Properties of reactively sputter-deposited TaN thin films

Thin Solid Films, 236 (1993) 347-351

347

Properties of reactively sputter-deposited T a - N thin films Xin Sun, Elzbieta Kolawa, Jen-Sue Chen, Jason S. Reid and Marc-A. Nicolet California Institute of Technology, Pasadena, CA 91125 (USA)

Abstract

We deposited T a - N films by reactive r.f. sputtering from a Ta target with a n N 2 - A r gas mixture. Alloys over a composition range 0-60 at.% N have been synthesized. We report on their composition, structure and electrical resistivity before and after vacuum annealing in the temperature range 500-800 °C. We found that the film growth rate decreases with increasing ratio of the nitrogen flow rate to the total flow rate, while the nitrogen content in the films first increases with the N2 partial flow rate and then saturates at about 60 at.%. B.c.c.-Ta, Ta2N, TaN and T a 5 N 6 appear in succession as the nitrogen content rises, with Ta2N being the only single-phase film obtained. The atomic density of the films generally increases with the nitrogen content in the film. Transmission electron micrographs show that the grain size decreases from about 25 to 4 nm as the nitrogen concentration increases from 20 to 50 at.%. The Ta2N phase can exist over a wide range of nitrogen concentration from about 25 to 45 at.%. For as-deposited films an amorphous phase exists along with polycrystaUine Ta2N in the center portion of that range. This phase crystallizes after vacuum annealing at 600 °C for 65 min. A diagram of stable and metastable phases for T a - N films based on X-ray diffraction and transmission electron microscopy results is constructed. The resistivity is below 0.3 m~ cm for films with 0-50 at.% N and changes little upon vacuum annealing at 800 °C.

I. Introduction

Thin films o f Ta and its various compounds have long been of practical and scientific interest [1]. In the recent past tantalum nitride has attracted attention for applications as a thin film resistor with a low temperature coefficient of resistivity [2, 3], as a stable Schottky contact to silicon [4] and as a thin film diffusion barrier between silicon and metal overlayers o f Ni [5], AI [6-9] and most recently Cu [10, 11]. As diffusion barriers in metal-semiconductor contacts, refractory metal nitrides have long been recognized as an attractive class o f material because o f their high stability and good conductivity [12]. As an impurity in polycrystalline transition metal films, nitrogen has also been shown to restrict diffusion and increase the barrier effectiveness of those films, presumably by decorating the extended defects that act as fast diffusion paths in polycrystalline metallic films [ 13]. In substantial atomic concentrations, nitrogen can also promote the formation o f amorphous metallic alloys with most early transition metals. The resulting films tend to combine the advantages of the high inertness of the respective metal nitrides with the absence of fast diffusion paths as they exist in polycrystalline films (for a review see ref. 14). To elucidate the influence of nitrogen on the diffusion barrier performance, it is essential to clarify how the deposition conditions and subsequent annealing treatment alter the structural and electrical properties of a film. The objective of the present study is to establish

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these facts for reactively sputter-deposited tantalum nitride films with a particular concern for the existence of amorphous T a - N . The study generally follows the approach of a similar investigation o f the W - N system [15].

2. Procedures

Silicon wafers, either covered with thermally grown S i O 2 o r patterned with photoresist, and segments of

carbon tape were used as substrates on a stationary platform. T a - N films were deposited on these substrates by reactive r.f. magnetron sputtering from a 7.5 cm planar Ta cathode in an N 2 - A r ambient. The substrate holder was placed about 7 cm below the target and was neither cooled nor heated externally. The background pressure was 5 x 10-TTorr or less prior to the film deposition. The flow rates of N2 and Ar and the total gas pressure were adjusted by mass flow controllers. The total pressure was monitored with a capacitive manometer in a feedback loop. We varied the N2 partial flow rate, defined as the ratio of the nitrogen flow rate to the total flow rate of argon and nitrogen, from 0% to 25% to get 10 sets of films with different compositions. The total flow rate was in the range 55-80 cm 3 min -t. The highest flow rate was used to maintain good control of the nitrogen flow when its fraction was small. The forward sputtering power and total gas pressure were kept at 300 W and 10 mTorr

