**4. Experimental methods**

der. Because of these advantages the semiconductor industry is adopting Cu pillar bump by electrodeposition for flip-chip attachment to replace the typical Pb solder [67, 68]. Power and thermal non-uniformity in devices are increasing steadily with each new device genera‐ tion leading to serious concerns for the industry regarding thermal issues. Mechanical stress on Cu bumps generated by the difference in thermal expansion coefficients between the chip and the substrate materials can lead to device failures. This differential thermal expansion also creates shearing forces at the bump. As a result bumps are most vulnerable to damage.

Cu/CNT composites could be a suitable candidate material to resolve these issues for next generation flip chip assembly. CNTs have high mechanical strength (10-60 GPa, c.f. Cu 70 MPa) and thermal conductivity (>3000 W/m.K, c.f. Cu 400 W/m.K) which may alleviate the issues related to die degradation and non-uniform temperature distribution in the pillars. CNTs have a negative temperature coefficient of resistivity (- 1.5 х10-3/ºC, c.f. Cu + 4 х10-3/ºC) and low coefficient of thermal expansion (- 1.5 ppm/ºC, c.f. Cu + 17 ppm/ºC) which can make the Cu/CNT composites material more reliable against thermal cycling and fatigue with less risk of stress induced failure. Typical photolithography techniques can be utilised to fabri‐ cate Cu/CNT pillar bumps on chip. Arai et al [64] recently demonstrated Cu/CNT pillar

Repeated thermal expansion and contraction leads to fatigue cracking of the bump.

emitters deposited by electrodeposition on a patterned substrate.

378 Syntheses and Applications of Carbon Nanotubes and Their Composites

**3. Cu and Cu/CNT in through Si via (TSV) for 3D interconnect**

Cu electrodeposition in TSV features is a key component of new 3D integration approaches that are of great interest in the semiconductor industry [69]. 3D integration increases per‐ formance and lowers power consumption due to reduced length of electrical connections. Cu has been selected as the TSV interconnect because of its low electrical resistance and compatibility with conventional multilayer interconnection in large-scale integration (LSI) and back-end processes. The key challenges for TSV plating processes are to fill the vias across the entire wafer and to complete the fill as fast as possible to minimize cost. TSV in‐ terconnect shortens the interconnect requirements and reduces signal delay. However, it is difficult to fill high aspect ratio vias without voids through conventional damascene electro‐ plating. Perfect filling without voids is required to minimise interconnect failure and relia‐ bility issues. TSVs have been extensively studied because of their ability to achieve chip stacking for enhanced system performance. This is a very promising technology that may replace wire bonding in chips or single chip solder bumping. Metal filled TSVs allow devi‐ ces to be connected using a 3D approach [69]. Cu is the best low cost conductor for TSV in‐ terconnect and an extension of the damascene plating in smaller features. Recently enormous attention has been given to bottom up filling of TSVs to fill high aspect ratio vias without voids like conventional damascene electroplating [70-72]. However, there are key issues that need to be resolved, such as process reliability, electrical continuity and thermal management. TSVs should have the ability to maintain operation over a wide range of tem‐ peratures and to withstand these temperatures in a cyclic manner. The TSV material proper‐ In this work the Cu/CNT composites codeposition process was assessed and the deposited materials characterised. Electrochemical analysis of the deposition requires an analysis of the nucleation and growth characteristics for the candidate materials. MWCNTs have been added to the typical Cu sulphate plating bath to achieve homogeneous Cu/MWCNT compo‐ sites. Here, we will report electrochemical analysis and kinetics of electrodeposited Cu when MWCNTs were present in the bath. Solubilisation or suspension of the CNTs in the Cu bath is also a key requirement. Composite plating bath chemistries for Cu/CNT deposition were investigated. The influence of typical additives in the Cu bath on the deposit characteristics was determined for optimised electrodeposition in vias and trenches. The influence of dif‐ ferent surfactants on the deposition and electrical properties of composite films was also an‐ alyzed. Cu and Cu/CNT composites were electrodeposited on planar and structured substrates. Microstructure characterization of the deposit employed scanning electron mi‐ croscopy (SEM), focussed ion beam microscopy (FIB) and x-ray diffraction (XRD). The sheet resistance of Cu/CNTs film and changes due to self-annealing and high temperature anneal‐ ing were monitored by 4 point probe resistivity techniques. Cu/CNT composites were also deposited in test structures. After chemical mechanical polishing of the test structures, the line resistance was measured using a Cascade probe station.

The amount of CNTs in the deposit was determined by dissolving the deposit in a concen‐ trate HNO3 solution. The Cu/CNT films were deposited on 1 cm X 1 cm thin film sputtered Cu on Si. The deposition current was 1 A and deposition time was 1 h. The concentration of CNTs in the bath was 10 or 100 mg/l. After deposition, the sample was dipped in hot con‐ centrate acidic solution (65% HNO3, 65°C). The diluted acid solution was then vacuum fil‐ tered using PTFE filter paper. The filtration process was repeated at least 5 times to ensure all CNTs were left on filter residue. After filtration, the PTFE membrane was dried in an oven at 80°C for at least 30 minutes to ensure the membrane was completely dried. The weight difference of the PTFE membrane before and after filtration gives the amount of

