JPH0676882B2 - Relative displacement measurement method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a relative displacement measuring method, and more particularly, it accurately measures the relative displacement of a plurality of objects by using the phenomenon of diffraction and interference of waves by a diffraction grating formed on the objects. It is about the method.

[0002]

2. Description of the Related Art A conventional relative displacement measuring method shown in FIGS. 1 and 2 is a method used for measuring a distance between a mask and a wafer in X-ray lithography. In the method of FIG. 1, the electromagnetic wave I is made incident on the two objects 1 and 2 arranged in parallel, and the intensity of the reflected wave is measured by the detector 4. In the method of FIG. 2, the diffraction grating G is provided on one of the two objects 1 and 2 and the intensity of the diffracted wave is measured by the detector 4, thereby eliminating the need for a semi-transparent mirror.

In these conventional methods, s is measured by utilizing that the intensity of a reflected wave or a diffracted wave periodically changes with the distance s between two objects.

[0004]

However, the detection intensity of the reflected wave or the diffracted wave is influenced by many factors, and is virtually unpredictable. Therefore, in order to measure the distance s by the conventional method as described above, it is necessary to actually displace the object, observe the intensity change, and determine the value of s at which the intensity becomes maximum or minimum in advance. It was Another drawback is that the intensity measurement is liable to be affected by slight changes in the characteristics of the light source, the optical system in the middle, and the detector, so that the accuracy that can be achieved is limited.

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks and to provide a method for measuring relative displacement with high accuracy.

[0006]

In order to achieve such an object, the present invention provides a diffraction grating only on one of two objects arranged in parallel with each other and directs the diffraction grating from one side toward the diffraction grating. An electromagnetic wave composed of a plurality of waves having mutually different frequencies and different polarization states is made incident, and the diffracted wave obtained by diffracting by the diffraction grating and the diffraction grating after being diffracted by the diffraction grating A diffracted wave reflected by the surface of an object that does not have, a diffracted wave diffracted by the diffraction grating after being reflected by the surface of an object that does not have a diffraction grating in the object, and diffracted by the diffraction grating After that, a composite wave including at least two diffracted waves among the diffracted waves reflected by the surface of the object having no diffraction grating in the object and further diffracted by the diffraction grating is taken out, From the beat phase of intensity, the two And measuring the distance between the body.

Further, as one form of the present invention, the plurality of waves having different polarization states are electromagnetic waves linearly polarized in different directions.

By adopting the configuration of the present invention, highly accurate heterodyne measurement can be realized with an extremely simple configuration without using any optical component other than the diffraction grating.

Here, the diffracted wave is not only the wave diffracted by one diffraction grating, but (1) it is diffracted by the diffraction grating provided on the first object and further reflected by the second object. Wave, (2) wave diffracted by the diffraction grating provided on the first object, and further diffracted by the diffraction grating provided on the second object, (3) reflected by the second object, and further Including a wave diffracted by a diffraction grating provided on one object, when the wave is incident on a plurality of objects provided with at least one diffraction grating, diffraction by the diffraction grating is at least 1
It is also used as a generic term for waves that are emitted in a specific direction after rotation.

[0010]

The present invention is based on the principle described below. Waves diffracted by the diffraction grating on the first object and reflected by the second object or diffracted by the diffraction grating provided on the object and on the first object after being reflected by the second object The phase of the wave diffracted by the diffraction grating of 1 changes according to the relative position of two objects. Therefore, the relative displacement between the two objects can be measured by measuring the phase of the diffracted wave.

The conventional method is based on the fact that the intensity of the diffracted wave changes due to the relative displacement of the two objects. However, the intensity of the diffracted wave is easily affected by various factors other than the relative displacement to be measured. For example, there are factors such as the overlapping state of the two diffraction gratings, the angle of view between the two diffraction gratings, the angle of view of the diffraction grating from the detector, and the intensity of the incident wave. On the other hand, the phase of the diffracted wave is basically an amount determined by the passage distance of the wave, and it is difficult to suffer the influence of the above-mentioned intensity fluctuation factor. Therefore, by measuring the phase, it is possible to perform the relative displacement measurement which is inherently highly accurate and stable against an external disturbance. Further, for the same reason as above, the measurement condition is relaxed and the measurable range can be expanded. Furthermore, if the frequency is several tens of GHz or less, it is easy to measure the wave phase with an accuracy of 1 ° or more, and for this reason, higher accuracy can be achieved than with intensity measurement.

If the frequency of the wave used for measurement exceeds several tens GHz, it becomes difficult to measure the phase with high accuracy. In this case, a certain wave is used as the wave used for measurement, and a wave having a frequency slightly different from that of the wave and coherent with each other is prepared, and the waves are caused to interfere with each other, and the phase of the beat is measured. Good. The heterodyne measurement using the beat of the two-frequency wave is particularly effective for the relative displacement measurement using the diffraction grating. This is because the beat signal can be obtained by devising the method of injecting the two waves into the diffraction grating without the need for other components. With such a configuration, the phase of the beat signal hardly depends on the distance between the diffraction grating and the detector. As a result, the adjustment of the measurement system is significantly facilitated and the stability against external disturbance is also improved.

