Integrated Microprobe Array and CMOS MEMS by TSV Technology for Bio-Signal Recording
Application
Lei-Chun Chou1, Shih-Wei Lee1, Po-Tsang Huang1, Chih-Wei Chang2, Shang-Lin Wu1,
Jin-Chern Chiou1,3*, Ching-Te Chuang1, Wei Hwang1,4, Chung-Hsi Wu4,
Kuo-Hua Chen 4, Chi-Tsung Chiu4, Ho-Ming Tong4, Kuan-Neng Chen 1**
1
National Chiao Tung University, Hsinchu, Taiwan; 2University of California, Los Angeles, USA; 3China Medical
University, Taichung, Taiwan; 4Advanced Semiconductor Engineering Group, Kaohsiung, Taiwan
Tel: *+886-3-571-2121#31881; ** +886-3-571-2121#31558;
Email: * chiou@mail.nctu.edu.tw; ** knchen@mail.nctu.edu.tw
Abstract
Bio-signal probes that provide stable observation with
high-quality signals are crucial for understanding how the
brain works and how the neural signal transmits. Because
bio-signals are weak and noisy, the length of the string
connecting the sensor and Complementary Metal–Oxide–
Semiconductor (CMOS) circuit significantly impacts biosignal quality. The collected weak signals from the sensor
must pass through a series of interconnections and interfaces
that introduce noise and lead to bulky packaged systems. This
work uses through-silicon via (TSV) technology to connect
the μ-probe array bio-sensor and CMOS circuit located on
opposite sides of a chip for brain neural sensing applications.
With the elimination of wire bonding and the reduction of the
soldering, bio-signal quality can be significantly improved.
Introduction
In
the
past
decade,
advances
in
micromachined/assembled micro probe arrays with electrical
stimulation/recording ability have played an essential role the
exploration of central neural systems. Simultaneous
observation of a large number of cell activities is required to
understand the nervous system [1]. Microelectrode arrays
give a method for accessing numerous neurons
simultaneously with high spatial resolution [2]. Extracellular
action potentials are recorded by surgically implanting neural
probes into neurons of interest, which result from neural
activities [3]. Probes that could insert a large number of
recording sites into neural tissues with minimal tissue
damage are therefore needed. Additionally, the design of
probe arrays should be optimized for experiment such
purpose that an electrode diameter of a few micrometers
could support single-unit recording [4].
Since the 1950s, microelectrodes combined with
electronic recording and signal processing began to allow for
meaningful studies of the central nervous system at the
cellular level [5]. A great deal was learned gradually about
how single neurons work. Serially moving sharpened wire
electrodes in tissue also acquired considerable amounts of
information about nervous system function at the circuit
level, especially in sensory areas. However, arrays of
electrodes, and perhaps large arrays, were clearly needed to
fully understand signal processing in complex neural
networks. Early experiments glued individual electrodes
together or used cutoff wire bundles to record simultaneously
from many points with some success, but were limited by
978-1-4799-2407-3/14/$31.00 ©2014 IEEE
their geometries and reproducibility [6,7]. Furthermore, they
caused considerable insertion damage, and typically spread
out in tissue, making exact placement difficult. Since they
were fabricated easily with technology, microwire electrode
arrays are still used extensively for both acute and chronic
extracellular recording [8 -10]. However, the length of the
connected string between the sensor and CMOS circuit has
significant impact on the quality of the inherently, weak and
noisy bio-signal. The collected weak signals from the sensor
need to pass through a string of interconnections and
interfaces that introduce noises and lead to bulky packaged
systems.
Highly integrated and miniaturized neural sensing
microsystems that provide stable observation, a small form
factor and biocompatible properties are crucial for brain
function investigation and neural prostheses realization by
acquiring accurate signals from an untethered subject in
his/her natural habitat [11,12]. Such biomedical devices
usually comprise sensors and Complementary Metal–Oxide–
Semiconductor (CMOS) circuits for biopotential acquisition,
signal conditioning, processing, and transmission. Many
approaches for solving these problems have been developed,
including stacked multichip [13,14], a microsystem with
separated neural sensors [15], and monolithic packaged
microsystem. Regardless of which scheme is used, signals
collected from a sensor are weak and must pass through a
string of interconnections, including wire bonds, flip-chip
bonds and welded or soldered bonds to processing circuits.
