JP2003240507A - Interferometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To surely improve S/N of data acquired by a fringe scan method even if disturbance is not suppressed. <P>SOLUTION: Fringe scan is performed to change optical path difference between reference light and the light to be inspected. Accumulated data that is a time integration value over a prescribed hours for intensity is sequentially sampled as the data representing changes in intensity of the interference light generated then. The timing representing minimum strength of the interference light is made to agree with the timing for starting sampling of any accumulated data. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference measuring apparatus to which a fringe scan method is applied, and more particularly to an interference measuring apparatus when a surface to be inspected has a relatively high surface accuracy. About. 2. Description of the Related Art In manufacturing a high-performance optical system, not only processing and adjustment of an internal optical element but also measurement of the surface shape of the optical element performed each time the processing and adjustment are performed. It must be performed with high precision. As an apparatus for measuring the shape of the surface to be measured with high precision, there is an interference measuring apparatus using a Twyman-Green interferometer, a Fizeau interferometer, or the like. As shown in FIG. 4 (a), the interference measuring device causes a test light beam 60b reflected on a test surface 60a to interfere with a reference light beam 61b reflected on a reference surface 61a, thereby obtaining a test light beam. It is intended to observe the difference between the respective equiphase plane distributions of the reference light beam 60b and the reference light beam 61b as interference fringes.
This observation is performed by an imaging device including a photoelectric conversion device such as a CCD. [0004] In an interference measurement apparatus, a fringe scan method is often employed in order to perform measurement with higher accuracy than the accuracy determined by the gradation of the photoelectric conversion element. In the fringe scan method, the optical path difference between the test light beam 60b and the reference light beam 61b is positively changed by moving the reference surface 61a (or the measurement surface 60b) in the optical axis direction (fringe scan), and imaging at that time is performed. The signal waveform of the output of the element is detected, and the phase (for example, initial phase) of the signal waveform that changes based on the change in the intensity of the interference light generated at each position is obtained. By determining the distribution of this phase, the shape of the test surface 60a is accurately represented. [0005] Here, the imaging device accumulates charges as much as incident light intensity at each pixel for each unit time T _c, accumulated data B ₀ is a time integral value over time T _c of the incident light intensity , B ₁ , B ₂ ,... Are sequentially output. For example, when the optical path difference between the test light beam 60b and the reference light beam 61b gradually changes, the intensity of the interference light repeatedly increases and decreases as shown in FIG. 4B, and the accumulated data B output from the image sensor at this time. _.. , ₀ , B ₁ , B ₂ ,... Are the area values of the regions hatched by oblique lines as shown in FIG. Some noise components are superimposed on the stored data B ₀ , B ₁ , B ₂ ,..., Which are output signals of the image sensor, by disturbance. If the S / N is low, the shape of the test surface 60a cannot be accurately obtained. In particular, (a diagram 4 (c) the accumulated data B _3.) Minimal accumulation data shown in FIG. 4 (c),
Since the signal component is small, other stored data (FIG. 4)
In (c), the S / N is lower than that of the accumulated data B ₀ , B ₁ , B ₂ , B ₄ ), which causes the calculation accuracy of the shape of the test surface 60a to be reduced (this specification). In this case, the accumulated data having the minimum signal component is referred to as “minimum accumulated data”.) However, it is extremely difficult to detect a disturbance that causes a noise component and suppress it to such an extent that the S / N of accumulated data is improved. Therefore, the present invention provides an S / N of data obtained by the fringe scan method,
It is an object of the present invention to provide an interference measurement device that can surely improve even if disturbance is not suppressed. An interference measuring apparatus according to claim 1 performs a fringe scan for changing an optical path difference between a reference light and a test light, and measures the intensity of the interference light generated at that time. In an interference measurement device that sequentially samples accumulated data that is a time integral value of the intensity over a predetermined time as data indicating a change, the timing at which the interference light shows the minimum intensity and the timing at which sampling of any of the accumulated data is started are determined. It has sampling control means for matching. An embodiment of the present invention will be described below with reference to the drawings. [First Embodiment] A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram of an interference measurement apparatus 10 according to the present embodiment. The present invention is particularly suitable when the surface accuracy of the test surface 12a (the degree of deviation from the design shape is small) is high (that is, when the uniformity of the interference fringes is high).
