Atomic-Layer Chemical-Vapor-Deposition of TiN Thin Films on Si(100) and Si(111)

Transcription

1 Journal of the Korean Physical Society, Vol. 37, No. 6, December 2000, pp Atomic-Layer Chemical-Vapor-Deposition of TiN Thin Films on Si(100) and Si(111) Young-Seok Kim, Hyeongtag Jeon and Young Do Kim Division of Materials Science and Engineering, Ceramic Processing Research Center, Hanyang University, Seoul Won Mok Kim Materials Design Laboratory, Materials Science and Technology Division, Korea Institute of Science and Technology, Seoul (Received 12 April 2000) An atomic-layer chemical vapor deposition (AL-CVD) system was used to deposit TiN thin films on Si(100) and Si(111) substrates by cyclic exposures of TiCl 4 and NH 3. The growth rate was measured by using the number of deposition cycles, and the physical properties were compared with those of TiN films grown by using conventional deposition methods. To investigate the growth mechanism, we suggest a growth model for TiN in order to calculate the growth rate per cycle with a Cerius program. The results of the calculation with the model were compared with the experimental values for the TiN film deposited using the AL-CVD method. The stoichiometry of the TiN film was examined by using Auger electron spectroscopy, and the chlorine and the oxygen impurities were examined. The x-ray diffraction and the transmission electron microscopy results for the TiN film exhibited a strong (200) peak and a randomly oriented columnar microstructure. The electrical resistivity was found to decrease with increasing deposition temperature. The densities of the TiN films measured by using Rutherford backscattering spectroscopy were between 4.2 g/cm 3 and 4.98 g/cm 3. I. INTRODUCTION The microelectronic industry has shown an increasing interest in TiN as a diffusion barrier for aluminum and copper metallization in ultra-large-scale integration (ULSI) devices because of its low electrical resistivity, high melting point, thermal stability, and good adhesion properties [1,2]. It is also used as a diffusion barrier for capacitor electrodes for high-k dielectric materials, such as Ta 2 O 5 or BST [3], and is used as both a barrier against WF 6 attack and a nucleation layer in the chemical vapor deposition (CVD) W plug filling process [4]. Both physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques have been applied to grow TiN film. Conventionally, TiN films have been deposited either by the reactive ion sputtering or by thermal nitridation of sputtered titanium. As integrated feature sizes have shrunk to submicron dimensions, the limitations of films produced by these physical techniques have become apparent. The poor step coverage of sputtered films, especially in submicron contact holes with hjeon@ .hanyang.ac.kr, Fax: high aspect ratios, has been shown to lead to barrier property degradation. However, TiN films produced by low-pressure chemical vapor deposition (LPCVD) and metalorganic chemical vapor deposition (MOCVD) provide good step coverage and good diffusion barrier characteristics [5,6]. However, there are problems such as contamination with residual elements from the precursors and particle generation by gas-phase reactions [7 9]. These contaminants degrade the barrier properties of TiN films. Therefore, a new deposition technology has been suggested to produce high-quality TiN films with improved barrier properties. In this study, an atomic-layer chemical vapor deposition (AL-CVD) method [10 13] was introduced for depositing TiN films. AL-CVD is a new deposition method that can control the deposition of a monolayer per cycle. Control of the growth in AL-CVD depends on saturating the surface reactions on the substrate, so the growth rate is controlled by the number of deposition cycles instead of by the reactant flow rate. The advantages of AL-CVD over other conventional CVD processes are excellent thickness uniformity over a large area, effective step coverage, low pinhole density in thin films, and elimination of particle generation by gas-phase re

