of 3
Silver diffusion bonding and layer transfer of lithium niobate to silicon
Kenneth Diest,
a

Melissa J. Archer, Jennifer A. Dionne, Young-Bae Park,
Matthew J. Czubakowski, and Harry A. Atwater
Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California
91125, USA

Received 7 July 2008; accepted 8 August 2008; published online 5 September 2008

A diffusion bonding method has been developed that enables layer transfer of single crystal lithium
niobate thin films to silicon substrates. A silver film was deposited onto both the silicon and lithium
niobate surfaces prior to bonding, and upon heating, a diffusion bond was formed. Transmission
electron microscopy confirms the interface evolution via diffusion bonding which combines
interfacial diffusion, power law creep, and growth of

111

silver grains to replace the as-bonded
interface by a single polycrystalline silver film. The transferred film composition was the same as
bulk lithium niobate. ©
2008 American Institute of Physics
.

DOI:
10.1063/1.2976560

Throughout the past two decades, wafer bonding has be-
come an important electronics fabrication method.
1
,
2
The
ability to directly bond two materials, with varying lattice
constants and crystal orientations, has facilitated the devel-
opment of a wide range of research, from microelectrome-
chanical systems to silicon-based photonics. Recently, wafer
bonding processes have been combined with thin-film layer
transfer induced by ion implantation. The combination of
these two methods enables thin-film single crystal layer
transfer of a wide variety of semiconductors
3
5
and
ferroelectrics.
6
8
By combining the flexibility of bottom-up
processing with the near-ideal optical and electronic
properties of single crystal films, these two techniques
have become the standard method for producing
silicon-on-insulator.
4
,
5
In addition, wafer bonding and layer
transfer has enabled ultrahigh efficiency, multijunction solar
cells to be fabricated by bonding lattice-mismatched semi-
conductors.
Major challenges with current layer transfer processes
concern the thermal mismatch and preparation of the two
bonding surfaces. Previous work with lithium niobate and
silicon bonding was done using laser-induced forward trans-
fer techniques.
8
This technique minimized thermal expansion
mismatch between the two bonding layers by inducing layer
transfer using a carbon dioxide laser rather than traditional
thermal cycling. In addition to thermal considerations, exten-
sive work is required to ensure surface planarity, smoothness,
and cleanliness of the two surfaces;
9
11
however, such pro-
cesses are expensive and inefficient. Further, many of the
methods used to produce bondable surfaces can ruin the im-
plantation process and prevent a layer from being trans-
ferred. This problem could be circumvented with a reliable
method for bonding two roughened or otherwise nonideal
surfaces.
In this communication, we report a method for bonding
lithium niobate thin films onto silicon substrates. The bond is
facilitated through a thin diffusion-bonded silver layer be-
tween the lithium niobate and silicon, and the technique al-
lows single crystal thin films of a wide variety of materials to
be transferred to virtually any type of substrate. We demon-
strate lithium niobate bonding to silicon substrates and elu-
cidate the silver-silver diffusion bonding mechanism. In the
context of direct wafer bonding, copper has already been
studied for possible applications in three-dimensional electri-
cal interconnects.
12
,
13
Our results indicate that silver exhibits
a diffusion bonding mechanism similar to that seen for cop-
per. Technologically, a silver bonded interface has the advan-
tage of lower losses for optical device applications. Indeed,
silver has been studied extensively for its plasmonic proper-
ties in metallodielectric waveguides, resonators, and
modulators.
14
17
Polished, x-cut lithium niobate single crystal samples
were obtained from MTI Corporation. These samples were
then coimplanted with hydrogen to 6

10
16
ions
/
cm
2
at 80
keV, followed by a 115 keV helium implantation with a dose
of 5

10
16
ions
/
cm
2
. All implantations were done at room
temperature. The lithium niobate and silicon

