Stress control of thin films in Plasma deposition

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Stress control of thin films in Plasma deposition
By Liwei Wang Professor Dr. Y. Tzeng For ELEC
7730 Date Nov 10, 2003
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• What is the microstructure formed during film
  growth cause the intrinsic tensile stress?
• How is dual frequency in PECVD control the
  stress of thin films?

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• Background Introduction
• ---Why it is important to control the
  stress in the
• thin films deposition?
• ---Where is the stress in the thin films
  come from?
• Thin film deposition techniques
• ---How to control the stress of Plasma
  deposited
• thin films?
• Case study

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• To control
• High stress in thin films can cause
• --- Adhesion problems.
• --- Long-term reliability.
• To make use of
• ---Use of the connection between stress and
• structure for producing nanostructure.

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• Extrinsic
• A non-uniform plastic deformation.
• Thermal stress
• Intrinsic
• Growth stresses
• Lattice misfit
• Substitutional or interstitial impurities
• Volume changes by phase transformation

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Thermal Stress results from the difference in the
thermal expansion coefficients of the substrate
and the film.
http//mmadou.eng.uci.edu/Classes/MSE621/MSE62101(
14).pdf.
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• Intrinsic stress results from the microstructure
  created in the film as atoms are deposited on the
  substrate.
• At substrate temperatures less than 20 of the
  melting point, intrinsic stress due to incomplete
  structural ordering dominates.

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• Biaxial stresses
• Compressive.
• Tensile.
• Shear and normal stresses
• Only important near edge.
• Important for fine line pattern in
  Microelectronic.

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Compressive stress
Tensile stress
The film wants to be "smaller" than the substrate
because it was "stretched" to fit.
The film wants to be "larger" than the substrate,
because it was "compressed" to fit.
Stress Control in NiV, Cr and TiW Thin Films used
in UBM and Backside Metallization, K. ODonnell,
J. Kostetsky, and R.S. Post
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Microvoids formed during film growth cause
attractive interaction of atoms across the voids.
Y. Fu et al. / Surface and Coatings Technology
167 (2003) 120128
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• Impact of heavy ions or energetic particles
  during deposition (like hitting film with a
  hammer), can pack atoms more tightly.
  Compressive stress.

• Atomic Peening - Energetic Particle Bombardment
  of growing film at low sputter pressures results
  in a dense film with compressive stress.

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Process parameters such as Sputtering
pressure Negative Bias voltage.
http//www.postech.ac.kr/mse/tfxs/2003_2/chapter3.
pdf
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Sputtering Film --TiW (high mass) --3000A
Figure. Stress control in TiW film by varying
sputtering pressures
Stress Control in NiV, Cr and TiW Thin Films used
in UBM and Backside Metallization, K. ODonnell,
J. Kostetsky, and R.S. Post
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Figure. AFM Imaging of TiW films processed at 2,
11, and 20 mtorr.
Stress Control in NiV, Cr and TiW Thin Films used
in UBM and Backside Metallization, K. ODonnell,
J. Kostetsky, and R.S. Post
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• The surface morphology indicate
• -?A dense microstructure in the TiW sputter
  deposited
• at 2 mTorr which is reflected in the
  large compressive
• stress measured for this film (- 1800
  MPa).
• -?A dense to a voided microstructure change
  in the
• TiW film deposited at 20 mTorr which is
  reflected
• in the tensile stress measured for this
  film (700 MPa).
• The increase in resistivity is consistent with
  the
• development of such a voided structure.

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Sputtering Film --NiV (low mass) --Cr (low
mass) --4000A
Figure. Stress control in NiV and Cr films by
varying RF bias voltage.
Stress Control in NiV, Cr and TiW Thin Films used
in UBM and Backside Metallization, K. ODonnell,
J. Kostetsky, and R.S. Post
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http//tftlcd.kyunghee.ac.kr/research/poly-Si/chap
ter4.html
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• Two main methods to control film stress in PECVD
• Dual frequency PECVD
• ---13.56MHz (HF) HF component used to control
• electron density.
• ---100-400kHz (LF) LF component used to
  control
• plasma sheath
  voltage ion energy
• distribution
• Gas flow

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In a conventional reactor, increase the RF power
to get a higher ion bombardment energy, the
plasma density will also increase.

• Two separate power supplies
• Each attached to one electrode
• Independent control of ion bombardment energy
  and plasma density.

http//www.batnet.com/enigmatics/semiconductor_pro
cessing/CVD_Fundamentals/plasmas/Cap_parameters.ht
ml
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• Films studied
• silicon nitride
• silicon dioxide
• Frequency interlacing 130 KHz (LF)
• 13,56 MHz (HF)
• Stress control
• tensile 40 MPa compressive120 MPa

http//www.oxfordplasma.de/process/sinsioka.htm
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• Gas flow ratio
• Neutral gas species

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• Film deposited
• Hydrogenated amorphous silicon nitride
  (a-SiNxH)
• Gas Species
• SiH4-NH3-H2

Stress control of hydrogenated amorphous silicon
nitride films deposited by PECVD, Hidenori Gamo1,
Hideyuki Eguchi1, Toshiaki Kurosu1,Kiyoharu
Nakagawa2,4, Mikka Nishitani-Gamo3, and Toshihiro
Ando4
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• Film studied
• Hydrogenated amorphous silicon nitride
  (a-SiNxH)
• Gas Species
• SiH4-NH3-N2
• Stress control
• Through dilution of the film precursors
  with an inert gas.
• Result
• The tensile stress in films without dilution
  reduces when argon
• is added to the plasma due to the Si-H
  plasma chemistry and
• film hydrogen bond density change.

Chemical influence of inert gas on the thin film
stress in plasma-enhanced chemical vapor
deposited a-SiN H films, M. J. Loboda and J. A.
Seifferly
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• Stress control of Thin film attract wide
  attention because the huge influence to
  Microelectronic performance and applications.
• Stress control can be achieved in sputtering by
  adjusting the sputtering pressure and negative
  bias voltage.
• Stress control can be achieved in PECVD by
  employing dual frequency and changing gas flow.

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• What is the microstructure formed during film
  growth cause the intrinsic tensile stress?
• --- Microvoids formed during film growth cause
  attractive interaction of atoms across the voids.
• How is dual frequency in PECVD control the
  stress of thin films?
• --- Independent control of ion bombardment
  energy and plasma density.