JPH10300442A - Shape measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To change NA at high speed without deteriorating S/N by providing the number of an opening changing means for electrically changing the aperture size of an objective lens and a control means for controlling the aperture and the exposing time of a detector. SOLUTION: A shape of an object 7 is measured by changing the positional relation between the focal plane of a two-dimensional alignment confocal optical system and the object 7 to store images every change in a signal processing device, and signal-processing a plurality of confocal images to determine the image providing the maximum value for each picture element. An NA of the objective lens 6 is changed by the number of an opening changing means 5. The number of an opening changing means 5 is arranged in the aperture position of the objective lens 6 to change the aperture size, whereby the NA is controlled. The output signal from a CCD sensor is outputted to a signal processing device 10 by an image signal output circuit. An exposure start signal and an exposure end signal are controlled by a control means 11 to provide an image in an optional exposing time. The relation between the aperture size and the exposing time of a detector array 9 is determined to control them by the control means 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical apparatus used for measuring the surface shape of an object, and more particularly to an apparatus for detecting a position of an object in an optical axis direction using a confocal optical system.

[0002]

2. Description of the Related Art When a confocal optical system is used, it is possible to accurately measure a position (hereinafter, referred to as a height) of an object in an optical axis direction (hereinafter, referred to as a Z direction). FIG. 6 shows the basic configuration of the confocal optical system. Light emitted from the point light source 601 is condensed by the objective lens 603 and projected on an object. Light reflected from the object and incident again on the objective lens 603 is incident on the pinhole 604 at the same optical position as the point light source 601 via the half mirror 602, and the amount of light passing through the pinhole 604 is detected by the detector. 605. This is the basic structure of the confocal optical system. With such an optical system, the height of a point on the surface of an object can be measured as follows. When the object surface is at a position conjugate to the point light source 601,
The reflected light converges on the pinhole 604 surface, which is also a conjugate position, and much reflected light passes through the pinhole 604. However, when the object surface moves away from the position conjugate to the point light source, the reflected light converges to a position away from the pinhole 604, and the amount of light passing through the pinhole 604 decreases rapidly.
This is shown in FIG. This figure is drawn considering the diffraction of light, so it waves around the foot of the mountain.
(This figure is hereinafter referred to as a vertical diffraction pattern.) From this, the distance between the object and the objective lens 603 is changed, and if the detector 605 finds a point showing the maximum output, the height of the object surface can be known. The above is the principle of height measurement by the confocal optical system.

The height measurement described here is a point measurement,
XY by laser scanning or Nippow disk scanning
If scanning is performed or a two-dimensional array type confocal optical system in which the above confocal optical systems are two-dimensionally arranged is used, planar height measurement, that is, three-dimensional measurement can be performed. Detector 605
It is common to move the object or optical system continuously in the direction of the optical axis to find the maximum output position of the optical axis. However, for faster three-dimensional measurement, move the object or the optical system stepwise in the direction of the optical axis. At each step position, a confocal image (an image obtained by XY scanning of the confocal optical system without moving in the optical axis direction) is obtained, and an image is obtained for each corresponding pixel between the confocal images obtained at each step. An apparatus for performing a process of estimating a maximum output position by an insertion operation is disclosed by the present inventor in Japanese Patent Application Laid-Open No. Hei 7-176931. The present invention will be described in detail with reference to FIG.

A confocal imaging system 403 comprising a two-dimensional array type confocal optical system 401 and a detector 402 can obtain a confocal image at high speed, and the obtained confocal image is sent to an image processing device 405. . The image processing device 405 has a capability of storing a plurality of confocal images, and executes a height calculation described later using the stored confocal images. The high-speed focus moving mechanism 404 can move the focal plane of the two-dimensional array type confocal optical system 401 stepwise at high speed as indicated by z1, z2, and z3 in the figure. The focal plane is moved step by step by the high-speed focus moving mechanism 404, and the confocal image for each step is stored in the image processing device 405. Corresponding points (points having the same coordinates) of each confocal image are obtained by sampling a vertical diffraction pattern at each point as shown by a dotted line in FIG. The model formula for the vertical diffraction pattern is intensity | V (z) | 2 = (| s
in kz (1-cosθ) | / | kz (1-cosθ)
θ) |) 2 (where k is the wave number, sin θ is the numerical aperture (hereinafter abbreviated as NA) of the objective lens, and z is the Z-axis coordinate). The maximum output position can be accurately estimated from. By performing this operation on all points in the image, height data of all points in the image, that is, three-dimensional data can be obtained. Compared to the method of finding the maximum output position by moving the object or optical system continuously in the direction of the optical axis, this device can obtain three-dimensional data from far less confocal images without greatly reducing the accuracy. The measurement time can be greatly reduced. This high speed makes it possible to apply it to online product appearance inspections and the like that were not possible before.