100

substrates
were then sequentially sonicated in methanol, acetone, iso-
propanol, and de-ionized water. Silver was concurrently
evaporated on both the implanted lithium niobate and silicon
substrates

without removal of the silicon native oxide layer

to a thickness of 400 nm at 2.5 Å
/
s. The roughness of the
deposited silver was measured using atomic force micros-
copy, and the contamination of the silver surfaces before
bonding was measured using x-ray photoelectron spectros-
copy. The silvered surfaces were then bonded together at
500°Cfor4hina
nitrogen environment. The high tempera-
ture anneal caused the silver layers to bond together. Simul-
taneously, the anneal induced crack formation within the
lithium niobate at the peak implantation depth. As a result, a
layer of lithium niobate, whose thickness corresponds to the
peak depth of ion implantation and whose area corresponded
to the entire 1 cm
2
sample, was transferred to the silicon
handle substrate.
Using a focused ion beam, transmission electron micros-
copy samples were extracted from both the bonded lithium
niobate/silver/silicon sample and a silicon

100

substrate
with 400 nm of silver evaporated on it under the same con-
ditions as the bonded sample. Before milling, a 1

m thick
platinum layer was locally deposited on the sample to pre-
vent ion-induced lattice damage. Thin sections were ex-
tracted from the substrate using focused ion beam patterned
microtweezers, as shown in Fig.
1

a

. After extraction, the
lamellae were thinned to 50 nm using gallium ions with an
accelerating voltage of 30 keV. The samples were character-
a

Author to whom correspondence should be addressed. Electronic mail:
diest@caltech.edu.
APPLIED PHYSICS LETTERS
93
, 092906

2008

0003-6951/2008/93

9

/092906/3/$23.00
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93
, 092906-1
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ized using a transmission electron microscope, which was
operated at 300 keV.
Figure
1

b

shows the lithium niobate sample with the
silicon substrate at the bottom, the silver layer as the dark
central region, the lithium niobate above it, and the 1

m
thick platinum layer in the top right section of the image. The
silver layer, which appears as a dark band through the middle
of the sample, appears to be continuous from one interface to
the other and shows no sign of the original interface between
the two silver layers. The lithium niobate layer is shown in
detail in Fig.
1

c

. From this image, strain contrast from the
bonding process can be seen at the interface between the
transferred layer and the silver layer

bottom left

. This can
be attributed to the mismatch in the coefficients of thermal
expansion between the silicon, silver, and x-cut lithium nio-
bate layers

4

10
−6
, 19.1

10
−6
, and 5

10
−6
/
K
−1
, respec-
tively, at room temperature

. Through the center of the trans-
ferred film, a defect-free single crystal region exists. This is
seen in the selected area diffraction pattern shown as an in-
set, in which the crystal orientation corresponds with the
x-cut orientation of the original lithium niobate sample.
Above the single crystal region, a region of lattice damage
remains as a result of the ion implantation and platinum
deposition processes. It has been shown in previous work
that a secondary annealing step, after the bonding step, can
significantly remove any residual lattice damage from the
layer transfer process and return the transferred layer to its
original, single crystal state.
7
X-ray photoelectron spectroscopy confirmed the exis-
tence of a thin oxide layer on the surface of the deposited
silver films before bonding and atomic force microscopy de-
termined that the root-mean-squared roughness of that sur-
face was

7.0 nm. These conditions were sufficient to pre-
vent bonding from occurring at room temperature. The
samples were initially brought together at room temperature,
such that the two silver films only contacted at the asperities
of each silver surface. Upon heating to 500 °C, the silver
oxide layer became thermodynamically unstable,
18
,
19
and dif-
fusion bonding took place. This process, which was previ-
ously investigated by Derby and Wallach,
20
,
21
consisted of an
initial, rapid process of plastic deformation of the asperities
of both silver films. Such deformation results in formation of
a series of elliptical voids at the fractionally bonded inter-
face. These voids are clearly visible in both scanning elec-
tron microscope and transmission electron microscope im-
ages, as shown in Figs.
2

a

and
2

b

, respectively.
5
μ
m
400nm
200nm
(a)
(b)
(c)
Si
LiNbO
3
Ag
Pt
Microtwezers
TEMlamella
FIG. 1.

