JPH11273612A - Inspection device using electron beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inspection device which can always acquire an erect sample image by applying an electron beam. SOLUTION: This inspection device comprises an application means 24 which applies an electron beam onto a sample surface, an electron detection means 35 which detects a secondary beam consisting of at least either secondary electrons or reflected electrons generated from the sample surface so as to create a sample image, projecting electron optical systems 28, 31, 33, 34 placed between the sample and the electron detection means and focusing the secondary beam on the detection surface of the electron detection means so as to project an image of the sample surface onto the detection surface, magnification control means 44, 42 which control the focusing action of the projecting electron optical systems to vary the number of times that the secondary beam is focused, and an image conversion means 40 whereby the inverted sample image generated according to the number of times of focusing of the secondary beam is converted to an erect image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection apparatus for obtaining a sample image of a semiconductor wafer or the like by irradiating an electron beam, and more particularly to an inspection apparatus capable of always obtaining an erect sample image.

[0002]

2. Description of the Related Art In general, there is known a scanning electron microscope which scans a sample surface with an electron beam focused in a spot shape, detects secondary electrons, reflected electrons and the like generated from the sample, and obtains a sample image. ing. In this scanning electron microscope, since the sample is scanned with a spot-shaped electron beam having a reduced beam diameter, there is a problem in that scanning takes a long time and detection of the sample image requires a large amount of time. As one means for solving this problem, it is conceivable to increase the beam scanning speed. However, this raises a new problem that the amount of injected electrons per pixel is reduced and the image contrast is reduced. Therefore, this problem could not be easily solved.

[0003] In order to solve the above problems,
JP-A-7-181297 or JP-A-7-249
No. 393 has proposed a defect detection device. In the above-described defect detection device, the electron beam generated from the rectangular cathode is shaped into a rectangular or elliptical cross section by a plurality of multipole lenses, and is irradiated on the sample surface for scanning. At this time, since the sample surface is scanned with a rectangular beam having a large irradiation area, the sample surface can be scanned in a shorter time as compared with the case of scanning with a conventional spot-shaped electron beam. Higher speed was achieved.

[0004]

In recent years, however, there has been a strong demand to further speed up the detection of sample images in order to improve the throughput of semiconductor wafers. In the above-described defect detection apparatus, the detection of the sample image is speeded up by scanning the surface of the sample with a beam having a constant irradiation area called a rectangular beam, but in order to further increase the detection speed of the sample image. Has nothing but to increase the scanning speed. However, increasing the scanning speed causes another problem of deteriorating the image quality of the detected image as described above. Therefore, the above-described defect inspection apparatus can cope with further increasing the detection speed. Did not.

Accordingly, an object of the present invention is to provide an inspection apparatus which can speed up the detection of a sample image in order to meet the above demand.

[0006]

According to a first aspect of the present invention, there is provided an inspection apparatus using an electron beam, comprising: an irradiating means for irradiating an electron beam onto a sample surface; An electron detecting means for detecting a secondary beam and generating a sample image, and being disposed between the sample and the electronic detecting means, forming an image of the secondary beam on a detection surface of the electron detecting means, and forming an image of the sample surface. A projection electron optical system that projects onto the detection surface, lens control means for controlling the focusing action of the projection electron optical system, and changing the number of times of imaging of the secondary beam; Inverted sample image
Image conversion means for converting the image into an erect image.

In such a configuration, since the image of the area irradiated with the electron beam is projected on the detection surface of the electron detection means, it is possible to collectively obtain a sample image.
Further, in this inspection apparatus, it is possible to change the magnification by changing the number of times of imaging of the secondary beam, but at this time, if the number of times of imaging changes, the sample image is inverted. In this case, the image conversion means can convert the inverted image into an erect image.
Note that the inverted image is an image in which, when a sample image facing in a certain direction is an erect image, the relationship between up, down, left, and right is opposite to the erect image.

