Sub-nanometer fabrication processes and advanced packaging solutions such as 2.5D stacked silicon interconnect technology (SSIT) facilitate the production of high-performance ICs, but make physical failure analysis and debugging more difficult. For example, at 16nm, most diagnostic tools reach their limitations in terms of spatial resolution and signal sensitivity and require complex modifications and adjustments. In addition, a higher level of precision and uniformity is required for sample preparation. This paper describes a fault isolation technique that combines solid immersion lens (SIL) technology with precision die thinning. Two failure analysis case studies are presented to demonstrate the method, one a low level negative current leakage failure caused by ESD testing, the other a scan chain failure traced to the input of a delay buffer circuit. In both cases, success is attributed to the resolution and sensitivity of the SIL lens and the ability to precisely control die thickness.

Conductive anodic filament (CAF) formation is a mechanism caused by an electrochemical migration of metals from a metal trace in ICs or in PCBs. This is commonly caused by the moisture build-up in the affected metal terminals in an IC package or PC board caused by critical temperature, high humidity and high voltage gradients conditions. This phenomenon is known to have caused catastrophic field failures on various OEMs electronic components in the past [1,7]. Most published articles on CAF described the formation of the filament in a lateral formation through the glass fiber interfaces between two adjacent metal planes [1-6, 8-12]. One common example is the CAF formation seen between PTH (Plated through Hole) in the laminated substrate with two different potentials causing shorts [1-6, 8-12]. In this paper, the Cu filament grows in a vertical fashion (z-axis formation) creating a vertical plane shorts between the upper and lower metal terminals in a laminated IC package substrate. The copper growth migration does not follow the fiber strands laterally or vertically through them. Instead, it grows through the stress created gaps between the impregnated carbon epoxy fillers from the upper metal trace to the lower metal trace with two different potentials, between the glass fibers. This vertical CAF mechanism creates a low resistive short that was sometimes found to be intermittent in nature. This paper presents some successful failure analysis approaches used to isolate and detect the failure locations for this type of failing devices. This paper also exposes the unique physical appearance of the vertical CAF formation.