JP5545987B2 - Optical interference measuring device and shape measuring device - Google Patents

Description

  The present invention relates to an optical interference measurement device and a shape measurement device, and more particularly to reduction of interference noise due to multiple reflected light.

  Conventionally, a probe with a sharp tip (also called a stylus) is brought into contact with the measurement object or scanned while periodically contacting the object to observe the surface shape of the measurement object with high spatial resolution, Many devices capable of measuring the shape with high accuracy are known. For example, there are surface roughness measuring machines that quantitatively measure the surface roughness of machined surfaces and scanning probe microscopes such as an atomic force microscope (AFM) that magnifies and observes surface irregularities at the atomic level. There are various measuring devices and microscopic devices according to the desired spatial resolution.

  There has been reported a method for directly measuring the displacement of a probe with a light wave interference displacement meter in a scanning probe microscope (see, for example, Patent Documents 1 and 2). This method will be briefly described with reference to FIG. FIG. 1 is a configuration diagram showing a main part of a conventional fine shape measuring apparatus 10. The measuring apparatus 10 includes a reference mirror 12 that is held so that the position and orientation of the workpiece 20 are not changed, a probe 14 that is displaced in the vertical direction according to the unevenness while scanning the surface of the workpiece 20, and the probe. A displacement meter for measuring the amount of displacement of the needle 14 and scanning means for scanning the probe 14 along the surface of the workpiece 20 are provided.

The position and posture of the reference mirror 12 with respect to the workpiece 20 do not change even when the probe 14 is scanned. Further, a reflecting surface 36 facing the reference mirror 12 is formed on the probe. A part of the laser from the laser light source 70 passes through the reference mirror 12 and is irradiated on the reflecting surface 36 of the probe. The measurement light 56 from the reflecting surface 36 interferes with the reference light 54 reflected from the reference mirror 12. The displacement meter has detection means 30 that detects interference light between the measurement light 56 and the reference light 54 (hereinafter, sometimes referred to as an interference signal for convenience), and the displacement meter has a probe 14 based on the detected interference light. This is a so-called light wave interferometer that acquires displacement in the vertical direction.
As described above, there has been known a method of detecting the amount of displacement of the probe 14 that is displaced according to the shape based on the position of the reference mirror 12 whose relative position with respect to the workpiece 20 is not changed, as a reference.

JP 2008-76221 A JP 2008-51602 A

<Multiple reflected light>
In the conventional fine shape measuring apparatus 100 described in Patent Documents 1 and 2, in order to accurately measure the displacement of the probe 14 from the detected interference light, an interference signal that is the interference light of the measurement light 56 and the reference light 54 is used. The S / N ratio needs to be sufficiently high. However, the reference mirror 12 and the reflecting surface 36 of the stylus 14 are arranged on a straight line. That is, the optical axis of the measurement light formed between the reference mirror 12 and the reflecting surface 36 of the stylus 14 always coincides with the movement axis of the stylus 14 in the displacement direction. For this reason, there is a problem that the multiple reflected light 75 generated between the reference mirror 12 and the stylus 14 becomes interference noise and enters the detection means 30 to reduce the S / N ratio of the interference signal. Details of the generation of interference noise will be described later.
Such a problem is not limited to the fine shape measuring apparatus 100 of FIG. 1, but is a problem common to the case where a light wave interference displacement meter is used in a conventional surface roughness measuring machine, a scanning probe microscope, or the like.

  The present invention has been made in view of the above-described prior art, and a problem to be solved is an optical interference capable of performing highly accurate optical interference measurement without reducing the S / N of an interference signal necessary for measurement. The object is to provide a measuring device and a shape measuring device.

In order to solve the above problem, an optical interference measuring apparatus according to claim 1 of the present invention is provided.
It has a first surface and a second surface that face each other, and a part of transmitted light that is transmitted through the first surface and incident at a right angle on the second surface is reflected by the second surface to become reference light. And a reference mirror that emits the transmitted light that does not reflect from the second surface;
A variable reflector that is provided in the traveling direction of the transmitted light emitted from the second surface, is displacable along the traveling direction, and reflects the transmitted light in the original direction;
Of the reflected light from the variable reflector, the light that passes through the reference mirror is used as measurement light, and the measurement light and the reference light traveling in the same direction from the first surface of the reference mirror are caused to interfere with each other, thereby the interference light. Interference light detecting means for detecting,
Displacement amount acquisition means for acquiring a displacement amount of the variable reflector with respect to the reference mirror based on detected interference light, and
Furthermore, it comprises a light attenuating means disposed on the optical axis between the reference mirror and the variable reflector,
The light attenuating means attenuates light reciprocating between the second surface of the reference mirror and the variable reflector, and detects the detection interference based on multiple reflected light generated between the reference mirror and the variable reflector. It is characterized by reducing interference noise in light.

