JPH01200312A - Automatic focus controller using laser light - Google Patents

Abstract

PURPOSE:To determine a best focus position by detecting the maximum and minimum values of the intensity of reflected light at two points which differ in focus-directional distance from each other and varying the focus-directional distance between an objective and a body to be measured. CONSTITUTION:Two light beams 105 and 106 which are reflected by the body 7 to be measured is changed in course by a beam splitter 101 and guided out as reflected light beams 107 and 108, which are put one over the other by a condenser lens 109 as converged reflected light 110, whose reflected light intensity is detected by a maximum and minimum intensity detector 11. In this case, a reflected light intensity pattern corresponding to the setting of the distance between points where the beams constituting the two beams 105 and 106 have maximum intensity is obtained, but a V-shaped pattern has the largest maximum-minimum intensity difference at the best focus point. A control direction deciding means 12 detects which side of the best focus position the focus position deviates to and performs focus control. Consequently, the best focus position is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an autofocus control device that is applied to measurement of minute dimensions in the submicrometer region using laser light.

[Background of the invention]

With recent advances in precision processing technology, the dimensions of workpieces have become smaller, reaching the order of submicrometers.

For example, in the fields of integrated circuits, magnetic heads, etc., highly accurate measurement of dimensions in the submicron region is required, and measurement by optical means has become mainstream.

In the case of optical dimension measurement, the main method used is to detect the reflected light of the light irradiated on the object to be measured and calculate the dimensions from the intensity of the reflected light or the shape of the reflected light. The state of light varies greatly depending on the distance between the objective lens that irradiates the light and the object to be measured.

Therefore, in order to carry out stable measurements, it is necessary to always set the object to be measured at the focal position of the objective lens, which requires autofocus control.

[Conventional technology]

The autofocus method often used in the field of optical measurement is called the astigmatism method. It's a method.

The astigmatism method uses a combination of a cylindrical lens and a 4-split optical sensor to detect the beam shape of the reflected light from the object to be measured and performs focus control.

When the object to be measured is set at the focal position of the objective lens, the light intensity distribution of the reflected light received by the 4-split photo sensor is circular, so the output voltage from the 4-split photo sensor is zero. Further, when the object to be measured is not at the focal position, the light intensity distribution of the reflected light becomes elliptical, and the output voltage from the 4-split optical sensor has positive and negative signs with absolute values greater than zero. Autofocus control is performed by determining the output voltage of the 4-split optical sensor in this manner.

[Problem to be solved by the invention]

The conventional autofocus method described above measures the beam shape of reflected light, and this reflected light shape does not change sensitively to the focus position.

Although it is easy to distinguish between an elliptical shape and a circular shape that have changed significantly, it is difficult to determine the difference between an elliptical shape that is close to a circular shape and a circular shape when close to the focus position.

This is because when the intensity distribution of the reflected light has a Gaussian intensity distribution, the output voltage from the 4-split optical sensor changes little because the shapes are compared in a region where the intensity level of the reflected light is low.

In order to measure submicron dimensions with dimensional stability on the order of 0.01 μm, the focus position must be controlled at 1 μm.
It is necessary to perform this on the order of 1 μm, and the conventional autofocus control described above cannot control on the order of 1 μm.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and provide an autofocus control device that is highly sensitive to changes in focus position.

[Means to solve the problem]

In order to achieve the above object, the present invention has the following configuration.

When the laser beam emitted from the laser light source is focused by an objective lens and irradiated onto the surface of the object to be measured, and the intensity of the reflected light is detected to perform autofocus control, the focused laser beam is focused on the surface of the object to be measured. a reflected light intensity pattern creation section for creating a reflected light intensity pattern by deflecting the reflected light at the top and detecting the reflected light intensity for each deflection state within the deflection region; a focal direction distance control means for changing a distance in a focal direction between the two; and a distance change of a first magnitude emitted by the focal direction distance control means;
The maximum and minimum values of the reflected light intensity in a preset area of the reflected light intensity pattern from the object to be measured are detected at at least two points having different distances in the focal direction, and the intensities of the maximum and minimum values are detected. control direction determining means for determining the direction in which to change the distance in the focal direction between the objective lens and the object to be measured based on a change in the distance; and a distance change of a second magnitude generated by the focal direction distance controlling means. The distance is changed in the direction determined by the control direction determining means, and the maximum and minimum values of the reflected light intensity in a preset area of the reflected light intensity pattern are detected for each distance state, and the It consists of an optimum intensity control means that stores the intensity of the maximum value and the minimum value, and sets the distance in the focal direction to the position where the intensity of the difference between the maximum value and the minimum value is maximum.

