JP2003331769A - Particle beam inspection device, inspection method and particle beam application device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-throughput particle beam inspection device. <P>SOLUTION: A characteristic frequency in correspondence with focal deviation volume is found from Fourier spectrum of a sample image by a focal deviation computing unit 60 in a particle beam length measurement device, an out-of-focus beam distribution function in accordance with the characteristic frequency is generated by a beam distribution generator 61, a component of the out-of- focus beam distribution function is removed from the sample image recorded by a one-dimensional image memory 71 by a deconvolution computing unit 51, and a size measurement of the sample image obtained is made by a length measurement computing unit 72. With this, a focusing time by particle beam scanning can be saved, which has an effect of saving an inspection time for sizes or appearance defects of semiconductor elements or the like. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a particle beam scanning microscope, and more particularly to a particle beam inspection method suitable for observing or inspecting small dimensions and external shapes of semiconductor devices and the like. Related to the device. 2. Description of the Related Art As an electron beam length measuring device for scanning a semiconductor sample with an electron beam, which is a particle beam beam, to obtain a sample image by secondary electrons and inspecting a dimension of a characteristic pattern in the image, JP-A-60-161514 is known. Japanese Patent Application Laid-Open No. 5-258703 is known as an electron beam appearance inspection apparatus for inspecting an abnormality of a sample by comparing an image of a scanning electron microscope with a reference pattern. These methods have higher image resolution than inspection methods using light, and are very effective for inspection of today's submicron semiconductor patterns. In the above-described conventional apparatus, before acquiring a sample image with an electron beam, signals of at least three different sample images obtained by scanning the sample with the electron beam while the focus of the electron lens system is shifted are obtained. The optimal focusing of the electron lens system is performed. In order to shorten this focusing time, Japanese Patent Laid-Open No. 8-27
In 3575, there is shown a means for detecting a sample height by a height sensor (Z sensor) using light and adjusting the electron lens system based on a table created in advance to adjust the height change to focus. ing. However, electron beam scanning is required because the surface of the sample measured by the Z sensor does not completely match the outermost surface of the sample, and the reproducibility of focus adjustment is degraded due to the magnetism hysteresis of the electron lens system. Omitting the focusing deteriorated the resolution of the sample image. [0003] On the other hand, a method of mathematically converting (integrating conversion) a sample image using a beam distribution measured in advance to increase the resolution of the sample image is disclosed in Japanese Patent Laid-Open No. 2-18 / 1990.
1639. However, this method does not consider that the beam distribution changes due to defocus at each location where a sample image is taken. [0004] In the inspection apparatus as described above, the inspection amount is increased by the improvement in the resolution, so that it is necessary to improve the throughput. However, in the above-described related art, it was not considered that the time of focusing accompanied by electron beam scanning significantly reduced the inspection throughput. Further, it has not been considered that a dimensional change of the sample occurs due to the electron beam irradiation. It is an object of the present invention to provide a high-resolution, high-throughput particle beam inspection apparatus. SUMMARY OF THE INVENTION The object of the present invention is to provide a particle beam inspection apparatus having a means for obtaining a sample image by scanning a particle beam on a sample, and a sample inspection means for arithmetically processing the sample image. An apparatus for detecting a defocus amount from the sample image; a means for generating a defocus beam distribution function corresponding to the spread of the particle beam from the defocus amount; Means for generating a separated image from which the component of the defocused beam distribution function has been removed or reduced, and the sample inspection means performs arithmetic processing on the separated image to solve the problem. In the particle beam inspection apparatus, the means for detecting the amount of defocus can surely solve the problem by detecting any spatial frequency in the Fourier spectrum of the sample image as the amount of defocus. Is done. Further, in the above-mentioned particle beam inspection apparatus, means for storing the sample image, the defocused beam distribution function and the separated image in association with each other, and a set of the sample image, the defocused beam distribution function and the separated image The problem is surely solved by providing the means for simultaneously displaying. In the inspection method using the above-described particle beam inspection apparatus, the sample is moved to an inspection position of the sample after the astigmatism adjustment of the particle beam is completed, and the particle beam is moved for each inspection position of the sample. The problem is solved by scanning the beam to acquire two or less different sample images and inspecting the sample based on the sample images. A means for forming a first image obtained by scanning the sample with the particle beam; a means for obtaining a defocus amount based on the first image; a means for obtaining a defocus amount based on the first image; The object is solved by means for forming a second image by using the image of (1) and a particle beam inspection apparatus for inspecting a sample based on the second image. First, the operation of the above-mentioned solving means will be described. Sample image i in a scanning electron microscope
