JP5093220B2 - Displacement measuring method and apparatus - Google Patents

Description

The present invention relates to a method and apparatus for measuring the displacement of an object using optical interference, and in particular, irradiates the object with laser light, causes the reflected light to interfere with reference light, and uses the obtained interference signal as a target. The present invention relates to a displacement measuring method and apparatus for measuring the amount of displacement of an object.

As a method for measuring the amount of displacement or movement of an object, a method using optical interference is widely known (Meas. Sci. Technol., 9 (1998), 1024-1030). An example is shown in FIG.

In the interferometer shown in FIG. 10, the polarization directions of the laser head 301 are orthogonal to each other.
In addition, a two-frequency orthogonally polarized beam 302 having an optical frequency different from each other by 20 MHz is emitted. This beam is separated into two polarization components by a polarization beam splitter 303. The S-polarized beam 303 is reflected by the polarizing beam splitter 303, then reflected by the right-angle prism 304, and enters the polarizing beam splitter 303 as reference light. The P-polarized beam 305 passes through the polarization beam splitter 303, is reflected by the right-angle prism 306 placed on the measurement object 400, and enters the polarization beam splitter 303. The both reflected beams are combined by the polarization beam splitter 303, and the polarizing plate 3 has a polarization angle in the direction of 45 ° with respect to the polarization direction of the both reflected beams.
Heterodyne interference occurs after transmitting 07.

This heterodyne interference light is received by the photoelectric conversion element 308 and converted into an electric signal 309. The frequency f _M of the heterodyne interference signal 309 is given by (Expression 1) to which a Doppler shift frequency corresponding to the moving speed V of the measurement object 400 is added.

f _M = f _B ± NV / λ (Equation 1)
Here, f _B = 20 MHz. λ is the wavelength of the laser beam. N = 2, 4, ...
The constant determined by the number of round trips of the optical path, and in the case of FIG. 10, N = 2. On the other hand, a beat signal 310 of f _B = 20 MHz is output from the laser head 301 as a reference signal.

The measured heterodyne interference signal 309 and the reference signal 310 are input to the phase detection circuit 311, and the moving speed V and the moving amount 400 d of the measurement object 400 are obtained from the phase difference between both signals, and output as the moving amount output signal 312. Is done.

Meas. Sci. Technol., 9 (1998), 1024-1030

In the interferometer shown in FIG. 10, a probe optical path, that is, a P-polarized beam 3 that is probe light.
Since the optical path through which the 05 passes and the reference optical path through which the S-polarized beam 303 as the reference light passes are spatially separated, when temperature distribution or refractive index distribution due to air fluctuations or mechanical vibration occurs, The optical path difference fluctuates between both optical paths, resulting in a measurement error on the order of nanometers. Positioning accuracy of sub-nanometer order or less is required for the probe position control of probe microscopes used for the future stage of exposure equipment and pattern dimension measuring equipment for semiconductor fine pattern manufacturing corresponding to 45nm node and 32nm node, or local characterization. Therefore, the prior art shown in FIG. 10 cannot meet this requirement. Although a method of controlling environmental factors such as temperature, humidity, and mechanical vibration with high accuracy can be considered, the economic effect is remarkably reduced in terms of apparatus cost, apparatus size, and usability.

The present invention solves the above-described problems and provides a displacement measuring method and apparatus capable of stably measuring the amount of displacement and the amount of movement of an object.

In the present invention, the light from the light source is separated into the first light and the second light, and the first light can be moved by bringing the optical axis of the first light and the optical axis of the second light close to each other. When irradiating the object and irradiating the reference light with the second light, the reflected light from the object interferes with the reflected light from the reference surface, and the amount of movement of the object is obtained from the interference light In addition, the movement amount of the object can be obtained with high accuracy without being affected by disturbance.

In the present invention, when the optical axis of the first light and the optical axis of the second light are brought close to each other, the physical property change of the medium around the optical axis of the first light and the optical axis of the second light is changed. Is set as the distance that acts equally on the optical axis of the first light and the optical axis of the second light, the influence of the disturbance acts on the two lights equally to cancel each other, and is affected by the disturbance. The amount of movement of the object can be obtained with high accuracy.

In the present invention, when the optical axis of the first light and the optical axis of the second light are brought close to each other, by matching the optical axis of the first light and the optical axis of the second light, The influence of the disturbance acts on the two lights equally to cancel each other, making it possible to determine the amount of movement of the object with high accuracy without being affected by the disturbance.

Further, in the present invention, by configuring the reference surface with a diffraction grating, it is possible to make the optical axis of the first light coincide with the optical axis of the second light, and the influence of the disturbance is equal to the two lights. It acts and cancels out, and it becomes possible to determine the amount of movement of the object with high accuracy without being affected by disturbance.

The present invention also separates the light from the light source into the first light and the second light, and the optical axis of the first light and the second light.
By irradiating the surface of the object with the first light with the optical axis of the light close to the object, and irradiating the reference surface with the second light, the reflected light from the surface of the object and the reflection from the reference surface When the shape of the surface of the object is obtained from the interference light by causing interference with light, the surface shape of the object can be obtained with high accuracy without being affected by disturbance.

According to the present invention, the influence of disturbance such as temperature distribution, refractive index distribution or mechanical vibration due to air fluctuations acts on the probe light (first light) and the reference light (second light) equally.
It becomes possible to cancel the influence of the disturbance when the two lights interfere. As a result, it is possible to obtain the displacement amount and the movement amount of the target object from the interference light with accuracy of sub-nanometer order or less without being affected by disturbance such as air fluctuation and mechanical vibration.

Further, by configuring the common optical path type interferometer, the displacement measuring device can be made small, and this device can be applied even when the space around the measurement object is small.

Furthermore, since it is not necessary to control environmental factors such as temperature, humidity, and mechanical vibration with high accuracy, there is an effect that the economic effect is remarkably improved in terms of apparatus cost, apparatus size, and usability.

