JP3858370B2 - Inspection device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inspection apparatus that uses an electron beam to acquire, for example, an integrated circuit pattern image of a semiconductor wafer, and more particularly to an inspection apparatus that can reduce aberration due to a Wien filter and unevenness in the intensity of an electron beam.
[0002]
[Prior art]
Unlike a conventional scanning electron microscope (SEM), the beam cross-sectional shape of the electron beam is irradiated with a primary optical system in a linear or rectangular shape, and the sample is irradiated with reflected electrons, secondary electrons, etc. There is a system in which a secondary beam is detected by an electron detector via a secondary optical system and a surface image of a sample is acquired. However, such a system is only described in a paper, and there is no practical known example yet.
[0003]
In addition, there is a paper using a Wien filter (K. Tsuno, Ultramicroscopy 55 (1994) 127-140 “Simulation of a Wien filter as beam separator in a low energy electron microscope”) as an example of the configuration of the secondary optical system. In this paper, only the calculation results are not put into practical use as a device.
Therefore, the present applicant has examined various problems when putting such a system into practical use as an inspection apparatus, and in the application of Japanese Patent Application Nos. 9-91421 and 9-91422, the specifics of this inspection apparatus are described. A typical configuration example was proposed.
[0004]
The inspection apparatus will be described with reference to FIG.
In FIG. 9, the inspection apparatus has a primary column 50, a Wien filter 51, a stage 52, and a secondary column 54.
A sample 53 is placed on the stage 52, and a cathode lens 55 and a secondary optical system 56 are arranged inside the secondary column 54. Further, a numerical aperture 57, which is an aperture stop, is arranged inside the secondary column 54, and a field aperture (field stop) 58 in which a plurality of field holes having different diameters are formed can be pulled out behind. Has been inserted.
[0005]
On the other hand, a detector 59 is arranged at a position where secondary electrons and reflected electrons are focused via the secondary optical system 56. The detector 59 converts the detected electrons into an optical signal, which is further converted back into a photoelectric signal by the image sensor. The photoelectric signal is input to the CPU 61 via the control unit 60.
The CPU 61 outputs control signals to the primary column control unit 62 and the secondary column control unit 63. The primary column control unit 62 controls the lens voltage for the primary optical system in the primary column 50, and the secondary column control unit 63 controls the lens voltage for the cathode lens 55 and the secondary optical system 56.
[0006]
Next, the operation of this inspection apparatus will be described.
The primary optical system in the primary column 50 causes an electron beam (primary beam) emitted from the electron gun to enter the Wien filter 51 at a constant incident angle α. The Wien filter 51 is a deflector that acts as an electromagnetic prism, and irradiates the sample 53 perpendicularly by bending an electron beam having specific energy from the primary column 50 at an angle determined by the acceleration voltage. On the other hand, the secondary beam generated from the sample 53 is caused to travel straight and enter the secondary optical system 56 in the secondary column 54. Details of the Wien filter will be described later.
[0007]
The secondary beam is focused by the secondary optical system 56, enlarged and projected at a predetermined magnification on the detection surface of the detector 59, and converted into an image signal. At this time, the numerical aperture 57 plays a role of suppressing the aberration of the secondary optical system 56 and also prevents the extra electrons scattered in the apparatus from being detected by the detector 59.
In addition, by selecting a field hole formed in the field aperture 58, the type of detection electrons can be selected. That is, since secondary electrons have more electrons than reflected electrons, a field hole having a small diameter is selected when it is desired to detect the secondary electrons as the center. On the other hand, when it is desired to detect many reflected electrons, a field hole having a large diameter is selected. Thereby, electrons with a high yield can be selected and detected according to the state of the sample.
[0008]
The control unit 60 takes out the image signal from the detector 59 and outputs it to the CPU 61. The CPU 61 performs pattern defect inspection from the extracted image signal by template matching or the like. The CPU 61 drives the stage 52, aligns the position with the next inspection location, and repeats the above inspection operation.
