Electroless Cu Deposition on a TiN barrier in CuSO 4 -HF Solution

Transcription

1 C466 Journal of The Electrochemical Society, C466-C /2005/152 7 /C466/8/$7.00 The Electrochemical Society, Inc. Electroless Cu Deposition on a TiN barrier in CuSO 4 -HF Solution Sheng Zhong, a Zhi-Gang Yang, a,z Jian Cai, b Hao-Jie He, a Jin-Sheng Wen, a and Chao Liu a a Department of Material Science and Engineering, Tsinghua University, Beijing , China b Institute of Microelectronics, Tsinghua University, Beijing , China Electroless Cu deposition on a TiN barrier in a CuSO 4 -HF solution by separated electrodes of a Si wafer and a TiN/Ti/SiO 2 /Si substrate has been studied. In this electroless Cu plating method, two substrates are fixed on a base made by polystyrene without an external circuit. During plating the Si wafer is oxidized by hydrofluoric acid and releases free electrons; F ions transmit the free electrons to the TiN/Ti/SiO 2 /Si substrate, and Cu 2+ ions accept the electrons and then directly deposit them on the TiN surface. The morphology, surface coverage, average grain size, crystallography, and adhesion of the as-deposited Cu films on the TiN surface are related to the distance between the two substrates, bath temperature, and concentration of HF and CuSO 4. During the plating, Cu is also deposited on the Si wafer and the properties of the Cu films on the Si wafer are discussed in the paper as well. The effect of HF on the TiN barrier in this electroless Cu deposition method is examined by X-ray photoelectron spectroscopy The Electrochemical Society. DOI: / All rights reserved. Manuscript submitted July 27, 2004; revised manuscript received January 18, Available electronically June 3, With the dimensions of integrated circuits ICs decreasing dramatically, the size of the interconnection of the ICs is decreased to deep subquartermicrometers. In this trend Cu is proposed to replace aluminum, 1,2 the last generation interconnection material, in ultralarge-scale integrated ULSI circuits for its lower electrical resistivity of 1.67 cm, higher electromigration resistance, and higher resistance to stress-induce voiding. 3 However, Cu easily diffuses into the silicon and affects the electric performance of the chip, thus TiN, TaN, Ta, and WN are employed as the diffusion barrier between Cu and the silicon wafer to prevent diffusion. 4-6 Electroless plating is regarded as one of the most attractive methods for metallization in ULSI for the simple chemistry of the process, high step coverage, and low cost. The alkaline solution composed of Cu sulfate, ethylenedinitroloacetate EDTA, formaldehyde, and tetramethylammonium hydroxide TMAH is widely used for this purpose. 7-9 Unfortunately Cu cannot deposit directly on diffusion barriers since they are not an autocatalytic surface for Cu 2+ ions, and thus the barriers are always activated before deposition. Traditionally, Pd deposited on the barrier either by chemical reaction in solution or by physical vapor deposition PVD works as the activation layer for Cu electroless deposition. 10,11 Simultaneously, many works suggest that the acid-based electroless plating process is a successful method for Cu metallization. In this method silicon is oxidized by hydrofluoric acid, and the released electrons are accepted by Cu 2+ ions, leading to the direct Cu deposition on the silicon surface. This redox process is based on the following two half-cell reactions Anodic 2Cu 2+ +4e =2Cu 1 Cathodic Si + 6F = SiF e 2 The method was initially employed in the metallization of microelectromechanical systems MEMS. 13 Tseng et al. 15,16 first applied the acid-based method to the Cu metallization on the TaN barrier. In their reports, the electroless Cu deposition was carried out in a HF-NH 4 F buffer solution, and CuCl 2 was the source of Cu 2+ ions and HNO 3 was added into the plating bath acted as the catalyst for the reaction. During the plating, F and NO 3 ions dissolved Si on the back and sides of the TaN/Ta/SiO 2 /Si substrate and formed complexions with Cu 2+ ions. The electrons released from Si were transmitted by F and Cl ions to the TaN surface and accepted by Cu 2+ ions. By this method, a Cu film with a resistivity of 2.35 cm can be achieved. However in this process the Cu 2+ ions z zgyang@tsinghua.edu.cn also were reduced on the naked Si surface, thus generating unwanted metallic contamination, which concentrates at the Si gate oxide interface and is responsible for defective final ULSI devices. 