# Rapid 3D-Imaging of Semiconductor Chips Using THz Time-of-Flight Technique
## 1 Introduction  
Terahertz (THz) spectroscopy has been widely adopted as a powerful tool for biomedical diagnosis, device inspection, and characterization of novel functional materials [1, 2, 3, 4, 5, 6]. THz waves are transparent to nonconductive materials frequently used for the encapsulation of devices such as semiconducting chips; therefore, THz imaging allows us to characterize the internal structures of objects in a nondestructive manner [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. Imaging using continuous-wave (CW) sources has recently increased in popularity due to recent technological advances in the development of sources and detectors [18, 19, 20, 21, 22]; however, CW systems lack the ability to obtain three-dimensional (3D) tomographic information [23, 24]. Although coherent THz tomography based on frequency-modulated CW (FMCW) methods have been developed using a tunable source or ensemble source, they are limited in terms of the depth and longitudinal resolution in general [25, 26, 27]. In contrast, the use of pulsed THz radiation from time-domain spectroscopy (TDS) enables the collection of information such as the depth and range of objects from reflected waves and the nature of scattering. In other words, THz pulsed imaging provides a direct 3D \"map\" of an object by using the time-of-flight (TOF) of the reflected THz pulses.  
Although imaging using the THz-TDS system has numerous advantages, it suffers from low average source power and long acquisition time, making it unsuitable for many applications. As two-dimensional (2D) array detectors are not available, most THz TOF imaging systems require sample scanning, followed by reconstruction of a series of single-point measurements for reflected or transmitted waves. Additionally, the spectral resolution is linked to the inverse of the temporal window during the acquisition step, and there is a tradeoff between measurement time and spectral resolution. Novel THz-TDS systems were introduced commercially based on compact femtosecond fiber laser technology without the mechanical delay stage. For example, techniques such as electronically-controlled opticalsampling (ECOPS), asynchronous optical sampling (ASOPS), and optical sampling by cavity tuning (OSCAT) deliver a rapid time delay of more than 100 Hz per scan with a wide temporal range (60 ps-10 ns) [12,28,29]. Recently, a novel 3D imaging technique based on the ECOPS system has been developed [12]. Artificial defects in glass fiber reinforced polymers have been imaged, proving their potential use for rapid, nondestructive inspection of various materials, components, and systems. However, a rapid imaging technique based on the galvano scanning system has not been introduced for inspection of defects in packaged chips, although packaged chips have been studied with the THz-TDS system using relatively slow sample scanning methods [3].  
In this work, we report high-speed imaging for packaged integrated circuit (IC) inspection with a high temporal resolution based on the THz-TDS technique using the OSCAT system. We obtained nondestructive TOF images of IC chips, whose 3D information was analyzed with respect to both the temporal and spectral domains. Further, the reconstruction of the 3D plots allows us to identify defects in the packaging, which is not possible using 2D integrated images.