Applications of SEM in Semiconductor Device Failure Analysis

Failure analysis (FA) is a crucial methodology employed by engineers during the development of semiconductor devices, such as the CPUs in smartphones and laptops. This process utilizes various analytical tools to pinpoint the root cause of specific failure modes, thereby preventing future occurrences. Failure modes refer to the ways in which a device fails to meet certain electrical, mechanical, or visual specifications.

These failure modes can be detected during the manufacturing process if a component fails an inline quality control test and is measured as "out-of-spec." When a significant number of units are affected, the manufacturing yield decreases, prompting FA to identify the causes and explore potential solutions. Additionally, failure modes can be discovered during reliability testing, where FA is used to determine the physical origins of electrical failures, such as delamination, buried cracks, or other physical defects.

Why is SEM essential for semiconductor FA?

High Resolution: SEM provides high-resolution imaging, allowing engineers to observe fine details of a semiconductor's surface and structure at the micro and nanoscale. This is critical for identifying defects that are not visible with traditional optical microscopy.

Depth of Field: SEM offers a greater depth of field compared to optical microscopes. This means more of the sample is in focus at one time, providing a clearer and more comprehensive view of the surface features.

Elemental Analysis: Equipped with Energy Dispersive X-ray Spectroscopy (EDS or EDX), SEM can perform elemental analysis of the sample. This is crucial for identifying contamination or compositional variations that may contribute to device failure.

Topographical and Morphological Insights: SEM can produce detailed topographical maps of the sample surface, revealing the morphology and texture of materials. This helps in understanding how physical features correlate with failure modes.

Failure Localization: SEM can pinpoint the exact location of failures such as cracks, voids, delamination, and other structural anomalies. This localization is vital for correlating failures with specific process steps or material properties.

Non-destructive Testing: SEM allows for non-destructive examination of samples, meaning that the same sample can be analyzed multiple times or subjected to further testing.

Versatility: SEM can be used to study a wide range of materials and structures within semiconductor devices, from metals and insulators to complex multilayered structures. This versatility makes it a valuable tool in FA.

Figure 1: SEM images of a wire-bond package assembly acquired on a Phenom ProX Desktop SEM using the BSD detectors.Image source: nanoScience

Figure 2: SEM images of a wire-bond package assembly acquired on a Phenom ProX Desktop SEM using the SED (middle) detectors.Image source: nanoScience

Figure 3: SEM images of a wire-bond package assembly acquired on a Phenom ProX Desktop SEM using the EDS (right) detectors.Image source: nanoScience

Scanning electron microscopy (SEM) is a standard tool used in FA not only because of its resolution and depth of field advantages but also because it can provide a comprehensive view of the sample using multiple detectors. The backscattered electron detector (BSD) and secondary electron detectors (SED) that are used to acquire images in the SEM provide complementary views of composition and surface topography, respectively. Another advantage of SEM is the ability to perform elemental analysis using an energy-dispersive X-ray (EDS) detector. EDS mapping is a great tool for detecting trace levels of contamination that can be correlated with structural characteristics.

What are some examples of how SEM can help with FA?

SEM is a powerful and versatile FA tool that is used for physical and compositional microanalysis. Some examples of how SEM is routinely used in semiconductor FA include:

Surface Inspection

SEM can be used to identify failure modes including cracks, delamination, or contamination. Since the escape depth of secondary electrons is close to the surface (~2-5 nm in metals), the SED is useful for visualizing surface features. If the SEM has a segmented BSD, then qualitative topographic images can be generated by applying signal subtraction to the top/bottom or left/right halves. Topographic imaging mode can highlight features that may be more difficult to distinguish using the SED, allowing for acute identification of physical defects that impact device functionality. All Phenom Desktop SEM models come standard with a segmented BSD that contains four quadrants.

Elemental Analysis

When combined with SEM imaging, EDS is highly valuable for semiconductor FA. EDS makes use of the X-rays emitted from the sample when it is irradiated with the scanned electron beam to identify the elemental composition of the sample. Using EDS analysis software, characteristic X-rays are identified, and the composition (in terms of atomic percentages or weight percentages) is calculated. By analyzing the elemental composition of the component materials, it is possible to identify impurities, alloy compositions, or any variation that may affect device performance or reliability.

Imaging of cross-sections

Semiconductor packages are often cross-sectioned at the suspected location of failure to reveal internal structures of interest. These samples are specifically prepared for SEM and EDS analysis. Some common examples of failures found via cross-sectioning are metallization issues, bridged bumps, interfacial delamination, voids in adhesives or underfill, and inconsistent metal plating.

Electron backscatter diffraction (EBSD)

EBSD is an SEM technique used to analyze the crystallographic orientation and microstructure of semiconductor and metallic materials. By mapping the crystal orientation and grain boundaries, EBSD can assist in identifying causes of corrosion or fracture in thin films, interconnects, and metal trace lines within semiconductor devices.

Particle analysis

SEM can readily identify and analyze the size, shape, and composition of individual particles that may have caused shorting or mechanical damage to a device. Understanding the composition and other characteristics of foreign particles can help in identifying the root cause of contamination, whether it be from upstream processing, handling/storage media, or some other cause.

Electrical Characterization

SEM can be combined with other techniques, such as focused ion beam (FIB) milling, to create electrical contacts on microscopic device features. The Phenom Desktop SEM is compatible with an Electrical Feedthrough Sample Holder, which has six quick-release pins that can be used to measure or apply voltages and currents to a sample during SEM imaging. These methods enable in-situ electrical testing and analysis of failed devices, providing insights into electrical failures such as opens, shorts, or leakage currents.

Summary

By leveraging these capabilities, SEM is indispensable in failure analysis within the semiconductor industry. It aids in identifying failure mechanisms, enhancing device reliability, and informing design and process improvements.