17,18 Liu et al. 19 also proposed a new displacement reaction for the production of a Cu seed layer on the TiN surface, by depositing an amorphous silicon layer on the TiN surface and then carrying out the displacement reaction in the HF solution. This method is interesting but was only performed in the preparation of the Cu seed layer for alkaline metallization. In our previous work, 20 an acid-based metallization using separated electrodes in the CuSO 4 -HF solution was discussed. A Si wafer and a Cu substrate were fixed at open circuit at different distances in the plating solution and achieved Cu deposition on both Si and Cu substrate. In this paper, a TiN/Ti/SiO 2 /Si substrate was employed instead of a Cu substrate to investigate the application of the direct Cu deposition on the TiN barrier using this acid-based metallization process. The plating structure is shown schematically in Fig. 1. The sides and back of the TiN/Ti/SiO 2 /Si substrate was smeared with silver paste to protect the Si surface of the substrate, and the Si wafer employed in the plating is used as the source of electrons. During plating, F ions dissolved the Si and then transmitted the electrons released from the oxidation of the Si to the TiN surface, resulting in the Cu deposition on TiN barrier at open circuit potential OCP. After plating, the surface morphology, crystallography, and adhesion of the as-deposited Cu films are discussed. Experimental In the experiments, a mm TiN/Ti/SiO 2 /Si substrate was used for electroless Cu deposition. The SiO 2 layer was thermally grown to 100 nm thick on a 3 in. Si 100 wafer and the 50 nm thick Ti and 150 nm thick amorphous TiN films were consecutively deposited on the SiO 2 layers by dc magnetron sputtering in Ar and N 2 ambient. Conductive silver paste was smeared on the back and sides of the substrate to prevent corrosion by hydrofluoric acid. In the experiments there is no Cu deposition on the Ag paste because the Ag is not the autocatalytic surface for Cu 2+ ions. A silicon 100 wafer the same size as the substrate was exploited as the reducing agent and fixed facing the TiN barrier at different distances, indicated as d in this paper, or bound at the back of the substrate during the plating. Both were cleaned by acetone, 10% HCl, and deionized water with an ultrasonic rinse at room temperature for 5 min before running the experiments. The electroless plating solution was composed of cupric sulfate CuSO 4 5H 2 O and hydrofluoric acid HF, diluted by deionized water of different concentrations. The conditions of the plating solution, the distance between the Si wafer, and

2 Journal of The Electrochemical Society, C466-C C467 Figure 1. Schematic of plating structure the TiN/Ti/SiO 2 /Si substrate and bath temperature are shown in Table I. For all the conditions, the plating time was 5 min. The morphology of plated Cu films was investigated by scanning electron microscopy SEM and X-ray diffraction XRD was employed to analyze the crystallization structure of the as-deposited Cu films; X-ray photoelectron spectroscopy XPS was used to study the effect of hydrofluoric acid on the TiN surface. The adhesion of the Cu films was examined by the tape peel test. Results and Discussion Surface morphology and grain size of Cu films in different plating conditions. The surface morphology of as-deposited Cu films on the TiN/Ti/SiO 2 /Si substrate in condition A is shown in Fig. 2, and the average grain size of all the as-deposited Cu films in this paper was determined from three areas of m each. Table I. Different conditions and parameters for electroless Cu plating. Condition no. CuSO 4 M HF wt % Bath temp. K Distance mm Condition A. Variation of d A A A a A Si is bound at the back of substrate Condition B. Variation of bath temperature B B B B Condition C. Variation of CuSO 4 C C C C Condition D. Variation