Hereinafter, for convenience, the shape measuring device 10 is of a Fizeau type, and has a reference surface 13a (here, the Fizeau lens 13).
A case in which the present invention is applied to a fringe scan method of moving the image will be described. The interference measuring apparatus 10 includes a light source 11, a beam expander 14a, a beam splitter 14b, a light beam diameter conversion optical system 14c, an image pickup device 16, a moving mechanism 17, a control circuit 18, an arithmetic circuit 19, etc. , Light source 11, beam expander 14a, beam splitter 1
4b, an optical system including the light beam diameter conversion optical system 14c, and the imaging element 16 is referred to as an interferometer 15). The interferometer 15 causes the test light beam 12b, which is reflected light from the test surface 12a, to interfere with the reference light beam 13b, which is light reflected from the reference surface 13a, and causes the test light beam 12b and the reference light beam 13b to intersect. The difference in the equiphase plane distribution is observed as interference fringes in the image sensor 16. Since the interferometer 10 is a Fizeau type, the Fizeau lens 13 having the reference surface 13a is disposed between the interferometer 15 and the test object 12 having the test surface 12a. The moving mechanism 17 performs a fringe scan (changes the optical path difference between the reference light flux 13b and the test light flux 12b) by a distance corresponding to the drive voltage given from the control circuit 18 to the Fizeau lens 13 Is moved in the optical axis direction. The imaging device 16 includes a control circuit 18
When the drive signal is input from, accumulated charges as much as incident light intensity at each pixel for each unit time T _c, accumulated data B ₀ is a time integral value over time T _c of the incident light intensity,
B _1, B _2, sequentially outputs .... The stored data groups B ₀ , B ₁ , B ₂ ,...
Are used as data for calculating the initial phase distribution of the signal waveform that changes with the change in the intensity of the interference light in the arithmetic circuit 19. The arithmetic circuit 19 may be provided outside the interference measurement device 10. The arithmetic circuit 1
Instead of 9, a computer that performs the same operation as the arithmetic circuit 19 may be used. Further, the arithmetic circuit 19 may perform a part or all of the operation of the control circuit 18. At the time of fringe scanning, the control circuit 18 controls the light source 11, the moving mechanism 17, and the image pickup device 16. The arithmetic circuit 19 analyzes the signal waveform obtained during the fringe scan based on the relationship between the moving speed of the reference surface 13a and the accumulation time _Tc of the image sensor 16. In order to simplify the arithmetic operation by the arithmetic circuit 19, , During which the intensity of the interference light changes by one period (a period in which the optical path difference is shifted by a distance of one wavelength)
Are set such that an integral number (hereinafter, referred to as four) of accumulated data is acquired. The arithmetic circuit 19 has at least one
A storage data group for a cycle (hereinafter, five storage data B ₀ , B ₁ , B ₂ , B ₃ , B ₄ for a ／ cycle) is provided. FIG. 2 is a diagram illustrating the operation of the interference measurement device 10 according to the present embodiment. FIGS. 2A and 2B show the relationship between the change in the intensity of the interference light at a certain pixel obtained by the interference measurement device of the present embodiment and the accumulated data, and FIG. The relationship between the change in the intensity of the interference light at a certain pixel obtained by the conventional interference measurement device and the accumulated data is shown (the differences between FIGS. 2A and 2B will be described later). Here, when the optical path difference between the test light beam 12b and the reference light beam 13b in the fringe scan gradually changes, the amplitude of the signal obtained by the intensity of the interference light changes in magnitude as shown in FIG. Therefore, the signal components of the accumulated data B ₀ , B ₁ , B ₂ , B ₃ , and B ₄ sampled at this time are various (however, the accumulated data B ₀ and B ₄ are shifted by one wavelength. , The signal components are almost the same.) At this time, the minimum accumulated data (accumulated data B ₀ , B ₃ , B ₄ in FIG. 2A, accumulated data B ₂ , B ₃ in FIG. 2B, accumulated data B in FIG. 2C). ₂ ) has a small signal component, so that S / S is compared with other stored data.
