The front end of line (FEOL) is the first portion of
IC fabrication where the individual components (
transistor
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...
s,
capacitor
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
s,
resistor
A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...
s, etc.) are patterned in a
semiconductor
A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
substrate. FEOL generally covers everything up to (but not including) the deposition of metal
interconnect
In telecommunications, interconnection is the physical linking of a carrier's network with equipment or facilities not belonging to that network. The term may refer to a connection between a carrier's facilities and the equipment belonging to its ...
layers.
Steps
For the
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss
", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...
process, FEOL contains all fabrication steps needed to form isolated CMOS elements:
# Selecting the type of
wafer to be used;
Chemical-mechanical planarization (CMP) and cleaning of the wafer.
#
Shallow trench isolation (STI) (or
LOCOS
''Locos: A Comedy of Gestures'' is the first novel of Spanish-born American writer Felipe Alfau (1902–1999), written in 1928 and published in 1936. The metafictional novel remained out of print until 1988 when it was reprinted by Dalkey Arch ...
in early processes with
feature size > 0.25 μm);
# Well formation;
#
Gate
A gate or gateway is a point of entry to or from a space enclosed by walls. The word is derived from Proto-Germanic language, Proto-Germanic ''*gatan'', meaning an opening or passageway. Synonyms include yett (which comes from the same root w ...
module formation;
#
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.
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.
Front-end of Line (FEOL) in Metrology
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.
See also
*
Back end of line (BEOL)
*
Integrated circuit
An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
References
Further reading
*"CMOS: Circuit Design, Layout, and Simulation" Wiley-IEEE, 2010.
pages 177-178(Chapter 7.2 CMOS Process Integration); pages 180-199 (7.2.1 Frontend-of-the-line integration)
by Lienig, Scheible, Springer, , 2020
Chapter 2: Technology Know-How: From Silicon to Devices pages 78-82 (2.9.3 FEOL: Creating Devices)
{{DEFAULTSORT:Feol
Electronics manufacturing
Semiconductor device fabrication