JPS60100007A - Measuring device of surface by optical heterodyne interference method - Google Patents

Abstract

PURPOSE:To enable measurement of high accuracy, by generating two light beams different in frequency by using an acoustooptical element, and by detecting the mutual phases of interfered beat signals while detecting the direction of the reflection of reflected lights from the surface of an object to be measured. CONSTITUTION:Two light beams different in frequency, which are emitted from an acoustooptical element (A.O)103, are grouped in one unit. The light beams in this group are split in two directions by a polarized beam splitter 102, and light beams 104' and 105' on one side turn to be reference lights which are not applied to the surface 107 of an object, while light beams on the other side are applied onto the surface 107 of the object. Lights reflected therefrom are turned to be reflected lights 120 and 121 by the splitter 102. Photoelectric converters 122 and 123 are provided for making the reference lights 104' and 105' interfere with the object-reflected lights 120 and 121. The reference lights 104' and 105' contain on informations on the surface of the object to be measured, while the object- reflected lights 120 and 121 contain informations on the surface, the angle of inclination, etc. of the object at the point to which light beams are applied. This constitution enables accurate measurement in a wide scope of measurement.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface measuring device using optical heterodyne interferometry using a laser. In the conventional measurement of surface conditions using optical interference, two
Due to the interference of two light waves. This is the so-called homodyne optical interference.
The measured quantity is on the order of 0.1 micrometer.

In recent years, as the accuracy of machining has improved, the demand for surface roughness measurement on the order of -0.01 micrometers has increased, and the development of new high-precision measuring instruments is awaited. Optical heterodyne interference is a method that can measure surface conditions more precisely than homodyne interference by two orders of magnitude. Optical heterodyne interference is 2
This is a method of interfering light with two different frequency components and photoelectrically converting the intensity to obtain a beat signal of the difference frequency.

For example, if the light wave with frequency f,, f2 is E,, E2, then E+ (tl=A,, (tlcos(2πf, t+daI(t))E,, (tl=A2(tlcos(2πf)
2 +6(t)) where -A, , A2 is the amplitude, Da1
．． , indicates the phase.

When these two light waves interfere - their intensity I (tl is I (tl= l E, (tl+ E2(tl l
2, and when this is converted into the amount of current (1) using a photodetector, i(t>cxA, ' +A2 ' +2A4 A,
, cos (2πΔft+Δg)where Δf2f,
-f2- ΔDag1-g.

An electrical signal is obtained.

Here, Δf is on the order of 105 to 106 Hz, which can be sufficiently electrically detected, and by detecting changes in the frequency and phase of this beat signal, information in the optical frequency domain that the original light wave has can be obtained. It can be extracted with high precision.

In the surface measuring device using optical heterodyne interferometry according to the present invention, two light beams having different frequencies are generated using an acousto-optic element (hereinafter abbreviated as A.0), and mutual phase detection of the interfered beat signals is performed. It also detects the reflection direction of the reflected light from the object surface of the object to be measured, and achieves measurement over a wider measurement range with higher precision than the conventional optical heterodyne interferometry. The present invention provides a surface measuring device suitable for measuring.

FIG. 1 shows a block diagram of a surface measuring device that is an embodiment of the present invention.

A single light beam 101 having a frequency fo emitted from a laser oscillation unit 100 such as a He-Ne laser tube or a semiconductor laser is incident on an acousto-optic element (Aφ0) 103. A-0106 generates an ultrasonic traveling wave internally by the A-0 driver 112 which inputs a sine wave oscillator 111 with a frequency fm, and the interaction between light and ultrasonic waves generates a frequency that is the basis of optical heterodyne interference. Two different light beams 104 and 105 are generated. Note that the light beam 108 is non-diffracted light and is not used for measurement.

Here, the A.0 driver 112 is generally composed of a voltage controlled oscillator (VCD), a balanced modulator, a high frequency power amplifier, etc.
It generates signals with frequency components a - f m and f a -4 - f m, and shifts the frequency of the light that passes through the inside of A-0103, and the light beam 104 is
The o+fa-fm-light beam 105 has a frequency of fo+fa+fm, and the difference in frequency is 2fm.

Here the distance between the light beams 104 and 105 is the frequency f
Proportional to m. The two light beams 104 and 105 are made into one unit as a probe light beam, and the probe light beam is divided into two directions.

