# A CMOS-Based Terahertz Reconfigurable Reflectarray with Amplitude Control: Design and Validation
## 1 Introduction  
The terahertz (THz) frequency range (0.1 THz to 10 THz) holds immense potential for next-generation high-speed wireless communication, sensing, and imaging applications [1, 2, 3, 4]. However, the severe free-space path loss of THz waves necessitates the development of high-gain beamforming with agile reconfigurability to adapt to dynamic channel conditions [5]. Traditional approaches, such as mechanical beam-steering [6] and phased arrays [7, 8], face limitations in THz regimes: mechanical systems suffer from high processing accuracy, complex servo system and slow scanning speed, while the traditional phased arrays require complex feeding networks and expensive active components. Alternatively, reconfigurable metasurfaces consists of subwavelength-scale tunable meta-units that enable agile wavefront modulation [9], offering advantages such as high integration, low cost, and low power consumption compared to traditional phased arrays [10, 11]. Therefore, reconfigurable metasurfaces have emerged as a promising technology for enabling high gain and programmable wireless environments in future networks [12, 13]. Reconfigurable metasurfaces typically rely on tuning devices such as a Complementary Metal-Oxide-Semiconductor (CMOS) [14, 15, 16, 17], High Electron Mobility Transistor (HEMT) [18, 19, 20, 21, 22], graphene [23, 24, 25, 26], liquid crystals [27, 28, 29, 30], vanadium dioxide (VO\({}_{2}\)) [31, 32, 33, 34], and Micro-Electro-Mechanical Systems (MEMS) [35, 36] to dynamically control the surface current distribution of subwavelength periodic structures and the amplitude and phasechange in electromagnetic waves. In the sub-THz band, HEMT devices generally achieve higher operating frequencies and lower noise than CMOS, yet face challenges in large-scale integration and higher fabrication complexity [20]. Graphene and liquid crystals, as tunable materials, are not limited by the cutoff frequencies of CMOS and HEMT devices and offer broad operational bandwidth, making them valuable for terahertz wave modulation. However, graphene faces challenges in fabrication and reliability for large-scale integration [24], while liquid crystals require high driving voltages and exhibit slow response times [28]. Other phase change materials, such as VO\({}_{2}\), are also candidate solutions for terahertz signal control [32]. They respond not only to electrical stimuli but also to thermal excitation, though their response times remain significantly slower than semiconductor-based approaches. Compared to CMOS processes, these control methods are often troubled by the difficulty of controlling each element individually and scaling up to large sizes in THz applications. MEMS enable terahertz wave manipulation by physically reconfiguring the metasurface structure [35; 36]. Compared to semiconductor-based methods, MEMS offer advantages such as low insertion loss and wide bandwidth, but suffer from limited operational lifetime. In this context, silicon-based CMOS technology presents a better solution [14; 15; 16; 17]. Mature CMOS processes, capable of micron-level precision, are suitable for fabricating THz metasurface element structures. Additionally, CMOS-based approaches offer multilayer interconnect capabilities and mass production feasibility, enabling individual element control and large-scale scalability, making them ideal for metasurface fabrication. Due to the parasitic characteristics of CMOS processes in high-frequency bands, it is unavoidable that the performance of terahertz switches decreases. One notable development used the parallel inductive loops for resonance with the parasitic capacitance for switch performance improvement [15]. The CMOS-based reconfigurable transmitarray antenna (RTA) [15] operated at 300 GHz with an eight-switch integrated metasurface element. Another CMOS-based RRA [16; 17], fabricated by advanced 22 nm process, employed two integrated Fin Field-Effect Transistor (FinFET)s as a polarization conversion method. A \(\pm\)45\({}^{\circ}\) beam scanning range is achieved at 265 GHz.