CMP Fill for Reduced STI Variability

1
CMP Fill for Reduced STI Variability
Student Puneet Sharma Advisors Andrew B. Kahng,
Alex Zelikovsky Theme System-Level Living
Roadmap To appear in ICCAD06
Introduction
Methodology
Experimental Study

• Our testcases 2 large designs created by
  assembling smaller ones
• Mixed 2mm x 2mm, 756K cells
• OpenRisc8 2.8mm x 3mm, 423K cells SRAM

• Shallow trench isolation (STI) mainstream
  inter-device electrical isolation technique used
  in all designs today
• Chemical mechanical planarization (CMP) critical
  process step in STI to remove excess deposited
  oxide
• Imperfect CMP ? Loss of functional and parametric
  yield
• Post-CMP topography variation ? process (esp.
  defocus) variation
• CMP is pattern dependent ? fill can reduce
  post-CMP variability
• Traditional fill tile based, used with expensive
  reverse etchback
• Our goal fill insertion for superior post-CMP
  topography characteristics without use of reverse
  etchback
• Results show proposed method achieves
  significantly better density and superior
  post-CMP topography (as predicted by CMP
  simulation)

• Objectives of fill insertion
• Minimize oxide density variation
• ? Overburden oxide uniformly removed from all
  regions
• ? Enlarges planarization window as oxide clears
  simultaneously
• Maximize nitride density
• ? Enlarges planarization window as nitride
  polishes slowly
• Shrinkage ? Oxide density depends on nitride
  density
• ? Insert fill (nitride features) to control
  nitride and oxide densities
• Dual-objective problem formulation Insert dummy
  fill
• Given STI regions where fill can be inserted,
  shrinkage a
• Constraint No DRC violations
• Objectives (1)min. oxide density variation,
  (2)max. nitride density
• Minimize oxide density variation
• Use previously proposed LP-based solution
• Inputs min. oxide density (OxideMin and and
  max. oxide density (OxideMax) per tile
• Output target oxide density (OxideTarget) per
  tile
• For min. oxide density shrink nitride features
  by a
• For max. oxide density insert max. fill, shrink
  nitride features by a
• Nitride maximization problem formulation Insert
  dummy fill

Background

• In STI, substrate trenches filled with oxide
  surround devices or group of devices that need to
  be isolated
• Relevant process steps
• Diffusion (OD) regions covered with nitride
• Trenches created where nitride absent and filled
  with oxide
• Chemical Mechanical Polishing (CMP) to remove
  excess oxide over nitride (overburden oxide)
• Unfortunately, CMP is not perfect
• Planarization window Time window in which CMP
  may be stopped. Stopping sooner leaves oxide over
  nitride, stopping later polishes silicon under
  nitride. Larger window desirable.

After Perfect CMP
Before CMP
Conclusions
Max. nitride fill (purple rectilinear region)

• Imperfect STI CMP causes functional and
  parametric yield loss
• Traditionally tile-based fill used with expensive
  reverse etchback to control post-CMP topography
  variability
• Our fill insertion approach focuses on (1) oxide
  density variation minimization, and (2) nitride
  density maximization
• Large nitride fill features contribute to nitride
  and oxide densities, small ones to nitride only ?
  shape fill to control both densities
• Proposed max. nitride fill insertion with holes
  to control oxide density and achieve high nitride
  density
• Results indicate significant decrease in oxide
  density variation and increase in nitride density
  over tile-based fill.
• CMP simulation shows superior CMP
  characteristics, planarization window increases
  by 17, and step height decreases by 9.

Optimal hexagon cover (beige)
Holes created in nitride at centers of hexagons
ensure zero oxide contribution