JPH084564Y2 - Optical displacement measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a focus error detection method for an optical displacement measuring instrument.

<Prior Art> FIG. 8 is a block diagram of a prior art relating to the present invention, which is filed by the applicant of the present application as Japanese Patent Application No. 01-19699 “Optical displacement measuring device”. The present invention is an improvement of the "optical displacement measuring device" of the above-mentioned application.
Before describing the present invention, this prior application will be outlined.

In FIG. 8, the light emitted from the laser diode 1 is collimated by the collimator lens 2 and reflected by the polarization beam splitter 3. The reflected light passes through the quarter-wave plate 4, is condensed by the objective lens 5, and is irradiated onto the DUT 6. The irradiated light is reflected by the DUT 6, passes through the objective lens 5, the quarter-wave plate 4, and the polarization beam splitter 3, and is split into two by the half mirror 7. The transmitted light is
A photodiode 10 which is condensed by the condenser lens 8 and is provided in front of the focal point of the condenser lens 8 at a position of a distance x from the focal point.
And a displacement signal A corresponding to the displacement of the DUT 6 is output. On the other hand, the reflected light is condensed by the condenser lens 9 and is incident on the photodiode 11 installed at the position of the distance x from the focal point behind the focal point of the condenser lens 9, and the displacement signal B is similarly output. .

Here, since the displacement signals A and B output from the photodiodes 10 and 11 are placed before and after the photodiodes 10 and 11 at the same distance x from the focal point,
As shown in the figure, a signal displaced with respect to the displacement of the DUT 6 is output. The displacement signals A and B include influences such as unevenness of reflectance of the DUT 6 and inclination of the surface. Therefore, by performing a calculation such as (A−B) that takes the difference between the displacement signals A and B, it is possible to obtain a focus error signal in which the influence of the unevenness of the reflectance of the DUT 6 or the inclination of the surface is eliminated. You can

<Problems to be Solved by the Invention> However, in the optical displacement measuring instrument shown in the above-mentioned prior art, the condensing lenses 8 and 9 have the same focal length, and the condensing lenses 8 and 9 have the same focal length before and after the focal length. The rate of change of the displacement signals A and B from the photodiodes 10 and 11 placed at the positions are not equal as shown in FIG. 9, and the difference (A−B) between the displacement signals A and B is taken into focus. The error signal is bent before and after the in-focus point as shown in FIG. Therefore, there is a problem that the gain of the focus servo changes and the servo system becomes unstable.

The present invention has been made in view of the above-mentioned problems of the prior art. By equalizing the change ratios of two displacement signals, it is possible to eliminate the change in gain before and after the focus error signal is focused. The purpose of the present invention is to provide a displacement measuring instrument.

<Means for Solving the Problems> The first configuration of the present invention for solving the above problems is to drive an objective lens in the focus direction to focus a light spot on an object to be measured and to reflect the light intensity of the reflected light. In an optical displacement measuring device for measuring a minute displacement of an object to be measured by measuring a pattern with a photodetector and performing arithmetic processing, an optical element for branching reflected light from the object to be measured into two directions, and the optical element Two condensing lenses that condense each of the two branched lights, and two photo lenses that are arranged at the same distance before and after the focal points of the two condensing lenses and into which the lights condensed by the condensing lenses are respectively incident. A configuration is provided in which a diode and two gain adjustment circuits for receiving the detection signals of the two photodiodes and equalizing the rate of change of the detection signals are provided, and the two gain adjustment circuits are provided. The second configuration is such that the focus error signal is obtained from the difference between the output signals of the two. Different condensing lenses are used, and two photodiodes on which the light condensed by the condensing lenses respectively enter are provided before and after the focus of the two condensing lenses, and the diameter of the photodiode is equal to the spot diameter on the photodiode. The focus error signal is obtained from the difference between the detection signals of the two photodiodes.

<Operation> According to the present invention, the gain of the arithmetic circuit is adjusted so that the change ratios of the output signals of the two photodiodes become equal, and the focal lengths of the two condenser lenses are changed so that The change ratios of the output signals of the two photodiodes are made equal to each other, so that the change of the gain before and after the focusing of the focus error signal can be eliminated.

<Example> Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of signal processing of an optical displacement measuring instrument according to the present invention. The optical system is the same as that of the conventional example shown in FIG.

In FIG. 1, the output signals A and B of the photodiodes 10 and 11 are input to the gain adjusting circuits 12 and 13, respectively,
The gain of G ₁ and G ₂ is adjusted. Here, 12 and 13 are gain adjustment circuits for equalizing the change ratios of the displacement signals of the photodiodes 10 and 11, for example, the change ratios and gain adjustments of the output signals A and B of the photodiodes 10 and 11. The relationships between the gains G ₁ and G ₂ of the circuits 12 and 13 satisfy the following conditions.

G ₁ · dA / dz = G ₂ · dB / dz However, z is the displacement amount of the DUT 6.

