JPS61259105A - Optical minute displacement meter - Google Patents

Abstract

PURPOSE:To make it possible to arbitrarily set resolving power and a measuring range, by changing NA of an incident side to a specimen by providing a means for changing NA behind an objective lens. CONSTITUTION:With respect to a minute displacement meter consisting of a laser diode 1, an objective lens 5, a beam splitter 7, critical angle prisms 8, 9 and photodiodes 10-13, for example, an iris diaphragm type luminous flux control iris 14 is provided between a collimation lens 2 and a deflection beam splitter 3. The iris 14 consisting of an iris blade 15, a rotary ring 16, a rack 17, a motor 18 and a pinion 19. When the large undulation or unevenness on the surface of a specimen 6 is desired to observe, NA is reduced by throttling the iris 14 to enlarge a measuring range and the measurement of a rough sur face profile is enabled and the measuring range is reduced as NA is made larger by opening the iris 14 but, because resolving power is enhanced, the use as a high resolving power minute displacement meter is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an optical minute displacement meter that measures the surface roughness, minute displacement (vertical displacement), etc. of a workpiece in a non-contact manner.

[Conventional technology]

With the remarkable development of precision processing technology in recent years, the number of products and parts with minute shapes processed on their surfaces has increased. LSI patterns, diffraction gratings, optical discs.

The surface of a roughness standard piece can be said to be a typical example of a surface with a regular fine shape. In addition, steps made by etching on a silicon wafer, cross-sections of groove-like scratches on aluminum surfaces, and mirror surfaces produced by ultra-precision processing on steel surfaces can also be considered microscopic shapes. For example, the surface roughness of an ultra-precision machined surface using a diamond cutting tool has achieved sub-μm or less, and the surface roughness of laser disks, magnetic tapes, films, etc. is on the same order. Currently, most measurements of surface roughness are performed using a stylus-type micro-displacement meter.

However, many of these products and parts are finished as commercial products, and it is desirable to be able to measure the fine shapes of their surfaces in detail without damaging them. Therefore, in order to satisfy these needs, development of non-contact minute displacement meters has been progressing. Among them, the optical type is currently considered to be practical.

Although there are various measurement principles, for example, one using the critical angle method is shown in FIG. This means that the infrared laser beam from the laser diode 1 passes through the collimator lens 2.
．． Polarizing beam subrifter 3°λ/4 plate 4. Objective lens 5
The reflected light is projected onto the sample 6 through the objective lens 5. λ/4 plate 4, polarizing beam splitter 3. The light reflected by the critical angle prism 8 or 9 through the beam splitter 7 passes through two photodiodes to, tt or 12., respectively.
13 (see FIG. 11). When the measurement surface of the sample 6 is at the focal point of the objective lens 5, the light reflected from the measurement surface becomes a parallel beam of light by the objective lens 5 and enters the critical angle prism 8 or 9, but this incident light is exactly If the critical angle prism 8 or 9 is set so that the incident light is near the critical angle, each of the two photodiodes 10.11 or 12.13 will have the following characteristics as shown in FIG.
As shown in b), the same amount of light arrives. In addition, when the measurement surface is located closer to the objective lens 5 than the focal position, the reflected light becomes a diverging light after passing through the objective lens 5 and enters the critical angle prism 8 or 9, but at this time, the incident angle is different on both sides of the optical axis. Because of the difference, the light on the side that does not satisfy the conditions for total reflection will go out of the prism 8 or 9, and the light on the side that satisfies the conditions for total reflection will be totally reflected. Thus, only a small amount of light reaches photodiode 10 or 12, and sufficient light reaches photodiode 11 or 13. If the measurement surface is located further from the objective lens 5 than the focal position, the above will be reversed, and sufficient light will reach the photodiode 10 or 12 and no photo will be detected, as shown in FIG. Only a small amount of light reaches the diodes 11 or 13, so each of the two photodiodes 1
Sample 6 while reading the output difference of 0°11 or 12.13.
By moving and scanning the surface, its surface roughness, minute displacement, etc. can be measured.

[Problem that the invention seeks to solve]

However, although micro-displacement meters using such a critical angle method have achieved a high resolution of 2n- or less, the measurement range is as narrow as 2 μm, and the measurement target has large-amplitude undulations or irregularities on the surface. It was limited to specular samples. That is, as mentioned above, it is required to realize the required resolution and measurement range for measuring a wide variety of samples, but conventional micro-displacement meters have not been able to meet these requirements.

On the other hand, one way to expand the measurement range is to replace the objective lens with a low-magnification lens with a small NA and deep depth of focus. In addition to being quite large in size, misalignment is likely to occur and it is difficult to maintain parfocality at the required accuracy. In addition, even with the method of replacing a single objective lens, it is difficult to suppress misalignment with high accuracy or maintain parfocality to the required accuracy, and the task of replacing the objective lens is cumbersome, especially when performing in-process measurements. This is extremely difficult. Also, multiple resolutions.

