JP5489488B2 - Ferroelectric sample observation method - Google Patents

Description

The present invention relates to a ferroelectric sample observation method for performing domain observation of the ferroelectric, and in particular a ferroelectric sample observation method using a reflection electron image of a scanning electron microscope. Hereinafter, a ferroelectric sample may be simply used as a sample.

There is a region in which directions are aligned polarization in size in the crystal particles of the ferroelectric materials are called domain. Conventionally, such a domain is observed using a polarizing microscope, a transmission electron microscope, a scanning electron microscope, a scanning probe microscope, or the like.

  Among these, the scanning electron microscope is attracting attention because it can cope with a wide range of magnifications, can be applied to various samples, and the pretreatment of the sample for observation is relatively easy. However, domains differ only in the direction of polarization, and there is almost no change in the surface state or composition. Therefore, domain observation cannot be performed by a normal observation method. Therefore, when observing with a scanning electron microscope, an electron back scatter diffraction (EBSD) method in which the crystal orientation is measured by irradiating an electron beam to a sample tilted about 70 degrees, It is necessary to use a method in which the observation surface is mirror-polished and then chemically etched, carbon is further deposited and observed with a secondary electron image, and the observation surface is mirror-polished and observed with a secondary electron image without vapor deposition There is.

“Basics and Applications of Polarization Inversion Devices”, Shintaro Miyazawa, Nao Kurimura, Optronics, 2005

  However, the EBSD method has a problem that the resolution is low and it cannot be observed depending on the size of the domain. Furthermore, depending on the observation magnification, only a very narrow region can be measured, and it takes a long time to perform observation over a wide region, which is not practical. Also, in the method of chemically etching the observation surface after mirror polishing, and further observing with a secondary electron image by vapor deposition of carbon, it is necessary to use chemicals such as acid, and it is necessary to search for etching conditions for each material In addition, the work requires skill and waste liquid treatment, and the sample pretreatment is complicated. The method of observing the observation surface with a secondary electron image without mirror polishing and without vapor deposition is a method of observing the insulating material without vapor deposition. There was a problem that it took.

  The present invention has been devised in view of the above-mentioned problems, and the purpose thereof is a sample that is highly versatile and capable of performing stable domain observation because it has a high degree of freedom in application sample and application magnification. It is to provide an observation method.

The sample observation method of the present invention is a ferroelectric sample observation method in which domain observation is performed with a reflected electron image obtained by irradiating an electron beam to an observation sample made of a ferroelectric material using a scanning electron microscope, A sample preparation step in which the sample is mirror-polished and a conductive deposition film is formed on the polished surface to produce the observation sample, and the observation sample is scanned with a scanning electron microscope and the acceleration voltage of the electron beam is 3 kV or more Observation is performed under conditions that enhance contrast as compared to contrast conditions that are set so that the luminance and distribution amount of the reflected electron scanning image are averagely distributed within a displayable luminance range. an observation step is performed following the observation step, the conditions of the contrast set to averagely distributed in the displayable intensity range the brightness and distribution of the reflected electron scanning image To integer, and has an image adjusting step.

  According to the sample observation method of the present invention, since chemical etching processing is not required, it is not necessary to search for etching conditions for each material, and pretreatment of the observation sample is facilitated. In addition, the waste liquid treatment such as acid generated when the chemical etching treatment is performed becomes unnecessary. Furthermore, since a vapor deposition film having conductivity is formed on the observation surface, charge-up does not occur even when irradiated with an electron beam, so that the observation sample can be observed stably. As described above, it is possible to provide a highly versatile sample observation method that facilitates pretreatment for performing stable observation.

  Moreover, since the acceleration voltage by the electron beam at the time of sample observation is set to 3 kV or more and 10 kV or less, reflected electrons having information on the domain of the outermost surface of the observation sample can be detected. And since these reflected electrons are measured under conditions that enhance the contrast, the domains that could not be observed in the reflected electron image because they are usually hidden behind noise and the difference in brightness cannot be recognized are scanned electrons. A sample observation method that can be observed with a microscope can be provided.

