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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Thermal chemical vapor deposition grown graphene heat spreader for thermal management of hot spots Zhaoli Gao
a,b
Yong Zhang
a,c,1
, Yifeng Fu d, Matthew M.F. Yuen b, Johan Liu
a,c,*
a
Department of Microtechnology and Nanoscience, Chalmers University of Technology, Kemiva¨gen 9, SE-412 96 Gothenburg, Sweden Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Special Administrative Region c SMIT Center, School of Automation and Mechanical Engineering and Key State Laboratory of New Displays and System Applications and Shanghai University, 149 Yanchang Rd., Shanghai 200072, China d SHT Smart High Tech AB, Fysikgra¨nd 3, SE-412 96 Gothenburg, Sweden b
A R T I C L E I N F O
A B S T R A C T
Article history:
Graphene of different layer numbers was fabricated using thermal chemical vapor deposi-
Received 10 December 2012
tion (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum
Accepted 7 May 2013
thermal evaluation chips were used to evaluate the thermal performance of the graphene
Available online 14 May 2013
heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm2 was decreased from 121 C to 108 C (DT 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. 2013 Elsevier Ltd. All rights reserved.
1.
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 m1 K1). 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 m1 K1[4] compared with the bulk value of 237 W m1 K1. At a thickness of 140 nm, the thermal conductivity of Cu thin films decreases to 220 W m1 K1[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 m1 K1[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
* Corresponding author: Fax: +46 31 7723622. E-mail address:
[email protected] (J. Liu). 1 Equally contributed to this work. 0008-6223/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.014
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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 [9–14], 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 [6,15–18], and TCVD allows layer number control
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of the fabricated graphene [14,19–22]. 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.
Fig. 1 – (a) Cold-wall TCVD furnace for graphene synthesis, (b) TCVD process profile for graphene synthesis, (c) schematic of graphene transfer process.
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2.
Experimental details
2.1.
Graphene synthesis
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A TCVD (Black Magic, AIXTRON Nanoinstruments Ltd.), as schematically shown in Fig. 1a, was used for monolayer graphene synthesis. 1 lm thick Cu thin films were prepared by electron beam (e-beam) evaporation (HVC600, AVAC) on the SiO2 substrates. They were then cleaned with acetone, isopropyl alcohol and distilled water, and were placed on the heating stage with a thermocouple attached on the stage surface. C2H2 and argon (Ar) were chosen respectively as the carbon precursor and the gas carrier. Fig. 1b shows the temperature profile during graphene synthesis. The reactor chamber was initially pumped down to a base pressure of 0.5 mbar. 1000 sccm Ar and 20 sccm H2 were then introduced into the furnace. The temperature was raised at a rate of 300 C/min to 900 C and maintained for 5 min to anneal the Cu films. C2H2 was introduced at a flow rate of 5 sccm for 5 min. Finally, all the gas supplies were turned off, and the residual gas was pumped to a pressure of 0.1 mbar. The reactor chamber was subsequently cooled to room temperature with 1000 sccm Ar and 20 sccm H2. Multilayer graphene was synthesized using the e-beam evaporated (Explorer 14, Denton Vacuum) 700 nm Ni thin films on SiO2 substrates. A TCVD furnace (CTF, CARBOLITE) with a base pressure of 0.1 mbar was implemented, and the synthesis temperature
was kept at 900 C. C2H2, H2 and Ar were introduced at a flow rate of 12 sccm, 24 sccm and 200 sccm, respectively. A micro-Raman spectrometer (RM3000, Renishaw) using a laser excitation wavelength of 514.5 nm and Transmission Electron Microscopy (TEM, JEM 2010 JEOL) were employed to identify the quality and layer numbers of the graphene. Optical microscope (MX50, Olympus) and field emission scanning electron microscopy (FESEM, JSM-6700F) were utilized to characterize the graphene transferred onto the thermal evaluation chips.
2.2.
Thermal evaluation chip fabrication
The micro-heaters and temperature sensors were made of titanium/platinum/gold (Ti/Pt/Au) with a thickness of 20/ 150/30 nm. A standard lift-off process was used, and the thin films were deposited using an e-beam evaporator (PVD 255, Kurt J. Lesker Company). Pt was chosen as the temperature sensing material due to its remarkably linear temperatureresistance relationship [26,27]. In addition, the high melting point of Pt (1770 C) allows it to be used as a micro-heater when driven by electric current. The designed dimension of the Pt micro-heater was 390 lm · 400 lm, and this functioned as a hot spot in the 1 cm · 1 cm thermal evaluation chip. The Ti acts as the adhesion layer between the Pt and the SiO2 substrate. The Au layer on top eased the soldering of the chip to the power supply and electric resistance sensing circuit. A
Fig. 2 – (a and b) TEM images of synthesized graphene on Cu thin film, (c) TEM images of synthesized graphene on Ni thin film, (d) Raman spectra of as synthesized graphene on Ni and Cu thin films and graphene on Cu after being transferred onto a thermal evaluation chip, (e) Raman mapping of graphene synthesized on Cu thin film, the color gradient bar represents the IG/ I2D intensity ratio.
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210 nm thick SiO2 insulation layer between the graphene and the Au was deposited on the chip using a sputter machine (MS150, FHR). The thermal evaluation chips were calibrated with a standard resistance temperature detector (LABFACILITY PT100 XF-316-FAR) at various temperatures in a furnace. In order to eliminate the wiring and contact resistance contribution, the four point probe methodology was implemented to measure the electric resistance of the Pt micro-heater, and it is described as RðTÞ ¼ aT þ R
ð1Þ
where a is the coefficient of electric resistance to temperature, and R0 is the electric resistance of the Pt micro-heater at 0 C.
2.3.
Graphene transfer
A wet chemical process was adopted to transfer the graphene onto the thermal evaluation chip. As illustrated in Fig. 1c, graphene synthesized on the Cu thin film was firstly spin coated with a polymethylmethacrylate (PMMA) layer (2% in anisole) at 3500 rpm for 90 s and baked at 130 C for 3 min. After the Cu thin film was etched in a 30% FeCl3 solution, the PMMA supported graphene was floated in the FeCl3 solution. The graphene was then transferred onto the calibrated thermal evaluation chip. Finally, the PMMA was dissolved in hot acetone, and the graphene was laid on top of the Pt hot spot, acting as a heat spreader. For multilayer graphene transfer, the same process was used, except for using the aqueous solution of 3 M HCl and 0.15 M Fe(NO3)3Æ9H2O for Ni thin film removal.
3.
Results
3.1.
Graphene synthesis
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Synthesis parameters were optimized in order to fabricate monolayer graphene. As shown in Fig. 2b, the monolayer graphene structure is clearly identified at the edge of the folded graphene in Fig. 2a. Due to the unique electron bands of graphene, Raman spectroscopy can distinguish a monolayer graphene from multilayer ones. For the as synthesized graphene on Cu thin film, the G band (1582 cm1) and the 2D band (2693 cm1) are identified (Fig. 2d), with an IG/I2D intensity ratio of 0.25, and the full width half-maximum (FWHM) of the symmetric 2D band is 29 cm1. These indicate monolayer graphene is obtained [28–30]. Raman mapping of the graphene (30 lm · 30 lm) is shown in Fig. 2e. Over 95% of the areas were covered by the monolayer graphene with an IG/I2D < 0.5 [13,31], and the rest was mainly bilayer ones (0.5 < IG/I2D < 1). For graphene transferred onto the thermal evaluation chip, the intensity of the D band (1350 cm1), which was hardly measurable (
