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Simulating wafer bow for integrated capacitors using a multiscale approach

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To simulate the bow of wafers with integrated capacitors in the form of pit arrays, various approaches were pursued. After unfruitful attempts to reliably obtain the wafer bow directly from simulating part of the wafer, a multi-scale approach was used. In this approach, the layer with the integrated capacitors was replaced by a homogeneous material having the same properties. Small-scale simulations of representative parts of the layer were performed to determine its effective stiffness tensor. Inclusion of the intrinsic strains of the grown and deposited dielectric and conductive layers enabled the volume change to be calculated of the layer with the integrated capacitors upon fabrication. Finally, the structure obtained was used in a full-wafer-scale model to simulate the bow of the wafers. Even for uncalibrated values for the coefficients of thermal expansion, most simulations agreed well with measurements. Post-print version available at publica.fraunhofer.de/documents/N-410511.html
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Simulating wafer bow for integrated capacitors using a multiscale approach
A. Wright
1
, F. Krach
2
, N. Thielen
1
, S. Grünler
1
, T. Erlbacher
1,2
, P. Pichler
1,2
1
Fraunhofer Institute for Integrated Systems and Device Technology IISB, Erlangen, Germany
2
Chair of Electron Devices, University of Erlangen-Nuremberg, Erlangen, Germany
Abstract
To simulate the bow of wafers with integrated
capacitors in the form of pit arrays, various approaches
were pursued. After unfruitful attempts to reliably
obtain the wafer bow directly from simulating part of
the wafer, a multi-scale approach was used. In this
approach, the layer with the integrated capacitors was
replaced by a homogeneous material having the same
properties. Small-scale simulations of representative
parts of the layer were performed to determine its
effective stiffness tensor. Inclusion of the intrinsic
strains of the grown and deposited dielectric and
conductive layers enabled the volume change to be
calculated of the layer with the integrated capacitors
upon fabrication. Finally, the structure obtained was
used in a full-wafer-scale model to simulate the bow of
the wafers. Even for uncalibrated values for the
coefficients of thermal expansion, most simulations
agreed well with measurements.
1. Introduction
Silicon integrated capacitors offer advantages over
discrete state-of-the-art capacitors: higher switching
speeds, excellent temperature stability, highly precise
capacitor values (by specifying the exact geometry
through mask patterning), and increasing overall circuit
integration density by eliminating external
components. The manufacture of high-voltage
integrated capacitors involves process steps that result
in the bowing of the wafers. This then affects their
further processing since lithography steps in particular
cannot be performed if the wafers are too deformed.
Each of the integrated capacitors being investigated
consists of an array of pits etched into silicon, see
Figure 1. These pits are lined with dielectric layers,
silicon dioxide and silicon nitride, followed by layers
of polysilicon and aluminium. Since these layers are
deposited at different temperatures, they induce strain
at room temperature due to their coefficients of thermal
expansion differing from that of silicon (and changes in
molar volume e.g. during thermal growth of oxide).
The amount of wafer bow is further influenced by the
arrangement of the pits, their depth, diameter and pitch
along with the wafer thickness. Simulation using
ANSYS has been employed to investigate different
geometries towards uncovering configurations with
minimal wafer bow.
A wafer patterned with millions of pits is unsuitable
for simulation in its entirety; the computational
requirements would be too much for current systems.
Figure 1 Integrated capacitor consisting of an array of pits: a)
view from above wafer, b) cross-section.
Modelling a small portion of the wafer with a low
number of pits was also found to be unsuitable due to
numerical inaccuracy from such a small amount of
displacement for any one pit and problems defining
appropriate boundary conditions for the outer faces of
the simulated structures. A feasible method was found
in taking a multiscale approach. The complete wafer
was virtually separated into a layer with the pits and
into the remainder which has properties of crystalline
silicon (see Figure 2).
Figure 2 Virtual separation of a integrated capacitor wafer
into substrate and pit array layers for multi-scale simulation
a
b
This is the post-print version of the original article published in the proceedings of the
2016 17th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in
Microelectronics and Microsystems (EuroSimE)
which can be found at IEEEXplore (http://dx.doi.org/10.1109/EuroSimE.2016.7463348) © 2016 IEEE
The layer containing the pit array was changed to a
homogenous layer which has the same macroscopic
mechanical properties as the layer with the pits. These
macroscopic mechanical properties are obtained from
small-scale simulations through taking advantage of
the periodic symmetry of the pits. The effective
mechanical parameters for the pit array layer were
determined by considering a unit cell, see Figure 3, and
adopting its effective mechanical properties to the
entire layer.
Figure 3 a) extraction of a repeating unit from the periodic
symmetry of the pit capacitor array, b) enlargement of the
unit cell. Note the curved entrance and bottom of the pits
were not considered, as explained in section 2.
