JPH03282205A - Thickness measuring apparatus - Google Patents

Abstract

PURPOSE:To eliminate disagreement in focal points due to the thicknesses of samples and to make it possible to measure the thickness highly acurately by arranging two Wollaston prisms in series in the reverse direction to each other, and thereby omitting a convex lens. CONSTITUTION:The output light from a Zeeman laser 4 is split into projecting light 10 and reference light 12 in a first Wollastone prism 8. The light beams becomes the parallel light beams to each other in a reversely dirrected second Wollastone prisim 27. The projecting light 10 is projected on a sample 25, and the reference light 12 is projected on a reference stage 26. The refelcted light beams are returned to the prisim 27 again. The projecting light 10 and the reference light 12 are synthesized and become measuring light 16. The measuring beat signal is inputted into a phase meter 23 through a measuring optical con verter 22. Meanwhile, 4% of the output light is reflected from a planar glass 7 and converted into a reference beat signal in a reference light converter 21. The signal is inputted into the phase meter 23. The thickness of the sample 25 is detected based on the phase difference between both waves.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thickness measuring device for measuring the thickness of highly precisely finished parts.

[Conventional technology]

Conventionally, when measuring the thickness of parts, etc., stylus-type thickness gauges and even photoelectric thickness gauges have been used.
In both cases, precision is insufficient when high-precision measurements are required.

It is also conceivable to measure the thickness by applying a high-precision surface roughness measuring device for different purposes, but there are problems as described below.

Figure 4 shows, for example, the monthly magazine "Obtronics", 1983.
FIG. 1 is an explanatory diagram showing the configuration of a conventional surface roughness measuring device shown in the monthly issue, which utilizes the interference phenomenon of a Zeeman laser. In the figure, (1) is the sample to be measured for roughness, and (2) is the rotating shaft (3) on which the sample is attached.
(2) is a Zeeman laser, and the output light f5] is polarized in two directions x and y perpendicular to the irradiation direction Z, and the frequencies in the two directions are slightly different. Outputs laser light. (7) is a flat glass that reflects about 4% of the output light (51), which is caused by a Wollaston prism whose refractive index differs depending on the polarization direction of the incident light.
The sample irradiation light α0) for irradiating the measurement point (9) on the surface of the sample (1) and the reference point (l) set on the surface on the rotation axis G)
The output light (51) is divided into two parts: the reference light (12) that irradiates the sample irradiation light (12) and the reference light (12) that irradiates the sample
It is a convex lens that makes the 01 and reference light (12) parallel to each other and focuses them on the measurement point (9) and the reference point (11), respectively.

(15) is the measuring point (9) on the sample (1) and the reference point (11).
) is a small mirror that reflects the sample irradiation light α0) and the reference light (12) that pass through the convex lens (14) and the Oraston prism 6, and then reflect the measurement light (16) that is condensed into a single beam. , the optical path after reflection at sample (1) is shown by a broken line in the figure. (17) and (18) are the output light (5)
A reference polarizing filter and a measuring polarizing filter (21), (22) are incident on the reference light (19), which is reflected by about 4% on the flat glass m, and the measurement light (16), which is reflected on the small mirror (15), respectively. ) are the reference light (19) and measurement light (16) that have passed through the reference and measurement polarizing filters.
), respectively, and a reference optical converter and a measuring optical converter generate beat signals from each of them, and (23) is a phase meter connected to the output ends of the reference and measuring optical converters (21) and (22).

Next, the operation will be explained. The output light (5) emitted from the Zeeman laser (2) is polarized in two directions x and y, which are perpendicular to the irradiation direction Z and perpendicular to each other.
It consists of a wave and an S wave, and this is shown schematically by the thick arrow in the vector diagram in Figure 5. Here, the amplitude of the light of the p wave and the S wave is almost the same, and the frequencies are slightly different from each other. It's different. Therefore, the reference light (19) reflected by the flat glass opening is also the same.

