The advantages of the THz spectrum (0.1 to 30 THz) make it an attractive option for next-generation communication systems. The spectrum offers highly secure, high-throughput communication with minimal interference. The broader bandwidth available in the THz range significantly enhances the potential for higher data transmission rates compared to traditional RF-based systems.1 The availability of components in the THz range is more restricted compared to the RF and optical spectral regions. This limitation poses challenges in developing and deploying THz-based systems, as the technology and infrastructure for THz components are still in the early stages of development. A key concern in accepting THz technologies for real-time applications is the cost of the system. The physical size of the THz component decreases with increasing operating frequency. Traditional manufacturing technologies for most microwave components will not be appropriate at THz frequencies. New developments in fabrication technologies are providing a path to meet this requirement. This article discusses some of the more interesting and important recent developments in advanced fabrication techniques suitable for the THz regime.
MATERIAL SELECTION AND CHARACTERIZATION
Material selection for fabrication is important. Studying the properties of microwave dielectric materials at terahertz frequencies is crucial for precise component design. Accurate characterization of dielectric properties, such as permittivity and loss tangent, helps in designing components with the desired performance characteristics, minimizing signal loss and achieving better overall efficiency in terahertz technology applications.2
This area of investigation has garnered interest and is seeing significant activity. For instance, the characterization of photopolymers for their dielectric properties is discussed by Nattapong Duangrit et al.3 Time-domain spectroscopy techniques have been used to study the dielectric properties of polymer samples. Material properties of the most commonly used copper, graphene and carbon nanotube (CNT) and their suitability for THz antenna are studied in a paper by Sasmita Dash et al.4 Poor radiation efficiency and increased propagation delay are observed for copper-based THz antennas. Conductivity and skin depth of copper tend to be lower at the THz range. CNT has better conductivity and graphene exhibits better performance compared to copper. Miniaturization and reconfigurability are claimed as the advantages of graphene-based antennas. A detailed study on graphene-based antenna geometries at THz frequency has been carried out and reported by Diego Correas-Serrano et al.5 The conductivity of graphene, which is strongly influenced by its band structure, has been mathematically modeled and analyzed. Dielectrics used in THz frequency metasurfaces are reviewed by Rajour Tanyi Ako et al.6 The use of dielectrics as substrates and spacers and appropriate fabrication techniques have been studied. Thin film dielectric sheets and liquid-coated and solidified dielectric materials have been studied and proposed for spacers. Thick film or bulk dielectric materials formed by etching or machining techniques have been studied and proposed as solutions.
Figure 1 Advances in high frequency component fabrication methods.
ADVANCED FABRICATION METHODS
Traditional fabrication methods require a cleanroom environment, advanced machinery and highly skilled personnel to produce nanoscale components for very high frequency applications. Precise and error-free fabrication techniques are needed to ensure performance in the THz band. Additive manufacturing methods address the limitations of conventional techniques and have been adapted for fabricating a broad range of active and passive components. Figure 1 shows a timeline of some of the important fabrication developments that have gotten the industry to its present state. Figure 2 shows a timeline of some of the processes that support high frequency components and system development.
Figure 2 Process techniques for high frequency component fabrication.
Figure 3 (a) On-chip antenna in 65nm CMOS process.8 (b) 1 × 2 antenna array on 65 nm CMOS.8 (c) 2 × 2 antenna array on 65 nm CMOS.8
CMOS FABRICATION OF THZ COMPONENTS
Reduced gate length in nanometer CMOS transistors, device scaling and the ease of integrating analog and digital components make silicon a viable, low-cost material for THz ICs. In the CMOS process, the antenna is fabricated on a silicon substrate, along with all the other components. This improves the antenna performance by reducing interconnects in on-chip antennas. A prototype test chip for terahertz reconfigurable metamaterial in 180 nm CMOS technology was developed and described by Zihan Ning.7 Added parasitics resulting from scaling bring additional complications in the design of THz circuits. A 65 nm CMOS process has been used to design a compact broadband antenna array with an operating frequency centered at 300 GHz. This effort has been reported by Changmin Lee and Jinho Jeong.8 Structures that exhibit improved gain and bandwidth as a result of this study have been fabricated and the antenna is shown in Figure 3a. As reported, a novel ground structure was developed to increase the isolation between elements. This new structure is shown for a 1 × 2 antenna array in Figure 3b and a 2 × 2 antenna array in Figure 3c.
