JPH04366704A - Position detecting method and position detecting device using diffraction grating - Google Patents

Abstract

PURPOSE:To detect an optical heterodyne interference beat signal from diffraction gratings at a high stability and at a high accuracy. CONSTITUTION:Single lights of three wavelengths of different frequencies each other are used, and the single lights are injected to diffraction gratings 7 and 6 formed on a wafer 8 and a mask 9 which are movable relatively, at the incident angles to obtain a composed diffracted light to detect the position slippage and the gap. By the diffraction gratings 7 and 6, beat signals for detecting the slippage and the gap with different frequencies are separated and produced by frequency selection circuits 44 and 45, from two sets of optical heterodyne interference lights obtained by diffracting and composing the single lights of two wavelengths, an optical heterodyne interference beat signal produced by using either one grating by a signal process controller 19 is made as a standard beat signal, and by calculating the phase difference with one side optical heterodyne interference beat signal, the relative position slippage amount between the two gratings 6 and 7 is detected.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a position detection method and a position detection apparatus used in exposure apparatuses, pattern evaluation apparatuses, etc. for manufacturing semiconductor ICs and LSIs.

0002

[Prior Art] In recent years, with the miniaturization of semiconductor ICs and LSIs, the development of X-ray exposure equipment is progressing as a device that can transfer submicron patterns with high productivity. and are aligned with high precision in a plane parallel to the mask surface and wafer surface, and the positional relationship in the normal direction of the mask surface and wafer surface, that is, the gap between the mask and wafer, is also set to a predetermined value. Establishment of technology is essential. Particularly when using a divergent X-ray source, highly accurate gap setting is required. Conventionally,
A method for performing this type of alignment is described in the Proceedings of the 34th Applied Physics Association Conference, p. 396 (198
There is an optical heterodyne interferometry method that uses a diffraction grating and is capable of highly accurate and highly stable position detection, as introduced in 7).

FIG. 2 shows an example of an apparatus for positioning using such a phase difference signal. In the figure, laser light emitted from a two-wavelength orthogonally polarized laser light source 1 passes through a cylindrical lens 2 and becomes an elliptical beam.The beam is divided into a horizontal component (p-polarized light component) or a vertical component (s-polarized light component) by a polarizing beam splitter 3, respectively. Directly polarized light having only one component) is separated into two wavelengths with slightly different frequencies. Of these, the p-polarized light component is the plane mirror 4a, 4b.
The incident light 5 enters the reflective diffraction gratings 6 and 7 from the direction of the first-order diffraction angle with respect to the normal direction (Z direction) perpendicular to the diffraction grating surface. Note that the light enters the diffraction grating 7 provided on the wafer 8 through a window 10 provided in the mask 9 . On the other hand, the s-polarized component is transmitted through beam splitter 1
1, one of which passes through a plane mirror 4c as incident light 12 from the direction of the first-order diffraction angle that is bilaterally symmetrical to the incident light 5 with respect to the Z direction, and the other passes through a plane mirror 4d,
Incident light 5 in the Z direction as incident light 13 through 4e
The light enters the reflective diffraction gratings 6 and 7 from the direction of the third-order diffraction angle on the same side. The incident light 5 enters the diffraction grating 7 through the window 10 in the same way as the incident light 5.

The two reflective diffraction gratings 6 and 7 are offset in the grating line direction (Y direction), and
Each wavelength of incident light is placed within the same elliptical beam spot. Further, the diffraction grating pitches of both the diffraction gratings 6 and 7 are set to be equal to each other.

The incident light beams 5 and 12 cause the first diffraction grating 6 to
The combined diffracted light obtained in the Z direction from
Combined diffracted light 14a with the first-order diffracted light and second diffraction grating 7
The composite diffracted light obtained in the Z direction from
The composite diffracted light 14b of the −1st-order diffracted light of the incident light 12 and the −1st-order diffracted light of the incident light 12 is separated by the prismatic mirror 15a after its direction is changed by the plane mirror 4f. Among them, the side of the combined diffracted light 14a is detected by the photodetector 18a via the polarizing plate 16a and the condensing lens 17a, and is detected by the photodetector 18a.
is input to the signal processing control unit 19 as an optical heterodyne interference beat signal. On the other hand, the side of the combined diffracted light 14b is connected to a photodetector 18b via a polarizing plate 16b and a condensing lens 17b.
and is input to the signal processing control section 19 as a third optical heterodyne interference beat signal.