It is the schematic which shows the apparatus structure of the displacement measuring device in Example 1 of this invention, the structure of a light source unit, the polarization direction of a 2 frequency orthogonal polarization beam, and a polarizing plate. It is the schematic which shows the structure of the reference mirror in this invention. It is the schematic which shows the structure of the light source unit using the 2 frequency He-Ne laser in Example 2 of this invention, and the polarization direction of a 2 frequency orthogonally polarized beam. It is the schematic which shows the apparatus structure of the displacement measuring device which transmits the 2 frequency orthogonal polarization beam radiate | emitted from the light source unit to the interferometer unit by a polarization plane preservation | save fiber in Example 3 of this invention. It is the schematic in the Example 4 of this invention which shows the apparatus structure of the displacement measuring device which produces | generates reference light using a birefringent prism. It is the schematic in the Example 5 of this invention which shows the apparatus structure of the displacement measuring device which produces | generates reference light using a polarization beam splitter and a reflective mirror. It is the schematic in the Example 6 of this invention which shows the apparatus structure of the displacement measuring device which probe light reciprocates 4 times in the optical path between a quarter wavelength plate and a target mirror. It is the schematic which shows the apparatus structure of the displacement measuring device which uses a homodyne common optical path type | mold interferometer in Example 7 of this invention, and the structure of a light source unit. It is the schematic which shows the apparatus structure of the displacement measuring device which uses a homodyne common optical path type | mold interferometer in Example 8 of this invention as a basic system. It is a figure for demonstrating the displacement measuring apparatus using the conventional optical interference.

  Embodiments of the present invention will be described with reference to the drawings.

A first embodiment of the present invention will be described with reference to FIG. The displacement measuring apparatus of this embodiment is shown in FIG.
), The light source unit 2, the interferometer unit 3, and the phase detection unit 18 are included. As shown in FIG. 1B, in the light source unit 2, four linearly polarized beams 22 from a linearly polarized laser light source 21 (for example, a frequency stabilized He-Ne laser having a wavelength of 632.8 nm) are provided.
The light is incident on the polarization beam splitter 23 with a polarization direction of 5 ° and separated into two polarization components. P
Polarized beam 24 passes through the polarization beam splitter 23, the acousto-optic modulation device 25 which is driven at a frequency _{f 1:} the (AOM Acousto-Optic Modulator), an optical frequency shifts by _{f 1.}

On the other hand, the S polarized light beam 28 is reflected by the polarization beam splitter 23, from acousto-optic modulation element 29 which is driven at a frequency _f 1 + _{f B,} the optical frequency is _f 1 + _{f B} shift. Where f _B
Is, for example, 100 kHz to several tens of MHz. P-polarized beam 24 and S-polarized beam 28
After being reflected by the mirrors 26 and 26 ', synthesized by the polarization beam splitter 27, then reflected by the mirror 26 "and emitted from the light source unit 2 as a two-frequency orthogonally polarized beam 4. FIG. 24P indicates the polarization direction of the P-polarized beam 24 of the two-frequency orthogonal polarization beam 4, and 28S indicates the polarization direction of the S-polarized beam 28.

The two-frequency orthogonal polarization beam 4 enters the interferometer unit 3 and is separated into two optical paths by the non-polarization beam splitter 5. The two-frequency orthogonally polarized beam 6 reflected by the non-polarizing beam splitter 5 is transmitted through a polarizing plate 7 having a polarization angle in a 45 ° direction with respect to both polarization directions as indicated by a broken line 7a in FIG. Heterodyne interference. This heterodyne interference light is converted into an electric signal 9 of a beat frequency f _B is received by a photoelectric conversion element 8 such as a photodiode, used as a reference signal. On the other hand, the two-frequency orthogonal polarization beam 10 transmitted through the non-polarization beam splitter 5 is incident on the reference mirror 11. As shown in FIG.
The diffraction grating 11g is formed of a metal material such as Al on the synthetic quartz substrate 11b.

As shown in the figure, such a diffraction grating reflects the S-polarized component 28S parallel to the longitudinal direction of the diffraction grating in the two-frequency orthogonally polarized beam 10, and transmits the orthogonal P-polarized component 24P as it is. The characteristic as a so-called polarizing element (Wire Grid Polarizer) is shown. In this embodiment, the pitch of the diffraction grating 11g is 144 nm, the line width is 65 nm, and the height is 165 nm. The S-polarized beam 10r reflected by the reference mirror 11 is used as reference light. The transmitted P-polarized beam 10m is used as probe light.

The P-polarized beam 10m becomes circularly polarized light after passing through the quarter-wave plate 12, reflected by the target mirror 13 placed on the measurement object 1, and again becomes S-polarized light after passing through the quarter-wave plate 12; The light is reflected by the reference mirror 11, passes through the quarter wavelength plate 12, is reflected as circularly polarized light by the target mirror 13, passes through the quarter wavelength plate 12, becomes P-polarized light, and passes through the reference mirror 11. That is, the probe light 10m reciprocates twice in the optical path between the reference mirror 11 and the target mirror 13, and the amount of movement 1d of the measurement object 1 is doubled and detected.

The S-polarized beam 10r reflected by the reference mirror 11 and the transmitted P-polarized beam 10m are 2
Reflected by the non-polarization beam splitter 5 as the frequency orthogonal polarization beam 14. This two-frequency orthogonal polarization beam 14 causes heterodyne interference by passing through a polarizing plate 15 having a polarization angle in a 45 ° direction with respect to both polarization directions as indicated by a broken line 15a in FIG. This heterodyne interference light is received by a photoelectric conversion element 16 such as a photodiode and converted into an electric signal 17. The frequency f _M of the heterodyne interference signal 17 is given by (Equation 1) to which a Doppler shift frequency corresponding to the moving speed V of the measurement object 1 is added.

