Delft University of Technology
MEMS enabled fast time-resolved X-ray diffraction characterization platform for copper
nanoparticle sintering in heterogeneous integration applications
Zhang, Boyao; Wei, Jia; Bottger, Amarante J.; van Zeijl, Henk W.; Sarro, Pasqualina M.; Zhang, GuoQi
DOI
10.1109/TRANSDUCERS.2019.8808192
Publication date
2019
Document Version
Final published version
Published in
2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems and Eurosensors
XXXIII, TRANSDUCERS 2019 and EUROSENSORS XXXIII
Citation (APA)
Zhang, B., Wei, J., Bottger, A. J., van Zeijl, H. W., Sarro, P. M., & Zhang, G. (2019). MEMS enabled fast
time-resolved X-ray diffraction characterization platform for copper nanoparticle sintering in heterogeneous
integration applications. In
2019 20th International Conference on Solid-State Sensors, Actuators and
Microsystems and Eurosensors XXXIII, TRANSDUCERS 2019 and EUROSENSORS XXXIII: Proceedings
(pp. 1772-1775). [8808192] (2019 20th International Conference on Solid-State Sensors, Actuators and
Microsystems and Eurosensors XXXIII, TRANSDUCERS 2019 and EUROSENSORS XXXIII). IEEE .
https://doi.org/10.1109/TRANSDUCERS.2019.8808192
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MEMS ENABLED FAST TIME-RESOLVED X-RAY DIFFRACTION
CHARACTERIZATION PLATFORM FOR COPPER NANOPARTICLE SINTERING IN
HETEROGENEOUS INTEGRATION APPLICATIONS
Boyao Zhang
1
, Jia Wei
2
, Amarante J. Böttger
3
, Henk W. van Zeijl
1
, Pasqualina M. Sarro
1
, and
Guoqi Zhang
1
1
Department of Microelectronics, Delft University of Technology, Delft, the Netherland
2
Else Kooi Lab, Delft University of Technology Delft, the Netherland
3
Department of Material Science and Engineering, Delft University of Technology, Delft, the
Netherland
ABSTRACT
We report the design, fabrication and experimental
investigation of a MEMS micro-hotplate (MHP) for fast
time-resolved X-ray diffraction (TRXRD) study of Cu
nanoparticle paste (nanoCu paste) sintering process. The
device and its system are designed to have a 60 ms
minimum time interval, uniform temperature distribution
and variant gas environments. A TRXRD study of nanoCu
paste sintering at 200 °C in H
2
-N
2
gas mixture was done
using this device. With 1 sec interval, Cu
8
O reduction and
Cu crystallization in sintering is observed. Results can be
combined with other studies to optimize material design
and process development.
KEYWORDS
MEMS, Microhotplate, TRXRD, Copper nanoparticle
paste
INTRODUCTION
Low temperature sintering of metallic nanoparticle
paste, such as nanoCu paste, has a great potential as
interconnect solution in heterogeneous integration [1].
Experimental time-dependent insight of the sintering
process is essential for both process optimization and
material development [2,3]. This time-dependent study is
however very limitedly reported, even for Ag nanoparticle
paste, which is already developed and applied in
interconnect solutions for high reliability and performance
driven applications. Xu used real-time electrical resistance
measurement to study sintering process of Ag nanoparticle
paste [2]. The corresponding time-dependent Ag
nanoparticle structure information was not reported and
correlated. Milhet applied an in-situ X-ray
nanotomography method to evaluate microstructure change
of Ag nanoparticle paste during thermal exposure after
sintering [3]. However, the experimental conditions and
specimen geometry are far from real conditions in the
envisioned applications. As an emerging material and
technology, more detailed and systematic study of nanoCu
paste sintering process under real application conditions
needs to be performed.
To mimic and study the sintering process as in real
applications, nanoCu paste (particle size < 100 nm) needs
to be processed with micrometer thickness, in gas
environment. Transmission electron microscope (TEM)
reveals the particle behavior at atomic level [4,5]. However,
it is limited to thin sample thickness and ultra-high vacuum
environment. On the other hand, traditional XRD is
affordable and penetrative to micrometer sample thickness.
However, it is time consuming (a few minutes to hours) to
collect precise material information. Due to size effect and
complexity of structure, nanoparticles study requires even
more time and sophisticated methods. Therefore, it is not
suitable to observe fast reaction processes. Modern
synchrotron techniques enables ultra-fast XRD detection
[6,7]. It brings time resolution down to femtoseconds.