[0005] In this apparatus, high-speed measurement is possible if the movement pitch of the step movement of the focal plane (hereinafter, referred to as step distance) is large, but the effect is diminished as the step distance becomes small. For this reason, it is necessary to measure at a step distance as large as possible, but this step distance is not freely determined. Since the peak position cannot be calculated unless two or three points are sampled inside the first dark part (inside the central mountain) of the vertical diffraction pattern shown in FIG. 5, this limits the step distance. The peak width of the vertical diffraction pattern that determines the upper limit of the step distance is determined by the NA of the objective lens of the two-dimensional array confocal optical system 401 as is clear from the above equation. Can be increased to increase the step distance. However, if the NA is reduced and the step distance is increased, the accuracy of the peak estimation calculation decreases substantially in proportion to the step distance. That is, there is a trade-off relationship between measurement accuracy and measurement speed.

[Problems to be solved by the invention]

For this reason, it is necessary to determine the optimum value of the NA of the objective lens from the required measurement accuracy of the object and the required measurement speed, but it is convenient if the NA can be dynamically changed according to the target object. For example, if you want to measure the lower part of the object (lower in the optical axis direction) with low accuracy, and you want to measure the upper part (upper in the optical axis direction) with high precision, you can set the NA and step distance to the upper part. If the measurement can be changed at the lower part, more optimal measurement will be possible. Changing the NA can be realized by changing the size of the aperture of the objective lens.
In this case, since the light amount changes with the change of NA,
There is a problem that an image obtained with a small NA becomes dark and S / N deteriorates.

Accordingly, an object of the present invention is to provide a shape measuring apparatus capable of changing NA at high speed without deteriorating S / N.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention provides an objective lens having an independent aperture stop, illumination means for irradiating an object with illumination light through the objective lens, and an object lens provided by the objective lens. The photoelectric conversion of the reflected light, a detector that can electrically control the exposure time, and a signal processing device that analyzes the electric signal obtained from the detector and calculates the position of the object in the optical axis direction of the objective lens, An apparatus is constituted by numerical aperture changing means for electrically changing the size of the aperture stop of the objective lens, and control means for controlling the numerical aperture changing means and the exposure time of the detector.

The above-mentioned numerical aperture changing means is disposed at an aperture stop position of the objective lens and has a liquid crystal cell having liquid crystal molecules sandwiched between two transparent electrode substrates, and a voltage application to a transparent electrode on the transparent electrode substrate. A transparent electrode on the transparent electrode substrate is patterned with at least one kind of opening shape, and the voltage of the liquid crystal molecules is controlled by controlling the voltage from the liquid crystal controller to the transparent electrode. The light passing through the aperture stop of the objective lens is blocked or transmitted according to the shape of the transparent electrode by utilizing the optical effect.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention. First, the structure of the two-dimensional array confocal optical system will be briefly described.

