Defect Simulation Methodology for iDDT Testing

The International Technology Roadmap for Semiconductors (ITRS) identifies two main challenges associated with the testing of manufactured ICs. First, the increase in complexity of semiconductor manufacturing process, physical properties of new materials, and the constraints imposed by resolution of lithography techniques etc., give rise to more complex failure mechanisms and hard-to-model defects that can no longer be abstracted using traditional fault models. Majority of defects, in today's technology, include resistive bridging and open defects with diverse electrical characteristics. Consequently, conventional fault models, and tools based on these models are becoming inadequate in addressing defects resulting from new failure mechanisms. Second, the defect detection resolution of main-stream IDDQ testing is challenged by significant elevation in off-state quiescent current and process variability in newer technologies. Overcoming these challenges demands innovative test solutions that are based on realistic fault models capable of targeting real defects and thus, providing high defect coverage. In prior works power supply transient current or iDDT testing has been shown to detect resistive bridging and open defects. The ability of transient currents to detect resistive opens and their insensitivity (virtually) to increase in static leakage current make iDDT testing all the more attractive. However, in order to integrate iDDT based methods into production test flows, it is necessary to develop a fault simulation strategy to assess the defect detection capability of test patterns and facilitate the ATPG process. The analog nature of the test observable, i.e., iDDT signals, entail compute intensive transient simulations that are prohibitive. In this work, we propose a practical fault simulation model that partitions the task of simulating the DUT (device under test) into linear and non-linear components, comprising of power/ground-grid and core-logic, respectively. Using divide-and-conquer strategy, this model replaces the transient simulations of power/ground-grid with simple convolution operations utilizing its impulse response characteristics. We propose a path isolation strategy for core-logic as a means of reducing the computational complexity involved in deriving iDDT signals in the non-linear portion. The methodology based on impulse response functions and isolated path simulation, can enable iDDT fault simulation without having to simulate the entire DUT. To our knowledge, no practical technique exists to perform fault simulation for iDDT based methods. The proposed fault simulation model offers two main advantages, first, it allows fault induction at geometric or layout level, thus providing a realistic representation of physical defects, and second, the current/voltage profile of power/ground-grid, derived for iDDT fault simulation, can be used to perform accurate timing verification of logic circuit, thus facilitating design verification. In summary, the proposed fault simulation framework not only enables the assessment of defect detection capabilities of iDDT test methodologies, but also establishes a platform for performing defect-based testing on practical designs.