[Effect]

When performing autofocus control using the above means, it is important to first determine the direction in which focus control will be performed.

When two beams are created from a laser beam and the two beams are deflected, a reflected light intensity pattern is obtained that corresponds to the setting of the distance between the points where the intensity of each beam making up the two beams is maximum. However, for example, in the case of a V-shaped reflected light intensity pattern, the difference in intensity between the maximum intensity and the minimum intensity is maximum at the best focus point, and as the focus position changes, the above-mentioned difference in intensity decreases, and The change in the difference intensity is represented by an upwardly convex quadratic curve when the pest focus point is the center.

At this time, there are at least two different distances in the focus direction.
The above-mentioned difference intensity is measured at the point to detect whether the focus position is shifted to the left or right of the curve with respect to the peak (best focus position) of the quadratic curve.

Next, focus control is performed in the detected direction to detect the best focus position. The above-mentioned difference intensity increases according to a quadratic change, and eventually passes a peak and reverses to a decreasing direction. The best focus position is determined by detecting a change in this difference intensity from increase to decrease.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a system block diagram illustrating an embodiment in which the autofocus control device of the present invention is applied to minute dimension measurement.

Reference numeral 1 denotes a laser light source, for example, an I-Ie-N e laser oscillation tube, which emits laser light 100 having a wavelength of 01633 μm and a Gaussian intensity distribution. 2 is laser beam 1
An optical system that converts the optical path, beam shape, etc. of 00, which includes an acousto-optic element (hereinafter abbreviated as KA, O) 6, a beam splitter 101, an objective lens 102, and a condenser lens 10.
9 and various other lenses and optical elements (not shown).

4 is an acousto-optic element driver for controlling the optical operation of A, 0.3 (hereinafter abbreviated as A, 0. driver)
The frequency generator 5 and the ramp wave generator 6 are operated as inputs, and an electrical signal in a high frequency band is outputted to A, 0.3.

The frequency generator 5 receives a laser beam 10 incident on A, 0.3.
Two beams of light 106 and 104 are generated that have an angle of 0 to 0 m, are separated from each other, and travel in different directions. At this time, the frequency f emitted from the frequency generator 5
The angle θm changes in proportion to m.

The ramp wave generator 6 controls the optical deflection of A, 0.3, and changes the traveling direction of the two-beam light 106.104 while keeping the angle θm formed by the two-beam light 106.1.04 at the same angle. At this time, the optical deflection position is determined in proportion to the voltage Va emitted from the ramp wave generator 6.

Two beams of light emitted by A, 0.3 106.10
4 passes through the beam splitter 101 and enters the objective lens 1.
02, the light is focused into a minute spot diameter, and is converted into two focused two-beam lights 105 and 106 that travel close to each other in parallel. The individual light beams of the two focused beams 105 and 106 have a Gaussian intensity distribution, and the distance between the maximum points of the intensity is defined as the peak-to-peak distance C
The peak-to-peak distance CW, which will be referred to as W, is set depending on the angle θm formed by the two beams 106 and 104 mentioned above, the focal lengths of various lenses constituting the optical system 2, and the focal length of the objective lens 102. be done.

The two focused beams of light 105 and 106 are irradiated onto the object 7 whose dimensions are to be measured. Reference numeral 8 denotes a distance control means in the focal direction, which is composed of, for example, a moving stage on which the object to be measured 7 is placed and moved in the focal direction. Control.

Two-beam light 105.10 reflected by the object to be measured 7
6 is changed in its course by the beam splitter r101 and taken out as reflected light 107, 108 and sent to the condenser lens 1.
09, the intensity of the reflected light is detected as condensed reflected light 110 superimposed at one point.

