DSpace at VNU: Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices

Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices Nguyen Van Chuc, Ngo Thi Thanh Tam, Nguyen Van Tu, Phan Ngoc

Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices

Nguyen Van Chuc, Ngo Thi Thanh Tam, Nguyen Van Tu, Phan Ngoc Hong and Than Xuan Tinh

Laboratory for Carbon Nanomaterials, Institute of Materials Science,

Vietnam Academy of Science and Technology,

18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam E-mail: chucnv@ims.vast.ac.vn E-mail: tamngo@ims.vast.ac.vn E-mail: tunv@ims.vast.ac.vn E-mail: hongpn@ims.vast.ac.vn E-mail: tinh.thx@gmail.com Tran Tien Dat College of Technology, Vietnam National University,

144 Xuan Thuy Road, Cau Giay District, Hanoi, Vietnam E-mail: tiendat3101@gmail.com Phan Ngoc Minh*

Laboratory for Carbon Nanomaterials, National Key Laboratory of Electronic Materials and Devices, Institute of Materials Science,

Vietnam Academy of Science and Technology,

18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam E-mail: minhpn@ims.vast.ac.vn

*Corresponding author

Abstract: Currently, most of the vertically aligned carbon nanotubes

(VA-CNTs) and diamond films are mainly synthesised on flat silicon (Si) substrate However, to achieve thermal dissipation in high-power electronic devices (HPEDs), the VA-CNTs and diamond films need to be attached to thermal dissipation metal substrates (like Cu, Ag, Al, etc.) In this paper, the fabrication process of the VA-CNTs and diamond films on Cu substrate is reported in detail The VA-CNTs were synthesised by the thermal chemical

vapour deposition (CVD) method The VA-CNTs on Cu substrates were fabricated by two different methods:

• directly growing the VA-CNTs using thin catalytic metal layers such as Fe/Al or Cr/Al as a catalyst

• transferring the VA-CNTs film that was pre-grown on Si substrate

to Cu substrate

The diamond films were also directly grown on the Cu substrate by microwave plasma chemical vapour deposition (MPCVD) The grown VA-CNTs and diamond films were tested as the thermal dissipation media on a 0.5W InGaN LED chip The VA-CNTs and diamond films greatly increased input current of the LED by more than 500 mA and 350 mA without reaching saturation

This is higher compared with that of the device packaged using normal commercial silver thermal paste Initial experiment results on the LED demonstrated that the VA-CNTs and diamond films greatly improve the light’s output power and that they are optimal choices for the thermal dissipation

of HPED

Keywords: vertically aligned carbon nanotubes; diamond; thermal dissipation;

high-power electronic device

Reference to this paper should be made as follows: Chuc, N.V., Tam, N.T.T.,

Tu, N.V., Hong, P.N., Tinh, T.X., Dat, T.T and Minh, P.N (2011) ‘Synthesis

of vertically aligned carbon nanotubes and diamond films on Cu substrates for

use in high-power electronic devices’, Int J Nanotechnol., Vol 8, Nos 3/4/5,

pp.188–200.

Biographical notes: Nguyen Van Chuc received a BS from the Hanoi

University of Science, Vietnam National University, Hanoi (VNUH), in 2003 and an MS from the College of Technology, VNUH, in 2006 He is currently pursuing his PhD in Electronic Materials and Devices at the Institute of Materials Science, Vietnam Academy of Science and Technology (VAST)

His current interests are physics, technology and the applications of carbon nanotube materials

Ngo Thi Thanh Tam received a BS in Electronics from the National Polytechnic College of Azerbaijan in 1983 and an MS in Physics of Semiconductors from the Institute of Physics, National Center of Natural Sciences and Technology of Vietnam, in 1994 She received PhD in Materials Sciences from the Institute of Materials Sciences, VAST, in 2002 At present, her research concentrates on the fabrication of carbon nanotubes and their application She is a co-author of more than 20 publications, which focus on the hydrogen-sensing structure of Pt (Pd)/Si diodes, the structural and optical properties of porous silicon, and the fabrication and application of carbon nanotubes.

