Summary 117 Figure 7.4. RBS spectra (3.7 MeV He +2 , 7° tilt) of the diffusion barriers before (broken line) and after (solid line) testing at 620°C for 30 minutes in flowing atmosphere N 2 -5% H 2 ambient [8] 7.3 Electromigration Resistance Electromigration (EM) resistance of Ag is the most critical interconnect reliability concern that would determine its suitability for integrated circuit technology. EM is the drift of metal ions as a result of either collision between the conductor electrons and/or the metal ions or high electrostatic field force when current is passed through a metal conductor. The direction of mass transport depends on the direction of the net force. When high current densities pass through the metal line, voids or hillocks are formed at a point of ion flux divergence (Figure 7.5). Voids and hillocks deform and grow until electrical failure is completed [10]. 118 Silver Metallization Figure 7.5. Schematic to illustrate metal conductor failure due to electromigration [17] Ho and Huntington [11], Patil and Huntington [12] reported that Ag migrates toward the anode for the temperature range 670–877°C. Klotsman et al. [13] and Breitling and Hummel [14] however, found that Ag ion transport is toward the cathode, and is opposite to the direction of the electron wind. Studies conducted by Hummel et al. showed that the dominating movement path is grain boundaries for temperatures in the range of 225–280°C (activation energy of 0.95 eV) and surface for the temperature range 160–225°C (activation energy of 0.3 eV) [15]. In this study the influence of an encapsulation process on the electromigration resistance of Ag has been investigated. The sample configurations used consisted of bare Ag patterns and encapsulated Ag patterns. Compared to bare Ag lines, TiN(O) encapsulated Ag lines exhibited much better electromigration resistance in terms of the time required for the formation of significant number of voids and hillocks. After the encapsulation process, the Ag surface is capped by a thin layer of TiN(O). The mobility of Ag atoms at the TiN(O)/Ag interface is substantially reduced. Therefore the electromigration resistance is improved significantly. By examination of the surface of the encapsulated Ag lines tested for a long time, almost no hillocks were observed. This implies that the surface diffusion has been hindered substantially by the presence of the TiN(O) encapsulation. Comparison of Tables 5.1 and 5.2 suggests that the encapsulation process improves the electromigration resistance of Ag metallization by at least one order of magnitude for the test structures and conditions used [16]. 7.4 Future Trends Ag metallization research has intensified significantly in recent years. Research work has been conducted mainly in the US. Increasing numbers of companies have already begun research and development (R&D) efforts in Ag-based metallization. However in this highly competitive era the industrial research has been propriety. Ag-based interconnects represents the future trend in the deep sub-micron regime. Ag is an attractive material for interconnect-metallization in future integrated circuits technologies due to its low bulk resistivity and high reliability against Summary 119 electromigration. Ag can be deposited by plating (electroless and electrolytic), sputtering (physical vapor deposition (PVD)), laser induced reflow and chemical vapor deposition (CVD) cluster tools, chemical