rus
Issue #1/2015
V.Gelever, E.Usachev, A.Manushkin
Nanodiagnostic educational scientific complex based on hybrid nanoscopes
Nanodiagnostic educational scientific complex based on hybrid nanoscopes
In MIREA is developed hybrid device – nanoscope that is designed to research of small nanostructured objects using different methods of microscopy and spectroscopy.
With the purpose of development of scientific-educational process in nanoindustry the Government of the Russian Federation since 2008 creates an educational nanotechnology network involving universities. Training centers with the modern, economical and relatively simple, focused on educational process equipment are necessary for ensuring comprehensive specialist training. The equipment should be integrated in teaching and laboratory facilities and classes with the appropriate methodological support of the educational process. Such systems must meet the following basic requirements [1]:
•modularity allows to select the equipment according to the needs and capabilities of the training institutions for a wide coverage of different areas;
•low price of equipment for institutions with small budgets (the average cost of the laboratory module is several times smaller than of the research equipment of the same type).
Currently, there are several solutions for educational and research tasks, for example, the classes NanoEducator and NanoEducator II, as well as educational laboratory complex, developed in Nanotechnologies SEC at Saint Petersburg Electrotechnical University "LETI".
Solutions based on similar devices
Educational research classes NanoEducator and NanoEducator II, allows to combine teaching with research activities [2]. Although the developer and manufacturer NT-MDT positions them as an integrated mini-laboratory, in fact, it is a complex of scanning probe microscopes (SPM) of entry level with accessories and manuals. Therefore, we can speak about the relatively low cost of such training and research solutions, but not about complexity, because only one research method of the surface is used. However, already in 2010, teaching and research complexes NanoEducator were installed in about 50 educational institutions abroad, and in nearly 130 scientific and educational centers in Russia and the CIS.
Training course on the development of SPM is taught in National Research University of Electronic Technology, and at workshop students assemled microscopes from 42 parts and receive on them the images of atoms [3]. In addition, practical classes of work on scanning probe and ion-conductive microscopes are available in the YICC "Nanotechnology" at the Lomonosov Moscow State University [4].
Complexes of the equipment
In Nanotechnologies SEC at Saint Petersburg Electrotechnical University "LETI" is developed low-cost educational laboratory complex [5], which provides broad coverage of scientific and educational areas and combines teaching and research laboratory for nanotechnology (six pieces of small-size equipment) and nano-measurement laboratory, including research and control of nano-objects by the methods of capillary electrophoresis, ellipsometry, scanning electron microscopy (SEM), electron paramagnetic resonance, atomic force microscopy with optical and electrical nano-probe measurements. Devices are completed with manuals and software. Unfortunately, in fact, this complex cannot be considered as low-budget, since it consists of 12 devices of various types. In addition, in the laboratory there are no high-resolution electron microscopes, apparently for economic reasons, as they have a high cost. The laboratory includes only desktop REM with average resolution, operating in a fixed mode. Formally, the complex has several diagnostic techniques include, but they are not integrated with each other. Complexity implies the possibility of the research of area of the object using various methods about the same time.
Complexes of equipment which is used, including, for educational purposes, also have nano-centres at universities. So, in MIREA at the Department of nano-diagnostic of materials and microelectronic devices, the structure unit of Engineering research and production center "Non-destructive testing", it is planned to train students to modern means of nanotechnology control of electronic component base. The department has an almost complete set of basic types of microscopes with high parameters that allows a comprehensive study of integrated circuits. However, due to the large volume of planned works to conduct laboratory studies can be allocated a relatively short time, so training will be most likely for informational purposes to obtain some practical skills.
Scanning probe microscopy: advantages and limitations
In principle, it is understandable why educational equipment in Russia is focused on SPM. Their principle of operation is quite simple: probe (tip) mechanically moves near the surface of the object and registers the current flowing through the surface and the probe when a small potential difference between them, or the forces acting on the probe from the surface. The resolution of the SPM is mainly determined by the tip radius of the probe. These microscopes are used to study the topology of surfaces at the atomic-molecular level and of properties of materials. SPM are small in size, do not require large operating costs, quite simple in design and relatively cheap, therefore, small companies can produce them at low capital cost.
