**1. Introduction**

[86] Ju S, Lee JM, Jung Y, Lee E, Lee W, Kim S-J. Highly sensitive hydrogen gas sensors using single-walled carbon nanotubes grafted with Pd nanoparticles. Sens Actuators,

[87] Rumiche F, Wang HH, Indacochea JE. Development of a fast-response/high-sensitivi‐ ty double wall carbon nanotube nanostructured hydrogen sensor. Sens Actuators, B.

[88] Sadek AZ, Bansal V, McCulloch DG, Spizzirri PG, Latham K, Lau DWM, et al. Facile, size-controlled deposition of highly dispersed gold nanoparticles on nitrogen carbon

[89] Penza M, Rossi R, Alvisi M, Serra E. Metal-modified and vertically aligned carbon nanotube sensors array for landfill gas monitoring applications. Nanotechnology.

[90] Penza M, Rossi R, Alvisi M, Suriano D, Serra E. Pt-modified carbon nanotube net‐ worked layers for enhanced gas microsensors. Thin Solid Films. 2011;520 (3):959-965.

[91] Zhou X, Tian WQ, Wang X-L. Adsorption sensitivity of Pd-doped SWCNTs to small

[92] Li K, Wang W, Cao D. Metal (Pd, Pt)-decorated carbon nanotubes for CO and NO sensing. Sens Actuators, B. 2011;In Press, Corrected Proof:doi: 10.1016/j.snb.

nanotubes for hydrogen sensing. Sens Actuators, B. 2011;160 (1):1034-1042.

B. 2010;146 (1):122-128.

366 Syntheses and Applications of Carbon Nanotubes and Their Composites

2012;163 (1):97-106.

2011.1006.1068.

2010;doi:10.1088/0957-4484/21/10/105501.

gas molecules. Sens Actuators, B. 2010;151 (1):56-64.

#### **1.1. Electronic interconnect**

In the electronics industry interconnect is defined as a conductive connection between two or more circuit elements. The interconnect connects elements (transistor, resistors, etc.) on an integrated circuit or between components on a printed circuit board. The main function of the interconnect is to contact the junctions and gates between device cells and input/output (I/O) signal pads. These functions require specific material properties. For performance or speed, the metallization structure should have low resistance and capacitance. For reliabili‐ ty, it is important to have the capability of carrying high current density, stability against thermal annealing, resistance against corrosion and good mechanical properties.

Over the past 40 years the continuous improvements in microcircuit density and perform‐ ance predicted by Moore's Law has led to reduced interconnect dimensions. According to Moore's law the number of transistors incorporated in a chip will approximately double ev‐ ery 18-24 months. The interconnect length increases with each generation, leading to higher resistances, while the distance between the adjacent interconnects decreases, leading to in‐ crease capacitance. Previously Al interconnect was used for VLSI processing [1]. Al and its alloys, suffer from the problems of high resistance-capacitance (RC) delay (a "time-delay" between the input and output, when a signal or voltage is applied to a circuit), poor electro‐ migration resistance and poor mechanical properties for application in ultra-large-scale inte‐ grated (ULSI) circuits [2].

Table 1 shows the comparison of different metals resistivity at room temperature. It can be seen from the table that only three metals have lower resistivity than Al, namely Ag, Au and Cu. Ag has the lowest resistivity but it has poor electromigration reliability. Electromigra‐

© 2013 Chowdhury and Rohan; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Chowdhury and Rohan; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

tion is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The resistivity of Cu is 1.67 µΩ.cm, which is about 40% better than Al. The self-diffusivity (the spontaneous movement of an atom to a new site in a crystal of its own species) of Cu is also the smallest among the four metals, resulting in improved reliability [3, 4].

interconnect is deposited. This can be achieved by conventional methods such as physical vapour deposition (PVD) [7] and chemical vapour deposition (CVD) [8]. However, PVD presents poor step coverage in sub-micrometer dimension vias and trenches. This technique deposits blanket films which would require further patterning. CVD also requires the use of combustible and toxic precursors at elevated temperatures which has limited the develop‐ ment of Cu deposition by CVD. In 1997 IBM developed the electrodeposition technique (du‐ al damascene) for Cu metallization [9]. Electrodeposition has become the standard method for Cu metallization with demonstrated uniformity, gap filling ability and low processing temperatures. In the dual damascene technique, lines and vias can be filled with electrode‐ posited Cu at the same time. Fig. 1 shows a schematic diagram of via filling with Cu and the requirement to achieve superfilling or bottom up deposition through the use of suitable ad‐ ditives in the plating bath rather than subconformal or conformal which result in voids or

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369

**Figure 1.** Cross section schematic of interconnect trench or via showing 'super-filling' or 'bottom-up filling' of features through the use of specific plating bath additives for optimum void-free profile evolution in damascene processing [9].

Semiconductor manufacturers have adopted the electroplating technique for Cu intercon‐ nect deposition in electronic devices and continue to work on miniaturization of device and feature sizes. Fig. 2 shows cross-sections from the International Technology Roadmap for Semiconductors (ITRS) of a typical microprocessor and application specific integrated circuit where the interconnect of different lines and vias between two adjacent layers are filled with Cu. As the feature sizes decrease and consequently the operating currents increase, electro‐

The effect is important in applications where high direct current densities are used, such as in high performance processors. Grain boundaries are the fastest diffusion path for Al elec‐ tromigration (activation energy 0.6 eV for grain boundary diffusion and 1 eV for interface diffusion) but an interface is the fastest diffusion path for Cu (activation energy 1.2 eV for grain boundary diffusion and 0.7 eV for interface diffusion) [14, 15]. The difference in elec‐ tromigration mechanism drives different focus areas for Cu and Al reliability improvement. The damascene process requires the removal of overdeposited Cu by chemical mechanical polishing (CMP). The CMP produced top Cu surface is the fast Cu diffusion path which needs to be reliably capped. A nonconductive barrier layer is generally applied as the cap layer (silicon nitride, silicon carbide, nitride silicon carbide etc) is used to cover the top sur‐ face of the Cu line. However, there are some issues with using dielectric caps to passivate

seams in the Cu. [10-12].

migration becomes a serious issue once more [13].


