JPH05256611A - Laser interferometer - Google Patents

Abstract

(57) [Abstract] [Object] The present invention relates to a laser interferometric length measuring apparatus, and an object thereof is to provide a laser interferometric length measuring apparatus that suppresses the influence of the atmosphere and performs highly accurate length measuring. A laser light irradiating means for irradiating a predetermined measuring object with laser light, a laser light reflecting means for reflecting laser light provided on the measuring object, and dividing the laser light into two or more laser lights. , After providing a predetermined optical path difference between the respective laser lights, the laser light interference means for causing the laser lights to interfere with each other and the light intensity of the interfered laser light obtained by the laser light interference means A laser light phase measuring means for measuring the phase of the laser light by monitoring in series, and an airtight tubular member covering all or part of the optical path of the laser light, and a unit inside the tubular member. The molecular density per volume is configured to be smaller than the molecular density per unit volume outside the tubular member.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser interferometric length measuring device, and more specifically, to a one-dimensional device suitable for use in the fields of various industrial length measuring devices, precision machine tools, semiconductor manufacturing equipment, etc. The present invention relates to a laser interferometer for measuring the position of a two-dimensional or three-dimensional moving stage. In recent years, a laser interferometer for measuring the stage position of an XY stage by laser interferometry, especially a dual frequency interferometer utilizing the Zeeman effect, is provided in large numbers because of its extremely high measurement accuracy. Has been done.

When manufacturing a semiconductor device, it is necessary to repeat a step of transferring a pattern formed on a mask or a reticle to a photosensitive material on a wafer and forming a predetermined pattern after developing the photosensitive material ten times or more. is there. At this time, alignment is performed between the steps of forming each pattern, but with the recent miniaturization of patterns, exposure light of a short wavelength is used, so that the mask or reticle and the wafer are directly observed. It is extremely difficult to perform the alignment, and for this reason, the relative alignment method is mainly used, and the positional accuracy of the stage on which the wafer is mounted becomes extremely important.

However, it is known that the interferometer for measuring the stage position has a large change in the measured value depending on the environmental conditions in which it is used. Therefore, it is necessary to suppress the fluctuation of the measured value due to the use environment. Become.

[0004]

2. Description of the Related Art A conventional laser interferometer of this type is, for example, an X-type semiconductor exposure apparatus as shown in FIG.
Some are used for the Y stage. This laser interference measuring device is roughly divided into a stage 1 and a measuring means 2.
The stage 1 includes a stage body 3, an X-axis plane mirror 4, a Y-axis plane mirror 5, an X-axis direction guide 6,
The measuring means 2 is composed of a Y-axis direction guide 7, and the length measuring means 2 includes a Zeeman laser tube 8, a beam splitter 9, an X-axis interferometer 10, an X-axis receiver 11, a Y-axis interferometer 12, and a Y-axis receiver. It is composed of 13.

The stage 1 moves in the X-axis direction and the Y-axis direction along the X-axis direction guide 6 and the Y-axis direction guide 7,
The object placed on the stage body 3 is conveyed to an arbitrary position on the XY two-dimensional plane. The length measuring means 2 measures the length by a dual frequency laser interference length measuring device which is widely used at present in a semiconductor exposure apparatus or the like.

The operation of the dual-frequency laser interferometer will be described below with reference to FIG. In the figure, 21 is a dual wavelength Zeeman laser light source, 22 is a beam splitter, 23 is a photodetector, 24 is an AC amplifier, 25 is a polarization beam splitter, 26 is a retroreflector for position reference, and 27
Is a retro reflector for position measurement, 28 is a photodetector, 29
Is an AC amplifier, and 30 is a pulse converter.

The dual wavelength Zeeman laser irradiated by the dual wavelength Zeeman laser light source 21 has two left and right circular circles whose oscillation frequencies are slightly different from each other due to the Zeeman effect that occurs when a direct current magnetic field is applied to the laser resonator. It is obtained by separating it into polarized laser light, and the laser light generated from such a resonator is linearly polarized with frequencies f ₁ and f ₂ orthogonal to each other by a predetermined optical system (telescopic optical system, retarder). Is converted to.

Incidentally, the difference between f ₁ and f ₂ depends on the strength of the DC magnetic field. For example, a He--Ne laser (λ = 6) is used.
33 nm) and about 2 MHz is often used. In the above configuration, the laser light generated from the dual wavelength Zeeman laser light source 21 is first split into the beat reference light α and the length measuring light β by the beam splitter 22, and the beat reference light α is frequency f by the photodetector 23. _2-
It is detected as an AC signal (reference signal) of f ₁ and output to the AC amplifier 24.

