C. Y. Wong

Formation of thin films of NiSi: Metastable structure, diffusion mechanisms in intermetallic compounds

The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.

Thermal stability of TiSi2 on mono- and polycrystalline silicon

Thermal stability of TiSi2 on mono‐ and polycrystalline silicon was investigated by cross‐sectional transmission electron microscopy and high‐resolution electron energy loss spectroscopy. Additional heat treatments after silicide formation result in a rough silicide/silicon interface, discontinuity of the metal silicide film, and a penetration of silicide into silicon/polycrystalline silicon substrates. Plausible explanations for these observations are presented.

The poly‐single crystalline silicon interface

Cross sectional transmission electron microscopy (TEM) reveals an amorphous interfacial region of the order of 2 nm thick between chemical vapor deposition‐(CVD) deposited polycrystalline silicon films and the single‐crystal silicon substrate. The continuity of this region varies from sample to sample and plays an important role in the effects produced by subsequent heat treatment. In cases where this interfacial layer is continuous, the deposited layer remains polycrystalline. When the region is discontinuous, complete epitaxial realignment of the poly is possible. The speed of realignment depends on the implanted arsenic dose and is much greater than reported for undoped films. Various impurities are also observed at the interface and correlate with the character of the interface.

Lattice imaging of metastable TiSi2

The microstucture of metastable C49‐TiSi2 was studied by high‐resolution transmission electron microscopy in a bilayer thin film of Ti and Si annealed at 700 °C. Large grains (300–400 nm) of C49‐TiSi2 phase with high density of stacking faults on (010) planes were observed. A preferred orientation among the C49‐TiSi2 grains was identified, and the grains were aligned along the [010] with slight misorientation. The diffraction pattern from the metastable TiSi2 showed continuous streaks superimposed by discrete spots along hk0 reciprocal lattice rows, indicating the presence of polytypism. Lattice imaging of polytypic TiSi2 has been obtained and it showed that the structure is nonperiodic and one dimensionally disordered due to the stacking faults. The finding of metastable TiSi2 associated with polytypism suggests that its structure could be conveniently represented by various stackings of atomic planes along the [010] direction. An atomic model is proposed to explain the origin of polytypism. It is shown ...

Effect of arsenic segregation on the electrical properties of grain boundaries in polycrystalline silicon

Equilibrium arsenic segregation to the grain boundaries of polycrystalline silicon was measured directly by x‐ray microanalysis in the temperature range 700–1000 °C. A direct link was observed between arsenic segregation and resistivity. Increasing arsenic segregation at the lower annealing temperatures is consistent with an observed increase in resistivity. Fitting the enhancement levels at various temperatures with the McLean segregation isotherm, a binding energy of 0.65 eV/atom and a boundary saturation limit of 12 at. % for arsenic was obtained. A model for the effect of the segregation of arsenic to silicon grain boundaries is proposed. Segregation to boundary defects that cause trapping states can remove these interfacial traps, and segregation to other boundary sites can create a degenerately‐doped interfacial layer. The electrical consequences of this segregation are considered, and by comparison of the measured resistivity changes with temperatures with those predicted from these simple models i...

As segregation to grain boundaries in Si

Abstract Scanning transmission electron microscopic (STEM) analysis has been applied to the study of equilibrium segregation of As to grain boundaries in Si. Direct quantitative evidence for the extant of segregation has been obtained in a number of specimens heated to temperatures between 1000 and 700°C. A value for the binding energy of an As atom to an Si grain boundary of 0·66 eV has been calculated. The experimentally observed saturation concentrations of As have also been shown to be consistent with the density of potential segregation sites in previously proposed models of grain boundary structure in Si.

Cross-sectional transmission electron microscopy investigation of Ti/Si reaction on phosphorus-doped polycrystalline silicon gate

Titanium interaction with phosphorus‐doped polycrystalline silicon gate electrodes was investigated by cross‐sectional transmission electron microscopy and correlated with sheet resistance measurements. Phosphorus concentration above 1×1016 ion/cm2 in the polycrystalline silicon leads to decreased TiSi2 formation, discontinuous metal silicide layer, and increased sheet resistance. A possible cause could be the formation of titanium phosphide at high phosphorus concentration in the polycrystalline silicon, competing with the total titanium available for silicide formation.

Grain growth study in aluminum films and electromigration implications

Studies of grain structure, size, distribution, and geometric arrangement are gaining importance especially from the point of view of thin‐film interconnects for micron and submicron semiconductor technologies. We have studied the effect of annealing on the grain growth characteristics of pure Al films. As‐deposited films exhibit a uniform grain size distribution between 500 and 800 A. With heat treatment, the average grain size increases with the largest grains exhibiting a parabolic growth with time, limited to a size in the neighborhood of 1 μ. Smaller grains, however, cannot be eliminated and about 30% of the grains are less than 2500 A across. The grain boundaries of the smaller grains have a higher oxygen content than the rest of the film. Lines defined by liftoff exhibit a characteristic grain boundary seam along the length of the line. The implications of the above study in terms of processibility and reliability with respect to electromigration are also discussed.

Enhanced grain growth of phosphorus‐doped polycrystalline silicon by titanium silicide formation

The influence of silicide reaction on the grain growth behavior of phosphorus‐doped polycrystalline silicon has been studied. Intrinsic and phosphorus‐doped polycrystalline silicon, with and without evaporated titanium overlayer, were prepared by low pressure chemical vapor deposition on thermally grown SiO2 on silicon wafer. These samples were then annealed in He ambient at 700 °C for various time. The Rutherford backscattering spectrum showed that TiSi2 formed completely after 30 min anneal. Cross‐sectional transmission electron microscopy showed that Kirkendall voids exist at the interface between TiSi2 and polycrystalline silicon, indicating that silicon is the predominant diffusing species. Enhanced grain growth as a result of silicide reaction was observed in phosphorus‐doped polycrystalline silicon. The enhanced grain growth of polycrystalline silicon is suggested to be due to the redistribution of dopant, analogous to the alloying and dealloying processes in diffusion induced grain boundary migration.

Arsenic segregation to silicon/silicon oxide interfaces

Arsenic segregation at polycrystalline silicon/silicon and polycrystalline silicon/silicon oxide interfaces was examined directly by transmission electron microscopy(TEM) and scanning transmission electron microscopy(STEM). Segregation occurring precisely at these interfaces was identified. A simple model was proposed based on arsenic segregation to structural units containing dangling bonds and consequent bond saturation. The removal of these dangling bonds will then play an important role in the electrical properties of these interfaces. Furthermore substitutional arsenic segregation at a degenerate level at these interfaces was also proposed. The subsequent dopantionization and localized charges at the interfaces was discussed.