JPH0326765B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is a three-dimensional measurement method that optically and non-contactly measures the surface shape of general free-form surfaces or aspherical lenses and mirrors with high precision. In particular, it focuses a laser beam onto the surface of an object to be measured using an objective lens, and measures the surface shape by detecting the Doppler shift of the frequency of the reflected light caused by movement of the surface of the object to be measured. The present invention relates to an optical measuring device.

Conventional configurations and their problems As a laser length measuring device using the optical heterodyne method, there is a product manufactured by Heuretsu Patscard Co., Ltd. (for example, HP5526A). This is currently simple and
It is known as the most accurate length measuring device. It is also known that this can be attached to a three-dimensional moving table and used as a three-dimensional measuring device or a precision lathe.

By the way, with conventional devices, a corner cube or mirror is attached to the moving table, and only the movement of the moving table is measured with a laser length measuring device. The moving table is moved along the surface shape. However, there are contact and non-contact types of measurement probes.
In either case, the measurement accuracy is about an order of magnitude lower than that of a laser measuring device.

OBJECTS OF THE INVENTION The present invention eliminates the above-mentioned conventional drawbacks, and makes it possible to directly measure the shape of the surface of an object to be measured using a laser length measurement method.

By focusing the measurement light onto the surface of the object to be measured using the objective lens and applying focus servo to keep the distance between the objective lens and the surface of the object to be measured constant,
The reflected light returns in the same direction as the incident light and therefore interferes with the reference light, detecting the beat frequency and
The displacement of the surface of the object to be measured can be measured.

Even if the measured object surface has a low reflectance such as a glass surface, the surface is not a somewhat smooth polished surface, or the surface is tilted, and a sufficient amount of reflected light is not returned. The purpose of the present invention is to obtain an optical measurement device that can measure the shapes of many types of surfaces with high precision by using a newly developed laser light source with high power.

Structure of the Invention In order to achieve the above object, the optical measurement device of the present invention uses a measurement light (frequency f ₁ ) and a reference light (frequency f ₂ ).
A synchrotron radiation source that generates synchrotron radiation whose two frequency fluctuations are sufficiently small, that is, the frequency fluctuation divided by the oscillation frequency is about 10 ^-9 ; An optical system that converts the synchrotron radiation into radiation having a spot size and a spread angle, a light separation means that separates the optical path of the synchrotron radiation into the measurement beam _f1 and the reference beam _f2 , and the measurement beam is focused onto the surface of the object to be measured. a second photodetector group that receives a portion of the reflected light from the surface of the object to be measured and detects a focus error signal when the focal point deviates from the surface of the object to be measured; An optical system installed in the optical path of the reflected light so as to generate an error signal, and a moving stage that moves the objective lens so that the distance between the objective lens and the surface of the object to be measured is constant according to the focus error signal. The measurement light reflected from the surface of the object to be measured and the reference light are made to interfere with each other, and the shape of the surface of the object to be measured can be measured with high precision from the fluctuations in their beat frequencies.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described based on the drawings.

In Fig. 1, the light source 1 is a He-Ne laser;
This is a laser that applies a magnetic field in a direction perpendicular to the tube axis, emits two lights with oscillation frequencies f ₁ and f ₂ with mutually orthogonal polarization directions, and stabilizes this oscillation frequency. The contents are described in the June and July issues. The laser power of this laser is stronger than that of conventional lasers, and the frequency stability is also quite good at 10 ^-9 .

2 and 3 are beam expanders which expand the beam into parallel light to the extent that the light enters the entire diameter of the entrance pupil of the objective lens 6. The light of frequency f ₁ has an electric field polarized in a direction parallel to the plane of the paper, and is completely transmitted through the polarizing beam splitter 3 . This is called measurement light. On the other hand, frequency f ₂
The light is polarized in a direction perpendicular to the plane of the paper and is totally reflected by the polarizing beam splitter 3. This is called the reference light. The measurement light completely passes through the polarizing prism 4,
After passing through the λ/4 plate 5, by the objective lens 6,
It is narrowed down to the surface of the object to be measured 7. The object to be measured 7 is fixed to a moving table 14 and can be moved in the X-Y directions. The reflected light from the surface of the object to be measured passes through the objective lens 6 again, and passes through the λ/4 plate, so that the polarization direction changes by 90° from that of the incident light. The polarizing prism 4 is designed to completely transmit P-polarized waves, partially transmit S-polarized waves, and partially reflect them.Since the reflected light from the surface of the object to be measured becomes S-polarized waves, A portion is transmitted through the polarizing prism 4, and a portion is reflected. The light reflected from the polarizing prism 4 is focused by a lens 17, divided into two by a half mirror 18, and passes through circular apertures 19 and 21 placed before and after the focal point.
The photodetectors 20 and 22 are reached. When the focal position deviates from the surface of the object to be measured 7, a portion of the light is blocked by either the aperture 19 or 21 depending on the direction of the deviation, resulting in a difference in the amount of light entering the photodetectors 20 and 22.
By using the difference between the outputs of the photodetectors 20 and 22 as an error signal and moving the objective lens 6 in the Z direction, it is possible to always focus on the surface of the object to be measured. This focus detection method was developed by Y. Fainman et.
Al Appl.Opt/Vol.21, No.17, P3200.