On the other hand, the length-measuring light β in which the two-frequency light beams whose polarization planes are orthogonal to each other overlap each other
The two frequency components f ₁ and f ₂ are separated, and the f ₂ component is directed to the position reference retro-reflector 26, and the f ₁ component is directed to the position measurement retro-reflector 27. Here, the position measuring retro-reflector 27 has a speed V (however,
If it is moving in the right direction in the figure), the length measurement light β ′ reflected and returned by the position measuring retro-reflector 27 undergoes frequency shift due to the Doppler effect, and the frequency after the shift occurs. Is f ₁ ^' , the frequency f
₁ ^' is calculated by Equation 1 (however, the relativistic effect is ignored).

[0010]

[Equation 1] Therefore, the shift amount Δf ₁ is

[0011]

[Equation 2] And the displacement amount d of the position measuring retro reflector 27
Is

[0012]

[Equation 3] It is represented by. That, lambda ₁ is from a constant determined by the Zeeman laser head, to know Delta] f ₁ continuously, by integrating the Delta] f ₁ time, it is possible to calculate the velocity V of the position measuring retroreflector 27.

Therefore, in the case of the system shown in FIG. 5, the position reference light having the frequency f ₂ returned from the position reference retro-reflector 26 and the frequency returned by the position measuring retro reflector 27 after receiving the Doppler shift. The light of f ₁ ± Δf ₁ is polarized beam splitter 25.
Are combined together again and input to the photodetector 28 for length measuring light.

Then, these interfering lights are converted into a Doppler signal having a beat of frequency f _2- (f ₁ ± Δf ₁ ) by the photodetector 28. Since the reference signal f ₂ −f ₁ which is the reference beat of the interferometer can be known by the beat reference light described above, Δf ₁ is counted by obtaining the difference between these beat frequencies.

In an actual system, in order to detect the position with higher accuracy, generally, a plurality of pulses are generated for each increment of the beat by one wavelength. Further, there are various variations in the interference optical system, and in addition to the plane mirror interferometer (see FIG. 6) used in FIG. 4, a linear interferometer (see FIG. 7) and a single beam interferometer (see FIG. 8)) etc.

[0016]

However, in such a conventional laser interference measuring device, the length is measured based on the interference of the laser light emitted from the Zeeman laser tube 8. Therefore, there were the following problems. That is, when the above-mentioned laser interferometer is used in the atmosphere, the measured value is affected by the temperature, pressure, humidity and the like of the atmosphere.

This is because the refractive index of the atmosphere is a function of these environmental parameters. Therefore, in order to reduce the influence of the atmosphere, put the entire length measurement system in a thermal chamber to stabilize the environment, and on the assumption that the environmental parameters are homogeneous in the vicinity of the length measurement system, appropriate means are provided. To reduce the influence of the atmosphere by correcting the measurement value using a theoretically given formula and by covering a part of the optical path of the laser light with a telescopic spring 31 as shown in FIG. And other measures are taken.

However, with the miniaturization of an approximate semiconductor device, extremely high accuracy in controlling the movement of the stage is required, and an accuracy of 10 ^-6 or more is to be achieved with respect to the length to be measured. In this case, the atmosphere is not homogeneous, so the influence of subtle atmospheric fluctuations on the measured values cannot be ignored. For example, it is empirically known that the reproducibility of measured values is 0.1 μm or more when a fixed distance of about 20 cm is continuously measured in an environment of a normal laboratory, and the resolution of the laser interferometer itself is low. As 0.01 μm
Nowadays, the products with the following resolutions are produced, and the deterioration of the measurement accuracy due to the influence of the atmosphere becomes a serious problem.

[Purpose] Therefore, an object of the present invention is to provide a laser interferometric length measuring apparatus which suppresses the influence of the atmosphere and performs high precision length measurement.

[0020]

In order to achieve the above object, a laser interference measuring apparatus according to the present invention is provided with a laser beam irradiating means for irradiating a predetermined measuring object with a laser beam, and a laser beam irradiating means provided on the measuring object. It is located between the laser light reflecting means for reflecting the laser light emitted from the laser light emitting means and the laser light emitting means and the laser light reflecting means, and divides the laser light into two or more laser lights. After providing a predetermined optical path difference between the laser beams, the laser beam interfering means for interfering the laser beams by putting them together and the optical intensity of the interfering laser beam obtained by the laser beam interfering means are set in time series. Laser light phase measuring means for measuring the phase of the laser light by monitoring, and an airtight cylindrical member covering all or part of the optical path of the laser light, per unit volume inside the cylindrical member The molecular density of the cylinder It is configured to be smaller than the number density per unit volume in the outer member.

