Researchers at Cornell University have modified a household microwave oven to demonstrate how microwave annealing – as an alternative to conventional rapid thermal annealing – could overcome a major fabrication challenge faced by the semiconductor industry. Producing the materials that make up transistors and other microchip components is similar to baking in that material ingredients must be mixed together and then heated, among other steps, in order to produce a desired electrical current.
For instance, phosphorus is added to silicon and then the mixture is annealed, or heated, to position the phosphorus atoms into the correct place so that they are active in current conduction. But as microchips continue to shrink, the silicon must be doped, or mixed, with higher concentrations of phosphorus to produce the desired current.
Semiconductor manufacturers are now approaching a critical limit in which heating the highly doped materials using traditional methods no longer produces consistently functional semiconductors, say the researchers.
“We need concentrations of phosphorus that are higher than its equilibrium solubility in silicon. That goes against nature,” says James Hwang, a research professor in the Department of Materials Science and Engineering. “The silicon crystal expands, causing immense strain and making it potentially useless for electronics.”
The Taiwan Semiconductor Manufacturing Company (TSMC) had theorized that microwaves could be used to activate the excess dopants, but just like with household microwave ovens that sometimes heat food unevenly, previous microwave annealers produced “standing waves” that prevented consistent dopant activation. So TSMC partnered with Hwang, who modified a microwave oven to selectively control where the standing waves occur. Such precision allows for the proper activation of the dopants without excessive heating or damage of the silicon crystal.
This discovery, says Hwang, could be used to produce semiconductor materials and electronics appearing around the year 2025. Hwang has filed two patents for the prototype microwave annealer with postdoctoral researcher Gianluca Fabi.
“A few manufacturers are currently producing semiconductor materials that are three nanometers,” Hwang says. “This new microwave approach can potentially enable leading manufacturers such as TSMC and Samsung to scale down to just two nanometers.”
The breakthrough could change the geometry of transistors used in microchips, say the researchers. For more than 20 years, transistors have been made to stand up like dorsal fins so that more can be packed on each microchip, but manufacturers have recently begun to experiment with a new architecture in which transistors are stacked horizontally as nanosheets that can further increase the density and control of transistors. The excessively doped materials enabled by microwave annealing would be key to the new architecture.