Stress Effect Model in BSIM3v3 Model
Stress effect models are now implemented in major models such as BSIM4 or HiSIM. The need for evermore accurate models with a strong relation to technology is accute. Since BSIM3v3 is still a widely-used model and has not been totally replaced by its successor, an improvement was made to the model in SmartSpice to fulfill customers need for stress effect equations.
Background
The stress effect became important for simulation because of more and more shrinking processes. The smaller devices now require efficient isolation techniques. One of them is Shallow Trench Isolation (STI), mainly used with strain channel materials.
Shallow Trench Isolation is used to replace LOCOS, as shown on the schematic in Figure 1.
This particular process induces a mechanical stress on the device structure. Because of this behavior, device performance is related to the dimension of the active area, as well as the location of the device.
It has been shown that :
- Stress has an influence on mobility
- Saturation velocity is also modified
- Dopant diffusion during processing is modified, leading to different doping profiles. This implies a threshold voltage shift as well as changes in second-order effect such as Drain-Induced Barrier Lowering (DIBL) and body effect
Berkeley University considered that the effect of stress is due to two main mechanisms: mobility variation induced by changes in the band structure, and influence on threshold voltage because of different doping profiles as explained above.
Both of these mechanisms have the same dependence on 1/LOD (invert of the Length of Oxide Definition), but show a different trend with regard to width and length of the device.
