Tailoring Strain in Microelectronic Devices

The central device of this thesis is the transistor. It acts like a faucet, but then for electric charge. There is a connection that is called the source, just like the water company. And the charge flows into the drain. Finally there is a handle, here called the gate, to control the flow of charge. The transistor is not an ideal faucet for electrons. For example, even when the gate is closed a very small current of electric charge flows through the device. This is the leakage current. In many modern electronics integrated circuits are used which may contain more than a billion of these transistors. Even if only a small leakage current flows through each of these transistors this may sum up to an altogether large leakage current. This leakage current is responsible for the static, or standby, power consumption of integrated circuits. Nowadays this static power is becoming one of the major energy consumers in integrated circuits. One way to reduce the static power consumption the use of transistors that have an equal maximum flow of current, however, a smaller leakage current. This is the aim of the so-called small subthreshold swing transistors. Many different small subthreshold swing concepts exist. They can be differentiated by either the way the charge is transported through the device, or the way in which the gate controls the potential of the channel. In this thesis we contribute to the latter group by proposing an innovative concept in which the control of the gate over the channel is amplified with mechanical strain. When compared to the faucet it is like the addition of a mechanical gearbox between the control knob and the actual piston blocking the water. To understand the concept we have to explain the fin shaped field effect transistor (FinFET) first. This device has been recently introduced into mass production. It is as a fin shaped conventional field effect transistor (FET). The gate surrounds the complete fin and thus has an excellent control over the charge in the channel. As a result the FinFET is able to achieve a subthreshold swing of 60 mV/dec at room temperature, which is the limit for conventional transistors. In these fin shaped field effect transistors mechanical strain is permanently present which enhances the carrier mobility. We studied a typical fin shaped field effect transistor from NXP-TSMC Research. During the front-end-of-line integration of these transistors a thin layer of titanium-