New solutions for materials science, comprehensive research and testing of materials and structures with high spatial resolution
SCANNING PROBE MICROSCOPY
Scanning probe microscopy (SPM) began to develop in 1966 as a result of the works of Russell Young and colleagues at the National Bureau of Standards, USA. Group of researchers at the IBM’s Swiss laboratory, which was led by Gerd Binnig and Heinrich Rohrer (Nobel laureates of 1986), has begun to develop it as one of the main methods of research of nanostructures. The development of SPM has become possible with the advent of personal computers as systems for control of devices, and also for collection and processing of the results.
The first key innovation proposed by Russell Young and colleagues was the use of piezoelectric ceramics for the implementation of displacements of the tip and sample relative to each other.
G. Binnig and H. Rohrer showed that with the help of scanning tunneling microscope, which construction was similar to the Yang’s topografiner, it is possible to image individual atoms. The instrument control and data processing have begun to use personal computers. Personal computers began to be used for controlling the device and data processing. Progress in the capabilities of the devices to date is largely determined by the capacity of computers.
In 1986, the same group of researchers has developed a tubular piezoceramic scanner. To register the surface topography it was proposed to use flexible beam with a sharp needle at the loose end, and devices with such cantilevers were called atomic force microscopes (AFM). AFM proposed by G. Binnig, C. Quate, and Ch. Gerber used a tunneling sensor to register normal movement of the cantilever, which is extremely inconvenient. A crucial innovation that has made AFM a reality, was the invention of G. Meyer and N. Amer – an optical positioning circuit, which with use of four-sectional photodiode allowed to register both normal and lateral interaction forces of the probe with the sample surface.
Innovation, preventing the capillary effect, was proposed as the fundamental possibility by G. Binnig in 1986, and W. Ducker, R. Cook and D. Clarke showed its real implementation. This system was integrated in the first commercial AFM by Virgil Elings as "tapping" or semi-contact mode.
In 1987, a group of researchers has proposed to manufacture the cantilevers using silicon technology, allows selective etching of grooves and needles with the angle at the vertex determined by properties of the crystal. The thickness of the beams was regulated by either the coating thickness or depth of doping with boron or phosphorus. The ability to manufacture cantilevers with use of industrial technology has made it an affordable consumables and has provided a possibility of wide use of the method.
In the late 1980-ies – early 1990-ies, the possibilities of use of SPM for registration of a number of physical properties of surfaces in various conditions, from ultrahigh vacuum to research at the interface of solid / liquid, and for surface modification (so-called SPM lithography) were shown. Currently, these modes are integrated in the most of the SPM-devices under different names, that does not change their physical nature.
In the late 1980-ies the works on creation of SPM started in the USSR. The MDT corporation has been founded, which has begun to develop the SPM with lithography capabilities for both study and modification of molecular structures, including Langmuir–Blodgett (LB) films. Fig.1 shows the first scanning tunneling microscope designed by MDT.
The corporation started a number of high-tech projects, the results of which can be found on the shelves of pharmacies (for example, Vetoron – the molecular-capsulated form of beta-carotene), in university labs, in high-tech companies around the world (SPM, NTEGRA Spectra spectral nanotechnology systems, SOLVER Pipe devices for surface inspection of large objects) and even in school classes and educational labs.
By 1995 an AFM was created, and by 1997 – the multi-mode devices (Fig.2) for comprehensive non-destructive study of surface structures, including the LB films.
Fig.3 shows the topography of the LB films of cadmium arachidate. Instead of smooth molecular structure, which was expected to see according to prevailing notions, the pronounced domain structure is observed. It turned out that proposed by Langmuir and Blodgett model of the structure of the films has nothing to do with reality. Method of formation was erroneously identified with the real molecular structure, but atomic force microscopy helped to understand that the Langmuir-Blodgett technique is just a method of supplying construction material, of surface active molecules, and the type of obtained structures depends on the nature of these molecules, and not on the way of applying them.
Development of the device combining the capabilities of the SPM and spectrometer began in 1998 with interaction with the Tokyo Instruments. Japanese colleagues needed the scanning microscope operating in the Raman scattering mode, and they turned to our company. The creation of such a microscope (Nanofinder) initiated the development of a whole family of devices combining the functions of scanning probe microscopy, Raman and luminescence spectroscopy, which led to the creation of NTEGRA Spectra system.
In 2003, NT-MDT took the second position among the 500 participants of the mega-projects competition, organized by the Ministry of industry and science of the Russian Federation. Significant investment has allowed the company to develop a range of competitive devices (Fig.4) and to be among the world's best manufacturers of this equipment. Devices were installed in 58 countries.
To date, scanning probe microscopy has become one of the classic methods of research of nanostructures and is widely used for qualitative assessment of physical-chemical properties and geometrical parameters of surfaces. However, it is considered that it requires special knowledge and skills to obtain the data and for their interpretation, which hinders wide dissemination of the method.
