Physical Design Interview Guide

The net with double width will generally have less delay.

Reasoning (RC Delay): Wire delay is primarily determined by the product of its resistance (R) and capacitance (C).

Resistance (R): Resistance is inversely proportional to the cross-sectional area of the wire. For a fixed thickness (T), resistance is inversely proportional to width (W). R ∝ 1 / (W * T). Doubling the width (W -> 2W) approximately halves the resistance (R -> R/2).

o Netlist vs. Layout: LVS clean. If design has shorts, it results in inaccurate extraction for those nets. So LVS clean is preferred.

SPEF File Validity: Check if any issues or critical warning in extraction flow. If there are ignored nets during extraction, paths with those nets will not have accurate timing.

Other consistency chek like Lib and SDC, no missing constraints.

Annotation Check (report_annotated_delay/check): After reading SPEF/SDF, verify that parasitic delays and checks have been successfully annotated onto the design nets and cells. Look for warnings about unannotated elements.

o Register-to-Register (Reg2Reg): Starts at the clock pin of a launch flip-flop/latch and ends at the data input pin (e.g., D) of a capture flip-flop/latch. Both launch and capture elements are controlled by related clocks (often the same clock). This is the most common type of path analyzed for setup/hold within a synchronous design.

Input-to-Register (In2Reg): Starts at a primary input port of the design and ends at the data input pin of a sequential element. Constrained by set_input_delay.

o Netlist:

Timing Libraries (.lib**,** .db**):**

Parasitics Information (SPEF/DSPF/RSPF/SBPF):

Timing Constraints (SDC - Synopsys Design Constraints):

Operating Conditions/MMMC file: Implicitly defined via the library corners selected, but sometimes explicitly set (e.g., specific voltage/temperature for analysis if not tied directly to a library corner).

(Optional) Power Intent (UPF/CPF): If the design has multiple power domains, the UPF/CPF file is needed to understand which supply nets power which cells, enabling power-aware STA.

o Via Pillar: A structure used in advanced process nodes (like FinFET nodes) to create a lower-resistance vertical connection between metal layers compared to traditional single vias or via arrays. It typically consists of:

Stacked, short metal segments (“fingers”) on intermediate metal layers, aligned vertically.

These segments are connected by vias above and below, essentially forming a “pillar” of alternating via and short metal bar segments running vertically through multiple layers.

oOCV analysis (like AOCV/POCV or simple derating) applies different delay values for ’early’ (fast) and ’late’ (slow) conditions.

For a setup check, the launch clock path uses late delays, and the capture clock path uses early delays. For a hold check, it’s reversed.

However, both clock paths often share a common segment starting from the clock root before diverging. Applying both early and late derates simultaneously to this same physical common path introduces artificial pessimism because the common path cannot physically be both fast and slow at the exact same instant.