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CMOS and Microfluidic Hybrid System on Chip for Molecule Detection
Bowei Zhang, Qiuchen Yuan, Zhenyu Li, Mona E. Zaghloul, IEEE Fellow
Dept. of Electrical & Computer Engineering, The George Washington University
SPAD Breakdown Simulation
Background and Motivation
Scientific and Medical Fluorescence Image System
needs to detect very weak photon signal
Lost Energy
hc
E=

Time Delay
f (t ) = a  e
Lifetime
Lifetime
Wavelength
 (1/ ) t
Fluorescence
excitation process
Traditional image system use 3
filters at each pixel to identify
different wavelength, which
waste 2/3 of the photon signal.
lifetime image system doesn’t
need filter. Each pixel use lifetime
to distinguish signals. Sensitivity
can be improved about 3 times.
SPAD Design & Modeling
For fluorescence image system, signal photon avalanche
diode(SPAD) can be used as the photon detector. It not only
can detect the photon numbers, but also can detect the
fluorescence lifetime. We used the CMOS 0.5μm technology
to design and fabricated the SPAD.
The top view of our design is as
shown in Fig.1 (a). A center n-well and
a surrounding n-well were designed
with certain gap between each other .
The guard ring gap can protect SPAD
from perimeter breakdown. A p+
doping is designed to cover the center
n-well to form a p+n- juction, which is
the area to detect photons.
Fig.1(a) SPAD design top view
For p+/n- junction
photodiode, when the
p+ doping density
fixed, the lower the ndoping density, the
higher the breakdown
voltage. When the
doping density at the
perimeter area of the
photodiode is low
enough, the junction
would breakdown at
the center area first.
Therefore, by control
the gap between two
n-wells, a low doping
n- guard ring can be
created to prevent the
perimeter breakdown.
Fig. 2&3 show the
simulation results.
Fig.2(a) Simulated current density
with proper n-wells gap(1.8um)
Quenching Circuit for SPAD
When a photon arrives, SPAD
breakdown, a quenching circuit
is required to pull down the
reverse
voltage
after
an
avalanche event, and after the
recombination of the electron in
the junction, the quenching
circuit will apply a high reverse
voltage on the SPAD again to
ready for detecting another
photon.
CMOS & Microfluidic Integration
“Delay control signal” controls the rising
delay of a slow rise inverter, which could
adjust the output pulse width
Traditional fluorescence image system is complex and
relative big. To build a portable system, we want to
integrated the CMOS & Microfluidic technology. The hybrid
CMOS & Microfluidic system is much smaller compare with
traditional optics setup. It also have the potential to reach
the similar sensitivity as the traditional setup.
we use the material call polydimethylsiloxane (PDMS) to
make the Microfluidic channels. PDMS is a soft polymer
widely used in microfluidic and micro-optics due to its low
cost, easy fabrication process, bio-compatibility and optical
transparency.
10μm
Layout design of Dynamic Quenching Circuit
Fabricated CMOS Dynamic Quenching Circuit
Fig.2(b) Simulated current density, when n-wells
gap too small, breakdown happened at perimeter
Design diagram of hybrid CMOS & Microfuidic System. Microfluidic
channels are used for electrical interconnection and sample delivery
Fig.3(a) Simulated I-V character of SPAD
Fig.3(b) Simulated quantum efficiency
Fabrication of SPAD in CMOS
After simulation, SPAD, Quenching circuit and I/O
Pads are designed in CMOS technology. The layout
of the chip design was sent to semiconductor
manufacturing company for fabrication.
Output from Dynamic Quenching Circuit
Each pulse indicate one photon event
Each pulse has around 45ns pulse width
FPGA Counting System
FGPA counting system is design to count the total
number of signal pulse from the quenching circuit,
which indicates the total photon number in certain
accumulation time. In addition, Time-to-Digital
Converter (TDC) is designed on FGPA to record the
photon arrive time. Which can be used to generated
the fluorescence lifetime after signal processing.
CMOS Foundry
500μm
Layout of our SPADs design
Silicon Wafer
100μm
80um microfluidic
channel delivering low
melting point solder
200μm
Preliminary CMOS & Microfluidic Hybrid System
The key innovations of this method are: 1. using
microfluidic channel to delivery low melting point solder
for CMOS die and PCB board bonding. 2. PDMS
Microfluidic channela put test sample directly contact with
CMOS sensor chip to achieve high signal collection
efficiency, thus improve the sensitivity.
Fabricated chip of SPAD design
Altera Cyclone II FGPA
board used for design
counting system. The
figure on the right shows
the basic function of Time
correlated Single Photon
Counting (TCSPC), which
embedded in FPGA
Fig.1(b) Device doping profile model based on CMOS 0.5 μm Technology .
f (t ) = a  e
 (1/ )t
Coated micro-beads that can
catch target molecule and
generate fluorescence signal.
25μm
9 SPAD Designs with different Guard Ring Gap length
Two samples with different
fluorescence lifetime
Microbead
fluorescence
signal
in
preliminary
experiment. A microfluidic channel trap is designed to
catch a 10um diameter micro-bead with fluorescence
signal. Our further work is to integrate this trap right on top
of the SPAD
Contact Information
Fig.1(c) Device electric field profile. High electric field area easier to breakdown.
Measured I-V character of one of the fabricated
SPAD with proper guard ring gap length(1.8um)
Measured I-V character of one of the
fabricated SPAD with wrong Gap length
20μm
Bowe Zhang, email: [email protected]