Deformation Behavior of Electrolessly Deposited Ultrafine Nanocrystalline Copper Films under Instrumented Nanoindentation

Grain size effect on nanomechanical properties and deformation behavior of copper under nanoindentation test

“…Under indentation, the four Cu specimens deformed elastically at the beginning stage and then elastoplastically at the late stage of loading, and only elastically during unloading. [23][24][25][26][27]31 The 700 600 500 400 300 200 100 400 300 200 None 200 100 None Grain size ͑nm͒ 40 100 20 500 14 300 12 300 501 266 140 4530 1560 560 43 25 16 10 STDV 5030 2640 2950 1520 55 56 18 532 352 73 19 11 9 8 Hardness ͑GPa͒ elastic and elastoplastic behaviors are discussed in detail in the following section. From the loading/unloading curves of nanoindentation tests and the Oliver-Pharr relation, [19][20][21] the hardnesses H and effective elastic moduli E * ͑the same as the reduced modulus used in contact mechanics 20,21 ͒ of the Cu specimens were obtained, as listed in Table I and plotted in Fig.…”

“…As the applied load exceeded about 0.13 mN ͑indentation depth ϳ5 nm͒, especially for bulk Cu, the load-depth curves deviated from the Hertzian elastic response, and the indentation depth jumped horizontally, recognized as a dislocation burst phenomenon. [23][24][25][26][27] During the curve deviation, it was expected that the stress intensity applied at the indenter tip had accumulated to the critical shear stress for the initiation of plastic deformation ͑yielding͒ of Cu, leading to sudden formation and rapid sliding of a large number of dislocations ͑dislocation cluster͒, thus resulting in the burst phenomenon in load-depth curves. [23][24][25][26][27] During each jump, the indentation depth proceeded for a few nanometers, and, thus, several to ten dislocations were predicted to form in the dislocation cluster by assuming that the Burgers vector of Cu was about 0.3 nm.…”

“…The plastic yielding behavior of the electroless Cu films with ultrafine grain sizes was expected to change, according to the inverse Hall-Petch effect, and no longer be dominated by dislocation activity but rather most probably by grain-boundary sliding and/or grain rotation. 2-6, [25][26][27] Therefore, no obvious dislocation burst behavior was recognized in the indentation curves of the electrolessly plated Cu films. Additionally, a smaller activation energy was required to trigger plastic deformation through grain-boundary sliding and/or grain rotation, and, consequently, achieved the grain softening effect and reduced hardness.…”

“…A smaller number of bonding was believed to exist in a large portion of the grain-boundary affected zones of the electroless Cu films with ultrafine grains, consequently reducing the required shear stresses to trigger plastic deformation through grain-boundary sliding and grain rotation rather than dislocation formation and sliding. 2-6, [25][26][27] Analyses of hardness, characteristic shear stress, and microstructural changes around indented regions effectively provide quantitative evaluations for representative mechanical properties, defect nucleation, and deformation mechanisms of Cu under a grain size effect. In practical applications to interconnect metallization in ULSI circuits as an example, Cu interconnects with different grain size ranges may exhibit differing mechanical performance and deformation behavior.…”

“…19,20 In addition to basic experimental data such as that for hardness and elastic modulus, true mechanical responses, including stress criteria and the onset of plasticity ͑dislocation burst phenomenon͒, have been also examined. [21][22][23][24][25][26][27] Furthermore, nanoindentation is a convenient and well-controlled method for introducing highly localized mechanical deformation into materials. 28 To determine how the microstructure changes around an indented region under ultrafine scale normal contact further facilitates effective examination of material deformation and defect nucleation.…”