JPH03105202A - Automatic focus controlling apparatus - Google Patents

Abstract

PURPOSE:To avoid influences of the change of an offset current by detecting a balancing part of a voltage after an offset correction during the focus searching time, and correcting each offset so that the voltage becomes zero. CONSTITUTION:A current flowing in a PIN photodiode 21 for detecting a focus error is converted at 75 to a voltage, and this voltage is added at 81 to a voltage for offset correction from a D/A converter 82 and guided to a focus error signal generating part 22. A switch SW2 is set first at a position indicated by a solid line. An objective lens 17 is set by a volume 77 to be adjacent to an S curve. The volume 77 is connected to a D/A converter 82b by a switch SW1. The lens 17 is moved up and down to search the S curve on the basis of signals output from a CPU 84. When a peak of the S curve is detected, the lens 17 is returned via a converter 82b. At this position of the lens 17, an output of an operational amplifier 81 is input to the CPU 84, and an offset correcting amount is changed so that each voltage becomes zero. When it enters a focus drawing range, the SW2 is set at a position indicated by a virtual line.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is an optical measuring device that uses a laser, and is configured to follow a focusing objective lens so that the focus of the laser is always on the surface of an object. This invention relates to automatic focus control devices and the like.

Conventional technology An automatic focus control device that applies a force servo to an objective lens that focuses a laser beam onto a reflective surface in an optical disk device, an optical measuring device, etc. is disclosed in Japanese Patent Application No. 60-39178 and Japanese Patent Application No. 62-76927. It is known for such things. this is,
A focus difference signal is generated based on the amount of error with respect to the focus position of the objective lens, and focus servo is applied so that this error signal becomes zero, and has a configuration as shown in FIG. The laser beam reflected by the surface 2 of the object 1 is guided to the two PIN photodiodes 21 for detecting focus errors, thereby converting the current flowing through each PIN photodiode 21 into the respective current-voltage conversion circuit 75.
After adjusting the offset using the respective volumes 80, the voltages are amplified to obtain the voltages 1 and V2. The V and v2 change as shown in FIG. 8 as the distance 1 between the objective lens 17 and the object 1 changes. The difference voltage Ve between 2 and v2 is as shown in FIG. The curve shown in FIG. 9 is hereinafter referred to as an S-shaped curve. The point at Ve10 in the middle of this S-shaped curve is the focus position, and the focus servo is applied to bring the focus to this position. At this time, SW2 (FIG. 7) is at the position shown in the virtual image. The S-curve is the 9th
As shown in the figure, the movable range of the objective lens 17 is very narrow, so the focus position is searched before applying focus servo. In other words, it is necessary to search for an S-shaped curve.

Therefore, at first, set SW2 to the position shown by the solid line, set the objective lens l7 near the S-curve using the volume 77, connect the oscillator 76 using SW1, move the objective lens 17 up and down, and enter the focus pull-in range. Switch SW2 to the virtual line position and apply focus servo.

Problems to be Solved by the Invention In the focus control device as described above, ideally the current flowing through the two PIN photodiodes is zero at a position away from the S-curve, but stray light from the laser or the current flowing through the PIN photodiodes may occur. Offset current exists due to stray current, etc. Laser stray light refers to light other than normal light reflected by an object, such as light that is reflected midway through the optical path and enters the PIN photodiode, and it is extremely difficult to eliminate this stray light. In order to perform highly accurate focus control, this offset current has a large effect and must be removed in the control circuit.

Conventionally, the offset was corrected by manually turning the volume as described above. Therefore, even if the offset has been corrected once, there is a problem in that the offset must be corrected again due to changes in the characteristics of the laser light source and the PIN photodiode over time.

In view of the above problems, the present invention provides an automatic focus control device having a function of automatically detecting offset deviation and correcting it.

Means for Solving the Problems In order to solve the above problems, the present invention provides a circuit that converts the current flowing through the two PIN photodiodes into voltages, a circuit that corrects the offset of each voltage, and a circuit that converts the current flowing through the two PIN photodiodes into voltages, a circuit that corrects the offset of each voltage, and The present invention is configured to include a circuit for reading each voltage and a focus search circuit, detect a balanced portion of the voltage after offset correction during focus search, and correct each offset so that each voltage becomes zero. It is characterized by

Function: The automatic focus control device of the present invention automatically corrects the offset current flowing through the PIN photodiode during focus search, so it not only eliminates the need for manual offset correction in the initial stage, but also eliminates the need for the laser light source and the PIN photodiode.
The feature is that it is completely unaffected by changes in offset current due to changes in photodiode characteristics over time.

Embodiment An optical measuring device according to an embodiment of the present invention will be explained with reference to FIGS. 1 to 6.

This device measures the x-y-z coordinate position based on the optical heterolog method, and uses semiconductor laser light (λ = 78
0nm) G on the surface to be measured 2 of the object to be measured 1, and based on the reflected light, focus servo is applied, and the measurement H
E-Ne Zeeman laser light (λ=633 nm) F is focused perpendicularly onto the surface to be measured 2, and the shape of the surface to be measured 2 is measured while applying tilt correction servo based on the reflected light.