Is the secondary particle emission rate distribution on the sample surface as S, and the particle beam intensity distribution at the optimal focus as b, and S as
It is represented by the convolution of b. ## EQU1 ## Here, (x, y) is a two-dimensional coordinate. The convolution operation is defined by the following integral transformation. ## EQU2 ## Here, assuming that the particle beam intensity distribution without focusing is B, the sample image I obtained is
It is expressed by the convolution of S and B as in the following equation. [Equation 3] Here, it is assumed that the particle beam B is obtained by blurring the particle beam b by a certain amount. This blur (spread)
Let D be the defocused beam distribution function that represents B. Then, B is expressed by the convolution of b and D as in the following equation. [Equation 4] Here, since the convolution operation has a binding law, the following equation is obtained from Equations 1, 4, and 4. (Equation 5) From Equation 5, it can be seen that if the out-of-focus beam distribution function D is known, the inverse convolution operation, that is, the sample image i at the optimum focus from which the blur has been removed by the deconvolution operation can be obtained. In addition, it can be seen from Expression 3 that an image with higher resolution can be obtained by using an amount close to B instead of D. Here, if a table of the relationship between the frequency spectrum shapes of various defocused sample images and the defocused beam distribution function D is obtained in advance, a defocused beam distribution function can be generated for any sample image. Further, the out-of-focus beam distribution function D itself acquires, for example, images of various out-of-focus states with respect to the reference sample in advance, and compares these with the image at the optimum focus (deconvolution operation is performed using Equation 5). Can be determined by: Therefore, according to the present invention, defocusing of the sample image of the particle beam scanning microscope can be removed by the introduced means, so that the focusing time by the particle beam scanning can be increased and the inspection time can be reduced. Note that the time of the above-described arithmetic processing can be reduced to about the sample image acquisition time by a parallel computer. Hereinafter, embodiments of the present invention will be described in detail. Embodiment 1 FIG. 1 is a configuration diagram of an electron beam length measuring apparatus according to a first embodiment of the present invention. The apparatus main body 1 mainly includes an FE electron source 2, a variable limiting aperture 4, electron lenses 5, 5 ', a deflector 7, a sample stage 9 for holding a sample 6 movably, and a secondary particle detector 8. You. Further, the apparatus body 1 has a laser 10, a lens system 11, 11 ', and a laser position detector 13 for detecting the height of the sample 6.
Is provided. Here, the vacuum container storing these is omitted. The secondary particle detector 8 includes a fluorescent plate and a photomultiplier, and detects secondary electrons. The apparatus main body 1 is connected to a control circuit 30 via a deflection control system 20 and a secondary particle signal control system 21. Here, the electron source 2, the variable aperture 4, the electron lens 5, and the sample stage 7
And the like are omitted. Focusing lens controller 14
Is connected to the laser position detector 13 and the electronic lens 5 ′. The control circuit 30 includes a display 31, a sample image memory 3
2, and a measurement position detector 70, a one-dimensional image memory 71, and a length measurement calculator 72, which constitute an inspection unit, are connected. Next, the operation of this device will be described. Electron source 2
The electron beam 3 emitted from the electron beam 3 is controlled by a variable limiting aperture 4 to control the beam amount, and is focused by electron lenses 5 and 5 ′.
Irradiate on top. The electron beam 3 'is decelerated by the electron lens 5', and the acceleration voltage of the electron beam 3 'on the sample is 800V. The beam diameter of the electron beam 3 'is 5 nm and the current is 10 nA
It is. When the sample 6 is irradiated with the electron beam 3 ′, secondary electrons and reflected electrons corresponding to the material and shape of the sample are generated from the irradiated portion. The secondary particle detector 8 detects secondary electrons and converts them into electric signals (secondary electron signals). The deflector 7 scans the sample 6 by deflecting the electron beam 3 ′ by a scan deflection signal generated by a scan circuit in the deflection control system 20. The control circuit 30 controls the secondary particle detection system 21 in conjunction with the scanning deflection signal.
