IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 6, JUNE 2010
615
1/f Noise of Silicon Nanowire BioFETs
Nitin K. Rajan, David A. Routenberg, Jin Chen, and Mark A. Reed, Fellow, IEEE
Abstract—The 1/f noise of silicon nanowire (NW) biological field-effect transistors (NW FETs with exposed channels) is
characterized and compared with various fabrication approaches,
specifically, a wet orientation-dependent etch (ODE) versus common plasma-based etching methods. The wet-etched devices are
shown to have significantly lower noise and subthreshold swing,
and the average extracted Hooge parameter for ODE wet-etched
devices (αH = 2.1 × 10−3 ) is comparable to the values reported
for submicrometer MOSFETs with a metal/HfO2 gate stack.
Index Terms—Etching, low-frequency noise (LFN), MOSFET,
nanowire (NW).
I. I NTRODUCTION
S
ILICON nanowire (Si NW) and nanometer-scale fieldeffect transistors (FETs) have proved to be quite useful as
chemical and biological FETs (bioFETs). However, efforts to
fabricate such devices by top-down lithographic methods have
often exhibited degraded electrical characteristics associated
with the exposed silicon surfaces and high surface-to-volume
ratios inherent in NW sensors [1]–[3]. These surfaces have most
often been prepared by plasma-etching techniques, long known
to cause surface states and bulk damage [4], [5]. The presence
of these defects has likely been the source of high levels of lowfrequency noise (LFN), diminished sensitivity, and threshold
voltage hysteresis in NW sensors.
Anisotropic wet etching has been previously proposed as
a method for producing bioFETs with high-quality surfaces
as compared to plasma-etched nanostructures. While devices
have been fabricated using tetramethylammonium hydroxide
(TMAH) to produce smoothly faceted NW devices [6], [7], it
has not been demonstrated conclusively and quantitatively that
the wet-etching process is responsible for the superior electrical
performance of these devices. Here, we verify these claims by
comparing nominally identical bioFETs fabricated by TMAH
anisotropic etching, as well as two common plasma-etching
methods. LFN is of particular importance in devices used for
chemical and biological sensing, where the changes in signal
levels are small and the signal-to-noise ratio limits the sensitivity of the sensor [7], [8]. The subthreshold swing determines
the maximum sensitivity of the devices to charged species since
the dependence of current on gate voltage is exponential in the
subthreshold region [9]. In both cases, we demonstrate signifiManuscript received February 10, 2010; revised March 10, 2010. Date
of publication May 14, 2010; date of current version May 26, 2010. This
work was supported in part by the National Institutes of Health under Grant
R01EB008260. The first two authors contributed equally to this work. The
review of this letter was arranged by Editor L. Selmi.
The authors are with Yale University, New Haven, CT 06520-1942 USA
(e-mail: nitin.rajan@yale.edu; davidroutenberg@yahoo.com; jin.chen@yale.
edu; mark.reed@yale.edu).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2010.2047000
Fig. 1. Si NW FET fabricated by TMAH etching. The inset shows the transfer
characteristics. TSI refers to the thickness of the silicon channel, whereas
TBOX refers to the thickness of the buried oxide layer.
cantly better performance from the TMAH-etched devices than
similar devices fabricated by plasma-etching techniques which
we attribute to a decrease in surface state density.
II. D EVICE FABRICATION AND E XPERIMENTAL M ETHODS
NW channel FETs were fabricated from SOI (Soitec). The
high-resistivity active layer was thinned to 25 nm using sacrificial oxidation. Source and drain regions were patterned by
contact lithography and doped by As+ ion implantation. NW
channels and large source and drain pads were patterned in
hydrogen silsesquioxane by electron beam lithography. The
pattern was transferred through the active silicon layer using
either a TMAH anisotropic wet etch (25% in H2 O at 50 ◦ C), a
Cl2 inductively coupled plasma etch (Oxford 100), or a CF4
reactive-ion etch (Oxford 80). The devices were then metallized by titanium/gold evaporation and patterned by liftoff. The
finished device is shown in Fig. 1. The NW channels were
nominally 100 nm wide and 2.5 μm long.
The transfer characteristics were measured on a parameter
analyzer, from which the subthreshold slopes and field-effect
mobilities were extracted. A typical transfer curve for our NW
FETs is shown in the inset in Fig. 1. Noise measurements
were performed under dc conditions using a Keithley 2636
source meter. The devices were biased at a source–drain voltage of 0.1 V, and the gate voltages were chosen so that the
devices were always operated in the linear region of the Id –Vg
(transfer) curve. The drain current was measured at a sampling
rate of 25 Hz, and 5 × 105 samples were stored for later
signal processing. The spectral power density was estimated
using Welch’s modified periodogram method implemented in
MATLAB [10].
The noise spectral analysis, as well as the calculation of the
field-effect mobility, requires an accurate determination of the
gate capacitance. Due to our device geometry and the absence
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616
IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 6, JUNE 2010
of a surrounding gate (because of sensing applications), the
voltage-dependent capacitance of the NW FET channel is many
orders of magnitude less than the capacitance of the source
and drain, as well as the metallic fan-in and contact structures.
Moreover, the back gate consists of the nondegenerately doped
handle layer of the SOI wafer, and this parasitic capacitance
is also voltage dependent, precluding an accurate experimental
determination of the gate-channel capacitance of an individual
device. Thus, we simulated the devices using ATLAS Device
3D to calculate the capacitance of the NW FETs based on
dimensions verified by SEM analysis.
