rus
Issue #4/2017
N.Gerasimenko, A.Volokhovsky, O.Zaporozhan
Accounting of features of changing of material properties in technology of silicon nanostructures
Accounting of features of changing of material properties in technology of silicon nanostructures
The features of change of physical properties of the structure elements when reducing their size to nanoscale and the prospects for using these effects in microelectronics technology are discussed.
Теги: nanosize effect nanostructure nanotechnology наноразмерный эффект наноструктура нанотехнология
Fashionable prefix "nano" often has an ambiguous meaning in the articles, theses and other scientific literature. Until now, many experts believe that it should be used when talking about the size parameter of 100 nm or less. In our view, this definition can be regarded as purely historical, and its scientific meaning is determined by the existence of a size threshold associated with the defined physical, chemical, and other parameters. For example, in discussing electrophysical and optical characteristics of semiconductors, the threshold of transition to the "nano" should be linked to the achievement of at least by one of the dimensional parameters of the wavelength of an electron (de Broglie wavelength). For mechanical characteristics as a size threshold the empirical parameter, which for metals and semiconductors is about 30 nm, is used. This parameter can be linked also with changes in certain physical quantities, caused by the proximity of the surface, etc.
The definition of the phenomenon of nano is complicated by the emergence of specific properties of materials that presents a major problem both in manufacturing and in the experimental study. For example, single crystal silicon, which is used in solid-state electronics, is difficult to obtain in a powdery state when we can talk about his nano-properties, because, after passing through the size threshold, the nanograins stick together and oxidize rapidly. At room conditions the latter may lead to an explosion.
Information about the fundamental properties of new really nanoscale materials in some cases is still fragmented and contradictory, hindering the development of real technologies, in particular, of solid-state nanoelectronics.
The development of a new scientific field is linked with the emergence of new areas in the scientific-technical terminology. For example, the term "nanoindustry" needed a special explanation in the literature [1]. On the other hand, seemingly clear terms can sometimes mislead. For example, terms "nanoconductor" and "nanowire" are often confused and incorrectly used.
Of course, the new area requires the HR development for research, education and technology taking into account the ultra-fast development of scientific bases of laboratory and technological approaches [2].
New scientific and technical sphere is acquiring a special importance, in our view, at the transition to commercialization of scientific results, ideas, new features and technologies. The difficulties arising force us to seek fundamentally new approaches to the research, creation of technological foundations, the design and production of new materials, technological environment and equipment. At the same time, existing technological capabilities require either excessive costs or new ways, and sometimes drastic changes. And even well-known and well enough mastered industry technologies during creation of nanoscale objects in the electronics show new opportunities, creating new challenges, requiring constant attention of researchers and bringing forward new principles of interaction between scientists and engineers that can be combined by the concept of "scientific support of technology". The latter is a continuous process, not fragmentary contacts.
In connection with the above considerations, let's consider two typical situations. The first one is connected with the use of photolithography in the field of ultra-small sizes, where optical devices have a new design features due to the move into a remote part of the UV range, which leads to a sharp rise in the cost of the equipment. In upcoming articles, we will tell you about developed in Russia industrial approaches, which are based on the use of a focused x-ray beams. This possibility has already been discussed at Mikron PJSC and can be very promising.
Another example concerns applications of the method and equipment for ion implantation and synthesis. This widespread technology is practically not used in creation of nanoscale (quantum-dimensional) structures to obtain essentially new devices and IC that combines features of integrated electronics and photonics. International literature contains quite a lot of information about this area, however, quantum-sized emitting structures on the basis of the germanium-silicon in the vast majority of cases are made using molecular beam epitaxy. The first and only works on the use of ion implantation [3, 4] showed that the preparation of the ion synthesis of Si–Ge nanostructures requires quite a long time, but now the process is ready to use in the industry. A number of the new features inherent only to an ion beam, which at the correct use allow to obtain record results, is revealed. However, ignorance of these features may lead to many practical difficulties [5].
Up-to-date production technology of microelectronic components develops towards reducing the topological sizes, increasing the degree of integration, introducing new materials, transitioning to advanced packaging and also creating of hybrid devices running on new physical principles [6]. The development of microelectronics occurs through its evolution to nanoelectronics, i.e. with the transition through the size thresholds that change sharply electrophysical, structural, mechanical, optical and other properties of materials and structures that need to be taken into account in the formation and implementation of the process cycle of their production, and in the design of the control system of parameters.
Electrophysical and optical properties of objects in a state of dimensional quantization and magnetic properties are not considered in this work, since their consideration and discussion are carried out at the stage of implementation of the principles of operation of the device and its design. Also such typical for nanostructures effects as tunneling, ballistic effects etc. aren't considered. The subject of the discussion are the features of change of structure and mechanical properties, which are considered from the point of view of their influence on the result of measuring operations, accompanying the process of formation.
The choice in favor of the study of these groups of parameters is caused by the fact that the majority of measurements in the production cycle of IC relate to the geometric and structural parameters of the obtained elements. The proposed approaches allow to approach the implementation of processes of nanoelectronics when the design rules pass through either empirical or theoretical size thresholds, and the accounting of such changes is necessary to obtain reliable results of the control of the geometric parameters of the structures.
CHANGES IN STRUCTURE
AND MECHANICAL PROPERTIES
AT NANOSCALE
We will consider some of the changes that occur in the development of microelectronics technology in the area of nanosizes and which in recent years are well established, however not always taken into account when forming the process cycles of manufacturing low-sized structures and even more rarely taken into account when developing the methodology of their control.
Features of the structure associated with changes in fundamental parameters of objects such as the binding energy of the atoms with a host of crystal lattice (or a node in an ordered quasi-liquid state), the equilibrium concentration for the temperature of structural defects (moving vacancies and interstitial atoms or their combinations), as well as the role of the surface in the formation of structural properties will be considered either from the viewpoint of currently known facts and ideas, either from the standpoint of a hypothetical description of the possible effects, which need experimental verification. The structural parameters of the nanoobject define many of its properties, including melting point, radiation resistance, phase transitions of the crystal-amorphous state, diffusion processes, etc. It should be noted that certain information on the above-mentioned parameters of nanostructures can be obtained from the analysis of the near surface properties of single crystals, discovered and described earlier.
