JP4681153B2 - CCD camera automatic switching method and system in scanning electron microscope with laser defect detection function - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of switching between a standard CCD camera and a high-sensitivity CCD camera in an optical microscope provided in a scanning electron microscope used for yield management in a semiconductor manufacturing process.
[0002]
[Prior art]
It is said that the main factor causing defective products in the recent manufacture of ultra-highly integrated LSI is due to minute foreign matter adhering to the wafer. In other words, the minute foreign matter becomes a contaminant and causes circuit pattern disconnection or short-circuiting, leading to the occurrence of defects in semiconductor chips and the deterioration of quality and reliability. Along with the miniaturization of circuit patterns, even fine foreign objects with a diameter of O.1 μm are targeted. For this reason, it is the most important issue to improve the yield in the manufacture of ultra-highly integrated LSIs by measuring, analyzing and analyzing the actual state of minute foreign matter quantitatively with high accuracy.
[0003]
At present, a particle inspection apparatus as shown in FIG. 1 is used as a method for quantitatively and accurately measuring and analyzing the actual state of the adhesion of minute foreign matters. This apparatus has three types of images: (1) scanning electron microscope image, (2) high sensitivity dark field image obtained through high sensitivity CCD camera 4, and (3) dark field image or bright field image by CCD camera 5. This is a device that can display on the CRT 9. The scanning electron microscope 1 includes a lens barrel portion 11 and a secondary electron detector (SED) 12, and is deflected and scanned by the deflection means of the lens barrel portion 11 and is ejected by an electron beam irradiated on the sample 6. Secondary electrons are detected by the SED 12, and the detected information is imaged in correspondence with the beam irradiation position to obtain a scanning electron microscope image (1). The bright field image of (3) irradiates light from the light 8 through the optical path to the sample surface from above, and is uniformly reflected when the sample surface is a mirror surface. Therefore, the image is obtained by photographing the sample surface from above with the CCD camera 5 based on the scattered light. In the dark field image (3), the sample surface is irradiated with a beam from the laser light source 7 obliquely from above, and when the sample surface is a mirror surface, it is totally reflected, and when there is a foreign object, the light is scattered there. This is an image obtained by photographing the surface of the sample with the CCD camera 5 from above. The high-sensitivity proposed field image (2) is basically the same as the dark-field image (2), but is an image obtained through the high-sensitivity CCD camera 4 in order to observe smaller particles. Since the object is an ultrafine particle, the scattered light becomes weak, so a highly sensitive CCD (ICCD) camera is used.
[0004]
In the conventional defect detection using this apparatus, first, the position of a foreign object is detected from a low-magnification bright-field image or dark-field image, and the number and distribution status thereof are grasped. When a bright field image is obtained, the control box 11 is operated via the computer 10 to turn on the light 8 to irradiate a spot on the sample surface from above, and the CCD camera 5 captures a microscope image. When a dark field image is captured, the control box 11 is operated via the computer 10 to emit a laser light source, irradiate the sample surface with a laser beam obliquely from above, and the CCD camera 5 captures a microscope image. Concerning the foreign matter whose presence was confirmed by this initial inspection, including a fine foreign particle that could not be observed in the initial inspection by a high-sensitivity CCD camera 4 and a high-magnification dark-field image or scanning electron microscope image, Observe the shape and evaluate it. Fine foreign particles can be observed at a high magnification after the position is specified by observation at a low magnification. Fine position information that cannot be handled as sample stage position information can also be handled as information on screen coordinates. In addition, the composition of the foreign particles of interest cannot be analyzed from a high-magnification optical image or a scanning microscope image of secondary electron detection, but for this, an electron microscope having a secondary X-ray detection (EDS) function is used. Can be analyzed.
By the way, when the laser scattered light is observed with an optical microscope, the amount of laser scattered light varies greatly depending on the size of the defect. This scattered light is imaged by a CCD camera, but a high sensitivity CCD (Image Intensifier CCD: ICCD) in which detection sensitivity is set high in order to observe weak scattered light caused by minute particles or the like is relatively large. When a strong scattered light from a foreign substance or the like is received or the stage is moved, a safety device is activated to stop the operation for high pressure protection. Therefore, there is a problem that work is interrupted each time and work efficiency is deteriorated.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to solve the above problem, that is, in a scanning electron microscope equipped with a high-sensitivity CCD camera that performs laser defect detection, when operating a high-sensitivity CCD camera, strong scattered light from a relatively large foreign object or the like is generated. It is to improve the work efficiency by constructing a system in which the safety device is not operated frequently by stopping the operation by receiving the light.