2 Journal of the Korean Physical Society, Vol. 37, No. 6, December 2000 Fig. 1. Hydrogen-terminated Si(100) substrate surface. actions. Here, we suggest a growth model for TiN films using the Cerius program, and we compare the growth rate of TiN film with that of a TiN film deposited by using AL-CVD. The purpose of this study is to investigate the effect of different orientations of the Si substrate on TiN films deposited by using the AL-CVD method and to achieve excellent film quality with low resistivity and low chlorine content. II. EXPERIMENT TiN thin film deposition using an AL-CVD system was performed in a Cyclic M2000 (Genitech Co., Ltd., Korea) operated under a pressure of Torr. In this study, B doped p-type Si(100) and Si(111) substrates with a resistivities of 4 to 10 Ωcm were used. The Si substrates were cleaned in a piranha solution (H 2 SO 4 : H 2 O 2 = 4 : 1) for 10 min and a dilute HF solution (HF : H 2 O = 1 : 50) for 1 min to remove organics and native oxides, respectively [14]. By this cleaning treatment, a hydrogen-terminated Si surface was obtained. After this treatment, the Si substrate was transferred into the deposition chamber. The source TiCl 4 vapor was generated in an external reservoir held at 15 C and was pulsed into the reactor with an Ar carrier gas. Reactant NH 3 gas was introduced into the reactor through a mass flow meter and solenoid valves. TiCl 4 and NH 3 gases were alternatively supplied, and an Ar purge gas was added between the source and the reactant gases to suppress direct reactions. Deposition of the TiN films was performed at substrate temperatures of 350 C, 400 C, and 450 C, and under a pressure of 2 Torr. The crystallinity of the AL-CVD TiN films was examined using an X-ray diffractometer (XRD). The N/Ti stoichiometry and the impurity content were measured using Auger electron spectroscopy (AES) and Rutherford backscattering spectroscopy (RBS), respectively. The sheet resistance and the film thickness were measured with a four-point probe and cross-sectional transmission Fig. 2. Growth model for the ALD TiN film per cycle on a Si(100) substrate. electron microscopy (XTEM), respectively. The growth

3 Atomic-Layer Chemical-Vapor-Deposition of Young-Seok Kim et al sorbed TiCl 3 or TiCl 2 [15]. In this study, the reactant and the source gases were injected into the reactor in the order of TiCl 4 pulse, Ar purge gas pulse, NH 3 gas pulse, and Ar purge gas pulse. These four pulses were defined as one cycle. To investigate the growth rate of the TiN films, we modelled the growth mechanism of a TiN film on a Si(100) substrate by the Cerius program. Figure 1 shows hydrogen-terminated Si(100) surface, which indicates the initial Si surface of the TiN thin-film growth. The proposed growth model for a TiN film on a Si(100) substrate is shown in Fig. 2. The mechanism for each step is based on an exchange reaction at the surface between Ti-Cl and N-H bonds having large electrostatic dipole moments. In the first cycle, the H-terminated Si surface, as shown in Fig. 1, was exposed to TiCl 4 gas. Because the reaction : Si H 2 (ad)+t i Cl(g) Si T i Cl x (ad)+hcl(g),(1) Fig. 3. AES spectra of TiN films deposited at temperatures of 350 C, 400 C, and 450 C on (a) Si(100) and (b) Si(111). rate was determined in terms of film thickness per cycle and was calculated by dividing the TiN film thickness deposited by the total number of cycles. The film resistivity was derived from the sheet resistance and the film thickness. The film density was measured by using the RUMP program with the film thickness from the XTEM analysis and the areal density from the RBS analysis. III. RESULTS AND DISCUSSION TiN films were successfully grown by using the AL- CVD method, and the film thicknesses were measured to be proportional to the number of cycles. The thickness control of the AL-CVD TiN films was affected not by the flow rate but by the number of cycles. The cycle was repeated to grow the TiN films. After processing 1000 cycles on Si(100) and Si(111), we were able to obtain the ideal linear relationship between the number of cycles and the film thickness, and the growth rates of the TiN films were about 0.25 monolayer/cycle. The growth rate of less than one monolayer per cycle implies a low surface density for the titanium species in the saturated chemisorbed layer formed during the TiCl 4 exposure. This is due to surface reconstruction creating binding sites and to steric hindrance between the ad- occurred at the surface, the H surface was removed as HCl, as shown in Fig. 2(a). However, all hydrogen atoms were not cleared off the substrate due to the steric hindrance effect of the relatively large size of the TiCl 4 molecule. The unreacted TiCl 4 gas and the by-products were purged by Ar gas. After complete evacuation of TiCl 4 gas and byproducts, the surface was exposed again to NH 3 gas, and the Cl surface was cleared off as HCl by the following reaction: Si T i Cl x (ad) + N H(g) Si T i N H y (ad) + HCl(g). (2) The surface was changed to Si-Ti-N-H y, as shown in Fig. 2(b), after the reaction. The unreacted NH 3 gas and the byproducts were purged by the Ar gas. After the first TiCl 4 and NH 3 adsorptions, the remaining Si-H 2 bonds had not been fully changed to Si-Ti-N-H y bonds during the first cycle. During the second cycle, two of the hydrogens on the Si surface or one hydrogen on the Si-Ti-N-Hy and two of the hydrogens on the Si surface reacted with the TiCl 4 gas, as shown in Fig. 2(c). The unreacted TiCl 4 gas and the HCl gas produced after the reaction were purged by the Ar gas. After complete evacuation of the TiCl 4 gas and the byproducts, the surface was exposed to NH 3 gas. Ti-Clx adsorbed on the surface reacted with the NH 3 gas. The surface was changed to Si-Ti-N-H y, as shown in Fig. 2(d). If this cyclic reaction was done repeatedly, one monolayer of the TiN film was grown after about four and a half cycles by this Cirius 2 program. A deposition rate of 0.23 monolayer per cycle was calculated by this program. The result agreed relatively well with the experimental result. Figure 3 shows the AES spectra of the AL-CVD TiN films. As shown in Fig. 3, the chlorine contents of the TiN films deposited on Si(100) and Si(111) were much lower than those of the TiN films grown by conventional CVD methods from TiCl 4 and NH 3 which were about 2 6 at.% of Cl [16]. This low chlorine content in AL-CVD TiN films was due to the complete surface

4 Journal of the Korean Physical Society, Vol. 37, No. 6, December 2000 Fig. 4. RBS spectra of TiN films deposited at temperatures of 350 C, 400 C, and 450 C on (a) Si(100) and (b) Si(111). reaction caused by the separate source supply and Ar purge. From a thermodynamic point of view, a direct reaction between TiCl 4 and NH 3 can occur at temperatures higher than 320 C. Therefore, the chlorine content in a TiN film should decrease with increasing deposition temperature. However in this AL-CVD system, the Cl content was reduced at low temperatures of 400 C and 450 C because of the complete surface reaction. This low impurity concentration in TiN films grown by using Fig. 5. Resistivity of TiN films as a function of the deposition temperature. Fig. 6. XRD patterns of TiN films deposited at temperatures of 350 C, 400 C, and 450 C on (a) Si(100) and (b) Si(111). the AL-CVD method can provide a high-quality diffusion barrier for Al and Cu metallization. The RBS spectra of the AL-CVD TiN films are shown in Fig. 4. From the RBS result, the chlorine contents of the TiN films deposited on Si(100) and Si(111) substrates were very low at temperatures of 350 C, 400 C, and 450 C. In this study, the densities of the TiN films deposited on Si(100) and Si(111) substrates were between 4.2 g/cm 3 and 4.98 g/cm 3 and increased with increasing deposition temperature. The low density of the TiN films was related with loosely packed grains. A diffusion barrier with a low density can fail much more easily due to the easy path of the grain boundaries because the diffusion path of a polycrystalline diffusion barrier is mostly a grain boundary area. However, the density of the TiN films formed by using the AL-CVD method is higher than that of other conventional CVD TiN films and is expected to exhibit excellent diffusion barrier performance for Al and Cu metallization. The composition of the TiN films was determined from the combined results of the RBS and the AES analyses. The stoichiometry of the TiN films deposited on Si(100) was found to be slightly Ti rich with N : Ti ratios between 0.94 and 0.96 whereas the stoichiometry of the TiN films deposited on Si(111) was found to be slightly N rich with N : Ti ratios

5 Atomic-Layer Chemical-Vapor-Deposition of Young-Seok Kim et al (111) planes, which are formed in the initial nucleation stage. Therefore, it is thought that the randomly oriented growth of the TiN films originates from a combination with low growth rate that can cause stress relief and complete surface reaction that can lower the surface energy. Figure 7 shows the XTEM images of the TiN film deposited at 450 C, which exhibits columnar TiN grains predominantly in the (200) plane. Also, the thin oxide layer at the interface between the TiN and the Si substrate was observed on both Si substrates. This oxide layer was amorphous with a thickness in the range of 1 to 2 nm. The TiN film deposited on the Si(111) substrate was thicker than the TiN film deposited on the Si(100) substrate. It is thought that the thickness difference is caused by the orientation of the Si substrate. We will subsequently investigate the reason for this result. IV. CONCLUSIONS Fig. 7. Cross-sectional TEM images of TiN films grown at 450 C on (a) Si(100) and (b) Si(111). between 1.01 and The resistivity of AL-CVD TiN films deposited at temperatures between 350 C and 450 C is shown in Fig. 5. As shown in Fig. 5, the resistivity decrease with increasing deposition temperature. It is thus, clear that a reduction in Cl content is one of the main reasons for the lower resistivity. The resistivity of TiN films obtained by using AL-CVD is significantly lower than those of TiN films grown by conventional CVD methods. Figure 6 shows the XRD patterns taken at various temperatures. The TiN film structure and orientation can significantly affect the barrier properties of the TiN films. As shown in Fig. 6, TiN with a B1-type NaCl structure exhibits a polycrystalline structure with peaks corresponding to 2θ = (111), (200), and (220), with the (200) orientation exhibiting the highest diffraction intensity. The growth orientation of the TiN films can be explained on the basis of two thermodynamic quantities, the surface-free energy and the strain energy. The surface and strain energies of the TiN films have the lowest values in the (200) and the TiN thin films were grown using a new deposition method, the AL-CVD method. The physical properties of AL-CVD TiN films deposited on Si(100) and Si(111) were investigated as functions of the deposition temperature between 350 C and 450 C. The growth rate had an ideal linear relationship with the number of deposition cycles. The growth rate of TiN films deposited by using the AL-CVD method was about 0.25 monolayer/cycle. In the model to prove the growth mechanism, the growth rate was 0.23 monolayer/cycle, which agreed relatively well with the experimental result. The chlorine content in the TiN films was below the detection limit of AES (< 1 at.%) at deposition temperatures between 350 C and 450 C, and the density of the TiN films increased with increasing deposition temperature. The TiN films had a columnar structure and showed a polycrystalline NaCl structure. The resistivity of the TiN films decreased with increasing deposition temperature. The TiN films deposited by using the AL-CVD method exhibited excellent physical and electrical properties. ACKNOWLEDGMENTS This work has been supported by the Ceramic Processing Research Center (97K ). REFERENCES [1] G. I. Grigorov, K. G. Grigorov, M. Stoyanova, J. L. Vignes, J. P. Langeron and P. Denjean, Appl. Phys. A57, 195 (1993). [2] Rama I. Hedge, Appl. Phys. Lett. 62, 2326 (1993). [3] P. C. Mclntyre and S. R. Summerfelt, J. Appl. Phys. 82, 4577 (1997). [4] I. Raaijmakers and R. Vrtis, VMIC Conference (1992), p. 260.

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