Reference numeral 9 denotes a photoelectric conversion section, which receives the condensed reflected light 110 with a photodetector such as a PiN photodiode and performs current-voltage conversion.

In the case of the configuration of the present invention, the frequencies of the two beam lights 106 and 104 emitted by A, O, and t have different frequencies depending on the configurations of A, O, and driver 4.
Since the photoelectrically converted electrical signal becomes a signal in which a DC voltage is superimposed on an AC voltage signal, the AC component is cut off and only the DC voltage is detected.

10 is a reflected light intensity pattern creation section, which includes a ramp wave generator 6;
In the region where the light is deflected by the voltage change, the photoelectrically converted voltage for each light deflection state is A/D converted and a voltage proportional to the reflected light intensity is stored as a function of the lamp wave voltage.

Reference numeral 11 denotes a maximum/minimum intensity detection unit that detects the maximum and minimum values of the reflected light intensity in a preset area of the reflected light intensity pattern. Two beams of light focused here 105.1
06 peak-to-peak distance CW is 30% of the diameter of each individual beam.
If set to a small value below 105.
The 106 combined light intensity distributions are superimposed on each other and become substantially equivalent to one beam of light, and the reflected light intensity pattern in this case has a V-shape. In addition, if the peak-to-peak distance CW is set to a value as large as the diameter of each beam, the reflected light intensity pattern becomes a W-shape, and the maximum and minimum intensities in the area where the ■-shape or W-shape pattern is set are detected. do.

Reference numeral 12 denotes a control direction determining means, which determines the direction when changing the distance in the focal direction between the objective lens 102 and the object to be measured 7. For example, if the object to be measured 7 is closer than the focal position of the objective lens 102, If the focal point is farther than the focal point, the control is performed in the direction of moving away from the focal point, and if the focal point is farther than the focal point, the control is performed in the direction of approaching the focal point.

For example, if the reflected light intensity pattern is V-shaped, the maximum intensity of the ■-shaped pattern is the maximum in the best focus state.
The minimum intensity becomes the minimum, and the difference intensity between the maximum intensity and the minimum intensity becomes the maximum. However, as the focus position shifts, the maximum intensity changes to a smaller value and the minimum intensity to a larger value, so the difference intensity becomes It becomes smaller compared to the best focus state.

At this time, the change in the difference intensity with respect to the distance in the focal direction is represented by a quadratic function curve with an upwardly convex peak or a higher-order even function curve, and the best focus point is at the peak position of the difference intensity. Become.

Now, even if the maximum intensity and minimum intensity of the V-shaped pattern Q are detected and the difference intensity is calculated, it is unclear which side of the curve it is located on, relative to the peak position of the quadratic function curve mentioned above. .

The control direction determination means 12 issues a signal 112, and the focal direction distance control means 8 causes a distance change of a first magnitude to change the distance between the objective lens 102 and the object to be measured 7. Since this first distance change is only for determining the direction, it may be a relatively large change, for example, a change of about 20 μm.

By performing this first distance change, the direction in which focus control should be performed is determined from the difference intensity change at at least two points having different distances in the focal direction.

The determination at this time will be described later.

Reference numeral 16 denotes optimum intensity control means, which moves the object 7 to be measured in the direction determined by the control direction determination means 12 to determine the best focus position. The signal 11 is controlled by the optimum intensity control means 13.
4, and the focal direction distance control means 8 causes the distance change of the second magnitude to change the distance between the objective lens 102 and the object to be measured 7. Since this second distance change is for best focus control, a small distance change, for example, a change of about 1 μm is required.

Each time the distance in the focal direction is changed by applying a second distance change, the reflected light intensity pattern of the set area is detected by deflecting the light, and the maximum/minimum intensity detection unit 11 detects the reflected light. The maximum and minimum intensity values are detected, and the maximum and minimum voltage values at that time are stored in the maximum and minimum intensity storage section 14.

As described above, the difference intensity between the maximum φ and minimum intensity changes quadratically with respect to a change in distance, but the difference intensity increases as the distance in the focal direction changes and eventually decreases.

