JPH06137814A - Minute displacement measuring method and its device - Google Patents

Abstract

PURPOSE:To measure the minute displacement of a body and its height at the same time in a minute displacement measuring method and its device which measure the minute displacement of the body by means of a diffraction grating. CONSTITUTION:In a minute displacement measuring method and its device which measure the minute displacement of a body by means of a diffraction grating, light having wavelength lambda1 and light having wavelength lambda2 are directly heterodyne-interfered to form a secondary reference signal, which is measured by a photodetector 93 to compute a phase difference between the secondary reference signal and a reference signal measured by a photodetector 92 for simultaneously measuring the minute displacement and height of the diffraction grating 7. Thus, in the case of measuring the minute displacement of the diffraction grating, its height can be measured simultaneously and accurately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a minute displacement measuring method and apparatus for measuring minute displacement of an object using a diffraction grating.

[0002]

2. Description of the Related Art Conventionally, a stepper for performing reduction projection exposure while sequentially moving a wafer has been used for manufacturing a semiconductor device. A semiconductor element is manufactured by sequentially superposing and exposing a circuit pattern on a wafer and a circuit pattern on a mask.

In recent years, this circuit pattern has been further miniaturized and highly densified with the improvement of the processing capability of semiconductor elements, so that the wafer and the mask need to be aligned with higher precision. First, it is necessary to measure the position of the circuit pattern on the wafer with high accuracy. One method for this is, for example, JP-A-61-215.
Methods using heterodyne interference as disclosed in Japanese Patent Application Laid-Open No. 905 and Japanese Patent Application Laid-Open No. 62-274216 are known. An example of the minute displacement measuring device using this method will be described below with reference to FIG.

In the figure, 1 is a dual wavelength orthogonal polarization laser,
2 is a half mirror, 3 is a polarization beam splitter, 4a,
4b is a mirror, 5 is a collimator lens, 6 is a reduction lens, 7 is a diffraction grating, 8a and 8b are polarizing plates, 9a and 9b are photodetectors, and 10 is a detection signal processing unit.

The two-wavelength orthogonal polarization laser 1 emits coherent light whose wavelengths are slightly different from each other and whose polarization directions are orthogonal to each other. These lights are half mirror 2
And is separated by the polarization beam splitter 3 into light of wavelength λ ₁ and wavelength λ ₂ . The polarization beam splitter 3, the mirror 4, and the collimator lens 5 allow these lights to enter the diffraction grating 7 on the wafer as parallel beams at the first-order diffraction angle θ by the reduction lens 6 via the optical paths 111 and 112, respectively. Is configured. The first-order diffracted light of wavelength λ _{1 and} the first-order diffracted light of wavelength λ ₂ generated by the diffraction grating 7 are respectively optical paths 121, 1 perpendicular to the plane of the diffraction grating 7.
Proceed to 22. The optical paths 121 and 122 are substantially the same optical path, and the heterodyne interference signal Im is measured by the photodetector 9a via the polarizing plate 8a. This heterodyne interference signal Im is expressed by the following equation (1).

[0006]

[Equation 1]

Where Am is the amplitude of the heterodyne interference signal, ω ₁ and ω ₂ are the angular frequencies of the wavelengths λ ₁ and λ ₂ , respectively, t is the time, P is the pitch of the diffraction grating 7, and ε is the movement of the diffraction grating 7. Is the amount. This heterodyne interference signal Im includes information on the amount of movement ε of the diffraction grating 7, and will be hereinafter referred to as a measurement signal. To obtain the movement amount ε of the diffraction grating 7,
It suffices to subtract the time term (ω ₁ −ω ₂ ) t from the phase of the measurement signal Im. Then, this time term is separately measured as a reference signal. That is, the light reflected by the half mirror 2 is measured by the photodetector 9b through the polarizing plate 8b, and the heterodyne interference signal Is is

[0008]

[Equation 2]

It is expressed as follows, and this becomes the reference signal. Here, As is the amplitude of the reference signal.

Therefore, if the detection signal processing unit 10 obtains the phase difference between the two heterodyne interference signals detected by the photodetectors 9a and 9b, the movement amount ε of the diffraction grating 7 can be obtained. In this method, since the phase of light is detected, compared with the conventional method of detecting the light intensity distribution of the alignment mark image,
It is possible to perform highly accurate position detection regardless of the light intensity distribution of the illumination light and the resolution of the detection optical system.

