JPH04102003A - Frequency modulation optical heterodyne interference measuring device - Google Patents

Abstract

PURPOSE:To enable frequency modulation in short period with small driving force, regardless of the weight of an article being measured by provising a lens driving device which vibrates an objective lens in the direction of optical axis as a means for frequency modulation of a measuring beam. CONSTITUTION:A reference beam reflected by a polarization beam splitter 28 is irradiated to a mirror 32 via a 1/4 wave length plate 30. The mirror 32 is vibrated in the direction of the reference beam optical axis by the expansion or shrinkage of a PZT piezoelectric modulation element 34. Change in optical path length caused by the vibration frequency-modulates the reference beam. A measuring beam, on the other hand, is condensed by an objective lens 46 then irradiated to the surface 50 of an article being measured. The article being measured 48 is moved on the plane in right angle to the measuring beam optical axis. The objective lens 46 is vibrated in the direction of the measuring beam optical axis, which frequency-modulates the measuring beam.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical heterodyne interference measurement device, and more particularly to a frequency modulated optical heterodyne interference measurement device that performs measurement by frequency modulating a measurement beam and a reference beam. be.

2. Description of the Related Art A type of device that measures surface topography using optical heterogain interference is a method that modulates the frequency of a measurement beam and a reference beam, which are emitted from the same light source and have different frequencies, at the same period. The measurement beam is focused by an objective lens on the surface of the object to be measured, which is relatively moved in a direction intersecting the optical axis, and the measurement beam reflected from the surface and the reference beam are caused to interfere and enter the optical sensor. By extracting the beat signal,
There is a frequency modulated optical heterogain interference measurement device that measures the uneven shape of the surface based on changes in the beat frequency of the beat signal. According to such a measuring device, it is difficult to be affected by amplitude noise due to changes in the light intensity of the light source, etc.
It has the advantage of providing high measurement accuracy. In addition, the measurement beam is usually frequency-modulated by vibrating the object to be measured in the optical axis direction of the objective lens, and the vibration of the objective lens causes fluctuations in the air and external vibrations. It will also be difficult to be affected by such factors.

Problems to be Solved by the Invention However, as mentioned above, vibrating the object to be measured requires a large driving force, and it is difficult to vibrate at a high frequency, in other words, to shorten the modulation period. There was a problem in that the change in the beat frequency of the beat signal due to the unevenness of the beat signal was small, and sufficient measurement sensitivity could not necessarily be obtained.

In addition, the vibration frequency of the object to be measured changes depending on the weight of the object to be measured, and as a result, the modulation period of the measurement beam and the modulation period of the reference beam deviate. Therefore, adjustment means or the like had to be provided to match the modulation periods.

The present invention was made against the background of the above circumstances, and its purpose is to enable frequency modulation of a measurement beam in a short period with a relatively small driving force without being affected by the weight of the object to be measured. It's about doing.

Means for Solving the Problems In order to achieve the above object, the present invention frequency-modulates a measurement beam and a reference beam, which are emitted from the same light source and have different frequencies from each other, at the same period, and modulates the light of the measurement beam. The measurement beam is focused by an objective lens on the surface of the object to be measured, which is relatively moved in a direction intersecting the axis, and the measurement beam reflected from the surface and the reference beam are caused to interfere and enter the optical sensor. In a frequency-modulated optical heterogain interference measurement device that extracts a beat signal by using a beat signal and measures the uneven shape of the surface based on a change in the beat frequency of the beat signal, the objective lens is used as a means for frequency-modulating the measurement beam. It is characterized by being provided with a lens drive device that vibrates in the axial direction.

Actions and effects of the invention, namely, when the objective lens is vibrated in the direction of its optical axis,
Although the optical path length on the optical axis does not change, the distance between the surface of the object to be measured on which the measurement beam is reflected and the beam waist of the measurement beam focused by the objective lens changes periodically as the objective lens vibrates. Changes to If the beam waist and the surface do not match, the measurement beam will be irradiated onto the surface of the measured object in the state of a spherical wave, and a phase shift will occur between the beam periphery and the beam center. The amount of deviation is the change in distance between the beam waist and the surface,
That is, it changes periodically as the objective lens vibrates.

Frequency modulation is applied to the measurement beam by this periodic change in the amount of phase shift.