According to a second aspect of the present invention, in the inspection apparatus using an electron beam according to the first aspect, the image conversion means is configured to detect a point on the sample image when the number of imaging times is either an even number or an odd number. It is characterized in that a symmetric inverted image is generated. In such a configuration, when the sample image is inverted, the image can be converted into an erect image by generating an inverted image of the sample image (an image obtained by inverting the image vertically and horizontally).

According to a third aspect of the present invention, in the inspection apparatus using an electron beam according to the first or second aspect,
The image conversion unit is configured to include a storage unit that stores pixel information constituting the sample image, a writing unit that writes the pixel information in the storage unit, and a reading unit that reads the pixel information from the storage unit. According to a feature of the present invention, the pixel information is read from the storage means in one of the order in which the pixel information is written to the storage means and the reverse order in accordance with the number of times of imaging.

With such a configuration, when the sample image is inverted, the image can be converted into an erect image by reading the pixel information from the storage means in the reverse order of the writing order.
According to a fourth aspect of the present invention, in the inspection apparatus using an electron beam according to the first or second aspect, the image conversion unit stores the pixel information constituting the sample image, and stores the pixel information in the storage unit. Writing means for writing pixel information, and reading means for reading pixel information from the storage means, wherein the writing means is configured to perform one of the order in which the pixel information is read from the storage means and the reverse order in accordance with the number of times of image formation. The pixel information is written in the storage means in the order of.

With such a configuration, when the sample image is inverted, the image can be converted into an erect image by writing the pixel information in the storage means in the reverse order of the reading order of the pixel information.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall configuration diagram of the present embodiment.
This embodiment corresponds to the first to third aspects of the present invention. In FIG. 1, the inspection device has a primary column 21, a secondary column 22, and a chamber 23. The primary column 21 is obliquely attached to a side surface of the secondary column 22, and a lower part of the secondary column 22 is provided with a chamber 2.
3 are arranged.

An electron gun 24 is provided inside the primary column 21, and a primary optical system 25 is arranged on the optical axis of an electron beam (primary beam) emitted from the electron gun 24. On the other hand, a stage 26 is provided inside the chamber 23, and a sample 27 is placed on the stage 26.
Inside the secondary column 22, a cathode lens 28, a numerical aperture 29, a Wien filter 30, a second lens 31, a field aperture 32, a third lens 33 are arranged on the optical axis of a secondary beam generated from the sample 27. The fourth lens 34 and the detector 35 are arranged. The cathode lens 28, the second lens 31 to the fourth lens 34
Constitutes a secondary optical system.

The detector 35 includes an MCP (micro channel plate) 36, an FOP (fiber optic plate) 38 having a fluorescent screen 37, and a CCD sensor 39. The detector 35 is connected to the image processing unit 40. The image processing unit 40 includes a control unit, an A / D conversion unit, a VR
It consists of an AM and D / A converter, and its output is C
It is input to the RT display 41.

The CPU 42 outputs control signals to an image processing unit 40, a primary optical system lens control unit 43, a secondary optical system lens control unit 44, and a stage control unit 45. The primary optical system lens control unit 43 controls the lens voltage of the primary optical system 25, and the secondary optical system lens control unit 44 controls the cathode lens 2
8 and the lens voltages of the second lens 31 to the fourth lens 34 are controlled, and the stage control unit 45
Drive control is performed in the Y direction.

The primary column 21, the secondary column 22, and the chamber 23 are connected to a vacuum evacuation system (not shown), and are evacuated by a vacuum pump of the vacuum evacuation system, so that the inside maintains a vacuum state. As for the correspondence between the first and second aspects of the present invention and the present embodiment, the irradiation means corresponds to the electron gun 24, the primary optical system 25, and the primary optical system lens control unit 43, and the electron detection means corresponds to , The detector 35, the projection electron optical system corresponds to the cathode lens 28, the second lens 31, the third lens 33, and the fourth lens 34, and the lens control means includes the CPU 42 and the secondary optical system lens control unit. The image conversion means corresponds to the image processing unit 40.

