Industrial Applications of THz Imaging Based on Resonant Slit-Type Probe

Article information

J. Electromagn. Eng. Sci. 2022;22(3):179-185
Publication date (electronic) : 2022 April 15
doi : https://doi.org/10.26866/jees.2022.3.r.75
1Korea Electrotechnology Research Institute (KERI), Ansan, Korea
2Pohang Accelerator Laboratory, Pohang, Korea
*Corresponding Author: Jung-il Kim (e-mail: sky@keri.re.kr)
Received 2021 June 10; Revised 2021 July 5; Accepted 2021 July 17.

Abstract

In this study, the possibility of the industrial application of terahertz (THz) imaging technology was verified. It was applied to the inspection of voids in multistack semiconductors that require safe inspection and to the high-resolution detection and inspection of foreign substances in tablets in the pharmaceutical field. To acquire a high-resolution THz image, a resonant slit probe operating in the THz region was designed, and a high-speed scanning system was established. For the inspection of a multistack semiconductor, a lateral scan method was proposed, and voids with a diameter of 0.5 mm in the multistack semiconductor were detected. In addition, the proposed probe even enables the distinguishment of the positions of voids in the multistack semiconductor. For pharmaceutical inspection, we investigated the application of THz imaging to detect mixed foreign objects frequently occurring in the tablet manufacturing process. For metals, plastics, and rubber, which are the most frequently mixed materials in the tablet manufacturing process, the foreign objects were identified in tablets using a transmission THz system. The measured THz image was compared with the conventional X-ray test result to confirm the potential of THz inspection. In the X-ray image, only metal and some polymer foreign objects were detected. In contrast, in the THz image, although the materials could not be distinguished, most foreign substances were detected. Consequently, the THz imaging test was verified as a possible new tool in fields where X-ray or existing tests are not possible.

I. Introduction

Terahertz (THz) technology has been proposed as a new inspection tool in various fields [1, 2]. Particularly, for medicine, foods, security screening, and nondestructive testing, a new inspection method is required due to the limitations of the existing inspection technology [37]. The transmission and absorption of THz waves for various materials, a high sensitivity to moisture, and low ionization energy are key to applying THz technology in these fields. Recently, the development of various high-power THz sources and high-sensitivity detectors has led to the expansion of THz technology into industrial applications.

For industrial applications of THz waves, THz imaging technology using compact continuous-wave (CW) high-power THz sources and high-sensitivity detectors is expected to be applied in many industrial fields requiring nondestructive, non-contact, and noninvasive methods [8, 9]. However, as the frequency increases, a dramatic decrease in the output of the THz source and a low detection sensitivity are obstacles to applying THz. For this reason, a THz source and a detector in the low-frequency region are preferred in many applications. A THz source in the low-frequency region has a high output power, so it has advantages in inspecting a thick sample or in scanning a large area. However, the low spatial resolution and diffraction due to the inherent wavelength make it difficult to apply the THz wave to an industrial field that requires a high resolution.

Recently, numerous studies on improving spatial resolution have been conducted [10, 11]. A resonant slit-type probe for the THz frequency range, which has a simple structure, high coupling efficiency, and linear polarization, has been proposed, and one with a slit height less than 1/10 of the wavelength and a slit width of approximately half the wavelength can easily achieve high spatial resolution [12, 13]. This is useful in applications where small defects or enclosed foreign objects must be detected.

In this study, THz technology was applied to industrial applications that require safe and high-resolution detection. For the latter, a resonant slit probe operated at 200 GHz was designed and manufactured, and it was applied to the inspection of voids in a multistack semiconductor, which is an issue in the semiconductor field. In addition, it was applied to the inspection of foreign objects that are frequently mixed in the tablet manufacturing process in the pharmaceutical field.

II. Resonant Slit-type Probe with A Rounded Matching Structure

According to Babinet’s principle, the impedance (Zs) of a slit antenna is

(1) Zs=Z0πh2w11-(λ02w)2

where λ0 is the free-space wavelength and w and h are the width and height of the slit antenna, respectively [14].