The incident light beams 5 and 13 cause the first diffraction grating 6 to
A composite diffracted light 20 obtained in the direction of the second-order diffraction angle with respect to the Z direction, that is, a composite diffracted light 20 of the +1st-order diffracted light of the incident light 5 by the first diffraction grating 6 and the −1st-order diffracted light of the incident light 13
a, the combined diffracted light similarly obtained from the second diffraction grating 7 in the direction of the second-order diffraction angle and taken out through the window 10, that is, the +1st-order diffracted light of the incident light 5 by the second diffraction grating 7,
The combined diffracted light 20b with the −1st-order diffracted light of the incident light 13 is
After the direction is changed by the plane mirror 4g, it is separated by the prismatic mirror 15b. Of these, the side of the combined diffracted light 20a is detected by the photodetector 18c via the polarizing plate 16c and the condensing lens 17c, and is input to the signal processing control unit 19 as a second optical heterodyne interference beat signal. On the other hand, the side of the combined diffracted light 20b is detected by the photodetector 18d via the polarizing plate 16d and the condensing lens 17d, and is sent to the signal processing controller 1 as a fourth optical heterodyne interference beat signal.
9 is input.

The signal processing control section 19 determines the phase difference between the first optical heterodyne interference beat signal and the third optical heterodyne interference beat signal. This phase difference Δφx is the amount of relative positional deviation between the first diffraction grating 6 and the second diffraction grating 7 in the direction (X direction) orthogonal to the grating line direction (Y direction) within the grating plane of the diffraction grating. Corresponding to Δx,
Δφx=360・Δx/(P/2) ・・・・・・・・・
......The relationship is as shown in (1). Here, P is the diffraction grating pitch. Therefore, by moving the mask stage 21 on which the mask 9 is placed or the wafer stage 22 on which the wafer 8 is placed in the X direction so that this phase difference becomes zero, the pattern on the mask surface can be moved to a predetermined position on the wafer surface. It can be aligned to the position.

[0008] Furthermore, the signal processing control unit 19 determines the phase difference Δφxz between the second optical heterodyne interference beat signal and the fourth optical heterodyne interference beat signal, and calculates the phase difference Δφxz between this and the above-mentioned first optical heterodyne interference beat signal. The phase difference Δφx with the third optical heterodyne interference beat signal is added. This addition signal Δφz corresponds to the gap Δz between the first diffraction grating 6 and the second diffraction grating 7 in the normal direction (Z direction) of the grating plane of the diffraction grating, and is expressed as Δφz=Δφx+Δφxz =360・Δz・( COSθ3-
COS θ1)/λ...The relationship is as shown in (2). Here, θ1 is the first-order diffraction angle, θ3 is the third-order diffraction angle, and λ is the wavelength of the laser beam. Therefore, by moving the mask stage 21 or wafer stage 22 in the Z direction so that this phase difference signal Δφz becomes zero, the gap can be set to a predetermined value.

However, in the above alignment optical system, the relative positional deviation amount detection optical system and the gap detection optical system share a part of the optical system with each other, so that the relative positional deviation amount detection and gap detection optical systems are There is a problem in that unnecessary diffracted light is mixed into the heterodyne interference light, degrading detection accuracy. This will be explained using FIG. 3.

FIG. 3 is a diagram showing in detail the incident light and the diffracted light at the mask diffraction grating 6 in FIG. 2. In FIG. From FIG. 3, the -3rd order diffracted light 25b of the incident light 13 overlaps with the optical heterodyne interference light 14a of the -1st order diffracted light for detecting the amount of relative positional deviation obtained from the incident light 5 and the incident light 12, causing unnecessary interference. bring about. On the other hand, the optical heterodyne interference light 20a of the ±1st order diffracted light for gap detection obtained from the incident light 5 and the incident light 13 is similarly overlapped with the -3rd order diffracted light 25a of the incident light 12, causing unnecessary interference. Generally, it is considered that the light intensity of the first-order diffracted light is sufficiently stronger than that of the third-order diffracted light, so that the influence of the third-order diffracted light hardly contributes to the optical heterodyne interference lights 14a and 20a.