The illuminating light emitted from the illuminating means 1 is condensed by a lens 2 to become parallel light, and irradiates a pinhole array 3. Each illumination light emitted from the pinhole of the pinhole array 3 spreads by the diffraction of the pinhole, passes through the half mirror 4, and is provided at the aperture position of the objective lens 6 including the lens 6a and the lens 6b. The light enters the number changing means 5 and the beam diameter is restricted. The numerical aperture changing means 5 will be described later in detail. The illumination light that has entered the objective lens 6 receives the image forming action of the objective lens 6 and projects an image (spot) of the pinhole array 3 onto the object 7. The light reflected from the object 7 is again subjected to an image forming operation by the objective lens 6 to form an image near the pinhole array 8.
The pinhole array 8 is optically equivalent to the pinhole array 3 by the half mirror 4, and the positional relationship of each pinhole in the direction perpendicular to the optical axis is exactly the same. Each pinhole of the pinhole array 8 is arranged in a one-to-one relationship with each detector of the detector array 9, and each detector detects the intensity of reflected light passing through each pinhole. I have.

The above structure is a structure in which the above-described confocal optical system (FIG. 6) is two-dimensionally arranged, and the role of each pinhole and each detector is exactly the same as that of the above-described confocal optical system. . The output from the detector array 9 is obtained by collecting the outputs of the detectors when the focal plane (object plane) of the confocal optical system and the object 7 are in a certain positional relationship (in the optical axis direction) and two-dimensionally collecting the outputs. It is conceivable (such an output signal is hereinafter referred to as a confocal image). The positional relationship between the focal plane (object plane) of the two-dimensional array type confocal optical system and the object 7 is changed, and a confocal image is stored in the signal processor 10 each time, and a plurality of obtained confocal images are obtained. If an image giving the maximum value for each pixel is obtained from the focus image by performing signal processing by the signal processing device 10, the focal plane (object plane) of the confocal optical system is obtained for each pixel (= each detector of the detector array 9). ) And the object 7 are determined, and the shape of the object can be measured.

The NA of the objective lens 6 is changed by the numerical aperture changing means 5. Details of the numerical aperture changing means 5 will be described with reference to FIGS. The numerical aperture changing means 5 is provided with an anti-reflection coating, and is provided between the polarizer 21 and the analyzer 22 having a perpendicular Nicol relationship with each other.
The liquid crystal cell 27 has a structure in which a twisted nematic (TN) liquid crystal 25 is interposed therebetween.
The transparent electrode substrates 23 and 24 are provided with a plurality of concentric segment electrodes as shown in FIG. The transparent electrodes are indium oxide (ITO) films, which transmit light and have conductivity, so that a voltage exceeding the threshold voltage of the TN liquid crystal 25 can be individually applied to each electrode by the liquid crystal control device 26. It has become. The TN liquid crystal 25 has an orientation in which liquid crystal molecules are twisted by 90 degrees from one electrode to the other electrode.
If the long axis direction of the liquid crystal molecules on the side is matched, the light that has become linearly polarized light through the polarizer 21 rotates 90 degrees in the liquid crystal along the twist of the liquid crystal molecules and passes through the analyzer 22. When a voltage is applied to the transparent electrode by the liquid crystal controller 26, the twist of the liquid crystal molecules is eliminated, and the light passing through the polarizer 21 passes through the liquid crystal without rotating, and in this case, the light cannot pass through the analyzer 22. . Since such a light-shielding effect occurs only in the transparent electrode pattern portion to which a voltage is applied, the size of the opening (light transmitting portion) of the numerical aperture changing means 5 can be electrically changed by the liquid crystal control device 26. become able to. Since the numerical aperture changing means 5 is arranged at the stop position of the objective lens 6, a change in the size of the aperture corresponds to a change in the size of the stop, and as a result, the NA of the objective lens 6 can be controlled. .

Next, the detector array 9 will be described. The detector array 9 is currently the most common two-dimensional detector C
It is a TV camera with a built-in CD sensor and has an electronic shutter function that can be controlled from the outside. This function will be described. The CCD sensor generally includes a photoelectric conversion unit and a charge transfer unit. The photoelectric conversion unit converts the received light into electric charge and temporarily stores the light, and sweeps out all the stored electric charge to the charge transfer unit according to the control signal, and starts new photocharge storage. The charge transfer unit transfers the charge swept out of the photoelectric conversion unit and serially outputs the charge. The signal output from the CCD sensor is converted into a television signal format by an image signal output circuit in the television camera and output to the signal processing device 10. The electronic shutter discharges the charge accumulated in the photoelectric conversion unit to the charge transfer unit in response to the exposure start signal, starts new photocharge storage, and uses the exposure end signal to store the charge accumulated from the exposure start signal to the exposure end signal in response to the exposure end signal. This is a function of sweeping out to the charge transfer section again and outputting the charge as a television signal through the charge transfer section and the charge readout section. In the present invention, by controlling the exposure start signal and the exposure end signal by the control means 11, an image can be obtained at an arbitrary timing and at an arbitrary exposure time.

