JPH01292821A - Optical apparatus - Google Patents

Abstract

PURPOSE:To reduce spatial coherence of a laser beam, and to enable the laser beam to be applied to an illuminating optical system for a mask with ease, by a structure wherein the distance between first and second reflecting surfaces, and the refractive index for laser light between the first and second reflecting surfaces are respectively specified. CONSTITUTION:Two inner surfaces 101, 102, opposite to each other, of a plane parallel plate 1 made of transparent material forms reflecting surfaces for a laser beam. Out of them, the edge section of the reflecting surface 102 on the side of a laser beam emitting source is provided with an incidence window 103 which transmits a laser beam. On the other hand, transmission window regions M1, M2,..., M5 are formed as a mask pattern on the reflecting surface 101. The distance (d) between the first and second reflecting surfaces 101, 102, and the refractive index (n) for a laser beam between the first and second reflecting surfaces 101, 102 are respectively set so that the phase difference of the laser beam passing through the transmission window regions adjacent to each other is larger than the coherence length of the laser beam. As a result, the laser beam passing through the adjacent transmission window out of the transmission window regions M1-M5 become incoherent mutually.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an optical device, and relates to an optical device suitable for, for example, an exposure device that projects a mask pattern onto a semiconductor wafer using an excimer laser. be.

[Prior Art] In recent years, in order to increase the degree of integration of semiconductor integrated circuits, there has been active research into exposure apparatuses for semiconductor manufacturing that use an excimer laser that oscillates at high output in the ultraviolet region as an exposure light source.

In particular, recently, it is expected that a reduction projection exposure apparatus using an F laser as an exposure light source for pulse oscillation at 248.5 nm will be developed and put into practical use as a mass-produced apparatus for processes requiring a resolution of 0.5 μm or less.

A problem with this type of exposure light source is that the coherence of the laser light causes an undesirable interference pattern that is superimposed on the image. Although excimer lasers are originally lasers with relatively low coherence both temporally and spatially, this inconvenience is unavoidable because they are not completely incoherent.

In addition, as a projection lens, it is preferable to use a monochromatic lens (consisting only of quartz) because it is easy to manufacture a lens with a relatively wide field. It is necessary to narrow the band to a certain degree, and an excimer laser with high coherence is inevitably required. As a result, the occurrence of the above-mentioned inconveniences is further aggravated.

Conventionally, as a method to solve the above problem, for example, by deflecting the illumination direction of the laser light on the mask for each oscillation pulse, the interference pattern is gradually shifted and irradiated multiple times, and as a result, the interference pattern is removed. That was considered.

[Problems to be Solved by the Invention] However, in the conventional interference pattern removal method as described above, when attempting to remove inconvenient interference patterns such as speckles, the number of irradiation pulses required for one exposure increases. However, there was a problem in that the throughput was significantly reduced.

This invention was made in view of such problems,
The objective is to provide an optical device that reduces the spatial coherence of the laser beam and can be easily applied to the illumination optical system of a mask.

[Means for Solving the Problems] In order to achieve the above object, an optical device of the present invention includes: a light source that outputs a laser beam; first and second reflective surfaces that are directly opposite to each other and are flat and parallel; and the light source. An entrance window is provided at one end of the second reflective surface so that the laser light output from the input window enters from the back side, and the laser light that enters at a predetermined angle from this entrance window irradiates the first reflective surface. After being reflected by the second reflective surface,
When the first reflecting surface is irradiated again and the reflection is repeated alternately on the first and second reflecting surfaces, the first reflecting surface or the second reflecting surface
The first reflecting surface or the second
An optical device comprising a plurality of transmission window regions sequentially arranged at predetermined intervals in correspondence to each irradiation position of a reflective surface, the optical device comprising: The distance between the first and second reflective surfaces and the refractive index for the laser beam between the first and second reflective surfaces are set so that the phase difference is longer than the coherence length of the laser beam.

[Function] The function of the present invention will be explained with reference to FIGS. 1 and 2.

In FIG. 1, a laser beam that enters from the entrance window 103 at a predetermined angle θ1 into the plane-parallel plate 1 with a thickness d and a refractive index n is refracted at an angle θ2 that satisfies the following: sin θ,=n sin θ2 (1). After being irradiated and reflected on the first reflecting surface 101, it is reflected on the second reflecting surface 102, and then the first
The first and second reflecting surfaces 10 are irradiated.
1 and 102, the reflection is repeated alternately. At that time, a part of the laser beam irradiating the first reflective surface 101 is
As shown in the figure, the light passes through a plurality of transmission window areas M, .about.M, which are successively arranged at predetermined intervals, corresponding to each irradiation position on the first reflective surface 101. Next, consider the phase difference between the laser beams transmitted through the plurality of transmission window regions M I"" M s.