However, scanning probe microscopy quite difficult to combine with other methods of surface investigation, therefore, are produced mainly specialized devices, although sometimes SPM work in pairs, for example, with modules of optical microscopy or Raman spectroscopy. In addition, the SPM have several disadvantages, which greatly reduce the possibilities of their use. Thus, the maximum scanning field does not exceed a few tens of microns, which corresponds to an magnification of several thousand-fold, but for a choice of place of study there is no possibility to view the object with less magnification. In addition, the image of surface can have up to 20 types of artifacts, and it is difficult to determine whether the image corresponds to reality.
According to some estimates, about 70% of the published studies are conducted on electronic microscopes, and on SPM – not more than 5%. Thus, probe microscopes cannot be considered as the main tool of modern nanotechnology, and they are not very suitable for the role of the base when creating a training and research complexes. By their nature and capabilities, scanning probe microscopy is more suitable to future technologies and manufacture at the atomic level.
Scanning electron microscopy
Originally under the nanotechnology (NT) understood atomically-precise methods, but now usually referred to them technologies to explore and produce objects smaller than 100 nm. In this area more opportunities for research and monitoring provides an electron beam, therefore, the role of the main research instrument is more suitable for scanning electron microscope (SEM).
The main tool of SEM is an electron beam, whose parameters (energy, size, current, speed and size of the scan area) can easily vary within wide limits by means of electronics. At interaction of a of high-energy electron beam with objects appear various processes and secondary radiation, effective registration of which gives a variety of information about these objects. Electron microscopes are more complex and expensive devices than SPM. The device must be provided with a high vacuum, and on the various elements of the SEM – coil magnetic lens, deflecting system and stigmator, e-gun, etc. – is necessary to apply voltages and currents of different magnitude (about half a dozen different values), some of which must be regulated in the process. In different modes of SEM can be measured practically any objects, and constraints associated only with dimensions. In SEM, practically nothing can break down mechanically, but can, as in any device with electronics, fail electronic components. The main consumable material is a tungsten filament cathode, which can be changed easily. In addition, it is necessary to periodically clean the elements inside the column, which are affected by the electron beam.
Universal SEM allow you to explore a variety of nanostructured objects. At the request of the customer they are equipped with different sets of modules set – top boxes, detectors, etc. However, these microscopes are expensive and often have a high degree of automation, which is inconvenient for educational purposes.
In most SEM main design element is a object camera. The camera and elements of the vacuum system are fixed on the frame – this basic part of the microscope has a fairly large size with high cost. If the SEM has been designed, including, for the semiconductor industry and metallurgy, and for measuring of wafers of 100-300 mm or of large samples of metal, the size and cost of design are additionally increased.
Due to the use of a large camera with holes for detectors and of multi-axis table with a large range of movement, SEM is sensitive to electromagnetic interference and vibration, so often it is necessary to place it in a special room. For universal devices is also very difficult to achieve simultaneously high settings in all modes. Therefore often choose one main operating mode of a microscope and optimize for it the focusing optics, but worsen parameters in other modes.
Hybrid nanoscopy: basic principles
It should be noted that most of the objects nanotechnology are small within a few millimeters, while some are nanoparticles. In this connection it is expedient in the development of devices to focus mainly on small objects with dimensions of a few millimeters, which greatly simplifies the design and reduces the cost of equipment. To better meet modern needs in studies of surface and structure of objects at the micro- and nanoscale in MIREA is developing a hybrid nanoscope (HN) of economy class, designed for research of small nanostructured objects using electron, x-ray, ion, probe and optical microscopy, as well as of different modes of spectroscopy.
The basic element of HN is the electron probe module (EPM), which is formed by a focused electron beam, characterized by a wide range of electron energy, currents and dimensions of the beam (probe). Power and control from a PC is provided by two blocks (about 15 cards).
EPM (fig.1) is a compact, desktop device, containing a system of magnetic lenses (column), electronic gun, the elements of vacuum system, detectors of electrons, as well as tables for small objects.