The kinetics of the metal nucleation and growth/dissolution can be analysed with a rotating disk electrode system. While acknowleding the limitations and complications in the kinetic analysis of Cu [12] the general trends indicated in the data are consistent with published da‐ ta for the Cu sulphate system and those with typical damascene additives. It can be seen from the kinetic data analysis below that the exchange current density, i0, for Cu nucleation

Carbon Nanotube Composites for Electronic Interconnect Applications

is - 406 mV. The exchange current value for Cu nucleation and growth in the literature var‐ ies from 1 to 15 mAcm-2 [73–75]. Addition of all typical additives in CuSO4 bath decreases

mA/cm2 and increases the E0 value from - 406 mV to - 417.5 mV. This result confirms that all additives together have a suppressor effect on Cu deposition. It can be observed that addi‐ tion of 1% nafion also has a minor suppressor type behaviour on Cu nucleation and growth

and increases E0 value from - 406 mV to - 410.5 mV. But addition of CNTs has an accelerator influence on Cu nucleation and growth increasing the exchange current density, i0 from 7.24 mA/cm2 to 10.23 mA/cm2 and decreasing the E0 value from - 406 mV to - 403.5 mV. It is also observed that all typical additives including nafion and nanotubes together in the solution have an overall suppressor effect on Cu deposition. The summary results are shown in table 7. It can be seen from the table that anodic slopes are in the range from 1/65 mV to 1/76 mV and cathodic slopes are in the range from 1/122 mV to 1/164 which are close to the theoreti‐ cal values when the reactions are two separate single electron transfer steps. The above re‐ sults show that baths containing nafion & CNTs are compatible with the existing typical

**Anode Cathode**

0.24 M CuSO4 + 1.8 M H2SO4 (Basic bath) 154 67 - 406.0 **7.24**

Basic bath + 1% nafion 161 66 - 410.5 **7.07**

the exchange current density, i0 for Cu nucleation and growth from 7.24 mA/cm2

and the E0 value

http://dx.doi.org/10.5772/52731

to 7.07 mA/cm2

**I0/ mAcm-2**

**E0/mV**

164 76 - 403.5 **10.23**

151 67 - 417.5 **0.54**

128 72 - 420.0 **1.70**

122 76 - 421.0 1.66

to 1.2

381

and growth from the basic CuSO4 bath without any additive is 7.24 mA/cm2

as it slightly decreases the exchange current density, i0 from 7.24 mA/cm2

**Conditions 1/slope, mV**

CuSO4 bath used in IC interconnect deposition.

Basic bath + 1% nafion + 10 ppm CNTs

ppm SPS

Basic bath + All additives + 1% nafion

Basic bath + All additives + 1% nafion + 10 ppm CNTs

+ 300 ppm PEG + 1

**Table 7.** Summary of Tafel analysis obtained from rotating disk system.

Basic bath + 50 ppm Cl-

**Figure 5.** CVs of Cu and Cu/CNTs deposition on a glassy carbon electrode (scan rate: 0.1 V/s) from 0.24 mol dm-3 CuSO4 + 1.8 mol dm-3 H2SO4 with/without CNTs and different surfactants in the bath.

CNTs in the deposit. The amount of CNTs in the deposit was approximately 2% by weight by using long CNTs (length 5-9 µm, diameter 110-170 nm) or short CNTs (length < 1 µm, diameter 9.5 nm) in the bath. The density of CNTs is close to 1.3 gm/cm3 and pure Cu is 8.89 gm/cm3 which indicates the CNTs in the deposit are up to 12% by volume.

#### **5. Results**

To utilise CNTs as a composite with Cu for interconnect applications it is necessary to verify the influence of the materials on the Cu plating chemistry. Fig. 5 shows the comparison of cyclic voltammetry of Cu and Cu/CNTs co-deposition from a simple CuSO4/H2SO4 bath (hereafter referred to as the basic bath) with/without CNTs and surfactant at a scan rate of 0.01 V/s. It can be seen that the addition of CTAB and CNT results in a cathodic peak poten‐ tial shift to a more negative value which represents a suppression influence on Cu deposi‐ tion. On the other hand the Cu deposition occurs at lower overpotential when the bath contains either nafion or SDS which represents an acceleration influence. The diffusion coef‐ ficient for Cu2+ ions estimated from chronoamperometry data using the Cottrel equation in the basic bath (0.24M CuSO4 + 1.8M H2SO4) is 4.5 x 10-6 cm2 /s. A similar value (4.6 x 10-6 cm2 /s) was found from the SDS containing Cu/CNTs bath. Upon addition of nafion or CTAB in the Cu/CNTs bath, the diffusion coefficient value of Cu2+ ions slightly increases to 5.1 x 10-6 cm2 /s and 5.3 x 10-6 cm2 /s, respectively. It is clear that CNTs and surfactants in the Cu bath do not have a significant influence on the diffusion coefficient value of Cu. These re‐ sults indicate that the CNT + surfactant is compatible with the basic Cu sulphate/sulphuric acid bath chemistry. An assessment of the influence of the composite materials on baths that contain the basic constituents and the necessary additives to achieve bottom-up fill or super‐ filling of interconnect features in silicon technology is also required.