The polarization direction r of the reference polarizing filter (17) is preset to the 45 degree direction as shown in FIG.
It is extracted as the output of the P wave and inputted to the reference optical converter (21). Since there is a slight difference in frequency between the P wave and the S wave, a sine wave beat signal is obtained from both waves, and the reference optical converter (21) vessel(
21) is input to the phase meter (23) as a reference beat signal.

On the other hand, most of the output light ((5) passes through the flat glass (7) and enters the Wollaston prism 6, where it is divided into two waves, p-wave and S-wave with different polarization directions, each of which is the sample irradiation light α0). and reference light (12). Both become diffused and enter the convex lens (14), but the convex lens (14)
4), they become parallel to each other, and each sample (1
) is focused on the measurement point (9) and the reference point (11) as minute points. Then, the sample irradiation light aα diffusely reflected by the sample (1) and the reference light (12) are reflected by the convex lens (
14), the light is guided to the Wollaston prism (to), and is combined with each other to form measurement light (16). The measurement light (16) emerging from the Wollaston prism is reflected by a small mirror (15), and converted into the aforementioned reference light (19) by a measurement polarizing filter (I8) and a measurement light converter (22).
P-wave and S-wave beat signals are generated in the same manner as above, and are input to the phase meter (23) as measurement beat signals.

FIG. 6 is a waveform diagram of a beat signal, and A and B are a reference beat signal and a measurement beat signal, respectively.

In Figure 4, if the optical path lengths of the sample irradiation light α0) and the reference light (12) are equal, the relative relationship between their waveforms will be the same as the relative relationship between the p-wave and the S-wave in the reference light (19). Therefore, A and B are in phase. If there is a difference between the optical path lengths, there will be a difference between the two sets of relative relationships, so the beat signals A and B determined by these relative relationships will be different.
A phase difference θ is generated between them, which can be detected as the output of the phase meter (23).The relationship between the phase difference θ and the optical path difference is determined by the wavelength of the laser beam used. This corresponds to 316.4 nanometers.

Considering the case where the rotary table (2) rotates around the rotation axis (3) and the sample (1) also rotates, the reference point (1)
1) is located on the rotation axis (3), so it remains unchanged regardless of rotation, and therefore the optical path length of the reference light (I2) also remains unchanged. On the other hand, since the measurement point (9) moves on the circumference of the surface of the sample (1) as it rotates, the optical path length of the sample irradiation light QOI changes depending on the surface unevenness, and therefore the reference light (12 ) and the waveforms of the sample irradiation light α0) change. Therefore, the phase of the measurement beat signal changes, which can be detected by the phase meter as a change in the phase difference θ in Figure 6, and the sample (1
) surface roughness can be determined.

Applying the above surface roughness measuring device as a thickness measuring device,
For example, consider the case where the thickness of something like a highly precisely machined block gauge is to be measured with high precision. Fourth
In the figure, point (1) is used as a reference stand, and the object to be measured is attached to measurement point (9). Note that the object to be measured is at the reference point (1
1), that is, the reference point (11) should be on the reference stand.

Further, rotation is not necessary.

In the above manner, a difference in optical path length occurs between the sample irradiation light αω and the reference light (12) depending on the thickness of the object to be measured, and therefore, the thickness It also seems possible to measure. However, in reality, a convex lens (14
), the sample irradiation light α [and the reference light (12) are simultaneously focused on the measurement point (9) and the reference point (11), respectively.
Since the distances from the convex lens (14) to both points differ by the thickness of the object to be measured, there is a problem that when the focus of the convex lens (14) is made to match one point, the focus does not match at the other point. . If the focus does not match, the reflected laser beam will be condensed by the convex lens (14) and the Wollaston prism, but it will still produce p-waves and S-waves, that is, the sample irradiation light α [and the reference light (1
2) are not superimposed or combined, so the measuring optical converter (
22), the measurement beat signal cannot be generated.Therefore, a method may be considered in which the convex lens (14) is not provided, but in that case, the sample irradiation light α0) and the reference light (12) are separated by the Oraston prism 8. have an angle of opening to each other, so after being reflected, they are reflected again into the Wollaston prism (
8), resulting in a problem that light detection cannot be performed.

