Hybrid nanoscope – versatile device for laboratories of various profiles
Most of the researches in the field of nanotechnologies are performed using scanning electronic microscopes (SEM). Universal (with different add-on devices), high-resolution SEM are basically used having large dimensions and high cost. Chambers and tables for objects in many microscopes are designed to work with samples that have dimensions from tens to hundreds of millimetres. SEM have narrowly specialized design and are not intended to investigate a wide range of materials in different modes. The series of desktop SEM with low resolution (about 20 nm) are available, which are mainly focused on certain modes of work with objects of a few tens of millimetres. With moderate cost and small dimensions, the resolution of scanning probe microscopes (SPM) operating at low scanning fields and high magnification is quite high. Electron and probe microscopes can effectively explore the surface of objects, but many properties of a large part of nanostructure materials are related to the internal structure. Chips and fractures prepared according to certain procedures are often used to get information about the internal structure. Also it is possible to obtain information about the internal structure of objects using layer by layer surface etching by the ion beam. But these destructive and expensive methods do not give full and operative information about internal structure of a studied sample. There is a number of problems, related both with the preparation of objects to be researched in these microscopes, and with the interpretation of obtained results.
Modern research needs of materials surface and structure at the micro- and nanoscale level are beyond the scope of one research method. One and the same microscope cannot be equally adapted for use with all variety of objects.
Combination of methodologies allows obtaining detailed information on the chemical composition and structure with sequential use of two or more methods without changing position of a sample. Other microscopes are frequently inserted as add-on devices in the base microscope, wherein the mode of operation of additional microscopes is far from optimal.
Operating principle and design of hybrid nanoscope
The hybrid nanoscope (HN) of economy class was designed to study the surface and structure of objects at micro- and nanoscale level, which optimally combines various types of microscopes and spectrometers adapted to operate in vacuum and in air.
Scanning electronic microscope (SEM) in the desktop version is the base microscope of the hybrid. The key element of HN is the electron probe module (EPM), which consists of the column with the electron beam projector and elements of the vacuum system (Fig.1). The column consists of magnetic lenses with beam-deflection systems inside. The objects chamber is traditionally the primary element of the SEM design. The remaining elements of the microscope are placed on the chamber, and the chamber along with elements of the pumping system is fixed to the frame of large size. Due to the large size of the chamber with holes for detectors and multi-axis table for objects with the large movement range, the SEM is sensitive to electromagnetic interference and vibration, so in many cases it must be placed in dedicated rooms. The significant disadvantage of universal complexes is that it is very difficult to provide simultaneously the boundary parameters in all modes. Frequently it is necessary to compromise: when developing the complex, the basic mode is selected and the focusing optics should be optimized for it.
In general, the column that provides the electron beam focusing is the primary functional element of SEM, and just it was made the basic element of EPM design for developed HN. Magnetic lens are fixed by two steel plates, using pins, located at the ends of columns with open access to the electron beam projector and to the objective lens (OL), which focuses the electron beam on an object. The plates have slots with the possibility to install two overlapping steel U-shaped screens into slots to shield the column against electromagnetic interference. If additional shielding is required, OL is closed with casing as well. Legs may be attached to any of the plates for mounting on the table, placing the electron beam projector at the bottom and the OL with an object at the top; or traditionally for electronic microscopy when the electron beam projector is at the top. As basic it was selected the option, when the electron beam projector is located at the bottom, and the objective lens – at the top.
Picking of secondary and backscattered electrons is performed through the objective lens by using built-in detectors in the column. The maximum density of the electron probe in the used range of energy and size of the probe is achieved by means of optimum focus lens system . The space in the upper half plane above the OL is free, and removable tables and objects chamber, electronic and X-ray detectors, microscopes and other devices can be placed behind it. In principle the objects chamber in the traditional sense can be absent, but if necessary, the chamber can be made with a table for certain objects and modes of research and EPM installed on it. To move small (a few mm) objects the set of vibro-damped tables is used, including for transmission microscopy. HN design provides high protection level against mechanical vibration and electromagnetic interference.
HN passes into the transmission X-ray microscope mode (TXM) during target (thin metal layer) installation under the electron beam on the leak-tight substrate, which let pass X-rays on the air to an object and to X-ray detectors. When the electron beam focusing, an area emitting X-rays is created on the target surface. The size of the emitting area (focal spot df) is determined by the electron beam diameter and by effective length of electrons path in the target, which depends on the accelerating voltage and density of a target material. With proper selection of the accelerating voltage, target’s material density and thickness, the focal spot can be obtained with a diameter close to the diameter of the electron probe. The detector of secondary electrons from target is used for accurate and operative electron beam focusing on a target, since with low x-ray intensity it is practically impossible to focus a Nano-sized electron beam . The X-ray microscope operates in projection mode when the electron beam stands at a point on the target, and the X-rays passing through an object are registered by coordinate-sensitive detector (Fig.2). Also, when scanning the beam over the target and using X-ray detectors with X-ray changing input aperture on the detector, it is possible to receive the scanning X-ray image of an object, which resolution is determined by aperture of the detector. The use of multiple detectors allows obtaining of several images at different angles, which when applied provide three-dimensional images with high resolution. The hybrid detector can be used with coordinate-sensitive detector for the projection mode on the axis and on each side – detectors at different angles record image in scanning mode. At the same time energy-dispersive detectors may be among them.