In addition, regarding the correspondence between the invention described in claim 3 and the present embodiment, the storage means includes an image processing unit 40
The writing unit and the reading unit correspond to a control unit inside the image processing unit 40, corresponding to the internal VRAM. Next, an operation for acquiring a sample image in the inspection apparatus of the present embodiment will be described. As shown in FIG. 2, the primary beam emitted from the electron gun 24 is accelerated by the acceleration voltage of the electron gun 24, and enters the Wien filter 30 under the lens action of the primary optical system 25.

Here, as the cathode of the electron gun, lanthanum hexaborite (LaB ₆ ) capable of extracting a large current with a rectangular cathode is used. In addition, the primary optical system 25 uses a quadrupole (or octupole) electrostatic lens (or electromagnetic lens) that is asymmetric about the rotation axis. This lens, like a so-called cylindrical lens, has a long axis (X
Convergence and divergence can be caused in each of the axis (axis) and the short axis (Y axis). FIG. 2 shows the trajectories of the electrons emitted in the X-direction section and the electrons emitted in the Y-direction section of the rectangular cathode.

As a specific lens configuration, as shown in FIG. 3, when an electrostatic lens is used, four cylindrical rods are used. The opposing electrodes are set at the same potential, and voltage characteristics opposite to each other (+ Vq for a and b and −Vq for c and d) are given. By arranging this lens in three stages and optimizing each lens condition, the beam irradiation area on the sample surface can be formed into an arbitrary rectangular or elliptical shape without losing irradiation electrons. .

The primary beam formed into a rectangular shape by the primary optical system 25 has its trajectory bent by the deflection action of the Wien filter 30, and forms an image at the opening of the numerical aperture 29. The Wien filter 30 makes the magnetic field and the electric field orthogonal, and sets the electric field to E, the magnetic field to B, and the velocity of the charged particle to v
In this case, only charged particles satisfying the Wien condition of E = vB are made to go straight, and the trajectories of the other charged particles are bent.

Further, the new mechanical aperture 29 is
The aperture angle of the cathode lens 28 is determined by an aperture stop. Its shape is made of metal (M
o), which prevents extra electron beams scattered in the apparatus from reaching the sample surface, thereby preventing charge-up and contamination of the sample 27. The primary beam imaged at the aperture of the numerical aperture 29 is irradiated perpendicularly onto the surface of the sample 27 via the cathode lens 28. When the primary beam is irradiated on the sample surface,
A secondary beam including at least one of secondary electrons and reflected electrons is generated from the beam irradiation area.

This secondary beam has two-dimensional image information of the beam irradiation area. In particular, since the primary beam is irradiated perpendicularly to the sample 27, the secondary beam is sharp and has no shadow. Image. As shown in FIG. 4, the secondary beam passes through the lens while undergoing lens action by the cathode lens 28. By the way, the cathode lens 28 is usually composed of 2 to 4 electrodes. Here, a configuration example of three electrodes is shown. Normally, in order to perform the lens action, a voltage is applied to the first and second electrodes from the bottom of the cathode lens 28, and the third electrode is set to zero potential.

A voltage is applied to the lowermost electrode of the cathode lens 28 and the stage 26, and a negative electric field is applied to the primary beam and a positive electric field is applied to the secondary beam between the electrode and the sample surface. Is formed. With this voltage, the cathode lens 28 slows down the primary beam to prevent charge-up and destruction of the sample, and draws electrons (especially secondary electrons with low directivity) for the secondary beam. , Accelerates and efficiently guides the lens.

The secondary beam that has passed through the cathode lens 28 and the numerical aperture 29 proceeds straight without being deflected by the Wien filter 30. At this time, by changing the electromagnetic field applied to the Wien filter 30, only electrons having a specific energy band (for example, secondary electrons or reflected electrons) can be guided to the detector 35 from the secondary beam.