Assume that a narrow slit is attached to the end of a waveguide. The maximum transmission occurs when the waveguide impedance and slit impedance match. If the width and height of the waveguide are a and b and the width and height of the slit are a′ and b′, then from Eq. (1),

(2) Z0πb2a11-(λ02a)2=Z0πb2a11-(λ02a)2

and

(3) ab1-(λ02a)2=ab1-(λ02a)2

In other words, the slit becomes transparent under the above conditions [15]. In Eq. (3), if b′ approaches 0 (zero), then this equation is satisfied at λ=2a. This means that if the width of the slit is half the wavelength, then the height of the slit can be infinitely reduced. That is, electromagnetic waves of a specific frequency according to the above conditions can pass without loss through a very narrow slit.

Figure 1(a) shows a schematic diagram of a resonant slit probe with a rounded matching structure for impedance matching. The proposed structure is optimized to have a resonant frequency at 200 GHz using CST simulation. The designed slit structure has a slit width of 740 μm (~λ/2) and a slit thickness of 250 μm, and the radius of the matching structure is 100 μm. The slit height is fixed at 150 μm (=λ/10). Fig. 1(b) shows the frequency response characteristics of the designed resonant slit probe. The resonant frequency is 199.5 GHz, and the return loss at the resonant frequency is calculated as 59.5 dB.

Fig. 1

(a) Resonant slit probe with a matching structure (b) Frequency response characteristics of the slit probe designed using CST simulation

III. Void Inspection in A Multistack Semiconductor using the Resonant Slit Probe

In recent years, such devices as mobile phones and laptops have been expected to offer high-speed processing technology while exhibiting compact and lightweight characteristics. To solve this problem, in recent semiconductor technology, multistack semiconductors, in which chips are multistacked by a chip packaging method, have been widely used. In the stacking method, die attach film (DAF) is commonly used. The bonding method using DAF has the advantage of the easy application of a certain amount and a simple bonding process [16, 17]. However, a problem arises due to voids occurring between the film and the chip in the bonding process. Internal voids larger than a certain size expand when the temperature of the device rises, causing cracks in the chips. Therefore, it is important to be able to detect voids in a multistack semiconductor in the chip packaging process.

The ultrasonic inspection method has been used to inspect voids in multistack semiconductors [18,19]. However, in underwater ultrasonic inspection, the semiconductor cannot be used after inspection due to the invasiveness issue. In addition, infrared (IR) inspection [20] cannot be used due to the high absorption rate of DAF polymer materials, and X-rays cannot be applied due to ionization of semiconductor materials under high X-ray energy. The THz wave is the most ideal light source for semiconductor inspection because it has high transmittance through silicon and polymers, which are semiconductor materials, and it does not ionize semiconductor materials with low photon energy.

Figure 2 shows a photograph of the void inspection system for a multistack semiconductor using the resonant THz slit probe. The complex metal pattern on the top of the semiconductor chip makes it difficult to transmit the THz wave through the chip. In contrast, polypropylene (PP) is mainly used as a DAF material for semiconductor lamination, so THz waves can easily pass through the films because PP has a small absorption coefficient of less than 5 cm−1 in the THz region. Therefore, we built an inspection system using the lateral scan method on the side of the multistack semiconductor. In addition, the slit probe was applied to the front end of the detector to identify the positions of the voids in the multistack semiconductor.

Fig. 2

THz experimental setup for multistack semiconductor package inspection using the lateral inspection method

A photograph of a multistack semiconductor package is shown in Fig. 3(a). The number of stacked chips is 13, and the chip thickness in the transmission direction is 8.6 mm. Chips with void diameters of 1 mm and 0.5 mm were fabricated and placed in stacked semiconductors. Fig. 3(b) shows THz images of the amplitude and phase of the measured signal for a package sample without voids and package samples with 1 mm and 0.5 mm voids.

