Sorin Dobre

Standard cell characterization considering lithography induced variations

As VLSI technology scales toward 65nm and beyond, both timing and power performance of integrated circuits are increasingly affected by process variations. In practice, people often treat systematic components of the variations, which are generally traceable according to process models, in the same way as random variations in process corner based methodologies. Consequently, the process corner models are unnecessarily pessimistic. In this paper, we propose a new cell characterization methodology which captures lithography induced gate length variations. A new technique of dummy poly insertion is suggested to shield inter-cell optical interferences. This technique together with standard cells characterized using our methodology will let current design flows comprehend the variations almost without any changes. Experimental results on industrial designs indicate that, our methodology can averagely reduce timing variation window by 8%-25%, power variation window by 55% when compared to a worst case approach. For an industrial low power design, over 300ps reduction on the path delay variation is obtained by using cells characterized according to our methodology

Mixed Cell-Height Implementation for Improved Design Quality in Advanced Nodes

In advanced nodes, standard-cell libraries can be developed with different cell heights (e.g., in FinFET technology, corresponding to different numbers of fins). Larger cell heights provide higher drive strengths, but at the cost of larger area and power consumption as well as pin capacitance. Cells with smaller heights are relatively smaller in area, but have weaker drive strengths and are more likely to suffer from routing congestion and pin accessibility issues. Existing design methodologies and tool flows are able to mix cells with different heights at the block level (i.e., each block contains cells of a particular cell height). To our knowledge, no design methodology in the literature mixes cells of different heights in a fine-grained manner. In this work, we propose a novel physical design optimization flow to implement design blocks with mixed cell heights in a fine-grained manner. Our optimization resolves the “chicken-and-egg” loop between floorplan site definition and the optimized choices of cell heights after placement. Comprehending the constraints and costs of mixing cells of different heights (e.g., the “breaker cell” area overheads of row alignment between sub-blocks of 8T and 12T cell rows), our optimization achieves 25% area reduction versus 12T-only implementation while maintaining the same performance, and 20% performance improvement versus 8T-only implementation while maintaining similar total cell area.

Resonance-aware methodology for system level power distribution network co-design

Power delivery network (PDN) design continues to be a major challenge because it demands a good portion of available silicon, package, and board routing resources. In this paper, we outline a frequency and time domain co-design flow that uses frequency domain results to construct time domain input vectors, resulting in a resonance aware time domain analyses flow that can highlight low and mid frequency behaviors dominated by board and package components and parasitics.

Improved signoff methodology with tightened BEOL corners

To ensure functional correctness, conventional chip implementation methodology signs off the SOC design at extreme process, voltage and temperature (PVT) conditions. At the 20nm node and beyond, the back end of line (BEOL) layers have become major sources of variation, which must be accounted for by signoff at various BEOL corners. Conventional signoff methodology uses extreme BEOL corners, in which all BEOL layers are skewed to the worst-case condition (e.g., all BEOL layers have the worst parasitic capacitance). However, such a BEOL condition is very pessimistic because the probability of having all BEOL layers skew towards the worst-case condition simultaneously is extremely small. Such pessimism results in longer chip implementation schedules and poorer design quality. In this paper, we propose a signoff methodology with tightened BEOL corners to recover the pessimism incurred by the conventional BEOL corners. This approach is based on the observation that most timing-critical paths use different BEOL layers. When the variations of BEOL layers are not fully correlated, the BEOL-induced timing variation is much smaller due to averaging of random variations. Our experimental results show that by using tightened BEOL corners, we can reduce timing-violation paths by up to 100% and improve the WNS and TNS by up to 101ps and 53ns, respectively.