The Mechanical Property Of Nitrogen-doped Czochralski (CZ) Silicon

Although the electronic and optical properties are the two main aspects of silicon material, its mechanical property is also a very important factor, as the damage, fracture and warpage increasing in the processing of fabrication and packaging of modern semiconductor devices.It's well known that silicon is a kind of classic brittle material at room temperature, meaning it is linear elastic until catastrophic fracture, and its fracture is not deterministically predictable. During the fabrication or service, if the applied tensile stresses exceed the probabilistic tensile strength of silicon, then failure will occur. Even a tiny crack will bring tremendous damage to devices and circuits. Especially nowadays, with the increasing of silicon wafer diameter, warpage in heat treatment, defects and dislocations generated in silicon often become critical problems in ULSI devices fabrication. Therefore, it is desirable to investigate and enhance the mechanical property of silicon single crystal.In this paper, the fracture property of Czochralshi(CZ) and nitrogen-doped silicon(NCZ) were investigated at room temperature by three-point bending method. It was found that nitrogen increased the fracture strength of silicon single crystal. We consider that the nitrogen may change the Shockley band on silicon surface and form complex to influence the fracture procedure. What's more, the amount of oxygen precipitates, dislocations in silicon and the orientation of the pressed surface also influence the fracture strength. Stress-strain curves were studied at room temperature.In another experiment, brittle-ductile transition (BDT) of NCZ and CZ were studied for first time. It seems the NCZ silicon has a higher BDT temperature compared with CZ silicon's. It is suggested that the elastic effects and the electronic effects of nitrogen doped in silicon made the BDT temperature higher. The observation of fracture surface showed that it was curves at high temperature in brittle fracture, but smooth planes at room temperature. Stress-strain curves were also analyzed at high temperatures.The warpage of silicon wafer of NCZ and CZ silicon were investigated. The relationship of thermal stress, oxygen precipitates and dislocations were analyzed too. It can be seen that nitrogen enhanced the oxygen precipitation, which seemed to pin the dislocations generated by thermal stresses, so that no warpage were found in the heat treatment processing. In another hand, oxygen precipitates were observed by TEM,polyhedral oxygen precipitates and thermal stresses induced dislocations were found, but no oxygen precipitates related dislocations. Moreover, the slip dislocations were generated in the direction of [110] and [112] on [111] plane, which were most possible for dislocations to slip.