of HF D D D D a The distance between Si and TiN is too close to be measured. From Fig. 2, the Cu films were successfully plated on the TiN/Ti/SiO 2 /Si substrate at various distances between the two sub strates, however, the surface morphology of as-deposited Cu films varied remarkably with the distance. The substrate plated at d = 1 mm contained a nonuniform Cu film with an average grain size of 204 nm and many asymmetric Cu grain clusters distributed on the surface, suggesting the poor surface coverage of the as-deposited Cu film. As d was reduced to 0.5 mm, the microstructure of asdeposited Cu film was improved. The Cu grain became more homogeneous, and the average grain size was reduced to 182 nm. Moreover, the surface coverage was enhanced with the decrease of voids and Cu grain clusters. In both condition A3 and A4, uniform Cu films with complete surface coverage were obtained, whereas the grain size and the voids on the as-deposited Cu film surface increased. As the TiN surface of the substrate and Si wafer was close condition A3, the average Cu grain size was increased to 507 nm with some large Cu grains scattered on the surface of the asdeposited Cu film. When the Si wafer was bound at the back of the substrate condition A4 the shape of as-deposited Cu grain was even, and the average grain size was reduced to 480 nm, whereas its uniformity still needed to be improved compared with the asdeposited Cu film in condition A2. The decrease of the average grain size as well as the coverage of the Cu films is associated with the reduction of the distance between the Si wafer and the substrate. When the Si wafer was apart from the TiN/Ti/SiO 2 /Si substrate, the electrons need to diffuse to the sub strate from the Si wafer, and thus the deposition of Cu on the TiN surface can be enhanced by decreasing the distance. In condition A4, while the Si wafer was fixed at the back of the substrate, the electrons were conveyed either by the F ions diffusing in the solution or by Cu adatom diffusion on surfaces through the hopping mechanism. 21 Therefore the electrons were much more easily transmitted to the surface of the TiN surface resulting in the rapid grain growth of the Cu films. In condition A3 the Si wafer was close to the TiN surface and hence the electrons released from the oxidation of the Si wafer can be accepted by Cu 2+ ions at the TiN surface directly, which accounts for the largest average Cu size of condition A. The effect of bath temperature on microstructure of as-deposited Cu films is displayed in Fig. 3. In Fig. 3a, large Cu clusters deposited at a bath temperature of 303 K were loosely distributed on the TiN surface. With the bath temperature increased to 313 K, the average grain size of the as-deposited Cu films was reduced greatly from 3250 to 246 nm with better and full surface coverage. The average grain size of the as-deposited Cu films in high temperature 303 K was changed comparatively slightly, which was reduced to 182 nm at 323 K and then reversed to 260 nm at 333 K. Raising the bath temperature can provide more thermal energy for the transmission of the ion complex with electrons, the grain nucleation, and the growth of the Cu grains. At low temperatures 303 K few electrons were transmitted to the TiN surface leading to the difficult nucleation of the Cu grains on the substrate and clearly in this case the growth of nuclei proceeded at a faster rate than the rate they nucleated, thus resulting in large Cu grains. From 313 to 323 K, the nucleation rate of Cu grains was increased and hence led to the decrease of the average Cu grain size. Yet at higher temperature, the raised thermal energy may accelerate the growth of the nuclei and the larger grain size of condition B4 displayed in Fig. 3d proves it. Additionally, there are many blisters on the surface of the asdeposited Cu films, revealing the formation of H 2 during the Cu deposition. Figure 4 presents the relationship of the microstructure of asdeposited Cu films on the TiN surface with the concentration of Cu 2+ ions. As expected, the average grain size of as-deposited Cu films and their surface coverage are proportional to the CuSO 4 con centration. As the CuSO 4 concentration was M with 8 wt % HF and a bath temperature of 323 K, shown in Fig. 4a, only small Cu grains scattered on the TiN surface with a poor surface coverage. As the SEM images in Fig. 4 show, the high growth rate and larger

