REACTION MECHANISM AND PROFILE
EVOLUTION FOR CLEANING AND SEALING
POROUS LOW-k DIELECTRICS USING He/H2 AND
Ar/NH3 PLASMAS
Juline Shoeba) and Mark J. Kushnerb)
a)
b)
Department of Electrical and Computer Engineering
Iowa State University, Ames, IA 50011
jshoeb@eecs.umich.edu
Department of Electrical Engineering and Computer Science
University of Michigan Ann Arbor, Ann Arbor, MI 48109
mjkush@umich.edu
http://uigelz.eecs.umich.edu
October 2010
AVS10_01
*Work supported by Semiconductor Research Corporation
AGENDA
 Sealing of Low-k Dielectrics
 Modeling Platforms
 Generation of Hot H
 Polymer Removal and PR Stripping In He/H2 Mixtures
 Sealing Mechanism Using Ar/NH3 Plasma Treatment
 Sealing Efficiency
 Pore Radius and Aspect Ratio
 Pulsing Effect On Etch Rate
AVS10_02
University of Michigan
Institute for Plasma Science & Engr.
POROUS LOW-k DIELECTRICS
 The capacitance of the
insulator contributes to RC
delays in interconnect
wiring.
 Low-k porous oxides, such
as C doped SiO2 (CHn lining
pores) reduce the RC delay.
 Porosity  0.5,
Interconnectivity  0.5.
Ref: http://www.necel.com/process/en/images/porous_low-k_e.gif
 Inter-connected pores open
to plasma may degrade kvalue by reactions with
plasma species.
 Desire to seal pores to
prevent diffusion into
porous network.
AVS10_03
University of Michigan
Institute for Plasma Science & Engr.
LOW-k
PROCESS INTEGRATION
 Typical porous SiO2 has CH3
lineing pores with Si-C bonding –
referred to as SiOCH.
 Ave pore radius: 0.8-1.1 nm
Mask
 Porosity: up to 50%
 Etching and sealing SiOCH is an
integrated, multistep process
Porous
Low-k
SiCOH
Si
AVS10_04
 Etch
Ar/C4F8/O2 CCP
 Clean
 Activate
Ar/O2 or He/H2 ICP
He/H2 ICP
 Seal
Ar/NH3 ICP
University of Michigan
Institute for Plasma Science & Engr.
PORE SEALING
PROCESS INTEGRATION
 Step 1: Ar/C4F8/O2 CCP
Etch trench leaving PR
mask and CFn polymer
 Step 2: Ar/O2 ICP
Remove PR and CFn
polymer with O radicals
 O atoms diffuse into
pore network to etch
CH3 groups.
 Degrades low-k
material.
AVS10_05
University of Michigan
Institute for Plasma Science & Engr.
PORE PLASMA
SEALING MECHANISM
 Step 3: He ICP
Activate surface by
sputtering and photodetachment to create
dangling bonds.
 Step 4: Ar/NH3 ICP
Seal pores with NHn
radicals by forming C-N
and Si-N bonds which
bridges opening.
AVS10_06
University of Michigan
Institute for Plasma Science & Engr.
He/H2 CLEAN-ACTIVATE
 Highly motivated to eliminate
Ar/O2 step as degradation of
SiOCH occurs.
 Possible alternative is He/H2
ICP plasma cleaning.
 Hot H atoms (> 1 eV) are
produced by dissociative
excitation and charge
exchange.
e  H2  H  H *  e
H *  Si : CFn   Si  HF, CHFn , H *   Si  CH3   Si  CH n   Hn
 H* remove PR and CFn while activating surface sites.
 Low mass of H reduces likelihood for sputter of CHn.
 Must optimize H* production
AVS10_07
University of Michigan
Institute for Plasma Science & Engr.
