JPH0298919A - Laser apparatus - Google Patents

Abstract

PURPOSE:To lower coherence in terms of space and time of a laser beam by a method wherein an optical system used to connect a cross-sectional shape of the beam is installed between a laser source and a first reflection member. CONSTITUTION:Optical systems 5a, 5b used to convert a cross-sectional shape of a beam are installed between a laser source 1 and a first reflection member 6. In this case, wavelength-narrowing elements 3a, 3b used to narrow a spectral width of the laser beam may be installed on a light path between the first reflection member 6 and a second reflection member 4. It is preferable that the first reflection member 6 is composed of an aggregate of a plurality of optical rods, of different lengths, which are arranged in parallel with a light-path direction of the beam; at least one end face of the plurality of optical rods is used as a reflecting face having a transmissive property; the aggregate as a whole is used as a stepped mirror. Thereby, the beam is expanded inside a resonator; it is possible to prevent an increase in coherence in terms of space and time by an expansion of the cross-sectional shape of the beam.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser device, and particularly to an excimer laser device used as a light source in an exposure device for semiconductor manufacturing.

[Prior Art] When using an excimer laser among laser devices as a light source of an exposure device, an excimer laser whose spectral width is in the If range is used due to the specifications of the projection lens.

Usually, this narrowing is achieved by inserting an etalon, grating, prism, etc. into the external mirror resonator.

Figure S4 shows an example of the configuration of a conventional projection exposure apparatus using an excimer laser.

Reference numeral 100 denotes an excimer laser cover body, which incorporates a flat low-reflectance front mirror 60 as a resonator, a rear mirror 4g, a laser chamber Ig, an etalon 3g for wavelength narrowing, and an etalon tilt drive section 30. There is.

A portion of the laser beam emitted from the excimer laser device main body 100 passes from the beam splitter 32 to the etalon 34 for wavelength measurement to form interference fringes on the light receiving surface of the image sensor 36. Since the bright and dark intervals of these interference fringes are shifted due to wavelength shift, this is processed by the calculation unit 38 to obtain the amount of wavelength shift, and the amount of tilt of the etalon 3g required to correct the shift amount is calculated. It is output to the drive section 30.

This attempts to stabilize the absolute wavelength.

Now, the excimer laser reflected by the mirror M enters a beam expander (BXP) that expands the beam diameter (cross-sectional shape) and aligns it into a square, and then cooperates with the condenser lens 26 to adjust the illuminance distribution on the reticle 27. Uniform fly eye lens (obtical integrator)
The light is incident on an illuminance uniform optical system (FOP) including a light source. The uniform illumination light irradiates the reticle 27, and the pattern of the reticle 27 is exposed to the resist layer of the wafer 29 through a projection lens (quartz only, a combination of quartz and fluorite) 28.

FIG. 2 shows an example of the optical system FOP. Expander BX
The parallel beam LB from P is the first fly-eye lens 2
The light enters each element 20a of 0 and is focused as spot lights SPI and SP2 in the space on the exit side of each element, and then is focused on the incident surface of the second fly's eye lens 24 by the lens system 22. A plurality of spot lights SP, SPb as a tertiary light source are focused on the exit side of each element lens of the fly-eye lens 24, and these spot lights SPa,
Each beam diverging from SPb is condensed by a condenser lens 26 and superimposed on a reticle 27. This makes the illuminance uniform.

Here, the first stage fly-eye lens 20 is, for example, 5×5.
If the light source is composed of 25 element lenses, there will be 25 spot lights S P i and S P 2 as secondary light sources. All of the light that diverges and travels from each of these 25 spots enters the 5×5 element lenses of the second-stage fly-eye lens 24.

Therefore, since light from 25 secondary light source spots simultaneously enters one element lens 24a, spot lights from 25 tertiary light sources are generated at the exit end of the element lens 24a, and as a result, 3 The total number of spot lights of the secondary light source is 25x25=625.

By the way, in the case of the above configuration, the spatial coherence of the beam LB is considerably increased by the etalon 3g, and furthermore, due to the magnification by the fly's eye lens 20.24, the tertiary light source sp.

The spatial coherence of the beam at SPb increases considerably, and in the case of Fig. 2, the coherence increases by about 5 times (the number of array sides in one direction in the first stage) at the tertiary light source surface. It will improve.

Therefore, light beams traveling from mutually adjacent spot lights among the tertiary light source spots may interfere with each other. Of course, depending on the degree of coherence of beam LB, 2
Next light source SP1. ．． There may also be conditions that cause interference at SP2.