When frequency modulating the measurement beam by vibrating the objective lens, the modulation period is not affected by the weight of the object to be measured, so the measurement beam and reference beam are always set at the same predetermined period. Frequency modulation can be performed, and there is no need to provide adjustment means or the like to match the modulation periods according to the weight of the object to be measured.

In addition, since the objective lens is lightweight compared to the object to be measured, it can be vibrated with a small driving force, and the lens driving device can be made small and compact.
By vibrating the objective lens at high speed and shortening the modulation period, it is possible to increase the change in the beat frequency of the beat signal due to surface irregularities and improve measurement sensitivity.

Furthermore, even when the measurement beam is frequency-modulated by vibrating the objective lens in this way, amplitude noise and air fluctuations occur due to changes in the light intensity of the light source.

It is not easily affected by external vibrations, etc., and high measurement accuracy can be obtained, similar to the conventional case in which frequency modulation is performed by vibrating the object to be measured.

EXAMPLE Hereinafter, an example of the present invention will be described in detail based on the drawings.

In FIG. 1, a laser light source 10 such as a He-Ne laser is shown.
The linearly polarized laser beam of frequency fo emitted from
After the returned light passes through an isolator 12 that prevents people from entering the laser light source 10, it is reflected by a mirror 14.
The light is made incident on the polarizing beam splitter 16. The attitude of the laser light source IO is set so that the plane of polarization of the laser beam (the plane of vibration of the electric vector) is inclined at an angle of 45° to the plane of the paper, and the plane of polarization of the laser beam is parallel to the plane of the paper. The P-polarized light component is passed through the polarizing beam splitter 16 as a measurement beam L, and the S-polarized light component whose plane of polarization is perpendicular to the plane of the drawing is reflected by the polarizing beam splitter 16 as a reference beam.

The measurement beam LM that has passed through the polarizing beam splitter 16 is subjected to a frequency shift of 10 f1 by the acousto-optic modulator 18 to have a frequency of fo + f, and is then made incident on the polarizing beam splitter 20.

Further, the reference beam L+t reflected by the polarization beam splinter 16 is further reflected by the mirror 22, and is subjected to a frequency shift of +f2 by the acousto-optic modulator 24, so that the frequency becomes f. After being set to +f2, the light is reflected by the mirror 26 and is made incident on the polarizing beam splitter 20. The above frequency shift amounts f1 and f! For example, 80MHz and 80. I
It is set to have a frequency difference of about several hundred kHz, such as MHz.

The measurement beam L and the reference beam Lm superimposed by the polarizing beam splitter 20 are then separated again by the polarizing beam splitter 28 depending on the direction of the plane of polarization, and the reference beam Le+ consisting of the S-polarized component is reflected by the polarizing beam splitter 28. Ru. The reference beam LR reflected by the polarizing beam splitter 28 passes through the 174-wave plate 3
0 to the mirror 32. Mirror 32 is PZ
It is attached to the end face of the T piezoelectric displacement element 34, and when the PZT piezoelectric displacement element 34 is expanded and contracted by the PZT modulation drive circuit 36, the reference beam L is
The reference beam LH is oscillated in the optical axis direction of R, and the reference beam LH is frequency-modulated by the change in optical path length accompanying this oscillation.

The vibration frequency of the PZT piezoelectric displacement element 34 is f.

When H2 and amplitude are d2, the optical path length change ΔZ2 of the reference beam L due to its vibration is expressed by the following equation (1), and the maximum frequency deviation Δf2 of frequency modulation due to the optical path length change is expressed by the following equation (2). It is expressed as Note that 2 in equation (2) is the wave number of the reference beam L.

ΔZ2 = d2・5in(2πfs t)...
11) Δf2=d2・f,・k2...(
2) The reference beams L and l, which have been reflected by the mirror 32 and frequency-modulated in this way, are transmitted through the 174-wave plate 30 again to become P-polarized light, and then transmitted through the polarizing beam splitter 28. After passing through the polarizing plate 38, the light is irradiated onto the light sensor 40.