Further, the new mechanical aperture 29 is
For the secondary beam, it plays the role of suppressing the lens aberration of the second to fourth lenses 31 to 34 at the subsequent stage. By the way, if the secondary beam is imaged only by the cathode lens 28, the lens action becomes strong and aberrations are likely to occur.
Therefore, one image formation is performed together with the second lens 31. The secondary beam obtains an intermediate image on the field aperture 32 by the cathode lens 28 and the second lens 31.

A lens for projecting an intermediate image is arranged at the subsequent stage, but in order to secure a projection magnification required as a secondary optical system, a configuration is provided in which two lenses, a third lens 33 and a fourth lens 34, are added. To Second lens 31 to fourth lens 3
Reference numeral 4 denotes a rotation axis symmetrical lens called a unipotential lens or an Einzel lens, and each lens includes three electrodes. Usually the outer 2
The lens action is controlled by setting the electrodes to zero potential and changing the voltage applied to the center electrode.

A field aperture 32 is arranged at an intermediate image forming point, and this field aperture 32 limits the field of view to a necessary range similarly to the field stop of the optical microscope. In particular, in the case of an electron beam, an unnecessary beam is cut off together with the third lens 33 and the fourth lens 34 at the subsequent stage, thereby preventing charge-up and contamination of the detector 35.

The secondary beam is re-imaged on the detection surface of the detector 35 by the third lens 33 and the fourth lens 34, and the image of the beam irradiation area is projected on the detection surface. Although details will be described later, by changing the focal length of each lens at this time,
The projection magnification can be changed. The secondary beam enters the MCP 36 inside the detector 35, is accelerated and amplified when passing through the MCP 36, and collides with the fluorescent screen 37.

The phosphor screen 37 converts electrons into an optical image,
The optical image passes through the FOP 38 and is captured by the CCD sensor 39. Here, the image size on the fluorescent screen 37 and C
To match the imaging size of the CD sensor 39, the FO
In P38, the image is reduced to about 1/3 and projected. The optical image is C
The photoelectric conversion is performed by the CD sensor 39, and the CCD sensor 39
Accumulates signal charges. The image processing unit 40 includes a CPU 42
The signal charge is read out serially from the CCD sensor 39 in accordance with the readout instruction. The image processing unit 40 creates a sample image and displays it on the CRT display 41.

As described above, in the present embodiment, the sample surface can be collectively obtained by irradiating the sample surface with the electron beam and projecting the image of the beam irradiation area onto the detection surface of the detector 35. Therefore, the detection speed of the sample image can be significantly increased. By the way, in the inspection device of the present invention, the projection magnification can be changed by changing the focal length of the secondary optical system as described above. At this time, the number of times of imaging of the secondary beam may change with the change of the focal length, and the sample image may be inverted.

For example, as shown in FIG.
By increasing the absolute value of the voltage applied to the central electrode of the third and fourth lenses 34, the focal length of both lenses is reduced, and the secondary beams are imaged by both lenses. At this time, the projection magnification is set to a high magnification. However, if the image obtained when the number of times of image formation shown in FIG. 4 is an even number (two times) is an erect image, the image obtained when the number of times of image formation is odd (three times) in FIG. Since the image becomes an inverted image in which the left and right relations are opposite, it is very difficult for the operator to observe. Therefore, inversion processing for converting an inverted image into an erect image is indispensable.

Hereinafter, the inversion process will be described. Here, an image obtained when the number of times of image formation is an even number is defined as an erect image,
An image obtained at an odd number of times is an inverted image. First, an arbitrary projection magnification is set by an instruction of the operator. CPU42
, The excitation voltage value of each lens corresponding to the projection magnification is prepared in advance as a conversion table.
At the set projection magnification, it can be determined in advance whether the number of times of imaging of the secondary beam is “even number” or “odd number”.

The CPU 42 controls each lens of the secondary optical system,
A predetermined excitation voltage value is set via the secondary optical system lens control unit 44. When the number of times of image formation is an even number, the image processing unit 40 displays the sample image on the CRT display 41 as it is.
1 is displayed. Hereinafter, this operation will be described in detail.

