1. Introduction

A terahertz (THz) wave has unique advantages of being applicable to the human body owing to its low energy level, and being able to inspect certain foreign substances such as bugs and rubbers contained in foods [1–3]. In addition, a THz wave can help perform security checks at public places such as the airport, post office, and subway. However, real-time, high-density, and low-cost THz imaging systems are required for such applications [4–7]. In order to satisfy these system requirements, two-dimensional (2D) or one-dimensional (1D) array-type detectors can be utilized. THz array detectors such as a 124 × 124 pyroelectric array camera, a 320 × 240 microbolometer focal-plane array operating at 4.3 THz, a 32 × 32 CMOS-based detector for 0.7-1.1 THz imaging applications, and a 1 × 20 InGaAs Schottky barrier diode (SBD) array detector operating at 250 GHz have been developed [8–11].

InGaAs-based SBD array detectors are promising devices for use in THz image scanners. These detectors have lower power consumption because of their zero-bias operation, and they have compact system configurations because of their small array size [12–15]. The square-spiral antenna with an antenna area of less than 200 × 200 µm has been adopted to realize the small-size SBD array [11]. In addition, the simple, plain SBD structure, which is composed of a dielectric layer instead of air in the air-bridge type structure, can be suitable for implementing larger scale arrays per chip because of the structure’s good uniformity and high yield. Furthermore, 1D array-type line detectors, compared to 2D array-type area detectors, have the following merits. The THz emitter power that forms the 1D line beam is needed less than that of the 2D beam, owing to a smaller beam area that has the same intensity. In addition, the array lengths of the line detectors can be as long as desired because they are scalable along only one direction.

For high-speed scanning to obtain real-time THz images, image-enhancement techniques are required. Various fine image acquisition methods using phase-shift interferometry, polarization control, compressed sensing with phase correction, and deconvolution algorithms have been reported [16–20]. Among them, the image enhancement technique that uses a polarization control can be a simple and easy method when determining the hardware system configuration.

In this paper, we present a broadband square-spiral antenna-integrated 1 × 240 InGaAs SBD array line detector module. In addition, a real-time and large-area THz line-scanning system using the InGaAs SBD array detector is demonstrated. The experimental results of the enhanced image performance using the polarization control of the THz source are also presented.

2. Fabrication of 1 × 240 SBD array detector

We fabricated a square-spiral antenna-integrated 1 × 240 InGaAs SBD array detector with an anode diameter of 3 μm. The detector has 240 pixels in a line, with a unit chip of 40 pixels and a pixel pitch of 0.5 mm. The total length in a scan line (the scan line length) was 12 cm; this value can be calculated simply by multiplying 240 pixels by 0.5 mm/pixel. A simple, plain structure comprising a silicon nitride (SiN_x) layer was designed to implement a THz array detector that would achieve good uniformity and high yield for 40 pixels [11]. The average responsivity was approximately 98 V/W at a center frequency of 250 GHz (Gunn diode). The average noise equivalent power (NEP) was approximately 106 pW/√Hz. These were measured values of SBD array samples used to fabricate a SBD array detector module. The responsivity was relatively low because we used a square-spiral broadband antenna; however, we can improve the responsivity by increasing the optimization via lower series resistance and lower capacitance. Nevertheless, the square-spiral broadband antenna was chosen because it has superior characteristics such as less influence on any polarization state of THz sources, a lower divergence angle (which results in lower crosstalk between pixels), and a smaller size (less than 200 × 200 µm). In addition, the maximum detectable frequency of the SBD array sample was approximately 630 GHz, which was measured by using a fiber-coupled continuous-wave THz spectroscopy system composed of dual-mode laser optical beat sources, photomixers, and THz emitter/detector modules that we developed in our previous studies [21–23].

