JPS633205A - Optical thickness measuring instrument - Google Patents

Abstract

PURPOSE:To select measurement resolution and a measurement range optionally by adding a member which varies the numerical aperture of the convergent luminous flux of an objective. CONSTITUTION:A variable diameter stop 20 which varies the numerical aperture NA0 is provided in the objective 4, and consequently even if the distance between the maximum points of on-axis center intensity on the top surface and reverse surface of a body 5 to be inspected becomes less than distance (a), the resolution is improved by increasing the numerical aperture NA0, thereby taking a measurement. Therefore, even when the thickness (t) of the body 5 to be inspect varies variously, it is measured by varying the numerical aperture NA0.

Description

DETAILED DESCRIPTION OF THE INVENTION a. Technical Field The present invention relates to an improvement in an optical thickness measuring device for thin glass.

b. Prior art and its problems There is an invention for which the present applicant filed a patent application on June 20, 1986, as a method for optically measuring thin glass. As shown in FIG. 5, in this method, light from a sufficiently small light source 1 is projected onto a transparent or semi-transparent object 5 using an objective lens 4 having a numerical aperture of NA0, and the condensed light beam is Move the condensing point or the object to be measured in the direction of the optical axis of the objective lens,
The position of the maximum axial central intensity of the condensed light beam on the front and back surfaces of the test object was measured, and the wall thickness was determined by calculation (the details of Figure 5 are explained later in the example of Figure 1). ) is the numerical aperture NA of the condensed light beam,
Since it was fixed, only the measurement resolution and measurement range determined by the numerical aperture NA could be obtained. Therefore, there is room for improvement in order to enable arbitrary measurement resolution and range.

C0 Purpose The present invention was developed in view of the above points, and by adding a member that makes the numerical aperture NA of the condensed light beam of the objective lens variable, it is possible to select any measurement resolution or measurement range. The purpose is to provide an optical thickness measuring device.

d. Effects and Functions of the Embodiment In order to achieve the above objects, an embodiment in which a laser is used as a light source in the wall thickness measuring device of the present invention will be described with reference to FIG.

The light from the linearly polarized laser 1 is expanded by a beam expander 2, passes through a quarter-wave plate 3, becomes circularly polarized light, and is turned into circularly polarized light by an objective lens 4 with a refractive index n.

The light beam is projected onto the object 5 having a wall thickness t as a condensed light beam having a numerical aperture NA. A variable-diameter diaphragm 20 is installed in the objective lens 4 so that the numerical aperture NA can be varied. The reflected light from the test object 5 becomes light whose polarization direction is perpendicular to that of the laser oscillation light, and is guided to the photoelectric conversion element 7 by the polarization beam splitter 6. A diaphragm 8 is installed in front of the photoelectric conversion element 7 for selecting the axial center intensity of the reflected light.

The analog signal from the photoelectric conversion element 7 becomes a digital signal through a sample hold circuit 9 and an A/D conversion circuit 1o, and is input to the microcomputer 11. - On the other hand, when the test object 5 is moved in the optical axis direction of the objective lens 4 using a moving means (not shown), the front surface 5-a and back surface 5-b of the test object 5 are brought into focus, and the photoelectric conversion element 7 is brought into focus. The number of signal engineers will be the maximum. The difference a between the focus positions on the front surface 5-a and the back surface 5-b is read by the length measuring scale 12, and the read signal is sent to the microcomputer 11.
Enter. The microcomputer 11 has a refractive index n2 and a numerical aperture A0.
The wall thickness t of the object to be inspected is calculated from values such as the difference a between the focus positions.

FIG. 2 is a diagram illustrating the calculation of the axial center strength (■) with respect to the out-of-focus Z. As shown in Figure 2-1, the numerical aperture N of the condensed beam is
Assuming A0, the axial center strength is maximum at the focus surface 13 of the front surface 5-a and the focus surface 14 of the back surface 5-b of the test object 5, and the distance between them (difference in focus position), a, is measured. Depending on the situation, the wall thickness t of the test object 5 may be reduced.

However, as the thickness of the object becomes thinner and the distance a gradually decreases (that is, when the focus surface 14 on the back surface approaches the focus surface 13 on the front surface), the distance on the axis of the front and back surfaces of the object becomes smaller. The position of the peak of the center intensity (the position of both focus planes 13 and 14) becomes indistinguishable, and measurement becomes impossible. This greatly depends on the depth of focus of the objective lens 4, that is, the resolution of the focus shift in the Z-axis direction. Now, let us assume that the wavelength of the light projected onto the test object 5 is λ, and the axial center intensity is 80, which is the maximum value.
%, the length in the Z direction of the focus shift is defined as the depth of focus z0, then 2Q can be expressed as λ Zo:±□ 2NAo''.