**Table 1.** Comparison of the bulk resistivity for different metals.

Table 2 shows a comparison of the activation energy (the minimum energy required for movement of an atom from a lattice position in a crystal) and melting temperature of Al vs. Cu. It can be seen from this table that Cu is a more reliable metal than Al with more energy required for diffusion of Cu atoms. The reason that Cu had not been used much earlier than its introduction in 1997 was because of device reliability concerns and processing difficul‐ ties. Cu diffuses rapidly through SiO2 in the presence of an electric field [5]. This causes deg‐ radation of transistor reliability by increasing metallic impurity levels in the Si. Another problem with Cu is that it oxidises at low temperatures but without self-passivation [6]. Cu is also difficult to etch unlike Al. This means that the classical approach where metal is de‐ posited over the entire surface, structures created in the metal and finally infilled with die‐ lectric (oxide) cannot be followed with Cu.


**Table 2.** Comparison of the active energy for diffusion of Al vs. Cu.

To overcome the problems of Cu integration the inter-level dielectric (ILD) is first deposited and patterned to define "trenches" into which the metal lines of the interconnect will be placed. A thin layer of barrier material (typically refractory metals or their alloys) is deposit‐ ed generally using a physical vapour deposition (PVD) process. This layer covers the entire surface to act as a barrier to Cu diffusion. After the deposition of the barrier layer the Cu

interconnect is deposited. This can be achieved by conventional methods such as physical vapour deposition (PVD) [7] and chemical vapour deposition (CVD) [8]. However, PVD presents poor step coverage in sub-micrometer dimension vias and trenches. This technique deposits blanket films which would require further patterning. CVD also requires the use of combustible and toxic precursors at elevated temperatures which has limited the develop‐ ment of Cu deposition by CVD. In 1997 IBM developed the electrodeposition technique (du‐ al damascene) for Cu metallization [9]. Electrodeposition has become the standard method for Cu metallization with demonstrated uniformity, gap filling ability and low processing temperatures. In the dual damascene technique, lines and vias can be filled with electrode‐ posited Cu at the same time. Fig. 1 shows a schematic diagram of via filling with Cu and the requirement to achieve superfilling or bottom up deposition through the use of suitable ad‐ ditives in the plating bath rather than subconformal or conformal which result in voids or seams in the Cu. [10-12].

tion is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The resistivity of Cu is 1.67 µΩ.cm, which is about 40% better than Al. The self-diffusivity (the spontaneous movement of an atom to a new site in a crystal of its own species) of Cu is

**Metal Bulk resistivity**

Table 2 shows a comparison of the activation energy (the minimum energy required for movement of an atom from a lattice position in a crystal) and melting temperature of Al vs. Cu. It can be seen from this table that Cu is a more reliable metal than Al with more energy required for diffusion of Cu atoms. The reason that Cu had not been used much earlier than its introduction in 1997 was because of device reliability concerns and processing difficul‐ ties. Cu diffuses rapidly through SiO2 in the presence of an electric field [5]. This causes deg‐ radation of transistor reliability by increasing metallic impurity levels in the Si. Another problem with Cu is that it oxidises at low temperatures but without self-passivation [6]. Cu is also difficult to etch unlike Al. This means that the classical approach where metal is de‐ posited over the entire surface, structures created in the metal and finally infilled with die‐

> **Ea for lattice diffusion eV**

Al 660 1.4 0.4 – 0.8 Cu 1083 2.2 0.7 – 1.2

To overcome the problems of Cu integration the inter-level dielectric (ILD) is first deposited and patterned to define "trenches" into which the metal lines of the interconnect will be placed. A thin layer of barrier material (typically refractory metals or their alloys) is deposit‐ ed generally using a physical vapour deposition (PVD) process. This layer covers the entire surface to act as a barrier to Cu diffusion. After the deposition of the barrier layer the Cu

**Ea for grain boundary diffusion eV**

Ag 1.63 Cu 1.67 Au 2.35 Al 2.67 W 5.65

**μΩ.cm**

also the smallest among the four metals, resulting in improved reliability [3, 4].

**Table 1.** Comparison of the bulk resistivity for different metals.

368 Syntheses and Applications of Carbon Nanotubes and Their Composites

lectric (oxide) cannot be followed with Cu.

**oC**

**Table 2.** Comparison of the active energy for diffusion of Al vs. Cu.

**Interconnect Metal Melting Point**

**Figure 1.** Cross section schematic of interconnect trench or via showing 'super-filling' or 'bottom-up filling' of features through the use of specific plating bath additives for optimum void-free profile evolution in damascene processing [9].

Semiconductor manufacturers have adopted the electroplating technique for Cu intercon‐ nect deposition in electronic devices and continue to work on miniaturization of device and feature sizes. Fig. 2 shows cross-sections from the International Technology Roadmap for Semiconductors (ITRS) of a typical microprocessor and application specific integrated circuit where the interconnect of different lines and vias between two adjacent layers are filled with Cu. As the feature sizes decrease and consequently the operating currents increase, electro‐ migration becomes a serious issue once more [13].

The effect is important in applications where high direct current densities are used, such as in high performance processors. Grain boundaries are the fastest diffusion path for Al elec‐ tromigration (activation energy 0.6 eV for grain boundary diffusion and 1 eV for interface diffusion) but an interface is the fastest diffusion path for Cu (activation energy 1.2 eV for grain boundary diffusion and 0.7 eV for interface diffusion) [14, 15]. The difference in elec‐ tromigration mechanism drives different focus areas for Cu and Al reliability improvement. The damascene process requires the removal of overdeposited Cu by chemical mechanical polishing (CMP). The CMP produced top Cu surface is the fast Cu diffusion path which needs to be reliably capped. A nonconductive barrier layer is generally applied as the cap layer (silicon nitride, silicon carbide, nitride silicon carbide etc) is used to cover the top sur‐ face of the Cu line. However, there are some issues with using dielectric caps to passivate Cu. As devices become smaller, the current density through the interconnect increases lead‐ ing to the requirement for better electromigration resistance. The dielectric cap generally has a higher dielectric constant than the interlevel dielectric, resulting in an increase in line-toline capacitance. Improved Cu electromigration resistance was reported when Cu lines were protected with thin conductive surface capping layers of self-aligned electrolessly deposited CoWP or CoSnP etc [16, 17]. Diffusion barrier layers such as Ta or TaN for Cu metallization act as redundant layers for current shunting as well as for uniform Cu seed deposition. It was reported that Cu vias are the weak link in the interconnect metallization [14]. The Cu via connects directly to the Cu metal below. If a void forms in the Cu underneath the via, there is no redundant layer available for current shunting. This is the primary cause of early failure distribution in Cu interconnects. For the 22 nm technology node or below, the inter‐ connect metal should have current carrying capability of more than 107 A/cm2 to overcome the electromigration issue but Cu is limited to 107 A/cm2 .

