Thermal chemical vapor deposition grown graphene heat spreader for thermal management of hot spots

Abstract

Graphene of different layer numbers was fabricated using thermal chemical vapor deposition (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum thermal evaluation chips were used to evaluate the thermal performance of the graphene heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm⁻² was decreased from 121 °C to 108 °C (ΔT ≈ 13 °C) with the insertion of the monolayer graphene heat spreader, compared with the multilayer (n = 6–10) ones’ temperature drop of ∼8 °C. Various parameters affecting the thermal performance of graphene heat spreaders were discussed, e.g. layer numbers of graphene, phonon scattering, thermal boundary resistance. We demonstrate the potentials of using a complementary metal oxide semiconductor compatible TCVD process to utilize graphene as a heat spreader for heat dissipation purposes.

Introduction

Thermal management of hot spots with localized high heat flux is critical for high power electronic devices. Non-uniform heat dissipation leads to the overheating of specific areas in chips, affecting the computing performance and reliability of electronic devices. Active solutions like thermoelectrics [1], [2], enable site-specific and on demand cooling in electronic devices. For passive solutions, a heat spreader without power consumption is widely used. Metallic materials such as Cu and Al, are utilized to dissipate heat from the hot spots owing to their high thermal conductivity (200–400 W m⁻¹ K⁻¹). While, due to the scattering of electrons from the film surfaces, the thermal conductivity of metal film decreases with the decrease of film thickness [3]. For instance, the thermal conductivity of 140 nm thick Al thin films was measured to be 94 W m⁻¹ K⁻¹[4] compared with the bulk value of ∼237 W m⁻¹ K⁻¹. At a thickness of ∼140 nm, the thermal conductivity of Cu thin films decreases to ∼220 W m⁻¹ K⁻¹[5], which is ∼55% of the bulk value. Graphene, a one atomic layer sheet of carbon atoms, is proposed as a promising heat spreader material, as its strong sp² bonds result in ultrahigh thermal conductivity of 5300 W m⁻¹ K⁻¹[6], [7]. Recently, Yan et al. [8] reported the application of exfoliated graphene quilts (few-layer graphene) in the thermal management of a high-power transistor. The temperature of the hot spot was reduced by ∼20 °C, extending the transistor’s lifetime by one order of magnitude. However, there are several issues to be addressed before graphene can be commercialized for this application. It is well known that the mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) provides the best quality graphene structure, but the layer numbers of graphene, exfoliated size and location are difficult to control. Complementary metal oxide semiconductor (CMOS) fabrication techniques require uniform graphene deposition for wafer scale processing [9], [10]. Thermal chemical vapor deposition (TCVD) is thus a feasible way to fabricate large area graphene [9], [10], [11], [12], [13], [14], allowing mass production of the graphene heat spreaders. Due to the nanostructures and the acoustic phonon transport properties, different layer numbers are required to maximize its thermal performance [6], [15], [16], [17], [18], and TCVD allows layer number control of the fabricated graphene [14], [19], [20], [21], [22]. Moreover, recent breakthroughs in graphene synthesis show that uniform, large-size and single-crystalline hexagonal graphene can be obtained from a controlled Cu surface [23], [24] and optimized synthesis parameters [25], making the quality of the CVD graphene comparable to the exfoliated ones.

In this paper, the application of TCVD prepared monolayer and multilayer graphene heat spreaders is demonstrated. The graphene was transferred onto a thermal evaluation chip, using a calibrated platinum (Pt) circuit driven by electric current as a hot spot, in which the temperature can be evaluated by measuring the electric resistance of the Pt circuit. Thermal performance of the graphene heat spreaders was evaluated by the temperature drop of the hot spots after the graphene transfer.