The excessive number of interfaces and connections
introduce noise and result in bulky packaged systems.
Integrated Bio-signal Recorder by TSV Technology
Fig. 1 shows the overall system architecture. The neural
signal comes from the brain tissue is modeled as a current
source, collected by the neural probe array on the back-side of
the chip. The raw signals pass through an electrode-tissue
interface, which is modeled by a Randles Cell [16]. The
signal is then transferred by TSV array through the chip to
the CMOS front- end circuits on the front- side of the chip
for signal conditioning and processing. On the back-side of
the chip, 480 neural probes are divided into a 4x4 array. A
total of 16 channels are designed in a chip for local area
mapping. For each channel, 42 TSVs are lumped together for
low impedance connection. It is possible to use one single
TSV for one recording channel. However, in this paper, the
main focus is to demonstrate the feasibility of double-side
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2014 Electronic Components & Technology Conference
single- chip integration concept, and therefore a group of
TSVs are lumped together to mitigate the variance caused by
laboratory fabrication process and to improve yield. 16
parallel analog front- end circuits are designed for 16
channel signal inputs. In the measurement setup, another
common reference electrode on scalp is required for
referencing. Besides, the proposed structure still allows
stacking of other CMOS chips fabricated with different
technologies onto the circuit side using 3D IC techniques.
TSVs that are 200μm deep (height) and 25-30 μm in
diameter. Next, the RDL is fabricated for connections
between TSV arrays and circuit input pads. An Inductive
Coupled Plasma (ICP) etching process is then applied on the
back side of the wafer to form the microprobe array.
Fig.1. Structure of the integrated microsystem.
Fig. 2 illustrates the detailed physical structure of
CMOS circuits, TSV and neural probe array. The CMOS
pads on the front- side of the chip are connected to TSV by
two layers of redistribution layers (RDLs). A passivation
layer is used to protect the RDL, TSV and CMOS circuitry.
On the back- side of the chip, parylene-platinum/titaniumparylene structure is designed to form the connection between
TSVs and neural probes. The metal layer on the tip is
exposed by etching process to serve as a sensing material of
the probe.
Fig.3. Detailed process flow including post processing.
Fig.2. Cross-section view of the structure.
Fabrication
Figure 3 shows the detailed process flow. In the front
process, CMOS circuits are fabricated using United
Microelectronics Corporation (UMC) 0.18-μm process
technology on an 8-inch Si wafer. Then a front-side, via-last,
and fully-filled Cu plating process is executed to fabricate Cu
At the start of the ICP etching process, isotropic etching
is used to etch the probe tip area into a hill (Fig 4(a)). Then,
anisotropic etching is used to etch the probe height to 150 μm
(Fig 4(b)). Finally, isotropic etching is applied again to etch
the tip of the probe. To insert the probe into the in vivo brain,
tip diameter must be <5μm (Fig. 4(c)).
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Figure 6 is a cross-sectional view of the device before postprocessing. The CMOS circuit and TSV has already
implemented by the United Microelectronics Corporation
(UMC) and Advanced Semiconductor Engineering (ASE)
Group. Figure 7 is a cross-sectional view of the device with a
probe array and TSV.
Fig.4. ICP etching process flow
After the ICP step of the post process flow, a 5-μm
parylene-C is deposited on the structure to isolate different
channels. Parylene-C is biocompatible, and is commonly used
in vivo body. The area of TSV must be open to transfer the
signal from the probe side to the circuit side using O2 plasma.
However, with the same etching rate for photoresist (PR) and
parylene-C, the area, especially that of parylene-C, near the
probe tip is over etched. A hard mask solves this problem
(Fig. 5). The hard mask is implemented using a standard 4inch glass mask. The TSV open area is drilled by laser to let
the O2 plasma pass the hard mask. The tip area of the probe
is protected by the hard mask, such that only the TSV open
area can be etched by O2 plasma.
Fig.6. Cross-section view of the device before the post
processing.
Fig.7. Cross-section view of the device with TSV array and
probe array.