According to the authors, there are as yet no numerical methods detailed consideration of unit cell parameters within the nanoobject. We will notice however that some of the observed effects can be directly associated with other experimentally observed and previously described phenomena. For example, a sharp decrease in the melting temperature of the nanoobject can be associated with previously observed effect of "anisotropic local melting" [7] in near-surface layers of monocrystalline silicon, which was explained by the presence of structural defects, a decrease of the binding energy of the atom with the lattice site and, consequently, an increase in the concentration of the equilibrium structural defects in these layers, as well as by the behavior of the structural defects inside the nanoobject, including their nucleation and annihilation. Especially it is necessary to consider changes of the equilibrium concentration of defects at a given temperature. Taking into account this parameter it is necessary to examine the effects associated with radiation exposure to nanoobjects, including improved radiation resistance [8] and radiation-induced amorphization of single crystals [9]. Here it is necessary to consider the role of neighbor Frenkel pairs and surface condition of the nanoobject.
First, let us dwell on polycrystalline materials play an important role in thin film semiconductor technology. Currently, it is established that the change in mechanical properties is largely caused by the nature of the microstructure of nanomaterials [10, 11]. For example, the value of microhardness and plasticity depend on the grain size and the dislocation density in a polycrystalline material. In the classic view, these regularities explicitly weren't considered, however, the experiments conducted in the last decade have convincingly shown that the mechanical and plastic properties of nanomaterials strongly depend on the nanoparticle size [12]. Determination of the mechanisms of the discovered regularities is a subject for scientific research. Both factors – microstructure of nanoscale material and size of the studied object play an important role in changing the mechanical characteristics.
The interaction of structural defects with the grain boundaries traditionally was of special interest for polycrystalline materials in case of the sequential decrease of the crystallite size. According to the classical law of Hall-Petch, the grain boundaries serve as a limiting factor for the movement of dislocations, resulting in the increase of microhardness and yield stress of polycrystalline material at the decreasing of grain size. However, when studying nanocrystalline materials with grain size less than several tens of nanometers, the description of plastic deformation requires consideration of a wide range of phenomena related to the interaction between structural defects and boundaries of the crystallites [13, 14].
Features of small crystalline objects taking into account the changes primarily of mechanical properties (microhardness, strength, etc.) are most clearly demonstrated in one of the first works on their modelling [15]. The paper clearly shows how mechanical properties change when passing through the size threshold. The results of the model experiment were further confirmed experimentally and will be discussed below. We will note influence of a surface of a nanoscale object on the electrical, mechanical and other characteristics. If the conventional technology allow to divide the material properties in the volume and surface areas, then in this case the influence of the surface state increases dramatically and in many cases as will be shown below determines the properties of the object itself, including during technological operations.
In [12] using high resolution transmission electron microscopy the behavior of silver nanoparticles (<10 nm in diameter) during deformation is investigated (Fig.1). In this study, the nanoparticle was placed on the tip of the tungsten needle, the pressure was created by the micromanipulator fabricated of single crystal ZrO2. The behavior of nanoparticles the authors call "pseudoelasticity" (quasi-liquid), implying that under pressure the particles are compressed but after its removal restore the original shape. At the same time, the substance remains crystalline during the whole process.
The shape memory is explained by minimization of the capillary energy, that is, the residual form is the energetic compromise between the surface energy of the nanoparticle and the energy of the Ag-W boundary. As a mechanism explaining the evolution of the shape of the particle, we can offer surface diffusion. The applied stress can be reduced by moving atoms from the surface of the nanoparticles to the interfaces of Ag-W or Ag-ZrO2 and by the growth of new atomic planes.
The authors of [16] have reported the spontaneous phase transition by nanoscale particles of PdSi from crystalline to amorphous states caused by only a decrease in the form of nanoparticle without application of any external stress (Fig.2). The authors explain such behavior with the help of classical ideas – from the condition of minimization of the Gibbs free energy, which they have represented as a superposition of the free energy of the surface (increasing with increasing size of nanoparticles) and the free energy of the bulk material (decreasing with increasing size of nanoparticles).
The transition through the size threshold leads to a sharp change in the structure and related properties of the object. First of all, it concerns the state of ordering of atoms, which in extreme cases can be crystalline or amorphous. However, as shown in [16], the size changes near the threshold level does not affect the structure.
The above experimental facts about the possibility of switching of nanoobjects into the new phase state at change of the size are brand new and require further testing. However, the proposals to use effect in microelectronic technology, in particular, on the application of quantum-dimensional liquid crystals already appear [17].
Other features of the material are related to the size of a crystallite. The possibility of reducing the melting temperature with decreasing of the crystallite size have been discussed widely [18]. A typical example is considered in [19]. It is appropriate to mention that the change in properties with decreasing size of the object after a threshold can be associated with experiments on "local anisotropic etching" [7]. By itself, the effect is associated with an increased number of structural defects near the surface and decrease of the energy of the atom in the lattice site. It is pertinent to recall also that the studies of radiation effects in the crystal lattice of silicon showed dependence of the energy of the radiation defect formation on temperature (decreases with heating of the crystal and becomes zero at the melting temperature) [20]. The experiments on reduction of energy of defect formation in nanocrystals upon heating have been not carried out. Another important effect related to the lattice parameters is the change of the radiation resistance and of radiation threshold of the phase transition "crystal–amorphous state" in case of reducing the object below the size threshold.
One of the notable effects associated with the change in the structure at the reducing of the size of the object is the radiation resistance. This phenomenon was first observed when studying the properties of nanoobjects (carbon nanotubes) on the spacecraft, where the effect of radiation damage is significant [21].