In (Expression 1), N = 4. The measured heterodyne interference signal I (t) 17 and the reference signal 9 obtained by the photoelectric conversion element 8 are input to the phase detection unit 18, and the moving speed V and the moving amount of the measuring object 1 are calculated from the phase difference between the two signals. 1d is obtained and output as the movement amount signal 19. As the phase detection unit 18, for example, a lock-in amplifier or the like can be used. The heterodyne interference signal I (t) 17 is given by (Equation 2).

I (t) = I _m + I _r
+2 (I _m · I _r ) ^1/2 cos (2πfBt ± 2πNVt / λ) (Equation 2)
Here, I _m is the detected intensity of the probe light, the detection intensity of I _r is the reference light, n is the refractive index of the air,
λ is the wavelength of the laser beam 22. The phase detection unit 18 outputs the second term: ± 2π NVt / λ in the cos component of (Equation 2) as a phase signal. For example, the phase signal is π / 1
In the case of 800, the movement amount 1d = 0.044 nm.

As is clear from FIG. 1, the probe light 10 m toward the target mirror 13 and the reference light 1
The two beams of 0r are emitted from the light source unit 2 and enter the interferometer unit 3 until they reach the reference mirror 11 and are further received by the photoelectric conversion element 16 from the reference mirror 11. Follow the light path. That is, the configuration is a common optical path type interferometer. Therefore, even if temperature distribution or refractive index distribution due to air fluctuations or mechanical vibration occurs in the optical path,
Since these disturbances affect both beams equally, when both beams interfere, the influences of these disturbances are completely canceled, and the interference light is not affected by the disturbance. Only the probe light 10m exists in the optical path between the reference mirror 11 and the target mirror 13, but for example, the stroke of the probe microscope or the like is about several hundred microns at most. The gap can be set to 1 mm or less, and the influence of disturbance in such a minute gap can be ignored.

Further, in the light source unit 2 shown in FIG. 1B, two orthogonal polarized beams 24 and 28 pass through different optical paths, and there is a possibility that the influence of disturbance is superimposed between the two optical paths. However, even if the influence of the disturbance is present, the influence is equal to both the measured heterodyne interference light and the reference light, so that they are canceled when the phase detection unit 18 detects the phase difference between them. .

With the configuration of the interferometer of the present embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are stabilized with sub-nanometer to picometer accuracy without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure. In this embodiment, a metal diffraction grating (Wire Grid Polarizer) is used as a reference mirror, so that probe light is generated from one of the two frequency orthogonal polarization beams, and reference light is generated from the other polarization beam on the same axis. Therefore, a common optical path type heterodyne interferometer can be configured.

In the first embodiment, two polarization beam splitters 23 and 27 and two acousto-optic modulation elements 25 and 29 are used to generate the two-frequency orthogonal polarization beam 4 as shown in FIG. However, in the second embodiment, instead of the light source unit 2 shown in FIG. 1B in the first embodiment described above, as shown in FIG. Ne laser 31
(Dual mode laser having two longitudinal mode oscillations) is used. The polarization direction of the obtained two-frequency orthogonal polarization beam 32 is as indicated by 32P and 32S in FIG. 3B, and a beat signal of about 640 MHz is obtained as an example. Since the configurations and functions of the interferometer unit 3 and the phase detection unit 18 are the same as those of the first embodiment, the description thereof is omitted. According to the present embodiment, as in the first embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are changed from sub-nanometer to pico without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure stably with meter accuracy.

Next, as a third embodiment of the present invention, a displacement measuring apparatus for transmitting the two-frequency orthogonally polarized beams 4 and 32 emitted from the light source unit 2 to the interferometer unit 3 using the polarization plane preserving fiber will be described with reference to FIG. . The two-frequency orthogonally polarized beams 4 and 32 are separated into a P-polarized beam 42 and an S-polarized beam 45 by a polarization beam splitter 41. The P-polarized beam 42 is condensed on the incident end face 44a of the polarization plane preserving fiber 44 by the condenser lens 43 and transmitted while maintaining the linearly polarized light. The P-polarized beam emitted from the exit end face 44 b of the polarization plane preserving fiber 44 is converted into a parallel beam by the collimating lens 46 and passes through the polarization beam splitter 41. Similarly, the S-polarized beam 45 is condensed on the incident end face 44′a of the polarization plane preserving fiber 44 ′ by the condenser lens 43 ′, and transmitted while maintaining the linearly polarized light.

The S-polarized beam emitted from the exit end face 44 ′ b of the polarization plane preserving fiber 44 ′ is converted into a parallel beam by the collimating lens 46 ′ and reflected by the polarization beam splitter 41. The P-polarized beam and the S-polarized beam are combined to form a two-frequency orthogonally polarized beam 48 again and enter the interferometer unit 3. Since the configurations and functions of the interferometer unit 3 and the phase detection unit 18 are the same as those of the first embodiment, the description thereof is omitted.

In this embodiment, the light source 2 ′ described in the second embodiment may be used instead of the light source 2.

According to the present embodiment, as in the first embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are changed from sub-nanometer to pico without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure stably with meter accuracy. Further, since the light source unit 2 is separated from the interferometer unit 3 and connected by knitting wavefront preserving fibers 44 and 44 ′, the light source unit 2 is disposed far away, and only the interferometer unit 3 is disposed in the vicinity of the measurement object 1. There is an advantage that it can be applied even when there is no space in the vicinity of the measuring object 1.

Next, as a fourth embodiment of the present invention, a method of generating reference light using a birefringent prism will be described with reference to FIG. In the displacement measuring apparatus of the present embodiment, the basic configurations and functions of the light source unit 2 and the phase detection unit 18 are the same as those of the first embodiment, and thus description thereof is omitted.
Also in this embodiment, the light source 2 ′ described in the second embodiment may be used instead of the light source 2.