Figure 8 shows the integrated microsystem, including
probe arrays, a Printed Circuit Board (PCB), and connector.
This work creates 4-channel and 16-channel designs. Total
area is 6 mm × 6 mm.
Fig.5. Hard mask design for O2 plasma etching.
After the TSV is opened by O2 plasma, a 3000-Å
platinum is sputtering and lifted off to define different
channels. In this step, platinum (Pt) is used instead of gold
because the body may erode the gold, breaking the biosensor.
In lift-off processing, the ultrasonic cleaner is used to
decrease lift-off processing time.
Fig.8. Photograph of the integrated probe microsystem.
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Electrical Characteristics
Daisy chain and comb structure are fabricated to
investigate the electrical characteristics of 25-μm and 50-μmdiameter TSV. Figure 9 lists resistance measurements of 30
Cu TSV arrays. The measured daisy chain resistance keeps
stable under current stressing. To ensure that the insulating
capability of the sidewall TSV is adequate, the comb
structure is designed to measure capacitance and current
leakage.
Fig.9. Two-point resistance measurement of an array with 30
Cu TSVs.
Capacitance is measured from -10 to 10V (Fig. 10).
Average measured capacitances for 25-μm-diameter Cu
TSVs are 0.74 pF and 0.88 pF, respectively. Current leakage
from the TSV structure is low and in the nA scale (Fig. 11).
These experiment results validate the electrical properties of
the TSV sidewall and filling.
X-ray microscopy images show no visible voids inside
Cu TSVs of the bio-chip, indicating TSVs are filled fully
with Cu (Fig. 12). These experiment results show that TSV
sidewall insulation, electrical performance, and fabrication
have adequate quality.
Fig.12. The bio-chip and TSV array under X-ray
microscopy
Figure 13 is the impedance measurement result,
including micro probe array and PCB/connector, in 0.9%
saline which emulates the in-vivo environment. Fig. 13(a)
shows 4-channel results and Fig. 13(b) shows 16-channel
results, respectively. The impedance for 4 channels is
441.1265Ω with standard deviation 56.5515Ω at 1KHz; for
16 channels is 1136.76Ω with standard deviation 691.89Ω at
1KHz.
Fig.10. C-V sweeping of 100 TSVs using comb structure.
Fig.13. Impedance measurement results of (a) 4 channels
and (b) 16 channels.
Fig.11. Leakage current measured between +/- 10V on 100
Cu TSVs.
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Besides, in order to ensure function of all channels of
device operate well. Figure 14(a) & 14(b) are signal
recording results of the micro probe array from a 1 V sine
wave (peak to peak) source signal. Notably, sine wave here is
used to ensure that all channels of the bio-signal package
operate well only.
Fig. 14. Characteristics of (a) the 4-channel and (b) 16channel μ-probe array.
Conclusions
In this paper, a special TSV-based double-side biosignal recording device is designed and tested. By TSV
technology, the signal travels the shortest path to the CMOS
device. By eliminating conventional wire bonding, bio-signal
quality can be improved significantly. The chip has only a
size of 5 mm × 5 mm with 3 × 8 TSV arrays for each
channel. Therefore, the rat survival rate increased due to the
small size of the device. There are 30 × 16 microprobes in 16
channels die and 140 × 4 microprobes in 4 channels die.
Since multiple channels can acquire different neural cell
signals, this design benefits the neural-signal analysis. For
this recording scheme, all the post processes have been
developed and the micro system is ready for bio-signal
investigation.
Acknowledgments
This work was supported in part by the National Science
Council, Taiwan, R.O.C. under Contract No. 102-2221-E-
009-160, No. 102-2220-E-009-014, No. 102-2220-E-009002, and "Aim for the Top University Plan" of the National
Chiao Tung University and Ministry of Education, Taiwan,
R.O.C.. This work was also particularly supported by R&D
Piloting Cooperation Projects between Industries and
Academia at Science Parks under Contract Number: 100A20
and the UST-UCSD International Center of Excellence in
Advanced Bioengineering sponsored by the Taiwan National
Science Council I-RiCE Program under Grant Number:
NSC-101-2911-I-009-101. The authors would like to thank
National Chip Implementation Center (CIC) for chip
fabrication.
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