SAN FRANCISCO -- At temperatures far hotter than the sun's surface, University at Buffalo chemists are generating new coatings and then dramatically cooling them to, or even below, room temperature before depositing them on electronic devices.
By using such extremely high temperatures and then quenching the heat, the new technique solves one of the trickier problems in computer-chip fabrication: how to coat them while avoiding high temperatures that can cause computer chip samples to fail. This has been a serious drawback for fabricators of expensive chips for research-grade supercomputers, such as the Cray.
The technique -- utilizing an approach similar to that used in snow-making machines -- also is an improvement because the coatings it produces are always uniform and it removes from the fabrication process toxic precursors utilized in conventional methods.
The new method has applications in fabricating a wide range of thin films for use in electronic and opto-electronic materials and in fabricating uniform nanopowders for electronics applications, according to James F. Garvey, Ph.D., professor of chemistry and principal investigator.
Garvey discussed the technique here today (April 17, 1997) in an invited talk at the American Chemical Society's annual meeting. He conducted the work with Robert DeLeon, Ph.D., UB adjunct associate professor of chemistry.
Funding for the work is being provided by several Small Business Innovation Research grants, funded by Wright Patterson Air Force Base through the UB researchers' cooperative effort with Structured Materials Industries, Inc. (SMI) of Piscataway, N.J.
With the support of SMI, the UB scientists envision their technique and the apparatus they devised to implement it as eventually being marketed and sold as an accessory to expand the capabilities of molecular beam epitaxy machines, which are used widely in materials fabrication.
"We have developed a hybrid technique that marries the advantages of the two conventional fabrication methods -- laser ablation and molecular beam epitaxy -- and overcomes their disadvantages," said Garvey.
The new method, called Laser Assisted Molecular Beam Deposition (LAMBD), also takes the process for producing coatings for electronic devices a step further.
"Instead of simply sputtering a target material from point A to point B, we're chemically modifying it at the same time," Garvey said. "What's key about our fabrication method is that it causes chemical reactions that would be impossible to generate otherwise, and it does it all in one step. It provides us with a way to conduct novel, high-temperature chemistry."
Like laser ablation and molecular beam epitaxy, LAMBD uses extremely high heat to remove particles of a target material from one surface and transport it to another, creating a thin film.
"With our technique, molecules are ablated off the target material," said Garvey, "producing a plasma that is heated up to 20,000 degrees Kelvin."
What makes LAMBD different is that those molecules then collide with a pulse of gas, the identity of which is determined by the type of chemical product desired. For example, to create copper oxide, copper atoms that are ablated off a copper rod collide with oxygen.
The molecules are then pushed through a tiny nozzle, where they undergo a process that Garvey says is very similar to what happens when ice water is turned into snow in snow-making machines.
"When you pass ice water slurry through a hose, it's going from high pressure to low," he said. "That change in pressure causes the molecules to mix at a higher rate and to expand and cool, forming snow.
"In our system, the material that was ablated off the target mixes with the gas and expands as it passes through the nozzle, from high pressure to low pressure," he said. "When they get through the nozzle, the molecules physically separate, causing them to cool and to disperse into a spray, depositing material onto the substrate in a uniform layer."
Garvey noted that the technique removes from the fabrication process certain toxic precursors that otherwise are necessary in generating thin films.
For example, electronic wafers often must be coated with titanium nitride to act as a diffusion barrier. The process requires a very toxic precursor, requiring extensive safety and disposal equipment.
"With our technique, we simply blow nitrogen gas over a rod of titanium, depositing titanium nitride directly on the substrate," said Garvey. "The process results in a thin film of protective titanium nitrate without the use of toxic precursors."
So far, Garvey and DeLeon have produced a number of unique materials using LAMBD.
In collaboration with Paras Prasad, Ph.D., UB professor of chemistry, they have produced hybrid organic/inorganic films where an organic material with good optical characteristics was encapsulated in silica glass. These materials have photonics applications for new computer devices. The work was funded by the National Science Foundation's Division of Material Research.