than any metal (e.g. 40 nm for Cu). In addition the covalent C-C bonding between the neigh‐ bouring atoms in the CNT is one of the strongest bonds reported in the literature [20] and C atoms will not migrate even under very high current density (activation energy 7.7 eV for atom movement). Thus the electromigration resistance of CNT is much better than other in‐ terconnects material such as Al and Cu. Because of these advantages over Cu, CNTs require consideration as the next generation interconnect material for specific applications, such as

However there are still significant scientific and engineering challenges to incorporate CNTs in devices. CNTs can deposit inside of vias on suitable catalyst like zeolite [21]. It is necessa‐ ry to ensure the selective growth of metallic CNTs in vias and lines to achieve better electri‐ cal conductivity. Alternatively CNTs can be first synthesized in a powder form and metallic CNTs separated from bulk growth CNTs (mixture of metallic and semiconducting). After that metallic CNTs would need to be transferred onto specific wafer locations. The scale of this task is obvious when considering that there are billions of transistors in a microproces‐ sor and the placement of CNTs inside of all vias and trenches on the wafer is unlikely.

Maximum Current density (A/cm2) 1 x 109 1 x 107 Electrical conductivity (S/m) 106-107 6 x 107 Thermal coefficient of resistivity (/oC) -1.5 x 10-3 4 x 10-3 Thermal conductivity (W/m K) 6,000 400 Coefficient of thermal expansion (ppm/oC) -1.5 17 Activation energy (eV) 7 2

The contact resistance between CNTs and metal is large (≥1 kΩ). The minimum resistance for a ballistic single-walled CNT is ~ 6.5 kΩ, Therefore, relatively dense arrays of nanotubes will be needed to replace Cu interconnects and these arrays will still only show reduced re‐ sistance by comparison with Cu interconnect for line lengths ≥ 1 µm. The ITRS [22] therefore predicts for the 22 nm node an estimated resistance for a 17 nm x 38.5 nm x 1.5 µm Cu inter‐ connect is RCu 145 Ω. An ensemble of ~ 45, 1 nm diameter defect-free metallic CNTs with mean inter-tube separations ~ 4 nm in a trench of these dimensions would have the same

Intrinsic voids between CNTs significantly reduce the electrical and thermal conductivity and bring reliability challenges for the use of CNTs as interconnects. Contact resistance between CNTs and interconnect metal like Cu becomes the dominant source of electrical and thermal resistance which significantly reduces the benefits of CNTs. The potential use of CNTs alone as

interconnect in semiconductor manufacturing is still open to debate at this time.

**Single CNT Cu at 22 nm node**

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371

through silicon vias (TSV) for stacked die.

**Table 3.** Comparison of the properties of single walled CNTs vs. Cu.

total resistance as the Cu line.

**Figure 2.** Typical cross section illustrating hierarchical scaling methodology [ITRS technology road map, 2011 update].

#### **1.2. Carbon nanotubes**

Carbon nanotubes (CNTs) have unique electrical, thermal and mechanical properties [18]. They can carry an electrical current density of ~ 4 × 109 A cm-2, which is three orders of mag‐ nitude higher than Cu [19]. CNT's have high aspect ratio and the mean free path of the carri‐ ers (or the probability of an electron transmitted from at one end of the CNT to the other without phonon scattering or other thermal effects) in the CNT at 10 µm is much longer than any metal (e.g. 40 nm for Cu). In addition the covalent C-C bonding between the neigh‐ bouring atoms in the CNT is one of the strongest bonds reported in the literature [20] and C atoms will not migrate even under very high current density (activation energy 7.7 eV for atom movement). Thus the electromigration resistance of CNT is much better than other in‐ terconnects material such as Al and Cu. Because of these advantages over Cu, CNTs require consideration as the next generation interconnect material for specific applications, such as through silicon vias (TSV) for stacked die.

However there are still significant scientific and engineering challenges to incorporate CNTs in devices. CNTs can deposit inside of vias on suitable catalyst like zeolite [21]. It is necessa‐ ry to ensure the selective growth of metallic CNTs in vias and lines to achieve better electri‐ cal conductivity. Alternatively CNTs can be first synthesized in a powder form and metallic CNTs separated from bulk growth CNTs (mixture of metallic and semiconducting). After that metallic CNTs would need to be transferred onto specific wafer locations. The scale of this task is obvious when considering that there are billions of transistors in a microproces‐ sor and the placement of CNTs inside of all vias and trenches on the wafer is unlikely.


**Table 3.** Comparison of the properties of single walled CNTs vs. Cu.

Cu. As devices become smaller, the current density through the interconnect increases lead‐ ing to the requirement for better electromigration resistance. The dielectric cap generally has a higher dielectric constant than the interlevel dielectric, resulting in an increase in line-toline capacitance. Improved Cu electromigration resistance was reported when Cu lines were protected with thin conductive surface capping layers of self-aligned electrolessly deposited CoWP or CoSnP etc [16, 17]. Diffusion barrier layers such as Ta or TaN for Cu metallization act as redundant layers for current shunting as well as for uniform Cu seed deposition. It was reported that Cu vias are the weak link in the interconnect metallization [14]. The Cu via connects directly to the Cu metal below. If a void forms in the Cu underneath the via, there is no redundant layer available for current shunting. This is the primary cause of early failure distribution in Cu interconnects. For the 22 nm technology node or below, the inter‐

connect metal should have current carrying capability of more than 107 A/cm2

 A/cm2 .

