Sn/SnO hybrid graphene for thermal interface material and interconnections with Sn hybrid carbon nanotubes
Graphical abstract
Introduction
Reducing chip size and increasing performance are among the most important research aspects for today’s electronics industries. Besides high thermal conductivity, the required material for small sized chip should have good electrical properties. Large amount of heat is generated in this process because of high circuit density. The life span and reliability of the gadgets also depend on the dissipation of heat [1]. Thermal interface material (TIM) provides a conductive path and improves heat flow across the interface [2]. Existing TIMs are found to be not effective for heat dissipation [3]. Poor or very high anisotropy in the thermal conductivity is a problem [4], [5], [6], [7], [8], [9], [10]. Therefore, search is going on for improving the thermal property of TIM.
Thermal conductivity of single layer graphene (SLG) at room temperature is ~5000 W/m.K [11], [12] which is much higher than the thermal conductivity of bulk graphite ≈500 W/m.K [13]. High thermal conductivity of graphene [11], due to large phonon mean free path in strong carbon sp2 bond network, renders graphene possible application for thermal management in future ULSI circuits. [12]. Few layer graphene (FLG) is preferred for application because of its lower cost. However, thermal conductivity ~1600 W/m.K of FLG is lower than SLG. This is due to the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering [14]. Graphene layers are easy to attach to heat sinks, which can solve the thermal contact resistance problems, Further, flat plane geometry of graphene is also suitable for integration with Si CMOS circuits for thermal management.
However, this high thermal conductivity value is reduced drastically when used in bulk. It has been observed in the current study that the average thermal conductivity of graphene with the film thickness from 1 mm is measured around 3.27 W/m K at room temperature. The reasons are very low thermal conductivity across the axis, existence of substantial defects and large gaps due to no interaction between the graphene. These problems restrict the applications of graphene as TIM. Therefore, graphene is studied as thermal interface material using formation composites with other materials like polymers and boron nitrides.
Reducing thermal conductivity of low-k dielectrics and increasing current density demands from small dimension interconnects raise the reliability concerns for Cu based interconnects [15]. The rise in metal temperature and electromigration (EM) exponentially degrades interconnect lifetime [16]. There are also environmental and economic concerns for the current copper-based technology in electronics. Due to exceptional electronic and other properties carbon nanotubes and graphene are considered to be the best materials for very small dimension interconnects [17], [18], [19]. CNTs are less sensitive to electromigration [20], and possible alternative interconnect material because of their high current carrying capacity and thermal conductivity [21], [22]. In our previous study, we showed the formation of interconnects using the Sn hybrid single multiwalled carbon nanotube on the copper substrate [23]. However, Cu involved in this interconnect also leads to the formation of Cu6Sn5 inter-metallic compound during the formation of connection [23]. Besides high thermal conductivity, graphene also shows extremely [24] low electrical resistivity of 7.5 × 10−7 Ω m along the plane so it is highly suitable for interconnect applications. However, high resistance in the vertical direction to the plane and very low interactions among the graphene are big hindrance for their applications.
One possibility for using the FLGs for both as thermal interface material and interconnect applications is to modify them by coating, doping, substitution or functionalisation [25], [26], [27], [28].
Sn metal is used for soldering purpose because of its high thermal, electrical conductivity, and availability. Coating and filling of CNTs with Sn have been studied for the bulk thermal conductivity and interconnect applications [23], [29] but few studies has been undertaken for Sn-graphene.
In the present work, a new way is used the FLGs as thermal interface material and interconnect applications. FLGs was coated with Sn/SnO using a easy method and studied for bulk thermal conductivity and electrical properties. Study also reports a possibility of forming an interconnect between Sn coated and filled single carbon nanotubes and Sn coated graphene.
Section snippets
Development of hybrid few layer graphene
The commercial few layer graphene (FLG), produced using chemical vapor deposition method, were used in the present study. The FLGs were heated in O2 at 250 °C for 15 min for surface oxidation. The oxidized FLGs were then stirred with SnCl2 solution in small amount of HCl at 70 °C for half an hour for coating of SnO2 on the surface of FLGs, similar to the earlier studies on carbon nanotubes [30], [31]. After the reaction, hybrid FLGs-SnO2 material was separated from solution using centrifugation
Crystal structure studies
X-ray diffraction of pristine FLGs in Fig. 1 displays only two peaks. The major peak at 2θ = 26.2° denoted as (G0 0 2) is due to (0 0 2) of graphite with the interplanner distance of 3.38 Å. The minor peak centered at 2θ = 54.3° with an interplanner distance of 1.69 Å is attributed to (G0 0 4). These peaks indicate the existence of multilayer structure in graphene. The interplanner distances of (0 0 2) and (0 0 4) peaks are consistent with previous work [34]. Broadness of G0 0 2 peak shows the
Conclusions
A simple method was used for coating the few layer graphene with Sn/SnO. Bulk thermal conductivity of the Sn/SnO hybrid FLGs pellet was surged >4 times than that of pristine graphene pallet. Study demonstrates the possibility of using Sn/SnO hybrid graphene as thermal interface materials. When compared, the thermal conductivity value of 14.41 Wm−1 K−1 of was higher than the existing thermal interface materials. Possibility of further possibility of improving this thermal conductivity is there
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The support of this study by the Ministry of Science and Technology (MOST) of the Republic of China (Taiwan) under Grant NSC101-2221-E006-117-MY3 is gratefully acknowledged. One of the authors (J. Mittal) is grateful to MOST for his financial support during the course of this work under Grant NSC102-2811--E-006-048.
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