ECS Journal of Solid State
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Room Temperature SiC-SiO2 Wafer Bonding
Enhanced by Using an Intermediate Si Nano Layer
To cite this article: F. Mu et al 2017 ECS J. Solid State Sci. Technol. 6 P227
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ECS Journal of Solid State Science and Technology, 6 (5) P227-P230 (2017)
2162-8769/2017/6(5)/P227/4/$37.00 © The Electrochemical Society
P227
Room Temperature SiC-SiO2 Wafer Bonding Enhanced by Using
an Intermediate Si Nano Layer
F. Mu,a,z K. Iguchi,b H. Nakazawa,b Y. Takahashi,b R. He,a M. Fujino,a and T. Sugaa,∗
a Department of Precision Engineering, School of Engineering,
b Fuji Electric Co., Ltd., Matsumoto, Nagano, Japan
The University of Tokyo, Bunkyo, Tokyo, Japan
In this work, compared with the direct wafer bonding by surface activated bonding (SAB) at room temperature, SiC-SiO2 wafer
bonding was effectively enhanced by using a Si nano layer deposited on SiO2 such as improvement of fracture surface energy and
reduction of bonding void. A uniform seamless bonding in bonded region was confirmed by interface analysis. The strong bonding
confirmed the strong bonding of SiC-Si in previous research and also demonstrated a strong adhesion of Si nano layer deposited on
SiO2 substrate, which is different from direct wafer bonding of Si-SiO2 by SAB. The possible mechanism of strong adhesion was
investigated by molecular dynamic simulation.
© 2017 The Electrochemical Society. [DOI: 10.1149/2.0081705jss] All rights reserved.
Manuscript submitted January 9, 2017; revised manuscript received March 3, 2017. Published March 10, 2017.
Wafer bonding is indispensable in semiconductor device field since
it is one key technology capable of novel structure formation, efficient fabrication, device integration and cost reduction.1–3 SiC is
a very promising candidate for next-generation power electronics
and MEMS/NEMS due to its superior properties like wide bandgap, high breakdown electric field, high thermal conductivity, high
thermal stability, and high chemical inertness.4–7 SiO2 is one of the
most widely used insulating material and often used as sacrificial
layer for structure formation by etching as well as used for surface
smoothing of extreme hard material.8 Wafer bonding of SiC-SiO2 is
necessary in a typical smart-cut process, which is widely used during
a permanent or temporary transfer of SiC thin layer to a SiO2 covered
substrate.9–13
To date, a few papers about SiC-SiO2 wafer bonding have been
published.10–13 There are mainly two kinds of bonding methods: hydrophilic bonding and plasma activation bonding. For the hydrophilic
bonding, the high temperature annealing at ∼1273 K is necessary
and the separation is common during annealing process owing to
the thermal mismatch and the low interface strength at the initial
stage.10–12,14 For the plasma activation bonding of SiC-SiO2 , the low
yield owing to long annealing time and limited bonded area are
undesired.13
Surface activated bonding (SAB) is a promising room temperature bonding method in ultrahigh vacuum (UHV).15 In this method,
the wafer surfaces are irradiated by Ar ion beam for surface activation, by which contaminations and oxide layer on the surfaces could
be removed and, as a result, the surfaces would become very active.
When these activated surfaces were contacted in UHV, strong bonding between these surfaces can be formed even at room temperature.15
However, SAB method has been found to be not effective for SiO2 .16
Although Fe nano layer has been proved to be effective for SiO2 bonding, the possible Fe diffusion is not desired in many applications.17 Recently, SiC-Si bonding by SAB methods has been demonstrated.18,19
It is expected that a Si nano adhesion layer could be beneficial to
SiC-SiO2 wafer bonding if the Si nano layer deposited on SiO2 had
a strong adhesion. Therefore, in this study, we performed room temperature SiC-SiO2 wafer bonding by using an intermediate Si nano
layer. To better illustrate the effect of Si nano adhesion layer, SiCSiO2 direct wafer bonding by SAB was also carried out for comparison. The bonding results of two methods characterized by scanning acoustic microscope (SAM) and razor blade method were compared. The bonding interface with a Si nano adhesion layer was
investigated by transmission electron microscopy (TEM). To make
the bonding mechanisms clear, molecular dynamic simulation was
performed to simulate the initial process of Si sputtering deposition
on SiO2 .
