Impact of the Cu-based substrates and catalyst deposition techniques on carbon nanotube growth at low temperature by PECVD

Microelectronic Engineering 84 (2007) 2501–2505 www.elsevier.com/locate/mee

Impact of the Cu-based substrates and catalyst deposition techniques on carbon nanotube growth at low temperature by PECVD M. Dubosc a,b,*, S. Casimirius a, M.-P. Besland a, C. Cardinaud a, A. Granier a, J.-L. Duvail a, A. Gohier a, T. Mine´a c, V. Arnal b, J. Torres b a

Universite´ de Nantes, IMN, UMR CNRS 6502, 2 rue de la Houssinie`re, F-44322 Nantes, France b STMicroelectronics, 850 rue Jean Monnet F-38920 Crolles, France c Universite´ Paris-Sud, LPGP, UMR CNRS 8578, Bat. 210, F-91405 Orsay, France Received 14 May 2007; accepted 21 May 2007 Available online 26 May 2007

Abstract This article reports on carbon nanotubes (CNT) grown on TiN/Cu stacks by plasma enhanced chemical vapor deposition (PECVD) at 450 C. Ni catalyst was deposited by two techniques – physical vapor deposition (PVD) and electrochemical deposition (ECD). First, the influence of the catalyst thickness and the catalyst deposition technique on grown CNTs is investigated. Second, the enhancement of the CNTs growth by use of electrodeposited catalysts is emphasized.  2007 Published by Elsevier B.V. Keywords: Carbon nanotubes (CNT); Plasma enhanced chemical vapor deposition (PECVD); Electrochemical deposition (ECD)

1. Introduction Carbon nanotubes (CNTs) have been the focus of interest of many research works within the scientific community since their discovery [1]. Due to their unique mechanical, thermal and electrical properties [2–4], they are an exciting potential candidate for many applications [5–14]. Among these, nanoscale active or passive devices in integrated circuit (IC) technology can take advantage of the combination between physical properties and mesoscopic size of the CNTs. In particular, the high current carrying density reported by Wei et al. [15] make them very interesting as conductive wires in future interconnects. A recent work reports on a model where multi-walled nanotube (MWNT) can outperform standard copper as local and global interconnects [16]. * Corresponding author. Address: STMicroelectronics, 850 rue Jean Monnet F-38920 Crolles, France. Tel.: +33 0 240 37 39 62; fax: +33 0 2 40 37 39 59. E-mail address: [email protected] (M. Dubosc).

0167-9317/$ - see front matter  2007 Published by Elsevier B.V. doi:10.1016/j.mee.2007.05.024

Toward integration in IC technology, CNTs must overcome several challenges such as in situ low-temperature growth process, consistency between materials required for CNT growth (mainly between catalyst and substrate) and the conventional CMOS processes, optimization of CNTs’ diameter and density to carry high current density while keeping low contact resistance. One of the most convenient CNTs low-temperature synthesis technique is PECVD. It allows the control of the vertical alignment of nanotubes due to the intrinsic electric field of the plasma sheath [17]. Using this feature, the first step for CNTs integration is to evaluate the PECVD performance to fill vias with CNTs, connecting copper metal lines. But, prior to this study, it is essential to perform CNTs growth on a Cu layer as substrate. This paper presents a preliminary study focused on CNTs synthesis on blanket wafers by a PECVD process for growth temperatures as low as 450 C, in order to be compatible with CMOS processes thermal budget. We also demonstrate the feasibility to grow CNTs on Cu by the use of a TiN interlayer between Cu and catalyst to prevent

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interdiffusion. Two ways for catalyst deposition have been investigated, namely a plasma technique, i.e. physical vapor deposition (PVD) and electrochemical deposition (ECD). We highlight the dependence of the CNT growth on catalyst deposition process and on catalyst thickness. Finally, taking advantage of electrochemically deposited catalyst film morphology, the CNT growth is enhanced by as-deposited catalyst particles.

