To reduce the transistor size when SRAM cells are created using conventional planar transistors, IC manufacturers generally adjust properties by doping more impurities into the device area. However, this adjustment creates undesirable variability and deteriorates the SRAM stability. This issue is becoming critical, especially at the 22nm technology node and beyond. The use of FinFETs -- vertical transistors with fin-shaped undoped silicon channels -- is an alternative approach to allow SRAM cell size reduction with less characteristic variation.
Researchers from the three companies fabricated a highly scaled FinFET SRAM cell using HKMG. It is the smallest nonplanar-FET SRAM cell yet achieved: at 0.128μm2, the integrated cell is more than 50 percent smaller than the 0.274μm2 nonplanar-FET cell previously reported. To achieve this goal, the team optimized the processes, especially for depositing and removing materials, including HKMG from vertical surfaces of the non-planar FinFET structure.
The researchers also investigated the stochastic variation of FinFET properties within the highly scaled SRAM cells and simulated SRAM cell variations at an even smaller cell size. They verified that FinFETs without channel doping improved transistor characteristic variability by more than 28 percent. In simulations of SRAM cells of 0.063μm2 area, equivalent to or beyond the cell scaling for the 22nm node, the results confirmed that the FinFET SRAM cell is expected to offer a significant advantage in stable operation compared to a planar-FET SRAM cell at this generation.
By successfully fabricating highly scaled FinFET SRAM cells with HKMG, the companies have positioned FinFETs as an attractive transistor structure for SRAMs in the 22nm node and beyond. The new technology is a step forward to more powerful practical devices.