The kinetics of the metal nucleation and growth/dissolution can be analysed with a rotating disk electrode system. While acknowleding the limitations and complications in the kinetic analysis of Cu [12] the general trends indicated in the data are consistent with published da‐ ta for the Cu sulphate system and those with typical damascene additives. It can be seen from the kinetic data analysis below that the exchange current density, i0, for Cu nucleation and growth from the basic CuSO4 bath without any additive is 7.24 mA/cm2 and the E0 value is - 406 mV. The exchange current value for Cu nucleation and growth in the literature var‐ ies from 1 to 15 mAcm-2 [73–75]. Addition of all typical additives in CuSO4 bath decreases the exchange current density, i0 for Cu nucleation and growth from 7.24 mA/cm2 to 1.2 mA/cm2 and increases the E0 value from - 406 mV to - 417.5 mV. This result confirms that all additives together have a suppressor effect on Cu deposition. It can be observed that addi‐ tion of 1% nafion also has a minor suppressor type behaviour on Cu nucleation and growth as it slightly decreases the exchange current density, i0 from 7.24 mA/cm2 to 7.07 mA/cm2 and increases E0 value from - 406 mV to - 410.5 mV. But addition of CNTs has an accelerator influence on Cu nucleation and growth increasing the exchange current density, i0 from 7.24 mA/cm2 to 10.23 mA/cm2 and decreasing the E0 value from - 406 mV to - 403.5 mV. It is also observed that all typical additives including nafion and nanotubes together in the solution have an overall suppressor effect on Cu deposition. The summary results are shown in table 7. It can be seen from the table that anodic slopes are in the range from 1/65 mV to 1/76 mV and cathodic slopes are in the range from 1/122 mV to 1/164 which are close to the theoreti‐ cal values when the reactions are two separate single electron transfer steps. The above re‐ sults show that baths containing nafion & CNTs are compatible with the existing typical CuSO4 bath used in IC interconnect deposition.


**Table 7.** Summary of Tafel analysis obtained from rotating disk system.

CNTs in the deposit. The amount of CNTs in the deposit was approximately 2% by weight by using long CNTs (length 5-9 µm, diameter 110-170 nm) or short CNTs (length < 1 µm, diameter 9.5 nm) in the bath. The density of CNTs is close to 1.3 gm/cm3 and pure Cu is 8.89

**Figure 5.** CVs of Cu and Cu/CNTs deposition on a glassy carbon electrode (scan rate: 0.1 V/s) from 0.24 mol dm-3

To utilise CNTs as a composite with Cu for interconnect applications it is necessary to verify the influence of the materials on the Cu plating chemistry. Fig. 5 shows the comparison of cyclic voltammetry of Cu and Cu/CNTs co-deposition from a simple CuSO4/H2SO4 bath (hereafter referred to as the basic bath) with/without CNTs and surfactant at a scan rate of 0.01 V/s. It can be seen that the addition of CTAB and CNT results in a cathodic peak poten‐ tial shift to a more negative value which represents a suppression influence on Cu deposi‐ tion. On the other hand the Cu deposition occurs at lower overpotential when the bath contains either nafion or SDS which represents an acceleration influence. The diffusion coef‐ ficient for Cu2+ ions estimated from chronoamperometry data using the Cottrel equation in

/s) was found from the SDS containing Cu/CNTs bath. Upon addition of nafion or CTAB in the Cu/CNTs bath, the diffusion coefficient value of Cu2+ ions slightly increases to 5.1 x

bath do not have a significant influence on the diffusion coefficient value of Cu. These re‐ sults indicate that the CNT + surfactant is compatible with the basic Cu sulphate/sulphuric acid bath chemistry. An assessment of the influence of the composite materials on baths that contain the basic constituents and the necessary additives to achieve bottom-up fill or super‐

/s, respectively. It is clear that CNTs and surfactants in the Cu

/s. A similar value (4.6 x 10-6

which indicates the CNTs in the deposit are up to 12% by volume.

CuSO4 + 1.8 mol dm-3 H2SO4 with/without CNTs and different surfactants in the bath.

380 Syntheses and Applications of Carbon Nanotubes and Their Composites

the basic bath (0.24M CuSO4 + 1.8M H2SO4) is 4.5 x 10-6 cm2

filling of interconnect features in silicon technology is also required.

gm/cm3

**5. Results**

cm2

10-6 cm2

/s and 5.3 x 10-6 cm2

CTAB was added). On the other hand, the percentage of CNTs was slightly higher (CNT content 1.64% by weight) when SDS or CTAB was used in the bath with short CNTs in the bath (CNT content 1.12% by weight when nafion was added). It can be seen from the table, the weight percentage of MWCNTs in the deposit was less than 2%. According to the litera‐ ture [25, 60] the maximum CNT concentration in Cu/CNT composites achieved has been ap‐ proximately 2.5 % by weight. So the value found in these experiments is quite reasonable. The density of MWCNTs is close to 1.3 g/cm3 and pure Cu is 8.89 g/cm3 which indicates that

MWCNTs Diameter

**Table 8.** The weight percentage comparison of CNTs in the composite films using different size of CNTs and

ness of film was measured by using surface profilometry to be approximately 660 nm.

the CTAB containing bath which significantly increased the resistivity.

