JPH0465601A - Optical heterodyne interference measuring instrument - Google Patents

Abstract

PURPOSE:To vary the measurement resolution and response speed by varying the frequency difference between measurement light and reference light according to the property of an object of measurement, the purpose of measurement, etc., so that the frequency difference can be varied by a frequency control means. CONSTITUTION:The measurement beam LM passed through a polarization beam splitter 16 is shifted in frequency by +f1 through an acoustooptic modulator 18 to have a frequency f0+f1 and then the beam is made incident on a polarization beam splitter 20. Further, the reference beam LR which is reflected by the polarization beam splitter 16 is further reflected by a mirror 22 and shifted in frequency by +f2 through the polarization beam splitter 16 to have a frequency f0+f2, and then the beam is reflected by a mirror 26 and made incident on the polarization beam splitter 20. The measurement beam LM and reference beam LR which are put one over the other by said polarization beam splitter 20 have their beats due to interference detected and a reference beat signal RBS of reference beat frequency fB represented as ¦f1-f2¦ is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical heterodyne interference measurement device, and particularly to a technique for making the measurement resolution and response speed variable.

Conventional technology and its challenges In addition to making the frequencies of the measurement light and reference light different from each other using optical frequency chic, the measurement light is irradiated onto the measurement target and then interfered with the reference light to enter the optical sensor, thereby generating the measurement beat signal. An optical heterodyne interference measurement device is known that measures physical quantities of the object to be measured, such as surface roughness, unevenness, moving speed, displacement amount, gas concentration, etc., based on the phase change of the measured beat signal. Conventionally, the measurement resolution and response speed of such measurement devices are fixed based on the frequency difference between the measurement light and the reference light, the time resolution of wavelength two-phase measurement, etc., and are changed depending on the characteristics of the measurement target, the measurement purpose, etc. I couldn't do that. For this reason, there are disadvantages such as measurement being performed with a resolution higher than necessary, resulting in a long measurement time, or measurement time being short but sufficient measurement resolution cannot be obtained.

The present invention was made against the background of the above circumstances.
The purpose is to be able to change measurement resolution and response speed.

Means for Solving the Problems In order to achieve the object, the present invention controls the optical frequency thick so as to change the frequency difference between the measurement light and the reference light in the optical heterodyne interference measurement device described above. It is characterized in that a frequency control means is provided.

Operation and Effects of the Invention In such an optical heterodyne interference measurement device, the frequency difference between the measurement light and the reference light can be changed by the frequency control means, so the frequency difference can be changed depending on the characteristics of the measurement object, the measurement purpose, etc. By changing it, measurement resolution and response speed can be changed. In other words, the above frequency difference corresponds to the reference beat frequency of the measurement beat signal (the frequency when there is no phase change in the measurement light), and while the measurement resolution decreases in proportion to the reference beat frequency, the response speed corresponds to the reference beat frequency. The speed increases in proportion to the beat frequency. Therefore, if other conditions such as the wavelength of the measurement light are the same, increasing the frequency difference will decrease the measurement resolution but increase the response speed, and decreasing the frequency difference will decrease the response speed but increase the measurement resolution. becomes higher.

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

The first part is an optical configuration diagram illustrating an example in which the present invention is applied to a fine shape measuring device using optical heterodyne interference. . The linearly polarized laser light passes through an isolator 12 that prevents the return light from entering the laser light source 10, is reflected upward by a mirror 14, and is made incident on a polarizing beam splitter I6. The attitude of the laser light source 10 is set so that the plane of polarization of the laser beam (the plane of vibration of the electric hector) is inclined at an angle of 45° with respect 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 reference beams L and I. The above measurement beam L, 4
corresponds to the measurement light, and the reference beam L corresponds to the reference destination.

The measurement beam toy that has passed through the polarization beam splitter 16 undergoes a frequency shift of +[l by the acousto-optic modulator 18 to have a frequency of fo+f, and is then made incident on the polarization beam splitter 20.

The reference beam reflected by the polarizing beam splitter 16 or further reflected by the mirror 22 is subjected to a frequency shift of 10 f by the acousto-optic modulator 24 to have a frequency of f0+f, and then reflected by the mirror 26. The polarized beam is incident on the polarized beam splinter 20. The acousto-optic modulator 18.24 corresponds to a frequency shifter, and the control device 28
(See Figure 2) Frequency control signal SFI output from
By controlling the frequency control circuit 30 according to SF2, the frequency shifts f,, f! provided by those acousto-optic modulators 18,24! are changed respectively.