For example, if λo is 63μ and NAo=0.4, then Z
. If phantom ±2μ, NA = 0.65, then Z0 = ±0.
The depth of focus, which is 8μ, varies greatly depending on the numerical aperture NA0 of the condensed light beam.

Therefore, as shown in Figure 2-2, the distance between the points 15 and 16 of the maximum axial center intensity on the front and back surfaces of the test object is 2-2.
Even if the distance a′ is smaller than the distance a in Figure 1, the numerical aperture N
By increasing A to NAa', the resolution improves and measurement becomes possible.

On the other hand, as shown in Figure 2-3, as the thickness of the object becomes thicker, the spherical aberration that occurs when the condensed light beam passes through the object increases, and the axial central intensity on the back surface becomes more focused. The rate of change with respect to the deviation Z becomes gradual, and the point 18 of the maximum axial center strength
becomes difficult to determine.

Therefore, the maximum point of axial center strength on the front and back surfaces 17.1
8, the measurement reproducibility of the distance a'' deteriorates, and the accuracy of wall thickness measurement decreases.Now, let W be the wavefront aberration due to spherical aberration that occurs when the condensed light beam passes through the test object, then W can be expressed as , the value is as shown in Figure 3.When the magnitude of the wavefront aberration becomes λ, the maximum axial center intensity becomes about 80% of that when there is no aberration, and the wall thickness at this time is set to be thicker. If the wall thickness measurement limit is n≠1.5, NA = 0.6
5+ too, 05nn+, N An” 0, 4
t is 0.47 mm, which varies greatly depending on the numerical aperture NAo. Therefore, even if the thickness of the object to be tested varies, it can be measured by changing the numerical aperture NA.

Furthermore, as shown in Fig. 4, if a zoom expander 19 is installed in the light source optical system to make the spread of the light flux variable, no matter how much the numerical aperture NA of the objective lens is changed, the numerical aperture of the light source side will change accordingly. Since the NA can be changed to NAoo or NAoz, the energy of the light source can be used effectively.
Since the received light energy on the photoelectric detection side can also be adjusted, it is also advantageous for signal processing circuits.

Of course, the objective lens side numerical aperture NA0 and the light source side numerical aperture NA
It goes without saying that the above-mentioned benefits will be even more pronounced if these changes occur in conjunction with each other.

e. Effects As explained above, by using the present invention, it is possible to measure objects with various wall thicknesses with the required resolution,
The scope of application of measurement equipment is expanded.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the axial center intensity on the front and back surfaces of the test object with respect to defocus, and FIG. 3 is a diagram showing the transmitted wavefront aberration W with respect to the wall thickness t.
FIG. 4 is a diagram showing that the zoom expander is installed on the light source side, and FIG. 5 is an explanatory diagram of the conventional installation. 1: Laser 2: Beam expander 3: 1/
4 wavelength plate 4: Objective lens 5: Test object 6: Polarizing beam splitter 7: Photoelectric conversion element 8: Aperture 9: Sample hold circuit 10: A/D conversion circuit 11: Microcomputer 12: ? III
I length scale 19: Zoom expander 20: Variable diameter diaphragm (member that makes the NAo of the condensed light beam variable) Patent applicant Asahi Optical Co., Ltd. Representative Toru Matsumoto Figure 4

Claims (1)

[Claims] 1. Projecting a condensed beam of light from a sufficiently small light source onto a transparent or semi-transparent object using an objective lens with a numerical aperture NA_0, and determining the focal point or object of the condensed beam. Optical thickness measurement that moves in the optical axis direction of the objective lens and measures and calculates the maximum axial center intensity position of each condensed beam on the front and back surfaces of the object to determine the wall thickness of the object. An optical thickness measuring device characterized in that the device further includes a member that changes the size of the numerical aperture NA_0 of the condensed light beam of the objective lens. 2. Optical thickness measurement according to claim 1, wherein the member that changes the size of the numerical aperture NA_0 of the condensed light beam of the objective lens is a variable diameter diaphragm provided within the objective lens. Device. 3. The optical thickness measuring device according to claim 1, characterized in that a zoom expander that makes the spread of the light beam variable is installed in the light source optical system.