Front end of line

The front end of line (FEOL) is the first portion of IC fabrication where the individual components (transistors, capacitors, resistors, etc.) are patterned in a semiconductor substrate.^[1] FEOL generally covers everything up to (but not including) the deposition of metal interconnect layers.^[2]

For the CMOS process, FEOL contains all fabrication steps needed to form isolated CMOS elements:^[3][4]

1. Selecting the type of wafer to be used; Chemical-mechanical planarization (CMP) and cleaning of the wafer.
2. Shallow trench isolation (STI) (or LOCOS in early processes with feature size > 0.25 μm);
3. Well formation;
4. Gate module formation;
5. Source and drain module formation.

Finally, the surface is treated to prepare the contacts for the subsequent metallization. This concludes the FEOL process, that is, all devices have been built.^[4]

Following these steps, the devices must be connected electrically as per the nets to build the electrical circuit. This is done in the back end of line (BEOL). BEOL is thus the second portion of IC fabrication where the individual devices are connected.^[4]

Advanced packaging involves wafer-level processing, and thus several Front-end of Line (FEOL) and Back-end of Line (BEOL) processes are related to it. Key steps such as bump inspection, die attachment, wafer cutting characterization, and CMP pad characterization are essential to ensure precise interconnect formation, defect-free surfaces, and robust mechanical integrity for heterogeneous integration.

• Bump inspection: Requires accurate height and coplanarity measurements to ensure electrical connections.
• Wafer cutting characterization: Focuses on evaluating edge quality to avoid microcracks and debris that may affect bonding and reliability.
• CMP pad characterization: Ensures that the polishing process maintains surface uniformity, which is critical for planarization before further processing.

Recent advances in optical metrology and interferometric techniques have enabled more efficient and accurate surface characterization at these stages. Studies have demonstrated the effectiveness of confocal microscopy for assessing surface topography in metal-based microstructures, and the feasibility of in situ interferometric monitoring of CMP pad conditions during planarization processes.^[5][6]

• Back end of line (BEOL)
• Integrated circuit

• "CMOS: Circuit Design, Layout, and Simulation" Wiley-IEEE, 2010. ISBN 978-0-470-88132-3. pages 177-178 (Chapter 7.2 CMOS Process Integration); pages 180-199 (7.2.1 Frontend-of-the-line integration)
• "Fundamentals of Layout Design for Electronic Circuits", by Lienig, Scheible, Springer, doi:10.1007/978-3-030-39284-0ISBN 978-3-030-39284-0, 2020. Chapter 2: Technology Know-How: From Silicon to Devices, pages 78-82 (2.9.3 FEOL: Creating Devices)