Nguyen Van Tu received a BS from the College of Technology, VNUH, in

2009 He is currently a Researcher in the Laboratory of Carbon Nanomaterials

at the Institute of Materials Science, VAST His research fields include the fabrication and application of carbon nanotubes (CNTs)

Phan Ngoc Hong received a BS from the Hanoi University of Science, VNUH, in 2005 and an MS from the College of Technology, VNUH, in 2009

He is currently working as a Researcher in the Laboratory of Carbon Nanomaterials, Institute of Materials Science He is currently pursuing his PhD

at the Universite Pierre et Marie Curie, France His research fields include the fabrication and application of carbon nanotubes, diamond materials, and nano photonics.

Than Xuan Tinh received a BS from the Hanoi University of Science, VNUH,

in 2005, and an MS from the College of Technology, VNUH, in 2008 He is a Researcher in the Laboratory of Carbon Nanomaterials, Institute of Materials Science He is currently pursuing his PhD at the University of Montpellier 2, France His research fields include the fabrication and application of carbon nanotubes.

Tran Tien Dat received a BS from the Hanoi University of Science, VNUH, in 2006 He is currently pursuing his MS in Nano Science and Technology at the College of Technology, Vietnam National University, Hanoi At present, his research focuses on the fabrication and application of diamond materials

Phan Ngoc Minh received his BS in Physics from Hanoi University, Vietnam,

in 1991; a PhD in Physics from the Institute of Physics, Vietnam Academy of Science and Technology (VAST), Hanoi, Vietnam, in 1996; a PhD in Engineering from Tohoku University, Japan, in 2001 From 2001 to 2004, he worked as a Post-Doctoral Researcher and then as an Assistant Professor

at the Graduate School of Engineering, Tohoku University, Japan From June 2007 to September 2009, he has been a Vice Director of the Institute

of Materials Science and Director of the National Key Laboratory of Electronic Materials and Devices at the Institute of Materials Science, VAST

Currently, he is an Associate Professor of the Institute of Materials Science, VAST and Visiting Lecturer of the College of Technology, Vietnam National University-Hanoi His current interests are physics, technology and applications

of nano-structured materials for electronic, photonic and optoelectronic applications; carbon-based nanomaterials; micro/nano electro mechanical systems

1 Introduction

The problem of thermal dissipation materials for use in high-power electronic devices (HPEDs) such as light emitting diode (LED) chips and laser has attracted special interest from scientists and technologists The inner temperature of high-power electronic devices increases cyclically as a consequence of their own operation Traditionally, heat dispersion from the HPED is carried out by passive strategies, which means using high thermal conductivity metals such as copper (Cu), silver (Ag) and aluminium (Al) as heat sinks In order to increase the rate of thermal dissipation and increase the lifetime of electronic devices, finding new materials with advanced thermal dissipation properties to replace Cu, Ag, and Al is necessary

Carbon nanotubes (CNTs), vertically aligned carbon nanotubes (VA-CNTs) and diamond materials are promising candidates to improve the thermal performance of the HPED due to their low thermal resistance as well as the ultra-high thermal conductivity

At room temperature, an individual multi-walled carbon nanotube (MWCNT) and diamond film have high thermal conductivity of about 600–3000 W/m.K [1,2], and about

2000 W/m.K [3,4], respectively Meanwhile, the thermal conductivity of traditional high

thermal conductivity metals is much lower compared with that of the individual MWCNT and diamond film The thermal conductivities of Ag, Cu and Al are 419 W/m.K, 380 W/m.K [5,6] and 238 W/m.K [7], respectively

However, most of the VA-CNTs and diamond films are mainly synthesised on flat silicon (Si) substrate Unfortunately, Si is not a good choice in terms of thermal conductivity Therefore to achieve thermal dissipation in the HPED, the VA-CNTs and diamond films need to be attached to thermal dissipation metallic substrates such as Cu,