[0011]

However, as shown in the above-mentioned diffraction grating 7 on the wafer in FIG.
Process layer for pattern formation, e.g. Al wiring layer, SiO
When the second insulating layer, resist layer, etc. are deposited and the aperture ratio of the diffraction grating 7 (the ratio of the grating line width to the diffraction grating pitch) changes, there is a condition in which the intensity of the first-order diffracted light and the intensity of the third-order diffracted light do not change much. appear. That is, the third-order diffracted light interferes with the first-order diffracted light having the same wavelength (or frequency) as the third-order diffracted light, resulting in amplitude fluctuations of the beat signal and affecting the detection signal. Further, the first-order diffracted light having a different wavelength (or frequency) from the third-order diffracted light undergoes optical heterodyne interference, generating a signal having a period different from that of the desired optical heterodyne interference light. Therefore, the optical heterodyne interference light emitted in the normal direction and the second-order diffraction angle direction becomes a phase difference signal that changes nonlinearly with respect to minute displacements of the diffraction grating due to the combination of these effects, making highly accurate alignment difficult. Become. Also,
There is a problem in that a desired optical heterodyne interference signal cannot be detected at all depending on the intensity of the third-order diffracted light.

Therefore, the present invention has been made to solve the above-mentioned conventional problems, and its purpose is to convert optical heterodyne interference beat signals from a diffraction grating into highly stable and
An object of the present invention is to provide a position detection method and a position detection device that enable highly accurate detection.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the position detection method according to the present invention uses monochromatic light of three wavelengths having different frequencies, and uses each monochromatic light to detect positional deviation and gaps. The synthesized diffracted light is incident on two diffraction gratings fixed or formed on two relatively movable objects at an incident angle to obtain the synthesized diffracted light, and the monochromatic light of two wavelengths is diffracted and synthesized by the diffraction gratings. A frequency selection circuit separates and generates beat signals for position shift and gap detection with different frequencies from the optical heterodyne interference light of the pair, and the optical heterodyne interference beat signal generated using one of the diffraction gratings is used as a reference beat signal. , the amount of relative positional shift between two diffraction gratings is detected by a method of calculating the phase difference with the other optical heterodyne interference beat signal. Further, the position detection device according to the present invention includes two relatively movable objects, two diffraction gratings fixed or formed on these two objects, and a light source that generates monochromatic light of three wavelengths with different frequencies. and an incident angle adjustment means for causing monochromatic light of three wavelengths emitted from the light source to enter the diffraction grating from a direction in which a synthesized diffracted light for position shift and gap detection is obtained; A photodetector that generates beat signals for position shift and gap detection with different frequencies from two sets of optical heterodyne interference lights obtained by diffracting and combining light, and these optical heterodyne interference beat signals generated by the photodetector. A frequency selection circuit that separates and generates a desired beat signal from the optical heterodyne interference beat signal from one of the diffraction gratings is used as a reference beat signal, and the phase difference between the optical heterodyne interference beat signal and the other optical heterodyne interference beat signal is determined. It consists of a signal processing device that performs calculation processing, and is configured to detect the amount of relative positional deviation between two diffraction gratings.

[0014]

[Operation] In the present invention, when a beam of three wavelengths is incident on a diffraction grating, out of the beat signals generated from the diffracted light emitted from the diffraction grating, an unnecessary beat signal and a desired beat signal are selected by a frequency screening circuit. Since the instability of the detection signal caused by high-order diffracted light can be completely removed by separating the signals, it is possible to detect the amount of relative positional deviation with high stability and high accuracy.

