JP5809935B2 - Scanning transmission electron microscope and sample observation method - Google Patents

Description

  The present invention relates to a scanning transmission electron microscope and a sample observation method, and more particularly to a scanning transmission electron microscope capable of observing a secondary electron atom resolution image of a thick film sample and a sample observation method thereof.

  Electron microscopes are used in a wide range of fields such as physics, chemistry, medicine, biology, and engineering because they can obtain images with higher resolution than optical microscopes. Scanning electron microscopes, transmission electron microscopes, and scanning transmission electron microscopes are known as electron microscopes, but transmission electron microscopes and scanning transmission electron microscopes that can obtain images using transmission electrons that have passed through the sample are more suitable for the sample. It is known that the resolution is higher than that of a scanning electron microscope that can obtain an image using secondary electrons generated from the electron beam. Conventionally, transmission electron microscopes and scanning transmission electron microscopes have been used for image observation at the atomic level.

  On the other hand, it is possible to mount an aberration corrector on the electron microscope in order to obtain an electron microscope image with higher resolution. In an electron microscope using an aberration corrector, it has been reported that the resolution is 0.6 nm at an acceleration voltage of 5 kV ([Non-Patent Document 1]). Further, there is a technique ([Patent Document 1]) that observes the internal structure of a sample three-dimensionally with a scanning transmission electron microscope based on SEM, which corresponds to the resolution of a transmission electron microscope and is easy to use.

JP 2005-32732 A

Kazumori, H., Honda, K., Matsuya, M., Date, M. & Nielsen, C. Field emission SEM with a spherical chromatic aberration corrector. Microsc. Microanal. 10, 1370-1371 (2004) Zhu, Y., Inada, H., Nakamura, K., & Wall, J., Nature Mater. 8, 808-812 (2009)

  Conventionally, transmission electron microscopes and scanning transmission electron microscopes have been used for image observation at the atomic level. However, in the case of image observation, preparation takes time and time from the viewpoint of beam adjustment and sample preparation.

  For example, when adjusting the crystal orientation, TEM and STEM confirm the crystal orientation with an electron beam diffraction image using transmitted electrons, so prepare a thin film sample to ensure a sufficient amount of transmitted electron signals. Had to do. In a thick film sample, the amount of transmitted electrons is reduced, making it difficult to observe an electron beam diffraction image. The sample actually observed had to be a thin film.

  As a method for producing a thin film sample, a focused ion beam (FIB) processing method is used. Some FIB processing devices can be processed while observing the processing position by mounting an SEM column on the FIB device, but the image resolution has not reached the atomic level, and processing accuracy, throughput, Furthermore, the processing area has become a problem. Observation of TEM or STEM at the atomic level requires that the thickness of the sample thin film be 20 nm or less, and it is difficult to produce an observation sample with high accuracy.

  On the other hand, in 2009, it was reported that atoms can be observed with a SEM image of a 200 kV scanning transmission electron microscope equipped with an aberration corrector ([Non-patent Document 2]). Thereby, even if it was a thick film sample, possibility that atomic resolution could be observed in the SEM image of a scanning electron microscope or a scanning transmission electron microscope increased. However, in the subsequent experiments by the present inventors, when a crystalline sample is targeted, crystal information of the sample can also be obtained, and the image quality changes depending on the crystal orientation as in TEM and STEM. It has also been found that adjustment is necessary.

  An object of the present invention is to provide an electron microscope apparatus, a sample preparation method, and an observation method for performing SEM observation of a thick film sample using an electron microscope with an atomic level image resolution and high magnification accuracy. .

  The configuration of the present invention that achieves the above object is as follows.

  In a sample observation method of irradiating a focused electron beam to an observation region having a crystal structure of a thick film sample and forming a sample image using secondary electrons generated from the thick film sample, the vicinity of the observation region In addition, an amorphous region thin film and a crystal region thin film are formed, and the electron beam aberration is adjusted based on a Ronchigram obtained by irradiating the amorphous region with the electron beam, and an electron beam diffraction image of the crystal region is obtained. And adjusting the irradiation direction of the electron beam to the sample, irradiating the adjusted electron beam to the observation region of the thick film sample, and the atoms in the observation region of the thick film sample based on the emitted secondary electrons. A sample observation method characterized by forming a resolution image.

  According to the present invention, even a thick film sample can be observed and measured with a magnification accuracy equivalent to that of a TEM. In addition, sample preparation is simplified, and an observation area with a large area can be obtained in a short time, leading to an improvement in throughput. Furthermore, for a crystalline sample, atomic resolution SEM observation can be performed from any crystal orientation, and analysis can be performed using the crystal structure information.

The schematic block diagram of an electron microscope. The schematic block diagram of the electron microscope carrying EBSD. The figure which shows the procedure of the Example at the time of setting a semiconductor device as a sample. An example of sample processing. An example of beam adjustment using Ronchigram. An example of sample inclination adjustment. The conceptual diagram of the sample inclination adjustment using EBSD. An example of magnification calibration using an SEM image. An example of high resolution SEM observation and length measurement.