**Figure 2.** Typical cross section illustrating hierarchical scaling methodology [ITRS technology road map, 2011 update].

Carbon nanotubes (CNTs) have unique electrical, thermal and mechanical properties [18].

nitude higher than Cu [19]. CNT's have high aspect ratio and the mean free path of the carri‐ ers (or the probability of an electron transmitted from at one end of the CNT to the other without phonon scattering or other thermal effects) in the CNT at 10 µm is much longer

A cm-2, which is three orders of mag‐

the electromigration issue but Cu is limited to 107

370 Syntheses and Applications of Carbon Nanotubes and Their Composites

**1.2. Carbon nanotubes**

They can carry an electrical current density of ~ 4 × 109

to overcome

The contact resistance between CNTs and metal is large (≥1 kΩ). The minimum resistance for a ballistic single-walled CNT is ~ 6.5 kΩ, Therefore, relatively dense arrays of nanotubes will be needed to replace Cu interconnects and these arrays will still only show reduced re‐ sistance by comparison with Cu interconnect for line lengths ≥ 1 µm. The ITRS [22] therefore predicts for the 22 nm node an estimated resistance for a 17 nm x 38.5 nm x 1.5 µm Cu inter‐ connect is RCu 145 Ω. An ensemble of ~ 45, 1 nm diameter defect-free metallic CNTs with mean inter-tube separations ~ 4 nm in a trench of these dimensions would have the same total resistance as the Cu line.

Intrinsic voids between CNTs significantly reduce the electrical and thermal conductivity and bring reliability challenges for the use of CNTs as interconnects. Contact resistance between CNTs and interconnect metal like Cu becomes the dominant source of electrical and thermal resistance which significantly reduces the benefits of CNTs. The potential use of CNTs alone as interconnect in semiconductor manufacturing is still open to debate at this time.

#### **1.3. Metal CNT composites**

To overcome some of the interconnect issues described above metal-CNT composites can be an alternative candidate material for future interconnects. The composite material would in‐ crease the contact area between vias and interconnect lines. There is also less chance of in‐ trinsic voids between CNTs as they would be metal filled. Among the different metals Cu is the best choice at this moment to use as a composite material with CNTs for interconnects applications because of superior electrical and thermal conductivity. Chai et al [23,24] and Yoo et al [25] reported that Cu fills the voids between neighbouring CNTs which results in a more densely packed structure. They reported that the addition of Cu increases the contact area between the nanotube (1 D) and the substrate (3 D contact) making it a mechanically strong material that can sustain high electrical or thermal stress cycling. To obtain superior properties of metal-CNT composites, it is necessary to achieve a homogeneous dispersion of CNT throughout the metal matrix. It is also necessary that the composites should be void free to obtain better electrical and thermal conductivity.

each other. They reported that C content in the composite increased by increasing CNT con‐ centration in the bath but it decreased with annealing. Chai [29] et al reported that the me‐ chanical strength of Cu/CNT nanocomposite was three times higher than that of pure Cu. Chen et al [26] observed good interfacial bonding between CNT and Cu when the nanocom‐ posites were codeposited by electrodeposition. They reported that for Cu/SWCNT nanocom‐ posites, the radial breathing mode (RBM) in the Raman was absent and the tangential or Gband had shifted and widened. Recently several patents on the metal/CNT composites codeposited by electrodeposition have been filed [30-32]. The comparison of electrical resistivity

> **electrodeposited Cu (thickness = 10.5 μm)**

**Cu/CNT composite (thickness = 22 μm)**

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Carbon Nanotube Composites for Electronic Interconnect Applications

of Cu and Cu/SWCNT film which was reported by Chan [30] are shown in table 4.

Resistivity (μΩ.cm) 1.72 1.22 Sheet resistance (mΩ/sq) 1.64 0.56

**1.4. Chlorosulphonic acid for CNT dissolution**

**1.5. Purification and functionalization of carbon nanotubes**

**Table 4.** Comparison of the electrical resistivity of electrodeposited Cu/SWCNT composite film vs. Cu alone [30].

Recently Davis et al [33] reported that CNTs can dissolve spontaneously in chlorosulphonic acid solution up to 0.5 wt % [5 g/l], which is much higher than previously reported in other acids (up to 80 mg/l). They reported that at higher concentrations, they form liquid-crystal phases that can be processed into fibres and sheets of controlled morphology. Their pro‐ posed phase diagram helps to identify the optimal starting fluid composition and determine micro and macrostructure of fibres and films such as plated fibres, straight fibres and smooth films. Plated fibres have potential application for hydrogen storage and sensors be‐ cause of high surface area. Straight fibres are of interest for structural reinforcement and smooth, dense films for electrical applications such as electrically conductive thin films.

A significant problem in dealing with CNTs is the difficulty to separate them as the individ‐ ual CNTs form bundles due to van der Waals attractive forces. Also in all of the synthesis techniques several impurities like catalyst particles, amorphous carbon etc. are also present in the bundles of CNTs. These impurities may deteriorate the properties of CNTs. To pre‐ pare stable and homogeneous dispersions of CNTs considerable efforts have been made [34-39] but the solubility of CNTs in water or organic solvent is relatively low. At room tem‐ perature the solubility of CNTs is in the range of 60 to 80 mg/l [34]. In order to achieve better stabilization, CNTs require additional hydrophilic groups directly on the CNT walls or pro‐ vided by surfactant molecules to impart ionic charge on the CNTs [40-42]. The most com‐ mon hydrophilic groups are –OH-, -COOH-, -SO3-, -NH2-. Functionalization of CNTs can be an important factor to manipulate the properties of CNTs. With functionalization CNTs mau be more easily separated. Several methods have been suggested for the purification and

Cu/CNT composites can be prepared by powder metallurgy, electroless plating or electrode‐ position techniques [25, 26]. Among these methods electrochemical routes are relatively straightfoward methods to produce defect free nanocomposites [24, 25]. Powder metallurgy requires sintering at elevated temperatures that may damage CNT's and the difficulty of composite placement remains an issue. Chen et al [26] observed a clear separation of CNTs and Cu matrix composites deposited by powder metallurgy. To achieve optimum perform‐ ance, CNTs need to be well-dispersed and aligned parallel rather than randomly oriented in the Cu matrix. Hjortstam et al [27] estimated the increase of effective conductivity as a func‐ tion of the volume fraction of CNT in a Cu matrix. Their calculation showed that 30-40% CNT is needed in the composite with a resistivity 50% lower than for Cu. Liu et al [28] found that electrical sheet resistance is lower in Cu/CNT composite films than Cu and also decreases due to annealing at 200 - 300ºC.

