Nucleation and growth of ECD Cu on PVD TiN from low acid sulfate electrolyte

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1 Microelectronic Engineering 76 (2004) Nucleation and growth of ECD Cu on PVD TiN from low acid sulfate electrolyte L. Magagnin a, *, A. Vicenzo a, M. Bain b, H.W. Toh b, H.S. Gamble b, P.L. Cavallotti a a Dipartimento di Chimica, Materiali e Ing. Chimica G. Natta, Politecnico di Milano, Via Mancinelli, 7, Milan 20131, Italy b School of Electrical Engineering, QueenÕs University Belfast, Northern Ireland Available online 6 August 2004 Abstract Direct copper deposition by electrochemical methods on diffusion barriers has been recently investigated. Structure and properties of copper films growing according to a three dimensional island growth mode are expected to be strongly affected by the mechanism and kinetics of nucleation. The electrodeposition of copper on PVD TiN from low acid sulfate electrolyte is studied. Nucleation and growth of copper films are evaluated to explore the feasibility of direct copper electrodeposition on TiN barriers as a reliable deposition method without the use of seed layers. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Copper; Titanium nitride; Electrodeposition; Diffusion barrier; Seed layer 1. Introduction Copper interconnects are going to replace the conventional aluminum technology. Low resistivity and high resistance to electromigration are critical key properties to improve device performance at the submicrometer level. Due to copper solubility in silicon, copper metallization requires the interposition of diffusion barriers onto the silicon * Corresponding author. Tel.: ; fax: address: luca.magagnin@polimi.it (L. Magagnin). substrate. Nitride materials, such as titanium nitride TiN, by chemical vapor deposition (CVD) or plasma vapor deposition (PVD), are effective in preventing copper diffusion. A copper seed layer is usually vapor deposited onto nitride barriers to overcome difficulties in direct copper plating. Then, copper electrodeposition is carried out as the most efficient method to deposit void-free copper with high electromigration resistance in high aspect ratio structures. Direct copper deposition by electrochemical methods on diffusion barriers has been recently investigated [1]. Direct electrochemical deposition /$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi: /j.mee

2 132 L. Magagnin et al. / Microelectronic Engineering 76 (2004) on TiN was investigated by Oskam et al. [2], by Graham et al. [3], and by Radisic et al. [4] from fluoboric acid, ammonia and pyrophosphate baths, respectively. Under appropriate experimental conditions, a high density of copper nuclei was obtained from the acidic and the pyrophosphate bath with formation of continuous films by deposition at constant potential. The Volmer Weber growth mechanism, involving formation of copper islands directly on top of the substrate, was observed in the above mentioned investigations, with potential-dependent instantaneous nucleation followed by diffusion limited growth. Instantaneous nucleation and high nuclei density should result in a fast complete coverage of the substrate; however, such favorable nucleation conditions can be often achieved only in complex bath and at high negative potential. In this paper, we report on the electrodeposition of copper on PVD TiN from ph 3 sulfate electrolyte. Nucleation and growth of copper films are studied to explore the feasibility of direct copper electrodeposition on TiN as a reliable deposition method without the use of a seed layer. A low acid electrolyte is evaluated to avoid highly stressed films with poor adhesion from acidic solutions. The low acid sulfate electrolyte was shown [5] to be a viable alternative to conventional acidic bath: improvements in microthrowing power and growth structure were observed. These characteristics were shown to be linked to the peculiarities of the electrokinetic and additives behavior in the low acid electrolyte. The combined addition of chloride 50 ppm and 1500 MW polyethylene glycol 300 ppm were particularly effective in promoting microstructure refinement and a high rate of nucleation, as shown by transient kinetics and AFM topography analysis. 2. Experimental TiN deposition was carried out on oxidized (0.5 lm) silicon substrates. The oxidized wafers were prepared under standard cleaning method, before being pumped down in a DC magnetron sputtering system equipped with water cooled pure titanium target. A turbo molecular pump, backed by a rotary pump was used to evacuate the system to a chamber pressure of mbar. An argon pressure of mbar was introduced into the system. The target was pre-cleaned for 10 min, before commencing the deposition. The nitrogen gas flow rate was maintained constant by a gas mass flow controller introduced into the sputtering system. A DC power of 340 W was applied. During deposition, the composition of the material was monitored by an optical emission spectroscopy system (OES), where the optical readout was feedback to the computer. Stoichiometry of the TiN materials was observed when the peaks of Ti (364 nm), N 2 (357 nm) and N 2 (390 nm) were approximately in equal counts or ratios, which coincided to a nitrogen flow rate of approximately 4.5 sccm. TiN layer was 100 nm thick with sheet resistance of about 32 X/cm 1. Copper deposition was carried out in CuSO M, Cl 50 ppm electrolyte at ph 3 with dilute H 2 SO 4 and 25 C with platinum counter electrode. Electrochemical measurements were performed using EG&G PAR 273 potentiostat/ galvanostat with a platinum counter electrode and Ag/AgCl (KCl 3 M) reference electrode. Working electrode area was 1 cm 2. Scanning Electron Microscopy (SEM) experiments were performed with a Cambridge Stereoscan 360 with Torr vacuum was used. X-ray diffraction (XRD) experiments with a Bragg Brentano configuration were performed in a Philips PW 1830 instrument, with a Philips PW 3020 goniometer and a Philips PW 3710 control unit (Cu Ka radiation Å). An estimate of the volume fraction of different orientation components was obtained by the calculation of the Harris texture index [6,7]: I hkl X I hkl F hkl ¼ n ð1þ I R hkl hkli R hkl where I hkl is the measured intensity of reflection (hkl) and I R hkl is the reflection intensity of a random powder sample; n is the number of measured reflections. The calculation is based on the assumption that only crystals with orientation along (111), (100), (110), (311) directions occur. Microhardness was measured on as-prepared samples by depth-sensing technique with a

