Michihiro Shintani

On-die parameter extraction from path-delay measurements

Device-parameter estimation through path-delay measurement, which facilitates fast on-die performance prediction and diagnosis, is proposed. With the proposed technique, delays of a set of paths consisting of different logic cells are monitored. Based on the pre-characterized parameter to delay sensitivity, the process variation of a chip is estimated as an inverse problem. Discussion of desirable logic cell combination to form paths that maximize estimation accuracy is presented. Measurement of ring oscillator arrays composed of standard and customized logic cells resulted in consistent estimation of threshold voltages. Measurement accuracy is greatly enhanced by the proposed good logic cell combinations.

An Adaptive Test for Parametric Faults Based on Statistical Timing Information

The continuing miniaturization of LSI dimension is causing the increase of process-related variations which significantly affects not only its design turn around time but also its manufacturing yield. Statistical static timing analysis (SSTA) is expected as a promising way to estimate the performance of circuits more accurately considering delay variations. However, LSIs designed using SSTA may have higher probability of parametric faults than the ones designed with deterministic timing analysis. In order to test these parametric faults, effective extraction techniques of critical paths are needed. In this paper, we discuss a general trend between the delay margin of LSIs designed by SSTA and their parametric fault ratio. Then we propose an adaptive test flow for parametric faults using statistical static timing information, and a concept of parametric fault coverage. Experimental results demonstrate the effectiveness of our approach.

Path clustering for adaptive test

Adaptive test is one of the most efficient techniques that practically ensure high yield and reliability of designed chips. In this paper, a novel path-clustering method suitable for the adaptive test, in which test paths are altered according to the monitored process-parameters, is proposed. Considering the probability function of the die-to-die systematic process variation, the proposed method clusters path sets so that the total number of test-paths are minimized. For quantitative evaluation of different clusterings, figure of merit for clustering, which represents the expected number of test-paths at a particular test coverage, is also proposed. The proposed clustering is experimentally evaluated by applying to an industrial circuit. With our clustering, the average test paths in the adaptive test have been reduced to less than 50% compared with the ones of the conventional test.

Small Delay Fault Model for Intra-Gate Resistive Open Defects

We propose the fault model considering weak resistive opens inside the gate which might cause pattern-sequence-dependent and timing-dependent malfunction of the circuit. We assume the fixed observation interval for the signal transition, and derive the minimum resistance of intra-gate resistive opens to be detected as a fault by SPICE simulation. Based on the simulation results, we establish three fault models, that is, the one considering the location of the resistance, the one considering both the location and the resistance distribution, and the simplified one where str and stf faults considering the signal transition of the input ports are assumed. The coverage calculation for the primitive gates and small benchmark circuit reveals that the proposed models have more accuracy on the detection of weak open defects.

A simulation model for SiC power MOSFET based on surface potential

In this paper, we propose a surface-potential-based simulation model of SiC power MOSFETs for accurate circuit simulation. By considering physical structure and behavior of vertical power SiC MOSFETs, the proposed model reproduces static and dynamic characteristics upon wide range of bias voltages. Through experiments using a commercial SiC power MOSFET, good agreements have been observed between measurement and simulation in I-V, C-V characteristics. Good match in transient behavior beyond 1 MHz is also confirmed.

A Variability-Aware Adaptive Test Flow for Test Quality Improvement

In this paper, we propose a process-variability-aware adaptive test flow that realizes efficient and comprehensive detection of parametric faults. A parametric fault is essentially a malfunction in a large-scale integration chip, which is caused by the variability in fabrication processes. In our adaptive test framework, test pattern sets are altered on individual chips in order to apply the optimal set of test patterns for each chip, and thus the test coverage is improved and the test time is reduced. The test pattern is chosen on the basis of parameter estimations measured using an on-chip sensor with respect to statistical timing information. We also propose a novel metric to quantize the test coverage suitable for evaluating the test quality of parametric faults. Our experimental results using an industrial design show that the proposed flow significantly improves the parametric fault coverage and test efficiency compared to conventional test flows.

Nonlinear delay-table approach for full-chip NBTI degradation prediction

As technology further scales semiconductor devices, aging-induced device degradation has become one of the major threats to device reliability. Hence, taking aging-induced degradation into account during the design phase can greatly improve the reliability of the manufactured devices. However, considering instance-dependent Vth degradations for extremely large circuits, like processors, is time-consuming. In this research, we focus on the negative bias temperature instability (NBTI) as the aging-induced degradation mechanism, and propose a fast and efficient way of estimating NBTI-induced delay degradation by utilizing static-timing analysis (STA) and simulation-based lookup table (LUT). Probability-based delay model (PBDM) for each gate is characterized in advance, having input slews, output capacitances and signal probabilities as the table indices. Using the PBDM modeled, path delays of arbitrary circuits can be efficiently estimated. With a typical five-stage pipelined processor as the design target, by comparing the calculated delay from LUT with the reference delay calculated by a commercial circuit simulator, we achieved 4114 times speedup within 5.6% delay error.

Runtime NBTI Mitigation for Processor Lifespan Extension via Selective Node Control

Negative bias temperature instability (NBTI) has become one of the major reliability concerns for nanoscale CMOS technology. The NBTI effect degrades pMOS transistors by stressing them with negatively biased voltage, while the transistors heal themselves as the negative bias is removed. In this paper, we propose a cross-layer mitigation technique for NBTI-induced timing degradation in processors. The NOP (No Operation) instruction is replaced by a custom NOP instruction for healing purpose. Cells that are likely to be stressed under negative bias are classified and their upstream cell will be replaced by the internal node control (INC) logics. Upon encountering a custom NOP instruction, the INC logics will force the NBTI-stressed cell to be in its healing mode. The optimal INC logic insertion through genetic programming approach achieves much greater delay mitigation of 44.3% than prior works in a 10-year span with less than 4% of power and negligible area overhead.

A high power curve tracer for characterizing full operational range of SiC power transistors

A curve tracer is proposed for measuring static characteristics of power devices at high voltage and large current range. Using a SiC-MOSFET as a switch for pulse-based measurement, high voltage tolerance and fast switching are simultaneously achieved. The proposed curve tracer facilitates current-voltage measurements for full I-V regions found in practical device operations. The measurement results provided by the proposed method contribute to build device models that can be used to design efficient power converters.

Workload-Aware Worst Path Analysis of Processor-Scale NBTI Degradation

As technology further scales semiconductor devices, aging-induced device degradation has become one of the major threats to device reliability. In addition, aging mechanisms like the negative bias temperature instability (NBTI) is known to be sensitive to workload (i.e., signal probability) that is hard to be assumed at design phase. In this work, we analyze the workload dependence of NBTI degradation using a processor, and propose a novel technique to estimate the worst-case paths. In our approach, with careful examination, we exploit the fact that the deterministic nature of circuit structure limits the amount of NBTI degradation on different paths, and proposes a two-stage path extraction algorithm to identify the invariable critical paths in the processor. Through numerical experiment on a MIPS32 processor, we performed a detailed signal probability analysis, and successfully extracted 85 invariable critical paths out of the 24,978 path candidates, achieving nearly 300× reduction in the sheer number of paths.