coherence length of the laser beam is given to each of the five divided laser beams. As a result, mutually incoherent beams are obtained. Furthermore, since each of the divided beams is arranged in the lateral direction of the beam cross section, the number of modes in the lateral direction increases, resulting in a beam with low spatial coherence.

Therefore, the beam formed by converging each of the divided beams to an appropriate diameter has no possibility of causing speckles as a result.

It goes without saying that this beam has high temporal coherence and excellent monochromaticity because it employs a laser light source equipped with means for narrowing the spectral width.

Therefore, if the illumination device according to the present invention is used in, for example, an exposure device for manufacturing integrated circuits, it is possible to obtain extremely uniform exposure.

Furthermore, since it is not necessary to vibrate the beam a considerable number of times to remove speckles, an appropriate exposure amount can be ensured without reducing throughput. For the same reason, since there is no need to provide a vibrating mirror or the like, the device configuration is also simple.

The lighting device according to the present invention has excellent effects as described above, and is suitable for use in an exposure device for manufacturing integrated circuits, a photo-CVD device, and the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIGS. 2 to 4 are schematic diagrams of conventional devices. [Explanation of symbols of main parts] 1...Laser beam 2...Beam splitter 4a, 4
b... Cylindrical Lens Agent Patent Attorney Tadashi Sato Figure 1

Claims (1)

[Claims] A laser light source equipped with a means for narrowing the spectral width of laser light to be output, and a laser beam outputted from this light source in parallel to its traveling direction and at approximately equal intervals. a means for dividing the plurality of laser beams into a plurality of laser beams arranged in a row and giving each of the divided laser beams an optical path difference larger than a coherence length of the laser beam; 1. An illumination device characterized by comprising: a means for gathering light to an appropriate diameter and illuminating an object to be irradiated.