Improved electromigration resistance is expected to result from the location of the alloy ele‐ ment at grain boundaries to prevent movement of Cu at those vulnerable points, which may lead to wiring voids (opens) or hillocks (shorts) during operation [25]. Cu/CNT composites may also improve thermal conductivity of lines and vias which also increases electromigra‐ tion resistance. Chai et al [24] reported that the Cu/CNT composite vias have lower electrical resistance than that of vias with CNT only. Their electromigration test results showed that the void growth rate for a Cu/CNT composite strip was four times lower than that of pure Cu strip. Their electromigration test of Cu and Cu/CNT composites which were carried out in the temperature range of 100 to 250ºC and current density from 5 × 105 to 2 × 106 A/cm2 using a conventional Blech-Kinsborn test structure showed that longer strips had larger void length, while no void formation was detected in the strips below 40 µm. Below the critical length the electromigration flux is balanced by the opposing backflow generated by the stress gradient in the test strip.

Yoo et al [25] fabricated Cu/MWCNT composite films by a pulsed electrodeposition techni‐ que with additives and obtained a dense structure without any voids. Their microstructure analysis showed that most of the MWCNTs exist at the Cu grain boundaries and cross-linked each other. They reported that C content in the composite increased by increasing CNT con‐ centration in the bath but it decreased with annealing. Chai [29] et al reported that the me‐ chanical strength of Cu/CNT nanocomposite was three times higher than that of pure Cu. Chen et al [26] observed good interfacial bonding between CNT and Cu when the nanocom‐ posites were codeposited by electrodeposition. They reported that for Cu/SWCNT nanocom‐ posites, the radial breathing mode (RBM) in the Raman was absent and the tangential or Gband had shifted and widened. Recently several patents on the metal/CNT composites codeposited by electrodeposition have been filed [30-32]. The comparison of electrical resistivity of Cu and Cu/SWCNT film which was reported by Chan [30] are shown in table 4.


**Table 4.** Comparison of the electrical resistivity of electrodeposited Cu/SWCNT composite film vs. Cu alone [30].

#### **1.4. Chlorosulphonic acid for CNT dissolution**

**1.3. Metal CNT composites**

free to obtain better electrical and thermal conductivity.

372 Syntheses and Applications of Carbon Nanotubes and Their Composites

decreases due to annealing at 200 - 300ºC.

stress gradient in the test strip.

To overcome some of the interconnect issues described above metal-CNT composites can be an alternative candidate material for future interconnects. The composite material would in‐ crease the contact area between vias and interconnect lines. There is also less chance of in‐ trinsic voids between CNTs as they would be metal filled. Among the different metals Cu is the best choice at this moment to use as a composite material with CNTs for interconnects applications because of superior electrical and thermal conductivity. Chai et al [23,24] and Yoo et al [25] reported that Cu fills the voids between neighbouring CNTs which results in a more densely packed structure. They reported that the addition of Cu increases the contact area between the nanotube (1 D) and the substrate (3 D contact) making it a mechanically strong material that can sustain high electrical or thermal stress cycling. To obtain superior properties of metal-CNT composites, it is necessary to achieve a homogeneous dispersion of CNT throughout the metal matrix. It is also necessary that the composites should be void

Cu/CNT composites can be prepared by powder metallurgy, electroless plating or electrode‐ position techniques [25, 26]. Among these methods electrochemical routes are relatively straightfoward methods to produce defect free nanocomposites [24, 25]. Powder metallurgy requires sintering at elevated temperatures that may damage CNT's and the difficulty of composite placement remains an issue. Chen et al [26] observed a clear separation of CNTs and Cu matrix composites deposited by powder metallurgy. To achieve optimum perform‐ ance, CNTs need to be well-dispersed and aligned parallel rather than randomly oriented in the Cu matrix. Hjortstam et al [27] estimated the increase of effective conductivity as a func‐ tion of the volume fraction of CNT in a Cu matrix. Their calculation showed that 30-40% CNT is needed in the composite with a resistivity 50% lower than for Cu. Liu et al [28] found that electrical sheet resistance is lower in Cu/CNT composite films than Cu and also

Improved electromigration resistance is expected to result from the location of the alloy ele‐ ment at grain boundaries to prevent movement of Cu at those vulnerable points, which may lead to wiring voids (opens) or hillocks (shorts) during operation [25]. Cu/CNT composites may also improve thermal conductivity of lines and vias which also increases electromigra‐ tion resistance. Chai et al [24] reported that the Cu/CNT composite vias have lower electrical resistance than that of vias with CNT only. Their electromigration test results showed that the void growth rate for a Cu/CNT composite strip was four times lower than that of pure Cu strip. Their electromigration test of Cu and Cu/CNT composites which were carried out in the temperature range of 100 to 250ºC and current density from 5 × 105 to 2 × 106 A/cm2 using a conventional Blech-Kinsborn test structure showed that longer strips had larger void length, while no void formation was detected in the strips below 40 µm. Below the critical length the electromigration flux is balanced by the opposing backflow generated by the

Yoo et al [25] fabricated Cu/MWCNT composite films by a pulsed electrodeposition techni‐ que with additives and obtained a dense structure without any voids. Their microstructure analysis showed that most of the MWCNTs exist at the Cu grain boundaries and cross-linked Recently Davis et al [33] reported that CNTs can dissolve spontaneously in chlorosulphonic acid solution up to 0.5 wt % [5 g/l], which is much higher than previously reported in other acids (up to 80 mg/l). They reported that at higher concentrations, they form liquid-crystal phases that can be processed into fibres and sheets of controlled morphology. Their pro‐ posed phase diagram helps to identify the optimal starting fluid composition and determine micro and macrostructure of fibres and films such as plated fibres, straight fibres and smooth films. Plated fibres have potential application for hydrogen storage and sensors be‐ cause of high surface area. Straight fibres are of interest for structural reinforcement and smooth, dense films for electrical applications such as electrically conductive thin films.

