High performance IC's have driven the semiconductor industry towards the sub-nanometer technology nodes for several years. At 16nm and beyond, the spatial resolution and sensitivity of some diagnostic equipment used for failure analysis have reached certain limitations. The accuracy of isolating a faulty signal in a tightly packed group of transistors in a die becomes more challenging. However, with the improvement of SIL (Solid Immersion Lens) based lens technology with higher N.A. (Numeric Aperture), combined with precision die thinning process, allowed some very promising results. This paper demonstrates successful diagnostic techniques utilizing the SIL lens and a variety of die thinning preparation techniques on 7nm and 16nm process nodes in both monolithic and 2.5D SSIT (Stacked Silicon Interconnect Technology) packages.
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.
The growing popularity of 2.5D SSIT (Stacked Silicon Interconnect Technology) & 3D package technology in the IC industry had made it more challenging for manufacturers and packaging assembly sites to perform failure analysis and identifying the root causes of failures. There had been some technical papers written on various failure analysis techniques on 2.5D SSIT and 3D IC packages using a variety of equipment for detecting and localizing failures [1, 2]. This paper explains a non-evasive, non-destructive approach of localizing failures on a 2.5D SSIT package by identifying and recognizing certain waveform patterns that the failing devices exhibit in the scanning acoustic microscope A-Scan and in Time domain reflectometry. There are noticeable waveform patterns that an analyst can recognize and used to determine certain types of failure mechanisms that may be present in the device. Please note that it is very important to use the exact same type of package sample when characterizing and comparing waveform patterns as package variability from vendor to vendor and material contents can certainly affect the results.
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