Technology /13/$31.00 c 2013 IEEE. Fu 3, Yan Zhang 2 and Johan Liu 2,4 No 149, Yanchang Road, Shanghai , China

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1 Graphene Based Heat Spreader for High Power Chip Cooling Using Flip-chip Technology Shirong Huang 1, Yong Zhang 2, 4, Shuangxi Sun 4, Xiaogang Fan 2, Ling Wang 2, Yifeng Fu 3, Yan Zhang 2 and Johan Liu 2,4 1 Dept. of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, No 149, Yanchang Road, Shanghai , China 2 SMIT Center, School of Mechatronics Engineering and Automation, Shanghai University, Box 282, No 149, Yanchang Road, Shanghai , P.R. China 3 SHT Smart High Tech AB, Fysikgränd 3, Se Göteborg, Sweden 4 SMIT Center and BioNano Systems Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, SE , Goteborg, Sweden Corresponding author: jliu@chalmers.se Abstract Monolayer graphene was synthesized through thermal chemical vapor deposition (TCVD) as heat spreader for chip cooling. Platinum (Pt) serpentine functioned as hot spot on the thermal testing chip. The thermal testing chip with monolayer graphene film attached was bonded using flip-chip technology. The temperature at the hot spot with a monolayer graphene film as heat spreader was decreased by about 12 o C and had a more uniform temperature compared to those without graphene heat spreader when driven by a heat flux of about 640W/cm 2. Further improvements to the cooling performance of graphene heat spreader could be made by optimizing the synthesis parameters and transfer process of graphene films. 1. Introduction The need for faster, smaller, more reliable and more efficient products has resulted in increasing heat generated in microelectronic components. With the noticeable increase of IC integrationn density, millions of transistors and CMOS cells are being fabricated on only the area of a few square centimeters. Power density on a single chip has reached a magnitude in the order of 100 W/cm 2 and is expected to continue rising according to the ITRS roadmap (fig. 1) [1]. This is even worsened due to the presence of hot spots, i.e. localized areas on the chip that reach tremendous power densities during operation [2, 3]. Dissipation of heat generated in electronic components has therefore /13/$31.00 c 2013 IEEE become an important issue as high hot spot temperature will result in semiconductor deterioration, including fractures, delamination, melting, creep, corrosion, electron migration etc. Fig.1 Microprocessor power dissipation (die area 140mm 2 ) and junction temperature evolution [1] Graphene, a recently-discovered new material, has exhibited a high thermal conductivity value of about 5300 W/m K at room temperature [4-6], and has become a promising material for thermal management. Yan et al. [7] reported the application of exfoliated graphene quilts for the thermal management of a high-power AlGaN/GaN transistor. The hot spot temperature was reduced by ~20 o C, raising the transistor s lifetime by at least one order. Subrina et al. [8] carried out a feasibility study on the use of graphene as material for lateral heat spreaders in SOI-based chips and showed that the incorporation of graphene or FLG under the insulating layer could to substantial reduction in hot spot temperature. Gao et.al [9-10] has reported graphene heat spreader applied for thermal 347