[0015]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 shows an embodiment of a position detection device according to the present invention. In the figure, a light source 40 that emits linearly polarized laser light of three wavelengths with slightly different frequencies
Of the three elliptical beams 41, 42, 43 emitted from the plane, the beam 41 passes through the plane mirrors 4a, 4b, and enters the reflective diffraction gratings 6, 7 as incident light 5 with a normal line perpendicular to the diffraction grating surface. The light is incident from the direction of the first-order diffraction angle with respect to the direction (Z direction). Note that the light enters the diffraction grating 7 provided on the wafer 8 through a window 10 provided in the mask 9 . The beam 43 passes through the plane mirrors 4d and 4e to the incident light 1.
3, the beam 42 is directed from the direction of the third-order diffraction angle on the same side as the incident light 5 with respect to the Z direction.
The incident light 12 enters the reflective diffraction gratings 6 and 7 from the direction of the first-order diffraction angle that is bilaterally symmetrical to the incident light 5 with respect to the Z direction. For the diffraction grating 7, the window 10
The incident light is similar to the incident light 5.

The two reflective diffraction gratings 6 and 7 are offset from each other in the grating line direction (Y direction) and are arranged within the same elliptical beam spot of each incident light beam of two wavelengths. Further, the diffraction grating pitches of both the diffraction gratings 6 and 7 are set to be equal to each other.

The incident light beams 5 and 12 cause the first diffraction grating 6 to
The combined diffracted light obtained in the Z direction from
Combined diffracted light 14a with the first-order diffracted light and second diffraction grating 7
The composite diffracted light obtained in the Z direction from
The composite diffracted light 14b of the −1st-order diffracted light of the incident light 12 and the −1st-order diffracted light of the incident light 12 is separated by the prismatic mirror 15a after its direction is changed by the plane mirror 4f. Among them, the side of the synthesized diffracted light 14a is detected by the photodetector 18a via the condensing lens 17a, and sent to the first frequency selection circuit 44, where unnecessary beat signals are separated and the first optical heterodyne interference is generated. Only a desired beat signal is input to the signal processing control circuit 19 as a beat signal. On the other hand, the side of the synthesized diffracted light 14b is detected by the photodetector 18b via the condensing lens 17b, and sent to the first frequency selection circuit 44, where unnecessary beat signals are separated and the third optical heterodyne interference is generated. Only a desired beat signal is input to the signal processing control section 19 as a beat signal.

On the other hand, the combined diffracted light obtained from the first diffraction grating 6 in the direction of the second-order diffraction angle with respect to the Z direction by the incident lights 5 and 13, that is, the +1st order of the incident light 5 by the first diffraction grating 6. A composite diffracted light 20a of the diffracted light and the −1st-order diffracted light of the incident light 13, and a composite diffracted light 20a similarly obtained from the second diffraction grating 7 in the direction of the second-order diffraction angle and taken out through the window 10, that is, the Synthetic diffracted light 20 of the +1st-order diffracted light of the incident light 5 and the -1st-order diffracted light of the incident light 13 by the diffraction grating 7 of No. 2
b is separated by a prismatic mirror 15b after its direction is changed by a plane mirror 4g. Among them, the combined diffracted light 20a side is detected by the photodetector 18c via the condensing lens 17c, and sent to the second frequency selection circuit 45, where unnecessary beat signals are separated and the second optical heterodyne interference is generated. Only a desired beat signal is input to the signal processing control section 19 as a beat signal. On the other hand, the combined diffracted light 20b
side is detected by a photodetector 18d via a condensing lens 17d and sent to a second frequency selection circuit 45, where unnecessary beat signals are separated and a desired beat signal is generated as a fourth optical heterodyne interference beat signal. Only the signal is processed by the signal processing control section 19
is input.

In the signal processing control unit 19, the phase difference Δφx between the first optical heterodyne interference beat signal and the third optical heterodyne interference beat signal and the second optical heterodyne interference beat signal and the fourth optical heterodyne interference beat signal are determined. Detects the phase difference Δφxz. Regarding the relationship between Δφx and Δx or the relationship between Δφxz and Δz,
The same relationship as the above-mentioned equations (1) and (2) holds true.