The frequency control circuit 30 is configured, for example, as shown in the third factor. The frequency synthesizers 32 and 34 output frequency multiplied signals based on the reference frequency signal supplied from the reference oscillator 36, and the frequency ff2 of the frequency multiplied signals is equal to the frequency control signal SF.
I, SF2, and this is the RF amplifier 384
0 through 7 to the acousto-optic modulator 18.24 so that the measurement beams L, 4. The reference beams L7 are frequency shifted by the frequencies f+ and fz, respectively.

The measurement beams L, 4 and the reference beam Lll superimposed by the polarizing beam splitter 20 are then split into two by a non-polarizing beam splitter 42, and the beams reflected by the non-polarizing beam splitter 42 are converted into interference light by a polarizing plate 44. After that, the light is irradiated onto the reference optical sensor 48 by the condensing lens 46. In this reference optical sensor 48, the measurement beam L. A beat due to interference between the reference beam LR and the reference beam LR is detected, and the difference in their frequencies, i.e., l (fO+j, I ) − (fo + ft )
Reference beat frequency f expressed as l = r, -r21
A reference beat signal RBS of B is output.

The measurement beam L and the reference beam Lx transmitted through the non-polarizing beam splitter 42 are transmitted to the polarizing beam splitter 5.
0, the reference beam LR consisting of the S polarization component is separated again by the direction of the polarization plane and is sent to the polarization beam splitter 50.
reflected by.

Reference beam Ll reflected by polarizing beam splitter 50
l is irradiated onto the mirror 54 via the 1/4 wavelength plate 52, reflected by the mirror 54, and then applied to the 174 wavelength plate 5 again.
2 is transmitted, and becomes P-polarized light. The P-polarized reference beam LR is transmitted through the polarizing beam splitter 50, passes through the polarizing plate 56, and then is irradiated onto the measuring optical sensor 60 by the condenser lens 58.

On the other hand, the measurement beam L1 consisting of the P-polarized component is passed through the polarizing beam splitter 50, passes through the non-polarizing beam splitter 62 and the quarter-wave plate 64, and is focused onto the surface 70 of the object to be measured 68 by the objective lens 66. be done. The object to be measured 68 is placed on a moving table 74 that can be moved in two dimensions in an X-Y plane perpendicular to the optical axis of the measurement beam LMO by a drive device 72 such as a motor. As the measurement beam LH moves, the surface 7
Doppler shift f corresponding to minute irregularities of 0.

receive. Therefore, the frequency of the measurement beam Lx reflected by the surface 70 of the object to be measured 68 is f0f++f. This Doppler shift f is the measurement beam L per unit time due to the change in optical path length due to the unevenness of the surface 70.
This corresponds to the amount of phase change in A.

The surface 70 of the object to be measured 68 is the object of measurement in this embodiment, and its unevenness is the physical quantity to be measured.

Further, the objective lens 66 can be moved in the optical axis direction by a Z-axis motor 76. This Z-axis top-ta 7
6 is configured such that the drive control device 78 is controlled in accordance with the focus position control signal DD output from the control device 28 so that the focal point of the measurement beam LM coincides with the surface 70, in other words, Measurement beam L. The objective lens 66 is moved so that the irradiation point coincides with the focal position of the objective lens 66.

The measurement beam L, reflected by the surface 70, is reflected by the objective lens 6.
6 and again passes through the 1/4 wavelength plate 64 to become linearly polarized light, that is, S-polarized light, with the plane of polarization rotated by 90 degrees with respect to the outward path, and a portion of the light is reflected by the non-polarizing beam splinter 62.

Measurement beam L reflected by non-polarizing beam splitter 62
. is irradiated onto a four-split optical sensor 84 via a condensing lens 80 and a cylindrical lens 82. The cylindrical lens 82 provides astigmatism to the measurement beam, and the 4-split optical sensor 8
4 is arranged at a position where the cross-sectional shape of the measurement beam LM becomes circular when the irradiation point of the measurement beam LH on the surface 7o coincides with the focal position of the objective lens 66, and the reference numeral 4 outputs a defocus signal SA corresponding to the defocus. Output to the control device 28.

