Vacuum electronics: renaissance or stagnation
Vacuum electronics is based on thermionic emission (Edison in 1883, Richardson in 1901)  and field emission (Lilienfeld in 1922, Fowler and Nordheim in 1929 )  – abilities of materials to emit carriers in vacuum, in particular, electrons. The St. Petersburg school of vacuum electrical engineering and electronics  began to develop from the end of the XIX century in the walls of the Electrotechnical institute (ETI, LETI, now – ETU "LETI") in cooperation with the Aivaz factory, which since 1913 produced incandescent light bulbs under the Svetlana trade mark. In 1928, Electrovacuum Plant Svetlana was created on the basis of Aivaz for the production of receiver-amplifier and generator lamps. S.A. Vekshinsky, the chief engineer of the Svetlana, used the experience gained in the development and production of vacuum electronic devices in organizing the Research Institute of Electro-Vacuum Equipment in 1943 (now – Istok RPC named after Shokin) , whose activities determined the development of a number of up to the present time demanded, relevant and in fact unique industrial devices for the generation of microwave radiation.
Along with the presentation of developments in the field of vacuum electronics implemented at the Department of Radio Electronics and at the Center for Microtechnology and Diagnostics of ETU "LETI", the purpose of this article is to consider the following issues:
• design and technological support for creating devices with previously unattainable parameters;
• role of micro- and nanoscale factors in the evolution of micro-power vacuum devices;
• demand for vacuum microelectronics and its competitiveness.
PHYSICAL AND TECHNOLOGICAL PROBLEMS OF VACUUM ELECTRONICS
Characterizing the current state of experimental developments in the field of vacuum microelectronics, it should be noted that for a set of parameters, including speed, the quality criterion (the product of output power, operating frequency and frequency band), the limiting operating frequency, noise level, resistance to radiation, temperature and electromagnetic influences, devices of vacuum emission electronics must exceed solid-state functional analogues. Let's define a number of actual physical and technological problems.
Providing efficiency and stability of cathodes, despite their diversity (dispenser, barium, scandate, tungsten, scandium-doped, carbon), requires research in the field of materials, structures and emission physics.
Losses in the retarding system during the transportation of electron beams determine the priority of providing the required aspect ratio of the retarding structures (looping, counter-pin waveguides) and, especially, the quality of their surfaces (roughness not worse than 30–50 nm). It should be noted that in the transition to the use of retarding structures of small length (of the order of 2 cm) with increasing generation frequency, the requirements to an increase in the current density with increasing cathode brightness and to electron beam formation systems, taking into account its scattering on the surface, sharply increase.
The introduction of new microtechnologies for the formation of waveguides with a developed geometry and low surface roughness is based on replacing the traditional spark erosion treatment with deep ion etching or the so-called LIGA technology based on thick-resist lithography using synchronous radiation followed by galvanic growth of retarding structures in a polymer matrix. To form a developed volumetric channel of complex geometry with a slowing 3D structure, it is possible to use the method of thermal or anodic splicing of two pseudo-planar channel parts with their precise alignment.
The creation of focusing systems with the magnitude and distribution of the magnetic field necessary for effective electron-wave interaction requires the design and materials science solutions with a high level of the focusing magnetic field, while minimizing its value outside the region where the beam passes. At this stage, the processes of numerical analysis dominate on the basis of modeling of nonlinear electron-wave interaction.
When a vacuum channel of a field effect transistor with ballistic transfer is formed, along with the use of a focused ion beam (FIB) for ultra-local removal and ion-stimulated deposition of a material, a technological realization based on the so-called "sacrificial" layer, which is widely used in the creation of microsystem objects, is possible.
Functioning of a miniature device with a set of functional 3D geometrically developed emission, transport and collector subsystems requires the achievement and long-term preservation of a sufficiently high level of vacuum. In the presence of vacuum micro- and nanocannels, that is, evacuation of heterogeneous 3D objects of extremely small sizes, along with the traditional pumping problems, there is a restriction on the use of sprayed getters. Reducing the requirements for the level of vacuum in micro- and nanoscale emission devices is predicted when the characteristic dimensions decrease to the level of tens of nanometers.
OF ETU "LETI"
Autoemission cathodes based on silicon carbide and diamond
Matrix field emission cathodes based on silicon carbide are considered as promising sources of electrons for vacuum microelectronic devices. Despite the fact that silicon carbide have a number of significant advantages in comparison with traditional materials of auto-cathodes, only a few laboratories in several leading countries are developing its technology.
The paper  presents the results of the formation of a matrix of nanoscale points with a reduced scatter of their heights. The results of the measurements show that, at high electric field strengths the inhomogeneity of the tip heights is a key factor of current instability.