In the above embodiment, the three-wavelength monochromatic light source 40 is a combination of a frequency-stabilized He-Ne laser light source and three acousto-optic elements such as Bragg cells to generate three beams with slightly different frequencies. , the beam was made into an elliptical beam using a cylindrical lens. The Bragg cell sets the frequency of beam 41 to f0+f1, the frequency of beam 42 to f0+f2, and the frequency of beam 43 to f
When converted to 0+f3, beam 41 and beam 4
The beat frequency of the synthesized diffracted light by 2 is Δfa=f1-f
2. The beat frequency of the combined diffracted light by beam 41 and beam 43 can obtain two different beat frequencies as Δfb=f1−f3. The former signal with the beat frequency Δfa is used for positional deviation detection, and the latter signal with the beat frequency fb is used for gap detection.

The former positional deviation detection signal includes an unnecessary beat signal generated when the −3rd-order diffracted light of the beam 43 diffracted by the diffraction grating interferes with the −1st-order diffracted light of the beams 41 and 42, as described above. It will be done. The frequencies of these unnecessary beat signals are respectively Δfb=f1-f3
, Δfd=f2−f3, which is different from the beat frequency Δfa of the positional deviation signal. Therefore, the frequency screening circuit 44 can easily extract only the positional deviation detection signal, and only the first beat signal and the third beat signal, which are true signals, can be sent to the signal processing control section 19. Similarly, the latter gap detection signal also includes an unnecessary beat signal that is generated when -3rd-order diffracted light from the beam 42 diffracted by the diffraction grating interferes with -1st-order diffracted light from the beams 41 and 43. The frequencies of these unnecessary beat signals are Δfa=f1-f2, Δfd=f2-f3, respectively.
This is different from the beat frequency Δfb of the gap signal. Therefore, the frequency screening circuit 45 can easily extract only the gap detection signal, and only the second beat signal and the fourth beat signal, which are true signals, can be sent to the signal processing control section 19.

In the above embodiment, the frequency stabilized light source 40
The case where a He-Ne laser is used has been explained, but the multichannel acousto-optic device shown in Utility Model Application No. 115951/1992 and the Proceedings of the 1990 Society for Precision Engineering Spring Conference, p. By combining it with a frequency-stabilized semiconductor laser shown in No. 429, it is possible to make a three-wavelength monochromatic light source compact.

[0023]Also, an example has been described in which the direction of light incident on the diffraction grating and the direction of diffracted light from the diffraction grating are included in the XY plane perpendicular to the plane of the diffraction grating. Even if the optical heterodyne interference beat signal is detected by optically combining two wavelengths of obliquely incident and obliquely output diffracted light that are not included in the XY plane perpendicular to the diffraction grating surface as the direction of the diffracted light from the grating. A similar effect can be obtained.

Furthermore, the diffraction grating in the present invention includes:
Either an absorption type diffraction grating or a phase type diffraction grating may be used, and various types of diffraction gratings such as not only a binary diffraction grating but also a sine wave diffraction grating and a phrase diffraction grating can be used. It is also possible to use a reflection type diffraction grating.

[0025]

As explained above, according to the present invention, monochromatic light of three wavelengths with different frequencies is incident on a diffraction grating, and an optical heterodyne interference beat signal generated using the combined diffracted light generated from the diffraction grating is diffracted. It is possible to completely remove the beat signal, which is noise caused by unnecessary interference between light beams, using a frequency selection circuit, and is extremely superior in that it is possible to detect the optical heterodyne interference beat signal from the diffraction grating with high stability and high precision. Effects can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of a position detection device according to the present invention.

FIG. 2 is a diagram showing the configuration of a conventional position detection device.

FIG. 3 is a diagram showing details of incident light and diffracted light to a diffraction grating section.

