Sample holder for scanning electron microscope, scanning electron microscope image observation system, and scanning electron microscope image observation method

A water solution in which an observation sample is, for example, dissolved is sandwiched on a first insulative thin film side provided under a conductive thin film. When an electron beam incident part is charged minus, electric dipoles of water molecules are arrayed along a potential gradient. Electric charges are also generated on the surface of a second insulative thin film. The electric charges are detected by a terminal section and changes to a measurement signal. In a state in which an electron beam is blocked, the minus potential disappears. Consequently, the electric charges on the surface of the first insulative thin film also disappear, and the measurement signal output from the terminal section changes to 0.

TECHNICAL FIELD

The present invention relates to an observation technique by a scanning electron microscope. More specifically, the present invention relates to a scanning electron microscope observation technique also capable of observing a biological sample in a living state.

BACKGROUND ART

A scanning electron microscope is widely used as a tool in observing a biological sample and an organic sample at high resolution. Conventionally, when the biological sample and the organic sample are observed by the scanning electron microscope, in order to reduce electron beam damage to an observation target sample and obtain an image with high contrast, it has been considered essential to perform treatment for, for example, after fixing the sample with formaldehyde or the like, coating the surface of the sample with gold, platinum, carbon, or the like or applying dying by heavy metal to the sample.

However, in recent years, a method with which a biological sample can be observed at high contrast without coating and dying has been developed (see Patent Literature 1 and Non Patent Literature 1).

In this new method, a sample is deposited on a lower surface of a thin sample supporting film such as a carbon film and an electron beam with a low acceleration voltage is irradiated on the sample from above the sample supporting film. The irradiated electron beam spreads while diffusing on the inside of the sample supporting film and reaches near the lower surface of the film. Secondary electrons are emitted from the lower surface of the sample supporting film. The secondary electrons are absorbed by the sample immediately below the sample supporting film. Consequently, it is possible to obtain an image with extremely high contrast.

In this method, a condition is set such that energy of the secondary electrons is approximately 10 eV. With such extremely weak secondary electrons, electron beam damage to the observation target sample is markedly low. Therefore, even in a sample susceptible to damage such as a biological sample, an original shape and an original structure of the sample can be observed or analyzed with an image with high contrast. Such an observation condition is called “indirect secondary electron contrast condition”.

Such an observation method is further promoted to also develop a method of forming a conductive film below an insulative thin film layer and further improve resolution and contrast by making use of a charging effect by electron beam incidence (see Patent Literature 2 and Non Patent Literature 2).

CITATION LIST

Patent Literature

Non Patent Literature

SUMMARY OF INVENTION

Technical Problem

Currently, an exclusive holder for enabling observation of a biological sample in a water solution (see Non Patent Literature 3) and an electron microscope system capable of performing biological sample observation under the atmospheric pressure have also been developed. However, damage to the biological sample by an electron beam is serious even in these methods. Moreover, since interaction between the electron beam and the biological sample is extremely weak, it is extremely difficult to observe the biological sample in the water solution in a living state at high contrast.

From such circumstances, in order to clearly observe the biological sample, it is necessary to apply dying treatment and fixation treatment, reduce the damage by the electron beam, and improve contrast of an observation image (Non Patent Literature 3).

However, when such treatment is applied, the biological sample, which is an observation target, dies out and the observation in the living state cannot be performed. Moreover, various artifacts involved in the dying treatment and the like occur. Reliability of an image obtained by the observation is damaged. In addition, such dying treatment and the like not only require expert skills but also are undesirable from the viewpoint of environmental protection because a toxic substance such as uranium acetate is used as a dying agent.

An image observed under the “indirect secondary electron contrast condition” has an advantage that contrast is extremely high and, on the other hand, has a problem in that resolution is relatively low. Further, usually, it is difficult to cause secondary electrons with low energy to penetrate into a water solution having thickness (depth) of several micrometers or more. Therefore, it is difficult to observe a biological sample present in the water solution.

In this way, in the conventional observation technique, it is extremely difficult to observe a biological sample in a water solution in a living state and obtain an image with high resolution on which an original form and an original structure of the biological sample are correctly reflected.

The present invention has been devised in view of such problems and it is an object of the present invention to provide a scanning electron microscope observation technique for making it possible to observe a biological sample in a living state at high resolution and high contrast using a scanning electron microscope without applying dying treatment and fixation treatment.