102 is an optical isolator composed of a polarizing beam splitter and a quarter wavelength plate, and is installed between A.0103 and the object plane 1070 to be measured.

The two light beams 104 and 105 are connected to the optical isolator 10
2 to split in two directions. One is used as a reference light 101105 that does not irradiate the object plane 107, and the other irradiates the object plane 107 through the condensing lens 106. Object plane 107
The reflected light from the object passes through the condensing lens 106 again, and the path is bent again by the optical isolator 102.The object reflected light 120
゜121. 122 and 126 are reference beams 104.1
05 and object reflected light 120-121, the photoelectric conversion unit 1 is composed of, for example, a PIN photodiode, a current-voltage converter, and the like.

The beat current signal obtained from the PIN photodiode is subjected to current-voltage conversion.

Here, the reference beam 101105 includes surface information of the surface of the object to be measured, and the single-object reflected beams 120 and 121 include surface information of the point irradiated with the light beam, that is, surface information such as surface roughness or surface inclination angle. . The photoelectric conversion unit 123 for the object reflected lights 120 and 121 is the first photoelectric conversion unit 123
1 and a second photoelectric conversion section 1231 and a second photoelectric conversion section 1233.

The first photoelectric conversion unit 1231 mainly detects the phase of the beat signal of the object reflected light, and the second photoelectric conversion unit 12
32.1263 mainly detects the amount of light reflected from an object depending on the direction of reflection, and detects differences in the direction of reflection of the irradiated probe light due to differences in shape, roughness, etc. of the object surface 107. For example, the shape of the object plane 107 is 1071
In such a case, the object reflected light enters the second photoelectric conversion unit 1262, and in the case where the shape of the object surface 107 is 1072, the object reflected light enters the second photoelectric conversion unit 1266,
The shape can be distinguished depending on which part of the object reflected light is present. If the object plane 107 is perpendicular to the direction of incidence, the photoelectric conversion unit 1231 of any Maki 2
The object reflected light is not incident on 1233 either, and the photoelectric conversion unit 12
61 and the surface shape is determined to be flat. Generally, light reflected from an object has a wide beam diameter.

It is desirable to install the first photoelectric conversion unit 1261 so that reflected light is incident on any two of the three photoelectric conversion units.

Although it is a line, the one-object reflected light beams 120 and 121 are at the object plane 10.
70 Due to the difference in optical path length between the two beams due to irregularities such as surface shape and surface roughness, the phase of the two beams differs, and this appears as a change in the phase of the interfered beat signal.

By cutting the DC component of the voltage signal that becomes the output signal of the photoelectric conversion units 122 and 126, the AC voltage signal 12 obtained is obtained.
4 and 125 are respectively A, 'cos (2yr fist 2fmt + θ1) and A2
'cos(2π*2fmt+θ2).

θ1 is the initial phase of the reference signal and is a constant amount, and θ2 is an amount that changes depending on the amount of surface unevenness of the object surface 107, and it is sufficient to detect a change in the difference θ2−θ.

A phase comparator 114 detects the phase difference between the signals from the reference light photoelectric conversion section 122 and the object reflected light photoelectric conversion section 123. 126 is an object surface 1070 based on data from the phase comparator 114 and the object reflected light photoelectric conversion unit 12311233.
This is a data processing unit that calculates surface conditions.

In this embodiment, an example is shown in which the phase difference between the output signal of the reference light and the output signal of the first photoelectric converter 1261 of the object reflected light photoelectric converter 123 is detected.

When the reflection angle of the object-reflected light is large and the reflected light does not enter the first photoelectric converter 1231 of the object-reflected light photoelectric converter 126, the output signal from the 20th photoelectric converter 1232 or 1236 is The phase difference may be detected. Furthermore, if the shape of the object surface 107 is concave or convex in only one direction, the second photoelectric conversion unit
There is no need to use both 262 and 1233 at the same time.
Only one of them may be used depending on the state of reflection.

Next, a data processing section 12 performs data calculation from the output of the phase comparator 114 and the output of the second photoelectric conversion section of the object reflected light.
Let me explain the operation of step 6.