© 1993-- Elsevier Sequoia. All rights reserved

348

X. Sun et al. / Reactively sputter-deposited T a - N thin films

respectively for all depositions. Sputter deposition was performed in a static mode for 5 min. The carbon substrates were used to determine the nitrogen concentration in the films by 2 MeV 4He2+ backscattering spectrometry. Photoresist-patterned films on Si substrates were obtained by lifting off in acetone and were subsequently used for measuring the film thickness with a Dektak profilometer. The atomic density of a film was calculated from its thickness and from the width of the Ta signal in the backscattering spectrum of the film. Films on oxidized Si substrates were annealed in a vacuum furnace at a pressure of about 5 x 10 -7 Torr and temperatures ranging from 500 to 800 °C. All annealings lasted 65 min. All as-deposited and annealed films on oxidized Si were analyzed by backscattering spectrometry, four-point probe sheet resistance measurement, Co K s X-ray diffraction using a stationary position-sensitive detector and by Cu K s X-rays using a Read camera. We also made special depositions on copper grids covered with holey carbon. Plan-view microstructures of these films were characterized by transmission electron microscopy (TEM) in a Philips EM430 microscope operating at 300 keV.

3. Results and discussion

3.1. As-deposited films Figure 1 shows the growth rate of films for various N2 partial flow rates in the chamber. The growth rate decreases rapidly as the amount of nitrogen increases. For films reactively sputtered in a gas mixture with 5% N2, the growth rate is only 70% of that in pure Ar. At the same time the nitrogen concentration in the films increases steeply at first and then tends to saturate at about 60 at.%, as shown in Fig. 2. The symbols in this and subsequent figures indicate that the films are typi-

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cally of multiphase composition, as determined by Xray diffraction. Similar trends in deposition rate and nitrogen concentration have been reported previously for d.c.-sputtered T a - N films [6], but the functional dependences on the N2 flow rate of those results and ours cannot readily be compared because the nitrogen flow rate was varied under different experimental constraints. X-ray analyses reveal that the film deposited in pure Ar is b.c.c.-Ta. For a nitrogen concentration between about 10 and 20at.%, Ta2N is present in addition to the b.c.c.-Ta phase. The only crystalline phase present between about 20 and 50 at.% N is Ta2N, but an amorphous component also appears in the center portion of that range. At 50 at.% N, TaN is present, together with TasN6 as the nitrogen concentration exceeds the 50 at.% value. The structure of tantalum nitrides can be described as close-packed arrangements of Ta atoms with N atoms inserted in interstitial sites. The space group of Ta2 N is P63/mmc [16], with equal numbers of sites for Ta and N atoms, while the nitrogen atoms occupy half of the sites randomly [ 17]. However, deviations from this occupancy ratio can occur, which explains the finite range of existence of Ta2N. The range here exceeds that reported for thermal equilibrium conditions [18] and is presumably due to the non-equilibrium aspects of the sputter deposition technique. The sequence of the phases observed here is consistent with the equilibrium phase diagram [18], where b.c.c.-Ta is known to have a low solubility for N, Ta2N exists over a range of about 10 at.% around its stoichiometric composition and TaN is a narrow phase. Various nitrogen-rich compounds (TasN6, Ta4Ns, Ta3Ns) are also reported [16, 18], of which only TasN6 fits the observed X-ray lines well. The atomic density of the as-deposited films vs. the nitrogen concentration in the films is plotted in Fig. 3. The Ta film sputter deposited in pure Ar has a density

X. Sun et al. / Reactively sputter-deposited T a - N thin films

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Fig. 3. Atomic density of reactively sputtered T a - N films with various nitrogen concentrations. Note that the Y axis starts from about 4.5 instead of zero. For reference, bulk densities are indicated with arrows on the Y axis.