The optimum intensity control means 16 detects the change in the difference intensity (increase→decrease), and when the difference intensity starts to decrease, the control signal 114 stops the movement of the distance by the focal direction distance control means 8, and the maximum/minimum The distance state when the difference intensity between the maximum and minimum reflected light intensity stored in the intensity storage unit 14 is maximum is calculated, and the distance in the focal direction is returned to the distance state at that time. . This state is the best focus state.

Reference numeral 15 denotes a dimension calculating section which calculates dimensions from the intensity of the maximum and minimum values in a preset area of the reflected light intensity pattern of the object to be measured 7 when controlled to the best focus position by the optimum intensity control means 16. The method of measuring minute dimensions from the intensity level of the reflected light intensity pattern by optical deflection of two beams is described in Japanese Patent Application No. 62-51617.
Since the details are described in Japanese Patent Application No. 62-85545, they will be omitted in this application.

FIG. 2 shows an example of a reflected light intensity pattern and a difference intensity pattern for explaining the operation of the control direction determining means 12.

The V-shaped waveform 21 in FIG. 2(a) is an example of a reflected light intensity pattern when the peak-to-peak distance CW of the two focused beams 105 and 106 is small, and is in the best focus state.

The horizontal axis of the graph is the lamp wave voltage that deflects the light, and two beams of light are irradiated at positions proportional to the voltage. The vertical axis of the graph is the reflected light intensity.

Reference numeral 210 denotes a calculation area set in advance for analyzing the waveform 21. 212 is the maximum intensity within the calculation area, and 214 is also the minimum intensity. V in Figure 2 (b)
The mold waveform 22 corresponds to the case where the focus position corresponding to the state shown in FIG. 2(a) is shifted. 216 is calculation area 2
10 is the maximum intensity, 218 is also the minimum intensity.

The W-shaped waveform 26 in FIG. 2 p1 is the condensed two-beam light 1.
This is an example of a reflected light intensity pattern when the peak-to-peak distance CW of 05.106 is large, and is in the best focus state. 2
20 is the maximum intensity within the calculation area 210, and 222 is the minimum intensity.

The W-shaped waveform 24 in FIG. 2 (2) is the case when the focus position corresponding to the state of FIG. 2 (/j) is shifted.

In both the waveforms of the ■-shaped pattern and the W-shaped pattern, the maximum intensity is the maximum and the minimum intensity is the minimum at the best focus position, so the difference in intensity between the maximum intensity and the minimum intensity is the maximum. On the other hand, if the focus position shifts, the maximum intensity decreases and the minimum intensity increases, so the difference intensity decreases.

In the following explanation, the case of the ■-shaped pattern will be explained, but the same applies to the case of the W-shaped pattern.

A waveform 25 in FIG. 2(e) is a waveform of the difference intensity between the maximum intensity and the minimum intensity of the ■-shaped pattern, and the horizontal axis of the graph is the distance toward the focal point, hereinafter referred to as the Z-direction distance. The vertical axis is strength. 2
60 is the maximum point of the difference intensity, and the distance in the Z direction is in the state of best focus, and it is assumed that Z=0 at this point.

The waveform 25 is expressed by a quadratic function having an upwardly convex peak centered on Z20. For example, the direction in which the object 7 approaches the objective lens is the CF direction of Z)O, and conversely, the direction in which the object 7 is moving away from the objective lens is Z
If this is called the CB direction of <0, the difference intensity decreases when the state of IZ+)0 (out of focus) occurs. Moreover, since the waveform 25 changes almost symmetrically with respect to Z=0, it is impossible to determine whether to control in the CF direction or in the CB direction from only the information on the difference intensity at the - point.

An example of control direction determination is shown in FIG.

In the case of FIG. 2(f), the initial focus shift is large and the level of the differential intensity is smaller than the peak 260 of the differential intensity. It is assumed that the initial movement direction to determine the control direction is the CF direction.

If the initial differential intensity is at the position of point 262, then if the differential intensity is moved, for example, 20 μm in the CF direction, and the differential intensity becomes the point 234, and the differential intensity increases, then the CF direction becomes the forward direction, and in the correct control direction. be.