On the other hand, the measurement beam L consisting of the P-polarized component is passed through the polarization beam splitter 28, and is passed through the 174-wave plate 4.
4, the light is focused by the objective lens 46, and is focused on the object to be measured 48.
is irradiated onto the surface 50 of. The object to be measured 48 is placed on a moving table 54 that can be moved in two dimensions within an XY plane perpendicular to the optical axis of the measurement beam L by a drive device 52 such as a motor. In addition, the objective lens 46
is attached to a Z-axis motor 56, and a motor modulation drive circuit 58 moves the measurement beam L in the Z-axis direction. The measurement beam L8 is caused to vibrate in the optical axis direction, thereby frequency modulating the measurement beam L8.

That is, when the objective lens 46 is vibrated in the direction of its optical axis, the distance between the beam waist of the measurement beam LM focused by the objective lens 46 and the surface 5° changes periodically as the objective lens 46 vibrates. If the beam waist and the surface 5o do not match, the measurement beam LM will be irradiated onto the surface 50 in the form of a spherical wave, which will cause a phase shift between the beam periphery and the beam center.
This amount of phase shift changes periodically as the distance between the beam waist and the surface 5o changes, that is, the vibration of the objective lens 46. Therefore, the periodic change in the amount of phase shift causes the measurement beam L to change. is subjected to frequency modulation.

For example, if the objective lens 46 is provided in a fixed position and the object to be measured 4 is
When measuring the uneven shape of the surface 50 by normal optical heterodyne interference while moving the lens 8 in the X-Y direction, the light is focused by the objective lens 46, as shown in FIG. 2(a). When the beam waist of the measurement beam LH to be measured is approximately aligned with the surface 50, there is almost no optical phase shift between the center and peripheral parts of the light flux of the measurement beam LM, as shown in Φ). If the surface 50 is located closer to the beam waist than the beam waist, the measurement beam toy will be irradiated onto the surface 50 in the form of a spherical wave.
A phase shift of light occurs between the center and the periphery of the light beam, and when this is averaged, a measurement error occurs. The one-dot chain line in FIG. 2 is an equivalent curve in which the phases of light are equal.

Analyzing this measurement error, we can find the spot diameter at the beam waist of measurement beam L4, that is, the diameter of the part where the light intensity of the measurement beam is 1/e'' (e: natural logarithm) at the center. 2w.

Then, the radial dimension W0 is calculated by the following formula (
3), and the radial dimension W(D) of the spot diameter and the radius of curvature R(D) of the wavefront at a position distanced from the beam waist are expressed by the following equations (4) and (5), respectively. Ru. Note that the convergence angle θ is determined based on the numerical aperture NA of the objective lens 46.

□. =λ/πθ...(3)w
(D) = wo(l + D”/Do”)””
...(4) R(D)=D+D,"/D
(5) However, D0=πw0'/λ Furthermore, if the error E between the average height of the wavefront at the above distance and D is calculated two-dimensionally, it is expressed by the following equation (6).

E= f (R-(R”-X”)””) d X/w
= (Rw w(R” w”)””/2(R”5i
n-'(w/R))/2]/w・・・・(
6) This error E corresponds to the measurement error caused by the phase shift. For example, using a sample in which a 30 nm convex portion is formed on the surface 50, multiple types of objective lenses 46 with different numerical apertures NA are used. While replacing the sample, after aligning the surface 50 with the beam waist, the sample is moved in a direction perpendicular to the optical axis and the above 30 nm convex portion is measured by normal optical heterodyne interference.The experimental value D'' is shown in Figure 3. The numerical aperture NA of the objective lens 46 is
Accordingly, the error E when D=30 nm was determined according to the above equation (6), and the theoretical value obtained by subtracting the error E from 30 nm was as indicated by "." in FIG. As is clear from FIG. 3, it can be seen that the error E calculated in (6) 2 has a good correlation with the error of the experimental value D1'. Note that the actual measurement beam L is a three-dimensional spherical wave, and strictly speaking, the error E must also be determined three-dimensionally, but if the error E is analyzed and determined in three dimensions, it will be larger than in the two-dimensional case. As a result, the difference between the experimental value and the experimental value increases.
A better correlation was obtained when the two-dimensional analysis was performed as described above. This is thought to be because the light intensity of the measurement beam L has a Gaussian distribution, and the light intensity is small in the peripheral portion where the phase shift is large, so that the influence of the phase shift is reduced.

Also, for the case where the numerical aperture NA is 0.95, D=30
．． When the error E at 60.90 nm is calculated according to (6) 2 above, it is 7.952 and 16.448.25, respectively.
．． 853, and it can be seen that the error E increases approximately in proportion to the distance. That is, if the unevenness of the surface 50 is the same, the error E will be the same regardless of the size of the deviation between the beam waist and the surface 50.