The electrical resistivity results showed that at room temperature the resistivity of Cu/CNTs composite films (2.46 µΩ-cm) when nafion was used for the surfactant of CNTs is close to the resistivity of Cu film deposited (2.15 µΩ-cm). The resistivity of Cu/CNTs composite film was higher when SDS (3.03 µΩ-cm) or CTAB (4.19 µΩ-cm) was used as a surfactant. The results are summarised in table 9. There was a larger scatter in the distribution of resistivity data in the CTAB case. This is probably a result of a less uniform and void rich deposit from

The resistivity of samples maintained at room temperature did not change significantly. The summary of the changes of the room temperature resistivity over time for Cu and Cu/CNT

The electrical properties of Cu/CNT composites were assessed by determining the resistivity of submicron films. Room temperature self-anneal phenomenon is usually observed in elec‐ trodeposited Cu films [76-80]. Due to large grain growth at room temperature and annihila‐ tion of the defects (void, vacancy, stacking fault, impurities redistribution etc), the electrical resistivity of Cu may change with time. It is therefore necessary to monitor the resistivity changes of Cu and Cu/CNT composite films over time after electrodeposition. The resistivity of Cu and Cu/CNTs composite films at room temperature was monitored using a four point probe apparatus (Keithley 2400 four point probe). In each case we took 4 samples and record‐ ed the average resistivity. The film was electrodeposited on a sputter Cu coated Si substrate. The deposition current density was 15 mAcm-2 and deposition time was 2 minutes. The thick‐

5 – 9 110 - 170 Nafion 1.12 5 – 9 110 - 170 CTAB 1.64 5 – 9 110 - 170 SDS 1.64 <1 9.5 Nafion 1.56 <1 9.5 CTAB 1.13 <1 9.5 SDS 1.69

nm **Surfactant % CNTs**

Carbon Nanotube Composites for Electronic Interconnect Applications

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383

the CNT content in the deposit is up to 12% by volume.

surfactants in the bath. PTFE membrane was used for filtration purpose.

MWCNTs Length μm

**Figure 6.** Comparison of Tafel plots for Cu deposition from with/without additives and CNTs in a standard Cu bath using a Cu rotating disk electrode which was rotated at 2000 rpm during experiments (Initial potential: 0 V, scan rate 0.1 V/s). Area of Cu RDE was 12.566 mm2

On a 1 cm2 Cu substrate (200 nm sputtered Cu on Si) the deposition current was 15 mA/cm2 and deposition time was 1 h. The concentration of CNTs in the bath was 100 mg/l. The acid solution was then vacuum filtered using 5 µm PTFE filter paper. The length and diameter of the MWCNTs were 5-9 µm and 110-170 nm respectively. Fig. 7 shows the SEM images of PTFE membrane after filtration of Cu/CNT deposits. The CNTs are clearly observed.

**Figure 7.** SEM image of CNTs on PTFE membrane after filtration of dissolved Cu/CNT composites. Image magnification 5000X on left and 40000X on right.

The amounts of CNTs in the deposit are shown in table 8 which compares the percentage of CNTs in the deposit when long or short CNTs with different surfactants were added in the bath. We found the percentage of CNTs was slightly higher (CNT content 1.69% by weight) when SDS was added with long CNTs in the bath (CNT content 1.13% by weight when CTAB was added). On the other hand, the percentage of CNTs was slightly higher (CNT content 1.64% by weight) when SDS or CTAB was used in the bath with short CNTs in the bath (CNT content 1.12% by weight when nafion was added). It can be seen from the table, the weight percentage of MWCNTs in the deposit was less than 2%. According to the litera‐ ture [25, 60] the maximum CNT concentration in Cu/CNT composites achieved has been ap‐ proximately 2.5 % by weight. So the value found in these experiments is quite reasonable. The density of MWCNTs is close to 1.3 g/cm3 and pure Cu is 8.89 g/cm3 which indicates that the CNT content in the deposit is up to 12% by volume.


**Table 8.** The weight percentage comparison of CNTs in the composite films using different size of CNTs and surfactants in the bath. PTFE membrane was used for filtration purpose.

**Figure 6.** Comparison of Tafel plots for Cu deposition from with/without additives and CNTs in a standard Cu bath using a Cu rotating disk electrode which was rotated at 2000 rpm during experiments (Initial potential: 0 V, scan rate

On a 1 cm2 Cu substrate (200 nm sputtered Cu on Si) the deposition current was 15 mA/cm2 and deposition time was 1 h. The concentration of CNTs in the bath was 100 mg/l. The acid solution was then vacuum filtered using 5 µm PTFE filter paper. The length and diameter of the MWCNTs were 5-9 µm and 110-170 nm respectively. Fig. 7 shows the SEM images of

**Figure 7.** SEM image of CNTs on PTFE membrane after filtration of dissolved Cu/CNT composites. Image magnification

The amounts of CNTs in the deposit are shown in table 8 which compares the percentage of CNTs in the deposit when long or short CNTs with different surfactants were added in the bath. We found the percentage of CNTs was slightly higher (CNT content 1.69% by weight) when SDS was added with long CNTs in the bath (CNT content 1.13% by weight when

PTFE membrane after filtration of Cu/CNT deposits. The CNTs are clearly observed.

0.1 V/s). Area of Cu RDE was 12.566 mm2

382 Syntheses and Applications of Carbon Nanotubes and Their Composites

5000X on left and 40000X on right.

