rus
Issue #6/2025
M. S. Kovalev, N. G. Stsepuro, E. V. Ulturgasheva, S. I. Kudryashov
Single-pulse Laser-Induced Al Transfer Onto the Si Wafer for Ohmic Contact Development
Single-pulse Laser-Induced Al Transfer Onto the Si Wafer for Ohmic Contact Development
DOI: 10.22184/1993-7296.FRos.2025.19.6.466.477
Bauman Moscow State Technical University (National Research University), Moscow, Russia
The article examines the aluminum contacts on silicon wafers developed by the laser-induced forward transfer. It is determined that a single-pulse regime with an energy density of 15 J/cm2 leads to an aluminum concentration of 18.6% and a minimum contact resistance of 439 ± 4 Ohm. The method simplifies the contact generation process and eliminates the need for complex cleaning, making it promising for future silicon-based electronic applications.
Bauman Moscow State Technical University (National Research University), Moscow, Russia
The article examines the aluminum contacts on silicon wafers developed by the laser-induced forward transfer. It is determined that a single-pulse regime with an energy density of 15 J/cm2 leads to an aluminum concentration of 18.6% and a minimum contact resistance of 439 ± 4 Ohm. The method simplifies the contact generation process and eliminates the need for complex cleaning, making it promising for future silicon-based electronic applications.
Теги: direct laser writing laser-induced forward transfer (lift) ohmic contacts optoelectronics silicon кремний лазерно-индуцированный прямой перенос (пипп) омические контакты оптоэлектроника прямая лазерная запись
Single-pulse
Laser-Induced
Al Transfer Onto the Si Wafer for Ohmic Contact Development
M. S. Kovalev 1, N. G. Stsepuro 1, 2, E. V. Ulturgasheva 1, 2, S. I. Kudryashov 1, 2
Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia
Bauman Moscow State Technical University (National Research University), Moscow, Russia
The article examines the aluminum contacts on silicon wafers developed by the laser-induced forward transfer. It is determined that a single-pulse regime with an energy density of 15 J/cm2 leads to an aluminum concentration of 18.6% and a minimum contact resistance of 439 ± 4 Ohm. The method simplifies the contact generation process and eliminates the need for complex cleaning, making it promising for future silicon-based electronic applications.
Key words: silicon, ohmic contacts, laser-induced forward transfer (LIFT), direct laser writing, optoelectronics
Article received: December 24, 2024
Article accepted: May 25, 2025
Introduction
The laser-induced forward transfer (LIFT) of functional materials is one of the key technologies in the field of microelectronics [1–3]. It is based on the use of ultrashort laser pulses to transfer the matter and create the conductive channels on various substrates, including silicon, polymers, and ceramics. This method is characterized by high precision that allows the production of small-scale components with complex geometry, as well as versatility of application [4, 5]. The localized nature of the process minimizes the heat affected zone to about 100 nm [6, 7], while demonstrating a significant improvement compared to the conventional laser technologies. This small heat affected zone is of key importance, because it helps to maintain the integrity of sensitive substrate materials and thereby reduces the risk of damage that can negatively affect the electronic performance of devices.
Surface irregularities of the materials used in electronics technologies can negatively affect the electrical performance and reliability of devices obtained by the LIFT method. Therefore, the development of electrical contacts on the uneven and rough surfaces represents a significant scientific and technical challenge. Advances in the field of laser annealing [6–8] have demonstrated the high potential of this method in overcoming this issue when developing the high-quality ohmic contacts. For example, it has been shown that the pulsed laser annealing efficiently develops such contacts on the GaN transistors with high electron mobility, even despite the surface roughness arising during the hot metallization process [8]. This efficiency is due to the non-thermal nature of laser annealing that minimizes the interdiffusion of metals, being a common drawback of conventional processing methods that can lead to the formation of rough surface morphology. Further studies have confirmed the laser annealing efficiency in developing the nickel-based ohmic contacts on n-type 4H-SiC taking into account the substrate surface characteristics [9]. The ability to treat the front side of devices prior to the contact development on the back side significantly improves the overall production process. In addition, studies of the laser annealing mechanisms have revealed its effect on the surface morphology and reduction of contact resistance due to the mixing of metal layers [10]. The LIFT technique is turning into an industrial technology for obtaining ohmic contacts that has already been demonstrated in the production of silicon solar cell panels [11].