Further, the remaining measurement beam LM that has passed through the non-polarizing beam splitter 62 is reflected by the polarizing beam splitter 50, and after being caused to interfere with the reference beam Lll by the polarizing plate 56, it is transmitted to the measuring optical sensor 60 by the condensing lens 58.
is made incident on the In this measurement optical sensor 60, the measurement beam L reflected by the reference beam and the surface 70 is
A beat due to interference with s is detected, and the difference in their frequencies, i.e., l (fo + f, + + fo )
A measurement beat signal MBS having a measurement beat frequency f expressed as (La+ft)=lfa+fol is output.

The measurement beat signal MBS and the reference beat signal RBS are each shaped into a rectangular pulse waveform by a waveform shaping circuit (not shown) and then supplied to the deviation counter 88 of the measurement circuit 86 shown in FIG. Ru. The deviation counter 88 calculates the number of beats C1 of the measurement beat signal MBS and the reference beat signal RBS according to a timing signal supplied from a timing signal generation circuit (not shown).
Count the deviation E (-CM Cm) from the beat number 08,
The deviation E is temporarily stored in the latch device 90 and then taken into the control device 28.

Also, the reference beat signal RB shaped into the above pulse waveform
S and measurement beat signal MBS are supplied to an AND circuit 94 together with a reference pulse signal KS of a constant clock frequency fc output from a crystal oscillator 92. The reference beat signal RBS is supplied to an AND circuit 94 via a NOT circuit 96, and a counter 98 counts the number of pulses CI of the reference pulse signal KS that has passed through the AND circuit 94. This pulse number 01 is the reference beat signal R
This corresponds to a phase difference of one phase 2π or less between BS and the measurement beat signal MBS, and is temporarily stored in the latch device 100 and then taken into the control device 28.

The control device 28 is configured to include a microcomputer, and performs signal processing according to a program stored in advance in the ROM while utilizing the temporary storage function of the RAM. In addition, as shown in FIG.
A switching signal SS is supplied from 2.

This changeover switch 102 is operated by the measurer according to the required measurement resolution, and a desired one is selected from a plurality of resolutions.

Further, the control device 28 is functionally equipped with three blocks as shown in FIG. The frequency control block 104 is for changing the measurement resolution according to the switching signal SS, and is for changing the measurement resolution according to the switching signal SS.
．． 24 to the difference between frequencies f1 and f2, in other words, the reference beat frequency f of the reference beat signal RBS.

The frequency control signals SFI and SF2 are controlled in relation to each other so that the frequency control signals SFI and SF2 are set to a value corresponding to the resolution represented by the switching signal SS from among a plurality of predetermined values. Specifically, if high-resolution measurement is selected, the frequency difference Δ
The frequency control signal SFI is set so that f becomes small and the frequency difference Δf becomes large when low resolution measurement is selected.
and SF2 are output. This frequency control block 104 constitutes frequency control means together with the frequency control circuit 30.

The measurement block 106 measures the deviation E and the number of pulses C1.
The unevenness dimension of the surface 70 is calculated based on the following. That is,
If the unevenness dimension of the surface 70 is ΔZ, the Toppler shift f is expressed by the following formula (]) from Doppler's formula, while the Doppler shift f0 is expressed as (rM rs). Based on this, the displacement amount Z1 is calculated according to the following equation (2). Equation (2) calculates the displacement Z1 based on the change in the beat frequency f of the measured beat signal MBS, that is, the Tonpraschiff fo, and the change in the beat frequency f, 4 is 1 per unit time. It corresponds to the phase change of the measurement beat signal MBS whose phase is on the order of 2π, and the displacement Z is λ
It is found on the order of /2. Further, the displacement amount Z2 of the portion smaller than λ/2 is calculated according to the following equation (3) based on the above pulse number 01 from the optical heterodyne interference calculation equation.

Note that V2 in equation (1) is the surface 7 due to the movement of the moving table 74.
0 is the displacement speed in the vertical direction of the unevenness, that is, in the optical axis direction of the measurement beam toy, and λ in equations (1) to (3) is the wavelength of the measurement beam L, respectively.

λ - E ・ ・ ・ (2) Then, by adding the above-mentioned displacement 1z- and 22, the displacement ΔZ of the surface 70 of the measured Th 68 due to the movement of the moving table 74 is calculated, and this is the predetermined amount. By repeating this at regular intervals, the uneven shape of the surface 70 is measured and displayed on the display 108 (see FIG. 2) using the display signal SD.