Only the reference light generation method and the interferometer unit 503 will be described below. Of the two-frequency orthogonal polarization beam 10 transmitted through the non-polarization beam splitter 5, the S polarization beam 10br is
Optical materials 51 and 51 ′ exhibiting birefringence characteristics, for example, a birefringent prism 50 in which two pieces of calcite are bonded together, the optical axis thereof is shifted by approximately 200 μm and reflected by a reflecting mirror 52 composed of a dielectric multilayer film. Then, the original optical axis is returned as reference light. On the other hand, P-polarized beam 1
0bm is transmitted through the birefringent prism 50 as it is, and the target mirror 13 is used as probe light.
After being reflected by the light beam, it returns to the original optical axis and is combined with the reference light 10br to be combined with the two-frequency orthogonal polarization beam 1
4 is reflected by the non-polarizing beam splitter 5. Similar to the first embodiment, the two-frequency orthogonal polarization beam 14 is transmitted through a polarizing plate 15 having a polarization angle in a 45 ° direction with respect to both polarization directions as indicated by a broken line 15a in FIG. Heterodyne interference. This heterodyne interference light is received by the photoelectric conversion element 16 and converted into an electric signal 67. Subsequent operations and signal processing are the same as those in the first embodiment, and a description thereof will be omitted.

As apparent from FIG. 5, the two beams of the probe light 10 bm and the reference light 10 br toward the target mirror 13 are emitted from the light source unit 2 and enter the interferometer unit 503, and reach the incident surface of the birefringent prism 50. Until the light is further received by the photoelectric conversion element 16 from the incident surface of the birefringent prism 50. That is, the configuration is a common optical path type interferometer. Therefore, even if temperature distribution or refractive index distribution due to air fluctuations or mechanical vibration occurs in the optical path, these disturbances affect both beams equally.
When both beams interfere with each other, the influence of these disturbances is completely canceled, and the interference light is not affected by the disturbances. Only the probe light 10bm exists in the optical path between the incident surface of the birefringent prism 50 and the target mirror 13, but for example, the stroke of the probe microscope or the like is about several hundred microns at most. The gap between the incident surface and the target mirror 13 can be set to several mm or less, and the influence of disturbance in such a minute gap can be ignored.

Therefore, according to the present embodiment, similarly to the first embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are set to sub-nanometers without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure stably with accuracy of picometer. In the present embodiment, by using the birefringent prism 50 and the reflecting mirror 52, the probe light is generated from one of the two frequency orthogonal polarization beams, and the reference light is generated substantially coaxially from the other polarization beam. Thus, a common optical path type heterodyne interferometer can be configured.

Next, as a fifth embodiment of the present invention, a method of generating reference light using a polarizing beam splitter and a reflecting mirror will be described with reference to FIG. In the displacement measuring apparatus of the present embodiment, the basic configurations and functions of the light source unit 2 and the phase detection unit 18 are the same as those of the first embodiment.
Description is omitted. In this embodiment, the light source 2 ′ described in the second embodiment may be used instead of the light source 2.

Only the reference light generation method and the interferometer unit 603 will be described below. Approximately 4% of the two-frequency orthogonally polarized beams 4 emitted from the light source unit 2 are beam splitters 6.
1 (transmittance 96%, reflectivity 4%), and the reflected two-frequency orthogonal polarization beam 62 has a polarization angle in a direction of 45 ° with respect to both polarization directions as indicated by a broken line 7a in FIG. Polarizing plate 7
Causes heterodyne interference. This heterodyne interference light is converted into the photoelectric conversion element 8.
In is converted into an electric signal 69 of the light receiving by the beat frequency f _B, it is used as a reference signal.

On the other hand, the two-frequency orthogonal polarization beam 63 transmitted through the beam splitter 61 is separated into an S-polarization beam 64 and a P-polarization beam 65 by the polarization beam splitter 60. S-polarized beam 64
Becomes a circularly polarized light after being transmitted through the quarter-wave plate 12 ′, is reflected by the reflection mirror 66 composed of a dielectric multilayer film, and then becomes a P-polarized light after being transmitted through the quarter-wave plate 12 ′. It returns through the optical axis and passes through the polarizing beam splitter 60.

The P-polarized beam 65 becomes circularly polarized light after being transmitted through the quarter-wave plate 12 ", reflected as the probe light by the target mirror 13, and again becomes S-polarized light after being transmitted through the 1/4 wavelength plate 12", returning to the original optical axis. Reflected by the polarization beam splitter 60. The two return lights are combined into a two-frequency orthogonally polarized beam 67, which is polarized light having a polarization angle in the 45 ° direction with respect to both polarization directions, as indicated by a broken line 15a in FIG. 1C, as in the first embodiment. Heterodyne interference occurs by passing through the plate 15. This heterodyne interference light is received by the photoelectric conversion element 16 and converted into an electric signal 68. Subsequent operations and signal processing are the same as those in the first embodiment, and a description thereof will be omitted.

As apparent from FIG. 6, the probe light 65 and the reference light 64 directed toward the target mirror 13.
These two beams are emitted from the light source unit 2, enter the interferometer unit 603, and pass through only the polarization beam splitter 60. Temperature distribution and refractive index distribution due to air fluctuations,
Or, even if mechanical vibration occurs, these disturbances are considered to affect both beams equally, and when both beams interfere, the influences of these disturbances are completely cancelled, and the interference light is not easily affected by the disturbances. . Only the probe light 65 passes through the air in the optical path between the quarter-wave plate 12 ″ and the target mirror 13, but for example, the stroke of the probe microscope or the like is about several hundred microns at most. The gap between the wave plate 12 "and the target mirror 13 is 1
It can be set to mm or less, and the influence of the disturbance in such a minute gap can be ignored. Therefore, according to the present embodiment, similarly to the first embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are set to sub-nanometers without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure stably with accuracy of picometer.