The secondary electron signal stored in the buffer memory in the memory is stored in the sample image memory 32 and sent to the display 31 to be displayed as an SEM image (scanning electron microscope image) of the sample in which the secondary electron signal is used as a luminance signal. . The control circuit 30
Sends the sample image to an inspection unit and selectively measures dimensions at a preset location. In this inspection section, a measurement position detector 70 having a pattern of a measurement place beforehand examines the sample image stored in the sample image memory 32, instructs the control circuit 30 on a place required for measurement, and re-enlarges the sample image. Then, a one-dimensional image required for measurement is generated from the acquired one, and stored in the one-dimensional image memory 71. The length measuring unit 72 extracts a characteristic position from the one-dimensional image stored in the one-dimensional image memory 71 and calculates a dimension. The control circuit 30 stores these calculation results in a storage device (not shown), and displays the results as necessary.
Display at 31. A change in the height of the sample 6 is detected from a change in the position at which the reflected light 12 ′ of the emitted light 12 from the laser 10 enters the laser position detector 13. Focusing lens controller
14 is an electronic lens based on a signal from the laser position detector 13
The intensity of 5 ′ is adjusted so that the electron beam 3 ′ is always focused on the sample 6. However, in this method, the sample surface sensed by light and the electron beam sense do not completely coincide with each other, and thus include an inconsistent error. Therefore, usually, several sample images with different intensities of the electron lens 5 ′ are obtained by scanning with an electron beam, and these are compared to determine the optimum intensity of the electron lens 5 ′. The features of this embodiment are (1) detecting the amount of defocus from the sample image, (2) generating a defocus beam distribution function corresponding to the defocus, and (3) detecting the defocus amount from the sample image. This is to provide a means for removing the influence of the defocused beam distribution function. Specifically, as shown in FIG. 1, a feature is that an image separation unit is connected to the control circuit 30. The defocus calculator 60 retrieves the sample image obtained by the electron beam 3 ′ from the sample image memory 32, detects the defocus amount from the shape of the Fourier spectrum of this image, and obtains a beam distribution generator.
Send to 61. The beam distribution generator 61 generates a defocus beam distribution function corresponding to the defocus amount by interpolating data of a table that holds the defocus beam function in advance, and stores the generated data in the beam distribution memory 50. The deconvolution calculator 51 generates a one-dimensional image of the sample at the optimum focus by performing a deconvolution operation for removing components of the defocused beam distribution function from the one-dimensional image, and stores the generated one-dimensional image in the separated image memory 52. . Conventionally, a one-dimensional image cut out from a sample image stored in a sample image memory 32 in an inspection unit was directly used to measure a minute dimension on the sample. However, in the present embodiment, a minute dimension is measured using the one-dimensional image stored in the separated image memory 52 after removing the influence of defocus. This eliminates the need to scan the sample with an extra electron beam to find the optimum focus. FIG. 2 shows a display screen on which the flow of the series of processes can be understood. First, a defocused beam distribution function 150 is generated from the sample image 132 that is slightly out of focus. Next, from the one-dimensional image 170 cut out from the sample image 132, the component of the defocus beam distribution function 150 is removed by a deconvolution operation to generate a separated image 152. Finally, the minute size of the sample (here, the diameter of the hole) is measured using the characteristic position of the separated image 152. Figure
The data shown in 2 can be collectively stored in a storage device (not shown) for user confirmation. FIG. 10 shows the flow of signals in these series of processes. Here, in order to reduce the calculation time, the deconvolution operation was performed on the one-dimensional image extracted from the sample image. Although the calculation time increases, a more accurate process is performed by a method as shown in FIG. That is, a deconvolution operation is performed on the sample image, which is a two-dimensional image, and a measurement position is extracted therefrom to create a one-dimensional image and measure the length. FIG. 3 shows a flow of a series of measurements.
