JPH04158320A - Focusing device - Google Patents

Abstract

PURPOSE:To obtain a focusing device with a simplified optical structure at a low cost by providing a light source radiating the desired measurement light flux, a drift detecting means and a separation changing means. CONSTITUTION:A shape measuring device is constituted of an optical unit 1 and a focusing control device 50. A semiconductor laser 9 outputs the light flux with an elliptic cross section perpendicular to the optical axis, outgoing positions of the light expanded in the long-axis direction and the light expanded in the short-axis direction are drifted along the optical axis, and the synthesized light flux has an elliptic cross section. When the light flux from the laser 9 is radiated to an object through the axis of an objective lens 5 and the reflected light forms an image on an image forming face (quartered light receiving element 17) through the axis of the lens 5, the formed image shape is changed according to the focal point drift. The device detects the change and changes the separation between an optical apparatus and the object. No cylindrical lens is required to be provided to generate astigmatism, and the structure of an optical system can be simplified at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a focusing device, and more particularly to a focusing device that focuses an optical device for observing or imaging the surface shape of an object and the object.

[Prior art] Conventionally, when observing the surface of an object using an optical instrument such as a microscope, focusing light (measuring light beam) is irradiated onto the object, and focusing is performed based on the position of the reflected light. is being carried out. For example, as shown in FIG. 5, a light beam L is incident on an objective lens T with an offset from the axis, and the light beam L reflected from the surface of an object W is reflected by a beam splitter S, and is divided into two light receiving elements P. irradiate. The spot position of the light beam L irradiated onto the two-split light receiving element P moves in the vertical direction along the dividing line P1 of the two-split light receiving element P, depending on the position of the surface of the object W.

Therefore, if we take the difference signal of each element of the two-split photodetector P, we get:
An output corresponding to the shift of focus can be obtained. Focusing is then performed based on this signal.

However, as shown in FIG. 5, for example, the object W
The light beam L is irradiated at point 2B, and when focusing is performed at point a, the position of the object is adjusted from 21 to 22, and the spot position moves to point b. For this reason, the position where you tried to focus (point a)
Then, a deviation d occurs between the actual focusing position (point b), and in the end, focusing is performed at point b. That is, even if focusing is performed by aiming at a spot position on the surface of the object W, the spot position moves, so focusing is only performed at a desired position. There is no problem if the heights of point a and point b are the same, but if the heights are different due to a step or the like, the focus at point a will be shifted.

In order to solve this problem, the applicant has devised the following device.

That is, as shown in FIG. 6, an optical unit 1 is supported on a frame F so as to be able to approach and separate from a stage 3 for placing an object W by means of a pulse motor PM. Objective lenses 5 and C for observing the object W
A CD camera 7 is provided. The optical unit 1 is provided with a light source (semiconductor laser) 9 for automatic focusing, and the semiconductor laser 9 emits a measuring beam having a wavelength in the infrared region. The emitted measurement light beam is corrected into parallel light by a collimating lens 91, and further by a cylindrical lens 92. The focusing lens 10. is corrected by a correction prism 93 so that its cross section in the direction orthogonal to the optical axis becomes a perfect circle.
After passing through the beam splitter 11, the dichroic mirror 1
3, the light is reflected so as to pass through the axis of the objective lens 5 and reaches the object W.

The light beam reflected on the surface of the object W passes through the objective lens 5 and is reflected by the dichroic mirror 13 toward the beam splitter 11. Here, since the dichroic mirror 13 has the property of reflecting infrared light and transmitting visible light, it does not interfere with the observation of the object W by the CCD camera 7.

Furthermore, the beam splitter 11 has a ratio of reflectance and transmittance of incident light of 5096.

The reflected light from the dichroic mirror 13 is reflected by the beam splitter 11, transmitted through the cylindrical lens 15, and then received by the four-split light receiving element 17.

The cross-sectional shape of the reflected light from the object W of the measuring light beam received by the four-part light receiving element 17 is as shown in FIG.
) to (c). That is, when the object is placed at the focal position of the objective lens 5, the four-division light receiving element 1
If the four-divided light receiving element 17 is positioned and fixed so that the cross-sectional shape of the reflected light received at 7 becomes a circle as shown in FIG. 7(b), when the object W is close to the focal point The cross-sectional shape is a vertically long ellipse, and conversely, when the object W is far from the focal point, the cross-sectional shape is a horizontally long ellipse.