Here, the reference beat frequency r, that is, the frequency difference Δf between the frequency shifts fI and f2 is set based on the frequency control signals SFI and SF2 output from the frequency control block 104. For example, if the frequency shift r by the acousto-optic modulator 18 controlled based on the frequency control signal SFI is set to 80 MHz, the frequency control signal SF2
If the frequency shift f2 by the acousto-optic modulator 24 controlled based on is 80.1 MHz, the frequency difference Δ
f("fl) is 100 kHz, and 100 kHz is set as f in equation (3).

Further, the clock frequency fe, that is, the measurement frequency when measuring the phase change is 100 MHz, and the wavelength λ is 632.
8 nm, the measurement resolution of the displacement ΔZ of the surface 70 is 0.3164 nm from the above equation (3), and the response speed C is 31.64 mm/sec from the following equation (4).

On the other hand, when low resolution measurement is selected by the changeover switch 102, the frequency shift f by the acousto-optic modulator 18 controlled based on the frequency control signal SF1 is 8
When the frequency shift f2 by the acousto-optic modulator 24 controlled based on the frequency control signal SF2 is changed to 80.5 MHz while keeping the frequency as 0 MHz, the frequency difference Δt (=r
*) is 50OkHz. Therefore, the clock frequency f.

and wavelength λ or 100MHz63 respectively as above
2.8 nm, the measurement resolution of the displacement Δ2 of the surface 70 is 1.582 nm, and the response speed C is 158.
2 mm/sec. That is, in the case of low-resolution measurement, although the measurement resolution decreases, the response speed C increases, so it is suitable even when the surface 70 has steep irregularities or when the movable table 74 is moved at high speed by the drive device 72. be able to take measurements.

Note that the reference beat frequency fi in equation (3) above may be set based on the switching signal SS instead of the frequency control signals SFI and SF2, or may be set based on the reference beat signal RBS.
There is no problem with incorporating and calculating U7.

Returning to FIG. 5, the focus position control block 110 outputs the focus position control signal DD so that the focus shift signal SA becomes zero, and performs feedback control on the Z-axis sensor 76. The focal position of the objective lens 66 and the surface 70 of the object to be measured 68 are always aligned, and the measurement beam L
, will be irradiated to one point on the surface 70 regardless of the unevenness of the surface 70.

In this way, in the fine shape measuring device of this example,
By switching the changeover switch 102, the frequency difference Δ of the frequency shift by the acousto-optic modulator 18.24 is changed.
Since f is changed and the measurement resolution and response speed C are changed accordingly, the desired measurement resolution and response speed C are selected according to the uneven shape of the surface 70 of the object to be measured 68, the purpose of measurement, etc. It is possible.

Furthermore, in this embodiment, since the focal position of the objective lens 66 is controlled to always coincide with the surface 70, the measurement beam LM is prevented from defocusing regardless of the unevenness of the surface 70, and the surface 70 is in the state of a spherical wave. This has the advantage of preventing measurement errors caused by irradiation.

Next, another embodiment of the present invention will be described. In the following embodiments, parts that are substantially the same as those in the first embodiment are given the same reference numerals and detailed explanations will be omitted.

In the 6th minute shape measuring device, the laser light source 10 is arranged to emit P-polarized laser light whose polarization plane is parallel to the plane of the paper, and the laser light is sent to the measurement beam toy by the non-polarizing beam splitter 112. reference beam L,
It is divided into The measurement beam L is further split into two by a non-polarizing beam splitter 114, and the beam reflected by the non-polarizing beam splitter 114 is incident on the reference optical sensor 48 via the non-polarizing beam splitter 116 and the condensing lens 46. It will be done. Furthermore, the measurement beam L that has passed through the non-polarizing beam splitter 114 is transmitted through the polarizing beam splitter 118, passes through the non-polarizing beam splitter 62, etc., and is irradiated onto the surface 70, and the measurement beam L is reflected from the surface 70. The LM is split by the non-polarizing beam splitter 62 and a part is made incident on the 4-split optical sensor 84, and the rest is sent to the polarizing beam splitter 1.
It is reflected by 18 and 120 and is made incident on the measuring optical sensor 60.

