JPH0418764B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention is a method of irradiating a laser beam into a minute gap formed between the surface of an object to be measured and the surface of an optical flat, which is a flat transparent body. The present invention relates to a micro-gap measuring device that measures the dimensions of the micro-gap using interference of laser light reflected from an optical flat surface and a surface of an object to be measured.

[Technical Background and Problems of the Invention] A micro-gap measuring device that measures the size of a micro-gap using laser light interference can be used, for example, to measure the gap in which a floating head slider floats in a magnetic disk device, or to perform mirror polishing. It is effective for measuring the roughness of a surface.

FIG. 4 is a diagram for explaining the measurement principle of such a micro gap measuring device. In the same figure, 1
3 is an optical flat, and 3 is an object to be measured. A minute gap 5 is formed between the surface 1a of the optical flat 1 and the surface 3a of the object 3 to be measured. A focused laser beam 111 generated from a laser light source (not shown) enters the optical flat 1 from above as shown in the figure.
, and further passes through a minute gap 5 to irradiate the surface 3a of the object to be measured 3. Then, this incident laser beam 11
1 is reflected by the surface 1a of the optical flat 1 as reflected light 113, and a part is reflected by the surface 3a of the measurement object 3 as reflected light 115. Since the relative phases of the reflected lights 113 and 115 that are reflected in this way change depending on the size of the minute gap 5, their interference light intensity is
As shown in the graph drawn at the top of the figure, if the wavelength of the laser beam is λ, the dimension of the minute gap 5 is λ/
It changes periodically between the bright level A and the dark level B every time it changes by 2. Optical flat surface 1
The wedge-shaped gap angle in the minute gap 5 formed between a and the surface 3a of the measurement object is actually very small, and the reflected light 115 reflected at the surface 3a of the measurement object
A part of the reflected light 11 is repeatedly reflected between the optical flat surface 1a and the measurement object surface 3a, and is reflected each time by the optical flat surface 1a.
Multiple interference with 3. In this case, the relationship between the multiple interference light intensity I of the laser beam and the dimension h of the minute gap 5 is defined by the following equation.

Here, R is the reflectance of the optical flat 1, S is the reflectance of the object to be measured 3, and λ is the wavelength of the laser beam. Therefore, the size of the minute gap 5 can be calculated by measuring the interference light intensity I in the above equation (1).

FIG. 5 is an example of a conventional micro gap measuring device that utilizes the above-mentioned measurement principle. In the figure, a laser beam generated from a linearly polarized laser oscillator 21 passes through a polarizing prism 23 and a polarizing plate 25 and is focused by a condensing lens 27 in a minute gap 5 formed between an optical flat 1 and an object to be measured 3. The laser beam 121 enters the laser beam as a minute spot laser beam 121. The interference light 123 between the reflected light from the optical flat surface 1a of the incident laser beam and the reflected light from the measurement object surface 3a is generated by the condensing lens 27 and the deflecting plate 2.
5, the plane of polarization is rotated by 90 degrees, reflected laterally by the polarizing prism 23, and incident on the laser light intensity detection element 29, where the light intensity is measured. Based on the light intensity thus measured, the dimensions of the minute gap 5 can be calculated from the above equation (1).

By the way, as described above, the interference light 123 that is reflected by the optical flat surface 1a and the measurement object surface 3a and enters the polarizing prism 23 is not completely reflected laterally at the polarizing prism 23; A part of the light is polarized by the polarizing prism 2.
3 and enters the laser oscillator 21 as shown by an arrow 125. As a result, the laser oscillator 21
The laser beam travels back and forth between the laser beam and the minute gap 5, and a beat, or beat, is generated due to relative positional fluctuations between the laser oscillator 21 and the optical flat 1 or the measurement object 3, and the laser beam There is a problem that the output of the light intensity detection element 29 fluctuates. Therefore, for this reason, the conventional apparatus has a problem in that the measurement accuracy of the minute gap 5, especially the measurement accuracy of dynamically changing gaps, is reduced.

[Object of the Invention] The present invention has been made in view of the above, and its purpose is to provide a minute gap measuring device that can measure minute gaps with high precision.