When the object to be measured is moved in the X-Y direction, the frequency of the reflected light of the measurement light undergoes a Doppler shift in accordance with the change in the thickness of the object to be measured. Here, the frequency of the reflected light is written as f ₁ +Δf+ε. Δf+ε is the amount of Doppler shift. The reason for writing Δf+ε is as follows. Since this measuring device aims at very high precision measurement of 0.1 to 0.01 μm, the moving table 14 also needs to be highly accurate.
When it is moved in the Z direction, it is also slightly displaced in the Z direction. Currently, even with the most highly accurate moving platforms such as air bearings and oil bearings, it is difficult to keep the straightness of movement below 0.2 μm. Therefore, the amount of Doppler shift of the reflected light is the amount caused by deviation in the Z direction due to insufficient straightness when the moving table 14 is moved in the X-Y direction, ε, the thickness of the surface of the object to be measured The amount due to the change is set as Δf. As described above, the reflected light of the measurement light has a frequency of f ₁ +Δf+ε, is totally reflected by the polarizing beam splitter 3, and passes through the polarizing plate 23,
The light passes through the lens 15 and reaches the photodetector 16.

On the other hand, the reference light passes through the λ/4 plate 8, the mirror 9, the lens 10, and the mirrors 11 and 12, and reaches the mirror 13 fixed on the movable table 14. mirror 13
has an extremely high flatness of about 0.01 μm. Therefore, the reflected light of the reference light is Doppler shifted only by the displacement in the Z direction when moving the moving table 14, and the frequency of the reflected light of the reference light is f ₂ + ε becomes. lens 10
The focal length of is long enough, so the depth of focus is deep,
Further, since the displacement of the moving table 14 in the Z direction is sufficiently small compared to the depth of focus, a focus servo is not necessary for the reference light. The reflected light of the reference light returns along the same path and passes through the λ/4 plate 8 again, and the polarization direction changes by 90 degrees from the incident light, so it completely passes through the polarizing beam splitter 3 and reaches the photodetector 16. . If a certain amount or more of the measurement light reflected from the object to be measured 7 or part of the reflected light from the mirror 13 returns to the laser, the laser oscillation becomes unstable and measurement becomes difficult. It is necessary to devise a way to make the return light extremely weak. In other words, the polarizing beam splitter 3 has the function of totally reflecting S-polarized light and completely transmitting P-polarized light, and the extinction ratios of these must be sufficiently large. In addition, it is necessary to use λ/4 plates 5 and 8 of good performance, and adjust them so that they are almost completely λ/4 with respect to the laser wavelength by fine adjustment of rotation and tilt. Since the polarization directions of the measurement light and the reference light are orthogonal to each other, only the components in the 45° direction are passed through the polarizing plate 23 to align the polarization directions, so that the beat frequency ( Detect f ₁ +Δf+ε)−(f ₂ +ε)=f ₁ −f ₂ +Δf. On the other hand, f ₁ - f ₂ can be detected by partially separating and detecting the emitted light from the laser 1 in advance and comparing these frequencies to detect information Δf about the thickness of the object to be measured.

The Doppler shift is as follows: When the reflecting surface moves in the Z direction at a speed of v, if the frequency of the incident light is f ₁ , the reflected light will be f ₁ (1-2v/c), and the amount of Doppler shift Δf = 2v/cf ₁ becomes. Therefore, the variation becomes ΔZ=∫vdt=c/2f ₁ ∫Δfdt. Here, the measurement accuracy depends on the stability of _f1 . In other words, the stability of f ₁ is the reliability of measurement.

Various surfaces can be considered as the surface of the object to be measured. These three factors are tilt, surface accuracy, and reflectance.
It is divided into points and described below.