Figure 1(a) shows the fabricated 1 × 240 InGaAs SBD array detector module with a volume as small as 155 × 85 × 25 mm³. This SBD detector module consists of a Teflon (or high-density polyethylene) cover, 1 × 240 SBD array chip, SBD chip board (CB), SBD data processing board (DPB), and module housing. Figure 1(b) shows six 1 × 40 SBD array chips bonded onto a SBD CB and vertically connected onto a SBD DPB in the SBD array detector module. The SBD CB was designed to carry out bonding six 1 × 40 SBD array chips onto the SBD CB using a flip-chip bonder (Kalsuss, FC150), and to connect with the SBD DPB by a small high-density stack connector. The solder pads of the 1 × 40 SBD chip and the solder balls of the SBD CB were bonded in active alignment at a heating temperature of 240°C and a heating time of 30 s [11]. This process was repeated six times under the same conditions. This process can bond four 1 × 60 SBD array chips or three 1 × 80 SBD array chips onto the SBD CB, if the uniformity of the SBD array chip is guaranteed. In addition, the board thickness of the SBD CB was made as thick as 3.2 mm to prevent bending after flip-chip bonding. In the SBD DPB, the following items were configured: eight 32 × 1 multiplexers (MUXs) to switch the pixels in the 1 × 240 SBD array, eight low-noise amplifiers (LNAs) with a gain of 30 dB to amplify the signal from the output port of each MUX, eight 16-bit analog-to-digital converters (ADCs) connected to the output port of each LNA, a microcontroller unit (MCU) to control all of the electronic devices, and an universal serial bus (USB) device to communicate with a personal computer (PC).

 

Fig. 1 Photographs of (a) 1 × 240 Schottky barrier diode (SBD) array detector module and (b) SBD array chips bonded onto a SBD chip board (CB), and vertically connected onto a SBD data processing board (DPB) in the SBD array detector module. The inset shows an SEM image of an SBD array chip sample.

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3. Terahertz line-scanning system and its experimental results

We set up a THz line-scanning system composed of a gyrotron THz source, 1 × 240 SBD array detector, conveyor system, and several optical components such as a THz wire-grid polarizer with a wire diameter of 15 µm and a wire spacing of 60 µm (Microtech Instruments), high-density polyethylene cylindrical lens, and metal cylindrical mirror, as shown in Fig. 2. The gyrotron source, with a center frequency of 200 GHz and employed with a compact cryogen-free magnet, was used as an illumination source for the THz line-scanning system, which has optical characteristics such as power of ~50 W, a Gaussian-like mode, and slightly elliptic polarization [24]. Test samples were transported by the conveyor system using a moving part made of Teflon material and a scan velocity of 25 cm/s, and transmitted by the THz line beam. The uniform THz line beam was achieved by using the cylindrical lens to extend a circular beam, and then using a cylindrical mirror to form the line beam. The SBD array detector, which was fixed on the conveyor system, obtained line-scanned images. Foreign substances, hidden behind crackers as test samples, were layered on the moving conveyor part of the system. A metal ring and paper clip were used as the foreign substances.

 

Fig. 2 THz line-scanning system setup composed of a gyrotron THz source, 1 × 240 SBD array detector, conveyor system, THz wire-grid polarizer, high-density polyethylene cylindrical lens, and metal cylindrical mirror.

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We adjusted the power of the 200-GHz gyrotron by several watts to provide optimal conditions that avoided the saturation of signal-detecting levels for all pixels of the SBD array detector. Then, we attempted to find a fine detecting condition of the SBD array detector by rotating the THz wire-grid linear polarizer in front of the gyrotron source, since the gyrotron source had slight elliptic polarization characteristics.

We measured the THz line-scanned images, which had a scan velocity of 25 cm/s and a pixel size of 0.5 × 0.5 mm², for three different focus degrees and polarizer availability configurations to determine the best imaging conditions. These three configurations were unfocused/polarizer-unused, focused/polarizer-unused, and focused/polarizer-used. Figure 3(a) is a photograph of three metal letters “THZ,” each with a width of 2.5 mm, which were used as the first test samples. Figures 3(b)–(d) show the THz images of the samples measured using the three imaging conditions. For the focused/polarizer-used condition, the “THZ” image was most sharply detected. Fine images for the transmitted samples can be experimentally obtained owing to the suppressed spill light that results from the selection of a major polarization state instead of a minor polarization state. We note that the polarization state control can play an important role as a kind of signal synchronization between the THz source and the detector.

