In Situ Electron Diffraction and Resistivity Characterization of Solid State Reaction Process in Cu/Al Bilayer Thin Film
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AT present, alloys and composite materials based on Al and Cu are being widely used in different areas of microelectronics and energy production. Thus, copper-cladding aluminum is widely applied for energy transmission.[1] The pair ‘‘copper–aluminum’’ has actively been studied for its application in soldered connections of wire bonding in microelectronic devices.[2,3] Copper and aluminum-based alloys have been used as junctions[4] and absorbers[5] in CMOS technology. An opportunity of using copper and aluminum-based materials for nanojoining has also been considered.[6–9] One of the most important issues concerning intermetallic compounds is the sequence of their formation and mechanisms occurring on the interface of the heterogeneous nanolayers. This is important since the formation of intermetallic compounds can result into a considerable change in the physical properties of the materials. Thus, for example, in Reference 10 it is shown that the formation of intermetallic compounds Al2Cu and Al4Cu9 on the interface of Cu/Al during the thermosonic Cu-wire bonding process results in the EVGENY T. MOISEENKO and ROMAN R. ALTUNIN are with the Siberian Federal University, 79 Svobodny pr., Krasnoyarsk, Russia 660041. SERGEY M. ZHARKOV is with the Siberian Federal University and also with the Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Akademgorodok 50/38, Krasnoyarsk, Russia 660036. Contact e-mail: [email protected]. Manuscript submitted July 20, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
increasing strength of the copper aluminum compound. In Reference 11 it is established that the Al4Cu9 phase makes the biggest contribution into the mechanical strength of the copper–aluminum compound, while the phase Al2Cu is extremely fragile and brittle at room temperature. As far as the electrical properties are concerned, in References 12 and 13 it is shown that the formation of intermetallic compounds on the interface of Cu/Al results in the increase of the electrical resistivity. According to the phase diagram,[14] in the system Al-Cu the following intermetallic compounds are formed: Al2Cu, AlCu, Al3Cu4, Al2Cu3, Al4Cu9, AlCu3. According to the model of the effective heat of formation (EHF),[15,16] on the interface of Cu/Al, the Al2Cu phase is the first to be formed, which was confirmed experimentally.[17–20] According to the calculation results for the effective heat of formation presented in Reference21 the following sequence of phase formation is to be observed on the interface of Cu/Al: Al2Cu fi AlCu fi Al3Cu4 fi Al2Cu3 fi Al4Cu9. However, in various experimental works, the researchers present various sets of phases formed on the interface of Cu/Al.[17,22–24] Besides, in spite of a great number of papers devoted to the processes of solid state reactions in Cu/Al thin films, for example,[17–19,24] the initiation temperature of the solid state reaction in the system Cu/Al has not been determined exactly. Different studies give different temperatures: 110 C,[18,19] 120 C,[24] 130 C,[20] 145 C[25] and 163
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