The Effectiveness of OBIRCH Based Fault Isolation for Sub-90nm CMOS Technologies

摘要

The various experiments presented in this study have shown that OBIRCH based fault isolation in latest generation CMOS devices can be significantly enhanced by using a pulsed laser in combination with a lock-in amplifier. Using such set-up with appropriate settings for laser pulse frequency, scan speed and phase shift off-set, a tenfold S/N ratio gain is achieved. This improved S/N ratio permits to detect faulty circuitry with higher sensitivity and to isolate faults that can not be detected with the traditional OBIRCH set-up. But it is the combination of S/N ratio and spatial resolution improvements that makes the lock-in set-up really attractive. The higher spatial resolution is a well appreciated bonus of the altered heat dissipation kinetics that results from the pulsing of the laser. It is shown that the heat spreading is significantly reduced even with a 5 KHz pulse frequency, suggesting that thermal heat-up of the defects in the devices takes typically 10-100 μs. "Freezing" the laser energy in a small, confined area permits to improve the analytical sensitivity for buried defects and to obtain sharper, smaller and better defined spots. It is demonstrated that for optimal results with the lock-in setup it is necessary to be able to adjust the amplifier phase shift off-set and/or the laser pulse duty cycle. From our experiments it is believed that by monitoring the OBIRCH signal as a function of phase shift off-set or duty cycle, information can be obtained on the depth at which the defect is located. Finally, NAIL optics are shown to further improve spatial resolution and sharpness, especially when NAIL is combined with S/N ratio enhancing lock-in amplifier. The benefits of the pulsed laser, lock-in amplifier and NAIL optics have been demonstrated in several case studies on real 120-65 nm device structures. With these added features the OBIRCH technique is shown to be effective for fault isolation in 65nm technology, however, whether it will be sufficiently sensitive and accurate for the future 45 nm and beyond devices still remains to be seen.