JPH04250407A - Automatic focusing device - Google Patents

Abstract

PURPOSE:To realize high-accuracy focusing without generating an error depending upon a way of lighting by performing arithmetic operation based upon a target movement quantity and the movement quantity of an object which is measured by a light wave interference type displacement measuring means. CONSTITUTION:A control means 6 outputs a signal required to elevate a stage 2 by a constant quantity to a stage driving mechanism 5 to elevate the stage 2. At this time, the movement quantity of the object 13 is measured by using the light wave interference type displacement measuring means 4. Then the control means 6 detects the place where the variation quantity of the measured value becomes minimum when the object 13 is displaced by a constant quantity and recognizes the position as a focus position. Then the control means 6 supplies the fed-back signal to the stage driving mechanism 5 to move the object 13 for automatic focusing.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an automatic focusing device, and more particularly, the present invention relates to an automatic focusing device, and more particularly, it detects the point where the amount of change in measured value is the minimum when an object to be measured is displaced by a certain amount. The present invention relates to an automatic focusing device that can perform highly accurate automatic focusing by recognizing the focal position as the focal position.

0002

2. Description of the Related Art Conventionally, as an automatic focusing device, there has been a device as disclosed in Japanese Patent Application Laid-open No. 10117/1983. According to this automatic focusing device, the object to be imaged is placed on a stage on the microscope table, and the inspection surface is magnified by the microscope through the objective lens using the proof light, and by the TV camera. Take an image. A brightness histogram representing the number of pixels with respect to brightness is determined from the image data, the brightness distribution of the brightness histogram is separated into two based on the brightness level within a predetermined brightness range, and a normal distribution is determined for each brightness distribution. Then, the overlapping area of the two normal distributions and the minimum value of the area are determined, and the distance is set using the point where the minimum value is the minimum based on the change when the distance of the optical system lens to the object to be measured is changed as the focal position. After that, a feedback signal is given to the drive device to move the stage and automatically focus the microscope.

0003

[Problems to be Solved by the Invention] However, with the above-mentioned method that utilizes the contrast between the object to be measured and its background, highly accurate focusing is not possible.
There is a drawback that errors may occur depending on the way the lighting is applied.

The present invention was made in order to solve the above-mentioned problems, and it is possible to calculate the illumination by calculating the movement amount of the object to be measured and the target movement amount measured by a light wave interference type displacement measuring means. It is an object of the present invention to provide an automatic focusing device capable of performing highly accurate focusing without causing errors due to the method of focusing.

[0005]

[Means for Solving the Problems] In order to achieve this object, an automatic focusing device of the present invention includes a stage drive mechanism, a control means for controlling the stage, and a target to be measured placed on the stage. The reflected light from the measured object is received by irradiating light toward the measured object to produce a minute displacement of the object, and the reflected light is calculated by using optical interference between the reflected light and a reference light or standard light. A light wave interference type displacement measuring means that measures based on frequency or phase shift, and a movement amount and a target movement amount of the object to be measured, which are measured by the displacement measuring means while controlling the first stage using the control means. and recognition means for recognizing the focal point position from the calculation.

[0006]

[Operation] According to the present invention having the above configuration, when the control means outputs a predetermined signal necessary for raising the stage by a fixed amount to the stage drive mechanism, the object to be measured on the stage is raised by a fixed amount. do. The amount of movement of the object to be measured at this time is measured using a light wave interference type displacement measuring means. Next, the recognition means detects the point where the amount of change in the measured value becomes the minimum when the object to be measured is displaced by a certain amount, and recognizes that position as the focal position. After that, the control means feeds back the signal to the stage drive mechanism to move the object to be measured and automatically adjust the focus.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the automatic focusing device of the present invention will be described below with reference to the drawings.

FIG. 1 shows the overall configuration of this device. A stage holding part 12 is provided at the left end of the base 1, and the stage 2 is attached to this stage holding part 12 so as to be movable in the vertical direction in the figure. A piezoelectric element 3 that can be compressed and expanded in the vertical direction and a coil spring 11 in an expanded state are attached between the base 1 and the stage 2. On the other hand, an object to be measured 13 is placed on the upper end surface of the stage 2 .

