50 GHz Operation of RSFQ Arithmetic Logic Unit Designed Using the Advanced Design Flow and the Dual RSFQ/ERSFQ Cell Library

Arithmetic Logic Unit (ALU) is an integral part of digital signal processing applications and computing systems. We used ALU, based on Kogge-Stone adder, as a reference circuit to experimentally validate the advanced design flow and the dual RSFQ/ERSFQ standard cell library. Using the advanced design flow, the ALU sub-blocks were optimized across multiple process corners using Monte-Carlo simulations incorporating statistical variations in the process parameters. The correct operation of ALU was verified in the digital HDL simulation using static timing analysis pre-fabrication. We designed the RSFQ 1-bit and 4-bit ALU using the standard cell library approach for MIT-LL 100 µA/µm
²
SFQ5ee fab node. For the ERSFQ 1-bit ALU, the bias current overhead for the feeding JTLs was designed to be 100% of the ERSFQ DUT bias current. The RSFQ 1-bit ALU, designed for low frequency functional testing, demonstrated greater than ±15% bias margins. We demonstrated successful operation of the RSFQ 4-bit ALU with greater than ±6% bias margins at 20 GHz, greater than ±5% bias margins at 40 GHz, and greater than ±4% bias margins at 50 GHz clock frequency with BER less than 10
⁻¹²
. The ERSFQ 1-bit ALU worked up to 30.72 GHz clock frequency with BER less than 10
⁻¹²
. In addition, we analyzed the model-to-hardware correlation for the INIT sub-block of the ALU. The simulations accounted for critical current density (
J_c
) and sheet resistance (
R_s
) process parameters. We noted the discrepancy in simulated and measured margins and studied that a 15% higher
J_c
in simulations results in better model-to-hardware correlation for the INIT sub-block.