JPH11142720A - Focus detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a focus detector constituted so that time required for a focusing action is shortened and focusing accuracy is enhanced. SOLUTION: In a semiconductor laser 21, a probe light beam modulated to AC by a modulator 36 is emitted. Then, a testee body 26 is irradiated with the probe light beams through a polarizing beam splitter 22, a 1/4 wavelength plate 23, an image forming lens 24 and an objective lens 25. The reflected light beam thereof is passed through an optical element and split into two beams by a beam splitter 27. By photodetectors 29 and 31, signals B and A corresponding to incident light quantity are outputted. By an HPF 33, the high-frequency components of the signals are passed so as to generate the signals AH and BH. By an LPF 34, the low-frequency components thereof are passed so as to generate the signals AL and BL. By a switching unit 35, the signals AL and BL are selected when the signal (A+B) is large and the signals AH and BH are selected when it is small. By a signal processing system 32, the amplitude of the signals AH and BH are defined as A and B and the values of the signals AL and BL are defined as A and B so as to execute the calculation of (A-B)/(A+B).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detecting device for focusing on a subject used in a microscope, an optical measuring instrument, or the like.

[0002]

2. Description of the Related Art There is known a focus detection apparatus which irradiates a probe light onto a surface of an object through an objective lens and detects a focus on the surface of the object based on reflected light. Several configurations are known as this type of focus detection device. Among them, a representative device will be described.

FIG. 4 shows the configuration of a focus detection device disclosed in Japanese Patent Application Laid-Open No. 8-304694. A laser light beam emitted from a semiconductor laser 1 is reflected by a polarizing beam splitter 2, and the reflected light beam
The light beam is converted into a parallel light beam by the imaging lens 4 via the 波長 wavelength plate 3 and is condensed on the surface 6 of the subject via the objective lens 5.
The light beam reflected by the subject surface 6 is incident on the polarization beam splitter 2 again via the objective lens 5, the imaging lens 4, and the quarter-wave plate 3. This incident light beam is 1 /
As a result of transmitting through the four-wavelength plate twice, the polarization direction is shifted by 90 degrees with respect to the first light beam, so that the light beam passes through the polarization beam splitter 2 this time. The light beam transmitted through the polarizing beam splitter 2 is divided in two directions by a beam splitter 7. One of the light beams enters the first light receiving element 9 via the first stop 8 disposed a distance L ahead of the focal point P of the imaging lens 4, and the other light beam forms an image. The light is incident on the second light receiving element 11 via the second stop 10 disposed behind the light condensing point P of the lens 4 by the distance L.

[0004] The first light receiving element 9 and the second light receiving element 11 each generate an electric signal corresponding to the amount of reflected light from the surface 6 of the subject and send it to the signal processing system 12. Signal processing system 1
2 performs a predetermined operation on each input signal,
A displacement signal according to the displacement of the subject surface 6 is output.

An electric signal B and an electric signal A having the characteristics shown in FIG. 5A are sent from the first light receiving element 9 and the second light receiving element 11 to the signal processing system 12, respectively. The calculation of (AB) / (A + B) is performed as a signal for detecting the displacement of the object surface 6 by the system 12, and a displacement signal which becomes 0 at the focal point F as shown in FIG. Desired. The focusing operation is performed by moving the subject surface 6 itself or the focus detection device so that the subject surface 6 comes to a position where the displacement signal becomes 0.

As is clear from FIG. 5 (a), when the subject surface 6 is located at a position distant from the focal point F, the output levels of the generated signals A and B are low, so that Susceptible to mechanical noise and optical noise. Accordingly, as shown in FIG. 5B, even when the subject surface 6 is located at a position other than the true focal point F, ｛(AB) /
In some cases, a determination result of (A + B)｝ = 0 may be produced (hereinafter, referred to as pseudo-focusing).

In order to prevent the occurrence of such false focusing,
As shown in FIG. 5C, a threshold value T is set for the signal (A + B), and when the light amount level falls below the threshold value T, that is, when the subject surface 6 is located When the distance is more than a certain amount, the focus determination is not performed, and the focus operation range is limited.

[0008]

The focus determination of the focus detecting device includes an error caused by electric noise, optical noise, adjustment of an optical member, or the like. This focusing error is determined by measuring the height dimension of the object by determining the position of the object lens or the height of the object when the focus detection is focused on the surface of the object. When used in a measuring instrument, there is a problem in measurement accuracy.