[0011]

According to the conventional heterodyne interference method, the minute displacement amount of the diffraction grating 7 on the wafer in the horizontal direction can be measured, but the height cannot be measured. Therefore, another height detection system is required to measure the height of the diffraction grating 7.

An object of the present invention is to solve the problems of the above-mentioned prior art, to measure a minute displacement amount of the diffraction grating 7 in the horizontal direction, and to measure a height of the diffraction grating 7 and a minute displacement measuring method thereof. To provide a device.

[0013]

In order to measure the height of a diffraction grating, a method for measuring a minute displacement of the present invention and an apparatus therefor are provided in a diffraction grating with two wavelength orthogonal polarization lasers from different directions.
Irradiating a light beam, diffracted light beams generated from the diffraction grating by the irradiation of the two light beams are heterodyne-interfered to generate a measurement signal, and then a reference signal corresponding to a wavelength difference between the two light beams is generated, and the reference signal is generated. In a minute displacement measuring method for measuring a minute displacement amount of the diffraction grating from a phase difference between a signal and the measurement signal, specularly reflected light beams from the diffraction grating are heterodyne-interfered to generate a second reference signal, The height of the diffraction grating is measured based on the second phase difference by causing the two reference signals and the reference signal to heterodyne interfere with each other.

As a second method, the micro-displacement measuring method and apparatus according to the present invention allow coherent light of wavelength λ ₁ to be incident on the diffraction grating at an angle of about 1/2 of the first-order diffraction angle and the optical path on the incident side. To the reverse direction to combine the first-order diffracted light and the coherent light of wavelength λ ₂ to generate a measurement signal, and to combine the regular reflection light generated at the time of incidence and the coherent light of wavelength λ ₂ to generate a reference signal, In the method and apparatus for measuring the displacement of the diffraction grating from the phase difference between these measurement signal and reference signal, the coherent light of wavelength λ _{1 and} the coherent light of wavelength λ ₂ are directly combined to generate the second reference signal. However, the height of the diffraction grating is measured from the phase difference between the reference signal and the second reference signal.

[0015]

In the minute displacement measuring method and the apparatus therefor according to the second method, the generated measurement signal I ₁ and reference signal I ₀ are expressed by the following equations (3) and (4).

[0016]

[Equation 3]

[0017]

[Equation 4]

Here, γ is the phase term of the measurement signal and the reference signal, which are caused by the movement of the diffraction grating in the height direction.

On the other hand, the second reference signal I ₀₀ is expressed by the following equation (5).

[0020]

[Equation 5]

Therefore, the phase term γ of the height of the diffraction grating can be extracted by taking the difference between the equations (4) and (5). By converting this into a displacement amount, the amount of movement of the height of the diffraction grating from the reference position can be calculated, and the accurate height of the diffraction grating can be measured.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS.
This will be described with reference to 0.

In FIG. 1, 11 is a linearly polarized laser, 21,
22, 23, 25, 28 are half mirrors, 24, 26, 29, 30 are mirrors, 31, 32, 33, 34, 35, 36 are AO (acousto-optic) modulators, 41, 42, 43, 44 are polarized light. Beam splitter, 45 beam spacing changing prism, 46 1/4 wavelength plate, 47 and 48 ND filter, 51 collimator lens, 52 mirror, 6 reduction lens, 7 diffraction grating, 81 and 82 polarizing plates , 85, 86 and 87 are pinholes, 91, 92 and 93 are photodetectors, and 101 is a control processing circuit.