The tubular member preferably expands and contracts according to the optical path length of the laser light, and the laser light interference means may be, for example, a dual frequency Zeeman laser interferometer. Further, as a specific example for making the molecular density per unit volume inside the tubular member smaller than the molecular density per unit volume outside the tubular member, the inside of the tubular member is subjected to normal pressure or reduced pressure. It is conceivable to fill it with helium gas, simply reduce the pressure, or make a vacuum.

[0022]

According to the present invention, the whole or part of the optical path of the laser beam is covered with the airtight tubular member, and the molecular density per unit volume inside the tubular member per unit volume outside the tubular member. The inside of the tubular member is filled with helium gas at normal pressure or reduced pressure, or reduced pressure or vacuum state so as to be smaller than the molecular density.

That is, the influence of the atmosphere is suppressed by the cylindrical member having a molecular density per unit volume inside that is smaller than the molecular density per unit volume outside, and highly accurate length measurement is performed without being affected by the atmosphere. ..

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. 1 and 2 are views showing a first embodiment of a laser interference measuring apparatus according to the present invention, FIG. 1 is a perspective view showing a configuration of a main part of the first embodiment, and FIG. 2 is an enlarged sectional view of a main part of FIG. Is. First, the configuration will be described.

In FIG. 1, the same numbers as the numbers given to the conventional example shown in FIG. 4 indicate the same parts. The laser interferometric length measuring apparatus of this embodiment is roughly divided into a stage 1 and a length measuring means 2. The stage 1 includes a stage body 3, an X-axis plane mirror 4 which is a laser beam reflecting means, and a Y-axis plane. It is composed of a mirror 5, an X-axis direction guide 6 and a Y-axis direction guide 7. The length measuring means 2 is a Zeeman laser tube 8 which is a laser light irradiation means, a beam splitter 9, and an X-axis interferometer which is a laser light interference means. Meter 10 and Y
It is composed of an axis interferometer 12, an X axis receiver 11 and a Y axis receiver 13 which are laser light phase measuring means.

Further, in FIG. 1, reference numeral 41 designates an expandable and contractible and airtight cylinder, and in this embodiment, the X-axis plane mirror 4 and the Y-axis plane mirror 5 that most affect the length measurement accuracy.
And the X-axis interferometer 10 and the Y-axis interferometer 12. FIG. 2 shows an enlarged cross section of the main part of the cylinder.

The cylinder 41 includes a cylinder body 42, an optical window 43, and an O window.
It is composed of a ring 44, a buffer tank 45, a piston 46, and a valve 47. The tube body 42 is closed at both ends by transparent optical windows 43, thereby blocking the outside and inside of the tube body 42. At the same time, the plurality of cylinders are engaged with each other through the O-ring 44 so as to be expandable and contractible in an airtight state.

Further, from the side wall of the cylinder body 42, the cylinder body 4
A large-capacity buffer tank 45 having a capacity sufficiently larger than the second volume is arranged, and a piston 46 of the buffer tank 46 compensates for a change in internal volume due to expansion and contraction of the cylinder main body 42. The valve 47 includes an external device (not shown) for adjusting the pressure in the cylinder body 42 and the helium gas, the cylinder body 42, and the address conversion buffer tank 45.
In the present embodiment, the helium gas having the same pressure as the atmospheric pressure is filled in the cylinder main body 42.

With the above-described structure, in this embodiment, as shown in FIG. 9, as compared with the conventional example in which a part of the optical path of the laser beam is simply covered with the telescopic spring 31,
By filling the inside of the cylinder body 42 with an inert helium gas having a smaller molecular weight than the atmosphere (air), the influence of the atmosphere is suppressed, and more accurate length measurement is possible.

FIG. 3 is a diagram showing a second embodiment of the laser interferometer according to the present invention, and is a perspective view showing the structure of the essential parts of the second embodiment. In FIG. 3, the same numbers as the numbers given to the first embodiment shown in FIG. 1 indicate the same parts. In the first embodiment, the cylinder 41 is provided between the X-axis plane mirror 4 and the Y-axis plane mirror 5, and the X-axis interferometer 10 and the Y-axis interferometer 12. The laser interferometer of the embodiment is provided for all the optical paths of laser light.

Therefore, in this embodiment, although the cost of the apparatus is higher than that of the first embodiment, more accurate measurement can be performed. Since the Zeeman laser tube 8, the X-axis receiver 11, and the Y-axis receiver 13 in this embodiment have a heat source inside, the characteristics of the internal equipment may change due to heat generation in a depressurized state or a vacuum state. There is. In this case, a cooling unit is required to be attached to the Zeeman laser tube 8, the X-axis receiver 11 and the Y-axis receiver 13.

