Heavily Boron-doped Silicon Substrate Doped Germanium Growth Of Misfit Dislocations At The P / P ~ + Silicon Epitaxial Wafers

Nowadays, p/p⁺ epitaxial silicon wafers are widely applied in CMOS and power devices. Unfortunately, a mass of misfit dislocations generally form in the p/p⁺ epitaxial silicon wafers due to the release of mismatch strain induced by the lattice mismatch between the heavily boron-doped substrate and lightly boron-doped epitaxial layer. The misfit dislocations definitely lead to significant leakage current of transistors. Therefore, how to reduce and even eliminate the misfit dislocations in p/p⁺ epitaxial silicon wafers is a stringent issue to be addressed. In this regard, herein, a worthwhile effort has been expended by using heavily germanium and boron (Ge-B) co-doped silicon wafer as the substrate for p/p⁺ epitaxial silicon wafer. Listed below are the main results achieved in this thesis.1. Growth of misfit dislocation-free p/p⁺ epitaxial silicon wafers by using Ge-B codoped silicon wafers as substratesThe Ge-B codoped Czochralski silicon crystal was firstly grown from the silicon melt doped with boron and germanium with concentrations of 2×10¹⁹ cm^-3 and 1.48×10²⁰cm^-3, respectively. Subsequently, a few tens polished wafers with the resistivities of about 6-6.5mΩ·cm were processed from this silicon ingot. Lastly, 10-100μm thick lightly boron-doped epitaxial layers were deposited on the polished Ge-B codoped wafers. According to expectation, misfit dislocation-free p/p⁺ epitaxial silicon wafers were achieved by using Ge-B codoped silicon wafers as the substrates.Generally, the lattice of heavily boron-doped silicon crystal is distorted to large extent because the tetrahedral covalent radius of boron atom is smaller than that of silicon atom. It is the lattice distortion of heavily boron-doped silicon crystal that leads to the mismatch dislocations in the p/p⁺ epitaxial silicon wafer. The basic idea underlying the growth of misfit dislocation-free p/p⁺ epitaxial silicon wafer using Ge-B codoped silicon substrate is that the introduction of enough amount of Ge atoms into the heavily boron-doped silicon crystal can considerably reduce the latticedistortion, because the tetrahedral covalent radius of germanium atom is larger than that of silicon atom. In practice, according to the customized thickness of epitaxial layer, the lattice distortion of whole Ge-B codoped silicon ingot can be well controlled within a desirable range through doping a certain amount of germanium atoms; therefore, using the polished wafers processed from this silicon ingot, misfit dislocation-free p/p⁺ epitaxial silicon wafer with certain thicknesses can be achieved.2. Thermal stability of p/p⁺ epitaxial silicon wafers grown on Ge-B codoped silicon substratesIt was found that no misfit dislocations appeared in the modified p/p⁺ epitaxial silicon wafers that were subjected to the simulated MOS device thermal cycles. Therefore, it can be believed that the modified p/p⁺ epitaxial silicon wafers are of superior thermal stability.3. Boron out-diffusion in p/p⁺ epitaxial silicon wafersDuring the growth of p/p⁺ epitaxial silicon wafers, boron in the substrate will out-diffuse into the epitaxial layer. It was shown that the out-diffusion of boron was to certain extent suppressed by codoping germanium into the heavily boron-doped substrate, which is probably due to the formation of Ge-B pairs. Therefore, the transitional zone in the modified p/p⁺ epitaxial silicon wafer is narrower than that in the conventional p/p⁺ epitaxial silicon wafers.4. Effects of germanium on oxygen precipitate in heavily boron-boped Czochralski (CZ) SiliconThe density of bulk micro-defects (BMDs) in the Ge-B codoped CZ silicon was much lower than that in the conventional heavily boron-doped CZ silicon when subjected to two step anneal of 650 °C/4h + 1000°C/16h. The origin of this phenomenon is most likely due to the tensile stress introduced by the doped germanium, which suppresses the growth of oxygen precipitates.When subjected to single step anneal at temperatures 800, 950 and 1050°C and two-step anneal (800°C/4¹28h+1000°C/16h), it was found that the BMD density of Ge-B codoped silicon was much lower than that of conventional heavily boron doped silicon. Based on this result, it is believed that the formation of boron-oxygencomplexes generally formed in the temperature range 800-1050°C, which are the heterogeneous nucleation sites for oxygen precipitation, is significantly suppressed by the doped germanium atoms.