Based on this change in shape, the amount of light received by each element of the 4-split light receiving element 17 changes, so the amount of light received by each element diagonally opposed to the 4-split light receiving element 17 is added up, and the magnitude of the two added values is determined. By doing this, it is possible to determine whether the object is far or near the focal point.

The device developed by the present applicant utilizes this determination result to drive the pulse motor PM to perform focusing, which makes it possible to keep the focusing position constant regardless of the focusing operation. Ta.

[Section 8 to Invent or Solve] However, the following problems were discovered in the above-mentioned developed device as well.

That is, the above-mentioned developed device corrects a beam of light (elliptical cross section) emitted from a semiconductor laser into a beam of light having a circular cross section using a group of correction lenses such as five reducing lenses, one cylindrical lens, and a correction prism. Since astigmatism is generated by transmitting the reflected light from the object of the light beam through the objective lens and the cylindrical lens, there is a problem that the structure of the optical system becomes complicated.

The present invention has been made to solve the above-mentioned problems, and its purpose is to provide a focusing device that can simplify the structure of a focusing optical system and is inexpensive.

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides an optical device for observing or imaging the surface shape of an object, and a focusing device for focusing on the object. a light source that irradiates the object with a measurement light beam whose optical axis is the axis of the objective lens of the optical device and whose cross section perpendicular to the optical axis is an ellipse;
a deviation detection means having an imaging section that forms an image after the reflected light from the object passes through the objective lens, and detects a deviation of the focus according to the shape of the image formed on the imaging section; The apparatus further includes distance changing means for changing the distance between the optical device and the object in accordance with the focal shift detected by the detection means.

[Function] In the present invention, the light source irradiates the object with a measurement light beam whose optical axis is the axis of the objective lens and whose cross section perpendicular to the optical axis is an ellipse, and the deviation detection means detects the reflected light from the object. The distance changing means detects the focal shift depending on the image formation shape when the reflected light from the object passes through the objective lens and forms an image on the imaging section, and the distance changing means detects the focal shift between the optical equipment and the object according to the detected focal shift. Change the distance from objects.

[Example] Hereinafter, details will be described with reference to an example in which the present invention is embodied in a shape measuring device. Note that the same members as in the conventional example shown in FIG. 6 are given the same reference numerals.

The shape measuring device includes an optical unit 1 and a focusing control device 50.
It is composed of.

The optical unit 1 carries out a workpiece placed on a stage 3 (
It images the surface of a target object) W, and includes a removable objective lens 5 provided opposite to the stage 3;
and a CCD camera 7 aligned with the axis of the camera. and,
In order to focus the CCD camera 7, the following optical mechanism is provided.

First, a light source 9 for focusing is provided. This light source 9
A semiconductor laser is used, and the wavelength of the emitted light is set in the infrared range.

A collimating lens 91 is provided in the emission direction of the semiconductor laser 9. A focusing lens 10 and a beam splitter 11 are provided, and a dichroic mirror 13 is further provided at a position intersecting the axis of the objective lens 5.

The light beam emitted from the semiconductor laser 9 passes through the beam splitter 11 and enters the dichroic mirror 13, where it is reflected in a 90 degree direction and passes through the objective lens 5. Therefore, the surface of the workpiece W is irradiated with the light beam from the semiconductor laser 9. Note that the optical mechanism described above is configured so that the axis of this light flux and the axis of the objective lens 5 coincide.

The light beam reflected from the surface of the workpiece W passes through the objective lens 5 and then passes through the dichroic mirror 13 to the beam splitter 11.
reflect towards. The dichroic mirror 13 has different spectral characteristics for reflected light and transmitted light, and if the light flux from the objective lens 5 is visible light, it is transmitted, and if it is infrared light, it is reflected. However, the reflectance of infrared light is not 100%, and a small percentage of the infrared light is transmitted. Therefore, visible light from the surface of the workpiece W and infrared light for focusing are incident on the CCD camera 7. And the monitor screen of the CCD camera 7 (as shown)
, the surface shape of the workpiece W and the spot of the focusing light beam are projected in an overlapping manner.

The light beam reflected from the dichroic mirror 13 is reflected by the beam splitter 11, and the optical path of this reflected light beam includes:
Four divided light receiving elements 17 are arranged at a predetermined distance apart.

Here, the semiconductor laser 9 has a property that the emitted light beam has an elliptical cross-sectional shape perpendicular to the optical axis. In other words, since the emission positions of the light that spreads in the long axis direction of the ellipse and the light that spreads in the short axis direction of the ellipse are shifted along the optical axis,
The combined light beam has an elliptical cross-sectional shape. This development is called astigmatism.