Direct laser transfer of aluminum onto the silicon wafers is of great technological importance for the efficient formation of electrical contacts, primarily due to the laser-induced melting of the aluminum layer that facilitates its diffusion into the silicon wafer. The microsecond laser pulses lead to the efficient electrical contacts between aluminum and the silicon wafer, while bypassing the need for high-temperature processing, thereby increasing the production capacity and reducing the risk of wafer damage [12]. The dynamics of temperature fields and velocities during the laser processing can significantly affect the geometry and electrical properties of the contacts, so it is important to control the operating parameters of the laser [13]. The interaction between aluminum and silicon during the laser transfer procedure is characterized by the aluminum-induced layer exchange that allows crystallization of amorphous silicon and the development of ohmic contacts at the lower temperatures than in the case of conventional methods [14]. However, despite the advantages of laser processing technologies, some problems remain. The uneven aluminum deposition on the semiconductor wafers can result in the uneven contact development that will lead to changes in the electrical properties in the contact area. In addition, the thermal effects from the laser pulses can cause defects in the silicon wafers, potentially compromising the integrity of ohmic contacts and increasing the contact resistance [15, 16]. The risk of aluminum oxidation during the transfer process further complicates the functional specifications of the contacts.
This article provides the study of the conductive aluminum layer deposition on a silicon wafer using the laser-induced forward transfer from a donor substrate. The influence of a distance between the donor and acceptor substrates and the influence of laser parameters on the generated contact’s resistance have been studied. This allowed us to select the optimal regime for achieving the minimum contact resistance. The surface topography and chemical composition were studied using the scanning and transmission electron microscopy, as well as the energy-dispersive X-ray spectroscopy. The electrical properties of the contacts were analyzed based on the current-voltage characteristics obtained using an adjustable DC source.
Methods and materials
To obtain the ohmic contacts on the acceptor silicon wafersurface, the method of laser-induced metal transfer from the donor substrate was used. To implement the transfer process (Fig. 1), an HTF MARK laser station (manufactured by Bulat Experimental Design Office, Zelenograd, Russia) with an ytterbium fiber laser, laser pulses with a duration of 120 ns and a central wavelength of 1064 nm was used. The maximum pulse energy was 1 mJ in the TEM00 mode, and the pulse repetition rate could vary in the range from f = 20 to 80 kHz. A galvanometric double-optical scanner equipped with an F-θ lens with a focal length of 160 mm was used for focusing and surface scanning. The diameter of the focused Gaussian laser beam was about 50 μm at the level of 1/e2 of its maximum intensity. This approach ensured the required energy concentration on the wafer surface that is a critical condition for the successful development of ohmic contacts with the specified electrical characteristics.
The Levenhuk G100 object glass with an aluminum metal film with the thickness of 250 nm applied by the magnetron sputtering method was used as a donor substrate. A plane-parallel monocrystalline p-type silicon wafer with a crystallographic orientation of <100>, specific resistance of 20 Ohm · cm and thickness of 350 μm served as an acceptor substrate. When generating the ohmic contacts, the donor film was oriented toward the acceptor substrate and located horizontally at a distance of 20 to 60 μm from each other.
The laser-induced transfer of metal donor film onto the silicon wafer was performed in a single-pulse mode at a radiation scanning speed over the donor substrate of 1 000 mm / s and a laser pulse repetition rate of 20 kHz. The calculated size of the light spot in the focusing plane was 50 μm at the 1/e² level with a beam waist of 2.8 mm. The energy density during the laser radiation scanning process in the lateral plane of the donor film varied from 5 to 45 J / cm2. The dimensions of the developed areas were 1 × 1 mm2, and the distance between them was 3 mm, as shown in Fig. 2a. It should be noted that prior to the ohmic contact formation, the acceptor silicon wafer surface was not subjected to the standard four-stage cleaning process of silicon chips [17].
The analysis of changes in the crater dimensions was performed using the functional dependence of the radius square on the laser pulse energy logarithm: R2 = f (ln E). The linear approximation of dependencies given in Fig. 4 allows one to determine the radius of the spot in focus (at the level of 1 / e2) and the threshold energy value for the ablation threshold generation at a given focus. Thus, it is possible to estimate the threshold energy density (fluence) of ablation. The calculation and approximation results are shown in Fig. 2b [18].