[Summary of the Invention] In order to achieve the above object, a laser light source, a polarizing prism, a polarization plane rotating means, and a condensing lens are sequentially arranged with their optical axes aligned in a straight line, and the laser light is reflected by the polarizing prism. the laser beam emitted from the laser light source is passed through the polarizing prism and the polarization plane rotation means in this order, and is focused by the condensing lens to form an optical flat. The laser beam is irradiated into a minute gap formed between the surface of the optical flat and the surface of the object to be measured, and the reflected light of the laser beam from the optical flat surface and the reflected light of the laser beam from the surface of the object to be measured are mutually combined. In the micro-gap measuring device that measures the dimensions of the micro-gap by causing interference and detecting the intensity of the interference light by the detection means, a beam diameter and a beam diameter of the laser light are provided between the laser light source and the polarizing prism. A diaphragm having approximately equal apertures is provided, and a straight line on which the laser light source, polarizing prism, polarization plane rotating means, and condensing lens are arranged is inclined at a predetermined angle with respect to the normal to the surface of the optical flat. The gist is:

[Embodiments of the Invention] Examples of the invention will be described below with reference to the drawings.

FIG. 1 is a configuration diagram of a micro gap measuring device according to an embodiment of the present invention. The microgap measuring device shown in the figure measures the microgap 5 between the optical flat 1 and the object to be measured 3, similar to the conventional device shown in FIG.
In order to measure
7 is not perpendicular to the minute gap 5, that is, the optical flat surface 1a and the measurement object surface 3a, but is slightly inclined. Specifically, the laser oscillator 7 is set so that when the normal line is perpendicular to the optical flat surface 1a, the laser incident light 117 from the laser oscillator 7 is inclined by a small angle θ with respect to the normal line. There is. Therefore, the reflected light 119 of the laser incident light 117 by the optical flat surface 1a is inclined at an angle θ with respect to the normal. Note that since the angle between the optical flat surface 1a and the measurement object surface 3a is very small, the normal line of the optical flat surface 1a is substantially the same as the measurement object surface 3a. It is also a normal line, and as mentioned above, the laser beam set to be inclined with respect to the normal line of the optical flat surface 1a is also inclined at an angle of approximately θ with respect to the normal line of the measurement object surface 3a. It is.

Further, a diaphragm 9 is disposed near the output end of the laser oscillator 7. Note that the distance between this diaphragm 9 and the minute gap 5 is l. The diameter of the aperture 9a formed at the center of the aperture 9 is approximately equivalent to the diameter of the laser beam generated from the laser oscillator 7. The laser beam generated from the laser oscillator 7 passes through the aperture 9a and then passes through the polarizing prism 1.
1. The light passes through the polarizing plate 13 and is focused by the condensing lens 15 to form a tiny spot of laser light 117 on the normal line.
The light enters the optical flat 1 at an incident angle θ inclined with respect to PQ, passes through the optical flat 1, and enters the minute gap 5. A portion of the laser beam 117 incident on the optical flat 1 is reflected by the optical flat surface 1a, and a portion of the laser beam 117 incident on the minute gap 5 is reflected by the measurement object surface 3a. The reflected light from the optical flat surface 1a and the reflected light from the measurement object surface 3a of the incident laser beam 117 interfere with each other and form interference light 119, which is reflected at an angle θ with respect to the normal line, and then passes through the condenser lens 15 and the polarizing plate. 13, most of the light is reflected laterally by the polarizing prism 11, enters the laser light intensity detection element 17, and its light intensity is measured. As a result, as described above, the dimensions of the minute gap 5 can be calculated from the above equation (1) based on the measured light intensity.

By the way, among the interference lights 119, the weak interference lights 119' that are transmitted through the polarization prism 11 without being reflected laterally by the polarization prism 11 are slightly smaller than the aperture 9a of the aperture 9 due to the above-mentioned inclination of the laser light. It reaches a position shifted to , where it is completely cut off. Therefore, this transmitted light 119' does not enter the laser oscillator 7 and go back and forth between the laser oscillator 7 and the minute gap 5 as in the conventional case.
Therefore, there is no beat, that is, beat, as in the conventional method, and stable interference light intensity detection can be performed.

FIG. 2 is a diagram for explaining the influence of tilting the laser beam at an angle θ in the micro gap measuring device shown in FIG. 1 on the measurement accuracy of the micro gap 5. Measurement object 3
The micro gap 5 is enlarged to show in detail the relationship between the laser incident light 117, the reflected light 119a from the optical flat surface 1a, and the reflected light 119b from the measurement object surface 3a.

In FIG. 2, point A is where the laser incident light 117 enters at an incident angle θ and is reflected by the optical flat surface 1a at a reflection angle θ, and the point A is the point where the laser incident light 117 passes through the optical flat 1 and passes through the small gap. 5, is refracted at a refraction angle θ by the refractive index n' of the air in the minute gap 5, and is reflected at the measurement object surface 3a at a point B, and the laser beam 119b reflected at point B is the optical flat 1. The point of incidence on the optical flat 1 is C, the foot of the perpendicular from point C to the reflected light 119a, that is, the intersection of the perpendicular and the reflected light 119a is N, the refractive index of the optical flat 1 is n, and the optical flat surface 1a is If the dimension between the measured object surface 3a and the measured object surface 3a is h, then reflected light 119a from the optical flat surface 1a and reflected light 11 from the measured object surface 3a.
The optical path difference ΔP at the same wavefront position N or C of 9b is expressed by the following equation.