Ag or Al

In this paper, the fabrication of the VA-CNTs and diamond films on Cu substrates is reported in detail The VA-CNTs on Cu substrates were fabricated by both directly growing the VA-CNTs on the Cu substrates and transferring the VA-CNTs layers that were pre-grown on Si substrate onto the Cu substrate The diamond films were synthesised on the Cu substrate by microwave plasma chemical vapour deposition (MPCVD) The light emission performance of the LED packages using the VA-CNTs and diamond films was tested on the 0.5 W InGaN LED chip The results indicated that the light output power of the LED chip was greatly improved with the use of the VA-CNTs and diamond films as thermal dissipation materials These initial results show that the VA-CNTs and diamond materials are optimal choices for thermal dissipation

of HPED

2 Experimental results and discussions

2.1 Fabrication of VA-CNTs on Cu substrates 2.1.1 Direct growth of VA-CNTs on Cu substrates

To successfully grow the VA-CNTs films, it is necessary to have a high density of catalytic particles on the surface of a substrate Al/Fe or Al/Cr catalyst films deposited onto thin Cu sheets with the purity of 99.9% were used An Al layer with a thickness of

15 nm was first deposited on the surface of the Cu substrate by thermal evaporation, and then Fe or Cr layers with thickness levels from 3 nm to 5.5 nm were deposited

by sputtering method at room temperature and a base pressure of about 8 × 10–7 Torr

Subsequently, the Cu substrates with Al/Fe or Al/Cr catalyst films were placed in a quartz boat and then inserted into the centre of a quartz tube reactor with a diameter of 2.7 cm housed in a furnace at 400°C The samples remained at 400°C in air for 10 min

Then, the furnace was heated to 750°C in Ar gas (300 sccm) The H2 gas (100 sccm) was introduced to deoxidise the Fe or Cr catalyst for 10 min The VA-CNTs were grown

at 750°C for 30 min in the mixture of C2H2/H2/Ar with flow rate ratios fixed at 30/100/300 sccm After finishing the growing process, H2 gas (100 sccm) was maintained for 10 min at growing temperature Then, the samples were cooled down to room temperature in the flow of Ar gas (300 sccm)

Figure 1 shows SEM images of the VA-CNTs films grown on Cu substrates with thickness of (A) Fe – 4 nm; (B) Cr – 4 nm We found that the Fe catalytic film with

a thickness ranging from 3 nm to 5.5 nm is suitable for growing VA-CNTs on Cu substrates In contrast, on the samples with the Cr thickness of lower than 4 nm, the CNTs are not aligned (not shown here) Meanwhile, on the samples with the

Cr thickness of higher than 4 nm, the CNTs are aligned (Figure 1(B)) The SEM images

(Figure 1) indicate that density of the VA-CNTs on Cu substrate is very high and the length of the CNTs is in the range of 20–30 µm Figure 2 shows that the CNTs grown on the Cu substrate are clean and the diameter of the CNTs is in the range of 15–25 nm

Figure 1 SEM images of VA-CNTs grown on (A) Fe/Al/Cu substrate with Fe thickness of 4 nm

and (B) Cr/Al/Cu substrate with Cr thickness of 4 nm

Figure 2 High magnification SEM images of CNTs grown on (A) Fe/Al/Cu substrates

and (B) Cr/Al/Cu substrates

2.1.2 Transferring VA-CNTs layer grown on Si/SiO 2 substrate to Cu substrate

Besides the method of directly growing the VA-CNTs on Cu substrate as mentioned above, we developed a technique to transfer the VA-CNTs layer from Si to Cu substrates

First, we synthesised the VA-CNTs films on the Si/SiO2 substrate Then, we transferred the VA-CNTs layer from the Si/SiO2 substrate to the Cu substrate The VA-CNTs films were synthesised on the Si/SiO2 substrate by CVD method using Fe3O4 particles as the catalyst The Fe3O4 nanoparticles were formed by the co-precipitation reaction of iron salts The Fe3O4 particles, which had diameters from 10 to 20 nm, were uniformly coated on the surface of Si/SiO2 substrate by spin-coating method The SEM image (Figure 3(A)) indicated that the Fe3O4 nanoparticles were located on the surface of the Si/SiO2 substrate with a high density of approximately 1010–1012 cm–2 The AFM image (Figure 3(B)) showed that the diameters of the Fe3O4 nanoparticles were in the range of 10–20 nm