3 C468 Journal of The Electrochemical Society, C466-C Figure 2. The surface morphology of as-deposited Cu films on TiN/Ti/SiO 2 /Si substrate at different distances. Distance was a 1, b 0.5, and c 0 mm, d Si wafer was bound at the back of substrate. The solution was M CuSO wt % HF. The bath temperature was 323 K and plating time was 5 min. grain structure are accompanied by more microvoids in the asdeposited Cu films, indicating that more H 2 bubbles were concentrated at the surface of the as-deposited Cu films. The effect of hydrofluoric acid HF on the electroless Cu deposition on the TiN surface was examined by experiments under condition D. Interestingly, only when the HF concentration of the plating solution was 8 wt %, a uniform and dense as-deposited Cu film could be obtained on the substrate refer to Fig. 2b, while in diluted solutions of D1, D2, and D3 wherein the HF concentration was range from 2 to 6 wt %, only a few Cu dots were scattered on the substrate, as displayed in Fig. 5. However in all the electroless plating baths, even including the solution with the lowest concentration of 2 wt % HF, continuous, thick, dense Cu films were observed to be electrolessly deposited on the Si wafer swiftly in about 30 s while no as-deposited Cu films were clearly formed on the TiN/Ti/SiO 2 /Si substrate, indicating that the mechanisms of electroless Cu deposition on the Si wafer and on the TiN/Ti/SiO 2 /Si substrate are different, and the Cu film is more easily electrolessly deposited on the Si wafer than on the substrate. The effects of HF in the solution. In condition D1, D2, and D3 no continuous Cu film was achieved on the TiN barrier and so this provides a convenient case to test the effects of HF on the TiN barrier. In previous work the HF is adopted as an etching agent on the fully oxidized TiN surface in pretreatment of the substrate for Pd activation 8,22 or direct electroless Cu deposition on the barriers. 23 As studied, an oxygen-rich layer on the barrier layer may result in no deposition of electroless-plated Cu. XPS was performed to test the etching effect of HF as well as its influence on the electroless Cu deposition on the TiN surface. The effect of different plating solutions etching on the Ti 2p XPS of the TiN surface is shown in Fig. 6. The spectrum shows a noticeable change in the line shape. Curve a in Fig. 6 reveals 24 the presence of a fully oxidized layer with the peak at ev corresponding to the Ti 2p 3/2 binding energy of TiO 2. The curve shape of b of Fig. 6 overlaps peaks of ev for the 2p 3/2 binding energy of Ti, ev for the 2p 3/2 binding energy of Ti 2 O 3, ev for the 2p 3/2 binding energy of TiO 2, and ev for the 2p 1/2 binding energy of Ti, indicating the partly removed oxygen-rich layer of the TiN surface. It may due to the poor etching effect of the 2 wt % HF to the TiN surface at 323 K for 5 min. In curves c and d a significant decrease of peaks representing the Ti 2 O 3 and TiO 2 is observed, which results from the total removal of the oxidized layer on the top of the TiN by HF with concentration of 4 and 6 wt % during plating. The atom ratios of oxygen to titanium and nitrogen to titanium in the substrates are shown in Table II. As expected the atom ratio of O 1s to Ti 2p decreases with the increase of the concentration of HF, indicating the enhanced etching effect on the oxidized layer of TiN in solution with higher HF concentration. The decreased atomic ratio of nitrogen to titanium suggests that during the plating, the TiN surface is