A plane S perpendicular to the laser beam transmitted through the plurality of transmission window regions M
Assuming that adjacent transmission window regions, e.g., M and M
The phase difference Δ at the surface S of the laser beam transmitted through the laser beam 2 is Δ=2nd cos θ2 (2).

On the other hand, if the half width of the spectrum of the laser light source (not shown) is Δλ, the coherence length (coherence length xc is given by Jlc=λ2/Δλ (3). Therefore, from (2) and (3), , the condition that the phase difference between the laser beams transmitted through the adjacent transmission window regions M, , M is longer than the coherence length of the laser beams, Δ>ft, C, that is, 2nd cos θ2>λ2/Δλ (4) is obtained.

Therefore, the distance sd between the first and second reflective surfaces 101 and 102 of the parallel plane plate 2 is set so that this conditional expression (4) is satisfied.
By setting the refractive index n for the laser beam between the first and second reflective surfaces 101 and 102, even if the initially incident laser beam is completely spatially coherent, the adjacent transmission window areas The light beams that have passed through M, , M2 become incoherent. In the above description, M l + M 2 was shown as an example of a combination of adjacent transmission window areas, but an incoherent light beam can be obtained similarly even if other combinations are selected.

[Examples] Examples of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, two opposing inner surfaces 101 and 102 of a parallel plane plate l made of a transparent material form reflective surfaces for laser light. Among them, a laser light source (not shown)
At the end of the side reflective surface 102 (second reflective surface), an entrance window 103 through which the laser beam enters is provided.

On the other hand, on the reflective surface 101 (first reflective surface), as shown in FIG. 2, transmission window regions M, , M, . . . M5 are formed as a mask pattern. In the transmission window region Ml on the left side in FIG. 2, the laser light incident at a predetermined angle θ1 from the entrance window 103 directly irradiates the first reflective surface 101 without being reflected by the second reflective surface 102. located at the location. Hereinafter, M2
．． . . M, are arranged sequentially at predetermined intervals.

In the above configuration, the first and second reflective surfaces 101.10
As explained in the above operation, the distance Md between the two and the refractive index n for the laser beam between the first and second reflective surfaces 101 and 102 are such that the phase difference between the laser beams transmitted through the adjacent transmission window areas is It is set to be longer than the coherence length of the laser beam. As a result, among the transmission window regions M1 to MS, the light beams that have passed through adjacent transmission window regions become incoherent with each other.

Note that the transmission window areas M, ~MS are the areas where the irradiated laser light is transmitted.
It is configured to transmit a part of the luminous flux. Next, an example of the configuration of the transmission window regions M, to M5 will be described.

If the width of the incident laser beam is , then the first reflecting surface 10
The width b2 of the laser beam at the cross section of 1 is b2=b,/co
It becomes sθ1. In order to prevent the transmission window regions Ml-MS from overlapping each other, the width b2 of this laser beam should satisfy the relationship: 2dtan θ2>b2.

Further, in each of the transmission window regions M1 to M, three rectangular transmission windows 104 each having a long side a and a short side 2 are provided.

The transmission windows 104 are arranged at a pitch of 5b, and the difference between the transmission window regions M1 to M is that they are sequentially shifted from each other. Considering the transmission window regions Ml-MS superimposed, the 15 transmission windows 104 are aX1
5b and do not overlap each other. Therefore, 15 b ”= b 2= b , / c
o sθ.

If it is set as , there will be no loss of light quantity.

Furthermore, the transmission window 104 within one transmission window region has the function of sampling the light beam from a partial portion within the cross section of the laser beam. Therefore, even if there is unevenness in the light intensity distribution of the incident laser beam, the difference in the light intensity between the light beams that have passed through each of the transmission window regions M1 to M5 is small.

Next, a description will be given of an example of a device configuration in which the device shown in FIG. 1 is expanded two-dimensionally.

FIG. 3(a) shows its plan view, and FIG. 3(b) shows its side view.

In the figure, a laser beam emitted from a laser light source (not shown) is adjusted to an appropriate diameter by an adjustment system 2. As the adjustment system 2, a cylinder and a trical expander are shown as examples. Of course, the system is not limited to this, and any system with different (or equal) beam expansion factors in orthogonal directions may be used.

The laser beam whose diameter has been adjusted appropriately is incident on the plane-parallel plate l. The plane-parallel plate 1 emits the incident laser beam as a group of mutually incoherent beams, as described with reference to FIG.