Solution to Problem

In order to solve such a problem, a sample holder for a scanning electron microscope according to the present invention includes a first insulative thin film, one principal plane of which is a holding surface for an observation sample, and a conductive thin film stacked on the other principal plane of the first insulative thin film. On the one principal plane side of the first insulative thin film, a terminal section that detects a signal based on the potential of the one principal plane of the first insulative thin film caused by an electron beam made incident from the conductive thin film side is provided.

For example, a second insulative thin film is provided between the one principal plane of the first insulative thin film and the terminal section. One principal plane of the second insulative thin film and the one principal plane of the first insulative thin film are disposed to have a gap of a predetermined interval. The terminal section detects the potential of the other principal plane of the second insulative thin film as a signal.

Preferably, the thickness of the first insulative thin film is 200 nm or less.

For example, the first insulative thin film is made of a silicon nitride film, a carbon film, or a polyimide film.

Preferably, the thickness of the conductive thin film is 100 nm or less.

Preferably, the conductive thin film is a metal thin film containing, as a main component, metal having a specific gravity of 10 g/cm3or more.

For example, the metal thin film contains any one of tantalum, tungsten, rhenium, molybdenum, osmium, gold, and platinum as a main component.

Preferably, the interval between the one principal plane of the first insulative thin film and the one principal plane of the second insulative thin film can be set to 40 μm or less.

Preferably, the sample holder for the scanning electron microscope includes an outer frame section that seals the inside of the sample holder, and an adjusting mechanism for internal pressure is provided in the outer frame section.

Further, preferably, in the sample holder for the scanning electron microscope, a channel for perfusing the water solution is provided in the gap of the predetermined interval between the one principal plane of the second insulative thin film and the one principal plane of the first insulative thin film.

A scanning electron microscope image observation system according to the present invention includes: a scanning electron microscope; the sample holder for the scanning electron microscope set in the scanning electron microscope; and an arithmetic unit that processes, as an output signal detected by the terminal section, a signal based on potential of the one principal plane of the first insulative thin film or a potential signal of the other principal plane of the second insulative thin film.

Preferably, the arithmetic unit processes the output signal, extracts a signal component having an intensity change frequency from the output signal, and forms an image on the basis of the extracted signal frequency having the intensity change frequency.

For example, the extraction of the signal component having the intensity change frequency is performed by any one of a band pass filter method, a lock-in amplifier method, an autocorrelation analysis method, and a Fourier transform analysis method.

Preferably, a plurality of the terminal sections are provided in different positions, and the arithmetic unit forms an image for each of signal components having the intensity change frequency respectively extracted from the plurality of terminal sections, calculates an inclination angle of the image from a relation between an incident position of an electron beam and the positions where the terminal sections are provided, applies correction to the image on the basis of the inclination angle, and analyzes three-dimensional structure information of an observation sample on the basis of a plurality of images after the correction.

A scanning electron microscope image observation method according to the present invention performs, using the scanning electron microscope image observation system, observation in a state in which the conductive thin film is set to a ground potential of the scanning electron microscope or a predetermined potential.

A scanning electron microscope image observation method according to the present invention performs, using the scanning electron microscope image observation system, observation with an acceleration voltage of the incident electron beam set to a voltage at which the incident electron beam is hardly transmitted through the first insulative thin film.

A scanning electron microscope image observation method according to the present invention performs, using the scanning electron microscope image observation system, observation with an acceleration voltage of the incident electron beam set to 10 kV or less.

A scanning electron microscope image observation method according to the present invention turns on and off, using the scanning electron microscope image observation system, the incident electron beam at a frequency of 1 kHz or more and performs observation with the extracted intensity change frequency set to 1 kHz or more.

Further, a scanning electron microscope image observation method according to the present invention performs, using the scanning electron microscope image observation system, observation in a state in which an observation sample is supported on the one principal plane of the first insulative thin film together with a water solution.

Advantageous Effects of Invention

According to the present invention, it is possible to easily observe a biological sample in a water solution at extremely high contrast without applying dying treatment and fixation treatment to the biological sample. In addition, since damage to the sample due to an electron beam does not occur, it is possible to learn original forms and structures concerning biological samples such as cells, bacteria, viruses, and protein complexes and organic materials susceptible to damage.

