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Research Papers

A Comprehensive Study of the Sintering of Copper Nanoparticles Using Femtosecond, Nanosecond, and Continuous Wave Lasers

[+] Author and Article Information
Nilabh K. Roy

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 4.164,
Austin, TX 78712
e-mail: nilabh.roy@utexas.edu

Obehi G. Dibua

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 4.164,
Austin, TX 78712
e-mail: ogodibua@utexas.edu

William Jou

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 4.164,
Austin, TX 78712
e-mail: williamjou@utexas.edu

Feng He

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 7.150,
Austin, TX 78712
e-mail: feng.he@utexas.edu

Jihoon Jeong

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 7.150,
Austin, TX 78712
e-mail: jihoonjeong@utexas.edu

Yaguo Wang

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 7.150,
Austin, TX 78712
e-mail: yaguo.wang@austin.utexas.edu

Michael A. Cullinan

Department of Mechanical Engineering,
The University of Texas at Austin,
204 E. Dean Keeton ETC 4.154,
Austin, TX 78712
e-mail: michael.cullinan@austin.utexas.edu

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received June 15, 2017; final manuscript received September 6, 2017; published online December 14, 2017. Assoc. Editor: Yayue Pan.

J. Micro Nano-Manuf 6(1), 010903 (Dec 14, 2017) (21 pages) Paper No: JMNM-17-1034; doi: 10.1115/1.4038455 History: Received June 15, 2017; Revised September 06, 2017

A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for the fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output pin count on electronic chips, and thus, minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns), and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs.

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Figures

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Fig. 1

Schematic of the FS laser sintering setup

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Fig. 2

CW laser sintering setup

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Fig. 3

Area estimation using ImageJ

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Fig. 4

Comparison of morphology of Cu sample at (a) room temperature and (b) after heating till 350 °C

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Fig. 5

Absorptivity of Cu ink layer on glass substrate

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Fig. 6

SEM images of spots sintered with (a) and (e) 12.6±2.9 mJ/cm2, (b) and ( f) 18.9±4.3 mJ/cm2, (c) and (g) 25.1±5.7 mJ/cm2, (d) and (h) 50.3±11.2 mJ/cm2 for 500 ms on 0.4 μm and 1.2 μm thick Cu layers on Al substrate

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Fig. 7

SEM images of spots sintered with 18.9±4.3 mJ/cm2 for (a) 50 ms, (b) 500 ms, and with 25.1±5.7 mJ/cm2 for (c) 50 ms, and (d) 500 ms on a 0.8 μm thick Cu layer on Al substrate

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Fig. 8

SEM images of FS sintered spots (a) and (d) 31.4±7.0 mJ/cm2, (b) and (e) 44.0±9.8 mJ/cm2, (c) and (f) 56.6±12.6 mJ/cm2 for 500 ms on 0.4 μm and 1.2 μm thick Cu layers on glass substrate

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Fig. 9

Processing window for different sintering regions for Cu samples on Al substrate: (a) 50 ms exposure and (b) 500 ms exposure

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Fig. 10

Processing window for different sintering regions for Cu samples on glass substrate: (a) 50 ms exposure and (b) 500 ms exposure

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Fig. 11

Morphology of pulsed laser sintered spots classified into different categories of processing window: (a) no sintering, (b) weak sintering, (c) good sintering, and (d) ablation/damage

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Fig. 12

SEM images of spots sintered with (a) and (d) 101.8±24.0 mJ/cm2, (b) and (e) 203.7±48.0 mJ/cm2, (c) and (f) 305.6±66.2 mJ/cm2 for 500 ms on 0.4 μm and 0.8 μm thick Cu layers on Al substrate

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Fig. 13

SEM images of spots sintered with (a) and (d) 101.8±24.0 mJ/cm2, (b) and (e) 203.7±48.0 mJ/cm2, (c) and (f) 305.6±66.2 mJ/cm2 for 500 ms on 0.4 μm and 0.8 μm thick Cu layers on glass substrate

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Fig. 14

Processing window for different sintering regions for Cu samples on Al substrate: (a) 50 ms exposure and (b) 500 ms exposure

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Fig. 15

Processing window for different sintering regions for Cu samples on glass substrate: (a) 50 ms exposure and (b) 500 ms exposure

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Fig. 16

SEM images of spots sintered with power density: (a) and (d) 99.5±37.6 kW/cm2, (b) and (e) 198.9±75.3 kW/cm2, (c) and (f) 298.4±112.3 kW/cm2 ms for 500 ms on 0.4 μm and 0.8 μm thick Cu layers on Al substrate

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Fig. 17

SEM images of spots sintered with power density: (a) 4.9±1.9 kW/cm2, (b) 9.7±3.6 kW/cm2, (c) 22.7±8.3 kW/cm2 on 0.4 μm thick Cu layer, (d) 4.9±1.9 kW/cm2, (e) 9.7±3.6 kW/cm2, and (f) 22.7±8.3 kW/cm2 for 500 ms on 0.8 μm thick Cu layer on glass substrate

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Fig. 18

Morphology of CW laser sintered spots classified into different categories of processing window: (a) no sintering, (b) weak sintering, (c) good sintering, and (d) melting