Therefore, the output signals G1A, G1 of the gain adjustment circuits 12, 13 are
2B has the same change rate as shown in FIG.
Each is input to the arithmetic circuit 14. By taking the difference (G ₁ A−G ₂ B) between the output signals G ₁ A and G ₂ B of the gain adjusting circuits 12 and 13 in this arithmetic circuit 14, as shown in FIG. Since a focus error signal having no linearity and good linearity is required, it is possible to eliminate a change in the gain of the focus servo.

FIG. 4 is a block diagram of an optical displacement measuring instrument showing a second embodiment of the present invention. In FIG. 4, the same elements as those in FIG. 8 are designated by the same reference numerals, and the duplicated description will be omitted.

In FIG. 4, reference numerals 15 and 16 are condenser lenses having different focal lengths. The photodiode 10 is located in front of the focus of the condenser lens 15 at a position x from the focus, and the photodiode 11 is located behind the focus of the condenser lens 16. They are installed at positions y away from the focal point. The distances x and y are the diameters of the photodiodes 10 and 11 and the photodiodes 10 and 11 respectively.
This is the distance at which the upper spot diameters are equal.

In such a configuration, the operation of the optical system is
It is omitted because it is similar to the device in the figure, but here, assuming that the spot diameters on the photodiodes 10 and 11 are W ₁₀ and W ₁₁ , and the displacement amount of the DUT 6 is z (the rightward direction in the figure is positive). Do),
The focal lengths F ₁₅ and F ₁₆ of the condenser lenses ₁₅ and ₁₆ are expressed as satisfying the following equation.

dW ₁₀ / dz = dW ₁₁ / dz ... Further, as shown in FIG. 11, the DUT 6 is the objective lens 5
When entering z (closer to the objective lens 5) from the focal position of (the position away from the center of the objective lens 5 by the distance F ₅ ),
The light radiated to the DUT 6 is folded back and focused on P ₁ to enter the objective lens 5, and from the objective lens 5 enters the focusing lens 15 as divergent light from the virtual image P ₂ .

Here, the effective diameter of the objective lens 5 is A ₅ , the spot diameter on the objective lens 5 is W ₅ , and the spot diameter on the condenser lens 15 is
W ₁₅ and the spot diameter on the photodiode 10 are W ₁₀ .

Distance l ₁ from the objective lens 5 to the converging position P ₁ is l ₁ = F ₅ -2z, distance l ₂ from the objective lens 5 to the virtual image P _{2 is, l 2 = 1 / (1} / F 5 −1 / l ₁ ) = {F ₅ · (F ₅ −2z)} / 2z.

The distance a ₁₅ from the condenser lens 15 to the virtual image P _{2 has, a 15 = l 2 + D} 15 However, D ₁₅ is the distance between the objective lens 5 and the condenser lens 15, the rear side from the condenser lens 15 Distance to focus position
b _{15 is, b 15 = 1 / (1} / F 15 -1 / a 15) = a is _{15 · F 15 / (a 15} -F 15).

Further, there is a relation of W ₁₅ = W ₅ · a ₁₅ / l ₅ · a ₁₅ / (a ₁₅ −D ₁₅ ) between the spot diameters of the objective lens 5, the condenser lens 15 and the photodiode 10. Thus the spot diameter W ₁₀ on the photodiode 10 is expressed by the following equation.

_{W 10 = W 15 · (b} 15 -f 15) / b 15 = W 5 · (a 15 / (a 15 -D 15)} · {(a 15 -F 15) / (a 15
・ F ₁₅ )} × {a ₁₅ · F ₁₅ / (a ₁₅ −F ₁₅ ) −f ₁₅ } = W ₅ · {a ₁₅ · F ₁₅ − (a ₁₅ −F ₁₅ ) · f ₁₅ } / {(a ₁₅ −D ₁₅ ) ・ F ₁₅ } ... On the other hand, the spot diameter W ₁₁ of the photodiode ₁₁ is
In the figure, the condenser lens 15 is the condenser lens 16, the photodiode 10 is the photodiode 11, and a ₁₅ , D ₁₅ , f ₁₅
Was comparable to that was changed to a _16, D _16, f _16, respectively, it is expressed by the following equation.

W ₁₁ = W ₅ · {a ₁₆ · F ₁₆ − (a ₁₆ −F ₁₆ ) · f ₁₆ } / {(a ₁₆ −D ₁₆ ) · F ₁₆ } ... In this formula and the formula, when W 5> A 5 Is the formula and W ₅ of the formula becomes A ₅ . Further, the distances between the objective lens 5 and the condenser lenses 15 and 16 are D ₁₅ and D ₁₆ , the focal length of the objective lens 5 is F ₅ , the condenser lens 15 and the photodiode 10, and the condenser lens 16 and the photodiode 11. The distances of f ₁₅ and f respectively
When _{16, a 15 = D 15 +} F 5 (F 5 -2 z) / 2 z ... f 15 = F 15 -x ... a 16 = D 16 + F 5 (F 5 -2 Z) / 2 z ... f 16 = F ₁₆ + y ... Here, the focal length of the condenser lenses 15 and 16
Spot diameters W ₁₀ and W 11 on the photodiodes 10 and ₁₁ and the DUT 6 when F ₁₅ and F ₁₆ are set to 15 mm and 60 mm, respectively.
FIG. 5 shows a simulation of the relationship with the displacement amount z of. In addition, in FIG. 5, the following numerical values are substituted using the above formulas to formulas.