2 management of hot spots. The temperature of hot spot driven at a heat flux of up to 430W/cm 2 was decreased from 121 o C to 108 o C according to the Micro-RTD measurement. In Gao s case, the chip was suspended in the air. Tun-Jen et al [15] reported a highly quantized G band and 2D band graphene achieved by dispersion technique and incorporated in an organic-inorganic acrylate emulsion to form a coating assembly on heat sink. The heat dissipation performance of the molecular fan coating applied on LED devices shows that the coated 50W LED gives an enhanced cooling of 20% at constant light brightness. In this paper, the thermal performance of graphene films on the hot spot of a high power chip was investigated by both IR camera technology and Micro-RTD method. Platinum (Pt) thermal testing chips were prepared to evaluate the precise thermal performance of the graphene heat spreaders after thermal testing chips were bonded to PCB test board using flip-chip technology. 2. Experiments 2.1. Graphene Synthesis A thermal CVD method was developed for graphene synthesis [9-11]. 1 m thickness Cu film was evaporated on a SiO 2 substrate cleaned with acetone, isopropyl alcohol (IPA) and deionized (DI) water. Then the substrate transferred onto the heating stage in the CVD chamber with a thermocouple attached to the substrate surface. C 2 H 2 and Ar gas were chosen as the carbon precursor and the gas carrier, respectively [11] Fabrication of Thermal Testing Chip The micro heater and temperature sensor were made up of titanium/platinum/ gold (Ti/Pt/Au) with thickness of 20/150/30 nm respectively. A lift-off process was used, and the thin films were deposited using an e-beam evaporator. Pt was chosen as the micro heater with designed dimension of 390 m 400 m. The micro heater functioned as a hot spot for the thermal testing chip. The Ti layer acted as the adhesion layer between the Pt and the SiO 2 substrate, and the Au layer on top was added to ease the soldering of the testing chip to the power supply and sensing circuits. The thermal testing chips were calibrated with a standard resistance temperature detector at different temperature in the thermostat. In order to eliminate the wiring and contact resistance contribution, four probe method was adopted to characterize the electric resistance of the fabricated thermal testing chip by multimeter (Keithley 2000). The Pt electric resistance R is R (T) = T + R 0 (1) Where is the coefficient of electric resistance to temperature, and R 0 is the electric resistance at 0. Fig.2 Graphene transferred onto Thermal testing chip process [12] IEEE 15th Electronics Packaging Technology Conference (EPTC 2013)

3 2.3. Graphene Transfer A wet chemical process, as shown in Fig.2, was employed to transfer the graphene onto the thermal testing chip [9-11]. Firstly, graphene synthesized on the thin Cu film was spin coated with PMMA at 3500 rpm and baked at 110 o C for 3 minutes. 30 wt % FeCl 3 solution was prepared for etching the thin Cu film. Then, the PMMA layer with graphene was floated in the FeCl 3 solution and was diluted with DI water afterwards. The grapheme with PMMA supporting layer was transferred onto the thermal testing chip. Lastly, the PMMA was dissolved in hot acetone, and the graphene laid on the top of chip, serving as a heat spreader Flip-chip bonding The thermal testing chip with graphene heat spreader was bonded onto PCB (FR4) using flip-chip bonder, as illustrated in Fig. 3. The PCB dimensions were 100 mm 100 mm 1 mm, and the thermal testing chip s plane size is 22 mm 22 mm. Sn solder ball was used to connect the PCB with the thermal testing chip. Measurement of another thermal testing chip without graphene layer was carried out as reference Measurement Steady-state measurement and transient test method, such as Micro-RTD method and Infrared temperature measurement, were utilized to study the thermal performance of graphene heat spreader attached on chip cooling. During the experimental measurement, the Infrared method (FLIR SC600, spatial resolution of temperature measurement was 25 m [12].) was used to record the heat spreading process of the hot spot and obtain the surface temperature distribution. Besides, a Micro-RTD measurement was also carried out to obtain the hot spot temperaturee in the thermal steady state. Fig.3 (a) Schematic of flip-chip case with graphene heat spreader (not scale) and (b) chip sample bonding structures (c) thermal testing chip structure 3. Results and Discussions Firstly, characterization of the monolayer graphene film was made. The optical images recorded by microscope (LV5000) and Raman spectrum (LabRam-1B, laser wavelength 632.8nm) characterization was gained, as illustrated in Fig.4, to verify the quality of graphene films. It was found that monolayer graphene had a quite smooth surface. The Raman spectrum shows a good quality except defect was observed on the edge IEEE 15th Electronics Packaging Technology Conference (EPTC 2013) 349