The shape measuring apparatus according to claim 2 of the present invention acquires a displacement amount of a probe that is displaced in a predetermined direction in accordance with a surface shape of a workpiece, and measures the shape of the unevenness of the workpiece surface from the acquired displacement amount. A device,
It has a first surface and a second surface that face each other, and a part of transmitted light that is transmitted through the first surface and incident at a right angle on the second surface is reflected by the second surface to become reference light. And a reference mirror that emits the non-reflected transmitted light from the second surface toward the probe;
A variable reflector that is displaceable integrally with the probe and reflects the transmitted light from the reference mirror in the original direction;
Of the reflected light from the variable reflector, the light that passes through the reference mirror is used as measurement light, and the measurement light and the reference light traveling in the same direction from the first surface of the reference mirror are caused to interfere with each other, thereby the interference light. Interference light detecting means for detecting,
Displacement amount acquisition means for acquiring a displacement amount of the probe with respect to the reference mirror based on detected interference light, and
Furthermore, it comprises a light attenuating means disposed on the optical axis between the reference mirror and the variable reflector,
The light attenuating means attenuates light reciprocating between the second surface of the reference mirror and the variable reflector, and detects the detection interference based on multiple reflected light generated between the reference mirror and the variable reflector. It is characterized by reducing interference noise in light.

Here, it is preferable that the variable reflector includes a reflecting surface formed on a surface of the cantilever having the probe, and the reflectance of the reflecting surface is a reflectance of the material of the cantilever. Alternatively, it is preferable that the reflectance of the reflecting surface of the variable reflector is smaller than the reflectance of the material of the cantilever due to the reduced reflection layer applied to the reflecting surface.
The reflectance of the reference mirror is preferably the reflectance of the material of the reference mirror.

Furthermore, it is preferable to set the transmittance of the light attenuating means in a range of 20% to 40%. More preferably, the transmittance of the light attenuating means is 30%.
In addition, it is preferable that incident light incident at a right angle on the first surface of the reference mirror is a laser, and the light attenuating means attenuates light having the same wavelength band as that of the laser. Furthermore, it is preferable to observe the observation image using the same optical path between the reference mirror and the displacement reflector.

  According to the configuration of the optical interference measuring apparatus according to the present invention, even if multiple reflected light is generated between the reference mirror and the variable reflector by arranging the light attenuating means between the reference mirror and the variable reflector. The generated multiple reflected light is reduced by the light attenuating means, and the interference noise included in the interference signal is attenuated. Accordingly, highly accurate optical interference measurement can be performed without reducing the S / N of the interference signal necessary for the measurement.

Since the measurement light makes one round trip between the reference mirror and the variable reflector, it passes through the light attenuating means twice. On the other hand, the cause of the generation of the multiple reflected light is the reflected light of the measurement light that is reflected by the reference mirror without being transmitted through the reference mirror, and travels toward the probe again without passing through the reference mirror. Including one round trip of the measurement light described above, the first-order multiple reflected light makes two round trips between the reference mirror and the variable reflector, and passes through the light attenuating means four times. The second-order multiple reflected light passes through the light attenuating means 6 times, the third-order multiple reflected light 8 times, and the n-order multiple reflected light 2 (n + 1) times. Here, attention is focused on the first-order multiple reflected light that has the greatest influence on the interference signal.
In this way, since the multiple reflected light passes through the light attenuating means four times with respect to the measurement light twice, the multiple reflected light is reduced at a larger rate than the measurement light, resulting in an interference signal. The interference noise contained in can be attenuated.

It is a block diagram which shows the principal part of the conventional fine shape measuring apparatus. It is a whole lineblock diagram showing the fine shape measuring device concerning a first embodiment of the present invention. It is a block diagram which shows the principal part of the said fine shape measuring apparatus. It is a graph explaining the influence of multiple reflected light. It is a characteristic curve figure explaining the performance characteristic of the said fine shape measuring apparatus. It is a whole block diagram which shows the fine shape measuring apparatus concerning 2nd embodiment of this invention.

Preferred embodiments of the present invention will be described below with reference to the drawings.
First Embodiment FIG. 2 shows the overall configuration of a fine shape measuring apparatus according to a first embodiment of the present invention.
The fine shape measuring apparatus 10 acquires the displacement amount of the probe 14 that is displaced in a predetermined direction according to the surface shape of the workpiece (measurement object) 20, and measures the fine uneven shape of the workpiece surface from the acquired displacement amount. The apparatus is characterized in that the amount of displacement is acquired by detecting the interference light that changes in accordance with the displacement of the probe 14.
That is, the measuring apparatus 10 is disposed so as to cover the probe 14 having a cantilever 21 that holds the probe 14 at the distal end so as to be displaceable in the Z-axis direction, and a first surface 12A and a second surface 12B that face each other. The reference mirror 12, the interference optical system 28 having the laser light source 70, the detection means 30 for detecting the interference light from the interference optical system 28, and the cantilever for moving the probe 14 along the work surface along with the cantilever 21. The scanning means 18 which scans the workpiece | work surface by moving 21 to an XY direction, the Z direction drive means 42 which moves the cantilever 21 to a Z direction, and the controller 44 as a control means are provided. Hereinafter, each configuration will be specifically described.

  The probe 14 is provided at the free end of the cantilever 21 having flexibility. The base end of the cantilever 21 is held by the Z-axis drive means 42 and is movable in the Z direction. The Z-axis driving means 42 is held by the scanning means 18 and can move in the XY directions. The probe 14 can trace the surface irregularities in the Z direction while scanning the workpiece 20 by the cantilever 21, the Z-axis drive means 42, and the scanning means 18. Further, the workpiece surface is scanned in a state where the displacement axis 38 indicating the displacement locus in the Z direction of the tip of the probe 14 and the measurement axis 40 formed by the interference optical system 28 and the reference mirror 12 coincide with each other. Thus, the displacement amount of the probe 14 can be acquired with high accuracy. The same effect can be obtained even if the scanning unit 18 is a scanning stage that moves the workpiece 21 in the XY directions.