Suppose that when the two light beams 104 and 105 are irradiated onto the object plane 107, there is a surface unevenness amount Z between the two light beams, and the output data of the first phase comparator 114 is θ=θ2−θ. It is expressed as Z-λ·θ/4π. However, λ
is the wavelength of the laser beam. For example, when λ=0.6328 micrometers, θ. = Z per 1° is 8.
It is 8 angstroms.

The phase comparator 114 outputs -π4θ<π or 0<θi2π
The phase angle can be measured in either mode - For example, when operating in Oi θ<2π mode - θ = 3π
is measured only as θ = π, and generally θ22nπ'-
θ. (n is an integer) where n is indefinite and θ. It is only measured.

The two-beam probe light 101105 has a measurement range in which the object plane has a phase angle O<θ≦2π or −π≦θiπ.

When the phase comparator 114 is in the mode of 1 π < θ < π and dynamic λ is displayed as -π ≦ θ ≦ 0, it is judged as a convex state where 0 ≦ Z ≦ -, which is an inconvenience that makes it impossible to accurately demodulate the surface state. be. That is, it is impossible to measure the surface state of IZI>2 using only the phase comparator 114.

4 Fig. 2 (al is a schematic diagram showing the relationship between incident light rays and reflected rays with respect to the object plane 107 and the photoelectric conversion unit 123, and Fig. 2 fbl is a side view showing an enlarged view of the object plane 107 in Fig. 2 (al) , FIG. 2(C) is a schematic diagram showing the phase angle.

In Figure 2 (al), 20 is the incident light ray.As mentioned above, there are actually two light beams, but in the figure it is omitted and described as one light beam.The same applies to the reflected light rays. When the incident light ray 20 is irradiated onto the flat part 1076 of the object surface 107, it is specularly reflected and the reflected light 2
5 travels in the opposite direction to the incident light beam 20 and passes through the optical isolator 10
2, the light changes its course at right angles and mainly enters the 1'th object-reflected light photoelectric conversion unit 1261. The phase state at this time is the state of phase angle 0 shown in FIG. 2(C). If the incident light ray 20 is also irradiated with the surface state 1072 of the object surface 107, the reflected light passes through the path 21, changes its course again at the optical isolator 102, and proceeds in the direction 22 to the second photoelectric conversion section of the object reflected light. 1266 is also assumed to be P of incidence (c).

At this time, if the phase comparator 114 is operating in a mode of -π≦θ≦π, one phase angle must be measured at the locus of OAP and 0BCP as θ=−(2π−θ.). The phase angle conversion shown above is performed by the second photoelectric conversion unit 1233.
This is done based on voltage data of the amount of incident light.

As mentioned above, since the object reflected light is a beam with a somewhat wide beam diameter, it is simultaneously incident on the photoelectric conversion units 1231 and 1232, and the phase is measured by the first photoelectric conversion unit 1261.
The phase angle θ is measured by detecting the direction of reflection in the second photoelectric conversion unit 1233. It is possible to determine whether or not to perform one line of conversion. Furthermore, for example, the incident ray 2
0 is irradiated onto the surface state 1071 of the object surface 107, the reflected light 23 changes its course by the optical isolator 102, proceeds in the direction of 24, and is also incident on the 20th photoelectric conversion unit 1232 of the object reflected light. (also incident on the photoelectric conversion unit 1231) in FIG. 2 (when the phase angle P is C1λ), it is determined that the surface state is +Z (- and corresponds to θ=00).

λ As shown in the above example, the conventional measurement range was Z〈-, but by detecting the direction of reflection of the object-reflected light according to the present invention, the absolute value of the phase angle can be changed to within the range of λ 2π ( That is, the surface state (IZI≦-) can be accurately demodulated.

Furthermore, the magnitude of the light reflected from the object also changes at the same time. That is, it is possible to detect how many rotations the phase angle has rotated around 2π from the change in the amount of light.

As Z increases, the direction of reflection of the object-reflected light increases, and for example, when the amount of light incident on the 20th photoelectric conversion section 1262 increases, the amount of light incident on the first photoelectric conversion section 1231 decreases, so the gain of both increases. is detected, and if the gain reaches a certain value go, it can be determined that the phase angle has made one rotation around 2π.For example, θ−2π10. Therefore, it can be determined that Z〉2.

Two beams of light incident on the object surface 107 are sequentially superimposed to perform line scanning or surface scanning on the object surface.