Fig. 3

(a) Sample of a multistack semiconductor package with voids (b) THz magnitude and phase images according to the diameter of voids

As shown in the figure, even though a light source of 200 GHz (λ=1.5 mm) was used, even voids with a diameter of 0.5 mm, smaller than the wavelength, were detected. Particularly, small voids are easier to detect in the phase image than in the amplitude image because the attenuation of the amplitude at the small-diameter voids is small, but diffraction of the THz wave easily occurs in the voids due to the short wavelength. In addition, the positions of the voids could be distinguished because the height of the slit probe used was less than the height of the DAF. Because the signal range detected with the long slit length was measured to be wider than the void diameter, accurately determining the size of the voids was difficult. However, void inspection using THz waves was verified as a possible solution for detecting voids larger than a certain size.

IV. Inspection of Foreign Objects in A Tablet using the THz Probe

In the pharmaceutical field, X-ray inspection is mainly used for foreign body inspection, but it is avoided because of concerns about the harm caused by high energy. In addition, detecting foreign objects, such as rubber and plastics, is difficult because of the low detection sensitivity of X-rays for soft materials. The photon energy of THz waves is approximately one million times less than that of X-rays, making them an ideal light source for applications requiring safe inspection [21]. Recently, a laser-based THz inspection system was developed [22], but it has not been applied to manufacturing due to the limitation of the inspection speed.

To improve the detection speed, we applied high-speed lock-in amplifier technology using TTL modulation of a semiconductor-based CW THz source. The applied high-speed lock-in amplifier can perform signal processing even under the high-speed modulation of several tens of kHz, so it can detect signals with a high signal-to-noise ratio (SNR) even under high-speed scanning of several tens of cm/s.

Figure 4 shows the THz system and control program for the inspection of foreign objects in tablets based on CW THz waves. The samples were placed in the holes of a tablet tray made of plastic. For 2D scanning, the tablet tray was fixed to an XY translation stage, and the source and detector were installed close to either side of the tablet tray. The THz signal transmitted through the tablets was displayed to identify foreign objects in the tablets.

Fig. 4

THz system (upper) and control program (bottom) for the inspection of foreign objects in tablets

Structure diagrams of the tablet samples are shown in Fig. 5(a). The samples were of two types: a round tablet with a diameter of 8.1 mm and a height of 3.7 mm (A-type) and a round tablet with a diameter of 8.7 mm and a height of 3.9 mm (B-type). Considering the applications of THz inspection, the tablet structures commonly encountered in pharmacies were selected. For A-type tablets, a PP piece, an acrylic piece of 1.0 mm or 0.8 mm, a rubber piece of 1.0 mm, or a Teflon piece of 0.8 mm was included in the manufacturing process. The foreign object included in the B-type tablets was a small metal rod with a diameter of 0.4 mm or 0.2 mm.

Fig. 5

Structural diagrams of tablets (upper) and positions of the tablet samples installed in the tablet tray

Figure 6(a) shows the transmitted THz image for the samples in Fig. 5, and Fig. 6(b) shows the X-ray image for comparison. Comparing the THz image with the X-ray image, most foreign objects were detected in the THz image, whereas only metal rods, PP, and the Teflon piece were detected in the X-ray image. First, rubber was not detected in the X-ray image because of the low sensitivity of X-ray to soft materials. Second, although acrylic is a polymer, similar to PP and Teflon, it was not detected by X-ray inspection. An X-ray image is an image produced according to the attenuation coefficient. The attenuation coefficient of acrylic according to the X-ray energy is located between those of PP and Teflon [23]. From the X-ray image, the reason the acrylic object in the tablet was not detected in the X-ray image can be determined to be the similarities in the attenuation coefficients of the tablet and acrylic. In contrast, THz images contain various information, such as the dielectric constant, absorption rate, and diffraction and refraction of materials. In the measured THz image, the amplitude of the THz signal decreased in all the foreign object regions in the tablets because, as in many studies, diffraction occurs at small foreign objects in the path of THz waves.