  The interference optical system 28 includes a laser light source 70 as light emitting means and a beam splitter 34. The laser light source 70 emits coherent light (laser) toward the beam splitter 34. The beam splitter 34 is a prism type, and has a joint surface that transmits the laser from the laser light source 70 and can reflect incident light from the opposite direction in a 45-degree direction.

  The reference mirror 12 is disposed between the interference optical system 28 and the probe 14 and is irradiated with a laser from the beam splitter 34. The reference mirror 12 is used as a reference for acquiring the displacement amount of the probe 14 and is also a light splitting unit that splits the laser from the beam splitter 34. Here, a planar beam splitter having a first surface 12A and a second surface 12B is used. The laser beam from the prism type beam splitter 34 is incident on the first surface 12A. A part of the laser which is transmitted through the reference mirror from the first surface 12A and is incident on the second surface 12B at a right angle reflects the second surface 12B to become reference light 54. The reference light 54 travels in a direction returning from the first surface 12A toward the beam splitter 34. Further, the laser emitted from the second surface to the outside of the reference mirror 12 without reflecting the second surface 12B travels toward the probe 14. The size of the reference mirror 12 is approximately the same as the measurement area of the workpiece 20. The reference mirror 12 is held so that the position and posture of the workpiece 20 do not change during scanning. In the present embodiment, the workpiece 20 is provided on the base 22, and a holding member (not shown) is fixed to the base 22. The reference mirror 12 is held by this holding member, and its position and posture are fixed with respect to the workpiece 20.

The laser transmitted through the reference mirror 12 travels along the measurement axis 40 and irradiates the probe 14 arranged on the measurement axis 40. On the surface of the probe 14, a reflection surface (specific part) 36 that reflects the laser in the original direction is formed. The normal direction of the reflecting surface 36 coincides with the measurement axis 40. That is, the laser beam reflected by the reflecting surface 36 of the probe 14 travels again toward the reference mirror 12 along the measurement axis 40. The reflected light from the reflecting surface 36 passes through the reference mirror 12 and irradiates the beam splitter 34. The reflected light from the reflecting surface 36 is used as the measurement light 56.
The beam splitter 34 reflects the measurement light 56 and the reference light 54 described above toward the detection unit 30. The detecting unit 30 detects the interference light between the measurement light 56 and the reference light 54 and outputs the intensity information of the detected interference light to the controller 44.

  Instead of the reflecting surface 36 formed on the probe 14, a reflecting surface formed on the central axis of the probe 14 on the back surface of the cantilever 21 in a horizontal state may be used. In addition, as the specific portion that reflects the laser, a laser reflector formed separately from the probe 14 may be used instead of the reflection surface 36 described above. If this laser reflector is attached to the free end of the probe 14 or the cantilever 21 and the laser reflector is displaced integrally with the probe 14, the reflective surface 36 can be substituted.

  In the present embodiment, the beam splitter 34, the reference mirror 12, the reflecting surface 36 of the probe 14, and the tip of the probe 14 are arranged in a straight line.

  The controller 44 includes a displacement amount acquisition unit 45 that acquires the displacement amount of the probe 14 in the Z direction based on the intensity information of the detected interference light from the detection unit 30, an XY axis drive circuit 46 that drives the scanning unit 18, and A Z-axis drive circuit 48 for driving the Z-axis drive means 42 and an analysis means 50 are provided. The Z-axis drive circuit 48 drives the Z-axis drive unit 42 based on the displacement information in the Z direction from the displacement amount acquisition unit 45 and outputs the displacement information to the analysis unit 50. The analysis means 50 is a computer that analyzes the uneven shape of the workpiece surface based on the coordinate information of the probe 14 in the X and Y directions and the displacement information in the Z direction.

The fine shape measuring apparatus 10 according to the present embodiment is configured as described above, and the operation thereof will be described below. In the following, a case will be described in which the probe 14 is scanned in the X-axis direction to measure the displacement in the Z direction.
In FIG. 2, while scanning, the displacement axis 38 of the probe 14 and the measurement axis 40 that is the optical path of the measurement light 56 are always matched, while tracing the surface of the workpiece 20 with the probe 14 of the cantilever 21. The Z displacement amount of the reflection surface 36 of the probe 14 is measured.

  FIG. 3 schematically shows the paths of the measurement light 54 and the reference light 56. The figure is an enlarged view of the interference light measurement portion of the measurement apparatus of FIG. In FIG. 3, a laser (coherent light) 52 emitted from a laser light source 70 is converted into a parallel light beam having an appropriate diameter by a collimator lens 72. A part of the laser beam that has passed through the beam splitter 34 passes through the first surface 12A of the reference mirror 12 and is reflected by the second surface 12B to become reference light 54. The reference light 54 is linearly polarized light having a predetermined polarization plane.

