Thermal Atomic Layer Deposition and Oxidation of ...

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(ALD) of TiN films followed by their dry oxidation in oxygen ambient. For both processes .... pressure, oxidation in the present work occurred much faster. -60- ...
Thermal Atomic Layer Deposition and Oxidation of TiN monitored by in-situ Spectroscopic Ellipsometry H. Van Bui, A.A.I. Aarnink, A.Y. Kovalgin, R.A.M. Wolters and J. Schmitz

Abstract—Thin TiN films have many important applications in Integrated Circuit (IC) technology. In spite of its chemical inertness, it is reported that TiN can be oxidized when exposed to oxidants (O2, H2O, etc.). To avoid an undesired oxidation of the metal-nitride layers, a study on this process is necessary. In this work, we present our study on thermal atomic layer deposition (ALD) of TiN films followed by their dry oxidation in oxygen ambient. For both processes, in-situ monitoring by spectroscopic ellipsometry is carried out. Index Terms—Titanium nitride, TiN, Atomic layer deposition, ALD, Dry oxidation, Titanium oxide, Spectroscopic ellipsometry.

I. INTRODUCTION

T

HE continued dimensional scaling down in semiconductor devices requires a deposition technique to produce conformal ultra-thin films. Thermal evaporation, sputtering, chemical vapor deposition (CVD) and atomic layer deposition (ALD) are conventional techniques used to deposit thin layers. Among these methods, ALD, which is carried out using sequential exposures of chemical reactants, can be an ideal choice for growing conformal films on high aspect ratio structures [1]. The ALD technique was first developed in the 1970s by Dr. Tuomo Suntola [2] and was originally called atomic layer epitaxy. Until the early 1990s, the applications of ALD were very limited because of its low growth rate that could not be acceptable in mass production in comparison with CVD or other physical vapor deposition (PVD) methods. With the trend in shrinking device dimensions, however, ALD is considered as a good candidate for producing ultra-thin layers with control of thickness and composition of the films [3]. Titanium nitride (TiN) is well known for its high thermodynamic stability, high corrosion resistance, low friction constant, and relatively low electrical resistivity. In IC technology, the most popular applications of TiN thin films

Manuscript received October 15, 2009. This work is supported by the Dutch Technology Foundation (STW), project 10017. H. Van Bui1, A.A.I. Aarnink, A.Y. Kovalgin, R.A.M. Wolters and J. Schmitz are with the MESA+ Institute for Nanotechnology, University of Twente, Chair of Semiconductor Components. P.O. 217, 7500 AE Enschede, the Netherlands. R. A. M. Wolters is also with NXP Semiconductors. 1 corresponding author: phone: +31 53 489 2727; fax: +31 53 489 1034; email: [email protected]

are diffusion barrier [4], metal gate material for dynamic random access memory (DRAM) [5], pseudo-electrodes for phase change random access memory (PC-RAM) [6] and complementary metal-oxide-semiconductor (CMOS) device [7]. The ALD and oxidation of TiN have been studied by many researchers. Spectroscopic ellipsometry (SE) has been used as a versatile tool for in-situ studies of ALD of TiN [8, 9, 10]. X-ray diffraction (XRD), Raman scattering spectroscopy and reflection high energy electron diffraction (RHEED) were used for either ex-situ or in-situ observations of the oxidation behavior of TiN [11, 12]. To our knowledge, a real time observation of oxidation of ALD TiN films has not been reported so far. In this paper, we present the thermal ALD and oxidation of very thin TiN films. These processes were in-situ monitored by a Woollam M2000 Spectroscopic Ellipsometer (SE) in the energy range 0.7-5 eV.