Thus, in this inspection apparatus, the electron beam is irradiated perpendicularly to the sample by the Wien filter 51. Therefore, since a shadow does not occur in the reflected image, a clear image can be acquired.
[0009]
[Problems to be solved by the invention]
However, the electromagnetic field applied to the Wien filter usually has a deviation or nonuniformity from the ideal distribution. As a result, the primary beam that has been deflected by the Wien filter has a so-called aberration in which it is irradiated with a deviation from the position or irradiation angle of the specimen, and the specimen is irradiated with such a beam. Then, unevenness of the beam intensity occurred in the irradiated area.
[0010]
When a secondary beam from a sample is detected in this state, such a beam intensity unevenness appears as fixed pattern noise, and the detected image becomes unclear.
In particular, if the incident angle α when the primary beam is incident on the Wien filter is large, the angle at which the beam trajectory is bent increases, and a large difference occurs in the optical path to the sample surface between the outer periphery and the inner periphery of the primary beam. As a result, aberration and unevenness of the beam intensity increased, and the S / N of the detected image was greatly deteriorated.
[0011]
Accordingly, an object of the present invention is to provide an inspection apparatus capable of acquiring an optimum image with reduced aberration and unevenness of beam intensity due to the Wien filter in order to solve the above-described problems.
[0012]
[Means for Solving the Problems]
1 and 2 are block diagrams showing the principle of the present invention.
[0013]
According to the first aspect of the present invention, there is provided an irradiation means 1 for irradiating a sample surface with an electron beam having a spread in a cross-sectional direction, and secondary electrons, reflected electrons or backscattered electrons generated from the sample by the irradiation of the electron beam. An electron detecting means 2 for detecting at least one kind as a secondary beam, an irradiating means 1 and an electron detecting means 2, and a sample are arranged on the sample surface by bending the electron beam irradiated from the irradiating means 1. A Wien filter 3 that vertically irradiates and emits a secondary beam generated from the sample to the electron detection means 2 is disposed between the irradiation means 1 and the Wien filter 3, and the electron beam enters the Wien filter 3. And deflection means 4 for changing the beam incident angle or beam incident position.
[0014]
Further, the inspection apparatus includes the contrast detection unit 5 that generates image information of the sample from the secondary beam detected by the electron detection unit 2 and obtains the contrast of the image information . Deflection means 4, by changing the angle of beam incidence or beam incidence position, maximize the contrast obtained by the contrast detection unit 5.
[0015]
(Function)
In the inspection apparatus according to claim 1, the electron beam irradiated from the irradiation unit 1 is incident on the Wien filter 3. At this time, the deflecting unit 4 receives a deflection action, and the incident angle or incident is incident on the Wien filter 3. The position to perform changes.
Therefore, the incident angle can be reduced without changing the device structure, the degree of bending of the electron beam by the Wien filter can be reduced, and the occurrence of unevenness of the beam intensity can be suppressed as much as possible.
[0016]
Further, the contrast detection means 5 generates image information of the sample from the secondary beam detected by the electron detection means 2, and obtains the contrast of the image information.
The deflecting means 4 sequentially changes the incident angle or incident position when the primary beam is incident on the Wien filter (by so-called hill-climbing method) to obtain the incident angle or incident position where the contrast is maximized.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 3 is an overall configuration diagram of the present embodiment. Further, FIG. 4 is a diagram showing the trajectory of the primary beam, FIG 5 is Ru FIG der showing the trajectory of the secondary beam.
[0018]
3 to 5, the inspection apparatus has a primary column 21, a secondary column 22, and a chamber 23. An electron gun 24 is provided inside the primary column 21, and a primary optical system 25, a deflector 26, a deflector 27, and a deflector are arranged on the optical axis of an electron beam (primary beam) emitted from the electron gun 24. 28 is arranged.
A stage 29 is installed inside the chamber 23, and a sample 30 is placed on the stage 29.
[0019]
On the other hand, inside the secondary column 22, on the optical axis of the secondary beam generated from the sample 30, a cathode lens 31, a numerical aperture 32, a Wien filter 33, a second lens 34, a field aperture 35, and a third lens. 36, a fourth lens 37 and a detector 38 are arranged.