The surface topography and chemical composition of the ohmic contacts obtained on the acceptor silicon wafer surface were analyzed during the scanning electron microscopy (SEM) process using the Vega Tescan equipment (Czech Republic). The Aztec One (Xplore) module produced by Oxford Instruments (United Kingdom) was used for the energy-dispersive X-ray spectroscopy (EDS). The studies were performed at an accelerating voltage of 5 keV [19]. The electrical properties of the contacts were analyzed based on the current-voltage characteristics using an adjustable DC source with a nominal value of 0.5, 5, 10, 100 μA and 1 mA. The measuring instruments used included a high-performance digital multimeter (model 34470A, produced by Keysight) that was applied in the circuit as a microammeter and microvoltmeter, as well as a six-dial resistance box with a reproducible resistance range from 100 mOhm to 99.9 kOhm.
Experimentation
The chemical composition and transfer continuity of the obtained ohmic contacts were studied using the SEM (Fig. 3) and analysis of the spectra obtained by the EDS. These spectra were recorded on an area of one of the contacts with the dimensions of 1×1 mm2 as a part of each series of experiments. In accordance with the previously specified parameters, the calculated radiation penetration depth was about 100 μm. This value corresponds to the X-ray radiation penetration depth into silicon at an accelerating voltage of 5 keV. The characteristic peaks were identified in the EDS spectra, confirming the presence of the chemical elements under study, on the basis of which a 2D map was constructed corresponding to the location of selected elements on the analyzed surface of the acceptor silicon substrate: Si (Fig. 4a), Al (Fig. 4c), oxygen O (Fig. 4e) and carbon C (Fig. 4g). The last two elements were selected as the combustion by-products.
Subsequently, the dependence of concentration of the metal transferred using the LIFT technology and the contact resistance on the distance between the donor and acceptor substrates was investigated at an energy density of 15.3 J/cm2. Fig. 4b, d, f, and h show the analysis results for the EDS spectra for each chemical element under study. It follows that with an increase in the distance between the substrates and with an increase in the number of pulses (ppd, pulses per dot), the Si concentration is increased (Fig. 4b). In the first case, this is directly related to the fact that the amount of transferred material becomes smaller since it is sputtered at such a distance. In the second case, this is due to the excess energy that is deposited in the acceptor substrate, as a result of which the transferred metal begins to melt due to its small thickness, and then completely evaporates. This fact is confirmed by the details provided in Fig. 4d. In this case, the plasma generated in the focusing region does not allow oxygen access to the area of interaction of laser radiation and Si. Due to this fact, the oxygen concentration is reduced with an increase in the number of pulses (ppd) (Fig. 4f).
After the optimal distance between the acceptor and donor substrates was determined to be 20 μm, the dependences of the single-pulse laser-induced Al transfer to the Si wafer (when developing the ohmic contacts) on the energy density were studied. The X-ray spectra analysis of the silicon acceptor wafer under the conditions of ohmic contact formation demonstrates the key information about any materials at various energy densities (Fig. 5). The Si concentration remains stable at a level of about 36.6% that indicates the preserved wafer integrity under the conditions of energy density changes. Moreover, a noticeable decrease in the aluminum concentration is observed that is related to the exponential nature of the concentration dependence on energy density: the higher density values lead to the increased aluminum layer ablation and, accordingly, to less transfer of the Al material to the silicon wafer. These data are confirmed by Fig. 4a that demonstrates the removal degree of the surface material from the substrate. In addition, an increased concentration of carbon C and other by-products is observed that is explained by exceeding the ablation threshold value of the aluminum film. The excess laser pulse energy not only facilitates the aluminum separation process, but also promotes its evaporation during the transfer that reduces its concentration on the acceptor substrate. The increased rates of ablation and evaporation processes also lead to the more significant roughness of the substrate surface that can negatively affect the electrical properties of ohmic contacts. Thus, the relations between the energy density, material transfer process and surface morphology are critical for optimizing the specifications of ohmic contacts.
Moreover, information on the analysis of EDS spectra correlates rather well with the measurement results of the resistance of the obtained ohmic contacts. The results shown in Fig. 6 are proportional to the Al concentration on the acceptor substrate. The best indicator of the ohmic resistance value was achieved at a distance between the contacts of 20 μm. Moreover, starting from 10 ppd, the contact resistance remains practically unchanged. This fact means that the pulse energy that is not spent on transferring the donor metal to the acceptor substrate is spent on annealing the obtained Al contact.