∗ Electrochemical Society Member.
z
E-mail: mu.fengwen@su.t.u-tokyo.ac.jp
Experimental
The used SiC wafers are n-type, 4-inch, 4◦ off-axis 4H-SiC with
a thickness of ∼350 μm. Since no orientation dependence was found
for SAB method in previous study,20 only the C-face of 4H-SiC wafers
with a root-mean-square (RMS) surface roughness of ∼0.18 nm were
used as bonding surface. The used SiO2 wafers are 4-inch Si (100)
wafers with a 100 nm-thick thermally grown SiO2 layer. The Si wafers
have a thickness of ∼500 μm. The bonding surface of SiO2 wafer has
an RMS roughness of ∼0.15 nm.
Wafer bonding was performed in a UHV bonding machine, which
consists of a load-lock chamber and a processing-bonding chamber.
At the beginning, wafers were set into the load-lock chamber and
then transferred to the processing-bonding chamber, where an Ar ion
beam and Si target was set for surface activation and Si sputteirng
depostion. Fig. 1 schematically shows the detailed process of room
temperature SiC-SiO2 wafer bonding by using a Si nano layer in the
processing-bonding chamber. First, Ar ion beam was used to activate
the SiC and SiO2 wafer surface. Then, an ∼4.8 nm Si layer was
deposited on the SiO2 wafer through Si sputtering depostion by Ar
ion beam. Then, both of the SiC wafer surface and the deposited Si
layer was activated again. ∼2.0 nm deposited Si layer on SiO2 was
etched. Finally, the wafers were bonded directly under ∼2.5 MPa for
300 s. During the surface activation and sputtering deposition of the
Si layer, the voltage and current of the ion beam source were 1 kV
and 100 mA, respectively. The base pressure is ∼5.0 × 10−6 Pa.
Figure 1. Detailed process of SiC-SiO2 wafer bonding by using a Si nano
layer in the processing-bonding chamber.
P228
ECS Journal of Solid State Science and Technology, 6 (5) P227-P230 (2017)
The direct wafer bonding of SiC-SiO2 by SAB was performed
using similar bonding parameters except none of Si layer deposition
step. Before bonding, the roughness of the SiC and SiO2 bonding
surfaces were measured by dynamic force microscopy (DFM; Hitachi
High Tech Science NanoNavi/L-trace II). After bonding, the bonded
wafers were immersed in distilled water and observed by SAM (Hitachi FineSATFS300) to examine the un-bonded areas. The acoustic
pixel size was set as 50 μm. The fracture surface energy was measured by razor blade method.21 The structure of bonding interface was
analyzed by TEM (JEOL JEM 2010F).
To investigate the bonding mechanisms, empirical molecular dynamics simulation performed by the code LAMMPS was used to study
the initial process of Si sputtering deposition on SiO2 to make the interface between SiO2 and deposited Si more clear.22,23 To model the
interactions between the atoms in SiO2 , Tersoff potential was used,
which allows for the breakage and creation of covalent bond and can
also take the no-bonded interactions into consideration.24 The interactions between sputtered Si atoms and the Si and O atoms in SiO2
are modeled by Tersoff/zbl potential. This potential is derived from
Tersoff potential, which is smoothly splined to the Ziegler-BiersackLittmark (ZBL) universal repulsive potential at short interatomic distances using a Fermi function to realistically describe the collisions
between energetic atoms.25,26 During the collisional process, only the
energy loss originated from nuclear stopping was considered because
the electronic stopping was relatively much weaker in the low energy
range and could be neglected.27,28 The simulation model of SiO2 substrate had a dimension about 100 Å × 100 Å × 70 Å. The bottom
layer of SiO2 was fixed to avoid the system being displaced. The Si
atoms initially located in a cylinder space, 100 Å above the SiO2 and
then deposited with a normal incident angle. This separation distance
could preclude any initial interactions between SiO2 substrate and the
incident Si atoms. The irradiated area is a circular region with a diameter 10 Å. Before the starting of Si deposition, the SiO2 substrate is
fully equilibrated at room temperature to get a relaxed configuration.
