JPH03134538A - Evaluating apparatus of lens - Google Patents

Abstract

PURPOSE:To measure the image revolving efficiency of a lens with high accuracy even in a high spatial frequency region by shutting the 0-order diffracting light among the diffracting lights from a first diffraction grating of a reticle by a spatial filter. CONSTITUTION:A light from a coherent light source 4 illuminates the whole of a projecting area 20 through an illuminating optical system comprised of a condensing optical system 5 and the like. As a result, a diffracting light is generated from all of the first diffraction gratings 21, which is incident upon a projecting lens 1. The diffracting light becomes a spectrum of points corresponding to the diffraction order on an entrance pupil 8 of the projecting lens 1. A shielding part of a spatial filter shuts off the 0-order diffracting light of the diffracting lights from the whole of the projecting area 20 on a reticle 2, so that the + or - first order diffracting lights penetrating the projecting lens 1 interfere with each other to form an interference fringe on an image forming surface of the projecting lens 1. A reference reticle 11 is moved to a normal image forming position within the projecting area 20 on 12. Accordingly, the interference fringe by a second diffraction graring 31 on the reference reticle 11 and the + or - primary diffracting lights from the projecting area 20 and a Moire's fringe, namely, the aberration efficiency of the whole of an exposing area 30 can be monitored.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION The present invention relates to a lens evaluation device for evaluating the optical characteristics of a high-resolution lens used for exposure of semiconductors and the like.

BACKGROUND OF THE INVENTION Recently, lens evaluation devices are required to have ultra-high resolution. In particular, lenses used in stepper devices that project fine patterns on semiconductors are required to have ultra-high resolution of 1 micron or less and spatial frequency of 500 lines per minute or less. performance is very important. Conventionally, the MTF method has been used as a lens evaluation method. As an example, "Photographic lenses and response functions" (Supervised by Hiroshi Kubota, Optical Technology Association Cotton, P2
8-P41, October 1960) is generally known.

The conventional MTF method will be explained below with reference to FIG. FIG. 9 is a configuration diagram of a measuring device showing the measurement principle of the MTF method using a scanning slit. In FIG. 9, 101 is an evaluation lens for evaluation;
102 is a grid chart placed on the object image plane of the evaluation lens 101, 103 is a light source, 104 is a narrow scanning slit placed on the image formation surface of the evaluation lens 101, and 105 is a photodetector made of a phototube. .

In the above configuration, when the lens magnification of the evaluation lens 101 is β, a lattice chart 102 of 1/β times the spatial frequency to be evaluated is placed on the object image plane. Then, this grid chart 102 is illuminated by a light source 103 behind it, and an image is formed through an evaluation lens 101. A scanning slit 10 whose width is sufficiently narrower than the spatial frequency to be evaluated is placed on the imaging plane.
4 is placed, and the intensity of the light passing through this scanning slit 104 is detected by a photodetector 105.

The light intensity detected by the photodetector 105 is modulated by scanning the slit 104, as shown in FIG. If the horizontal axis in Fig. 10 is the slit movement amount ■ and the vertical axis is the light intensity (I), then the MTF is (Im
ax-Imin)/(Imax+lm4n) At this time, if the resolving power of the evaluation lens 101 is sufficient, the detected MTF is close to 1 as shown by the broken line, but as the resolving power deteriorates, it approaches O. . By this method, the resolution of the evaluation lens 101 can be measured.

Problems to be Solved by the Invention However, in the conventional configuration as described above, as the spatial frequency increases, the slit width becomes narrower, especially 1 μm.
When trying to measure the following resolution performance, the slit width becomes sub-μm. Therefore, it is not possible to manufacture 104 (Surin), and direct measurement of MTF is impossible. Therefore, conventionally, when measuring resolution performance of 1 μm or less, a method was adopted in which a grating image was enlarged with a lens and the enlarged image was scanned with a slit 104 for measurement. The measurement includes aberrations, making it difficult to obtain accurate values.

The present invention solves the problems of the prior art as described above, and aims to provide a lens evaluation device that can measure the resolution performance of a lens with high precision even in a high spatial frequency region. This is the purpose.

