rus
Issue #6/2018
G.Baranov, A.Italyantsev, N.Gerasimenko, A.Seletskiy
Physical features of formation of local submicron ion-implanted regions
Physical features of formation of local submicron ion-implanted regions
The continuous technology node scaling of Si microelectronics prompts to face with new difficulties in the formation of nanoscale ion-implanted regions. Starting from a certain size the formation of such regions goes under strong electrical and mechanical fields associated with the presence of a masking layer. In this paper, we perform numerical evaluations of size factors wherein the action of force fields is significant and require corrections in the design of Si microelectronics devices.
Теги: ion-doped region internal mechanical stresses ion implantation size factor внутренние механические напряжения ионная имплантация ионно-имплантируемая область размерный эффект
Ion implantation as a technique for doping semiconductors has been successfully used in microelectronics technology since the 1970s until now. Despite the fact that during this time a huge amount of papers has been published on defect formation and redistribution of impurity atoms, the study of the physics of processes accompanying ion irradiation remains relevant today. This is primarily caused by the fact that current trends in the miniaturization of electronics devices determine the need to form doped regions in semiconductors with dimensions in the nanometer range [1].
With the approach of the design rules to the quantum-size regime, the active regions become not just smaller, the physics of the processes of their formation has a fundamentally different character [2]. In particular, the proximity of the surface and phase boundaries begins to have a significant impact and opens up new possibilities for the engineering of defects [3]. When a certain size is reached, the implantation process takes place under conditions of strong electrical and mechanical fields generated by the masking layer, which are no longer regional in nature, but their action extends to the entire area of formation of small ion-doped regions. This means that the phenomenon of radiation physics, electrical activation and the redistribution of impurities occur in fundamentally different conditions in such areas.
The study of the physical features of the formation of ion-doped regions with critically small planar dimensions will allow not only to predict technological difficulties in the way of the constant striving of modern microelectronics for miniaturization, but also to find ways to overcome them, mechanisms for controlling the geometry and properties of nanometer-doped areas.
PHENOMENOLOGY OF PARTICLE SIZE
The decrease in the design rules of discrete devices and integrated circuits of silicon microelectronics is inevitably accompanied by a decrease in the depth of the ion-doped regions and their planar dimensions. Reducing the thickness of the layers will contribute to a qualitative change in the background of non-equilibrium radiation defects, in which the formation of ion-doped regions takes place. One of the most important factors causing such features is the enhancement of electric and, in part, mechanical field gradients associated with the presence of a masking layer through which the implantation is carried out. These effects are manifested already in the process of irradiation by impurity atoms and can lead to practically significant correction of the boundaries of the ion-implanted region due to the kinetics of impurity atoms. Although the implantation temperature is not essential for impurity diffusion, the presence of diffusion and drift flows of non-equilibrium defects intensively generated during implantation will lead to the directional movement of impurity atoms through the effects of vacancy and interstitial wind [4].
It is known that point defects are electrically active and effectively interact with charged interfaces and masking layers [5]. Ion implantation into Si through a dielectric coating layer always creates a long-term electrostatic charge in the dielectric. Without discussing the nature of its appearance, we only note that the charging efficiency of the dielectric is high and, for example, for the case of the SiO2 mask is 0.1 qF, where q is the elementary electric charge and F is the implantation dose [6]. This charge will generate electric fields and lead to directional drift of charged radiation defects. In case of ion implantation into sufficiently wide windows of the masking layer, the propagation of electric fields in the region being formed is of an edge character. With a decrease in the size of the window, the fields begin to substantially overlap, and the formation of the ion-doped region occurs in fundamentally different conditions (Fig.1).
This is typical not only for electrostatic, but also for mechanical fields. The process of growing and deposition of thin films on a Si substrate is almost always accompanied by the appearance of internal mechanical stresses at the interface. Opening the window in the masking layer will cause them to splash on the edges of the mask breakage. As the window width decreases, the propagation regions of these stresses will also begin to fill the entire volume of the formed region leading to a drift of radiation vacancies into the compressed Si region, and of its own interstitial atoms – into the stretched one.