ΔP=n′(+)−n・…(2) Here,,, are the minute gaps 5, respectively.
It is expressed as follows using the dimension h, the incident angle θ, and the refraction angle θ'.

==h/cosθ'...(3) =・sinθ=2h tanθ'・sinθ...(4) Therefore, by substituting equations (3) and (4) into equation (2), the optical path difference ΔP is calculated by the following equation. It is expressed as follows.

ΔP=2n′・hcosθ′ …(5) From this result, to calculate the difference in optical path difference in the case of oblique incidence and the case of normal incidence, we now set the distance between the aperture 9 and the minute gap 5 to 100 mm, and the laser beam The diameter is 1 mm, and the position in the aperture 9 when the reflected light 119 returns to the aperture 9 is the aperture 9a of the aperture 9.
When the position is 3 mm away from the laser beam, the incident angle θ of the laser beam is approximately 1°, and the refraction angle θ' is approximately the same, and the difference in optical path difference is 0.2% or less. Therefore,
It can be seen that there is no problem even if the interference light intensity characteristics with respect to changes in the minute gap 5 are treated as normal incidence.

Figures 3a and 3b show oscilloscope waveforms of the interference light intensity output for a fixed microgap measured by a conventional vertical incidence microgap measurement device and an oblique incidence microgap measurement device shown in this example, respectively. It is. In the figure, the horizontal axis is time;
The vertical axis indicates the light intensity output, A indicates the bright level, and B indicates the dark level. Further, the size of the minute gap to be measured is set to be an intermediate level between the bright level A and the dark level B, which have the highest measurement sensitivity. As can be seen from the figure, it is clear that the beat component is removed better in the present embodiment shown in FIG. 3B than in the conventional method shown in FIG. 3A.

[Effects of the Invention] As explained above, according to the present invention, a laser beam is irradiated into a minute gap at an angle with respect to the normal to the optical flat surface and the surface of the object to be measured,
In addition, since the aperture is provided, the reflected light from the optical flat surface or the surface of the object to be measured is completely blocked from re-entering the laser light source, so that the laser light is not transmitted between the laser light source and the minute gap. The beats, that is, the beats generated by the back and forth movements of the laser beam, are eliminated, and a stable interference light intensity can be detected, thereby improving the measurement accuracy of minute gaps, especially the measurement accuracy of dynamically fluctuating minute gaps.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a micro-gap measuring device showing an embodiment of the present invention, and Fig. 2 is a partial enlargement of the device shown in Fig. 1 to explain the influence of oblique incidence of laser light on the device shown in Fig. 1. Figures 3a and 3b show the oscilloscope waveforms of the interference light intensity output for a fixed microgap measured with a conventional vertical incidence microgap measurement device and an oblique incidence microgap measurement device shown in Fig. 1, respectively. Figure 4 is a diagram for explaining the measurement principle of the micro clearance measuring device,
FIG. 5 is a configuration diagram of a conventional micro-gap measuring device. DESCRIPTION OF SYMBOLS 1... Optical flat, 1a... Optical flat surface, 3... Measured object, 3a... Measured object surface, 5... Minute gap, 7... Laser oscillator, 9... Aperture, 11... Polarizing prism, 13... Polarizing plate, 15...
Condensing lens, 17...Laser light intensity detection element.

Claims (1)

[Claims] 1. A laser light source, a polarizing prism, a polarization plane rotating means, and a condensing lens are sequentially arranged with their optical axes aligned in a straight line, and a detecting means for detecting the light reflected by the polarizing prism. , the laser light emitted from the laser light source is passed through the polarizing prism and the polarization plane rotating means in this order, and is focused by the condensing lens to form a laser beam between the surface of the optical flat and the surface of the object to be measured. The reflected light of the laser light from the optical flat surface and the reflected light of the laser light from the surface of the object to be measured mutually interfere with each other, and the intensity of the interference light is detected as described above. A micro-gap measuring device for measuring the dimensions of the micro-gap by detecting the micro-gap by means of a device, comprising: a diaphragm having an aperture approximately equal to the beam diameter of the laser light between the laser light source and the polarizing prism; A micro-gap measuring device characterized in that a straight line on which a light source, a polarizing prism, a polarizing plane rotating means, and a condensing lens are arranged is inclined at a predetermined angle with respect to the normal to the surface of the optical flat.