Portland, Ore. Understanding the surface process called wetting has become essential to the bottom-up assembly of atomically precise semiconductors as well as to the functioning of chips and boards. Now researchers at the Technion-Israel Institute of Technology (Haifa, Israel) have modified a transmission electron microscope (TEM) to reveal new details about wetting.
At the atomic level, wetting is the movement of atoms at the interface between a solid and a liquid. Understanding the phenomenon is key to understanding crystal growth on silicon wafers, for soldering chips to boards, brazing flip-chips and controlling liquid flow through microfluidic chips.
In measuring "the degree of wetting and the interface energy between liquid aluminum and sapphire [single-crystal aluminum oxide], we have identified a decrease in interface energy associated with growth of the sapphire into the liquid aluminum," said Technion professor Wayne Kaplan. "The decrease in interface energy means better wetting and improved adhesion."
Kaplan and Yaron Kauffmann, a doctoral candidate at Technion, performed the work in cooperation with Germany's Max Planck Institute. The modified TEM was engineered at Technion's Russell Berrie Nanotechnology Institute in cooperation with the microscope's maker, FEI Co. (Hillsboro, Ore.).
The aluminum-sapphire interface has been extensively studied worldwide in an effort to understand the high-temperature wetting processes, but the Technion group has uncovered evidence to back a hitherto unproven theory that a transition region exists at the interface. In this transition region, the precise atomic structure of the solid causes the liquid atoms likewise to become highly organized, resulting in layers of crystalline-like metal adjacent to the sapphire.
"Now that we know the transition region exists, we need to understand the exact role it plays in influencing processes," said Kaplan. "This transition region at the interface, where the liquid atoms are partially ordered, certainly affects the properties of the interface. Under the correct conditions, we believe we can cause the liquid to solidify into a crystal structure not normally adopted by the liquid, which in turn means its properties can be different."
Transmission electron microscopes work by passing an electron beam through 200-nanometer-thick samples and rendering the resulting scattered electrons into images with a resolution of less than half an angstrom. The researchers modified the usual TEM process by using only 20-nm-thick slices of sapphire and by adding a heating stage that enabled them to increase the temperature of the slice past the melting point of aluminum. By filming the process as the sample heated, the researchers were able to follow the movement of individual atoms at the interface. In a nutshell, the electron beam knocked oxygen atoms out of the aluminum oxide lattice, thereby enabling droplets of pure aluminum to form and be studied on the sapphire surface.
"We believe that the improvement of wetting was due to oxygen reaching the interface," said Kaplan. "Oxygen is transported along the interface in the region of partially ordered liquid aluminum atoms. If more oxygen arrives to the interface [from the outside environment], the region then becomes saturated with oxygen, and new solid sapphire forms."
The Russell Berrie institue, the German-Israel Fund and the German Science Foundation funded the project.