In addition, ion implantation is always accompanied by spraying the surface of the mask. With a decrease in the implantation energy that accompanies the scaling of the active regions, the efficiency of this process will only increase, since the primary ions will increasingly lose energy due to elastic losses on the surface atoms of the target. The dissolution of recoil atoms in the zone of formation of the active region becomes significant, which leads to contamination, which in some cases may contribute to the degradation of the electrical-physical parameters of the region (Fig. 2).
As long as all the considered effects are marginal, they do not significantly affect the formation of ion-doped regions. However, the tendency to decrease in design rules observed in microelectronics no longer allows them to be ignored and requires quantitative estimates of the size threshold of this transition.
QUANTITATIVE ESTIMATES
Electric field factor
Based on the data of [6] on the magnitude of the electrostatic charge arising in the SiO2 layer when ions are implanted into it, computer simulation in the TCAD Sentaurus are used to calculate the 2D distribution of electric fields in Si in the region of a window opened in a mask. The process of ion implantation of As+ with an energy of 20 keV and a dose of 1016 cm–2 into the topological structure of SiO2–Si with a mask thickness of 60 nm in the window size range of 22–180 nm was simulated. The maximum values of the electrostatic fields were observed directly in the region of the targeted implantation of impurity atoms, reaching values of about 103–104 V/cm (Fig.3). Moreover, with a decrease in the window width from 180 nm to 22 nm the field strength changes 17 times in various points in the formed region, which inevitably leads to a change in the flow of various radiation processes and, ultimately, in the impurity distribution profile.
In the case of small implantation doses, the concentration of charged radiation defects is determined by the initial position of the Fermi level in Si. Using the provisions of the theory of statistics of charge carriers in a solid, it is easy to determine that the maximum level of charged defects and hence the drift flows will be observed in silicon with the following specifications: KDB-0.002 (V+ predominance), KDB-0.05 (V++ predominance) and KEF-0.01 (V= predominance).
The depth of correction of the boundaries of the ion-implanted region due to the action of the electric fields of the mask can be estimated directly at the boundary of the region from the ratio of the drift flow of defects to the diffusion one. We assume that the criterion of a significant contribution of the drift flow to the correction of the region’s boundaries will be its achievement of 10% of the diffusion flow:
, (1)
where μ and D are the mobility and diffusion coefficient of defects; n is the concentration of defects at a localized point of the region; E is the electric field strength.
Assuming that the distribution of radiation defects n(x) is described by the Gauss law with a maximum in the region of 1.2Rp, the relation (1) will be satisfied at an electric field strength of ~10 kV/cm. In accordance with Fig.3, correction of the depth of the region due to the action of the electric field is achieved at a window size of 45 nm and increases as it decreases. Lateral blurring of the ion-implanted region, on the contrary, is significant with a large width of the window and appears up to a value of 65 nm.
Mechanical field factor
A theoretical analysis of the distribution of the tangential and normal components of the elastic fields at the edge of the opened window in the mask was performed in [7]. The results of the calculations showed that the normal components of the fields tend to be separated by the sign of the stresses at the edge of the mask leading to the formation of multidirectional drift flows of radiation vacancies (V) and internodes (I) (Fig.4). If we consider the entrainment of impurities by fluxes of point defects, the diffusion of impurity atoms is accelerated in the direction of the flow I (interstitial atoms) and against the flow V (vacancies). This means that regardless of the type and mechanism of impurity diffusion, the nature of the distribution of mechanical fields in the region of the mask edge will always contribute to the side diffusion blurring of the profile of impurity atoms.
By analogy with electrostatic fields, the criterion of a significant correction of the area was taken to achieve a drift flow of 10% of the diffusion one:
, (2)
where n and D are the concentration and diffusion coefficient of a specific type of defects; ω is the silicon atom volume; f is a correlation factor taking into account the possibility of a defect to make a reverse jump (for Si f = 0.781); ∆σ is the gradient of mechanical stresses.