  Further, at least a part of the remaining laser 52 from the beam splitter 34 goes out of the reference mirror 12 without being reflected by the second surface 12B after passing through the reference mirror 12. The laser 52 that has passed through the reference mirror 12 passes through the λ / 4 wavelength plate 74 and becomes circularly polarized light, and is condensed on the reflection surface 36 of the probe 14 by the condenser lens 60. The circularly polarized light reflected by the reflecting surface 36 is transmitted again through the λ / 4 wavelength plate 74 and becomes linearly polarized light. Thus, the laser that has passed through the λ / 4 wavelength plate 74 twice becomes linearly polarized light having a polarization plane orthogonal to the reference light 54 and is used as the measurement light 56. The measurement light 56 passes through the reference mirror 12 and thus overlaps the reference light 54 to become interference light. This interference light is reflected by the beam splitter 34 and observed by the detection means 30. In the present embodiment, this interference light is referred to as an interference signal 58.

  The interference signal 58 indicates a change in interference intensity (brightness) according to the Z displacement of the reflecting surface 36 of the probe 14. An interference signal 58 having such interference intensity information (brightness / darkness information) is photoelectrically converted by the detection means 30. For this reason, the output of the detection means 30 changes according to the Z displacement of the reflecting surface 36 of the probe 14, that is, according to the change of the interference intensity. As a result, the displacement amount acquisition unit 45 acquires the distance between the reference mirror 12 and the reflection surface 36 by analyzing the interference intensity change included in the output change of the detection unit 30, and the reflection surface of the probe 14. A Z displacement of 36 can be calculated.

Here, in the present embodiment, the displacement observation position is set so that the Z displacement of the reflection surface 36 of the probe 14 is observed from directly above the reflection surface 36 of the probe 14 during scanning of the cantilever 21. It is not fixed with respect to 20, but moves according to the scanning of the probe 14 in the X direction (lateral direction). In the present embodiment, the probe 14 and the beam splitter 34 are scanned while the workpiece 20 and the reference mirror 12 are stationary. Even by such scanning, the displacement axis 38 and the measurement axis 40 are always one. I'm doing it.
As a result, in the present embodiment, the displacement of the reflecting surface 36 of the probe 14 can be measured from directly above at the time of scanning. For this reason, in this embodiment, since the occurrence of cosine errors can be greatly reduced, highly accurate displacement measurement of the probe 14 can be performed.

In the present embodiment, the reference mirror 12 having the same size as the measurement area of the workpiece 20 is fixedly installed on the workpiece 20. The Z displacement of the reflecting surface 36 of the probe 14 is measured based on the reference mirror 12 whose position and posture are fixed with respect to the workpiece 20.
As a result, in this embodiment, even if a motion error occurs during scanning of the cantilever 21, if the probe 14 is in contact with the surface of the workpiece 20, the cantilever 21 is only bent. Thus, the motion error can be prevented from being superimposed on the measurement result of the workpiece 20. For this reason, in this embodiment, the highly accurate displacement measurement of the probe 14 can be performed.

  Based on the relative displacement amount of the probe 14 in the Z direction and the feed amount in the X direction from the scanning unit 18 thus obtained, the surface shape of the workpiece 20 is obtained as a relative shape with respect to the reference mirror 12. Therefore, the fine shape on the surface of the workpiece 20 can be measured with high accuracy.

<About multiple reflected light>
On the other hand, in order to accurately measure the displacement of the probe 14 from the interference signal 58, the S / N ratio of the interference signal 58 needs to be sufficiently high. Further, as described above, there is an advantage that the displacement axis 38 and the measurement axis 40 always coincide with each other and the cosine error can be reduced. For this reason, each optical element is installed on a straight line. This optical element indicates the beam splitter 34, the reference mirror 12, and the reflecting surface 36 of the probe 14. Therefore, the reflected light generated between the second surface 12B of the reference mirror 12 and the reflecting surface 36 of the probe 14 becomes noise and enters the detecting means 30, and there is a problem that the S / N ratio of the interference signal is lowered. there were. This will be described in detail with reference to FIG. 1 showing a conventional apparatus.

  In FIG. 1, a part of the laser (measurement light 56) reflected by the reflecting surface 36 is reflected by the reference mirror 12, followed by the same optical path, and again reflected by the reflecting surface 36, thereby becoming multiple reflected light 75. When the multiple reflected light 75 passes through the reference mirror 12 and travels toward the beam splitter 34 side, it is detected by the detection means 30 together with the reference light 54 and the measurement light 56. Since the multiple reflected light 75 interferes with the reference light 54 having the same polarization plane, the interference light becomes interference noise and degrades the interference signal 58.

In the apparatus configuration of FIG. 1, the intensity of the multiple reflected light 75 is calculated. In order to increase the contrast of the interference signal 58 in the interference measurement, it is necessary to make the intensities of the reference light 54 and the measurement light 56 equal.
The reference light intensity _{I R} is determined by the reflectivity _{R ref} of the reference mirror 12. When the amount of incident light from the beam splitter 34 to the first surface 12A of the reference mirror 12 is normalized as 1, the reference light intensity can be expressed as I _R = R _ref . Next, the measurement light intensity I _M is the intensity of the laser after the light transmitted through the reference mirror 12 is reflected by the reflecting surface 36 and again transmitted through the reference mirror 12, and the reflectance of the reflecting surface 36 is denoted by R _C. Then, the following equation is obtained.