Means for Solving the Problems Technical means of the present invention for solving the above problems include a reticle placed at the object image position of the lens to be evaluated and on which a first diffraction grating is formed, and a reticle on which this reticle is placed. a coherent light source that illuminates the entire area of the reticle where the first diffraction grating is formed; and a coherent light source that is removably arranged on a spectral plane inside the evaluation lens that re-diffracts the diffracted light from the reticle. a spatial filter that blocks zero-order light among the diffracted light from the first diffraction grating; a reference reticle arranged at the imaging position of the evaluation lens and on which a second diffraction grating is formed; and the reference reticle. a stage for moving the reticle within the projection range of the evaluation lens, and a means for observing moiré fringes generated by the interference fringes formed on the reference reticle by re-diffraction by the evaluation lens and the second diffraction grating. It is something.

and forming the first diffraction grating into a linear diffraction grating,
When the numerical aperture of the evaluation lens is NA and the wavelength used is λ, the pitch P is set so that P≧λ/NA, and the second diffraction grating is formed as a linear diffraction grating, and the pitch is p, and when the magnification of the evaluation lens is β, p=n・
If the second diffraction grating is formed into a linear diffraction grating, and the pitch is set to p1, where the magnification of the evaluation lens is β, then p-1/n.
It can be made to be β·P (where n is an integer).

According to the above structure, the present invention blocks the 0th-order diffracted light from the first diffraction grating of the reticle by a spatial filter, and re-diffracts the ±1st-order diffracted light by an evaluation lens to produce a light beam on the imaging plane. By interfering with each other, interference fringes are formed, and by observing moiré fringes between these interference fringes and the second diffraction grating pattern of the reference reticle placed on the imaging plane, the high spatial frequency of the re-diffraction image to be evaluated is determined. can be converted to the low frequency region of moiré fringes.

EXAMPLES Hereinafter, examples of the present invention will be described with reference to the drawings.

First, a first embodiment of the present invention will be described.

FIG. 1 is an overall configuration diagram showing a lens evaluation apparatus according to a first embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes a projection lens whose resolution performance is to be evaluated, and in this example, the projection magnification is 115, the wavelength used is λ=248 nm, and the numerical aperture NA=0°355. 2
As will be described later, is a reticle on which a first diffraction grating is formed, and is placed on the object image plane of the projection lens 1. 3
is a mask stage on which a reticle 2 is placed and held movably within a horizontal plane. 4 is a coherent light source for illuminating the reticle 2, and has the same wavelength as the wavelength used by the projection lens 1. Reference numerals 5, 6, and 7 denote a condensing optical system, an optical path conversion mirror, and a condenser lens, respectively, and these collectively constitute an illumination optical system. 8 is the entrance pupil of the projection lens 1; 9 is a spatial filter that is detachably provided on the 8 surfaces (spectral plane) of the entrance pupil of the projection lens 10; , has an aperture 10 that selectively transmits only the ±1st-order light. Reference numeral 11 denotes a reference reticle on which a second diffraction grating is formed, as will be described later, and is arranged on the imaging plane of the projection lens l.

12 is a stage on which a reference reticle 11 is placed and held movably within the projection range of the projection lens 1 in the horizontal plane and in the optical axis direction;
have. 14.15.16.17 is stage 12
This is an objective lens, an optical path conversion mirror, an imaging lens, and an imaging device that is disposed below the aperture 13 of the reference reticle 11 and is sensitive to the wavelength of the coherent light source.

Next, details of the reticle 2 and the reference reticle 11 will be explained.

FIG. 2 is a plan view of the reticle 2. FIG. As shown in FIG. 2, the projection lens 1 needs to be evaluated over the entire projection area 20, so the projection area 20 of the reticle 2 has a pitch of, for example, 3.5 μm (P≧λ/NA).
A first linear diffraction grating 21 is formed.

FIG. 3 is a plan view of the reference reticle 11. As shown in FIG. 3, the reference reticle 11 has the magnification of the projection lens I, that is, ×115, and the
．． A second linear diffraction grating 31 with a pitch of 0.7 μm resulting from a pitch of 5 μm is formed over the entire exposure area 30 corresponding exactly to 115 of the projection area 20 .

In the above configuration, the measurement principle will be explained in the fourth section below.
This will be explained with reference to the explanatory diagram shown in the figure.