DESCRIPTION OF EMBODIMENTS

Modes for carrying out the present invention are explained below with reference to the drawings.

FIG. 1is a block diagram for explaining a configuration example of a scanning electron microscope image observation system according to the present invention.

In an example shown in the figure, the system includes a scanning electron microscope100, a sample holder200for a scanning electron microscope set on the inside of the scanning electron microscope100, a function generator300, a lock-in amplifier400, a data recorder500, and a PC600functioning as an arithmetic unit.

In the scanning electron microscope100, a beam blanking device103for controlling irradiation intensity on an observation sample of an electron beam102emitted from an electron gun101is provided. The beam blanking device103is a device for obtaining an intensity change of an incident electron beam on the observation sample. For example, an ON/OFF signal having a frequency of 1 kHz or more is input to the beam blanking device103as a control signal having a rectangular wave form from the function generator300provided outside a microscope chamber. The function generator300outputs a reference signal to the lock-in amplifier400.

When a control signal for OFF is input from the function generator300, the electron beam102emitted from the electron gun101travels forward. The entire electron beam102is transmitted through a diaphragm104and irradiated on an observation sample (not shown in the figure) stored in the sample holder200.

On the other hand, when a control signal for ON is input from the function generator300, an electric field is generated in the vicinity of the beam blanking device103and a track of the electron beam102emitted from the electron gun101is bent. The entire (or a part of) electron beam102is blocked by the diaphragm104.

As a result, when the control signal has plus potential, an electric field is generated in the beam blanking device103. An electron beam is bent by a Coulomb force and blocked by the diaphragm104. The electron beam made incident on the observation sample is turned off. On the other hand, when the control signal has zero potential, the electron beam is transmitted through the diaphragm104and irradiated on the observation sample.

When ON/OFF by such a control signal is repeated, the intensity of the electron beam irradiated on the observation sample changes. The frequency of the control signal at this point is suitably 1 kHz or more. In general, the frequency is set in a range of 20 to 100 kHz.

That is, according to the control signal input from the function generator300, the intensity of the electron beam irradiated on the observation sample (not shown in the figure) stored in the sample holder200changes at a frequency same as the frequency of the control signal.

When the electron beam is irradiated on the observation sample stored in the sample holder200, because of a reason explained below, a signal based on potential caused by the incidence of the electron beam is generated in the sample holder200. The signal is detected by a terminal section210provided under the sample holder200, amplified by an amplifier105, and output to the lock-in amplifier400as a measurement signal. That is, the reference signal from the function generator300and the measurement signal from the amplifier105are input to the lock-in amplifier400.

The lock-in amplifier400extracts only a frequency component of the reference signal of the function generator300out of the measurement signal using the reference signal and transmits the frequency component to the data recorder500as an output signal.

The PC600functioning as the arithmetic unit processes the output signal, extracts a signal component having an intensity change frequency from the output signal, and forms an image on the basis of the extracted signal component having the intensity change frequency according to the scanning signal of the electron beam. The extraction of the signal component having the intensity change frequency can be performed by a method such as a band pass filter method, a lock-in amplifier method, an autocorrelation analysis method, or a Fourier transform analysis method.

FIG. 2is a schematic sectional view for explaining a configuration example of a sample holder for the scanning electron microscope according to the present invention.

The sample holder200includes a first insulative thin film203, one principal plane of which is a holding surface for an observation sample10, and a conductive thin film201stacked on the other principal plane of the first insulative thin film203. The stacked body has pressure resistance enough for withstanding observation in a vacuum. That is, in an electron microscope chamber, an atmospheric pressure state can be retained in the holder. On the one principal plane side of the first insulative thin film203, the terminal section210that detects a signal based on the potential of the one principal plane of the first insulative thin film203caused by the electron beam102made incident from the conductive thin film201side is provided.

In the configuration example shown in the figure, a second insulative thin film204is provided between the one principal plane of the first insulative thin film203and the terminal section210. One principal plane of the second insulative thin film204and the one principal plane of the first insulative thin film203are disposed to have a gap of a predetermined interval. Therefore, in the case of this configuration, the terminal section210detects the potential of the other principal plane of the second insulative thin film204as a signal. In a state in which the terminal section210is electrically insulated from the first insulative thin film203and the second insulative thin film204functioning as sample supporting films, the terminal section210is provided to be separated from these thin films.