It is troublesome to prepare minute displacement meters with different measurement ranges and replace them depending on the measurement target, and it is almost impossible to measure the same location even if the measurement target is the same, resulting in economic loss. The problem was that there were too many.

In view of the above-mentioned problems, the present invention allows a single micro-displacement meter to arbitrarily set a resolution 5 measurement range depending on the object to be measured and the purpose of tm determination, and prepares a plurality of micro-displacement meters for different uses. The object of the present invention is to provide an optical micro-displacement meter that can measure the same object at different ranges almost simultaneously, and can also be applied to in-process measurement.

[Means and effects for solving the problem] The optical micro-displacement meter according to the present invention has a means for changing the NA at the rear of the objective lens, and changes the NA on the side of incidence to the sample, thereby increasing the resolution. , the measurement range can be set arbitrarily.

(Embodiments) The present invention will be described in detail below based on the illustrated embodiments, with the same reference numerals assigned to the same members as in the conventional example described above.

FIG. 1 shows the optical system of the first embodiment, which is the same as the conventional example except that a light flux limiting aperture 14 is disposed between the collimator lens 2 and the polarizing beam splitter 3. The light flux limiting diaphragm 14 may be, for example, an iris diaphragm used in a camera as shown in FIG. In addition, a pinion 19 that meshes with the rack 17 is fixed to the ring of the encoder DC motor 18.

FIG. 3 shows the signal processing system of the above embodiment, where 20 is a measurement range setting circuit, 21 is a control circuit, 22 is a motor drive circuit, 23 is an aperture diameter detection circuit, and 24 is a laser diode drive circuit.

Since the optical minute displacement meter according to the present invention is configured as described above, when the measurement range is set by the measurement range setting circuit 20, the motor drive circuit 22
is controlled, and the motor 18 is driven. When the motor 18 is driven, the rotating ring 16 is rotated via the pinion 19 and the rack 17, and the aperture blades 15 are opened or closed. At this time, a signal from the encoder section of the motor 18 is sent to the aperture diameter detection circuit 23 to detect the aperture diameter. When the aperture diameter corresponds to the range set by the measurement range setting circuit, the motor 18 is stopped via the control circuit 21 and the motor drive circuit 22.

Assuming that the aperture 14 is now closed, the luminous flux at this time becomes the luminous flux b (shaded area) shown in FIG. 1. This state is compared to the state of the luminous flux a when the aperture 14 is not narrowed down. In a state where the NA of the luminous flux incident on sample 6 is reduced,
This is equivalent to reducing the NA of the objective lens 5. The expansion of the measurement range when the NA is reduced is shown in the calculation results, ie, the displacement-output relationship, in FIG. 4, which agrees with the experimental results. Therefore, by changing the diameter of the light flux limiting diaphragm 14, the measurement range can be arbitrarily set. However, if you reduce the NA and expand the measurement range, the resolution will decrease accordingly, and the size of the measurement spot on the sample 6 will also increase as shown in the table in Figure 5. It is necessary to set the measurement range. Furthermore, when the aperture 14 is stopped down, the light intensity decreases, resulting in a decrease in the S/N ratio. Therefore, the laser diode drive circuit 24 is controlled by the control circuit 21 using the signal from the aperture diameter detection circuit 23 to adjust the aperture diameter. It is also possible to set the amount of laser light to increase when the value becomes smaller.

Although the iris diaphragm has been explained here as an example, the luminous flux limiting diaphragm 14 may be a liquid crystal diaphragm with transparent electrodes arranged concentrically, or a plurality of diaphragms with different diameters arranged on the circumference as shown in Fig. 6. Any device that can limit the luminous flux may be used, such as a turret-type diaphragm 25 or a switching diaphragm 26 in which a plurality of diaphragms with different diameters are arranged in a straight line as shown in FIG.
8 is used to change the aperture diameter, but it goes without saying that it can also be done manually.

Further, although the light flux limiting diaphragm 14 is located between the collimator lens 2 and the polarizing beam splinter 3, it can also be placed between the polarizing beam splinter 3 and the objective lens 5. However, in this case, the effective NA will also be reduced for the light reflected back from the sample 6, and if the sample 6 is tilted, vignetting will occur due to the aperture 14, making it extremely weak against tilt. Become something.

As described above, according to the optical micro-displacement meter of the present invention, when it is desired to see large undulations or irregularities on the surface of a measurement sample, the measurement range can be expanded by narrowing down the aperture 14 and reducing the NA. This makes it possible to measure profiles. Furthermore, since the measurement spot size also increases at this time, measurements can be made without being affected by small irregularities such as dust on the surface.