4 Journal of The Electrochemical Society, C466-C C469 Figure 3. The surface morphology of as-deposited Cu films on TiN/Ti/SiO 2 /Si substrate at different bath temperatures. Bath temperature was a 303, b 313, c 323, and d 333 K. The solution was M CuSO wt % HF. The plating time was 5 min, and the distance was 0.5 mm. also destroyed by HF by its highly corrosive effect. Therefore, HF acts as the etching agent to clean the oxidized layer of the TiN surface and its etching ability and corrosion of the TiN surface are both enhanced with this concentration. However the decrease of the atom ratio of Cu 2p3 to Ti 2p with the concentration of HF presented in Table II is unexpected, because in previous reports 15,16 F ions were deduced as the transmitter of electrons, and thus as the concentration of F ions increases it is reasonable to suppose that more electrons can be transmitted to the TiN surface by F ions and more Cu should be deposited on the TiN surface. It is possible to attribute the decrease of the atom ratio of Cu 2p3 to Ti 2p with the concentration of HF to the acid corrosion. By Reaction 3 2Cu + O 2 +4HF 2CuF 2 +2H 2 O 3 As a result, as the HF concentration increases the enhanced acid corrosion can eliminate the tiny Cu nuclei on the TiN surface, leading to the reduction of the atom ratio of Cu to Ti. The other conditions in which a Cu film with a full coverage on the TiN surface reveals that in the high HF concentration in this paper it is about 8wt% the deposition of Cu on the TiN surface suppresses the Cu corrosion. The as-deposited Cu grains on the TiN/Ti/SiO 2 /Si substrate were also analyzed by XPS, and the results are shown in Fig. 7. Curve a in Fig. 7 reveals that Cu ions with high valence existed in the Cu grains deposited in low HF concentration of 2 wt %. Compared with curve b and c of Fig. 7, the peaks at and ev represent the Cu 2p 3/2 and 2p 1/2 binding energy of Cu, respectively. This suggests that in the plating solution with low HF concentration in this paper it is 2 wt % the Cu grains deposited on the TiN surface are easily oxidized and the reasons need to be further studied. In higher concentrations about 4 wt % of HF, pure Cu can be obtained. So we concluded that the effects of the HF in this electroless Cu deposition system include transmission of the free electrons, removal of the oxidized layer of TiN to activate the surface, and corrosion of the as-deposited Cu grains and TiN substrate. Crystallography of as-deposited Cu films on TiN barrier and Si wafer. Figure 8 presents the typical XRD pattern of as-deposited Cu film on TiN/Ti/SiO 2 /Si substrate in condition A2. The crystalline Cu peaks are clearly shown in the figure. The peaks of Ag 111 and Ag 200 come from the silver paste smeared at the sides of the substrate. Table III shows the peak intensity ratio I 111 /I 200 and the full width at half maximum fwhm of peak Cu 111 of the as-deposited Cu films plated on the TiN barrier in all the conditions. Compared with the intensity ratio I 111 /I 200 of as-deposited Cu films displayed in Table III and the intensity ratio of 2.17 of standard powder Cu listed in the JCPDS card, no specific texture was found in the as-deposited Cu films on the TiN/Ti/SiO 2 /Si substrate. The intensity ratio I 111 /I 200 of as-deposited Cu films in condition A decreased as well as the stability of the fwhm of the peak Cu 111 with the reduction of distance, suggesting that the Cu 111 texture

5 C470 Journal of The Electrochemical Society, C466-C Figure 5. The morphology of the Cu dots on TiN/Ti/SiO 2 /Si substrate. The solution was M CuSO wt % HF. The bath temperature was 323 K, and the plating time was 5 min. The distance was 0.5 mm. Figure 4. The morphology of as-deposited Cu films at different CuSO 4 concentrations. CuSO 4 was a 0.030, b 0.035, and c M. The morphology of condition C4 is shown in Fig. 2c. In solution HF was 8 wt %. The bath temperature was 323 K and plating time was 5 min. The distance was 0.5 mm. was enhanced by the large distance between the two substrates. Also, the increase of the fwhm of peak Cu 111 and reduction of the intensity ratio I 111 /I 200 with the growth of as-deposited Cu grains were observed in condition B. It has been reported 25 that the very weak texture of Cu deposition on TiN layer because Cu grains nucleate incoherently on the TiN surface to minimize the interfacial energy resulting from the large lattice mismatch between Cu and TiN. It is noted that the TiN surface was amorphous as shown in Fig. 9, and therefore resulting in the randomly oriented grain growth of the electrolessly deposited Cu films. It has been proved that the 111 texture strongly prevents stress-induced voids in Al interconnects 26,27 and the 111 textured Cu film has a higher resistance to electromigration 28 inasmuch as the highly textured microstructure suppressed the grain boundary and interfacial diffusion of metal atoms. 