Effective diameter of objective lens A ₅ = 4.3mm Focal length of objective lens 5 F ₅ = 4.6mm Distance between objective lens 5 and condenser lens 15 D ₁₅ = 135.8mm Distance between objective lens 5 and condenser lens 16 D ₁₆ = 142.8 mm Distance between condenser lens 15 and photodiode 10 f ₁₅ = 11.51 mm Distance between condenser lens 16 and photodiode 11 f ₁₆ = 73.95 mm As shown in Fig. 5, the displacement amount of the DUT 6 z Spot diameter W on photodiodes 10 and 11 before and after focusing at zero
The rates of change of ₁₀ and W ₁₁ are almost equal. Therefore, the displacement signals A, B from the photodiodes 10, 11 are
As shown in FIG. 6, the change rates become equal, and the focus error signal obtained by taking the difference between the displacement signals A and B becomes a straight line without bending before and after the in-focus point as shown in FIG. , The focus servo gain does not change.

When the displacement signals A and B do not completely match, the focus error signal does not become a perfect straight line, but some bending does not hinder the focus servo operation.

Further, in the configuration diagram of the conventional example shown in FIG. 8, the same effect as that of the device of FIG. 4 can be obtained by making the photodiodes 10 and 11 have different diameters.

Further, the optical element for branching the reflected light from the DUT 6 is not limited to the half mirror shown in the above embodiment,
For example, the reflected light may be split by a diffraction grating, a polarization beam splitter, or a combination of a half-wave plate and a polarization beam splitter, and the point is that parallel light can be split in two directions. good.

As a displacement measuring method, a focus servo is applied based on the focus error signal to measure the lens displacement amount of the objective lens 5, and the servo error is taken into consideration for output, or an input is used instead of the lens displacement amount. You may make it measure by the structure using the interference of a laser beam.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, a gain adjusting circuit for equalizing the change ratios of the outputs of the two photodiodes is provided, and the difference between the outputs is calculated. Focus error can be obtained by arranging the photodiode such that the focus error signal is obtained or two condenser lenses with different focal lengths are used and the photodiode is arranged at a distance where the diameter of the photodiode and the spot diameter on the photodiode are equal. Since the linearity of the signal before and after focusing is improved, the gain change of the focus servo becomes constant, and an optical displacement measuring instrument with a stable servo system can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of signal processing of an optical displacement measuring device according to the present invention, FIG. 2 is a diagram showing an output signal of a gain adjusting circuit which constitutes the signal processing of FIG. 1, and FIG. FIG. 4 is a diagram showing a focus error signal obtained from the signal processing of FIG. 1, FIG. 4 is a block diagram showing another embodiment of an optical displacement measuring device according to the present invention, and FIG. 5 is an apparatus of FIG. FIG. 6 is a simulation diagram showing the relationship between the spot diameter and the amount of displacement of the object to be measured, FIG. 6 is a diagram showing the output signal of the photodiode obtained from the apparatus of FIG. 4, and FIG. 7 is a diagram of FIG. FIG. 8 is a diagram showing a focus error signal obtained from the apparatus, FIG. 8 is a configuration diagram of a conventional example, FIG. 9 is a diagram showing an output signal of a photodiode obtained from the conventional apparatus of FIG. 8, and FIG.
FIG. 11 is a diagram showing a focus error signal obtained from the conventional device shown in FIG. 11, and FIG. 11 is an explanatory diagram relating to the optical system in the configuration of FIG. 1 ... Laser diode, 2 ... Collimator lens, 3
...... Polarizing beam splitter, 4 …… 1/4 wavelength plate, 5 ……
Objective lens, 6 ... DUT, 7 ... Half mirror, 1
0, 11 ... Photodiode, 12, 13 ... Gain adjustment circuit, 14 ... Arithmetic circuit, 15, 16 ... Condensing lens.

Claims (2)

[Scope of utility model registration request] 1. A small displacement of an object to be measured by driving an objective lens in a focus direction, converging a light spot on the object to be measured, measuring a light intensity pattern of the reflected light with a photodetector, and performing arithmetic processing. In an optical displacement measuring device for measuring, an optical element for branching reflected light from an object to be measured into two directions,
Two condensing lenses that condense the two lights branched by the optical element, and the lights that are arranged at the same distance before and after the focal points of the two condensing lenses and are condensed by the condensing lenses respectively enter. And two gain adjusting circuits for inputting the detection signals of the two photodiodes and equalizing the rate of change of the detection signals, respectively. An optical displacement measuring device characterized in that a focus error signal is obtained from a difference between output signals. 2. The two condensing lenses are condensing lenses having different focal lengths for equalizing the rate of change of the detection signal, and two photons into which the lights condensed by the condensing lenses are respectively incident. The diodes are arranged before and after the focus of the two condenser lenses and at a distance where the diameter of the photodiode and the spot diameter on the photodiode are equal, and a focus error signal is obtained from the difference between the detection signals of the two photodiodes. An optical displacement measuring instrument characterized in that