As shown in Figure 4, the light emitted from the coherent light source is
It passes through the first diffraction grating 21 of the reticle 2 and is diffracted,
+1st order diffraction light 41A, 0th order diffraction light IC11th order diffraction light old B
After entering the projection lens 1, the light passes through the spatial filter 9. The spatial filter 9 cuts the O-order diffracted light 41C,
Only the ±1st-order diffracted lights 41A and 41B are transmitted. The ±1st-order diffracted lights 41A and 41B that pass through the projection lens 1 are diffracted again and form interference fringes on the second diffraction grating 31 on the reference reticle 11.

The intensity I(x) of the interference fringe at this time is determined by the "optics of optical equipment" (
Written by Yoshisada Hayami, Optical Technology] Contact Vo123, N03
(1985) P 174-P183), it is given by the following formula.

Hx)=b”/2 (1+cos (4πx/p))
b: Transmission amplitude of the first diffraction grating 21 p: The value obtained by multiplying the pitch P of the first diffraction grating 21 by the magnification β of the projection lens 1.The pitch of the interference fringes formed is based on the original geometrical optics. The contrast is 1/2 of the normal image, and the contrast is 1. That is, by forming the pitch of the second diffraction grating 31 to be 1/2 times the pitch given by βxp formed by original geometrical optics, the interference fringes on the reference reticle 11 and the second diffraction It becomes possible to observe moiré fringes due to the superposition of the gratings 31.

Furthermore, if the projection lens 1 has no aberration, the interference fringes formed will be the interference of ideal plane waves of the ±1st-order diffracted lights 41A and 41B, so the interference fringes will be straight lines, as shown in FIG. , the moire fringes are also straight lines, but if the projection lens 1 has aberrations, the ±1st-order diffracted lights 41A and 41B do not become plane waves, so the interference fringes are curved, resulting in moiré fringes as shown in FIG. A bend will also occur. By quantitatively measuring the curvature of the moiré fringes, it becomes possible to measure the aberrations of the projection lens 1.

In order to measure the bending of moiré fringes, the pitch of the second diffraction grating 31 is practically nXβxP S or 1/
It may be n×βXP (n is an integer), and in this embodiment, it is set to 07 μm as described above.

Next, the evaluation procedure of the above embodiment will be explained with reference to FIG.

The light emitted from the coherent light source 4 is transmitted through an illumination optical system consisting of a condensing optical system 5, an optical path conversion mirror 6, and a condenser lens 7 to a projection area 20 (the first diffraction grating 21 of the reticle 2). (see Figure 2). As a result, diffracted lights from all the first diffraction gratings 21 are generated and enter the projection lens 1. The diffracted light incident on the projection lens 1 forms a point spectrum on the entrance pupil 8 of the projection lens 1 according to the order of diffraction. Since a spatial filter 9 having an aperture 10 that selectively transmits only the ±1st-order diffracted light is provided on the entrance pupil plane 8, the image forming surface of the projection lens 1 is provided with a spatial filter 9 that selectively transmits only the ±1st-order diffracted light. ±1st order diffracted light 41A,
41B (see FIG. 4) interfere with each other, resulting in interference fringes. The reference reticle 11 is the first reticle in the projection area 20.
The second diffraction grating 31 on the reference reticle 11 is moved by the stage 12 to a position where the diffraction grating 31 is properly imaged by the projection lens 1, and interference fringes are formed by the second diffraction grating 31 on the reference reticle 11 and the ±1st-order diffracted lights 41A and 41B from the projection area 20. It becomes possible to observe moiré fringes, that is, the aberration performance of the entire exposure area 30 (see FIG. 3). Objective lens 14, optical path conversion mirror 1
5. An observation optical system including an imaging lens 16 and an imaging device 17 moves the moire fringes on the reference reticle 11 to the center of the observation field, and observes the shape of the moire fringes.

Next, a second embodiment of the present invention will be described.

7 and 8 are plan views of a reticle and a reference reticle, respectively, used in a lens evaluation apparatus according to a second embodiment of the present invention.

The overall configuration diagram of this embodiment is the same as the configuration shown in FIG. 1, but the reticle and the reference reticle are different. As shown in FIG. 7, the first diffraction grating 21 of the reticle 2 is a group of diffraction gratings separated into AXA minute regions in the projection area 20.

On the other hand, as shown in FIG. 8, one second diffraction grating 31 of the reference reticle 11 is formed with a size corresponding to 115 of the AXA minute region 21a in the first diffraction grating 21.