In this work, all materials were assumed to show
elastic behaviour. However, the method described in
this work could be extended to non-elastic models. The
cell’s effective elastic properties were obtained by
determining the coefficients of the elastic stiffness
tensor. This required a number of simulations to
calculate strains in component directions from forces
from different directions as well as shear strains acting
upon the unit cell.
Following this, the volume change during
manufacture of the pit capacitors was determined. This
is the overall strain of the unit cell in the ‘x’, ‘y’ and
‘z’ coordinate directions resulting from the deposition
of the pit liner layers. For this purpose, intrinsic strains
were applied to each deposited layer with the amount
of strain calculated from the deposition temperature
and the coefficient of thermal expansion (CTE). The
exception was the thermally grown silicon dioxide for
which values reported in the literature were used.
With parameters obtained for the unit cell, a larger-
scale simulation of the full wafer could then proceed.
The unit cell parameters (mechanical and volume
change) were applied to the upper layer representing
the pit array with the lower layer assigned as unstrained
silicon, see Figure 4.
Figure 4 Full wafer scale simulation. The pit array layer had
the parameters assigned from the small scale simualtions of a
unit cell of the pit array.
The depth of the pits could be taken into
consideration by changing the thickness of the effective
pit array layer. The following sections describe how the
mechanical properties and volume change of the unit
cell were obtained followed by results of the full wafer
simulations.
2. Obtaining equivalent mechanical material
parameters
The first step was to define the unit cell geometry,
see Figure 3. The tops and bottoms of the pits
containing curved features were not considered.
Considering them is well within scope of this
approach: this would involve two additional unit cells
and two corresponding additional layers in the full
scale model. The upper aluminium layer was not
considered as measurements made have shown that it
does not have a significant effect on wafer bow. The
unit cell contains the spaced holes with their linings of
different materials: SiO
2
, Si
3
N
4
and polysilicon. Each
material has unique mechanical parameters such as
Young’s modulus, Poisson’s ratio, and coefficient of
thermal expansion (CTE), see Table 1.
Table 1 Elastic material parameters and CTEs
Material Young’s
modulus
(GPa)
Poisson’s
ratio
CTE
(ppm/K)
SiO
2
69 [1] 0.14 [2] 0.5 [3]
Si
3
N
4
290 [4] 0.27 [5] 3.4 [6]
Polysilicon 169 [7] 0.22 [7] 2.8 [8]
What was then required were the effective
mechanical parameters for the unit cell. The unit cell
was subjected to multiple simulations to determine
these. The unit cell shown in Figure 5 has three
mutually perpendicular planes of symmetry.
Figure 5 three symmetry planes corresponding to the faces of
the unit cell.
The two symmetry planes ‘xy’ and ‘yz’ differ from
one another due to the hexagonally arrayed pits. There
is then no long-range isotropy in the material and the
three symmetry planes are independent. The array of
pits is thus an orthotropic material. As such, its
stiffness is dependent on the direction in which a force
is applied to the material. To determine the unit cell’s
stiffness, the coefficients of the elastic stiffness tensor
C were calculated, from Hook’s law in the form:
σ = Cε (1)
where σ and ε are the stress and strain tensors
respectively. For an orthotropic material the stiffness
tensor has nine independent components which then
gives Hook’s in the form:
(2)
Calculation began with determining coefficients for
longitudinal compression starting with C
11
from the
following equation:
σ
xx
= C
11
ε
xx
+ C
12
ε
yy
+ C
13
ε
zz
(3)
This was realised in a simulation as depicted in Figure
6. Zero displacement boundary conditions were applied
to the outer-faces parallel to the ‘x’ axis (along the
force direction) and the face of the unit cell opposite to
the force applied. Since the displacement was confined
to be along the ‘x’ coordinate axis, the strains ε
yy
and
ε
zz
were reduced to zero. Next, the plane upon which
the force was applied was set as a remote boundary
condition with ‘coupled’ behaviour in ANSYS.
Importantly, all boundary conditions ensured that the
nodes remained in their respective planes, thus
maintaining the periodic symmetry of the unit cell. If
this were not the case, as would be if a free boundary
condition was used, the deformation of the unit cell
would not behave as if it were part of a larger whole.
Figure 6 Longitudinal compression simulation undertaken to
determine coefficient C
11.
a) undeformed unit cell b) after
force applied. Deformation is exaggerated.
The resulting displacement along the ‘x’ coordinate
axis and stress upon action of the force were used to
calculate C
11
. Similar simulation setups were made for
the calculation of C
22
and C
33
with longitudinal
compression occurring instead along the ‘y’ and ‘z’
axes, respectively.