In fact, there is no objects camera in the EPM, in the traditional sense. The main element of EPM is a installed on legs column, to which are attached all other elements. This ensures access to the latest magnetic lens, which focuses the beam on the object that allows the use of removable tips, the geometry of which (the width of the gap and the diameter of the pin holes) can be optimized for different modes. In this design column and two detector are placed in the same half-plane of the object, and the second half-plane is free, so it can accommodate various detectors, scanning probe modules and optical microscopy. In some cases the user will be able to install them yourself, in accordance with their needs and capabilities.
If the object is installed under the beam, EPM works as an electron microscope with the ability to obtain the minimum diameter of the electron beam 1 nm.
The mode of transmission
x-ray microscope
If the target (a thin layer of metal on a vacuum-tight substrate) is placed under the beam, and the object and the detector are placed on the air, it is possible to conduct research in x-rays. In transmission x-ray microscope (TXM), when the electron beam is focused on the target, a surface area is created that emits X-rays. The size of the emitting region (focal spot) is determined by the diameter of the electron beam and the effective path length of the electrons in the target, which depends on the accelerating voltage and the density of the target material. With a suitable choice of these parameters it is possible to obtain a focal spot size close to the size of the electron probe. The projective and raster modes of formation of x-ray images of internal structure of objects are possible. In projection mode, the electron beam is still focused at the point of the target, and in raster mode it scans the surface.
To obtain nanometer-size in TXM-mode you should to use the accelerating voltage in the range 5-15kV. At the soft x-ray radiation nanoscale details of objects will have rather high contrast. Electronic probes and focal spot sizes in the tens of nanometers can be obtained with currents less than 10-9A. However, the yield coefficient of x-ray quanta under the influence of electron beam is on five to six orders of magnitude lower than that of secondary electrons, so there are problems with the registration of low-intensity x-ray fluxes, timely and accurate focusing of the electron beam on the target and with maintaining the focus of the beam at large times of x-ray emitting.
For efficient and accurate focusing of the electron beam on the target were asked to use a detector of secondary electrons in the space between the two last lenses [6]. When operating in SEM mode, the raster electron microscopy mode is used as an auxiliary to obtain images of the target surface in the secondary electrons. Secondary electrons from the target are registered by built-in detector that allows quickly and accurately to focus the nanoscale beam on the target, to control the condition of the surface of the target and to choose the focus point for TXM. In the result, it is possible to realize big times of obtaining x-ray images without the control of the focusing of x-rays.
To provide the level of x-ray flux sufficient for forming high-quality images with nanoscale focal spots with small currents, it is advisable to bring the detector to the target. Thus for receiving high magnifications and resolutions it is necessary to pass into the short focal mode when micron and submicronic distances between object and a focal spot (do-f) are provided. In this case, the increase and the density of the x-ray flux on object and detector increases proportionally to 1/d2o-f. The latter allows to compensate reduction of power of x-ray radiation in the transition to a focal spot in a few tens of nanometers and get resolution on the level of 20-30 nm.
In most modern x-ray microscopes do-f ranges from hundreds of microns to several millimeters. At the same time, many researched and produced nano-objects such as thin films and nanoparticles, can simply be placed on a substrate (membrane) from the air. Then the thickness of the membrane determines the minimum do-f, and optimal is design, when the electron gun is at the bottom, and at the top the membrane with the target is placed. Modern technologies allow to obtain a vacuum-tight micron and submicron membrane of Be, Si, Si3N4, C, and other materials.
Combining of methods
The proposed design of EPM is an electron x-ray microscope [7], which optimally combined two types of high-resolution devices and is possible a comprehensive study of one area of the object in different kinds of electrons and x-rays. In addition, the coordinate-sensitive detectors can be combined with detectors of x-ray analysis that will allow to simultaneously obtain information about the chemical composition of objects.
It should be noted that actually for the first time for wide application in nano-researches the high-resolution (20-30 nm) x-ray microscopy is offered. Currently, such resolution in the x-ray radiation reach only at synchrotrons with the use of expensive and complex elements of x-ray optics [8]. In addition, there are foreign x-ray microscopes based on x-ray tubes with a resolution of 50-100 nm, but these devices are expensive (some up to $1-3 million) and have fairly large dimensions.
