JPH05126534A - Method for calibrating line width measuring instrument - Google Patents

Abstract

PURPOSE:To calibrate the title instrument by finding one picture element of a CCD element in terms of mum from the finely moved amount of a precise positioning stage for measuring the line width of an image taken with the CCD element by utilizing the stage. CONSTITUTION:By combining a precise positioning stage 1 and line width measuring instrument 4, luminance distribution on a sample 3 is first measured and an arbitrary reference point A is set on the luminance distribution of light irradiating the screen of a CCD element 8. By moving the stage 1 on which the sample 3 is set, the moving amount of the stage 1 is read and the measuring instrument 4 is calibrated by detecting the pitch corresponding to one picture element of a picture sensor 1 from the moved amount of the stage 1 and moved amount of the reference point A proportional to the moved amount of the stage 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calibrating a line width measuring device for measuring the line width of a sample from an image obtained by magnifying and projecting the sample.

[0002]

2. Description of the Related Art Generally, in a calibration method of a line width measuring device, an image magnification rate for enlarging an image has a variation of several percent.
An image is captured by a light receiving element such as a CCD element. However, if the image captured by a microscope changes depending on the magnification, it is necessary to calibrate the length corresponding to one pixel of the light receiving element in a certain image. Then, the image is measured based on the calibration value. However, in the calibration method of the prior art, it is necessary to prepare a reference sample, and the reference sample must have a so-called high-accuracy line and space with an accurate pitch. There was a problem. Furthermore, there are comparatively high-precision reference samples for micron-order ones, but it is very difficult to make highly reliable sub-micron-order reference samples that the optical system supports at high magnifications. .. For this reason, the reliability cannot be ensured for the calibration in the submicron order (digits in the range of 0.1 to 0.9 μm), which causes a measurement error of the line width measuring instrument. Further, since the distortion of the vicinity of the screen periphery due to the aberration of the optical system causes the pitch or length of the reference sample to be applied at high magnification, there is also the problem that an accurate sample length cannot be obtained on the image captured by the CCD element. is there.

[0003]

In particular, when the total magnification changes depending on the combination of the image pickup lens and the objective lens as in a microscope, the CCD in the image taken by the CCD element is changed.
When calibrating how many μm a pixel of an element corresponds to, it is possible to easily make a sample without requiring a reference sample having a known pitch with high accuracy as in the past, and to perform CC
It is an object of the present invention to enable the calibration of a line width measuring instrument on the order of submicron (0.1 μm) of D element one pixel length.

[0004]

According to the present invention, a line width measuring device for magnifying and projecting a sample using an optical system for magnifying an image and electrically detecting and measuring the image, and a precise line holding device for holding the sample. By combining the positioning stage, measuring the luminance distribution of the sample on the screen, setting an arbitrary reference point on the distribution, and moving the positioning stage in one direction by a known amount, the amount of movement of the reference point It is characterized in that the pitch corresponding to one pixel is detected and the measuring instrument is calibrated.

[0005]

EXAMPLE A first example will be described with reference to FIGS. 1 (a), 1 (b) and 2. In FIG. 2, a piezo actuator type stage is used as the positioning stage. The positioning stage 1 can be moved by a linear distance (horizontal direction in the drawing) ranging from sub-micron order to several-micron order by the actuator 2, and the amount of movement can be detected. A reference sample 3 is set on the positioning stage 1. The line width measuring device 4 is composed of an objective lens 5, an image forming lens 6, a photographing lens 7 and a CCD camera element 8 as a CCD element, and the image of the sample 3 is an objective lens 5, an image forming lens 6, a photographing lens 7, etc. An image is formed on the CCD element 8 for the camera via the. In the case where the total magnification changes depending on the combination of the objective lens 5 and the photographing lens 7 as described above, the pitch corresponding to one pixel of the CCD element in each image changes, so that the line width measuring instrument must be calibrated. I have to.
Here, a pitch corresponding to one pixel is obtained from the absolute movement amount of the stage using the precision positioning stage.

