Before some metals can be incorporated into electronics and displays, they must first be evaporated and made into thin films.
But some metals are difficult.
Materials like platinum, iridium, ruthenium, and tungsten, for example, call for extremely high temperatures — over 2,000 degrees Celsius — before they can be converted into thin films.
A team at the University of Minnesota has found a way to work with the “stubborn” materials at lower temperatures so that they can be more easily integrated into today's electronics devices.
By adding combinations of carbon, hydrogen, and oxygen atoms to the hard-to-evaporate metals like tungsten and platinum, University of Minnesota Twin Cities researchers were able to transform the elements into thin films in a cheaper and safer way.
A common method for synthesizing metal films is a technique known as electron beam evaporation. The process melts metals at a high temperature, allowing a film to then form on top of wafers. The conventional method, however, is very expensive and uses a lot of energy, and a lot of voltage.
The University of Minnesota researchers evaporated these metals at significantly lower temperatures — fewer than 200 degrees Celsius instead of several thousands.
By designing and adding organic ligands — combinations of carbon, hydrogen, and oxygen atoms — to the metals, the researchers were able to substantially increase the materials’ vapor pressures, making them easier to evaporate at lower temperatures.
The new technique offers a greater scalability for metal thin films, according to the project's lead researcher.
"The ability to make new materials with ease and control is essential to transition into a new era of energy economy,” said University of Minnesota professor and senior author of the study Dr. Bharat Jalan , in a recent news release. (Read the full paper entitled, “Novel Synthesis Approach for “Stubborn” Metals and Metal Oxides” on the PNAS website .)
In a short Q&A with Tech Briefs below, Dr. Jalan explains more about how the team's breakthrough could lead to better electronics, and better cars.
Tech Briefs: What is so “stubborn” about the metals that you’ve been able to make into thin films, and how does your technique overcome that stubbornness?
Dr. Bharat Jalan: We have referred to the metals used as “stubborn” due to their ultra-low vapor pressures and low oxidation potentials. The former means that in typical physical vapor deposition process like evaporation, these metals require extremely high temperatures for evaporation, sometimes over 2000°C. The latter indicates that these metals are difficult to oxidize and, consequently, require strong oxidants to achieve the desired oxidation state.
Our technique overcomes these issues by supplying these metals bonded to organic ligands simply made up of carbon, hydrogen, and oxygen. These metal-organic precursors are sublimed, as they are solids, in-vacuum at temperatures as low as 100°C. This process makes the delivery of the metal simpler and less expensive than conventional methods like the electron-beam evaporation of the corresponding metal. With a suitable choice of ligands, the metal can also be supplied in a “pre-oxidized” state which makes the oxidation process more favorable. These two characteristics of the solid metal-organic precursor are key to the technique.
Tech Briefs: What inspired you to add carbon, hydrogen, and oxygen atoms to the metals?
Dr. Bharat Jalan: The use of organic ligands is not new — chemical vapor deposition processes widely use metal-organics as precursors. These metal-organic-based chemical and physical deposition techniques motivated us to benefit from their key characteristic of a higher vapor pressure than that of the metal. However, finding metal-organics with large enough vapor pressures for these techniques can be difficult, and it is for many of these metals. The key innovation was making use of solid metal-organics with more intermediate vapor pressures which can be used directly in-vacuum like traditional evaporation processes instead of requiring carrier gases.
Tech Briefs: How are these added? And in what amounts? This addition seems a bit complicated, no? What makes your technique simpler?
Dr. Bharat Jalan: The design and synthesis of these metal-organic precursors is an active field of research. In our case, the precursors we used were commercially available. Our technique which involves low-temperature sublimation of these metal-organic precursors is more cost-effective, less complicated, and safer than the conventional electron-beam evaporation. We also show great control over the deposition, atomic-layer control, as most molecular beam epitaxy approaches do, the technique that we have modified here. This process is capable of growing films one atomic layer at a time, and our process is no different.
Tech Briefs: In what applications do you imagine this technique being used and being most valuable?
Dr. Bharat Jalan: We imagine that the cost-effectiveness and simplicity of this technique will prompt its use in technological applications like catalytic converters and fuel cells that use platinum or as metallic electrodes of SrRuO3 for oxide electronics. We also envision that with the optimization of the growth parameters, this technique will further the fundamental study of the electromagnetic phenomena that are associated with some of these materials.
Tech Briefs: What’s next with this research?
Dr. Bharat Jalan: We plan to explore the thin film deposition and the electromagnetic behavior of a suite of materials with “stubborn” elements, particularly ruthenium and iridium, using this technique.
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