Also, assuming that the initial difference intensity is at the position of point 266,
If the differential intensity is moved by 20 μm in the CF direction and the differential intensity is reduced to the position of point 268, the CF direction is the opposite direction, and it is determined that the control direction is to be controlled in the CB direction because this is an incorrect control direction.

In this manner, when the intensity level of the difference intensity is low, the control direction can be determined by comparing the difference intensities at two points having different distances Z in the focal direction. In order to set the size of the movement distance and the number of movements in this case, a slice level 261 is provided for the intensity level of the difference speed, and if the initial speed difference is lower than the slice level 261, the intensity of two points is set. Direction judgment is performed with a large distance change at the level.

If the initial speed difference is greater than the slice level 261, an error may occur in determining the control direction as shown in FIG. 2(G). When the initial differential velocity is at point 240, for example, by moving 20 μm in the CF direction, the differential velocity becomes point 24.
Suppose you are in position 2.

In this case, since the differential speed changes in an increasing direction, it is determined that the CF direction is the forward direction.

Since the differential velocity at point 242 is lower than the differential velocity peak 230, it is actually necessary to control in the CB direction.

Figure 2 (7) shows the slice level 2 where the initial differential velocity is determined.
An example of determining the control direction when the value is larger than 61 is shown.

Assume that the initial differential velocity is at point 250 in FIG. 2H, and that the initial control direction is the CF direction. In this example, unlike the previous case, the difference speeds are compared between three points. Move the distance in the Z direction by 15 μm to point 252
A state with a differential velocity of
If the differential speed state of 54 is obtained, the change in differential speed will be from increase to decrease, and if this is the case, the differential speed will exceed the peak 260, so control should be carried out in the CB direction from point 254. It is determined that it is good.

In the case of this example, it is necessary to set the magnitude of the change in the distance in the Z direction in consideration of the change in differential speed with respect to the change in Z of the waveform 25 and the intensity of the slice level 261. That is, each Z
It is necessary that the change in the differential speed be large to some extent with respect to the movement of the distance in the direction.

Further, if the intensity change in the waveform 25 in the Z direction is broad, the difference velocity may be compared between four points.

As described above, it is possible to easily determine the control direction in the Z direction by detecting increases and decreases in the differential speed with respect to changes in the distance in the Z direction.

The operation of the optimal intensity control means 16 will be explained with reference to a differential velocity waveform in FIG.

FIG. 3(A) shows a case where the differential speed is lower than the slice level 261 when it is determined that the direction to be controlled is the CF direction. Assume that the differential speed when the direction is determined is at a point 260. Since the optimum intensity control means 16 needs to control pest focus, it is necessary to sequentially change the Z direction distance in the set direction. At this time, if the differential speed is lower than the slice level 261, for example, 10 μm.
When the differential velocity exceeds the slice level 261 and reaches the point 262, the CF
The differential velocity is detected by changing the amount of movement in the direction, for example, in steps of 1 μm.

From point 262 to point 264, the differential velocity increases as the Z direction distance changes. From point 264 onwards, the change in differential speed decreases, and movement in the CF direction is stopped at the differential speed at point 266 where the decrease occurs twice in succession.

The maximum value, minimum value, and difference of reflected light intensity stored in the maximum/minimum intensity storage unit 14 for each distance state when moving the Z direction distance from the state of point 260 to the state of point 266 The speed is determined, the distance when the differential speed is maximum is calculated, and the vehicle is moved back in the CB direction. This state is the best focus state (corresponding to point 264).

FIG. 3 (b) shows the point 268 when the initial differential velocity when the direction is determined is greater than the slice level 261.
This is the case in the state of In this example, control in the CF direction may be performed in steps of 1 μm from the beginning. The determination of the maximum point of differential velocity is the same as that described in FIG. 3(a).

FIG. 3H shows a third embodiment when performing optimal intensity control.

Assume that the differential speed when the control direction is determined is at point 268a, and it is determined that the direction control should be performed in the CF direction. If the state of differential velocity at point 270 is reached when the movement is performed twice in 1 μm steps in the CF direction, the differential velocity decreases compared to the initial state.

