tailieunhanh - The Materials Science of Thin Films Episode 11

Tham khảo tài liệu 'the materials science of thin films episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 478 Electrical and Magnetic Properties of Thin Films reason that technological interest has centered on insulating films employed in microelectronics notably the gate oxide where dielectric breakdown is a serious reliability concern. The remainder of this section will therefore be devoted to SiO2 films Ref. 19 . It is generally agreed that electron impact ionization is responsible for intrinsic breakdown in SiO2 films. In this process electrons colliding with lattice atoms break valence bonds creating electron-hole pairs. These new electrons accelerate in the field and through repeated impacts generate more electrons. Ultimately a current avalanche develops that rapidly and uncontrollably leads to excessive local heating and dielectric failure. Typical of theoretical modeling Refs. 19 20 of breakdown is the consideration of three interdependent issues 1. Electrode charge injection into the insulator . by tunneling Eq. 10-22 . This formula connects current and applied field. 2. The local electric field that is controlled by the relatively immobile hole density through the Poisson equation. 3. A time-dependent change in hole density that increases with extent of impact ionization but decreases with amount of hole recombination or drift away. The resulting rate equation depends on both current and field. Simultaneous satisfaction of these coupled relationships leads to the prediction that the current-voltage characteristics display negative resistance. This appears as a knee in the response above a critical applied voltage and reflects a current runaway instability. Another prediction is that the average breakdown field rises sharply as the film thickness decreases. Reliability concerns for thin SiO2 films in MOS transistors have fostered much statistical analysis of life-testing results and some typical experimental findings include 1. The histogram of the number of breakdown failures due to intrinsic causes peaks sharply at about X 107 v cm Fig. 10-14a . The