It was believed that the mask is experiencing compressive stress of 300 MPa, and its thickness is 60 nm. The area of the substrate was analyzed in various planes at depths of 0–50 nm, considering that the implantation of a functional impurity will be carried out within these boundaries.
The calculation showed that the area of propagation of the gradients of elastic fields in the lateral direction is very long and ranges from 2 to 3.5 μm depending on the depth of the plane under consideration. In other words, the edge elastic fields will completely fill the ion-implanted area upon implantation even into windows of 4–7 µm wide. In this case, the maximum of the stress gradient is reached directly near the edge of the mask, and the ratio jmech/jdif in this area exceeds 1000%. Such a pattern will probably be observed until the drain of point defects leads to a relaxation of stress sources due to the well-known effect of low doses.
Sputtering factor and contamination of mask atoms
The degree of influence of Si contamination by sputtered atoms of the mask was carried out on a particular example of the implantation of As+ ions into a topological window of a SiO2–Si structure with an energy of 15 keV and a dose of 1016 cm–2.
An estimate of the maximum dose of contamination can be made under the following assumptions. The small value of the mask window size makes it possible to consider a cloud of atoms over the active region homogeneous in composition with an equal concentration in the region above the mask. We will also assume that all atoms sprayed above the window experience elastic interaction with the primary ion beam leading to their introduction into the volume of the Si crystal lattice.
The coefficient of cathode sputtering in accordance with the empirical formula obtained in [8] for the normal incidence of an ion beam is approximately 0.8. This means that the average depth of the SiO2 sputtering region reaches 0.5 nm, and the density of the atoms sprayed above the window and introduced into the crystal is 1.14 · 1015 cm–2.
It should be noted that the sputtering coefficient has a strong dependence on the angle of incidence of the ions and will have a maximum value at the naturally rounded edges of the mask, which will increase the calculated value somewhat. But even without taking into account this condition, the upper estimate for the dose of contamination is about 10% of the implantation dose.
It is difficult to predict for which window sizes model assumptions are valid, but it is important that the role of contamination of mask’s atoms becomes an important factor in the formation of local submicron ion-implanted areas and acts as an additional constraint on the choice of material for the masking layer.
CONCLUSION
In this paper, the effects of controlling the ion-implanted profile of impurity atoms during the formation of local submicron active regions in Si under the action of electric and mechanical fields associated with the presence of a masking layer are considered. Estimates of the critical dimensions of such areas in which the action of the force fields is significant and require consideration in the process design of silicon microelectronics devices are made. In addition, the role of contamination of sputtered mask’s atoms in the volume of ion-doped regions is discussed, which becomes a serious factor in the contamination of the substrate as the linear dimensions of the regions scale. ■
With the approach of the design rules to the quantum-size regime, the active regions become not just smaller, the physics of the processes of their formation has a fundamentally different character [2]. In particular, the proximity of the surface and phase boundaries begins to have a significant impact and opens up new possibilities for the engineering of defects [3]. When a certain size is reached, the implantation process takes place under conditions of strong electrical and mechanical fields generated by the masking layer, which are no longer regional in nature, but their action extends to the entire area of formation of small ion-doped regions. This means that the phenomenon of radiation physics, electrical activation and the redistribution of impurities occur in fundamentally different conditions in such areas.
The study of the physical features of the formation of ion-doped regions with critically small planar dimensions will allow not only to predict technological difficulties in the way of the constant striving of modern microelectronics for miniaturization, but also to find ways to overcome them, mechanisms for controlling the geometry and properties of nanometer-doped areas.
PHENOMENOLOGY OF PARTICLE SIZE
The decrease in the design rules of discrete devices and integrated circuits of silicon microelectronics is inevitably accompanied by a decrease in the depth of the ion-doped regions and their planar dimensions. Reducing the thickness of the layers will contribute to a qualitative change in the background of non-equilibrium radiation defects, in which the formation of ion-doped regions takes place. One of the most important factors causing such features is the enhancement of electric and, in part, mechanical field gradients associated with the presence of a masking layer through which the implantation is carried out. These effects are manifested already in the process of irradiation by impurity atoms and can lead to practically significant correction of the boundaries of the ion-implanted region due to the kinetics of impurity atoms. Although the implantation temperature is not essential for impurity diffusion, the presence of diffusion and drift flows of non-equilibrium defects intensively generated during implantation will lead to the directional movement of impurity atoms through the effects of vacancy and interstitial wind [4].
