Trench microcapacitor for chip power
Researchers in the US have developed a microcapacitor that can be integrated into chips as a 3D trench to provide power.
The team at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have achieved record-high energy and power densities in a microcapacitor made with engineered thin films of hafnium oxide and zirconium oxide.
This uses materials and fabrication techniques already widespread in chip manufacturing to make a microcapacitor with nine times higher energy density and 170 times higher power density at 80 mJ/cm2 and 300 kW/cm2, respectively.
“We’ve shown that it’s possible to store a lot of energy in a microcapacitor made from engineered thin films, much more than what is possible with ordinary dielectrics,” said Sayeef Salahuddin, the Berkeley Lab faculty senior scientist and UC Berkeley professor who led the project. “What’s more, we’re doing this with a material that can be processed directly on top of microprocessors.”
The thin films of HfO2-ZrO2 are engineered to achieve a negative capacitance effect. Normally, layering one dielectric material on top of another results in an overall lower capacitance. However, if one of those layers is a negative-capacitance material, then the overall capacitance actually increases.
Previous work demonstrated the use of negative capacitance materials to produce transistors that can be operated at substantially lower voltages than conventional MOSFET transistors. Here, the used the negative capacitance to store larger amounts of charge.
The crystalline films are made from a mix of HfO2 and ZrO2 grown by atomic layer deposition, using standard materials and techniques from industrial chip fabrication. Depending on the ratio of the two components, the films can be ferroelectric, where the crystal structure has a built-in electric polarization, or antiferroelectric, where the structure can be nudged into a polar state by applying an electric field.
When the composition is tuned just right, the electric field created by charging the capacitor balances the films at the tipping point between ferroelectric and antiferroelectric order, and this instability gives rise to the negative capacitance effect where the material can be very easily polarized by even a small electric field.
“That unit cell really wants to be polarized during the phase transition, which helps produce extra charge in response to an electric field,” said Suraj Cheema, a postdoc in Salahuddin’s group. “This phenomena is one example of a negative capacitance effect but you can think of it as a way of capturing way more charge than you normally would have.” Nirmaan Shanker, a graduate student in Salahuddin’s group, is co-lead author.
To scale up the energy storage capability of the films, the team needed to increase the film thickness without allowing it to relax out of the frustrated antiferroelectric-ferroelectric state. Interspersing atomically thin layers of aluminum oxide after every few layers of HfO2-ZrO2 allowed the films to be up to 100 nm thick while still retaining the desired properties.
Researchers at the MIT Lincoln Laboratory integrated the films into three-dimensional microcapacitor structures, growing the precisely layered films in deep trenches cut into silicon with aspect ratios up to 100:1.
“The energy and power density we got are much higher than we expected,” said Salahuddin. “We’ve been developing negative capacitance materials for many years, but these results were quite surprising.”
The microcapacitor could be used for Internet-of-Things sensors, edge computing systems, and artificial intelligence processors. The researchers are now working on scaling up the technology and integrating it into full-size microchips, as well as pushing the fundamental materials science forward to improve the negative capacitance of these films even more.
“With this technology, we can finally start to realize energy storage and power delivery seamlessly integrated on-chip in very small sizes,” said Cheema. “It can open up a new realm of energy technologies for microelectronics.”
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