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DOI: 10.17516/1997-1397-2022-15-4-493-499 УДК 537.9
Solid-state Synthesis of Cu6Sn5 Intermetallic in Sn/Cu Thin Films
Liudmila E. Bykova* Victor G. Myagkov^ Yuri Yu. Balashov Victor S. Zhigalov
Kirensky Institute of Physics Federal Research Center KSC SB RAS Krasnoyarsk, Russian Federation
Gennady S. Patrin
Siberian Federal University Krasnoyarsk, Russian Federation Kirensky Institute of Physics Federal Research Center KSC SB RAS Krasnoyarsk, Russian Federation
Received 16.12.2021, received in revised form 11.02.2022, accepted 20.03.2022
Abstract. The study results of solid-state synthesis of the CueSng intermetallic in the Sn/Cu thin-film systems during vacuum annealing from room temperature to 300 °C are presented. The initiation and finishing temperatures of the solid-state reaction between the Cu and Sn nanolayers and the phase composition of the reaction products were determined. The synthesized thin films were monophasic and consisted of the hexagonal ^-CueSng phase. It is assumed that the initiation temperature of the solidstate reaction in Sn/Cu thin films is associated with the start temperature of the reverse polymorphic transformation n' ^ П between the monoclinic and hexagonal CueSng phases.
Keywords: thin films, solid-state synthesis, CueSng intermetallic, n ^ n' reversible polymorphic transformations.
Citation: L.E. Bykova, V.G. Myagkov, Y.Y. Balashov, V.S. Zhigalov, G.S. Patrin, Solid-state Synthesis of CuaSn5 Intermetallic in Sn/Cu Thin Films, J. Sib. Fed. Univ. Math. Phys., 2022, 15(4), 493-499. DOI: 10.17516/1997-1397-2022-15-4-493-499.
Introduction
Cu6Sn5 is an important intermetallic for electric interconnects and anode materials of Li-ion batteries, with n ^ n' reversible polymorphic transformations between the hexagonal and monoclinic Cu6Sn5 phases. The polymorphic transformation of Cu6Sn5 has the potential to generate internal stresses in soldered joints during both the soldering process and the subsequent device operation involving thermal cycling [1]. It is known that a layer of Cu6Sn5 forms instantaneously between the Sn rich solder and the Cu substrate during the early stages of soldering [2-4]. However, fundamental understanding of intermetallic compound formation and the associated interface motion between the Cu and Sn nanolayers during the reaction is still very limited. An understanding of these fundamental properties is essential for better control and improvement of the process of nano-soldering and Cu6Sn5 intermetallic synthesis. The Sn/Cu thin films were chosen as a model system for Cu6Sn5 intermetallic synthesis.
* lebyk@iph.krasn.ru http://orcid/0000-0002-6569-2533 t http://orcid/0000-0002-3276-0309 © Siberian Federal University. All rights reserved
Numerous studies of solid-state synthesis in nanofilms show that as the annealing temperature at the interface of the film reagents is increased, a certain temperature Tin (initiation temperature) is reached where only one phase, called the first phase, is formed. As the annealing temperature is further increased other phases can also occur, forming a phase sequence [5-7]. The formation of only one first phase among many equilibrium phases, the low initiation temperatures and the migration of only one element during the reaction [8,9] are unique and inexplicable phenomena of solid-state synthesis in thin films.
Our previous works on solid-state reactions in thin films showed that the first phase is the phase that, according to the phase diagram, has the lowest temperature of structural phase transformation (TK) and the initiation temperature (Tin) of the solid-state reaction coincides with the temperature of the solid-state structural transformation of the first phase (Tin = TK) [10, and references therein]. The equality Tin = TK indicates the general nature of chemical interactions that control both solid-state transformations and solid-state reactions in bilayers. Consequently, low-temperature reactions in A/B bilayers occur only in binary systems A-B, which have corresponding low-temperature solid-state transformations. Therefore, if the A-B phase equilibrium diagram is well established then the equality Tin = TK makes it possible to predict the first phase and its initiation temperature. Conversely, the study of reactions in A/B bilayers with different layer ratios is the study and refinement of the low-temperature part of the A-B phase diagram [10].
This work presents the results of studying the synthesis of Cu6Sn5 thin films obtained by initiating a solid-state reaction in bilayer Sn/Cu film systems. The synthesis was carried out by vacuum annealing from room temperature to 300 °C. The initiation and finishing temperatures of the solid-state reaction between the Cu and Sn nanolayers and the phase composition of the reaction products were determined. Based on the analysis of the thin-film solid-state reactions, the general chemical mechanisms controlling both the synthesis of Cu6Sn5 in the Sn/Cu bilayer and the reversible polymorphic transformations n ^ n' between hexagonal and monoclinic phases Cu6Sn5 are affirmed.