In addition to topography, SPM allow to measure a number of physical properties:
• distribution of the surface electric potential;
• distribution of surface conductivity;
• distribution of capacitance of the "probe – surface" system C (x, y) and dC / dz, dC / dV;
• distribution of magnetic forces in the "probe with a given magnetization – surface" system;
• piezoelectric properties of materials and their spatial distribution with the ability to register phase transitions during heating and cooling;
• distribution of thermal conductivity;
• distribution of mechanical properties (Young modulus, hardness);
• distribution of adhesiveness.
Measurements can be made on the air, in a gas atmosphere of predetermined composition, in liquid, in low, high or ultrahigh vacuum with the possibility of heating and cooling in the temperature range from liquid-helium to 1 000 ⁰C.
To implement the listed above capabilities a various constructive solutions are offered. In particular, during the implementation of the mega-project, NT-MDT has developed research complex NTEGRA, which is similar to the "assembly kit" with ready units (Fig.5) and is easily adaptable to user requirements with possibility of further upgrade by installing new controllers or additional systems to solve specific problems. NTEGRA is offered in two versions. NTEGRA Prima is a multifunctional system for operation in air and in controlled atmosphere. NTEGRA Aura is designed for measurements in vacuum conditions. The level of vacuum depends on the type of dry vacuum system. It is possible to obtain pressure down to 10–5–10–6 torr, but usually it is limited to 10–3–10–4 torr at which the quality factor of the cantilever does not depend on the level of vacuum.
NEW AFM MODES
In 1993, to minimize the influence of lateral forces, a jumping mode has been offered, in which during scanning the force dependence on the distance could be measured at the cyclical supply and withdrawal of the probe to / from the surface.
Until recently, registration of force curve in each point of scanning took too much time and jumping mode was not demanded. But at the present time, the relevant circuitry became available, and versions of jumping mode with the measurement of the force curve were implemented in Peak Force mode by Bruker and in HybriD Mode (HD-AFM) by NT-MDT Spectrum Instruments [5–7]. HD-AFM allows to investigate the topography, stiffness, potential distribution, adhesive forces at normal for SPM scanning frequency of 1–2 Hz. At the same time, measurement are greatly simplified, it is possible to automate settings, which drastically reduces the staff skill requirements. The introduction of new technology significantly changes the consumer properties of the SPM, giving them a new quality of instrumentation for wide use for quantitative characterization of nanostructures.
Implementation of so-called multi-pass methods (often is limited to two passes), when during the first line scan, for example, topography or dependence of force on distance (hybrid mode) are measured, and repeated scan along the same trajectory is carried out, for example, in the surface potential or magnetic interference measuring modes allows to measure a whole group of surface physical properties during a single scan. Fig.6 shows the topography and the elastic modulus measured in the HD mode, and the surface potential measured during second scanning pass. Thus, you can easily get information about different surface properties in the stability conditions, which dramatically reduces the possibility of incorrect interpretation of results.
The development of hybrid mode has made possible the development of non-resonant jumping piezoelectric force microscopy, allowing to measure the piezoelectric properties of brittle materials with a small adhesion to the surface, for example, of diphenylalanine nanotubes .
NEW DESIGNS OF SPM
The development of HD-mode and cartridges greatly improve and expand the capabilities of the automated SPM, which led to the creation of NEXT-II, TITANIUM and VEGA. Their design is close to the SOLVER Next, but the internal design features has made possible the integration of cartridges and HD-mode. New VEGA allows to work with 200 mm wafers obtaining atomic resolution that is provided by the excellent resonance characteristics of the device in combination with a system for thermal and acoustic stabilization .
It should be noted that SPM are highly sensitive to external acoustic noise, temperature changes, humidity changes, which lead to drift of the probe relative to the sample, uncontrolled displacements and failures during the scanning. To minimize these factors, it is necessary to use devices inside the acoustic-protected enclosures with active or passive vibration protection and systems for temperature and humidity maintainance. Providing all of these conditions is currently not expensive task, and it is solved for the whole range of SPM of NT-MDT Spectrum Instruments (Fig.7). Drift less than 10 nm/hour allows to obtain atomic resolution even at relatively low frequencies of scanning.
SCANNING PROBE SPECTROSCOPY WITH NANOMETER SPATIAL RESOLUTION
Integrated SPM and spectrometers that combine the techniques of high resolution measurements of the topography and of various physical properties of the surface structures, have a powerful development. The devices allow to obtain information both about the physical properties of the surface structures and their qualitative composition using luminescence spectroscopy, Raman spectroscopy and IR spectroscopy with high spatial resolution.
The development of these methods began in 1998 with the development of Nanofinder scanning Raman spectrometer (NT-MDT and Tokyo Instruments), after which NTEGRA Spectra, a combination of SPM and Raman spectrometer, was created. In 2004, came the idea of using of light concentration effect on the tip of a needle made of materials with plasmonic spectra in the visible region (Tip-enhanced Raman spectroscopy, Renato Zenobi, Switzerland), which provided the possibility in principle of registration of Raman scattering from single molecules [6, 9]. This gave new impetus to the development of the devices, and in 2006, NTEGRA Spectra (Fig.8) was included in the top hundred of the best developments according to the Research & Developments magazine.