II. EXPERIMENTAL A. ALD and oxidation of TiN films TiN films were deposited on standard p-type monocrystalline silicon (100) wafers covered by a 100 nm thermally grown SiO2 layer. The depositions were performed in our home-built cluster system [13]. Prior to deposition, the wafers were cleaned in ozone steam cleaning chamber. One wafer was then loaded into the single-wafer ALD reactor and heated up in nitrogen ambient at a pressure of 10 mbar for 30 min. Titanium chloride (TiCl4) and ammonia (NH3) were used as precursors. The ALD cycle started with the exposure to TiCl4 for 2 seconds. Then, the excess TiCl4 and the by-products were purged out of the reactor by a nitrogen flow for 4 seconds. In the next step, NH3 vapor was introduced to the chamber for 2 seconds. The cycle ended with a nitrogen purge for 4 seconds. To study the dry oxidation behavior of the TiN films in oxygen ambient, the TiN layers with a thickness of 5 nm were deposited at 350 oC. Directly after deposition, the wafer was heated up to 400 oC in N2 at a pressure of 10 mbar for 30 min. After evacuating the reactor, dry oxygen with a flow of 25 sccm was introduced. The pressure inside the reactor was kept at 10 mbar.

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B. Material characterizations The deposition and oxidation of TiN were in-situ monitored by using a Woollam M2000 SE operating in the wavelength range between 245 and 1688 nm. The film thicknesses were measured by either high resolution scanning electron microscopy (HR-SEM) or X-ray reflectivity techniques. The crystalline structure of the oxide layers was analyzed with grazing incidence X-ray diffraction (GI-XRD). The composition of the layers was verified by X-ray photoelectron spectroscopy (XPS).

A similar drop of the growth rate was observed by Heil et al [9]. In the temperature range between 300 and 375 oC, the rate is nearly constant.

III. RESULTS AND DISCUSSION A. ALD of TiN Figure 1 shows the initial growth of TiN film deposited at 350 oC, monitored in-situ by SE. The inset shows a schematic representation of the optical model used for data analysis. A double layer Drude-Lorentz oscillator [8, 10] with the known dielectric function of ALD TiN was used for analyzing the SE data.

Fig. 2. HR-SEM image of a TiN layer grown on SiO2/Si wafer at 350 oC for 1800 cycles.

Fig. 3. The dependence of growth rate on temperature obtained by SE.

Fig. 1. The initial growth of ALD TiN deposited at 350 oC, as observed by SE. The inset shows the optical model: TiN was deposited on silicon wafer with a 100 nm thermally grown SiO2.

The growth of TiN layer basically consists of two stages: a nucleation at the beginning (the first 100 cycles), and then a linear increase in film thickness. The limited number of nucleation centers related to surface defect sites and –OH groups, which are necessary for TiCl4 to adsorb, causes the slow nucleation [10, 14]. The thickness of a TiN layer deposited at 350 oC for 1650 cycles was measured by HR-SEM, as shown in Figure 2. The film was ~ 40 nm thick with a good uniformity over the substrate. To study the effect of temperature on deposition rate, several experiments at temperatures in the range of 300-425 o C were done. The growth rate as a function of temperature is shown in Figure 3. The rate reaches a value of ~0.67 Å/cycle at 425 oC and decreases drastically to 0.25 Å/cycle at 400 oC.

B. Dry oxidation of TiN in oxygen ambient As mentioned above, the oxidation of TiN was studied by using 5 nm thick TiN layers deposited at 350 oC. The real time behavior of the oxidation of the film at 400 oC was observed by SE (Figure 4). The oxidation followed the linear-parabolic law accordingly to the classic Deal-Grove oxidation model of silicon [15]. Starting with a linear regime, controlled by the reaction kinetics, the thickness of the oxide layer increased quickly. The process continued with a parabolic regime, where the oxidation was limited by the diffusion of oxygen through the oxide layer. In a few minutes, the TiN layer was completely oxidized, forming a 10 nm thick oxide layer. In comparison with the results obtained from our previous work [16, 17] where the process was carried out at atmospheric pressure, oxidation in the present work occurred much faster.

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broad reflection around 20° originates from the amorphous silicon oxide. The remaining reflections suggest the presence of a NaCl type structure, which can either be TiN or titanium monoxide (TiO). The weak peak shoulder at 26° could originate from the presence of a small amount of TiO2 (anatase).

Fig. 4. The oxidation behavior of a 5 nm thick as-deposited TiN film in oxygen. During the process, the thickness of the nitride (red line) decreased and that of oxide (black line) increased. The TiN layer was completely oxidized in a few minutes.

The measurement of the oxide thickness by XRR is shown in Figure 5. The blue and the red line in the graph represent the measured and simulated data, respectively. The thickness of the film resulting in this rather accurate fit is given in Table 1. The result shows a good agreement with the oxide thickness measured by SE.

