Built-in miniaturized micro-supercapacitor powers silicon chip

Replaces bulky batteries in wearable electronics, mobile internet-of-things (IoT) devices, and autonomous sensor networks
June 8, 2016

In-chip porous silicon-titanium nitride supercapacitor. (a) Scanning electron microscopy (SEM) inset of the trenches separating the electrodes (dark gray). (b) Rotated schematic illustration of the cross-section of two opposite electrodes of a device (titanium nitride-coated porous silicon layer with aluminum contact pads on the back side), with electrolyte shown in orange. (c) Higher-magnification SEM picture of the porous silicon regions. (d) Device trench side. (e) metallization side containing aluminum contacts for electrodes. (f) 3D illustration of two atomic-layer-deposition cycles of titanium-nitride growth. (credit: adapted from Kestutis Grigoras et al./Nano Energy)

Finnish researchers have developed a method for building highly efficient miniaturized micro-supercapacitor energy storage directly inside a silicon microcircuit chip, making it possible to power autonomous sensor networks, wearable electronics, and mobile internet-of-things (IoT) devices.

Supercapacitors function similar to standard batteries, but store electrostatic energy instead of chemical energy.

The researchers at VTT Technical Research Centre of Finland have developed a hybrid nano-electrode that’s only a few nanometers thick. It consists of porous silicon coated with a titanium nitride layer formed by atomic layer deposition.

The nano-electrode design features the highest-ever conductive surface-to-volume ratio. That combined with an ionic liquid (in a microchannel formed in between two electrodes), results in an extremely small form factor and efficient energy storage. That design makes it possible for a silicon-based micro-supercapacitor to achieve higher energy storage (energy density) and faster charge/discharge (power density) than the leading carbon- and graphene-based supercapacitors, according to the researchers.

The micro-supercapacitor can store 0.2 joule (55 microwatts of power for one hour) on a one-square-centimeter silicon chip. This design also leaves the surface of the chip available for active integrated microcircuits and sensors.

Micro-supercapacitors can also be integrated directly with active microelectronic devices to store electrical energy generated by thermal, light, and vibration energy harvesters to supply electrical energy (see, for example, Wireless device converts ‘lost’ microwave energy into electric power).

An open-access paper on the research has been published in Nano Energy journal.

Abstract of Conformal titanium nitride in a porous silicon matrix: A nanomaterial for in-chip supercapacitors

Today’s supercapacitor energy storages are typically discrete devices aimed for printed boards and power applications. The development of autonomous sensor networks and wearable electronics and the miniaturization of mobile devices would benefit substantially from solutions in which the energy storage is integrated with the active device. Nanostructures based on porous silicon (PS) provide a route towards integration due to the very high inherent surface area to volume ratio and compatibility with microelectronics fabrication processes. Unfortunately, pristine PS has limited wettability and poor chemical stability in electrolytes and the high resistance of the PS matrix severely limits the power efficiency. In this work, we demonstrate that excellent wettability and electro-chemical properties in aqueous and organic electrolytes can be obtained by coating the PS matrix with an ultra-thin layer of titanium nitride by atomic layer deposition. Our approach leads to very high specific capacitance (15 F cm−3), energy density (1.3 mWh cm−3), power density (up to 214 W cm−3) and excellent stability (more than 13,000 cycles). Furthermore, we show that the PS–TiN nanomaterial can be integrated inside a silicon chip monolithically by combining MEMS and nanofabrication techniques. This leads to realization of in-chip supercapacitor, i.e., it opens a new way to exploit the otherwise inactive volume of a silicon chip to store energy.