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Copper–Titanium Thin Film Interaction

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Copper–Titanium Thin Film Interaction Microelectronic Engineering 76 (2004) 153–159 www.elsevier.com/locate/mee Copper–titanium thin film interaction L. Castoldi a, G. Visalli a, S. Morin a, P. Ferrari a, S. Alberici b, G. Ottaviani c,d,*, F. Corni c,d, R. Tonini c,d, C. Nobili c,d, M. Bersani e a ST Microelectronis, Cornaredo, Milano, Italy b Central R&D, Physics and Material Characterization Laboratory, ST Microelectronis, Via Olivetti 2, I-20041 Agrate Brianza, Milano, Italy c Dipartimento di Fisica, via Campi 213/a 41100 Modena, Italy d MASEM, Materiali Avanzati per Sistemi, lElettroMeccanici, via Vivaldi 70, 41100 Modena, Italy e ITC-irst, via Sommarive 18, 38050 Povo, Trento, Italy Available online 12 August 2004 Abstract Interaction between 5 lm thick copper and 50 nm thin titanium films was investigated as a function of annealing temperature and time using MeV 4He+ Rutherford backscattering, X-ray diffraction and dynamic Secondary Ion Mass Spectrometry. Samples were made by depositing 10 nm of titanium on a PECVD silicon oxynitride, followed by 50 nm of titanium nitride and 50 nm of titanium in the said order. In the same system 100 nm of copper were subsequently sputtered; finally 5 lm of copper were grown by electroplating. This complex structure was chosen in order to inves- tigate the possibility of using copper interconnects also in power devices. To investigate the composition and growth of Ti–Cu compound on the buried interface, it was necessary to develop a special procedure. The results of the investiga- tion show the formation of a laterally non-uniform layer of TiCu4, which is presumably preceded by the formation of CuTi. The growth of the compound is kinetically controlled by means of a diffusion coefficient having 1.7 eV activation energy and a 5 · 10À2 cm2/s pre-exponential factor. The formation of a titanium–copper compound ensures a reliable and low resistance electrical contact especially at the vias. The Ti/TiN/Ti acts efficiently as a sacrificial and inert diffu- sion barrier. No copper was detected on the silicon oxynitride surface even after a 20-min 500 °C heat treatment. Ó 2004 Published by Elsevier B.V. PACS: 68.35.Fx; 6630.Xj; 68.55.Nq Keywords: Copper-titanium compounds; Power devices; Electrical contact; Diffusion barrier; Interconnect; Vias 1. Introduction * Corresponding author. Tel.: +39 039 6035168; fax: +39 039 6035530. Copper metallization is considered as a E-mail address: [email protected] substitute for aluminum-based interconnects for (G. Ottaviani). 0.1–0.15 lm advanced devices [1]. For these 0167-9317/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.mee.2004.07.043 154 L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 applications, a few hundredths nanometer thick and each piece was annealed (at the desired tem- films, generally in contact with low-k oxides, are perature–time couples) in a 10À6 mbar vacuum considered. To prevent copper diffusion, barriers furnace. are necessary; polycrystalline tantalum and amor- During the heat treatments Ti and Cu react; in phous tantalum nitride films are currently used [2]. order to observe the buried reacted film, the unre- The stability of such barriers against copper diffu- acted copper was selectively removed by a wet sion is very good, since, according to the equilib- chemical etching at room temperature with a mix- rium phase diagrams, no Ta–Cu compounds are ture of inorganic acids (phosphoric acid and acetic expected, and amorphous TaN inhibits grain acid) and hydrogen peroxide, diluted in water boundaries diffusion. (H2O2:H3PO4:CH3COOH:H2O = 1:1:1:20). Due to the advantage offered by copper in terms The surface morphology of the copper film was of resistance, electromigration properties, and unveiled by means of an atomic force microscope integrability in the process, its use has been consid- (AFM) and of a focused ion beam (FIB). Heat ered also for power devices. For these applications, treatments induce grain growth but no correlation mostly due to the necessity to deliver high power, was found with the Ti–Cu interaction process. Nu- the properties required to the interconnection clear techniques, Rutherford backscattering scheme, and as a consequence also the structures spectrometry (RBS) and elastic recoil detection and materials, are different than those necessary (ERD) techniques [3] with a 2–2.5 MeV 4He+ for memory or microprocessor applications. Actu- beam, were used to investigate the Cu–Ti interac- ally, for power devices the metal films are thicker, tions and the hydrogen redistribution. The experi- around a few