The electrical properties of Cu/CNT composites were assessed by determining the resistivity of submicron films. Room temperature self-anneal phenomenon is usually observed in elec‐ trodeposited Cu films [76-80]. Due to large grain growth at room temperature and annihila‐ tion of the defects (void, vacancy, stacking fault, impurities redistribution etc), the electrical resistivity of Cu may change with time. It is therefore necessary to monitor the resistivity changes of Cu and Cu/CNT composite films over time after electrodeposition. The resistivity of Cu and Cu/CNTs composite films at room temperature was monitored using a four point probe apparatus (Keithley 2400 four point probe). In each case we took 4 samples and record‐ ed the average resistivity. The film was electrodeposited on a sputter Cu coated Si substrate. The deposition current density was 15 mAcm-2 and deposition time was 2 minutes. The thick‐ ness of film was measured by using surface profilometry to be approximately 660 nm.

The electrical resistivity results showed that at room temperature the resistivity of Cu/CNTs composite films (2.46 µΩ-cm) when nafion was used for the surfactant of CNTs is close to the resistivity of Cu film deposited (2.15 µΩ-cm). The resistivity of Cu/CNTs composite film was higher when SDS (3.03 µΩ-cm) or CTAB (4.19 µΩ-cm) was used as a surfactant. The results are summarised in table 9. There was a larger scatter in the distribution of resistivity data in the CTAB case. This is probably a result of a less uniform and void rich deposit from the CTAB containing bath which significantly increased the resistivity.

The resistivity of samples maintained at room temperature did not change significantly. The summary of the changes of the room temperature resistivity over time for Cu and Cu/CNT composite films deposited from different surfactant containing baths are shown in table 10. Osaka et al [66] reported that the resistivity of a deposit from an additive free bath and Cl- + PEG containing bath was unchanged with time. But when SPS was present in the bath, the resistivity decreased over time due to self-annealing. Lee and Park [82] reported that selfannealing is caused by Cu grain boundary diffusion. They mentioned that locally high stress originated from the trapped large molecule PEG which can accelerate grain boundary diffu‐ sion of Cu. There is a lack of consensus about the cause of self-annealing [81-86]. Among the suggested possible causes for self-annealing of electrodeposited Cu film are bath composi‐ tions [83], additives [77, 81, 82], film thickness [79, 80, 84], barrier layers [86, 87], impurities [79, 85] and deposition current [80].

cm and 2.10 µΩ-cm respectively when the sample was annealed at 315 °C. The conductivity increase of the composite films was probably due to a decrease the interface resistance be‐ tween CNTs and Cu matrix at the higher temperature, grain refinement and elimination of

**Basic + Nafion + CNT**

Carbon Nanotube Composites for Electronic Interconnect Applications

**Basic + CTAB + CNT**

http://dx.doi.org/10.5772/52731

385

**Basic bath Resistivity / μΩ-cm**

**Table 11.** Comparison of the resistivity of Cu and Cu/CNT composite films at room temperature and higher

The influence of CNT concentration in the Cu/CNT composite bath was investigated. The electrical resistivity results showed (fig. 8) that at room temperature the resistivity increased 10% when the concentration of CNT in the bath was increased from 10 mg/l to 100 mg/l. This data also shows no evidence of self annealing at room temperature for the composite material.

**Figure 8.** Comparison of the resistivity of Cu/CNT composite film at room temperature over time using 10 mg/l and

It can be seen from fig. 9 that when the samples were annealed at higher temperature up to 315ºC for 20 minutes the resistivity decreased from 2.46 µΩ-cm to 1.89 µΩ-cm for 10 mg/l CNT and 2.7 µΩ-cm to 2.19 µΩ-cm for 100 mg/l CNT in the bath. It is expected that CNT content in the composite is higher when deposited from higher CNT concentration containing bath [25, 60]. The resistivity increase of the higher CNT content bath is probably due to increased CNTs

content and higher contact resistance between CNTs and Cu in the composites.

temperatures 312 hours after deposition using different surfactants in the bath.

No anneal 2.15 2.46 4.19 1.78 2.14 2.72 1.92 2.04 2.45 1.67 1.88 2.10

defects under high temperature annealing.

**Annealing temperature oC**

100 mg/l CNT in the bath.


**Table 9.** Comparison of the resistivity of Cu and Cu/CNT composite film at room temperature 1 hour after deposition using different surfactants in the bath.


**Table 10.** Comparison of the resistivity changes of Cu and Cu/CNT composite film at room temperature and 311 to 313 hours after deposition.

It is well known that through annealing at higher temperature a reduced defect Cu micro‐ structure can be obtained [76-87]. Cu/CNT composite films (660 nm in thickness) were an‐ nealed in nitrogen at 215°C, 265°C and 315°C for 20 minutes. It can be seen that a clear decrease of sample resistivity was observed with increasing annealing temperature which is shown in table 11. The resistivity value of Cu film approaches that of bulk Cu value (1.67 µΩ-cm) after annealing at 315°C for 20 minutes. Also the resistivity of Cu/CNTs composite films decreased with increasing annealing temperature. The electrical resistivity of the Cu/ CNTs composite films deposited from a nafion and CTAB containing bath became 1.88 µΩ- cm and 2.10 µΩ-cm respectively when the sample was annealed at 315 °C. The conductivity increase of the composite films was probably due to a decrease the interface resistance be‐ tween CNTs and Cu matrix at the higher temperature, grain refinement and elimination of defects under high temperature annealing.

