Research Article www.acsami.org
Carbon Nanotube Interconnects Realized through Functionalization and Sintered Silver Attachment V. Gopee,†,‡ O. Thomas,† C. Hunt,† V. Stolojan,‡ J. Allam,‡ and S. R. P. Silva*,‡ †
National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom Advanced Technology Institute, University of Surrey, Guildford GU2 7XH, United Kingdom
‡
ABSTRACT: Carbon nanotubes (CNTs) in the form of interconnects have many potential applications, and their ability to perform at high temperatures gives them a unique capability. We show the development of a novel transfer process using CNTs and sintered silver that offers a unique high-temperature, high-conductivity, and potentially flexible interconnect solution. Arrays of vertically aligned multiwalled carbon nanotubes of approximately 200 μm in length were grown on silicon substrates, using low-temperature photothermal chemical vapor deposition. Oxygen plasma treatment was used to introduce defects, in the form of hydroxyl, carbonyl, and carboxyl groups, on the walls of the carbon nanotubes so that they could bond to palladium (Pd). Nanoparticle silver was then used to bind the Pd-coated multiwalled CNTs to a copper substrate. The silver−CNT−silver interconnects were found to be ohmic conductors, with resistivity of 6.2 × 10−4 Ωm; the interconnects were heated to temperatures exceeding 300 °C (where common solders fail) and were found to maintain their electrical performance. KEYWORDS: carbon nanotubes, sintered silver, electrical interconnect, functionalized, plasma oxidation individual MWCNTs.9 These defect-free CNTs are typically synthesized at high temperatures by the arc-discharge method. Synthesis by chemical vapor deposition (CVD) methods (typically at temperatures above 700 °C) produces CNTs with a higher number of defects and therefore higher resistances. The high temperatures make both methods unsuitable for CMOS applications. Growth at lower temperatures often yields CNTs with defects, although some novel approaches are now reporting growth of CNTs with few defects at low temperatures.10 A major challenge for the application of CNTs as interconnects has been to overcome the high contact resistances at the interface between CNTs and metals due to Schottky barrier effects.11 Previous studies of CNTs as interconnects have mainly focused on growing CNTs in vias with metal contacts added after a planarization process.12,13 A difference in the work function between metal and CNTs gives rise to an energy barrier that impedes electron tunnelling, causing a high contact resistance.14,15 High contact resistance can also occur due to interfacial defects.16 Another challenge is promoting adhesion of the CNTs to substrates. Due to the unreactive nature of CNTs, the surface needs to be modified to add chemically active groups necessary for promoting adhesion to target substrates. In particular, oxygen plasma treatment of the surface has been used both as a purification method and as a way to make the surface more chemically reactive.17,18 The surface chemical composition of
1. INTRODUCTION The electronics industry has been working on interconnect technology to comply with the restriction of hazardous substances and the waste electrical and electronic equipment directives1 while meeting the demands for ever smaller components and connections and lead-free interconnections. Although the high-temperature lead-rich alloys are legally permitted, there is a widely held belief that this exemption has a limited life and new interconnect solutions are required. Many novel alternatives, such as sintered silver,1,2 are being considered. Silver nanoparticles have been shown to sinter at temperatures as low as 150 °C.3,4 An advantage of using silver as the interconnect material is that it has a melting point of 961 °C, which is ideal for high-temperature electronic applications. In contrast, solder interconnects reflow in the 180−250 °C range, thus constraining the manufacture and working temperatures of the interconnect. However, like solder interconnects, sintered silver interconnects are still susceptible to thermal coefficient of expansion mismatch when operated at high temperatures and have not been proven to work with all metal finishes. Carbon nanotubes (CNTs) are a potential replacement for copper interconnects due to their high current carrying5 capabilities, high thermal transport capabilities,6 and resistance to electromigration.7,8 Unlike single-wall CNTs, which have nanometer diameters and can be metallic or semiconducting, multiwall carbon nanotubes (MWCNTs) consist of tightly packed concentric tubes and are predominantly metallic. Defect-free MWCNTs may act as ballistic conductors at room temperature with mean free paths that can exceed tens of microns and resistance per unit length of ∼100 Ω μm−1 for © 2016 American Chemical Society