Then the effect was discovered and investigated in detail on silicon nanoscale structures (powders, porous silicon) [22], and multilayer metallic objects [23]. In the latter study, the authors irradiated a series of thin-film multilayer Nb-Cu structures with different thickness of layers (2.5, 5, 40 and 100 nm) by helium ions with energies of 33 and 150 keV, dose of 6 ∙ 1016–1.5 ∙ 1017 cm–2 at room temperature. When the thickness of layers was less than 20 nm, there was no "blistering" after exposure. The study was conducted with the help of high resolution TEM. The effect is explained by the fact that the energy of formation of point defects (vacancies) at the interface of Cu–Nb nano-composite is much smaller than in the single-crystal material. The interface is an effective area of drain of mobile radiation defects.
It was also investigated the effect of surface on radiation resistance. The effect itself is associated with the passage of the components of Frenkel pairs through the interface, and its influence on the accumulation of radiation defects can also be associated with published results for single crystals [9], where it was shown that the fields of elastic stresses caused by the presence of dielectric films of SiO2 or Si3N4 on the surface of the silicon, due to the opposite effects (tension, compression) can control the separation of the defects born during irradiation, in particular during ion implantation [24].
We will note also that when considering radiation damage of nanocrystals, it is necessary to take into account the role of the components of the neighbor Frenkel pairs, which in the description of radiation effects in bulk single crystals are not considered [25].
The effect of increasing radiation resistance is associated with a decrease in the size of the object below the threshold level. However, by itself such an increase of resistance should be linked to the phase transition of "single crystal–amorphous state" during irradiation with particles, in particular during the ion bombardment. This phase transition is considered in detail in [26] where it is experimentally shown that the dose of amorphization of ions depending on their mass is significantly less than for the same ions used for the amorphization of single crystals. This result would seem contradicts the hypothesis about the accumulation of radiation defects in nanocrystals, because the existing models of amorphization are associated with the accumulation of point defects, in particular of vacancy complexes of the order of 1020 cm–3. The model used in this work presupposes the existence of near-surface pre-amorphizated areas, which is confirmed by subsequent studies, as the near surface areas contains the increased concentration of equilibrium defects. It should be noted that the silicon nanocrystals formed in a dielectric SiO2 matrix, which in itself can stimulate the phase transition, were studied. Reducing the size of the objects with the formation of nanocrystals, as already noted, can lead to amorphization without radiation exposure, which, in our opinion, is associated with a higher concentration of equilibrium defects in such objects.
Let us now consider the radiation amorphization during the ion bombardment. In recent years there have been several works published which have investigated electrophysical properties of ion-implanted nanowires [27, 28], however the effects of ion irradiation that is related to the introduction of defects of the crystal structure, which could limit the electrophysical characteristics of the implanted impurities ions, still remain practically unstudied. Bending of gallium arsenide nanowires under the influence of ion irradiation was recently discovered. Presumably, the main mechanism of this bending is the spatial separation of vacancy and interstitial defects [29]. In this regard, we should pay attention to the results of observation of amorphization in [30], where the deformation of a silicon nanowire under ion irradiation was experimentally investigated. The bending mechanism has not been fully explored, however it has been shown that the deformation of the wires is caused by mechanical stress arising at the interface of the amorphous (obtained in the irradiation) and crystalline phases (Fig.3). It is shown that the reduction of the size (the diameter of the whiskers) leads to a decrease in the dose of amorphization.
Currently, the main method of mechanical treatment of samples at the nanoscale is the focused ion beam (FIB) technique. Focused beams of ions, typically of gallium (Ga+), allow to carry out local etching and deposition of the material in nanoscale areas. FIB is used, for example, in IC prototyping [31], three-dimensional analysis of materials and structures [32, 33], extraction and preparation of samples for electron microscopy [34, 35], study of the threshold dimensional properties of materials and structures in nanotechnology [36, 37], as well as in many other areas. However, FIB treatment not only removes the surface atoms, but also shifts the atoms of the underlying layers from their equilibrium state, causing a cascade of collisions and structural damages [36]. These effects that are typical for the processes of ion emission are caused by bombardment of the surface by the flow of heavy high-energy ions, however, the fact that the beams are tightly focused brings its own peculiarities into the process of calculation of the trajectories of ions and the resulting structural damages [38, 39]. The effects associated with the FIB treatment can also be the cause of the observed changes in the mechanical properties of nanoobjects [36, 40–42].
Damages caused by FIB treatment include amorphization of the surface [43], the generation of crystal lattice defects [44] and the formation of links between the sample material and ions of the beam [45]. These data should be considered when preparing samples to study using FIP, especially if we are talking about structural or phase analysis. As it will be shown below, by itself, the FIB treatment can contribute distortion in the observed result, and these distortions can be observed at a sufficiently large distance from the treated area.
The change in material properties when exposed by FIB was recently described in [49] on the example of nanocrystals of gold. It is concluded that this process leads to significant changes in the structure of the single crystal and should be discussed from the standpoint of applications in electronics nanotechnology. Found in [49] structural changes were discussed at the ConFab 2017 conference [46]. Results of [47] that have been discussed at this conference (presented in detail in [49]) are the most important according to the publication in a recent issue of the Solid State Technology journal. However, it should be noted that a similar phenomenon was described by us in details in the work carried out on silicon [50] and is confirmed by the results that will be described in detail below. It is possible that the reference to these results is missing due to the fact that they were obtained for the actually used technology of silicon-based electronics.
STUDY OF PHYSICAL PROPERTIES
OF SILICON NANOSTRUCTURES
In [50], an one-dimensional extended structure was etched in a single-crystal silicon with the use of the FEI Quanta 200 3D system and FIB technique. The ion energy of 5 Kev at beam current of 1 nA were used. Etching of the sample was carried out in scanning mode with a specified time of 20 µs at each point of the trajectory. The probe diameter was around 2.75 µm. The wafers of monocrystalline silicon with orientation (100) were used in the project. Etching of the sample was carried out along parallel trajectories so that between the two grooves a nano-sized gap (Fig.4) remained, the value of which can be set by changing the coordinates of the initial point of the scanning trajectory in the direction that is perpendicular to the scanning direction. The control was conducted in such a way that this gap reduced from groove to groove.