However, the equipment and accessories are costly and not
frequently available in most laboratories.
In this paper, a MEMS-enabled fast time-resolved
XRD (TRXRD) method is proposed to obtain insight
knowledge about nanoCu paste sintering.
DESCRIPTION OF THE NEW METHOD
AND SYSTEM
Figure 1: The concept of MEMS enabled time-resolved X-
ray diffraction for material characterization.
TRXRD is defined as a method for detecting dynamic
position and intensity of diffracted XRD pattern as a
function of time, by using a short pulse X-ray beam at the
sample [7]. High intensity synchrotron radiation beams
enables the necessary time interval to reveal intermediate
states of materials. Instead of using short pulses of
synchrotron X-ray beam, a MEMS micro-hotplate (MHP)
device is introduced to enable TRXRD in milli-seconds
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(ms), using conventional XRD facilities. Fine, accurate
XRD scans are performed during frozen states (Fig. 1). By
switching alternatively between activated and frozen states,
a series of XRD patterns, thus material details, for a specific
dynamic process can be obtained as a function of time. To
enable a milli-seconds TRXRD, the device contains an on-
chip MHP to enable fast temperature control (typically tens
of milli-seconds) on micro-sized samples placed on top of
the MHP surface. This allows a fast switching of
temperature sensitive reactions between activated and
frozen states.
Typically, the size of a MHP with suspended is below
0.5 mm in diameter. A temperature difference of 30-50 °C
from edge to the center, when the MHP temperature is
between 300 – 400 °C, is generated [8-10]. For a sintering
process study under TRXRD, a 0.5 mm diameter heating
area hardly provides enough signal for detection. In
addition, nanoCu paste sintering takes place below 300 °C,
more than 10% of temperature variation can have a strong
effect on the final characterization results. To ensure a
homogeneous reaction across a large XRD detection area,
a 50 µm thick single crystal silicon membrane (1 mm in
diameter for typical X-ray beam size) is introduced to have
uniform horizontal thermal distribution (Fig. 2b).
Furthermore, thick oxide blocks as horizontal thermal
isolation is included in the design to reduce power
consumption, and constrain temperature uniformity within
MHP area [11]. Thus, a millisecond-level fast temperature
response needs to be achieved to ensure a fast temperature
switching. By accurate control of current value, device can
switch between activated and frozen states.
Finite element simulation
Table 1: Micro-hotplate average temperature and
deviation from FEM simulation results
T
avg
(°C) T
max
(°C) T
min
(°C) Deviation
41.67 41.97 41.17
1.39%
106.64 107.83 104.64
2.18%
214.82 217.51 210.34
2.43%
366.25 371.03 358.28
2.54%
Figure 2: (a) Schematic structure of MHP. (b) FEM
simulation result of MHP temperature distribution when
input current is 0.2A.
To evaluate the temperature distribution along the
MHP area, a finite element modeling (FEM) simulation
was performed using COMSOL Multiphysics, v5.2.
The simulated temperature distribution with different
average temperature and deviations are shown in Table 1
and Figure 2(b). The temperature deviation from the
average value is 2.54% within the hotplate area, up to a
maximum average working temperature of 366.25 °C.
Typically, the sintering of nanoCu paste is between 200 –
300 °C. In this temperature range, MHP has 2 – 2.5% (± 5
- 7 °C) deviation from the average temperature. This is a
relatively small deviation for sintering process.
Temperature uniformity in this device achieves
requirement for sintering process study.
FABRICATION
Figure 3: The main steps of the fabrication process. (a-b)
Formation of thick silicon oxide blocks for thermal
isolation by using DRIE and thermal oxidation. (c-d)
Surface smoothening and metallization. (e) Passivation
layer deposition and contact pads opening. (f) Backside
cavity etching in a KOH solution
Figure 4: (a) Optical image (close-up) of a fabricated MHP.
(b) SEM cross-section image of the thick oxide blocks used
for lateral thermal isolation.
