|M.Sc Student||Rodes Yael|
|Subject||Investigation and Optimization of MIM Silicon Nitride|
Capacitors Embedded in Cu BEOL Technology
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Moshe Eizenberg|
|Dr. Eitan Shauly|
|Full Thesis text|
A Metal-Insulator-Metal (MIM) capacitor is commonly used in analog mixed-mode and RF integrated circuits due to its ability to provide high accuracy in analog signals. For many years silicon nitride film, which is deposited using a plasma-enhanced chemical vapor deposition (PECVD) method, was used as the metal-insulator-metal capacitor dielectric. However, to the best of our knowledge a comprehensive optimization work, considering electrical performance such as leakage current and voltage coefficient of capacitance (Vcc) was never done.
This study focuses on improving the electrical performances of a MIM capacitor, which has a silicon nitride dielectric and is embedded in a copper (Cu) back-end-of-line (BEOL), by controlling the structure and composition of the silicon nitride.
Controlling the structure and composition of the silicon nitride was achieved by changing the SiH4 and NH3 flow rates and the NH3/SiH4 flow rates ratio inside the PECVD chamber, during silicon nitride deposition process.
A linear dependence of the N/Si ratio values on the deposition rates is reported. This dependence shows a significant increase in N/Si ratio towards stoichiometric composition as the deposition rate decreases. Therefore, this study presents for the first time, the ability to create a significant change of the silicon nitride film stoichiometry, by deposition rate tuning alone. In addition, an increase in the silicon nitride film density when the deposition rate decreases was demonstrated.
A significant increase in the silicon nitride’s dielectric constant and in the capacitor’s failure threshold voltage was achieved by increasing the film densities and N/Si ratio values. This increase in the film densities and N/Si ratio values was achieved by reduction in precursors’ flow rates followed by lower deposition rates.
Leakage current reduction by more than one order of magnitude was accomplished using deposition optimization. The lowest leakage currents were achieved by using the silicon nitride film with the highest density. This film was created by applying the lowest deposition rate. This study proposes a model for controlling the leakage currents by deposition rates tuning. A high density film will show a lower leakage current compared to a lower density film at the same electric field, due to lower density of initial traps and lower rate of traps generation under stress.
To the best of our knowledge, this was the first time a control of the capacitance dependence on voltage was achieved by deposition rates tuning. The dependence of the capacitance on voltage was reduced by more than 5% with deposition rate decrease.
The main conclusion of the study is that improved electrical performances of MIM capacitor embedded in Cu BEOL, in terms of capacitance density, failure threshold voltage, leakage current and voltage linearity, can be achieved by applying low flow rates of SiH4 and NH3 and low NH3/SiH4 flow rates ratio during silicon nitride deposition in the PECVD system.