Coating-boosted interfacial thermal transport for carbon nanotube array nano-thermal interface materials

Abstract

The interfacial thermal transport (ITT) between vertically aligned carbon nanotube (VACNT) arrays and heat sink is the dominant barrier blocking the path towards practical application of VACNT arrays as nano-thermal interface materials (nTIMs). Although developing VACNT arrays with homogeneous heights and larger diameters could lower the thermal contact resistance between the arrays and heat sink (R_c), little effect is achieved at present stage. Here, by using Plasma Enhanced Chemical Vapor Deposition approach, we attain DLC/TiN-coated VACNT arrays, which gives up to 50 times reduction in R_c from 15 mm² K/W to 0.3 mm² K/W. Microscopic morphological analyses confirm that the remarkably expanded contact area brought by coatings can promote the ITT and also retain the high phonon transmission rate within individual CNTs. These novel structures are significantly in favor of fulfilling a nTIM function. It is also intriguing to note that R_c is no longer linearly dependent on CNT height variations once CNT diameters become large enough. This indicates that the contact area with heat sink is dominant in influencing R_c instead of the surface roughness. The above findings fuel future effort towards industrial realization of high-performance VACNT array-based nTIM and high-efficiency thermal management in microelectronic and nanoenergy fields.

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 (R_c) between VACNT array and heat sink [10,11]. A series of work focusing on the reduction of R_c 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 R_c reduction. Experimental measurements elucidate that R_c 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 R_c 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 R_c 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 R_c 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 R_c 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 R_c, 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 R_c at CNT-heat sink interface. This aims to shrink the R_c after applying VACNT array as nTIM between heat spreader and microelectronic devices (see Fig. 1).