  As described above, according to the present invention, it is possible to provide a sample observation method using a scanning electron microscope and capable of domain observation with high versatility.

It is the reflected electron image which observed the observation sample which consists of ferroelectrics by the sample observation method of this invention.

Embodiment of the sample observation method of this invention is demonstrated for every process.
[Sample preparation process]
First, an observation sample is prepared. The sample is not particularly limited as long as it has a domain in the crystal. For example, a ferroelectric material such as lead zirconate titanate (PZT) or barium titanate can be used.

  Such a sample is mirror-polished to produce a polished surface. Specifically, the sample is polished using a polishing machine in the same manner as normal sample polishing for a scanning electron microscope. First, roughing is performed using a diamond polishing plate to flatten the sample surface. Thereafter, polishing is performed by gradually reducing the particle size of the combination of the polishing liquid and the polishing plate. A diamond suspension or the like can be used as the polishing liquid. Finally, as final polishing, an alumina suspension or a colloidal silica suspension is used as a polishing liquid, and a buff is used as a polishing plate to perform buffing and mirror finishing.

  Note that the method of holding the sample in the polishing machine when performing a series of polishing operations can be appropriately selected in consideration of the sample size and the like. For example, the sample may be directly bonded to a polishing machine, or the sample embedded in a resin or fixed to a sample stage for a scanning electron microscope with an adhesive or the like may be held using a jig. good.

  In addition, as a surface processing method for the observation sample of the scanning electron microscope, there is ion polishing using an ion beam. However, as described above, mirror polishing is performed by mechanical polishing. No damage phase formation or domain alteration due to thermal damage. For this reason, domain polishing in the sample pretreatment stage can be suppressed by mechanically polishing the sample.

  By producing a mirror-polished polished surface in this way, when observing a backscattered electron image in a later observation step, the contrast change caused by the unevenness of the observation sample surface is suppressed, and the signal amount change due to the domain ( Change in contrast) can be extracted.

  Next, a vapor-deposited film having conductivity is formed on the polished surface of the mirror-polished sample. The material of the vapor deposition film is not particularly limited as long as it has conductivity in order to suppress charge-up when the electron gun is irradiated with a scanning electron microscope. For example, carbon, Pt, Au, etc. may be used. Can be used. Among these, in order to obtain information on the extreme surface of the domain, it is preferable to use carbon in order to suppress the absorption of the electron beam and the reflection electron reflected from the observation surface. The vapor deposition film made of carbon may be formed using a vapor deposition machine that evaporates carbon as a vapor deposition source by resistance heating in a vacuum chamber. The film thickness of the vapor deposition film can be controlled by the distance between the vapor deposition source and the sample, the opening / closing of a shutter provided between the vapor deposition source and the sample in the vapor deposition machine, or the like. Note that the carbon film produced by such a method is amorphous and weakens the information (reflected electrons) from the sample. Therefore, it is desirable that the film thickness is thin, and it is formed with the minimum necessary thickness that can suppress charge-up. To do. Specifically, if the film thickness is 10 nm or less, domain observation can be performed while preventing charge-up, but it is more preferably 5 nm or less.

  In this way, by using carbon as the deposited film and setting the thickness to 10 nm or less, the signal from the domain of the observation sample reaches the detector of reflected electrons without being weakened, and a clear domain image is observed. be able to.

As described above, an observation sample can be easily produced without damaging the domain of the observation surface by mirror-polishing the sample and forming a deposited film on the polished surface.
[Observation process]
Next, the observation sample produced in the sample preparation step is observed with a scanning electron microscope. The method of placing the observation sample in the scanning electron microscope, the degree of vacuum in the sample chamber of the scanning electron microscope, and the like are performed in the same manner as the sample observation method using a normal scanning electron microscope. The distance between the observation surface of the observation sample and the electron gun of the scanning electron microscope can be appropriately set according to the observation magnification and the observation sample.