Furthermore, since only the NA on the incident side to the sample is made small, even if the sample is tilted, it is less affected than when the NA of the objective lens itself is made small, and the NA can be increased by opening the aperture 14. As the measurement range decreases, the resolution improves, so it can be used as a conventional high-resolution micro-displacement meter.

Therefore, by adjusting the aperture of the aperture 14, one micro-displacement meter can be freely changed from a high-resolution micro-displacement meter to a micro-displacement meter with a wide measurement range. The performance of the meter can be set arbitrarily.

FIG. 8 shows the optical system of the second embodiment, which is the same as the conventional example except that a variable focal length zoom lens system 27 is arranged in place of the collimator lens 2. The zoom lens system 27 functions as a collimator lens with a variable focal length, and by arbitrarily changing this focal length, the laser beam can be converted into a luminous flux having an arbitrary diameter, such as a luminous flux b (shaded area). shall be. This zooming may be done manually or by some kind of motorized system.

In this example, the NA on the incident side to the sample 6 can be reduced by simply setting the zoom lens system 27.
This has the same effect as the first embodiment.

Furthermore, since the amount of light is not limited, there is also the advantage that the S/N ratio can be kept constant.

FIG. 9 shows an optical system of the third embodiment, in which a polarizing plate 28 is arranged at the position of the light flux limiting aperture 14 of the first embodiment in accordance with the direction of polarization of the laser diode 1, and a polarizing beam splitter 3 is replaced with a non-polarizing beam splitter 29, a new non-polarizing beam splitter 30 is installed between the beam splitter 29 and the beam splitter 7, and the beam splitter 30. New polarizing plate 31. Collimator lens 3
2. The laser diode 33 is connected to a beam splitter 29, a polarizing plate 28. Collimator lens 2. It is the same as the conventional example except that it is arranged in an optically similar positional relationship to the laser diode 1. However, since the focal length of the collimator lens 32 is different from the focal length of the collimator lens 2 and is shorter in the case of FIG. 9, the luminous flux C passing through the collimator lens 32
(The shaded area) is narrower than the luminous flux a, and the NA on the incident side is smaller.

Note that the component of the reflected light from the sample 6 that is reflected by the beam splitter γ 29 or 30 will proceed to the laser diode 1 or 33 side, but it passes through the λ/4 spider plate 4 twice and the polarization direction is changed. Since it is perpendicular to the polarizing plate 28 or 31,
There is no bank talk noise that returns to the laser diode 1 or 33, and since there are many reflective surfaces, there is a lot of loss, especially on the beam C side, but the beam splitter 29 and 30
By setting the reflectance to about 25%, the reflection loss on the luminous flux C side can be alleviated to some extent. In addition, the luminous flux is not particularly eclipsed, and the number of light projection systems consisting of laser diodes, collimator lenses, polarizing plates, and beam splitters can be set to two as long as the reflection loss can be covered by the laser light intensity. You may provide more than one.

In the case of this example, by switching the laser to be oscillated by the laser diode and 33, there is an effect that the characteristics of the minute displacement meter can be instantaneously changed, which is not available in the first and second embodiments. Furthermore, by modulating the laser diodes 1 and 33 at sufficiently fast but different frequencies, or by lighting them alternately at high speed at the same frequency, the micro-displacement meter can have both high resolution and a wide measurement range. It is also possible to have them at the same time.

〔Effect of the invention〕

As mentioned above, in the optical micro-displacement meter according to the present invention, the resolution and measurement range can be arbitrarily set according to the measurement target and measurement purpose with one micro-displacement meter, so multiple micro-displacement meters can be prepared for each purpose. This has the important practical advantage of eliminating the need for multiple measurements, allowing measurements of the same object at different ranges almost simultaneously, and being applicable to in-process measurements.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the optical system of the first embodiment of the optical micro-displacement meter with zero exposure, Fig. 2 is a perspective view of the luminous flux limiting diaphragm of the first embodiment, and Fig. 3 is a diagram of the first embodiment. A block diagram of the signal processing system, Fig. 4 is a diagram showing the displacement-output relationship according to NA, Fig. 6
7 and 7 are perspective views showing other examples of the luminous flux limiting diaphragm, respectively, FIGS. 8 and 9 are views showing optical systems of the second and third embodiments, respectively, and FIGS. 10 and 11 The figures are diagrams showing the state of light reception on a conventional optical system and a photodiode, respectively. 1... Laser diode, 2... Collimator lens, 3... Polarizing beam splitter, 4... λ
/4 plate, 5...objective lens, 6...sample, 7...
...beam splitter, 8,9...critical angle prism, 10.11.12.13...photodiode, 14...luminous flux limiting aperture, '''' Figure 2: Figures 1-4 Figure 16 Off Figure 8 Figure 9 Figure 10 Figure 11 Procedural Amendment (Method) 6 September 13, 1985

Claims (1)

[Claims] An optical micro-displacement meter comprising means for changing NA behind an objective lens.