JP2935548B2 - Automatic focus control device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an auto-focusing system that causes an objective lens to follow a focal point of a laser beam on a surface of an object in an optical disk device or an optical measuring device using a laser. The present invention relates to a control device.

2. Description of the Related Art As a conventional automatic focus control device in an optical disc device or an optical measurement device, an error amount with respect to a focus position of the objective lens in order to match a focus position of a laser beam focused by an objective lens with a surface of an object. There is known a device which generates a focus error signal on the basis of a focus servo and applies a focus servo so that the error signal becomes zero.

As shown in FIG. 8, the focus error signal is obtained by converting the laser beam reflected by the surface b of the object a into two focus error detection signals.
The current is guided to the PIN photodiode c, and the current flowing according to the light amount is converted into a voltage by the current-voltage conversion circuit d, and then differentially amplified by the focus error signal generation unit e. According to the change in the distance between the lens f and the object a, as shown in FIG. 6, the shape changes in an S-shaped curve. Ve in the middle of this S-curve
The point where = 0 is the focus position, and the focus pull-in range is between two positions where the focus error signal has a peak. Therefore, when the position of the objective lens f is in the focus pull-in range, focus servo is performed so that the focus error signal becomes zero, that is, the objective lens comes to the focus position.

Since the focus pull-in range (about 10 μm) is very narrow as compared with the movable range (about 20 mm) of the objective lens f, it is necessary to search for the focus pull-in range before applying the focus servo. Therefore, in the conventional automatic focus control device, first, the switches SW ₂ and SW ₃ are set to the positions shown by solid lines, and the position of the objective lens f in the Z direction is set to a position near the focus pull-in range by manual operation using the volume g. After the switch
The SW ₁ is connected an oscillator h in accordance with the amplitude of the waveform signal by vertically moving the objective lens f in a certain range, the switch SW ₂ Once it has entered the focus capture range is confirmed by detecting a focusing error signal Is switched to the position of the imaginary line, and focus servo is applied. When the entire optical system is moved in the X-Y direction with the focus servo applied, the focal position of the laser light is The objective lens f follows in the Z-axis direction so as to fit on the surface b.

If there is a step beyond the focus pull-in range on the surface b of the object a while the focus servo is applied, the objective lens f goes out of the focus pull-in range. Switch from sample state to hold state, and switch SW ₃
After switching the position of the virtual line, than to switch the switch SW ₂ to the position indicated by the solid line, first holding the position in the Z direction of the objective lens f the focus position before departing from the focus capture range, followed by a switch SW ₁ , The oscillator h is connected, the objective lens f is moved up and down within a certain range, the focus pull-in range is searched again, and the focus lens is returned to the focus position.

However, in the above conventional example, however, since the objective lens moves up and down according to a constant amplitude, the position of the objective lens in the Z direction is adjusted by, for example, a volume in order to automate the operation of searching for the focus pull-in range. If an oscillator is connected with the objective lens set at the highest position in the movable range of the objective lens, if the focus pull-in range cannot be found due to an abnormality such as a failure or disconnection of the PIN photodiode, the objective lens at the time of downward movement collides with the object There is a possibility that.

Also, even if the focus servo is applied, if the objective lens is too close to the object due to a step that exceeds the focus pull-in range on the surface of the object, if the objective lens is moved up and down according to a certain amplitude, There is also a risk of the objective lens colliding with the object before the focus pull-in range is found.

In view of the above problems, the present invention can prevent the objective lens from colliding with the object even when the focus pull-in range is not found or even when there is a step beyond the focus pull-in range on the surface of the object, An object of the present invention is to provide an automatic focus control device capable of automating a task of searching for a focus pull-in range.

Means for Solving the Problems In order to achieve the above object, the present invention provides a light detecting means for detecting reflected light of laser light condensed on a surface of an object by an objective lens, and an output from the light detecting means. A focus error signal generating means for generating a focus error signal in response thereto; and an oscillating means for oscillating a waveform signal and setting a movable range of the objective lens based on its amplitude. In an automatic focus control device configured such that a light position is located on a surface of an object, height detection means for detecting the height of the object is provided, and the waveform signal is output in accordance with an output of the height detection means. Characterized in that the amplitude is controlled.

According to the above configuration, the amplitude of the waveform signal of the oscillating means for setting the movable range of the objective lens is controlled in accordance with the height of the object, so that the lower limit of the movable range can be set by the objective lens. Since the object lens can be set at a position where it does not collide with the object, the objective lens can be moved up and down within a range where it does not collide with the object.

