Effects Of Impurities On The Mechanical Properties Of Czochralski Silicon

Czochralksi (Cz) silicon, as the base material of intergrated circuits (ICs), has been extensively and intensively studied for decades. Mechanical properties of Cz silicon have been important subjects in the research and development. With the increasing diameter of silicon wafer used for ICs, increasingly stringent requirements on silicon wafer processing and fast development of silicon based micro-electro-mechanical systems (MEMS), the mechanical properties of Cz silicon are receiving much more attention lately. It has been well recognized that oxygen and nitrogen can substantially improve mechanical performances of silicon. However, the effects of other impurities on the mechanical properties of Cz silicon have been rarely addressed. This place limits on further improvement of the mechanical performances of Cz silicon. In this context, systematic and in-depth investigation of impurity effects on the mechanical properties of Cz silicon is of significance.In this dissertation, the influcences of heavily germanium (Ge), phosphorus (P), arsenic-(As) doping and high density oxygen precipitates on the mechanical properties of Cz silicon have been detailedly investigated by means of nanoindentation, microindentation, high temperature bending and so on. The significant results achieved in this dissertation are listed as follows:(1) The mechanical properties of Ge-doped Czochralski (GCz) silicon have been investigated using instrumented nanoindentation, microindentation and three-point bending technique. The GCz silicon samples show higher Young's modulus and hardness than germanium-free Cz silicon samples in nanoindentation tests. We believe this is caused by the Ge enhanced phase transition from the Si-I (diamond cubic) phase to the stiffer Si-II (β-Sn) phase in GCz silicon under contact load during indentation, which is confirmed by micro-Raman spectroscopy characterization. The indentation fracture toughness of GCz silicon is slightly lower than that of Cz silicon. Ge-doping with concentration up to 1.4×1020cm-3 does not substantially change the critical resolved shear stress (CRSS) or the activation energy of dislocation glide, but increases the dislocation velocity slightly.(2) The mechanical properties of heavily P- and As-doped Cz silicon have been investigated using nanoindentation, microindentation and three-point bending technique, taking lightly P-doped Cz silicon as reference. Heavy P-doping does not change the hardness of silicon, while it decreases the Young's modulus of silicon to a certain extent. The indentation fracture toughness of the heavily P-doped Cz silicon is a little higher than that of the lightly P-doped Cz silicon. Dislocation velocities increase by two orders of magnitude and the activation energies are reduced by 0.6-0.7 eV in heavily P- and As-doped Cz silicon over the temperature range 400～650℃. The CRSS for dislocation glide in heavily P-doped silicon is higher than that in lightly P-doped Cz silicon by a factor of～2, while the CRSS of heavily As-doped Cz silicon hardly changes. The origin of CRSS has been discussed. The remarkable increase of CRSS in heavily P-doped silicon may be attributed to the strengthening effects of the grown-in precipitates.(3) The dislocation locking and transport of oxygen in Cz silicon with high concentrations of impurities (Ge, As and P) have been studied. We have studied oxygen transport in Cz silicon by analyzing data on the locking of dislocations by oxygen as a function of time and temperature. The experimental data have been analyzed, together with the reported data for silicon with a high boron concentration, to further the understanding of the mechanism by which high impurity concentrations affect oxygen transport at temperatures at which the oxygen dimer dominates transport (up to 550℃). It is shown that a high level of boron doping (3×1018cm-3) enhances the effective diffusivity of oxygen by a factor of～8 to～25 relative to low doped material with the same oxygen concentration. High levels of Ge-doping (8×1019cm-3) and As-doping (2×1019cm-3) can both have a slight retardation effect on oxygen transport. The magnitude of the reduction measured is less than a factor of～4 in the heavily Ge-doped specimens and less than a factor of～5 in the heavily As-doped specimens, and in most cases is significantly less than this. Ge-doping introduces considerable strain into the silicon lattice without affecting the Fermi level position, so data from these samples show that lattice strain affects oxygen dimer transport. The arsenic and boron doping levels in the materials studied give rise to lattice strain with a smaller magnitude and opposite sign to that in the germanium doped samples. It is therefore suggested that the Fermi level position also affects the transport of oxygen dimers.(4) The influence of high density (～109cm-3) oxygen precipitates on mechanical properties of Cz silicon has been studied. The oxygen precipitates exhibit significant retardation effect on dislocation motion in Cz silicon. Such an effect becomes stronger as the density and average size of oxygen precipitate increase. The correlation of rosette sizes with oxygen precipitate densities and sizes has been investigated quantitatively in terms of dispersion and precipitation strengthening mechanism respectively. It is found that the precipitation strengthening mechanism is much more appropriate to account for the immobilization effect of oxygen precipitates on dislocations as compared to the dispersion strengthening mechanism. This suggests the strain field interaction between oxygen precipitates and dislocations plays an important role in the retardation of dislocation glide. It is found that the oxygen precipitates increase Young's modulus of silicon to a certain extent, but hardly affect the hardness and indentation fracture toughness of Cz silicon. Compared with Cz silicon without any heat treatment, the pop-out tends to occur at a lower load during the unloading process of nanoindentation in the annealed Cz silicon with high density oxygen precipitates.