  An observation sample placed in a scanning electron microscope is irradiated with an electron beam. Here, the acceleration voltage of the electron beam is set to 3 kV or more and 10 kV or less. If it is less than 3 kV, a reflected electron image having a signal amount sufficient for observation cannot be obtained, which is not preferable. Exceeding 10 kV is not preferable because the depth of escape of reflected electrons from the observation sample surface becomes deep, the reflected electron image becomes complicated, and the signal from the surface domain decreases. By setting it to 3 kV or more and 10 kV or less, it is possible to efficiently detect the reflected electrons having information on the domain of the outermost surface of the observation sample.

  The contrast of the reflected electron image measured under these conditions is compared with the contrast condition in which the brightness and the distribution amount of the reflected electron scanning image are set to be averagely distributed within the displayable luminance range. Observe under emphasized conditions. In normal measurement, the brightness and distribution amount of the reflected electronic scanning image are set so as to be averagely distributed within the displayable brightness range regardless of whether the contrast is adjusted automatically or manually. Specifically, when the displayable luminance range is from 0 to 255, the maximum and minimum luminance values of the reflected electronic scanning image can be obtained and divided into 256, or the luminance and distribution amount can be displayed by the histogram method. A plurality of numerical values (usually two, 50, 150, etc.) representative of luminance information of the reflected electron scanning image are extracted so that the signals are not averaged within the luminance range, and signals deviating from the representative value are reduced. Therefore, parameters such as the voltage of the photomultiplier tube of the backscattered electron detector are adjusted.

  On the other hand, in the present invention, measurement is performed under such a condition that the contrast is exaggerated and the contrast is further enhanced. Specifically, the contrast is set to the maximum within a range where the reflected electron signal is not saturated on the observation screen. Such conditions are not usually used because the observation screen is rough and the image cannot be confirmed. In this way, by observing the contrast-enhanced conditions, it is possible to observe a domain that could not be observed in a reflected electron image with a scanning electron microscope because it was hidden behind noise or could not recognize the difference in brightness. It is possible to provide a sample observation method capable of

  Here, a mechanism in which domain observation by a reflected electron image, which has been considered impossible in the past, has been made possible by the present invention will be considered.

  The mechanism of domain observation in a transmission electron microscope capable of observing a ferroelectric domain is considered to be caused by diffraction contrast in the 90 ° domain and by Friedel's law violation in the 180 ° domain. The violation of Friedel's rule is a phenomenon in which when an electron beam is reflected on a certain crystal surface, the reflection intensity differs depending on whether the reflection occurs on the front side or the back side of the crystal surface. When the sample to be observed has no central symmetry and the sample is thick and multiple reflection occurs, the Friedel rule is broken. In the transmission electron microscope, the location where the 180 ° domain can be observed is limited to the location where the sample is thick and multiple reflection occurs, and the 180 ° domain cannot be observed where the sample is thin.

  Considering the domain observation method in the transmission electron microscope as described above, the 90 ° domain has a slightly tilted crystal orientation between domains. Therefore, in the scanning electron microscope, the channeling corresponding to the diffraction contrast of the transmission electron microscope is used. Contrast can be obtained. On the other hand, for the 180 ° domain, the channeling contrast seen in the backscattered electron image of the scanning electron microscope also uses the diffraction phenomenon of the electron beam in the bulk sample. It seems that the domain can be observed.

  However, the reflected electron image in the scanning electron microscope is for observing the difference in contrast caused by the difference in the composition of the observation sample. Although domains have different polarities, there is no major cause of contrast differences because there is no change in composition.

  For this reason, the signal amount change that causes the difference in contrast depending on the domain is very small and may be hidden by noise. Therefore, the acceleration voltage is set large enough to observe the Friedel law violation and not too large so that it contains a lot of domain information on the surface, ensuring the change in the signal amount due to the domain, and contrast conditions. As compared with normal conditions, it is considered that the contrast is greatly emphasized, and it is considered that the domain observation by the reflected electron image is made possible by picking up and enhancing the difference in the signal amount hidden in the noise in the normal case.