[0009] To reduce the focus determination error, ie,
To increase the focusing accuracy, as shown in FIG. 6B, the inclination of the displacement signal near the focal point F is increased so that the displacement of the surface of the subject can be controlled to be smaller with respect to the same amount of noise. I just need. As a device configuration for realizing this, the magnification of the optical system provided in the focus detection device, that is, in the device of FIG.
The magnification of the optical system may be set large. Accordingly, the amount of movement of the focal point P of the imaging lens 4 on the light receiving element 11 side with respect to the displacement of the subject surface 6 increases, and the inclination of the displacement signal near the focal point F increases.

Increasing the magnification of the optical system of the focus detecting device reduces errors in focus determination, but reduces the peak width of the output of the light receiving element as shown in FIG. A + B) exceeds the threshold, that is, the focusing operation range is narrowed. As described above, when the focus determination error is reduced, that is, the focus accuracy is improved, the focus operation range is reduced.

The subject is not always guaranteed to be located inside the focusing operation range, and may be located outside the focusing operation range. In such a case, the focus detection device moves the subject to the inside of the focusing operation range. The movement of the subject inside the focusing operation range is performed as follows.

By comparing the signal (A + B) with the threshold value T, it is determined whether or not the subject is located inside the focusing operation range, and the subject is moved relative to the objective lens in the optical axis direction. Move to The relative movement of the subject with respect to the objective lens is performed by first increasing the distance between the objective lens and the subject, as shown in FIG. 7, in order to avoid collision between the objective lens and the subject. If the subject does not enter the focusing operation while the interval between the subjects is maximized, the distance between the objective lens and the subject is reduced on the contrary.

Since the movement of the object with respect to the objective lens always starts to increase the distance between the object and the objective lens, the distance between the objective lens and the object is maximized when the focal point is located in the opposite direction. The operation until the space is returned to the initial interval after being spread out is completely useless. This useless operation increases the time required for the focusing operation by the operation time.

That is, it is desirable to move the subject with respect to the objective lens from the beginning in a direction approaching the focal point, but in some cases, the subject is once moved away from the objective lens, and in that case, much time is required until focusing. Resulting in.

In particular, in a focus detection device with high focusing accuracy, as described above, since the focusing operation range is narrow, the subject often falls out of the focusing operation range. For this reason,
The time required for the focusing operation tends to be long. The present invention has been made in view of such a situation, and an object of the present invention is to provide a focus detection device having high focusing accuracy and a short time required for a focusing operation.

[0016]

SUMMARY OF THE INVENTION A focus detecting device according to the present invention comprises a light source for emitting probe light, a light source modulating means for modulating the light source to emit an alternating probe light, and a probe light from the light source. An optical system that emits light and also condenses reflected light from the subject, and a focus detection unit that detects a displacement between the converging point of the probe light and the surface of the subject, and the optical system includes: A light splitting unit that splits the reflected light into two beams, wherein the focus detection unit includes a first light receiving element disposed in front of a converging point of one beam, and a focusing unit of the other beam. It includes a second light receiving element disposed rearward, and signal processing means for obtaining a displacement signal by calculation based on output signals of the first and second light receiving elements.

For example, in one embodiment, the focus detecting section includes a high-pass filter for extracting high-frequency components of output signals of the first and second light receiving elements, and a high-pass filter for extracting output signals of the first and second light receiving elements. It has a low-pass filter for extracting low-frequency components, and signal switching means for selectively supplying output signals of the high-pass filter and the low-pass filter to the signal processing means.

Further, in another embodiment, the light source modulating means includes a switching means for switching whether or not to modulate the light source, and the switching means is controlled by the focus detecting section.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. First, a focus detection device according to the first embodiment will be described with reference to FIG. FIG.
, A light source such as a semiconductor laser 21 is
A probe light modulated into alternating current by 6 is emitted. The laser beam, which is the probe light emitted from the semiconductor laser 21, is s-polarized with respect to the polarization beam splitter 22, and is reflected by the polarization beam splitter 22. The reflected light passes through the 波長 wavelength plate 23, is converted into a parallel beam by the imaging lens 24, is condensed by the objective lens 25, and is irradiated on the subject 26.

The light reflected on the surface of the subject 26 passes through the objective lens 25, the imaging lens 24, and the quarter-wave plate 23, and enters the polarization beam splitter 22. This light is p-polarized with respect to the polarizing beam splitter 22 by twice passing through the quarter-wave plate 23, and thus passes through the polarizing beam splitter 22.