On the other hand, the reference beam LR is connected to the acousto-optic modulators 18 and 2.
After receiving a frequency shift of 10[, and -f2 by 4, it is split into two by a non-polarizing beam splitter 122, and a part is combined with the measurement beam LM by a non-polarizing beam splitter 116 to be sent to a reference optical sensor 48. is made incident on. The remaining portion is reflected by the mirror 124, and then superimposed by the polarizing beam splitter 120 on the measurement beam L reflected by the surface 70, and is caused to interfere with the measurement beam LM by the polarizing plate 56, and is sent to the measurement optical sensor 60. is made incident on.

Also in such a measuring device, the frequency f of the measurement beam Lx is changed by changing the value of the frequency shift (f−f2) of the reference beam L by the frequency control circuit 30. and the frequency f0+fl−f2 of the reference beam L3, the frequency difference Δf
(=f,=f.

f2) is changed, and the same effects as in the previous embodiment can be obtained.

Further, in the embodiment of FIG. 7, the measurement beam L1. Z-axis moving table 1 that is moved in the Z-axis direction, which is the optical axis direction of
This is a device that measures the amount of displacement and movement speed of 26.
Compared to the example, the measurement beam L transmitted through the polarizing plate 64
H is directly irradiated onto a mirror 128 provided on a Z-axis moving table 126 at right angles to the Z-axis. In this case as well, although the object to be measured is different, the measurement principle is the same as in the first embodiment, and the same effects can be obtained.

Although several embodiments of the present invention have 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, focusing is performed by moving the objective lens 66 in the Z-axis direction, but it is also possible to perform focusing by moving the moving stage 74 in the Z-axis direction. . Further, defocus may be detected by a method other than the astigmatism method. However, such focusing is not necessarily necessary in carrying out the present invention.

Further, in the embodiment described above, the displacement amount ΔZ of the surface 70 is calculated from the phase change amount of the measurement beat signal MBS, but the moving table 74 is moved to Z so that the phase change amount becomes zero.
It may be moved in the axial direction and the amount of movement may be determined as the unevenness dimension of the surface 70.

Further, in the embodiment described above, the reference beat signal RBS is output from the reference optical sensor 48, but the reference beat signal RBS can also be generated from the output signals of the frequency synthesizers 32, 34, etc.

Further, in the above embodiment, the frequency shifts of the acousto-optic modulators 18.24 can be changed together by the frequency control circuit 30, but the pair of acousto-optic modulators 18.24
It is only necessary that the frequency shift of either one of them can be changed.

Further, in the embodiment described above, a pair of acousto-optic modulators 18 and 24 are used as optical frequency shifters, but other optical elements having a frequency conversion function may also be employed.

Further, in the above embodiment, only the measurement beam L+4 is connected to the surface 70.
However, the reference beam LR is irradiated to a relatively wide range of the surface 70, that is, a range that is not affected by Doppler shift due to the minute unevenness of the surface 70, It can also be configured to measure the uneven shape of the surface 70 by optical heterodyne interference with the measurement beam L7, which is affected by Doppler shift due to the uneven shape.

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 optical configuration of a fine shape measuring device using optical heterodyne interference, which is an embodiment of the present invention. The second figure is a block diagram showing a measurement control circuit included in the apparatus of FIG. 1. The third panel is a block diagram illustrating the frequency control circuit of the device shown in FIG. 1. FIG. 4 is a block diagram illustrating the measurement circuit of FIG. 2. FIG. 5 is a functional block diagram of the control device shown in FIG. 2. FIG. 6 is a diagram illustrating the optical configuration of another embodiment of the present invention. FIG. 7 is a diagram illustrating the optical configuration of yet another embodiment of the present invention. 18° 24: Acousto-optic modulator (light frequency shifter) 60: Measurement optical sensor 70: Surface Lo: Measurement beam (measurement light) LR: Reference beam (reference light) MBS: Measurement beat signal (measurement target)

Claims (1)

[Scope of Claims] A measurement beat signal is generated by making the frequencies of measurement light and reference light different from each other using an optical frequency shifter, and by irradiating the measurement light onto a measurement object, making it interfere with the reference light, and making it enter the optical sensor. In an optical heterodyne interference measurement device that extracts a signal and measures a physical quantity of the measurement target based on a phase change of the measurement beat signal, the optical frequency shifter is controlled to change the frequency difference between the measurement light and the reference light. An optical heterodyne interference measurement device characterized by being provided with frequency control means.