Design
Materials
Process
Pulsed Laser Deposition of Nanocrystalline Metal Films
A. Kulovits, J.M.K. Wiezorek, J.P. Leonard April 2010
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh PA 15261
With a goal of producing high quality metallic nanocrystalline thin films, this research has centered on gaining a fundamental understanding of the kinetic processes occurring during vapor deposition on amorphous substrates. Using the PLD systems that Dr. Leonard designed and built, we have deposited nanocrystalline Au, Al, and Cr films under various conditions and have developed a unified process model to interpret the results.
Figure 1. Microstructure of polycrystalline gold films deposited on amorphous SiO2 substrates for different substrate temperatures and flux kinetic energy (eV). Symbols circle, square and triangle correspond deposition via evaporation, sputtering, and PLD respectively as found in the literature. Crosses indicate the present experiments we have conducted using PLD.
Our model is a unique extension to current models for polycrystalline film formation. We postulate that there are three fundamental parameters common to all physical vapor deposition: 1) Flux kinetic energy, 2) substrate temperature, and 3) deposition rate. This has enabled us to consider the 3 previously disparate techniques (evaporation, PLD, and sputtering) in a unified process map, as shown Figure 1. With this model, it is predicted that nanocrystalline films can only be formed in the limits of low temperature, flux, and high deposition rate.
We selected pure gold as the primary system to study, as gold is non-reactive in most atmospheres, and is particularly difficult to oxidize. Amorphous SiO2 is easily prepared as a substrate using thermal oxidation of standard silicon wafers, resulting in a high purity, very smooth surface without crystalline order that would affect orientation selection in the early stages of film growth. The deposited films are analyzed with X-ray diffraction and SEM to determine texture and grain morphology, which are found to fit well within in the process maps.
Figure 2. Microstructure of polycrystalline gold films deposited on amorphous SiO2 substrates for different substrate temperatures and deposition rate (A/sec). Symbols circle, square and triangle correspond deposition via evaporation, sputtering, and PLD respectively as found in the literature. Crosses indicate the present experiments we have conducted using PLD.
Although much in the field of epitaxial physical vapor deposition is understood today, the details of polycrystalline film formation on amorphous (glass) substrates is less complete. Gaining a fundamental understanding particularly important for future applications such as wide area sputter deposition for photovoltaics, flat panel displays and architectural glass. Going forward, there are several areas that must be explored.
- Rate Effects. A detailed investigation of rate effects is needed, as it relates to the instantaneous adatom concentration, island/grain nucleation rates, and ability to bury defects. Our work currently shows that in the Au/SiO2 system there is a critical transition from random to textured nanocrystalline grains at a mean deposition rate of 1-3Ĺ/sec. A further complication is that in PLD the flux is pulsed, with the material arriving at the surface within a few µs, followed by a long period without flux. Comparison to sputtering under similar flux energies but continuous deposition rate is needed.
- Deposition Stress. There are a number of models, and some experimental work concerning stresses arising during vapor deposition of thin films. The system is complex, involving surface diffusion, grain boundary zipping and migration, and bulk diffusion. With gold, under the most extreme conditions associated with the production of random nanocrystalline films, we see evidence of a large tensile stress that produces fissures in the film on a timescale of minutes to days after deposition. The mechanisms are not well understood at this time.
Publications relating to this project
- A.K. Kulovits, J.P. Leonard, J.M.K. Wiezorek, “Microstructure of pulsed laser deposited FePd thin films on amorphous and crystalline substrates”, Intermetallics 15, 1606 (2007).
- A.K. Kulovits, J.P. Leonard, J.M.K. Wiezorek, “Influence of processing parameters on microstructure in pulsed laser deposited Au thin films”, Mat. Res. Soc. Symp. Proc., 0979-HH11-20 (2006).
- A.K. Kulovits, J.P. Leonard, J.M.K. Wiezorek, “Microstructural Evolution during Post Deposition Annealing of Pulsed Laser Deposited Fe100-xPdx thin films”, Mat. Res. Soc. Symp. Proc., 0980-II03-08 (2006).