When two waves having different frequencies used for measurement are electromagnetic waves having different polarization states, the fact that the diffraction efficiency by the diffraction grating and the reflectance at the object are different depending on the polarization state The incidence method can be significantly simplified.
In this case, a beat signal necessary for measurement can be obtained by simply entering two electromagnetic waves as the same light flux without separating them. However, at this time, a larger beat signal can be obtained by limiting the polarization state of the incident wave to the detector using an appropriate polarizer. In this method, the phase of the beat signal is largely independent of the distance between the source and the grating and the distance between the grating and the detector, since the two waves forming the heterodyne measurement travel in exactly the same path. Therefore, the adjustment of the measurement system becomes much easier, the stability against external disturbance is dramatically improved, and the reliability is increased.

[0014]

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

FIG. 3 shows an embodiment in which the method of the present invention is applied to the measurement of relative displacement in the direction perpendicular to the diffraction grating surface. Figure 3
As shown in FIG. 3, a diffraction grating G is formed on one of two objects arranged in parallel with each other, and an electromagnetic wave I is vertically incident on the diffraction grating G. At this time, the diffracted wave D is a combination of a wave diffracted by the upper surface of the diffraction grating G, a wave diffracted by the diffraction grating G and then reflected by the object 2 and a wave reflected by the object 2 and then diffracted by the diffraction grating G. Become a wave.

In the conventional method, the intensity of D is measured to measure the object 1
-2 The distance S was measured. Therefore, in order to specify the value of S, it is necessary to change S to change the intensity of D. In addition, the measurement accuracy is λ / It was limited to 20 (λ is the wavelength of I).

It is easy to improve the accuracy by applying the method of the present invention to the example of FIG. An electromagnetic wave having two components I ₁ and I ₂ having different frequencies and polarization states may be used as the incident wave I, and the phase of the beat component of the diffracted wave D may be measured. Such electromagnetic waves can be easily obtained by using a Zeeman laser, for example. Here, a stronger beat can be obtained by inserting the polarizer 21 in an appropriate direction in front of the detector 4. Here, it is preferable that the plurality of waves having different polarization states used as incident waves are electromagnetic waves linearly polarized in different directions.
The change in the beat phase due to the change in the distance s between the objects becomes more remarkable. The reason is that the present invention utilizes the fact that the diffraction efficiency of the diffraction grating and the reflectance on the object surface differ depending on the polarization state. Generally, however, such a change becomes the largest when the polarization direction of linearly polarized light is changed. Is. Such coherent electromagnetic waves having different frequencies which are linearly polarized in different directions can be easily obtained by, for example, a transverse Zeeman laser.

The reason why S can be measured by such a measurement is as follows.
Due to reasons. D is a composite wave of diffracted waves having different paths. This
The polarization state dependence of the amplitude diffraction efficiency for each diffraction component of
different. The electromagnetic waves that traveled back and forth between objects 1-2 responded to S
There is a phase delay. This phase delay amount is also calculated for each diffraction component.
It is different for each. Therefore, I that composes D₁ And I ₂ 
The S dependence of the phase delay of the origin component is different,
The beat phase changes according to S.

When the beat phase is measured with an accuracy of about 1 ° by the method of the present invention, the measurement accuracy of S is easily λ / 100.
The above can be done.

[0020]

As described in detail above, the present invention can be applied to various relative displacement measuring methods using the diffraction effect of waves by a diffraction grating, and its characteristics can be remarkably improved. Further, in the above embodiments, only an example of relative displacement measurement between two objects is described, but it is easy to extend the method of the present invention to measure relative displacement of three or more objects. Therefore, the present invention can be widely applied in the industrial field that requires highly accurate relative displacement measurement, and particularly, the relative displacement measurement of the exposure mask and the semiconductor wafer in the lithography process which is frequently used in the electronic device manufacturing industry. It is extremely effective when applied to.

[Brief description of drawings]

FIG. 1 is an arrangement configuration diagram showing a configuration of a conventional example.

FIG. 2 is a layout configuration diagram showing another configuration of a conventional example.

FIG. 3 is a layout configuration diagram showing a configuration of an embodiment of the present invention.

[Explanation of symbols]

1, 2 Object 4 Detector 21 Polarizer G Diffraction grating D Diffraction wave I, I ₁ , I ₂ Incident wave R (i) Wave D (i), D (i, j ) Object 2 receives the i-th order diffraction by the diffraction grating
Waves diffracted by the diffraction grating and then diffracted again in the j-th order θ Diffraction angle s Distance

Claims (2)

[Claims] 1. A diffraction grating is provided only on one of two objects arranged in parallel with each other, and an electromagnetic wave composed of a plurality of waves having different frequencies and different polarization states from one side toward the diffraction grating is interfered with each other. A diffracted wave that is incident and diffracted by the diffraction grating; a diffracted wave that is diffracted by the diffraction grating and then reflected by a surface of an object that does not have a diffraction grating in the object; And a diffracted wave diffracted by the diffraction grating after being reflected by the surface of an object that does not have a diffraction grating, and a diffraction wave diffracted by the diffraction grating and then reflected by the surface of an object that does not have a diffraction grating And extracting a combined wave including at least two diffracted waves from the diffracted waves diffracted by the diffraction grating, and measuring the distance between the two objects from the phase of the beat of the intensity of the combined wave. Displacement measurement method characterized by . 2. The plurality of waves having different polarization states are
The relative displacement measuring method according to claim 1, wherein the electromagnetic waves are linearly polarized in different directions.