Note that the second insulative thin film204is not essential. For example, if an observation sample can be, for example, dissolved in an extremely thin layer of water, the observation sample is held by the surface tension of the water layer. Therefore, even if the second insulative thin film204is absent, a signal based on the potential of the one principal plane of the first insulative thin film203can be detected by the terminal section210.

The observation sample10may be a biological sample present in a water solution20. The observation sample10is encapsulated between the first insulative thin film203and the second insulative thin film204and stored in a conductive outer frame section208, the inside of which is sealed by a conductive gasket206and an O-ring207. That is, observation in a state in which the biological sample is supported together with the water solution is possible.

Note that, as explained below, a mechanism for adjusting the pressure on the inside may be provided in the outer frame section208. A member denoted by reference numeral209is an insulating member for insulating the terminal section210from the outer frame section208. Sections denoted by reference numerals202and205are frame sections provided for the purpose of strength maintenance. Reference numeral30denotes a diffusion region of the incident electron beam102.

In the example shown inFIG. 2, both of the gasket206and the outer frame section208are conductive. The conductive thin film201is set to a ground potential of the scanning electron microscope100. Observation is performed in this state. Note that the observation may be performed in a state in which the potential of the conductive thin film201is set to a predetermined potential rather than the ground potential.

An acceleration voltage of the electron beam102emitted from the electron gun101is desirably set to a voltage at which an incident electron beam is hardly transmitted through the first insulative thin film203. Specifically, the acceleration voltage is desirably set to an acceleration voltage at which the incident electron beam is almost scattered or absorbed on the inside of the conductive thin film201. According to such voltage setting, primary electrons are hardly transmitted to the first insulative thin film203side. It is possible to completely prevent electron beam damage to the observation sample10.

In general, if the acceleration voltage of the incident electron beam is set to 10 kV or less, the condition is realized.

If the first insulative thin film203is too thick, the intensity of a signal detected by the terminal section210decreases. Therefore, the thickness of the first insulative thin film203is desirably set to 200 nm or less.

As the material of the first insulative thin film203, a silicon nitride film, a carbon film, and a polyimide film can be illustrated.

If the conductive thin film201is too thick, the intensity of the signal detected by the terminal section210also decreases. Therefore, the thickness of the conductive thin film201is desirably set to 100 nm or less.

The conductive thin film201is desirably a metal thin film containing, as a main component, metal having a specific gravity of 10 g/cm3or more. This is for the purpose of efficiently suppressing blocking properties of the electron beams and internal diffusion of incident electrons. As such a metal thin film, a metal thin film containing any one of tantalum, tungsten, rhenium, molybdenum, osmium, gold, and platinum as a main component can be illustrated.

FIG. 3is a result obtained by stacking nickel having thickness of 5 nm and tungsten having thickness of 20 nm on a silicon nitride thin film having thickness of 50 nm and calculating, with a Monte Carlo simulation, a scattering state at the time when an electron beam having an acceleration voltage of 3 kV is made incident on a stacked film of nickel/tungsten.

Most of incident electrons are scattered or absorbed in a tungsten film having a specific gravity of 19.3 g/cm3. Only a part of the electrons scatter to the silicon nitride thin film. However, all of the electrons are absorbed in the silicon nitride thin film. That is, the primary electrons are hardly transmitted through the silicon nitride thin film, which is the first insulative thin film.

When the interval between the one principal plane of the first insulative thin film203and the one principal plane of the second insulative thin film204is too thick, the intensity of the signal detected by the terminal section210decreases. Therefore, the interval can be desirably set to 40 μm or less.

The observation sample10is stored in the sample holder200. The electron beam102is made incident while being turned on and off at a frequency of 1 kHz or more. Observation is performed with an intensity change frequency extracted by the lock-in amplifier400set to the frequency (1 kHz or more) of the electron beam102.

FIG. 4is a diagram for conceptually explaining a mechanism in which a signal is detected by the terminal section210when the electron beam102, intensity of which changes in a rectangular wave shape, is made incident on the sample holder200according to the present invention.FIG. 4(a)shows a state in which the electron beam is made incident (an ON state) andFIG. 4(b)shows a state in which the electron beam is blocked by a diaphragm and is not made incident (an OFF state).