4 Fig. 4 characterization of monolayer graphhene film (a) (b) monolayer graphene optical image (c) Temperaturee and resistance calibration curve of bare chip (d) Raman sppectrum of monolayer graphene film after being transferred on therm mal testing chip A series of power loads weree applied on the minutes. Temperature distribuution images of chips thermal testing chips with monolayer graphene film. with and without graphene film m were compared as in The infrared (IR) images of both thee bare chip and the Fig.5 (b) and (c). Not only a sm maller hot spot size was chip with monolayer graphene weere then taken, as observed, but also a more uniform temperature shown in Fig.5. Fig.5 (a) indiicates the chip s distribution was shown in thhe thermal testing chip temperature had reached a steaddy state after 15 with single layer graphene. Fig. 5 Temperature-Time profile (a) and Temperature distribution of chips by IR camera with graphene (b) andd without graphene(c) when heat flux at 640W/cm IEEE 15th Electronics Packaging Technology Conference (EPTC 2013)

5 Fig. 6 shows the relation between the hot spot temperature and the heat flux of the thermal testing chip with monolayer graphene film attached, where the data were obtained by the Micro-RTD measurement. The hot spot temperature was calculated according to the formula (1), with calibration shown in Fig.4 (c). As the heat flux increases, the chip with monolayer graphene heat spreader exhibits better thermal performance. The hot spot temperature can decrease from 124 o C to 112 o C at 640W/cm 2 with a 12 o C improvement compared with the bare chip. Gao et.al [10], who has reported the temperature of hot spot driven at a heat flux of 430W/cm 2 was decreased 13 o C when adopting graphene heat spreader. In Gao s case, the chip was suspended in the air. Huang et al [14] have also reported that the temperature of hot spot driven at a heat flux of 1280 W/cm 2 using Infrared camera measurement technology decreased 5 o C when adopting graphene heat spreader. In this case, the chip was bonded on a huge heat sink. Therefore, a larger heat flux is needed to generate enough amount of heat to study the effect of graphene. In the present case, the chip was bonded on PCB using flip-chip technology. Fig. 6 Thermal testing Chip s hot spot temperature relationship with heat flux by Micro-RTD method Further improvements to the cooling performance of graphene heat spreader could be made by optimizing the synthesis parameters and transfer process of graphene films. Because Small size grain can increase grain boundaries and the phonon grain boundary scattering lowers the in-plane thermal conductivity [14], which will in turn undermine the thermal performance of the graphene films. Additionally, there are many other parameters that can affect the heat spreading performance of graphene including fabrication process, defect amount and ambient conditions. 4. Conclusions In this paper, the thermal performance of monolayer graphene films on the high power chip s hot spot as heat spreader was investigated using both IR camera and Micro-RTD method. It was found that the temperature at hot spot of thermal testing chip with monolayer graphene film heat spreader was decreased by about 12 o C when driven by a heat flux of about 640W/cm 2 and has a more uniform temperature distribution compared to those without graphene. It was also noticed that cooling performance of monolayer graphene can be significantly affected by its defects, such as curl, size, grain etc. Acknowledgments This work was supported by EU programs Smartpower, Nanotherm, Nano-RF and the SSF program Scalable Nanomaterials and Solution Processable Thermoelectric Generators, contract NO. EM This work was also carried out as a part of the Sustainable Production Initiative and the Production Area of Advance at Chalmers. In addition, the work was also supported by the Shanghai Science and Technology Program (12JC , 12ZR ) and NSFC ( , ). References [1]. ITRS, "International Technology Roadmap for Semiconductors 2007 Edition-Assembly and Packaging, [2]. R. Mahajan, C.-P. Chiu, and G. Chrysler, Cooling a Microprocessor Chip, Proceedings of the IEEE, vol. 94, no. 8 (2006), pp [3]. R. Prasher, Thermal interface materials: Historical perspective, status, and future directions, Proceedings of the IEEE, vol. 94, no. 8 (2006), pp IEEE 15th Electronics Packaging Technology Conference (EPTC 2013) 351

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