The light that has passed through the polarizing beam splitter 22 is split into two beams by a beam splitter 27. The beam that has passed through the beam splitter 27 passes through a first stop 28 disposed a distance L ahead of the converging point P of the imaging lens 24 and passes through the first light receiving element 2.
9 is incident. On the other hand, the beam reflected by the beam splitter 27 enters the second light receiving element 31 via the second stop 30 disposed behind the focal point P of the imaging lens 24 by a distance L.

The first light receiving element 29 outputs an electric signal B corresponding to the amount of incident light, and the second light receiving element 31 outputs an electric signal A corresponding to the amount of incident light. High pass filter (HP
F) 33 passes the high frequency components to the electric signal A and the electric signal B to generate the electric signal AH and the electric signal BH.
The low-pass filter (LPF) 34 outputs the electric signal A
And an electric signal B to pass a low frequency component to generate an electric signal AL and an electric signal BL. The switch 35 compares the signal (A + B) with a preset threshold value and outputs the signal (A
+ B) is larger than the threshold value, selects and outputs the electric signal AL and the electric signal BL, and when the signal (A + B) is smaller than the threshold value, selects and outputs the electric signal AH and the electric signal BH. I do.

The signal processing system 32 performs a predetermined operation on the electric signals AH and BH or the electric signals AL and BL supplied from the switch 35. This calculation is based on the electric signal A
For the supply of H and BH, their amplitudes are A
And B, and for the supply of electrical signals AL and BL, their values are A and B, respectively, and (AB) /
The operation of (A + B) is performed. This calculation result, that is, the displacement signal, corresponds to the relative shift along the optical axis of the converging point F of the probe light and the surface of the subject 26.

Next, the operation of the focus detection device of this embodiment will be described with reference to FIG. At the start of focus detection,
The switch 35 outputs the signal AH and the signal BH, and the signal processing system 32 performs the arithmetic processing of (A−B) / (A + B) with the amplitude of the signal AH as A and the amplitude of the signal BH as B. Thus, a displacement signal is generated. Since this displacement signal is generated based on the amplitude of the signal AH and the signal BH, it is not affected by an electrical or optical offset (DC level signal).

Since the semiconductor laser 21 is modulated into an alternating current, the amplitude of the probe light fluctuates periodically. As a result, as shown in FIG. 2A, the outputs of the light receiving elements 29 and 31 vary in amplitude and center of vibration according to the amount of incident light. Therefore, the signal AH near the converging point F
And the signal BH both have sufficiently detectable amplitudes.

Also, at a position relatively far from the focal point F (for example, at a position outside the focusing operation range of the conventional example), one of the signal AH and the signal BH has a sufficiently detectable amplitude. . Therefore, even when the surface of the subject is located far from the focal point F, that is, when the amount of reflected light from the surface of the subject is small, the direction of the relative displacement between the two is determined according to the sign of the displacement signal. it can. Thereby, the subject 26 or the objective lens 25 is moved in an appropriate direction along the optical axis.

As shown in FIG. 2B, the switch 35 outputs the output signal (A +) of the adder 32a in the signal processing system 32.
B) is compared with a predetermined threshold, and when the signal (A + B) exceeds the threshold, the output is switched from the signal AH and the signal BH to the signal AL and the signal BL. The threshold is the signal AH
Alternatively, the value is set to a value between the value corresponding to the amplitude of the signal BH and the value corresponding to the sum of the amplitudes of the signal AH and the signal BH, preferably a value near the middle.

In the range where the signal (AH + BH) exceeds the threshold value, the signal processing system 32 sets the value of the signal AL to A and the signal B to
Assuming that the value of L is B, a displacement signal is generated by performing an arithmetic process of (AB) / (A + B). Since the modulated components of the laser beam are removed by the LPF 34, the signal AL and the signal BL have substantially the same DC signal waveform as the conventional example, as shown in FIG. 2C. Therefore, the displacement signal obtained based on the DC component in this manner is substantially the same as the displacement signal obtained in the conventional example, and the focusing operation is performed in exactly the same manner as in the conventional example.

As can be seen from the above description, in this embodiment, the focusing operation range is substantially expanded as compared with that of the conventional example. Thereby, the range in which the appropriate movement direction of the subject 26 or the objective lens 25 can be detected is widened, and the time required for the focusing operation is reduced.