As shown inFIG. 4(a), in the state in which the electron beam102is made incident (the ON state), almost all incident electrons are scattered or absorbed in the conductive thin film201. Consequently, electrons are accumulated in an incident part of the electron beam. The part changes to minus potential.

The water solution20in which the observation sample10is, for example, dissolved is sandwiched between the first insulative thin film203and the second insulative thin film204. Since water molecules themselves of the water solution20are polarized, when the electron beam incident part is charged minus, electric dipoles of the water molecules are arrayed along a potential gradient. According to the electric dipole array, electric charges are also generated on the surface of the second insulative thin film204present on the lower side of the water solution20. The electric charges are detected by the terminal section210as a potential signal generated on the principal plane of the second insulative thin film204and changes to a measurement signal.

On the other hand, as shown inFIG. 4(b), in the state in which the electron beams102is blocked (the OFF state), the incident electrons in the conductive thin film201immediately flows into the GND. The minus potential disappears. Consequently, the array of the electric dipoles in the water solution20come apart, the electric charges on the surface of the first insulative thin film203disappear, and the measurement signal output from the terminal section210changes to 0.

By repeating ON/OFF of the electron beam at a frequency of 1 kHz or more, it is possible to extract a signal of a frequency component same as the frequency from the terminal section210.

FIG. 5is a diagram for conceptually explaining a mechanism in which a signal is detected in the terminal section210when the electron beam102is irradiated on a biological sample, which is the observation sample10.

When the electron beam102is irradiated at a frequency of 1 kHz or more of ON/OFF, since the dielectric constant of the biological sample is extremely low compared with water, the array of the electric dipoles is weakened and the intensity of the measurement signal decreases.

Since the dielectric constant of the water molecules is as large as approximately 80, if a potential change occurs on the one principal plane of the first insulative thin film203, the potential change can be used as a signal with a strong propagating force in the water solution20. On the other hand, in general, the dielectric constant of the biological sample10is low. For example, the dielectric constant of protein is 2 to 3. Therefore, the propagation force of the potential change signal in the biological sample is weak. Therefore, it is possible to obtain high contrast according to such a large difference in the dielectric constants (the difference in the propagating forces).

As a result, in the biological sample10and the water solution20, a difference occurs in the propagation force of the potential change signal due to the difference in the dielectric constants. The difference is detected by the terminal section210provided on the one principal plane side of the first insulative thin film203. Consequently, it is possible to observe the biological sample at high contrast without dying the biological sample. Moreover, since the electron beam is not directly irradiated on the biological sample10, the observation sample10is not damaged by the electron beam irradiation.

Note that the resolution of an image obtained by observation generally depends on the irradiation diameter of the electron beam. Therefore, if the irradiation diameter of the electron beam is narrowed to approximately 1 nm, it is also possible to attain resolute (1 nm) substantially equal to the irradiation diameter. As a result, a biological sample including bacteria, viruses, protein, or a protein complex can also be observed in a living state at high resolution and high contrast.

EXAMPLES

An example in which bacteria and yeast, which were observation samples, were dissolved in a water solution and encapsulated in the sample holder200and scanning electron microscope observation was performed is explained.

FIG. 6is an observation image obtained by storing bacteria10dissolved in the water solution20in the sample holder200for the scanning electron microscope according to the present invention and observing the bacteria. The acceleration voltage of the electron beam during the observation is 3 kV and an ON/OFF frequency of the incident electron beam is 30 kHz.

The first insulative thin film203, which is a sample supporting film, is a silicon nitride film having thickness of 50 nm. The conductive thin film201obtained by forming thin films of nickel having thickness of 5 nm and tungsten having thickness of 20 nm is stacked on the first insulative thin film203as shown inFIG. 3.

The water solution sample was sandwiched between the first insulative thin film203and the second insulative thin film204having thickness of 50 nm. The terminal section210was set below the second insulative thin film204with an air gap provided therebetween.

From this observation image, elongated bacteria of approximately 5 nm and spherical bacteria can be observed. In the observation, although dying treatment and fixation treatment are not applied at all, an image with extremely high contrast is obtained.

FIG. 7is an observation image obtained by storing yeast10dissolved in the water solution20in the sample holder200for the scanning electron microscope according to the present invention and observing the yeast10.FIG. 7(a)is an observation image in the case in which an ON/OFF frequency of an incident electron beam is 30 kHz andFIG. 7(b)is an observation image in the case in which the ON/OFF frequency of the incident electron beam is 80 kHz. Note that the acceleration voltage of the electron beam during the observation is 3 kV.

