ENERGY STORAGE Microcapacitors to offer on-chip power supply

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Do you know that capacitors could become as small as a transistor integrated into a chip? Two scientists have fabricated “microcapacitors” on a chip with exceptional energy storage and power densities. Check out the article to learn about the record-breaking technology expected to offer on-chip power solutions and compete with lithium-ion batteries.

Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley published a nature study and called their experiment a “microcapacitor”. Most people think of capacitors as standalone small electronic components, mostly in cylindrical shape. However, capacitors can be integrated into a semiconductor chip. In such a case, a capacitor becomes a “microcapacitor”!

Capacitors use insulating dielectric materials to create electric fields and store energy through electrostatic charge instead of chemical energy in batteries. It enables capacitors to charge quickly. However, the energy storage density of capacitors is relatively lower than that of batteries. Smaller batteries store the same charge compared to bigger capacitors.

Conventionally, energy storage devices trade off energy storage density for power density. The amount of power a storage device can deliver relative to its physical size is called the power-to-size ratio. Ideally, the power-to-size ratio must be higher for capacitors or any energy storage device.

In simple words, increasing charge storage capacity decreases the speed of charge/discharge and vice versa. Microcapacitors solve this problem by showcasing a balanced approach with higher values of energy storage and power densities. As a result, microcapacitors have improved power-to-size ratio.

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How were microcapacitors built?

The foundation of the microcapacitor lies in its thin film construction. This section explores the materials and techniques used to create this critical component.

Thin-films

Initially, there is a 2 nm ultra-thin film— it is exactly what they call a “microcapacitor”. The ultra-thin film consists of a composite material made from hafnium dioxide and zirconium dioxide. These two compounds are dielectric materials.

An amorphous aluminum oxide layer is added to maintain the crystal structure of the microcapacitor in between different layers. In addition, this layer is important to scale up storing capabilities. The final structure has a thickness of about 100 nm.

The thin film is integrated into a silicon-based chip using the ALD (Atomic Layer Deposition) method. The microcapacitor fabrication procedure is compatible with all classic fabrication methods. A similar method is used in DRAM memory as well.

The superlattice

The microcapacitor structure represents a ‘superlattice’- an alternating layer structure of thin materials. The alternating layer structure reduces overall capacitance. This superlattice structure reduces film-related thickness limitations and increases total energy storage more than a single-layer construction would.

Antiferroelectric to ferroelectric

The integrated thin film is antiferroelectric in nature. These layers are engineered close to the field-driven ferroelectric phase transition state. In simple words, the antiferroelectric layers are engineered to a point where they can turn into ferroelectric to enhance intrinsic energy storage.

Both antiferroelectric and ferroelectric materials are used in memories and capacitors. Antiferroelectric materials have aligned electric dipoles in opposite directions, resulting in no net polarization. On the other hand, ferroelectric materials exhibit spontaneous polarization in the absence of an electric field.

A ferroelectric material has a high dielectric constant to allow capacitors to store more charge. The phase change into a ferroelectric state enables the microcapacitor to exhibit a negative capacitance effect. A negative capacitance effect is a phenomenon where reduced applied voltage can store substantial amounts of charge.

Antiferroelectric materials are compatible with manufacturing processes. The antiferroelectric to ferroelectric phase transition is possible by adjusting the hafnium dioxide and zirconium dioxide ratio in the composite film. Simply put, the antiferroelectric phase transition to ferroelectric amplifies the charge storage capacity and energy storage density of the microcapacitor.

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3D trench structures

The antiferroelectric superlattices are engineered into 3D trench capacitor structures to increase energy storage per footprint. The thin films grow in the deep trenches cut into silicon- A process used in modern DRAM cells. The shape of the energy storing area becomes a three-dimensional structure instead of planar.

Result

Energy storage density = 80 mJ/cm²

Power density = 300 kW/cm²

Boosting the area-specific and volumetric energy storage density with power density capabilities does not mean taking up more space on a chip. As a result, microcapacitors become more efficient and compact.

These microcapacitors were able to achieve area-specific energy storage density about nine times that of traditional capacitors. Similarly, microcapacitors achieved a record-breaking power density of about 170 times that of best-known traditional capacitors.

How can microcapacitors shape the energy storage industry?

The energy stored by a microcapacitor on a chip is called on-chip energy. Unlike “big” energy storage systems, on-chip energy needs to deliver power over a small area. The on-chip power supply gives hope for lesser losses and faster charging solutions for applications. In addition, there is no risk of chemical leakage like batteries.

Applications

Beyond their impressive performance on a chip, microcapacitors hold immense promise for a wide range of applications. These applications span various sectors, from powering miniature electronics and boosting memory performance to potentially revolutionizing energy storage in electric vehicles and enabling advanced Internet of Things (IoT) functionalities.

• Micro supercapacitors and micro batteries in EVs.

• On-chip power supply to FPGAs, processors, and peripherals.

• As mentioned above, DRAM already uses capacitors. Integrating microcapacitors could improve DRAM performance.

• Consumer electronics: smartphones, laptops, tablets, wearables, and home appliances.

• Medical devices: Pacemakers, insulin pumps, and health monitors.

• IoT (Internet of Things): Remote sensors and actuators

• 5G Telecommunications.

• Robotics, especially miniature robots.

• Industrial power electronics applications: UPS (Uninterrupted Power Supply), energy management systems, motor drivers, power equipment, etc.

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Experiment Summary

Scientists meticulously crafted these groundbreaking microcapacitors through a series of innovative steps.

• Scientists used a 2 nm ultra-thin film made from a composite material consisting of hafnium dioxide and zirconium dioxide.

• An additional amorphous aluminum oxide layer was added to increase the thickness to 100 nm.

• The alternating layers of hafnium dioxide and zirconium dioxide created a structure called a “superlattice”.

• Superlattice eliminates thickness limitations and improves the total energy storage capacity of the microcapacitor.

• The integrated thin film is antiferroelectric.

• Thin film is engineered close to the ferroelectric phase transition state.

• Phase transition increases intrinsic energy storage capacity and negative capacitance.

• The most important point in summarizing the whole experiment is negative capacitance.

• Negative capacitance is a counterintuitive effect that stores a greater amount of charge at a lower voltage.

• The thin film is engineered into 3D trench capacitor structures to increase energy storage per footprint.

• Microcapacitors were able to achieve area-specific energy storage density of about nine times and a power density of about 170 times that of traditional capacitors.

References

• newscenter.lbl.gov

• www.nature.com

• www.allaboutcircuits.com

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