Effect of the Crystallinity on the Electromigration Resistance of Electroplated Copper Thin-Film Interconnections

[+] Author and Article Information
Takeru Kato

Department of Finemechanics,
Graduate School of Engineering,
Tohoku University,
6-6-11-716, Aoba Aramaki, Aobaku,
Sendai, Miyagi 980-8579, Japan
e-mail: takeru.kato@rift.mech.tohoku.ac.jp

Ken Suzuki

Fracture and Reliability Research Institute,
Graduate School of Engineering,
Tohoku University,
6-6-11-716, Aoba Aramaki, Aobaku,
Sendai, Miyagi 980-8579, Japan

Hideo Miura

Fracture and Reliability Research Institute,
Graduate School of Engineering,
Tohoku University,
6-6-11-716, Aoba Aramaki, Aobaku,
Sendai, Miyagi 980-8579, Japan
e-mail: hmiura@rift.mech.tohoku.ac.jp

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received December 17, 2016; final manuscript received April 7, 2017; published online June 12, 2017. Assoc. Editor: S. Ravi Annapragada.

J. Electron. Packag 139(2), 020911 (Jun 12, 2017) (7 pages) Paper No: EP-16-1144; doi: 10.1115/1.4036442 History: Received December 17, 2016; Revised April 07, 2017

Dominant factors of electromigration (EM) resistance of electroplated copper thin-film interconnections were investigated from the viewpoint of temperature and crystallinity of the interconnection. The EM test under the constant current density of 7 mA/cm2 was performed to observe the degradation such as accumulation of copper atoms and voids. Formation of voids and the accumulation occurred along grain boundaries during the EM test, and finally the interconnection was fractured at the not cathode side but at the center part of the interconnection. From the monitoring of temperature of the interconnection by using thermography during the EM test, this abnormal fracture was caused by large Joule heating of itself under high current density. In order to investigate the effect of grain boundaries on the degradation by EM, the crystallinity of grain boundaries in the interconnection was evaluated by using image quality (IQ) value obtained from electron backscatter diffraction (EBSD) analysis. The crystallinity of grain boundaries before the EM test had wide distribution, and the grain boundaries damaged under the EM loading mainly were random grain boundaries with low crystallinity. Thus, high density of Joule heating and high-speed diffusion of copper atoms along low crystallinity grain boundaries accelerated the EM degradation of the interconnection. The change of Joule heating density and activation energy for the EM damage were evaluated by using the interconnection annealed at 400 °C for 3 h. The annealing of the interconnection increased not only average grain size but also crystallinity of grains and grain boundaries drastically. The average IQ value of the interconnection was increased from 4100 to 6200 by the annealing. The improvement of the crystallinity decreased the maximum temperature of the interconnection during the EM test and increased the activation energy from 0.72 eV to 1.07 eV. The estimated lifetime of interconnections is increased about 100 times by these changes. Since the atomic diffusion is accelerated by not only the current density but also temperature and low crystallinity grain boundaries, the lifetime of the interconnections under EM loading is a strong function of their crystallinity. Therefore, it is necessary to evaluate and control the crystallinity of interconnections quantitatively using IQ value to assure their long-term reliability.

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Fig. 1

Schematic structure of the electroplated copper thin-film interconnection: (a) cross-sectional structure and (b) planar structure

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Fig. 2

Definition of IQ value of grain boundary

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Fig. 3

Experimental setup for the measurement of resistance and temperature during the EM test: (a) general view of EM test equipment and (b) enlarged view near interconnection sample

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Fig. 4

Change in the resistance of the interconnection during the EM test at 7 mA/cm2

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Fig. 5

Change in the surface morphology of the interconnection after the EM test: (a) left edge, (b) center (1000 μm from the left side), and (c) right edge of the interconnection

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Fig. 6

Distribution of number of newly grown voids during the EM test

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Fig. 7

Temperature distribution along the interconnection at the current density of 7 mA/cm2

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Fig. 8

Change of the surface morphology during the EM test: (a) SEM image before the EM test and (b) SEM image after the EM test

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Fig. 9

Distribution of IQ values in CSL and random grain boundaries

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Fig. 10

Effect of grain boundary character on the accumulation of copper atoms under EM loading: (a) grain boundary distribution and (b) random grain boundaries on SEM image

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Fig. 11

Effect of grain boundary character on the formation of voids under EM loading: (a) SEM image and (b) random grain boundaries on SEM image

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Fig. 12

Change in the crystallinity of interconnections: (a) as-electroplated, (b) annealed for 30 mins at 200 °C, and (c) annealed for 3 h at 400 °C (see color figure online)

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Fig. 13

Change in the IQ distribution of random grain boundaries by annealing

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Fig. 14

Change in the average temperature of the interconnections with applied current density

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Fig. 15

Change of the activation energy depending on the crystallinity of the interconnection

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Fig. 16

Estimated lifetime of as-electroplated (low crystallinity) and annealed (high crystallinity) interconnections



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