It is known that point defects are electrically active and effectively interact with charged interfaces and masking layers [5]. Ion implantation into Si through a dielectric coating layer always creates a long-term electrostatic charge in the dielectric. Without discussing the nature of its appearance, we only note that the charging efficiency of the dielectric is high and, for example, for the case of the SiO2 mask is 0.1 qF, where q is the elementary electric charge and F is the implantation dose [6]. This charge will generate electric fields and lead to directional drift of charged radiation defects. In case of ion implantation into sufficiently wide windows of the masking layer, the propagation of electric fields in the region being formed is of an edge character. With a decrease in the size of the window, the fields begin to substantially overlap, and the formation of the ion-doped region occurs in fundamentally different conditions (Fig.1).
This is typical not only for electrostatic, but also for mechanical fields. The process of growing and deposition of thin films on a Si substrate is almost always accompanied by the appearance of internal mechanical stresses at the interface. Opening the window in the masking layer will cause them to splash on the edges of the mask breakage. As the window width decreases, the propagation regions of these stresses will also begin to fill the entire volume of the formed region leading to a drift of radiation vacancies into the compressed Si region, and of its own interstitial atoms – into the stretched one.
In addition, ion implantation is always accompanied by spraying the surface of the mask. With a decrease in the implantation energy that accompanies the scaling of the active regions, the efficiency of this process will only increase, since the primary ions will increasingly lose energy due to elastic losses on the surface atoms of the target. The dissolution of recoil atoms in the zone of formation of the active region becomes significant, which leads to contamination, which in some cases may contribute to the degradation of the electrical-physical parameters of the region (Fig. 2).
As long as all the considered effects are marginal, they do not significantly affect the formation of ion-doped regions. However, the tendency to decrease in design rules observed in microelectronics no longer allows them to be ignored and requires quantitative estimates of the size threshold of this transition.
QUANTITATIVE ESTIMATES
Electric field factor
Based on the data of [6] on the magnitude of the electrostatic charge arising in the SiO2 layer when ions are implanted into it, computer simulation in the TCAD Sentaurus are used to calculate the 2D distribution of electric fields in Si in the region of a window opened in a mask. The process of ion implantation of As+ with an energy of 20 keV and a dose of 1016 cm–2 into the topological structure of SiO2–Si with a mask thickness of 60 nm in the window size range of 22–180 nm was simulated. The maximum values of the electrostatic fields were observed directly in the region of the targeted implantation of impurity atoms, reaching values of about 103–104 V/cm (Fig.3). Moreover, with a decrease in the window width from 180 nm to 22 nm the field strength changes 17 times in various points in the formed region, which inevitably leads to a change in the flow of various radiation processes and, ultimately, in the impurity distribution profile.
In the case of small implantation doses, the concentration of charged radiation defects is determined by the initial position of the Fermi level in Si. Using the provisions of the theory of statistics of charge carriers in a solid, it is easy to determine that the maximum level of charged defects and hence the drift flows will be observed in silicon with the following specifications: KDB-0.002 (V+ predominance), KDB-0.05 (V++ predominance) and KEF-0.01 (V= predominance).
The depth of correction of the boundaries of the ion-implanted region due to the action of the electric fields of the mask can be estimated directly at the boundary of the region from the ratio of the drift flow of defects to the diffusion one. We assume that the criterion of a significant contribution of the drift flow to the correction of the region’s boundaries will be its achievement of 10% of the diffusion flow:
, (1)
where μ and D are the mobility and diffusion coefficient of defects; n is the concentration of defects at a localized point of the region; E is the electric field strength.