Next, as a sixth embodiment of the present invention, the case where the probe light reciprocates four times in the optical path between the quarter-wave plate and the target mirror will be described with reference to FIG. In the displacement measuring apparatus of this embodiment,
Since the basic configurations and functions of the light source unit 2 and the phase detection unit 18 are the same as those in the first embodiment, description thereof will be omitted. In this embodiment, the light source 2 ′ described in the second embodiment may be used instead of the light source 2.

Only the interferometer unit 703 will be described below. The two-frequency orthogonally polarized beam 4 or 32 emitted from the light source unit 2 enters the interferometer unit 703, is reflected by the mirror 71, and is then separated into two optical paths by the non-polarizing beam splitter 70. The dual-frequency orthogonally polarized beam 72 that has passed through the non-polarizing beam splitter 70 passes through the polarizing plate 7 having a polarization angle in the direction of 45 ° with respect to both polarization directions as shown by the broken line 7a in FIG. have a finger in the pie. This heterodyne interference light is received by the photoelectric conversion element 8 and beat frequency f _B
Is converted into an electrical signal 79 and used as a reference signal.

On the other hand, the two-frequency orthogonal polarization beam 73 reflected by the non-polarization beam splitter 70 enters the reference mirror 11. As shown in FIG. 2, the reference mirror 11 has a structure in which a diffraction grating is formed of a metal material such as Al on a synthetic quartz substrate 11b as shown in FIG. The polarizing element (Wire Grid Polarize) is exactly the same as the embodiment.
r). The S-polarized beam 10r reflected by the reference mirror 11 passes through the non-polarizing beam splitter 70 as reference light, is reflected by the reflecting surfaces 70m and 70n, and then passes through the non-polarizing beam splitter 70 again and is reflected by the reference mirror 11. Then, the light is reflected by the non-polarizing beam splitter 70.

On the other hand, the P-polarized beam 10m transmitted through the reference mirror 11 is used as probe light. The P-polarized beam 10m passes through the quarter-wave plate 12, becomes circularly polarized light, is reflected by the target mirror 13, is again transmitted through the quarter-wave plate 12, becomes S-polarized light, is reflected by the reference mirror 11, and is reflected by 1
After being transmitted through the / 4 wavelength plate 12, it is reflected by the target mirror 13 as circularly polarized light, and the ¼ wavelength plate 1
After passing through 2, it becomes P-polarized light and passes through the reference mirror 11.

This P-polarized beam 10m passes through the same optical path as the reference light, passes through the non-polarizing beam splitter 70, is reflected by the reflecting surfaces 70m and 70n, then passes through the non-polarizing beam splitter 70 again, and again becomes a quarter wavelength. After passing through the plate 12, it becomes circularly polarized light, reflected by the target mirror 13, again transmitted through the quarter wavelength plate 12, then becomes S polarized light, reflected by the reference mirror 11, and transmitted through the quarter wavelength plate 12 as circularly polarized light after transmission. The light is reflected by the target mirror 13, passes through the quarter-wave plate 12, becomes P-polarized light, and passes through the reference mirror 11. That is, the probe light 10m is emitted from the reference mirror 1
Thus, the optical path between 1 and the target mirror 13 is reciprocated four times, and the amount of movement 1d of the measuring object 1 is detected by enlarging it four times.

This P-polarized beam 10m is combined with the S-polarized beam 10r as the reference light to become a two-frequency orthogonally polarized beam 74, reflected by the non-polarized beam splitter 70, and then reflected by the mirror 71. The two-frequency orthogonal polarization beam 74 causes heterodyne interference by passing through the polarizing plate 15 having a polarization angle in the direction of 45 ° with respect to both polarization directions as indicated by a broken line 15a in FIG. This heterodyne interference light is received by the photoelectric conversion element 16 and converted into an electric signal 77.
Subsequent operations and signal processing are the same as those in the first embodiment, and a description thereof will be omitted.

As is clear from FIG. 7, as in the first embodiment, the two beams of the probe light 10m and the reference light 10r directed to the target mirror 13 are emitted from the light source unit 2 and incident on the interferometer unit 703. The optical path completely passes through the same optical path from the reference mirror 11 until the light is received by the photoelectric conversion element 16 until reaching the mirror 11. That is, the configuration is a common optical path type interferometer.

Therefore, even if a temperature distribution, refractive index distribution, or mechanical vibration occurs due to air fluctuations in the optical path, these disturbances affect both beams equally, so when both beams interfere, The influence is completely cancelled, and the interference light is not affected by the disturbance. Only the probe light 10m exists in the optical path between the reference mirror 11 and the target mirror 13, but for example, the stroke of the probe microscope or the like is about several hundred microns at most. The gap can be set to 1 mm or less, and the influence of disturbance in such a minute gap can be ignored.

Further, in the light source unit 2 shown in FIG. 1B, two orthogonal polarized beams 24 and 28 pass through different optical paths, and there is a possibility that the influence of disturbance is superimposed between the two optical paths. However, even if the influence of the disturbance is present, the influence is equal to both the measured heterodyne interference light and the reference light, so that they are canceled when the phase detection unit 18 detects the phase difference between them. .

With the configuration of the interferometer of the present embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are stabilized with sub-nanometer to picometer accuracy without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure. In this embodiment, a metal diffraction grating (Wire Grid Polarizer) is used as a reference mirror, so that probe light is generated from one of the two frequency orthogonal polarization beams, and reference light is generated from the other polarization beam on the same axis. Therefore, a common optical path type heterodyne interferometer can be configured. Furthermore, in this embodiment, the probe light 10m reciprocates four times in the optical path between the reference mirror 11 and the target mirror 13, and the movement amount 1d of the measurement object 1 is detected by expanding it four times. Thus, twice the displacement measurement sensitivity can be obtained compared to the first embodiment.