Next, a focus detection device according to a second embodiment will be described with reference to FIG. 3, the same reference numerals as in FIG. 1 indicate the same members as in the first embodiment. As shown in FIG. 3, the signal B from the light receiving element 29 and the signal A from the light receiving element 31 are directly input to the signal processing system 42, and the output signal of the adding unit 42 a of the signal processing system 42 ( A + B) is input to the modulator 41.

The modulator 41 compares the signal (A + B) supplied from the signal processing system 42 with a predetermined threshold value, and modulates the semiconductor laser 21 as appropriate according to the comparison result. That is, the modulator 41 modulates the semiconductor laser 21 when the signal (A + B) is smaller than the threshold, and modulates the semiconductor laser 21 when the signal (A + B) is larger than the threshold. And turn on as usual.

When the semiconductor laser 21 is modulated by the modulator 41, the signals A and B are handled in the same manner as the signal passed through the HPF in the first embodiment, and the semiconductor laser 21 is modulated by the modulator 41. If not, the signals A and B are the LPFs in the first embodiment.
The same function as in the first embodiment can be realized by treating the signal in the same way as the signal that has passed through.

This embodiment realizes the same function as that of the first embodiment by adding a function of switching between modulation and non-modulation to the modulator. Since the HPF, the LPF, and the switch are not required, it is very useful in terms of simplifying the configuration.

The present invention is not limited to the above-described embodiment, but includes all embodiments carried out without departing from the gist of the invention. For example, instead of the method described in the embodiment, the focus detection may be performed using another known method such as a pupil division method or an astigmatism method.

[0035]

According to the focus detecting device of the present invention, the focusing operation range is substantially expanded, and the focus can be determined even outside the conventional focusing operation range, and the accuracy of the focus determination can be improved. Focusing operation can be performed over a wide range while securing. Therefore, a focusing operation can be performed in a short time even on a subject having various shapes.

[Brief description of the drawings]

FIG. 1 shows a configuration of a focus detection device according to a first embodiment.

FIGS. 2A and 2B show signal waveforms obtained in the apparatus of FIG. 1, wherein FIG. 2A shows an output waveform of a light receiving element, FIG. 2B shows a waveform of a signal (A + B) based on an HPF output, and FIG. 3 shows the waveforms of FIG.

FIG. 3 shows a configuration of a focus detection device according to a second embodiment.

FIG. 4 shows a configuration of a focus detection device according to a conventional example.

5A and 5B show signal waveforms obtained in the apparatus of FIG. 4, wherein FIG. 5A shows an output waveform of a light receiving element, FIG. 5B shows a waveform of a displacement signal, and FIG. 5C shows a waveform of a signal (A + B). I have.

6A and 6B show a change in a waveform due to a difference in magnification of an optical system. FIG. 6A shows an output waveform of a light receiving element at low magnification and high magnification, and FIG. 6B shows displacement at low magnification and high magnification. 4 shows a signal waveform.

FIG. 7 shows how the subject moves with respect to a focal point F during a focusing operation by the apparatus of FIG.

[Explanation of symbols]

 Reference Signs List 21 semiconductor laser 24 imaging lens 25 objective lens 27 beam splitter 29, 31 light receiving element 32 signal processing system 33 high-pass filter 34 low-pass filter 35 switch 36 modulator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI G03B 13/36 G02B 7/11 N G03B 3/00 A

Claims (3)

[Claims] A light source for emitting probe light; a light source modulating means for modulating the light source to emit AC probe light; condensing probe light from the light source; and collecting reflected light from the subject. An optical system that emits light; and a focus detection unit that detects a positional shift between a converging point of the probe light and the surface of the subject. The optical system splits reflected light into two beams. Including means, the focus detection unit, a first light receiving element disposed in front of the focal point of one beam, and a second light receiving element disposed behind the focal point of the other beam, A focus detection device including signal processing means for obtaining a displacement signal by calculation based on output signals of the first and second light receiving elements. 2. The method according to claim 1, wherein the focus detection unit includes:
A high-pass filter that extracts high-frequency components of the output signals of the first and second light-receiving elements, a low-pass filter that extracts low-frequency components of the output signals of the first and second light-receiving elements, and output signals of the high-pass filter and the low-pass filter. A focus detection device having signal switching means for selectively supplying the signal processing means. 3. The focus detecting device according to claim 1, wherein the light source modulating means includes switching means for switching whether or not to modulate the light source, and the switching means is controlled by the focus detecting unit.