JPH06347418A - Laser scanner and image forming method - Google Patents

Abstract

PURPOSE:To carry out observation equivalent to a case in which an object to be inspected is irradiated with a laser beam having the uniform intensity distribution notwithstanding that laser beam intensity presents the gauss distribution by moving light flux along plural cross sections while radiating the laser beam, whose light intensity presents the gauss distribution in the radial direction of the light flux, to the object to be inspected. CONSTITUTION:A laser device 201 radiates a laser beam 203 whose light intensity presents the gauss distribution in the radial direction of light flux. The laser beam 203 is condensed by a condensing lens 207 so as to have a prescribed light flux radius in an illuminating part 205. A stage device 211 moves light flux of the laser beam in the illuminating part 205 along plural cross sections of an object 209 to be inspected having an interval almost equal to the radius. Scattered light from the illuminating part 205 forms an image by a microscope 215, and an image signal is outputted by a TV camera 217. When the illuminating part 205 is set in an initial position on a desired cross section of the object 209 to be inspected, a computer 213 takes in and stores a signal from the camera 217 while moving the device 211.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for scanning a certain area of an object to be inspected by using a laser beam in a laser tomography apparatus and a method for forming an image by the scanning.

[0002]

2. Description of the Related Art Conventionally, in a laser tomography apparatus for evaluating a defect or the like of an object to be inspected such as a semiconductor wafer, a laser beam is moved along an observation cross section of the object to be inspected and the scattered light is sequentially photoelectrically converted. A cross-sectional image of an internal defect or the like is obtained by conversion (Japanese Patent Laid-Open No. 1-151243). Then, in order to obtain a three-dimensional scattered image, the laser beam is sliced at a constant interval so that a plurality of cross-sectional images as described above are obtained at a constant interval necessary to form a stereoscopic image. (JP-A-62-1674
No. 1).

[0003]

However, in this prior art, since the intensity I of the laser light exhibits a Gaussian distribution in the radial direction of the light flux as shown in FIG. 4, internal defects of the same size are detected. Even if they exist uniformly in the object, as shown in FIG. 5, on the obtained cross-sectional image, with different intensities according to the distance from the center of the laser light of the laser light portion that illuminates individual defects and the like. appear. Therefore, there is a problem that an error occurs when trying to measure the particle size of internal defects or the like based on this image data. In order to solve this, it is possible to use a laser beam having a rectangular intensity distribution as shown in FIG.
Such a laser beam diverges immediately and is not suitable for forming a cross-sectional image.

In view of the problems of the prior art, it is an object of the present invention to make it possible to perform the same observation as when illuminated with laser light having a uniform intensity distribution, even though the laser light intensity exhibits a Gaussian distribution. Is to

[0005]

In order to achieve this object, a laser scanning device of the present invention comprises means for irradiating a laser beam whose light intensity exhibits a Gaussian distribution in the radial direction of the beam, and a beam of this laser beam. It is characterized in that it is provided with means for moving the object to be inspected along a plurality of cross-sections having intervals substantially equal to the radius. Further, the image forming method of the present invention irradiates the object to be inspected with laser light whose light intensity exhibits a Gaussian distribution in the radial direction of the light flux, and the laser light is irradiated along a plurality of cross sections having an interval substantially equal to the radius. While moving in the object to be inspected, scattered light due to the laser light is detected to obtain a plurality of tomographic images. Forming the image includes, for example, forming a single cross-sectional image by superimposing a plurality of adjacent to each other among the plurality of tomographic images. In addition, the position of an image can be specified on the basis of the image of a certain defective portion in the object to be detected, which is found in a plurality of adjacent and continuous images of the plurality of tomographic images.

[0006]

In this structure, the object to be inspected is irradiated with laser light whose light intensity exhibits a Gaussian distribution in the radial direction of the light beam, and a plurality of the object to be inspected having the light beams of the laser light mutually at intervals substantially equal to the radius thereof. When moved along the cross section, the distribution of the light flux I and the light intensity I in the Z direction perpendicular to the plurality of cross sections of the laser light irradiating each cross section is shown by a curve 103 in each cross section as shown in FIG. Although it has a Gaussian distribution as shown in the above, when it is overlapped for each cross section, it becomes trapezoidal as shown by the curve 101, and the light intensity becomes constant in the portion C corresponding to the upper bottom thereof. Therefore, at least with respect to this portion C, when the scattering intensities in each cross section are overlapped, observation with uniform illumination light intensity is performed. Therefore, the laser light having a high light intensity and a small luminous flux can be applied to the image formation in a wide range without any trouble, and the detection sensitivity can be improved.

In the following, the other effects and the like will be described in detail using examples.

[0008]

FIG. 2 is a schematic diagram showing the configuration of a laser tomography apparatus for evaluating internal defects in a semiconductor wafer or the like according to an embodiment of the present invention. As shown in the figure, the laser scanning device in this device is configured such that a laser device 201 for irradiating a laser beam whose light intensity exhibits a Gaussian distribution in the radial direction of the luminous flux and a laser beam 203 emitted by the laser device 201 are condensed and illuminated at an illumination portion 205. A condenser lens 207 that condenses light having a predetermined luminous flux radius a, and an illumination portion 20.
5 is provided with a stage device 211 for moving the light flux of the laser light along a plurality of cross sections of the object 209 to be inspected having a distance substantially equal to its radius a, and a computer 213 for controlling the stage device 211. In this case, the luminous flux radius a is set to any of the ranges of 3 to 10 μm. In addition, the illumination part 205 is ±
The range is 100 to 500 μm, and the range is selected so that the luminous flux diameter a is substantially constant.

