Design
Materials
Process
Nucleation Rate Measurement in the Si-SiO2 Thin Film System
J.P. Leonard, J.S. Im October 2000
Program in Materials Science, Columbia University, New York, NY 10027
Laser melting of thin silicon films on amorphous substrates such as SiO2 often results in rapid quenching and supercooling of the molten film Under these far-from equilibrium conditions, nucleation of solid crystallites has been shown to be an important factor in determining the final solidified microstructure. In this research, quantitative measurements of nucleation rates are made using patterned films and in-situ observation as well as a newly developed numerical model to predict nucleant densities and microstructures. Important insights into the mechanism of nucleation in the Si/SiO2 system have been gained-highly relevant both in the technical field of laser crystallization as well as the fundamentals of classical nucleation theory.
Figure 1. (a) Schematic view of sample used in nucleation rate measurements. Si film is subdivided into identically sized regions, which are observed to solidify randomly after melting. (b) Schematic of experimental setup showing heater and transient reflectance probe.
Figure 2. Summary of nucleation rate measurements for molten Si. Circles are direct measurements of isothermal nucleation rates, crosses are values inferred from simulation, boxes a-d represent data from the literature, interpreted in the context of heterogeneous nucleation kinetics.
The research centers on the measurement of nucleation rates of solid in supercooled liquid Silicon. In these experiments, an excimer laser is used to melt a polycrystalline or amorphous thin film on a SiO2 substrate while heated to ~1200C similar to rapid thermal processing conditions. In-situ surface reflectivity techniques are used to measure the rates of appearance of crystal nuclei through nucleation during the subsequent rapid quench. In the course of this work, I have been able to make direct measurements of liquid-to-solid nucleation rates on a microsecond time scale, and have identified a heterogeneous nucleation mechanism relevant to these laser crystallization processes. In addition, we have developed a 3D numerical model for nucleation and growth in rapidly solidified thin films that shows great promise for studying polycrystalline microstructures arising from nucleation.
Publications relating to this project
- J.S. Im, M.A. Crowder, R.S. Sposili, J.P. Leonard, H.J. Kim, J.H. Yoon, V.V. Gupta, H.J. Song, H.S. Cho, "Controlled Super-Lateral Growth of Si Films for Microstructural Manipulation and Optimization", Physica Status Solidi 166, 603, (1998)
- J.P. Leonard, "Nucleation Rate Measurement in the Si-SiO2 Thin Film System", Ph.D. Thesis, (2000)