composite films deposited from different surfactant containing baths are shown in table 10. Osaka et al [66] reported that the resistivity of a deposit from an additive free bath and Cl-

PEG containing bath was unchanged with time. But when SPS was present in the bath, the resistivity decreased over time due to self-annealing. Lee and Park [82] reported that selfannealing is caused by Cu grain boundary diffusion. They mentioned that locally high stress originated from the trapped large molecule PEG which can accelerate grain boundary diffu‐ sion of Cu. There is a lack of consensus about the cause of self-annealing [81-86]. Among the suggested possible causes for self-annealing of electrodeposited Cu film are bath composi‐ tions [83], additives [77, 81, 82], film thickness [79, 80, 84], barrier layers [86, 87], impurities

> Basic (0.24 M CuSO4 + 1.8 M H2SO4) 2.17 Basic + Nafion + CNT 2.43 Basic + SDS + CNT 3.03 Basic + CTAB + CNT 4.69

**Table 9.** Comparison of the resistivity of Cu and Cu/CNT composite film at room temperature 1 hour after deposition

**/ hour**

**Table 10.** Comparison of the resistivity changes of Cu and Cu/CNT composite film at room temperature and 311 to

It is well known that through annealing at higher temperature a reduced defect Cu micro‐ structure can be obtained [76-87]. Cu/CNT composite films (660 nm in thickness) were an‐ nealed in nitrogen at 215°C, 265°C and 315°C for 20 minutes. It can be seen that a clear decrease of sample resistivity was observed with increasing annealing temperature which is shown in table 11. The resistivity value of Cu film approaches that of bulk Cu value (1.67 µΩ-cm) after annealing at 315°C for 20 minutes. Also the resistivity of Cu/CNTs composite films decreased with increasing annealing temperature. The electrical resistivity of the Cu/ CNTs composite films deposited from a nafion and CTAB containing bath became 1.88 µΩ-

**Bath Time after deposition**

**Bath Resistivity / μΩ-cm**

1 2.17 312 2.15

1 2.43 311 2.47

1 4.09 313 4.19

**Resistivity / μΩ-cm**

[79, 85] and deposition current [80].

384 Syntheses and Applications of Carbon Nanotubes and Their Composites

using different surfactants in the bath.

Basic

Basic + Nafion + CNT

Basic + CTAB + CNT

313 hours after deposition.

+


**Table 11.** Comparison of the resistivity of Cu and Cu/CNT composite films at room temperature and higher temperatures 312 hours after deposition using different surfactants in the bath.

The influence of CNT concentration in the Cu/CNT composite bath was investigated. The electrical resistivity results showed (fig. 8) that at room temperature the resistivity increased 10% when the concentration of CNT in the bath was increased from 10 mg/l to 100 mg/l. This data also shows no evidence of self annealing at room temperature for the composite material.

**Figure 8.** Comparison of the resistivity of Cu/CNT composite film at room temperature over time using 10 mg/l and 100 mg/l CNT in the bath.

It can be seen from fig. 9 that when the samples were annealed at higher temperature up to 315ºC for 20 minutes the resistivity decreased from 2.46 µΩ-cm to 1.89 µΩ-cm for 10 mg/l CNT and 2.7 µΩ-cm to 2.19 µΩ-cm for 100 mg/l CNT in the bath. It is expected that CNT content in the composite is higher when deposited from higher CNT concentration containing bath [25, 60]. The resistivity increase of the higher CNT content bath is probably due to increased CNTs content and higher contact resistance between CNTs and Cu in the composites.

deposited line was linear and the resistance value was estimated to be 284 Ω which is to be

Carbon Nanotube Composites for Electronic Interconnect Applications

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387

**Figure 10.** Plan view SEM image of test structure after complete CMP. The structure was filled by electrodeposited Cu

**-0.015 -0.01 -0.005 0 0.005 0.01 0.015 Voltage (V)**

**Figure 11.** Current vs. voltage curve of the 110 nm line width connected with four 110 μm x 80 μm pads. The features

The next samples investigated contained CNTs in the Cu deposited from the nafion contain‐ ing bath. The concentrations of CNTs and nafion were 50 mg/l and 0.5% respectively. Fig. 12 shows the electrical measurement of 110 nm line width filled with Cu/MWCNT in the test structure. It can be seen from the measurement that the I-V curve of the Cu/MWCNT depos‐ ited line was linear and the resistance value was 29.7 kΩ. The resistance of individual MWCNT is hundreds of kΩ (minimum resistance for a ballistic single-walled CNT is ~ 6.5 kΩ). The high resistance of individual CNTs is due to high contact and quantum resistance. Therefore, relatively dense arrays of CNTs will be needed to replace Cu interconnects. As Cu and MWCNTs were codeposited in the narrow line with 110 nm width, so the resistance

expected for lines of that dimension.

(lighter colour in image). Image magnification 40000X.

**-40 -30 -20 -10 0 10 20 30 40 50**

**Current (**m**A)**

were filled with electrodeposited Cu.

**Figure 9.** Comparison of the resistivity change of Cu/CNT composite film after annealing for 20 minutes at higher temperature using 10 mg/l and 100 mg/l CNT in the bath.