Results and discussion
Based on the averaged data obtained as a result of ten experiments, it can be concluded that with increasing energy density, the resistance between the areas under study is increased. Moreover, the dispersion of results obtained increases, varying from 0.5% to 5.37%. These changes can be explained, first of all, by the decreased amount of metal transferred from the donor substrate to the acceptor one that contributes to the greater preservation of the natural oxide surface layer during the laser-induced transfer process. Secondly, this is due to an increased roughness coefficient of the acceptor substrate. Despite the fact that with increasing energy density, the aluminum concentration is decreased, the oxygen percentage remains at the level of 26.4% throughout the entire studied range. In this case, the current-voltage characteristic of the ohmic contacts of aluminum on the Si substrate is linear and symmetrical, as shown in Fig. 7.
Conclusions
This paper investigates the chemical, structural, and electrical characteristics of aluminum contacts generated on the silicon wafers using the laser-induced forward transfer method. As a result of a systematic series of experiments, the optimal processing parameters were selected, including the energy density F = 15 J / cm² in the single-pulse regime. This experimental setup allowed to obtain the aluminum concentration of 18.6% and provided a minimum contact resistance of 439 ± 4 Ohm. The laser-induced forward transfer method has significant advantages over the conventional methods, since it eliminates the need for a standard four-step cleaning procedure that usually includes immersion in the hydrofluoric acid to remove the silicon oxide layer. This simplification not only optimizes the contact generation process, but also increases the efficiency of ohmic contact development.
The results of our study demonstrate the high potential of using the laser-induced forward transfer method in the processes of developing aluminum-silicon contacts for the industrial silicon-based electronic applications. The ability to obtain high-quality contacts while eliminating the stages of a comprehensive surface cleaning procedure makes this method a realistic alternative in the field of semiconductor technologies.
Funding
The results were obtained as a part of the state assignment of the Ministry of Science and Higher Education of the Russian Federation No. FSFN‑2024-0019.
AUTHORS
Kovalev M. S., Cand.of Sc.(Tech.), Senior Researcher, Lebedev Physical Institute of the Russian Academy of Sciences (LPI), e-mail: kovalevms@lebedev.ru, Moscow, Russia.
ORCID: 0000-0001-5074-0718
Stsepuro N. G., Junior Research Fellow, LPI, Senior Lecturer, Bauman Moscow State Technical University (National Research University (Bauman Moscow State Technical University), Moscow, Russia.
ORCID: 0000-0002-8383-7396
Ulturgasheva E. V., Junior Research Fellow, LPI, Junior Research Fellow, Bauman Moscow State Technical University, Moscow, Russia.
Kudryashov S. I., Dr. of Sc.(Phys.&Math.), Leading Researcher at the LPI, Leading Researcher at the Bauman Moscow State Technical University.
ORCID: 0000-0001-6657-2739
AUTHOR CONTRIBUTION
Kovalev M. S.: idea of the work; Scepuro N. G.: processing of the results; Ulturgasheva E. V.: suggestions and comments; Kudryashov S. I.: organization of the work.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest in the author team. The suggestions and comments received during the work have been accepted by all authors.
Laser-Induced
Al Transfer Onto the Si Wafer for Ohmic Contact Development
M. S. Kovalev 1, N. G. Stsepuro 1, 2, E. V. Ulturgasheva 1, 2, S. I. Kudryashov 1, 2
Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia
Bauman Moscow State Technical University (National Research University), Moscow, Russia
The article examines the aluminum contacts on silicon wafers developed by the laser-induced forward transfer. It is determined that a single-pulse regime with an energy density of 15 J/cm2 leads to an aluminum concentration of 18.6% and a minimum contact resistance of 439 ± 4 Ohm. The method simplifies the contact generation process and eliminates the need for complex cleaning, making it promising for future silicon-based electronic applications.
Key words: silicon, ohmic contacts, laser-induced forward transfer (LIFT), direct laser writing, optoelectronics
Article received: December 24, 2024
Article accepted: May 25, 2025
Introduction
The laser-induced forward transfer (LIFT) of functional materials is one of the key technologies in the field of microelectronics [1–3]. It is based on the use of ultrashort laser pulses to transfer the matter and create the conductive channels on various substrates, including silicon, polymers, and ceramics. This method is characterized by high precision that allows the production of small-scale components with complex geometry, as well as versatility of application [4, 5]. The localized nature of the process minimizes the heat affected zone to about 100 nm [6, 7], while demonstrating a significant improvement compared to the conventional laser technologies. This small heat affected zone is of key importance, because it helps to maintain the integrity of sensitive substrate materials and thereby reduces the risk of damage that can negatively affect the electronic performance of devices.
