To be effective, the on-chip energy storage device must be able to store a large amount of energy in a compact space and output it quickly. However, existing technologies cannot meet these requirements. Researchers are working to make electronic devices smaller and more energy-efficient by integrating energy storage directly onto microchips. This method minimizes the energy loss that occurs when transferring electrical energy between different device components.
A breakthrough in microcapacitor technology
Scientists at Lawrence Berkeley National Laboratory (Berkeley Laboratory) and the University of California, Berkeley, have taken an important step forward in overcoming these challenges, recently achieving record-high energy and high power density in microcapacitors. These capacitors are made from engineered films of hafnium oxide and zirconia, employing materials and manufacturing techniques commonly found in chip manufacturing. Their findings, published in the journal Nature, could revolutionize on-chip energy storage and power transmission for the next generation of electronics.
Sayeef Salahuddin, a senior scientist at the Berkeley Laboratory and a professor and project leader at the University of California, Berkeley, said: "We have demonstrated that it is possible to store large amounts of energy in microcapacitors made of engineered films, much more than ordinary dielectrics. What's more, the materials we use can be processed directly on a microprocessor. "
The research is part of a broader effort by the Berkeley lab to develop more efficient new materials and technologies for microelectronics.
Miniature capacitors made using engineered hafnium/zirconia film in three-dimensional trench capacitor structures – the same structure used in modern microelectronics – achieve record-high energy storage and power density, paving the way for on-chip energy storage. Photo credit: Nirmaan Shanker/Suraj Cheema
Capacitor Basics and Challenges
Capacitors are one of the basic components of an electrical circuit, but they can also be used to store energy. Unlike batteries, which store energy through electrochemical reactions, capacitors store energy through an electric field established between two metal plates separated by an insulating material. When needed, the capacitors can be discharged quickly, allowing for a quick power supply. In addition, capacitors do not deteriorate due to repeated charge/discharge cycles, so their life span is much longer than that of batteries. Capacitors typically have a much lower energy density than batteries, though, which means they can store less energy per unit volume or weight, and this problem only gets worse when trying to shrink them down to the size of a tiny capacitor for on-chip energy storage.
Saif Salahadin(左)和 Nirman Schenker 在实验室. 图片来源:Marilyn Sergeant/伯克利实验室
Research Methods and Results:
Researchers have created revolutionary microcapacitors by carefully designing HfO2-ZrO2 films to achieve a negative capacitance effect. Typically, layering one dielectric material on top of another will result in a reduction in overall capacitance. However, if one of the layers is a negative capacitance material, then the overall capacitance actually increases. In earlier research, Salah al-Din and colleagues demonstrated a way to produce transistors from negative-capacitive materials that operate at much lower voltages than conventional MOSFET transistors. Here, they use negative capacitance to produce capacitors that can store more charge and, therefore, more energy.
These films are made from a mixture of HfO2 and ZrO2 and are subjected to atomic layer deposition using standard materials and techniques for industrial chip manufacturing. Depending on the ratio of these two components, the film can be ferroelectric, i.e., the crystal structure has built-in polarization; It can also be antiferroelectric, i.e., the crystal structure can be polarized by applying an electric field. When the composition is adjusted just right, the electric field generated by charging the capacitor will bring the film to equilibrium at the critical point between the ferroelectric and antiferroelectric orders, and this instability can create a negative capacitive effect that can easily polarize the material even with a small electric field.
Suraj Cheema, a postdoctoral fellow in Salah al-Din's group and one of the paper's lead authors, said: "During the phase transition, the cells do want to be polarized, which helps to generate an additional charge under the action of an electric field. This phenomenon is an example of a negative capacitance effect, but it can be seen as a way to trap more charges than normal. "
In order to increase the energy storage capacity of the film, the research team needed to increase the thickness of the film without allowing it to relax into a frustrated antiferroelectric-ferroelectric state. They found that by interspersing thin layers of atomic-scale alumina after every few layers of HfO2-ZrO2, it was possible to increase the film thickness to 100 nanometers while maintaining the desired properties.
Finally, the researchers collaborated with collaborators at MIT's Lincoln Laboratory to integrate the film into a three-dimensional microcapacitor structure to grow precisely layered films in deep grooves cut on silicon wafers with up to 100:1 aspect ratios. These three-dimensional trench capacitor structures can be used with today's DRAM capacitors, which have a much higher capacitance per unit area compared to planar capacitors, allowing for greater miniaturization and design flexibility. The resulting devices have record-breaking characteristics: these miniature capacitors have a 9x higher energy density and a 170 times higher power density (80 mJ-cm-2 and 300 kW-cm-2, respectively) than today's best electrostatic capacitors.
"The energy and power density we get is much higher than we expected," Salah al-Din said. We have been developing negative capacitance materials for many years, but these results are quite surprising. "
Future directions
These high-performance microcapacitors help meet the growing demand for efficient, miniaturized energy storage for tiny devices such as IoT sensors, edge computing systems, and AI processors. Researchers are currently working to scale up the technology to integrate it into full-scale microchips and advance fundamental materials science to further increase the negative capacitance of these films.
"With this technology, we can finally start to seamlessly integrate extremely small size energy storage and power transmission on a chip," says Cheema. "It can open up new areas of microelectronic energy technology."
编译来源：ScitechDaily