ABSTRACT
CONTENTS 16.1 Introduction ...................................................................................................................... 465 16.2 Defect Generation and Dielectric BD............................................................................ 468
16.2.1 Monitoring Defect Generation during Electrical Stress Experiments ......... 469 16.2.2 Physics of Defect Generation and BD.............................................................. 473
16.2.2.1 Thermochemical Model of Oxide BD............................................... 474 16.2.2.2 AHI Model............................................................................................ 475
16.2.3 Relationship between Defect Generation and Breakdown Statistics................................................................................... 476
16.3 HR Model of Defect Generation and BD ..................................................................... 480 16.4 Conclusions....................................................................................................................... 490 Acknowledgments ..................................................................................................................... 491 References.................................................................................................................................... 491
The focus of this chapter is the breakdown (BD) of the thin SiO2 films used as gate insulators in metal-oxide-semiconductor (MOS) field-effect transistors (FETs). This includes the nitrided silicon oxides that have replaced pure SiO2 for ultrathin gate oxide applications because the incorporation of nitrogen allows a reduction in the gate leakage current. Although the main concepts presented in this chapter might also (at least partially) apply to the BD of the high-K dielectrics required to further proceed with MOS technology downscaling, we will not explicitly deal with these types of dielectrics. In the process of reducing the transistor dimensions, the gate oxide thickness (TOX) has been reduced from hundreds of nanometers to roughly 1 nm. Here, we will deal with oxides thinner than 10 nm, with emphasis on ultrathin oxides with TOX< 3 nm. Specification of the thickness range is important because the phenomenology associated with defect generation and BD changes with TOX. When an oxide is subjected to a constant-voltage (CV) stress, leakage current flows
through the oxide by tunneling. If the stress continues for a certain time, the current is found to evolve due to the continuous degradation of the oxide (generation of defects in the oxide bulk and the interfaces), and finally a current jump reveals the occurrence of the
dielectric BD. This is illustrated in Figure 16.1 for a 6.2 nm oxide. Analogously, if the stress is performed under constant current conditions, the BD signature is an abrupt voltage drop. The abruptness of the BD event tends to disappear in the case of ultrathin dielectrics, and this introduces serious difficulties not only for the BD detection but also for its definition [1,2]. Figure 16.2 shows an example of a nonabrupt BD event measured in a p-channel FET (PFET) with TOX¼ 1.25 nm. In general, we can define the BD as a local loss of the insulating properties of the dielectric. Whether this loss is abrupt or progressive depends mainly on the oxide thickness and the stress conditions.