Even when the ON/OFF frequency of the incident electron beam is 30 kHz, yeast having a size around 10 μm can be observed at extremely high contrast. However, when the ON/OFF frequency of the incident electron beam is set to 80 kHz, the observation image is clearer.

FIG. 8is a configuration example of a scanning electron microscope image observation system having a form in which a plurality of terminal sections210ato210care provided in different positions. A plurality of amplifiers105ato105cand a plurality of lock-in amplifiers400ato400care also provided because the plurality of terminal sections210ato210care provided.

The arithmetic unit forms an image for each of signal components having the intensity change frequency respectively extracted from the plurality of terminal sections, calculates an inclination angle of the image from a relation between an incident position of the electron beam and the positions where the terminal sections are provided, applies correction to the image (an inclined image) on the basis of the inclination angle, and analyzes three-dimensional structure information of an observation sample on the basis of a plurality of images after the correction.

For example, when three terminal sections are provided, an inclined image from the terminal section210aprovided on the left side, an inclined image from the terminal section210cprovided on the right side, and an inclined image from the terminal section210bat the center present in a position where the observation sample is observed from the front can be obtained in one observation. These three images are constructed as a three-dimensional image using a three-dimensional reconfiguration algorithm according to inclination angles of the images.

FIG. 9is a diagram for explaining the structure of the sample holder200having a form in which a mechanism for adjusting the pressure on the inside of the outer frame section208is provided therein. InFIG. 9(a), a state under the atmospheric pressure is shown. InFIG. 9(b), a state in a vacuum (in a microscope chamber) is shown. In an example shown in the figure, a decompression film211is formed as a pressure adjusting mechanism and provided on the lower surface side of the outer frame section208.

The decompression film211of the sample holder200expands to the outer side in the microscope chamber. The pressure in the holder decreases. The conductive thin film201side also bends to the outer side. However, a bending degree is relaxed by the effect of the decompression film211.

Note that a region below the second insulative thin film204is sealed by the O-ring207to maintain the atmospheric pressure. Therefore, the second insulative thin film204bends upward rather than downward. The gap between the one principal plane of the first insulative thin film203and the one principal plane of the second insulative thin film204expands. Therefore, no problem occurs in holding the observation sample10.

In the sample holder200, a channel for perfusing the water solution may be provided in the gap of the predetermined interval between the one principal plane of the first insulative thin film203and the one principal plane of the second insulative thin film204. A plurality of kinds of solutions may be encapsulated in the sample holder200and observed while changing the solutions.

FIG. 10is a configuration example of a part of the sample holder200in which channels for perfusing the water solution are provided in the gap of the predetermined interval between the one principal plane of the second insulative thin film204and the one principal plane of the first insulative thin film203.FIGS. 10(a) to 10(c)are respectively a perspective view, a top view, and a sectional view.

In the example shown in the figure, three channels for perfusing the water solution are provided. The outer frame section208of the sample holder200includes an upper portion208aand a lower portion208b. In the upper portion208a, injection holes213associated with three channels212are formed. A solution introduced into these channels212is introduced into the gap of the predetermined interval between the one principal plane of the second insulative thin film204and the one principal plane of the first insulative thin film203by action of surface tension.

Dampers214functioning as pressure applying sections for pushing out the solution from the injection holes213are provided on a side of the sample holder200. Pressure applying valves215are provided at the distal ends of the dampers214.

When such a plurality of channels212are provided, it is easy to conduct, for example, an experiment for, for example, feeding a reagent or the like anew into the sample holder200, in which the observation sample10such as cells or bacteria are stored in advance, and observing reaction by the reagent in detail. Note that a unit denoted by reference numeral216in the figure is a discharge liquid tank. Such feeding of the solution may be performed electrophorentically rather than being performed by the pressure applying section.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, it is possible to easily observe a biological sample in a water solution at extremely high contrast without applying dying treatment and fixation treatment to the biological sample. In addition, since damage to the sample due to an electron beam does not occur, it is possible to learn original forms and structures concerning biological samples such as cells, bacteria, viruses, and protein complexes and organic materials susceptible to damage.

REFERENCE SIGNS LIST