# Selected Applications of Terahertz Pulses in Medicine and Industry
## 7 Applications of Terahertz Pulses to Failure Analysis within the Semiconductor Packaging Sector  
One of the first successful industrial markets for terahertz technology has been the area of failure analysis (FA) and fault isolation for advanced integrated circuits in the semiconductor industry. This success is largely due to drivers from the semiconductor industry itself to develop terahertz pulses as a FA tool. Over the past two decades, semiconductor packaging technologies have increased their complexity and importance [108]. When combined with advances in lithography applied to silicon, this has resulted in complex devices, with an ever-growing need for more rapid and accurate FA capabilities. To fulfil these requirements and to enable 2.5-D and 3-D product solutions in which traditional processor and memory devices are combined, chip-scale packages (CSP) and similar structures are increasingly being used. More complex packaging architectures, such as through-silicon vias (TSVs), micropillars, and stacked-dice devices, are becoming standard, leading to a more compact design and higher functionality density. An example of some of the structures encountered in advanced packaging that require new FA techniques include solder ball cracking and partial delamination, cracking in redistribution layers in doped regions of semiconductors, and defects in through-silicon-vias, which are increasingly used to interconnect devices. The traditional approach to FA workflow involves a variety of techniques. DC electrical testers, combined with X-ray or in some cases acoustic imaging, are one approach. However, all have sensitivity limitations in some applications, and it can be difficult to rapidly isolate the position of a fault. Physical failure analysis is frequently used to underpin these techniques but is destructive in that it involves cutting and exposing faults inside a chip and identifying these faults with microscopy. Until around 2010, the main alternative was microwave TDR (time domain reflectometry), which uses microwave wavelengths (e.g., circa 20-70 GHz) in the form of a step function voltage injected into the bond pads on integrated circuits via high frequency probes. Measuring the time gated response (reflection) arising at changes in impedance in the circuit (such as those arising in the presence of an open or short circuit) allowed the position of a fault in the integrated circuit to be identified from the time-of-flight data, as shown in Figure 6. However, because of the long wavelengths used in microwave TDR as well as the step nature of the voltage, the accuracy is limited to 100-500 \(\upmu\)m, depending upon the system used and the device under test. Figure 6: Terahertz EOTPR compared to microwave TDR for fault isolation. Electro optical terahertz pulse reflectometry (EOTPR) was developed by TeraView to address the limitations cited above. EOTPR is now a well-established, non-destructive fault isolation tool for advanced integrated circuit devices [108]. As can be observed in Figure 7, EOTPR is an implementation of the TDR technique at terahertz frequencies, proven to enhance fault isolation accuracy to better than 10 \(\upmu\)m [109]. The fault detection accuracy of EOTPR is a function of the rise time of the incident pulse, the wavelengths of radiation used, the time-based jitter, and signal-to-noise ratio (SNR). The EOTPR instrument generates a terahertz pulse using an ultrafast laser and a pair of photoconductive switches for signal generation and detection, resulting in a system with (i) high measurement bandwidth, (ii) low time-base jitter, and (iii) a high time-base resolution [108]. Compared to conventional TDR, EOTPR generates signals with faster rise time, has greater SNR, and much-reduced time base jitter-system properties that offer the potential for significantly increased distance-to-defect accuracy. An example is given in Figure 7, where EOTPR has been used to detect the position of faults in a device under test and where microwave TDR proved to be less accurate.