As is clear from the above explanation, an error E occurs depending on the deviation distance between the surface 50 and the beam waist.
This means that when L is moved by a distance in the optical axis direction, a phase change occurs in the measurement beam L by an amount corresponding to the error E. In other words, the same effect as moving the surface 50 by the error E without moving the objective lens 46 can be obtained. For example, when the objective lens 46 with a numerical aperture NA of 0.95 is moved by 30 nm, the object to be measured 48 is moved by about 8 nm.
The same effect as displacement can be obtained.

Therefore, the objective lens 4 is moved by the Z-axis motor 56.
6 is vibrated at the frequency fsHz and amplitude d1, the optical path length change ΔZ1 of the measurement beam LM due to the vibration is expressed by the following equation (7), and the maximum frequency deviation Δft of frequency modulation due to the optical path length change is as follows. It is expressed by equation (8). Here, d ma in the equation (7) corresponds to the error width when the distance is defined as the amplitude d, and is determined by the equation (6) above. Moreover, 1 in equation (8) is the wave number of the measurement beam LM. Note that the amplitude d is adjusted so that the maximum frequency deviation Δf1 matches the aforementioned Δf2.

and d2 are set in advance.

ΔZI=dna・5in(2x fs t)...
・(7) Δf, = d, ・f, ・k, ・・
(8) In this embodiment, a Z-axis motor 56 and a motor modulation drive circuit 58 that vibrate the objective lens 46 in the optical axis direction
A lens driving device is constructed by the following.

The measurement beam L, which is frequency-modulated by the vibration of the objective lens 46 and reflected by the surface 50, is polarized with respect to the outward path by passing through the objective lens 46 and the 174-wave plate 44 again. wave front is 90
The rotated linearly polarized light, that is, S polarized light, is reflected by the polarizing beam splitter 28, and is reflected by the polarizing plate 3.
8, the light beam is irradiated onto the halo sensor 40 which is caused to interfere with the reference beam L. From the optical sensor 40, measurement beams L are each frequency-modulated with the same period of 1/f. Due to the interference between the reference beams L and I, a beat signal BS that generates a beat with a frequency difference between them is output to the measuring means 60.

To explain specifically about the beat frequency f of the beat signal BS, for example, the frequency of the measurement beam L at each part between the position S1 of the surface 50 and the position S3 of the optical sensor 40 at time t0 is shown by the solid line in FIG. The position S of the mirror 32 at the same time t0 is as shown in
2 to the position S3 of the optical sensor 40 is as shown by the dashed line in FIG. 4, the frequency difference between the two at the position S3 is the beat at that time L0 The frequency becomes f.

As shown in FIG. 5, this beat frequency f changes at the same period as the modulation period 1/f of both beams LM and LR, and the maximum value f% of the beat frequency [,
Both beams L.

The optical path difference between L and l, that is, the distance Z and S from Sl to 33
If the difference Z + Z z from distance Z 2 to S 2 is constant, it will not change. As the optical path difference Z+
When Zt changes, in other words, when the measurement beam Lx shown in FIG.
f b ” also changes.

The measuring means 60 is constituted by a microcomputer or the like, and extracts the maximum value f% of the beat frequency of the beat signal BS at a period of 1/f, and calculates the maximum value f% of the beat frequency of the beat signal BS to a predetermined maximum value f%.
The amount of change in the optical path difference Z, Z2, that is, the unevenness dimension of the surface 50, is determined from the change in the maximum value f, 11 based on an arithmetic expression or data map, etc. expressing the relationship between to be displayed.

Here, in the frequency modulated optical heterodyne interference measurement device of this embodiment, the objective lens 46 is vibrated to frequency modulate the measurement beam L.
The modulation period is completely unaffected by the weight of the object to be measured 48, and the measurement beam L and the reference beam Lll can always be frequency modulated at a predetermined constant period of 1/f. There is no need for adjusting means or the like to match the modulation periods accordingly.