The obtained relief structure had a trapezoidal cross-section (Fig.4b). When the size of the relief has approached 40 nm (at the top), his spontaneous bending has occured (Fig.4a). Thus, in this experiment, the material has shown plastic properties that are not typical for monocrystalline silicon with high hardness. Under the influence of FIB two processes occur simultaneously in the material: the sputtering of atoms of the surface layer and the plastic flow associated with the introduction of an excessive concentration of radiation mobile defects. Properties of the near-surface layer, which is formed including, on walls of a crater of FIB etching, differ from the properties of the basic material.
Let us consider the case when two craters of etching are located in close proximity to each other. The walls of each include a regions with a high concentration of nonequilibrium radiation defects. In case of approaching of these areas, begins their mutual influence on each other, and, in the end, they completely overlap. The total concentration of defects increases, which is accompanied by the growth of mechanical stresses in this layer and, as a result, by plastic flow of material. In case of overcoming threshold concentration of defects, a complete amorphization of the material in this area can also occurs.
The causes of this phenomenon, at first glance, are associated with the introduction of an excess concentration of defects at the FIB processing. However, it should be noted that the lower part of the structure in Fig.4, and a part having large lateral dimensions, hasn’t shown plastic properties, although they are also exposed by FIB. Traces of cracks in the lower part of Fig.4 indicate that the structure showed the properties of solids. Thus, we can conclude that the the size parameter had decisive importance for the manifestation of plastic properties in this structure. Part of the structure, passed through the size threshold, has plastic properties, while the part that have not passed it showed increased hardness in accordance with classical concepts of the Hall–Petch [11].
In [50], scientists from MIET have investigated features of formation of nanoscale topography on the walls of the crater at the etching of silicon by focused ion beam. It is discovered that depending on the experimental conditions, the formed surfaces of the cylinders differ from perfectly smooth, as ring-shaped formations located from one another at equal distances arise (Fig.5). In this work, the etching by Ga+ ions with varying the energy from 5 to 30 keV and the beam current from 1 to 20 nA have been carried out. The etching was carried out at a fixed position of the beam perpendicular to the sample surface. Time of etching was varied from 1 to 50 µs. In this project we have investigated the dependence of the observed plastic phenomena on the orientation of monocrystalline silicon. For wafers with orientation (111) and (100) the dependence of the observed phenomena on the crystallographic orientation was not identified. At a current density of the ion beam of 2.16 µA/cm2 the pronounced formations on the walls of the cylinder (Fig.5) were registered, while at reducing the current density to 1.07 µA/cm2 the formations were absent.
To investigate the structural properties of the considered samples, the study of the structure of the formations was performed. Images in Fig.6 show that the Kikuchi lines are poorly visible in the field of ion irradiation (Fig.6a), in contrast to the region without irradiation (Fig.6b). Residual visibility in the non-irradiated region is caused by the fact that diffraction occurs in the surface silicon layer with a greater thickness than amorphizated layer. Study of fast electron diffraction was conducted at an accelerating voltage of 30 kV, a current of 9.7 nA, a slope of surface of 70°. Under these conditions the electrons are mostly reflected from the surface, and only a small part of them with high energy penetrates to a depth of about 150–200 nm.
Thus, it is experimentally shown that the surface of the crater is amorphizated. This implies that the predominant mechanism in the formation of the walls of the cylinder is not the resputtering, but a plastic flow associated with the introduction of an excessive concentration of mobile radiation defects. It can be concluded that in the described experiment that is carried out in the framework of the study [50], the dependence of the process of formation of periodic structures during ion etching on the current density of the ion beam and the independence on time of etching have been registered. The obtained results are in contradiction with the classical views on the formation of topography during ion etching by resputtering of material, which makes necessary the discussion of alternative points of view on this process. An alternative mechanism that is associated with plastic flow [48] have been discussed in the world scientific literature recently. Also, it should be noted that a similar effect was observed for metals in a recent article [49], however, our team has discovered this effect a few years earlier [50].
According to modern concepts, in case of the irradiation of the monocrystalline substrate by an ion beam that is directed to the surface at some angle θ less than critical, the cascades of radiative offsets arise, leading to directional motion of significant flows of interstitial and vacancy defects near the surface accompanied by plastic flow of material [48]. The density of defects in these radiative cascades is high, which leads to complete amorphization of the surface layer.
CONCLUSION
The considered results show that the reduction in size of the elements of the structure down to the nanoscale can lead to dramatic changes in its mechanical properties, therefore we should pay attention to the results on nanostructure formation of record small sizes, as have been demonstrated recently by IBM (FinFet transistor structure with 7 nm node) [51].
Special attention to the condition of the structure of nanoscale objects should be given when considering the results of radiation effects on the nanoobjects. The sharp increase in radiation resistance in nanoobjects [8], which is reflected in a change in the functional properties of the devices, contradicts to the facts of reduction of the dose of amorphization of nano-objects [26]. These issues were discussed in [52], and also in the number of studies carried out in our laboratory [7, 54]. It should be noted that the phase transition at a lower (in comparison with the single crystal) doses of irradiation is associated, in our opinion, with the accumulation of critical doses of defects, in particular of vacancy V-V centres [53], which consists of the equilibrium concentration and non-equilibrium radiation component. In this case, the expected equilibrium concentration at a given temperature is increased [54]. In this case, it is expected that the equilibrium concentration at a given temperature is increased [54].
Under the proposed unified approach to the consideration of properties of nanoscale objects, taking into account the interaction of moving defects with the boundaries of nanosized objects and the influence of surfaces and interfaces, it is possible to develop a description of various physical phenomena such as amorphization and plastic flow of solid materials. The transition of the object through the size threshold leads to a sharp change in the structure and related properties. First of all, it concerns the state of ordering of atoms, which in extreme cases can be crystalline or amorphous. In our paper [50] it is shown that radiative processes can lead to so significant changes in the structure, that on the borders of etching the formations associated with the formation of areas with plastic flow can be observed. The same effect was discussed in detail at a recent ConFab conference [47], and in [49] it was considered in more detail, but the results practically coincide with results of previously published work [50].