This means that the differential speed has deviated from the peak point 260, which means that the control direction determining means 12 has made an error in the determination.

In this way, for example, when the initial movement is performed twice, if the difference strength decreases both times compared to the previous state, it is assumed that control is being performed in the opposite direction, and the control direction is reversed and C
Control is performed in the B direction. If you move from point 270 in the CB direction, the change in the difference intensity will increase with respect to the change in distance in the Z direction, and then it will change to decrease, and point 2
The movement is stopped when the difference intensity is 74, and the maximum point 272 of the difference intensity is determined.

In this way, the change in the difference intensity always increases as the distance changes →
It is necessary to bring it into a state of decline. Note that it is necessary to determine two consecutive decreases in the difference strength because if the difference strength detection is affected by external disturbances such as vibrations, a single determination may result in an erroneous determination. .

The determination operations of the control direction determination means 12 and the optimum strength control means 16 described above are performed by software calculation processing using a microcomputer.

The calculation software according to the present invention will be explained below.

FIG. 4 shows an example of a flowchart of the operation of the control direction determining means 12.

In step 400, when the object 7 to be measured is initially set, light is deflected to obtain a reflected light intensity pattern. Step 400 detects the maximum intensity and minimum intensity in the area where the reflected light intensity pattern is set, and calculates the difference intensity.

Step 404 determines the magnitude of the difference intensity calculated in step 402 and the magnitude of the difference intensity to be set as a preset slice level.

In step 406, in accordance with the determination in step 404, the number of times the object to be measured 7 is to be moved a distance in the Z direction is determined in order to determine the control direction.

Step 408 similarly determines the direction in which the movement is to be performed in the Z direction (this may be set initially) and the magnitude of the distance change.

Step 410 operates the focal direction distance control means 8 to change the distance in the Z direction. When the movement is completed, step 412 deflects the light to create a reflected light intensity pattern, and step 414 similarly detects the maximum and minimum intensities, calculates the difference intensity, and stores it in the memory device.

In step 416, it is determined whether the number of movements set in step 406 has been reached. If the number of movements has not been reached, the operation from step 410 is repeated in a loop 417, and when the number of movements has been reached, in step 418, the distance in the focal direction is The operation of the control means 8 is stopped.

When the movement is completed, a change in the difference intensity is then determined. In step 420, a change in the differential intensity is determined to determine in which direction the control should be performed in the future, and then the operation moves to optimal intensity control.

FIG. 5 shows an example of a flowchart explaining the operation of the optimum strength control means 16.

Step 500 sets the magnitude of distance change when moving from the maximum and minimum values of reflected light intensity and difference intensity detected when the control direction was determined in step 420 of FIG. 4.

In step 502, the focus direction distance control means 8 moves the focal point by the amount set in step 500.

Step 504 performs light deflection to create a reflected light intensity pattern. Step 506 detects the maximum and minimum values of the reflected light intensity and calculates the difference intensity.

In loop 508, the same operations as steps 502, 504, and 506 are repeated again.

In step 510, the control direction is determined to be correct by comparing the change in the difference intensity when the direction was determined in step 420 of FIG. Determine whether If the change in the difference intensity between each point increases twice in succession, the direction of control is forward, so control is performed in the direction set in step 420.

If the change in the difference intensity between each point decreases twice in a row or changes from increase to decrease, the direction of control is in the opposite direction, so in step 512, the direction opposite to that set in step 420 is changed. Change to direction control.

Step 514 compares the magnitude of the intensity level of the difference intensity detected when the second distance movement was performed and the intensity of the slice level 231 described above. The magnitude of the distance change in the Z direction is set, and the distance in the Z direction is moved in step 518.

Steps 520 and 522 perform the same operations as steps 504 and 506 described above.

Step 524 detects the change in differential intensity before and after the distance movement. If the difference intensity after movement is larger than the difference intensity before movement, the difference intensity changes in an increasing direction. If the change in the difference intensity is increasing in step 526, the above-described distance movement operation is repeated in loop 528.

