## 6 Fabrication Techniques and Challenges  
Integrated photonics utilizes a range of fabrication techniques to create compact and highly efficient photonic devices as shown in Figure 6[105; 149]. Among these methods, lithography emerges as a fundamental approach, facilitating the precise formation of optical elements on semiconductor substrates. Noteworthy examples include photolithography, electron beam lithography (EBL) [150], and nanoimprint lithography (NIL) [151; 152], each offering unique benefits in terms of resolution and scalability. Additionally, techniques such as ion implantation and plasma etching are instrumental in shaping WGs and fine-tuning material properties with exceptional accuracy. Moreover, epitaxial growth methods like MBE [153] and metal-organic chemical vapor deposition (MOCVD) [154] are essential for depositing semiconductor layers with customized optical and electrical properties. Bonding techniques play a critical role in integrated photonics, facilitating the seamless integration of diverse materials and components onto a single photonic chip. These techniques are essential for ensuring efficient light propagation, minimizing losses, and maintaining alignment precision between various photonic elements [155]. Effective bonding enables the combination of different functional materials, such as Si, InP and LN, each offering unique optical properties [156]. Etching techniques are vital in integrated photonics as they enable the precise structuring of photonic materials, crucial for fabricating components like WGs, resonators, and photonic crystals. These techniques ensure high-resolution definition and smooth sidewalls, which are essential for minimizing optical losses and enhancing device performance. By using various etching methods, such as reactive ion etching (RIE) and wet etching, manufacturers can create intricate photonic structures with high accuracy and repeatability [157; 158]. This precision is fundamental for developing complex photonic circuits and devices that are integral to advanced applications in optical communication, sensing, and signal processing. Integrated photonics holds immense promise for advancing various fields, yet its progression is impeded by several adversities. A significant obstacle is the development of compact, efficient light sources and detectors suitable for integration on a chip scale, essential for enabling intricate photonic functionalities. Fabricating lasers directly on a Si platform has been challenging due to its intrinsic properties. Si, while widely used in electronics, is an indirect bandgap semiconductor, meaning it inefficiently emits light when excited. This property severely limits its ability to generate laser light efficiently. Additionally, integrating diverse photonic elements like WGs, modulators, and detectors demands precise fabrication techniques and material compatibility, often presenting manufacturing complexities. Another obstacle involves minimizing light propagation losses and maintaining high signal fidelity, particularly at the nanoscale. Additionally,issues with standardization and scalability persist, limiting the widespread adoption and commercialization of integrated photonic technologies. Lowering the cost of PICs poses significant challenges despite their potential for revolutionizing various industries [159; 160]. One major obstacle lies in the complexity of the fabrication processes involved, which often demand specialized equipment and expertise, driving up production costs. In addition, the limited availability of materials suitable for photonic applications further complicates cost reduction efforts. Furthermore, achieving economies of scale remains a challenge due to relatively low production volumes compared to conventional electronic integrated circuits. The intricacies of PIC design and the need for stringent quality control add to manufacturing costs.