Further, in the above-described first and second embodiments, since the laser optical path also changes simultaneously with the movement of the stage 1 in the two-dimensional direction, the cylinder 41 expands and contracts in synchronization with the movement of the stage 1. I have to. As described above, in the present embodiment, the entire or part of the optical path of the laser light is covered with the airtight cylinder 41 filled with the atmospheric pressure helium gas, so that highly accurate measurement without being affected by the atmosphere can be performed. You can

Specifically, the environmental conditions are temperature: 2
3.0 ± 0.2 ° C., pressure: 1013 ± 5 mb, humidity: 5
0 ± 10%, breeze: ≤10 mm / sec, distance between interferometer and plane mirror: 300 mm, measuring time:
The experimental result of the fluctuation of the laser measurement value in the case of 10 hours is about 70 nm when nothing is done, about 20 nm when the tube covers the interferometer and the plane mirror, and the inside of the interferometer and the plane mirror is covered. Decompression (200T
When it is covered with a cylinder that has been turned off, a very good value of about 7 nm is obtained.

In the above embodiment, the case where the cylinder is filled with helium gas at normal pressure has been described as an example. However, the present invention is not limited to this, and if the influence of the helium gas is desired to be suppressed, the cylinder is evacuated. Most preferably, it is effective to reduce the pressure of the helium gas to be filled, or even simply reduce the pressure. In short, it suffices that the molecular density per unit volume inside the cylinder is set to be smaller than the molecular density per unit volume outside the cylinder, for example, if safety measures are taken,
The helium gas in the cylinder may be replaced with hydrogen gas.

[0035]

According to the present invention, the entire or part of the optical path of laser light is covered with an airtight tubular member, and the molecular density per unit volume inside the tubular member is measured per unit volume outside the tubular member. The inside of the cylindrical member is filled with helium gas at normal pressure or reduced pressure so as to be smaller than the molecular density of, or reduced in pressure or in a vacuum state to suppress the influence of the atmosphere in the optical path of the laser light. It is possible to measure with high accuracy.

[Brief description of drawings]

FIG. 1 is a perspective view showing a main configuration of a first embodiment.

FIG. 2 is an enlarged sectional view of a main part of FIG.

FIG. 3 is a perspective view showing a main configuration of a second embodiment.

FIG. 4 is a perspective view showing a configuration of a main part of a conventional example.

FIG. 5 is a diagram for explaining the operation of the dual frequency laser interferometer.

FIG. 6 is a diagram showing a plane mirror interferometer.

FIG. 7 is a diagram showing a linear interferometer.

FIG. 8 is a diagram showing a single beam interferometer.

FIG. 9 is a perspective view showing a main part configuration of another conventional example.

[Explanation of symbols]

1 stage 2 length measuring means 3 stage body 4 X-axis plane mirror (laser light reflecting means) 5 Y-axis plane mirror (laser light reflecting means) 6 X-axis direction guide 7 Y-axis direction guide 8 Zeeman laser tube (laser light) Irradiation means 9 Beam splitter 10 X-axis interferometer (laser light interference means) 11 X-axis receiver (laser light phase measurement means) 12 Y-axis interferometer (laser light interference means) 13 Y-axis receiver ( Laser light phase measuring means) 21 Dual wavelength Zeeman laser light source 22 Beam splitter 23 Photodetector 24 AC amplifier 25 Polarizing beam splitter 26 Retroreflector for position reference 27 Retroreflector for position measurement 28 Photodetector 29 AC amplifier 30 Pulse converter 31 de Rescopic spring 41 yen Cylinder (tubular member) 42 Cylinder body 43 Optical window 44 O-ring 45 Buffer tank 46 Piston 47 Valve

Claims (5)

[Claims] 1. A laser light irradiating means for irradiating a predetermined measuring object with laser light, and a laser light reflecting means provided on the measuring object for reflecting the laser light irradiated from the laser light irradiating means. It is positioned between the laser light irradiating means and the laser light reflecting means, divides the laser light into two or more laser lights, and after providing a predetermined optical path difference between the respective laser lights, they are put together. Laser light interfering means for interfering the laser light, and laser light phase measuring means for measuring the phase of the laser light by monitoring the light intensity of the interfering laser light obtained by the laser light interfering means in time series, An airtight tubular member that covers all or part of the optical path of the laser beam, and a molecular density per unit volume inside the tubular member is calculated from a molecular density per unit volume outside the tubular member. Be smaller The laser interferometer system according to claim. 2. The laser interferometer according to claim 1, wherein the tubular member expands and contracts according to the optical path length of the laser light. 3. The laser interferometer according to claim 1, wherein the laser light interfering means is a dual frequency Zeeman laser interferometer. 4. The laser interferometer according to claim 1, wherein the inside of the cylindrical member is filled with helium gas which is at normal pressure or reduced pressure. 5. The laser interferometer according to claim 1, wherein the inside of the cylindrical member is decompressed or evacuated.