Study Of Oxygen-related Defects In Czochralski Silicon

Oxygen atoms are inevitably incorporated into the Czochralski (CZ) silicon during crystal growth, typically with a concentration in the range (5-10)×1017 cm-3. The interstitial oxygen atoms can combine with the vacancies or impurity atoms in silicon to form oxygen-related complexes such as vacancy-oxygen complexes and electrically active nitrogen-oxygen complexes, which may exhibit detrimental effects on the devices. During the device fabrication process, the supersaturated oxygen atoms tend to form oxygen precipitates. It is well recognized that oxygen precipitation (OP) is a double-edged sword in terms of its influences on the devices. Concretely, the internal gettering of detrimental metal contaminants, which is associated with OP, is advantageous to increase the manufacturing yield of integrated circuits. While, the leakage current would increase if there were oxygen precipitates acting as the carrier recombination and generation centers in the active area of a device. Therefore, manipulation of oxygen-related defects in CZ silicon is of technological significance in terms of their influences on the devices. In this dissertation, the grown-in oxygen precipitates and the effect of nitrogen (N) or germanium (Ge) impurity on the formation of oxygen-related complexes and oxygen precipitation in CZ silicon have been studied. Besides, the influence of OP on the carrier transportation in CZ silicon has also been investigated. The achieved results in this dissertation will broaden and deepen the understanding of the interactions among oxygen atom, vacancy, and N or Ge atom and the OP behavior in CZ silicon. Listed below are the primary achievements in this work.(1) The radial distribution of grown-in oxygen precipitates in the 300 mm silicon wafer has been investigated. The density of grown-in oxygen precipitates decreases from the center to edge of the silicon wafer. Moreover, it is revealed that the density of Cu precipitates also decreases along the radial direction as mentioned above. Therefore, it is apparent that the Cu precipitate density is positively correlative to the grown-in oxygen precipitate density. This is due to that the grown-in oxygen precipitates can serve as the heterogeneous nucleation centers for Cu precipitation. It is suggested that the Cu decoration in combination with preferential etching can be used to indirectly evaluate the radial distribution of grown-in oxygen precipitates in the silicon wafers.(2) The effects of germanium (Ge)-doping at the level of 1020 cm-3 on formation of vacancy-oxygen complexes and oxygen precipitation (OP) in Czochralski (CZ) silicon have been investigated. It is found that Ge-doping remarkably suppresses oxygen precipitation (OP) in CZ silicon subjected to low-high two-step anneals without prior RTP. However, Ge-doping significantly enhances OP in CZ silicon with prior RTP at 1250 ℃. The suppressed OP in the case of the absence of prior RTP is primarily due to the fact that the 1020 cm-3 Ge-doping introduces compressive strain into silicon crystal lattice, which increases the critical size of oxygen precipitate nuclei for a given nucleation temperature. Moreover, it is revealed that the 1020 cm-3 Ge-doping facilitates the formation of vacancy-oxygen (V-O) complexes in CZ silicon during high temperature RTP. More vacancy-related complexes in CZ silicon not only reduce the critical size of oxygen precipitate nuclei but also provide more precursors for oxygen precipitate nucleation. Therefore, the 1020 cm-3 Ge-doping enhances OP in CZ silicon subjected to the two-step anneals following high temperature RTP.(3) The formation of VOm complexes in Cz-Si1-xGex wafers during the high temperature RTP has been investigated. The Cz-Si1-xGex wafers are found to exhibit much weaker OP than Cz-Si counterparts when subjected to the same two-step (nucleation-growth) anneal following the high temperature RTP, indicating that the formation of VOm complexes during the RTP is suppressed in Cz-Si1-xGex wafers. The ’gold-marker method’ in combination with deep level transient spectroscopy (DLTS) further confirms the suppressed formation of VOm complexes in Cz-Si1-xGex wafers during the RTP. Besides, the characterization of the oxygen out-diffusion depth profiles in Cz-Si1-xGex and Cz-Si wafers annealed at different temperatures in the range 650-1000 ℃ indicates that the oxygen diffusivity in Cz-Si1-xGex is a little smaller. The calculations based on density functional theory reveal that in Cz-Si1-xGex the vacancies incline to locate at the first nearest neighboring sites of the Ge atoms and the capture of Oi atoms by the vacancies is not energetically favored. The aforementioned adverse conditions are believed to be responsible for the suppressed formation of VOm complexes in Cz-Si1-xGex.(4) The nitrogen-oxygen complexes in nitrogen-doped CZ silicon have been investigated by DLTS. It is found that in CZ silicon a small amouont of nitrogen atoms are captured by oxygen atoms to form nitrogen and oxygen (N-O)-related centers featuring an energy level at Ec-84 meV, which is deeper than the well-documented N-O shallow donor centers [EC-(34～37) meV]. The two kinds of N-0 complexes as mentioned above exhibit the similar thermal stability, which both can survive at temperatures lower than 800 ℃. It is suggested that the N-0 complexes featuring the energy level at Ec-84 meV may be formed by the interaction between the nitrogen pairs and oxygen atoms.(5) The effects of nitrogen-doping on oxygen precipitation (OP) and associated internal gettering (IG) in the heavily arsenic (As)-doped Czochralski (CZ) silicon substrates of n/n+ epitaxial silicon wafers have been investigated. It is found that nitrogen atoms of high concentration are injected into the heavily As-doped silicon substrate by RTP in N2 atmosphere. It is believed that the vacancy-and nitrogen-assisted heterogeneous nucleation mechanisms are simultaneously operative for OP in the substrate subjected to the two-step anneal of 650 ℃/16 h+1000 ℃/16 h, thus leading to the more pronounced OP and therefore the better IG capability. It is expected that the present work offers a strategy feasible for improving the IG capability of n/n+ epitaxial silicon wafers.(6) The influence of OP on the carrier transportation in CZ silicon has been investigated. It is found that the carrier mobility is decreased to a certain extent while the carrier concentration is nearly unchanged due to significant OP in CZ silicon. Interestingly, such a decrease in mobility can be offset by copper (Cu) decoration of oxygen precipitates via Cu drive-in anneal at appropriate temperatures. Meanwhile, the oxygen precipitate/silicon interface states are substantially reduced. It is clarified that the charges associated with the oxygen precipitate/silicon interface states exert additional scattering effect on the carrier transportation, leading to the decrease of carrier mobility as mentioned above.