The VA-CNTs were grown on Si/SiO2 substrate at different growing temperatures using a mixture of N2/H2/C2H2 gases with ratio of 300/100/30 sccm We found that the alignment of the CNTs strongly depend on the growth temperature At a temperature of lower than 650°C, the CNTs were less well aligned (not shown here) The orientation

of CNTs changed from a random spaghetti-like distribution for CNTs grown at 650°C to

a vertical forest-like alignment for CNTs grown at 750°C Figure 4(A) shows a typical SEM image of the CNTs on SiO2/Si substrate grown at 750°C for 30 min It is clear that the nanotubes are well aligned and uniform in height The height of the VA-CNTs

is approximately 15 µm A typical TEM image (Figure 4(B)) of the CNTs sample grown for 30 min at 750°C shows that the CNTs are clean with diameters of approximately 15 nm

Figure 3 (A) SEM and (B) AFM images of the Fe3 O 4 nanoparticles on the Si/SiO 2 surface

(see online version for colours)

Figure 4 (A) SEM and (B) TEM images of the VA-CNTs grown for 30 min at temperature

of 750 °C [1]

Figure 5 is a schematic diagram of the process to transfer the VA-CNTs layer from the Si/SiO2 substrate to the Cu substrate The synthesised VA-CNT films were detached from the Si/SiO2 substrate by directly immersing the sample into distilled water at a temperature of 60°C with the slow rate of 2~5 mm/s The process of detaching the VA-CNTs films from the Si/SiO2 substrate is also reported in detail in [8] The floating

VA-CNTs film was then attached to the silver conductive epoxy coated Cu substrate

By using this technique, the VA-CNTs were successfully transferred to the Cu substrate

Figure 5 Schematic view of the transfer of the VA-CNTs layer from Si/SiO2 substrate

to Cu substrate (see online version for colours)

2.2 Fabrication of diamond films on Cu substrates

The process of growing the diamond films on the Cu substrate was carried out using an AsTeX plasma chemical vapour deposition system with a 1.5 kW microwave source and vertically confined plasma excited at 2.45 GHz A substrate holder was made of molybdenum The substrate was heated by the plasma using a control heater

During the deposition, the temperature of the substrate was monitored by a two-colour pyrometer

The size of the copper substrates was 25 mm × 25 mm in area and 1 mm in thickness

The surface of the Cu substrates was polished by mechanical abrasion using sand paper, and then wet etched in a 40% HF acid solution for 2–5 min Subsequently, the Cu substrates were abraded with diamond slurry by ultrasonic treatment for 12 h to create and enhance the diamond nucleation density After mounting on the target chamber, the Cu substrates were exposed to H+ plasma at 900°C for 3 h to remove the residual surface oxide For the growth stage, the experimental parameters were the gas pressure

of 25 Torr, the substrate temperature of 700°C, the total gas flow rate of 100 sccm, the gas flow ratio of CH4/H2 of 1%, the microwave power of 500 W and the deposition time of 5 h

Due to the poor adherence between the grown diamond film and the Cu substrate (called diamond/Cu sample), it was necessary to transfer the synthesised diamond film to another Cu substrate used as a heat sink The transfer process of the diamond film was as follows First, the good heat-conductive film (3 M thermal bonding film) was adhered onto the Cu heat sink surface Then, the diamond/Cu sample was placed on the Cu heat sink surface so that the diamond film was in contact with the Cu heat sink surface

We lightly pressed the diamond film/Cu sample into the Cu heat sink surface Next, the

Cu substrate of the diamond/Cu sample was removed Finally, to improve the adherence between the Cu heat sink and diamond film, the whole sample was annealed in air for

1 h at 75°C for polymerisation heat-conductive film The schematic procedure of the transfer of the diamond film to the Cu heat sink is shown in Figure 6

Figure 6 Schematic view of the transfer of the diamond film to Cu heat sink (see online version

for colours)

The diamond samples were characterised by Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) Raman spectroscopy was used

to characterise the diamond films for sp3– and sp2–bonded carbon content using

17 mW He-Ne laser at the excitation wavelength of 632.8 nm The X-ray diffraction patterns from the films were obtained using a Siemens D5000 X-Ray diffractometer and

Cu–KĮ radiation (Ȝ = 1.5406 Å) The surface morphologies of the films were studied

using a field emission scanning electron microscope (Hitachi FESEM S–4800)

