The effect of thermal treatment on the stress state and evolving microstructure of Cu/W nano-multilayers

Abstract

The functionality and reliability of nano-multilayered devices and components are largely affected by the stress evolution during fabrication, processing, and operation. The impact of thermal treatment on the stress state and evolving microstructure of Cu/W nano-multilayers, as deposited on different substrates (i.e., Si(001), Al 2 O 3-C, and Al 2 O 3-R) by magnetron sputtering, was investigated by in-situ high temperature X-ray diffraction and high-resolution scanning electron microcopy. The as-deposited Cu and W nanolayers exhibit an out-of-plane orientation relationship according to Cu h111ijj W h110i. On the Al 2 O 3-C and Al 2 O 3-R substrates, the Cu/W nanomultilayers also develop a pronounced in-plane texture given by Cu f111gh10 1ijj W f110gh00 1i. The stress state of the Cu nanolayers in the as-deposited state and upon heating, investigated ex-situ, is largely imposed by the accumulated stresses in the much stiffer W nanolayers. In the as-deposited state, the W nanolayers exhibit a much larger in-plane compressive stress than the Cu nanolayers (i.e., À3.5 GPa versus À1.5 GPa), which both mainly originate from growth stresses generated during the deposition process. The growth stresses in the as-deposited Cu nanolayers are relaxed after annealing at 500 C. Relief of compressive stresses in the W nanolayers is accompanied by grain coarsening which only occurs upon degradation of the nano-multilayered structure. The degradation of the periodic layer structure proceeds in the range of 750 À 900 C and is independent of the substrate.

Nano-Structured Cu/W Brazing Fillers for Advanced Joining Applications

“…Annealing up to 500 ºC fully releases the stresses of the Cu nanolayers [19], which also coincides with the temperature at which a high number of Cu protrusions appear on the surface [18]. At T > 700 ºC, degradation of the NML structure occurs [18,19]. Consequently, it is suggested for W 25nm + (Cu 5nm /W 5nm ) × 100 that upon heating (> 500 ºC) Cu penetrates through the W layers to the bond interface.…”

“…As shown by these studies, in the as deposited state very high compressive stresses (related mainly to growth stresses initiate by the PVD process) were observed in the confined Cu (≈ -1.5 GPa) and W layers (≈ -3.5 GPa) [19]. Annealing up to 500 ºC fully releases the stresses of the Cu nanolayers [19], which also coincides with the temperature at which a high number of Cu protrusions appear on the surface [18]. At T > 700 ºC, degradation of the NML structure occurs [18,19].…”

“…The microstructural evolution upon heating of the same Cu/W NML system (deposited on sapphire) has been studied in detail [18] and the corresponding stress evolution was comprehensively investigated [19]. As shown by these studies, in the as deposited state very high compressive stresses (related mainly to growth stresses initiate by the PVD process) were observed in the confined Cu (≈ -1.5 GPa) and W layers (≈ -3.5 GPa) [19]. Annealing up to 500 ºC fully releases the stresses of the Cu nanolayers [19], which also coincides with the temperature at which a high number of Cu protrusions appear on the surface [18].…”

Strain depth profiles in thin films extracted from in-plane X-ray diffraction

“…Various kinetic models have been proposed to describe the stress generation mechanisms during the different stages of thin-film growth, for example, due to island nucleation and growth, due to island coalescence and grain boundary formation ('zipping'), and due to the incorporation of adatoms into grain boundaries during the post-coalescence growth stage (Chason et al, 2012). The growth stresses in functional thin films and multilayers affect their thermal (Moszner et al, 2016;Druzhinin et al, 2019;Cancellieri et al, 2016), chemical (Delph & Jaccodine, 1999) and mechanical properties (Streitz et al, 1994;Wen et al, 2007). Hence fundamental knowledge on stress generating and relaxation mechanisms in thin films and multilayers is needed to tune their functional properties.…”

Synthesis, characterisation and thermal behaviour of Cu-based nano-multilayer

“…The study of its thermal behaviour revealed the degradation of the multilayer structure, transforming into a spheroidised nanocomposite at 700°C [29]. The migration of Cu to the surface of the multilayer and the feasibility of joint formation was attributed to the relaxation of the compression stresses accumulated in each layer mainly during the deposition process [30].…”