#### **1.5. Purification and functionalization of carbon nanotubes**

A significant problem in dealing with CNTs is the difficulty to separate them as the individ‐ ual CNTs form bundles due to van der Waals attractive forces. Also in all of the synthesis techniques several impurities like catalyst particles, amorphous carbon etc. are also present in the bundles of CNTs. These impurities may deteriorate the properties of CNTs. To pre‐ pare stable and homogeneous dispersions of CNTs considerable efforts have been made [34-39] but the solubility of CNTs in water or organic solvent is relatively low. At room tem‐ perature the solubility of CNTs is in the range of 60 to 80 mg/l [34]. In order to achieve better stabilization, CNTs require additional hydrophilic groups directly on the CNT walls or pro‐ vided by surfactant molecules to impart ionic charge on the CNTs [40-42]. The most com‐ mon hydrophilic groups are –OH-, -COOH-, -SO3-, -NH2-. Functionalization of CNTs can be an important factor to manipulate the properties of CNTs. With functionalization CNTs mau be more easily separated. Several methods have been suggested for the purification and functionalization of CNTs mainly based on covalent and noncovalent functionalization. Functionalized CNTs are easily dispersed and highly ionized in contract with water [43].

tants not only separate individual CNTs but also carry charge to the surface of CNTs so that the CNTs can be codeposited with Cu by electrodeposition. Examples of surfactants which

Carbon Nanotube Composites for Electronic Interconnect Applications

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375

In our study we primarily used nafion, a polymer surfactant for the dispersion of CNT bun‐ dles. It is a sulfonated tetrafluorethylene co-polymer with ionic properties which bears a po‐ lar side chain (-SO3H) and hydrophobic backbone (-CF2-CF2). It has unique ionic properties because of the incorporation of perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (Teflon) backbone. The hydrophobic backbone strongly anchors to the hydrophobic side-wall of CNTs. On the other hand the polar side-chain of the polymer imparts sufficient ionic charge to the CNT surface which enhances the solubility of

Cetyl trimethyl ammonium bromide (CTAB) is a cationic surfactant. Chen et al [51, 52] used CTAB for the dispersion of CNTs to prepare CNTs/Ni composites by an electroless deposi‐ tion technique. The chemical structure of CTAB is shown in fig. 4 (a). As a cationic surfac‐ tant it can make the CNT surface positively charged to assist the codeposition of CNT on the cathodic surface [52]. The CNT with negative charge readily adsorbs the cationic surfactant. This adsorption develops a net positive charge on the CNT, which prevents them from ag‐ glomerating and leads to electrostatic attraction to the cathode surface with negative poten‐ tial [51]. The net positive charge on the CNT increases the amount of CNT in the deposits. To calculate the volume fraction of CNTs, they dissolved the deposits in nitric acid. The CNTs in the deposits were filtered and the quantity of the CNTs in the deposits determined [52]. They reported that the content of CNTs in the deposit increases with an increase of CNT concentration in the bath, up to a maximum value at the CNT concentration of 1.1 g/l and then decreases. They explained this as a result of the CNT agglomeration in solution at higher concentration which reduces the content of CNT in the deposit. They also reported that the saturation concentration increases with decrease of length of CNTs because the lon‐

Sodium dodecyl sulfate (SDS) is an anionic surfactant used to improve the surface uniformi‐

ty of the composite deposit [53]. The chemical structure of SDS is shown in fig. 4 (b).

CNTs in liquid solvents. Fig. 3 shows the chemical structure of nafion.

have been investigated are given below.

**1.** Nafion® as a surfactant

**Figure 3.** Chemical structure of Nafion.

ger CNTs tend to agglomerate more readily.

**3.** SDS as a surfactant

**2.** CTAB as a surfactant

#### *1.5.1. Covalent functionalization of carbon nanotubes*

Several methods have been suggested for covalent functionalization of CNTs. The most common technique is to functionalize CNTs in concentrated acid by refluxing. In this proc‐ ess raw materials are sonicated followed by refluxing at 120-130°C. This process requires long processing times. After cooling at room temperature, the mixture is then centrifuged, leaving a black precipitate and a clear brownish yellow supernatant acid. Ko et al [44] re‐ ported that the presence of metal impurities in the MWCNTs is reduced significantly using this method. The purification process usually requires two repeat processing steps. The first step is acid reflux which washes metal catalyst and carbon impurities and the second step is annealing which burns the defective tubes and carbon particles. Ko et al [44] also used a mi‐ crowave oven technique to purify MWCNTs. Chen and Mitra [45] reported that MWCNTs were less reactive and had lower solubility than the SWNTs. Li and Grennberg [46] also found that microwave heating is highly useful for side wall functionalization of MWCNTs.

Lau et al [47] reported that the electrical conductivity of MWCNTs increased with different functionalization techniques such as oxidation, acid reflux, dry UV-ozonolysis. They ex‐ plained that the new functionalized groups increase the number of bands near the Fermi level, promoting electron transfer between the carbon atoms. They have claimed that CNT functionalization by UV-ozonolyzed technique significantly increases the electrical conduc‐ tivity of CNTs. Agarwal et al [48] reported that controlled defect creation could be an attrac‐ tive strategy to induce an electrical conductivity increase in MWCNTs. They reported that the outermost shell of MWCNTs is semiconducting so it is difficult to make electrical con‐ tacts to the inner shells of MWCNTs. Functionalization of CNTs may promote cross-shell bridging via sp3 bond formation. They proposed that intershell bridging facilitates charge carrier hopping to inner shells which can serve as additional charge carrier transport path‐ ways. Tantang et al [49] also reported that acid treatment increases the conductivity of CNT electrodes.