With this configuration, the light emitted from the linearly polarized laser 11 having vertically polarized light (S polarized light) is reflected by the half mirror 2.
1, 22, 23, and 25 split the light into five light beams, and two of these light beams enter the AO modulators 31 and 32. This AO modulator
Reference numerals 31 and 32 shift the frequency of the incident light by the drive frequency f ₁ input from the control processing circuit 101. In the first state, the AO modulator 31 is turned on and the AO modulator 32 is turned on.
When is turned off, the light LB1 having the wavelength λ ₁ is emitted from the AO modulator 31. Since the light LB1 having the wavelength λ ₁ is incident on the polarization beam splitter 41 as S-polarized light, most of it is reflected and travels along the optical path 130, passes through the beam interval changing prism 45, and is transmitted by the ¼ wavelength plate 46 to the light LB1 having the wavelength λ ₁ . Is circularly polarized light, travels along the optical path 141 through the collimator lens 51 and the mirror 52, and further follows the optical path 151 through the reduction lens 6, and forms a predetermined angle on the diffraction grating 7 on the wafer, that is, about 1 / th of the first-order diffraction angle. It is incident at an angle of 2. The focal position 511 of the collimator lens 51 and the pupil position 61 of the reduction lens 6 are conjugate. The first-order diffracted light generated by the diffraction grating 7 returns to the original optical path 151, the specularly reflected light advances to the optical path 152, and returns again to 141 and 142 via the reduction lens 6, and the mirror 52.
And it returns to the 1/4 wavelength plate 46 through the collimator lens 51. The 1/4 wavelength plate 46 converts these lights into linearly polarized light in a direction orthogonal to the time of incidence, and then enters the polarization beam splitter 41 as P-polarized light. After passing through this, the first-order diffracted light travels along the optical path 131 and is positive. The reflected light travels along the optical path 132, enters the polarization beam splitter 42, and passes through it.

On the other hand, of the light emitted from the linearly polarized laser 11, the light flux reflected by the half mirror 21 is the half mirror 22.
Is split into two light fluxes and enters the AO modulators 33 and 34. Since the AO modulators 33 and 34 are driven at the frequency f _2, they emit lights LB3 and LB4 having a wavelength λ ₂ slightly different from the wavelength λ ₁ . Lights LB3 and LB4 of wavelength λ ₂ are ND filters 4
The light is dimmed by 7, 48 and enters the polarization beam splitters 43, 44 as S-polarized light, and then enters the polarizing plates 81, 82, respectively. The lights LB3 and LB4 having the wavelength λ ₂ travel on optical paths that are close to each other.
When the transmission axes of the polarizing plates 81 and 82 are installed at an angle of about 45 degrees with respect to the horizontal direction, the photodetector 91 at the tip of the polarizing plate 81 and 82 has the above optical path.
The heterodyne interference signal generated through the polarization beam splitter 43 of the first-order diffracted light of wavelength λ ₁ from 131 and the light of wavelength λ ₂ from the ND filter 47 passes through the pinhole 85 for stray light cut and is a measurement signal. And the heterodyne interference signal generated by the photodetector 92 through the polarization beam splitter 44 of the specularly reflected light of wavelength λ ₁ from the optical path 132 and the light of wavelength λ ₂ from the ND filter 48 is stray light. It passes through the pinhole 86 for cutting and is detected as a reference signal.
The heterodyne interference signals detected by these photodetectors 91 and 92 are used as a measurement signal and a reference signal, respectively, as a control processing circuit.
Sent to 101. The control processing circuit 101 calculates the phase difference between these measurement signals and the reference signal and outputs the minute displacement amount of the diffraction grating 7.

Next, the light transmitted through the half mirror 25 is further divided into two by the half mirror 27, the light transmitted through the half mirror 27 goes to the AO modulator 35, and the light reflected by the half mirror 27 passes through the mirror 29 to AO. Each is incident on the modulator 36. A
Since the O modulators 35 and 36 are driven at the frequencies f ₁ and f ₂ , respectively, the lights incident on the AO modulators 35 and 36 are emitted as lights having wavelengths λ ₁ and λ ₂ , respectively. Light of wavelengths λ ₁ and λ ₂ emitted from the AO modulators 35 and 36 is combined by the half mirror 28 via the mirror 30, passes through the pinhole 87 for cutting stray light, and enters the photodetector 93. Since the optical path lengths of the wavelengths λ ₁ and λ ₂ from the AO modulators 35 and 36 to the photodetector 93 are equal, the heterodyne interference signals of the wavelengths λ ₁ and λ ₂ combined by the half mirror 28 have no phase term. Detected as a signal. Hereinafter, this heterodyne interference signal will be referred to as a second reference signal. This second reference signal is sent to the control processing circuit 101. The control processing circuit 101 calculates the phase difference between the second reference signal and the reference signal to obtain the amount of movement of the height of the diffraction grating 7 from the reference position. The height of the diffraction grating 7 can be obtained from the amount of movement of the diffraction grating 7 from the reference position.

Next, a method for obtaining the height of the diffraction grating 7 will be described with reference to FIGS. FIG. 2 is a diagram showing the relationship between the phases of the reference signal and the second reference signal when the height of the diffraction grating 7 is at the reference position. When the height of the diffraction grating 7 is at the reference position as shown in FIG. 2, the phase difference Δφ between the reference signal and the second reference signal becomes zero.