[0006]
[Means for Solving the Problems]
The CCD camera automatic switching method in a scanning electron microscope having a laser defect detection function of the present invention is a wafer including a laser optical system for irradiating a laser at a defect position obliquely from above and an optical microscope for observing scattered light from the defect. In a scanning electron microscope for surface observation, a high-sensitivity and standard CCD camera is attached to the optical microscope, the optical axis is positioned based on the defect position information measured in advance, and the defect is observed with the standard CCD camera of the optical microscope. A technique is adopted in which a step of performing defect observation is performed by switching to a high-sensitivity CCD camera when defect observation cannot be observed with the standard CCD camera.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
When using a scanning electron microscope equipped with a laser defect detection function to detect minute foreign objects existing on the surface of a flat sample such as a silicon wafer and to inspect and evaluate them, prior to observation with a scanning electron microscope, minute foreign objects When a dark field image by laser irradiation is observed with a high-sensitivity CCD camera and its position is to be specified, the operation is stopped immediately after the safety device is activated and the operation is interrupted. The reason for this is that, as described above, in the case of a high-sensitivity CCD camera, the detection sensitivity is set high, so when strong light from a relatively large foreign object is received, the output signal becomes high voltage, so the high-voltage protection of the circuit is safe. Due to the operation of the device. Therefore, the present invention intends to operate the high-sensitivity CCD camera only when irregularly reflected light is not received in advance by observation with a standard CCD camera. That is, the high-sensitivity CCD camera is operated after confirming that there is no relatively large foreign object by observation with a standard CCD camera in which the output signal does not become high voltage even with a large defect size. The output of the high sensitivity CCD camera does not become high pressure. Therefore, even when a high sensitivity CCD camera is used, the work is not interrupted and the work efficiency can be increased.
[0008]
The operation of the CCD camera switching method in the scanning electron microscope having the laser defect detection function of the present invention will be described with reference to FIG. First, (ST1) The relative positional relationship between the optical axis of the optical microscope and the electron optical axis of the scanning electron microscope in the scanning electron microscope to be used is stored. (ST2) Subsequently, defect position information measured by another inspection apparatus is stored. This is the preliminary preparation work. (ST3) A sample to be inspected such as a wafer is placed on the sample stage, and the environment in the chamber is adjusted. Note that the order of the above steps may be changed. (ST4) Based on the defect position information stored in advance, the driving mechanism of the sample stage (generally, the x, y axis driving mechanism) is driven to position the defect of interest at the optical axis position of the optical microscope. (ST5) First, a defect image is observed with a standard CCD camera. (ST6) It is determined whether or not a defect image can be seen in the observation image of the standard CCD camera. (ST7) When a defect image can be observed, the defect position information with respect to the reference position of the apparatus is measured and stored on the image. (ST8) When the defect image cannot be observed, the defect is observed with a high sensitivity CCD camera. (ST9) Subsequently, the defect position information with respect to the reference position of the apparatus is measured and stored on the observed image. (ST10) Based on the position information measured in step 7 or 9, the sample stage drive mechanism (generally, the x and y axis drive mechanisms) is driven to position the defect of interest at the optical axis position of the scanning electron microscope. (ST11) Observation / analysis with a scanning electron microscope is executed. (ST12) When the observation / analysis has not been completed for all the defects, the process returns to step 4 and the operation is repeated. When the observation / analysis has been completed for all the defects, the series of operations is terminated.
[0009]
【Example】
Furthermore, the present invention realizes a system that automatically executes the above-described CCD camera automatic switching method in a minute defect inspection performed using a scanning electron microscope equipped with a laser defect detection function, and automatically performs defect recognition. Provide a system.
As shown in FIG. 1, the system of the present invention is for observing a wafer surface provided with a laser optical system for irradiating a defect position with a laser beam from a laser light source 7 and an optical microscope 3 for observing scattered light from the defect. In the scanning electron microscope, the optical microscope 3 includes a high-sensitivity CCD camera 4 and a standard type CCD camera 5, and positions the optical axis of the optical microscope based on defect position information measured in advance by another inspection apparatus. There is a means to do. This means is based on the defect position information stored in the storage device as a function in the computer 10, and outputs a control signal to the x and y drive mechanism of the sample stage to operate the x and y drive mechanism of the sample stage. Positioning is performed so that the defect of interest comes to the optical axis position of the microscope 3. In this state, the computer 10 activates the standard CCD camera of the optical microscope to execute defect observation. Upon receiving this, the standard CCD camera starts operation and images the sample surface. This image signal is transmitted to the computer 10 and further transmitted to the display 9 such as a CRT for image display. Although it is possible to manually detect the presence or absence of laser scattered light from the defective portion on the display screen, in the present invention, it is preferable whether or not the computer 10 has a luminance signal greater than a threshold in the image information from the standard CCD camera. Provide a function to detect By providing this function as a system, it is possible to automatically determine whether or not there is defect information in the image.
[0010]
If a defect signal is recognized in the above step, the defect position is measured as position information with respect to the reference point of the system, and the position information is stored in the storage device of the computer 10. Subsequently, the defect shifts to observation with the scanning electron microscope 1, but since the optical axis of the optical microscope 3 and the optical axis of the electron microscope 1 are separated by several tens mm, the recognized defect of interest is regarded as an electron. In order to observe with the microscope 1, the sample stage must be moved by that distance. The relative positional relationship between the two optical systems is a fixed value that is uniquely determined when the apparatus is assembled, but this information is stored in advance in the storage device of the computer 10. Position information with respect to the reference point of this system is measured and stored by observation with the previous optical microscope, so that a control signal to be moved from the stored information to the x and y drive mechanism of the sample stage is output to the sample stage. The x and y drive mechanisms are operated so that the target defect comes to the position of the electron optical axis of the scanning electron microscope 1.
