Tiny capacitors integrated onto chip surfaces could make computing more energy efficient, extend the life of implanted medical devices like pacemakers, and help power small robots. Thanks to a materials-science trick, engineers made capacitors that store 9 times as much energy and provide 170 times the power in a given area. And these tiny but mighty capacitors can be made using the same materials and techniques already used to make DRAM.
Capacitors are made of dielectric materials and store energy in electric fields. They’re very durable and can provide high power levels and rapid recharging compared to batteries, which use chemical reactions to store energy. But the energy density (the amount of energy they can store in a given area) of capacitors is usually far lower than that of batteries. That makes scaling them down to chip sizes particularly challenging.
A team of engineers got around this limit by embracing some weird electronic properties that emerge in composite materials. They made a composite thin film of hafnium oxide and zirconium oxide, which exhibits spontaneous electrical polarization. Some regions are ferroelectric, with all dipoles pointing in the same directions, and others are antiferroelectric, with dipoles pointing in multiple directions, so that these regions can’t store charge. When an electrical field is applied to these materials, the antiferroelectric regions transition, becoming ferroelectric, and the film can store a huge amount of charge—much more than materials that are strictly ferroelectric.
This so-called negative capacitance effect means “you get way more charge storage,” says Suraj Cheema, a materials scientist at MIT. Cheema was part of a team that developed the new microcapacitor devices during his postdoc with Sayeef Salahuddin, a professor of electrical engineering and computer sciences at the University of California, Berkeley.
But negative capacitance alone isn’t enough to make a microcapacitor with high energy density—the layers are only 2 nanometers thick. The team had to figure out how to make these films thicker, while maintaining the unique crystal structure that underlies their negative capacitance. They were able to build up 100-nm-thick capacitors by layering in some amorphous aluminum oxide. This interrupting layer “hides” the structure of the dielectric materials from each overlying layer, ensuring that the correct crystal structure is maintained throughout the material.
To further increase the energy density of these devices without increasing their area, the researchers used a design common in today’s DRAM cell capacitors. These 3D structures are U-shaped trenches dug into the silicon chip’s surface. The design packs more charge-storing material in a given footprint. The trench capacitors can be made by atomic layer deposition (ALD). That technique is compatible with semiconductor manufacturing but would be hard to scale up to make larger capacitors for things like electric vehicles.
The microcapacitors can store 80 millijoules per square centimeter—only an order of magnitude less than a lithium-ion battery, says Cheema. But while microbatteries can be recharged only 1,000 times on the high end, these microcapacitors can be recharged billions of times. And they charge 100 million times faster, says Cheema.
Very impressive.
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