3 L. Magagnin et al. / Microelectronic Engineering 76 (2004) FISCHERSCOPE Ò H100 computer-controlled stress-limited system with Vickers indenter. The load-time waveform was triangular, ramping from zero to full load in 10 s, then ramping to zero load in 10 s. Three readings were performed for each load. 3. Results and discussion The nucleation and growth process in the electrodeposition of a metal on foreign substrate is assumed to take place at active sites at the surface. The shape of the current response to a potential step is characterized by a peak, corresponding to nucleation and three-dimensional diffusioncontrolled growth, and then by a decrease associated to one-dimensional diffusion-controlled growth to a planar surface [2]. In order to determine whether nucleation is instantaneous or progressive, the current response can be analyzed in terms of the maximum current i max and the time t max at which i max occurs. The reduced forms of the current transient as a function of the deposition time for the instantaneous, Eq. (2), or progressive, Eq. (3), nucleation are [8]: i 2 ¼ 1:9542 t max 1 exp 1:2564 t 2 ; i 2 max t t 2 max ð2þ i 2 i 2 max ¼ 1:2254 t 2 max 1 exp 2:3367 t2 : t t 2 max ð3þ Fig. 1(a) shows representative current transients recorded at potentials from 50 to 150 mv. The open circuit potential OCP for TiN was around 214 mv and for potentials more cathodic than OCP up to 50 mv, the deposition current density did not achieve significant values for nucleation and growth. Figs. 1(b) (d) show the deposition transients of Fig. 1(a) replotted in reduced form along with the theoretical curves for instantaneous and progressive nucleation given in Eqs. (2) and (3). Current transients follow the theoretical curve for instantaneous nucleation up to t slightly higher than t max. At longer times, experimental transients show higher values of current than those predicted by the models. In the potential range investigated, this effect cannot be attributed to the onset of the hydrogen discharge reaction, but could be related to growth conditions under prevailing activation control. Fig. 2 shows series of images of copper nuclei obtained at different potentials up to 500 mv for deposition time t/t max 3. At low potential, nuclei density is essentially independent of potential and copper nuclei are characterized by needle-like shape and faceting. For more cathodic potentials, i.e. 500 mv as in Fig. 2(d), copper nuclei are randomly distributed over the surface and have uniform size, consistently with instantaneous growth. Deposition of thick copper films on TiN from low acid electrolyte was also investigated. The deposition current density was varied in a wide range aiming at defining growth conditions leading to bright and adherent copper deposits. At high current densities, i.e. 200 ma cm 2, and for deposition times up to 60 s, homogeneous and bright copper films were obtained. As shown in Fig. 3, the structure of Cu deposits is only slightly affected by deposition time in the range s, corresponding to average thickness of 0.5, 1 and 3 lm, respectively. The estimated volume fraction M hkl of crystallites with different orientation remains unchanged at 0.5 and 1 lm thickness: M 111 is 50%, M 200 is 24%, M 220 is 11% and M 311 is 13%. The orientation distribution changes at 3 lm thickness, with increase of the volume fraction of crystallite with: (1 1 0), M 220 about 16% and (311), M 311 about 18% orientation, while M 111 decreases to 40%. Layer morphology changes accordingly (see Fig. 4), showing a smooth and almost featureless surface structure at 0.5 and 1 lm thickness, while at 3 lm thickness grain size increases and growth morphology becomes distinctly pyramidal in character with grains in the shape of unequally sided three to fivefold pyramids, related to the different orientation components. Crystallite size and microdeformation of Cu films were also estimated by the Williamson Hall method [9]. The first and second reflection order