In the above configuration, a method for evaluating aberrations in a specific region of the projection lens 1 will be explained below, taking the AXA micro region 21a shown in FIG. 7 as an example. Of the diffracted lights from the entire area 20, the 0th order diffracted light is blocked, ±1
Only the next diffracted light is re-diffracted onto the reference reticle 11 by the projection lens 1. Therefore, interference fringes are generated on the imaging plane of the projection lens 1 at a position corresponding to the entire projection area 20 of the reticle 2. However, since the second diffraction grating 31 is engraved only in a portion of the reference reticle 11 with a size corresponding to the minute region 21a, the moiré fringes are caused by the interference fringes and the second diffraction grating on the reference reticle ll. This occurs only in the area where the grids 31 overlap. The reference reticle 11 is a minute area 2
1a is properly imaged by the projection lens 1.
moiré fringes between the second diffraction grating 31 on the reference reticle 11 and the interference fringes caused by the ±1st-order diffracted light from the minute region 21a can be observed.

As in the first embodiment, the observation optical system includes a reference reticle 1
The moire fringes on 1 are moved to the center of the observation field, and the shape of the moire fringes is observed.

Effects of the Invention As described above, according to the present invention, of the diffracted light from the first diffraction grating of the reticle, the 0th-order diffracted light is blocked by the spatial filter, and only the ±1st-order diffracted light is re-diffracted by the evaluation lens. This method forms interference fringes on the imaging plane, and observes the moiré fringes between these interference fringes and the second diffraction grating on the reference reticle placed on the imaging plane, so the re-diffraction that should be evaluated is The high spatial frequency of the image can be converted into the low frequency region of moiré fringes, and therefore, it is possible to measure the aberration of an evaluation lens such as a projection lens with high precision even in the spatial frequency region of 1 μm or less.

[Brief explanation of the drawing]

1 to 4 show a lens evaluation device according to a first embodiment of the present invention, in which FIG. 1 is an overall configuration diagram, FIG. 2 is a plan view of a reticle, and FIG. 3 is a plan view of a reference reticle. , Fig. 4 is an explanatory diagram of the measurement principle, Fig. 5 is a diagram showing the moire fringe shape when the lens has no aberration, Fig. 6 is a diagram showing the moire fringe shape when the lens has aberration, and Fig. 7 8 shows a lens evaluation device according to a second embodiment of the present invention, FIG. 7 is a plan view of a reticle, FIG. 8 is a plan view of a reference reticle, and FIGS. MTF
FIG. 2 is a configuration diagram and measurement data diagram showing a method for measuring lens resolution according to the method. DESCRIPTION OF SYMBOLS 1... Projection lens (evaluation lens), 2... Reticle, 3... Stage, 4... Coherent light source, 9...
...Spatial filter, 11...Reference reticle, 12...
-Stage, 17...imaging device, 21...first
diffraction grating, 31... second diffraction grating.

Claims (1)

[Scope of Claims] (1) A reticle placed at the object image position of the lens to be evaluated and on which a first diffraction grating is formed, a stage on which this reticle is placed, and a first diffraction grating of the reticle. a coherent light source that illuminates the entire area formed; and a coherent light source that is removably disposed on a spectral plane inside the evaluation lens that re-diffracts the diffracted light from the reticle, and that includes diffracted light from the first diffraction grating. A spatial filter that blocks zero-order light,
a reference reticle arranged at the imaging position of the evaluation lens and on which a second diffraction grating is formed; a stage for moving the reference reticle within the projection range of the evaluation lens; A lens evaluation device comprising means for observing moiré fringes generated by interference fringes formed on a reticle and the second diffraction grating. (2) First
If the diffraction grating is a linear diffraction grating, the pitch P is the numerical aperture of the evaluation lens, and the wavelength used is λ,
The lens evaluation device according to claim 1, wherein P≧λ/NA. (3) If the second diffraction grating is a linear diffraction grating, its pitch is p, and the magnification of the evaluation lens is β, then p=n・β
- P (where n is an integer), the lens evaluation device according to claim 1. (4) If the second diffraction grating is a linear diffraction grating, its pitch is p, and the magnification of the evaluation lens is β, then p=1/n
-β·P (where n is an integer), the lens evaluation device according to claim 1.