Coating-boosted interfacial thermal transport for carbon nanotube array nano-thermal interface materials
Graphical abstract
Introduction
Since its discovery, vertically aligned carbon nanotube (VACNT) arrays have exhibited unique advantages as nano-thermal interface materials (nTIMs) for integrated circuit packaging and electronic cooling [[1], [2], [3], [4], [5], [6]]. Ultra-high thermal conductivity (κ), highly ordered orientation and adaptable morphology to the roughness of the contact surface enable CNT to become an ideal thermal conductive filler, which surpasses the disadvantages of commercial silicone grease such as frequently incurred leakage, contamination to electronic devices and limited κ value. However, it is still challenging to attain large-scale application of VACNT arrays because of a dominant thermal transport barrier caused by the uneven height of VACNTs [[7], [8], [9]]. This barrier is generally described as the thermal contact resistance (Rc) between VACNT array and heat sink [10,11]. A series of work focusing on the reduction of Rc for VACNT arrays have been consistently reported, shedding light on its potential for efficient heat dissipation application. Metallic bonding with Ti/Au/In/Sn on free-end VACNT tips is proven a feasible approach to achieve a Rc reduction. Experimental measurements elucidate that Rc can be shrunk to 1–3.5 mm2 K/W, nearly an order of magnitude smaller than that of the non-bonding counterpart (10–16 mm2 K/W) [4,[12], [13], [14]]. A recent progress has been reported on the successful bridging between metal surfaces and VACNT arrays where short, covalently bonded cysteamine and aminopropyl-trialkoxy-silane molecules demonstrate that Rc for VACNT array-gold and VACNT array-aluminium interfaces can be reduced to 0.8 and 0.6 mm2 K/W, respectively [15]. This minimized Rc is nearly equivalent to the thermal resistance of VACNT arrays (0.1–0.4 mm2 K/W for VACNT height of approximately 10 μm) [8,16]. To attain to this, a series of complicated chemical functionalization processes are required. Therefore, the quest to find better approaches to reduce Rc for VACNT array-based nTIM is still on-going. In pursuit of a new strategy for boosting interfacial thermal transport (ITT) between VACNT array and heat sink, we endeavor to investigate a possible optimum method to improve the interface contact conditions for VACNT array-heat sink structures in this work.
Previous studies have found that Rc is strongly dependent on the VACNT array top roughness and effective contact area with the heat sink. Specifically, the array top roughness originates from the CNT's height variation and the contact area is determined by CNT's diameters. Therefore, CNT arrays with homogeneous heights and larger diameters generally render lower Rc, owing to the decreased roughness and increased contact area with the heat sink [8], shedding light on the potential for improving the ITT between CNT arrays and heat sink. In the light of the limited enhancement of ITT through optimizing the growth conditions [9,16], we are seeking for better approaches by coating the top of the VACNT array with nanofilms, with the purpose of enlarging the contact areas between the array and heat sink and reducing the height variation of the CNTs. In this study, we have successfully developed VACNT arrays coated with diamond-like carbon (DLC, defined as amorphous carbon with higher than 70% sp3 bonds) and titanium nitride (TiN) nanofilms to achieve experimentally an extremely low Rc at CNT-heat sink interface. This aims to shrink the Rc after applying VACNT array as nTIM between heat spreader and microelectronic devices (see Fig. 1).
Section snippets
Sample preparation
Vertically aligned multi-walled CNT (MWCNT) arrays which serves as a platform for investigating the tunable Rc were successfully synthesized based on an Aixtron Blackmagic Plasma Enhanced Chemical Vapor Deposition (PECVD) approach [17,18]. In brief, a 20 nm thick Ni catalyst layer was e-beam evaporated onto a diced SiO2/Si substrate (length × width: 1 cm × 1 cm, thickness: 525 μm/380 nm). The whole substrate was placed in a vacuum chamber with pressure of 5 mbar. A radio frequency electric
Microscopic morphologies and structures
Scanning Electron Microscope (SEM) imaging confirmed that DLC coatings manage to fully cover the whole CNT walls instead of only the CNT tips (Fig. 4a–d). When the deposition time increases, more DLC coatings accumulated on the upper section of CNTs and form a continuous layer of coating. Statistical analysis of the heights of CNTs showed that there is no significant increase with increased DLC deposition timings. However, diameter distributions for the top and bottom of the CNT arrays clearly
Conclusions
In summary, this study reports a novel quantitative comparison of the thermal contact resistances Rc of VACNT array-heat sink interfaces from two main-stream VACNT array synthesis approaches, i.e., PECVD and FCCVD, and additional coatings on top of CNT arrays as compared to its uncoated counterpart. According to the measurement results for Rc, this study demonstrates that DLC and TiN-coated PECVD-synthesized CNT arrays possess excellent interfacial thermal transport characteristics when
Acknowledgements
This research receives financial support from Projects 51876008 and 51876007 supported by National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities from University of Science and Technology Beijing (L. Qiu), MOE2014-T2-2-105, SIM Tech-NTU PA RCA_16/174 and COE Research Grant (M4081666), Singapore (B.K. Tay). The authors are also grateful to Dr. Luo Jun from Beijing Normal University for his useful help on coating deposition.
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