Assuming that the distribution of radiation defects n(x) is described by the Gauss law with a maximum in the region of 1.2Rp, the relation (1) will be satisfied at an electric field strength of ~10 kV/cm. In accordance with Fig.3, correction of the depth of the region due to the action of the electric field is achieved at a window size of 45 nm and increases as it decreases. Lateral blurring of the ion-implanted region, on the contrary, is significant with a large width of the window and appears up to a value of 65 nm.
Mechanical field factor
A theoretical analysis of the distribution of the tangential and normal components of the elastic fields at the edge of the opened window in the mask was performed in [7]. The results of the calculations showed that the normal components of the fields tend to be separated by the sign of the stresses at the edge of the mask leading to the formation of multidirectional drift flows of radiation vacancies (V) and internodes (I) (Fig.4). If we consider the entrainment of impurities by fluxes of point defects, the diffusion of impurity atoms is accelerated in the direction of the flow I (interstitial atoms) and against the flow V (vacancies). This means that regardless of the type and mechanism of impurity diffusion, the nature of the distribution of mechanical fields in the region of the mask edge will always contribute to the side diffusion blurring of the profile of impurity atoms.
By analogy with electrostatic fields, the criterion of a significant correction of the area was taken to achieve a drift flow of 10% of the diffusion one:
, (2)
where n and D are the concentration and diffusion coefficient of a specific type of defects; ω is the silicon atom volume; f is a correlation factor taking into account the possibility of a defect to make a reverse jump (for Si f = 0.781); ∆σ is the gradient of mechanical stresses.
It was believed that the mask is experiencing compressive stress of 300 MPa, and its thickness is 60 nm. The area of the substrate was analyzed in various planes at depths of 0–50 nm, considering that the implantation of a functional impurity will be carried out within these boundaries.
The calculation showed that the area of propagation of the gradients of elastic fields in the lateral direction is very long and ranges from 2 to 3.5 μm depending on the depth of the plane under consideration. In other words, the edge elastic fields will completely fill the ion-implanted area upon implantation even into windows of 4–7 µm wide. In this case, the maximum of the stress gradient is reached directly near the edge of the mask, and the ratio jmech/jdif in this area exceeds 1000%. Such a pattern will probably be observed until the drain of point defects leads to a relaxation of stress sources due to the well-known effect of low doses.
Sputtering factor and contamination of mask atoms
The degree of influence of Si contamination by sputtered atoms of the mask was carried out on a particular example of the implantation of As+ ions into a topological window of a SiO2–Si structure with an energy of 15 keV and a dose of 1016 cm–2.
An estimate of the maximum dose of contamination can be made under the following assumptions. The small value of the mask window size makes it possible to consider a cloud of atoms over the active region homogeneous in composition with an equal concentration in the region above the mask. We will also assume that all atoms sprayed above the window experience elastic interaction with the primary ion beam leading to their introduction into the volume of the Si crystal lattice.
The coefficient of cathode sputtering in accordance with the empirical formula obtained in [8] for the normal incidence of an ion beam is approximately 0.8. This means that the average depth of the SiO2 sputtering region reaches 0.5 nm, and the density of the atoms sprayed above the window and introduced into the crystal is 1.14 · 1015 cm–2.
It should be noted that the sputtering coefficient has a strong dependence on the angle of incidence of the ions and will have a maximum value at the naturally rounded edges of the mask, which will increase the calculated value somewhat. But even without taking into account this condition, the upper estimate for the dose of contamination is about 10% of the implantation dose.
It is difficult to predict for which window sizes model assumptions are valid, but it is important that the role of contamination of mask’s atoms becomes an important factor in the formation of local submicron ion-implanted areas and acts as an additional constraint on the choice of material for the masking layer.
CONCLUSION
In this paper, the effects of controlling the ion-implanted profile of impurity atoms during the formation of local submicron active regions in Si under the action of electric and mechanical fields associated with the presence of a masking layer are considered. Estimates of the critical dimensions of such areas in which the action of the force fields is significant and require consideration in the process design of silicon microelectronics devices are made. In addition, the role of contamination of sputtered mask’s atoms in the volume of ion-doped regions is discussed, which becomes a serious factor in the contamination of the substrate as the linear dimensions of the regions scale. ■
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