A Case Study: Observation of Counter Doping of Gate Poly and Its Validation in High Density 90nm CMOS SRAM Bitcell

Threshold voltage (Vt) shift was measured, using atomic force probing (AFP) technique, in the pullup PFETs of high density SRAM bitcell arrays in 90nm CMOS bulk technology. This shift caused catastrophic yield loss. The direct measurements of dopant distribution both in plan view and x-section using Scanning Capacitance Microscopy (SCM) technique suggested counter doping of the P-poly had occurred. A single mask modification was shown to validate the observation and eliminated the counter doping resulting in drastic yield enhancement to about 60% from nearly no yield.