In addition, since the objective lens 46 is lightweight compared to the object to be measured 48, it can be vibrated with a small driving force, and a small and compact Z-axis motor 56 can be used, and the objective lens 46 can be vibrated at high speed. Let the modulation period 1/f
By shortening , it is possible to increase the change in the beat frequency f% of the beat signal BS due to the unevenness of the surface 50, and improve the measurement sensitivity. That is, modulation period 1
When /f is shortened, the slope of the frequency change of the measurement beam L and the reference beam L in FIG. 4 increases,
Optical path difference 2. Even if the change in -22 is small, the maximum value fb'' of the beat frequency will change greatly.

Furthermore, even when the measurement beam Lx is frequency-modulated by vibrating the objective lens 46 in this way, it is difficult to be affected by amplitude noise due to changes in the light intensity of the laser light source 10, air fluctuations, external vibrations, etc. The fact that high measurement accuracy can be obtained is the same as in the conventional case in which frequency modulation is performed by vibrating the object to be measured 48.

Although one embodiment of the present invention has been described above in detail based on the drawings, the present invention can also be implemented in other embodiments.

For example, in the embodiment described above, the measurement beam L is generated by the acousto-optic modulator 18. While shifting the frequency by +f1,
The acousto-optic modulator 24 shifts the frequency of the reference beams L and l by +f2, but as shown in FIG. Even if the frequency of only the reference beam Lm is shifted by +f2-f,
This is substantially the same as the previous embodiment. Note that, conversely, only the measurement beams L and 4 may be frequency-shifted, or a transverse Zeeman laser may be used to directly extract measurement beams and reference beams having different frequencies.

Further, a motor and a motor modulation drive circuit may be used instead of the PZT piezoelectric displacement element 34 and the PZT modulation drive circuit 36, or a PZT piezoelectric displacement element and a PZT modulation drive circuit may be used instead of the Z-axis motor 56 and the motor modulation drive circuit 58. It is also possible to employ other vibration drive devices, such as using

Further, the reference beam L may also be frequency modulated using the same principle as the measurement beam LH by arranging a condensing lens in front of the mirror 32 and vibrating it.

In addition, an optical system for detecting defocus is incorporated, and an autofocus mechanism is configured using the Z-axis motor 56 and the motor modulation drive circuit 58 as actuators, so that the distance between the objective lens 46 and the object to be measured 48 is detected at the beginning of measurement. The distance may also be adjusted.

Furthermore, in the embodiment described above, the unevenness dimension of the surface 50 is determined from the change in the maximum values f and ' of the beat frequency, but in order to maintain the maximum value fb1 constant, it is covered with a PZT piezoelectric displacement element or the like. The measurement object 48 may be moved in the Z-axis direction, and the amount of movement may be detected as the unevenness dimension of the surface 50.

Although other examples are not provided, the present invention can be implemented with various modifications and improvements based on the knowledge of those skilled in the art.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the basic configuration of a frequency modulated optical heterodyne interference measuring device which is an embodiment of the present invention. FIG. 2 is a diagram illustrating a phase shift when the measurement beam is irradiated in the form of a spherical wave onto the surface of the object to be measured. FIG. 3 is a diagram showing the correlation between the theoretical value and experimental value of the error caused by the measurement beam being irradiated onto the surface of the object to be measured in the state of a spherical wave. FIG. 4 is a diagram illustrating an example of the frequency distribution of the measurement beam and reference beam in the embodiment of FIG. 1. FIG. 5 is a diagram illustrating an example of the beat frequency of the beat signal in the embodiment of FIG. 1. FIG. 6 is a block diagram illustrating another embodiment of the present invention, and corresponds to FIG. 1. 10: Laser light source 4o: Optical sensor 46: Objective lens 48: Object to be measured 50: Surface area: Measurement beam BS: Beat signal: Reference beam

Claims (1)

[Scope of Claims] A measurement beam and a reference beam emitted from the same light source and having different frequencies are modulated in frequency at the same period, and the object to be measured is relatively moved in a direction intersecting the optical axis of the measurement beam. The measurement beam is focused on the surface of the object by an objective lens, and the measurement beam reflected from the surface and the reference beam are made to interfere and enter the optical sensor, thereby extracting a beat signal, and determining the beat frequency of the beat signal. In a frequency modulated optical heterodyne interference measurement device that measures the uneven shape of the surface based on changes, a lens driving device that vibrates the objective lens in the optical axis direction is provided as a means for frequency modulating the measurement beam. A frequency modulated optical heterodyne interference measurement device characterized by: