DEPOSITION AND CHARACTERISTICS OF TANTALUM NITRIDE FILMS BY PLASMA ASSISTED ATOMIC LAYER DEPOSITION AS CU DIFFUSION BARRIER

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1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society E DEPOSITION AND CHARACTERISTICS OF TANTALUM NITRIDE FILMS BY PLASMA ASSISTED ATOMIC LAYER DEPOSITION AS CU DIFFUSION BARRIER Kyoung-Il Na 1, Se-Jong Park, Woo-Cheol Jeong 2, Se-Hoon Kim 2, Sung-Eun Boo 2, Nam-Jin Bae 2, and Jung-Hee Lee The School of Electronic and Electrical Engineering, Kyungpook National University, Daegu , Korea 1 The School of Sensor Engineering, Kyungpook National University, Daegu , Korea 2 Comtesc Incorporated, Daegu, Korea ABSTRACT For a diffusion barrier against Cu, tantalum nitride (TaN) films have been successfully deposited by both conventional thermal atomic layer deposition (ALD) and plasma assisted atomic layer deposition (PAALD), using pentakis (ethylmethlyamino) tantalum (PEMAT) and ammonia (NH 3 ) as precursors. The growth rate of PAALD TaN at substrate temperature 250 was slightly higher than that of ALD TaN (0.80 A /cycle for PAALD and 0.75 A /cycle for ALD). Density of TaN films deposited by PAALD was as high as 11.0 g/cm 3, considerably higher compared to the value of 8.3 g/cm 3 obtained by ALD. The N : Ta ratio for ALD TaN was 44 : 37 in composition and the film contained approximately 8 10 atomic % carbon and 11 atomic % oxygen impurities. On the other hand, the ratio for PAALD TaN layers was 47 : 44 and the respective carbon and oxygen contents of TaN layers decreased to 3 atomic % and 4 atomic %. The stability of 10 nm-thick TaN films as a Cu diffusion barrier was tested through thermal annealing for 30 minutes in N 2 ambient and characterized by XRD, which proves the PAALD deposited TaN film to maintain better barrier properties against Cu below 800. INTRODUCTION Copper (Cu) interconnection is the most promising candidate for next-generation high-speed ultra large integrated circuit (ULSI), because copper exhibits lower resistivity and larger electro/stress migration resistance than conventional Al-based materials [1]. However, it is well known that Cu can diffuse easily into Si and SiO 2 to form copper silicide at a temperature as low as 200. Moreover, Cu acts as a deep-level contaminant in Si that deteriorates the device performance [2]. Tantalum nitride (TaN) has many good advantages as a diffusion barrier. It shows not only relatively high melting point (2100 ) but also thermo-dynamical stability with respect to Cu. Atomic layer deposition (ALD) processing method is based on self-limiting

2 E surface reaction occurring between substrate surface and the depositing film, ensuring monolayer thickness control, and therefore enabling the growth of perfectly conformal, continuous, and atomically smooth films [3]. In this work, we investigated two different of TaN layers, which were deposited by conventional thermal ALD and plasma assisted ALD (PAALD), on the thermal stability as diffusion barrier for the structures of Cu/10 nm-thick TaN/SiO 2 /Si. EXPERIMENTAL DETAILS TaN films were deposited on 80nm-thick SiO 2 /Si wafers at a deposition temperature of 250, where the temperature was proven to be ALD window for TaN deposition from our previous experiments. 10 nm-thick TaN films were then deposited by both ALD and PAALD. Schematic configuration of reactor was shown in Fig. 1. The quartz tube was wrapped with a multiple-turns coil connected to RF generator set at MHz with a power level up to 250W (Inductility coupled plasma type). Figure 1. Schematic diagram of horizontal/vertical flow ALD reactor employed in this study. Pentakis (ethylmethlyamino) tantalum (PEMAT) as a tantalum precursor and NH 3 reactant gases were alternately fed into the quartz chamber with Ar as a carrier gas. One cycle of ALD consisted of an exposure to PEMAT, Ar purge, NH 3 reactant, and another Ar purge, where pulse length was 4sec for each step. On the other hand, one PAALD reaction cycle consisted of an exposure to Ar and subsequent NH 3 +Ar plasma instead of the direct exposure to NH 3 reactant gas in ALD cycle, where pulse lengths were 8 sec and 2 sec, respectively. At this time, plasma power was 250W. Finally, 200 nm-thick Cu film was deposited by DC sputtering on TaN/SiO 2 /Si structure, followed by annealing at temperature range from 400 to 800 with

3 E interval 100 for 30 min in N2 ambient. To evaluate the quality of as-deposited TaN film and Cu/TaN(deposited both ALD and PAALD)/SiO 2 /Si before and after annealing, the resistivity of the films were analyzed by fourpoint probe sheet resistance measurement. Density and thickness were analyzed by High resolution X-ray diffraction (HRXRD). X-ray diffraction (XRD) was used to investigate the crystallographic orientations of TaN before annealing and new products (CuO, Cu 2 O, Cu 5 Ta 11 O 30, and etc) after each annealing. DISCUSSION Fig. 2 shows the thickness variation of the TaN film as a function of deposition cycle. The film thickness is linearly dependent on deposition cycle with growth rates of 0.75A /cycle and 0.8A /cycle for ALD and PAALD, respectively, which explains the deposition is controlled by ALD with atomic layer level accuracy. Figure 2. The dependence of TaN films thickness on the number of deposition cycles. The properties of as-deposited PAALD and ALD TaN films are summarized in Table 1. A TaN film deposited by PAALD has higher density and lower impurity concentration than those obtained from thermal ALD TaN film. This explains, as shown in Fig. 3 (a) and (b), the PAALD deposited TaN films experience negligible aging effect on sheet resistance over 1000 hours after deposition. On the other hand, the sheet resistance of the thermal ALD deposited TaN film was rapidly increased when the exposure time of the film in the air exceeds 140 hours deposition after.

4 E Table 1. Properties for PAALD and T-ALD TaN films. Items Results PAALD TaN T-ALD TaN Measurement Stoichiometry Ta : N 1:1 (44:47) Ta : N 4.2:5 (37:44) AES, RBS Structure Amorphous Amorphous XRD Density g/cm g/cm 3 HR-XRD, RBS Impurity 3 atomic % carbon 8~10 atomic % carbon 4 atomic % oxygen 11 atomic % oxygen AES, RBS (a) (b) Figure 3. Sheet Resistance of PAALD TaN film as a function of time. (a)t-ald TaN, (b) PAALD TaN The annealing was performed at temperatures ranged from 400 to 800 for 30 min in N 2 ambient. As shown in Fig. 4(a) and (b), XRD analysis identified the intermixing and new phase formation for the Cu(200nm)/TaN(10nm)/SiO 2 (80nm)/Si structure. For as-deposited samples (both ALD and PAALD deposited), only a pure cubic-cu (111), (200), (220) and (311) peaks were observed at , , , and , respectively [4]. Any reaction involving Cu, Ta, O or Si was not observed. When annealing temperature increased to 400, Cu 2 O phase [5] was formed due to reaction between Cu and residual oxygen or water vapor in the annealing furnace. As shown in Fig. 4(a) for ALD deposited TaN film, at annealing temperature of 700, Cu 3 Ta 11 O 30 phase [6] appeared due to the reaction between Cu and TaN under-layer. However, Cu 3 Ta 11 O 30 phase appeared at 800 for PAALD deposited TaN film, which explains the PAALD TaN has higher thermal stability than thermal ALD TaN.

5 E (a) (b) Figure 4. XRD pattern of the Cu/TaN(10nm)/SiO 2 /Si structure annealed at various temperature for 30min in N 2 under atmospheric pressure. (a) T-ALD TaN, (b) PAALD TaN Fig. 5 indicates the change in sheet resistance measured on the both Cu/ALD TaN(10nm)/SiO 2 /Si and Cu/PAALD TaN(10nm)/SiO 2 /Si structures as a function of annealing temperature. The measured sheet resistance was dominated by the Cu thin film since the copper film (200nm and ~6.0µΩcm ) is much thicker and has a markedly lower resistivity than that of tantalum nitride films and any reaction products. Because the most of the sensor current flows through the upper-most Cu film of 200 nm-thick, the sheet resistance measurements can evaluate the condition and the quality of the Cu film. The sheet resistance slightly decreased with increasing annealing temperature up to 400 due to the reduction of crystal defects and grain Figure 5. Sheet resistance of Cu/TaN(10nm)/ SiO 2 /Si structure as a function of annealing temperature in N 2 ambient.

6 E growth in the copper film, and then remained almost unchanged below 600. At 700 there is a rapid increase in resistivity, which seems to be either increase of resistivity due to the formation of Cu 2 O or sudden degradation of diffusion barrier properties. However, as shown in figure 4(a) and (b), it was confirmed that the intensity of Cu 2 O increased with increasing annealing temperature, but Cu 3 Ta 11 O 30 phase appeared at 700 and 800 for thermal ALD and PAALD TaN film, respectively. This explains that the increase in sheet resistance is mainly due to the formation of Cu 2 O around 600 and the barrier properties against Cu is maintained up to 700 and 800 for thermal ALD and PAALD TaN film, respectively. A similar behavior was reported by W. H. Tech [7]. CONCLUSIONS We have investigated the deposition and characteristic of TaN films as copper diffusion barrier, deposited by ALD and PAALD on SiO 2 (80 nm)/si. A TaN film deposited by PAALD has higher density and lower impurity concentration than those from ALD. The failure of the metallization structure was induced by the penetration of Cu through the TaN layer and the resultant Cu 5 Ta 11 O 30 compound formation in both Cu/ALD and PAALD TaN/SiO 2 /Si structure. From XRD analysis, PAALD TaN film (10nm) was found to be thermally stable below 800 (100 higher than ALD TaN). ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 project. XRD and HR-XRD measurement supported by Sang-gel Lee in Korea Basic Science Institute (KBSI) Daegu Branch. REFERENCES 1. T. Takewaki, R. Kaihara, T. Ohmi and T. Nitta: Tech. Dig. Int. Electron Device Meet. (1995) pp D. H. Zhang, S. W. Loh, C. Y. Li, IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 1, NO. 4, DECEMBER T. Suntola, Thin Solid Films 216, 84-89, JCPDS Number: JCPDS Number: JCPDS Number: W. H. Tech, ELECTRONICS LETTERS 7th, vol. 36, No. 25, December 2000,