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Rapid Lateral Solidification

R. Zhong, J.P. Leonard April 2010
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh PA 15261

Rapid lateral solidification (RLS) is a pulsed laser melt-mediated process by which large sheet-like grains can be formed in thin metallic films on amorphous substrates such as SiO₂. A schematic of the process is shown in Figure 1.

The key to the process is laser projection irradiation in a spatially limited region of the surface, typically a line of width 1-60 μm. In this region, the film absorbs the energy of the laser pulse, which is sufficient to completely melt the metal down to the underlying amorphous substrate. Upon cooling, solidification is initiated from the unmelted regions adjacent to the melt pool, and proceeds laterally toward the center of the pool. The resulting microstructure consists of large sheet like grains extending from the edge of the laser-melted region to the center line (typically up to 15 μm depending on the line width). These grains are typically 1 μm wide in the transverse direction, and extend through the full thickness of the film (typically between 100 and 500 nm). This morphology is similar to bulk cast or directionally solidified materials, but has a number of unique characteristics that are opening up an exciting new area of research in fundamental science and also has the potential to provide new applications in electronic interconnect structures.

This process has previously been used for the melting and recrystallization of silicon thin films on SiO₂, while it was believed that such processing was impossible in metals due to the severe dewetting that occurs while the film is molten. We set out to overcome this problem by exploring various process conditions, materials, and thin film configurations. After some time we found that the addition of a capping layer of SiO₂ suppressed dewetting and allowed the film to maintain continuity during melting and resolidification.

Several metal systems have been successfully processed, including Au, Cu, Cr, Al, and Cu-Nb. All show a characteristic columnar structure as shown in Figure 2. Although this resembles other solidification structures such as in casting, the time scale, spatial extent, and thin film geometry make this microstructure unique. The present research has been directed toward understanding several fundamental aspects of this process.

• Texture formation. It is found that under some conditions, Cu and Ag films produce columnar grains (Zone II) with a strong <100> texture along the growth direction. For pure materials this has often been attributed to a slight growth anisotropy along <100> coupled with a thermal (dendritic) instability at the interface due to heat flow that magnifies perturbations associated with <100> fast growth. Our results appear to support this, as do recent MD simulations in the literature. We believe that crystallites of random orientation originating from unmelted seeds at the left side of Zone I (pictured above) undergo an occlusion process that reduces the number of grains and favors the fast-growth <100> oriented grains. What is not yet known, however, is the magnitude of thermal dendritic instability, or why under some process conditions the <100> texture is absent.
• Interface velocity and undercooling. This is perhaps the most challenging experimental aspect, as the timescales (300 ns) and extent (20 um) make direct observation of the moving interface extremely difficult. It is expected that laser reflectance spectroscopy and in-situ dynamic TEM techniques will eventually be used to solve this problem. Additionally, we have now adapted our finite differences code (3DNS) to effectively model the solidification process. It predicts lateral interface velocities in excess of 100 m/sec under conditions corresponding to RLS, but experimental verification is critically needed.
• Thermal stress effects. It is now believed that a large number of the defects found in RLS microstructures arise from post-solidification thermal stress effects. As the freshly solidified film cools, the thermal expansion mismatch between the metal and underlying SiO₂/Si substrate produces a biaxial tensile stress that we estimate may be as high as 1 GPa. Most stacking faults and dislocations found in the Zone II columnar grains are produced during this cooling. The challenge here is to come up with material and thin-film configurations that minimize the thermal mismatch and reduce stresses.
• Nucleation kinetics. Zone IV of the RLS microstructure consists of large radial structures resulting from spontaneous nucleation in the undercooled liquid pool. We believe that nucleation occurs at some critical level of undercooling, typically at about 300 ns, or when the lateral solidification has progressed about 20 µm. Initially we thought this would correspond to heterogeneous nucleation of solid at the liquid-Cu/SiO₂ interface, but it now appears that the undercooling is far in excess of 200K. Quantitative measurements of interface velocity are needed to explore this further.
• Defects from solidification. As liquid-solid interfaces move faster, kinetic limitations associated with atom attachment to the solid lattice can produce vacancy point defects. This can lead to vacancy concentrations in the solid far in excess of equilibrium. It is believed that there is some uphill diffusion of vacancies to the interface during solidification, as well as vacancy aggregation to form stacking fault tetrahedra and dislocation loops in the newly formed solid. We have observed dislocation loops in the RLS processed material, and it is likely that quantitative measurement can yield new insight into the kinetics of vacancy trapping at the interface.
• Alloy effects. RLS provides a nearly ideal configuration to study segregation and defect formation in rapid solidification of alloys. The solidification microstructure extends 20 µm and is easily analyzed via electron microscopy without sectioning. It is possible to co-sputter virtually any alloy composition under well controlled and high purity conditions. Preliminary work with Cu-Nb alloy solidification shows a significant segregation effect and changes to the Zone III microstructure. We plan to conduct many more experiments on alloys of interest in the near future.
• Interconnect applications. Microelectronic interconnects today consist of electrochemically deposited copper lines are embedded in an insulating layer (e.g. SiO₂ or low-k dielectrics), using various Ta or TaN diffusion barriers and a PVD Cu seed layer. Scaling to line widths below 100nm raise new challenges relating to increased resistance losses due to grain boundary and interface scattering as well as electromigration problems. The ability of RLS to provide large directionally solidified grains at pre-determined locations opens up the possibility for obtaining single-crystal lines. Patterning, as well as electron transport properties and electromigration resistance are critical steps we are working on to verifying the suitability of RLS materials for these applications.

Publications relating to this project

• R. Zhong, A. Kulovits, J.M.K. Wiezorek and J.P. Leonard, "Four-zone solidification microstructure formed by laser melting of copper thin films", Appl. Surf. Sci. 256, 105 (2009).
• A. Kulovits, R. Zhong, J.M.K. Wiezorek and J.P. Leonard, "Electron microscopy of geometrically confined copper thin films after rapid solidification", Thin Solid Films 517, 3629 (2009).
• R. Zhong, J.M.K. Wiezorek, J.P. Leonard, “Laser-Induced Microstructural Modification in Polycrystalline Cu and Ag Films Encapsulated in SiO₂”, Mat. Res. Soc. Symp. Proc., 0990-B09-07 (2007).
• R. Zhong, J.M.K. Wiezorek and J.P. Leonard, "Microstructural investigation of excimer laser-crystallized metallic thin films", IMID/IDMC Digest No. 49-1, 1739 (2006).
• J.P. Leonard, "Determination of the absolute fluence profile in pulsed laser processing using melt-induced phase changes in an amorphous silicon thin film", Review of Scientific Instruments 77, 53101 (2006).
• J.E. Kline and J.P. Leonard, "Rapid lateral solidification of pure Cu and Au thin films encapsulated in SiO₂", Appl. Phys. Lett. 86, 201902 (2005).
• J.E. Kline and J.P. Leonard, "Suppression of dewetting phenomena during excimer laser melting of thin metal films on SiO₂", Thin Solid Films 488, 306 (2005).