FIG. 3 is a diagram showing the relationship between the phase of the reference signal and the phase of the second reference signal when the height of the diffraction grating 7 moves from the reference position. As shown in FIG. 3, when the height of the diffraction grating 7 moves from the reference position, the phase of the second reference signal does not change, but the optical path length of the specularly reflected light that generates the reference signal changes, so the reference signal The phase of changes. The phase difference between the reference signal and the second reference signal is given by the equation (6).

[0029]

[Equation 6]

Here, γ is the phase change amount of the reference signal corresponding to the change of the optical path length of the specular reflection light which generates the reference signal.

FIG. 4 is a diagram showing changes in the optical path length of specularly reflected light when the height of the diffraction grating 7 moves from the reference position. α is the incident angle and the outgoing angle of the specularly reflected light. The optical paths 151a and 152a of the regular reflection light are shifted to the optical paths 151u and 152u when the height of the diffraction grating 7 is moved by h from the reference position. The change in the optical path length at this time is FE + EG, and is represented by the following equation (Equation 7).

[0032]

[Equation 7]

Now, assuming that the incident angle α is 3 °, the pitch of the diffraction grating 7 is 6 μm, and the movement amount of the height of the diffraction grating 7 is 1 μm, the change in the optical path length of the specular reflection light is 1.997 μm from the formula (7). Can be calculated. This corresponds to a phase difference of about 120 ° and is a value that can be measured sufficiently by the control processing circuit 101. Therefore, by monitoring the phase difference between the reference signal and the second reference signal, the amount of movement of the height of the diffraction grating 7 from the reference position can be calculated, and the height of the diffraction grating 7 can be obtained.

FIG. 5 shows the displacement and optical path of the diffraction grating 7 of FIG.
It is a detailed view showing 151 and 152. Here, the relationship between the displacement amount ε of the rotating grating 7 in FIG. 1 and the phase difference Δφ between the measurement signals of the photodetectors 91 and 92 and the reference signal will be described with reference to FIGS. 5A and 5B. First, the phase change of the measurement signal of the photodetector 91 will be described with reference to FIG. When the coherent light of wavelength λ ₁ travels along the optical path 151a and is incident on the diffraction grating 7 at an angle θ ′, the first-order diffracted light returns to the original optical path 151a. However, θ ′ satisfies the following equation.

[0035]

[Equation 8]

[0036]

[Equation 9]

Here, P is the pitch of the diffraction grating 7, and θ is the first-order diffraction angle. Here, the position of the diffraction grating 7 when the diffraction grating 7 moves by the displacement amount ε is shown by a broken line. At this time, the optical path length difference between the optical path 151b of the incident light and the first-order diffracted light and the original optical path 151a is 2DC. However,

[0038]

[Equation 10]

And the diffraction grating 7 has a displacement amount ε.
The phase change φ ₁ of the measurement signal when moving is (Equation 11)
It can be expressed by a formula.

[0040]

[Equation 11]

Next, the phase change of the reference signal of the photodetector 92 will be described with reference to FIG. When the coherent light of wavelength λ ₁ travels along the optical path 151a and enters the diffraction grating 7 at an angle θ ′, the specularly reflected light travels along the optical path 152a at a reflection angle θ ′.
Here, similarly to the above, the position of the diffraction grating 7 when the diffraction grating 7 moves by the displacement amount ε is shown by a broken line. At this time, the incident optical path becomes the optical path 151b, the reflected optical path becomes the optical path 152b, and the optical path length difference before and after the movement of the diffraction grating 7 becomes GE-FE '.
However, since the incident angle θ ′ and the reflection angle θ ′ are equal, GE-F
E '= 0. Therefore, the phase change φ ₀ of the reference signal when the diffraction grating 7 moves by the displacement amount ε also becomes zero. From this result, the phase difference Δφ between the measurement signal and the reference signal is

[0042]

[Equation 12]

It becomes

FIG. 6 is a detailed view showing a symmetrical resist distribution on the diffraction grating 7 of FIG. Next, a method of measuring the displacement amount ε when the resist is present on the diffraction grating 7 which is the alignment mark will be described. First, a case where a resist 71 having a symmetrical distribution is present on the diffraction grating 7 which is an alignment mark as shown in FIG. 6 will be described. In this case, the phase difference Δφa between the measurement signals of the photodetectors 91 and 92 and the reference signal is

[0045]

[Equation 13]

It becomes However, a and c are the phases of the measurement signal and the reference signal generated by the multiple reflection by the resist 71. That is, when the resist 71 is present on the diffraction grating 7, the offset of ac is added to the phase difference. A method of removing this offset will be described below.