microns for copper, the insulator is mental apparatus allows to acquire two spectra at generally PECVD deposited silicon oxynitride, two different scattering angles at the same time; and above all, the contact resistance in the vias 160° and 120° for RBS and 15° and 160° for should be as low as possible, as well as reproduci- ERD. In this way a double check can be per- ble. For the last requirement a metallization formed. Titanium and copper interact forming a scheme ensuring reproducibility through metallur- compound, to establish the nature of which RBS gical interaction is desirable. and X-ray diffraction have been used. Secondary The purpose of this paper is to investigate the ion mass spectrometry (SIMS) has been performed properties of a multilayer metallic structure in con- to monitor the diffusion of copper trough the Ti/ tact with PECVD deposited silicon oxynitride film. TiN/Ti layer [4,5]. The estimated sensitivity of The multilayer being considered consists of cop- such technique is 1017 atoms/cm3 of copper in tita- per, titanium and titanium nitride films. nium. The situation in the TiN layer is different, as the copper signal, mostly the 63 amu signal, inter- feres with the TiN signal and the sensitivity is not 2. Experimental better than a few atomic per cent. The results showed that, within the sensitivity of the tech- Samples have been prepared by depositing 1000 nique, there is no copper contamination at the nm of PECVD silicon oxynitride on p-type silicon Ti-oxynitride interface as far as all the annealed wafer, followed by 10 nm of titanium, 50 nm of conditions considered. titanium nitride and 50 nm of titanium. Subse- quently, 100 nm of copper were sputter-deposited, and finally 5 lm of copper were grown by electro- 3. Results plating in the same system. Before each heat treat- ment the copper films were kept at room Fig. 1 shows RBS spectra made with 2.2 MeV temperature. The resistivity decreased from 2.3 to 4He+ and the detector at the scattering angle of 1.8 lX-cm in 10 hrs, reaching its steady state value. 120° with respect to the incoming ion beam ob- To study in detail the Ti–Cu interaction processes tained from various samples after chemical etching 10 · 10 mm2 pieces where cut from a single wafer of copper. The positions of Ti and Cu on the sur- L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 155 Energy (MeV) Energy (MeV) 1.6 1.7 1.8 0.6 0.8 1.0 1.2 1.4 1.6 35 60 as-deposited 500C 20 min 30 350 C 10 min 400 C 5 min 400 C 5 min 50 25 40 20 30 15 20 NormalizedYield 10 Normalized Yield 5 10 Ti Cu N O Si Ti Cu 0 0 400 420 440 460 480 500 100 150 200 250 300 350 400 450 Channel Channel Fig. 1. 2.2 MeV 4He+ RBS spectra taken on as-deposited and Fig. 2. 2 MeV 4He+ RBS spectra taken in two samples post-annealing samples. The unreacted copper film has been annealed at two different temperatures, after unreacted copper removed by chemical etching. The spectra show only the copper etched. The signals from Ti and Cu are consistent with a layer and titanium portions; the position of the elements, if on the having a composition of Cu4Ti. surface, is shown. face are indicated. The signal between channels senting measurable thickness of the compound. 400 and 460 is due to Ti atoms, whereas above Fig. 2 shows the spectrum obtained from the sam- channel 460 the signal comes from Cu. The signal ple annealed at 500 °C 20 min with 2 MeV 4He+. from silicon oxynitride is not shown. The area un- The spectrum obtained from the sample annealed der the Ti peak is proportional to the total amount at 400 °C 5 min is plotted for comparison. The of titanium atoms in the sample for unit area. In position of the various elements, if located on the the as-deposited sample this quantity is 5.3 · 1017 surface, is indicated. On the surface, a slight oxy- atoms/cm2; within a small atomic percentage, the gen contamination of about 2 · 1016 atoms/cm2 is same value was found in all samples, independ- present. The simulation of the spectrum taken at ently of the heat treatments undergone and of 500 °C is quite complex due to the multilayer the quantity of compound formed. No copper is structure of the sample and to the fact that the present in the spectrum of the as-deposited sample, edges are not sharp, indicating a laterally non-uni- indicating that chemical etching is extremely effi- form sample, as the AFM pictures have also cient in removing metallic copper. The spectra of shown. Further complication in the analysis is the samples annealed at 350 °C for 10 min and due to the presence of hydrogen. at 400 °C for 5 min, show 5 · 1015 and 22 · 1015 By combining the results from RBS and ERD copper atoms/cm2 respectively.
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