Most of microscopy techniques give information about the topology and chemical composition of the surface of nanostructured objects. However, many of the properties of nanomaterials are determined by the distribution of particles and pores in the volume, so you should to use costly and time-consuming sample preparation (to do the splits and kinks), but in these cases it is not always possible to obtain complete information about the internal structure of objects. X-ray microscopy allows to research the internal structure in air and in liquids without destroying objects and with minimal sample preparation. In some cases, you may receive 3D or layered images. In the future we should expect the development of x-ray microscopes with a resolution on the order of 5-10 nm.
Very effective may be the combination of x-ray, scanning probe and optical microscopy. Sometimes optical microscopes are used to control movement of the probe and supply it to the specified point with high accuracy. In this case it is possible to see an edge in an optical lens and in real time to watch scanning and nano-manipulations. If to work with a probe microscope with nano-objects on a thin film, with placement of targets on the reverse side of the film it will be possible to see the tip of the probe and the object in x-rays.
Improvement and implementation
of hybrid nanoscope
Selected design of HN, in which the basic element is a column with integrated electronic detectors and a set of tables for small objects, not only allows to optimally combine different research approaches, but also reduces the cost of the device. For the manufacture of HN it is enough mechanical assembly production of the middle level with low capital costs. The use of domestic power supplies and controls that are well developed and manufactured for many years also reduces the price of nanoscope – even when manufacturing in single instances these blocks are two to three times cheaper than foreign ones.
Development of HN is conducted on a voluntary basis for several years. To date, electron x-ray microscope is well developed [7], a small experimental series are made, which tested different variants of EPM design, good preliminary results in terms of resolution are received. Images of the same object at medium magnification (fig.2) show the possibility of obtaining images in x-rays and electron beams at close levels. Thus the modes of x-ray imaging were far from optimum in parameters of detectors, substrates and test objects. In the projection mode, the imaging was carried out on x-ray film, and in raster mode – to one semiconductor detector. Now developed a modification with 20 detectors that simultaneously record images at different angles to obtain a layered images of objects.
Developed research complex provides integration of the basic types of microscopes with maximum settings for each of them. Its functionality with minimal cost can be expanded in the right directions, and the specialization of a particular device can be easily modified by replacing and/or adding individual modules. HN actually allows you to combine the capabilities of most imported microscopes of various types at a cost level of desktop electron microscopes. This high-level device is optimal for import substitution and has no analogues.
The combination of high technical capabilities with reasonable price enables to use it for solving scientific and educational tasks at the present stage of development of nanotechnology. On the basis of hybrid nanoscopes can be created nanotechnology scientific-educational laboratories and workshops. The best may be a set of three to five hybrid nanoscopes with a mixed set in other modules (probe microscope, optical detectors of different types, and others). On one HN, you can research objects in different kinds of electrons, on the other to combine electrons and x-rays, on third to combine x-ray, scanning probe and optical methods. If the combinations of all kinds of microscopes in one set is necessary, you can gradually build up the capacity of HN, complementing it with additional modules.
In practice, it is not necessary to have a complete set of power supplies and controls for each EPM. Because the vacuum system is made on the basis of magnetic discharge pump, for which only for the period of start the booster pump is necessary, it is possible to use a single pump for a few EPMs. Since it is unlikely that all EPMs were be used at the same time throughout the working day, for several HN you can use one power source and to change the cables connecting of EPM with blocks.
HN (fig.3-4) are already used for education in MIREA. So, in the summer on these devices students of the first courses did practical training, and now two students pass externship with the subsequent implementation of the diploma thesis. Initially, they will perform a complete assembly of EPM, including the winding coils of the lenses and deflection systems, mechanical assembly of magnetic lenses system, of vacuum and detector systems, obtaining the vacuum, connection of power supply units, ensuring the transmission of the electron beam through the lens, obtaining first images of objects, the preliminary alignment of the lens system, the optimization of modes of electronic detectors and obtain the limiting parameters for the test objects. Then each of them will conduct research on nanostructured objects. As a result, they will become in practice familiar with the design of the instrument and learn the methods of work on it.
On the basis of manufactured HNs will operate the scientific-educational class, largely satisfying modern requirements for training and research centres. The value of such a class would be higher with the inclusion of probe and optical microscopes as well as x-ray detectors. Vacuum prototype of EPM to work in conjunction with an electron beam, as well as atmospheric prototype to work in conjunction with TXM are fundamentally already worked out.