FIG. 2 is an explanatory diagram of the optical path of reflected light when the surface to be measured is tilted by θ. Objective lens numerical aperture NA
= sinα. If the inclination θ is within α, the reflected light returns into the objective lens. However, even if the slope is within α, as shown in FIG. 4, the amplitude of the detected beat frequency is lower than when the slope is zero. As shown in FIG. 3, the amplitude of the beat frequency is proportional to the area of the overlapping portion C between the aperture A of the objective lens and the distribution B of the reflected light. Therefore,
The higher the NA of the objective lens, the more sloped surfaces can be measured. By allowing the incident light to fill the objective lens aperture, the amount of tilt tolerable increases.

Regarding surface accuracy, if the surface shape is rougher than the spot diameter of the measurement light focused by the objective lens, it can be considered in the same way as the above-mentioned inclination. Also,
If the surface shape is equal to or smaller than the spot diameter, the reflected light will be diffracted by the surface and a portion will be scattered outside the objective lens. However, the shape of the part irradiated by the measurement light has a slope of α at all positions.
Unless the amount of light exceeds 1, some of the light always returns to the objective lens, and the beat frequency can be detected using this light.

Regarding the reflectance of the surface, for example, when measuring the surface shape of a lens, since it is a glass surface, the reflectance is about 4%. Therefore, it is necessary to irradiate measurement light with sufficient power. According to experiments, if the laser power was 0.5 mW or more, it was possible to measure a polished glass surface without any inclination.

However, if the above-mentioned inclination, surface precision, and reflectance are all poor, the amount of reflected light decreases, making measurement difficult. Therefore, it is better to increase the detection sensitivity of the photodetector, use a laser with as strong power as possible, and use a high NA of the objective lens. Objective lenses with an NA of up to 0.95 are manufactured, but the distance between the objective lens exit end and the surface of the object to be measured,
That is, when the working distance (WD) becomes short and the surface of the object to be measured is tilted, the objective lens and the object to be measured come into contact, making measurement impossible. Considering this point,
This measuring device used had an NA of 0.5 to 0.6. However, depending on the application, other NAs may be appropriate. For example, if the surface has a small slope, but you want to measure a fine surface shape, you can use a higher
I use a lens with a NA, and when measuring surfaces that I can't get very close to, I use a lens with a lower NA.

In this embodiment, a moving stage 1 for fixing the object to be measured is used.
4 was moved in the X-Y direction, and the objective lens was moved in the Z direction. However, even if the optical system including the objective lens, laser, etc. is moved in the X-Y direction, or even if the object to be measured is moved in the Z direction, the principle of the present invention does not apply due to the degree of freedom in design. Needless to say, they are the same.

By moving the relative position of the object to be measured and the objective lens in the X-Y direction, the thickness of the object to be measured can be measured at a wide range of X-Y positions. There are cases where the reflected light does not return sufficiently to the objective lens due to heat or dust, making measurement impossible.
Since the optical heterodyne method measures the thickness by integrating continuous changes in thickness, if it becomes impossible to measure at even one point, it becomes impossible to measure from now on. As a solution to this problem, as shown in Figure 5, when the position (X _a , Y _a ) becomes unmeasurable, it is returned to the immediately previous measurement position (X _a , Y _b ), and
The value of the measured value Z _b at (X _b , Y _b ) is input again. After that, avoid the unmeasurable points (X _a , Y _a ),
Measurement can be continued by taking a detour as shown in FIG.

When measuring the thickness Z of an object to be measured, in order to increase accuracy, the X-Y coordinates of the measurement position must also be measured with high accuracy. For this purpose, a part of the light from the frequency-stabilized Zeeman laser may be separated, or the light from another similar light source may be transmitted to a mirror 27 fixed with the objective lens as shown in FIG.
Mirror 2 fixed together with the object to be measured
9, the beat signal of the reflected light is detected by the photodetector 32, and the amount of movement in the X and Y directions can be measured with high accuracy. Due to the functions of the λ/4 plates 25 and 26 and the corner cube prism 30, the light is reflected twice by the mirrors 27 and 29, respectively. This structure is mirror 27,2
It has a structure that is resistant to slopes of 9.

