Atom beam sputtered Mo2C films as a diffusion barrier for copper metallization

The saddle field fast atom beam sputtered (ABS) 50 nm thick molybdenum carbide (Mo₂C) films as a diffusion barrier for copper metallization were investigated. To study the diffusion barrier properties of Mo₂C films, the as-deposited and annealed samples were characterized using four probes, X-ray diffraction, field enhanced scanning electron microscopy, energy dispersive X-ray analysis, atomic force microscopy and Rutherford back scattering techniques. The amorphous structure of the barrier films along with presence of carbon atoms at the molybdenum carbide-silicon interface is understood to reduce effective grain boundaries and responsible for increased thermal stability of Cu/Mo₂C/Si structure. The lowest resistivity of the as-deposited molybdenum carbide barrier films was ∼29 μΩ cm. The low carbon containing molybdenum carbide was found thermally stable up to 700 °C, therefore can potentially be used as a diffusion barrier for copper metallization.     

Fine line printed silicon solar cells exceeding 20% efficiency

Silicon solar cells with passivated rear side and laser‐fired contacts were produced on float zone material. The front side contacts are built up in two steps, seed and plate. The seed layer is printed using an aerosol jet printer and a silver ink. After firing this seed layer through the silicon nitride layer, the conductive layer is grown by light induced plating. The contact formation is studied on different emitter sheet resistances, 55 Ω/sq, 70 Ω/sq, and on 110 Ω/sq. These emitters are passivated with a PECVD silicon nitride layer which also acts as an anti‐reflection coating. Even on the 110 Ω/sq emitters it was possible to reach a fill factor of 80·1%. The electrical properties i.e., the contact resistance of the front side contacts are studied by transfer length model (TLM) measurements. On a cell area of 4 cm² and emitter sheet resistance of 110 Ω/sq, a record efficiency of 20·3% was achieved. Excellent open‐circuit voltage (V_oc) and short‐circuit current (j_sc) values of 661 mV and 38·4 mA/cm² were obtained due to the low recombination in the 110 Ω/sq emitter and at the passivated rear surface. These results show impressively that it is possible to contact emitter profiles with a very high efficiency potential using optimized printing technologies. Copyright © 2008 John Wiley & Sons, Ltd.     

Development of under bump metallizations for flip chip bonding to organic substrates

Several under bump metallization (UBM) schemes using CuNi alloys as the solderable layer were investigated. Nickel slows down dissolution of the UBM into the solder and formation of intermetallics during reflow. To study the intermetallic reaction, CuNi foils of different concentrations were immersed in a eutectic PbSn solder bath for reaction times ranging from 30 seconds to 30 minutes. It was observed that when 10% and 20% Ni is added into copper, the intermetallic forms a continuous layer, instead of the discrete scallops seen in pure Cu/solder interfaces. However, the thickness of the intermetallic remained about the same. For 30% and 45% Ni alloys a definite decrease in the intermetallic thickness was observed compared to the lower Ni alloys. Actual under bump metallizations were also made on Si wafers to study the reactions when there is a limited supply of CuNi available. Cr or Ti was used as the adhesion layer, and the solderable layer was a copper-nickel alloy, instead of pure copper used in the conventional UBM scheme. The metal layers were deposited on a wafer by evaporation and patterned into contact pads. Eutectic PbSn solder balls were reflowed on top of the pads. SEM micrographs of the intermetallic that forms at the UBM/solder interface show the refining effect of Ni in the interfacial microstructure. Since nickel metallizations often have high stresses, stress in the UBMs was measured by the wafer curvature method. Stress vs Ni content plots show that while stresses increase somewhat with the Ni content, the adhesion layer under the CuNi layer has a much larger effect on the stress. UBMs with Cr/CrCu adhesion layer had stresses ranging from about 300 to 600 MPa, while the stresses in UBMs with Ti/TiNi layers were between 70 and 350 MPa.     

Study on ta diffusion barrier in Al/Si system

The property of Ta as a diffusion barrier is studied for Al/Ta/Si structure. Interfacial reactions of Al(180 nm)/Ta(130 nm)/Si and Al(180 nm)/Ta(24 nm)/Si, in the temperature range 450∼600°C for 30 min, have been investigated. In Al/Ta(130 nm)/Si system, which is Ta-excess case, Al₃Ta is formed at 500°C. At 575°C, TaSi₂ is formed at the interface of Ta Si. At 600°C, after Al₃Ta decomposes at the interface of Al₃Ta TaSi₂, free Ta is bonded to TaSi₂ with the supply of Si from Si substrate and free Al diffuses through TaSi₂, resulting in Al spiking. In Al/Ta(24 nm)/Si system, which is Al-excess case, Al₃Ta is formed at 500°C. At the same temperature of 500°C, after Al₃Ta decomposes at the interface of Al₃Ta/Si, free Ta reacts with Si to form TaSi₂ and free Al diffuses to Si substrate, resulting in Al spiking. The results of interfacial reactions can be understood from the calculated Al-Si-Ta ternary phase diagram. It can be concluded that the reaction at Al/Ta should be suppressed to improve the performance of Ta diffusion barrier in Al/Si system.     

