Please cite this article in press as: X.T. Vu, et al., Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Fabrication and application of silicon nanowire transistor arrays for
biomolecular detection
X.T. Vu, R. GhoshMoulick, J.F. Eschermann , R. Stockmann, A. Offenhäusser, S. Ingebrandt
∗
Institute of Bio- and Nanosystems and CNI – Centre of Nanoelectronic Systems for Information Technology, Forschungszentrum Jülich GmbH,
Leo-Brandt-Str., D-52428 Jülich, Germany
article info
Article history:
Available online xxx
Keywords:
Biosensor
Silicon nanowire transistor arrays
Field-effect sensors
Nanoimprint lithography
abstract
We present a novel approach for large-scale silicon nanowire (SiNW) array fabrication for bioelectronic
applications. Nanoimprint lithography was combined with standard CMOS processing on 4in. SOI wafers
in order to produce highly integrated arrays of siliconnanowire field-effect transistors (SiNW-FET). Witha
very smooth surface due to wet anisotropic etching, SiNW-FET arrays show a good electronic performance
with a subthreshold slope of about 85 mV/decade. When applying a front-gate control of the wires via an
electrochemical reference electrode, reliable electronic performance inside an electrolyte solution can be
achieved. Our SiNW-FET sensors exhibit almost no electronic hysteresis on forward and backward bias

E-mail address: [email protected] (S. Ingebrandt).
nanofabrication, the nanoimprint lithography [7], in combina-
tion with anisotropic wet etching with tetramethylammonium
hydroxide (TMAH) [8,9]. In addition, our process includes stan-
dard CMOS processes like wet, dry etching and conventional
photolithography techniques. We improved the device perfor-
mance by boron doping on the conducting lines to reduce the
serial resistance, while retaining the high charge mobility inside
the SiNW-FETs. Chips were passivated by a layer of low pres-
sure chemical vapor deposited (LPCVD) SiO
2
. As gate oxide of
the SiNW-FETs, a thin thermal SiO
2
(6–8 nm) was chosen, which
serves as input dielectric. Main advantage of our process flow is
that mass production with reproducible devices can be achieved.
We developed a portable electronic readout system for the use
of the SiNW-FET arrays in biosensing experiments [10,11]. With
this system, the simultaneous readout of all 16-channels can be
achieved.
We describe in this article the electrical and electrochemical
characterization of the SiNW transistors. The devices can be oper-
ated by applying a back gate voltage through theburied oxide (BOX)
layer as well asto the front-gatethrough anelectrolyte solution con-
tacted by a liquid-junction Ag/AgCl reference electrode. The wires
showed good pH sensitivity with little hysteresis. As a first proof-
of-principle experiments for biomolecular detection we covalently
immobilized short DNA molecules or biotin molecules at the wire
surfaces. The biomolecules were site-selectively attached at the

after imprinting and to increase the aspect ratio of the small struc-
tures (down to 100 nm), we used a monolayer of fluorsilane as
anti-adhesion layer on the mold surface.
2.2. Si-nanowire process
For fabrication of the devices we used 4 in. silicon-on-insulator
(SOI) wafers (SOITEC, France) with a BOX thickness of 40 0 nm
and a top Si layer of 360 nm thickness (Si
100 , boron doped
14–22  cm). The wafer carried three different layouts of nanowire
arrays (4 × 4-common source, 16 × 16 and 32 × 32-cross contacts).
The length of the wires was 3 ␮m in all three designs. For investi-
gation of possible size effects we varied the widths of the starting
structures for wet etching by 100 nm, 200 nm, 500 nm, and 1 ␮m
(mask measures), respectively. In Fig. 1a the layout of the 32 × 32
SiNW array and a scanning electron micrograph of a sensor spot
including six wires are shown (Fig. 1b).
A schematic of the process flow is shown in Fig. 2. Firstly the
top silicon layer of the SOI wafer was thinned out down to about
60 nm (Fig. 2, steps 1 and 2). Then the starting structures were trans-
ferred from the mold to the 4 in. SOI wafers by thermal nanoimprint
(Nanonex NX-2000, USA) (Fig. 2, step 3). After imprinting, RIE was
used to etch the residual resist layer and to etch off the SiO
2
layer
between the contact lines (Fig. 2, step 4). Then the device struc-
tures were transferred to the top Si layer by anisotropic wet etching
with TMAH (25%, 90
◦
C) [8,9]. Due to the large etch rate difference
between Si and SiO

