A 7nm FinFET SRAM using EUV lithography with dual write-driver-assist circuitry for low-voltage applications

SRAM plays an integral role in the power, performance, and area of a mobile system-on-a-chip. To achieve low power and high density, extreme ultraviolet (EUV) technology is adopted for the 7nm FinFET technology [3-4]. Conventional ArF immersion with a single exposure for an extreme high-resolution patterning shows the limitation of lithographic patterning. Therefore, multi-patterning lithographic technique is applied to support a high-resolution lithography. However, this also includes process variations due to using multi-pattering masks. Alternatively, EUV offers competitive scaling with a single-mask with the benefit of smaller wavelength, which provides smaller process variation with less additional pattering. Figure 11.2.1 shows a 7nm EUV FinFET 6T high-density (HD) SRAM bitcell with an area of 0.026μm². The pull-up, pass-gate, and pull-down ratios are 1:1:1 for high-density and low-power applications. Another benefit of EUV technology also features a bi-directional metal layer with a scaled pitch that provides an extra degree of freedom for signal and power routing. Figure 11.2.2 highlights EUV benefits in accordance with bi-directional metals. A uni-directional metal layer requires different metal layer to connect two nets, and have no choice but to support the limited via between two perpendicular metal lines with the limited metal width. A wider metal allows placement of more vias between the metal lines, but it does not demonstrate optimum Power, Performance, and Area (PPA) with redundant parasitic capacitance. However, EUV provides bi-directional metal lines, where the different layers of metal are coherent in the same direction. Therefore, more vias can be placed to reduce the IR-drop with smaller parasitic capacitance and resistance. Figure 11.2.2 illustrates the delay impact versus stacked-via distance in a standard cell array. It shows that the timing penalty is directly proportional to the stacked via distance in a uni-directional metal routing.