The numerical aperture 32 corresponds to an aperture stop, and is a metal (Mo or the like) thin film with a circular hole.
[0020]
Further, the output of the detector 38 is input to the image processing unit 39.
Further, the CPU 40 outputs control signals to the primary column control unit 41, the secondary column control unit 42, the deflector control unit 43, the Wien filter control unit 44, and the stage voltage control unit 45.
The primary column control unit 41 controls the lens voltage of the primary optical system 25, and the secondary column control unit 42 controls the lens voltages of the cathode lens 31 and the second lens 34 to the fourth lens 37, and deflector control. The unit 43 controls the voltage (or current) applied to the deflectors 26 to 28, the Wien filter control unit 44 performs electromagnetic field control applied to the Wien filter 33, and the stage voltage control unit 45 The voltage applied to 29 is controlled.
[0021]
The primary column 21, the secondary column 22, and the chamber 23 are connected to an evacuation system (not shown), and are evacuated by a turbo pump of the evacuation system, and the inside is maintained in a vacuum state.
[0022]
As for the correspondence relationship between the invention described in claim 1 and the present embodiment, the irradiation means 1 corresponds to the electron gun 24, the primary optical system 25 and the primary column control unit 41, and the electron detection means 2 The Wien filter 3 corresponds to the Wien filter 33 and the Wien filter control unit 44 corresponding to the second lens 34, the third lens 36, the fourth lens 37, the detector 38 and the secondary column control unit 42. Corresponds to the deflector 26, the deflector 27, the deflector 28, the CPU 40 and the deflector control unit 43, and the contrast detection means 5 corresponds to the image processing unit 39.
[0023]
As shown in FIG. 4, the primary beam receives the lens action of the primary optical system 25, is irradiated with the acceleration voltage Vacc, and enters the Wien filter 33 at an incident angle α ₀ . At this time, the deflecting action by the deflectors 26 to 28 is not received.
The trajectory of the primary beam is bent by the deflection action of the Wien filter 33. The Wien filter 33 orthogonally crosses the magnetic field and electric field, and when the electric field is E, the magnetic field is B, and the speed of the charged particles is v, only the charged particles that satisfy the Wien condition of E = vB travel straight. Bend the trajectory. As shown in FIG. 7, for the primary beam, a force FB caused by a magnetic field and a force FE caused by an electric field are generated, and the beam trajectory is bent. On the other hand, since the force FB and the force FE work in opposite directions with respect to the secondary beam, they cancel each other and the secondary beam goes straight as it is.
[0024]
The primary beam whose trajectory is bent by the Wien filter 33 forms an image at the opening of the numerical aperture 32.
The numerical aperture 32 prevents an extra electron beam scattered in the apparatus from reaching the sample surface and prevents the sample 30 from being charged up or contaminated.
When the primary beam is irradiated onto the sample via the cathode lens 31, secondary electrons, reflected electrons, or backscattered electrons are generated as a secondary beam from the beam irradiation surface of the sample.
[0025]
The secondary beam passes through the cathode lens 31 while receiving the lens action of the cathode lens 31.
At this time, a voltage is applied to the stage 29 by the stage voltage control unit 45, and a primary beam is provided between the lowermost electrode of the cathode lens 31 constituted by three electrodes and the sample surface. A negative electric field (this is called a retarding voltage Vr) is formed. With this retarding voltage Vr, the cathode lens 31 decelerates the primary beam to prevent charge-up or destruction of the sample, and the secondary beam has electrons (especially secondary electrons with small directivity). ) Is pulled in, accelerated, and efficiently guided into the lens.
[0026]
The lens action of the cathode lens 31 is performed by applying a voltage to the first and second electrodes of the cathode lens 31 and setting the third electrode to zero potential.