Fig. 6. XRD pattern of the layer. The film has a cubic structure which could originate from TiO.

The XPS analyses, however, indicate that the oxidized sample contains titanium in the TiO2 state (Figure 7). This is probably due to the fact that the oxide layer mainly comprises amorphous TiO2 and a fraction of crystalline TiO. The XRD measurement could indicate the presence of crystalline TiO while the XPS analyses gave the correct oxidation state of titanium in the oxide layer.

Fig. 5. The measurement of oxide thickness by XRR. The blue curve shows the measured data from the sample, the red one indicates the simulated data from the model. TABLE 1 Material Silicon SiO2 Ti(O,N)

Density 2.24 1.99 3.74

Thickness Substrate 107.5 nm 9.7 nm

Roughness 0.5 nm 1.1 nm

The crystalline structure and composition of the oxide were analyzed by GI-XRD and XPS, respectively. The GI-XRD scan is shown in Figure 6. All sharp reflections and also the wide reflection at 55° originate from the silicon substrate. The

Fig. 7. XPS analyses of the titanium oxide layer. The peaks at 464.31 and 458.63 eV indicate that titanium is in TiO2 state.

IV. CONCLUSIONS The thermal ALD of TiN using TiCl4 and NH3 as precursors was presented. The growth of TiN on the substrate

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occurs in two stages: a nucleation at the beginning and a linear growth later. The deposition rate was nearly constant at the temperatures in the range of 300-375 oC and was significantly changed when the temperature increased from 400 to 425 oC. We have also investigated the oxidation behavior of TiN in dry oxygen ambient by means of in-situ monitoring by SE. A 5 nm thick TiN film deposited at 350 oC could be completely oxidized at 400 oC in a few minutes, forming an oxide layer with a thickness of 10 nm. The XRD and XPS analyses indicated that the oxide layer mainly comprised of amorphous TiO2 with a small fraction of crystalline TiO. The result showed that the real time SE measurements were able to provide a reliable study of the oxidation behavior.

ACKNOWLEDGMENT The authors would like to thank M. Smithers (MESA+) for the HR-SEM measurement, Gerard Kip (MESA+) for the XPS analysis and H. J. Wondergem (Philips Eindhoven) for the XRR and XRD measurements.

[12] S. X. Wu, Y. J. Liu, X. J. Xing, X. L. Yu, L. M. Xu, Y. P. Yu, and S. W. Li, “Surface reconstruction evolution and anatase formation in the process of oxidation of titanium nitride film”, Journal of Applied Physics, vol. 103, pp. 063517, 2008. [13] A. Boogaard, A. Kovalgin, T. Aarnink, R. Wolters, J. Holleman, I. Brunets, and J. Schmitz, "Langmuir-probe Characterization of an Inductively-Coupled Remote Plasma System intended for CVD and ALD," ECS Transactions, vol. 2, pp. 181-191, 2007. [14] E. Langereis, S. B. S. Heil, M. C. M. van de Sanden, and W. M. M. Kessels, “In situ spectroscopic ellipsometry study on the growth of ultrathin TiN films by plasma-assisted atomic layer deposition”, Journal of Applied Physics, vol. 100, pp. 023534, 2006. [15] B. E. Deal, and A. S. Grove, "General Relationship for the Thermal Oxidation of Silicon," Journal of Applied Physics, vol. 36, pp. 37703778, 1965. [16] Brunets, A.W. Groenland, A. Boogaard, T. Aarnink, and A.Y. Kovalgin, "A Study of Thermal Oxidation and Plasma-Enhanced Oxidation/Reduction of ALD TiN Layers", Book of Abstracts, 8th International Conference on Atomic Layer Deposition, June 29 - July 2, 2008 - Bruges, Belgium, P-54. [17] A. W. Groenland, I. Brunets, A. Boogaard, A.A.I. Aarnink, A. Y. Kovalgin, and J. Schmitz, "Thermal and plasma-enhanced oxidation of ALD TiN", Proceedings of Semiconductor Advances for Future Electronics (SAFE) Conference (November 27-28 2008, Veldhoven, The Netherlands), pp. 468-471, 2008.

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