The considered size effects of the change of mechanical properties and structure of nanoscale objects should be taken into account in the manufacture of samples, sample preparation, and in conducting research because they can contribute distortions in the observed result. Manifestations of quasi-liquid (amorphous) properties are possible both in production and in the study of samples. This affects not only the processing and inspection in the laboratory but also process control in situ. ■
This work was supported by grant of the Russian Science Foundation, project No. 15-19-10054.
The definition of the phenomenon of nano is complicated by the emergence of specific properties of materials that presents a major problem both in manufacturing and in the experimental study. For example, single crystal silicon, which is used in solid-state electronics, is difficult to obtain in a powdery state when we can talk about his nano-properties, because, after passing through the size threshold, the nanograins stick together and oxidize rapidly. At room conditions the latter may lead to an explosion.
Information about the fundamental properties of new really nanoscale materials in some cases is still fragmented and contradictory, hindering the development of real technologies, in particular, of solid-state nanoelectronics.
The development of a new scientific field is linked with the emergence of new areas in the scientific-technical terminology. For example, the term "nanoindustry" needed a special explanation in the literature [1]. On the other hand, seemingly clear terms can sometimes mislead. For example, terms "nanoconductor" and "nanowire" are often confused and incorrectly used.
Of course, the new area requires the HR development for research, education and technology taking into account the ultra-fast development of scientific bases of laboratory and technological approaches [2].
New scientific and technical sphere is acquiring a special importance, in our view, at the transition to commercialization of scientific results, ideas, new features and technologies. The difficulties arising force us to seek fundamentally new approaches to the research, creation of technological foundations, the design and production of new materials, technological environment and equipment. At the same time, existing technological capabilities require either excessive costs or new ways, and sometimes drastic changes. And even well-known and well enough mastered industry technologies during creation of nanoscale objects in the electronics show new opportunities, creating new challenges, requiring constant attention of researchers and bringing forward new principles of interaction between scientists and engineers that can be combined by the concept of "scientific support of technology". The latter is a continuous process, not fragmentary contacts.
In connection with the above considerations, let's consider two typical situations. The first one is connected with the use of photolithography in the field of ultra-small sizes, where optical devices have a new design features due to the move into a remote part of the UV range, which leads to a sharp rise in the cost of the equipment. In upcoming articles, we will tell you about developed in Russia industrial approaches, which are based on the use of a focused x-ray beams. This possibility has already been discussed at Mikron PJSC and can be very promising.
Another example concerns applications of the method and equipment for ion implantation and synthesis. This widespread technology is practically not used in creation of nanoscale (quantum-dimensional) structures to obtain essentially new devices and IC that combines features of integrated electronics and photonics. International literature contains quite a lot of information about this area, however, quantum-sized emitting structures on the basis of the germanium-silicon in the vast majority of cases are made using molecular beam epitaxy. The first and only works on the use of ion implantation [3, 4] showed that the preparation of the ion synthesis of Si–Ge nanostructures requires quite a long time, but now the process is ready to use in the industry. A number of the new features inherent only to an ion beam, which at the correct use allow to obtain record results, is revealed. However, ignorance of these features may lead to many practical difficulties [5].
Up-to-date production technology of microelectronic components develops towards reducing the topological sizes, increasing the degree of integration, introducing new materials, transitioning to advanced packaging and also creating of hybrid devices running on new physical principles [6]. The development of microelectronics occurs through its evolution to nanoelectronics, i.e. with the transition through the size thresholds that change sharply electrophysical, structural, mechanical, optical and other properties of materials and structures that need to be taken into account in the formation and implementation of the process cycle of their production, and in the design of the control system of parameters.
Electrophysical and optical properties of objects in a state of dimensional quantization and magnetic properties are not considered in this work, since their consideration and discussion are carried out at the stage of implementation of the principles of operation of the device and its design. Also such typical for nanostructures effects as tunneling, ballistic effects etc. aren't considered. The subject of the discussion are the features of change of structure and mechanical properties, which are considered from the point of view of their influence on the result of measuring operations, accompanying the process of formation.
The choice in favor of the study of these groups of parameters is caused by the fact that the majority of measurements in the production cycle of IC relate to the geometric and structural parameters of the obtained elements. The proposed approaches allow to approach the implementation of processes of nanoelectronics when the design rules pass through either empirical or theoretical size thresholds, and the accounting of such changes is necessary to obtain reliable results of the control of the geometric parameters of the structures.
CHANGES IN STRUCTURE
AND MECHANICAL PROPERTIES
AT NANOSCALE
We will consider some of the changes that occur in the development of microelectronics technology in the area of nanosizes and which in recent years are well established, however not always taken into account when forming the process cycles of manufacturing low-sized structures and even more rarely taken into account when developing the methodology of their control.
Features of the structure associated with changes in fundamental parameters of objects such as the binding energy of the atoms with a host of crystal lattice (or a node in an ordered quasi-liquid state), the equilibrium concentration for the temperature of structural defects (moving vacancies and interstitial atoms or their combinations), as well as the role of the surface in the formation of structural properties will be considered either from the viewpoint of currently known facts and ideas, either from the standpoint of a hypothetical description of the possible effects, which need experimental verification. The structural parameters of the nanoobject define many of its properties, including melting point, radiation resistance, phase transitions of the crystal-amorphous state, diffusion processes, etc. It should be noted that certain information on the above-mentioned parameters of nanostructures can be obtained from the analysis of the near surface properties of single crystals, discovered and described earlier.