In the above, in FIG. 1, the AO modulator 31 is turned on and the AO modulator 32 is turned off as the first state. On the contrary, consider the case where the AO modulator 31 is turned off and the AO modulator 32 is turned on as the second state. Light LB of wavelength λ ₁ emitted from the AO modulator 32
2 travels along an optical path symmetrical to the light LB1 of wavelength λ ₁ emitted from the AO modulator 31 with respect to the optical axis of the optical system of FIG. That is, the incident light enters the diffraction grating 7 from the optical path 152, the first-order diffracted light returns to the optical path 152 in the opposite direction, and the specularly reflected light travels to the optical path 151. Therefore, contrary to the above case, the photodetector 91 detects the heterodyne interference signal of the specularly reflected light of the wavelength λ ₁ and the light of the wavelength λ ₂ which is the reference signal, and the photodetector 92 has the wavelength λ which is the measurement signal.
Detecting _one of a heterodyne interference signal order diffracted light and the wavelength lambda ₂ of light. Phase difference Δφ between the measured signal and the reference signal in this case
b is

[0048]

[Equation 14]

It becomes Therefore, the formula (13) and the formula (1)
Taking the difference of 4),

[0050]

[Equation 15]

Therefore, the offsets ac can be canceled. In this way, the states of the AO modulators 31 and 32 are temporally switched from the first state to the second state, the phase difference between each measurement signal and the reference signal is obtained, and the difference between these phase differences is calculated. Even when the symmetrical resist 71 is present, the displacement amount ε can be measured without an offset error.

FIG. 7 is a detailed view showing an asymmetric resist distribution on the diffraction grating 7 of FIG. Next, a case where the resist 72 on the diffraction grating 7 is asymmetrical as shown in FIG. 7 will be described. Since the resist 72 is spin-coated, the distribution of the resist 72 tends to be asymmetric with respect to the diffraction grating 7. For example,
The center line of the concave portion of the resist 72 is shifted by the shift amount δ with respect to the center line of the concave portion of the diffraction grating 7. In such a case, the phase difference Δφ between the measurement signal and the reference signal by the photodetectors 91 and 92 whose AO modulators 31 and 32 are in the first state.
e, and the phase difference Δ between the measurement signal and the reference signal in the second state
φf is

[0053]

[Equation 16]

[0054]

[Equation 17]

It becomes However, e, f, and g are the first
And the phase of the measurement signal in the second state, and the phase of the reference signal in the first and second states. Here, in the same way as above, if the difference between the equations (16) and (17) is taken,

[0056]

[Equation 18]

It becomes However, as shown in FIG. 7, when the distribution of the resist 72 is asymmetric, since e ≠ f, the offset error e−f remains.

Next, a method for indirectly measuring the offset error e-f and substituting it into the equation (18) to obtain an accurate value of the displacement amount ε will be described with reference to FIGS. 8 to 10. As shown in FIG. 7, the asymmetry of the resist 72 can be represented by the shift amount δ of the center line of the concave portion of the resist 72 with respect to the center line of the diffraction grating 7.

The values of the phases e and f of the measurement signals in the first state and the second state with respect to the shift amount δ are, for example, Journal of the Optical Society of America, A, 5th. Volume, Issue 8 (1988), 1270
Pp. 1280 (J. Opt. Soc. Am, A. Vol.
5, No. 8 (1988), pp1270-1280).

FIG. 8 is a calculation example diagram showing the relationship between the shift amount δ of the resist 72 of FIG. 7 and the phases e and f of the measurement signal in the first state and the second state. Here, an example of the calculation result by the method of the above literature is shown, the horizontal axis shows the shift amount δ, and the vertical axis shows the phases e and f of the measurement signal. When the shift amount δ of the resist 72 increases, the difference between the phases e and f of the measurement signal increases.