Received: December 10, 2015 Accepted: February 2, 2016 Published: February 2, 2016 5563
DOI: 10.1021/acsami.5b12057 ACS Appl. Mater. Interfaces 2016, 8, 5563−5570
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic of the process for the fabrication of Ag−MWCNT−Ag interconnect structures: step 1, patterning and catalyst deposition; step 2, PTCVD synthesis of MWCNTs; step 3, oxygen plasma functionalization; step 4, sputtering of Pd interface layer; step 5, sintering of MWCNTs at 230 °C to Ag paste printed onto a Cu substrate; step 6, removal of Si growth substrate; step 7, functionalization and metallization of the exposed side of the MWCNTs; step 8, printing of Ag paste on the tip of the MWCNTs followed by sintering at 230 °C. °C10,20,21 using acetylene as the carbon feedstock and hydrogen carrier gas. 2.2. CNT Functionalization and Metallization for Adhesion. Several methods are available for the functionalization of MWCNTs in the gas and liquid phases.22 Liquid phase functionalization was not considered to be appropriate as this would involve disrupting the vertical alignment of the MWCNTs crucial to the rest of our process. The MWCNTs were functionalized (Figure 1, step 3) in an Emitech K1050X plasma chamber using a frequency of 13.56 MHz. The plasma treatments were carried out at a set power of 50 W and pressure of 5 Torr. Oxygen was introduced in the chamber with a constant flow rate of 15 sccm. The CNTs were exposed to the oxygen plasma for increasing lengths of time, varying from 0 to 60 s, before Raman spectra and XP spectra were taken. The functionalized MWCNTs were subsequently coated with 50 nm of palladium (Pd) (Figure 1, step 4). Evaporation was carried out under high vacuum (∼8 × 10−5 mbar) with a deposition rate of ∼0.2 nm s−1 using 99.99% Pd, which was chosen as the metal contact, as it has been shown to form good electrical contacts with MWCNTs.23 2.3. Sintering to the Silver Paste. Heraeus silver paste (LTS 295-26P2) was screen-printed onto a copper substrate using a 25 μm thick stencil. The metallized CNT arrays were subsequently placed onto the wet silver paste before the sintering process (Figure 1, steps 5 and 6). Sintering was carried out using a temperature profile as recommended by the manufacturers, which involved holding the assembly at 165 °C for 1 h, to allow the evaporation of solvents, before heating to 230 °C and maintaining at that temperature for 1 h. The assembly was allowed to cool to room temperature before the Si growth substrates of the CNTs were gently removed. The functionalization and metallization processes were then repeated for the freshly exposed side of the CNT arrays (Figure 1, step 7). Silver paste was printed on top of the MWNCT arrays and the sintering process repeated to give Ag−MWCNT−Ag interconnects (Figure 1, step 8). 2.4. Electrical Measurements. A Keithley 4200 source-measure unit attached to a Suss-Microtec probe station was used to measure the resistance of individual arrays. The four-wire resistance technique was used to eliminate the resistance of the test leads leading to the
the CNTs changes during oxygen plasma treatment due to the addition of hydroxyl, carbonyl, and carboxyl groups.19 Here we investigate a fabrication process yielding freestanding Ag−CNT−Ag microstructures transferred onto a copper substrate. We demonstrate that the use of oxygen functionalization and deposition of thin metal layers on the tips of CNTs are key steps toward improving adhesion of CNTs to metals during the fabrication process. Sintered silver nanoparticles are shown to be effective in bonding metal-coated CNTs to a copper substrate. Finally, we show that the CNTs retain their electrical properties following exposure to an oxygen plasma and after heating to a temperature of 300 °C; this demonstrates the potential benefits of the interconnect for high-temperature applications.