Therefore, the light beam from this semiconductor laser 9 is irradiated onto the object through the axis of the objective lens 5, and the reflected light is passed through the axis of the objective lens 5 to the imaging plane (4 reflected light is passed through the axis of the objective lens 5).
When an image is formed on the imaging plane (quadrant light-receiving element) through the axis of , the image formation shape on the imaging plane changes as follows depending on the shift of focus.

As shown in FIG. 2(A), among the light beams from the semiconductor laser 9, light that spreads in the χ direction in a plane perpendicular to the optical axis (indicated by a solid line) is transmitted to an object W placed at position b. Let q be the focal point where the light is reflected by and forms an image. On the other hand, among the light beams from the semiconductor laser 9, light that spreads in the y direction (orthogonal to the χ direction) in a plane perpendicular to the optical axis (indicated by a broken line)
is reflected by the object W placed at position b to form an image, and let P be the focal position. In this case, the spot shape of the light beam formed on the object W is circular, and when the imaging surface is placed at the S position, the cross-sectional shape of the light beam also becomes circular as shown in FIG. Become.

Therefore, when the surface of the object W is shifted to position a, the focal position of the light spreading in the χ direction moves from position q to S, and the focal position of the light spreading in the y direction moves to position P.
from there to the objective lens 5 side. Therefore, an elliptical spot whose width in the y direction is longer than the width in the χ direction is formed on the imaging plane located at the position S, as shown in FIG. 2(A).

On the other hand, when the surface of the object W is shifted from position b to position C, the focal position of the light spreading in the χ direction moves away from the objective lens 5 from the position y
The focal position of the light that spreads in the direction moves from position P to S.

Therefore, the image forming plane located at position S has the image shown in Fig. 2 (1).
As shown in FIG. 2, an elliptical spot whose width in the χ direction is longer than the width in the y direction is formed.

Therefore, the spot shape of the light beam irradiated onto the four-division light-receiving element 17 arranged at the position S changes depending on the position of the object W.

As shown in FIG. 3, the four-division light receiving element 17 has four independent light receiving surfaces 17a and 17 arranged in a matrix.
It is a four-divided photodiode having light receiving surfaces 17a, 17b, 17c, and 17d, and outputs an electric signal according to the amount of light irradiated to each light receiving surface 17a, 17b, 17c, and 17d. In addition, the arrangement of the four-split light receiving element 17 is such that the axis of the irradiated light beam coincides with the intersection point O at the boundary of each light receiving surface 17a, 17b, 17c, and 17d, and the major axis of the elliptical cross section of the light beam coincides with this boundary. Nito 4
They are set to intersect at a fifth degree. Therefore, the spot shape of the light beam can be determined based on the output signal of the four-division light receiving element 17. Then, based on each spot shape, the position of the surface of the object W,
That is, a shift in focus is detected.

The optical unit 1 includes a stage 3 on which an object W is placed.
It is provided so as to be slidable in the vertical direction (in the direction of the arrow in FIG. 1). The position of this optical unit I is set by a stepping motor 19 fixed to the frame F. That is, the rotation of the stepping motor 19 is transmitted to the ball screw 21 to drive the optical unit 1 itself in the vertical direction. Therefore, by rotating the stepping motor 19, the distance between the stage 3 and the optical unit 1, that is, the distance between the surface of the object W and the objective lens 5 is changed, and focusing can be performed.

The focus control device 50 includes a CPU 51 . ROM52. A RAM 53 and a video RAM (V
- RAM) 54, an input/output interface 55, a bus 56 that interconnects these, and a stepping motor 1.
A motor driver 57 for driving the motor 9 is provided. The input/output interface 55 also includes CCD cameras 7 and 4.
The divided light receiving elements 17 are connected.

Next is the focus control system! The focus control process executed by '50 will be explained with reference to the flowchart in FIG.

This control routine is started by operating an operation switch (not shown). When the operation switch is turned on, C
The PU 51 waits for input of an offset value corresponding to the type of objective lens 5 attached to the optical unit 1 (step Sl). Inputting this offset value is necessary because the chromatic aberration differs depending on the objective lens used, that is, the focal position of visible light and the focal position of infrared light used for focusing control are different for each objective lens. The amount is input as an offset value. This input method is as follows:
There are methods in which the amount of deviation is entered directly using a numeric keypad, etc., methods in which the type code and amount of deviation are stored in memory in advance for each objective lens used, and the type code is inputted using a numeric keypad, etc., and a method in which the type code is entered using a numeric keypad. Instead of manual labor, various methods can be adopted, such as a method in which a mark is attached to the holding tube of the objective lens and the type code is input by detecting the mark.