  Hereinafter, the electron microscope apparatus, sample preparation and observation method of the present invention will be described in detail with reference to the accompanying drawings. 1 to 9 are diagrams illustrating an embodiment of the present invention.

  FIG. 1 is a conceptual diagram of a scanning transmission electron microscope used in the present invention. The scanning transmission electron microscope 1 includes an electron gun 2, a converging lens 3, a spherical aberration corrector 4, an objective lens 5, a projection lens 6, and a scanning electrode 7. The sample is inserted into the objective lens 5. A secondary electron / backscattered electron detector 9 is incorporated between the scanning electrode and the objective lens. Further, an annular dark field electron detector 11 and a bright field scan are provided below the projection lens 6 so that the detector input / output control unit 10 can enter and exit the optical axis of the electron beam for dark field scanning transmission microscope image observation. A bright field electron detector 12 for observing a transmission microscope image is arranged. A CCD camera 13 for observing a Ronchigram or an electron beam diffraction image is disposed below the transmission electron detector.

  Further, the crystal orientation of the sample 8 may be confirmed by incorporating an EBSD detector 14 between the objective lens 5 and the sample 8. In FIG. 2, the conceptual diagram of the electron microscope 1 carrying the EBSD detector 14 is shown. In addition, to confirm the crystal orientation of the sample, a reflection high energy electron diffraction (RHEED) method may be used.

  The sample 8 is mounted on a sample holder 15 and a sample stage 16 having an inclination mechanism. By using the sample holder 15 in common with the FIB apparatus, the throughput is improved and the sample holder 15 can be observed from multiple directions such as the surface and cross section of the sample. The movement of the sample is performed by the connected sample control unit 17. The sample controller 17 is connected to the CPU processing device 18.

  Each signal is connected to the image display unit 19 via the CPU processing device 18. The image display unit 19 can simultaneously display images from the respective detectors in separate windows. The electron beam 20 emitted from the electron gun 2 is converged by the converging lens 3, irradiated to the sample 8 through the spherical aberration corrector 4, and scanned on the sample surface by the scanning electrode 7.

  When the sample 8 is irradiated with the electron beam 20, secondary electrons and backscattered electrons are emitted from the sample 8. This is detected by the secondary electron / backscattered electron detector 9, and the surface structure of the sample is observed using an image (SEM image) based on the detection signal. Further, electrons scattered back by the sample are detected by the dark field electron detector 11, and the internal structure and composition information of the sample 8 are observed using an image (dark field STEM image) based on the detection signal. Then, the electrons transmitted through the sample 8 are detected by the bright field electron detector 12, and the internal structure and composition information observation of the sample 8 are observed using an image (bright field STEM image) based on the detection signal from the bright field electron detector 12. I do.

  FIG. 3 is a diagram showing an example of the procedure of the sample preparation and observation method of the present invention when a semiconductor device is used as a sample. The procedure will be described below with reference to FIG.

  (A) Using a dicer process, FIB microsampling method, etc., an attention location is extracted from a silicon wafer. (B) Next, a thin film is prepared by FIB so that an electron diffraction pattern can be observed in the amorphous region and the Si substrate region in the vicinity of the observation region. (C) Next, the cross section of the observation area is processed by FIB. (D) Thereafter, beam adjustment using an amorphous region and sample orientation adjustment using an electron beam diffraction image are performed with an electron microscope. (E) Imaging in the observation area. (F) The magnification of the electron microscope is calibrated using the crystal information of the Si substrate of the captured SEM image, and the length is measured. (G) Perform additional machining with FIB as necessary.

  FIG. 4 shows an example of the processed sample. The dicer processed sample is processed in the vicinity of the observation region 22 (thick film region 25) by FIB, and an amorphous region (Ronchigram observation region 23) is used for beam adjustment of an electron microscope, and Si single unit is used for sample orientation adjustment. The adjustment region 21 having a thin film region (electron beam diffraction image observation region 24) that can observe an electron beam diffraction image in the crystal region is produced. FIG. 4 shows a scanning ion microscope (SIM) image of the prepared sample 8 and a partially enlarged dark field STEM image. Here, as the amorphous region, in addition to the region existing in the sample 8, a damage layer caused by FIB processing, an ion beam assisted deposition film, a redeposition region generated during FIB processing, or the like may be used.

  In order to observe a clear Ronchigram, it is desirable that the amorphous region (Ronchigram observation region 23) has a thickness of about 100 nm or less. Since the Ronchigram uses transmission electrons, the image quality deteriorates when the sample becomes thick and cannot be observed with a bulk sample. Further, the Ronchigram can be observed even in the thinned crystal region (electron diffraction image observation region 24), but since contrast is generated by crystal structure information, skill is required for beam adjustment. Since the electron beam diffraction image observation region also uses transmission electrons, the thickness is sufficient to detect an electron beam diffraction image, and the thickness and area do not bend so as not to deviate from the bulk region. It is desirable.

  Next, the cross section of the target area is processed. At this time, by using a FIB and SEM compound machine, the processing can be performed while observing the FIB processing cross section by SEM, so that the end point can be detected with high positional accuracy. Further, since it is not necessary to produce a thin film in the target region, the processing volume is small and the processing time can be shortened. Furthermore, since it is not necessary to consider the deflection of the sample which becomes a problem in thinning, a wide field of view can be processed at a time, and the throughput from processing to observation is improved. For finishing the cross section, it is desirable to reduce the damage layer by using FIB or the like having a low acceleration voltage.

