Special Session: Panel: Variation-aware analyzes of Mega-MOSFET Memories, Challenges and Solutions
ABSTRACT
Designing large memories under manufacturing variability requires statistical approaches that rely on SPICE simulations at different Process, Voltage, Temperature operating points to verify that yield requirements will be met. Variation-aware simulations of full memories that consist of millions of transistors is a challenging task for both SPICE simulators and statistical methodology to achieve accurate results. The ideal solution for variation-aware verifications of full memories would be to run Monte Carlo simulations through SPICE simulators to assess that all the addressable elements enable successful write and read operations. However, this classical approach suffers from practical issues and prevent it to be used. Indeed, for large memory arrays (e.g. MB and more) the number of SPICE simulations to perform would be intractable to achieve a descent statistical precision. Moreover, the SPICE simulation of a single sample of the full-memory netlist that involve millions or billions of MOSFETs and parasitic elements might be very long or impossible because of the netlist size. Unfortunately, Fast-SPICE simulations are not a palatable solution for final verification because the loss of accuracy compared to pure SPICE simulations is difficult to evaluate for such netlists. So far, most of the variation-aware methodologies to analyze and validate Mega-MOSFETs memories rely on the assumption that the sub-blocks of the system (e.g. control unit, IOs, row decoders, column circuitries, memory cells) might be assessed independently. Doing so memory designers apply dedicated statistical approaches for each individual sub-block to reduce the overall simulation time to achieve variation-aware closure. When considering that each element of the memory is independent of its neighborhood, the simulation of the memory is drastically reduced to few MOSFETs on the critical paths (longest paths for read or write memory operation), the other sub-blocks being idealized and estimations being derived under Gaussian assumption. Using such an approach, memory designers avoid the usual statistical simulations of the full memory that is, most of the time, unpractical in terms of duration and load. Although the aforementioned approach has been widely used by memory designers, these methods reach their limits when designing memory for low-power and advanced-node technologies where non idealities arise. The consequence of less reliable results is that the memory designers compensate by increasing security margins at the expense of performances to achieve satisfactory yield. In this context sub-blocks can no longer be considered individually and Gaussianity no longer prevails, other practical simulation flows are required to verify full memories with satisfying performances. New statistical approaches and simulation flows must handle memory slices or critical paths with all relevant sub-blocks in order to consider element interactions to be more realistic. Additionally, these approaches must handle the hierarchy of the memory to respect variation ranges of each sub-block, from low sigma for control units and IOs to high sigma for highly replicated blocks. Using a virtual reconstruction of the full memory the yield can be asserted without relying on the assumptions of individual sub-block analyzes. With accurate estimation over the full memory, no more security margins are required, and better performances will be reached."
