Question

What role does nucleation and kinetics in general have for making a single crystal silicon wafer? Describe the process to make this, highlighting the steps where kinetics have a strong role.

Answer 1

The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced either by thermal annealing or by ion beam irradiation at high substrate temperatures.

The structural and thermo dynamical properties of the starting phase are emphasized. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermo dynamical properties of amorphous silicon and to the kinetical mechanisms of crystal grain growth ion at high substrate temperatures

Amorphous Si is described ideally as a covalently bonded fourfold coordinated continuous random network of Si Atoms. Its real structure, however, can be perturbed and its properties have been found to critically depend on the thermal history and preparation conditions. Thermal annealing produces large variations in Enthalpy, Conductivity, Electron-spin density, Atomic structure and of the optical & vibrational proper-ties. All of these changes have been attributed to ‘‘structural relaxation,’’ a process in which the whole network re-arranges upon annealing in order to decrease its free energy and the tetrahedral bond-angle distortion decreases and short range ordering occurs.

Defects can be passivated by high concentration H doping. Indeed, this passivation is responsible for the strong reduction of the defect-related band gap states in a-Si H layers and allows their doping in pure a-Si defect concentration can be reduced by a factor of upon annealing at 500 °C compared to that of as implanted a-Si

Calorimetric measurements demonstrated that during the relaxation process defect annihilation and bond rearrangement are associated to a heat release of 0.04 eV/atom. Moreover relaxed a-Si has in turn a free energy difference of 0.11 eV/atom with respect to the crystalline phase. The free energy of a-Si ~in both the relaxed and unrelaxed states and of liquid Si with respect to crystal Si (c-Si) shows that a-Si always has a higher free energy than c-Si and hence it exists as a kinetically frozen metastable phase. The melting temperature of a-Si is predicted to be smaller than that of c-Si and to depend on the relaxation state. These last properties have indeed been experimentally verified by ultra shortpulsed laser melting. Ultrashort pulses are needed to melt a-Si in order to kinetically inhibit solid phase crystallization

Heating at temperatures above 500 °C usually induces a solid phase transition of a-Si into the thermodynamically stable crystalline Si phase. a-Si layers on top of a single crystal substrate this transition occurs by planar motion of the crystal–amorphous (c–a)interface from the interior towards the surface. This process is referred to as solid phase epitaxy (SPE). The growth rate of epitaxial crystallization is strongly dependent on temperature and it presents an Arrhenius like behavior with a unique activation energy of 2.6860.05 eV over a growth rate range of more than 10 orders of magnitude. Defects present in the amorphous phase are hence responsible for the crystallization process.

Since ion beam irradiation introduces a non equilibrium defect concentration in the amorphous, ion bombardment is expected to stimulate crystallization. Ion beam induced epitaxial crystallization has been observed at temperatures as low as 200 °C with an enhancement in growth rate by about two orders of magnitude over pure thermal values. The crystallization rate has been observed to depend on temperature with an apparent activation energy of about 0.3 Ev energy lost by the impinging ions into elastic nuclear collisions, dose rate, dopant concentration and orientation. Further decreasing the temperature below a critical value a reverse phenomenon i.e., planar layer by layer amorphization has been observed

The thermodynamical driving force and growth kinetics are linked in the dynamical evolution of the system formed by a population of crystal clusters growing within the amorphous volume.

The characteristic crystallization time defined as the time needed to crystallize 67% of the a-Si layer starting from the instant at which nucleation takes place exhibits Arrenhius behavior.

The nucleation and growth kinetics are strongly influenced by impurities that are eventually dissolved in the amorphous layer.

In particular the range of concentrations between 0.1 and 1 at. % the group III and group V impurities all decrease the nucleation rate, whereas the reactive species O and F produce an increased nucleation rate. The presence of combined impurities may produce a compensating effect upon nucleation.

The kinetics of the amorphous to polycrystal phase transition in silicon can be described by ‘‘classical’’ nucleation theory, based on capillarity effects at the crystal amorphous interface. A crystalline cluster can nucleate homogeneously within the amorphous film or heterogeneously on discontinuities such as precipitates, extended defects, interfaces, etc.

Ion beam irradiation of silicon can stimulate the transition from the amorphous to the crystal phase in the presence of a pre-existing c–a interface. The energy released by the passing ions enhances the kinetics of the epitaxial crystallization which is observed at temperatures as low as 200 °C, well below the temperature required in a pure thermal recrystallization at 500 °C.

Ion irradiation produces defects within the amorphous network which can be monitored by Raman spectroscopy, calorimetry and conductivity measurements. Their annealing kinetics indicates that several classes of defects exist in a-Si and that they are characterized by different activation energies.

The thermodynamical parameters of a-Si are also strongly affected both by the irradiation and by the annealing conditions