2. MATERIALS AND METHODS The interconnect manufacture process that was developed has eight stages, as shown in Figure 1. This consisted of synthesis, functionalization, metallization, and sintering stages to adhere the grown MWCNTs to a copper substrate. Functionalization, metallization, and sintering were then repeated on the other side of the CNTs to form an interconnect. These stages are discussed in more detail in the following sections. 2.1. CNT Synthesis. Silicon substrates were coated with a thin layer of alumina, which acted as a thermal and diffusion barrier, using electron beam evaporation. Standard photolithography techniques were used to produce square patterns using photoresist on the silicon wafers. Five nanometers of Fe was deposited using magnetron sputtering, followed by lift-off of the photoresist (Figure 1, step 1). For the purposes of this study, the patterns produced were of four different sizes, 50 μm × 50 μm, 100 μm × 100 μm, 200 μm × 200 μm, and 400 μm × 400 μm. CNTs were subsequently synthesized (Figure 1, step 2) using a Surrey Nanosystems Photo Thermal Chemical Vapor Deposition (PTCVD) growth system. This technique allowed growth of highquality vertically aligned MWCNT arrays at temperatures below 400 5564
DOI: 10.1021/acsami.5b12057 ACS Appl. Mater. Interfaces 2016, 8, 5563−5570
Research Article
ACS Applied Materials & Interfaces
tubes cm−2. The average growth rate was found to be ∼10 μm min−1. 3.2. Raman Spectroscopy. The treatment of MWCNTs using oxygen plasma is known to introduce hydroxyl, carboxyl, and carbonyl groups, considered to be defects, on the surface of the CNTs.19 Raman spectroscopy is a fast and nondestructive method used to assess the quality of MWCNTs.24 All graphitic materials, including MWCNTs, have three first-order peaks around 1350 cm−1 (D), 1580 cm−1 (G), and 1620 cm−1 (D′).25 The origin of the D-band is attributed to double-resonant Raman scattering,26,27 and both the G and D bands are associated with graphitic sp2 bonded carbon.28,29 The ratio of the intensity of the D-band (ID) to intensity the G-band (IG) is a commonly used parameter to assess the quality of the MWCNTs,30 with a lower ID/IG ratio indicating higher quality MWCNTs. Raman spectra of the MWCNTs were taken with a Renishaw Raman spectrometer (model 2000 using an argon laser of wavelength 514.5 nm), and the ID/IG ratios were found to be 0.9. This value is low compared to values reported for MWCNTs grown at similar temperatures by other methods.30 The plots in Figure 4a show the first-order Raman spectra of
probes. The test leads were terminated by two probes, which were positioned onto the copper substrate and onto the sintered silver layer on the top side of the array as shown in Figure 2a. The resistance
Figure 2. (a) Schematic diagram of the structure of the Ag−CNT−Ag interconnect and arrangement for electrical measurements; (b) circuit diagram of resistances involved within the Ag−CNT−Ag interconnect structure. measured was a series-resistance combination of the sintered silver, CNT, and copper substrate as shown in the schematic in Figure 2b. Thus, the measured electrical properties represent the interconnect as a whole.
3. RESULTS AND DISCUSSION 3.1. MWCNT Synthesis. The PTCVD growth resulted in vertically aligned CNTs of length ∼200 μm, contained within the patterned areas, as shown in Figure 3. Scanning electron
Figure 4. (a) Normalized Raman spectra of MWCNT pretreatment and after oxygen plasma treatment showing an increase in intensity of the D-peak relative to the intensity of the G-peak for increasing exposure time; (b) ID/IG of samples exposed for various lengths of time showing an increase in the ID/IG ratio with increasing exposure time.
