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JUNE 2024 I PCB007 MAGAZINE 73 The rapid advancement in photonic integrated circuits (PICs), which combine multiple optical devices and functionalities on a single chip, has revolutionized optical communications and comput- ing systems. Researchers at EPFL have developed scalable photonic integrated circuits, based on lith- ium tantalate, marking a significant advancement in optical technologies with the potential to wide- spread commercial applications. Recently, the lithium niobate-on-insulator wafer platform has emerged as a superior material for photonic integrated electro-optical modulators due to its strong Pockels coef- ficient, which is essential for high-speed optical modulation. Nonetheless, high costs and complex production require- ments, have kept lithium nio- bate from being adopted more widely, limiting its commercial integration. Scientists led by Professor Tobias J. Kippenberg at EPFL and Professor Xin Ou at the Shanghai Institute of Microsystem and Information Technology (SIMIT) have created a new PIC platform based on lithium tantalate, a close relative of lithium niobate. The PIC leverages the material's inherent advantages and can transform the field by making high-quality PICs more economically viable. The researchers devel- oped a wafer-bonding method for lithium tantalate, which is compatible with silicon-on-insulator pro- duction lines. They then masked the thin-film lith- ium tantalate wafer with diamond-like carbon and proceeded to etch optical waveguides, modula- tors, and ultra-high quality fac- tor micro resonators. With this approach, the team was able to fabricate high-effi- ciency lithium tantalate PICs with an optical loss rate of just 5.6 dB/m at telecom wave- length. The work paves the way for scalable, cost-effective man- ufacturing of advanced electro- optical PICs. (Source: EPFL ) A New, Low-cost, High-efficiency Photonic Integrated Circuit every 10 mv. Typical ORP controls will hold to within ±10 mv of the setpoint, so the potential change in etch rate is 0.5 µm/min. However, the etch vs. ORP graph for cupric chloride in this range shows a plateau between 540 mv and 580 mv, where the etch rate remains fairly flat at around 25 µm/min. e best con- trol point for the ORP is 560 mv, where the ORP controller can maintain ±10 mv setpoint with virtually no change in etch rate. Most etch- ers will have an ORP monitor as standard, but it will cost $15,000–$20,000 to add the equip- ment needed to make it a controller. If you are contemplating doing high-density circuits, an ORP controller is a must. ORP monitors can also be useful for alkaline etch, but we will discuss this in more detail next month. For cupric chloride, the free acid level (HCl) must also be controlled closely. e free acid is necessary to prevent oxide buildup on the cop- per surface (which slows the etch rate) and to keep the Cu +1 ions in solution. With too little free acid, the Cu +1 ions will precipitate out of solution leading to an inaccurate ORP reading. e recommended operating range for free acid is from 0.5N (18 grams/liter HCl) to 3N (110 gpl HCl). Within this range, the change in etch rate is 0.75 µm per min/ for each 0.1N increase or decrease in free acid content so free acid must be controlled to at least ±0.1 N. An acid controller will mean an additional $8,000– $10,000 to the cost of the etcher but is vitally important for high-density circuits. Next month, I will continue with more alka- line etchant control issues and controls for developers and strippers. PCB007 Don Ball is a process engineer at Chemcut. To read past columns or contact Ball, click here.