One way to overcome this instability is the use of diamond films, since the diamond is recognized as one of the best materials for obtaining auto emission. A serious technological problem with the use of diamond is the creation of points with a high aspect factor. It can be solved by applying a nanocrystalline undoped thin film (0.5 μm) to a matrix autoemission cathode of silicon carbide. The film "smoothes out" the virtual emitting surface that connects the points, reduces the spread of heights and increases the stability of the current.
Fig.1 shows a matrix auto-emission cathode formed on a single crystal 6H-SiC wafer by two-stage reactive ion etching in a fluorine-containing atmosphere. At the first stage, a regular structure of the pedestals was formed, at the second stage – a nanoscale tips.
The obtained autoemission cathodes were placed in experimental diodes with an interelectrode gap of 40 μm. The diodes were preheated at 200°C for 50 minutes, and were evacuated to a residual pressure of 7.5 ∙ 10-8 Pa. The current characteristics were studied for 48 hours at an anode voltage of 3.75 kV. Fig.2 shows the experimental plots of current density versus time for two diodes. In a diode with an autoemission cathode without a diamond film, during the first 4 hours, the current density increased by 19%, but by the end of the observation period it decreased by 23% relative to the initial value. In a diode with a diamond film, the current density fluctuations did not exceed 5% during the entire experiment.
Thus, an autoemission from the surface of a nanocrystalline undoped thin film on a silicon carbide cathode was experimentally demonstrated. The formation of the combined structure stabilized the emission properties due to the formation of a chemically inert emitting surface, free from unstable sorbed contaminants.
Millimeter rage traveling-wave tube
for 5G wireless communication systems
The paper  presents the results of designing certain components of W-band traveling wave tube with a 2 GHz working band, an accelerating voltage of 6 kV, a gain of 25 dB and an output power of 6 watts. The design was carried out taking into account the possibility of using the results obtained at higher frequencies, in particular, a ribbon electron beam and a retarding system in the form of a looping waveguide was used (Fig.3). Basic parameters of the TWT node configuration are obtained.
The total length of the retarding system was 28 mm (40 periods). The width of the wide wall of the waveguide is 2.2 mm, the narrow wall is 0.1 mm in size. The dimensions (WЧH) of the transit canal are 1.2Ч0.1 mm. The instantaneous bandwidth of the amplified frequencies, determined at –1 dB, for all values of the accelerating voltage exceeded 3 GHz. The change in the accelerating voltage by ±0.2 kV ensured the tuning of the central frequency in the range from 92 to 102 GHz while maintaining a gain of more than 25 dB.
The electron gun was designed on the basis of a thermionic impregnated cathode with an allowable current density of 20 A/cm2. The coefficient of linear compression of the beam was 2.5. As a prototype, the Mueller gun was used. The possibility of using in the instrument of an autoemission cathode based on silicon carbide with a current density of up to 10 A/cm2 was considered.
The magnetic system was designed on the basis of a single-period configuration of rectangular cross-section bars of permanent axially magnetized magnets in SmCo with an energy of 15 MGOe. The polepiece gap was 0.33 mm, their thickness was 2.5 mm. The axial value of the induction B0 = 0.25 T was achieved. The magnetic system was able to fit into a volume of 30Ч15Ч11 mm.
The results of designing a W-band amplification klystron with an output power of 1 kW, a working frequency band of 0.38 GHz, an accelerating voltage of 10 kV, and a gain of 50 dB are presented in . The problems of choosing the accelerating voltage, the shape of the electron beam, and the working mode of resonator oscillations are considered.
The model of the projected klystron is shown in Fig.4 (without gun and collector). To implement the grouping of electrons, a tuning system is used: the first resonator is tuned to the lower edge of the band of operating frequencies, the intermediate resonators – to frequencies above the operating frequencies of the klystron, the output device – to the center of the band. The tuning system is sharply asymmetrical relative to the center frequency, but the final frequency characteristic of the output power can be aligned by increasing the resistance of the input resonator or by tuning the second resonator closer to the first within the band.
To simulate the process of interaction of electrons with a high-frequency electromagnetic field, the large particle method was used. Fig.5 shows the positions of large particles at a fixed time in a steady state (simulation time of 15 ns) at a frequency of 94.2 GHz in the output part of the klystron. The beam current was 0.7 A, the input power was 5 mW, the total number of particles was 46,485.
Amplitude and amplitude-frequency characteristics of the klystron, obtained as a result of the simulation, are shown in Fig.6. At a working frequency of 94.2 GHz with a beam current of 0.8 A, the output power reached 1070 watts. The efficiency at this point is 13%, and the gain factor is 50 dB. The width of the gain band at –3 dB is 380 MHz, which is somewhat less than the preset.
The design of the electrodynamic and electron-optical systems of the W-band amplification klystron with an output power of 1 kW has shown that the selected models of components allow to provide the specified parameters with the exception of the bandwidth of the amplified frequencies. The expansion of the band can be achieved by using passive resonators.
