What determines step coverage in

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Nov 15, 2013 (3 years and 1 month ago)

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What determines step coverage in
sputtering?

Petteri Kilpinen

S
-
69.4114 Postgraduate Course in Electron
Physics II

7.5.2010

Contents


sputtering (Argon ion bomardment) PVD process


3 different simulation methods for sputtering process


W
hat

good or bad

step coverage mean?


step coverage of sputtering process 1
-

11


Conlusion & why sputtering process is good?


References

Sputtering (Argon ion bomardment) PVD process


Most important Physical
Vapour Depositon process


Dominat method for thin
film deposition of various
materials in IC prosessing


Low substrate temperature
= ideal method to deposit
contact metals for thin
transistors


Sputtering is also used to
metalize plastics such as
potato chip bags.

http://en.wikipedia.org/wiki/Sputter_deposition
.

3 different simulation methods for sputtering process


Topogaphy or string
simulators, 2D (SAMPLE
2D, EVOLVE)




Atomistic Monte Carlo
model with ballistic
trajectories, 2D or 3D
(SIMBAD, SIMSPUD)





A
tomistic

Monte Carlo

simulator

with
the deposition

event and surface diffusion
,
3D (ADEPT)

S. Franssila, Introduction to Micro Fabrication. Wiley, 2004
.

step coverage of sputtering process

What good or bad step coverage mean?


Gap filling is possible with conformal step coverage.


Voids and cusps are fromed with poor step coverage.


Goog step coverage in metallization is essential for reliability.

S. Franssila, Introduction to Micro Fabrication. Wiley, 2004
.

step coverage of sputtering process


1.

Angle of the arriving atoms:


on horizontal free surfaces = 180
°


in convex corners = 270
°


in the bottom concave corners =
90
°


”Visible” area:


The growth rate of the film at each
point of the interface

is determined
by the ‘‘visible’’ area of the sputter
target


”Visible” area is

determined at a
given point on the feature by

marching radially outwards along
each sector and checking

for
tangency of the ray emanating
from that point
.

270
°

180
°

90
°

Arrival angles of depositing specie at different positions

S. Franssila, Introduction to Micro Fabrication. Wiley, 2004
.

P. L. O’Sullivan Et al
,
2000, Journal of Applied Physics, Vol 88, No 7, pp 4062


4068.

step coverage of sputtering process


2.

Aspect ratio:


Step coverage is usually no
major problem for low aspect
ratio stuctures (<0.5:1), but at
1:1 and higher
-
aspect ratios,
the step coverage rapidly
deteriorates.

1:1

2:1

P. L. O’Sullivan Et al
,
2000, Journal of Applied Physics, Vol 88, No 7, pp 4062


4068.

E. Bär, J. Et al., 2002, Microelectric Engineering, 64, pp 321


328.

step coverage of sputtering process


3.

3D geometries

:


In real microdevices, there
are always structures of
various shapes and variable
spacing and the film
deposition over all these
spaces needs to be
considered.


a), b) and c) are different
dual damascene
topographies


VIA figure d) is a baseline as
it is a worse case scenario
with regard to film coverage
of the side walls and bottom
of the via.

T. Smy,
Et al.,

2002, Thin Solid Films, 415, pp 32


45.

step coverage of sputtering process


4.

Wall taper

:


The tapering of the contact or via walls has a very considerable effect on the
coverage and quality of the film deposited in down the feature walls.


The thickness of the film on the side wall is much more uniform from top to
bottom and has increased from 10% to 25% as the wall is altered from
vertical to 82.5
8
°
.


An obvious drawback of this method of altering the side wall coverage is the
increase in effective size of the feature.


T. Smy,
Et al.,

2002, Thin Solid Films, 415, pp 32


45.

step coverage of sputtering process


5.

Overhangs:


An undesirable feature of direct sputtering is the formation of an overhang at the
top corners of vias and trenches.


The presence of an overhang can cause poor side wall and bottom coverage and
discontinuous film immediately below the overhang.


Rounding of the corner clearly removes the overhang and results in much better
side wall coverage.

T. Smy,
Et al.,

2002, Thin Solid Films, 415, pp 32


45.

step coverage of sputtering process


6.

Resputtering
:


The introduction of a energetic flux by increasing the substrate bias during
sputtering cause some degree of film resputtering.


This is effective because
(
a
)
it reducing overhangs; and
(
b
)
resputtered material
is preferentially trapped within topography, especially along side walls.


T. Smy,
Et al.,

2002, Thin Solid
Films, 415, pp 32


45.

step coverage of sputtering process


7.

Flux distribution
:


Another source of
asymmetry in step coverage
is an non
-
cylindrically
symmetric incident flux
distribution.


This will occur at the edge
of a wafer.


The flux distribution will be
shifted off normal resulting
in a non
-
isotropic flux
impingement.


U. H. Kwon, W. J. Lee,
2006, Japanese Journal of Applied Physics, Vol 45, No 11, pp 8629


8638.

step coverage of sputtering process


8.

Surface diffusivity
:


It determines how much
lateral movement the
impinging specie is allowed
before it is ’frozen’ in the
growing film.