In the overall structure of the device shown in Fig. 2, 3 is a lower stone surface plate as the base of the main body, and 4 is an X-table with an X table 5 and a Y table 6 interposed between it and this lower stone surface plate 3 in the X-Y direction. an upper stone surface plate that is movable; 7 is a Z moving part that is provided in front of the upper stone surface plate 4 and is supported movably in the Z direction; 8 is an L-shaped holding base that holds the object to be measured 1; 9 is this holding stand 8
The air spindle 85, which is rotated around the axis P in the Z direction (see FIG. 12), is a swivel base that supports the air spindle 9 so that it can move up and down, and can rotate around the axis Q in the Z direction.

As shown in FIG. 1, the Z moving section 7 includes a linear motor 10
It is suspended from the upper stone surface plate 4 by a spring 11 via. As shown in FIG. 3, a semiconductor laser 12 that emits semiconductor laser light G is installed inside the second moving unit 7. As shown in FIG. Semiconductor laser light G emitted from semiconductor laser 12
is a lens 13, a polarizing prism 14, and a λ/4 wavelength plate 15.
It is totally reflected downward by the microchroic mirror 16, enters the open low cup of the objective lens 17, and is reflected onto the object to be measured l.
The light is focused on the surface 2 to be measured. The condensing position of the semiconductor laser beam G substantially coincides with the irradiation position of the measurement light FZl used for measuring the Z coordinate of the Zeeman laser beam F. If the surface to be measured 2 is tilted, part of the reflected light of the semiconductor laser light G is reflected outside the aperture of the objective lens 17, but the remaining part is reflected into the aperture of the objective lens 17. The reflected light returning to the objective lens 17 is totally reflected by the guichroic mirror 16 and the polarizing prism 14, condensed by the lens 18, and divided into two by the half mirror 19. Each of the divided reflected lights passes through each pinhole 20 installed before and after the focus, and is irradiated onto each PIN photodiode 21 . If the focusing position of the objective lens 17 is on the surface to be measured 2, the amount of light detected by each PIN photodiode 21 will be equal.

When the distance (Z direction) between the object to be measured 2 and the objective lens 17 changes, the light condensing positions before and after the two bin holes 20 shift in the optical axis direction, resulting in a difference in the amount of light irradiated onto each PIN photodiode 21. .

Based on the difference in the outputs of these PIN photodiodes 21, the first
A focus error signal is generated in a focus error signal generating section 22 shown in the figure. As shown in Fig. 5, even if the irradiation position on the surface to be measured 2 is tilted, the amount of light to the two PIN photodiodes 21 will decrease, but no difference in the amount of light to the two PIN photodiodes 2 will occur. , no focus error occurs.

When the switch SW2 is in the position shown by the imaginary line in FIG. 1, the drive circuit 23 controls the linear motor 10 so that this focus error signal becomes zero, and moves the Z moving unit 7 in the Z direction. In this way, focus servo is applied so that the focusing position of the semiconductor laser beam G and the measurement light FZl of the Zeeman laser beam F, which will be described below, is always on the surface to be measured 2.

A part of the laser beam F emitted from the He-Ne Zeeman frequency stabilized laser 24 that oscillates at two frequencies f, f2 is separated by the second half mirror 26 after passing through the first half mirror 25. and used to measure the X-Y coordinates of the measurement position.

On the other hand, the laser beam F2 reflected by the first half mirror 25
is used to measure the Z coordinate of the measurement position.

This laser beam F2 is separated by a polarizing prism 27 into a measuring beam F2 and a reference beam FZ2. Frequency f of measurement light FZIZ1 and reference light F2. The difference between the frequency f2 and the frequency f2 is several hundred kHz, and the light is linearly polarized perpendicular to each other.

Note that the laser beams F and F used for X-Y coordinate measurement are also connected to the measurement beams Fxi and Fy1 and the reference beams Fx2 and Fy2 by the respective corner cubes 44V in the middle of each optical path.
It is separated into

As shown in FIG. 6, the measurement light F used for Z coordinate measurement is transmitted through a special polarizing prism 28 that completely transmits P polarized waves and partially transmits S polarized waves, a Faraday element 29, and a λ/2 plate 30.
The light passes through the polarization prism 31, becomes S-polarized light, and is totally reflected by the polarizing prism 31. The measurement light F21 passes through the λ/4 plate 32 and the condensing lens 33, is focused on the mirror 34, and is reflected. The measurement light F21 becomes P-polarized by the λ/4 plate 32, and completely passes through the polarizing prism 31. The light enters the objective lens 17 and is focused perpendicularly to the surface 2 to be measured. The reflected light from the surface to be measured 2 returns along the same optical path as the above-mentioned incident light, but becomes S-polarized wave, is partially reflected by the special polarizing prism 28, and then totally reflected by the polarizing prism 27, and is sent to the Z-axis photodetector. Reach 35. When measuring the shape of the surface to be measured 2, the frequency of the reflected light changes according to the rate of variation of the Z coordinate of the measurement point on the surface to be measured 2.
, ten△. Note that when the optical path of the reflected light is about to deviate depending on the inclination of the surface to be measured 2, the 4-split photodetector 3G detects the reflected light partially reflected by the special polarizing prism 28, and the condenser lens is moved. By moving the condensing lens 33 in the X-Y direction using the thousand steps 37 and changing the position of incidence of the incident light on the objective lens 17, tilt correction servo is applied so that the reflected light always returns along the same optical path.

