JPS61215905A - Position detecting device - Google Patents

Abstract

PURPOSE:To execute the highly accurate position detection by obtaining a prescribed frequency difference at two beams of luminous fluxe, generating the optical beat at the interference of two or more diffracted light beams and detecting the phase difference with a reference signal having the frequency of the difference between the optical beat signal and the frequency of two beams of luminous fluxe. CONSTITUTION:A laser beam LB from a laser light source 1 makes incident on an acoustic optical modulator 2 and a zero-order beam 10 parallel to the beam LB irradiates a substrate 4 from diagonally. A beam 12 frequency-modulated by the modulator 2 is deflected by a certain angle only to the beam 10 and irradiates the substrate 4 from diagonally. Since for the beam 10 and the beam 12, only the frequency (f) is different, the photoelectric signal goes to be an optical beat signal, and goes to be the sine waveform of the frequency (f) similar to a reference signal MS. The phase difference between the signal MS and the photoelectric signal is obtained by the level of a phase difference signal PDS from a phase difference detecting circuit 8. Here, presently, when the substrate 4 is positioned for two beams of luminous fluxe 10 and 12, the coarse positioning is executed by using a stage 5, after this is completed, a main control device 22 reads the signal PDS and the direction of dislocation is detected by the polarity and the dislocation quantity is detected by the size.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a position detection device for a wafer, mask, etc. using a diffraction grating, and particularly to a position detection device suitable for alignment called a holographic alignment method.

(Background of the Invention) In recent years, the miniaturization and densification of semiconductor devices have progressed significantly, and alignment of masks (reticles) and wafers is necessary for manufacturing semiconductor devices (especially VLSI).
There is also a need to perform this with higher precision. As one method for increasing the precision, for example, Japanese Patent Application Laid-Open No. 59-19291
A method called a more holographic alignment method disclosed in Japanese Patent No. 7 has been proposed. In this method, two beams of coherent light of equal frequency are incident from the t-2 direction, and a wafer with a diffraction grating arranged parallel to the interference fringes obtained by interference of the two beams is is placed in the optical path of the diffraction grating, and the light reflected or transmitted by the diffraction grating, or the diffracted light, is interfered again,
The relative position between the interference fringes of the two light beams and the diffraction grating is detected based on a signal obtained by photoelectrically detecting the interference intensity. Looking at this method from another perspective, when one of the two light beams A is irradiated onto the diffraction grating, the diffracted light (a) generated in a specific direction, and the other light beam B is irradiated onto the diffraction grating. , the phase of the nine-diffracted light (-) generated in that specific direction is
This means that a change in the interference intensity of the diffracted lights (a) and (b) caused by a change due to the relative movement of the diffraction grating and the two light beams is detected. According to such a detection method, it is certainly possible to detect a position with high precision, but there are major problems when it comes to actual device implementation. Errors in position detection are caused by changes in the intensity of the light source that generates two coherent beams, changes in the intensity ratio of the two beams, and changes in the intensity of the diffracted lights (a) and (b) caused by irregularities in the diffraction grating. It is to become. Although changes in the intensity of the light source and changes in the intensity ratio between the two beams can be suppressed to a considerable extent during the design and manufacture of the device, changes over time still result in errors. In addition, the intensity change of the diffracted light (-1(b) occurs depending on the shape and surface condition of the diffraction grating, so
It is difficult to deal with this on the equipment side.

Furthermore, unless the interference fringes created by the two light beams and the diffraction grating on the wafer are moved relative to each other, a photoelectric signal whose level changes depending on the positional shift cannot be obtained. This means that accurate alignment cannot be achieved until the interference fringes and the diffraction grating are moved relative to each other by 8 degrees per cycle, which results in the drawback that when integrated into a device, the alignment time cannot be fundamentally shortened.

(Objective of the Invention) The present invention solves these drawbacks, enables highly accurate position detection regardless of changes in light intensity, and detects the position within one period without relatively moving the two light beams and the diffraction grating. An object of the present invention is to obtain a position detection device that can detect the amount of deviation.