MODELING : LOW-k PORE SEALING
He/H2
PLASMA
Coils
Plasma
Metal
Wafer
Porous Low-k
Substrate
 Hybrid Plasma
Equipment Model
(HPEM)
AVS10_08
Energy and
angular
distributions
for ions and
neutrals
Ar/NH3
PLASMAS
 Plasma Chemistry
Monte Carlo Module
(PCMCM)
 Monte Carlo
Feature Profile
Model (MCFPM)
University of Michigan
Institute for Plasma Science & Engr.
MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
HPEM
PCMCM
Energy and angular
distributions for ions
and neutrals
MCFPM
Provides etch rate
And predicts etch
profile
AVS10_09
 The MCFPM resolves the surface
topology on a 2D Cartesian mesh to
predict etch profiles.
 Each cell in the mesh has a material
identity. (Cells are 4 x 4 A ).
 Gas phase species are represented by
Monte Carlo pseuodoparticles.
 Pseuodoparticles are launched towards
the wafer with energies and angles
sampled from the distributions obtained
from the PCMCM.
 Cells identities changed, removed, added
for reactions, etching, and deposition.
University of Michigan
Institute for Plasma Science & Engr.
TYPICAL PLASMA PROPERTIES: H2/He ICP
 Total ion density (cm-3):
1.5 x 1011
 Neutral densities (cm-3):
H
H2
H2(v=1,5)
9 x 1012
7 x 1013
1.5 x 1012
 Major fluxes to the
substrate (cm-2 s-1):
H
H2
H2(v=1,3)
H+
AVS10_10
6 x 1017
3 x 1018
6 x 1016
2 x 1015

Conditions: H2/He = 25/75,
10 mTorr, 300 W ICP
University of Michigan
Institute for Plasma Science & Engr.
Ar/C4F8/O2 CCP TRENCH ETCH
Photo-Resist
 CCP for trench etch.
 Ar/C4F8/O2 = 80/15/5
 40 mTorr, 300 sccm
 10 MHz
 5 kW
 CFx polymer deposited on the sidewalls efficiently seal the open
pores. CFx polymers are harmful to
diffusion barrier metals such as Ti
and Ta.
Porous
Low-k
SiCOH
 Polymer layers can be removed by:
 He/H2 plasmas without surface
damage.
 O2 plasmas that etch the CH3
groups.
AVS10_11
Animation Slide-GIF
Si
University of Michigan
Institute for Plasma Science & Engr.
HOT H GENERATION: He/H2 ICP
 Vibrational Excitation
e + H2(v=0)  H2(v=1)
+e
e + H2(v=n)  H2(v=n+1) + e
 Hot H Generation
e + H2(v=n)  H** + H** + e
 Charge Exchange Reactions
H2(v=n) + H2+  H2(v=n)** + H2+
H2(v=n) + H2+  H**
H
+ H2+  H2(v=0)** + H+
H2(v=n) + H+  H**
H
AVS10_12
+ H 3+
+ H+  H**
**Translationally
+ H 2+
+ H+
hot

Conditions: H2/He =
25/75, 10 mTorr, 300 W
ICP
University of Michigan
Institute for Plasma Science & Engr.
POLYMER REMOVAL AND PR STRIPPING
 He/H2 plasma used for both polymer (P)
removal and photoresist (PR) stripping.
PR
 Hot H, H2, H+ and H2+ remove polymer
and masking PR layers as CH4, HF, and
CxHyFz
H** + P(s)
 CF + HF
H** + P(s)
 CHF2
H2** + P(s)
 CH2F2
Porous
Low-k
SiCOH
H** + PR(s)  CH4
H2** + PR(s)  CH4.
 CHn groups are also activated by H
removal
Si
H** + CHn(s)  CHn-1 + H2.
Animation Slide-GIF
AVS10_13
**Translationally
hot
University of Michigan
Institute for Plasma Science & Engr.
POLYMER REMOVAL, CH3 DEPLETION
 Ar/O2 plasma efficiently
removes polymer.
 Also removes CH3 groups
in pores as O atoms
diffuse into the porous
network.
Low-k
SiCOH
 Net result is increase in
pore size.
 Pore openings can get too
large to easily seal.