Surface irregularities of the materials used in electronics technologies can negatively affect the electrical performance and reliability of devices obtained by the LIFT method. Therefore, the development of electrical contacts on the uneven and rough surfaces represents a significant scientific and technical challenge. Advances in the field of laser annealing [6–8] have demonstrated the high potential of this method in overcoming this issue when developing the high-quality ohmic contacts. For example, it has been shown that the pulsed laser annealing efficiently develops such contacts on the GaN transistors with high electron mobility, even despite the surface roughness arising during the hot metallization process [8]. This efficiency is due to the non-thermal nature of laser annealing that minimizes the interdiffusion of metals, being a common drawback of conventional processing methods that can lead to the formation of rough surface morphology. Further studies have confirmed the laser annealing efficiency in developing the nickel-based ohmic contacts on n-type 4H-SiC taking into account the substrate surface characteristics [9]. The ability to treat the front side of devices prior to the contact development on the back side significantly improves the overall production process. In addition, studies of the laser annealing mechanisms have revealed its effect on the surface morphology and reduction of contact resistance due to the mixing of metal layers [10]. The LIFT technique is turning into an industrial technology for obtaining ohmic contacts that has already been demonstrated in the production of silicon solar cell panels [11].
Direct laser transfer of aluminum onto the silicon wafers is of great technological importance for the efficient formation of electrical contacts, primarily due to the laser-induced melting of the aluminum layer that facilitates its diffusion into the silicon wafer. The microsecond laser pulses lead to the efficient electrical contacts between aluminum and the silicon wafer, while bypassing the need for high-temperature processing, thereby increasing the production capacity and reducing the risk of wafer damage [12]. The dynamics of temperature fields and velocities during the laser processing can significantly affect the geometry and electrical properties of the contacts, so it is important to control the operating parameters of the laser [13]. The interaction between aluminum and silicon during the laser transfer procedure is characterized by the aluminum-induced layer exchange that allows crystallization of amorphous silicon and the development of ohmic contacts at the lower temperatures than in the case of conventional methods [14]. However, despite the advantages of laser processing technologies, some problems remain. The uneven aluminum deposition on the semiconductor wafers can result in the uneven contact development that will lead to changes in the electrical properties in the contact area. In addition, the thermal effects from the laser pulses can cause defects in the silicon wafers, potentially compromising the integrity of ohmic contacts and increasing the contact resistance [15, 16]. The risk of aluminum oxidation during the transfer process further complicates the functional specifications of the contacts.
This article provides the study of the conductive aluminum layer deposition on a silicon wafer using the laser-induced forward transfer from a donor substrate. The influence of a distance between the donor and acceptor substrates and the influence of laser parameters on the generated contact’s resistance have been studied. This allowed us to select the optimal regime for achieving the minimum contact resistance. The surface topography and chemical composition were studied using the scanning and transmission electron microscopy, as well as the energy-dispersive X-ray spectroscopy. The electrical properties of the contacts were analyzed based on the current-voltage characteristics obtained using an adjustable DC source.
Methods and materials
To obtain the ohmic contacts on the acceptor silicon wafersurface, the method of laser-induced metal transfer from the donor substrate was used. To implement the transfer process (Fig. 1), an HTF MARK laser station (manufactured by Bulat Experimental Design Office, Zelenograd, Russia) with an ytterbium fiber laser, laser pulses with a duration of 120 ns and a central wavelength of 1064 nm was used. The maximum pulse energy was 1 mJ in the TEM00 mode, and the pulse repetition rate could vary in the range from f = 20 to 80 kHz. A galvanometric double-optical scanner equipped with an F-θ lens with a focal length of 160 mm was used for focusing and surface scanning. The diameter of the focused Gaussian laser beam was about 50 μm at the level of 1/e2 of its maximum intensity. This approach ensured the required energy concentration on the wafer surface that is a critical condition for the successful development of ohmic contacts with the specified electrical characteristics.
The Levenhuk G100 object glass with an aluminum metal film with the thickness of 250 nm applied by the magnetron sputtering method was used as a donor substrate. A plane-parallel monocrystalline p-type silicon wafer with a crystallographic orientation of <100>, specific resistance of 20 Ohm · cm and thickness of 350 μm served as an acceptor substrate. When generating the ohmic contacts, the donor film was oriented toward the acceptor substrate and located horizontally at a distance of 20 to 60 μm from each other.