  Since the observation is performed by the mechanism as described above, the 180 ° domain that could not be observed except for a limited place in the transmission electron microscope can be observed according to the present invention without limiting the place. This is because a bulk sample is observed with the scanning electron microscope, and therefore the entire sample surface corresponds to a place where the sample in the transmission electron microscope is thick and multiple reflection occurs.

  By observing the observation sample through such a process, a highly versatile domain observation method can be provided. By using this method, it becomes possible to observe over a wide range at a high magnification of 5000 times or more, which could not be observed by the conventional EBSD method, in a short time, and submicron order and nanometer order domains can also be observed. The present invention is particularly effective in that domain observation at such a high magnification can be applied to any sample.

  Next, a more preferable example of the sample observation method according to the present invention will be described.

  In the observation step, in the above-described example, the distance between the electron gun that irradiates the electron beam of the scanning electron microscope and the observation sample and the irradiation current of the electron gun are not particularly limited, but the electron gun that irradiates the electron beam The distance from the observation sample (working distance, hereinafter referred to as WD) is preferably 10 mm or more, and the irradiation current of the electron beam is preferably 7 nA or more.

  WD is usually set as small as possible in order to measure with high resolution. However, as a result of the inventor's intensive efforts, the reason is not clear. However, when performing domain observation, if the WD is reduced, the contrast of the crystal particles is enhanced, but the contrast of the domain is lowered. It was confirmed that when the crystal size was increased, the contrast of the crystal grains was weakened and the contrast of the domain was enhanced. For this reason, it is preferable that WD shall be 10 mm or more.

  On the other hand, the irradiation current is usually set as small as possible in order to measure with high resolution. However, as a result of the inventor's intensive efforts, it was confirmed that the domain contrast is enhanced as the irradiation current is increased when performing domain observation.

  Such measurement conditions for WD and irradiation current are opposite to those for normal high-resolution observation, and are conditions specific to domain observation. In particular, when observing at a high magnification, the WD is usually made stricter and the irradiation current is made smaller. However, in the case of domain observation, even in the case of high magnification observation, this unique condition It is preferable to observe with.

  Further, in the observation step, it is preferable to perform integral shooting or to perform a low-speed scan with a single field of view for 5 minutes or more.

  According to the method of the present invention, a fine domain can be observed in order to emphasize and observe the difference in contrast that may be hidden by noise during the reflection electron image shooting. On the other hand, however, image noise increases when observed under conditions that enhance contrast. For this reason, it is preferable to suppress noise by performing integral shooting or by performing a low-speed scan with a single field of view for 5 minutes or more. By adopting such an observation method, it is possible to suppress noise at the time of shooting (observation), and it is possible to observe an image with high contrast and low noise.

  Note that some control software for an energy dispersive X-ray spectrometer, which is one of the accessory equipment of a scanning electron microscope, can perform integrated photographing of scanning electron microscope images while correcting the drift of the sample. When performing integral shooting, it takes a long time to shoot, and the image may become unclear due to a shift in the electron beam irradiation position (drifting). Since the drift is corrected every time integration is performed even if the total imaging is performed, it is possible to suppress the image from becoming unclear due to the integration.

  Further, following the observation step, there may be further provided an image adjustment step of adjusting the brightness and distribution amount of the reflected electron scanning image to a contrast condition set so as to be averagely distributed within a displayable luminance range. .

  In the image observation process, observation is performed under the condition of enhancing the contrast in order to extract a slight change in signal amount that may be hidden by noise. On the other hand, in order to enhance the contrast while further suppressing noise, it is possible to display the luminance and distribution amount of the obtained image again after extracting the contrast of the domain by performing the shooting by the integrated shooting or the low speed scan. By adjusting to the contrast condition set so as to be averagely distributed within the luminance range, the domain can be observed more clearly. Note that the contrast adjustment method is not particularly limited, and may be realized by a normal image processing automatic adjustment function or the like.

