Meaning
Thermal treatment conducted immediately after the formation of a dielectric layer modifies the chemical structure and reduces the density of thin-film defects. This thermal process, known as post deposition annealing, is employed for improving the dielectric quality in modern integrated circuits. It is performed at elevated temperatures in controlled atmospheres such as nitrogen, oxygen or formyl gas.
The operation stabilizes the material and establishes the required interface qualities for the gate stack. Proper optimization of the thermal budget ensures that the dielectric achieves high performance without degrading the underlying semiconductor substrate.
Atomic Reconstruction
Atomic rearrangement is promoted by exposing the newly deposited dielectric film to high temperatures. During post deposition annealing, the density of the film increases as sub-stoichiometric oxides are converted into a more uniform molecular structure. This densification reduces the leakage current density of the dielectric layer under high electric fields.
Defect Passivation
Chemical reaction with the annealing gas passivates dangling bonds and neutralizes charge trapping centers. In cases where the gas contains oxygen, post deposition annealing repairs oxygen vacancies that would otherwise lead to electron traps. This chemical repair reduces the slow-trap density, thereby improving the breakdown field of the oxide.
Process Integration
Careful temperature budget management represents a constraint when planning this thermal step within a complete fabrication flow. Since excessive temperatures can cause unwanted dopant diffusion or silicide formation, the post deposition annealing must be carefully scheduled before low-temperature metallization steps. The process parameters are verified using sweep measurements and capacitance-voltage curves during in-line wafer monitoring.
