Development of bulk nanostructured copper with superior hardness for use as an interconnect material in electronic packaging

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Abstract

In the present study, an attempt is made to synthesize bulk nanostructured copper by optimization of compaction pressure followed by low temperature sintering (200 °C). The selection of compaction and sintering parameters was made keeping in consideration the capability of silicon wafer and temperatures encountered during electronic packaging, respectively. The results revealed that grain size and porosity reduces while microhardness increases with an increase in compaction pressure. The microhardness obtained at limiting 1 GPa pressure was found to be superior when compared to the published values in open literature.

Introduction

Over the last decade there has been a steep rise in research investigations on the processing, characterization, properties and potential applications of nanostructured metals [1], [2], [3]. The principle reason for the interest arises from their novel mechanical, electrical and magnetic properties compared to conventional polycrystalline counterparts. The enhanced properties have been mainly attributed to the grain size refinement and increased grain boundary area [4]. Copper is one of the interesting materials for investigation due to its ability to exhibit attractive properties when the microstructural length scale is in nanometric level. The results of literature search indicate that nanostructured bulk copper has been successfully synthesized by various methods such as gas phase condensation coupled with in situ compaction [5], severe plastic deformation [6], solution-phase synthesis [7], plasma pressure compaction [8], microwave sintering [9], and pulsed electrodeposition method [10], [11].

Significant amount of work has been done in the past on the fabrication, mechanical and electrical property characterization of the nanostructured bulk copper for its use in many engineering applications such as an interconnect material in electronic packaging [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Electronic packaging refers to interconnection, powering, cooling and protecting semiconductor chips for reliable systems while the main purpose of an interconnect material or interconnection is to join two electrical terminals with low parasitics such as electrical resistance, inductance, etc. and to be reliable in field use. The processing temperature in the aforementioned techniques, however, remains higher when compared with solder reflow temperature in packaging area. Besides, no systematic data about the mechanical properties of the nanostructured bulk copper obtained by conventional cold pressing and low temperature sintering in inert atmosphere is available.

In the present study nanostructured bulk copper samples were synthesized using the technique of compaction of nanosize copper powders followed by low temperature sintering. The temperature of sintering was kept at 200 °C which is significantly lower than the solder reflow temperature used currently. Sintered specimens were characterized principally in terms of microstructure and microhardness. Particular emphasis was placed to correlate the effect of compaction pressure with the microstructural evolution and microhardness of the sintered nanocrystalline copper compacts.

Section snippets

Experimental

Elemental copper powders with purity of 99% or above and average powder particle size of ∼100 nm, were procured from Argonide Corporation (FL, USA) and compacted using a rigid die with a 10-mm diameter. Compaction pressure ranged from 0.25 to 1.00 GPa and its upper limit was governed by the limitation posed by silicon wafer [13]. The green compacts were then sintered at 200 °C for 2 h under argon atmosphere to minimize the oxidation of compacts.

Density was measured using Archimedes principle. The

Results

The density results are listed in Table 1. The porosity results obtained using optical image analysis are also listed in Table 1.

Fig. 1 shows the representative optical micrographs of bulk nanostructured copper synthesized using aforementioned method. The average grain sizes of bulk nanostructured copper obtained by compacting at different pressures and subsequent sintering using Scherrer equation are shown in Table 1. Diffractograms of powder compacts obtained with different compaction

Discussion

Synthesis of bulk nanostructured copper was successfully accomplished using cold uniaxial compaction and subsequent low temperature sintering. Cu2O was found to be present in the as-received nanocopper powder and hence its presence was expected in the compacts. No conclusive evidence of further oxidation was observed from diffractograms (see Fig. 2). It may be noted that formation of Cu2O during the synthesis of nanocopper powder synthesis cannot be avoided even under the vacuum level of 10−7 

Conclusions

The following conclusions can be made from the investigation of the bulk nanostructured copper prepared by cold compaction and low temperature sintering:

  • 1.

    The grain size and porosity of nanostructured copper decreases with an increase in compaction pressure.

  • 2.

    The density and microhardness of the nanostructured copper specimens increases with an increase in compaction pressure.

  • 3.

    The increase in microhardness exhibited by low temperature sintered powder compacts exceeded that of the one fabricated by

Acknowledgements

The authors wish to acknowledge NUS RP R-265-000-112-305 which is a collaboration project on Nano Wafer Level Packaging between the National University of Singapore (NUS), Institute of Microelectronics. Singapore (IME) and Packaging Research Center of the Georgia Institute of Technology, USA, funded by the Agency for Science, Technology and Research (A-STAR), Singapore as a Temasek Professorship project.

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