(When the number of times of image formation is an even number) As shown in FIG. 6, an image of a beam irradiation area (here, as an example, an “F” mark) is projected on the detection surface of the detector 35, and CC
The D sensor 39 captures this image. Pixel information (signal charge) is accumulated in the CCD sensor 39, and the pixel information is sequentially read out to the image processing unit 40.

The A / D converter in the image processing unit 40 converts the pixel information into a digital signal, and the control unit converts the pixel information into the start address (for example, address 0000) of the VRAM.
To the last address (for example, address FFFF). Next, the control unit reads the pixel information in the order in which the pixel information is written, that is, in the order from the start address (address 0000) to the end address (address FFFF).

The read pixel information is converted into an analog signal by a D / A conversion unit, and is converted into an analog signal.
Is displayed in. (When the number of image formations is an odd number) As shown in FIG. 7, when the number of image formations is an odd number, the image projected on the detection surface of the detector 35 is inverted. The CCD sensor 39 captures this inverted image. Pixel information is accumulated in the CCD sensor 39, and the pixel information is sequentially read out to the image processing unit 40.

The A / D converter converts the pixel information into a digital signal, and the controller sequentially stores the pixel information from the start address (address 0000) of the VRAM to the end address (address FFFF). Next, the control unit reads the pixel information in the reverse order in which the pixel information is written, that is, in the order from the last address (address FFFF) to the start address (address 0000).

The read pixel information is converted into an analog signal by a D / A conversion unit, and is converted to a CRT display 41.
Is displayed in. At this time, the CRT display 4
In FIG. 1, an inverted image is converted into an erect image and displayed. As described above, in the inspection apparatus according to the present embodiment, even if the number of times of imaging of the secondary beam changes with the change in the projection magnification, and the sample image becomes an inverted image due to the influence, the image processing unit 40 sets the erect image. Therefore, an erect sample image can always be obtained regardless of the number of times the secondary beam is imaged.

The inversion process is not limited to the method according to the present embodiment. Another method (corresponding to the invention described in claim 4) will be described. As shown in FIG. 8, an inverted image is projected on the detection surface of the detector 35. The CCD sensor 39 captures this inverted image. C
Pixel information is accumulated in the CD sensor 39, and the pixel information is sequentially read out to the image processing unit 40.

The A / D converter converts the pixel information into a digital signal, and the controller stores the pixel information in order from the last address (address FFFF) to the start address (address 0000) of the VRAM. Next, the control unit sets the pixel information to VR
The AM is read in order from the first address (address 0000) to the last address (address FFFF).

The D / A converter converts the pixel information into an analog signal, and the inverted image is converted into an erect image and displayed on the CRT display 41. When the sample image is an erect image, the pixel information is stored in the head address (000) of the VRAM.
0 address) to the final address (FFFF address). As another inversion processing method, for example, the stage 26 may be rotated 180 degrees via the stage control unit 45 according to the number of times of image formation. Further, a drive mechanism for driving the detector 35 to rotate may be provided, and the detector 35 itself may be rotated by 180 degrees. Furthermore, the order in which pixel information is read from the CCD sensor 39 may be changed according to the number of times of image formation.

Further, the configuration of the secondary optical system is not limited to the configuration of the present embodiment, but may be a configuration further including a lens. In addition, the change of the number of times of image formation is the third
The focal length of the cathode lens 28 and the second lens 31 is not limited to the lens 33 and the fourth lens 34, and may be changed. Further, in the present embodiment, the inversion processing is performed using an image obtained when forming an odd number of times as an inverted image, but the inversion processing may be performed using an image obtained when forming an even number of times as an inverted image.

