Capacitors tweak the leak

A new method to stop current leaking from capacitors could vastly improve computer memory chips.

Researchers from Seoul National University and Ewha Woman’s University in Korea have significantly improved the performance of a capacitor used in computer memories.¹ Their new design includes an ultra-thin layer of aluminum-doped titanium dioxide to maximize capacitance while preventing current leaking from the capacitor.

The main type of memory in modern computers is called dynamic random access memory, or DRAM. Individual bits of information are stored in capacitors which hold either positive or negative charges corresponding to binary ‘1’ or ‘0’ states.

The capacitors in DRAM cells contain a layer of dielectric —insulator—which should be as thin as possible to maximize charge build up in the capacitor, but not too thin that it allows current to leak out of the capacitor.

Cheol Seong Hwang at Seoul National University has spent several years developing DRAM capacitors with a layer of titanium dioxide, which maintains strong dielectric properties even in the very thin film form. “The only trouble with titanium dioxide dielectric films is the high leakage current,” he says, “which can seriously degrade the charge storing capability of the capacitors.”

Titanium dioxide is an insulator because there is an energy gap between the highest stable electron energy level—the valence band—and the conduction band energy level that is required for free electron movement. However there are two possible ways that electrons can leak through the layer. Firstly, free electrons from the surrounding metal can gain enough energy to reach the titanium dioxide conduction band. Secondly, electrons in the valence band of the titanium dioxide can be excited to the same energy level as the metal electrons.

Fig. 1: Pictorial representation of energy bands in layers of titanium dioxide (TiO₂) and aluminum-doped titanium dioxide (Al-doped TiO₂. The aluminum doping moves the electron energy level in the surrounding platinum (Pt) to the midpoint between the conduction band (CB) and the valence band (VB) so that current leakage via either band is equally unlikely.Copyright © Cheol Seong Hwang 2008

The researchers showed that by adding aluminum to the titanium dioxide, the energy levels were adjusted so that the average electron energy in the capacitor was right at the midpoint between the conduction and valence bands (Fig. 1). This minimizes both types of current leakage.

The new DRAM capacitor is made by a cheap and easy process called atomic layer deposition and further improvements in capacitor performance will be made by modifying the crystal structure of titanium dioxide.