Abstract Various detector chips in optocoupler devices have a thin indium tin oxide (ITO) film deposited over the passivation. This transparent, conductive film is found over the photodetecting area of the die. When this film is electrically connected to ground potential through contacts, it acts as a shield to avoid inversion failures by sinking any charge buildup to ground. In order to perform a full electrical failure analysis on these optocoupler detector chips, the ITO layer must be removed. An extensive search found numerous papers on etching this film over glass substrates, but no known technique was found to selectively remove the ITO layer on a packaged die. This paper discusses an approach to remove this film using an argon gas etch technique. The ideal characteristics of any process used to remove this film on a finished die would be to completely etch the ITO layer, electrically isolating it from ground, while leaving the underlying passivation and metallization fully intact. This would allow for further electrical failure analysis of the die without causing additional damage or affecting the failure mechanism. The results of an experiment using various chemical and gas etchants found that an argon gas etch would remove the ITO layer while only slightly etching the phosphosilicate glass (PSG) passivation beneath. Electrical failure analysis of the die continued at this point, and a subsequent buffered oxide etch (BOE) removed the remaining passivation, leaving the exposed metallization and oxide completely intact. This technique has been used successfully on device failures to find passivation contamination shorting the aluminum metallization to the ITO film.

Abstract Thermal laser signal injection microscopy (T-LSIM) (aka TIVA and OBIRCH) has shown considerable promise in stateof- the-art digital integrated circuits. The technique has been utilized to locate shorts, leakage currents, problem vias, and timing issues in these devices. However, little has been published on the utility of this technique for analog and mixed signal devices. In this paper we demonstrate the application of T-LSIM on two different analog devices with defects that conventional FA technology and fault isolation techniques were unable to locate. Analog devices produce several unique challenges to the basic T-LSIM technique as typically utilized in the digital regime. Extensions of the basic T-LSIM technique were utilized to locate the failures, which produced unexpected results. The T-LSIM technique has proved essential in the quick identification and localization of failure sites. The T-LSIM technique provides the failure analyst with the analytical power not previously available on conventional fault isolation tools such as emission microscopy and liquid crystal.