What is characteristic here is that the focus adjustment and the astigmatism adjustment of the electron beam are first performed using the reference sample set on the sample stage. Conversely, it is characterized in that dimension measurement cannot be performed unless this adjustment is performed. This is a countermeasure against a situation where the out-of-focus beam distribution function is not normally generated unless the astigmatism adjustment is performed, and the accuracy of the separated image generated by the deconvolution operation is reduced. In the electron beam length measuring apparatus according to the present embodiment, the program is programmed so that, when the above adjustment is performed in the normal automatic measurement and the operation fails, the operation is abnormally terminated and displayed to the user. When the above adjustment is performed normally, the sample is moved so that an arbitrary position on the sample registered in advance is placed immediately below the electron beam. Simultaneously with this movement, the height of the sample is detected by the Z sensor to adjust the focus of the electron lens. Next, a sample image is obtained by scanning the sample with an electron beam, and after storing the sample image, a one-dimensional image necessary for dimension measurement is cut out and stored. Here, in the stage of acquiring the sample image, it is often the case that the enlarged sample image is acquired again in order to adjust to the required magnification. Therefore, the number of sample images acquired by electron beam scanning in the procedure up to the following dimension measurement is 1
One or two. Next, a separated image is generated from the stored sample image and one-dimensional image, and dimension measurement is performed on the separated image. When these series of operations are completed, the operation moves to the next sample position and the operations are repeated. In FIG. 3, the acquisition of the sample image and the measurement of the dimensions are performed all at once, but these can be performed separately and collectively. Conventionally, it took 10 seconds to measure the length of one sample. The breakdown is 2 seconds for moving the sample and focusing with the Z sensor, 5 seconds for focusing by beam scanning, 2 seconds for acquiring the sample image and detecting the feature position, and 1 second for generating and inspecting the one-dimensional image. . In the present embodiment, the time required for generating the separated image is 1 second instead of the need for focusing by beam scanning, so that the measurement is completed in 6 seconds in total. The generation time of the separated image may be further reduced by parallelization of the calculation. FIG. 4 shows a flow of a portion for generating a separated image in FIG.
Shown in Here, detecting the amount of defocus from the sample image, that is, the contents of the defocus calculator 60 in FIG. 1 will be described in more detail. First, all the sample images called out from the sample image memory are integrated in a one-dimensional direction. Next, this signal is Fourier-transformed. However, the intensity is normalized by the total signal amount. The Fourier-transformed image (ie, Fourier spectrum) is as shown in FIG. Here, an appropriate intensity close to the noise level is set. That is line 1 in FIG. Further, a straight line asymptotic to the changing part of the Fourier spectrum, such as line 2 in FIG. 5, is obtained. Eventually lines 1 and 2
The spatial frequency F0 obtained from the intersection is detected as the defocus amount. Here, the defocus calculation is performed on the sample image, but may be performed on a one-dimensional image. In this case, the SN (signal-to-noise ratio) of the signal is slightly deteriorated, so that the calculation accuracy is reduced but the calculation time can be shortened. FIG. 6 shows a sample image and its Fourier spectrum when various defocuses are artificially set for the reference sample. Here, the sample images are already one-dimensionally integrated and the intensity is normalized. This defocus amount is a level that cannot be corrected by the Z sensor, and is small. Since the spatial frequency F0 changes according to the defocus, it can be understood that this can be read as the defocus amount. It can also be seen that a defocused beam distribution function can be obtained by performing a deconvolution operation on the sample image at the optimum focus from the sample image at an arbitrary focus, and the relationship with the spatial frequency F0 can be created as a table. (In order to obtain the out-of-focus beam distribution function as accurately as possible, it is necessary to increase the SN of the sample image and increase the dynamic range.) The beam distribution generator 61 in FIG.
Holds this table, and generates a defocused beam distribution function by interpolating the data in the table for an arbitrary spatial frequency. By the way, the height of the line 1 and the inclination of the line 2 in FIG. 6 differ greatly depending on the model of the electron beam measuring device, and slightly differ depending on the device.