oxygen plasma treated MWNCTs for increasing exposure times. The ID/IG ratios are plotted as a function of exposure time in Figure 4b, showing an increase from 0.9 to 1.4, which indicates31 that the crystalline structure has been damaged by high-energy oxygen ions introducing defects to the surface of the CNTs. Cancado et al. also report an increase in the ID/IG ratio32 of graphite-based materials after argon ion bombardment. However, Raman spectroscopy does not offer any information on whether the defects are simply in the form of vacancies in the crystalline structure or if bonds are formed with the oxygen atoms. 3.3. X-ray Photoelectron Spectroscopy (XPS). XPS was used to perform quantitative analysis of the surface functional groups present on the MWCNTs exposed to oxygen plasma for various lengths of time. This is discussed in detail in the next section. XPS can be used to detect and differentiate between surface functional groups on MWCNTs to a depth of between 1 and 5 nm33 as well as providing information about the concentration of these groups. An asymmetric peak is generally observed, as is generally seen for highly oriented pyrolytic graphite, centered at 284.4 eV.34 Additional peaks are observed
Figure 3. SEM micrograph of 200 μm × 200 μm MWCNT arrays synthesized by PTCVD on patterned Si substrates using Fe catalyst and acetylene feedstock. (Inset) TEM micrograph of an individual nanotube at high magnification.
microscopy (SEM) micrographs (Figure 3) of the substrates showed dense arrays of vertically aligned MWCNTs with a length of ∼200 μm. The average diameters of the MWCNTs, measured using TEM, were found to be 7.3 ± 0.2 nm with, onaverage, between three and four walls per MWCNT. It is estimated that each array of MWCNTs has 1010 individual 5565
DOI: 10.1021/acsami.5b12057 ACS Appl. Mater. Interfaces 2016, 8, 5563−5570
Research Article
ACS Applied Materials & Interfaces
Figure 5. XPS survey scans of (a) pristine MWCNTs (A) and oxygen plasma functionalized MWCNTs (B) showing the change in intensity of the O 1s peak after oxygen plasma treatment and high-resolution scans of the C 1s peak for (b) pristine MWCNTs and (c) oxygen plasma functionalized MWCNTs.
when functional groups are induced on the surface of the CNTs. XPS analyses were performed on a ThermoFisher Scientific Theta Probe spectrometer. XPS spectra were acquired using a monochromated Al Kα X-ray source (hν = 1486.6 eV). An Xray spot of ∼400 μm radius was employed. Survey spectra were acquired using a pass energy of 300 eV. C 1s high-resolution core level spectra were acquired using a pass energy of 50 eV. All spectra were charge referenced against the C 1s peak at 285 eV to correct for charging effects during acquisition. Quantitative surface chemical analyses were calculated from the high-resolution core level spectra following the removal of a nonlinear (Shirley) background. A convolution of Gaussian− Lorentzian function and Shirley background subtraction was used for XPS spectra deconvolution. An upper limit of 1.5 eV for the full width at half-maximum was used during peak fitting. From the XPS spectrum (Figure 5), an increase in atomic percentage of oxygen from 0.3% for pristine MWCNTs (presence attributed to adsorption of atmospheric oxygen) to 26.2% for oxygen plasma treated MWCNT samples was noted. A high-resolution scan of the C 1s peak for oxygen plasma treated samples is shown in Figure 5b,c with fitted C 1s peak (1) at 284.6 ± 0.2 eV and the sp3 carbon peak (2) at 285.1 ± 0.2 eV. Three more peaks were fitted at 286.2 ± 0.2 eV (3), 287.2 ± 0.2 eV (4), and 288.9 ± 0.2 eV (5), corresponding to hydroxyl, carbonyl, and carboxyl groups, respectively.33 Quantitative analysis was performed on samples treated between 0 and 60 s in oxygen plasma, and the resulting C 1s spectra were deconvoluted to obtain the relative concentrations of the sp2, sp3, −CO−, −CO, and −COO− groups. The relative concentrations as a function of exposure time are represented in graphical form in Figure 6. First, we observed that around 14% of the total amount of carbon consisted of sp3 carbon; this is attributed to the amorphous carbon from the growth phase. The concentration of sp3 carbon remained almost constant at around 12−18% even after exposure to the plasma for 60 s. This confirms that the plasma did not convert sp2 bonded carbons into amorphous carbon and also that the amorphous carbon was not oxidized and removed from the MWCNTs. The concentration of sp2 carbons, however, decreased from 60 to ∼30% after 60 s of
Figure 6. Atomic concentration as a function of time of exposure to the oxygen plasma showing variations in atomic concentrations of sp2, sp3, −CO−, −CO, and −COO− groups and obtained from the deconvolution of C 1s XP spectra.