Fig. 6

(a) THz image and (b) X-ray image for foreign object inspection in a tablet

V. Conclusion

In this study, the industrial applicability of THz inspection technology was verified. THz probe-based inspection technology was applied to the semiconductor and pharmaceutical inspection fields, which require safe, high-resolution inspection. In semiconductor inspection using THz waves, voids with a diameter of 500 μm were detected in stacked semiconductors. Moreover, by using the resonance-type slit probe, even the positions of the voids in the stacked structures were distinguished. In the tablet inspection, all foreign objects in the prepared tablet samples were detected. In contrast, some plastics and rubber, a soft foreign object, were not detected in the X-ray inspection, as a comparison. From the results, the THz inspection technique can be confirmed applicable as a new inspection tool in areas where X-ray inspection is avoided.

Acknowledgments

This research was supported by the Korea Electrotechnology Research Institute (KERI) Primary Research Program through the National Research Council of Science and Technology (NST) funded by the Ministry of Science and ICT (MSIT) (No. 17A01068, 22A01043) and consignment research program (14A03014).

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Biography

Geun-Ju Kim received a Ph.D. in electrical and electronics engineering from the Korea Maritime & Ocean University (KMOU), Busan, Korea, in 2018. He joined the Korea Electrotechnology Research Institute, Korea, in 2004. He is currently working at the Electro-Medical Device Research Center of the Ansan Branch, Korea. His current position is Principal Researcher. His current research interests include THz imaging and applications, high-power electromagnetic wave devices and applications, and radiation therapy.

Sanghoon Kim received B.S. and M.S. degrees in electrical and electronics engineering at Korea Maritime and Ocean University in 2008 and 2010. Prior to joining the Korea Electrotechnology Research Institute (KERI) in 2016, he worked as a Researcher in the Medical Convergence Research Institute of Yonsei University College of Medicine. He is currently a Research Associate at KERI, where he specializes in developing Terahertz imaging technology and control systems of radiation therapy devices.

Jeong-Hun Lee received a bachelor’s degree in applied physics from Dankook University, Gyeonggi-do, Korea, in 2014. He has been working as a researcher at the KERI since 2014. His current research interests include applications of electron beams, electromagnetic device technology, and applications of high-power RF devices.

Insoo S. Kim received his B.S. and M.S. degrees in electronic engineering from Dong-A University, Korea, in 1984 and 1986, respectively. In 2016, he completed his doctoral studies (ABD) in electronic engineering at Namseoul University, Korea. Since 1986, he has been working as a principal researcher at the KERI. From 1997 to 2000, he was a visiting scientist at the Shanghai 803 Research Institute of China Aerospace Science and Technology Corporation (CASC) and Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS). His research areas focus on biomedical applications of electromagnetic waves, ubiquitous healthcare systems, biomedical optics, and optical technology.

Yong-Seok Lee received a Ph.D. in electronic electrical engineering from Sungkyunkwan University, Suwon, Korea, in 2015. From 2015 to 2020, he was a senior researcher at the KERI, Ansan, Korea. He joined the Pohang Accelerator Laboratory (PAL), Pohang, Korea, in 2020. His current position is Staff Scientist, and his current research interests include fast feedback control systems and superconducting RF cavities for accelerator applications.

Jung-Il Kim received a Ph.D. from the School of Physics, Seoul National University (SNU), Seoul, Korea, in 2006. Since 2006, he has been with the KERI, Korea. His current position is Principal Researcher. His research interests include high-power vacuum electronic devices, from microwave to terahertz wave; terahertz imaging systems; medical linear accelerators (LINACs); and high-precision radiotherapy systems.

Article information Continued

Fig. 1

(a) Resonant slit probe with a matching structure (b) Frequency response characteristics of the slit probe designed using CST simulation

Fig. 2

THz experimental setup for multistack semiconductor package inspection using the lateral inspection method

Fig. 3

(a) Sample of a multistack semiconductor package with voids (b) THz magnitude and phase images according to the diameter of voids

Fig. 4

THz system (upper) and control program (bottom) for the inspection of foreign objects in tablets

Fig. 5

Structural diagrams of tablets (upper) and positions of the tablet samples installed in the tablet tray

Fig. 6

(a) THz image and (b) X-ray image for foreign object inspection in a tablet