[0044]

As described above, in the electron beam inspection apparatus according to the first aspect, since the image on the sample surface is projected on the detection surface of the electron detection means, the sample image is obtained collectively. It is possible to greatly improve the detection speed of the sample image. Further, in this inspection apparatus, it is possible to change the magnification by changing the number of times of imaging of the secondary beam, but at this time, if the number of times of imaging changes, the sample image may become an inverted image. In this case, since the inverted image of the sample can be converted into an erect image by the image conversion means, the operator can always observe the erect image.

In the inspection apparatus using an electron beam according to the second aspect, the image conversion means can generate an inverted image which is point-symmetric with the sample image. Therefore, when the sample image is inverted, it can be converted into an erect image. In the inspection apparatus using an electron beam according to claim 3,
When the sample image is inverted, the inverted image can be converted into an erect image by reading out the pixel information in the reverse order of the order in which the pixel information is written into the storage means.

In the inspection apparatus using an electron beam according to the fourth aspect, when the sample image is inverted, the inverted image is written by writing the pixel information in the reverse order in which the pixel information is read from the storage means. It can be converted to an erect image.
In this manner, the inspection apparatus to which the present invention is applied can always observe the erect image regardless of the projection magnification, and can reliably detect a defect portion of the sample image.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of the present embodiment.

FIG. 2 is a diagram showing a trajectory of a primary beam.

FIG. 3 is a configuration diagram of a primary optical system.

FIG. 4 is a diagram showing a trajectory of a secondary beam.

FIG. 5 is a diagram illustrating a trajectory of a secondary beam when the number of times of imaging is changed.

FIG. 6 is a diagram illustrating an operation of the image processing unit when the number of times of image formation is an even number.

FIG. 7 is a diagram illustrating an operation of the image processing unit when the number of times of imaging is an odd number.

FIG. 8 is a diagram illustrating another reverse operation.

[Explanation of symbols]

 21 Primary Column 22 Secondary Column 23 Chamber 24 Electron Gun 25 Primary Optical System 26 Stage 27 Sample 28 Cathode Lens 29 New Mechanical Aperture 30 Wien Filter 31 Second Lens 32 Field Aperture 33 Third Lens 34 Fourth Lens 35 Detector 36 MCP 37 phosphor screen 38 FOP 39 CCD sensor 40 image processing unit 41 CRT display 42 CPU 43 primary optical system lens control unit 44 secondary optical system lens control unit 45 stage control unit

Claims (4)

[Claims] An irradiation unit configured to irradiate an electron beam onto a sample surface; and an electron detection unit configured to detect a secondary beam including at least one of a secondary electron and a reflected electron generated from the sample surface to generate a sample image. A projection electron optical system disposed between the sample and the electron detection means, for imaging the secondary beam on a detection surface of the electron detection means, and projecting an image of the sample surface on the detection surface; Lens control means for controlling the focusing action of the projection electron optical system to change the number of times the secondary beam is imaged; and inverting the inverted sample image generated according to the number of times the secondary beam is imaged. An inspection apparatus using an electron beam, comprising: an image conversion unit that converts the image into a standing image. 2. The inspection apparatus using an electron beam according to claim 1, wherein the image conversion means is point-symmetric with respect to the sample image when the number of times of imaging is either an even number or an odd number. An inspection apparatus using an electron beam, which generates an image. 3. The inspection apparatus using an electron beam according to claim 1, wherein the image conversion unit stores a pixel information of the sample image, and stores the pixel information in the storage unit. A writing unit for writing, and a reading unit for reading out the pixel information from the storage unit, wherein the reading unit is configured to write the pixel information into the storage unit in a reverse order according to the number of times of imaging. Wherein the pixel information is read from the storage means in any one of the following order. 4. The inspection device using an electron beam according to claim 1, wherein the image conversion unit stores pixel information constituting the sample image, and stores the pixel information in the storage unit. A writing unit for writing, and a reading unit for reading out pixel information from the storage unit, wherein the writing unit is configured to read the pixel information from the storage unit according to the number of times of imaging, and the reverse order. An inspection apparatus using an electron beam, wherein pixel information is written in the storage means in any one of the order.