nanowire array. (b) SEM image of one sensor spot with six nanowires. The open-
ing of the passivation layers on top of the nanowire area can be seen. (c) Scanning
electron micrograph of a single silicon wire (<100 nm). One can see the Si
100
and Si 111 surfaces of the trapezoid wire structure.
pads (Fig. 2, step 8). For the 16 × 16 and 32 × 32 arrays this metal
layer served as second contact line inside the grid array (Fig. 1a).
To enable operation of these devices in an electrolyte solution, a
nitride-oxide stack was deposited by plasma enhanced chemical
vapor deposition (PECVD) (at the clean room facilities of the Uni-
versity of Applied Sciences Kaiserslautern - Campus Zweibrücken,
Germany) and the bond pads were re-opened.
2.3. Electronic readout and detection methods
For measurements in a liquid environment, devices were wire
bonded on 68-pin LCC carriers (LCC0850, Spectrum, USA) and
encapsulated using glass rings and a biocompatible epoxy glue
Please cite this article in press as: X.T. Vu, et al., Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens.
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Fig. 2. Main process steps for the fabrication of the SiNW-FET arrays. Our wafer-scale
process for SOI wafers is combining nanoimprint lithography with wet etching using
TMAH. Contact lines are p-doped for reliable operation of the devices. The finalized
structure in step 8 shows back gate contact, bond pad, contact line and open wire
(from left to right).
(U300 8OZ, Epo-TEK, USA) (Fig. 3b). Recording was done on a wafer
probe station or with our previously described 16-channel FET
amplifier system for dc and for ac readout [10–13] (Fig. 3a). To record

biomolecules.
As a second effect, the biomolecular layer on the surface of
the SiNWs is acting as an additional, passive RC element inside
Please cite this article in press as: X.T. Vu, et al., Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens.
Actuators B: Chem. (2009), doi:10.1016/j.snb.2008.11.048
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the readout circuit (resistance R
mem
and capacitance C
mem
of the
biomembrane in Fig. 3c). The DNA hybridization reaction or the
protein binding is leading to a change of the input impe dance of
the device. This change can be accessed by using an impedimetric
readout method utilizing the transistor transfer function (TTF) prin-
ciple [10,11,18,20,21]. For this detection method the combination of
sensor, its first amplification circuit with feedback resistor R
FB
, the
reference electrode resistance R
RE
, the liquid solution resistance
R
sol
, the contact line capacitance C
CL
, the gate oxide capacitance

was done in oxygen plasma (100E Plasma System from Techniques
Plasma GmbH, 1.4 mbar, 200 W, and 1 min). For biofunctionaliza-
tion of the wire surfaces, we used a vapor phase silanization
protocol with 3-glycidoxypropyltrimethoxysilane (GPTES) [28,29].
The chips were placed in a desiccator containing a few drops of
silane (300 ␮l). The desiccator was sealed, heated and the reac-
tion was allowed for 1 h. The complete silanization procedure was
performed inside a glove box containing a water- and oxygen-free
argon atmosphere. The silanization procedure was finalized by rins-
ing several times with ultra pure water in order to remove unbound
silane molecules. Finally all samples were dried with argon. For
micro-spotting with our single-nozzle system with aiming option
[13], the amino-modified 20 base-pair (bp) DNA probes (MWG-
Biotech AG, Germany) were prepared in a concentration of 1 ␮Min
a 0.1 M phosphate buffer of pH 8.5. After micro-spotting, the immo-
bilization process was performed by overnight incubation at 37
◦
C
in a humid atmosphere.
For covalent immobilization of biotin to the SiNW-FETs, the chip
surface was functionalized with 3-aminopropyl-triethoxysilane
(APTES) [30–32]. Chips were wet-chemically cleaned in three steps
including ethanol for 2 min, HCl (2%, v/v) for 2 min, and piranha
solution for 2 min. The chip surfaces were then activated by H
2
SO
4
(20% (v/v), 80
◦
C for 10 min). After each step, the chips were carefully