N decreases. However, in the interference measurement apparatus 10 of the present embodiment, the interference light has the minimum intensity so that the signal component of the minimum accumulated data of the accumulated data group (B ₀ , B ₁ , B ₂ , B ₃ , B ₄ ) is as large as possible. The sampling start timing of any of the stored data is made to coincide with the timing shown. For this reason, the control circuit 18 of the present embodiment
At the beginning of the fringe scan, the change in the intensity of the interference light is directly monitored. For this purpose, a monitoring circuit indicated by reference numeral 100 in FIG. 1 is provided. The monitoring circuit 100 includes, for example, a half mirror 1 for monitoring a change in intensity.
00a, photodetector 100b, peak hold circuit 10
0c, a comparator circuit 100d is provided. The half mirror 100a is inserted into a parallel light beam (a parallel light beam after the integration of the test light beam 12b and the reference light beam 13b) forming an interference fringe in the interferometer 15, and at least a part of the parallel light beam ( For example, a part of the light near the center of the light beam) is guided in the direction of the light detection element 100b.
The light detection element 100b is an optical sensor such as a silicon photodiode, and converts the intensity of incident light into an electric signal. Therefore, the output of the light detection element 100b changes according to the change in the intensity of the interference light. The output of the light detecting element 100b is connected to one input terminal of a comparator circuit 100d.
It is connected to the other input terminal of the comparator circuit 100d via the peak hold circuit 100c. Therefore,
During the fringe scan, a sine wave corresponding to a change in the intensity of the interference light is input to one input terminal of the comparator circuit 100d, and a high level corresponding to the peak of the intensity of the interference light is input to the other input terminal of the comparator circuit 100d. Is input. Therefore, a pulse wave is generated at the output of the comparator circuit 100d at a timing corresponding to the appearance of the peak of the intensity of the interference light. Control circuit 18 of the present embodiment
Monitors the output of the comparator circuit 100d at the start of the fringe scan, supplies a trigger signal to the image sensor 16 in accordance with the generated pulse wave, and stores the stored data group (B ₀ ,
The acquisition of B ₁ , B ₂ , B ₃ , and B ₄ ) is started. At this time, as shown by arrows in FIGS. 2A and 2B, a predetermined time T from the pulse wave generation time T _a (this can be regarded as the time when the intensity of the interference light becomes the highest). ₀
Only elapsed time T _b to the storage data group _{B 0, B 1, B 2} , B
_3, the acquisition of B ₄ is started. In the present embodiment, the time T ₀ is set to be equal to an integral multiple of the sampling period T _c. FIG. 2A shows that this time T ₀ is 2T _c
2B, the time T shown in FIG.
₀ is when it is set to 3T _c. FIG. 2 (c)
Is a case where the time T ₀ is other than an integral multiple of the sampling period T _c (conventional). As is apparent from a comparison between FIGS. 2A and 2B and FIG. 2C, in the present embodiment, the timing at which the intensity of the interference light becomes minimum (open arrows in the drawing) , Any of the stored data (in FIG. 2A, the stored data B ₀ , B ₄ ,
In FIG. 2B, the timing at which the storage data B ₃ ) starts to be stored coincides. Therefore, the signal components of the minimum accumulated data (accumulated data B ₀ , B ₃ , B ₄ in FIG. 2A, and accumulated data B ₂ , B ₃ in FIG. 2B) are the conventional minimum accumulated data (FIG. 2). In the case of (c), it is surely larger than the accumulated data B ₂ ). Therefore, the S / N is surely improved even if disturbance is not suppressed. As described above, in the interference measuring apparatus 10 of the present embodiment, instead of suppressing the disturbance, the change in the intensity of the interference light is directly monitored, and the timing of acquiring the accumulated data group is controlled accordingly. The S / N of the data is surely improved without suppressing the disturbance. In the present embodiment, the sampling timing of the stored data is controlled in order to increase the signal component of the minimum stored data as much as possible. However, the movement start position of the reference surface 13a (or the test surface 12a) is controlled. It may be good. However, at present, it is preferable to control the data sampling timing as in the first embodiment because it can be performed with high accuracy. In this embodiment, the surface (the surface to be inspected 12
