Over the last 40 years, the number of transistors per unit area and the speed at which they switch have doubled every 2 years. To achieve this, the transistors have been scaled down in each successive generation. Designing small MOSEFT transistors that still turn on and off properly at high speed is difficult because the various device parameters, such as junction depth, oxide thickness, and substrate doping density, that govern the transistor operation in precise portion must also be scaled down. For example, the gate oxide thickness should decrease with each technology node to improve the on-current of a transistor, which can charge the node capacitors more easily. However, as the gate oxide gets thinner, ensuring its reliability becomes increasingly difficult.
As transistors are used over long periods of time, the stress imparted to the thin gate insulators by applied voltages increases the probability of hard breakdown. At hard breakdown, an ohmic pathway exists across the gate dielectric and transistor function can be completely destroyed. Thus, operational lifetime of the devices will continue to be a major concern.
However, as operating voltages are reduced, the amount of power required to cause ohmic or hard breakdown may not be available. When a path of defects forms across a dielectric, but power to cause melting of silicon is not available, this is called soft breakdown. The resulting leakage across the dielectric is larger than the leakage across an undamaged dielectric, but smaller than the leakage resulting from hard breakdown.
In some cases, dielectric breakdown proceeds quickly from soft to hard breakdown. In other cases, the transistor remains in the soft breakdown condition and continues to function acceptably as a transistor. The operational lifetime for a transistor can be extended if oxide breakdown is kept soft and hard breakdown does not immediately ensue.
Dielectric breakdown is a statistical process and manufacturers use statistical reliability specifications. Therefore, the overall operational lifetime of a product can be extended if the operational lifetime of a majority of the components in a product is extended.
Accordingly, what is needed in the art is an electronics device and a process for making that device that extends the operational lifetime.