[Problems to be Solved by the Invention] Therefore, in such a state, strong interference fringes will be generated on the reticle 27 and eventually on the wafer 29, which is inappropriate for the above-mentioned exposure.

Here, the beam LB incident on the fly eye lens 20,
Alternatively, it is considered that the interference fringes are averaged (smoothed) in the resist layer by oscillating excimer laser pulses while changing the incident angle of the beam incident on the fly's eye lens 24, but the structure The problem is that it becomes complicated and costs increase.

Further, in a general laser device, a laser beam generated only from a laser chamber has a large divergence angle and is an extremely large multi-mode laser beam, and the beam itself has extremely low spatial coherence.

However, especially in the case shown in Fig. 5 where band narrowing is performed by the etalon 3g within the resonator, the etalon 3g has the property of aggressively selecting only a certain spectral width (for example, 4/1000r+n+). , rear mirror 4
By repeating the beam reflection multiple times between Since the divergence angle becomes smaller, the temporal coherence of the beam generated from the resonator naturally increases, but the spatial coherence also increases. (In other words, it can no longer be said to be multimode.) For this reason, when expanding and converting the cross-sectional shape of the laser beam emitted from the resonator (especially when an expander BXP is installed outside the front mirror 60 as shown in FIG. In this case, since the excimer laser beam is not in car mode, it is considered to have partial coherence), and the spatial coherence will be increased according to its magnification.

Here, in particular, since the beam cross-sectional shape of the excimer laser is a rectangle elongated in the direction of the discharge electrode, it is necessary to expand the beam with a beam expander or the like and shape it into a square in the optical path leading to the exposure device.

At this time, as mentioned above, the spatial coherence of the laser improves by the magnification, causing the problem that speckles, that is, interference fringes, are more likely to occur.

That is, in the case of FIG. 4 in which the exposure apparatus illuminates the reticle 27 by enlarging the beam diameter, interference fringes are generated to an extent that cannot be ignored.

Furthermore, since the illumination system of the exposure apparatus includes two fly's eye lenses as described above, the spatial coherence is further improved (both in the short side direction and the long side direction).

The present invention aims to reduce the spatial and temporal coherence of a laser beam generated in a laser device, and particularly aims to reduce speckles generated by the above-mentioned coherence improvement in an excimer laser. .

[Means for solving the problem] This invention has been made in view of the above problems, and includes a laser source that generates a laser beam, and a laser source that is placed in front and behind the laser source on the optical path of the beam. In a laser device that has a first reflecting member and a second reflecting member that form a resonator, the first reflecting member has transparency and extracts the laser beam, an optical system that converts the cross-sectional shape of the beam,
It is provided between the laser source and the first reflecting member.

In this case, a wavelength narrowing element for narrowing the spectral width of the laser beam may be provided on the optical path between the first reflecting member and the second reflecting member.

The first reflecting member is preferably made of an aggregate of a plurality of optical rods of different lengths arranged in parallel to the optical path direction of the beam, and has transparency through at least one end surface of the plurality of optical rods. The reflection surface is used as a reflection surface, and the aggregate as a whole is used as a stepped mirror.

In addition, especially when the laser source is a laser source that generates an excimer laser beam, in order to shape the beam cross-sectional shape of the excimer laser beam, which has a rectangular cross-sectional aspect ratio other than 1:1, to make it approximately square. Preferably, the beam cross-sectional shape converting optical system is an optical system including a cylindrical lens.

[Function] In the present invention, the beam is expanded within the resonator by providing an optical system for converting the cross-sectional shape of the laser beam in the resonator on the above line, so that the cross-sectional shape of the beam is not changed as in the conventional method. There is no increase in spatial coherence due to expansion.

As mentioned above, the laser beam generated only from the laser source is
Originally, the spatial coherence is extremely low, and the temporal coherence is also low, so even if the optical system is placed inside the resonator as in the present invention, the beam expansion by this optical system will not affect the spatial coherence at all. never change.

Figure 5 schematically represents this situation.
Spatial coherence is considered in terms of whether light at adjacent points in the cross section of the beam interfere with each other.
The wider the distance between the points, the higher the spatial coherence.

FIG. 5(^) shows a beam L B o having a rectangular cross section that has only been narrowed by an etalon, and each point on the X-axis and the y-axis represents the point at which they begin to interfere with each other. Here, point P1 on x glaze. When P2 (points that are adjacent to each other) is narrower than the interval shown in the figure, the light from those points will interfere with each other. The coherence in the y-axis direction is at point P3. It is assumed that no interference occurs between two points separated by 67 points or more, such as P4.