With some support from the state the results of the development of HN can be used more widely, for example, for production of instruments within the fablabs – industrial laboratories in universities – to attract students, which contribute to the development and production of marketable innovative products and service. Thus, it will be possible not only to teach students, but also to solve the problem in production of domestic research equipment for science and industry.
•modularity allows to select the equipment according to the needs and capabilities of the training institutions for a wide coverage of different areas;
•low price of equipment for institutions with small budgets (the average cost of the laboratory module is several times smaller than of the research equipment of the same type).
Currently, there are several solutions for educational and research tasks, for example, the classes NanoEducator and NanoEducator II, as well as educational laboratory complex, developed in Nanotechnologies SEC at Saint Petersburg Electrotechnical University "LETI".
Solutions based on similar devices
Educational research classes NanoEducator and NanoEducator II, allows to combine teaching with research activities [2]. Although the developer and manufacturer NT-MDT positions them as an integrated mini-laboratory, in fact, it is a complex of scanning probe microscopes (SPM) of entry level with accessories and manuals. Therefore, we can speak about the relatively low cost of such training and research solutions, but not about complexity, because only one research method of the surface is used. However, already in 2010, teaching and research complexes NanoEducator were installed in about 50 educational institutions abroad, and in nearly 130 scientific and educational centers in Russia and the CIS.
Training course on the development of SPM is taught in National Research University of Electronic Technology, and at workshop students assemled microscopes from 42 parts and receive on them the images of atoms [3]. In addition, practical classes of work on scanning probe and ion-conductive microscopes are available in the YICC "Nanotechnology" at the Lomonosov Moscow State University [4].
Complexes of the equipment
In Nanotechnologies SEC at Saint Petersburg Electrotechnical University "LETI" is developed low-cost educational laboratory complex [5], which provides broad coverage of scientific and educational areas and combines teaching and research laboratory for nanotechnology (six pieces of small-size equipment) and nano-measurement laboratory, including research and control of nano-objects by the methods of capillary electrophoresis, ellipsometry, scanning electron microscopy (SEM), electron paramagnetic resonance, atomic force microscopy with optical and electrical nano-probe measurements. Devices are completed with manuals and software. Unfortunately, in fact, this complex cannot be considered as low-budget, since it consists of 12 devices of various types. In addition, in the laboratory there are no high-resolution electron microscopes, apparently for economic reasons, as they have a high cost. The laboratory includes only desktop REM with average resolution, operating in a fixed mode. Formally, the complex has several diagnostic techniques include, but they are not integrated with each other. Complexity implies the possibility of the research of area of the object using various methods about the same time.
Complexes of equipment which is used, including, for educational purposes, also have nano-centres at universities. So, in MIREA at the Department of nano-diagnostic of materials and microelectronic devices, the structure unit of Engineering research and production center "Non-destructive testing", it is planned to train students to modern means of nanotechnology control of electronic component base. The department has an almost complete set of basic types of microscopes with high parameters that allows a comprehensive study of integrated circuits. However, due to the large volume of planned works to conduct laboratory studies can be allocated a relatively short time, so training will be most likely for informational purposes to obtain some practical skills.
Scanning probe microscopy: advantages and limitations
In principle, it is understandable why educational equipment in Russia is focused on SPM. Their principle of operation is quite simple: probe (tip) mechanically moves near the surface of the object and registers the current flowing through the surface and the probe when a small potential difference between them, or the forces acting on the probe from the surface. The resolution of the SPM is mainly determined by the tip radius of the probe. These microscopes are used to study the topology of surfaces at the atomic-molecular level and of properties of materials. SPM are small in size, do not require large operating costs, quite simple in design and relatively cheap, therefore, small companies can produce them at low capital cost.
However, scanning probe microscopy quite difficult to combine with other methods of surface investigation, therefore, are produced mainly specialized devices, although sometimes SPM work in pairs, for example, with modules of optical microscopy or Raman spectroscopy. In addition, the SPM have several disadvantages, which greatly reduce the possibilities of their use. Thus, the maximum scanning field does not exceed a few tens of microns, which corresponds to an magnification of several thousand-fold, but for a choice of place of study there is no possibility to view the object with less magnification. In addition, the image of surface can have up to 20 types of artifacts, and it is difficult to determine whether the image corresponds to reality.