[Problem to be solved by the invention]

Conventional thickness measuring devices, such as stylus and photoelectric thickness gauges, cannot perform highly accurate measurements, and high-precision surface roughness meters for other purposes are configured as described above. When applied as a thickness measurement device, the beat signal for measurement cannot be generated properly due to the mismatch in focus between the sample irradiation light and the reference light and the aperture angle between them, and therefore thickness measurement cannot be performed. There were problems such as:

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide a thickness measuring device that can measure thickness with high accuracy.

[Means to solve the problem]

The thickness measuring device according to the present invention uses a Zeeman laser, and has two Wollaston prisms arranged in series in opposite directions.

[Effect]

The thickness measuring device according to the present invention divides the laser beam emitted from the Zeeman laser into a sample irradiation light and a reference beam using one Wollaston prism, and divides the sample irradiation light and the reference beam, which have an angle of divergence from each other, into the other. parallel to each other using a Wollaston prism.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure is an explanatory diagram showing the configuration of a thickness measuring device according to an embodiment of the present invention, and in the figure, (2), (9, (to),
Since θ, α[, (12) and (15) to (23) are the same as in the case of FIG. 4, their explanation will be omitted. However, 6 shall be referred to as the first Wollaston prism. <
25) is the sample whose thickness is to be measured, and (26) is the reference stand on which the sample (25) is mounted. Both the sample (25) and the reference stand (26) are processed with high precision and have a mirror finish. ing.

(27) is a second Wollaston prism that has the same separation angle of p-wave and S-wave as the first Wollaston prism, and is arranged in series in the opposite direction, (29) is a reference stand. (2
6) and a one-sided stage that can move in the X direction, (
30) is a base that supports the unidirectional stage; (31) is a position sensor that detects the amount of movement of the unidirectional stage (29) with respect to the base (30); (32) is a conversion amplifier for the position sensor (3I); 33) is an X-Y recorder, in which the output of the conversion amplifier (32) is input to the X axis, and the output of the phase meter (23) is input to the Y axis.

Next, the operation will be explained. First, the thickness of the sample (25) is determined with an accuracy of 0.1 to 0.2 microns using an electronic micrometer or a glass scale dimension measuring device (none of which are shown). Measure the thickness with high precision as follows.

The sample (25) is mounted on the reference stand (26), and when the output light (51) is emitted from the Zeeman laser (2), the sample irradiation light 001 and the reference light (51) having an angle of divergence from each other are separated by the first Oraston prism θ. 12).Since the second Wollaston prism (27) is provided in the opposite direction to the first Wollaston prism 6, the sample irradiation light α0
) and reference light (12) are incident, they are emitted parallel to each other. The sample irradiation light αl is applied to the sample (25
), the reference light (12) illuminates the reference stand (26). The material (25) and the reference stand (26) are processed with high precision and have a mirror finish, so the sample irradiation light α0) and the reference light (12)
The beam is reflected as a round beam at the measuring point (9) and the reference point (11) without being diffusely reflected, and returns to the second Oraston prism (27) almost along the outward path as shown by the broken line. . The separation and focusing effects of the first and second Wollaston prisms 6, (27) on the laser beam are symmetrical when the laser beam travels to the right and to the left in the figure. The irradiation light α0) and the reference light (12) are combined to become the measurement light (16), which is then combined with a small mirror (15), a measurement polarizing filter (18), and a measurement light converter ( 22) is processed by the diameter, becomes a measurement beat signal, and is input to the phase meter (23). In addition, in order to prevent the optical axes of the outward and return paths from overlapping, the first and second Wollaston prisms (to), (2
7) may be arranged with a slight inclination to the optical axis.