Therefore, even when the focus pull-in range is not found, or even when the focus pull-out range is found and focus servo is applied and the focus position deviates from the focus pull-in range, a situation where the objective lens collides with the object is avoided. be able to.

Embodiment FIGS. 1 to 7 show an embodiment in which the present invention is applied to an optical measuring device.

This apparatus measures the XYZ coordinate position based on the optical heterodyne method, and uses a semiconductor laser beam (λ = 780n).
m) G is condensed on the surface (surface) 2 to be measured of the object 1 and focus servo is performed based on the reflected light.
He-Ne Zeeman laser beam (λ = 633nm) F
And the shape of the surface to be measured 2 is measured while performing a tilt correction servo based on the reflected light.

In the overall configuration of the apparatus shown in FIG. 2, reference numeral 3 denotes a lower stone surface plate as a main body base, and 4 denotes an X-axis through an X table 5 and a Y table 6 disposed between the lower stone surface plate 3 and the lower surface plate.
Upper stone surface plate 7 that can be moved in the Y direction.
, A Z-moving unit provided on the front surface of the unit and instructed to be movable in the Z-direction; 8, an L-shaped holding table for holding the object 1; 9, air for rotating the holding table 8 around an axis P in the Y-direction The spindle 10 is a turntable that supports the air spindle 9 so as to be able to move up and down and can turn around the axis Q in the Z direction.

As shown in FIG. 1, the Z moving unit 7 includes a linear motor 10
And is suspended on the upper stone surface plate 4 by a spring 11 through the spring.
As shown in FIG. 3, a semiconductor laser 12 that emits a laser beam G for performing focus servo is installed inside the Z moving unit 7. The laser light G emitted from the semiconductor laser 12 passes through the lens 13, the polarizing prism 14, and the λ / 4 wavelength plate 15, is totally reflected downward by the dichroic mirror 16, enters the entire opening of the objective lens 17, and enters the target. The light is focused on the surface 2 to be measured of the object 1. The focusing position of the laser light G is
Measurement light Fz used for Z coordinate measurement of Zeeman laser light F
_This is almost the same as the irradiation position of ₁ . If the surface 2 to be measured is inclined, a part of the reflected light of the laser light G is reflected toward the outside of the opening of the objective lens 17, but the rest is reflected toward the inside of the opening of the objective lens 17. The reflected light that has returned to the objective lens 17 is totally reflected by the dichroic mirror 16 and the polarizing prism 14, respectively, then condensed by the lens 18, and is split into two by the half mirror 19 in the optical path. Each of the divided reflected lights passes through the respective pinholes 20 installed at the pre-focal position and the post-focal position of the lens 18, and the respective PINs
The light is irradiated to a photodiode (light detecting means) 21.

The size and position of these pinholes 20
When the focal point of 17 is on the surface 2 to be measured, as shown in FIGS. 4 and 5, the amounts of light detected by the PIN photodiodes 21 are set to be equal to each other. Therefore, when the distance (Z direction) between the surface 2 to be measured and the objective lens 17 changes, the light condensing positions before and after the pinhole 20 shift in the optical axis direction, resulting in a difference in the amount of light applied to each PIN photodiode 21. For example, when the distance between the surface 2 to be measured and the objective lens 17 is shorter than the focal length of the objective lens 17, each condensing position is closer to the respective PIN photodiode 21 than the focal point of the lens 18. Accordingly, in the PIN photodiode 21 in which the pinhole 20 is provided at the front position on the focal point, the amount of reflected light blocked by the pinhole 20 increases and the amount of light decreases, and the pinhole 20 is provided on the rear position on the focal point. PIN
In the photodiode 21, the reflected light is not blocked by the pinhole 20 and the light amount does not change, or the amount of the reflected light that is blocked decreases and the light amount increases. The reverse is true when the distance increases. As a result, both PIN photodiodes 21 and 2 are changed according to the distance between the surface 2 to be measured and the objective lens 17.
There is an output difference between one.

Depending on the difference between the outputs of these PIN photodiodes 21,
A focus error signal is generated by a focus error signal generator (focus error signal generator) 22. The current flowing through the PIN photodiode 21 is a current-voltage conversion circuit 75
After that, the voltage is converted into a voltage, and then guided to the focus error signal generator 22. As shown in FIG. 5, when the light-collecting surface on the measured surface 2 is inclined, the amount of light to the two PIN photodiodes 21 decreases, but the amount of light between the two PIN photodiodes 21 and 21 decreases. No difference occurs and no focus error occurs.