The secondary beam that has passed through the cathode lens 31 and the numerical aperture 32 passes straight through without being subjected to the deflection action of the Wien filter 33. At this time, the electromagnetic field (E, B) applied to the Wien filter 33 is determined by three parameters of the acceleration voltage Vacc, the incident angle α, and the retarding voltage Vr.
[0027]
Note that by changing the electromagnetic field applied to the Wien filter 33, only electrons having a specific energy (for example, secondary electrons, reflected electrons, or backscattered electrons) can be guided to the detector 38 from the secondary beam. it can.
On the other hand, when the secondary beam is focused only by the cathode lens 31, the lens action becomes strong and aberrations are likely to occur. Therefore, as shown in FIG. 5, one image formation is performed together with the second lens 34. The secondary beam obtains an intermediate image on the field aperture 35 by the cathode lens 31 and the second lens 34.
[0028]
In this case, since the magnification magnification necessary for the secondary optical system is usually insufficient, the third lens 36 and the fourth lens 37 are added as lenses for enlarging the intermediate image. The secondary beam is enlarged and imaged by the third lens 36 and the fourth lens 37, respectively, and is imaged three times in total here. Note that the third lens 36 and the fourth lens 37 may be combined and imaged once (total twice).
[0029]
The second lens 34 to the fourth lens 37 are all rotationally symmetrical lenses called unipotential lenses or Einzel lenses. Each lens has a configuration of three electrodes. Usually, the outer two electrodes are set to zero potential, and the lens action is performed with a voltage applied to the center electrode.
A field aperture 35 is disposed at the intermediate image point. The field aperture 35 limits the field of view to the required range, similar to the field stop of the optical microscope, but in the case of an electron beam, the extra beam is blocked together with the third lens 36 and the fourth lens 37 in the subsequent stage, The charge-up and contamination of the detector 38 are prevented.
[0030]
The secondary beam is enlarged and projected by the secondary optical system and forms an image on the detection surface of the detector 38.
The detector 38 includes an MCP that amplifies electrons, a fluorescent plate that converts electrons to light, a relay between the vacuum system and the outside, a lens and other optical elements for transmitting an optical image, and an image sensor (CCD or the like). It consists of. The secondary beam forms an image on the MCP detection surface and is amplified, and the electrons are converted into an optical signal by the fluorescent plate and converted into a photoelectric signal by the imaging device.
[0031]
Next, the operation related to the optimum image acquisition which is a feature point of the present embodiment will be described with reference to FIG.
The image processing unit 39 reads the image signal of the sample from the detector 38. The image processing unit 39 includes a high-pass filter, a rectifier circuit, an A / D conversion circuit, and an integration circuit. The high-pass filter extracts a high-band signal from the image signal. Further, the integrator circuit integrates the output rectified by the rectifier circuit, A / D converts it, and uses the value as the contrast value of the image (step S1).
[0032]
The image processing unit 39 transmits the contrast value to the CPU 40.
Next, the CPU 40 changes the incident angle α from α ₀ to α ₁ and obtains the contrast value of the image again. In general, as the value of the incident angle α is smaller, the influence of aberration can be reduced. Therefore, as shown in FIG. 4, the incident angle α ₀ of the primary beam is set to α ₁ that is reduced by a predetermined step width. The CPU 40 and the deflector control unit 43 control the voltages of the deflectors 26 to 28 so that the primary beam is incident on the Wien filter at the incident angle α ₁ (step S2).
[0033]
The CPU 40 calculates an electromagnetic field (E, B) to be applied to the Wien filter 33 from the incident angle α ₁ , the acceleration voltage Vacc, and the retarding voltage Vr, and the Wien filter control unit 44 applies the electromagnetic field to the Wien filter 33. Apply.
In this state, the electron beam is irradiated to detect the secondary beam, and the contrast value of the sample image is obtained (step S3).
[0034]
The CPU 40 compares the previous contrast value with the current contrast value (step S4). If the current contrast value is large, the CPU 40 further reduces the incident angle by a predetermined step size, and again the contrast value of the image. Ask for.
When the current contrast value is small, the previous contrast value has a peak value, and the incident angle α at that time is the optimum incident angle (step S5).