FIG. 9 is a calculation example diagram showing the relationship between the shift amount δ of the resist 72 of FIG. 7 and the intensities of the first-order diffracted light in the first state and the second state. Here, an example is shown in which the relationship between the shift amount δ and the intensities of the first-order diffracted light in the first state and the first-order diffracted light in the second state is calculated in the same manner as in FIG. Also in this relationship, if the shift amount δ increases, the intensity difference between the first-order diffracted light in the first state and the first-order diffracted light in the second state increases. Therefore, there is a correlation between the difference between the phase e and the phase f of the measurement signal and the difference between the intensities of the first-order diffracted light in the two states. Therefore, the above-mentioned offset e
-F can be measured indirectly.

FIG. 10 shows the relationship between the intensity difference S of the first-order diffracted light in the first state and the second state of FIGS. 9 and 8 and the offset error ef which is the phase difference between the measurement signals in the two states. It is a calculation example figure. Here, the relationship between the difference S in the intensity of the first-order diffracted light in the two states and the offset error ef is shown.
The vertical axis represents the offset error ef, and the horizontal axis represents the intensity difference S of the first-order diffracted light in the two states represented by the following equation (Equation 19).

[0063]

[Formula 19]

However, Ia is the intensity of the first-order diffracted light in the first state, and Ib is the intensity of the first-order diffracted light in the second state.
The intensity Ia of the first-order diffracted light is substituted with the amplitude of the heterodyne interference signal detected by the photodetector 91 in the first state in FIG. 1, and the intensity Ib of the first-order diffracted light is detected in the second state. The amplitude of the heterodyne interference signal detected by the device 92 may be substituted. The control processing circuit 101 calculates the intensity difference S of the first-order diffracted light from the intensities Ia and Ib of the first-order diffracted light according to the equation (19).
Is calculated, and the intensity difference S and the offset error e−f obtained in advance are calculated.
The offset error e−f is calculated from the relationship of ## EQU1 ## and this is substituted into the equation (19) to calculate the accurate displacement amount ε.

According to the above procedure, the accurate displacement amount ε can be measured even when the distribution of the resist 72 in FIG. 7 is asymmetric with respect to the diffraction grating 7 which is the alignment mark. Regarding a method of correcting an error due to asymmetry of a resist from the intensity difference between two first-order diffracted lights, Japanese Patent Laid-Open No. 1-2429
It is also disclosed in Japanese Patent Publication No. 04. However, according to the embodiment of the present invention, the detection of the heterodyne interference signal and the detection of the intensity of the first-order diffracted light can be performed by the same photodetector, so that the signal processing system is simplified as compared with the above-described method. The effect is that you can do it.

FIG. 11 is a diagram showing a second embodiment of the minute displacement measuring method and apparatus according to the present invention.

In the figure, 1 is a dual wavelength orthogonal polarization laser,
2a, 2b, 2c and 2d are half mirrors, 3 is a polarization beam splitter, 4a, 4b, 4c, 4d and 4e are mirrors, 5 is a collimator lens, 6 is a reduction lens, 7 is a diffraction grating, and 8a, 8b and 8c are Polarizing plates, 9a, 9b and 9c are photodetectors, and 10 is a detection signal processing unit.

The two-wavelength orthogonal polarization laser 1 emits coherent light whose wavelengths are slightly different and whose polarization directions are orthogonal to each other. These lights are transmitted through the half mirror 2a and are transmitted by the polarization beam splitter 3 to the wavelengths λ ₁ and λ _2.
Is separated into light. In the polarization beam splitter 3, the mirror 4a and the collimator lens 5, these lights pass through the optical paths 111 and 112, and the reduction lens 6 forms the first-order diffraction angle θ, respectively, and the parallel beams form the diffraction grating 7 on the wafer.
Is configured to be incident on. The light having the wavelength λ ₁ and the light having the wavelength λ ₂ passes through the half mirrors 2b and 2c. The first-order diffracted light of the wavelength λ _{1 and} the first-order diffracted light of the wavelength λ ₂ generated by the diffraction grating 7 are respectively optical paths perpendicular to the plane of the diffraction grating 7.
Proceed to 121 and 122. The optical paths 121 and 122 are substantially the same optical path, and the heterodyne interference signal Im is measured by the photodetector 9a via the polarizing plate 8a. This heterodyne interference signal Im is expressed by the equation (1).