In summary, the Cu/CNT composites were codeposited with the aid of a surfactants at differ‐ ent current densities. As a comparative study, three surfactants (nafion, CTAB and SDS) were used separately to disperse CNTs in the bath and Cu/CNT composite films were electrodepos‐ ited. SDS in the bath results in a smoother deposition whereas CTAB leads to rougher deposi‐ tion of the composite. The maximum CNT concentration in Cu/CNT composites achieved in our study was approximately 2 % by weight deposited from 100 mg/l CNT containing compo‐ site baths. The electrical resistivity results show that at room temperature the resistivity of Cu/ MWCNT composite film (2.47 µΩ-cm) is close to the resistivity of Cu film (2.15 µΩ-cm) when nafion was used in the deposition bath for the surfactant of MWCNTs. With the use of CTAB or SDS in the bath, the resistivity of Cu/MWCNT film was higher [deposited from 10 mg/l CNT containing composite baths]. A clear decrease in sample resistivity was observed with increasing annealing temperature. The resistivity also increased when the concentration of MWCNTs was increased from 10 mg/l to 100 mg/l in the bath.

The line resistance of Cu filled and Cu/CNT filled test chip structures was measured using a Cascade probe station. The test chip structure consisted of 110 µm x 80 µm pads connected with metal lines of different widths. A Cu seed (12 nm) and a barrier Ta/TaN (25 nm) were PVD deposited. To achieve a uniform deposit the plated substrates were planarised with a CMP process. The test chip coupon was mounted on a 4 inch Si carrier-wafer. A Logitech CDP51 was used for the CMP. The polishing slurry used was a Cabot Microelectronics prod‐ uct, Eterpol 2362, which was mixed with H2O2 (30%), the ratio of H2O2 to slurry was 5% by volume. During CMP, the rotation of wafer holder and polishing pad was 50 rpm and the applied pressure was 2-3 psi. Fig. 10 shows the SEM image of the test structure after CMP for 2 minutes. It can be seen that excess deposits were completely removed by the devel‐ oped CMP process. Before probing, the test sample was vacuum attached in a dedicated holder. Four micro-probes were placed on four pads in the structure. The pads were con‐ nected with Cu filled interconnect lines. The Cu was deposited from a damascene additive containing sulphate based bath. Fig. 11 shows the electrical measurement of 110 nm width Cu line in the test structure. It can be seen from the measurement that the I-V curves of Cu deposited line was linear and the resistance value was estimated to be 284 Ω which is to be expected for lines of that dimension.

**Figure 9.** Comparison of the resistivity change of Cu/CNT composite film after annealing for 20 minutes at higher

In summary, the Cu/CNT composites were codeposited with the aid of a surfactants at differ‐ ent current densities. As a comparative study, three surfactants (nafion, CTAB and SDS) were used separately to disperse CNTs in the bath and Cu/CNT composite films were electrodepos‐ ited. SDS in the bath results in a smoother deposition whereas CTAB leads to rougher deposi‐ tion of the composite. The maximum CNT concentration in Cu/CNT composites achieved in our study was approximately 2 % by weight deposited from 100 mg/l CNT containing compo‐ site baths. The electrical resistivity results show that at room temperature the resistivity of Cu/ MWCNT composite film (2.47 µΩ-cm) is close to the resistivity of Cu film (2.15 µΩ-cm) when nafion was used in the deposition bath for the surfactant of MWCNTs. With the use of CTAB or SDS in the bath, the resistivity of Cu/MWCNT film was higher [deposited from 10 mg/l CNT containing composite baths]. A clear decrease in sample resistivity was observed with increasing annealing temperature. The resistivity also increased when the concentration of

The line resistance of Cu filled and Cu/CNT filled test chip structures was measured using a Cascade probe station. The test chip structure consisted of 110 µm x 80 µm pads connected with metal lines of different widths. A Cu seed (12 nm) and a barrier Ta/TaN (25 nm) were PVD deposited. To achieve a uniform deposit the plated substrates were planarised with a CMP process. The test chip coupon was mounted on a 4 inch Si carrier-wafer. A Logitech CDP51 was used for the CMP. The polishing slurry used was a Cabot Microelectronics prod‐ uct, Eterpol 2362, which was mixed with H2O2 (30%), the ratio of H2O2 to slurry was 5% by volume. During CMP, the rotation of wafer holder and polishing pad was 50 rpm and the applied pressure was 2-3 psi. Fig. 10 shows the SEM image of the test structure after CMP for 2 minutes. It can be seen that excess deposits were completely removed by the devel‐ oped CMP process. Before probing, the test sample was vacuum attached in a dedicated holder. Four micro-probes were placed on four pads in the structure. The pads were con‐ nected with Cu filled interconnect lines. The Cu was deposited from a damascene additive containing sulphate based bath. Fig. 11 shows the electrical measurement of 110 nm width Cu line in the test structure. It can be seen from the measurement that the I-V curves of Cu

temperature using 10 mg/l and 100 mg/l CNT in the bath.

386 Syntheses and Applications of Carbon Nanotubes and Their Composites

MWCNTs was increased from 10 mg/l to 100 mg/l in the bath.

**Figure 10.** Plan view SEM image of test structure after complete CMP. The structure was filled by electrodeposited Cu (lighter colour in image). Image magnification 40000X.

**Figure 11.** Current vs. voltage curve of the 110 nm line width connected with four 110 μm x 80 μm pads. The features were filled with electrodeposited Cu.