When the input of the offset value in step S1 is completed, CP
U51 reads the output signal of the four-division light receiving element 17 (step S2), and calculates the following equation (step S3).

α+(Al+A3)-(A2+A4))/ΣAiHere, A1-A4 indicates the output value of each element (light-receiving surface 17a, 17b, 17c, 17d) of the 4-split light-receiving element 17, and the focus Sometimes the value of α is approximately equal to zero,
When the object W is further away from the focal point, the value of α is positive, and when the object W is closer, the value of α is negative.

It is determined whether the value of α obtained in step S3 is larger than -r (r is a predetermined positive number close to zero) and smaller than r (step S4), and if YES, the CPU 5
1 determines that the measurement laser beam (infrared region) is in focus, and drives the stepping motor 19 to move the optical unit 1 relative to the stage 3 by the offset value input in step S1. (Step S5). CCD camera 7 that receives visible light as a result
can capture a clear image of the surface of the object W.

On the other hand, in the case of NO in step S4, the CPU 51
It is determined whether the value of α is 1 or more (step S6), Y
In the case of ES, it is determined that the object is located further away than the focal position, and the stepping motor 19 is driven one step in the direction of reducing the distance between the object W and the objective lens 5 (step S7), and the process of step S2 is performed. return.

On the other hand, if NO in step S6, C
The PU 51 determines that the object W is closer than the focal position and drives the stepping motor 19 one step in the direction of increasing the distance between the object W and the objective lens 5 (step S8), and returns to the process of step S2. .

In this way, step S2. S3. S4゜S6. S
7. Focusing control is performed by repeating S8.

It should be noted that the present invention is not limited to the above-described embodiments, and various changes can be made within the scope of the invention. For example, in this embodiment, focusing is achieved by moving the optical unit 1 up and down, but this can be changed to a system in which focusing is achieved by moving the stage 3 up and down. Furthermore, when using the objective lens 5 that has chromatic aberration corrected to zero over the wavelengths of visible light and infrared light, the offset values in steps Sl and S5 may be set to zero.

[Effects of the Invention] As described in detail above, the present invention enables measurement by using a light source that emits a beam of light for measurement whose optical axis is the axis of the objective lens and whose cross section perpendicular to the optical axis is an ellipse. When a target object is irradiated with a beam of light and the reflected light from the object passes through the objective lens and forms an image on the imaging section, the shape of the image changes depending on the distance between the object and the objective lens. The shift detecting means detects the change, and the distance changing means changes the distance between the optical device and the object in accordance with the detected focus shift.

Therefore, there is no need to provide a cylindrical lens or the like to generate astigmatism as in the prior art, and the structure of the focusing optical system can be simplified and the cost can be reduced.

[Brief explanation of drawings]

Fig. 1 shows a shape measuring device according to an embodiment of the present invention, and its schematic configuration is shown. Fig. 2 is an explanatory diagram showing the principle of detecting defocus, and Fig. 3 shows a four-part light receiving system. 4 is a flowchart showing a focusing control routine, FIG. 5 is an explanatory diagram showing the principle of a conventional focusing device, and FIG. 6 is a schematic configuration diagram of an improved conventional focusing device. , FIG. 7 is an explanatory diagram showing the principle of focusing in the improved device of FIG. 6. In the figure, 1 is an optical unit, 3 is a stage, 5 is an objective lens, 9 is a semiconductor laser (light source), 17 is a 4-split light receiving element, 19 is a stepping motor, 50 is a focusing control device,
W is a work (object).

Claims (1)

[Claims] 1. A focusing device that focuses an optical device for observing or imaging the surface shape of an object and the object, wherein the axis of the objective lens of the optical device is the optical axis; It has a light source that irradiates the object with a measuring beam having an elliptical cross section perpendicular to the optical axis, and an imaging section that forms an image after the reflected light from the object passes through the objective lens. a displacement detection means for detecting a focal displacement according to the shape of the image formed on the imaging section; and a separation for changing the separation between the optical device and the object according to the focal displacement detected by the displacement detection means. A focusing device comprising: a changing means;