Color online

a

Scanning electron microscope image of the
lamella structure that was extracted using the focused ion beam.

b

Trans-
mission electron image of the extracted lamella. From bottom to top the
layers are silicon, the bonded silver layer, lithium niobate, and the protective
platinum layer.

c

At higher magnification, the lithium niobate layer shows
a strained interface, followed by a single crystalline region, and finally an
amorphous damage region. The diffraction pattern

inset

was taken in the
region enclosed by the red box.
(a)
1m
Pt
LiNbO
3
Ag
Si
500nm
200nm
(b)
FIG. 2.

a

Scanning electron microscope image of the lithium niobate
bonded to silicon with a silver bonding layer. Ellipsoidal voids can be seen
half way through the partially bonded silver layer.

b

A transmission elec-
tron microscope image of one of the voids shows an intermediate stage
between initial contact of the two silver layers and complete bonding, Fig.
3

d

.
092906-2 Diest
etal.
Appl. Phys. Lett.
93
, 092906

2008

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After the initial stage of plastic deformation, the interfa-
cial voids are removed during the remainder of the bonding
process. Voids are removed through a combination of surface
diffusion at the void/silver interface, bulk diffusion and grain
growth, and power law creep.
20
,
21
All three mechanisms con-
tribute to the removal of the bonding interface simulta-
neously, and the degree to which each mechanism contrib-
utes is determined by the bonding material, time,
temperature, pressure, and initial surface conditions.
During the bonding process, significant grain growth oc-
curs through the bonding interface, as well as in the plane of
the silver film. Focused ion beam images of silver films on Si
before and after annealing show that the average as-
deposited grain size was 53 nm, whereas after the annealing
the average grain size was 400 nm, as shown in Figs.
3

a

and
3

b

, respectively. X-ray diffraction analysis of identical
samples

not shown

of the silver films before and after the
annealing process show that the grain growth observed is a
result of preferential growth of silver

111

grains during the
annealing process. Transmission electron microscope images
of the silver layer before and after bonding are shown in
Figs.
3

c

and
3

d

. The sample prior to bonding in Fig.
3

c

exhibits multiple grains whose diameters are small enough
that they do not extend throughout the thickness of the film.
In contrast, grains extend across the original silver-silver in-
terface after bonding, as shown in Fig.
3

d

. Three grains
within the silver film extending from the silicon interface to
the lithium niobate interface are shown. Note that the in-
plane grain size in Fig.
3

d

is also significantly larger than in
3

c

. As in Fig.
1

b

, the interface between the two unbonded
silver layers is completely removed after bonding. Also,
within each grain the film is single crystalline from the sili-
con interface to the lithium niobate interface.
After the initial bonding process is complete, if further
high temperature processing steps are required, the diffusion
bonding process would continue based on the new time, tem-
perature, and pressure conditions. Assuming that the initial
heating cycle produces a void-free bond, the bonding would
occur through a combination of bulk diffusion and grain
growth, along with power law creep. This would further in-
crease the size of the grains that extend from the silicon/
silver interface to the lithium niobate/silver interface.
Finally, we note that the use of this method could gen-
erate deep level traps within the silicon substrate.
22
With a
diffusivity on the order of 10
15
cm
2
/
s in silicon,
23
certain
device applications that are dependent on minority carrier
lifetimes could be affected. For those devices where silver
concentration within the silicon is a concern, a thin diffusion
barrier could be deposited between the silver and silicon.
24
Provided the processing temperature of such a device is kept
below 500–600 °C, these layers could be used to prevent
adverse silver diffusion.
In conclusion, silver bonding in combination with ion
implantation-induced layer transfer has been demonstrated as
a method for layer transfer of single crystal lithium niobate
onto silicon substrates. Bonding between the two deposited
silver layers occurred as a result of diffusion bonding. Scan-
ning electron microscopy and transmission electron micros-
copy images show that this process completely removed the
bonding interface, and within individual grains, the silver
was single crystalline from the lithium niobate/silver inter-
face to the silicon/silver interface.
This work was supported by the Army Research Office

ARO-MURI

under Grant No. DAAD 19-01-1-0517 and the
Caltech KNI and Materials Science TEM facilities supported
by the MRSEC Program of the National Science Foundation
under Award No. DMR-0520565.
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ore annealing
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f
ter annealing
1m
5m
60nm
(a)
(c)
(b)
500nm
(d)
FIG. 3. Focused ion beam images of the deposited silver films before and
after annealing,

a

and

b

, respectively. The average grain sizes were 53
nm in

a

and 400 nm in

b

. Transmission electron microscope images of
the silver layer before and after bonding,

c

and

d

, respectively, show that
grain growth during the annealing process has completely removed the in-
terface between the original silver bonding layers.
092906-3 Diest
etal.
Appl. Phys. Lett.
93
, 092906

2008

Downloaded 05 Sep 2008 to 131.215.225.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp