JPH07280555A - Focusing type displacement measuring apparatus - Google Patents

Abstract

PURPOSE:To enable prevention of erroneous measurement in the measurement of a transparent object by controlling the operation of a servo circuit so as to limit a moving range of an objective lens. CONSTITUTION:An output light beam of a semiconductor laser 1 is focused on a subject 6 with an objective lens 5 through a polarization beam splitter 2 or the like. Outputs A and B of photodetectors 9 and 10 receiving the reflected light are com-puted with a focus error signal generation circuit 21 to generate an S curve signal as given by (A-B)/(A+B). A servo circuit 22 which inputs the output together with an output of a displacement detector 15 for detecting a displacement value of the lens 5 drives a coil 11 to control the focus of the lens 5. Here, when the subject 6 is a transparent object, a zone judging circuit 231 of a control circuit 23 defines a focus zone near either the surface or the rear thereof to control the circuit 22 so that the focus is not off the range. A limit judging circuit 232 controls the circuit 22 so that the movement of the focus is limited within a physically movable range of the lens 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-focus type displacement measuring device which performs displacement measurement optically without contact.

[0002]

2. Description of the Related Art A focusing type displacement measuring apparatus is an optical system having a light beam irradiating means for irradiating an object to be measured with a light beam through an objective lens and a light receiving element for receiving a reflected light beam from the object to be measured. Displacement detector that detects the amount of displacement of the objective lens in conjunction with this optical system, and processes the output signal of the light receiving element to obtain the focus error signal, and based on this, moves the objective lens to the in-focus position. And a driving servo circuit.

When a transparent body such as a thin glass plate is measured by using this type of focal point displacement measuring apparatus, there is reflected light from the front surface as well as reflected light from the front surface. Focus on. Therefore, erroneous measurement may be performed. Specifically, when the front surface is scanned to inspect the surface properties and the like, if there is a scratch or the like on the front surface, the back surface may be erroneously focused on.

In this type of focusing displacement measuring apparatus, the movable range of the actuator portion including the objective lens is usually physically limited. For example, the upper limit of the movable range of the actuator is the lower end of the displacement detector, and the lower limit is the case containing the optical system. Therefore, when the actuator vigorously moves and collides with the displacement detector or the case, the displacement detector malfunctions or the optical system is displaced due to loosening of the screw or the like.

[0005]

As described above, in the conventional focusing type displacement measuring device, when measuring a thin transparent body,
There are problems that erroneous measurement occurs due to focusing on the front surface and back surface, and that the actuator collides with the upper and lower limits of the physical movable range, resulting in erroneous measurement and deviation of the optical system.

The present invention prevents erroneous measurement in transparent body measurement,
It is also an object of the present invention to provide an in-focus point displacement measuring device capable of improving measurement reliability.

[0007]

A focal point displacement measuring apparatus according to the present invention is a light beam irradiating means for irradiating an object to be measured with a light beam through an objective lens, and a reflected light beam from the object to be measured. An optical system having a light receiving means for detecting the displacement amount of the objective lens, a focus error signal generating means for processing an output signal of the light receiving means to generate a focus error signal, and the focus error signal. And servo circuit means for driving the objective lens to the in-focus position based on the output of the displacement detecting means, and operation of the servo circuit means for limiting the moving range of the objective lens based on setting information from the outside. It has a control means to control.

Specifically, the control means (1) determines the focus zone only in the vicinity of either one of the front and back sides when the object to be measured is a transparent body, so as not to deviate from the range. Zone determining means for controlling the servo circuit means, or (2) Limit determining means for controlling the servo circuit means so as to limit the movement of the objective lens within the upper and lower limits of the physical movable range of the objective lens. , At least one of It is more preferable to provide both the functions of the zone determination means and the limit determination means so that they can be switched.

[0009]

According to the present invention, by providing the function of softly limiting the moving range of the objective lens, for example, in the case of thin transparent body measurement, only the front surface or the back surface is focused so that erroneous measurement is prevented. Can be prevented. It is also possible to prevent erroneous measurement and deviation of the optical system by preventing the actuator from hitting the upper and lower limits of the physical movable range.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an in-focus point displacement measuring device according to an embodiment of the present invention. The optical system includes a semiconductor laser 1, a polarization beam splitter 2 for converging an output light beam of the semiconductor laser 1 on a DUT 6, a quarter-wave plate 3, a collimator lens 4, and an objective lens 5. An imaging lens 7 for detecting a reflected light beam from the beam splitter 6, a beam splitter 8, and two diaphragms (pins) respectively arranged before and after the focus position of each reflected light split by the beam splitter 8. Holes) 24, 25, and these diaphragms 2
Light receiving element 9, 10 for receiving the reflected light transmitted through 4, 25
Have.

The objective lens 5 is separated from the case to which other optical members are fixed, the coil 11 is integrally provided, and the magnet 12 provided on the case side faces the objective lens 5 so that the objective A voice coil that drives the lens 5 is configured. Further, the scale 13 is provided integrally with the objective lens 5 that moves up and down, the encoder 14 that is arranged to face the scale 13 and reads the scale 13, and the displacement detector 15.
Is configured.

