Introduction: Molecular Dynamic Simulation of Point Lattice Defects in Silicon Nitride
Silicon nitride is a ceramic material, used as a dielectric barrier in the silicon based device industry. Silicon Nitride is
used both in CMOS technology based devices as a high-K dielectric and high dielectric strength barrier, and in MEMS device
fabrication as sacrificial layer for suspended structure production and as dielectric barrier. Silicon nitride can be deposited
onto the substrate via several deposition techniques:
Two most common techniques are the LPCVD and PEVCD thin film deposition methods. LPCVD (Low Pressure Chemical Vapoe Deposition) employs a high temperature (around 800 degrees celcius) as an activation energy source for a chemical reaction on the surface of the silicon wafers. In the PECVD (Plasma Enhanced Chemical Vapor Deposition) a RF plasma source is used for the activation energy for the same chemical reaction. The base temperature is only around 300 degrees celcius.
During those processes, a variety of lattice defect can form. These defects can influence the mechanical properties of the layer.
The Silicon Nitride Lattice
There are three known crystallographic structures for silicon nitride, designated as α, β and γ phases. The α and β phases are the most common forms of Si3N4, and can be produced under normal pressure conditions. The γ phase can only be synthesized under high pressures and temperatures. The α - and β - Si3N4 have trigonal (Pearson symbol hP28, space group P31c, No. 159) and hexagonal (hP14, P63, No. 173) structures, respectively, which are built up by corner-sharing SiN4 tetrahedra. They can be regarded as consisting of layers of silicon and nitrogen atoms in the sequence ABAB... or ABCDABCD... in β - Si3N4 and α - Si3N4, respectively. The AB layer is the same in the α and β phases.
Fig 2. A Silicon nitride thin film on a silicon wafer.
[from filmetrics, http://www.filmetrics.com/dielectrics]
The aim of this project is to reproduce the silicon nitride lattice using the LAMMPS code and to simulate the diffusion of the lattice defects, which include:
- Silicon vacancy
- Nitrogen vacancy
- Hydrogen inclusion 
Furthermore, a calculation of the average internal stress will be made for the lattice defects.
A visualization of the lattice will be realized using AViz.
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