Here, Am is the amplitude of the heterodyne interference signal, ω ₁ and ω ₂ are the angular frequencies of the wavelengths λ ₁ and λ ₂ , respectively, t is time, P is the pitch of the diffraction grating 7, and ε is the movement of the diffraction grating 7. Is the amount. This heterodyne interference signal Im includes information on the amount of movement ε of the diffraction grating 7, and will be hereinafter referred to as a measurement signal. To obtain the movement amount ε of the diffraction grating 7,
It suffices to subtract the time term (ω ₁ −ω ₂ ) t from the phase of the measurement signal Im. Then, this time term is separately measured as a reference signal. That is, the heterodyne interference signal Is measured by the photodetector 9b through the polarizing plate 8b for the light reflected by the half mirror 2a is expressed by the equation (2),
This becomes the reference signal. Here, As is the amplitude of the reference signal.

Therefore, by the detection signal processing unit 10,
If the phase difference between the two heterodyne interference signals detected by the photodetectors 9a and 9b is obtained, the movement amount ε of the diffraction grating 7 can be obtained.

On the other hand, the light reflected by the half mirror 2c passes through the mirror 4e and enters the half mirror 2d. The light that has passed through the half mirror 2b once and is reflected by the diffraction grating 7 is reflected by the half mirror 2b and then enters the half mirror 2d through the mirrors 4c and 4d. The two light fluxes incident on the half mirror 2d are measured by the photodetector 9c through the polarizing plate 8c to determine the heterodyne interference signal Is ₂ . The heterodyne interference signal Is ₂ is represented by the following equation (Equation 20).

[0072]

[Equation 20]

Here, As ₂ is the amplitude of the heterodyne interference signal, and γ ′ is the phase term associated with the movement of the diffraction grating 7 in the height direction. Hereinafter, this heterodyne interference signal will be referred to as a second reference signal. Therefore, if the detection signal processing unit 10 calculates the phase difference between the reference signal and the second reference signal, the height of the diffraction grating 7 can be measured at the same time as the minute displacement amount according to the principle described with reference to FIGS.

According to the present invention, in FIG. 1, light having a wavelength λ ₁ is directly incident on the diffraction grating 7 from the collimator lens 51 without passing through the reduction lens 6, so that a small displacement of the proximity exposure apparatus or the electron beam drawing apparatus is obtained. It can also be applied to a measuring method and its device.

[0075]

According to the present invention, the phase difference between the second reference signal and the reference signal generated by directly heterodyne-interfering the light of the first wavelength and the light of the second wavelength causes the diffraction grating Since the height can be measured, there is an effect that the minute displacement amount and the height of the diffraction grating can be measured simultaneously and accurately.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of a minute displacement measuring method and an apparatus therefor according to the present invention.

FIG. 2 is a diagram showing a relationship between phases of a reference signal and a second reference signal when the height of the diffraction grating is at the reference position.

FIG. 3 is a diagram showing a relationship between phases of a reference signal and a second reference signal when the height of the diffraction grating is moving from the reference position.

FIG. 4 is a diagram showing a change in optical path length of specularly reflected light when the height of the diffraction grating is moving from a reference position.

5 is a detailed view showing a displacement and an optical path of the diffraction grating of FIG.

6 is a detailed view showing a symmetrical resist distribution on the diffraction grating of FIG.

7 is a detailed view showing an asymmetric resist distribution on the diffraction grating of FIG.

8 is a calculation example diagram showing the relationship between the resist shift amount and the phase of the measurement signal in the first state and the second state in FIG.

9 is a calculation example diagram showing the relationship between the resist shift amount and the intensity of first-order diffracted light in the first state and the second state in FIG. 7. FIG.

FIG. 10 is a calculation example diagram showing the relationship between the intensity difference of the first-order diffracted light in the first state and the second state of FIGS. 9 and 8 and the phase difference of the measurement signal in the two states.

FIG. 11 is a schematic configuration diagram showing a second embodiment of the method for measuring a minute displacement and the apparatus therefor according to the present invention.

FIG. 12 is a schematic configuration diagram showing an example of a conventional minute displacement measuring method and its apparatus.

[Explanation of symbols]

 11 ... Linearly polarized laser, 21, 22, 23, 25 ... Half mirror, 24, 26, 29, 30 ... Mirror, 31, 32, 33, 34, 35, 36 ... AO modulator, 41, 42, 43, 44 … Polarizing beam splitter, 45… Beam interval changing prism, 46… 1/4 wavelength plate, 47,48… ND filter, 51… Collimator lens, 52… Mirror, 6… Reduction lens, 7… Diffraction grating, 71,72… Resist, 81, 82 ... Polarizing plate, 85, 86, 87 ... Pinhole, 91, 92, 93 ... Photodetector, 101 ... Control processing circuit.

