Terahertz radiation for inspection and fault detection has been of interest for the semiconductor industry since the first generation and detection of THz signals. Until recent hardware advances, THz systems lacked the signal quality and reliability for use as an effective nondestructive testing (NDT) method. Incremental advances in THz sources, detectors, and signal processing resulted in the successful applied-industrial use of THz NDT techniques on carbon fiber laminates, automotive coatings, and for detection of counterfeit pharmaceutical tablets. Semiconductor inspection and verification methods ensure the functionality and thereby safety of vital electronics for several critical industries. For this reason, the reliability and verification of a THz NDT method must exceed currently used inspection systems. With recent laboratory access to THz radiation, THz inspection methods are often compared with existing optical, electrical, and volumetric semiconductor verification techniques for their production monitoring and failure analysis viability. This review will cover THz techniques and their applications at the printed circuit board (PCB), integrated circuit (IC), and transistor/gate scales. The THz radiation gap spans between optical and electronic ranges with a millimeter-sized wavelength allowing for adequate penetration of plastic and ceramic and semiconductor materials. THz radiation can be used to determine structural features, electrical signatures in the THz range, and chemical information simultaneously. Cost and environmental limitations restricted the ability for THz NDT semiconductor inspection methods to escape the lab and succeed in the dynamic environment of a semiconductor fabrication environment. Hybridized metrology methods incorporating information from multiple inspection tools are a regime where THz spectral and structural data can be combined with existing methods such as optical, x-ray, or E-beam. THz can be used initially to offer support to the complex failure analysis and verification requirements of the semiconductor industry from nanoscale to macroscale features and components. For THz systems to become independent inspection tools used for semiconductor production monitoring, in the lab or fab, this will require a confident level of statistical process control for THz signal generation, detection, or processing. Applied industrial semiconductor device inspection will likely be a result of a combination of research into THz hardware, reconstruction techniques, and the widespread application of machine learning techniques. Many breakthroughs occurred over the years to enable successful nondestructive characterization and inspection of semiconductor devices from the nanoscale transistors to fully packaged integrated circuits and assembled PCBs.

Charge-injection devices (CIDs) are versatile detectors having a number of features which recommend them for use in the imaging of X-ray diffraction patterns. They have a flexible nondestructive readout allowing for analysis of image quality during data collection and rapid readout of selected portions of the device. CIDs have full-well capacities in the range of 10(6) charge carriers giving them a high dynamic range for both direct and indirect imaging of X-rays. CIDs have peak quantum efficiencies in the optical region over 50% allowing for their incorporation into indirect detection systems. Rapid random single-pixel address allows for their use as single X-ray photon counters with energy discrimination. Three types of position-sensitive detectors for X-rays have been developed using CIDs. Two CID formats, the CID 17PPRA (388 x 256) and the CID 38SG (512 x 512), were incorporated into systems performing indirect imaging, direct imaging and single X-ray photon counting with energy discrimination. Indirect images of the Laue diffraction patterns from tetraphenylphosphonium tetrachlorooxomolybdenum(V) and natural MoS(2) were collected using a phosphor sheet to convert X-rays into optical photons which were detected with the CID 38SG. Directly detected images of spots from the Laue diffraction pattern of MoS(2) were recorded with the CID 17PPRA. Single photon counting with energy discrimination is demonstrated with the CID 17PPRA using a reflection from the Laue diffraction pattern of MoS(2). Useful information could be obtained from a single pixel at read rates over 7 kHz. Complete energy-dispersive analysis suitable for determination of space groups from Laue diffraction is currently limited due to incomplete charge collection and/or split events.

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Electronic assemblies such as printed circuit boards (PCBs) and integrated circuits (ICs) require precise fabrication and process control of the electrical and mechanical properties. Microelectronic fabrication is a combination of additive methods, such as deposition of metal layers [18]; or subtractive, such as etching or milling [19]. This stack of layers in PCB or IC design creates the connectivity required for each device to operate functionally within a range of design specifications such as electrical resistance or impedance. The non-destructive verification of final assembled devices has become widespread with access to highenergy metrology equipment, demand for trusted devices [13], and to mitigate the complex defect-prone multilayer designs in PCB ( >8 Layer Lay-up) and IC (2.5D & 3D). Volumetric tools such as X-ray Micro Computed Tomography CT and Nano CT [9] are used to analyze the internal features of PCBs [5] and ICs [10] non-destructively. The automated quantification of these internal structures requires highresolution data to provide enough spatial resolution throughout the sample's "sliced" volume. Each slice's resolution in the volume represents the spatial resolution of the X-ray scan collection. To quantify internal structures such as signal traces or via connections the internal feature must be processed to separate or segment individual components enabling automated dimensional analysis or material property characterization. As the miniaturization of device geometries and the density of components increases, the labor resources required for manual segmentation of layered volumetric data becomes impossible.

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Semiconductor manufacturing has been outsourced to un-trusted regions due to globalization. The complex multistep fabrication of micro-scale integrated circuits (ICs) and the tedious assembly of macro-scale Printed Circuit Boards (PCBs) are vulnerable to malicious attacks from design to final delivery. PCBs provide the functional connections of Integrated Circuits (ICs), sensors, power supplies, etc. of many critical electronic systems for consumers, corporations, and governments. The feature sizes of PCB signal traces in 2D and vias in 3D are an order of magnitude larger than IC devices, and are thereby more vulnerable to non-destructive attacks such as X-ray or probing. Active and passive countermeasures have been successfully developed for IC devices, however PCBs devices are difficult to wholly secure from all attacks. Passive countermeasures for X-ray attacks using high-z materials to block and scatter X-rays are effective, but there is a lack of active and passive countermeasures for PCB. In this paper, a framework for passively obfuscating a PCB's critical connections between components, such as ICs, from non-destructive attacks is demonstrated. This framework can be further extended to incorporate active countermeasures in future work. A proof of concept for a PCB electronic design automation (EDA) tool which combines the small features of micro electro-mechanical systems (MEMS), simulation of X-ray, and 3D PCB Manufacturing to iteratively optimize PCB design to thwart reverse engineering and probing attacks. Index Terms—Additive Manufacturing, MEMS, Hardware Assurance, Physical Inspection, Non-Destructive Technology

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