Another parallel plane plate 3 is arranged on the exit side. This parallel plane plate 1 basically has the same structure as the parallel plane plate 2, and is provided with first and second reflective surfaces 301 and 302 on its directly opposing inner surfaces.

However, this parallel plane plate 3 is wider in the laser incident direction than the parallel plane plate 1 in order to collectively reduce the coherence of the light beams spread in the lateral direction. Further, the size of the transmission window area (not shown) of the second reflective surface 302 is also longer in the longitudinal direction than M1 to M11 shown in FIG.

It enters from the entrance window 303 of the parallel plane plate 3 and each reflection surface 30
The laser beam emitted while being reflected by 1,302 is
It is converted into a 5×5 mutually incoherent beam group. Even if these groups of light beams are appropriately condensed by a condenser lens into a single light beam, they no longer interfere with each other, and no interference pattern is generated on the desired object.

As a result, a laser beam with relatively high spatial coherence is converted into a group of globally uniform beams with low spatial coherence.

However, locally, within each unit light beam, there is an intensity distribution due to the transmission window pattern. Therefore, when this optical device is applied to the illumination system of a mask of a semiconductor exposure device, known means for uniformizing illumination by using the group of light beams emitted from this device as a secondary light source, such as a fly-eye lens and a condenser lens, are used. Note that in FIGS. 1 to 3 above, the parallel plane plate 1
Although the number of light flux groups that are converted into incoherent light fluxes according to (3) has been explained as five, it is not limited to this. Of course, the number of light beam groups can be arbitrarily selected by changing the length of the parallel plane plate 1 (3) and the number of transmission window areas. For example, in the configuration shown in FIG. 3, the number of light flux groups converted by each of the parallel plane plates 1 and 3 is N.

It is possible to configure it so that it can be converted into NXM luminous flux groups as M (Nf-M).

Further, an adjustment system such as a beam expander may be installed for each beam transmitted through each transmission window region.

Furthermore, the first reflective surface 101 (301) and the second reflective surface 101 (301)
The reflective surface 102 (302) is shown as the inner surface of the parallel plane plate 1 (3) made of a transparent material with a refractive index of n.
Instead of the parallel plane plate 1 (3), the first and second reflecting surfaces may be configured by simply opposing two reflecting mirrors.

Furthermore, the same effect can be obtained even if the second reflecting surfaces 102 and 302 are used as the reflecting surfaces for extracting mutually incoherent light beam groups.

[Effects of the Invention] As described above, the optical device of the present invention has the effect that the irradiated laser beam can be converted into a globally uniform beam with low spatial coherence.

Therefore, when this optical device is applied to a mask illumination system of an exposure apparatus for semiconductor manufacturing, it is possible to prevent the occurrence of undesirable interference patterns such as speckles. Furthermore, in this case, if a pulse laser such as an excimer laser is used as the laser light, it is possible to obtain light with reduced spatial coherence for each pulse. Therefore, compared to the conventional interference pattern prevention method using multiple times of polarized illumination, the number of pulses required for exposure is smaller, and throughput can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical device according to an embodiment of the present invention, and FIG.
The figure is a plan view showing a transmission window area, FIG. 3(a) is a plan view of an optical device according to an embodiment of the present invention, and FIG. 3(b) is a side view corresponding to FIG. 3(a). . [Explanation of symbols of main parts] 1.3...Parallel plane plate 101.301...First reflecting surface 102
．． 302...Second reflective surface M, ~M,...
......Transparent window area 103...
Entrance window 104...Transmission window Agent Patent attorney Masatoshi Sato

Claims (1)

[Scope of Claims] A light source that outputs a laser beam; first and second reflecting surfaces that are directly opposite to each other and parallel to each other; and a second reflecting surface that allows the laser beam output from the light source to enter from the back side. an entrance window provided at one end of the surface, and a laser beam that enters at a predetermined angle from the entrance window, irradiates and reflects the first reflective surface, and then reflects at the second reflective surface,
When the first reflecting surface is irradiated again and the reflection is repeated alternately on the first and second reflecting surfaces, the first reflecting surface or the second reflecting surface
The first reflective surface or the second
An optical device comprising a plurality of transmission window regions sequentially arranged at predetermined intervals in correspondence to each irradiation position of a reflective surface, the optical device comprising: The distance between the first and second reflecting surfaces and the refractive index for the laser beam between the first and second reflecting surfaces are set so that the phase difference is longer than the coherence length of the laser beam. optical equipment.