450 C. In this study, the grid bias was fixed at 100 V, and the synthesis pressure was 1.4 m Torr. The PECVD chamber is fed with acetylene for 60 min at the required temperature. Ni deposits and grown CNTs were characterized by scanning electron microscopy (SEM) with a JEOL JSM 6400 F1. 3. Results and discussion

2. Experiment Blanket SiO2/Si wafers covered with 400 nm-thick copper layer were used as substrates. In order to investigate the impact of a diffusion barrier layer on carbon nanotube growth, 50 nm of TiN was deposited by physical vapor deposition (PVD) on the top of the stack. 2.1. Catalyst preparation Known as an efficient catalyst for carbon nanotube synthesis, Ni was here chosen due to its compatibility with standard CMOS processes. Two techniques were used for Ni deposition: PVD and electrochemical deposition (ECD). Physical vapor deposition was performed in a standard industrial reactor on 300 mm diameter wafers. ECD process is a standard three electrodes system, used in the chronoamperometry mode. An aqueous nickel sulfate hexahydrate bath (15 g/L), with boric acid as buffer, was used in order to adjust the pH value to 4. In the first part of this work, Ni catalyst was electrochemically deposited on Cu/SiO2/Si substrates. A negative working potential of 0.8 V was applied on the copper side, high enough to reduce Ni ions into metallic Ni that deposits on the Cu surface. By varying the deposition time, several thicknesses were obtained. These thicknesses were estimated by calculating the electrodeposited charge and assuming an electrochemical yield of 100%. For the samples covered by the TiN layer, Ni catalyst was deposited by ECD, while negatively biasing the Cu layer at 1 V. 2.2. CNTs growth Once the catalyst was deposited on the samples, the synthesis of carbon nanotubes was performed using electron cyclotron resonance PECVD (ECR-PECVD). This experimental setup has been previously detailed [18]. Briefly, our standard conditions were the following. We used a gas mixture of C2H2 carbonaceous precursor and NH3 as diluting and etching gas (1:2 ratio). The residual pressure is typically of 3 · 10 5 Torr. A typical ECR low pressure (1.4 m Torr)–high density (1011 cm 3) plasma can be created through a microwave antenna (2.45 GHz, 250 W). A direct current biased stainless steel grid controls the energy of charged plasma species reaching the substrate. The substrate holder was grounded and heated up in order to keep the samples temperature at

On the basis of published works [19,20], the most commonly accepted CNT growth sequence and mechanism is as follows: (i) A catalyst film is deposited onto a substrate, (ii) during annealing, the catalyst thin film rearranges into submicronic catalyst particles, (iii) the carbonaceous precursors are decomposed at the outer surface of the catalytic particles, (iv) the carbon atoms diffuse through or around the particle and finally (v) a CNT is formed either at the bottom or at the top of the particle. 3.1. Carbon deposition on Cu/SiO2/Si and TiN/Cu/SiO2/Si stacks We first used blanket silicon wafers covered with 400 nm-thick Cu layer. We deposited Ni by ECD and performed ECR-PECVD at 450 C using our standard plasma conditions described in the previous section for CNT growth. Independent of the catalyst thickness, CNT synthesis has never succeeded (catalyst thickness was between 3 and 8 nm). We think that the Ni catalyst activity is inhibited by the copper poisoning coming from the substrate. In order to prove and overcome it, we intercalated, prior to catalyst deposition, a diffusion barrier layer on the copper film. TiN was chosen as barrier material due to its compatibility with CMOS processes and its good electrical conductivity. Thus, 90 nm of TiN was deposited over Cu. The same Ni thickness (i.e. 8 nm) was deposited on top of the TiN layer and followed by the same PECVD process as previously. In such conditions, CNTs were grown by PECVD on TiN at 450 C, as shown on SEM images in the next section. Hence we demonstrated that the catalyst activity is recovered by deposition of a TiN layer between Ni and Cu, which acts as a barrier to Ni–Cu interdiffusion in our growth temperature range. 3.2. Influence of the catalyst thickness on CNT growth After succeeding the synthesis on Cu-based substrates at 450 C, we studied the impact of catalyst thickness on the CNT growth. Three different Ni thicknesses of 3, 8 and 14 nm were deposited on TiN covered wafers by varying the ECD deposition time from 40 to 160 s. Fig. 1a shows the corresponding chronoamperograms. Then ECRPECVD process was performed at 450 C in our standard plasma conditions for CNT growth. CNTs syntheses are

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Fig. 1. (a) Chronoamperograms of catalyst deposition from which the Ni thicknesses are estimated. SEM micrographs (45 tilt) of samples after CNT synthesis by PECVD at 450 C on electrodeposited Ni films for different Ni thicknesses (b) 3 nm, (c) 8 nm, (d) 14 nm.