(Summary of the Invention) The present invention provides a predetermined frequency difference between two beams of light, generates an optical beat in interference between the diffracted lights (a) and (b) from a diffraction grating, and combines the optical beat signal with the frequency of the two beams. The technical point is to detect the position of the substrate from the phase difference using the so-called original heterodyne interferometry, which detects the phase difference with a reference signal having a frequency equal to the difference in frequency.

(Embodiment) FIG. 1 is a diagram showing a schematic configuration of a position detection device according to a first embodiment of the present invention. The laser beam LB from the laser light source 1 is transmitted through an acousto-optic modulator 2 as a two-beam generating means.
The parallel zero-order light 10 of the laser beam LB is incident on the mirror 3
The light is reflected by the beam and irradiates the substrate 4 such as a wafer from an oblique direction. Frequency modulated by acousto-optic modulator 2 (hereinafter simply referred to as optical modulator 2)! The (+1st order light) 12 is deflected by a certain angle with respect to the zero order light 10, and irradiates the substrate 4 obliquely as a parallel light beam. The optical modulator 2 receives a reference signal (modulation signal) MS- of a frequency f from an output circuit 6 as a reference signal generating means.
i is input to make the frequency of the modulated light 12 different from the frequency of the zero-order light by f.

The substrate 4 is provided with a diffraction grating 4a in which elongated gratings extending in a direction perpendicular to the plane of the drawing are formed parallel to each other at a constant pitch in the left-right direction in the plane of the drawing.

Then, so that the interference fringes obtained by interference between the zero-order light 10 and the modulated light 12 are parallel to the grating of the diffraction grating 4a,
Inject two Altlo, 12. At this time, zero-order light 1
0 and the modulated light 12 have different frequencies, the interference fringes formed by the two light beams do not remain stationary with respect to the substrate 4, but flow at a frequency f, and this is a so-called optical beat. still,
The pitch of the interference fringes and the pitch of the diffraction grating 4a are determined to be integral multiples. Now, the substrate 4 is placed on a stage 5 for positioning, and this stage 5 is moved by a drive motor 21 in the rightward direction in the plane of the drawing. Further, the position of the stage 5 is sequentially detected by a length measuring device 20 such as a laser beam interferometer.

By the way, when the zero-order light 10 is irradiated onto the diffraction grating 4a, diffracted lights of various orders are generated at respective diffraction angles.

Among them, diffracted light 11 of a certain order generated at an angle β is received by a photoelectric detector 7 such as a photomultiplier via a slit plate 7a having slits parallel to the grating. The angle β is the angle formed by the zero-order light 10 and the diffracted light 11. At the same time, since the modulated light 12 is irradiating the diffraction grating 4a,
Diffracted light of various orders due to t is generated at each diffraction angle. The photoelectric detector 7 receives the diffracted light 13 of a certain order, which is generated at an angle α and travels along substantially the same optical path as the diffracted light 11, through the diffracted light 13 (Surin) & 7a.

On the light receiving surface of the photoelectric detector 7, a change in brightness occurs due to interference between the diffracted lights 11 and 13, and the brightness changes with the frequency of the optical beat, that is, the frequency f of the reference signal MS.

Therefore, the photoelectric signal from the photoelectric detector 7 also has a sinusoidal waveform of frequency f. After the photoelectric signal is amplified by an amplifier 9,
The signal is input to a phase difference detection circuit 8 as a stage of phase difference detectors. The phase difference detection circuit 8 also receives the reference signal MS from the oscillation circuit 6, detects a phase shift of the photoelectric signal with respect to the reference signal MS, and outputs a phase difference signal PDS corresponding to the amount of shift.