Electroplated indium bump arrays and the bonding reliability

A novel electroplating indium bumping process is described,as a result of which indium bump arrays with a pitch of 100μm and a diameter of 40μm were successfully prepared.UBM(under bump metallization) for indium bumping was investigated with an XRD technique.The experimental results indicate that Ti/Pt(300(?)/200(?)) has an excellent barrier effect both at room temperature and at 200℃.The bonding reliability of the indium bumps was evaluated by a shear test.Results show that the shear strength of the ind...     

Microstructure and barrier properties of reactively sputtered Ti-W nitride

The materials properties of Ti-W, sputter deposited in pure Ar, and Ti-W-N deposited in Ar-N₂ mixtures containing up to 50% N₂ have been reported. Films with N contents between 0 and 45 at.% have been produced. The W/Ti ratio in the nitrided alloy has been varied from that corresponding to pure W, to that of pure Ti. The metastable ter-nary phase diagram Ti-W-N was composed from data presented in this paper and lit-erature data. For low N contents (up to about 20 at.%) the film consists of a metastable solid solution of N in bcc Ti-W. For high N contents (from about 35 to 45 at.%). We observed the film to consist of an fcc ternary Ti-W nitride. The barrier performance was evaluated in an Al-Cu metallization to TiSi₂/Si contacts by Auger electron spectroscopy and junction leakage measurements. In clean sputtering environments, we need to cre-ate the ternary nitride to form a reliable barrier. Adding some N to “stuff” the grain boundaries of the bcc phase, or contaminating the Ti-W surface by an exposure to at-mosphere, was found to be less effective in producing a reliable barrier.     

Information storage using nanoscale electrodeposition of metal in solid electrolytes

Programmable metallization cell (PMC) memory is based on the electrochemical control of nanoscale quantities of metal in thin films of solid electrolyte. It shows great promise as an ultra-scalable solid state non-volatile memory as it requires low programming voltage and current, and has the ability for the storage cells to be physically sized at minimum lithographically defined dimensions. Scalability issues will be discussed in the context of recent findings relating to the nanostructure of the electrolyte and results obtained from small-geometry devices based on Ag–Ge–Se nano-phase separated material.     

Microstructure and properties of multilayer-derived tungsten silicide

In metallization, peeling and oxidation of tungsten silicide are the most serious problems of tungsten rich silicide. In this study, multilayer-derived silicon rich tungsten silicide with the silicon film on the outermost surface is investigated to avoid these problems. The dependence of sheet resistance on the annealing conditions is studied. X-ray diffraction results indicate that silicide formation is nearly completed after 30 min annealing at 750° C. Microstructures of silicide and polycides are investigated by electron microscopy. Silicide deposited on SiO₂ has smaller grains that deposited on poly-Si. A resistivity of 60 μΩ-cm is obtained for multilayer-derived WSi_2.3.     

Stability of metal/GaAs-lnterfaces: A phase diagram survey

Calculated phase diagrams of ternary Ga-As-metal systems for the metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Re, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au are presented. The predictive calculations are based on the following simplifications: Ternary phases and solid solubilities are disregarded and the Gibbs energy of formation of binary compounds is estimated by the enthalpy of formation and calculated from Miedema’s model. The predicted diagrams agree surprisingly well with experimental data and they may be useful for the many cases where data are lacking or fragmentary. The phase diagrams and the thermodynamic data are shown to be a powerful tool for the understanding of interface reactions of metallic contacts to GaAs and hence for the development of improved contact materials.     

ULSI铜多层布线中钽阻挡层CMP抛光液的研究与优化

以高浓度纳米SiO2水溶胶为磨料,H2O2为氧化剂的碱性抛光液,研究了适用于终抛铜/钽的CMP抛光液.通过调节pH值,降低抛光液的氧化,增强有机碱的作用,来降低铜的去除速率并提高钽的去除速率,得到了很好的铜/钽抛光选择性.