For this reason, in the electron beam length measuring apparatus of the present embodiment, the data held inside the defocus calculator 60 and the beam distribution generator 61 in FIG. 1 are divided into a common standard part and a correction part, and the correction part Data is re-entered for each model. In order to perform such a calibration work, a reference sample is provided on the sample stage, and a program for automatically performing the calibration work using the reference sample is mounted on the control circuit 30. Here, the generation of the out-of-focus beam distribution is performed based on the table, but it is also possible to generate the distribution using an appropriate approximation function. In that case, the calculation time can be reduced. Here, the contents of the deconvolution operation performed by the deconvolution operation unit 51 in FIG. 1 will be described in more detail. This embodiment is characterized in that a method using a convolution operation and an iterative comparison is used instead of a simple deconvolution operation.
This method was adopted because it is resistant to noise. FIG. 7 shows the flow of this processing. First, (1) assuming an appropriate separated image (first separated image) at the initial stage, (2) obtaining a result of calculating a convolution of the first separated image and the defocus beam distribution function,
This result is compared with the actual sample image to obtain these difference images, and (3) the first separated image is corrected so that the intensity of the difference image is reduced, and the process returns to (2). Repeat. This operation converges after several tens of iterations, and the operation time is about 10 times using a dedicated circuit that parallels the product-sum operation.
0 ms. As described above, according to the present embodiment, since focusing by electron beam scanning is broken, the time required for dimension measurement can be shortened, and the sample, that is, the semiconductor element, etc., can be reduced in damage due to electron beam irradiation. Embodiment 2 FIG. 8 is a configuration diagram of an electron beam visual inspection apparatus according to a second embodiment of the present invention. The entire configuration is almost the same as the electron beam length measuring apparatus shown in FIG. 1, and the internal configuration of the inspection unit connected to the control circuit 30 and the configuration of the program incorporated in the control circuit 30 are different. Although the details of the design of the electron optical system and the secondary particle detector are different, the difference is not important here, and the description is omitted. Beam diameter of electron beam 3 'is 100nm
And the current is 100 nA. The inspection unit includes a reference image memory 40,
An image difference calculator 41 and a difference image memory 42 are provided. The control circuit 30 reads the sample image obtained by scanning the sample 6 with the electron beam 3 'from the sample image memory 32, sends the sample image to the inspection unit, and detects the sample abnormality such as a foreign substance by comparing with the reference image. . In this inspection unit, the sample image stored in the sample image memory 32 and the reference image previously stored in the reference image memory 40 are compared by the image difference calculator 41, and the result is stored in the difference image memory 42, If the difference image satisfies an arbitrary criterion, it is stored in a storage device (not shown) as abnormal and displayed on the display 31. The features of this embodiment are as follows: (1) detecting the amount of defocus from the sample image; (2) generating a defocus beam distribution function corresponding to the defocus; This is to provide a means for removing the influence of the defocused beam distribution function. Specifically, as shown in FIG. 8, an image separating unit is connected to the control circuit 30. The defocus calculator 60 retrieves the sample image from the sample image memory 32, detects the defocus amount from the shape of the Fourier spectrum of the sample image, and sends it to the beam distribution generator 61. The beam distribution generator 61 generates a defocus beam distribution function corresponding to the defocus amount by interpolating data of a table that holds the defocus beam function in advance, and stores the generated data in the beam distribution memory 50. Deconvolution operator
The 51 generates a sample image at the optimum focus by performing a deconvolution operation for removing the component of the defocused beam distribution function from the sample image, and stores this in the separation image memory 52. The appearance inspection of the sample is performed by comparing the sample image stored in the separated image memory 52 with the reference image stored in the reference image memory 40 by removing the influence of defocus. As a result, the inspection can be performed using the sample image of the optimal focus without scanning the electron beam on the sample. FIG. 9 shows a flow of a series of measurements. As in the first embodiment, the appearance inspection cannot be performed unless the focus adjustment and the astigmatism adjustment of the electron beam are first performed. When the above adjustment is performed normally, the sample is moved so that an arbitrary position on the sample registered in advance is placed immediately below the electron beam.