Although the case of measuring a) has been described, the fringe scan method is also applied to the case of measuring the wavefront (transmitted wavefront) of a light beam formed by transmitting an object such as a lens, and the accuracy of the transmitted wavefront is sufficiently high. If it is higher, the present invention can be applied. Further, it goes without saying that the present invention can be applied to various interferometers such as a Twyman-Green interferometer other than the Fizeau interferometer. [Second Embodiment] A second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a schematic configuration diagram of the projection exposure apparatus according to the present embodiment. At least one optical element constituting the projection optical system L mounted on the projection exposure apparatus, when manufactured, has its surface shape and / or its transmitted wavefront measured by any of the interference measurements according to the above embodiments. ing. Then, it is assumed that at least one surface of the projection optical system L has been processed and / or adjusted according to the measurement result. According to the above embodiment, since the measurement is performed with high accuracy, the projection lens is manufactured with high accuracy even if the processing (and / or adjustment) method is the same as the conventional method. The projection exposure apparatus includes at least a wafer stage 108 and a light source unit 101 for supplying light.
And a projection optical system L. Here, the wafer stage 1
08 denotes a substrate (wafer) W coated with a photosensitive agent
8a. The stage control system 10
Reference numeral 7 controls the position of the wafer stage 108. The projection optical system L is a high-precision projection lens manufactured using the interferometer according to the embodiment as described above. Further, the projection optical system L is disposed between an object plane P1 on which the reticle (mask) R is disposed and an image plane P2 matched with the surface of the wafer W. Further, the projection optical system L has an alignment optical system applied to a scan type projection exposure apparatus. Further, the illumination optical system 102 includes a reticle R and a wafer W
And an alignment optical system 103 for adjusting the relative position between. The reticle R is for projecting an image of the pattern of the reticle R onto the wafer W,
It is arranged on a reticle stage 105 that can move in parallel with the surface 108 a of the wafer stage 108.
The reticle exchanging system 104 includes the reticle stage 10
The reticle R set on 5 is exchanged and transported. Further, reticle exchange system 104 includes a stage driver (not shown) for moving reticle stage 105 in parallel with surface 108 a of wafer stage 108. Further, the main control unit 109 controls a series of processes from the alignment to the exposure. As described above, according to the present invention, there is provided an interference measuring apparatus capable of surely improving the S / N of data obtained by the fringe scan method without suppressing disturbance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of an interference measurement device 10 according to a first embodiment. FIG. 2 is a diagram illustrating an operation of the interference measurement apparatus 10 according to the first embodiment. FIG. 3 is a schematic configuration diagram of a projection exposure apparatus according to a second embodiment. FIG. 4 is a diagram illustrating a conventional interference measurement. DESCRIPTION OF SYMBOLS 10 Interferometer 12 Test object 12a Test surface 12b Test light beam 13 Fizeau lens 13a Reference surface 13b Reference light beam 14a Beam expander 14b Beam splitter 14c Light beam diameter conversion optical system 15 Interferometer 16 Image sensor 17 Moving mechanism 18 Control circuit 19 Arithmetic circuit 100 Monitoring circuit 100a Half mirror 100b Photodetector 100c Peak hold circuit 100d Comparator circuit 101 Light source unit 102 Illumination optical system 103 Alignment optical system 104 Reticle exchange system 105 Reticle stage 107 Stage control system 108 Wafer stage 109 Main control unit L Projection optical system W Wafer R Reticle

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Claims (1)

Claims: 1. A fringe scan for changing an optical path difference between a reference light and a test light is performed, and as data indicating a change in the intensity of the interference light generated at that time, the data is generated within a predetermined time of the intensity. An interference measurement apparatus for sequentially sampling accumulated data that is a time integrated value over a period of time, characterized by having sampling control means for making the timing at which the interference light shows the minimum intensity coincide with the timing of starting sampling of any accumulated data. Interferometer.