The electronic control device 6 includes a CPU 7, a ROM 8,
It is a so-called microcomputer composed of RAM 9 and the like, and includes stage control means and focus position recognition means. CPU7 is ROM8 or RAM
9, the input signal is processed according to a program stored in advance in the ROM 8, and an output signal is output to the stage drive circuit 5. Within the stage drive circuit 5, the digital signal processed by the CPU 7 is sent to a D/A converter (
(not shown) into an analog voltage, and that voltage is applied to the piezoelectric element 3.

The light wave interference type displacement meter 4 includes an objective lens 14 that performs an irradiation action, a light source (not shown) that irradiates the object to be measured 13 through the objective lens 14, and the objective lens 14.
A photosensor (not shown) receives reflected light from the object to be measured 13 through a photo sensor (not shown), and detects the frequency and phase shift of the reflected light by using light wave interference between the reflected light and the reference light or reference light. The detection circuit 10 detects the amount.

In this embodiment, the light wave interference type displacement meter 4 is one that can detect the displacement of the object to be measured 13 with a resolution of several angstroms, for example,
The one described in the specification of No. 31152 is adopted. The detection circuit 10 counts the frequency and phase shift amount of the reflected light and can read it as displacement information from the electronic control device 6.

The configuration of the detection circuit 10 will be explained with reference to FIG. In the figure, the measurement beat signal DS1 and the reference beat signal BS1 are shaped into rectangular pulse waveforms by waveform shapers 66 and 68 having an amplification function and a waveform shaping function, and then supplied to a first counter 70 and a second counter 72. and counted. That is, the first counter 7
0, the measurement beat signal DS1 having the measurement beat frequency fhd is counted, and the second counter 72 counts the reference beat signal BS1 having the reference beat frequency fhb.

The counted value Cd of the first counter 70 and the second
The count value Cb of the counter 72 is temporarily stored in the first latch 74 and the second latch 76 at the same time as the predetermined counting period ends, and the stored content of the second latch 76 is inverted in sign inversion circuit 78. and the first
The contents stored in latch 74 are added in adder 80 . That is, the storage contents of the second latch 76 are subtracted from the storage contents of the first latch 74, and the result of the subtraction (Cd-
Cb) is temporarily stored in the third latch 82. This stored content (Cd-Cb) is the difference between the measured beat frequency fhd and the reference beat frequency fhb, and represents the amount of movement of the stage 2 per unit time. Here, if the laser wavelength used is λ, the amount of movement of stage 2 is as follows (1
) is calculated using the formula.

(λ/2)×(Cd−Cb) (1) Next, the operation of this embodiment will be explained according to the flowchart of FIG. First, in step S1 (abbreviated as S1, the same applies hereinafter), the CPU 7
outputs a digital signal to the stage drive circuit 5 to move the stage 2 to the origin. The stage drive circuit 5 applies an analog voltage corresponding to the signal to the piezoelectric element 3 to move the stage 2 to the origin. In S2, RAM
9 is stored in the one-dimensional array Za[0] defined above. Here, Za represents the actual amount of movement of the object to be measured 13 from the origin. In S3, 0 is stored in a one-dimensional array Zb[0] defined on the RAM9. Here, Z
b represents the amount of movement of the object to be measured 13 from the origin measured using the light wave interference type displacement meter 4. In S4, the subscript i of the array is initialized to 1 and the incremental displacement amount ΔZ is initialized to 0.01 μm. In S5, the CPU 7 selects the object to be measured 2 by i×ΔZ
A digital signal is output to the stage drive circuit 5 to move the stage. The stage drive circuit 5 applies an analog voltage corresponding to the signal to the piezoelectric element 3, and drives the stage 2 to i.
Move to ×ΔZ. In S6, Za[i] has i×ΔZ
remember. In S7, the CPU 7 uses the light wave interference type displacement meter 4.
The contents of the third latch 82 are read and stored in Zb[i] as the amount of movement of the object to be measured 13 from the origin. In S8, 1 is added to the subscript i. In S9, i is 200
It compares whether it is larger, and if it is larger, it ends, and if it is not larger, it returns to S5. In other words, the operations in S5 to S9 are performed 2 times.
Repeat 00 times to move the object to be measured 13 by a total of 2 μm.