The outputs A and B of the two light receiving elements 9 and 10 are input to the focus error signal generating circuit 21. The focus error signal generation circuit 21 uses two light reception signals A and B.
The difference operation, the sum operation, and the division operation of (A−B) / (A
A focus error signal (so-called S-curve signal) of + B) is generated. This focus error signal generation circuit 2
The output of 1 is the servo circuit 2 together with the output of the displacement detector 15.
Entered in 2. Then, the coil 11 is driven by the output of the servo circuit 22 and focus control is performed.

The servo circuit 22 is controlled by a control circuit 23 including a CPU and a memory. The control circuit 23 functionally has a zone determination circuit 231, a limit determination circuit 232, and a switching circuit 233 for switching the outputs of these determination circuits and supplying them to the servo circuit 22 as a control signal. The zone determination circuit 231 measures the front surface or the back surface when the DUT 6 is a transparent body,
This is for defining the focus zone only in the vicinity of either one of them and controlling the servo circuit 22 so as not to deviate from the range. The limit determination circuit 232 controls the servo circuit 22 so as to limit the movement of the objective lens 5 within the upper limit and the lower limit of the physical movable range.

The zone determination circuit 231 stores the upper limit value and the lower limit value of the focus zone input from the outside using an input device or the like. Similarly, the limit determination circuit 232 stores the upper limit value and the lower limit value of the limit input from the outside. Then, when the zone determination circuit 231 and the limit determination circuit 232 reach the upper limit value or the lower limit value of the set focus zone or limit, for example, the position control of the servo circuit 22 is performed to stop the further displacement of the objective lens 5. Control. For example, in normal operation, the switching circuit 233 causes the limit determination circuit 23 to operate.
Select 2 outputs. When the measurement is performed with the focus zone set on the transparent body, the output of the zone determination circuit 231 is output by the servo circuit 2 by the switching control from the outside.
It will be sent to 2.

2 (a) and 2 (b) show a concrete configuration example of the servo circuit 22. As shown in FIG. An adder 221, which obtains an error signal from the displacement of the measurement surface and the displacement of the objective lens, an amplifier 222 which amplifies the error signal, a phase compensation circuit 223, and an amplitude limiting circuit 22.
4, the drive amplifier 225 and the coil 11 driven by the drive amplifier 225 form a control loop. Amplitude limiting circuit 22
4 is provided to limit the function of the servo circuit 22, and the control signal amplitude is limited here according to the determination result of the focus zone determination circuit 231 or the limit determination circuit 232, and the position control is performed. Will be done. The loop is turned on / off before the amplitude limiting circuit 224 in the control loop for limit setting and focus zone setting.
A switch SW1 for turning off is provided, and a drive signal circuit 228 for driving the actuator according to a drive command from the outside when the focusing position determination circuit 227 and the servo circuit are off is provided.

The amplitude limiter circuit 224, as shown in FIG. 2B, has a limit / focus zone determination circuit 231,
Two comparison circuits 233 and 234 that compare the upper limit value and the lower limit value from 232 with the output of the phase compensation circuit 223, and the phase compensation circuit 223 is switched depending on the comparison result.
Switch 235 that outputs the upper limit value when the output of is larger than the upper limit value, and outputs the lower limit value when the output is smaller than the lower limit value.
236 is provided. Further, in the limit / focus zone setting mode, the output of the drive signal circuit 228 is directly set to the drive amplifier 225 by setting the amplitude limiting circuit in the through state.
A changeover switch SW2 is provided so as to send to the.

FIG. 3 shows the objective lens 5 of the apparatus of this embodiment.
3 is a schematic side view showing a movable range of a portion of an actuator 31 including an arrow. The upper limit of the movable range of the actuator 31 is physically determined by the displacement detector 15, and the lower limit thereof is physically determined by the case 32. In this embodiment, the upper limit value and the lower limit value are set as shown so that the vertically moving actuator 31 is limited to a range slightly narrower than the movable range. Further, when the front road surface measurement or the back surface measurement of the transparent body is performed, the focus zone upper limit value and the focus zone lower limit value as shown are set as the focus zones.

Next, the operation of specific limit setting and focus zone setting will be described. These limit setting and focus zone setting operations are performed by turning off the servo loop by the switch SW1 of FIG. 2A and invalidating the setting of the switch SW2 of FIG. 2B (through state). FIG. 4 is an operation flow of limit setting. First, the actuator 31 is slowly moved downward and the lower end thereof is brought into contact with the case 32 (S11). In this state, the displacement detector 15 is set to 0 (S12). Next, the actuator 31 is slowly moved upward and stopped at a predetermined count value, and the drive signal output value is set as the lower limit limit value (S13). Subsequently, the actuator 31 is moved upward and applied to the displacement detector 15 (S14). Then slowly move the actuator 31 downward,
It stops at a predetermined count value and sets the drive signal output value as the upper limit limit value (S15). With the above, the limit setting is completed, the switch SW1 is switched to the upper position to turn on the servo mechanism, the switch SW2 is switched to the lower position to enter the setting valid state, and the normal measurement operation is started (S16).