The short focal mode  is used for obtaining nanoresolutions in X-rays, when using micron and submicron substrates for targets, reduce to the maximum the distance between the focal spot (electron beam on the target) and an object. Samples can be placed directly onto the target’s substrate. Herewith, the flux density of X-ray radiation significantly increases on the object and detector, compared with conventional for X-ray microscopy distances object – focus of the order of hundreds of microns. It compensates reduction of intensity of the X-ray source at nanoscale level focal spots. Modern technologies allow obtaining leak-tight, micron and submicron membranes of Be, Si, Si3N4, C, etc. So, RMT ltd commercially produces beryllium windows with diameter ≈ 6 mm and thickness of 8 microns. It is also possible to use silicon windows with thickness of a few microns or submicron windows of silicon nitride film. For film thicknesses of 0,1 microns electrons with energies of 10–30 keV pass into the air to objects and with registration by special detector of backscattered electrons from an object that passing through the film back into the vacuum, the mode of atmospheric SEM is realised as in Japanese microscope JASM-6200 with the window of silicon nitride of 250 × 250 micrometers (Fig.3). Basic characteristics of HN are shown in Table.
Furthermore, probe, optical and confocal Raman microscopes can be placed in HN, as well as energy-dispersive X-ray spectrometers with specially selected parameters. Moreover, it would be optimal to have probe and confocal Raman microscopes in the vacuum version, operatring in conjunction with the electron beam, as well as the same microscopes in the atmospheric version, operating in conjunction with backscattered electrons and X-rays. Basic types of electron and X-ray microscopes that HN can replace by main parameters and functions are shown in Fig.3.
HN provides resolution much higher than the desktop SEM operating at fixed modes, with approximately the same dimensions and cost. Compared with multifunctional SEM, HN has slightly higher the limit resolution and more functionality at 2–3 times smaller size and cost. HN has the mode of backscattered electrons passing from the object in the air, like a Japanese atmospheric SEM with the optical microscope JASM-6200, which only with these modes costs about 1 mln. USD. Like the Czech desktop microscope, HN has scanning mode of transmitted electrons, not just at 5 kV for biological objects, but in the range of 0–40 kW. At the same time they have similar overall size and cost. X-ray microscopes have only one X-ray mode with large overall size, high cost and resolution about 50 nm.
When comparing with the prototype, it becomes obvious that HN significantly surpasses them in functionality combined with maximum resolution parameters at a minimum cost. HN is unique: it optimally combines study of surface and internal structure (X-rays) of objects in combination with spectroscopic data on chemical composition. Advantages in image informativeness in the X-ray are seen by comparing images of various size zinc particles in the organic film of 270 microns thickness in transmitted X-rays and in secondary electrons in SEM (Fig.4). By picture it is possible to assess the resolution of particle with the size of 0,2–0,3 microns. A higher resolution can be expected in passing to high quality X-ray detectors, thinner substrates and objects with nanoparticles sizes about tens of nanometers. Pictures of rhenium films (Fig.5) show the possibility of obtaining similar in quality images in X-rays in Projection X-rays Microscope and in electron beams in SEM.
Prospects of HN application
The same object space with dimensions of several millimeters can be investigated in HN in different combinations using electron beams, X-rays, the tip of the probe microscope, light, and spectral detectors. HN has modular design that provides optimal (with high resolution settings) combination of basic types of microscopes and spectroscopic detectors. Its specialization changes with minimal costs by excluding and (or) adding individual modules. The device in complete package has the wide range and operates in the magnification range from a few to several millions with micron or atomic resolutions respectively.
With development of the nanoindustry, it is growing the number of companies and organizations involved in research, not only developing nanostructured materials, but producing and using them. Therefore, there is a need for control and measurement devices at affordable prices and in various versions for solution of specific production tasks. Designed analytical device due to its technical and economic characteristics may become the basic tool for national nanoindustry. It is simple in design, management and operation. HN can be easily modified in versions adapted to operate with various nanostructured materials.
HN development on the initiative order has being performed for several years and it focuses on improving the design of EPM [3, 4], providing optimal formation of nanoscale electron beams in the range of 1–40 kW. Small experimental series (Fig.6, 7) was produced for testing different options of EPM design. Good preliminary results were obtained on resolution, both in X-rays and in electron beams. The scientific and educational center is formed on the basis of three HN with combined modular assembling. It is possible to explore objects in different kinds of electrons, using one HN, in the other HN – combine electrons and X-rays, in the third – combine X-rays, probe and optical microscopes. At the same time it is possible to increase gradually capabilities of devices, completing them with additional modules in the process of use.
It is expedient in cooperation with different organizations to perform works on the study of their objects using HN. In principle, it will be necessary to optimize parameters of HN, especially for X-ray mode for various types of nanostructured materials depending on their structure, chemical composition and thickness. According to the results of joint researches it would be possible to assess capabilities of the new device in various areas of science and technology and to manufacture devices for specific orders. Thus, for optimum performance with organic objects HN has sufficient range of accelerating voltages of 1–15 kV, which will allow significant reduction of the device overall size and cost.