26,29 Furthermore, a lower oxidation rate in the 111 oriented Cu film was reported. Hence we need to improve the 111 texture of the as-deposited Cu films. Figure 10 is the typical XRD pattern of the crystalline plated Cu film on the Si wafer. The peak Cu 111 is the preferred orientation of the as-deposited Cu film and the 200, 220 orientations are also observed. The peak Cu 2 O 111 was detected, resulting from the reduction of Cu 2+ ions to Cu + ions at the surface of Si wafer. In addition, the Cu 111 is the predominant texture and Cu 2 O is also found in the as-deposited Cu films on the Si wafer, regardless of the plating solution, bath temperature, and distance between the two electrodes. An important finding is that there is no Cu 2 O detected in the as-deposited Cu films on the TiN surface shown in Fig. 8, though the peak Cu 2 O 111 was detected in the Cu films on the Si wafer. This reveals that Cu 2+ ions are more easily reduced to Cu + ions on the Si wafer than on the TiN barrier. In the alkaline electroless Cu deposition method, the Cu 2 O can be reduced by formaldehyde from Cu 2+ ions to Cu + ions according to the following reaction 7,30 2Cu 2+ +5OH +CH 2 O Cu 2 O + HCOO +3H 2 O 4 or oxidized by the oxygen dissolved in the plating bath. Cu 2 Omay cause the increase of resistivity of the as-deposited Cu film up to 5.1 cm. 31 The as-deposited Cu film on the TiN surface without Cu 2 O indicates the stability of the acid-based plating solution and high resistance to oxidation of the Cu films plated by this method. Adhesion of as-deposited Cu films to the TiN surface. The adhesion of as-deposited Cu films to the TiN barrier layer is an essential issue in consideration of electroless deposition method. The as-deposited Cu film needs to endure chemicalmechanical polishing to reduce the surface roughness in ULSI metallization 7 and hence demands a high adhesion between the as-deposited Cu film and the barriers. It is apparently known

6 Journal of The Electrochemical Society, C466-C C471 Figure 7. XPS Cu 2p 3/2 and Cu 2p 1/2 core-level spectra of as-deposited Cu in condition D1 a, condition D2 b, condition D3 c. Bath temperature was 323 K, and plating time was 5 min. in Table IV. Interestingly, the samples with excellent adhesion have poor surface coverage. Adhesion was decreased with the improvement of the surface coverage of the as-deposited Cu films on TiN surface, which indicates that many released H 2 were accumulated at the Cu/TiN interfaces and weakened the adhesion of Cu and TiN. The as-deposited Cu film on the Si wafer all had poor adhesion to the Si wafer, thus inferring the intense formation of H 2 at the Cu/Si interface. Figure 6. a XPS Ti 2p 3/2 and Ti 2p 1/2 core-level spectra of as-deposited PVD TiN; The TiN surface after etching by condition D1 b, condition D2 c, condition D3 d. Bath temperature was 323 K, and plating time was 5 min. The distance was 0.5 mm. that during the plating the H 2 trapped at the interface of the Cu/barrier can strongly reduce the adhesion. 32 Thus the tape peel test introduced by Tseng et al. was employed to test the adhesion of the as-deposited Cu films to TiN layer, and it was repeated three times on each sample. Those without any visible Cu film damage after tests are classified as excellent, those with some spots left on the film after tests are defined as good, those which peeled off easily are decided as poor. 15,16 The results are listed Table II. The atom ratio of the substrates etched by different solution. Sample no. Atom ratio ofo1sto Ti 2p Atom ratio ofn1sto Ti 2p Atom ratio of Cu 2p3 to Ti 2p D D D Figure 8. XRD patterns of as-deposited Cu film on TiN/Ti/SiO 2 /Si substrate in condition A2. The solution was M CuSO wt % HF. Bath temperature was 323K, and plating time was 5 min. The distance of two electrodes was 0.5 mm.