Effects of the Invention As explained above, according to the present invention, the structure is such that the shape of the surface of the object to be measured can be directly measured by the interferometric length measurement method using the optical heterodyne method, and other technically possible mechanical precision deficiencies can be overcome. It has almost surpassed the limits of measurement accuracy of conventional devices, and can measure the shape of surfaces with various surface shapes with the precision of interferometric length measurement, that is, the precision of 0.1 to 0.01 μm, and its industrial utility value is extremely high. It is.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an optical measuring device according to an embodiment of the present invention, and FIGS. 2, 3, and 4 are explanations of the case where the surface of the object to be measured is tilted in the optical measuring device of the present invention. Figure 5 is an explanatory diagram of the measurement sequence when there is an abnormality on the surface to be measured.
FIG. 6 is a block diagram of main parts of an embodiment of the present invention. 1... Laser light source, 2... Beam expander, 3, 24... Polarizing beam splitter, 4...
Special specification polarizing prisms, 5, 8, 25, 26...
...λ/4 plate, 6... Objective lens, 7... Object to be measured, 9, 11, 12, 13, 27, 28, 29...
...Mirror, 10, 15, 17... Lens, 14...
...Moving table, 16, 20, 22, 32... Photodetector, 18... Half mirror, 19, 21... Circular aperture, 23, 31... Polarizing plate, 30... Corner cube.

Claims (1)

[Scope of Claims] 1. Light emitting means for generating a measurement light of frequency f ₁ and a reference light of frequency f ₂ ; a first light separation means for separating these lights into separate optical paths; An objective lens for condensing light onto the surface of the object to be measured, measurement light reflected from the surface of the object to be measured and passed through the objective lens again, and irradiated onto a mirror fixed on the same mount as the object to be measured. an optical system that causes the reflected reference light to interfere on a first photodetector, and detects fluctuations in the beat frequency generated on the photodetector, making it possible to measure displacement of the surface of the object to be measured. A signal processing means for determining the position of the object to be measured and the position of the optical system including the light source, objective lens, etc., with the direction of the optical axis of the measurement light, in mutually perpendicular coordinates X-Y-
In Z, when the Z direction is set, a moving means that is relatively movable in the X-Y direction, and a part of the measurement light reflected from the surface of the object to be measured is separated by a second light separation means. , or the light from another second light source is irradiated onto the surface of the object to be measured through the objective lens, and the reflected light is separated from the reflected light of the measurement light by a third light separation means. a second photodetector group for detecting a focus error signal that receives reflected light; and a second photodetector group located between the second or third light separation means and the second photodetector group, comprising optical means for converting the optical path into a form capable of obtaining a suitable focus error signal on the second group of photodetectors, by means of the focus error signal obtained from the group of photodetectors; An optical measuring device comprising means for moving the objective lens or the object to be measured in the Z-axis direction so as to maintain a constant distance between the objective lens and the surface of the object to be measured. 2. The optical measuring device according to claim 1, wherein the beam diameter of the emitted light immediately before it enters the objective lens is equal to or larger than the entrance pupil of the objective lens. 3 The reference light is fixed on a mount on which the object to be measured is fixed, and is reflected after being incident on a mirror placed so that its surface is perpendicular to the optical axis (Z-axis) direction of the measurement light. The optical measuring device according to claim 1, characterized in that the optical measuring device reaches above a photodetector. 4 As a radiation means for generating measurement light and reference light, a magnetic field is applied in a direction perpendicular to the laser tube axis of the He-Ne laser, and two lights with different oscillation frequencies are generated by the Zeeman effect, and each The optical measuring device according to claim 1, characterized in that a transverse Zeeman frequency stabilized laser whose oscillation frequency is stabilized is used. 5 Total output power of measurement light and reference light is 0.5m
The optical measuring device according to claim 1, wherein the optical measuring device is W or more. 6. The optical measuring device according to claim 1, wherein the objective lens has a numerical aperture (NA) of 0.5 or more. 7 Determine the relative position between the objective lens and the object to be measured in X-Y
When the beat frequency on the photodetector temporarily becomes undetectable during the process of continuously or discretely measuring the thickness Z of the surface of the object to be measured, the previously measured X - Return the relative position to the Y coordinate position, output the Z measurement value at that time again, and then avoid the X-Y coordinate position that could not be detected,
The optical measuring device according to claim 1, characterized in that a sequence is performed in which relative positions in the X-Y directions are moved and measurements are continued. 8 A _{mirror M} , or a mirror M _Y2 having a reflective surface in a direction perpendicular to the Y-axis direction, and separates the emitted light from the light emitting means into the measuring light and the reference _light . or the radiation light from a third light emitting means similar to the light emitting means described above is divided into measurement light and reference light, and the measurement light is sent to the mirror M _X1 in the X direction, and the reference light is sent to the mirror M _X2 . Alternatively, in the Y direction, the measurement light is made incident on mirror M _Y1 and the reference light is made incident on mirror M _Y2 , and these reflected lights are made to interfere on the photodetector.
The optical measuring device according to claim 1, characterized in that the relative position of the objective lens and the object to be irradiated is measured in the X or Y direction.