According to some estimates, about 70% of the published studies are conducted on electronic microscopes, and on SPM – not more than 5%. Thus, probe microscopes cannot be considered as the main tool of modern nanotechnology, and they are not very suitable for the role of the base when creating a training and research complexes. By their nature and capabilities, scanning probe microscopy is more suitable to future technologies and manufacture at the atomic level.
Scanning electron microscopy
Originally under the nanotechnology (NT) understood atomically-precise methods, but now usually referred to them technologies to explore and produce objects smaller than 100 nm. In this area more opportunities for research and monitoring provides an electron beam, therefore, the role of the main research instrument is more suitable for scanning electron microscope (SEM).
The main tool of SEM is an electron beam, whose parameters (energy, size, current, speed and size of the scan area) can easily vary within wide limits by means of electronics. At interaction of a of high-energy electron beam with objects appear various processes and secondary radiation, effective registration of which gives a variety of information about these objects. Electron microscopes are more complex and expensive devices than SPM. The device must be provided with a high vacuum, and on the various elements of the SEM – coil magnetic lens, deflecting system and stigmator, e-gun, etc. – is necessary to apply voltages and currents of different magnitude (about half a dozen different values), some of which must be regulated in the process. In different modes of SEM can be measured practically any objects, and constraints associated only with dimensions. In SEM, practically nothing can break down mechanically, but can, as in any device with electronics, fail electronic components. The main consumable material is a tungsten filament cathode, which can be changed easily. In addition, it is necessary to periodically clean the elements inside the column, which are affected by the electron beam.
Universal SEM allow you to explore a variety of nanostructured objects. At the request of the customer they are equipped with different sets of modules set – top boxes, detectors, etc. However, these microscopes are expensive and often have a high degree of automation, which is inconvenient for educational purposes.
In most SEM main design element is a object camera. The camera and elements of the vacuum system are fixed on the frame – this basic part of the microscope has a fairly large size with high cost. If the SEM has been designed, including, for the semiconductor industry and metallurgy, and for measuring of wafers of 100-300 mm or of large samples of metal, the size and cost of design are additionally increased.
Due to the use of a large camera with holes for detectors and of multi-axis table with a large range of movement, SEM is sensitive to electromagnetic interference and vibration, so often it is necessary to place it in a special room. For universal devices is also very difficult to achieve simultaneously high settings in all modes. Therefore often choose one main operating mode of a microscope and optimize for it the focusing optics, but worsen parameters in other modes.
Hybrid nanoscopy: basic principles
It should be noted that most of the objects nanotechnology are small within a few millimeters, while some are nanoparticles. In this connection it is expedient in the development of devices to focus mainly on small objects with dimensions of a few millimeters, which greatly simplifies the design and reduces the cost of equipment. To better meet modern needs in studies of surface and structure of objects at the micro- and nanoscale in MIREA is developing a hybrid nanoscope (HN) of economy class, designed for research of small nanostructured objects using electron, x-ray, ion, probe and optical microscopy, as well as of different modes of spectroscopy.
The basic element of HN is the electron probe module (EPM), which is formed by a focused electron beam, characterized by a wide range of electron energy, currents and dimensions of the beam (probe). Power and control from a PC is provided by two blocks (about 15 cards).
EPM (fig.1) is a compact, desktop device, containing a system of magnetic lenses (column), electronic gun, the elements of vacuum system, detectors of electrons, as well as tables for small objects.
In fact, there is no objects camera in the EPM, in the traditional sense. The main element of EPM is a installed on legs column, to which are attached all other elements. This ensures access to the latest magnetic lens, which focuses the beam on the object that allows the use of removable tips, the geometry of which (the width of the gap and the diameter of the pin holes) can be optimized for different modes. In this design column and two detector are placed in the same half-plane of the object, and the second half-plane is free, so it can accommodate various detectors, scanning probe modules and optical microscopy. In some cases the user will be able to install them yourself, in accordance with their needs and capabilities.