Metamaterial antennas with substrate-integrated waveguide (SIW) via holes are demonstrated by Mohammad Alibakhshikenari et al.9 The antenna described in this paper is fabricated using stacked layers for a compact antenna structure. To reduce surface waves and substrate loss, metallic via holes are formed in the top silicon layer. A standard 0.18 μm CMOS process has been used in a 0.4 THz on-chip antenna design with a slotted radiator.10 SIW vias connect the top and bottom metal layers and create a back cavity to reduce surface waves. The cavity directs the radiation toward a low-resistivity substrate that improves the broadside radiation. Appropriately designed SIW sidewalls compensate for the effects of the lossy silicon substrate.
TERAFETS
Antenna-coupled field-effect transistors (TeraFETs) are specialized FETs designed for use in the THz frequency range. These devices combine the high speed electronic properties of FETs with antenna structures, making them capable of detecting, generating and manipulating electromagnetic waves in the THz regime. The modeling and experimental characterization of silicon CMOS detectors for THz radiation using TeraFETs has been extensively studied in the paper from F. Ludwig et al.11 TeraFET detectors have garnered increasing attention due to their high responsivity, ease of fabrication, ultra-fast response times and significant tunability through adjustments to the gate, doping levels and channel structures.12
SILICON MICROMACHINING
After selecting a suitable material, part of the substrate or a thin film is removed by different etching techniques to obtain micromachined structures. Silicon micromachining uses two main methods: surface micromachining and bulk micromachining. Surface micromachining works on thin films deposited on a silicon wafer to create surface structures. In contrast, bulk micromachining carves features directly from the silicon substrate. Etching is crucial for patterning silicon substrates and includes isotropic and anisotropic types. Isotropic etching, performed wet or dry, creates uniform etching in all directions, leading to rounded features. Anisotropic etching, mainly using dry methods, etches in specific directions, producing sharp-edged features. Both etching techniques are vital for achieving precise and structured results in silicon micromachining. Silicon micromachining has advantages like low fabrication cost, higher tolerance limits, smaller feature sizes and easy integration of components. Micromachined components have reduced surface roughness and insertion loss. Silicon micromachining has been demonstrated as a suitable method for THz frequency components.
Figure 4 (a) Dual-line antenna array.15 (b) Silicon-micromachined interposer.16
A. Madannejad et al.13 present an innovative silicon-micromachined, low-profile, high gain antenna designed for wideband performance across the entire 500 to 750 GHz waveguide band. The fabrication process is conducted on a silicon-on-insulator (SOI) wafer. A compact silicon-micromachined crossover switch prototype consisting of two hybrid couplers and two SPST switches is demonstrated by A. Karimi et al.14 The crossover switch prototype is composed of four vertically stacked SOI chips. A silicon-micromachined dual-port, dual-line antenna array with unbalanced feeding is discussed by A. Karimi et al.15 The fabrication of the antenna array designed in this paper, as shown in Figure 4a, employs advanced silicon micromachining techniques, specifically deep reactive ion etching (DRIE) and sidewall metallization. DRIE allows for precise etching of deep, vertical structures in the silicon. At the same time, sidewall metallization ensures uniform conductive coating along the etched features, enhancing the performance and integrity of the antenna array. These techniques enable the creation of complex, high-precision structures required for the antenna’s optimal operation in high frequency applications.
Figure 4b shows a silicon-micromachined interposer that has been used in multiport components discussed by Adrian Gomez et al.16 Silicon-micromachined test interposers offer higher accuracy, lower cost, greater versatility and the ability to integrate loads. These factors make them a crucial tool for advancing the development of sub-THz waveguide components.
Despite the advantages of silicon micromachining, several limitations challenge its application in complex circuit elements and signal chains:
Scalloped Sidewalls: The DRIE process can cause scalloping on etched sidewalls, increasing surface roughness and potentially affecting device performance and efficiency.
Sloped Sidewalls:Due to varying ion flow and density during deep trench etching, sidewalls may become sloped rather than vertical. Controlling this slope is crucial for maintaining precise dimensions and performance, especially in MEMS systems.
Aspect Ratio Dependent Etching (ARDE):ARDE causes variability in sidewall profiles depending on trench aspect ratios, leading to diverse profiles for different device geometries.
Surface Roughness: Surface roughness is a critical factor contributing to insertion loss in rectangular waveguides. Minimizing this roughness is essential for low loss, especially at high frequencies.
IC Integrability: The fabrication method should be compatible with integrated circuit processes, allowing simultaneous fabrication of active and passive components.