depicted in Fig. 1b–d, respectively for Ni thicknesses of 3, 8 and 14 nm. On the 3 nm-thick sample shown in Fig. 1b, almost no CNTs growth is observed. The sample is wholly covered by an amorphous carbon layer. Increasing the catalyst thickness to 8 nm (Fig. 1c), CNTs grow with a relatively high density. CNTs diameter and height are ranging, respectively from 30 to 50 nm and from a few tens to more than 350 nm. Concerning the 14 nm Ni-thick sample a very high surface density of CNTs is achieved (Fig. 1d). They are 1 lm long and have a wide spread of diameters from 6 to 80 nm. Thus by increasing the catalyst deposition time and subsequently the catalyst amount on the sample, a high density of CNTs can be achieved. This result is particularly attractive concerning the interconnect application since CNTs density is one of the main challenge for the effectiveness of the CNT filled via. 3.3. Influence of the catalyst deposition technique on CNT growth Two techniques were used for Ni deposition on TiN covered samples, namely PVD and ECD. Both deposition processes are compatible with standard interconnects technology since they are already used for Cu seed deposition and via filling. The difference can come from their ability to assist CNTs growth.

Fig. 2a and b are typical SEM micrographs of Ni deposit using, respectively ECD and PVD. Fig. 2c shows a SEM micrograph of CNTs grown on Ni deposited by ECD and Fig. 2d shows CNTs grown on 10 nm-thick sputtered Ni film. The amount of ECD deposited Ni is estimated to be equal to a 14 nm-thick film. The deposit is not a continuous thin film but island-shaped. One can observe that CNTs grown on PVD deposited Ni film are shorter (length < 400 nm) with diameters ranging from 30 nm to 80 nm. The CNTs surface density and length are remarkably larger when the catalyst material was electrodeposited, as compared to the sputtered one. Two arguments can be advanced to explain such a difference. The first one is the catalyst thickness. As discussed in the previous section, by tuning the Ni thickness, one can achieve very dense CNTs carpet-like synthesis. Hence, the huge improvement in the CNTs height and surface density, observed in Fig. 2(c and d), may be due to the Ni amount difference between the 10 nm-thick sputtered and 14 nm-thick electrodeposited Ni films. However, as a second origin for such different behaviour, we can consider the growth mechanisms involved in the case of sputtered and electrodeposited catalyst films. Indeed, in our ECD deposition conditions (working potential and deposition time), the electrodeposited catalyst film is still in the nucleation phase. It means that even after 160 s of electrodeposition,

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Fig. 2. SEM micrographs (45 tilt) of samples (a) 14 nm-thick electrodeposited Ni, (b) 10 nm-thick sputtered Ni film (c) after CNT synthesis PECVD process at 450 C on 14 nm-thick electrodeposited Ni, (d) after CNT synthesis PECVD process at 450 C on 10 nm-thick sputtered Ni film.

Ni does not cover the whole substrate and only small islands are formed at the surface. On one hand, prior to CNT growth, the electrodeposited catalyst exhibits Ni particles with size very closed to the subsequently grown CNTs’ diameter (80 nm). On the other hand, the as deposited Ni PVD film is smooth and continuous (Fig. 2b) and the film is assumed to segregate into small catalyst particles during the substrate heating-up to the growth temperature, which acts as an annealing treatment for the catalyst layer. At growth temperatures as low as 450 C, the metal diffusion at the substrate surface is still reduced and the 3D Ni film structuring can be uncompleted. Consequently, the nano-islands are not well separated and one or more monolayers can survive the heating-up phase of the process and afterwards be directly exposed to the hydrocarbon plasma. Therefore, pre-formed particles like those obtained by ECD technique appear more suitable for CNTs catalysis.

sequent CNTs growth was investigated by comparing two processes ECD and PVD. Our results showed the interest of using ECD for the purpose of increasing CNTs length and density, due to the islands morphology of the ECD deposited Ni. Finally, we emphasized that our 450 C CNT synthesis PECVD process is compatible with the CMOS thermal budget limitation.

4. Conclusion

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CNTs have been successfully grown on copper blanket wafers by adding a thin TiN layer, acting as diffusion barrier layer and preserving the catalyst activity. We highlighted the fact that by tuning the Ni deposition thickness, high surface density of CNTs can be achieved. The impact of the catalyst deposition technique on the sub-

Acknowledgments This work is supported by the French National Research Agency under the grant ‘‘2005 National Nanosciences and Nanotechnologies Program – nanoreseaux 3D’’ SEM imaging have been performed at the Centre Commun de Micro-Caracte´risation (CMC) of University of Nantes: A. Barreau is gratefully acknowledged. The authors are grateful to N. Langlois, F. Petitgas, J. Guillet for technical support.

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