fore the RIE etching of this residual layer was strictly controlled to
maintain the high aspect ratio of the structures.
The anisotropic TMAH etching created a trapezoidal SiNWs
structure having Si
111 sidewalls in an angle of 54.7
◦
with
100 top and bottom surfaces (Fig. 1c). By further etching, the size
of the top and bottom
100 planes will be slowly reduced under
the top oxide mask. The etching rate for the Si
111
direction
was about 20 nm/min for our process. Using this process SiNWs
with very smooth surfaces were achieved (Fig. 1c). Since the contact
lines of our chips were passivated by a high quality LPCVD oxide,
a reliable performance in electrolyte solution was achieved. Addi-
tionally, our chips can be re-used for many experiments by the use
of a standard cleaning protocol [30,31,33].
Fig. 4. When the contact lines of the SiNW-FET array are additionally implanted by
boron, a reliable p-FET operation of the wires can be achieved. (a) Transfer charac-
teristics of a SiNW-FET with p-doped contact lines. (b) Subthreshold characteristics
of a SiNW-FET.
Please cite this article in press as: X.T. Vu, et al., Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens.
Actuators B: Chem. (2009), doi:10.1016/j.snb.2008.11.048
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Fig. 5. Characterization of the pH sensitivity of the SiNW-FETs (six wires of 400 nm

etching in contrast to the TMAH etching, the wire surfaces were
much rougher resulting in a strong electronic hysteresis (data not
Fig. 7. Detection of immobilized DNA on the SiNW-FETs. Transfer characteristics
before (solid lines) and after DNA immobilization (dashed lines) are shown. DNA
was site-selectively immobilized on some channels out of the same array using a
micro-spotter. (a) One channel out of the same array having no DNA attached. (b)
Another channel out of the same array having 20-bp DNA attached with a high
grafting density. For the DNA-modified sensor a shift of 250 mV of the flat band
voltage was recorded.
Please cite this article in press as: X.T. Vu, et al., Fabrication and application of silicon nanowire transistor arrays for biomolecular detection, Sens.
Actuators B: Chem. (2009), doi:10.1016/j.snb.2008.11.048
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6 X.T. Vu et al. / Sensors and Actuators B xxx (2009) xxx–xxx
shown). For our current device types the transfer characteristics
was shifting to the left side for smaller pH values and to the
right side for larger pH values independent if p-type or n-type
devices were used. The sensitivity in both cases was measured
to 38–41 mV/pH, which is a typical value for silicon oxide sur-
faces.
With our lock-in based amplifier system the SiNW-FET devices
can also be used as impedimetric sensors like recently reported
with our standard, micro-sized FETs [10,11].InFig. 6 the trans-
fer characteristics of a SiNW-FET in buf fer solutions with different
concentrations of NaCl (pH 7) is shown. Similar to what we previ-
ously reported for our micro-sized FETs, the transfer characteristics
is shifting, because the solution resistance R
sol
in the electronic cir-

SiNW-FET arrays is shown. In the biomolecular-free channels a
minor shift was recorded, whereas in the channels spotted with
biotin a shift of 33 mV was recorded (Fig. 8b). For this measurement
a sodium phosphate buffer (2.5 mM, pH 8.2) was used as electrolyte
solution. Again with several chips containing many channels a sim-
ilar, reliable behavior was observed.
4. Conclusions
We present a robust, wafer-scale fabrication process for SiNW
transistor arrays. With a combination of nanoimprint and TMAH
wet anisotropic etching, we produced smooth surfaces for the SiNW
transistors. We achieved a wire width down to 20 nm on top and
100 nm at the bottom of the trapezoid nanowire structure having
a height of about 60 nm. In future designs the width of the wire
could be even reduced using longer etching times or a thinner start
layer. The devices were successfully operated in a liquid environ-
ment in different kinds of electrolyte solutions. SiNW-FETs with
their silicon oxide surface had a linear pH response with a typi-
cal sensitivity of about 40 mV/pH. The electronic performance was
stable and forward and backward bias sweeps of the transfer char-
acteristics revealed almost no hysteresis. When applying the TTF
method, we showed that the devices are sensitive to different ionic
strengths of the buffer electrolyte similarly to what we previously
reported for our micro-sized FET arrays. For biomolecular experi-
ments the devices were silanized with either APTES or GPTES using
our standard protocols. We present first biomolecular detection
experiments with the SiNW-FET arrays, where we site-selectively
and covalently attached single-stranded DNA molecules and biotin
molecules at the wire surfaces. In the case of DNA we recorded
a very large shift of the flat band voltage of 250 mV. For biotin a
smaller, but still large shift of 33 mV was measured.

