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Conductive or insulating substrate ?
How resolution and accuracy is affected by the substrate material (silicon, quartz or plastic)
Marco SampietroHIGH RESOLUTION ELECTRONIC MEASUREMENTS IN NANO-BIO SCIENCE
AmplifierContact pad
Experimental area
Conductiveelectrolyte (PBS, …)
Connecting strip
Experiments with cartridges in liquid
CE
Affinity biosensors
Binding detection Proximity detection
Electric field lines
Free channel
Cell detection
Electric field lines
Low Impedance
High Impedance
Advantages:• No need for optical instrumentation• Label-free technique
ElectrodeS S
O OH
SS
O OH
S S
O OH
S S
O OH
S S
O OH
S S
O OH
S S
O OH
S S
O OH
Enzyme
molecule
Counter electrode
IMPEDANCE MEASUREMENT
Rf
)(tvi
DUT )(tvo
Cp2Cp1
)(tii
)()(
SISVZ
i
iDUT =
In phase ⇒ resistance
In quadrature ⇒ capacitance
Transimpedance amplifier
vi(t)
Experiments with cartridges in liquid
Rf
+ -
VCE
Vout
- Absolute error !
Insulating substrate
Electrolyte
200 nm
SignalCE
Spurious signal
The thicker the better
AmplifierContact pad
Experimental area
Connecting strip covered by insulator
Good and spurious signals – top view
CE
Signal
Spurious signal
Practical tips
∅ 5 µm
C = 3 pF
1 cm x 100µm
C = 330 pF
2 mm x 3 mm
f
+ -
Vout
C = 3 pFC = 3 pF
f
+ -
Vout
10µm1 mm
Y
∆=2%
∆=0.02%Y
∆=2%
∆=1%
Beware of conductive substrate !
Rf
+ -
VCE
Vout
SiO2
Electrolyte
200 nm
CE
Conductive
C very big (short circuit)
ADDITIONALspurious signal.
Very big !
Amplifier
Contact pad
Experimental area
Contact pad contributes !
CE
Signal
All metal deposited on the chip contributes
Comparison btwn QUARTZ & SILICON
Rf
CE
Measurement of at various frequency
V
I
CIV
ω1
=
Comparison btwn QUARTZ & SILICON
Electrolyte
CE
Insulator
Electrolyte
CE
Conductive
Conductive
Comparison btwn QUARTZ & SILICON
With a silicon cartridge:
10% modulation of Cdl
(∆Cdl=0.8pF)
350ppm modulation of the total signal (0.8pF/2.2nF)
Difficult to be measured !
M. Carminati et al., Sensors and Actuators B, Chemical, vol.174 (2012) 168– 175
Conductive substrate held to GND
Rf
+ -
VCE
Vout
SiO2
200 nm
CE
Conductive
Silicon substrate connected to GROUND
Much easier to be measured !
SiO2
200 nm
CE
Insulating
NOISE EFFECT : quartz substrate
Electrolyteis at ground
Rf
+ -
Vout
Small sensor capacitance small noise
Csensor = ?
SiO2
200 nm
CE
Conducting
NOISE EFFECT : silicon substrate
Electrolyteis at ground
Rf
+ -
Vout
Big sensor capacitance big noise
Csensor = ?
Also the pad contributes !
Comparison Quartz vs Silicon : Noise
M. Carminati et al., Sensors and Actuators B, Chemical, vol.174 (2012) 168– 175
Current sensing : molecules through nanopore
DNA Molecule
Low-Pass Filter
(160Hz)
Low-Pass Filter
(160Hz)
Ag/AgCl
Ag/AgCl
Headstage
A
Si
Nanopore Device
Amplifier
Amplifier
Ionic Current
Tunneling Current
V1
V3
V4
V2
iTUN
iION
Cstray
Tunneling Electrodes
D. Branton et al., Nature biotechnology, 2008, 26, 1146-53. T. Albrecht et al., Nature Communications, 2012, 3, 829.
DNA sequencing
A. Ivanov et al., Nano letters, 2011, 11, 279-85.
Noise in nanopore on conductive Silicon
Lg=1mm
Lm=50µm
SiN (t=20 nm)
Si Si
gasket
Cm
gasket
electrolyte
electrolyte
Condition B=10kHz B=100kHzCm = 5pF 2.3pA 7.3pACm+ (CSI in series with CSiN) = 3.3 nF 133pA 1.5nA
pF5t
LxLC mmSiNm =ε≅
CSi
CSiN
nF3.3CCCxCC
SiNSi
SiNSistray ≅
+≅
(Idc= 100pA; Camp = 1pF)
Insulating substrate
Conductive substrate !J.K.Roosenstein et al. «Integrated nanopore sensing platform with
sub-microsecond temporal resolution», Nature 2012
Microfluidic system … on top of a microelectronic system
Ways through in Microfluidics
Experiments on the microfluidic gasket can interferewith the reading of microelectrodes elsewhere ?
Rf
+ -
VCE
Vout
SiO2
Electrolyte
200 nm
CE
Conductive
ADDITIONALspurious signal.
Very big !
Beware of conductive substrate !
THINGS to REMEMBER (1)
Conductive substrate promotes additional path for spurious signal (BAD)
THINGS to REMEMBER (2)
Accuracy and resolution can be affected very hard (bad)
SiO2
200 nm
CE
Conducting
Electrolyteis at ground
Rf
+ -
Vout
Cdetvery BIG
Also the pad contributes !
Beware of conductive substrate !
BIG noise
THINGS to REMEMBER (3)
Use Si cartridge only when you take advantage of Si for multielectrodes and integrated electronics.
Beware of sneaky paths !