Each of the first to sixth embodiments is based on a heterodyne common optical path type interferometer,
Next, as a seventh embodiment of the present invention, a case where a homodyne common optical path type interferometer is used as a basic system will be described with reference to FIG. As shown in FIG. 8A, the displacement measuring apparatus of this embodiment includes a light source unit 802, an interferometer unit 803, and a displacement detection unit 102. FIG.
In the light source unit 802, a linearly polarized laser 821 (for example, wavelength 6) is shown.
The outgoing beam 822 from a 32.8 nm frequency stabilized He-Ne laser) is incident on a polarizing beam splitter 823 with a polarization direction of 45 °, and two polarization components, a P-polarized beam 824 and S
Separated into a polarized beam 828. The P-polarized beam 824 is transmitted through the polarizing beam splitter 823, and the S-polarized beam 828 is reflected by the polarizing beam splitter 823, and each mirror 826 is reflected.
, 826 ′ and then synthesized by the polarization beam splitter 827, the mirror 826 ″
And output as an orthogonal polarization beam 881.

That is, in this embodiment, unlike the first embodiment, no optical frequency shift is given to the orthogonal polarization beam. In FIG. 1C, 24P indicates the polarization direction of the P-polarized beam 824 of the orthogonal polarization beam 4, and 28S indicates the polarization direction of the S-polarized beam 828.

As shown in FIG. 8A, the orthogonally polarized beam 881 enters the interferometer unit 803.
In the interferometer unit 803, the orthogonal polarization beam 882 that has passed through the non-polarization beam splitter 805 enters the reference mirror 811. As in the first embodiment, the reference mirror 811 has a configuration in which a diffraction grating is formed of a metal material such as Al on a synthetic quartz substrate 11b as shown in FIG. This is exactly the same as the embodiment, and the polarizing element (Wire Gri
d Polarizer).

The S-polarized beam 10r reflected by the reference mirror 811 is used as reference light. P penetrated
The polarized beam 10m is used as probe light. P-polarized beam 10m is a quarter wave plate 812
After passing through the light, it becomes circularly polarized light, is reflected by the target mirror 13, and is again a quarter wavelength plate 812.
After being transmitted, it becomes S-polarized light, reflected by the reference mirror 811, transmitted through the quarter-wave plate 812 and then reflected by the target mirror 13 as circularly polarized light, and after passing through the quarter-wave plate 812, becomes P-polarized light
The light passes through the reference mirror 811. That is, the probe light 10m reciprocates twice in the optical path between the reference mirror 811 and the target mirror 13, and the amount of movement 1d of the measurement object 1 is doubled and detected.

The S-polarized beam 10r reflected by the reference mirror 811 and the transmitted P-polarized beam 10m are combined and reflected by the non-polarized beam splitter 805 as an orthogonally polarized beam 883. The orthogonally polarized beam 883 is transmitted through the half-wave plate 884 and rotated in the polarization direction by 45 °, and is separated into two beams by the non-polarized beam splitter 885. The orthogonal polarization beam 886 reflected by the non-polarization beam splitter 885 is incident on the polarization beam splitter 887 and separated into two homodyne interference lights 888 and 890 whose phases are shifted by 180 °. The homodyne interference light 888 is received by a photoelectric conversion element 889 such as a photodiode and converted into an electric signal 92. The homodyne interference light 890 whose phase is shifted by 180 ° is received by the photoelectric conversion element 891 and converted into an electric signal 93.

The orthogonal polarization beam 894 that has passed through the non-polarization beam splitter 885 is a quarter wave plate 895.
After transmission, a phase difference of ± 90 ° is added, and the light enters the polarization beam splitter 887 and is further separated into two homodyne interference lights 896 and 898 whose phases are shifted by 180 °. The homodyne interference light 896 is received by a photoelectric conversion element 897 such as a photodiode, and the electric signal 1
Converted to 00. The homodyne interference light 898 whose phase is shifted by 180 ° is the photoelectric conversion element 8.
The light is received at 99 and converted into an electric signal 101.

Four homodyne interference signals 92, 93, 100, and 101 are given by (Equation 3) to (Equation 6), respectively.

I ₁ = I _m + I _r +2 (I _m · I _r ) ^1/2 cos (4πnD / λ) (Equation 3)
I ₂ = I _m + I _r +2 (I _m · I _r ) ^1/2 cos (4πnD / λ + π)
= I _m + I _r −2 (I _m · I _r ) ^1/2 cos (4πnD / λ) (Equation 4)
I ₃ = I _m + I _r +2 (I _m · I _r ) ^1/2 cos (4πnD / λ + π / 2)
= I _m + I _r −2 (I _m · I _r ) ^1/2 sin (4πnD / λ) (Equation 5)
I ₄ = I _m + I _r +2 (I _m · I _r ) ^1/2 cos (4πnD / λ + 3π / 2)
= I _m + I _r +2 (I _m · I _r ) ^1/2 sin (4πnD / λ) (Equation 6)
Here, I _m is the detected intensity of the probe light, the detection intensity of I _r is the reference light, n is the refractive index of the air, D
Is the amount of movement 1d of the measuring object 1, and λ is the wavelength of the laser beam 822. Displacement detection unit 10
2, the movement amount D of the measurement object 1 is calculated from (Equation 3) to (Equation 6) based on (Equation 7), and is output as the movement amount signal 103.

D = (λ / 4πn) tan ⁻¹ {(I ₄ −I ₃ ) / (I ₁ −I ₂ )} (Expression 7)
As is clear from FIG. 8A, the two beams of the probe light 10m and the reference light 10r toward the target mirror 13 are emitted from the light source unit 802 and are interferometer unit 803.
From the reference mirror 811 to the photoelectric conversion element 889 until it reaches the reference mirror 811.
, 891, 897, and 899, completely pass through the same optical path. That is, the configuration is a common optical path type interferometer.