that is almost the same as that of bulk Ta (5.52 x 1022cm-3). The atomic density of the films generally increases as the amount of nitrogen increases. This. change in density of the films corresponds to the successive appearance of Ta2N, TaN and TasN6 in the films as the nitrogen concentration increases. In the figure, the atomic densities of the films with high nitrogen concentration are typically larger than those of their corresponding bulk compounds. Although this is unusual, these films contain some phases that are far from their exact stoichiometric composition, which might at least partly explain the results. The remaining discrepancy, if any, may be attributed to experimental uncertainties, which are due to the lateral non-uniformity of the film thickness and to the high background encountered in the backscattering signal of nitrogen. To classify the microstructure of the as-deposited films, special depositions (labelled A - E on the bottom abscissa of Fig. 5) were made on copper grids covered with holey-carbon. Correspondingly labelled brightfield micrographs are shown in Fig. 4. The grains of all crystalline phases are small and their size decreases with increasing nitrogen concentration from about 25 nm for sample A to about 4 nm for sample E. They were measured from dark-field micrographs not shown here. The same trend has been reported by Gerstenburg and Calbick [2]. This reduction in grain size is also expressed in the broadening of the corresponding X-ray diffraction lines. Because the sputtering time is kept constant for films of various compositions and growth rates, the thicknesses of the films differ. It is possible that the observed trend in the grain size reflects different stages of their evolution with film thickness rather than with nitrogen content. The micrographs of samples C and D clearly reveal the additional presence of a featureless (amorphous) phase in which small grains of

Fig. 4. Plan-view bright field transmission electron micrographs of five T a - N films as deposited on copper grids covered with holeycarbon. The compositions and phases are indicated in Fig. 5.

a crystalline Ta2N are imbedded. This finding confirms the X-ray results. In transition metal nitrides, deviations from ideal stoichiometry are quite common owing to their defect structure, which is similar to that of Ta2N [17, 19]. In W - N films the W2N phase also exists in a finite range of film composition, but only at excess nitrogen concentration. A range with a mixture of W2N and an amorphous phase does not exist there. Instead, there is a relatively broad region from about 20 to 40 at.% N with a pure amorphous phase [ 15].

3.2. Annealed films Isochronal heat treatments in vacuum (65 min at 500, 600, 700 and 800 °C) for all sample compositions indicated by open circles in Fig. 5 were performed to

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Fig. 5. Diagram of stable and metastable phases of T a - N thin films prepared for this study. Open circles indicate all data points. Arrows on the bottom abscissa labelled A - E indicate compositions chosen for the TEM study (Fig. 4).