For the sake of simplicity, assuming that the reflectance of the reflecting surface 36 of the probe 14 is R _C = 1 and the reference light intensity I _R is equal to the measured light intensity I _M , I _R = I _M is given by the following equation: expressed.

From this equation, R _ref = 0.38 (<1) is obtained, so that the maximum contrast is obtained when the reflectance of the reference mirror is 38%.
The interference signal amplitude A is expressed by the following equation.

Here, under conditions that the reflectance of the reflection surface _{36 100% (R C = 1} ) and then, the reflectivity of the reference mirror to _{38% (R ref = 0.38)} , calculated intensity _{I N} of the multiple reflected light 75 To do.
As shown in FIG. 1, the multiple reflected light 75 is reflected by the reflecting surface 36 of the laser 52 that has passed through the reference mirror 12, reflected by the reference mirror 12, reflected again by the reflecting surface 36, and then transmitted through the reference mirror 12. Light. That is, since the multiple reflected light 75 is light that has reciprocated between the reference mirror 12 and the reflecting surface 36 only once more than the measuring light 56, the intensity thereof is expressed by the following equation.

The multiple reflected light 75 passes through the reference mirror 12 and then interferes with the reference light 54 having the same polarization plane to become interference noise. The interference intensity amplitude A _N of the interference noise expressed by the following equation.

These interference intensity amplitude A, with _{A N,} and calculates the ratio _{N ratio} of the interference noise in the interference signal 58. The ratio N _{ratio of the} interference noise is a condition in which the reflectance of the reference mirror 12 is R _ref = 0.38 and the reflectance of the reflecting surface 36 is R _C = 1 in order to maximize the contrast of the interference signal 58 as described above. Then it becomes as follows.

As a result, the magnitude of the interference signal amplitude A _N of the interference noise, it can be seen that even reach 61.6% of the interference signal amplitude A which is originally required. FIG. 4 shows a simulation result of the interference signal and the interference noise signal. In the figure, the amplitudes of the interference signal (waveform indicated by the fundamental wave) and the interference noise signal (waveform indicated by the double wave) necessary for the measurement are shown using the ratio (61.6%) obtained above.
The optical path difference between the measurement light 56 and the reference light 54 that forms an interference signal corresponds to the optical path length for one round trip between the reference mirror 12 and the reflecting surface 36. On the other hand, the optical path difference between the multiple reflected light 75 and the reference light 54 that forms interference noise corresponds to the optical path length for two round trips. Therefore, the period of interference noise is twice the frequency of the interference signal required for measurement. The fundamental wave and the harmonic wave are detected by the detection means 30 as a combined wave in the figure in which these are superimposed. It has been difficult to make accurate measurements from such interference signals.

<Configuration with an attenuation filter between the reference mirror and the cantilever>
What is characteristic of the present invention is that an attenuation filter is provided on the measurement axis line between the reference mirror 12 and the cantilever 21, as shown in FIG. 3, so that interference noise based on multiple reflected light can be reduced. It is. The attenuation filter 76 limits the amount of transmitted light and corresponds to the light attenuation means of the present invention. Usually, it is called an ND filter (neutral density filter).

The attenuation filter 76 may be installed at any position between the reference mirror 12 and the reflecting surface 36 of the probe 14. In the present embodiment, the attenuation filter 76 is disposed between the λ / 4 wavelength plate 74 and the condenser lens 60. When the transmittance of the attenuation filter 76 and T _ND, for measuring light 56 as shown in FIG. 3 is transmitted through the attenuation filter 76 twice, the light quantity is attenuated to a rate equivalent to 2 Article transmittance (T _{ND ²⁾} To do. However, since the multiple reflected light 75 is transmitted through the attenuation filter 76 four times, the amount of light is attenuated to a ratio corresponding to ^four transmittances (T _ND ⁴ ). Compared with the measurement light 56, the attenuation amount of the multiple reflected light 75 is very large.
The intensity I _M of the measurement light 56 when the attenuation filter 76 is installed is expressed by the following equation.

Further, the interference signal amplitude A is expressed by the following equation as described above.

The multi-reflected light 75 passes through the reference mirror 12, passes through the attenuation filter 76, is reflected by the reflection surface 36, and passes through the attenuation filter 76 again. Thereafter, the light that has passed through the attenuation filter 76 twice is further reflected by the second surface 12 </ b> B of the reference mirror 12, passes through the attenuation filter 76, and is reflected again by the reflection surface 36. The reflected light from the reflecting surface 36 passes through the attenuation filter 76 and passes through the reference mirror 12. Thus, since the multiple reflected light 75 is light that has passed through the attenuation filter 76 four times, its intensity _IN is expressed by the following equation.

The multiple reflected light 75 interferes with the reference light 54 having the same polarization plane and becomes interference noise. Interference intensity amplitude A _N of the interference noise, the following equation.

Accordingly, the ratio N _{ratio of the} interference noise included in the interference signal 58 when the attenuation filter 76 is installed is as follows.

As in the case without the attenuation filter 76, in order to maximize the contrast of the interference signal 58, the reflectance of the reference mirror 12 is R _ref = 0.38, and the reflectance of the reflecting surface 36 is R _C = 1. When the condition of transmittance T _ND = 0.5 of the installed attenuation filter is added, the above-described interference noise ratio N _ratio becomes the following value.