mechanical polishing (CMP) polishers and low temperature etchers. There are still many critical issues that remain to be resolved in the development of Ag-based metallization. For instance, to meet the throughput requirement in device manufacturing, there is a need to demonstrate a Ag deposition technique with a high deposition rate (>250 nm/min) at low substrate temperatures without simultaneously sacrificing low resistivity, good step coverage and complete via fill. The adequate use of thin diffusion barriers in Ag-based metallization will be critical for the 0.25 mm technology. Other materials science issues in Ag-based metallization need to be address: (1) microstructural control (e.g. grain size and texture); (2) contamination control (C, O and CI); (3) oxidation and corrosion control; (4) prevention of Ag diffusion in metals and dielectric materials; and (5) mechanical properties (e.g. stress migration, adhesion improvement). In addition, the development of Ag-based on-chip interconnects technology will also lead to new advancements in electronic packaging technology. The implementation of Ag-based interconnects will represent the future trend in the deep submicron regime. 7.5 References [1] S. P. Murarka, R. J. Guttman, A. E. Kaloyeros, W. A. Lanford, Thin Solid Films 236, 257(1993). [2] M. E. C. Willemsen, A. E. T. Kuiper, A. H. Reader, R. Hokke, J. C. Barbour, J. Vac. Sci. Technol. B 6, 53(1988). [3] S. Q. Wang, I. Raaijmakers, B. J. Burrow, S. Suthar, K. B. Kim, J. Appl. Phys. 68, 176(1990). [4] T. L. Alford, D. Adams, T. Laursen, B. M. Ullrich, Appl. Phys. Lett. 68, 3251(1996). [5] Y. L. Zou, T. L. Alford, D. Adams, T. Laursen, K. N. Tu, R. Morton, S. S. Lau, MRS Proc. 427, 355(1996). [6] J. Li, J. W. Mayer, E. G. Colgan, J. Appl. Phys. 70, 2820(1991). [7] D. Adams, T. L. Alford, N. D. Theodore, S. W. Russell, R. L. Spreitzer, J. W. Mayer, Thin Solid Films 262, 199(1995). [8] Y. Wang, T. L. Alford, Appl. Phys. Lett. 74(1), 32(1999). [9] D. Adams, Ph.D. dissertation, Arizona State University, 1996. [10] M. Mahadevan, R. M. Bradley, J. M. Bebierre, Europhys. Lett. 45, 80(1999). [11] P. S. Ho, H. B. Huntington, J. Phys. Chem. Solids 307, 1319(1966). [12] H. R. Patil, H. B. Huntington, J. Phys. Chem. Solids 31, 463(1970). [13] S. M. Klotsman, A. N. Timofeyev, I. S. Trakhtenberg, Phys. Metal Metallogr. 14, 140(1962). [14] H.M. Breitling, R. E. Hummel, J. Phys. Chem. Solids 33, 845(1972). 120 Silver Metallization [15] R. E. Hummel, H. J. Geier, Thin Solid Films 25, 335(1975). [16] Y. Zeng, L. H. Chen, Y. L. Zou, P. A. Nguyen, J. D. Hensen, T. L. Alford, Mater. Lett. 45, 157(2000). [17] D. Adams, T. L. Alford, Materials Science and Engineering R 40, 207(2003). Index Adhesion analysis, 54 tape test, 55 scratch test, 54 Ag on Pa-n, 51 compositional changes, 51 sheet resistance variation, 52 Ag on SiO 2 , 70 Ag(Ti) alloys, 20 Ag/barrier/silicide/silicon, 105 Ag/Ti-O-N/CoSi 2 /Si, 105 Ag/Ti-O-N/NiSi/Si, 105 Agglomeration, 2, 43–48, 69–73 Al(Cu), 75 Aluminum oxide, 59, 62, 84, 86, 93, 97 Aluminum properties, 5 bulk resistivity, 5 thin film resistivity, 5 diffusivity in Si, 5 self-diffusivity, 5 electromigration, 5 Young's modulus, 5 TCR, 5 mean free path of electron, 5 melting point, 5 thermal conductivity, 5 Ag sheet resistance, 36, 37 Amorphous-to-crystalline transition, 40 Annealing ambients, 57 argon ambient, 59 He-H ambient, 62 ammonia ambient, 63, 93 Annealing temperatures, 46 Auger depth profiles, 