Simultaneously with this movement, the height of the sample is detected by the Z sensor to adjust the focus of the electron lens. Next, the sample is scanned with an electron beam to obtain a sample image, which is stored. Next, a separated image is generated from the stored sample image, and a visual inspection of the sample is performed on the separated image to detect foreign matter on the sample. When these series of operations are completed, the operation moves to the next sample position and the operations are repeated. In FIG. 9, the acquisition of the sample image and the appearance inspection are performed all at once, but these can be performed separately and collectively. Conventionally, it took 100 ms to inspect the appearance of one place of the sample. This time is almost equal to the sample image acquisition time because the sample image is acquired while moving the stage. The resolution of the sample image at the optimum focus obtained by actually scanning the electron beam many times is 100 nm, but the resolution of the sample image when focusing only with the Z sensor is 200 nm on average.
It is. In the present embodiment, the sample image at the effective optimum focus is obtained, and the external appearance is inspected, so that the inspection resolution is doubled. The time required for the operation in the image separation unit shown in FIG. 9 is 100 ms using a dedicated circuit in which the product-sum operation is parallelized. However, in practice, this part is prepared by multiplexing only two channels. As a result, the effect on the actual inspection time has been completely eliminated. As described above, according to the present embodiment, since the appearance inspection can be performed effectively using the sample image at the optimum focus, there is an effect of improving the resolution of the inspection. According to the present invention, since the focusing time by the scanning of the particle beam can be shortened, there is an effect that the inspection time of the dimensions and the appearance abnormality of the semiconductor element and the like can be shortened. Also,
According to the present invention, a failure analysis of a semiconductor integrated circuit can be performed in a short time, so that the yield can be improved at an early stage.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an electron beam length measuring device according to the present invention. FIG. 2 is a diagram showing an example of a display screen. FIG. 3 is a diagram showing a flow for measuring dimensions. FIG. 4 is a diagram showing a flow of generating a separated image. FIG. 5 is a diagram showing an example of a Fourier spectrum of a sample image. FIG. 6 is a view showing an example of a sample image at various defocuses and its Fourier spectrum. FIG. 7 is a diagram showing a flow of a deconvolution operation. FIG. 8 is a block diagram showing an electron beam visual inspection device of the present invention. FIG. 9 is a diagram showing a flow of performing an appearance inspection. FIG. 10 is a view showing a signal flow in the electron beam length measuring device of the present invention. FIG. 11 is a diagram showing another method of a signal flow in the electron beam length measuring device of the present invention. [Description of Signs] 1 ... Electron beam device main body, 2 ... Electron source, 3, 3 '... Electron beam, 4 ... Aperture, 5, 5' ... Electronic lens, 6 ... Sample, 7 ... 2-stage electrostatic deflector, 8… Secondary electron detector, 9… Sample stage, 10…
Laser light source, 11, 11 '... laser light, 12, 12' ... lens system,
13: Laser position detector, 14: Focusing lens controller,
20: deflection control system, 21: secondary particle detection system, 30: control circuit,
31: display, 32: sample image memory, 40: reference image memory, 41: image difference calculator, 42: difference image memory, 50: beam distribution memory, 51: deconvolution calculator, 52 ...
Separated image memory, 60: defocus calculator, 61: beam distribution generator, 70: measurement position detector, 71: one-dimensional image memory, 72 ...
Length measurement calculator, 132: sample image, 140: reference image, 142: difference image, 150: beam distribution, 152: separated image, 170: one-dimensional image, 1
52… Isolated image.

Continuation of front page    (72) Inventor Tadashi Odaka             882 Ma, Hitachinaka City, Ibaraki Pref.             Hitachi, Ltd.Measurement Division F term (reference) 2F067 AA00 AA21 AA41 EE10 HH06                       JJ05 KK04 LL00 LL14 LL16                       LL17 RR35                 2G001 AA03 BA07 CA03 FA08 GA01                       GA06 KA03 LA11 MA05                 5C033 UU05 UU06 UU08

Claims (1)

Claims: 1. A particle beam inspection apparatus comprising: means for scanning a particle beam on a sample to obtain a sample image; and sample inspection means for arithmetically processing the sample image. Means for detecting the defocus amount from the sample image, means for generating a defocus beam distribution function corresponding to the spread of the particle beam from the defocus amount, and the defocus beam for the sample image or a part thereof. Means for generating a separated image from which components of the distribution function have been removed or reduced, and wherein the separated image is arithmetically processed by the sample inspection means.