As deposition rate decreases at
a given nominal deposition
temperature, the step coverage
increases.


The increase in step coverage
with decreasing deposition
rates especially at high
temperatures is due to higher
surface diffusion rates

D. S. Taylor, Et. al., 1998, J. Vac. Sci. Technol. A 16 (5), pp 3123


3126.

The films

shown were deposited at 448 K at rates of 20
(
Fig. 1
a)
, 220

(
Fig. 1b
)
,
and 140
(
Fig. 1c
)

Å/s, and at 523 K at rates of20
(
Fig. 1d
)

and 220
(
Fig. 1e
)

Å/s.

step coverage of sputtering process


9.

Distance between target and wafer
:


When increasing the distance between
target and wafer a point will be reached
where the reduction of solid angle
impacts not only the flux in the field
regions but also the flux at the inner
portions of the feature.


This is the point where the maximum step
coverage is achieved.


Further increase of the distance target

wafer will reduce step coverage because
of the increasing directionality of the flux
which leads to smaller angles of metal
atom impingement at the sidewalls than
at the top surface.


P. L. O’Sullivan Et al
,
2000, Journal of Applied Physics, Vol 88, No 7, pp 4062


4068.

step coverage of sputtering process


10.

Material to be deposited
:


The probability of suffering a
collision with the background gas
varies like exp(
2
x
/
l
), where
x
is
distance traveled and
l
is the mean
free path.


For instance Cu

gas
-
phase
scattering has the effect of
collimating the arriving flux


Since the atomic mass of Ti is less
than that of Cu, this collimation
effect is more pronounced at Ti


And for instance Ta has also
higher order effects such as
reflection and re
-
sputtering from
the substrate.

P. L. O’Sullivan Et al
,
2000, Journal of Applied Physics, Vol 88, No 7, pp 4062


4068.

step coverage of sputtering process


11.

Surface topography already made:


Without planarization between sputtering, patterning and
sputterning the step coverage will be poor.

P.B. Zantye et al. / Materials Science and Engineering R 45 (2004)
p.
95

S. Franssila, Introduction to Micro Fabrication. Wiley, 2004
.

Conclusion

why sputtering process is good?


a
n important advantage of sputter
deposition is that even the highest
melting point materials are easily
sputtered while evaporation of these
materials in a resistance evaporator
is problematic or impossible



deposited films have a composition
close to that of the source material



is a conceptually simple technique


S
tep coverage in sputtering

is determined by:

1.
Angle of the arriving atoms & ”Visible” area

2.
Aspect ratio

3.
3D geometries

4.
Wall taper

5.
Overhangs

6.
Resputtering

7.
Flux distribution

8.
Surface diffusivity

9.
Distance between target and wafer

10.
Material to be deposited

11.
Surface topography already made

http://www.tf.uni
-
kiel.de/matwis/amat/elmat_en/kap_6/backbone/r6_4_1.html

http://en.wikipedia.org/wiki/Sputter_deposition
.

References

1.
http://en.wikipedia.org/wiki/Sputter_deposition.

2.
S. Franssila,
Introduction to Micro Fabrication
. Wiley, 2004.

3.
H. Huang, G.H. Gilmer, T. Díaz de la Rubia,
An atomistic simulator for thin film deposition in
three dimensions
, 1998, Journal of Applid Physics, Vol 84, No 7, pp 3636


3649.

4.
D. S. Taylor, M.K. Jain, T.S. Cale,
Deposition rate dependence of step coverage of sputter
deposited

aluminum
-
.1.5%. copper films
, 1998, J. Vac. Sci. Technol. A 16 (5), pp 3123


3126.

5.
T. Smy, S.K. Dew
,
R.V. Joshic
,
Modeling 3D effects of substrate topography on step coverage
and film

morphology of thin metal films
,
2002, Thin Solid Films, 415, pp 32


45.

6.
D.G. Coronell, E.W. Egan, G. Hamilton, A. Jain, R. Venkatraman, B. Weitzman
,
Monte
Carlo simulations of sputter deposition and step coverageof thin
films
, 1998, Thin
Solid Films, 333, pp 77


81.

7.
U. H. Kwon, W. J. Lee,
Multiscale Monte Carlo Simulation of Circular DC Magnetron
Sputtering:

Influence of Magnetron Design on Target Erosion and Film Deposition
,
2006, Japanese Journal of Applied Physics, Vol 45, No 11, pp 8629


8638.

8.
P. L. O’Sullivan, F. H. Baumann, G. H. Gilmer,
Simulation of physical vapor deposition into
trenches and vias:

Validation and comparison with experiment
,
2000, Journal of Applied
Physics, Vol 88, No 7, pp 4062


4068.

9.
E. Bär, J. Lorenz, H. Ryssel,
Simulation of the influence of via sidewall tapering on step

coverage of sputter
-
deposited barrier layers
, 2002, Microelectric Engineering, 64, pp 321


328.

10.
P.B. Zantye et al.
Chemical mechanical planarization for microelectronics

applications
,

Materials Science and Engineering R 45 (2004)
pp
89

220