Therefore, even if a temperature distribution, refractive index distribution, or mechanical vibration occurs due to air fluctuations in the optical path, these disturbances affect both beams equally, so when both beams interfere, The influence is completely cancelled, and the interference light is not affected by the disturbance. Only the probe light 10m exists in the optical path between the reference mirror 811 and the target mirror 13, but the stroke of the probe microscope or the like is about several hundred microns at most.
The gap between the reference mirror 811 and the target mirror 13 can be set to 1 mm or less, and the influence of disturbance in such a minute gap can be ignored. In addition, in the light source unit 802 shown in FIG. 8B, two orthogonally polarized light beams 824 and 828 pass through different optical paths, and there is a possibility that the influence of disturbance is superimposed between the two optical paths. However, even if there is an influence of disturbance, the influence is equal to the measured four homodyne interference lights, and is canceled in the processing of (Equation 7) in the displacement detection unit 102.

With the configuration of the interferometer of the present embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are stabilized with sub-nanometer to picometer accuracy without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure. In this embodiment, a metal diffraction grating (Wire Grid Polarizer) is used as a reference mirror, so that probe light can be generated from one polarization beam of orthogonal polarization beams, and reference light can be generated coaxially from the other polarization beam. Thus, a common optical path type homodyne interferometer can be configured.

Next, as an eighth embodiment of the present invention, a case where a homodyne interferometer is used as a basic system as in the seventh embodiment will be described with reference to FIG. The displacement measuring device of the present embodiment is similar to the seventh embodiment,
The light source unit 802, the interferometer unit 903, and the displacement detection unit 102 are configured. However, the configurations and functions of the light source unit 802 and the displacement detection unit 102 are the same as those in the seventh embodiment, and thus the description thereof is omitted. The orthogonal polarization beam 881 emitted from the light source unit 802 enters the interferometer unit 903. In the interferometer unit 903, the orthogonal polarization beam 982 that has passed through the non-polarization beam splitter 905 enters the reference mirror 911. Reference mirror 91
As in the reference mirror 11 of the first embodiment, 1 has a configuration in which a diffraction grating is formed of a metal material such as Al on a synthetic quartz substrate 11b as shown in FIG. This embodiment is exactly the same as the first embodiment, and functions as a polarizing element (Wire Grid Polarizer). The S-polarized beam 10r reflected by the reference mirror 911 is used as reference light.

The transmitted P-polarized beam 10m is used as probe light. P polarized beam 10m is 1/4
After passing through the wave plate 912, it becomes circularly polarized light, reflected by the target mirror 13, and again 1/4.
After passing through the wave plate 912, it becomes S-polarized light, reflected by the reference mirror 911, and ¼ wave plate 912.
Is reflected by the target mirror 13 as circularly polarized light after passing through, and after passing through the quarter-wave plate 912, P
Polarized light is transmitted through the reference mirror 911. That is, the probe light 10m is transmitted from the reference mirror 911.
The optical path between the target mirror 13 and the target mirror 13 is reciprocated twice, and the amount of movement 1 of the measurement object 1 is 1
d is magnified twice and detected. S-polarized beam 1 reflected by reference mirror 911
The P-polarized beam 10m transmitted through 0r is combined and reflected by the non-polarized beam splitter 905 as an orthogonally polarized beam 983. This orthogonally polarized beam 983 is the beam expander 2.
It is enlarged by 01.

This expanded beam 202 is a DOE 203 (Differential Optical E
element: diffractive optical element), four orthogonally polarized beams 204, 205, 206, 2
It is divided into 07 and enters a phase shift mask 208 made of a birefringent material. The phase shift mask 208 includes four regions 20 corresponding to the four orthogonally polarized beams 204 to 207.
It is divided into 8a, 208b, 208c, and 208d, and phase shifts of 0 °, 90 °, 180 °, and 270 ° are given between mutually orthogonal polarized beams that pass through each region. The four orthogonally polarized beams to which the phase shift is given further pass through a polarizing plate 209 having a polarization angle in a 45 ° direction with respect to both polarization directions, thereby causing homodyne interference.

The four homodyne interference lights 210 to 213 are received by the four-divided photoelectric conversion element 214 and converted into electric signals 215 to 218. The four homodyne interference signals 215-218 are
As in the seventh embodiment, each is given by (Equation 3) to (Equation 6). In the displacement detection unit 102, the movement amount D of the measurement object 1 is calculated based on (Equation 7) from (Equation 3) to (Equation 6), and is output as the movement amount signal 103.

As is clear from FIG. 9, the probe light 10 m toward the target mirror 13 and the reference light 1
The two beams of 0r are emitted from the light source unit 902, enter the interferometer unit 903, pass through the same optical path until reaching the reference mirror 911, and further receiving from the reference mirror 911 by the photoelectric conversion element 214. . That is, the configuration is a common optical path type interferometer. Therefore, even if a temperature distribution, refractive index distribution, or mechanical vibration occurs due to air fluctuations in the optical path, these disturbances affect both beams equally, so when both beams interfere, The influence is completely cancelled, and the interference light is not affected by the disturbance. The only reference mirror 91
Only the probe light 10m exists in the optical path between 1 and the target mirror 13, but for example, the stroke of the probe microscope or the like is about several hundred microns at most.
11 and the target mirror 13 can be set to 1 mm or less, and the influence of disturbance in such a minute gap can be ignored.