exposure. This decrease in sp2 carbon explains the increase of the ID/IG ratios of their Raman spectra. Increases in the concentrations of −CO−, −CO, and −COO− groups from 6, 3, and 8% to 15, 26, and 16%, respectively, were observed, confirming that functional groups were grafted onto the surface of the MWCNTs. Felten et al. reported detecting the presence of catalyst nanoparticles in their XP survey spectra after 10 min of exposure to oxygen plasma,19 implying that the end-caps of their MWCNTs had been etched away by the plasma, hence opening the tubes to expose the inner walls. Catalyst nanoparticles were not detected in our XP spectra, which could indicate that the MWCNTs retain their caps during the functionalization process. The importance of these defects is discussed in section 3.4. An et al. reported that increasing the number of functional groups on the surface of the MWCNTs caused an increase in their resistivity.35 The effect of increasing the exposure time on resistivity is not investigated here, but we chose to expose the MWCNTs to the plasma for 30 s before metallization. From Figure 6 we can see that the concentration of functional groups induced on the MWCNTs did not change significantly after that point and also that the concentration of sp2 carbon is also 5566
DOI: 10.1021/acsami.5b12057 ACS Appl. Mater. Interfaces 2016, 8, 5563−5570
Research Article
ACS Applied Materials & Interfaces
functionalized and metallized CNT arrays. The pristine, functionalized, and metallized CNT arrays all showed minimal adhesion to the sintered silver material. However, the functionalized and metallized CNT arrays showed an improvement in adhesion. We observed that over 95 of 100 arrays adhered to the sintered silver material when the arrays were both functionalized and metallized, whereas fewer than 5 of 100 arrays adhered when either just functionalized, just metallized, or pristine (these arrays may have only been loosely attached to the growth substrate). Figure 8 is a SEM micrograph of
almost constant. It is also not possible, at this stage, to accurately predict which functional groups on the surface of the CNTs are preferable to the others with respect to the adhesion of evaporated metals. 3.4. Metallization of MWCNTs. Metallization of the MWCNTs is an essential step to promoting adhesion to sintered silver. During our experiments, it was observed that pristine and functionalized MWCNTs did not adhere to sintered silver after step 5 of Figure 1 if no metal was deposited. The choice of metal is crucial to both the adhesion and the electrical conductivity, as discussed below. The adsorption of metal atoms facilitates the growth of metal crystals on the surface of CNTs. Adsorbed atoms diffuse and coalesce, forming nucleation sites, the density of which depends on the incoming vapor flux, the rate of diffusion on the surface, and the interaction between the atoms and the surface.36 Atoms stick to the surface with a coefficient37 that depends on their binding energy Eb and diffuse along the surface of the CNTs at a rate proportional to exp(−Ediff/kT), where Ediff is the diffusion activation energy and Ediff is ∼ Eb/4. Hence, it can be seen that weak binding energies allow fast diffusion across the surface. The formation of vacancies on the surface of the CNT during oxygen plasma treatment causes an increase in Ediff. Pd atoms get trapped in the vacancies and the rate of diffusion across the surface decreases, causing the density of nucleation sites to increase.38 Oxygen plasma treatment is therefore expected to be beneficial to the adhesion of metals to the surface of the MWCNTs. Figure 7 shows a SEM micrograph of vertically aligned MWCNTs metallized with Pd and Au, acquired after stage 4 in
Figure 8. SEM micrograph of multiple carbon nanotube arrays after stage 8 of the process showing MWCNTs sintered to a copper substrate and with sintered silver paste on top. Slight densification of the arrays can be observed due to evaporation of absorbed solvents.