Monte Carlo simulation29 was used to determine the incident energy
of Si atoms sputtered from the Si target by Ar ion beam, which is 14–
17 eV. In this work, 1000 independent Si atoms with the minimum
incident energy of 14 eV at randomly chosen initial location in the
cylinder space, 100 Å above the SiO2 , were deposited to simulate the
initial stage of the deposition process.
(a)
RMS: ~0.25nm
(b)
RMS: ~0.33nm
Figure 2. DFM images of (a) C-face of SiC and (b) SiO2 surface coated by
Si layer ready for bonding.
Results and Discussion
Figure 3. Comparison of the SAM images of SiC-SiO2 wafer bonded by (a)
SAB and (b) using a Si nano layer.
Bonding energy (J/m2)
Fig. 2 shows the DFM images of SiC and SiO2 surfaces ready for
bonding in the bonding process shown in Fig. 1. The C-face of SiC,
shown in Fig. 2a, has an RMS surface roughness of ∼0.25 nm. The
SiO2 surface coated by Si deposited layer, shown in Fig. 2b, has an
RMS surface roughness of ∼0.33 nm. These results reveal that the
bonding surfaces after surface processing are still very smooth.
The SAM images of SiC-SiO2 wafer bonded by SAB and by using
a Si nano layer were compared in Fig. 3, in which the white part
is un-boned area. It can be clearly found the SiC-SiO2 wafer directly
bonded by SAB has many big voids, while the SiC-SiO2 wafer bonded
by using a Si nano adhesion layer has much fewer and smaller voids.
As a result of calculation, the ratio of bonded area of SiC-SiO2 wafer
bonded by a Si nano layer is 96.8%, very close to completely bonded,
while, the ratio for the SiC-SiO2 wafer directly bonded by SAB is
just 87.8%. Fig. 4 also compares the average fracture surface energies
of SiC-SiO2 wafer bonded by SAB and by using a Si nano layer.
The average fracture surface energy of the SiC-SiO2 wafer directly
bonded by SAB is only ∼0.2 J/m2 , which is similar as the Si-SiO2
direct wafer bonding by SAB method. It was argued that both of the
trapped particle contaminations and the weak bonding resulted in the
formation of big bonding voids, as shown in Fig. 3a. Fortunately,
the average fracture surface energy of the SiC-SiO2 wafer bonded
by using Si nano layer reached ∼2.4 J/m2 , very close to 2.5 J/m2 ,
which is the bonding strength of SiC-Si obtained by SAB methods
or the theoretical surface energy of Si (100) surface.19,30 Actually,
2.5
2
1.5
1
0.5
0
SAB
Using Si nano layer
Figure 4. Comparison of the average fracture surface energies of SiC-SiO2
wafer bonded by SAB and using a Si nano layer.
ECS Journal of Solid State Science and Technology, 6 (5) P227-P230 (2017)
P229
Broken edge of
SiO2 wafer
Intact SiC
5mm
Figure 5. Two fractured positions in the SiC-SiO2 wafer after razor blade test.
The wafer was bonded by using a Si nano layer.
most of the blade insertions during test resulted in the fracture in Si
wafer, which indicates the bonding could be as robust as bulk Si. Two
of the fractured positions are shown in Fig. 5. According to such a
strong bonding, the strong bonding of SiC-Si by SAB methods was
confirmed again. Also, the strong adhesion of Si layer deposited on
SiO2 could be demonstrated, which is very different from direct wafer
bonding of Si-SiO2 by SAB method.
