Date of Award
Doctor of Philosophy (PhD)
During the Czochralski growth of silicon, it is inevitable for oxygen to be incorporated into the silicon crystal from the quartz crucible. Interstitial oxygen improves the mechanical strength of silicon by pinning and locking dislocations, but also generates thermal donors during device processes, shifting the electrical resistivity. For silicon wafers used in radio frequency (RF) applications, it is important to ensure the high resistivity of the substrates for good RF characteristics. Therefore, the oxygen level in these high resistivity silicon wafers is kept very low (< 2.5 × 1017 atoms/cm3) by carefully controlling the Czochralski growth conditions, in order to reduce the thermal donor concentration to an acceptable level. Silicon on insulator (SOI) substrates made from high resistivity wafers have been widely used for RF applications. SOI manufacturing includes multiple high temperature thermal cycles (1000 – 1100 °C), during which the high resistivity wafers are prone to slip and warpage. Therefore, it is technologically important to recover some of the lost mechanical strength due to the lack of oxygen by introducing electrically inactive impurities to suppress the dislocation generation and mobility in silicon. Germanium (Ge) as an isovalent impurity is 4% larger in size and forms a solid solution with silicon in the entire concentration range. Previous works have shown Ge doping at high concentrations above 6 × 1019 atoms/cm3 increased mechanical strength of silicon with high oxygen concentration (~ 1 × 1018 atoms/cm3). In this work, we explore the effect of Ge doping (7 - 9 × 1019 atoms/cm3) on the mechanical strength of low oxygen concentration (< 2 × 1017 atoms/cm3) silicon, where the oxygen associated dislocation locking and pinning are very low. A mechanical bending test was used to study the average dislocation migration velocity and the critical shear stress of dislocations motion at 600 – 750 °C for Ge doped, nitrogen doped, and undoped low oxygen samples, as well as nitrogen doped float-zone and un-doped high oxygen concentration samples. Next, we fabricated SOI substrates using these high resistivity wafers and compared their slip generation rates and the slip-free epitaxial grow temperature windows after the high temperature thermal cycles (> 1000 °C). Our results indicate at lower temperature Ge doesn’t affect the dislocation mobility or generation. Whereas at higher temperature, Ge reduces the slip occurrence rate and increases the critical shear stress for slip generation, compared to that of the undoped control group. The temperature-independent solid solution strengthening of Ge becomes a larger percentage of the overall critical shear stress at higher temperature. Ge may also interact with vacancies and self-interstitials to retard dislocation creep at high temperature. Our study suggests Ge alone can modestly strengthen the high resistivity silicon wafers during high temperature manufacturing. Although nitrogen has been shown to effectively lock dislocations even at a much lower concentration, it forms shallow thermal donors with oxygen, precluding this approach for high resistivity silicon wafers. In the last part of the thesis, we switch gears and use density functional theory calculation to investigate two possible reaction pathways for the N-O complex (the core structure of nitrogen shallow donor) formation. Instead of nitrogen dimers (N2) dissociating into individual nitrogen monomers and then binding with oxygen, we find N-O complex can form more easily from the dissociation of oxidized nitrogen dimer (N2O). The activation energy for the second path is lowered by 0.6 eV due to oxygen stabilizing the metastable N2 configurations on the dissociation pathways. This result also explains the experimentally observed higher nitrogen diffusivity in Czochralski silicon, compared to that of float zone silicon or epitaxial film, where oxygen concentration is close to zero. We propose co-doping nitrogen with impurities that can trap N2 or retard oxygen diffusion as promising avenues for suppression of nitrogen shallow donor generation.
Katharine M. Flores Robert Standley
Katharine M. Flores, Robert W. Standley, Rohan Mishra, Kenneth F. Kelton,