MWCNT arrays that have been transferred to a copper substrate after stage 8 in Figure 1, forming the finished interconnect structure. It can be observed in Figure 8 that the arrays have densified, and this is attributed to the evaporation of solvents, absorbed by the arrays from the silver paste, before sintering.39 3.6. Electrical Measurements. Four-probe electrical measurements were carried out on the Ag−CNT−Ag interconnects, and typical I/V curves are shown in Figure 9a. The current increased linearly with an increase in voltage, showing we have achieved ohmic interconnects despite the introduction of defects during the oxygen plasma treatment. The average resistances, measured from 100 interconnects each, of four different size arrays are shown in Table 1. The resistance for the 50 μm × 50 μm interconnect is 41.1 ± 1.6 Ω, reducing by a factor of 4 with every doubling of the interconnect width, down to 0.7 ± 0.1 Ω for the 400 μm × 400 μm array. We state in section 3.5 that there may be a small amount of palladium contamination on the sides of the CNTs. However, if the resistance RPd of the palladium contamination film contributes to the total resistance, then the resistance of the interconnect should increase with the width of the interconnect by a factor of 2 (scaling with the perimeter of the interconnect). As the resistance of the interconnect increases by a factor of 4 (scaling with the area of the interconnect), we conclude that RPd has negligible contribution to the total resistance of the interconnect. The calculated resistivity value of ∼6.2 × 10−4 Ωm for each Ag−MWCNT−Ag interconnect was compared to the reported value for copper, which is 1.6 × 10−8 Ωcm. Jiang et al. reported
Figure 7. SEM micrograph of vertically aligned MWCNTs metallized with 50 nm of Pd taken from the middle of an array showing that Pd does not penetrate more than a few micrometers within the array and coats mostly the tip of the CNTs.
Figure 1 from the middle of an array. The metallization occurred primarily in the top few micrometers of the MWCNTs, where the MWCNTs have been exposed to the oxygen plasma. The tips of the MWCNTs and the Pd are in intimate contact. 3.5. Sintering Process. Sintering of the silver nanoparticles was carried out to bind (1) pristine, (2) functionalized, (3) metallized (without functionalization), and (4) combined 5567
DOI: 10.1021/acsami.5b12057 ACS Appl. Mater. Interfaces 2016, 8, 5563−5570
Research Article
ACS Applied Materials & Interfaces
Figure 9. (a) I/V graphs of a single interconnect at room temperature and 300 °C and (b) average resistance of 25 interconnect structures as a function of temperature of Ag−CNT−Ag interconnects heated from room temperature to 300 °C showing little variation in electrical resistance of the interconnects across the temperature range.
Using eq 1, the mean free path of the electrons in the interconnect is calculated to be 860 nm; high-quality MWCNTs have been shown to have mean electron mean free paths of up to 1 μm. This value is an improvement on that reported by Ahmad et al.21 on PTCVD-grown MWCNTs in vias with mean free paths of 500 nm. Oxygen plasma oxidation, at high power and long exposure times, removes the caps of MWCNTs and exposes the inner walls, which then provide additional channels for conduction.19 If all of the walls of the MWCNTs are exposed due to the oxygen plasma, this would lead to a reduction of the lmfp by an average factor of 3.5 for our carbon nanotubes, down to ∼245 nm. The contact resistance between the palladium and the MWCNTs can be considered to be negligible as palladium has been shown to form near ohmic contacts with MWCNTs23,47 The interconnect structures were heated to 300 °C; this is past the point where common solder interconnect structures fail. The resistance was measured at room temperature and then at 50 °C intervals between 50 and 300 °C (Figure 9b) for 25 different interconnect structures. The structures were maintained at each temperature for 30 min to allow thermal equilibrium to be reached. Small variations in resistance were observed, but the interconnect was shown to operate as an ohmic contact with a nearly constant resistance of 4 Ω across the whole temperature range. The resistance of CNTs is known to decrease with an increase in temperature,48 but this effect is not seen in our case. This may be due to the fact that the resistance of the structure is dominated by the contact resistance at the interface between the Pd and the CNTs, with the expected resistance contributions from pure copper and silver being