First, the luminance distribution of Sample 3 is measured.
FIG. 1A is a luminance distribution diagram in which the vertical axis is the luminance L with which the sample is illuminated on the screen of the CCD 8 and the horizontal axis is the CCD horizontal pixel number X. A threshold value (threshold value) is arbitrarily set and used as a reference point A. Figure 1
The position of the stage 1 is shown in (b). Next, using the actuator 2, move the stage 1 in one direction along the X axis on the order of submicron (0.1 μm) to several μm, for example, in the right direction of FIG. The distance Δx is detected and the amount of movement Δx is detected. Corresponding to the movement of the stage 1, the reference point A set on the screen of the CCD camera element 8 also moves by the number of a pixels and comes to the position A '. Since the number of pixels a of the reference point A moving on the screen is proportional to the Δx movement of the stage 1, the number a of pixels of the reference point moving amount is uniquely determined with respect to the stage movement amount Δx. And the amount of movement of stage 1 Δx
Can be read by the positioning stage. This gives 1
The pitch P per pixel can be calibrated by the following equation. △ x / a = Pμm / pixel

FIG. 3 shows another example 1 in which the stage 1 is moved as a positioning stage by the laser length-measuring device 21 based on an interference system. Regarding the combination with the line width measuring device 4,
It is similar to FIG. In this case as well, the stage 1 is moved on the order of several μm to several μm while being detected by the laser length-measuring device 21, and the amount of movement Δx is calibrated.

FIG. 4 shows the positioning stage 1 and the encoder 2
It is another example 2 which showed the apparatus moved by 2. In this case as well, the line width measuring device 4 is calibrated by the movement amount Δx as in the case described above.

Although the above-mentioned apparatus uses the CCD element 8 on the light receiving side, a line width measuring device 10 having a CCD line sensor 9 may be used as shown in FIG. Also in the case of this device, similarly to the above-mentioned one, the stage 1 is moved on the order of sub μm to several μm, and the line width measuring instrument is calibrated from the amount of movement Δx and the number a of moving pixels of the reference point A. ..

As described above, in the method for calibrating the line width measuring device according to the present invention, any sample can be used as long as the sample can be imaged by the CCD element or the image sensor and produces a luminance distribution. Calibration of one pixel length of the CCD element of the line width measuring device is performed based on the pixel movement amount of the reference point of the luminance distribution of the sample image and the detected movement amount of the stage. Further, the reference point set on the luminance distribution of the sensor can be set also on the edge portion represented by the primary differential image and the secondary differential image.
That is, the boundary where the brightness changes becomes clear by performing the first derivative and the second derivative. Moreover, since this image corresponds to the edge portion in the sample 3, the edge-enhanced image can be used regardless of the high magnification, which contributes to the improvement of the calibration accuracy. Further, the positioning stage 1 may have a mechanism capable of detecting a known movement amount of the stage, and the laser length measuring device 21, the encoder 22 and the piezo actuator 2 described above can correspond to a sub-μm order and can be highly accurately calibrated. Is possible. In addition, since calibration can be performed at the reference point of the luminance distribution regardless of the shape of the sample and the threshold value, calibration is easy.

[0011]

According to the present invention, the reference point and the threshold value can be set independently of the shape of the sample as long as they are on the luminance distribution of the sample, and the movement amount of the stage by moving the sample by a predetermined amount. It is extremely easy to calibrate the measuring device by detecting the pitch corresponding to one pixel of the pixel sensor from the movement amount of the reference point, and highly reliable calibration can be performed. Furthermore, if a precise positioning stage is used, it is possible to support up to the submicron order, and even at a high magnification at which errors due to aberrations are likely to occur, it is possible to calibrate because samples can be taken into consideration. At this time, the calibration can be performed at a resolution not lower than the resolution of the sample image.

[Brief description of drawings]

FIG. 1A is a luminance distribution chart of a sample. (B)
Is the stage movement amount.

FIG. 2 is a line width measuring device and a positioning stage according to the first embodiment.

FIG. 3 is another example 1 of the positioning stage.

FIG. 4 is another example 2 of the positioning stage.

FIG. 5 shows a line width measuring device and a positioning stage according to a second embodiment.

FIG. 6A is a luminance distribution diagram of a first-order differential image.
(B) is a luminance distribution diagram of the second-order differential image.

[Explanation of symbols]

 1 Positioning Stage 2 Actuator 3 Reference Sample 4 Line Width Measuring Device 5 Objective Lens 6 Imaging Lens 7 Photographing Lens 8 CCD Element 9 CCD Line Sensor 10 Line Width Measuring Device 21 Laser Length Measuring Machine 22 Encoder

Claims (1)

[Claims] 1. A line width measuring instrument for magnifying and projecting a sample using an optical system for magnifying an image, electrically detecting and measuring this image, and a precise positioning stage for holding and moving the sample are combined. By measuring the luminance distribution of the sample on the screen, setting an arbitrary reference point on the distribution, and moving the positioning stage in one direction by a known amount, the amount of movement of the reference point corresponds to one pixel of the image sensor. A method for calibrating a line width measuring instrument, characterized in that the measuring instrument is calibrated by detecting the pitch.