FIG. 5(B) shows a beam LB" which is made into a square beam by expanding the beam LBo in the y direction using an external expander BXP.
, and the y@force direction coherence is the point Ps, P4
If the two points are separated by more than n·ΔY, interference will occur between the two points.

Here, n is the magnification rate of the beam in the y direction. As is clear, in this case the spatial coherence is n times higher in the y direction.

On the other hand, if an expander BXP is provided inside, the fifth
As shown in figure (C), the coherence in the y direction at the original beam LB is preserved, and two points p, which are separated by more than △Y in the y direction,
, p, will not interfere with each other.

That is, the spatial coherence of the beam extracted from the front mirror without the expander BXP inside the resonator, and the beam extracted with the expander BXP inside the resonator (the beam diameter has been expanded). It can be considered that the spatial coherence of

Therefore, even if the temporal coherence is improved to some extent by providing an etalon inside the resonator, if the expander BXP is provided externally to obtain a beam diameter of, for example, 20×20IIIn, then if it is provided internally to obtain a beam with the same diameter. The spatial coherence is clearly lower in the latter case.

Note that the beam shape conversion optical system described above is not limited to one that expands only in one direction as described above, and there are various types, such as one that expands the entire beam uniformly, one that reduces the longitudinal direction and expands after shaping. This includes a conversion method.

Furthermore, a similar effect can be achieved even in a resonator that does not include a wavelength narrowing element within the resonator.

Here, let us consider the degree of coherence by assuming that the first reflecting member (front mirror) is an ordinary plane mirror.

As mentioned above, especially when wavelength narrowing is performed within a resonator, the laser beam emitted from the resonator is extracted with an extremely sharp wavelength spectrum. Naturally, when temporal coherence increases, spatial coherence also increases.

In the present invention, since the first reflecting member is a stepped mirror,
Since the output beams have different wavefronts (phases) due to the path length difference when passing through each step element, spatial coherence decreases and coherence within the beams weakens.

The phase difference given by each step element may be random, but
A phase difference relationship of λ/2 will result in a loss of light quantity and should be avoided as much as possible.

In the present invention, the stepped mirror is made up of an assembly of a plurality of optical rods of different lengths arranged parallel to the optical path direction of the beam generated from the laser source, and at least one end surface of the plurality of optical rods is made of a transparent material. It is created by making it a reflective surface (reflectance of about several percent).

Therefore, it is possible to easily form the step of the mirror by simply changing the length of each loft, and the outer diameter of the loft can be adjusted to 1
A front mirror that matches the cross-sectional shape of the laser beam can be easily created by simply changing the number or arrangement of the mirrors.

Also, AR (anti-flexing) is applied to the end surface of each rod.
By applying a coating, etc., it becomes a more transparent mirror. Here, if one end of each rod is aligned to form the same plane, AR coating etc. can be easily performed.

[Embodiment] FIG. 1 is a schematic diagram showing an entire embodiment of the present invention. 1 is a laser gas chamber (laser source), 2a,
2b is a window. 4 is a mirror with a reflectance of 100% (second reflecting member), and 6 is a front mirror with a step and a low reflectance.

Laser band narrowing is achieved by etalons (wavelength narrowing elements) 3a, 3.
5a and 5b are expanders B that expand only the short side direction of the laser beam and shape the beam into a square.
This is an XP (beam cross-sectional shape conversion optical system).

The size (cross-sectional shape) of the stepped mirror 6 corresponds to the cross-sectional shape of the beam expanded to a square shape (for example, for a shaped beam of 20 x 20 (mm), the number of elements is 4 x 4 = + 6 (consisting of prismatic rods of 5×5+ nm each), each of which has its longitudinal direction parallel to the optical path direction of the beam, and the length of each prismatic rod is different.

The end face of each element is coated with an AR coating, and has a reflectance of about several percent.

In the laser chamber 1, an active medium for excimer laser such as KrF or Xecl is sealed under a predetermined pressure and gas mixture.

Between the front window 2a and the rear window 2b (quartz), there are two electric panels parallel to each other along the optical axis AX.
is arranged, and by causing a high voltage discharge between the electrodes 1c and Id, a pulse of excimer laser beam LB0 is generated.

Note that la and Ib are suction and exhaust vibes for exchanging internal gas, and are closed by valves except when exchanging gas.

The beam LBO from the window 2a has a long rectangular cross section in the X direction of the beam cross section. This is electric Jilc, ld
However, it is usually difficult to make the cross-sectional shape of the beam LBO square, and the aspect ratio becomes a value other than 1:1.