[Explanation of symbols]

1 Two-wavelength orthogonally polarized laser light source 2 Cylindrical lens 3 Polarizing beam splitter 4a Plane mirror 4b Plane mirror 4c Plane mirror 4d Plane mirror 4e Plane mirror 4f Plane mirror 4g Plane mirror 4h Plane mirror 5 Incident light 6 Reflection type diffraction grating 7 Reflection type diffraction Grating 8 Wafer 9 Mask 10 Window 11 Beam splitter 12 Incident light 13 Incident light 14a Combined diffracted light 14b Combined diffracted light 15a Prism mirror 15b Prism mirror 16a Polarizing plate 16b Polarizing plate 16c Polarizing plate 16d Polarizing plate 17a Condensing lens 17b Optical lens 17c Condensing lens 17d Condensing lens 18a Photodetector 18b Photodetector 18c Photodetector 18d Photodetector 19 Signal processing control section 20a Combined diffracted light 20b Combined diffracted light 21 Mask stage 22 Wafer stage 23 Mask 24 Opaque thin film 25a Third-order diffracted light 25b Third-order diffracted light 40 Three-wavelength linearly polarized laser light source 41
Laser beam 42 Laser beam 43 Laser beam 44 Frequency selection circuit 45 Frequency selection circuit

Claims (2)

[Claims] Claim 1: A predetermined frequency is set for each of a first diffraction grating provided on a first object and a second diffraction grating provided on a second object whose grating lines are parallel to the first diffraction grating. A first monochromatic light having a different frequency from the first monochromatic light and a second monochromatic light having a different frequency from the first monochromatic light are incident on the diffraction grating from mutually symmetrical directions, and the first monochromatic light and the second monochromatic light A third monochromatic light having a different frequency is incident on the diffraction grating from a direction asymmetrical to the first monochromatic light in the same direction as the second monochromatic light, and A first optical heterodyne interference beat signal and a third optical heterodyne interference beat signal necessary for detecting positional deviation are received by receiving two sets of combined diffracted lights generated by diffraction and combination of the first monochromatic light and the second monochromatic light. a frequency selection circuit, and receive two sets of combined diffracted light generated by diffraction and combination of the second monochromatic light and the third monochromatic light by the first diffraction grating and the second diffraction grating, A frequency selection circuit extracts only a second optical heterodyne interference beat signal and a fourth optical heterodyne interference beat signal that have different frequencies from the first optical heterodyne interference beat signal and the third optical heterodyne interference beat signal necessary for gap detection. From the phase difference between the first optical heterodyne interference beat signal and the third optical heterodyne interference beat signal, a positional shift in the direction parallel to the grating plane and orthogonal to the grating line of the first diffraction grating and the second diffraction grating is determined. The normal direction of the grating surfaces of the first diffraction grating and the second diffraction grating is detected from the phase difference and the phase difference between the second optical heterodyne interference beat signal and the fourth optical heterodyne interference beat signal. A position detection method using a diffraction grating, which is characterized by detecting a gap between. [Claim 2] First monochromatic light having mutually different frequencies;
a light source device that outputs a second monochromatic light and a third monochromatic light; a first diffraction grating provided on the first object and a second diffraction grating provided on the second object; The first monochromatic light and the second monochromatic light are diffracted and synthesized by an incident angle adjusting means for making each light incident at a predetermined angle, and a first diffraction grating and a second diffraction grating. a first optical detection means and a third optical detection means that are guided to a detector and produce a first optical heterodyne interference beat signal and a third optical heterodyne interference beat signal; and a first diffraction grating and a second diffraction grating. The second monochromatic light and the third monochromatic light are diffracted and synthesized to produce a combined diffracted light, which is then guided to a photodetector. a second optical detection means and a fourth optical detection means for producing an optical heterodyne interference beat signal and a fourth optical heterodyne interference beat signal, and a beat generated by the first optical detection means and the third optical detection means. a first frequency selection circuit that extracts only a desired beat signal from the signal; and a second frequency selection circuit that extracts only a desired beat signal from the beat signals generated by the second and fourth light detection means.
and a phase difference signal between the first optical heterodyne interference beat signal and the third optical heterodyne interference beat signal, which is parallel to the grating plane of the first diffraction grating and the second diffraction grating and perpendicular to the grating line. the first diffraction grating and the second diffraction grating from the phase difference signal and the phase difference signal between the second optical heterodyne interference beat signal and the fourth optical heterodyne interference beat signal. 1. A position detection device using a diffraction grating, comprising at least a signal processing means for calculating a gap in a direction normal to a grating surface of the grating.