Low K materials

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Low K materials
Kejun Xia Auburn University,AL Nov 20, 2003
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outline

• Why Low k ?
• Questions
• Requirements for Low k
• General ways to gain Low k
• Low k materials
• Plasma process in Low k materials
• Conclusions
• Answers

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Questions

• Why k value of Carbon-doped Silicon Oxide
  increases after O2 plasma treatment?
• How to reduce the damage from Fluorine diffusion
  in SiOxCy film during plasma etching?

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Why Low k

• Speed
• The goal of dielectric material in advancement
  from one Tech Node to the flowed node is 20
  interconnect capacitance improvement.
• Power
• Cross talk
• The crossing effects between different connection
  lines contain signals.

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Interconnect crisis

• 200nm -gt130nm -gt90nm -gt 65nm
• SiO2 FSG ? ?
• K4.5 k3.6 klt2.5 ?
• Debate A
• Spin on Deposition Japan Korea
• CVD US Europe
• Debate B
• Evolutionary Dense materials as before
• Revolutionary Porous materials

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Requirements

• Electrical
• Klt3
• Thermal
• Stability up to 400 0C
• High glass transition temperature
• Chemical
• Etching compatibility
• Mechanical
• Hardness, modulus
• Structural
• Moisture absorption
• Adhesion to caps, hard masks, lines etc.

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Fundamental physics of k

• K originates from the polarization

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Basic approaches to reduce k

• Optimization of molecular structrue
• Minimize configurational and dipole
  polarizability, e.g. use of C-C and C-F bonds
• Reduce density and incorporation of porosity
• Add uniform and microscopic pores with k of 1
• Limitation Both approaches degrade the
  thermomechanical properties
• Proper tradeoff between dielectric constant and
  thermomechanical prosperities is important

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Electronic Polarizability vs. Strength of
Chemical Bonds
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Materials
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Materials
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Carbondoped Silicon Oxide (CDO)

• Klt2.9, candidate in the 130nm technology and
  beyond
• PECVD
• 400 0C, RF(13.56 MHz)
• C4H16O4Si4O2
• Disadvantage
• Oxygen plasma ashing (cnt )

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O2 plasma ashing

• O2 plasma is used in photoresist stripping which
  degrades CDO.
• Thickness 484.4nm -gt 396.4nm
• K 2.8 -gt3.6
• O2 plasma removes the entire C and part of Si
  contents in the film. The longer treatment time,
  the worse

Before
After
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Fourier Transform Infrared Spectroscopy (FTIS)
analysis on O2 plasma treatment

• Increasing of Si-OH leads to moisture uptaking,
  which is responsible for the increase of k value
  and leakage current

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Reducing O2 plasma damage by postdeposition He
plasma treatment

• He plasma treatment which is carried out before
  O2 plasma process does not effect the thickness
  and K value itself
• PECVD chamber, 700W, 20s, He 8Torr 1300sccm
• He plasma treatment reduces thickness loss and
  remain k value as that after deposition, i.e. 2.8
  
• Origin of the effect of He plasma treatment is
  not known yet

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Effect of He plasma treatment
He plasma treatment After deposition
After He and O2 plasma treatment
Without He plasma treatment
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Hydrocarbon material (SiLKTM)

• Cross linked Silica based materials
• Si-O network provides rigidity
• Organic groups lower k to 2.5-3.3

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SiLKTM dielectric properties
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Issues encounted

• Material
• SiLKTM semiconductor dielectric evaluated against
  a stringent set of requirements
• Process development
• New dielectric material required development of
  new unit process (i.e. dual hardmask patterning)
• New structures and ground rules were developed in
  order to deal with low modulus materials

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SiLKTM Plasma Etching and problem

• ICP, O2/N2/CH4 gas
• Problem and solution
• Formation of bow in high ratio contact holes
  which originates from the deflection of ions on
  the sidewalls, generating some etching. The
  distortion is explained by differential charging
  on the feature sidewalls.
• One solution is formation of a passivation layer
  on the sidewalls preventing the spontaneous
  attacks by oxygen reactives species in the
  plasma.

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Ultra Low k (ULK) material Nanoporous SiOxCy

• klt2.2
• Formation
• Porogen approach combined with spin-on deposition
  or PECVD
• Postdeposition treatment with H2 plasma or
  supercritical CO2 process or E-beam
• Inherent issues
• Moisture uptake or chemical absorption due to
  porosity
• Mechanical fragility, material are soft and
  brittle

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Diffusion barrier for Cu metallization

• Thickness consideration
• Barrier will increase overall k value
• Barrier should be thinner as possible without
  compromising its integrity
• A good step coverage
• Different materials (TaN, TiN, TiNSi) and
  different deposition techniques (PVD, CVD, ALD)
  are in competition

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CVD TiN on porous SiOC

• 10nm CVD TiN are required for a continuous
  barrier layer

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Mechanical behavior

• Hardness, modulus
• Insures the capability to support all the process
  steps including metal re-crystallization and
  packaging
• Adhesion strength between dielectric or metallic
  layers
• Insures stack stability during local or global
  stress variations including thermal treatments or
  process such as CMP

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Etching issue

• Fluorine species diffusion during plasma etching
• Penetrate into pores of SiOxCy film and react
  with hydrogen species in the course of Cu
  electroplating process and form HF molecules. HF
  then make larger void in the film
• Surface oxidized and k value increase

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One solution to etching issue

• It is indicated that the diffusion of the
  fluorine species is more significant for the
  films with fully interconnected pores
• Tune the pore-connectivity by varying the content
  of porogen.
• A trade off between k value and integrity ability

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Conclusions

• Low k materials has been one of the bottle neck
  in fulfill semiconductor roadmap for the nodes
  beyond 130nm. Many materials are still under
  research.
• CDO is one candidate for 130nm node and beyond.
• SiLKTM need a new set of process, many works have
  been done.
• ULK material has its unique low k value but has
  its weakness in mechanical behavior. Its
  integrity ability is under debate and research.

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Answer to Q1

• Why k value of Carbon-doped Silicon Oxide
  increases after O2 plasma treatment?
• O2 plasma treatment increases Si-OH which is
  hydrophilic in Carbon-doped Silicon film thus
  leads to moisture uptaking, which is responsible
  for the increase of k value and leakage current

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Answer to Q2

• How to reduce the damage from Fluorine diffusion
  in SiOxCy film during plasma etching?
• Control the content of porogen during film
  deposition

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References

• Introducing advanced ULK dielectric materials
  interconnects Performance and Intergration
  F.Fusalba, C.Le cornec, P.maury etal.
• Investigation of the Plasma Etching-Induced Pore
  structure transformation and diffusion of
  fluorine in porous Low-k thin films Kwang Hee
  Lee, Ji-Hoon Rhee, Sang Kook, et al.
• Etching of Low-k interconnect materials for next
  generation devices T.chevolleau, OlJOubert,
  N.Posseme, et al.
• The stability of Carbon-Doped silicon Oxide low
  dielectric constant thin films Y.H.Wang, R.Kumar
  
• Reduction of oxygen plasma damage by
  postdeposition Helium plasma treatment for
  Carbon-Doped Silicon Oxide Low Dielectric
  constant films Y.H.Wang, D.Gui, R.Kumar and
  P.D.Foo., Electrochemical and Solid-state
  Letters, 6(1) F1-F3 (2003)