Rapid detection of charging damage with non-contact electrical analysis

As concerns about plasma damage have grown in recent years, so have efforts to detect problems in a timely fashion. Ideally, the electrical test structures used to detect plasma damage should closely mimic the design rules and routing of the final product. This enables them to detect device-specific electron shading or topography-dependent charging that might otherwise be missed. Recently, a short-loop technique was introduced that uses an oxidized, unpatterned test wafer as a plasma damage process monitor. The tool (a Keithley Quantox^R Oxide Charge Monitoring System) has been extensively compared to state-of-the-art test chip methods developed at the Texas Instruments R&D facility and a subset of the findings is presented here. With this tool, noncontact electrical testing is performed both before and after exposure to a plasma process. The change in oxide electrical properties is used to measure the amount of oxide damage. Shifts in flatband voltage (V_fb) and density of interface traps (D_it) quantitatively measure the creation of broken bonds at the Si-SiO₂ interface, while changes in oxide resistivity (ρ_ox) indicate the formation of bulk oxide defects. Less quantitative (but faster) methods such as surface voltage (V_s) mapping are also performed, and examples are given here of both the successes and failures of these high-speed techniques