**1. Introduction**

Terahertz (THz) waves possess many unique properties, such as penetrating nonconductive materials, which are opaque in visible and infrared bands [1]. They could identify specific materials according to their characteristic THz signatures [2]. THz waves are safe for biological tissue because of low photon energy and non-ionizing attributes, in contrast to X-rays, and promise higher resolution compared with microwave bands [3,4]. The above characteristics of THz waves have promoted THz technology to make great progress in medical detection [5], security inspection [6], non-destructive testing [7], wireless communication [8], atmospheric monitoring [9], astronomical observation [10], and so on. However, the interest in a wide range of commercial THz applications is the main driver for the development of widely accessible room-temperature THz detectors. In addition, as the same material is imaged at different frequencies, the image sharpness would vary with different transmission rates. Hence, multiband detectors could dramatically improve the overall sensing and imaging ability by means of obtaining more informative images through fusion technology [11]. Besides, multiband detectors also have the advantages of enhanced detection probability, increased calibration capability, and reduced influences of standing waves or scattering, showing great potential for further development of THz applications [11–14].

With continuous developments in CMOS technology, which is considered as an attractive device technology because of its low cost, high yield, and high integration ability [15], various kinds of room-temperature CMOS multiband THz detectors have attracted more attention and have gradually received in-depth research in the last couple of years [16–20].

**Citation:** Wang, X.; Li, T.-P.; Yan, S.-X.; Wang, J. Room-Temperature CMOS Monolithic Resonant Triple-Band Terahertz Thermal Detector. *Micromachines* **2023**, *14*, 627. https://doi.org/10.3390/mi14030627

Academic Editors: Lu Zhang, Xiaodan Pang and Prakash Pitchappa

Received: 20 February 2023 Revised: 7 March 2023 Accepted: 7 March 2023 Published: 9 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Multiband active detectors use a higher harmonic, resulting in sharply increased noise figures and fixed operation frequencies, which are determined by fundamental and harmonic frequencies [21]. Similar trends do not exist for passive devices that are more suitable for human vision and image processing [22]. Therefore, several multiband passive detectors consisting of antennas and MOSFETs have been proposed [16–20]. The above FET-based THz detectors, whose operation frequencies are seriously restricted and influenced by transistors, achieve better characteristic results below 1 THz, and their performances degrade dramatically as the frequency exceeds 1 THz owing to frequency-dependent parasitic elements [19]. Besides, the detectors in [19,20] are composed of multiple discrete antennas and multiple FETs, resulting in lower integration levels, a larger chip area, and higher cost, thus it is necessary to design compact multiband THz detectors. However, it is hard to obtain compact FET-based detectors because the input impedance of transistors differs at multiple frequencies [16,21]. Compared with FET-based detectors, THz thermal detectors constitute promising options as they allow wideband detection, support high-frequency THz detection, and show performance advantages in higher THz bands because their output signals are independent of frequencies [23–25]. Therefore, several room-temperature CMOS multiband THz thermal detectors have been proposed. A room-temperature CMOS multiband THz thermal detector composed of an antenna and an NMOS sensor is proposed, but it operates at 0.546 THz, 0.688 THz, 0.78 THz, and 0.912 THz [26]. It is necessary to design room-temperature CMOS multiband THz thermal detectors that could detect sub 1 THz waves and above 1 THz waves to possess good sensitivity and high resolution [11]. Previous works have described two kinds of CMOS triple-band THz thermal detectors, which mainly concentrate on modules' designs, including designs of receiving structures and temperature sensors, thus they lacked the concept of collaborative designs between modules, such as completing the layout of a temperature sensor according to the raised temperature distribution of receiving structures [27,28]. Besides, these triple-band detectors just completed the performance characterizations at two frequencies.

This paper presents a compact room-temperature triple-band THz thermal detector made up of a strong octagonal ring antenna and a sensitive PTAT sensor using a Global Foundry 55 nm CMOS process. Because lower THz waves have a greater penetration depth and higher THz waves provide better spatial resolution, the proposed detector is chosen to operate at 0.91 THz, 2.58 THz, and 4.2 THz for available THz sources so as to obtain better penetration and greater spatial resolution. It achieves relatively better measurement results at three operation frequencies with detailed analysis, exactly presenting an uncooled, compact, cost-effective, easy-integration, and mass-production multiband detection system.

#### **2. Detector Structure and Operation Principle**

Antennas are ready to shape the radiation pattern and tune the impedance match within a wider bandwidth [29], while octagonal rings are used to constitute antennas because they have advantages of smaller chip area occupation and less coupling effect than other structures [30]. In addition, PTAT sensors as a type of common CMOS temperature sensor show great application potential owing to their better linearity and accuracy [31]. Based on this, Figure 1 shows the schematic diagram of the proposed detector, which consists of a compact triple-band octagonal ring antenna, a polysilicon resistor at the termination of the antenna, and a sensitive PTAT sensor. As THz waves interact with the antenna, an instantaneous frequency-dependent current is excited and flows through the resistor; by this means, incident THz waves are frequency-selective absorbed. Thus, electromagnetic (EM) energy is immediately transformed into thermal energy through ohmic loss and conductive loss, leading to the localized temperature increment depending on the magnitude of the radiation [24]. In addition, the PTAT sensor transforms the rising temperature into an increased output voltage, so the sensor is located below the antenna and in close proximity to the resistor in order to reduce heat loss and sense an increased temperature as fast as possible. The triple-band detection is accomplished as THz waves of three frequencies are incident on the detector successively.