On the other hand, the reference light Fz2 is totally reflected by the polarizing prism 27 and then condensed onto the Z-axis mirror 39 by the lens 38, reflected, and reaches the Z-axis photodetector 35. The frequency of the reflected light becomes f2+δ due to errors in the straightness of movement of the X, Y tables 5, 6, etc. Therefore, the Z-axis photodetector 35
Then, (f, +Δ)−(f2+δ) is detected as a beat signal, and the Z coordinate of the measurement position on the surface to be measured 2 is accurately obtained by the Z coordinate detection device 37.

Note that the X and Y coordinates of the measurement position on the surface to be measured 2 are determined by the Z moving unit 7.
The X. FF focused on Y-axis mirrors 38 and 39
Due to the difference in frequency between the reflected light of
It is detected by X and Y axis photodetectors 42 and 43.

Next, the focus servo system will be explained in detail using FIG.

In FIG. 1, 73 is a position detector that detects the amount of displacement of the Z moving unit 7 from the equilibrium position in the Z direction, 74 is a position signal generation circuit that generates a position signal based on the output from the position detector 73, and 76 is a 0.3Hz frequency oscillator, 77
is a volume for moving the Z moving unit 7 in two directions,
78 is a differential amplifier that amplifies the position signal based on the operation setting voltage of the volume 77 so as to eliminate the influence of the restoring force of the spring 11 and sends the signal to the drive circuit 23, and 79 is a gain control circuit.

The current flowing through the two PIN photodiodes 21 for focus error detection is converted into a voltage by a current-voltage conversion circuit 75, and the voltage is outputted from the CPU 84 through the D/A converter 82 and an operational amplifier. 81 and leads to the focus error signal generating section 22.

As shown in FIG. 9, the focus pull-in range is much narrower than the movable range of the objective lens 17, so it is necessary to search for the focus pull-in range before applying the focus servo. Therefore, initially, SW2 is set to the position shown by the solid line, the objective lens 17 is set near the S-curve using the volume 77, the D/A converter 82b is connected using SW1, and the Move the objective lens 17 up and down to search for an S-shaped curve. When the peak of the S-curve is detected, the CPU 84 returns the objective lens 17 through the D/A converter 82b to the equilibrium point of the S-curve, that is, a point away from the S-curve, and at that position the output voltage of the operational amplifier 8 is set to A. The voltages are inputted to the CPU 84 through the /D converter 83, and the offset correction amount is changed through the D/A converter 82 so that each voltage becomes zero. When the output voltages of the operational amplifier 81 both become zero, the offset correction value is held, and the command value to the D/A converter 82b is changed again to search for an S-shaped curve. When the focus pull-in range is entered, SW2 is set to the virtual line. Switch to the position and apply focus servo.

Effects of the Invention The present invention allows focus control to be performed without being affected by stray light from the laser or changes in the offset current of the PIN photodiode due to changes in the leakage current of the PIN photodiode.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a focus servo system in an optical measuring device according to an embodiment of the present invention, FIG. 2 is a schematic perspective view showing the overall structure of the device, and FIG. 3 is an optical path of a focus servo system in the same device. Figure 4 is an optical path diagram when the objective lens is in the focus position, Figure 5 is an optical path diagram when the object surface is tilted, Figure 6 is an optical path diagram of the tilt servo system in the same device, and Figure 7 The figure is a block diagram of a conventional focus servo system, Figure 8 is a graph of signals detected by two PIN photodiodes versus the distance between the objective lens and the object, and Figure 9 is a graph of the two signals in Figure 8 as a differential It is a graph of a focus error signal obtained by amplification. 1...Object to be measured, 2...Surface to be measured, 1
7... Objective lens, 22... Focus error signal generation section, 75... Current voltage conversion circuit, 81... Operational amplifier, 82, 82b...・・・
...D/A converter, 84...CPU. A7 Light system゜Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] In an automatic focus control device configured to follow a condensing objective lens so that the laser focus is always on the surface of the object, the current flowing through two PIN photodiodes for detecting focus errors is converted into a voltage. A circuit to convert,
It is equipped with a circuit that corrects the offset of each voltage, a circuit that reads each voltage after offset correction, and a focus search circuit. During focus search, the balanced part of the voltage after offset correction is detected, and each voltage is set to zero. An automatic focus control device characterized in that the automatic focus control device is configured to correct each offset so that the following results are obtained.