The main controller 22 uses the phase difference signal PDS and the length measuring device 20.
For example, the phase difference signal PDS is input by inputting the position information from
The drive motor 21 is servo-controlled so that the phase difference becomes zero (the phase shift is zero). This allows one-dimensional positioning of the substrate 4. Incidentally, as the phase difference detection circuit 8, an FM detection circuit, a needs meter 7, etc. can be used as is. Further, a frequency adjuster 30 for changing the oscillation frequency f is connected to the oscillation circuit 6. This is because the deflection angle of the modulated light 12 with respect to the zero-order light 10 emitted from the optical modulator 2 is made variable to adjust the angle α. Generally, once the pitch of the diffraction grating, the wavelength of the coherent light, and the angle of incidence of the coherent light are determined, the direction of the generated diffracted light of each order is uniquely determined. Therefore, the diffracted light 13 of a certain order is generated in an optical path parallel to the other diffracted light 11, and is not necessarily transmitted accurately through the slit plate 7afe.

Therefore, by adjusting the incident angle of the modulated light 12, the optical adjustment J! ! This is what we do.

By making the frequency f of the oscillation circuit 6 variable in this way, you can freely adjust it later.Note: The laser light source 1, optical modulator 2, mirror 3, and optical components can be moved mechanically. Because there's no need.

Errors due to changes over time are also less likely to occur. Also, the oscillation frequency f
The fine adjustment can be made by detecting the amplitude (for example, peak-to-peak) of the photoelectric signal from the photoelectric detector 7 and adjusting the amplitude to the maximum value. In this case, the adjuster 30 is provided with a circuit for detecting the image width of the photoelectric signal, a circuit for storing the maximum value of the amplitude value, a circuit for sweeping the oscillation frequency within a certain range, etc. During the second frequency sweep, the maximum amplitude value is compared with the detected amplitude value, and when the two match, the sweep is stopped and fixed at frequency 7. If you do this, the device will always be automatically set to the ideal condition (
You can also get the effect of being able to self-set.

Although FIG. 1 shows that the modulated light 12 and the zero-order light 10 are separated by a relatively large deflection angle, in reality the deflection angle is not that large, so a mirror or the like is used. The optical path of the modulated light 12 is bent to create an incident state as shown in FIG. Also, zero order light 10
Since the frequency of the modulated light 12 and the modulated light 12 differ by f, the wavelength itself is naturally different between the two, but the wavelength difference with respect to the absolute value of the wavelength of both light beams is extremely small (about io' to lO) and extremely They can be regarded as having the same wavelength.

Next, the operation of this embodiment will be explained with reference to the waveform diagrams of FIGS. 2(a), (bl, and (c)). FIG. 2(b) is a waveform diagram of the photoelectric signal I from the photoelectric detector 7, in which the horizontal axis represents time t and the vertical axis represents the level of each signal.As mentioned earlier, zero-order light 10 and the modulated light 12 differ by the frequency f, the photoelectric signal I becomes a so-called optical beat signal, and has a sine waveform with a frequency f similar to the reference signal MS.
Assuming that one light beam 10.12 is stopped at a certain position, the phase difference 0 between the reference signal MS and the photoelectric signal l becomes a constant value. This phase difference l is immediately determined by the level of the phase difference signal PDS from the phase difference detection circuit 8. Furthermore, when the substrate 4 is moved by the stage 5, the phase difference θ
changes continuously in proportion to the amount of movement. Of course, the range in which the phase difference can be detected is within the range of ±π if the direction of the phase shift is taken into account in the range 2-1c. Figure 2 (
c) is an output characteristic diagram of the phase difference signal PDS, in which the vertical axis represents the output level of the signal PDS, and the horizontal axis represents the two luminous fluxes 10
．． 12 and the substrate 4 relative to each other. When aligning the substrate 4 with respect to the two light beams 10.12,
Rough positioning is performed in advance using the stage 5 using known clear alignment means or crowbar alignment means so that one positioning error is within the range LP (±π in terms of phase difference). The range LP is expressed by the following equation, where λ is the wavelength of the two light beams 10.12.

LP=λ/(@inα+sinβ) - For example, wavelength λft600 nm (0,6pm)
If both the angles α and β are 30·, the range LPFi is 0.6μ from the above formula. Assuming that the correct position for alignment is the zero point of the phase difference signal PDS, the rough positioning accuracy needs to be ±0.3 μm.