**Figure 1.** Schematic diagram of the proposed triple-band detector.

Furthermore, a lower operating frequency leads to larger antenna sizes, so the temperature distribution caused by conductor loss is far from that caused by ohmic loss. The resistor becomes the main heat source and presents a strong, uniform, and raised temperature distribution in a certain area, because temperature sensing elements of the sensor should sense the same temperature and the received EM energy is mainly converted into joule heat through the resistor. Therefore, the increased temperature is approximately equal to the temperature increment generated by the resistor. However, a higher operating frequency leads to smaller antenna sizes, so the temperature distribution generated by the antenna and the resistor is closed or even overlapped and, finally, a strong, uniform, and raised temperature distribution is generated in a certain area centered on the resistor. In this way, the perceived temperature increment is approximately equal to the sum temperature increment caused by the resistor and the conductor of the antenna.

According to the operation principle of the detector, its design task not only includes the independent design of the antenna and the PTAT sensor, but also contains the co-design between the antenna and the PTAT sensor based on the temperature distribution of the antenna caused by the incident THz waves.

#### *2.1. Design of Octagonal Ring Antenna*

The structure diagram and optimized geometric parameters of a compact triple-band octagonal ring antenna (sample A) using HFSS tools are shown in Figure 2a,b. Nested octagonal rings are composed of three concentric rings with different sizes, and the smaller ring is embedded in the larger ring. As perimeters of the outer ring, the middle ring, and the inner ring are about dielectric wavelengths of 0.91 THz waves, 2.58 THz waves, and 4.2 THz waves, respectively, the fundamental modes of the outer octagonal ring, the middle octagonal ring, and the inner octagonal ring could radiate 0.91 THz waves, 2.58 THz waves, and 4.2 THz waves, correspondingly. Based on the reciprocity theorem, sample A could also receive THz waves of 0.91 THz, 2.58 THz, and 4.2 THz, respectively.

As shown in Figure 2a,b, sample A was made up of three nested octagonal rings, connection structures, a ground plane, two transmission lines, and a grounded wall. The outer octagonal ring was constructed in the metal 9 layer of the 55 nm CMOS process, while the middle and inner octagonal rings were fabricated in the metal 8 layer. The connection structures between the outer octagonal ring and the middle octagonal ring were realized in metal 8. The metal 3 layer was used to fabricate the metallic ground plane, which could effectively prevent the waves from being exposed to the lossy substrate because metal 1 and metal 2 were used for the electronics routing of the PTAT sensor. The resistance of the polysilicon resistor was 100 Ω for impedance matching and transmission lines were formed from the octagonal ring antenna down to the resistor. A grounded wall composed

of metal layers and vias layers around the antenna was applied to trap EM energy and prevent external interference. Besides, metal layers and inter-metal dielectric regions were modeled as aluminum and SiO2, respectively, and they were fixed by the CMOS process. In addition, sample B with only a ground plane in the metal 3 layer was also simulated for verifying the frequency-selective absorption of sample A.

**Figure 2.** Designed antenna: (**a**) top view; (**b**) side view; (**c**) simulated return loss.

Figure 2c shows the simulated return loss curves, where sample A could resonate at 0.91 THz, 2.58 THz, and 4.2 THz, while sample B does not have frequency−selective characteristics. Within the observation frequency range, three octagonal rings radiate 0.91 THz waves, 2.58 THz waves, and 4.2 THz waves, while the second-order mode and fourth-order mode of the outer octagonal ring correspond to radiating 1.82 THz waves and 3.74 THz waves, respectively. Although sample A could radiate THz waves of five frequencies, it is still considered that a triple-band THz antenna is obtained instead of a fiveband antenna. Besides, the fundamental modes of three octagonal rings are still applied to radiate 0.91 THz waves, 2.58 THz waves, and 4.2 THz waves, instead of only constructing

an outer octagonal ring to obtain a THz antenna operating in multiple bands through its fundamental modes and higher modes. This is because, compared with fundamental modes, higher order modes of the antenna are unstable, and have less energy and greater loss. In addition, as the antenna operates with higher order modes, the directional pattern usually has a large domain change because of the multi-periodic current distribution. Therefore, the application of higher order modes of the antenna is generally not recommended. As sample A obtains simulated gains of 3.9 dBi, 4.24 dBi, and 3.13 dBi in the *z*-axis direction with simulated radiation efficiencies of 63.5%, 82.6%, and 83.4% at 0.91 THz, 2.58 THz, and 4.2 THz, respectively, the receiving efficiencies of sample A towards 0.91 THz waves, 2.58 THz waves, and 4.2 THz waves are 63.5%, 82.6%, and 83.4%, respectively.