According to the authors, there are as yet no numerical methods detailed consideration of unit cell parameters within the nanoobject. We will notice however that some of the observed effects can be directly associated with other experimentally observed and previously described phenomena. For example, a sharp decrease in the melting temperature of the nanoobject can be associated with previously observed effect of "anisotropic local melting" [7] in near-surface layers of monocrystalline silicon, which was explained by the presence of structural defects, a decrease of the binding energy of the atom with the lattice site and, consequently, an increase in the concentration of the equilibrium structural defects in these layers, as well as by the behavior of the structural defects inside the nanoobject, including their nucleation and annihilation. Especially it is necessary to consider changes of the equilibrium concentration of defects at a given temperature. Taking into account this parameter it is necessary to examine the effects associated with radiation exposure to nanoobjects, including improved radiation resistance [8] and radiation-induced amorphization of single crystals [9]. Here it is necessary to consider the role of neighbor Frenkel pairs and surface condition of the nanoobject.
First, let us dwell on polycrystalline materials play an important role in thin film semiconductor technology. Currently, it is established that the change in mechanical properties is largely caused by the nature of the microstructure of nanomaterials [10, 11]. For example, the value of microhardness and plasticity depend on the grain size and the dislocation density in a polycrystalline material. In the classic view, these regularities explicitly weren't considered, however, the experiments conducted in the last decade have convincingly shown that the mechanical and plastic properties of nanomaterials strongly depend on the nanoparticle size [12]. Determination of the mechanisms of the discovered regularities is a subject for scientific research. Both factors – microstructure of nanoscale material and size of the studied object play an important role in changing the mechanical characteristics.
The interaction of structural defects with the grain boundaries traditionally was of special interest for polycrystalline materials in case of the sequential decrease of the crystallite size. According to the classical law of Hall-Petch, the grain boundaries serve as a limiting factor for the movement of dislocations, resulting in the increase of microhardness and yield stress of polycrystalline material at the decreasing of grain size. However, when studying nanocrystalline materials with grain size less than several tens of nanometers, the description of plastic deformation requires consideration of a wide range of phenomena related to the interaction between structural defects and boundaries of the crystallites [13, 14].
Features of small crystalline objects taking into account the changes primarily of mechanical properties (microhardness, strength, etc.) are most clearly demonstrated in one of the first works on their modelling [15]. The paper clearly shows how mechanical properties change when passing through the size threshold. The results of the model experiment were further confirmed experimentally and will be discussed below. We will note influence of a surface of a nanoscale object on the electrical, mechanical and other characteristics. If the conventional technology allow to divide the material properties in the volume and surface areas, then in this case the influence of the surface state increases dramatically and in many cases as will be shown below determines the properties of the object itself, including during technological operations.
In [12] using high resolution transmission electron microscopy the behavior of silver nanoparticles (<10 nm in diameter) during deformation is investigated (Fig.1). In this study, the nanoparticle was placed on the tip of the tungsten needle, the pressure was created by the micromanipulator fabricated of single crystal ZrO2. The behavior of nanoparticles the authors call "pseudoelasticity" (quasi-liquid), implying that under pressure the particles are compressed but after its removal restore the original shape. At the same time, the substance remains crystalline during the whole process.
The shape memory is explained by minimization of the capillary energy, that is, the residual form is the energetic compromise between the surface energy of the nanoparticle and the energy of the Ag-W boundary. As a mechanism explaining the evolution of the shape of the particle, we can offer surface diffusion. The applied stress can be reduced by moving atoms from the surface of the nanoparticles to the interfaces of Ag-W or Ag-ZrO2 and by the growth of new atomic planes.
The authors of [16] have reported the spontaneous phase transition by nanoscale particles of PdSi from crystalline to amorphous states caused by only a decrease in the form of nanoparticle without application of any external stress (Fig.2). The authors explain such behavior with the help of classical ideas – from the condition of minimization of the Gibbs free energy, which they have represented as a superposition of the free energy of the surface (increasing with increasing size of nanoparticles) and the free energy of the bulk material (decreasing with increasing size of nanoparticles).
The transition through the size threshold leads to a sharp change in the structure and related properties of the object. First of all, it concerns the state of ordering of atoms, which in extreme cases can be crystalline or amorphous. However, as shown in [16], the size changes near the threshold level does not affect the structure.
The above experimental facts about the possibility of switching of nanoobjects into the new phase state at change of the size are brand new and require further testing. However, the proposals to use effect in microelectronic technology, in particular, on the application of quantum-dimensional liquid crystals already appear [17].
Other features of the material are related to the size of a crystallite. The possibility of reducing the melting temperature with decreasing of the crystallite size have been discussed widely [18]. A typical example is considered in [19]. It is appropriate to mention that the change in properties with decreasing size of the object after a threshold can be associated with experiments on "local anisotropic etching" [7]. By itself, the effect is associated with an increased number of structural defects near the surface and decrease of the energy of the atom in the lattice site. It is pertinent to recall also that the studies of radiation effects in the crystal lattice of silicon showed dependence of the energy of the radiation defect formation on temperature (decreases with heating of the crystal and becomes zero at the melting temperature) [20]. The experiments on reduction of energy of defect formation in nanocrystals upon heating have been not carried out. Another important effect related to the lattice parameters is the change of the radiation resistance and of radiation threshold of the phase transition "crystal–amorphous state" in case of reducing the object below the size threshold.
One of the notable effects associated with the change in the structure at the reducing of the size of the object is the radiation resistance. This phenomenon was first observed when studying the properties of nanoobjects (carbon nanotubes) on the spacecraft, where the effect of radiation damage is significant [21].
Then the effect was discovered and investigated in detail on silicon nanoscale structures (powders, porous silicon) [22], and multilayer metallic objects [23]. In the latter study, the authors irradiated a series of thin-film multilayer Nb-Cu structures with different thickness of layers (2.5, 5, 40 and 100 nm) by helium ions with energies of 33 and 150 keV, dose of 6 ∙ 1016–1.5 ∙ 1017 cm–2 at room temperature. When the thickness of layers was less than 20 nm, there was no "blistering" after exposure. The study was conducted with the help of high resolution TEM. The effect is explained by the fact that the energy of formation of point defects (vacancies) at the interface of Cu–Nb nano-composite is much smaller than in the single-crystal material. The interface is an effective area of drain of mobile radiation defects.