61 AES spectrum, 76 Bragg’s Law, 12 Bulk resistivity, 5 Cobalt silicide, 104 CoSi 2, 104 Composition of Ta-N, 31 Copper metallization, 4 Copper properties, 5 bulk resistivity, 5 thin film resistivity, 5 diffusivity in Si, 5 self-diffusivity, 5 electromigration, 5 Young's modulus, 5 TCR, 5 mean free path of electron, 5 melting point, 5 thermal conductivity, 5 Corrosion, 2, 22 encapsulated silver films, 22 H 2 S ambient, 29 Cu(Ti) alloys, 21 Copper (Cu), 5 Current density, 76 Dealloying kinetics, 18 Depth scale, 10 122 Index Diffusion barriers, 15, 34 Diffusivity in Si, 5 in SiO 2 , 1 Electromigration, 1, 5, 75, 76, 78–81 Electronegativity, 102 Encapsulation, 80, 81, 83 Energy resolution, 8 Failure mechanisms, 109 TiN(O)-encapsulated Ag lines, 80 electromigration resistance, 1, 75, 76 Formation of voids, 45 Four-point-probe, 49, 50, 52 van der Pauw, 70 Gold, 98, 101 H 2 S ambient, 24 Heat of formation, 93 Hillocks, 69, 75–81, 117, 118 Hole formation, 69 Hydrogen sulfide, 24 H 2 S ambient, 24, 25, 28, 29 Identification of phases, 39 Impurity scattering factor, 46 Interfacial energy barrier, 94 Ion resonances, 11 Joule heating, 78, 80 Kinematic factor, 8 Kinetics in Ag/Al bilayer systems, 83, 97 reaction kinetics, 83 outdiffusion of Al through Ag, 83 Kinetics of oxide, 89, 93 growth kinetics, 89 Law of Reflectivity, 12 Mass transport, 43 Melting point, 5 Nitridation, 20, 28 Nitrogen, 41 N 2 flow, 30, 34–40 interstitial nitrogen, 40 Onset temperature, 70–73 Outdiffusion of Al, 60, 62, 67 Oxide surface layer, 84, 89, 93, 95 Parylene, 48, 49 Pa-n, 48, 49 dielectric, 48, 49 reliability issues, 549 phase change, 50 Polyimides, 2, 548 Recoil energy, 8 Refractory metal nitrides, 15 Resistivity, 5, 17, 34, 39 Resonances, 11 RUMP, 17, 18 Rutherford backscattering spectrometry (RBS), 8 Scattering cross section, 9 Scattering kinematics, 8 Self-diffusivity, 5 Self-encapsulation, 15 Silicide, 40 Silver, 1, 5 Ag, 1, 2, 5, 29, 79 Silver properties, 5 bulk resistivity, 5 thin film resistivity, 5 diffusivity in Si, 5 self-diffusivity, 5 electromigration, 5 Young's modulus, 5 TCR, 5 mean free path of electron, 5 Index 123 melting point, 5 thermal conductivity, 5 Silver-aluminum films, 46 Silver sulfide, 25 Stress, 67, 69 compressive stresses, 67, 69 thermal stresses, 67, 69 thermal expansion coefficients, 67 Surface energies, 67 Surface oxide layer, 102 Surface peak Al, 98 Tantalum nitride, 30 Ta-N films, 30, 39, 41 Ta-N diffusion barriers, 30, 34, 36 Ta-silicide, 40, 41 Test structures, 79 Thermal conductivity, 5 Thermal stability, 4, 43 Thermodynamic data, 93 Gibbs free energy, 93 enthalpy, 95 Thin film characterization, 7 Thin film resistivity, 5 TiN encapsulation, 24 TiN(O), 18, 117 Titanium nitride, 16, 113, 114 Titanium nitride self-encapsulation, 16 Transport of aluminum, 91 X-ray diffractometry, 12 XRD, 13, 30, 33, 37–41, 44, 46–50, 52 . (e.g. grain size and texture); (2) contamination control (C, O and CI); (3) oxidation and corrosion control; (4) prevention of Ag diffusion in metals and dielectric materials; and (5) mechanical. electromigration [17] Ho and Huntington [11], Patil and Huntington [12] reported that Ag migrates toward the anode for the temperature range 670–877°C. Klotsman et al. [13] and Breitling and Hummel [14]. formed at a point of ion flux divergence (Figure 7.5). Voids and hillocks deform and grow until electrical failure is completed [10] . 118 Silver Metallization Figure 7.5. Schematic to