Figure 7(A) is a typical SEM image of the diamond film surface deposited for 5 h and Figure 7(B) is its corresponding cross-section SEM image Figure 7(A) shows that the diamond film had a well-defined crystal shape and most of the grains had a well-faceted pyramidal shape The thickness of this film is approximately 700 nm (Figure 7(B))

The growth rate of the diamond film is approximately 140 nm/h

Figure 7 (A) SEM images of the surface and (B) cross-section of the diamond film deposited for

5 h on Cu substrate

Figure 8(A) is the Raman spectrum of the diamond film The spectrum contains an intense diamond peak at around 1332 cm–1 and a typical diamond-like carbon (DLC) peak at around 1550 cm–1 [9] Normally, the typical DLC spectrum contains a broad single peak at around 1550 cm–1 with a ‘shoulder’ at around 1350 cm–1, namely the G and

D peaks, respectively [10] The G-peak is attributed to the crystalline graphite; the D peak is attributed to the disordered graphite In our sample, the sharp peak at 1332 cm–1

is the characteristic line of crystalline diamond and wide enough to overlap with the

D peak at around 1350 cm–1 Hence, there are two peaks in the Raman spectrum: the G peak at around 1550 cmí1 and the diamond peak at 1332 cm–1 It is clear that the diamond peak at 1332 cm–1 is extremely sharp It demonstrates that the film has a good diamond structure [11]

Figure 8 (A) Raman spectrum and (B) X-ray diffraction pattern of the diamond film deposited

for 5 h

Figure 8(B) is the XRD pattern of the diamond film deposited for 5 h at 25 Torr

The XRD pattern exhibits two peaks at 43.93° and 75.29°, indexed to the diffraction from (111) and (220) crystal planes of the diamond, respectively [11] The large

I(111)/I(220) ratio indicates the preferred <111> textured growth of the diamond film

The Raman spectrum and XRD pattern indicated that the diamond and DLC thin films have been deposited on the copper substrates by the MPCVD method They are polycrystalline diamond films The result is acceptable because the synthesis of diamond

on copper is a difficult endeavour

2.3 Application of the VA-CNTs and diamond films for LED chips

The synthesised VA-CNTs and diamond thin films were utilised as heat spreaders to reduce the local temperature of high-power electronic devices The LED chip used in this work was an InGaN on sapphire with an active area; emitting light wavelength and working power of 0.5 mm × 0.5 mm, 460 nm and 0.5 W, respectively Figure 9 shows a schematic view of the LED package using the VA-CNTs or diamond films as thermal dissipation materials They were inserted as a heat spreader between the device and the copper heat sink The VA-CNTs and diamond films were adhered to the Cu substrate by good thermal conductive material (Arctic silver 5 M or 3 M thermal bonding film)

It is expected that the diamond heat spreader will reduce the local temperature in high-power LEDs

Figure 9 Schematic view of the LED using thermal dispersive VA-CNTs or diamond films

(see online version for colours)

The SEM images of the VA-CNTs and diamond film on Cu substrate before assembling the LED chip are shown in Figure 10(A) and 10(B), respectively Figure 10(C) is a typical SEM image of the VA-CNTs on Cu substrate after assembling and wiring the LED chip on the VA-CNTs or diamond/Cu substrate

Figure 10 SEM images of (A) the VA-CNTs layer; (B) the diamond layer lifted off and pasted on

the Cu substrates; (C) the LED chip adhered to the VA-CNTs or diamond film

The output light power of the LED packages should ideally maintain a linear relationship with the electrical input current if the heat generated from the LED modules can be effectively dissipated However, heat generated by high input power would degrade the LED optical performance and result in a saturation of output light power Normally, for the InGaN LED chip used in this experiment, the light power of the LED packages using the commercial thermal dissipation material starts to deviate from a linear relationship with the input current about 300 mA and reaches a peak value at 350 mA

By using VA-CNTs or diamond films instead of the commercial thermal dissipation material, the output light power of the LED packages retains a linear profile without reaching saturation even if the input current is higher than 500 mA and 350 mA, respectively Figure 11 shows the light emission from the modified LED/VA-CNTs/Cu