[4] Y. Cui, Q.Q. Wei, H.K. Park, C.M. Lieber, Nanowire nanosensors for highly sen-
sitive and selective detection of biological and chemical species, Science 293
(5533) (2001) 1289–1292.
[5] F. Patolsky, G.F. Zheng, O. Hayden, M. Lakadamyali, X.W. Zhuang, C.M. Lieber,
Electrical detection of single viruses, Proc. Natl. Acad. Sci. U.S.A. 101 (39) (2004)
14017–14022.
[6] F. Patolsky, B.P. Timko, G.H. Yu, Y. Fang, A.B. Greytak, G.F. Zheng, C.M. Lieber,
Detection, stimulation, and inhibition of neuronal signals with high-density
nanowire transistor arrays, Science 313 (5790) (2006) 1100–1104.
[7] L.J. Guo, P.R. Krauss, S.Y. Chou, Nanoscale silicon field effect transistors fabri-
cated using imprint lithography, Appl. Phys. Lett. 71 (13) (1997) 1881–1883.
[8] Y.X. Liu, K. Ishii, T. Tsutsumi, M. Masahara, E. Suzuki, Ideal rectangu-
lar cross-section Si-Fin channel double-gate MOSFETs fabricated using
orientation-dependent wet etching, IEEE Electron Device Lett. 24 (7) (2003)
484–486.
[9] K. Sato, M. Shikida, T. Yamashiro, K. Asaumi, Y. Iriye, M. Yamamoto, Anisotropic
etching rates of single-crystal silicon for TMAH water solution as a function of
crystallographic orientation, Sens. Actuators A: Phys. 73 (1–2) (1999) 131–137.
[10] S. Ingebrandt, Y. Han, F. Nakamura, A. Poghossian, M.J. Schöning, A. Offen-
häusser, Label-free detection of single nucleotide polymorphisms utilizing the
differential transfer function of field-effect transistors, Biosens. Bioelectron. 22
(12) (2007) 2834–2840.
[11] S. Schäfer, S. Eick, B. Hofmann, T. Dufaux, R. Stockmann, G. Wrobel, A.
Offenhäusser, S. Ingebrandt, Time-dependent observation of individual cel-
lular binding events to field-effect transistors. Biosens. Bioelectron., in press,
doi:10.1016/j.bios.2008.1007.1003.
[12] X.T. Vu, J.F. Eschermann, R. Stockmann, R. GhoshMoulick, A. Offenhäusser, S.
Ingebrandt, Top-down processed silicon nanowire transistor arrays for biosens-
ing. Phys. Status Solidi A: Appl. Mater., in press.
[13] S. Ingebrandt, Y.H. Han, M.R. Sakkari, R. Stockmann, O. Belinskyy, A. Offen-

