Copper/low-k process characterization for 90nm technology using SEM and TEM imaging
Artificial vertical and lateral shrinkage of different carbon-doped oxide (CDO) low-k materials is observed for electron beam exposure. The effect depends on SEM and TEM imaging conditions and sample preparation procedures. Typical artifacts include layer thinning and delamination of the CDO material from surrounding materials. TEM imaging is less critical compared to SEM. This is partly due to the favorable conditions during TEM sample preparation employing ion milling steps, which avoid the mechanical stress of conventional SEM sample cross-sectioning. Samples also absorb less energy from the electron beam during TEM scanning, which minimizes the impact of CDO shrink. Some artifacts are difficult to distinguish from real process issues, e.g. de-lamination caused by poor layer adhesion. Therefore, careful sample preparation and imaging are necessary to provide meaningful information.
14nm BEOL TDDB reliability testing and defect analysis
14nm BEOL (back end of line) TDDB (time to dielectric defect breakdown) test site structures successfully detect reliability defects but pose significant challenges in defect analysis At these advanced technology nodes, the reduction in copper land cross sectional area is accompanied by increased current density and electromigration failure rates. TDDB reliability test structures must be sensitive to capturing reliability defects. These same TDDB test site structures combined with porous ultra low-k (ULK) dielectric films represent real challenges in localizing and then determining BEOL reliability defects. Defect localization is difficult due to the complexity of these multiple metal layers along with the presence of the porous, low k dielectric films which exhibit shrinkage or void formation when exposed to an e-beam/FIB ion beam > 1 keV. Due to the porosity of these ULK dielectric films, they are especially susceptible to gallium ion implantation. It has been reported elsewhere that suppressing copper diffusion at the copper land/cap interface can be achieved by depositing a thin layer of CoWP and doping the copper seed layer with manganese [15, 16, 17]. However, a method for analytically confirming that these approaches for suppressing the copper diffusion do not affect TDDB performance/electromigration behavior must be demonstrated.
Electrical biasing and voltage contrast imaging in a focused ion beam system
We present two new techniques that enhance conventional focused ion beam (FIB) system capabilities for integrated circuit (IC) analysis: in situ electrical biasing and voltage contrast imaging. We have used in situ electrical biasing to enable a number of advanced failure analysis applications including (1) real time evaluation of device electrical behavior during milling and deposition, (2) verification of IC functional modifications without removal from the FIB system, and (3) ultraprecision control for cross sectioning of deep submicron structures, such as programmed amorphous silicon antifuses. We have also developed FIB system voltage contrast imaging that can be used for a variety of failure analysis applications. The use of passive voltage contrast imaging for defect localization and for navigation on planarized devices will be illustrated. In addition, we describe new, biased voltage contrast imaging techniques and provide examples of their application to the failure analysis of complex ICs. We discuss the necessary changes in system operating parameters to perform biased voltage contrast imaging.