It was also investigated the effect of surface on radiation resistance. The effect itself is associated with the passage of the components of Frenkel pairs through the interface, and its influence on the accumulation of radiation defects can also be associated with published results for single crystals [9], where it was shown that the fields of elastic stresses caused by the presence of dielectric films of SiO2 or Si3N4 on the surface of the silicon, due to the opposite effects (tension, compression) can control the separation of the defects born during irradiation, in particular during ion implantation [24].
We will note also that when considering radiation damage of nanocrystals, it is necessary to take into account the role of the components of the neighbor Frenkel pairs, which in the description of radiation effects in bulk single crystals are not considered [25].
The effect of increasing radiation resistance is associated with a decrease in the size of the object below the threshold level. However, by itself such an increase of resistance should be linked to the phase transition of "single crystal–amorphous state" during irradiation with particles, in particular during the ion bombardment. This phase transition is considered in detail in [26] where it is experimentally shown that the dose of amorphization of ions depending on their mass is significantly less than for the same ions used for the amorphization of single crystals. This result would seem contradicts the hypothesis about the accumulation of radiation defects in nanocrystals, because the existing models of amorphization are associated with the accumulation of point defects, in particular of vacancy complexes of the order of 1020 cm–3. The model used in this work presupposes the existence of near-surface pre-amorphizated areas, which is confirmed by subsequent studies, as the near surface areas contains the increased concentration of equilibrium defects. It should be noted that the silicon nanocrystals formed in a dielectric SiO2 matrix, which in itself can stimulate the phase transition, were studied. Reducing the size of the objects with the formation of nanocrystals, as already noted, can lead to amorphization without radiation exposure, which, in our opinion, is associated with a higher concentration of equilibrium defects in such objects.
Let us now consider the radiation amorphization during the ion bombardment. In recent years there have been several works published which have investigated electrophysical properties of ion-implanted nanowires [27, 28], however the effects of ion irradiation that is related to the introduction of defects of the crystal structure, which could limit the electrophysical characteristics of the implanted impurities ions, still remain practically unstudied. Bending of gallium arsenide nanowires under the influence of ion irradiation was recently discovered. Presumably, the main mechanism of this bending is the spatial separation of vacancy and interstitial defects [29]. In this regard, we should pay attention to the results of observation of amorphization in [30], where the deformation of a silicon nanowire under ion irradiation was experimentally investigated. The bending mechanism has not been fully explored, however it has been shown that the deformation of the wires is caused by mechanical stress arising at the interface of the amorphous (obtained in the irradiation) and crystalline phases (Fig.3). It is shown that the reduction of the size (the diameter of the whiskers) leads to a decrease in the dose of amorphization.
Currently, the main method of mechanical treatment of samples at the nanoscale is the focused ion beam (FIB) technique. Focused beams of ions, typically of gallium (Ga+), allow to carry out local etching and deposition of the material in nanoscale areas. FIB is used, for example, in IC prototyping [31], three-dimensional analysis of materials and structures [32, 33], extraction and preparation of samples for electron microscopy [34, 35], study of the threshold dimensional properties of materials and structures in nanotechnology [36, 37], as well as in many other areas. However, FIB treatment not only removes the surface atoms, but also shifts the atoms of the underlying layers from their equilibrium state, causing a cascade of collisions and structural damages [36]. These effects that are typical for the processes of ion emission are caused by bombardment of the surface by the flow of heavy high-energy ions, however, the fact that the beams are tightly focused brings its own peculiarities into the process of calculation of the trajectories of ions and the resulting structural damages [38, 39]. The effects associated with the FIB treatment can also be the cause of the observed changes in the mechanical properties of nanoobjects [36, 40–42].
Damages caused by FIB treatment include amorphization of the surface [43], the generation of crystal lattice defects [44] and the formation of links between the sample material and ions of the beam [45]. These data should be considered when preparing samples to study using FIP, especially if we are talking about structural or phase analysis. As it will be shown below, by itself, the FIB treatment can contribute distortion in the observed result, and these distortions can be observed at a sufficiently large distance from the treated area.
The change in material properties when exposed by FIB was recently described in [49] on the example of nanocrystals of gold. It is concluded that this process leads to significant changes in the structure of the single crystal and should be discussed from the standpoint of applications in electronics nanotechnology. Found in [49] structural changes were discussed at the ConFab 2017 conference [46]. Results of [47] that have been discussed at this conference (presented in detail in [49]) are the most important according to the publication in a recent issue of the Solid State Technology journal. However, it should be noted that a similar phenomenon was described by us in details in the work carried out on silicon [50] and is confirmed by the results that will be described in detail below. It is possible that the reference to these results is missing due to the fact that they were obtained for the actually used technology of silicon-based electronics.
STUDY OF PHYSICAL PROPERTIES
OF SILICON NANOSTRUCTURES
In [50], an one-dimensional extended structure was etched in a single-crystal silicon with the use of the FEI Quanta 200 3D system and FIB technique. The ion energy of 5 Kev at beam current of 1 nA were used. Etching of the sample was carried out in scanning mode with a specified time of 20 µs at each point of the trajectory. The probe diameter was around 2.75 µm. The wafers of monocrystalline silicon with orientation (100) were used in the project. Etching of the sample was carried out along parallel trajectories so that between the two grooves a nano-sized gap (Fig.4) remained, the value of which can be set by changing the coordinates of the initial point of the scanning trajectory in the direction that is perpendicular to the scanning direction. The control was conducted in such a way that this gap reduced from groove to groove.
The obtained relief structure had a trapezoidal cross-section (Fig.4b). When the size of the relief has approached 40 nm (at the top), his spontaneous bending has occured (Fig.4a). Thus, in this experiment, the material has shown plastic properties that are not typical for monocrystalline silicon with high hardness. Under the influence of FIB two processes occur simultaneously in the material: the sputtering of atoms of the surface layer and the plastic flow associated with the introduction of an excessive concentration of radiation mobile defects. Properties of the near-surface layer, which is formed including, on walls of a crater of FIB etching, differ from the properties of the basic material.