 

Fig. 3 (a) Photograph of three metal letters “THZ” and their THz images measured with the THz line-scanning system using a scan velocity of 25 cm/s and a pixel size of 0.5 × 0.5 mm², for (b) unfocused/polarizer-unused, (c) focused/polarizer-unused, and (d) focused/polarizer-used configurations.

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We also measured THz line-scanned images with the same scan velocity and pixel size as above, using the same three configurations. Figure 4(a) is a photograph of three crackers (50 × 50 × 4 mm³) with invisible foreign substances hidden under the crackers. The invisible foreign substances are as follows: empty (left), a metal ring (middle, 4 mm wide), and paper clip (right, 1 mm wide). Figures 4(b)–(d) show THz transmitted images measured by using the THz line-scanning system for unfocused/polarizer-unused, focused/polarizer-unused, and focused/polarizer-used configurations, respectively. The crackers and foreign substances were simultaneously observed in all three cases. When the foreign substance under the cracker was the metal ring, all three cases detected it accurately. When the foreign substance under the cracker was the paper clip; however, the shape of the paper clip was most finely detected in the focused/polarizer-used case, while the shape was not recognized in the other two cases. This capability of detecting the paper clip means that the spatial resolution is 1 mm.

 

Fig. 4 (a) Photograph of three crackers with invisible foreign substances under the crackers: empty (left), a metal ring (middle), and paper clip (right). Their THz images were measured by using the THz line-scanning system with a scan velocity of 25 cm/s and a pixel size of 0.5 × 0.5 mm², for (b) unfocused/polarizer-unused, (c) focused/polarizer-unused, and (d) focused/polarizer-used configurations (Media 1).

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Although the foreign substance hidden under a soft substance is made of metal, if the THz polarizer is not used, a paper clip with a diameter as thin as 1 mm cannot be found, as shown in Figs. 4(b) and 4(c). Thus, polarization control using the THz polarizer should be useful for enhancing the signal-to-noise ratio (SNR) when various substances are stacked. We note that the THz polarizer acts as a noise-rejection filter that results from seeing only the desired polarization while concealing the undesired polarization, as shown in Fig. 4(d). We can see that the high-speed and large-area-scanning capabilities of the SBD array line detector are suitable for examining foreign substances hidden under soft substances such as crackers. In addition, a more resolved image can be achieved if the center frequency of the THz source is changed to a higher frequency.

4. Summary

We fabricated a broadband square-spiral antenna-integrated 1 × 240 InGaAs SBD array line detector module with a volume as small as 155 × 85 × 25 mm³. The module has 240 pixels in a line, a unit chip of 40 pixels, and a pixel pitch of 0.5 mm. A simple, plain SBD structure comprising a SiN_x layer was adopted to achieve good uniformity and high yield. In the assembly process, six 1 × 40 SBD array chips bonded onto a SBD CB and then the SBD DPB, and then, they were vertically connected onto a SBD DPB. The SBD CB was made as thick as 3.2 mm to maintain safety after bonding 240 SBD pixels and had an easy connection structure. The SBD DPB was subsequently configured as eight 32 × 1 MUXs, eight 30-dB gain LNAs, eight 16-bit ADCs, an MCU, and a USB.

We demonstrated a real-time and high-density THz line-scanning system with a scan velocity of 25 cm/s, a scan line length of 12 cm, and a pixel size of 0.5 × 0.5 mm². This system was composed of a gyrotron THz source, 1 × 240 SBD array detector, conveyor system, THz wire-grid polarizer, high-density polyethylene cylindrical lens, and metal cylindrical mirror. The THz line-scanning system using the fabricated SBD array detector could detect a foreign substance such as a paper clip with a diameter as small as 1 mm hidden underneath a cracker, as well as simultaneously measuring a foreign substance underneath a cracker. In addition, the image performance when using a THz polarizer improved when compared with that of a nonpolarized system.