5a is a concave cylindrical lens (cylindrical lens), 5
b is a convex cylindrical lens, which expands the width of the beam L Bo (in the lateral direction and y direction) and converts it into a beam LBI having a square cross section.

The beam LBI is determined to be a nearly parallel beam, but this does not necessarily have to be strict, and in particular in the X direction of the beam LB, the cylindrical lens 5a
, 5b, the divergence angle of the beam LB0 emitted from the laser chamber 1 is preserved as it is.

Beam L B + (20ma+X 20 mml
enters the stepped mirror 6, but several percent of it is reflected and reaches the rear mirror 4.

By the way, in the case of FIG. 1, the cross-sectional shape of the beam LBO is rectangular, and the spatial coherence is relatively high in the lateral direction compared to the longitudinal direction. However, cylindrical lens 5 as expander BXP
Since a and 5b act to expand the width direction,
Beam LB finally obtained (front mirror emitted light)
The spatial coherence in the y direction of is improved to be closer to the spatial coherence in the x direction.

On the other hand, as shown in FIG. 3, if the stepped mirror 6 is a collection of prismatic lofts (quartz) with different lengths (for example, beam LB), if the cross-sectional area is 20 x 20 mm, each rod 6a,
If the tip area of 6b, 6c, .

If the phase difference generated for each rod is made slightly different (for example, all 16 rods are made to have different lengths), the spatial coherence of the beam LB finally emitted from this laser device will be the same in the x and y directions. Both will be further reduced.

Further, although the stepped mirror 6 serving as a front mirror has its stepped surface facing the laser chamber 1 side, it may be arranged with its stepped surface facing the opposite direction. In other words, the same result can be achieved even if the end surfaces of the rods are aligned on the same plane and are used as a front mirror as a resonator.

In Fig. 1, one etalon 3a has an extremely narrow spectrum width and is used for precise wavelength setting, and etalon 3b
The spectrum width is set to be relatively wider than the spectrum width narrowed in 3a. The need for two sets of etalons is due to the relationship between the original spectral width of the excimer laser and the etalon finesse, and both are driven by the etalon tilt drive section 10 to stabilize the absolute wavelength. .

In this embodiment, the inclination of etalons 3a and 3b corresponds to the beam LB.
This is done centering on an axis parallel to the longitudinal direction of O.

Since the excimer laser oscillates in pulses (100 to several hundred H2), it is recommended to servo control the etalons 3a and 3b during the exposure operation and fix the tilt angles of the etalons 3a and 3b during non-exposure. good.

Furthermore, as shown in FIG. 6, in another embodiment, as an expander BXP, lenses 5c and 5d reduce and shape the beam cross-sectional shape in the longitudinal direction, and a spherical lens 5e enlarges the entire beam.
, 5f is used. The conversion optical system also provides the same effect as described above.

In this case, in order to adjust the optical state, the position of the lens 5e is moved by a drive system 40 that slightly moves the lens 5e in the optical axis direction, and the divergence angle of the expanded beam L B c reaching the front mirror is adjusted. The state of the system can be adjusted by the divergence angle of the beam LBt between 5d and 5e.

In addition, as shown in Fig. 7, a convex plano lens 5G is used instead of the lens 5f.
By applying an AR coating to the flat side of the lens, the lens can also be used as a front mirror.

In this case, the laser i? Beam LBd from il or beam LB squared by internal expander BXP
, + are emitted as the second stage beam LBa by the concave lens 5H1 and the convex plano lens 5G.

In addition, by using an afocal zoom expander system as a conversion optical system, the beam diameter can be arbitrarily varied, making it easy to adjust the beam diameter according to the illumination area without changing the degree of spatial coherence. There is also the advantage that it can be done.

Furthermore, although the laser device becomes larger, it is possible to make the diameter of the emitted beam the same as that of the reticle, and after emitting the beam, reduce the beam diameter to the desired diameter using a lens system before guiding it to the illumination optical system. [Effects of the Invention] As described above, according to the present invention, an optical system for converting the cross-sectional shape is provided in the resonator of the laser device, and the beam is expanded within the resonator, so that it is possible to achieve the same effect as before. There is no increase in spatial coherence due to beam expansion.

Therefore, when the beam diameter generated from the laser source is enlarged and used, a laser beam with lower coherence than before can be obtained.

This also applies when a wavelength narrowing element is provided within the resonator.