By performing an integral of the measured phase set.

General shape measurement is possible.

FIG. 3 is a schematic diagram showing the details of the optical heterodyne interference optical system used in FIG. 1, and 160 and 134 are cylindrical lenses each having a focal length of 11°. 1
31 and 162 are plano-convex lenses each having a focal length of 12. 133 is a polarizing beam splitter - 165 is a 1/
A four-wave plate, 106 is a laser condenser lens with a focal length of l.

shall be.

Generally, A-0106 modulates light waves through the interaction of light and ultrasonic waves, and it is preferable that the beam width of the light incident on A-0106 is wide.

The combination of the cylindrical lens 130 and the plano-convex lens 161 generates a wide elliptical beam.

Further, by using a linearly polarized laser, the reference light and the object light are separated from the combination of the polarizing beam splitter 133 and the quarter-wave plate 165.

The beat signals 124 and 125 from the photoelectric converters 122 and 123 generally have different amplitudes, and it is preferable to input electrical signals with amplitudes as close as possible to the phase comparator 114. For example, the axis of linearly polarized light may be adjusted by rotating the laser tube. Alternatively, the linear polarization axis may be rotated by rotating the polarizing plate.

Furthermore, in order to improve the S/N ratio of the beat signals 124 and 125, it is preferable to install the polarizing beam splitter 133 at a location where the interference light becomes an elliptical beam.

In the embodiment of FIG. 3, the two separated by A.0106
Although the two light beams are not shown, they are actually separated into two very close beams. Furthermore, undiffracted light is omitted from the figure.

When the frequency that provides this two-beam separation is fm, the separation distance d on one object plane is given by:

However, ■ is the speed of the ultrasonic wave transmitted through A.0103. Moreover, ■ is determined by the medium of A.0106 - for example, V
=3.8km/5ec=A', =15g-12=5
If 00+w-lo =7mm.

f m = 1. At OOkHz, it is d-7 micrometers.

Also, the beam spot diameter on the object irradiation surface is determined by the condenser lens 10.
6. The diameter of the beam incident on 6 (converted to a circular Gaussian beam in this case) and the focal length l of the lens. However, in order to make the beam diameter smaller and the distance d between the two beams smaller, a beam expander may be inserted between the cylindrical lens 134 and the focusing lens 106.

As described above, the surface measuring device using the optical heterodyne interferometry according to the present invention is capable of accurate measurement over a wider measurement range than the conventional optical heterodyne interferometry.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a surface measurement device using optical heterodyne interferometry according to an embodiment of the present invention, and FIG. Figure (b) is a side view showing an enlarged view of the object plane in Figure 2 (a), Figure 2 (c) is a schematic diagram showing the phase angle, and Figure 3 is the optical heterodyne interferometry used in Figure 1. Schematic diagram showing details of the optical system 100... Laser oscillation unit. 103... Acousto-optic element. 104-105... Reference light, 107...
・Object plane, 114... Phase comparator, 120-121... Object reflected light, 122...
...Reference light photoelectric conversion section. 126...Object reflected light photoelectric conversion unit. 126...Data processing unit. 1261... 10th photoelectric conversion unit of object reflected light,
1262.1263... Second photoelectric conversion unit for object reflected light. Figure 2 (C) JP-A-Sho GO-100007 (6)

Claims (1)

[Claims] A surface produced by optical heterodyne interferometry, in which the light emitted from a laser oscillation unit is split into two light beams with different frequencies by an acousto-optic element, and the two light beams having the two frequency components are irradiated onto the object surface, which is the surface to be measured. In the measuring device, two light beams with different frequencies emitted from the acousto-optic element are treated as one unit of globe light beam, the probe beam is divided into two directions, and one probe beam is irradiated onto the object surface. The other probe beam irradiates the object surface, the reflected light of the light irradiated to the object surface is used as the object reflected light, and the reference light is photoelectrically converted to create a beat signal. a light-photoelectric converter; a plurality of first and second object-reflected light photoelectric converters for photoelectrically converting the object-reflected light to create a beat signal; the reference-light photoelectric converter; and the object-reflected light photoelectric converter. and a data processing section that calculates the surface state of the object surface based on data from the phase comparator and the object reflected light photoelectric conversion section. Surface measurement device using optical heterodyne interferometry.