The laser-induced transfer of metal donor film onto the silicon wafer was performed in a single-pulse mode at a radiation scanning speed over the donor substrate of 1 000 mm / s and a laser pulse repetition rate of 20 kHz. The calculated size of the light spot in the focusing plane was 50 μm at the 1/e² level with a beam waist of 2.8 mm. The energy density during the laser radiation scanning process in the lateral plane of the donor film varied from 5 to 45 J / cm2. The dimensions of the developed areas were 1 × 1 mm2, and the distance between them was 3 mm, as shown in Fig. 2a. It should be noted that prior to the ohmic contact formation, the acceptor silicon wafer surface was not subjected to the standard four-stage cleaning process of silicon chips [17].
The analysis of changes in the crater dimensions was performed using the functional dependence of the radius square on the laser pulse energy logarithm: R2 = f (ln E). The linear approximation of dependencies given in Fig. 4 allows one to determine the radius of the spot in focus (at the level of 1 / e2) and the threshold energy value for the ablation threshold generation at a given focus. Thus, it is possible to estimate the threshold energy density (fluence) of ablation. The calculation and approximation results are shown in Fig. 2b [18].
The surface topography and chemical composition of the ohmic contacts obtained on the acceptor silicon wafer surface were analyzed during the scanning electron microscopy (SEM) process using the Vega Tescan equipment (Czech Republic). The Aztec One (Xplore) module produced by Oxford Instruments (United Kingdom) was used for the energy-dispersive X-ray spectroscopy (EDS). The studies were performed at an accelerating voltage of 5 keV [19]. The electrical properties of the contacts were analyzed based on the current-voltage characteristics using an adjustable DC source with a nominal value of 0.5, 5, 10, 100 μA and 1 mA. The measuring instruments used included a high-performance digital multimeter (model 34470A, produced by Keysight) that was applied in the circuit as a microammeter and microvoltmeter, as well as a six-dial resistance box with a reproducible resistance range from 100 mOhm to 99.9 kOhm.
Experimentation
The chemical composition and transfer continuity of the obtained ohmic contacts were studied using the SEM (Fig. 3) and analysis of the spectra obtained by the EDS. These spectra were recorded on an area of one of the contacts with the dimensions of 1×1 mm2 as a part of each series of experiments. In accordance with the previously specified parameters, the calculated radiation penetration depth was about 100 μm. This value corresponds to the X-ray radiation penetration depth into silicon at an accelerating voltage of 5 keV. The characteristic peaks were identified in the EDS spectra, confirming the presence of the chemical elements under study, on the basis of which a 2D map was constructed corresponding to the location of selected elements on the analyzed surface of the acceptor silicon substrate: Si (Fig. 4a), Al (Fig. 4c), oxygen O (Fig. 4e) and carbon C (Fig. 4g). The last two elements were selected as the combustion by-products.
Subsequently, the dependence of concentration of the metal transferred using the LIFT technology and the contact resistance on the distance between the donor and acceptor substrates was investigated at an energy density of 15.3 J/cm2. Fig. 4b, d, f, and h show the analysis results for the EDS spectra for each chemical element under study. It follows that with an increase in the distance between the substrates and with an increase in the number of pulses (ppd, pulses per dot), the Si concentration is increased (Fig. 4b). In the first case, this is directly related to the fact that the amount of transferred material becomes smaller since it is sputtered at such a distance. In the second case, this is due to the excess energy that is deposited in the acceptor substrate, as a result of which the transferred metal begins to melt due to its small thickness, and then completely evaporates. This fact is confirmed by the details provided in Fig. 4d. In this case, the plasma generated in the focusing region does not allow oxygen access to the area of interaction of laser radiation and Si. Due to this fact, the oxygen concentration is reduced with an increase in the number of pulses (ppd) (Fig. 4f).