Currently, cantilevers with special coating of the tips made of gold-silver alloys are created, providing signal amplification of Raman scattering in TERS mode up to 104, which allows high spatial resolution when measuring molecular objects. Using the HD-AFM modes provides high-quality reproducible results.
The first versions of the devices for Aperturless Scanning Near-Field Optical Microscopy (ASNOM) with lateral resolution up to 10 nm are created. Currently, the CO2 laser with the Michelson interferometer is used as a source of IR radiation with the possibility of tuning the wavelength in the range of 10.3–10.8 µm. Probes with a conductive coating are applied to initiate scattering. The system allows to register the inelastic scattering caused by interaction of radiation at the approaching of the probe with the sample, modulated by frequency of the probe on the background of the reflected laser signal. Such systems allow to record changes in dielectric conductivity of the samples and signals of inelastic interaction caused by the excitation of vibrational modes of molecules on the surface of the sample. Further development of the devices with the capabilities of atomic force microscopy and spectroscopy involves combining methods of AFM, fluorescent and Raman spectroscopy and ASNOM with the extension of the spectral range of the latest by the use of cascading lasers that will give the opportunity to obtain comprehensive information about topography, physical properties of surfaces and chemical composition of the surface layers.
SPM IN SCHOOL
AND UNIVERSITY EDUCATION
Training of professionals is a critical task for the development of modern technology. This process should start at school. The ability to see and to influence the molecular structure dramatically changes and enhances the depth of understanding of physics, chemistry, biology. The idea of development of this market segment began in 2000 and was financially supported by the Fund of assistance to development of small forms of enterprises in scientific-technical sphere (now Foundation for the promotion of innovation), which enabled the development of NANOEDUCATOR SPM. Fig.9 illustrates the capabilities of the latest version of the device, which is used in dozens of training classes in Russia and other countries. This device was among the world's best developments according to the Research & Developments magazine in 2011.
Currently, Quantoniums specializing in nanotechnology, and Sirius educational centers are equipped with such devices.
OF CANTILEVERS FOR AFM
Depending on the type of samples and measurement modes it is required to use different types of probes with different stiffness, coatings, parameters of tips. Replacement of the probe also requires special training, which complicates the use of the device. Slightest carelessness may damage the probe, the price of which can reach several hundred dollars.
Development of MEMS manufacturing technologies significantly increased the cantilevers yield (almost 100%) with repeatability of the frequency resonance characteristics of beams better than 10%, which created the preconditions for the implementation of a multi-probe cartridges for AFM.
Cartridge (Fig.10) is a multi-probe contour-type sensor with a diameter of 8 mm, containing 38 cantilevers. The choice of the current cantilever is controlled by software with optical control. The cartridge is replaced manually, which is not a complicated procedure. A special measuring heads are developed for cartridge, which are integrated into the new devices of NT-MDT Spectrum Instruments.
Any modern device is a combination of precision mechanics, electronic unit (controller), electronic devices which are built into the mechanical unit, and software. The functionality of devices depends on the quality of all the components of the triad "mechanics-electronics-software" the most dynamic of which are software and electronics that are very closely related to each other. The basis of electronic devices is the circuitry, permanent progress of which makes it necessary to improve controllers and software periodically (with a period of about three years). That's what caused the obsolescence of equipment and development of new technologies for the realization of ideas, constantly generating in scientific community. It should be noted that scientific instrument engineering, by definition, requires the use of the most modern element base and components of the best world manufacturers.
The market for each type of scientific devices is determined by the capabilities and number of research groups, and for separate country is very limited. On the other hand, the small volume of production leads to significant higher cost and lower quality, i.e. low reliability, errors in the software. That is why manufacturers of scientific instruments can work, develop, competing with each other, only on the world market, which is at the same time a source of ideas for new products. On the one hand, this caused that the market for scientific instrumentation is unsaturated, and on the other hand leads to the constant need for new projects. The competitiveness of companies is largely determined by the strength of their teams of researchers and developers.
Rapid obsolescence of devices makes extremely important the rapid launch of the new products to the world market, which is only possible with effective advertising policies and a strong system of sales and service support. Unfortunately, the domestic policy of the customs control complicates this process and makes necessary the opening of service centers for international users outside of Russia. In addition, a number of European countries require work permits for the engineers installing the devices. All of this creates an additional burden for the budget of the companies and reduces their profitability. Another problem for development of business of Russian companies is the high cost of borrowed funds and the complexity of their involvement even in the presence of the contracts. Russian banks do not accept contracts as guarantees for attraction of credits, and company-suppliers require prepayment for the execution of contractor's works. In spite of this, a number of Russian companies are successfully developing in the sector of scientific instrument engineering, although their growth and position could be significantly higher when the solution to the above issues.
Consistent innovative development of scanning probe microscopes has allowed repositioning of these devices, significantly reducing user requirements. The new devices of NT-MDT Spectrum Instruments can be used by technicians, engineers to control the technological parameters, materials scientists, the goal of which is to obtain well-interpretable information on the physical and physical-chemical characteristics of the object.