  As described above, according to the sample observation method of the present invention, in the domain observation method using a scanning electron microscope, an etching process using a chemical such as an acid is not required, and stable observation can be performed without the influence of charge-up. And the thing with high versatility about magnification, a material, etc. can be provided.

  Note that the present invention is not limited to the above-described embodiments and examples, and various modifications may be made without departing from the scope of the present invention.

  Mirror finish was performed using lead zirconate titanate (PZT) as a sample. Specific mirror finishing conditions are rough polishing using a diamond polishing plate, and after flattening the sample surface, polishing is performed by gradually reducing the particle size of the combination of polishing liquid and polishing plate. Then, a colloidal silica suspension is used as the polishing liquid, and a buff is used as a polishing plate to perform buffing and mirror finishing. In order to suppress the fluctuation of the reflected electron signal due to the change of the domain due to polishing and the slight change of the surface state due to polishing, the pressure of the sample against the polishing plate was set low at the time of final polishing.

  The observation sample thus produced was measured by integrating 50 times with a scanning electron microscope at a WD of 15 mm, an acceleration voltage of 5 kV, an irradiation current of 8 nA, a scanning speed of 8 seconds / image.

  As a result, as shown in FIG. 1, a nanometer-order domain could be clearly observed.

  Note that the domain could not be observed when the contrast was set such that the luminance and distribution amount of the reflected electron scanning image were averagely distributed within the displayable luminance range.

  Further, even when the acceleration voltage was 2.5 kV, the domain could not be observed.

  It was confirmed that when the irradiation current was less than 7 nA or the WD was less than 10 mm, the domain contrast was weakened.

  In addition, the observation of the domain as described above can obtain an equivalent image even with a scanning electron microscope having a different format such as a thermal field emission type (FE) or a cold FE type. It was confirmed that also can be observed. As a result, it was confirmed that the observation method was highly versatile, regardless of the type of scanning electron microscope and the material of the sample.

Claims (8)

A ferroelectric sample observation method for observing a domain by a reflected electron image obtained by irradiating an electron beam to an observation sample made of a ferroelectric material using a scanning electron microscope,
A sample preparation step of mirror-polishing the sample and forming the observation sample by forming a vapor-deposited film having conductivity on the polished surface;
The observation sample is set with a scanning electron microscope so that the acceleration voltage of the electron beam is 3 kV or more and 10 kV or less, and the luminance and distribution amount of the reflected electron scanning image are averaged within a luminance range in which display is possible. An observation step of observing under a condition that enhances contrast compared to a contrast condition set to be distributed;
Performed following the observation step,
Adjusting the conditions of the contrast set to averagely distributed reflection electron intensity of the scanned image and the distribution amount can display brightness range, the ferroelectric sample observation method comprising the image adjustment step. The ferroelectric sample observation method according to claim 1, wherein in the observation step, an observation magnification is set to 5000 times or more. 3. The ferroelectric sample observation according to claim 1, wherein in the observation step, a distance between the electron gun that irradiates the electron beam and the observation sample is 10 mm or more, and an irradiation current of the electron beam is 7 nA or more. Method. The ferroelectric sample observation method according to claim 1, wherein integrated imaging is performed in the observation step. The ferroelectric sample observation method according to any one of claims 1 to 4, wherein in the observation step, a low-speed scan of one field of view is performed for 5 minutes or more. In the sample preparation step, the mirror-polishing was carried out at mechanical polishing is performed using an alumina suspension or a colloidal silica suspension, the ferroelectric sample observation method according to any one of claims 1 to 5. In the sample preparation step, the deposited film consists of the following carbon film thickness 10 nm, a ferroelectric sample observation method according to any one of claims 1 to 6. 8. The ferroelectric according to claim 1, wherein in the observing step, the contrast enhancement condition is such that the contrast is maximized in a range in which the reflected electron signal is not saturated on the reflected electron scanning image. Body sample observation method.