OBIST Method for Fault Detection in CMOS Complex Digital Circuits

Abstract

-The paper deals with an oscillation based built-in self-test (OBIST) technique to test faults in complex CMOS digital circuits (CCDCs). It focuses on stuck-at-faults, open or short faults, parametric gate delay faults. The method converts complex CMOS digital circuit under test (CCDCUT) to an oscillator and the output pulses are measured for fix time duration. Discrepancy in the number of pulses is used to judge circuits with catastrophic faults as well as parametric gate delays beyond the threshold limit which is set by designer. The advantage of this method is it does not need external test vector input or complex response analyzer. Universal gates and CCDCs are used to assess and substantiate the usefulness of proposed method. The simulation results show that the proposed method is quite proficient to improve diagnostic accuracy. Fault coverage for catastrophic faults is almost 100%. For ±10% deviation in parameters outside tolerance range limit 100% fault coverage is obtained. The yield loss is around 5% in the tolerance range limit. Keywords -Built-in self-test, catastrophic faults, complex CMOS digital circuit, complex CMOS digital circuit under test, fault-free complex CMOS digital circuit, oscillation based built-in self-test. I. INTRODUCTIONManufacturing very high speed CCDCs is the aftereffect of incessant advancement in semiconductor technology. The advantages of CCDCs are they need fewer MOSFETs and are fast due to single level circuits. The main desideratum of the manufacturer is to provide fault-free high performance circuits at low price. Faultfree chips are needed to reduce the IC cost which is essential to increase the product yield. In mixed signal ICs majority of circuits are digital and testing them is vital to improve yield. To ensure fault-free integrated circuits, a prototype is developed and tested vigorously before high-volume production [1]. In chips with high density of complex circuits, many internal sub-circuits are not accessible through the external input-output pins of IC. Such circuits can be tested by built-in self-test (BIST) which is fast and does not need advanced expensive automatic test equipments.OBIST method for analog circuits is discussed in [1]- [13]. Conversion of CMOS inverter into oscillator is discussed in [1]. In this paper it is used for testing CMOS complex digital VLSI circuits. The method proposes change in oscillation frequency to detect catastrophic as well as parametric faults present in the circuit. Analog as well as digital circuits are manufactured on the same wafer in case of mixed signal IC. To test mixed signal circuits is a costly and challenging process [1]. The method reduces area overhead and cost if it is used to test both analog as well as digital parts in a mixed signal chip.Quiescent current ( I DDQ ) testing approach and traditional approach of applying different input test vectors are used for testing digital circuits. These methods are used for easily detecting faults in simple digital circuits. These faults include, the catastrop...