A bulk-micromachining process is developed (Fig. 3)
for the device fabrication. A fabricated device is shown in
Figure 4. A 525 µm thick Si wafer was used as starting
material (Fig. 3a). A thick silicon oxide lateral thermal
isolation structure (50 µm deep, 100 µm wide) is fabricated
using deep reactive ion etching (DRIE) and wet thermal
oxidation (Fig. 3b). Figure 4b shows the cross section of
lateral thermal isolation structure. 300 nm of SiN
x
was
deposited by low pressure chemical vapor deposition
(LPCVD). It is used as isolation layer between Si and Pt,
diffusion layer between Si and nanoCu and as a hard mask
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for Si cavity etching on the backside (Fig. 3c). A 200 nm
Pt metal layer, which is the MHP metal layer, was
evaporated and patterned by lift-off method (Fig. 3d). A
SiO
2
passivation layer of was deposited by plasma
enhanced chemical vapor deposition (PECVD). Contact
pads were opened by using reactive ion etching, with
photoresist as mask layer (Fig 3e). Si cavity on the backside
was etched by isotropic wet etching with KOH solution
(Fig. 3f). It landed on predefined 50 µm thick SiO
2
thermal
isolation block. A 50 µm thick Si membrane was created.
EXPERIMENTAL RESULTS
Transient measurement of stabilization time
Figure 5: Transient measurement of MHP device
resistance variation during heating and cooling
To characterize the time constant of device
temperature response, the transient change of MHP
resistance is measured as an indication of the average
temperature in the heating area. By giving a short current
pulse, transient change of MHP voltage, namely the MHP
resistance, as a function of time, can be measured.
Fabricated MHP is probed with a probe station (Cascade
Microtech). SMU (Agilent 4156C parameter analyzer) and
an oscilloscope are connected to the probe station. The
SMU is used to supply accurate current pulse. An
oscilloscope is used to measure transient change of MHP
resistance in both heating and cooling process. The results
are shown in Figure 5. The time to reach 90% of stabilized
value is determined as heating or cooling time. In the
resistance measurement, it takes approximately 35 ms and
25 ms to stabilize for heating and cooling respectively to
reach 90% of total resistance change. Consequently, the
time resolution of the device for time-resolved
characterization is 60 ms.
Sintering study
To validate the concept, a layer of nanoCu paste
(particle size <100 nm) with an average thickness of 1.3 um
was applied on the MHP surface using drop-casting method
(Fig. 6). Controlled gas environment is created by using a
gas cell mounted on XRD equipment (Bruker D8, Co Kα
radiation, λ = 1.79 ). On top of the gas cell, A
polypropeiene film (thickness <13 µm) is used as X-ray
transparent film to allow conventional XRD measurement,
while keeping gas cell properly sealed. A SMU (Keithley
2602B) is connected to the device. Controlled current
pulses were given to obtain multiple activated states with
desired temperature and duration. A dedicated sintering
process, consisting of several activated and frozen states in
alternative sequences, is performed under a controlled
forming gas environment (5% H
2
+ 95% N
2
). Heat pulses
applied in this experiment is 1 sec at 200 °C. XRD scans
with 0.02° increment, 1 sec step time, were performed
during the frozen states (25 °C). Each XRD scan takes
around 10 min. Three heat pulses were applied in this
experiment, which correspond to series of scan patterns in
Figure 7.
Figure 6: Time resolved XRD experiment set-up. Chip is
connected to source-measurement unit (SMU) for
temperature control and real-time measurement. The SEM
image on the right corner shows a MHP device covered
with nanoCu paste.
In Figure 7, “0 s” XRD pattern, which represents the
status of the nanoCu paste before sintering, indicating the
existence of a small amount of surface oxide (Cu
8
O). In
general, small amounts of surface oxides might be
introduced from solution based nanoparticle synthesis.
There are two inserts in Figure 7, presenting detail peak
information about Cu
8
O and Cu (111) respectively. Based
on the series of diffraction patterns, a full-width-half-
maximum (FWHM) analysis of Cu (111) peak and
corresponding crystal size is performed and results are
presented in Figure 8. According to the insert of Cu
8
O peak,
it decreased after first heat pulse at 200 °C. It indicates that
Cu
8
O is reduced in the first second heat pulse. In addition,
Cu (111) peak width (FWHM) decreased due to crystal
growth (Fig. 8) in the first heat pulse. With reduction of
Cu
8
O, Cu atoms can diffuse between neighboring
nanoparticles. This probably leads to the initial state of
neck formation in sintering process. Cu (111) crystalline
size also experienced an increase, consequently. In the
following heat pulses (1-3 s), FWHM does not decrease
much. In contrast, it increases slightly. This is probably
contributed by overlapping of background at 2θ = 50 ° and
2θ = 52 °. These background signals are possibly obtained
from the SiO
2
on the substrate of MHP.
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