#### *1.5.2. Non-covalent functionalization of carbon nanotubes*

Covalent functionalization may deteriorate the unique ionic properties of CNTs by the for‐ mation of new covalent bonds on the CNTs wall. To overcome this disadvantage non-cova‐ lent functionalization mainly based on polymer surfactant interaction was developed that can disperse nanotubes easily but not degrade the CNT's unique properties [50]. The pro‐ posed mechanism for this solubilisaiton is through an individual CNT being wrapped by the polymer which acts as a surfactant in the solution to achieve separation. A surfactant is a wetting agent which lowers surface tension of liquids. It is usually an organic compound that contains both hydrophobic and hydrophilic groups. As a result, they are soluble both in organic solvents and water. Surfactants are classified based on the presence of a charged group. The head of an ionic surfactant carries a net charge. If the charge is negative, the sur‐ factant is more specifically called anionic; if the charge is positive, it is cationic. Ionic surfac‐ tants not only separate individual CNTs but also carry charge to the surface of CNTs so that the CNTs can be codeposited with Cu by electrodeposition. Examples of surfactants which have been investigated are given below.

**1.** Nafion® as a surfactant

functionalization of CNTs mainly based on covalent and noncovalent functionalization. Functionalized CNTs are easily dispersed and highly ionized in contract with water [43].

Several methods have been suggested for covalent functionalization of CNTs. The most common technique is to functionalize CNTs in concentrated acid by refluxing. In this proc‐ ess raw materials are sonicated followed by refluxing at 120-130°C. This process requires long processing times. After cooling at room temperature, the mixture is then centrifuged, leaving a black precipitate and a clear brownish yellow supernatant acid. Ko et al [44] re‐ ported that the presence of metal impurities in the MWCNTs is reduced significantly using this method. The purification process usually requires two repeat processing steps. The first step is acid reflux which washes metal catalyst and carbon impurities and the second step is annealing which burns the defective tubes and carbon particles. Ko et al [44] also used a mi‐ crowave oven technique to purify MWCNTs. Chen and Mitra [45] reported that MWCNTs were less reactive and had lower solubility than the SWNTs. Li and Grennberg [46] also found that microwave heating is highly useful for side wall functionalization of MWCNTs.

Lau et al [47] reported that the electrical conductivity of MWCNTs increased with different functionalization techniques such as oxidation, acid reflux, dry UV-ozonolysis. They ex‐ plained that the new functionalized groups increase the number of bands near the Fermi level, promoting electron transfer between the carbon atoms. They have claimed that CNT functionalization by UV-ozonolyzed technique significantly increases the electrical conduc‐ tivity of CNTs. Agarwal et al [48] reported that controlled defect creation could be an attrac‐ tive strategy to induce an electrical conductivity increase in MWCNTs. They reported that the outermost shell of MWCNTs is semiconducting so it is difficult to make electrical con‐ tacts to the inner shells of MWCNTs. Functionalization of CNTs may promote cross-shell

carrier hopping to inner shells which can serve as additional charge carrier transport path‐ ways. Tantang et al [49] also reported that acid treatment increases the conductivity of CNT

Covalent functionalization may deteriorate the unique ionic properties of CNTs by the for‐ mation of new covalent bonds on the CNTs wall. To overcome this disadvantage non-cova‐ lent functionalization mainly based on polymer surfactant interaction was developed that can disperse nanotubes easily but not degrade the CNT's unique properties [50]. The pro‐ posed mechanism for this solubilisaiton is through an individual CNT being wrapped by the polymer which acts as a surfactant in the solution to achieve separation. A surfactant is a wetting agent which lowers surface tension of liquids. It is usually an organic compound that contains both hydrophobic and hydrophilic groups. As a result, they are soluble both in organic solvents and water. Surfactants are classified based on the presence of a charged group. The head of an ionic surfactant carries a net charge. If the charge is negative, the sur‐ factant is more specifically called anionic; if the charge is positive, it is cationic. Ionic surfac‐

bond formation. They proposed that intershell bridging facilitates charge

*1.5.1. Covalent functionalization of carbon nanotubes*

374 Syntheses and Applications of Carbon Nanotubes and Their Composites

*1.5.2. Non-covalent functionalization of carbon nanotubes*

bridging via sp3

electrodes.

In our study we primarily used nafion, a polymer surfactant for the dispersion of CNT bun‐ dles. It is a sulfonated tetrafluorethylene co-polymer with ionic properties which bears a po‐ lar side chain (-SO3H) and hydrophobic backbone (-CF2-CF2). It has unique ionic properties because of the incorporation of perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (Teflon) backbone. The hydrophobic backbone strongly anchors to the hydrophobic side-wall of CNTs. On the other hand the polar side-chain of the polymer imparts sufficient ionic charge to the CNT surface which enhances the solubility of CNTs in liquid solvents. Fig. 3 shows the chemical structure of nafion.

**Figure 3.** Chemical structure of Nafion.

#### **2.** CTAB as a surfactant

Cetyl trimethyl ammonium bromide (CTAB) is a cationic surfactant. Chen et al [51, 52] used CTAB for the dispersion of CNTs to prepare CNTs/Ni composites by an electroless deposi‐ tion technique. The chemical structure of CTAB is shown in fig. 4 (a). As a cationic surfac‐ tant it can make the CNT surface positively charged to assist the codeposition of CNT on the cathodic surface [52]. The CNT with negative charge readily adsorbs the cationic surfactant. This adsorption develops a net positive charge on the CNT, which prevents them from ag‐ glomerating and leads to electrostatic attraction to the cathode surface with negative poten‐ tial [51]. The net positive charge on the CNT increases the amount of CNT in the deposits. To calculate the volume fraction of CNTs, they dissolved the deposits in nitric acid. The CNTs in the deposits were filtered and the quantity of the CNTs in the deposits determined [52]. They reported that the content of CNTs in the deposit increases with an increase of CNT concentration in the bath, up to a maximum value at the CNT concentration of 1.1 g/l and then decreases. They explained this as a result of the CNT agglomeration in solution at higher concentration which reduces the content of CNT in the deposit. They also reported that the saturation concentration increases with decrease of length of CNTs because the lon‐ ger CNTs tend to agglomerate more readily.