7 C472 Journal of The Electrochemical Society, C466-C Table III. FWHM peak (111) and peak intensity ratio I 111 /I 200. Sample no. FWHM of 111 I 111 /I 200 A A A A B B B B C C C C Figure 9. XRD pattern of TiN of the substrate. Figure 10. XRD patterns of as-electrolessly plated Cu films on Si wafer. The solution was M CuSO wt % HF. Bath temperature was 323 K and plating time was 30 s. Table IV. Adhesion of the as-electrolessly plated Cu film. Sample No. A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 Adhesion Excellent Good Excellent Good Excellent Good Good Conclusions Cu films with preferred 111 orientation was directly deposited on the TiN surface using the separated electrodes of the Si wafer and the TiN/Ti/SiO 2 /Si substrate in the CuSO 4 -HF solution. The uniformity of the as-deposited Cu films was improved by reducing the distance between the Si wafer and the substrate and increasing the concentrate of HF and CuSO 4 and the bath temperature. The grain size of the Cu films is closely related with the distance, the concentration, and the bath temperature as well. No Cu 2 O was detected in the as-deposited Cu films on the TiN surface, though it appeared in the Cu films plated on the Si wafer. The poor adhesion of the asdeposited Cu films to the TiN surface, resulting from the formation of the H 2 bubbles at the Cu/TiN interface, needs to be improved. During plating, hydrofluoric acid activates the TiN surface by etching the oxidized layer on top of the TiN surface, transmits the free electrons from the Si wafer to the substrate, and corrodes the as-deposited Cu grains. Because of the acid corrosion, there needs to be a high HF concentration about 8 wt % in this work to achieve the Cu deposition on the TiN surface. Acknowledgments This work was supported by the Specialized Research Fund for the Doctoral Program in Higher Education in China. The authors thank Professor Gong Zhang and Dr. Le Huang for the sample preparation and valuable discussions. Tsinghua University assisted in meeting the publication costs of this article. References 1. D. C. Edelstein, G. A. Sai-Halasz, and Y. J. Mii, IBM J. Res. Dev., 38, D. Edelstein, J. Heidenreich, R. Goldblatt, W. Cote, C. Uzoh, N. Lustig, P. Roper, T. McDvitt, W. Motsfiff, A. Simon, J. Dukovic, R. Wachnik, H. Rathore, R. Shultz, L. Su, S. Luce, and J. Slattery, Tech. Dig. - Int. Electron Devices Meet., 1997, J. C. Lin and C. Lee, J. Electrochem. Soc., 146, Y. Shacham-Diamand, J. Electron. Mater., 30, H. Ono, T. Nakano, and T. Ohta, Appl. Phys. Lett., 64, B. Chin, P. Ding, B. Sun, T. Chiang, D. Angelo, I. Hashim, Z. Xu, S. Edelstein, and F. Chen, Solid State Technol., 41, Y. Shacham-Diamand, V. Dubin, and M. Angyal, Thin Solid Films, 262, H. H. Hsu, C. C. Hsieh, M. H. Chen, S. J. Lin, and J. W. Yeh, J. Electrochem. Soc., 148, C A. Radisic, Y. Cao, P. Taephaisitphongse, A. C. West, and P. C. Searson, J. Electrochem. Soc., 150, C L. A. Nakahara, T. Ohmori, K. Hashimoto, and F. Fujishima, J. Electroanal. Chem., 333, J. C. Patterson, M. O Relly, G. M. Crean, and J. Barrett, Microelectron. Eng., 33, S. G. dos Santos F, L. F. O. Martins, P. C. T. D Ajello, A. A. Pasa, and C. M. Hasenack, Microelectron. Eng., 33, C. Carrao, L. Magagnin, and R. Maboudian, Electrochim. Acta, 47, L. Magagnin, R. Maboudian, and C. Carraro, Electrochem. Solid-State Lett., 4, C W. T. Tseng, C. H. Lo, and S. C. Lee, J. Electrochem. Soc., 148, C W. T. Tseng, C. H. Lo, and S. C. Lee, J. Electrochem. Soc., 148, C V. Bertagna, F. Rouelle, G. Revel, and M. Chemla, J. Electrochem. Soc., 144, M. L. Polignano, A. Giussani, D. Caputo, C. Clementi, G. Pavia, and F. Priolo, J.

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