Nanosheet Channel Formation is the process of creating suspended horizontal silicon sheets that form the transistor channel in Gate-All-Around (GAA) transistors — enabling the gate to wrap fully around the channel for superior electrostatic control at sub-3nm.

Why Nanosheets?

• FinFET limit: At < 3nm gate length, fin width must be < 6nm → manufacturing variability dominates.
• GAAFET nanosheet: Gate wraps all four sides → better SCE control, allows wider channel for more current.

Nanosheet Stack Formation

1. Superlattice Growth: Alternating SiGe and Si layers grown epitaxially: `` Si (nanosheet channel, 5-8nm thick) SiGe (sacrificial layer, 8-10nm thick) Si (channel) SiGe (sacrificial) Si (channel) [3-5 pairs typical] `` 2. Fin Patterning: SADP/SAQP to pattern fin pitch (same as FinFET). 3. Fin Etch: Etch through entire superlattice to form nanosheet "stack fin".

Dummy Gate Formation (Same as Gate-Last Flow)

1. Gate oxide + poly gate deposited over stack fin. 2. Poly gate patterned, spacers formed. 3. S/D recess, SiGe S/D epi, PMD deposit, CMP.

Inner Spacer Formation

1. SiGe layers laterally recessed through dummy gate-adjacent region: H2O2 or HCl. 2. Inner spacer material (SiN or SiCO) deposited by ALD — fills recess. 3. Etch back inner spacer to leave only the lateral recess filled. 4. Inner spacers isolate SiGe sacrificial from future metal gate.

Channel Release (Nanosheet Release)

1. Remove dummy poly gate (replacement gate flow). 2. Selective SiGe etch inside gate cavity: H2O2 or HCl removes SiGe, not Si. 3. SiGe:Si selectivity > 100:1 — leaves free-standing Si nanosheets between inner spacers. 4. Nanosheets now suspended — gate wraps all four sides.

Gate Fill

• ALD HfO2 conformal around all nanosheets.
• ALD TiN work function metal wraps each sheet.
• WN or W fill metal completes gate stack.

Nanosheet GAA transistor fabrication is the most complex process sequence in the history of CMOS — requiring precise SiGe/Si superlattice growth, inner spacer formation, and selective channel release to create floating silicon bridges at nanometer scale.