4 134 L. Magagnin et al. / Microelectronic Engineering 76 (2004) Fig. 1. Current transients for potential steps (a) and deposition transients at 50 mv (b) 100 mv (c) and 150 mv (d) in reduced form () along with the theoretical curves for instantaneous ( ) and progressive ( ) nucleation. Fig. 2. SEM images of copper nuclei at 100 mv (a), 150 mv (b), 250 mv (c) and 500 mv (d) for t/t max 3.

5 L. Magagnin et al. / Microelectronic Engineering 76 (2004) Fig. 3. X-ray patterns for copper electrodeposits on TiN. Thickness: 0.5 lm (A) 1 m (B) and 3 lm (C). from {111} planes were used in the calculation. Both crystallite size, in the range from 100 to 120 nm, and microdeformation, in the range , remain practically unchanged with thickness increase in the above given range. Adhesion remains a key issue because most of the samples did not pass the qualitative scotch tape test. Microindentation measurements were used to evaluate quantitatively the adhesion. With the aid of a composite hardness model for soft films on hard substrates, the critical reduced depth b (the ratio between the radius of the plastic zone beneath the indentation and the indentation depth) was evaluated [10]. Indentation results are shown in Fig. 5, where the difference between substrate hardness H s and Cu/TiN system composite hardness H c is reported as a function of the ratio between film thickness t and indentation diagonal d. The calculated b values are higher than three, suggesting the formation of an extended plastic zone beneath the indentation, due to increasing adhesion of the copper film to the substrate. 4. Conclusions Nucleation and growth of copper films from ph 3 sulfate electrolyte on PVD TiN have been studied as a possible route to direct copper deposition Fig. 4. SEM images of copper electrodeposits on TiN: 200 ma cm 2 t = 10 s (a); 200 ma cm 2 t = 20 s (b); 200 ma cm 2 t = 60 s (c). on wafers without the use of a seed layer. Current transients follow the theoretical curve for instantaneous nucleation up to t max. At longer times, experimental transients show higher values of current than those predicted by the model, as a consequence of growth kinetics controlled by activation overpotential. Homogeneous and bright thick copper films were obtained. The structure of Cu

6 136 L. Magagnin et al. / Microelectronic Engineering 76 (2004) size increases and growth morphology becomes distinctly pyramidal. Adhesion was generally found not satisfactory, 1 3 lm thick films failing the scotch tape test, but could be improved as suggested by microindentation measurements; in fact, the composite hardness model for soft films on hard substrate suggests the formation of an extended plastic zone beneath the indentation, due to increasing adhesion of the copper film to the substrate. Fig. 5. Difference of substrate and composite hardness as a function of the thickness/indentation diagonal ratio for 1 lm thick copper electrodeposit on TiN. deposits is only slightly affected by deposition time in the range s, corresponding to average thickness of 0.5, 1 and 3 lm, respectively. The estimated volume fraction M hkl of crystallites with different orientation remains unchanged at 0.5 and 1 lm thickness, while the orientation distribution changes at 3 lm thickness, with increase of the volume fraction of crystallite with (110) and (311) orientation. Layer morphology shows a smooth and almost featureless surface structure at 0.5 and 1 lm thickness, while at 3 lm thickness grain References [1] A. Radisic, Y. Cao, P. Taephaisitphongse, A.C. West, P.C. Searson, J. Electrochem. Soc. 150 (2003) C362. [2] G. Oskam, P.M. Vereecken, P.C. Searson, J. Electrochem. Soc. 146 (1999) [3] L. Graham, C. Steinbrüchel, D.J. Duquette, J. Electrochem. Soc. 149 (2002) C390. [4] A. Radisic, J.G. Long, P.M. Hoffmann, P.C. Searson, J. Electrochem. Soc. 148 (2001) C41. [5] A. Vicenzo, P.L. Cavallotti, J. Appl. Electrochem. 32 (2002) 743. [6] G.B. Harris, Phil. Mag. 43 (1952) 113. [7] M.H. Mueller, W.P. Chernock, P.A. Beck, AIME Trans. 212 (1958) 39. [8] B. Scharifker, G. Hills, Electrochim. Acta 28 (1983) 879. [9] G.K. Williamson, W.H. Hall, Acta Metall. 1 (1953) 22. [10] L. Magagnin, R. Maboudian, C. Carraro, Thin Solid Films 434 (2003) 100.