When the switch SW ₂ is in the position shown in phantom in FIG. 1, the driving circuit 23 controls the linear motor 10 so that the focus error signal becomes zero, moves the Z moving unit 7 in the Z-direction. In this way, the focus servo is applied so that the focal point of the objective lens 17 is always on the surface to be measured 2.

Next, the focus servo system will be described in detail with reference to FIG.

In the figure, reference numeral 73 denotes a position detector for detecting the amount of displacement of the Z moving unit 7 from the equilibrium position in the Z direction; 74, a position signal generating circuit for generating a position signal in accordance with the output from the position detector 73; Is an oscillator (oscillating means) that oscillates a frequency of 0.3 Hz. 78 is a drive circuit 23 that amplifies the position signal based on the reference voltage output from the reference voltage generator 82 so as to eliminate the influence of the restoring force of the spring 11. , 79 is a gain control circuit, 80 is a height measuring device for measuring the height of the surface 2 to be measured of the object 1, and 81 is the amplitude of the frequency according to the height. Is a height signal generating circuit for outputting a height signal for the sample, and 83 is a sample and hold circuit.

As shown in FIG. 6, the focus pull-in range is very narrow (about 20 mm) as compared with the movable range (about 20 mm) of the objective lens 17.
10 μm), it is necessary to search the focus pull-in range before applying the focus servo. So before setting the object 1, the uppermost position of the movable range in the Z moving unit 7 have opened the switch SW _1, the reference voltage generating unit 82 as well as sets the position indicating the switch SW ₂ and the switch SW ₃ by a solid line Set to. When the object 1 is set, first, a signal obtained by the height measuring device 80 is converted into a position signal of the highest point of the object 1 by the height signal generation circuit 81, and the oscillator 76 operates in accordance with the position signal. After the amplitude is set to a value slightly smaller than the object lens 17 colliding with the object 1, the switch SW
₁ is closed. Accordingly, the Z moving unit 7 starts descending from the uppermost position of the movable range toward the position immediately above the highest point of the object 1. Lowering the way the focus when the error signal is detected i.e. enters the focus capture range switch SW ₂ is switched to the position of the virtual line, the focus servo is. If the focus error signal is not detected due to an abnormality such as a failure or disconnection of the PIN photodiode 21 during the downward movement, the Z moving unit 7 stops at a position where the objective lens 17 is slightly above the highest point of the object 1. , Switch SW again
₁ is opened and the Z moving unit 7 is retracted to the uppermost position of the movable range. In this way, even when the focus pull-in range cannot be found, the operation of searching for the focus pull-in range can be automated without causing the objective lens 17 to collide with the object 1.

With the focus servo on, the entire optical system
When the laser beam G is moved in the -Y direction, the Z moving section 7 follows the Z-axis direction so that the focal position of the laser beam G always matches the measured surface 2 of the object 1. However, if the measured surface 2 of the object 1 has a step beyond the focus pull-in range, the objective lens
The position 17 is out of the focus pull-in range, and no focus error signal is detected. In this case, after the sample and hold circuit 83 switches from the sample state to the hold state, and furthermore, the switch SW ₃ switches to the position of the virtual line,
Switched switch SW ₂ is in the solid line position. Thus, the objective lens 17 can be held at the focus position before the focus falls out of the focus pull-in range. Here, if the previous focus position is too close to the measured surface 2, the amplitude of the oscillator 76 is reset to a value slightly smaller than the previous time. Then the switch SW ₁ is moved downward in the Z moving unit 7 is closed. When the middle into the focus new focus pull range error signal is detected, the switch SW ₂ is switched to the position of the virtual line, the focus servo again consuming. If the focus error signal is not detected even if the focus error signal is detected even if the focus position is lowered to the reset lowest position, the Z moving unit 7 moves upward. If the focus error signal is not detected yet, the Z movement unit 7 stops at the highest position in the movable range. In this manner, even when the measured surface 2 of the object 1 has a step beyond the focus pull-in range and the objective lens 17 is out of the focus pull-in range, the operation of searching for the focus pull-in range is performed by the objective lens 17. Automation can be performed without causing collision with the object 1.