Reference numeral 215 denotes an illumination portion 2 by the laser light 203.
A microscope for forming a scattered image based on scattered light from 05, 21
Reference numeral 7 denotes a TV camera that photoelectrically converts the image formed by the microscope 215 and outputs a video signal. Reference numeral 219 captures the video signal output by the TV camera 217, performs predetermined image processing such as A / D conversion, and performs computer 213. The image processing device 221 that stores the image in the storage device is a display device that displays the image processed by the image processing device 219. The computer 213 also controls the operation of the image processing device 219.

In this configuration, when the illuminating portion 205 is positioned at the initial position on the desired observation section of the object to be inspected 209 by the positioning means (not shown), the computer 213
While moving the stage device 211 in the x direction, the image processing device 219 captures the output of the TV camera 217 based on the scattered light from the illumination portion 205 and stores the image data in the storage device. When the acquisition of the image data for one observation section is completed in this way, the object 209 to be inspected is then moved in the z direction by the radius a by the stage device 211, and the illumination portion 205 is moved to the initial position of the next observation section.
Position, then the object to be measured 2 in the opposite x direction.
Image data is captured in the same manner while moving 09.

When the image data acquisition for a plurality of observation sections is completed in this way, the image processing device 219
The image data of each observation section can be displayed on the display device 221 by superimposing them in the z direction by associating the x and y coordinates with each other. The superimposed image data obtained in this manner can be said to be a cross-sectional image obtained by integrating the scatter image included in a range having a thickness approximately equal to the radius a multiplied by the number of observation cross-sections.

According to this, in the considerably wide range C corresponding to the upper bottom portion shown in FIG. 1, since the superimposed intensity of the illumination light is uniform, from the data of the cross-sectional image based on the scattered light from that range, The resulting scattering intensity from a given defect particle will indicate the size of that particle. This is because if the intensity of the illumination light is constant, the radius of the particle and the intensity of scattering from it have a constant relationship. However, with respect to the image data corresponding to the range S of the light flux diameters on both sides, the same error problem as in the case where there is only one observation cross section occurs. However, this error can be made relatively small by increasing the number of observation cross sections. Further, if the number of observation cross sections is increased, the detection efficiency in detecting particles having a low particle density and existing only sparsely can be improved. Furthermore, for example, when a certain particle is detected in the image data of four continuous observation sections adjacent to each other, as shown in FIG. 3, the intensity of the particle in each observation section is shown for the z coordinate of each observation section. The z-coordinate of the particle can be obtained as the vertex position z ₀ of the Gaussian curve connecting the points.

Although the scattered image is obtained by photoelectric conversion here, the invention is not limited to this, and the scattered image may be obtained using a photosensitive material or the like. In that case, one superimposed cross-sectional image can be obtained by overlapping and exposing the scattered images of each observation cross-section. Further, the image data of the observation cross section may be captured for a larger number of observation cross sections, and may be directly used for various analyzes as three-dimensional scattered image data with uniform illumination light intensity without being superimposed. Further, the scanning direction in each observation cross section is not limited to the x direction, but may be performed in the y direction to scan a larger observation cross section. Further, the scanning may be performed using parallel light instead of the focused laser light. Further, instead of moving the object to be inspected, the laser beam may be moved. Further, the laser scanning device of the present invention can be applied not only to laser tomography but also to illumination of various observation devices.

[0014]

According to the present invention, since the object to be inspected can be illuminated with the laser beam having a narrow luminous flux, the light intensity of the laser beam can be increased and the detection sensitivity can be enhanced.
Further, the object to be inspected can be illuminated over a wider range. Therefore, when measuring the defect density of the test object, the lower limit of the measurable defect density can be lowered.

[Brief description of drawings]

FIG. 1 is a graph showing a distribution of a light flux and a light intensity I in a direction Z perpendicular to the plurality of cross sections when the laser scanning device of the present invention scans the plurality of cross sections.

FIG. 2 is a schematic diagram showing a configuration of a laser tomography apparatus for evaluating an internal defect of a semiconductor wafer or the like according to an embodiment of the present invention.

FIG. 3 is a graph showing a principle of determining a particle position in the apparatus of FIG.

FIG. 4 is a graph showing how the intensity I of laser light exhibits a Gaussian distribution in the radial direction of a light beam.

FIG. 5 is a schematic diagram showing a cross-sectional image obtained by a conventional device.

FIG. 6 is a graph showing an intensity distribution of laser light having a rectangular intensity distribution.

[Explanation of symbols]

Reference numeral 201: laser device, 203: laser light, 205: illuminated portion, 207: condenser lens, 209: object to be inspected, 21
1: Stage device, 213: Computer, 215: Microscope, 217: TV camera, 219: Image processing device, 2
21: Display device.

Claims (4)

[Claims] 1. A means for irradiating a laser beam whose light intensity exhibits a Gaussian distribution in the radial direction of the luminous flux, and a plurality of cross sections having the luminous flux of the laser beam mutually at intervals substantially equal to the radius of the object to be inspected. A laser scanning device comprising means for moving the laser. 2. An object is irradiated with laser light whose light intensity exhibits a Gaussian distribution in the radial direction of the light beam, and the laser light is scanned in the object to detect scattered light due to defects to form an image. In the method, an image forming method is characterized in that a plurality of tomographic images in an object to be inspected are mutually provided with intervals substantially equal to a radius of a light flux of the laser light. 3. The image forming method according to claim 2, wherein forming the image includes forming a single sectional image by superimposing a plurality of adjacent to each other among the plurality of tomographic images. 4. A certain one in the object to be detected, which is found in a plurality of adjacent and continuous images of the plurality of tomographic images.
The image forming method according to claim 2, wherein the position of the image is specified based on the image of one defect portion.