[0035]
The CPU 40 performs a pattern defect inspection by template matching or the like on the image signal acquired at the optimum incident angle.
As described above, the inspection apparatus according to the present embodiment is optimal when the contrast of the sample image is maximized while using the deflectors 26 to 28 while changing the incident angle α when the primary beam is incident on the Wien filter. The incident angle is obtained. Therefore, it is possible to obtain an optimal image in which the aberration and the beam intensity unevenness due to the Wien filter are minimized.
[0036]
Note that contrast detection is not limited to the method of the present embodiment. For example, edge detection is performed from an image using a spatial differential filter, edge strength is calculated, and the point where the edge strength is maximized is the maximum contrast. You may judge.
[0037]
In the present embodiment, the incident angle α is changed. However, the present invention is not limited to this, and as shown in FIG. 8, by controlling the voltages of the deflectors 26 to 28, the incident angle α remains constant and the Wien filter is used. The position where the beam is incident on 33 may be shifted in parallel. At this time, generally, when the beam is incident on the center of the Wien filter 33, the contrast of the image becomes maximum.
[0038]
In the present embodiment, the direction in which the angle of incidence α is variable is limited to the direction in which the angle is reduced. However, the present invention is not limited to this. You may make it the direction to do.
In the present embodiment, a configuration using three stages of deflectors as the deflecting means 4 is adopted, but the present invention is not limited to this configuration.
[0039]
【The invention's effect】
In the inspection apparatus according to the first aspect, the incident angle or the incident position when the electron beam is incident on the Wien filter can be changed by the deflecting unit.
Therefore, the incident angle can be reduced without changing the device structure, and the occurrence of unevenness of the beam intensity can be suppressed as much as possible by relaxing the bending of the electron beam by the Wien filter.
[0040]
Further, since the beam incident angle can be adjusted later by the deflecting unit, the arrangement relationship between the irradiation unit and the Wien filter is not required to be as strict as the conventional one, and the degree of freedom in design is improved. .
Further, since this inspection apparatus can determine the incident angle or the incident position that maximizes the contrast of the sample image, aberrations and beam intensity unevenness due to the Wien filter can be minimized, and the S / N can be improved. Can be planned.
[0041]
Thus, in the inspection apparatus to which the present invention is applied, aberrations and unevenness of beam intensity can be suppressed as much as possible, and an optimal sample image can be acquired. Therefore, it is possible to provide an inspection apparatus with high inspection reliability.
[Brief description of the drawings]
FIG. 1 is a principle block diagram of the present invention.
FIG. 2 is a principle block diagram of the present invention.
FIG. 3 is an overall configuration diagram of the present embodiment.
FIG. 4 is a diagram (1) showing a trajectory of a primary beam.
FIG. 5 is a diagram showing a trajectory of a secondary beam.
FIG. 6 is a flowchart illustrating an operation related to acquisition of an optimal image.
FIG. 7 is a diagram illustrating a Wien filter.
FIG. 8 is a diagram (2) showing a trajectory of a primary beam.
FIG. 9 is a diagram illustrating an inspection apparatus using a Wien filter.

Claims (1)

Irradiation means for irradiating the sample surface with an electron beam having a spread in the cross-sectional direction;
Electron detection means for detecting at least one of secondary electrons, reflected electrons or backscattered electrons generated from the sample as a secondary beam by irradiation of the electron beam;
A secondary beam that is arranged between the irradiation means and the electron detection means and the sample, bends the electron beam irradiated from the irradiation means and irradiates the sample surface vertically, and is generated from the sample. A Wien filter leading to the electronic detection means,
A deflecting unit that is disposed between the irradiation unit and the Wien filter, and changes a beam incident angle or a beam incident position when the electron beam is incident on the Wien filter ;
Contrast detection means for generating image information of the sample from the secondary beam detected by the electron detection means and obtaining contrast of the image information,
The said deflection | deviation means maximizes the contrast calculated | required by the said contrast detection means by changing the said beam incident angle or beam incident position .

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