[0011]
When there is no defect information in the image information from the previous standard CCD camera 5, the operation of the standard CCD camera 5 is stopped by a command from the computer 10, and the high sensitivity CCD camera 4 is operated instead. The high-sensitivity CCD camera 4 performs defect observation, and it has been confirmed that the laser scattered light in this region has a low light amount level, that is, no large defect, so that the detection sensitivity is set high. The dark field image output signal from the high sensitivity CCD camera 4 does not become a high voltage. This image information is transmitted to the computer 10 and further transmitted to the display 9 for image display. From this observation image, the position information with respect to the reference point of this system is measured and stored in the same manner as described above. Further, since the relative position information between the optical axis of the optical microscope 3 and the optical axis of the electron microscope 1 is stored in the storage device, the control to be moved from these stored information to the x and y drive mechanism of the sample stage. The operation of outputting a signal and operating the x and y drive mechanism of the sample stage to position the target optical defect at the position of the electron optical axis of the scanning electron microscope 1 is the same as in the previous case.
[0012]
【The invention's effect】
A CCD camera automatic switching method in a scanning electron microscope having a laser defect detection function according to the present invention is a wafer including a laser optical system that irradiates a laser at a defect position obliquely from above and an optical microscope that observes scattered light from the defect. In a scanning electron microscope for surface observation, a high-sensitivity and standard CCD camera is attached to the optical microscope, the optical axis is positioned based on the defect position information measured in advance, and the defect is observed with the standard CCD camera of the optical microscope. When the defect observation cannot be observed with the standard CCD camera, the step of switching to the high-sensitivity CCD camera and taking the defect observation are taken. It is no longer necessary to operate the circuit protection layer device due to high voltage and interrupt the work, and this type of inspection analysis work can be made more efficient. It can be current.
[0013]
In addition, the CCD camera automatic switching system in the scanning electron microscope having the laser defect detection function of the present invention includes a laser optical system that irradiates a laser at a defect position obliquely from above and an optical microscope that observes scattered light from the defect. A scanning electron microscope for observing a wafer surface, the optical microscope comprising a high-sensitivity and standard CCD camera, and means for positioning the optical axis of the optical microscope based on defect position information measured in advance. A means for operating a standard CCD camera of an optical microscope to perform defect observation, a means for detecting the presence or absence of defect information in the image information of the standard CCD camera, and when the defect information cannot be detected by the detection means Since it consists of a means for observing defects by operating a high-sensitivity CCD camera, automatic switching of the CCD camera can be performed automatically. Ease of work and reduction of time can be promoted by providing means for detecting whether or not there is defect information in the image information of the standard CCD camera, and whether or not there is a luminance signal exceeding the threshold in the image information. Efficiency can be increased. Moreover, it is not necessary to add special hardware to the conventional apparatus, and the system configuration can be realized simply by providing software.
[Brief description of the drawings]
FIG. 1 is a diagram showing a basic configuration of a scanning electron microscope system having a laser defect detection function for realizing an autofocus method of the present invention.
FIG. 2 is a flowchart showing the operation of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Scanning electron microscope 7 Laser light source 11 Barrel part 8 Light 12 Secondary electron detector 9 CRT
3 Optical microscope 10 Computer 4 High sensitivity CCD camera 15 Control box 5 CCD camera 6 Sample

Claims (3)

In a CCD camera automatic switching method in a scanning electron microscope having a laser defect detection function comprising a laser optical system for irradiating a laser to a defect position obliquely from above and an optical microscope for observing scattered light from the defect,
Reading defect position information of the defect from a storage device;
Moving the sample stage and placing the defect position on the optical axis of the optical microscope;
Imaging the defect at the defect location with a first CCD camera of the optical microscope;
When there is no defect information in the image information imaged by the first CCD camera, a laser defect having a step of switching to a second CCD camera having higher sensitivity than the first CCD camera and imaging the defect A CCD camera automatic switching method in a scanning electron microscope having a detection function. In a CCD camera automatic switching system in a scanning electron microscope having a laser defect detection function comprising a laser optical system that irradiates a laser at a defect position obliquely from above and an optical microscope that observes scattered light from the defect,
A storage device for storing defect position information of the defect;
A first CCD camera of the optical microscope;
A second CCD camera having higher sensitivity than the first CCD camera;
Sample stage moving means for placing the defect position on the optical axis of the optical microscope based on the defect position information;
A detection function for detecting the presence or absence of defect information in the image information captured by the first CCD camera at the defect position ;
A function of outputting a signal for switching from the first CCD camera to the second CCD camera in order to image the defect by the second CCD camera when the defect information is absent in the detection function; And a CCD camera automatic switching system in a scanning electron microscope having a laser defect detection function.   3. The CCD camera automatic switching in a scanning electron microscope having a laser defect detection function according to claim 2, wherein the detection function detects presence / absence of a luminance signal equal to or higher than a threshold in image information captured by the first CCD camera. system.

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