The signal amplitude A _N of the interference noise corresponds to 30.8% of the originally required interference signal amplitude A, and it can be seen that the interference noise signal amplitude A _N is greatly reduced compared to 61.8% when the attenuation filter 76 is not provided. .
Normally, when the attenuation filter 76 is arranged on the optical path of the measurement light, even the measurement light 56 is attenuated. Therefore, it is a general idea to avoid the arrangement of the attenuation filter 76 on the optical path of the measurement light as much as possible. there were.

According to the present embodiment, the attenuating filter 76 is intentionally arranged on the optical path of the measurement light, so that the multiple reflected light that causes interference noise can be greatly attenuated. The influence can be reduced.
The attenuation filter 76 can be freely selected to some extent from commercially available ones. According to the above equation, when the transmittance T _ND of the attenuation filter is reduced, the interference noise ratio N _ratio is also reduced. However, as will be described later, when the condenser lens 60 also serves as an objective lens for magnifying observation of the object 20, and the interference optical system and the observation optical system are formed on the same axis, the illumination light of the object 20 Therefore, it is desirable to increase the transmittance T _ND of the attenuation filter as much as possible.

<Setting the reflectance of the reflective surface of the probe>
Further, according to the above equation, it can be seen that the multiple reflected light 75 can be reduced by changing the reflectance _RC of the reflecting surface 36 of the probe 14.
The cantilever 21 is generally formed of silicon (Si). When the cantilever 21 with the probe 14 formed integrally is used, the reflectance of the reflecting surface 36 is determined by the reflectance of Si and is about 35%. Conventionally, the surface of the cantilever 21 has been coated with aluminum (Al) or gold (Au) in order to increase the reflectance of the reflecting surface 36. The reflectance _RC is increased to about 75 to 90% by the coating.

However, in this embodiment, it is preferable to keep the reflectance _RC as the reflectance of silicon (Si) (about 35%) as the material. The ratio of noise included in the interference signal is calculated using the above equation. When the reflectance of the reference mirror 12 is R _ref = 0.38, the transmittance of the attenuation filter is T _ND = 0.5, and the reflectance of the reflecting surface 36 is R _C = 0.35, the _ratio of noise N _ratio included in the interference signal is as follows: Value.

According to this embodiment, the signal amplitude of interference noise is reduced to 18.2% of the originally required interference signal amplitude. By simply installing the attenuation filter 76, it is possible to further reduce the interference noise as compared with the case where the reflectance of the reflection surface 36 is kept as it is (R _C = 0.9). Further, since it is not necessary to apply the coating for increasing reflection to the cantilever 21, a cost merit can be obtained.

<Reflector reflectivity setting>
Further, according to the above equation, the multiple reflected light 75 can be reduced by changing the reflectance R _ref of the reference mirror 12. That is, interference noise can be further reduced by using the reference mirror 12 having a reflectance R _ref smaller than 38%. For example, if the reflectance of the reference mirror is R _ref = 0.1, the transmittance of the attenuation filter is T _ND = 0.5, and the reflectance of the reflecting surface 36 is R _C = 0.35, then the _ratio of noise N _ratio included in the interference signal is The following values are obtained.

The signal amplitude of the interference noise is reduced to 9.3% of the originally required interference signal amplitude, and the interference noise can be reduced. Here, the lower limit of the reflectance R _ref will be described. The reflectance R _ref of the reference mirror 12 is determined by the reflectance of the material if no reflective coating is applied. The reference mirror 12 is made of a glass material such as synthetic quartz (SiO2) or general BK7 (borosilicate glass). For example, when the reference mirror 12 is formed of synthetic quartz (SiO ₂ ), the reflectance of the synthetic quartz itself is about 4%. If the reflectance of the reference mirror is used as R _ref = 0.04, it is not necessary to apply a reflective coating. Considering the cost merit of the reference mirror 12, the lower limit of the optimum reflectivity R _ref is about 4%.

Further, when it is desired to make the reflectance higher than that of synthetic quartz, an increased reflection coating is performed, and when a lower reflectance is desired, the reflectance R _ref of the reference mirror 12 is set to an arbitrary value to some extent by performing a reduced reflection coating. It is possible to set.
Meanwhile, placing the attenuating filter 76, to reduce the reflectivity R _C of the reflective surface 36 of the probe, or to lower the reflectivity R _ref of the reference mirror 12, if measures were taken, such as, the interference signal 58 Contrast and interference signal strength are reduced. However, if the ratio N _{ratio of} noise included in the interference signal 58 is lowered, the interference signal 58 can be electrically amplified, and the problem of the contrast of the interference signal 58 and the decrease in the interference signal intensity can be avoided.