On the other hand, the reference light (19) also becomes a reference beat signal and is input to the phase meter (23) in the same manner as in the case of FIG. By the way, there is a difference between the optical path length of the sample irradiation light 00) and the optical path length of the reference light (11) depending on the thickness of the sample (25).
Therefore, as explained in FIGS. 4 to 6, a phase difference θ occurs between the reference beat signal and the measurement beat signal, and the DC voltage corresponding to this phase difference θ becomes the output of the phase meter. −
It is input to the Y axis of the Y recorder (33). Furthermore, when the one-way stage (29) is moved in the X direction on the base (30), the measurement point (9) and reference point (11) also move accordingly. The amount of movement of the one-way stage (29) is detected by a position sensor (31), and a DC voltage corresponding to this amount of movement is sent to an X-Y recorder (3) via a conversion amplifier (32).
3) is input to the X axis.

FIG. 2 is an explanatory diagram showing the movement of the measurement point. When the direction stage (29) moves, for example, in the direction of the solid line arrow, the measurement point (9) moves from (9A) to (9B) as shown by the two-dot chain arrow.
), (9C), and the reference point (11) moves from (IIA) to <IIB) and (IIC> as indicated by the - dotted chain arrow. Figure 3 shows the phase difference θ at that time. This is a graph, which changes from θA to θB and θC on the X-Y recorder (33) as shown by the broken line.The thickness t of the sample (25) is determined from the difference θ between θA and θB or θB and θC. The relationship is shown in the following equation.

Here, N is O or a positive integer, and λ is the wavelength of the laser light, which is 632.8 nanometers in the case of helium-neon type. For one value of θ, there are many values of t at intervals of 1/2λ, but the thickness of the sample (25) is set to 0.1 in advance.
Since it is determined with an accuracy within ~0.2 microns, t in the above equation can be set to one value, and the thickness of the sample (25) can be determined with extremely high accuracy. In reality, accuracy within a few nanometers was achieved.

In the above embodiment, the first and second Wollaston prisms f81, (27) are slightly tilted to shift the reciprocating optical path so that the small mirror (15) captures the measurement light (16). Match the optical paths of the measurement light (
16), or a one-way stage (29) may be provided so that the thickness of the sample (25) can be continuously detected in the X direction by the X-Y recorder (33). If the thickness of one or several points on the sample (25) needs to be determined, the one-way stage (29) and the x-y recorder (33) can be omitted.

〔Effect of the invention〕

As described above, according to the present invention, two Oraston prisms are installed in series in opposite directions, eliminating the need for a convex lens, eliminating focal mismatches caused by sample thickness, and achieving high precision. Thickness measurements can be taken.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the configuration of a thickness measuring device according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the movement of measurement points in the thickness measuring device of FIG. 1, and FIG. Figure 4 is an explanatory diagram showing the configuration of a conventional surface roughness measuring device, Figure 5 is a vector diagram showing p-waves and S-waves, and Figure 6 is a standard. It is a graph showing a beat signal and a measured beat signal. In the figure, (2) is the Zeeman laser, [51 is the output light, (8), (27) is the first laser. The second Wollaston prism, α0) is the sample irradiation light, (I2) is the reference light, (I
9) is the reference light, (21) is the reference light converter, (22) is the measurement light converter, (23) is the phase meter, (25) is the sample, (
26) is a reference stand. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] A reference stand on which a sample to be measured for thickness is attached, a Zeeman laser that outputs laser light polarized in two directions, a reference light converter that generates a reference beat signal from the laser light, and a sample irradiation light that irradiates the sample. Two Wollaston prisms arranged in series in opposite directions split the laser beam into a reference beam that irradiates the reference stand, and measurement is performed from the sample irradiation light and the reference beam that are reflected by the sample and the reference stand, respectively. measurement optical converter that generates beat signals for
and a thickness measuring device including a phase meter that measures the phase difference between the reference beat signal and the measurement beat signal.