 He/H2 plasma removes
polymer without
significantly depleting
CH3.
AVS10_14
Si
University of Michigan
Institute for Plasma Science & Engr.
SEALING MECHANISM IN Ar/NH3 PLASMA
 N/NHx species are adsorbed by activated sites forming Si-N and C-N
bonds to seal pores.
 Further Bond Breaking
 N/NHx Adsorption
M+ + SiO2(s)
 SiO(s) + O(s) + M
M+ + SiO(s)
 Si(s)
NHx + SiOn(s)
 SiOnNHx(s)
NHx + Si(s)
 SiNHx(s)
NHx + CHn-1 (s)
 CHn-1NHx(s)
NHx + P*(s)
 P(s) + NHx(s)
+ O(s) + M
 SiNHx-NHy/CNHx-NHy compounds seal the pores where end N are
bonded to Si or C by C-N/Si-N
NHy + SiNHx(s)
 SiNHx-NHy(s)
NHy + CHn-1NHx(s)  CHn-1NHx-NHy(s)
AVS10_15
University of Michigan
Institute for Plasma Science & Engr.
PORE-SEALING BY SUCCESSIVE He/H2 AND NH3/Ar
TREATMENT
·Initial Surface Pores
·He/H2 Plasma
Site Activation
·Ar/NH3 Plasma
Pore Sealing
 Surface pore sites are activated by 610s He/H2 plasma treatment.
 Ar/NH3 plasma treatment seals the pores by forming bridging Si-N,
N-N and Si-N bonds.
Animation Slide-GIF
AVS10_16
University of Michigan
Institute for Plasma Science & Engr.
SEALING: WITH POLYMER REMOVAL AND PR STRIP
He/H2 Activation
Sealing
 Ar/O2 Clean: additional
He treatment is
required for surface
activation, followed by
NH3 plasma sealing.
 He/H2 Clean: Performs
both activation and
cleaning in a single
step. Can seal with NH3
just after the clean.
He/H2
Activation
Si
AVS10_17
Animation Slide-GIF
Sealing
Si
University of Michigan
Institute for Plasma Science & Engr.
SEALING EFFICIENCY: PORE RADIUS
 Ar/O2 Clean: Sealing efficiency
decreases with increasing pore
size.
Ar/O2
Clean
He/H2
Clean
 H2/He Clean: Sealing is less
sensitive to pore radius.
Good
Sealing
Poor
Sealing
Animation Slide-GIF
AVS10_18
University of Michigan
Institute for Plasma Science & Engr.
SEALING EFFICIENCY: ASPECT RATIO
 O2 Clean: sealing
efficiency on
sidewalls decreases
with increasing
aspect ratio.
 He/H2 Clean: sealing
does not degrade with
higher aspect ratio.
 Hot H activates all of
the surface sites due
to its broad angular
distribution.
AVS10_19
University of Michigan
Institute for Plasma Science & Engr.
PULSING EFFECT ON PR REMOVAL: He/H2 ICP
PR
SiCOH
 Duty cycle reduction increases ion to
neutral flux ratios.

AVS10_20
Conditions: H2/He = 25/75,  A low duty cycle can increase PR
removal rate.
10mTorr, 300 W ICP
CONCLUDING REMARKS
 Integrated porous low-k material sealing was investigated
 Ar/C4F8/O2 Etch
 H2/He Clean, PR Strip, and Surface Activation
 Ar/NH3 Sealing
 He/H2 plasmas clean polymer, strips off PR and activates
surface sites in a single step. Higher activation and lower
damage seal the surface better.
 Pulsing can enhance the PR removal rate.
 Si-N and C-N bonds formed by adsorption on active sites
followed by one N-N bond linking C or Si atoms from opposite
pore walls.
 For Ar/O2 clean, sealing efficiency degrades when pore radius
is >1 nm and aspect ratio >10. He/H2 clean enables sealing of
larger pores and higher aspect ratio trenches.
AVS10_21
University of Michigan
Institute for Plasma Science & Engr.