Experimental procedures
To prepare the bilayer Sn(110 nm)/Cu(60 nm) films, the thicknesses of the Cu and Sn layers were chosen from the composition ratio of the Cu6Sn5 intermetallic compound (= 54.5 at.% Cu, = 45.5 at.% Sn). The fabrication of the initial bilayer Sn/Cu films was carried out by thermal deposition of the Sn and Cu, with thicknesses of 110 nm and 60 nm, respectively, on a glass substrate in a vacuum at a residual pressure of 1.3 x 10~4 Pa. To prevent an uncontrolled reaction between the Sn and Cu layers, the deposition was carried out at room temperature. Materials of a high level of purity were used for the evaporation: Cu (99.99 %), and Sn (99.995 %). The Sn and Cu layer thicknesses were determined by X-ray fluorescence analysis. The total thicknesses of the bilayer films under study were ~ 170 nm.
The initial samples were annealed under a vacuum of 1.3 x10~4 Pa Pa at temperatures from room temperature to 300 °C in intervals of 50 °C, holding for 30 minutes at each temperature. The phase formations and structural changes occurring at the interface between the Sn and Cu layers as the annealing temperature increased were identified with a DR0N-4-07 diffractometer (Cu^a radiation). The electron microscopy studies were performed on a Hitachi HT7700 transmission electron microscope equipped with a scanning transmission electron microscopy system and with an energy dispersion X-ray spectrometer.
Results and discussion
The initial Sn/Cu samples were bilayer thin films consisting of Sn and Cu nanolayers. The diffraction pattern (Fig. 1a), obtained from the initial Sn/Cu film showed diffraction reflections
characteristic of the phases: ,0-Sn (the space group I4i/amd, lattice constants: a = 5.831 A, b = 5.831A, c = 3.182 A PDF Card #00-004-0673) and Cu (the space group Fm-3m, lattice constant a = 3.615 A PDF Card # 00-004-0836). On Fig. 1c,d,e the diffraction patterns obtained at 200 °C, 250°C and 300°C are shown.
Fig. 1. X-ray diffraction patterns of Sn(110nm)/Cu(60nm) thin-film systems under vacuum annealing from room temperature to 300 °C
The initial samples were annealed under a vacuum of 1.3 x 10~4 Pa at temperatures from room temperature to 300°C. The diffraction pattern did not change until the temperature reached 100 °C, when, in addition to the reflections corresponding to the Cu and ,0-Sn phases, the diffraction reflections from the ^-Cu6Sn5 phase appeared (the space group P63/mmc, lattice constants a = 4.206 A, c = 5.097 A, PDF Card #00-047-1575), indicating the beginning of the reaction between the Cu and Sn nanolayers (Fig. 1b). On Fig. 1c the electron diffraction patterns obtained at 200 °C are shown. Reflections corresponding to the ^-Cu6Sn5 phase are observed, and the intensities of the reflections corresponding to the Cu and Sn phases decrease, which indicates the continuation of the reaction between Cu and Sn. During the annealing process at 250 °C and 300 °C and after cooling the sample to 25 °C (Fig. 1d, e), the diffraction pattern contained
only the reflections from the n-Cu6Sn5 phase, confirming that the Cu and Sn have reacted completely. A small number of low-intensity reflections from the unreacted tin were also present in the diffractogram. When the film was cooled under a vacuum from 300 °C to 25 °C, no phase transformation n ^ n' was observed.
From the foregoing, it follows that the reaction between the copper and tin nanolayers in the film begins at an initiation temperature of 100 ° C and proceeds up to a temperature of 250 ° C over a length equal to the total film thickness d = 170 nm. The synthesized thin films were monophasic and consisted of the hexagonal n-Cu6 Sn5 phase with a small amount of unreacted tin. The n-Cu6Sn5 grain size was estimated from the width of the Cu6Sn5 (110) reflection (Fig. 1d, inset) by the Scherrer formula d = kA/fi cos 0, where d is the mean crystal grain size, f3 is the diffraction maximum width measured at half the maximum, A is the X-ray radiation wavelength (0.15418 nm), 0 is the diffraction angle corresponding to the maximum of the peak, and k = 0.9. The calculated size of the n-Cu6Sn5 crystal grains was 55 ± 5 nm, which agrees well with the data obtained by the electron microscope (Fig. 2).