If the object is installed under the beam, EPM works as an electron microscope with the ability to obtain the minimum diameter of the electron beam 1 nm.
The mode of transmission
x-ray microscope
If the target (a thin layer of metal on a vacuum-tight substrate) is placed under the beam, and the object and the detector are placed on the air, it is possible to conduct research in x-rays. In transmission x-ray microscope (TXM), when the electron beam is focused on the target, a surface area is created that emits X-rays. The size of the emitting region (focal spot) is determined by the diameter of the electron beam and the effective path length of the electrons in the target, which depends on the accelerating voltage and the density of the target material. With a suitable choice of these parameters it is possible to obtain a focal spot size close to the size of the electron probe. The projective and raster modes of formation of x-ray images of internal structure of objects are possible. In projection mode, the electron beam is still focused at the point of the target, and in raster mode it scans the surface.
To obtain nanometer-size in TXM-mode you should to use the accelerating voltage in the range 5-15kV. At the soft x-ray radiation nanoscale details of objects will have rather high contrast. Electronic probes and focal spot sizes in the tens of nanometers can be obtained with currents less than 10-9A. However, the yield coefficient of x-ray quanta under the influence of electron beam is on five to six orders of magnitude lower than that of secondary electrons, so there are problems with the registration of low-intensity x-ray fluxes, timely and accurate focusing of the electron beam on the target and with maintaining the focus of the beam at large times of x-ray emitting.
For efficient and accurate focusing of the electron beam on the target were asked to use a detector of secondary electrons in the space between the two last lenses [6]. When operating in SEM mode, the raster electron microscopy mode is used as an auxiliary to obtain images of the target surface in the secondary electrons. Secondary electrons from the target are registered by built-in detector that allows quickly and accurately to focus the nanoscale beam on the target, to control the condition of the surface of the target and to choose the focus point for TXM. In the result, it is possible to realize big times of obtaining x-ray images without the control of the focusing of x-rays.
To provide the level of x-ray flux sufficient for forming high-quality images with nanoscale focal spots with small currents, it is advisable to bring the detector to the target. Thus for receiving high magnifications and resolutions it is necessary to pass into the short focal mode when micron and submicronic distances between object and a focal spot (do-f) are provided. In this case, the increase and the density of the x-ray flux on object and detector increases proportionally to 1/d2o-f. The latter allows to compensate reduction of power of x-ray radiation in the transition to a focal spot in a few tens of nanometers and get resolution on the level of 20-30 nm.
In most modern x-ray microscopes do-f ranges from hundreds of microns to several millimeters. At the same time, many researched and produced nano-objects such as thin films and nanoparticles, can simply be placed on a substrate (membrane) from the air. Then the thickness of the membrane determines the minimum do-f, and optimal is design, when the electron gun is at the bottom, and at the top the membrane with the target is placed. Modern technologies allow to obtain a vacuum-tight micron and submicron membrane of Be, Si, Si3N4, C, and other materials.
Combining of methods
The proposed design of EPM is an electron x-ray microscope [7], which optimally combined two types of high-resolution devices and is possible a comprehensive study of one area of the object in different kinds of electrons and x-rays. In addition, the coordinate-sensitive detectors can be combined with detectors of x-ray analysis that will allow to simultaneously obtain information about the chemical composition of objects.
It should be noted that actually for the first time for wide application in nano-researches the high-resolution (20-30 nm) x-ray microscopy is offered. Currently, such resolution in the x-ray radiation reach only at synchrotrons with the use of expensive and complex elements of x-ray optics [8]. In addition, there are foreign x-ray microscopes based on x-ray tubes with a resolution of 50-100 nm, but these devices are expensive (some up to $1-3 million) and have fairly large dimensions.
Most of microscopy techniques give information about the topology and chemical composition of the surface of nanostructured objects. However, many of the properties of nanomaterials are determined by the distribution of particles and pores in the volume, so you should to use costly and time-consuming sample preparation (to do the splits and kinks), but in these cases it is not always possible to obtain complete information about the internal structure of objects. X-ray microscopy allows to research the internal structure in air and in liquids without destroying objects and with minimal sample preparation. In some cases, you may receive 3D or layered images. In the future we should expect the development of x-ray microscopes with a resolution on the order of 5-10 nm.