The TEM images of the bonding interface of SiC-SiO2 wafer
bonded by using Si nano layer was shown in Fig. 6. Fig. 6a is the bonding interface in low magnificaiton. It could be seen that the bonding
interface is uniform without distinguishable cracks or voids in bonded
area. Fig. 6b is the magnification of the square area in Fig. 6a. Based
on the contrast difference, the thickness of interfacial layer is determined to be ∼7 nm. The interfaces among the SiO2 , deposited Si layer
and activated SiC layer cannot be recognized, since their bonding is
seamless and all of them are amorphous.
Since weak direct wafer bonding of Si-SiO2 by SAB method indicates the activated SiO2 surface is not active enough, seamless contact
between the Si layer and SiO2 substrate may be not the only reason
of strong adhesion of Si nano layer deposited on SiO2 . To make the
mechanism clear, molecular dynamic simulation was used to investigate the initial stage of the Si sputtering deposition on SiO2 . Fig.
7a shows the model of SiO2 substrate before the Si deposition. Fig.
7b describes the SiO2 with 1000 deposited Si atoms. The magnified
cross-section of the interface between deposited Si and SiO2 is shown
in Fig. 7c. The green spheres, blue spheres and red spheres represent for O atoms in SiO2 , Si atoms in SiO2 and incident Si atoms,
respectively. From the magnified cross-section of the interface, it can
be seen that the deposited Si atoms could be implanted into SiO2
substrate and a Si-rich transitional layer was formed at the interface.
In addition, some bonds between the deposited Si atoms and the O
atoms in SiO2 were found at the interface, which indicates the Si-O
bonds in the original SiO2 surface layer could be broken by the implantation of sputtered Si atoms and new covalent bonds are capable
of being formed. This may be the reason of strong bonding between
(a)
SiC
40nm
the deposited Si and SiO2 , which is very different from the Si-SiO2
direct bonding by SAB. Both of a strong bonding of SiC-Si and the
strong adhesion of Si nano layer deposited on SiO2 contribute to the
desired strong SiC-SiO2 bonding in this work.
Conclusions
In this study, different from the weak direct bonding by SAB, room
temperature SiC-SiO2 wafer bonding by using an intermediate Si nano
layer could be as robust as bulk Si and has much less voids. Besides,
it was confirmed to be a uniform seamless bonding in bonded region
by TEM observation. In accordance with the strong SiC-SiO2 wafer
bonding by using an intermediate Si nano layer, the strong bonding of
SiC-Si was confirmed again and the strong adhesion of Si nano layer
deposited on SiO2 substrate was demonstrated, which is different
from direct wafer bonding of Si-SiO2 by SAB. Based on the results of
molecular dynamic simulation, the strong adhesion of Si nano layer
deposited on SiO2 substrate might originate from the implantation of
deposited Si atoms into SiO2 surface layer, which causes the broken
of the Si-O bonds in SiO2 and the formation of new covalent bonds.
Further study on the dependence of bonding characterizations on the
thickness of deposited Si layer and the wafer bonding of SiC to other
insulating materials by using a Si nano layer will be continued.
Acknowledgment
This research was partially supported by a Grant-in-Aid for Scientific Research (A), 2011, 23246125 from the Ministry of Education,
Culture, Sports, Science and Technology.
(b)
SiC
~7
nm
SiO2
Si
Figure 7. (a) The original model of SiO2 substrate before Si deposition and
(b) the model of SiO2 with 1000 deposited Si atoms as well as (c) the magnified
cross-section of the interface between deposited Si and SiO2 substrate in (b).
The green spheres, blue spheres and red spheres represent for O atoms in SiO2 ,
Si atoms in SiO2 and incident Si atoms, respectively.
SiO2
4nm
Figure 6. TEM images of the bonding interface of SiC-SiO2 wafer bonded
by using a Si nano layer: (a) low magnification and (b) high magnification of
the square area in (a).
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