In addition, the first reflecting member is made of an aggregate of a plurality of optical rods having different lengths and arranged in parallel to the optical path direction of the beam, and the first reflecting member is configured to have a transmissive shape, and at least one end surface of the plurality of optical rods is made of a transparent material. Because it is a stepped mirror with a reflective surface,
Since the beams output from each stepped rod have different wavefronts (phases), spatial coherence decreases and coherence within the beams weakens.

In addition, especially when the laser source is a laser source that generates an excimer laser beam, the beam cross-sectional shape of the excimer laser beam whose cross-sectional shape has a rectangular 1iHM ratio other than 1:1 may be shaped to be approximately square. Furthermore, the beam cross-sectional shape converting optical system is an optical system including a cylindrical lens.

Therefore, when enlarging and shaping the cross-sectional shape of an excimer laser beam into a square of a desired size, a beam with low coherence can be obtained with a simple device.

In particular, when the laser device is used as a light source for an exposure device for semiconductor manufacturing, it is possible to obtain a good exposure device that does not cause speckles on reticles or wafers.

FIG.
CB) is a diagram showing the spatial coherence in the expanded state outside the resonator, and (C) is a diagram showing the spatial coherence in the expanded state inside the resonator. FIG. 6 is an optical system for converting the cross-sectional shape according to another embodiment of the present invention. FIG. 7 is a diagram showing a first reflecting member mirror according to another embodiment of the present invention.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the entire embodiment of the apparatus according to the present invention, Fig. 2 is a diagram showing an example of an FOP used in an exposure apparatus for semiconductor manufacturing, and Fig. FIG. 4 is an enlarged perspective view showing such a stepped mirror; FIG. 4 is a diagram showing an example of the configuration of a projection exposure apparatus using a conventional excimer laser; FIG. Schematic representation of the situation [Explanation of symbols of main parts] 1.1g... Laser chamber, la, b... Gas intake and exhaust pipes, Ic, d... Electrodes, 2a, b...
Window, 3a, b, g... Etalon, 4.4g... Rear mirror 5... Beam expander 5a, b, c, d --- Cylindrical lens, 5e, fg, h
... Spherical lens, 6 ... Stepped mirror, 6a, b, c - ... Prismatic loft 10 ... Etalon tilt drive section, LB, LB, ... Laser beam (state emerging from resonator) , LBO, LB, ... state as it is emitted from the laser source, LB,, LBe ... enlarged and shaped state, LBb,
LBd: Shaped state, AX: Optical axis of the laser beam, P+, Pt, P3, Pa: For convenience, to represent spatial coherence, a point representing the position where they start to interfere with each other, X, y, z...Axes indicating relative directions, 40... Drive system for slightly moving the lens in the axial direction, 41.
...AR coat. 20.24-・Fly eye lens, 20a, 24a・
...Fly eye lens element, S P ,, S
Px, SP-, SP6... Spot light generated in the space on the exit side of each element, 22... Lens system, 26.
...Condenser lens, Patent attorney: Tadashi Sato 27...Reticle, 28...Projection lens, 29...
- Wafer, 32... Beam splitter, 34... Etalon for wavelength shift measurement, 36... Image sensor, 38... Wavelength shift calculation section, 60... Planar front mirror, 30... Etalon tilt drive unit, 100・
...Wavelength narrow band stable excimer laser device, θ Fig. 2 Fig. 8 Fig. 4

Claims (4)

[Claims] (1) A laser source that generates a laser beam, and a first reflecting member and a second reflecting member that are arranged in front and behind the laser source on the optical path of the beam to form a resonator, and the first reflecting member In the laser device, the laser beam is extracted by imparting transparency to the reflective member, characterized in that an optical system for converting the cross-sectional shape of the beam is provided between the laser source and the first reflective member. laser equipment. (2) A wavelength narrowing element for narrowing the spectral width of the laser beam is provided on the optical path between the first reflecting member and the second reflecting member.
1) The laser device described above. (3) The first reflecting member is composed of an assembly of a plurality of optical rods of different lengths arranged in parallel to the optical path direction of the beam, and at least one end surface of the plurality of optical rods has transparency. Claim (1) characterized in that the aggregate is a reflective surface and the aggregate constitutes a stepped mirror as a whole.
The laser device according to 1) or (2). (4) The laser source is a laser source that generates an excimer laser beam, and the beam cross-sectional shape conversion optical system is configured to generate a transverse dimension of the excimer laser beam such that the cross-sectional shape has a rectangular aspect ratio other than 1:1. The laser device according to claim 1, wherein the laser device is an optical system including a cylindrical lens that enlarges and shapes the laser beam.