Examples of measurement results obtained according to the flowchart in FIG. 3 are shown in FIGS. 5 and 6. FIG. 5 is a diagram showing the actual amount of movement of the object to be measured 13 with respect to the subscript i, and FIG.
It is a figure showing the value measured using. As is clear from FIG. 6, the rate of increase in the measured value is smaller than the rate of increase in the actual amount of movement near the focal position. This phenomenon is described in the literature E. Ingelstam, “Pro
blems Related to the Accu
rate Interpretation of Mi
crointerferograms. ”in In
terferometry, National Ph.
ysical Laboratory Sympos
ium No. 11, pp. 141-163, it has been known for a long time, but since phase measurement was not performed under computer control, its characteristics could not be measured with high precision. Therefore, this phenomenon has not been utilized for focus detection.

Continuing from the flowchart in FIG. 3, in the flowchart in FIG. 4, the object to be measured 13 is
The procedure for automatically moving the camera to the focal position will be explained. S1
1 initializes the array index i to 1. In S12,
The one-dimensional array Zc[i] defined on RAM9 is expressed by the following formula (
2) and store it. Note that Zc represents the rate of increase in the measured value.

Zc[i]=Zb[i]−Z
b[i-1] (2) In S13, 1 is added to the subscript i. In S14, i is 200
It is compared to see if it is larger, and if it is larger, the process proceeds to S15, and if it is not larger, the process returns to S12. In other words, S12-S
Repeat step 14 200 times to find the rate of increase in the measured value. Proceeding to S15, the minimum increase rate Zcmi of the measured value
Initialize n to Zc[1], and initialize the subscript imin at the minimum increase rate of the measured value to 1. In S16, the subscript i
Initialize to 2. In S17, Zc[i] is Zcmi
It is compared whether it is smaller than n, and if it is smaller, the process proceeds to S18, and if it is not smaller, the process proceeds to S19. When proceeding to S18, the minimum increase rate Zcmin is set to Zc[i], and the subscript imin in the minimum increase rate is set to i. In S19,
Add 1 to subscript i. In S20, it is compared whether i is larger than 200, and if it is larger, the process proceeds to S21,
If it is not large, the process returns to S17. In other words, S17 to S2
Repeat the operation 0 200 times and find the minimum increase rate Zc of the measured value.
Find min and the subscript imin at the minimum increase rate of the measured value. Finally, the CPU 7 converts stage 2 to imin×Δ
The digital signal is sent to the stage drive circuit 5 in order to move it to Z.
The object to be measured 13 is moved to the position where the increase rate of the measured value is the minimum, that is, the focal position, and the process ends.

It should be noted that various changes can be made to this embodiment without departing from the spirit thereof. For example, if an XY-axis stage is provided in addition to a height stage, a level difference measurement device that focuses on each object to be measured with a step difference and calculates the step difference based on the amount of movement of the stage in the height direction at that time. is obtained.

[0019]

[Effect of the invention] As is clear from the detailed description above,
According to the present invention, when the object to be measured is displaced by a certain amount in the height direction, the point where the amount of change in the measured value is the minimum is detected, and that position is recognized as the focal point, so the illumination direction is adjusted accordingly. There are no errors due to Furthermore, since a light wave interference type displacement meter with a resolution of several angstroms is used as the displacement measuring means, a highly accurate focal position is required.

[Brief explanation of the drawing]

FIG. 1 is a system configuration diagram.

FIG. 2 is a configuration diagram of a detection circuit.

FIG. 3 is a flowchart for explaining a procedure for storing measurement values when an object to be measured is moved by 2 μm using the present apparatus.

FIG. 4 is a flowchart for explaining a procedure for recognizing a focal position based on data measured according to the flowchart in FIG. 3;

FIG. 5 is a diagram showing the actual movement amount of the object to be measured with respect to the subscript i.

FIG. 6 is a diagram showing the amount of movement of the object to be measured with respect to the subscript i, measured using a light wave interferometric displacement meter.

[Explanation of symbols]

2 Stage 4 Light wave interference type displacement meter (displacement measurement means) 5 Stage drive circuit

Claims (1)

[Claims] 1. A stage drive mechanism; a control means for controlling the stage; a light wave interference type displacement measuring means that receives reflected light from an object and measures the frequency or phase shift of the reflected light using optical interference between the reflected light and a reference light or standard light; automatic recognition means for recognizing the focal position from calculation of the movement amount of the object to be measured measured by the displacement measurement means and the target movement amount while controlling the former stage using the control means; Focusing device.