(Electron beam adjustment)
The sample moved to the electron microscope is subjected to beam adjustment using an amorphous region and sample orientation adjustment using a region such as a Si single crystal substrate.

  An example of beam adjustment using a Ronchigram is shown in FIG. Ronchigram is used for beam adjustment, residual aberration is calculated from its shape, correction up to third-order aberration is performed, and an optical system such as an aberration corrector and an objective lens is adjusted so that the flat area is maximized. In addition to the Ronchigram, manual adjustment can be easily performed while observing a secondary electron image, a dark-field electron image, and a bright-field electron image in the case of a primary aberration.

  An example of sample tilt adjustment is shown in FIG. The tilt adjustment of the sample is performed by adjusting the sample tilt so that the spots of the electron beam diffraction image are symmetric. By making such adjustments, clear crystal structure information can be obtained from the Si single crystal substrate region during electron microscope observation, and the interface of the sample stack structure can be aligned with the optical axis of the electron beam. .

FIG. 7 shows a conceptual diagram of crystal orientation adjustment using the EBSD detector 14. Normally, when using the E BSD detector 14, the measurement performed by tilting the 60 to 70 ° about the sample from the incident direction of the electron beam 20. Therefore, in a single crystal sample, the orientation may be confirmed on a plane different from the plane actually observed, and the tilt angle may be obtained from the obtained result. If EBSD is used, it can be applied not only to semiconductor devices but also to confirmation and adjustment of the crystal orientation of a single crystal material.

(About observation and length measurement)
After that, the image is moved to the observation area, the magnification is set, and the focus is finely adjusted to take an image. At this time, if an autofocus function is used, variation by a measurer can be reduced. Since the area that can be observed at a time becomes narrower as the magnification becomes higher, a function for displaying a plurality of images as a connected picture is provided. In addition, the position information of the sample stage is also stored, and the stage is automatically moved to the area stored when re-observing.

Acquired images, performs magnification Calibration using crystal structure information obtained from such single-crystal Si region of the substrate. An example is shown in FIG. Extracting a known surface distance sights FFT of SEM images perform calibration of the SEM image.

  An example of high-resolution SEM observation of a semiconductor sample is shown in FIG. This is an example of observation and length measurement around the gate. By using the sample preparation and observation method of the present invention, it is possible to perform SEM observation and length measurement in the sub-nanometer order, which has been difficult until now.

  TEM and STEM can only observe the thinned region of the sample, but if the method of the present invention is used, atomic level SEM observation can be performed in any crystal orientation and region regardless of the thickness of the sample. This enables measurement with high magnification accuracy equivalent to that of TEM or STEM. In addition, for an unknown crystalline sample, structural analysis can be performed using crystal information obtained from an SEM image.

DESCRIPTION OF SYMBOLS 1 Scanning transmission electron microscope 2 Electron gun 3 Convergent lens 4 Spherical aberration corrector 5 Objective lens 6 Projection lens 7 Scanning electrode 8 Sample 9 Secondary electron / reflected electron detector 10 Detector input / output control unit 11 Dark field electron detector 12 Bright Field electron detector 13 CCD camera 14 EBSD detector 15 Sample holder 16 Sample stage 17 Sample control unit 18 CPU processing unit 19 Image display unit 20 Electron beam 21 Adjustment region 22 Observation region 23 Ronchigram observation region 24 Electron diffraction image observation region 25 Thick film region 26 Observation surface 27 Sample surface for crystal orientation confirmation

Claims (7)

In a sample observation method of irradiating a focused electron beam to an observation region having a crystal structure of a thick film sample, and forming and observing a sample image using secondary electrons generated from the thick film sample,
Forming a thin film of an amorphous region and a thin film of a crystal region in the vicinity of the observation region;
Adjust the aberration of the electron beam based on the Ronchigram acquired by irradiating the electron beam to the amorphous region, adjust the irradiation direction of the electron beam to the sample based on the electron beam diffraction image of the crystal region,
Sample observation method characterized by the adjusted electron beam irradiated to the observation region of the thick film sample, to form an atomic resolution image of the observation area of the thick film sample based on emitted secondary electrons. According to claim 1, sample observation method characterized by the magnification of the sample image to calibration using a crystal information of the sample image produced on the basis of a signal of the secondary electrons. 3. The sample observation method according to claim 1, wherein an optical system is adjusted so that a flat area of the Ronchigram is maximized. The sample observation method according to claim 1, wherein the sample is tilted so that the spots of the electron beam diffraction image are symmetric.   The sample observation method according to claim 1, wherein the aberration of the electron beam is adjusted using a Ronchigram acquired by a camera.   6. The sample observation method according to claim 1, wherein an irradiation direction of the electron beam on the sample is adjusted using an electron diffraction image acquired by a camera.   6. The sample observation method according to claim 1, wherein a crystal orientation of the thick film sample is confirmed using an EBSD detector.