The relationship between the aperture size of the liquid crystal cell 27 of the numerical aperture changing means 5 and the exposure time is obtained in advance by calculation or experiment, and the aperture size of the liquid crystal cell 27 of the numerical aperture changing means 5 and the exposure time of the detector array 9 are determined. Is controlled by the control unit 11 so that the light amount (brightness of the image) does not change even when the NA is changed. Moreover,
If a liquid crystal material having a high response speed is used as the liquid crystal material of the numerical aperture changing means 5, the numerical aperture can be changed in real time (every 1/30 seconds), and the light amount can be kept constant.

In this example, the liquid crystal rotation (TN liquid crystal) utilizing the optical rotation of the liquid crystal is used as the numerical aperture changing means 5. However, other liquid crystal may be used as long as the light shielding and transmission can be switched for each segment transparent electrode. Good. For example, exactly the same can be achieved by using the birefringence (electric field control birefringence effect) of the liquid crystal. In the case where a decrease in the amount of light by the polarizer 21 and the analyzer 22 becomes a problem, It is also possible to perform light blocking and transmission only with liquid crystal by using the optical scattering effect, and in this case, it is not necessary to use a polarizing element.

According to the present invention, for example, the lower part of the object (the lower part in the optical axis direction) may be measured with low accuracy, and the upper part (the upper part in the optical axis direction) may be measured with high accuracy. In such a case, it is possible to change the NA and the step distance in real time between the upper part and the lower part, and to measure without changing the light amount (that is, without deteriorating the S / N).

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining a numerical aperture changing unit according to the embodiment of the present invention.

FIG. 3 is a diagram illustrating a transparent electrode pattern of a liquid crystal according to the embodiment of the present invention.

FIG. 4 is a diagram for explaining a conventional technique of the present invention.

FIG. 5 is a diagram illustrating characteristics of the confocal optical system with respect to a change in the Z coordinate position of an object.

FIG. 6 is a diagram for explaining a confocal optical system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Illumination means 2 Lens 3 Pinhole array 4 Half mirror 5 Numerical aperture changing means 6 Objective lens 7 Object 8 Pinhole array 9 Detector array 10 Signal processing device 11 Control means 21 Polarizer 22 Analyzer 23 Transparent electrode substrate 24 Transparent electrode Substrate 25 TN liquid crystal 26 Liquid crystal controller 27 Liquid crystal cell

Claims (2)

[Claims] 1. An apparatus for detecting an object position in an optical axis direction using focusing information, comprising: an objective lens having an independent aperture stop; illuminating means for irradiating an object with illumination light through the objective lens; A detector that photoelectrically converts reflected light from the object condensed by the objective lens, and that can electrically control the exposure time; and analyzes an electric signal obtained from the detector to analyze an object in an optical axis direction of the object lens. A signal processing device for calculating the position of the objective lens, a numerical aperture changing means for electrically changing the size of the aperture stop of the objective lens, and a control means for controlling the exposure time of the numerical aperture changing means and the detector. A shape measuring device comprising: 2. A liquid crystal cell having a numerical aperture changing means disposed at an aperture stop position of an objective lens and interposing liquid crystal molecules between two transparent electrode substrates, and applying a voltage to a transparent electrode on the transparent electrode substrate. And a liquid crystal control device for controlling the
The transparent electrode on the transparent electrode substrate is patterned with at least one kind of opening shape, and an aperture stop of an objective lens using an electro-optical effect of the liquid crystal molecules by controlling a voltage to the transparent electrode from the liquid crystal control device. 2. The shape measuring device according to claim 1, wherein the light passing through is blocked or transmitted according to the shape of the transparent electrode.