Of course, even if the wavelength is the same, the smaller both angles α and β become, the wider the range LP becomes, and the accuracy of rough positioning becomes looser. After rough positioning is completed, main controller 2
2 reads the phase difference signal PDS, detects the direction of shift based on its polarity, detects the amount of shift based on its magnitude, and controls the drive motor 21 to slightly move the stage 5 so that the phase difference signal PDS becomes zero. Alternatively, since the phase difference signal PDS changes linearly, the phase difference signal PDS can be directly used as an error signal in the feedback loop of the motor control circuit.
It may also be possible to incorporate In this case, using the previous example, the drive motor 21 is controlled based on the position information of the length measuring device 20 up to ±0.3μI11, and when it becomes within ±0.3μm, the drive motor 21 is driven based on the phase difference signal PDS. If the control system is automatically switched to control the motor 21, the stage 5 will automatically move until it is servo-locked to the zero point of the phase difference signal PDS. This is effective in that both high precision and high speed can be achieved in auto-alignment.

In the above embodiment, one-dimensional alignment was taken as an example, but when performing two-dimensional alignment, the diffraction grating 4a
Another diffraction grating extending in a direction perpendicular to the radial direction may be provided at base &4, and light beams of different frequencies may be irradiated from two directions in the same manner. Moreover, the slit of the slit plate 7a is the diffraction grating 4λ.
Although the openings were set to be parallel to each grid, other than slits, openings such as pinholes may also be used. Furthermore, a partial image of the diffraction grating 4a is placed between the slit plate 7a and the substrate 4 by the slit plate 7a.
J) An optical system for enlarging and projecting the image onto the plate 7a may be provided.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, the same members as in FIG. 1 are given the same reference numerals. In this embodiment, a Zeeman laser source 4 using the Zeeman effect is used as a coherent light source.
Use 0. Zeeman laser LB from laser light source 40
' includes both P-polarized and S-polarized waves, and the P-polarized and S-polarized waves have slightly different frequencies.

The Zeeman laser LB' is split into two by a half mirror 41, and one of the transmitted light beams is further separated by a polarizing beam splitter 42 into P polarized light 10' and S polarized light 12'. The P-polarized light 10' is reflected by mirrors 43, 44 and obliquely illuminates the diffraction grating 4a of the substrate 40. S polarized light 1
2' is reflected by the mirror 45 and irradiates the diffraction grating 4a obliquely. The conditions of incidence of the P-polarized light 10' and the S-polarized light 12' are the same as in the case of FIG.
Using an analyzer, it can be observed that interference fringes are flowing on the diffraction grating 4a at the frequency of the optical beat due to the interference. In the case of this embodiment, since polarized light is used, an analyzer is provided in each optical path of the Pm source 10' and the S polarized light 12', although not shown, or between the diffraction grating 4a and the optical eye detector 7. It is necessary to install an analyzer in the optical path of the On the other hand, it was reflected by half mirror 41 and Zeeman V-
The LB' passes through the analyzer 46 and then enters the photomultiplier 47. The intensity of the light that has passed through the analyzer 46 changes at a frequency corresponding to the difference in frequency between the P-polarized light and the S-polarized light of the Seman laser LB'. Therefore, when the photoelectric signal from the photomultiplier 47 is amplified by the amplifier 48, a reference signal MS similar to that shown in FIG. 1 is obtained.

Therefore, if the phase difference detection circuit 8 detects the phase difference between the reference signal MS and the photoelectric signal I, the relative position of the substrate 4 can be detected. In this embodiment, Zeeman laser light source 40. 'The polarizing beam splitter 42 and the mirrors 43, 44, and 45 constitute the two-beam generating means, and the half mirror 41, the analyzer 46, the photomultiplier 47, and the amplifier 48 constitute the reference signal generating means.