The next samples investigated contained CNTs in the Cu deposited from the nafion contain‐ ing bath. The concentrations of CNTs and nafion were 50 mg/l and 0.5% respectively. Fig. 12 shows the electrical measurement of 110 nm line width filled with Cu/MWCNT in the test structure. It can be seen from the measurement that the I-V curve of the Cu/MWCNT depos‐ ited line was linear and the resistance value was 29.7 kΩ. The resistance of individual MWCNT is hundreds of kΩ (minimum resistance for a ballistic single-walled CNT is ~ 6.5 kΩ). The high resistance of individual CNTs is due to high contact and quantum resistance. Therefore, relatively dense arrays of CNTs will be needed to replace Cu interconnects. As Cu and MWCNTs were codeposited in the narrow line with 110 nm width, so the resistance in Cu/MWCNT composites is expected to be between the resistance value of Cu (284 Ω) and MWCNT (hundreds kΩ). The resistance of the line filled with Cu/CNTs could be improved by using SWCNT instead of the MWCNTs used in this composite.

ues will require lower resistance SWCNTs or the improvement of the density of aligned nanotubes in the composite structure. This may be more feasible in larger dimension fea‐ tures such as those required for TSV interconnect at the chip scale rather the use of compo‐

Carbon Nanotube Composites for Electronic Interconnect Applications

http://dx.doi.org/10.5772/52731

389

This research was supported by the Irish Research Council for Science, Engineering and Technology (IRCSET) postgraduate scholarship Enterprise Partnership scheme in collabora‐

[1] L. L. Vadasz, A. S. Grove, T. A. Rowe and G. E. Moore, Silicon gate technology. IEEE

[3] S. Venkatesan, A. Gelatos, S. Hisra, B. Smith, R. Islam, J. Cope, B. Wilson, D. Tuttle, R. Cardwell, S. Anderson, M. Angyal, R. Bajaj, C. Capasso, P. Crabtree, S. Das, J. Far‐ kas, S. Filipiak, B. Fiordalice, M. Freeman, P. Gilbert, M. Herrick, A. Jain, H. Kawasa‐ ki, C. King, J. Klein, T. Lii, K. Reid, T. Saaranen, C. Simpson, T. Sparks, P. Tsui, R. Venkatraman, D. Watts, E. Weitzman, R. Woodruff, I. Yang, N. Bhat, G. Hamilton

[4] D. Edelstein, J. Heidenreich, R. Goldblatt, W. Cote, C. Uzoh, N. Lustig, P. Roper, T. McDevitt, W. Motsiff, A. Simon, J. Dukovic, R. Wachnik, H. Rathore, R. Schulz, L .Su,

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tion with Intel Ireland Ltd., funded under the National Development Plan.

Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland

[2] R. Solanki and B. Pathangey, Electrochem. Solid St., 3, 479 (2000).

and Y. Yu, Proc. IEEE-IEDM, 97, 769 (1997).

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sites for IC interconnect at deep sub micron dimensions.

**Acknowledgements**

**Author details**

**References**

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**Figure 12.** Current vs. voltage curve of the 110 nm line width which was filled with electrodeposited Cu/MWCNT composite. The concentrations of nafion and MWCNTs in the bath were 0.5% and 50 mg/l respectively.

#### **6. Conclusion**

In this chapter we have reported the influence of surfactants on the properties of Cu/CNT composites on Si substrates. Cu/CNTs composite films were co-deposited by electrodeposi‐ tion. Before electrodeposition, CNTs were dispersed by a suitable surfactant. Electrochemi‐ cal data shows that nafion or SDS accelerates the co-deposition where as CTAB suppresses the deposition. Nafion and SDS surfactants result in a relatively smooth deposit whereas CTAB surfactant leads to rougher deposition of the composite. The amount of CNTs in the deposit was up to 2 % by weight using different surfactants and different length/diameter of CNTs. Our electrical analysis showed that for Cu/CNT composite samples maintained at room temperature, the resistivity over time did not change significantly. The electrical resis‐ tivity results also showed that at room temperature the resistivity of the Cu/CNT compo‐ sites film (2.43 µΩ cm) is close to the resistivity of Cu film (2.17 µΩ cm) when nafion was used in the bath to disperse the CNTs. The resistivity of Cu/CNTs film was higher when CTAB or SDS were used instead of nafion as a surfactant. The electrical resistivity results showed that at room temperature the resistivity increased 10% when the concentration of CNT in the bath was increased from 10 mg/l to 100 mg/l. A clear decrease of sample resistiv‐ ity of composite films was observed with increasing annealing temperature. Cu/CNT com‐ posites deposited at a test structure with submicron lines and vias with Cu/CNT composites was only possible from the nafion surfactant containing damascene. The electrical measure‐ ment of 110 nm line width filled with Cu/MWCNT showed that the I-V curves of the Cu/ MWCNT deposited line was linear and the resistance value was 29.7 kΩ which was signifi‐ cantly higher that the resistance value of Cu (284 Ω) deposited. Improvements on these val‐ ues will require lower resistance SWCNTs or the improvement of the density of aligned nanotubes in the composite structure. This may be more feasible in larger dimension fea‐ tures such as those required for TSV interconnect at the chip scale rather the use of compo‐ sites for IC interconnect at deep sub micron dimensions.