<Optimal combination of characteristic values of each optical element>
Hereinafter, an optimum combination of optical characteristic values of optical elements (reference mirror 12, attenuation filter 76, probe reflecting surface 36) used in the measuring apparatus 10 of the present invention will be described.
FIG. 5 is a graph showing a change in the ratio of noise when the characteristic values of the optical elements are combined. According to this, it can be seen that when the reflectance R _ref of the reference mirror 12 is lowered, the noise ratio N _ratio becomes smaller. Then, when the reflectance of the reference mirror is fixed to 4% of the reflectance of synthetic quartz that does not require a reflective coating (R _ref = 0.04), the reflectance of the reflecting surface 36 is R _C = 0.35, and the transmittance of the attenuation filter is When T _ND = 0.2 (third curve from the bottom in the figure), the noise ratio N _ratio is 2.4%, when the reflectance is R _C = 0.9 and the transmittance is T _ND = 0.1 (in the figure) The noise ratio N _{ratio in the} second curve from the bottom is 1.9%. That is, although the noise ratio N _ratio is only 0.5% (= 2.4% −1.9%), the transmittance T _ND of the attenuation filter can be changed from 10% to 20%, which is twice as high. When the condenser lens 60 is used as an objective lens for magnifying and observing the object 20 and the illumination light for observation is passed through the reference mirror 12 and the beam splitter 34, it is advantageous that the transmittance _TND is higher.
Regarding the reflectivity _RC of the cantilever reflecting surface 36, the reflectivity (35%) of silicon (Si) as a material may be used without a reflective coating. Coating may be performed to form a reduced reflection layer that further reduces the reflectivity of the silicon substrate. By making the reflectance _RC smaller than 35% of the raw material, the noise ratio N _ratio can be further reduced even if other conditions are the same.

Further, for example, as is clear from comparison between the fourth curve and the fifth curve from the top in FIG. 5, even if the transmittance T _ND is set high, the noise ratio N _ratio can be reduced depending on the setting of the reflectance Rc. I understand that. The fourth curve shows a case where the reflectance _RC is 90% and the transmittance _TND is 20%, and the fifth curve is a reflectance _RC of 35% and the transmittance _TND is 30%. The case is shown. When comparing the noise ratio N _ratio when the reflectance R _ref of the reference mirror 12 is 4% in these curves, the fifth curve has a higher transmittance T _ND (30%). The noise ratio N _ratio can be reduced by 0.25% compared to the fourth curve having a low transmittance T _ND (20%). It can be seen that the same effect is obtained when the second and third curves from the top in FIG. 5 are compared. Further, as the reflectance R _ref of the reference mirror 12 is increased, the effect of reducing the noise ratio N _ratio is increased.
The transmittance T _ND of the attenuation filter 76 is preferably set in the range of 10 to 50%. More preferably, the range is 20 to 40%. When it is desired to reduce the noise ratio, the transmittance T _ND is decreased. Conversely, if it is desired to transmit a large amount of observation light, the transmittance T _ND may be increased within the allowable range of the noise ratio.
As can be seen from FIG. 5, by using the reflectivity 4% of synthetic quartz as it is as the reflectivity R _ref of the reference mirror 12, a reflective coating becomes unnecessary and the cost is reduced. On the other hand, the upper limit of the reflectance R _ref is determined by how much the noise ratio N _ratio is allowed. For example, under the condition of the third curve from the bottom in FIG. 5, if the reflectance R _ref is set to approximately 15% or more, the noise ratio N _ratio exceeds 5%. Therefore, depending on the conditions of the reflectivity R _C and the transmittance T _ND, reflectivity R _ref cities of the reference mirror 12 may be set in the range of 4 to 40%, more preferably, 4 to It should be set within the range of 15%.
Further, as can be seen from FIG. 5, since the selection range of the combination of characteristic values that can reduce the noise ratio is wide, the reflectivity of each optical element depends on the arrangement configuration of the optical elements used in the measuring apparatus 10. And a combination of transmittances can be selected as appropriate.

Second Embodiment FIG. 6 is a sectional view showing a fine shape measuring apparatus with a magnification observation function according to a second embodiment of the present invention.
The fine shape measuring apparatus 110 of the present embodiment has substantially the same configuration as the measuring apparatus 10 shown in FIG. 3 described above, except that an optical system for magnifying observation is newly added, and the attenuation filter 76 is used instead. The difference is that the dichroic filter 77 is disposed. Here, members different from those described above will be described.

<Magnification observation function>
A second beam splitter 78 is installed between the first beam splitter 34 and the detection means 58. The second beam splitter 78 branches the light beam from the first beam splitter 34. That is, a part of the light beam is transmitted and received by the detecting means 58, and the other part of the light beam is reflected by the joint surface. The reflected light of the second beam splitter 78 travels toward the camera 81 for magnified observation. An imaging lens 79, a third beam splitter 80, and a camera 81 are sequentially arranged in the traveling direction of the reflected light.

The camera 81 is installed at the focal position of the imaging lens 79 so that the illuminated workpiece 20 can be enlarged and observed. The magnification is determined by the ratio of the focal lengths of the condenser lens 60 and the imaging lens 79. For example, if the focal length of the condenser lens 60 is 25 mm and the focal length of the imaging lens 79 is 250 mm, the enlargement magnification is 10 times.
The third beam splitter 80 is irradiated with illumination light from the illumination light source 82. The illumination light is reflected in the order of the third beam splitter 80, the second beam splitter 78, and the first beam splitter 34, and the illumination light is introduced into the measurement axis. The illumination light source 82 may be a white light source using a general halogen lamp. The illumination light is introduced into the interference optical system, is condensed by the condenser lens 60, and irradiates the object 20.