**3.** SDS as a surfactant

Sodium dodecyl sulfate (SDS) is an anionic surfactant used to improve the surface uniformi‐ ty of the composite deposit [53]. The chemical structure of SDS is shown in fig. 4 (b).

composite film by dissolving the deposit in hot nitric acid. The CNTs in the nitric acid solu‐ tion were filtered, dried and weighed. Osaka et al [66] reported the carbon content in the deposit was analyzed by the combustion infrared absorption method (CS 444, LECO) as this element analyzer is capable of analysing for carbon. The summary of CNT content in the de‐

**g/l**

Ni PA 6 Up to 1 [64] Cu PA 2 0.4 [60] Ni SDS 0.3 Up to 7 [53] Cu PA Up to 6 Up to 2.5 [25]

Historically, IC chips have been electrically connected to the substrate by a wire bond meth‐ od. In this method, the chip faces up and is attached to the package via wires. This connec‐ tion has limited electrical performance and reliability problems in addition to requiring pad location at the edge of the die. Flip chip, also known as 'Controlled Collapse Chip Connec‐ tion, C4', replaced the traditional wire bond method. In this method, solder bumps are de‐ posited on the chip pads over the full area of the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry (e.g., a circuit board or an‐ other chip or wafer), it is flipped over so that its top side faces down, aligned to the sub‐ strate and then the solder is reflowed to complete the interconnect. Generally, Sn-Pb based solder bumps have been used in flip chip packaging to connect chips to external circuitry. According to the International Technology Roadmap for Semiconductors the total number of I/Os will reach up to 10,000 cm2 chip area by 2014 which require finer interconnect with a pitch size less than 20 µm. To fabricate such fine pitch interconnect, conventional solder bump requires fine solder deposition or paste particle which are not readily available [67]. It is also important to reduce lead-based solders for environmental concerns (RoHS compli‐ ance). As circuit density increases, devices are also more vulnerable to non-uniform thermal

Cu has higher thermal conductivity than most binary or ternary solders. Cu bumps in flip chip assembly offer increased reliability, extended temperature range capability, greater me‐ chanical strength, higher connection density, improved manufacturability, better electrical and heat dissipating performance over Pb-Sn solder. It is also less expensive and decreases the amount of solder needed to create bumps. Cu pillars do not change shape during reflow so they do not encounter any volumetric redistribution which can lead to voids in the sol‐

**Weight % CNT in deposit Reference**

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377

Carbon Nanotube Composites for Electronic Interconnect Applications

posit obtained from the literature is shown in table 2.5.

**Bath Surfactant CNT**

**Table 6.** Literature data of CNT content in the plating bath and deposit.

distribution.

**2. Cu and Cu/CNT pillars for flip chip interconnect assembly**

**Figure 4.** Chemical structure of (a) CTAB and (b) SDS.

A comparison of hardness and XRD patterns of Ni/CNT composites by using SDS and CTAB surfactant in the bath with Ni was performed [53]. The hardness changes for the com‐ posite films and depends on the concentration of CNTs in the bath as well as surfactant. The composite from the CTAB bath showed an increase of hardness unlike that of the composite from the SDS containing bath. It can be also seen from the XRD data that (111) is the prefer‐ red plane of Ni when the bath contains CTAB like pure Ni deposition. On the other hand, SDS in the bath reduces the preferred Ni (111) orientation significantly in the composite. A summary of surfactants which are commonly used for the dispersion of CNTs are reported in table 5.


**Table 5.** Literature review of CNT dispersion surfactants.

#### **1.6. Analysis of carbon nanotubes in deposit**

There is very little in the literature on quantifying CNT content in the deposits [48, 51, 52]. Arai et al [60, 64, 65] measured the content of multi-walled CNTs in the electrodeposited composite film by dissolving the deposit in hot nitric acid. The CNTs in the nitric acid solu‐ tion were filtered, dried and weighed. Osaka et al [66] reported the carbon content in the deposit was analyzed by the combustion infrared absorption method (CS 444, LECO) as this element analyzer is capable of analysing for carbon. The summary of CNT content in the de‐ posit obtained from the literature is shown in table 2.5.


**Table 6.** Literature data of CNT content in the plating bath and deposit.

**O O S O O**

(a) (b)

A comparison of hardness and XRD patterns of Ni/CNT composites by using SDS and CTAB surfactant in the bath with Ni was performed [53]. The hardness changes for the com‐ posite films and depends on the concentration of CNTs in the bath as well as surfactant. The composite from the CTAB bath showed an increase of hardness unlike that of the composite from the SDS containing bath. It can be also seen from the XRD data that (111) is the prefer‐ red plane of Ni when the bath contains CTAB like pure Ni deposition. On the other hand, SDS in the bath reduces the preferred Ni (111) orientation significantly in the composite. A summary of surfactants which are commonly used for the dispersion of CNTs are reported

**CNT CNT: g/l Surfactant Surfactant: g/l Composites References** MW 0.3 CTAB 0.6 Ni/CNT [53] MW 0.1 Mg(NO3)2 Cu/CNT [26] MW 6 PA 0.5 Cu/CNT [23] SW 3 CTAC 3 Cu/CNT [54] MW 1 CTAB 4 Cu/Zn [55] SW 0.002 Nafion 0.4 Nafion/CNT [56] MW 0.6 Gelatine 0.4 CNT/Cu [57] MW 2 CTAB Ni/CNT [58] SW 2 Cu/CNT [24] MW 1 Nafion 0.01 Nafion/CNT [59] MW 6 PA 0.1 Cu/CNT [60] MW 1 Ni-Co/CNT [61] MW 1 Nafion 5 Nafion/CNT film [62] SW 0.2 Nafion Nafion/CNT [63] SW 0.05 SDS 0.1 PVP/CNT [50]

There is very little in the literature on quantifying CNT content in the deposits [48, 51, 52]. Arai et al [60, 64, 65] measured the content of multi-walled CNTs in the electrodeposited

**Figure 4.** Chemical structure of (a) CTAB and (b) SDS.

376 Syntheses and Applications of Carbon Nanotubes and Their Composites

**Table 5.** Literature review of CNT dispersion surfactants.

**1.6. Analysis of carbon nanotubes in deposit**

in table 5.

**Na <sup>+</sup> Na+**