Next, a third embodiment of the present invention will be described based on FIG. This embodiment is a modification of the two-beam generating means, and the laser beam L is applied to a mechanically rotating radial grating plate 50.
By making light B incident, two coherent light beams A and B whose frequencies differ by a very small amount are obtained. The luminous flux A is equivalent to the zero-order light lO in Fig. 1, and the luminous flux B is the first
This is equivalent to the modulated light 12 in the figure. At this time, two luminous fluxes A
, H is proportional to the rotational speed of the radial grating plate 50, specifically, the moving speed of the radial grating with respect to the laser beam LB. Therefore, the reference signal can be easily obtained from a speed sensor or the like that detects the rotation of the radial grating plate 50. When obtaining two luminous fluxes mechanically in this way, the difference in frequency between the two luminous fluxes is at most several tens of kilohertz, so a semiconductor photodetector element with lower responsiveness than a photomultiglayer is used as the photoelectric detector 7. It also has the advantage of being available.

As another modification, two semiconductor lasers of the same type may be used as means for generating respective luminous fluxes. In general, even if semiconductor lasers are of the same type, there are considerable individual variations in their characteristics, so the frequencies of their laser beams will differ slightly.

This rose 1! ! This means that two luminous fluxes are obtained by actively utilizing the light. In this case, since the difference in frequency may not be constant, a polarizer is provided to convert the light flux from each semiconductor laser into vertically polarized light and horizontally polarized light, respectively, and the two elements t-1 given different polarizations are Combined into one light beam, it enters an analyzer like the one shown in Figure 3.
Try to get a reference signal.

In this way, an extremely compact position detection device can be obtained. However, each semiconductor laser must have a sufficiently stable oscillation frequency and no mode jumps.

(Effects of the Invention) According to the present invention, the so-called optical heterodyne interference method is used in which the frequencies of two light beams are made different and the optical beat generated by the difference is used. Can be directly detected using the phase difference between the reference signals. Phase differences can be electrically detected with considerable resolution and with high precision. Therefore, stable positioning is possible regardless of fluctuations in light intensity. Since the amount of positional deviation can be detected without moving the substrate relative to the fc2 light beam, it is possible to directly servo control the stage etc. using a phase difference signal when positioning the substrate, and also to control the positioning and unloading. Therefore, it is no longer necessary to move the board preliminarily, and as a result, automatic alignment with both high precision and high speed is possible1.
It becomes Hi.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a schematic configuration of a position detection device according to a first embodiment of the present invention, FIG. 2 (al (bl is a waveform diagram of each signal in FIG. 1, and FIG. 3 is a diagram showing the output characteristics of the phase difference signal, FIG. 3 is a diagram showing the schematic configuration of the position detection device according to the second embodiment, and FIG. 4 is a diagram showing the main parts of the two-beam generating means according to the third embodiment. It is a perspective view showing the configuration. [Explanation of symbols of main parts]

Claims (3)

[Claims] (1) A diffraction grating formed on a substrate whose position is to be detected is irradiated with two coherent beams from different directions, each of the two beams causes the diffracted beams generated from the diffraction grating to interfere with each other, and the interference intensity is measured. A device for detecting the position of the substrate based on a photoelectrically detected signal, comprising: two beam generating means for irradiating the diffraction grating with the frequencies of the two beams differing by a predetermined value; a difference in frequency between the two beams; a reference signal generating means for outputting a reference signal with a frequency corresponding to the frequency; detecting a phase difference between the interference beat signal and the reference signal, intensity modulated at a frequency corresponding to the difference by irradiation with the two light beams; A position detection device comprising: a phase difference detection means for detecting the position of the substrate based on the phase difference. (2) The reference signal generation means has an oscillation circuit that generates a reference signal with an arbitrary frequency f, and the two-beam generation means inputs the light flux from the coherent light source and the reference signal from the oscillation circuit. 2. The device according to claim 1, further comprising an optical modulator that emits two light beams having frequencies different by f. (3) The two beam generating means includes a polarization separation element that receives a Zeeman laser having different frequencies depending on the polarization direction and separates the beam into the two beams by polarization. The device described.