Very effective may be the combination of x-ray, scanning probe and optical microscopy. Sometimes optical microscopes are used to control movement of the probe and supply it to the specified point with high accuracy. In this case it is possible to see an edge in an optical lens and in real time to watch scanning and nano-manipulations. If to work with a probe microscope with nano-objects on a thin film, with placement of targets on the reverse side of the film it will be possible to see the tip of the probe and the object in x-rays.
Improvement and implementation
of hybrid nanoscope
Selected design of HN, in which the basic element is a column with integrated electronic detectors and a set of tables for small objects, not only allows to optimally combine different research approaches, but also reduces the cost of the device. For the manufacture of HN it is enough mechanical assembly production of the middle level with low capital costs. The use of domestic power supplies and controls that are well developed and manufactured for many years also reduces the price of nanoscope – even when manufacturing in single instances these blocks are two to three times cheaper than foreign ones.
Development of HN is conducted on a voluntary basis for several years. To date, electron x-ray microscope is well developed [7], a small experimental series are made, which tested different variants of EPM design, good preliminary results in terms of resolution are received. Images of the same object at medium magnification (fig.2) show the possibility of obtaining images in x-rays and electron beams at close levels. Thus the modes of x-ray imaging were far from optimum in parameters of detectors, substrates and test objects. In the projection mode, the imaging was carried out on x-ray film, and in raster mode – to one semiconductor detector. Now developed a modification with 20 detectors that simultaneously record images at different angles to obtain a layered images of objects.
Developed research complex provides integration of the basic types of microscopes with maximum settings for each of them. Its functionality with minimal cost can be expanded in the right directions, and the specialization of a particular device can be easily modified by replacing and/or adding individual modules. HN actually allows you to combine the capabilities of most imported microscopes of various types at a cost level of desktop electron microscopes. This high-level device is optimal for import substitution and has no analogues.
The combination of high technical capabilities with reasonable price enables to use it for solving scientific and educational tasks at the present stage of development of nanotechnology. On the basis of hybrid nanoscopes can be created nanotechnology scientific-educational laboratories and workshops. The best may be a set of three to five hybrid nanoscopes with a mixed set in other modules (probe microscope, optical detectors of different types, and others). On one HN, you can research objects in different kinds of electrons, on the other to combine electrons and x-rays, on third to combine x-ray, scanning probe and optical methods. If the combinations of all kinds of microscopes in one set is necessary, you can gradually build up the capacity of HN, complementing it with additional modules.
In practice, it is not necessary to have a complete set of power supplies and controls for each EPM. Because the vacuum system is made on the basis of magnetic discharge pump, for which only for the period of start the booster pump is necessary, it is possible to use a single pump for a few EPMs. Since it is unlikely that all EPMs were be used at the same time throughout the working day, for several HN you can use one power source and to change the cables connecting of EPM with blocks.
HN (fig.3-4) are already used for education in MIREA. So, in the summer on these devices students of the first courses did practical training, and now two students pass externship with the subsequent implementation of the diploma thesis. Initially, they will perform a complete assembly of EPM, including the winding coils of the lenses and deflection systems, mechanical assembly of magnetic lenses system, of vacuum and detector systems, obtaining the vacuum, connection of power supply units, ensuring the transmission of the electron beam through the lens, obtaining first images of objects, the preliminary alignment of the lens system, the optimization of modes of electronic detectors and obtain the limiting parameters for the test objects. Then each of them will conduct research on nanostructured objects. As a result, they will become in practice familiar with the design of the instrument and learn the methods of work on it.
On the basis of manufactured HNs will operate the scientific-educational class, largely satisfying modern requirements for training and research centres. The value of such a class would be higher with the inclusion of probe and optical microscopes as well as x-ray detectors. Vacuum prototype of EPM to work in conjunction with an electron beam, as well as atmospheric prototype to work in conjunction with TXM are fundamentally already worked out.
With some support from the state the results of the development of HN can be used more widely, for example, for production of instruments within the fablabs – industrial laboratories in universities – to attract students, which contribute to the development and production of marketable innovative products and service. Thus, it will be possible not only to teach students, but also to solve the problem in production of domestic research equipment for science and industry.
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