Portland, Ore. - A transmission electron-microscope procedure from IBM Corp. lets researchers create real-time videos of liquid deposition processes to explore their mechanisms at work. The experimental technique allows researchers to quantify electrodeposition fine-tuning instead of depending on trial and error.
Before the IBM breakthrough, nucleation, growth and coalescence in electrodeposition processes could only be observed indirectly, by measuring the current transient and analyzing with electrochemical models. But orders of magnitude of difference were found between the parameters obtained from models of current-transient analysis and those confirmed by post-growth microscopy, IBM said.
"There are a lot of reactions which take place in a liquid environment," said Frances Ross, a researcher at the Thomas J. Watson Research Center (Yorktown Heights, N.Y.) and winner of this year's Burton Medal for contributions to microscopy. "For instance, our technique will be useful for engineers who want to make videos of materials being deposited on the electrodes of rechargeable batteries."
The Burton Medal is awarded annually by the Microscopy Society of America to a scientist who is not yet 40 years old and who has made distinguished contributions to microscopy and microanalysis.
"I received the Burton Medal this week, because I am under 40, but certainly my most important research culminates in our most recent work at IBM, which is observing liquid processes in real-time, such as electrodeposition of copper for interconnects," said Ross.
Much to their frustration, semiconductor engineers perfecting copper-on-silicon processes have had to rely on slow imaging techniques, such as atomic-force microscopy, which takes as long as 30 seconds per still image. Fast acquisition methods that work only at step edges have been developed, but they run at only a few frames per second, too slow for electrodeposition, which occurs in milliseconds. In addition, they do not work for the three-dimensional growth of copper on silicon, IBM said.
Now, using Ross' imaging technique, chip engineers can make 30-frame-per-second movies of the three-dimensional growth of copper on silicon using a conventional transmission electron microscope.
When chip makers went from aluminum to copper interconnects, they found the best way to deposit copper was not sputtering or evaporation but electrodeposition-a liquid process in which an entire wafer is immersed in a bath of copper sulfate and sulfuric acid and voltage is applied. The copper grows in both the trenches and on the surface of the wafer. Later, a mechanical polishing step removes all copper except for that in the trenches.
"Engineers have discovered many ways to optimize copper growth, using various chemical additives to the copper sulfate and sulfuric acid bath, but even now it's not clear how they work. . . . That's what gave us the motivation to do our experiment," said Ross.
By directly observing additives under various conditions and parameter settings, Ross hopes to come up with confirmed rules that tell engineers what it is about the surface that causes copper to begin growing there.
"With that knowledge we may be able to design surfaces that have nucleation sites in the places where we want them, so that we have perfect control of the film that gets grown," said Ross.
IBM's current experiment had been meant only to prove the concept of the company's new real-time observation methodology, but researchers have already discovered things about the electrodeposition of copper that were unknown before.
"Conventional models would have us think that there are preferred nucleation sites," Ross said. "So what we thought would happen is if we watched where some of those sites were on the real-time video, then reversed the current to remove the copper, and did it again, we thought that the copper would start growing at the same places again. But what we found was that there are many different places where nucleation can begin to grow an island, and not all are successful, because they are all competing to get the copper instead of it being obvious which places the copper should grow. This is something that has not been put into the models of copper electrodeposition before, because until now there haven't been any demonstrations that this was what happens."
While theorists scramble to integrate this new observation into existing models of electrodeposition, Ross is forging ahead with new discoveries. Encouraged, IBM researchers have already begun characterizing all aspects of copper electrodeposition on silicon. For instance, they measured the rate of growth of the islands and found that the larger they are, the faster they grow.
"We are very encouraged, because basically the first experiment we did showed us something we didn't know before," said Ross. "Next we are going to try depositing copper on crystals that have various very particular defects in their structure, in an attempt to see what kind of sites foster nucleation," said Ross.
So far the team members have only looked at the early stages of copper growth-nucleation and the first few seconds of growth-so now they are looking to extend their observations to how islands coalesce to form a continuous film. They are also looking to quantify every step of the process by evolving the model with confirmed observations.
"When we# see the copper growing, we can also measure how much current is flowing, so we have an average measure of how much copper ought to be growing, because to deposit each atom of copper you need to add one electron, so at each step along the way we can relate how much copper is growing with how much current is flowing," said Ross.Observation window
To repeat IBM's experiment, engineers have to make a sandwiched observation window especially for their application, but the electron microscope used is conventional, so potentially Ross's procedure for making videos of real-time molecular processes in liquid phase can be performed at any university or commercial lab.
"Our invention should be useful to many outside the field of electronics too, such as biologists trying to observe biological materials in action. Today they have to dehydrate biological samples before observing them, which not only eliminates the possibility of real-time observations, but significantly changes the structure of the materials," said Ross.