Let us consider the case when two craters of etching are located in close proximity to each other. The walls of each include a regions with a high concentration of nonequilibrium radiation defects. In case of approaching of these areas, begins their mutual influence on each other, and, in the end, they completely overlap. The total concentration of defects increases, which is accompanied by the growth of mechanical stresses in this layer and, as a result, by plastic flow of material. In case of overcoming threshold concentration of defects, a complete amorphization of the material in this area can also occurs.
The causes of this phenomenon, at first glance, are associated with the introduction of an excess concentration of defects at the FIB processing. However, it should be noted that the lower part of the structure in Fig.4, and a part having large lateral dimensions, hasn’t shown plastic properties, although they are also exposed by FIB. Traces of cracks in the lower part of Fig.4 indicate that the structure showed the properties of solids. Thus, we can conclude that the the size parameter had decisive importance for the manifestation of plastic properties in this structure. Part of the structure, passed through the size threshold, has plastic properties, while the part that have not passed it showed increased hardness in accordance with classical concepts of the Hall–Petch [11].
In [50], scientists from MIET have investigated features of formation of nanoscale topography on the walls of the crater at the etching of silicon by focused ion beam. It is discovered that depending on the experimental conditions, the formed surfaces of the cylinders differ from perfectly smooth, as ring-shaped formations located from one another at equal distances arise (Fig.5). In this work, the etching by Ga+ ions with varying the energy from 5 to 30 keV and the beam current from 1 to 20 nA have been carried out. The etching was carried out at a fixed position of the beam perpendicular to the sample surface. Time of etching was varied from 1 to 50 µs. In this project we have investigated the dependence of the observed plastic phenomena on the orientation of monocrystalline silicon. For wafers with orientation (111) and (100) the dependence of the observed phenomena on the crystallographic orientation was not identified. At a current density of the ion beam of 2.16 µA/cm2 the pronounced formations on the walls of the cylinder (Fig.5) were registered, while at reducing the current density to 1.07 µA/cm2 the formations were absent.
To investigate the structural properties of the considered samples, the study of the structure of the formations was performed. Images in Fig.6 show that the Kikuchi lines are poorly visible in the field of ion irradiation (Fig.6a), in contrast to the region without irradiation (Fig.6b). Residual visibility in the non-irradiated region is caused by the fact that diffraction occurs in the surface silicon layer with a greater thickness than amorphizated layer. Study of fast electron diffraction was conducted at an accelerating voltage of 30 kV, a current of 9.7 nA, a slope of surface of 70°. Under these conditions the electrons are mostly reflected from the surface, and only a small part of them with high energy penetrates to a depth of about 150–200 nm.
Thus, it is experimentally shown that the surface of the crater is amorphizated. This implies that the predominant mechanism in the formation of the walls of the cylinder is not the resputtering, but a plastic flow associated with the introduction of an excessive concentration of mobile radiation defects. It can be concluded that in the described experiment that is carried out in the framework of the study [50], the dependence of the process of formation of periodic structures during ion etching on the current density of the ion beam and the independence on time of etching have been registered. The obtained results are in contradiction with the classical views on the formation of topography during ion etching by resputtering of material, which makes necessary the discussion of alternative points of view on this process. An alternative mechanism that is associated with plastic flow [48] have been discussed in the world scientific literature recently. Also, it should be noted that a similar effect was observed for metals in a recent article [49], however, our team has discovered this effect a few years earlier [50].
According to modern concepts, in case of the irradiation of the monocrystalline substrate by an ion beam that is directed to the surface at some angle θ less than critical, the cascades of radiative offsets arise, leading to directional motion of significant flows of interstitial and vacancy defects near the surface accompanied by plastic flow of material [48]. The density of defects in these radiative cascades is high, which leads to complete amorphization of the surface layer.
CONCLUSION
The considered results show that the reduction in size of the elements of the structure down to the nanoscale can lead to dramatic changes in its mechanical properties, therefore we should pay attention to the results on nanostructure formation of record small sizes, as have been demonstrated recently by IBM (FinFet transistor structure with 7 nm node) [51].
Special attention to the condition of the structure of nanoscale objects should be given when considering the results of radiation effects on the nanoobjects. The sharp increase in radiation resistance in nanoobjects [8], which is reflected in a change in the functional properties of the devices, contradicts to the facts of reduction of the dose of amorphization of nano-objects [26]. These issues were discussed in [52], and also in the number of studies carried out in our laboratory [7, 54]. It should be noted that the phase transition at a lower (in comparison with the single crystal) doses of irradiation is associated, in our opinion, with the accumulation of critical doses of defects, in particular of vacancy V-V centres [53], which consists of the equilibrium concentration and non-equilibrium radiation component. In this case, the expected equilibrium concentration at a given temperature is increased [54]. In this case, it is expected that the equilibrium concentration at a given temperature is increased [54].
Under the proposed unified approach to the consideration of properties of nanoscale objects, taking into account the interaction of moving defects with the boundaries of nanosized objects and the influence of surfaces and interfaces, it is possible to develop a description of various physical phenomena such as amorphization and plastic flow of solid materials. The transition of the object through the size threshold leads to a sharp change in the structure and related properties. First of all, it concerns the state of ordering of atoms, which in extreme cases can be crystalline or amorphous. In our paper [50] it is shown that radiative processes can lead to so significant changes in the structure, that on the borders of etching the formations associated with the formation of areas with plastic flow can be observed. The same effect was discussed in detail at a recent ConFab conference [47], and in [49] it was considered in more detail, but the results practically coincide with results of previously published work [50].
The considered size effects of the change of mechanical properties and structure of nanoscale objects should be taken into account in the manufacture of samples, sample preparation, and in conducting research because they can contribute distortions in the observed result. Manifestations of quasi-liquid (amorphous) properties are possible both in production and in the study of samples. This affects not only the processing and inspection in the laboratory but also process control in situ. ■
This work was supported by grant of the Russian Science Foundation, project No. 15-19-10054.
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