350

x. Sun et al./ Reactively sputter-deposited T a - N thin films

establish the diagram of stable and unstable phases given in t h a t figure, while open circles on the bottom abscissa indicate the compositions of all the as-deposited samples discussed in Section 3.1. Dashed lines have been drawn to indicate approximate boundaries between regions with different predominant phases. After annealing at 600 °C and above, the b.c.c.-Ta film sputtered deposited in pure argon partly transforms to the tetragonal 13-Ta phase. For nitrogen concentrations between about 10 and 20 a t . ° , the Ta2N and b.c.c.-Ta phases that are observed at room temperature persist after all annealings. The linewidths of the X-ray spectra reflect only minor increases in the initial sizes of the grains of both phases. The as-deposited Ta2N phase initially formed at 25 at.% N is unstable and dissociates into b.c.c.-Ta and a Ta2N phase with presumably increased nitrogen content which places it nearer to its stoichiometric composition. In the composition range 30-35 at.%N, the amorphous phase dominates in the as-deposited samples (C and D in Fig. 4), but disappears from the X-ray spectra above 500 °C, with the formation of grains that are the largest of the whole set investigated. When the composition approaches 50 at.% N, the Ta2N phase again is metastable upon thermal annealing, but the decomposition now favors the increase in the nitrogen content in the phase by forming TaN. Above 50 at.%N, a very-fine-grained mixture of TaN and TasN6 persists at all temperatures, with an increase in the grain size by only about 60% after annealing at 800 °C, as estimated from X-ray linewidths. As far as can be concluded from a comparison of the Ta signal heights of the as-deposited and 800 °Cannealed films in baekscattering spectra, none of the films loses any nitrogen upon annealing. This is dissimilar to what occurs for reactively sputtered W - N films, where the loss of nitrogen upon thermal annealing is common. At 800 °C all tungsten nitride phases change to ¢-W [ 15]. The dissimilarities between reactively sputter-deposited Ta and W nitride films are related to the lesser stability resulting from the less negative heats of formation of the nitrides of the Cr, Mo, W transition metal group compared with those of the Ti, Zr, Hf and V, Nb, Ta groups [19]. The bonding character in W - N also changes to a partial covalency from the metallic T a - N bond. As shown in Fig. 6, the room temperature resistivity of both as-deposited and 800 °C-annealed T a - N films always exceeds that of pure Ta films and gradually rises to a weak maximum of about 0.25 m~ cm as nitrogen in the film increases. Beyond 50 at.% N the resistivity rises very steeply. Annealing has its most pronounced effect in this composition range and noticeably increases the resistivity of a film. The general trend of

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Fig. 6. Resistivityvs. nitrogen concentrationfor films as deposited and after annealing at 800 °C for 65 min in vacuum. For reference, the resistivityof bulk Ta is 13.1 ~ cm. these results can be readily explained. The resistivities of crystalline Ta2N and TaN are variously quoted as being in the ranges 190-250pf~cm [2, 20] and 130250 Ixf~cm [2, 20-22] respectively. These values are close to those of amorphous T a - N observed in the range 25-35 at.% N, which explains why the transition from the amorphous T a - N phase to Ta2N upon thermal annealing does not alter the resistivity substantially. The decrease in the resistivity values at low nitrogen content reflects the admixture of Ta (bulk resistivity 13.1 ~tf~cm). The sharp rise above 50 at.% N coincides with the appearance of the TasN 6 phase, the resistivity of which is about 0.5 m [ cm [20]. It is the presence of this poorly conducting phase in the form of very small crystalline grains that degrades the conductivity of the film. After annealing at 800 °C, the content of the TasN6 phase increases as seen in the X-ray spectra, while the amount of nitrogen in the film remains constant as stated before. This phase transformation is responsible for the increase in resistivity. Gerstenberg and Calbick [2] and Mehrotra and Stimmell [6] have reported quite similar resistivity trends for films deposited by reactive sputtering of Ta in an Ar-N2 gas mixture, whose data start at 50 ~ cm for b.c.c.-Ta and level off at 250 ~ cm in the Ta2N to TaN composition range in ref. 2 and start at 80 ttQ cm for Ta and level off at 220 laf~cm in ref. 8.

4. Conclusions

Reactively sputter-deposited tantalum nitride films are similar in many respects to those of reactively sputter-deposited tungsten nitride films. The principal dissimilarity is the superior stability of the T a - N films over the W - N films upon thermal annealing. After an 800 °C heat treatment the W - N films lose most of their nitrogen and have a resistivity close to that of pure W

X. Sun et al. / Reactively sputter-deposited T a - N thin films

films, while the T a - N films retain their nitrogen and the correspondingly higher resistivity of Ta2N and TaN. In several cases amorphous thin films have been proven to be effective diffusion barriers compared with polycrystalline films [23, 24]. Thus it is likely that the best diffusion barrier obtainable with T a - N thin films is that which has the most amorphous material (around 30 at.% N concentration).

Acknowledgments Financial support for this work was provided by the Army Research Office (DAAL03-92). The Inel positionsensitive X-ray diffractometer at Caltech is supported by DOE (#DEFGO589ER75511). We also thank R. Gorris and M. Easterbrook for their technical assistance.

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