There are the following two methods for the focus zone setting operation. In the first method, preliminary measurement is actually performed to set the upper limit value and the lower limit value of the focus zone shown in FIG. The second method sets an upper limit value and a lower limit value of the focus zone in the form of ± Z with respect to an arbitrary current value.

In the case of the first method, in the preliminary measurement, as shown in FIG. 5, the gauge block 51 whose surface position is slightly lower than the surface position (broken line) of the transparent body to be measured and the surface position slightly A high gauge block 52 is used. Then, as shown in FIG. 6, the block 51 is first focused on the surface of the measurement table, and the lower limit value of the focus zone is set (S21). Then, the block 52 is focused on the surface of the measurement table, and the upper limit value of the focus zone is set (S22). Then, the servo mechanism is turned on to start the normal operation (S23).

In the case of the second method, as shown in FIG. 7, the actuator is moved by -Z [μm] from the current position to set the lower limit value of the focus zone (S31). Then move the actuator to + 2Z [μm], that is, from the current position +
It is moved to the position of Z [μm] to set the upper limit value of the focus zone (S32). Then, the servo mechanism is turned on to start the normal operation (S33).

In this embodiment, when the limit setting and the focus zone setting are performed at the same time, either one can be prioritized by switching. The case where the scan inspection of the surface of the transparent body is performed by giving priority to the focus zone setting will be described as follows. If the focus zone is not set, the focus error signal obtained by the displacement has zero values at the front surface position and the back surface position of the transparent body as shown in FIG. 8, and there may be two focus positions. Therefore, there is a possibility that the back surface is erroneously focused when there is a scratch or the like as described above. On the other hand, if the focus zone is limited to the range shown in FIG. 8, malfunction can be prevented. If the limit function is prioritized, the range of movement of the objective lens will be limited to software, and the situation of violent collision with the case or the like will be prevented, and the reliability of the measuring device will be high.

Although the upper limit of the movable range of the actuator is the displacement detector and the lower limit is the case in the embodiment, when the movable range is physically limited by other than these, for example, the upper limit is the coil and the magnet. In some cases, the present invention is effective. Further, although the pinhole method is used in the embodiment, the present invention can be similarly applied to other focusing methods.

[0024]

As described above, according to the present invention, in the focus displacement measuring apparatus, by providing the means for controlling the servo circuit operation so as to limit the moving range of the objective lens, the error in the transparent body measurement can be prevented. It is possible to prevent the measurement and improve the reliability of the measurement.

[Brief description of drawings]

FIG. 1 shows the configuration of a focusing point displacement measuring device according to an embodiment of the present invention.

FIG. 2 shows a servo circuit configuration of the same embodiment.

FIG. 3 shows how limit / focus zones are set in the same embodiment.

FIG. 4 shows an operation flow of limit setting of the embodiment.

FIG. 5 shows a preliminary measurement method for setting a focus zone in the same example.

FIG. 6 shows an operation flow of focus zone setting by the first method.

FIG. 7 shows an operation flow of focus zone setting by the second method.

FIG. 8 shows an operation of focus zone control according to the same embodiment.

[Explanation of symbols]

1 ... Semiconductor laser, 2 ... Polarization beam splitter, 3 ... 1
/ 4 wavelength plate, 4 ... Collimating lens, 5 ... Objective lens,
6 ... Object to be measured, 7 ... Imaging lens, 8 ... Beam splitter, 9, 10 ... Light receiving element, 11 ... Coil, 12 ... Magnet, 13 ... Scale, 14 ... Encoder, 15 ... Displacement detector, 21 ... Focus error Signal generation circuit, 22 ...
Servo circuit, 23 ... Control circuit, 24, 25 ... Aperture, 23
1 ... Focus zone judging circuit, 232 ... Limit judging circuit, 233 ... Switching circuit.

Claims (3)

[Claims] 1. An optical system having a light beam irradiating means for irradiating an object to be measured with a light beam through an objective lens and a light receiving means for receiving a reflected light beam from the object to be measured, and a displacement amount of the objective lens. Displacement detection means, a focus error signal generation means for processing an output signal of the light receiving means to generate a focus error signal, and a combination of the objective lens based on the focus error signal and the output of the displacement detection means. Focusing type characterized by having servo circuit means for driving to a focal position and control means for controlling the operation of the servo circuit means for limiting the movement range of the objective lens based on setting information from the outside. Displacement measuring device. 2. The servo circuit, when measuring the front surface or the back surface when the object to be measured is a transparent body, defines a focus zone only in the vicinity of either one of them and prevents the servo circuit from deviating from the range. 2. The in-focus type displacement measuring device according to claim 1, further comprising zone determining means for controlling the means. 3. The control means includes limit determination means for controlling the servo circuit means so as to limit the movement of the objective lens within a range of an upper limit and a lower limit of a physical movable range of the objective lens. The in-focus displacement measuring device according to claim 1.