Author:
Ally Winning, European Editor, PSD
Date
06/22/2024
PDF
Microcapacitors made with engineered hafnium oxide/zirconium oxide films in 3D trench capacitor structures achieve record-high energy storage and power density, paving the way for on-chip energy storage.
As we try to pack more transistors into smaller packages, delivering power across the chip becomes increasingly difficult. To get around this problem, researchers have been attempting to integrate energy storage directly onto the chips, which will potentially reduce losses. However, for on-chip storage to be effective, it has to be able to store a large amount of energy in a very small space and deliver it quickly.
Among the scientists trying to achieve this goal, a team of researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley have achieved record-high power densities in microcapacitors. The new microcapacitors were fabricated from thin films of hafnium oxide and zirconium oxide, using materials and fabrication techniques already prevailent in chip manufacturing.
Capacitors can be discharged rapidly, delivering power quickly, and unlike batteries they do not degrade with repeated charge-discharge cycles. Their main drawback is that they have lower energy density than batteries, and that problem gets worse when they are shrunk to microcapacitor size for on-chip energy storage.
The researchers got around that issue by using a negative capacitance effect. Usually, layering one dielectric material on top of another results in an overall lower capacitance. However, if one of the layers is a negative-capacitance material, then the overall capacitance increases. The same researchers have used negative capacitance materials in the past to produce transistors that can be operated at lower voltages than conventional MOSFETs.
The new microcapacitors use a mix of HfO2 and ZrO2 films that have been grown by atomic layer deposition. The ratio of the two materials affects whether the films are ferroelectric, where the crystal structure has a built-in electric polarization, or antiferroelectric, where the structure can be changed into a polar state by the application of an electric field. When the composition is right, the electric field created by charging the capacitor balances the films at the tipping point between ferroelectric and antiferroelectric behaviour, resulting in a negative capacitance effect where the material can be very easily polarized by even a small electric field.
To increase the energy storage capability of the microcapacitors, the film had to be made thicker without it changing from that antiferroelectric-ferroelectric balance. Interspersing atomically thin layers of aluminum oxide after every few layers of HfO2-ZrO2 allowed the researchers to grow the films up to 100 nm thick while still retaining the desired properties.
Along with partners at MIT’s Lincoln Laboratory, the researchers 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. These 3D trench capacitor structures are also used in today’s DRAM capacitors and can achieve much higher capacitance per unit footprint compared to planar capacitors. The properties of the resulting microcapacitor devices have nine-times higher energy density and 170-times higher power density (80 mJ-cm-2 and 300 kW-cm-2, respectively) than today’s best microcapacitors.
The findings from the research were published in the journal Nature.