Claims (10)

[Claims] 1. A diffraction grating formed on an object whose position is to be detected is irradiated with two light beams of a two-wavelength orthogonal polarization laser from different directions, and the diffracted light beams generated from the diffraction grating by the irradiation of the two light beams. A heterodyne interference to generate a measurement signal, then generate a reference signal corresponding to the difference in wavelength of the two light beams, and measure a minute displacement of the diffraction grating from the phase difference between the reference signal and the measurement signal. In the displacement measuring method, the specularly reflected lights from the diffraction grating are heterodyne-interfered to generate a second reference signal, and the second reference signal and the reference signal are heterodyne-interfered to each other from the second phase difference. A small displacement measuring method characterized by simultaneously measuring the height of a diffraction grating. 2. Specular reflection light generated when light of a first wavelength is incident on a diffraction grating formed on an object whose position is to be detected at a predetermined angle, and diffracted light generated at this time. Is subjected to heterodyne interference with light having a second wavelength slightly different from each other to generate a reference signal and a measurement signal, and a minute displacement of the object is measured from a phase difference obtained from the reference signal and the measurement signal. In the displacement measurement method,
The first wavelength light is directly heterodyne-interfered with the second wavelength light to generate a second reference signal, and the object is calculated from a second phase difference obtained from the reference signal and the second reference signal. A method for measuring minute displacement, characterized in that the height of each of them is measured at the same time. 3. The minute displacement measuring method according to claim 2, wherein the predetermined angle is about half of a first-order diffraction angle. 4. The minute displacement measuring method according to claim 2 or 3, wherein the incident direction of the light of the first wavelength can be switched to the opposite direction of the reflection direction of the specular reflection light generated at this time, A minute displacement measuring method characterized in that both the minute displacement measuring amount before the direction switching and the minute displacement measuring amount after the incident direction switching can be measured. 5. The micro-displacement measuring method according to claim 4, wherein the measurement of the micro-displacement measurement amount before switching the incident direction and the measurement of the micro-displacement measurement amount after switching the incident direction are temporally different. Displacement measurement method. 6. The small displacement measuring method according to claim 4, wherein the small displacement measurement amount of the object is corrected from the difference in amplitude between the measurement signal before switching the incident direction and the measurement signal after switching the incident direction. Displacement measuring method characterized by. 7. A diffraction grating fixed on an object, a light source for generating light of a first wavelength and light of a second wavelength whose wavelengths are slightly different from each other, and the light of the first wavelength. Means for making the light incident on the diffraction grating at a predetermined angle; means for heterodyne interference between the specularly reflected light of the first wavelength and the light of the second wavelength generated from the diffraction grating to generate a reference signal; Means for heterodyne interfering diffracted light of a first wavelength and light of the second wavelength to generate a measurement signal; first light detection means for measuring the time change of the reference signal; Second photo-detecting means for measuring a time change, and a phase difference between the reference signal and the measurement signal detected by the first and second photo-detecting means is calculated and processed to be converted into the displacement of the object. In a small displacement measuring device equipped with a signal processing circuit, Note: means for directly interfering the first wavelength with the second wavelength to generate a second reference signal; third light detecting means for measuring a time change of the second reference signal; And calculating a phase difference between the second reference signal and the reference signal detected by the third light detecting means,
A small displacement measuring device comprising a signal processing circuit for processing and converting the height into the height of the object. 8. The micro-displacement measuring device according to claim 7, wherein the predetermined angle is about half the first-order diffraction angle. 9. The micro-displacement measuring device according to claim 7, wherein the incident direction of the light of the first wavelength is switched to the opposite direction to the specularly reflected light generated at this time, and the incident direction is switched. A minute displacement measuring device comprising a control circuit for controlling 10. The micro-displacement measuring device according to claim 9, wherein the diffracted light of the first wavelength and the light of the second wavelength before the switching of the incident direction and the diffracted light of the first wavelength after the switching of the incident direction and the diffracted light of the first wavelength are selected. Means for interfering light of two wavelengths with each other by heterodyne to generate a first measurement signal before switching the incident direction and a second measurement signal after switching the incident direction;
A microdisplacement measuring device, comprising means for correcting the microdisplacement measurement amount from the difference in amplitude between the measurement signal and the second measurement signal.