[22] D. Landheer, G. Aers, W.R. McKinnon, M.J. Deen, J.C. Ranuarez, Model for the
field effect from layers of biological macromolecules on the gates of metal-
oxide–semiconductor transistors, J. Appl. Phys. 98 (4) (2005).
[23] D. Landheer, W.R. McKinnon, W.H. Jiang, G. Aers, Effect of screening on the
sensitivity of field-effect devices used to detect oligonucleotides, Appl. Phys.
Lett. 92 (25) (2008).
[24] M.W. Shinwari, M.J. Deen, D. Landheer, Study of the electrolyte–insulator–
semiconductor field-effect transistor (EISFET) with applications in biosensor
design, Microelectron. Reliab. 47 (12) (2007) 2025–2057.
[25] D. Landheer, W.R. McKinnon, G. Aers, W. Jiang, M.J. Deen, M.W. Shinwari, Calcu-
lation of the response of field-effect transistors to charged biological molecules,
IEEE Sens. J. 7 (9–10) (2007) 1233–1242.
[26] W.R. McKinnon, D. Landheer, Sensitivity of a field-effect transistor in detecting
DNA hybridization, calculated from the cylindrical Poisson–Boltzmann equa-
tion, J. Appl. Phys. 10 0 (5) (2006).
[27] A. Poghossian, A. Cherstvy, S. Ingebrandt, A. Offenhäusser, M.J. Schöning, Pos-
sibilities and limitations of label-free detection of DNA hybridization with
field-effect-based devices, Sens. Actuators B: Chem. 111 (2005) 470–480.
[28] C. Consolandi, B. Castiglioni, R. Bordoni, E. Busti, C. Battaglia, L.R. Bernardi, G. De
Bellis, Two efficient polymeric chemical platforms for oligonucleotide microar-
ray preparation, Nucleos. Nucleot. Nucleic Acids 21 (8–9) (2002) 561–580.
[29] J. Piehler, A. Brecht, R. Valiokas, B. Liedberg, G. Gauglitz, A high-density
poly(ethylene glycol) polymer brush for immobilization on glass-type surfaces,
Biosens. Bioelectron. 15 (9–10) (2000) 473–481.
[30] Y. Han, D. Mayer, A. Offenhäusser, S. Ingebrandt, Surface activation of thin sili-
con oxides by wet cleaning and silanization, Thin Solid Films 510 (1–2) (2006)
175–180.
[31] Y. Han, A. Offenhäusser, S. Ingebrandt, Detection of DNA hybridization by a field-
effect transistor with covalently attached catcher molecules, Surf. Interf. Anal.
38 (4) (2006) 176–181.

rent research interests are SiNW transistor array design, fabrication and simulation
for bioelectronic applications and extracellular recording from electrogenic cells.
Regina Stockmann was born in Aachen, Germany, in 1969. She graduated in Applied
Chemistry at the Aachen University of Applied Sciences in 1996. After years of
experience in clean room processing she joined in 2002 the Institute of Bio- and
Nanosystems (IBN), Institute 2: Bioelectronics, at the Forschungszentrum Jülich,
Germany. From then on her main focus was on optimizing semiconductor chips
to achieve better sensors for bioelectronic measurements. Her current interests are
silicon nanowire design, fabrication and optimization for biosensor applications.
Andreas Offenhäusser was born in Heidenheim, Germany in 1959. He graduated
in physics (Diplom) from the University of Ulm in 1985 and completed a Ph.D. at
the University of Ulm in 1989. From 1990 to 1992 he worked as an engineer at
Robert Bosch GmbH, Reutlingen. From 1992 to 1994 he joined the Frontier Research
Program, RIKEN, Japan. From 1994 to 2001 he worked at the Max Planck Institute for
Polymer Research, Mainz, as a group leader. In 2000 he received his “habilitation”. He
moved to the Forschungszentrum Jülich in 2001 where he is presently director of the
Institute of Bio- and Nanosystems (IBN), Institute 2: Bioelectronics. He is a professor
for experimental physics at the RWTH-Aachen University, Germany. The focus of his
work is the functional coupling of sensory cells and neurons with microelectronic
devices, signal processing in biological neuronal networks, electronic DNA-Chip, and
biophysics of lipid bilayers and membrane receptors.
Sven Ingebrandt was born in Alzey, Germany, in 1971. He graduated in physics
(Diplom) in 1998 at the Johannes Gutenberg University Mainz, Germany. From 1998
to 2001 he was working as Ph.D. student at the Max Planck Institute for Polymer
Research in Mainz, Germany. He received his Ph.D. degree in physical chemistry in
2001 from the Johannes Gutenberg University Mainz, Germany. In 2001 and 2002
he was working as postdoctoral researcher at the Frontier Research Program, RIKEN,
Japan. From 2002 to 2008 he was working as group leader in the Institute of Bio-
and Nanosystems (IBN), Institute 2: Bioelectronics, at the Forschungszentrum Jülich,
Germany. Recently he moved to the Kaiserslautern University of Applied Sciences as