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Fig. 19

Processing window for different sintering regions for Cu samples on Al substrate: (a) 0.4 μm, (b) 0.8 μm, and (c) 1.2 μm thick Cu layer

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Fig. 20

Processing window for different sintering regions for Cu samples on glass substrate: (a) 0.4 μm, (b) 0.8 μm, and (c) 1.2 μm thick Cu layer

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Fig. 21

Processing window for different sintering regions for Cu samples on Al substrate for 200 ms

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Fig. 22

Processing window for different sintering regions for Cu samples on glass substrate for 200 ms

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Fig. 23

SEM images of spots sintered with power density 248.7±93.8 kW/cm2 for (a) 10 ms, (b) 50 ms, (c) 200 ms, and (d) 500 ms on 1.2 μm thick Cu layer on Al and with 9.7±3.6 kW/cm2 for (e) 10 ms, (f) 50 ms, (g) 200 ms, and (h) 500 ms on 0.8 μm thick Cu layer on glass substrate

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Fig. 24

SEM images of cross sections of an unisntered and CW laser sintered spot

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Fig. 25

Schematic of the NS laser sintering setup

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Fig. 26

Absorptivity of Cu ink layer on Al substrate

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Fig. 27

SEM images of spots sintered using FS laser with (a) 9.4±2.3 mJ/cm2, (b) 12.6±2.9 mJ/cm2, (c) 15.7±3.7 mJ/cm2, (d) 18.9±4.3 mJ/cm2, (e) 22.1±5.0 mJ/cm2, (f) 25.1±5.7 mJ/cm2, (g) 37.7±8.4 mJ/cm2, and (h) 50.3±11.2 mJ/cm2 for 500 ms on a 0.4 μm thick Cu layer on Al substrate

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Fig. 28

SEM images of spots sintered using FS laser with (a) 9.4±2.3 mJ/cm2, (b) 12.6±2.9 mJ/cm2, (c) 15.7±3.7 mJ/cm2, (d) 18.9±4.3 mJ/cm2, (e) 22.1±5.0 mJ/cm2, (f) 25.1±5.7 mJ/cm2, (g) 37.7±8.4 mJ/cm2, and (h) 50.3±11.2 mJ/cm2 for 500 ms on a 1.2 μm thick Cu layer on Al substrate

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Fig. 29

SEM images of spots sintered using FS laser with (a) 12.6±2.9 mJ/cm2, (b) 18.9±4.3 mJ/cm2, (c) 25.1±5.7 mJ/cm2, (d) 37.7±8.4 mJ/cm2 for 50 ms, (e) 12.6±2.9 mJ/cm2, (f) 18.9±4.3 mJ/cm2, (g) 25.1±5.7 mJ/cm2, and (h) 37.7±8.4 mJ/cm2 for 500 ms on a 0.8 μm thick Cu layer on Al substrate

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Fig. 30

Comparison of morphology of FS laser sintered spot with a fluence of (a) 15.7±3.7 mJ/cm2 for 50 ms and (b) unsintered spots-Al substrate

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Fig. 31

SEM images of FS laser sintered spots: (a) 18.9±4.3 mJ/cm2, (b) 37.7±8.4 mJ/cm2, (c) 56.6±12.6 mJ/cm2 for 50 ms and (d) 18.9±4.3 mJ/cm2, (e) 37.7±8.4 mJ/cm2, and (f) 56.6±12.6 mJ/cm2 for 500 ms on 0.8 μm thick Cu layer on glass substrate

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Fig. 32

SEM images of NS laser sintered spots with (a) 152.8±39.8 mJ/cm2, (b) 254.6±56.9 mJ/cm2, (c) 357.0±75.7 mJ/cm2 for 50 ms, (d) 152.8±39.8 mJ/cm2, (e) 254.6±56.9 mJ/cm2, and (f) 357.0±75.7 mJ/cm2 for 500 ms on 0.8 μm thick Cu layer on Al substrate

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Fig. 33

Morphology of spots sintered using NS laser with (a) 152.8±39.8 mJ/cm2, (b) 254.6±56.9 mJ/cm2, and (c) unsintered for 50 ms on 1.2 μm thick Cu layer on Al substrate

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Fig. 34

SEM images of spots sintered using NS laser with (a) 152.8±39.8 mJ/cm2, (b) 254.6±56.9 mJ/cm2, (c) 357.0±75.7 mJ/cm2 for 50 ms, (d) 152.8±39.8 mJ/cm2, (e) 254.6±56.9 mJ/cm2, and (f) 357.0±75.7 mJ/cm2 for 500 ms on 1.2 μm thick Cu layer on glass substrate

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Fig. 35

Morphology of spots sintered using NS laser with (a) 152.8±39.8 mJ/cm2 and (b) 254.6±56.9 mJ/cm2 for 500 ms on 1.2 μm thick Cu layer on glass substrate

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Fig. 36

SEM images of spots sintered using CW laser with power density 6.5±2.5 kW/cm2 for (a) 10 ms, (b) 50 ms, (c) 200 ms, (d) 500 ms and 9.7±3.6 kW/cm2 for (e) 10 ms, (f) 50 ms, (g) 200 ms, and (h) 500 ms on 0.8 μm thick Cu layer on glass substrate

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