500 nm
Fig. 2. The electron microscope image of Sn(110nm)/Cu(60nm) thin-film after vacuum annealing at 300 °C
Our approach predicts that the solid-state reaction at the Sn/Cu interface should start with the formation of the first Cu6Sn5 phase at an initiation temperature Tin, regardless of the ratio of layer thicknesses, coinciding with the temperature of structural transformations in the Cu6Sn5 phase. According to the Cu-Sn phase equilibrium diagram the structural phase transition between the high-temperature hexagonal n-Cu6Sn5 and low-temperature monoclinic n'-Cu6Sn5 phases, which has a minimum temperature of 186°C, is well established [11]. However, as is known, reversible phase transformations have a temperature hysteresis. For example, martensitic transformations are characterized by the temperature of the start (Ms) and the finish (Mf) of the direct transformation upon cooling and the temperature of the start (As) and the finish (Af) of the reverse transformation upon heating. Therefore, it is reasonable to assume that the initiation temperature Tin coincides with the reverse transformation start temperature As (Tin = As). The equality Tin = As is satisfied by reactions in bilayers of Al/Ni [12], Ti/Ni [13], Mn/Fe [14] and
Cd/Au [15,16], in which austenitic and martensitic phases are formed in the reaction products. Similarly, the n ^ n' reversible transformation can be characterized by the start (Ts(n ^ n')) and finish (Tf (n ^ n') temperatures of the direct transformation during cooling and the start (Ts(n' ^ n)) and finish (Tf (n' ^ n)) temperatures of the reverse transformation during heating. As the temperature rises, the transition temperature Ts(n' ^ n) can be defined as the starting temperature of the heating flow change using differential scanning calorimetry curves, which for Cu6Sn5 bulk samples is approximately equal to ~ 150°C [17]. Studies of nanoscale systems have revealed that their phase transition temperatures are normally lower than those of bulk materials, which is due to the large surface area-to-volume ratio and the large number of defects [18,19]. These factors and the extremely slow kinetics of the n' ^ n phase transformations may lead to the decreased start and finish temperatures of the direct and reverse transformations in the Cu6Sn5 thin films.
This study gives reason to assume that the temperature Ts(n' ^ n) coincides with the initiation temperature Tin (Ts(n' ^ n) = Tin = 100 °C). However, the real temperature Ts(n' ^ n) can be even lower. So K.N. Tu reports the growth of the n'-Cu6Sn5 phase at temperatures below room temperature [20]. This suggests a general scenario for solid-state synthesis in Sn/Cu bilayer films and reversible n ^ n' phase transformations: as the temperature increases to Tin = Ts (n' ^ n) the Sn/Cu interface remains sharp and at temperatures above Tin = Ts (n' ^ n) chemical interactions arise between the Sn and Cu atoms that destroy the chemical bonds in the Cu layer (Cu is the dominant diffusing species [20]) and transfer Cu atoms to the Sn layer to form the first n-Cu6Sn5 phase.
Conclusion
The solid-state synthesis of the Cu6Sn5 intermetallic compound was carried out by vacuum annealing of Sn(110 nm)/Cu(60 nm) thin films from room temperature to 300 °C. The thicknesses of the Cu and Sn layers were chosen to correspond to the composition ratio of Cu6Sn5 (= 54.5 at.% Cu, = 45.5 at.% Sn). The synthesis of the n-Cu6Sn5 phase begins at the initiation temperature Tin ~ 100°C and proceeds up to a temperature of ~ 250 °C over the entire film thickness d = 170 nm. No phase transformation n ^ n' was observed when the film was cooled in the vacuum from 300 °C to 25 °C. The synthesized thin films were monophase and consisted of the hexagonal n-Cu6Sn5 phase with a small amount of unreacted tin. It is assumed that the initiation temperature of the solid-state reaction in Sn/Cu thin films is associated with the start temperature of the reverse polymorphic transformation n' ^ n between the monoclinic and hexagonal Cu6Sn5 phases. The analysis of the results obtained in this and many other works shed new light on the nature of atomic migration and first phase formation in the process of solid-state reactions in nanomaterials at low temperatures.
This study was supported by the Russian Foundation for Basic Research, Government of Krasnoyarsk Territory, Krasnoyarsk Regional Fund of Science to the research projects no. 194-3-24-0003. The electron microscopy investigations were conducted in the Krasnoyarsk Regional Center of Research Equipment of the Federal Research Center "Krasnoyarsk Science Center SB RAS". We thank G. N. Bondarenko for carrying out the X-ray structural experiments.
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Твердофазный синтез интерметаллида CueSns в тонких пленках Sn/Cu
Людмила Е. Быкова Виктор Г. Мягков Юрий Ю. Балашов Виктор С. ^Кигалов
Институт физики им. Киренского, ФИЦ КНЦ СО РАН Красноярск, Российская Федерация
Геннадий С. Патрин
Сибирский федеральный университет Красноярск, Российская Федерация Институт физики им. Киренского, ФИЦ КНЦ СО РАН Красноярск, Российская Федерация
Аннотация. Приведены результаты исследований твердофазного синтеза интерметаллида СибЯщ в тонкопленочной системе Яп/Си при отжиге в вакууме от комнатной температуры до 300 °С. Определены температуры начала и окончания твердофазной реакции между нанослоями Си и Яп, а также фазовый состав продуктов реакции. Синтезированные тонкие пленки были однофазными и состояли из гексагональной фазы п-Си6Вп5. Сделано предположение, что температура начала твердофазной реакции в тонких пленках Яп/Си связана с температурой начала обратного полиморфного превращения п' ^ П между моноклинной и гексагональной фазами СщВпб.
Ключевые слова: тонкие плёнки, твердофазный синтез, интерметаллид СибВп5, обратимый фазовый переход п ^ п'.