If the change in the differential intensity decreases, it is determined in step 530 whether the decreasing change in the differential intensity occurs twice in a row. If this does not occur continuously, the distance movement operation is repeated again in loop 528, but if the change in the difference intensity decreases twice in succession, the distance movement is stopped in step 562.

In step 564, step 506 and step 522
The maximum in each distance state detected and stored in
The state of distance from the minimum intensity to the maximum difference intensity is detected.

In step 566, the direction of the control that has been performed so far is reversed to the opposite direction, and the distance is moved to the position where the difference intensity calculated in step 564 is maximum.

This position is the best focus position, and the light is deflected again at this position to create a reflected light intensity pattern, and the maximum and minimum intensities at that time are detected.

The dimensions of the object to be measured are calculated from the minimum intensity and maximum intensity of the reflected light intensity pattern controlled to the best focus position.

In this case, determining the dimensions from the absolute values of the maximum and minimum strengths is not a good method.

This is because erroneous measurements may be made if the light intensity of the laser beam varies. Therefore, the dimensions may be calculated from the value of the ratio of the minimum strength to the maximum strength.

〔Effect of the invention〕

As is clear from the above explanation, by performing autofocus control according to the present invention, the object to be measured can always be set at the best focus position at high speed and with high precision, and with high resolution on the order of 0.01 μm. It becomes possible to stably measure dimensions in the submicron region.

Furthermore, the best focus position control can be executed with simple software, and it becomes possible to implement the device with an inexpensive configuration.

[Brief explanation of the drawing]

Fig. 1 is a system block diagram for explaining an embodiment of the autofocus control device of the present invention applied to minute dimension measurement, and Fig. 2 A) to C are control directions of the autofocus control device of the present invention. Figure 3 is a waveform diagram showing the reflected light intensity pattern and the change in the difference intensity with respect to the distance and the state of the change in the difference intensity depending on the control position to explain the setting operation.
B) ~ (/→ is a waveform diagram illustrating the state of change in differential intensity due to change in control position to explain the optimal intensity control operation of the autofocus control device of the present invention, and Fig. 4 is a waveform diagram illustrating the state of change in differential intensity due to change in control position. FIG. 5 is a flowchart explaining the operation of determining the control direction of the control device. FIG. ... Acousto-optic element, 7 ... Measurement object, 8 ... Focal direction distance control means, 10 ...
・Reflected light intensity pattern creation section, 11... Maximum
Minimum intensity detection unit, 12...Control direction determining means, 16...Optimum intensity control means, 14...Maximum/minimum intensity storage unit, 15...
...Dimension calculation section. Figure 3 (A) (B) CB
CF (c) Fig. 4 Kantsubin 1 secondary leg

Claims (1)

[Claims] In an autofocus control device that performs autofocus control by focusing the laser light emitted from a laser light source into a minute spot diameter using an objective lens and irradiating it onto the surface of the object to be measured, the intensity of the reflected light is detected. a reflected light intensity pattern creation unit that deflects the reflected laser light on the object surface to be measured and detects the reflected light intensity for each deflection state within the deflection region to create a reflected light intensity pattern; and the objective lens; A focal direction distance control means for changing the distance in the focal direction from the surface of the object to be measured, and a distance change of a first magnitude emitted by the focal direction distance control means, so that the distances in the focal direction are different from each other. The maximum and minimum values of the reflected light intensity in a preset area of the reflected light intensity pattern from the object to be measured are detected at at least two points, and the objective lens and the object are detected based on the change in the intensity of the maximum and minimum values. control direction determining means for determining the direction in which the distance in the focal direction between objects to be measured is changed; and a second control direction emitted by the focal direction distance controlling means.
The distance is changed in the direction determined by the control direction determining means by a distance change of the magnitude, and the maximum value of the reflected light intensity in a preset area of the reflected light intensity pattern is determined for each distance state. It is characterized by comprising an optimum intensity control means that detects the minimum value, stores the intensity of the maximum value and the minimum value, and sets the distance in the focal direction to a position where the intensity of the difference between the maximum value and the minimum value is maximum. Autofocus control device using laser light.