Carbon nanotubes
Introduction
Carbon nanotubes (CNTs) receive much attention for various applications. At IMEC, the application for CNTs is towards interconnects. Crucial for the integration of these CNTs in full CMOS processes is selective catalyst placement and growth on 200mm silicon wafers, either with or without topography. Therefore growth processes for CNTs on these selected places with very-large scale integration (VLSI)-compatible technology for electronic devices should be developed. Growth can be achieved by a range of processes, among other by means of catalyst-activated processes.
Nucleation and growth of carbon nanotubes
In an ongoing effort to gain insight into the catalytic activity of various metals for the nucleation and growth of CNTs, IMEC has studied the growth of CNTs using `typical' (Ni, Co, and Fe) and `atypical' (Al, In, Pt, Ti, Mg, Pd, K, Cs, Na, W, Mn, Mo, Ir, and Ni3C) catalysts by chemical vapor deposition (CVD). Based on a systematic X-ray diffraction (XRD) analysis of the metal carbides formed during the very early stage of the reaction, a model was found that explains why different metals catalyze the growth of CNTs, and, in particular, why Ni, Co, and Fe display the highest catalytic activity. The catalytic activity of a metal strongly depends on its electronic structure. Metals with few d-vacancies such as Ni, Co, or Fe are the best catalysts to nucleate and grow CNTs since they can both form metastable carbides and release carbon atoms under typical CNT synthesis conditions. Metals with a large number of d-vacancies are active catalysts if their stable carbides can decompose carbon sources. Metals with full d-orbitals are active catalysts only at the nanoscale (nanoparticles ~1nm). Further results show that the enthalpy of formation of the carbon source controls the formation of metal carbides and the activation of the catalyst. Due to its very high enthalpy of formation, C2H2 is one of the most reactive carbon sources. In forming metal carbides, C2H2 produces the highest change of Gibbs free energy, especially with Ni, Co, and Fe.
Regardless of the metal used, the growth mechanism involves a transition during which the system evolves from carbon source decomposition (induction) to a combination of continuous carbon source decomposition and carbon precipitation (nucleation and growth) as shown in figure 1a. The induction period varies from metal to metal and is influenced by the synthesis conditions. Figure 1 depicts induction and growth for the reaction between Ni and C2H4. Following annealing at 650ºC, a 1nm Ni film transforms into nanoparticles with a lateral size distribution from ~7 to 10nm and a number density of ~1012/cm2 (figure 1b). Upon exposure to 100 ml/min. of C2H4, the time of induction averages 45 (± 5) sec. At this time, the first carbon nanostructures start to nucleate and grow (figure 1c). Probably due to differences in nanoparticle size, not all nanostructures grow simultaneously; but a few seconds later, the sample becomes fully covered by CNTs (figure 1d).
Figure 1: (a) Schematic diagram of the reaction sequence as a function of time. (b) Ni nanoparticles formed after annealing of a 1nm Ni film at 650°C for 5 min. under N2/H2 ambient and atmospheric pressure. (c) Appearance of the first CNTs at the end of the induction time (nucleation) by exposing the described nanoparticles to previous synthesis conditions and a C2H4 flow of 100ml/min. (d) Massive growth of CNTs after 5 min. of growth and same growth conditions.
CNT integration into Si technology
For integration of CNTs into Si technology, approaches need to be developed to achieve selective catalysis of CNT growth at lithographically predefined locations on patterned structures. The catalysis and growth conditions explored and optimized for CNT growth on unpatterned, so-called blanket surfaces, have to be fine-tuned for transfer to patterned substrates suitable for integration and eventual electrical testing of individual CNTs and arrays. IMEC has demonstrated scalable and Si-compatible selective growth of CNTs using conventional processing materials and methods. To achieve this, standard arrays of via holes, lithographically defined, were used as templates for catalyst placement. CNTs were grown inside the holes in a conventional CVD system. A schematic of the process flow is shown in figure 2 . Not only does this scheme achieve selective CNT growth in via holes suitable for further integration, but also tailoring the synthesis parameters allows control over CNT properties and growth density as shown in figure 3.
Figure 2: Schematic diagram of CNT growth in via holes: (a) topography on SiO2; (b) 10nm Ni deposition; (c) formation of nanoparticles by annealing; (d) protection of inner nanoparticles with a sacrificial polymer layer; (e) CMP for removal of external nanoparticles and removal of the sacrificial protection layer; (f) as-grown CNT by CVD.
Figure 3: Selected top-down SEM images of CNTs selectively grown in via holes following the process flow shown schematically in figure 1; (a) CNT bundles resulting from 4nm Ni with growth at 700ºC for 1 min. using 0.2 L min-1 C2H4 and (b) individual CNTs resulting from 2nm Ni with growth at 900ºC for 30s using 0.2 L min-1 C2H4.