Strain-Induced Grain Growth during Rapid Thermal Cycling of Aluminum Interconnects

Abstract

We demonstrate the rapid growth of grains in nonpassivated, sputtered Al-1 at. pct Si interconnects during 200 Hz thermal cycling induced by alternating current. Mean grain diameters were observed by use of automated electron backscatter diffraction (EBSD) to increase by more than 70 pct after an accumulated cycling time of less than 6 minutes over a temperature range of 200°C, which corresponded to a total strain range of 4 · 10 )3 . Plasticity in growing grains primarily took the form of topography formation at the free surface and grain rotation, while consumed grains tended to retain relatively high dislocation content. Grain growth was characterized by means of pairwise comparisons in EBSD pattern quality across moving boundaries. Out of 92 cases where a grain was observed to grow into its neighbor, 61 cases indicated that the growing grain had a higher average pattern quality factor than that of the consumed grain, at the 95 pct confidence level. The results are consistent with a strain-induced boundary migration mechanism, wherein stored plastic strain energy differences from grain to grain drive growth, some of which was observed after only 10 seconds of cycling.

Cyclic bending experiments on free-standing Cu micron lines observed by electron backscatter diffraction

“…However, in practical applications the applied load is predominantly cyclic with up to 10 10 cycles, making it necessary to develop cyclic micromechanical experiments. In the past, cyclic thermo-mechanical techniques have been developed such as repeated thermal cycling of thin films on a substrate [1][2][3][4][5][6] or Joule heating by using AC currents to cyclically vary the thermal stress in a conduction line [7,8]. In these experiments the influence of temperature and stress cannot be separated.…”

Micrometer-width damascene copper conductors at ≥10 MA/cm²

“…Grain growth in aluminum resulting from cyclic heating with AC has been discussed in the literature [15]. Fig.…”

“…A possible mechanism for the acceleration of both grain growth and void generation may be dislocation motion. The cyclic stresses and strains in these lines are believed to be sufficient to cause dislocation motion [17], and plastic deformation by dislocations was related to grain growth in aluminum lines tested under high amplitude AC [15]. The generation of vacancies and "cavitation" in metals by dislocation motion was studied decades ago, in observations of voids at grain boundaries [18], electrical resistance changes [19], and flow stress changes [20] in different pure metals.…”

Externally constrained plastic flow in miniaturized metallic structures: A continuum-based approach to thin films, lines, and joints

“…In fact, even in a given line the properties can vary from segment to segment (such as in a line with a near bamboo grain structure) so there may not be a unique stress-strain relationship for the whole line [282]. The true cyclic constitutive response of confined lines, which is of importance to device reliability, is essentially unknown (experimental probing of local strain evolution and microscopic observations of defect mechanisms during cyclic deformation of metal lines have just started to receive attention [344][345][346]). For the purpose of conducting a straightforward quantitative analysis, one has to resort to a reasonable estimate of the overall line behavior.…”