Next, the tilt servo system will be described. Two frequencies
He-Ne Zeeman frequency-stabilized laser 24 oscillating at f ₁ and f ₂
Is transmitted through the first half mirror 25 and is used for measuring the XY coordinates of the measurement position, and the Z coordinate of the measurement position reflected by the first half mirror 25. It is separated into laser light Fz used for measurement.
Further, the laser light Fxy is reflected by the second half mirror 26 and used for measuring the X coordinate of the measurement position with the laser light Fx;
The light passes through the second half mirror 26 and is separated into laser light Fy used for measuring the Y coordinate of the measurement position. Thereafter, the laser beam Fz is a polarizing prism 27, the measurement beam Fz ₁ and the reference beam Fz ₂
And separated. The difference between the frequency f ₂ of the frequency f ₁ and the reference light Fz ₂ of the measuring beam Fz ₁ has a linear polarization perpendicular to each other a few hundred KHz. Note that the laser beams Fx and Fy are also measured by the respective corner cubes 44 in the respective optical paths, and the measurement beams Fx ₁ and Fy ₁ and the reference beam Fx
_2, is separated into the Fy _2.

Measuring beam Fz ₁ used for Z coordinate measurement, as shown in FIG. 7, a special polarizing prism 28 which partially transmits the S polarized totally transmits the P polarization, the Faraday element 29, lambda / 2 plate 30 , And becomes S-polarized light, and is totally reflected by the polarizing prism 31.
The lambda / 4 plate 32, passes through the condenser lens 33, the measurement light Fz ₁ reflected by condensing on the mirror 34 becomes a P polarization by the lambda / 4 plate 32, all transmitted through the polarizing prism 31 Then, the light enters the objective lens 17 and is condensed perpendicularly to the surface 2 to be measured. The reflected light from the measured surface 2 returns along the same optical path as the incident light,
After being partially polarized by the special polarizing prism 28 as S-polarized light, it is totally reflected by the polarizing prism 27 and reaches the Z-axis photodetector 35. When measuring the shape of the surface 2 to be measured, the frequency of the reflected light is Doppler-shifted according to the speed of change of the Z coordinate of the measurement point on the surface 2 to be measured, and becomes f ₁ + Δ. When the optical path of the reflected light is to be shifted in accordance with the inclination of the surface 2 to be measured, the reflected light partially reflected by the special polarizing prism 28 is converted into a four-divided photodetector 36.
Is detected, and the condenser lens 33 is moved in the XY directions by the condenser lens moving means 37 to change the incident position of the incident light to the objective lens 17 so that the reflected light always returns to the same optical path. The tilt correction servo is applied.

On the other hand, the reference beam Fz _2, after being totally reflected by the polarizing prism 27, is condensed on the Z-axis mirror 39 by the lens 38, is reflected reaches the Z-axis light detector 35. The frequency of the reflected light is f ₂ + δ due to errors such as the straightness of movement of the X and Y tables 5 and 6. Therefore, in the Z-axis photodetector 35, (f ₁ +
Δ) − (f ₂ + δ) is detected as a beat signal, and the Z coordinate of the measurement position on the surface 2 to be measured is accurately obtained by the Z coordinate detection device 37.

The X and Y coordinates of the measurement position on the surface to be measured 2 are the reflected light of the measurement light beams Fx ₁ and Fy ₁ collected on the X and Y axis mirrors 38 and 39 installed on the outer wall surface of the Z moving unit 7 and the lower portion. The X and Y axis photodetectors 42 and 43 are provided by the difference in frequency between the reference lights Fx ₂ and Fy ₂ reflected on the X and Y axis mirrors 40 and 41 installed on the stone surface plate 1.
Is detected by

Since the present invention has the above-described configuration and operation, the objective lens can be moved up and down in a range where the objective lens does not collide with the object, and when the focus pull-in range is not found or when the focus pull-in range is found, the focus servo is applied. Even in this state, even when the focus position deviates from the focus pull-in range, it is possible to avoid a situation in which the objective lens collides with the object. As a result, the operation of searching for the focus pull-in range can be automated.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a perspective view of an optical measuring apparatus in the present embodiment, FIG. 3 is an optical path diagram of a focus servo system in the apparatus, and FIG. 5 is an optical path diagram when the surface of the object is tilted, FIG. 6 is a graph of a focus error signal, FIG. 7 is an optical path diagram of a tilt servo system in the same device, and FIG. The figure is a block diagram of a conventional example. 17 Objective lens 21 Light detecting means 22 Focus error signal generating means 76 Oscillating means 80, 81 Height detecting means

Claims (1)

(57) [Claims] 1. A light detecting means for detecting reflected light of laser light converged on a surface of an object by an objective lens, and a focus error signal generating means for generating a focus error signal in accordance with an output from the light detecting means. Means for generating a waveform signal and oscillating means for setting the movable range of the objective lens by its amplitude, so that the objective lens is pulled into a focus position so that the focal position of the laser light is positioned on the surface of the object. Wherein the height of the target object is detected, and the amplitude of the waveform signal is controlled in accordance with the output of the height detection means. Automatic focus control device.