Calculation of the coefficients C
12
, C
13
, C
23
involved transverse expansion i.e. force being applied
along one component direction and observing
deformation along another component direction. This is
shown in Figure 7. Ensuring that nodes on the outer
faces stayed in-plane, whilst allowing the faces to
expand or contract required the use of special boundary
conditions. These needed to allow the outer face nodes
to slide against the walls whilst being able to move in
the direction of any expansion or contraction. A
solution for such a boundary condition was found in
including ‘wall’ bodies in the simulation. These had
their faces in contact with the unit cell set to rigid and
the contact condition set to ‘no-separation’ (or ‘sliding-
only’). The former ensures a rigid plane and the latter
that the nodes remain in this plane whilst being able to
slide within it.
Note the sliding-only contact ensures that the nodes
remain in contact with the wall whether the solid is
expanding or contracting. Figure 7 shows the walls
with hashed lines indicating the faces which have been
made rigid and red lines showing the ‘sliding-only’
contact. It can be seen that upon application of the
force along the ‘x’ coordinate axis the unit cell deforms
upwards along the ‘y’ coordinate axis and surface
nodes slide along both walls while staying in-plane.
Both walls move to accommodate deformation
perpendicular to their faces. In order to accommodate
for expansion upon deformation, the walls need to be
made larger than the unit cell. Therefore, walls must
travel within each other during the simulation. This is
facilitated by not defining any contact-conditions
between the walls. This boundary condition will be
referred to as the ‘sliding-wall’ boundary condition.
Figure 7 boundary condition using ‘walls’ with sliding-only
contact (red lines), enables deformation with preservation of
periodic symmetry (deformation not to scale).
The deformation of the unit cell with the sliding-wall
boundary conditions is shown in Figure 8.
Figure 8 Deformation of the unit cell upon application of
force along the ‘x’ coordinate axis, a) undeformed and b)
with deformation along the ‘x’ and ‘y’ coordinate axes.
Undeformed geometry is shown by the white outline.
Calculation of C
12
proceeded using equation 3 with C
11
already known from the previous calculation. Zero
displacement boundary conditions were added to
prevent displacement along the ‘z’ coordinate axis thus
reducing ε
zz
to zero. Similar simulation setups were
then used to calculate the coefficients C
13
and C
23
.
For the shear moduli C
44
, C
55
and C
66
, simulations
to shear the unit cell were carried out. Shear forces
were applied to the unit cell face, see Figure 9, and a
coupled boundary condition was applied to the same
face to ensure that the nodes in the face remained
together in the same configuration. In order to ensure
that nodes remain in plane on the faces ‘a’ and ‘b’
indicated in Figure 9, boundary conditions were
applied using remote points in ANSYS.
Figure 9 Shear force applied to the unit cell for calculation of
C
66
. Remote point boundary conditions are applied to faces
‘a’ and ‘b’.
The remote points were located at the pivot of the
respective faces (where the faces meet the ‘y’ axis) and
all degrees of freedom were restricted except for
rotation about the local ‘z coordinate axis. The faces
were further set to rigid to maintain that the nodes
remain in plane. These ensured that the internal
cylindrical features behaved as though they were still
part of the large pit array, see Figure 10 (b). Without
such boundary conditions, internal features deform
unrealistically as in Figure 10 (a).
Figure 10 Shear simulation of the unit cell: without (a) and
(b) with remote point boundary conditions applied. The
internal features behave as if part of a larger pit array in (b).
Similar boundary conditions were applied to the
remaining planes, with all degrees of freedom
restricted except for translation along the ‘y’ coordinate
axis. Calculation of C66 was then:

 


 
∙tan∅
(4)
where

and

are the shear stress and strain
respectively, F the shear force applied to the unit cell,
A the area of the face upon which the force is applied,
and the shear angle. Similar simulation setups were
made for the shear moduli C
44
and C
55
.
3. Determining overall strain from fabrication
To obtain the expected volume change during
fabrication, the unit cell was simulated with the
combined contraction of the various materials within.
Specifically, the strain upon each material was
calculated according to the difference from the
deposition temperature to room temperature and
corresponding CTE, see Table 2.
Table 2 Material deposition temperatures and CTEs
Material T deposition °C CTE
ppm/K
SiO
2
1000 0.5 [3]
Si
3
N
4
780 3.2 [6]
Polysilicon 570 2.8 [8]
The exception however was the thermally grown SiO2
known to expand upon growth due to the change in molar
volume. Thus, a value of 300 MPa of intrinsic stress at room
temperature as reported in [8] was applied to the SiO
2
. The
strains upon the materials were applied in ANSYS using the
‘inistate’ command. The simulation setup required the use of
three sliding-wall boundary conditions, see-Figure 11.
Figure 11 Simulating volume change from fabrication of the
unit cell. a) undeformed unit cell b) contracted unit cell. Note
three sliding-wall boundary conditions were used to maintain
periodic symmetry. All remaining planes were set to zero
displacement.