ON ROLE OF MICRO- AND NANOSCALE FACTORS IN EVOLUTION OF VACUUM MICRO DEVICES
The new functional capabilities of vacuum emission micro- and nanoelectronics devices are determined by the following factors:
• super-small time (10-9 – 10-12 sec) of processes (the minimum mean free path, the propagation velocity of an electromagnetic pulse in a vacuum of 2.9979 · 108 m/sec);
• ultra-small capacity due to micrometer sizes of structural elements and low value of relative permittivity of vacuum (ε = 1);
• high inductions of magnetic fields under conditions of extremely small dimensions (a weak manifestation of the effect of the magnetic field decay from distance – about 1/R6);
• ultrahigh electric field strengths (more than 108 V/cm) due to extremely small inter-electrode distances and nanoscale local curvature of structural elements, for example, cathodes;
• ultrahigh current densities (more than 108 A/cm2) due to the possibility of nano- and micro-localization of emission processes;
• high densities of elements (more than 108/mm2) due to the possibility of their implementation on the basis of integrated group micro- and nanotechnologies.
Along with this, there are the following possibilities:
• improvement of the power/volume index, reduction of the mass-dimensional characteristics of power supplies, reduction of operating voltages;
• improving the efficiency of controlling the current transmission and amplification of signals by means of magnetic fields in the conditions of their localization in extremely small volumes;
• implementation of structures that ensure the functioning of micro devices with reduced requirements for the depth of the vacuum;
• effective use of integrated-group microtechnologies of solid-state electronics and microsystem technology in the creation of miniature vacuum devices;
• integration of solid and vacuum micro devices into products with previously unattainable functional parameters.
Progressive design and technological solutions in the field of development of emission systems allow to solve a set of problems in the development of non-traditional micro- and nanotechnology of a new generation using the creation in small volumes of local zones of ultrahigh temperatures, creation and local confinement of plasma in ultra-small volumes, as well as stimulation of super-localized (including topologically ordered) X-ray radiation.
DEMAND FOR VACUUM MICROELECTRONICS AND ITS COMPETITIVENESS
Current trends and possible markets for vacuum microelectronics are determined by the demand for devices of the short-wave part of the millimeter range with an output power of tens of watts and more. A generalized structure of possible applications for such devices, taking into account the significance of the basic parameters is given in the table. Thus, vacuum microelectronics can be in demand in the following areas:
• communication systems with previously unreachable frequencies and power and bandwidth products, the creation of which is stimulated by the development of wireless broadband high-speed 5G communication, which requires an increase in the operating frequency and, as a result, an increase in signal power due to the factor of increasing its absorption in the atmosphere;
• space-based communication systems, the development of which may require a discrete increase in the operating frequency in the atmospheric transparency windows (90, 220, 460, 670, 850, 1030 GHz) for integration with terrestrial communication objects, increase of the information transfer rate and ensure its noise immunity, as well as the use of the X-ray frequency range due to the minimization of the radiation absorption factor in outer space;
• space-based navigation systems that require increasing the power and frequency of radio navigation systems in order to achieve higher resolution, positioning accuracy, maximizing coverage of areas, increasing reliability and durability of operation in conditions of external electromagnetic and radiation effects;
• high-precision systems of radio-electronic detection, guidance and electronic countermeasures with high energy-frequency parameters, spatial and 3D resolution, resistance to external electromagnetic influences;
• inspection equipment, oriented to operate in the frequency range of "terahertz slits" (300 GHz – 3 THz), which provides remote non-destructive tomographic detection and multispectral identification of a wide range of explosives and narcotic substances while minimizing the negative impact of the control and diagnostic procedure on the person;
• devices for scientific research and certification of products based on the analysis of vibrational spectra of molecules of organic and inorganic nature in the terahertz frequency range, providing rapid identification of the composition of objects, including without violating the integrity of the package;
• new generation medical equipment for harmless radio tomography express diagnostics of individual human organs with the purpose of early detection of pathological changes;
• security systems for nuclear and technogenic-hazardous facilities based on electronic devices for ultra-extreme temperature and radiation operating conditions.
An important incentive for the development of vacuum microelectronics should be considered the development of millimeter wavelengths and the terahertz frequency range, when it is necessary to provide the required values of the most important criterion for the quality of wireless communication systems, radar and radioelectronic countermeasures – product of output power, operating frequency and frequency band.
The use of basic and modified processes of micro- and nanotechnology and the infrastructure of integrated production of solid-state electronics and microsystem equipment create the preconditions for a renaissance of vacuum electronics with evolution into the micro- and nanoscale region.
The integration of solid-state and vacuum micro-devices is a rational economically effective way of harmonizing design and technological solutions at the breakthrough nature of integrated functional microwave and terahertz systems with previously unattainable parameters. ■