With the configuration of the interferometer of the present embodiment, the moving speed V and the moving amount 1d of the measuring object 1 are stabilized with sub-nanometer to picometer accuracy without controlling environmental factors such as temperature, humidity, and acoustic vibration with high accuracy. It is possible to measure. In this embodiment, a metal diffraction grating (Wire Grid Polarizer) is used as a reference mirror, so that probe light can be generated from one polarization beam of orthogonal polarization beams, and reference light can be generated coaxially from the other polarization beam. Thus, a common optical path type homodyne interferometer can be configured. In this embodiment, since the DOE 203 and the planar phase shift mask 208 are used to generate the four phase-shifted interference lights, the configuration of the interferometer unit 3 is simplified and the stability is further increased. Further, there is an advantage that the size is further reduced. Therefore, the measurement object 1
It is applicable even when there is no space around.

The embodiment of the present invention has been described above by taking the case of measuring the movement amount 1d of the measurement object 1 as an example. As the measurement object 1, a stage such as a semiconductor exposure apparatus or an inspection apparatus, a probe of a probe microscope or a measurement sample mounting stage, or a processing tool (such as a tool) is assumed. Further, the present invention is not limited to these examples. For example, the target mirror 13 is removed, the surface of the object 1 is directly irradiated with the probe light 10m, and the object 1 is perpendicular to the probe light 10m. If the measurement is performed while moving, it is also possible to measure the minute irregularities on the surface of the object 1 with high resolution with sub-nanometer to picometer resolution. Possible objects in this case include the surface of the magnetic disk, the surface roughness of the air bearing surface, and MEMS (Micro-Electro-Mechanical Systems) parts such as microlenses. In addition, if a condensing lens is inserted between the reference mirror 11 and the object 1, the in-plane spatial resolution will reach submicron order.

  It is also obvious that the third embodiment of the present invention can be combined with the fourth to eighth embodiments.

1, 400 ... measurement object 2, 2 ', 802 ... light source unit 3, 503,
603, 703, 803, 903 ... Interferometer unit 5, 85 ... Non-polarizing beam splitter 7, 15, 209, 307 ... Polarizing plate 8, 16, 89, 91, 97
, 99, 214, 308 ... photoelectric conversion element 11 ... reference mirror 11b
.... Synthetic quartz substrate 11g ... Diffraction grating 12, 95 ... 1/4 wavelength plate 13
... Target mirror 18 ... Phase detection unit 21 ... Linearly polarized laser 23, 27, 41, 87, 303 ... Polarizing beam splitter 25, 29 ...
Acoustooptic modulator 31... 2 frequency He-Ne laser 44... Polarization maintaining fiber 50... Birefringent prism 51 .. birefringent material 52 and 66 .. reflective mirror 60. Splitter 70: Non-polarizing beam splitter
84 ... 1/2 wavelength plate 102 ... displacement detection unit 201 ... beam expander 203 ... DOE 208 ... phase shift mask 304,306
... Right-angle prism 311 ... Phase detection circuit

Claims (11)

The light of the first polarization state and the light with respect to the polarization element that transmits the light of the first polarization state and reflects the light of the second polarization state whose polarization direction is orthogonal to the light of the first polarization state Irradiating light from a light source including light in the second polarization state;
A first polarization state which is transmitted by the first polarization state transmitted through the polarizing element to an object disposed with a minute gap of 1 mm or less formed between the polarization element and reflected by the object. Interference light and the reflected light of the second polarization state reflected by the polarizing element to generate interference light,
Displacement information of the object is obtained from information contained in the interference light and information from the light source that has not been applied to the polarizing element, and the optical axis of the transmitted light in the first polarization state; The displacement measuring method, wherein the optical axis of the reflected light in the first polarization state and the optical axis of the reflected light in the second polarization state are coaxial. A displacement measuring method according to claim 1 Symbol placement,
The polarization measuring device includes a reference mirror and a quarter-wave plate. The displacement measuring method according to claim 1 or 2,
The transmitted light in the first polarization state that has passed through the polarizing element travels back and forth between the polarizing element and the object, then passes through the polarizing element and is reflected by the polarizing element. Displacement measuring method characterized by causing interference with reflected light in the polarization state. The displacement measuring method according to claim 1 or 2 ,
The reference mirror is composed of a diffraction grating. The displacement measuring method according to claim 1 or 2 ,
The displacement measurement method according to claim 1, wherein the reference mirror is composed of a wire grid polarizer. A displacement measuring method according to any one of claims 1 to 5 ,
The displacement measuring method characterized in that the method of causing the reflected light in the first polarization state reflected by the object to interfere with the reflected light in the second polarization state reflected by the polarizing element is heterodyne interference. . A light source that emits light including light in a first polarization state, light in a second polarization state in which the polarization direction is orthogonal to the light in the first polarization state, and
A polarizing element that transmits the light in the first polarization state and reflects the light in the second polarization state;
A first polarization state which is transmitted by the first polarization state transmitted through the polarizing element to an object disposed with a minute gap of 1 mm or less formed between the polarization element and reflected by the object. And the reflected light of the second polarization state reflected by the polarizing element to generate interference light, the optical axis of the transmitted light of the first polarization state, and the first polarization Interference means configured such that the optical axis of the reflected light in the state and the optical axis of the reflected light in the second polarization state are coaxial,
A displacement measuring apparatus comprising: displacement information extracting means for detecting interference light generated by the interference means and light that has not been applied to the polarizing element to obtain displacement information of the object. The displacement measuring device according to claim 7,
The said polarizing element has a reference mirror and a quarter wavelength plate, The displacement measuring device characterized by the above-mentioned. The displacement measuring device according to claim 7 or 8 ,
The displacement measuring apparatus, wherein the polarizing element is formed of a diffraction grating. The displacement measuring device according to claim 7 or 8 ,
The displacement measuring apparatus according to claim 1, wherein the polarizing element is formed of a wire grid polarizer. A displacement measuring device according to any one of claims 7 to 1 0,
The interference means has a configuration in which the reflected light in the first polarization state reflected by the object interferes with the reflected light in the second polarization state reflected by the polarizing element by a heterodyne interferometer. Displacement measuring device.

Images