  In the measurement apparatus 10 of FIG. 3, the attenuation filter 76 for attenuating the multiple reflected light 75 is arranged in the interference optical system. However, the measurement apparatus of the present embodiment provided with the optical system for enlarged observation of FIG. If the attenuation filter 76 is similarly used for 110, the attenuation filter 76 also attenuates illumination light, and the observation image becomes dark.

<Configuration with a dichroic filter placed between the reference mirror and the cantilever>
What is characteristic in this embodiment is that a dichroic filter 77 for attenuating only the wavelength band of the laser 52 is provided in place of the attenuation filter 76. Since the wavelength band of the multiple reflected light by the laser 52 naturally matches the wavelength band of the laser, the dichroic filter 77 as the light attenuating means attenuates the multiple reflected light and does not attenuate illumination light that is not in the laser wavelength band.
Therefore, according to the present embodiment, interference noise can be reduced as in the above-described embodiment, and the observation image does not become darker than when an attenuation filter is used. The attenuation factor of the dichroic filter 77 can be determined in the same manner as the attenuation filter 76.

The reference mirror of the present invention may be a planar beam splitter in which the two surfaces (12A, 12B) are parallel to each other as shown in the embodiment. For example, the first surface 12A is arranged with respect to the optical axis of the laser. A wedge-shaped beam splitter slightly inclined may be used. If a wedge-shaped reference mirror is used, light from the cantilever 21 side (back side) reflecting the first surface 12A can be escaped, so that the influence of noise caused by the reflected light reflecting the first surface 12A is prevented. can do.
In addition, the present invention can be widely applied to a measuring apparatus that can observe the surface shape of a measurement object with high spatial resolution or measure the surface shape with high accuracy. For example, it is effective for a surface roughness measuring machine, a scanning probe microscope such as an atomic force microscope (AFM) that magnifies and observes surface irregularities at an atomic level, and the like.

10: Fine shape measuring device (optical interference measuring device, shape measuring device)
12: Reference mirror 12A: First surface 12B: Second surface 14: Probe 18: Scanning means 20: Workpiece (measurement object)
21: Cantilever 30: Detection means (interference light detection means)
34: Beam splitter 36: Reflecting surface of the probe (variable reflector)
38: displacement axis 40: measurement axis 45: displacement amount acquisition means 52: laser 54: reference light 56: measurement light 58: interference light (interference signal)
60: condensing lens 70: laser light source 72: collimating lens 74: λ / 4 wavelength plate 75: multiple reflected light 76: attenuation filter (light attenuation means)
77: Dichroic filter (light attenuation means)
78: Second beam splitter 79: Imaging lens 80: Third beam splitter 81: Camera 82: Illumination light source

Claims (5)

A shape measuring device that acquires a displacement amount of a probe that is displaced in a predetermined direction in accordance with a surface shape of a workpiece, and measures an uneven shape of the workpiece surface from the acquired displacement amount,
It has a first surface and a second surface that face each other, and a part of transmitted light that is transmitted through the first surface and incident at a right angle on the second surface is reflected by the second surface to become reference light. And a reference mirror that emits the non-reflected transmitted light from the second surface toward the probe;
A variable reflector that has a reflecting surface that is displaceable integrally with the probe and reflects the transmitted light from the reference mirror in the original direction;
Of the reflected light from the variable reflector, the light that passes through the reference mirror is used as measurement light, and the measurement light and the reference light traveling in the same direction from the first surface of the reference mirror are caused to interfere with each other, thereby the interference light. Interference light detecting means for detecting,
Displacement amount acquisition means for acquiring a displacement amount of the probe with respect to the reference mirror based on detected interference light, and
Furthermore, it comprises a light attenuating means disposed on the optical axis between the reference mirror and the variable reflector,
The light attenuating means attenuates light reciprocating between the second surface of the reference mirror and the variable reflector, and detects the detection interference based on multiple reflected light generated between the reference mirror and the variable reflector. It is one that reduces the interference noise in the light
N _ratio which is the said interference noise is determined according to following formula (1), The shape measuring apparatus characterized by the above-mentioned.
(Formula 1)

( Wherein , R _ref means the reflectance of the reference mirror, T _ND means the transmittance of the reflectance attenuating means, and _RC means the reflectance of the reflecting surface of the variable reflector.) In the shape measuring apparatus according to claim 1,
The reflectance R _ref of the reference mirror is set in the range of 4 to 40%,
The transmittance T _ND of the reflectance attenuating means is set in the range of 10 to 20%,
Furthermore, the reflectance _RC of the reflecting surface of the variable reflector is set in a range of 35 to less than 90%. In the shape measuring apparatus according to claim 1,
The reflectance R _ref of the reference mirror is set in the range of 4 to 40%,
The transmittance T _ND of the reflectance attenuation means is set in the range of 30-40%,
Furthermore, the reflectance _RC of the reflecting surface of the variable reflector is set in a range of 35 to less than 90%.   4. The shape measuring apparatus according to claim 1, wherein the incident light incident at a right angle on the first surface of the reference mirror is a laser, and the light attenuating means emits light in the same wavelength band as the laser. 5. A shape measuring device characterized by attenuation.   5. The shape measuring apparatus according to claim 4, wherein an observation image is observed using the same optical path between the reference mirror and the displacement reflector.

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