Superconformal Electrodeposition in Vias
D. Josell,
z
D. Wheeler, and T. P. Moffat*
National Institute of Standards and Technology, Metallurgy Division, Gaithersburg, Maryland 20899, USA
Conditions for which superconformal filling of vias can be expected are predicted using the curvature enhanced accelerator
coverage mechanism to model the effect of accelerator accumulation and area change on local copper deposition rate. Supercon-
formal filling of vias is predicted to occur over a more limited range of electrodeposition conditions than in trenches of similar
aspect ratio with significant implications for dual damascene processing. Parameters for the model describing both the accumu-
lation of accelerator on the copper/electrolyte interface and the impact of the accumulated accelerator on the local deposition rate
come from voltammetry experiments on planar electrodes. An idealized geometry permits reduction of the 3D filling problem to
solution of a system of coupled first-order, nonlinear ordinary differential equations.
© 2002 The Electrochemical Society. DOI: 10.1149/1.1452485 All rights reserved.
Manuscript submitted September 21, 2001; revised manuscript received December 4, 2001. Available electronically February 6,
2002.
Dual-damascene processing of semiconductor devices involves
simultaneous electrodepositon of copper for both trenches and vias.
Until recently, such processing has proceeded both with proprietary
operational parameters and in the absence of a robust physical de-
scription of the feature filling process. This combination of factors
has slowed scientific assessment of future prospects for this
technology.
Modeling of via filling in particular has been limited. One study
detailed the effects of geometry on cupric ion depletion in an addi-
tive free electrolyte.
1
That study did not address superconformal
filling i.e., superfilling, which requires the use of both deposition
rate inhibiting and accelerating additives in the electrolyte. Early
models of superfilling assumed location-dependent growth rates de-
rived from diffusion limited accumulation of only an inhibiting spe-
cies in trenches
2
and vias.
3
Such models were unable to predict
several key experimental observations of filling, including the initial
period of conformal growth, general fill geometry during supercon-
formal filling, and subsequent development of an overfill bump.
4-7
Recently, however, modeling has advanced significantly with the
publication of both a model electrolyte for the study of superconfor-
mal electrodeposition
7
and a curvature enhanced accelerator cover-
age CEAC mechanism that permits a quantitative description of
superconformal deposition in trenches.
8,9
The first part of the mechanism is that a dilute accelerating spe-
cies thiol or disulfide derived from a 3-mercapto-1-
propanosulfonate additive MPSA adsorbs strongly on the depos-
iting metal surface, thereby displacing the more weakly bound
inhibiting species derived from polyethylene glycol and chloride
PEG-Cl additives. All adsorbed species are presumed to remain
on or float at the surface during deposition. The second part of the
mechanism involves the compression of adsorbed accelerator with
reduction of surface area during growth, such as occurs at points of
high positive curvature like the bottoms of small vias, resulting in
increased local velocity. Models based on the CEAC mechanism
have been shown to yield predictions that agree well with experi-
mental results, including a period of conformal growth, bottom-up
filling or void formation, and creation of overfill bumps, for filling
of trenches between 350 and 100 nm wide and 500 nm deep over
a wide range of processing conditions.
8-11
One such model used an idealized geometry and simplified cu-
pric depletion to reduce the trench filling problem to a first-order
ordinary differential equation that could predict the potential and
concentration dependence of filling over a range of aspect ratios
height/width.
11
Predictions were compared with experimental re-
sults as well as results of a model that solves for the space and time
dependent cupric ion and accelerator concentrations in the electro-
lyte using the actual interface shape.
9,10
Agreement was good in both
cases for the range of parameter space studied.
This work is the first to extend a model that successfully predicts
all aspects of trench filling, in this case a CEAC-based model, to
superconformal filling of vias. The time-dependent copper/
electrolyte interface shape is approximated by a cylinder for the side
wall of the via and a plane for the bottom, and a cupric ion concen-
tration varying linearly with distance down the via is assumed. The
CEAC mechanism is then applied to the via geometry. Superconfor-
mal deposition by the CEAC mechanism might be anticipated to be
enhanced as compared to that for trenches because the bottoms of
vias have two nonzero radii of curvature while the bottoms of
trenches have only one. However, unlike the sidewalls of trenches,
the sidewalls of vias have nonzero curvature. This causes deposition
on the sidewalls of vias to also be affected accelerated by the
CEAC mechanism, to the detriment of superfilling.
Model
Determining the equations of evolution.—The time dependent
interface shape of the copper/electrolyte interface is approximated
for all times by a cylindrical surface, shown schematically in Fig. 1.
The validity of this approximation and other approximations to
come has been discussed previously in the context of filling of
trenches.
11
Filling of a via of initial radius R and height h is moni-
tored by tracking the motion of the bottom and side surfaces. The
velocity v is given by
v , C , =
C
C
Cu
v
o
exp
-
F
R
B
T
1
* Electrochemical Society Active Member.
z
E-mail: daniel.josell@nist.gov
Figure 1. A schematic of the idealized geometry used to model filling of
vias, viewed as a cross section through the midplane.
Electrochemical and Solid-State Letters, 5 4 C49-C52 2002
0013-4651/2002/54/C49/4/$7.00 © The Electrochemical Society, Inc.
C49