Since the tech boom began decades ago, we’ve seen a dramatic transformation of electronics. Today, some technological dreamers are talking about “smart environments” where electronics are seamlessly integrated into our environment, providing comfort and convenience.
For these dreams to be achieved, we need to get electronics—not just the chips—miniaturized to the point where sensors can be pervasive. This involves developing high-performance electrochemical storage devices to enable long-lived sensors and radio frequency identification (RFID) tags. But efficient miniaturized energy storage devices have proven to be challenging to create; it can be done, but it's hard to integrate the results with other electronics.
According to an article in Science, an international team of scientists has now reported some progress in this area—specifically with the design of micro-supercapacitors. Supercapacitors are a class of materials that can store energy through accumulation of charge at the surface of a high-surface-area carbon sheet. They typically have a good cycle life, moderate energy density (6 Wh/kg), and high power densities (> 10 kW/kg). Supercapacitors are a great replacement for batteries in applications that require high power delivery and uptake with a very long charge-discharge cycle life; micro-supercapacitors are the same kind of material but much, much smaller.
Though a variety of materials have been explored for development of micro-supercapacitors, the processing techniques involved are not fully compatible with semiconductor device manufacturing processes. This means that you need separate hardware for energy storage, complicating any devices.
Scientists have overcome this obstacle through development of a new wafer-scale process that enables manufacturing of micro-supercapacitors right on the chip. In this new process, titanium carbide (TiC) coatings are deposited onto a silicon dioxide (SiO2) coated silicon wafer, producing carbon films. Through alteration of the deposition parameters, the resistivity, thickness, and mechanical strength of the TiC could be controlled. The team fabricated films up to 20 µm thick with controlled roughness.
The films were then treated with chlorine at 450 degrees Celsius in a furnace. This chlorination step transformed the films into carbide-derived carbon layers while releasing titanium tetrachloride (TiCl4). The process was fast—carbon layers grew at a rate of 1 µm/min. A cross-section of one of these structures revealed a 5µm-thick carbon layer covering a 1.3 µm-thick film of metallic, conducting titanium carbide.
This process is advantageous because this transformation of TiC into porous carbon was achieved without any of the materials detaching from their interfaces (TiC/SiO2 or carbon/TiC). As a result, the TiC/SiO2 interface remains identical after the chlorination step. The remaining TiC layer functions as an adhesive that is able to absorb mechanical stresses. It can also be used as a current collector.
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