After the optimal distance between the acceptor and donor substrates was determined to be 20 μm, the dependences of the single-pulse laser-induced Al transfer to the Si wafer (when developing the ohmic contacts) on the energy density were studied. The X-ray spectra analysis of the silicon acceptor wafer under the conditions of ohmic contact formation demonstrates the key information about any materials at various energy densities (Fig. 5). The Si concentration remains stable at a level of about 36.6% that indicates the preserved wafer integrity under the conditions of energy density changes. Moreover, a noticeable decrease in the aluminum concentration is observed that is related to the exponential nature of the concentration dependence on energy density: the higher density values lead to the increased aluminum layer ablation and, accordingly, to less transfer of the Al material to the silicon wafer. These data are confirmed by Fig. 4a that demonstrates the removal degree of the surface material from the substrate. In addition, an increased concentration of carbon C and other by-products is observed that is explained by exceeding the ablation threshold value of the aluminum film. The excess laser pulse energy not only facilitates the aluminum separation process, but also promotes its evaporation during the transfer that reduces its concentration on the acceptor substrate. The increased rates of ablation and evaporation processes also lead to the more significant roughness of the substrate surface that can negatively affect the electrical properties of ohmic contacts. Thus, the relations between the energy density, material transfer process and surface morphology are critical for optimizing the specifications of ohmic contacts.
Moreover, information on the analysis of EDS spectra correlates rather well with the measurement results of the resistance of the obtained ohmic contacts. The results shown in Fig. 6 are proportional to the Al concentration on the acceptor substrate. The best indicator of the ohmic resistance value was achieved at a distance between the contacts of 20 μm. Moreover, starting from 10 ppd, the contact resistance remains practically unchanged. This fact means that the pulse energy that is not spent on transferring the donor metal to the acceptor substrate is spent on annealing the obtained Al contact.
Results and discussion
Based on the averaged data obtained as a result of ten experiments, it can be concluded that with increasing energy density, the resistance between the areas under study is increased. Moreover, the dispersion of results obtained increases, varying from 0.5% to 5.37%. These changes can be explained, first of all, by the decreased amount of metal transferred from the donor substrate to the acceptor one that contributes to the greater preservation of the natural oxide surface layer during the laser-induced transfer process. Secondly, this is due to an increased roughness coefficient of the acceptor substrate. Despite the fact that with increasing energy density, the aluminum concentration is decreased, the oxygen percentage remains at the level of 26.4% throughout the entire studied range. In this case, the current-voltage characteristic of the ohmic contacts of aluminum on the Si substrate is linear and symmetrical, as shown in Fig. 7.
Conclusions
This paper investigates the chemical, structural, and electrical characteristics of aluminum contacts generated on the silicon wafers using the laser-induced forward transfer method. As a result of a systematic series of experiments, the optimal processing parameters were selected, including the energy density F = 15 J / cm² in the single-pulse regime. This experimental setup allowed to obtain the aluminum concentration of 18.6% and provided a minimum contact resistance of 439 ± 4 Ohm. The laser-induced forward transfer method has significant advantages over the conventional methods, since it eliminates the need for a standard four-step cleaning procedure that usually includes immersion in the hydrofluoric acid to remove the silicon oxide layer. This simplification not only optimizes the contact generation process, but also increases the efficiency of ohmic contact development.
The results of our study demonstrate the high potential of using the laser-induced forward transfer method in the processes of developing aluminum-silicon contacts for the industrial silicon-based electronic applications. The ability to obtain high-quality contacts while eliminating the stages of a comprehensive surface cleaning procedure makes this method a realistic alternative in the field of semiconductor technologies.
Funding
The results were obtained as a part of the state assignment of the Ministry of Science and Higher Education of the Russian Federation No. FSFN‑2024-0019.
AUTHORS
Kovalev M. S., Cand.of Sc.(Tech.), Senior Researcher, Lebedev Physical Institute of the Russian Academy of Sciences (LPI), e-mail: kovalevms@lebedev.ru, Moscow, Russia.
ORCID: 0000-0001-5074-0718
Stsepuro N. G., Junior Research Fellow, LPI, Senior Lecturer, Bauman Moscow State Technical University (National Research University (Bauman Moscow State Technical University), Moscow, Russia.
ORCID: 0000-0002-8383-7396
Ulturgasheva E. V., Junior Research Fellow, LPI, Junior Research Fellow, Bauman Moscow State Technical University, Moscow, Russia.
Kudryashov S. I., Dr. of Sc.(Phys.&Math.), Leading Researcher at the LPI, Leading Researcher at the Bauman Moscow State Technical University.
ORCID: 0000-0001-6657-2739
AUTHOR CONTRIBUTION
Kovalev M. S.: idea of the work; Scepuro N. G.: processing of the results; Ulturgasheva E. V.: suggestions and comments; Kudryashov S. I.: organization of the work.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest in the author team. The suggestions and comments received during the work have been accepted by all authors.
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