Upon simulation, the unit cell within the simulation
model contracted. The results of the simulation were
then the strains along the ‘x’, ‘y’ and ‘z’ coordinate
axes of the unit cell. These constituted the effective
volume change of the unit cell. Along with the
previously calculated elastic stiffness tensor, the small-
scale simulation results were transferred to a larger-
scale simulation model.
4. Wafer scale simulation
Full scale simulation of the pit-capacitor wafer
proceeded with the geometry as shown in Figure 4. For
the silicon substrate layer the mechanical parameters of
(100)–oriented silicon were applied. The effective
properties of the unit cell were applied to the pit-array
layer. The strains from the unit cell were applied using
again the ‘inistate’ command in ANSYS.
Results from simulations showed concave wafer
bow, with all forms having some asymmetry, see
Figure 12.
Figure 12 Simulated wafer bow
Simulations were carried out to investigate the effect of
the wafer thickness on wafer bow and compare results
with measured wafers. The details of the pit array
were: 5.9 µ m pit diameter, 7 µm pitch and 20 µm
depth. The liner layers and thicknesses were: SiO
2
0.33 µm, Si
3
N
4
1 µ m, and polysilicon 0.5 µ m. The
results shown in Figure 13 reveal that measurements of
675 µm wafers agree well to simulation results,
particularly when considering that the CTEs were taken
directly from literature. Measurements made of 675 µ m
thick wafers with different thicknesses of the dielectric
layers are shown in Figure 14. For these, wafer bow
was considered at a radius of 60 mm of the 150 mm
diameter wafers. The results from simulating 0.5 and
1.0 µm thick silicon nitride layers above a 0.33 µm
thick silicon dioxide layer agree with measurements.
However, the simulation of a 1.0 µm nitride layer
above a 20 nm silicon dioxide layer overestimates
measurements. The reason for which is subject to
further investigation.
Figure 13 Wafer bow simulated for different wafer
thicknesses compared with measurements. Measurements of
675 µ m wafers are similar to simulation results. Pits are
5.9 µm in diameter with 7 µm pitch and 20 µ m depth which
is noted as ‘5.9/7/20’. The liner layers and thicknesses were:
SiO
2
0.33 µm, Si
3
N
4
1 µm and polysilicon 0.5 µm.
Figure 14 Simulation of the wafer bow at a radius of 60 mm
for combinations of different liner layer thicknesses. Pits are
5.9 µm in diameter with 7 µm pitch and 20 µm depth.
Figure 15 Pit capacitance vs. wafer bow for various depths
and decreasing the feature size of the pits (whilst maintaining
a constant ratio of diameter to pitch). The liner layers and
thicknesses are the same as in Figure 13.
Additionally, simulations have been carried out to
indicate conditions for the lowest wafer bow whilst
maximising capacitance, see Figure 15. Here, various
pit depths were simulated along with changing the
feature size of the pits and their pitch (whilst
maintaining the same ratio of pit diameter to pitch).
Simulated results indicate that simultaneously
decreasing both the diameter and pitch yields lower
wafer bow.
5. Conclusion
In this work, a methodology for simulating the bow
of wafers with integrated capacitors in the form of pit
arrays was described. Simulation of the entire wafer
features was found to be computationally unfeasible.
The multi-scale methodology used instead works as
follows: The portion of the wafer containing the
integrated capacitors was replaced by a solid with the
same effective mechanical parameters and volume
change from fabrication. The effective stiffness tensor
was obtained by small-scale simulations on a portion of
the pit array through taking advantage of its periodic
symmetry. Calculation of the volume change of the pit
array upon fabrication additionally required
considering the different strains upon the various
constituent materials from their different deposition
temperatures and CTEs. The layer structure obtained
this way was finally applied to a full-wafer-scale model
to simulate the bow of the wafer. Even for uncalibrated
CTEs, most simulations agreed well with
measurements. Finally, simulation suggests that
decreasing both the feature sizes of pit diameter and
pitch results in decreased wafer bow.
Acknowledgments
The research leading to these results has received
funding from the European Union Seventh Framework
Programme (FP7/2007-2013) under grant agreement n°
619246 (ATHENIS_3D).
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  • K Kawahata
  • S Sugiyama
  • I Igarashi
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  • W N Sharpe
  • Bin Yuan
  • R Vaidyanathan
  • R L Edwards
Sharpe, W. N., Bin Yuan, Vaidyanathan, R., and Edwards, R. L., "Measurements of Young's modulus, Poisson's ratio, and tensile strength of polysilicon," in IEEE The Tenth Annual International Workshop on Micro Electro Mechanical Systems. An Investigation of Micro Structures, Sensors, Actuators, Machines and Robots, Nagoya, Japan, pp. 424-429.