III. R ESULTS AND D ISCUSSION
The LFN of our devices follows Hooge’s equation
SI =
αH Id2
fβN
(1)
given by the mobility fluctuation model [11], [12], where
N is the number of carriers and αH is the Hooge constant
which is used to quantitatively assess and compare the noise
performance of our devices.
The exponents β for our devices were found to lie in
the range 0.8 < β < 1.5. LFN is usually characterized by an
exponent 0.8 < β < 1.2 [13]. The slightly higher exponents
encountered in our case can be understood by the presence
of RTS signals superimposed on the mobility fluctuation 1/f
noise [14]. In the present situation, we do not expect to be
able to observe RTS on top of 1/f noise since we will show
that our devices satisfy the “rule of thumb” N > 1/αH [15].
Exceptions to the rule of thumb are possible in the case of
inhomogeneous samples, which would explain additional RTS
noise giving rise to a higher 1/f exponent than expected for
pure 1/f noise [16], [17]. Nonetheless, we confirm the validity
of the mobility fluctuation model by observing the invariance of
the noise amplitude A = αH /N on the drain current, measured
at a fixed gate voltage and varying Vds , as well as the linear
dependence of 1/A on the gate voltage at fixed Vds .
Since, here, we operate the devices in the linear region of the
transfer curve, the number of carriers is given by
C
N = (Vg − Vth )
e
(2)
where C is the gate capacitance which is simulated using
ATLAS Device 3D and Vth is the threshold voltage.
Fig. 2 shows the typical normalized drain current noise power
spectra (SI /Id2 ) for a Si NW FET at various gate voltages. The
noise amplitude A can be obtained at f = 1 Hz from a least
squares linear fit of the noise spectra. From the linear dependence of 1/A on Vg , a value for αH can be extracted (shown
in the inset in Fig. 2). The Hooge constant αH is determined
in this manner for several devices from each etch method,
and the results are shown in Fig. 3(a). The TMAH-etched
devices have a considerably lower Hooge constant (close to an
order of magnitude lower) than the plasma-etched devices and
show smaller device-to-device variations. The average Hooge
constant for the TMAH-etched devices is given by αH =
2.1 × 10−3 , which is comparable to the Hooge constant values
reported for submicrometer MOSFETs with a metal/HfO2 gate
stack (αH = 1.6 × 10−3 for NMOS and αH = 6.9 × 10−3 for
Fig. 2. Typical dependence of 1/f noise spectra on gate voltage for a
TMAH-etched device, from which the noise amplitude A, at each gate voltage
(13–25 V), can be extracted. The inset shows 1/A plotted as a function of Vg ,
where the slope of the solid line (linear fit) is used to calculate αH .
Fig. 3. (a) Measured Hooge parameters for three sets of devices. Each set
was etched using either TMAH or Cl2 or CF4 . The box plot shows the 25th
percentile, the median, and the 75th percentile (the mean is indicated by a
square marker). The average values of αH were 0.0021 for the TMAH devices,
0.015 for the Cl2 devices, and 0.017 for the CF4 -etched devices. (b) Measured
subthreshold swing for three sets of devices, etched using either TMAH or Cl2
or CF4 . The average value for the TMAH devices was 1.0 V/decade. For Cl2 etched devices, the average was 2.6 V/decade, and for CF4 devices, the average
was 3.0 V/decade.
PMOS) [18]. The average Hooge parameters for the plasmaetched devices are 1.5 × 10−2 for Cl2 devices and 1.7 × 10−2
for CF4 devices.
An alternative approach based on a trapping–detrapping
model [19], [20] to analyze the noise measurement data makes
use of the empirical relationship
SId =
2
M gm
2 W Lf β
Cox
where M is the parameter to be extracted. W and L are
the width and length of the channel, respectively, gm is the
transconductance at the operating point of the measurement,
and Cox is the gate capacitance per unit area. Using the aforementioned model, the average M parameter for the TMAHetched devices is M = (6 ± 2) × 10−24 . Likewise, the average
values for the plasma-etched devices are M = (22 ± 10) ×
10−24 for the Cl2 devices and M = (12 ± 4) × 10−24 for the
CF4 devices. A similar trend is observed for the noise parameter
M , where the TMAH-etched devices exhibit a lower noise
figure than the plasma-etched ones.
RAJAN et al.: 1/f NOISE OF SILICON NANOWIRE BioFETs
TABLE I
AVERAGE F IELD -E FFECT M OBILITY AND C ORRESPONDING
S TANDARD D EVIATIONS FOR TMAH-, Cl2 -, AND
CF4 -E TCHED D EVICES A RE C OMPARED
The subthreshold swing measurements for each type of device are shown in Fig. 3(b). These values are all quite high
due to the thickness of the buried oxide serving as the gate
insulator. However, the TMAH devices exhibit significantly
lower average subthreshold swing and smaller device-to-device
variation than either of the plasma-etched devices.
We attribute the lower noise figure and lower subthreshold
swing, in the case of the wet-etched devices, to a lower density
of surface states at the etched sidewalls. The difference in
capacitance values is negligible and cannot account for the
differences in the measured subthreshold swing, which we
ascribe to variations in the interface state density. The noise
and subthreshold swing measurements, therefore, quantitatively
confirm that wet-etching-based methods yield smoother surfaces and consequently better electrical characteristics.
We also measured the field-effect electron mobility for devices fabricated using each of the etching methods. Field-effect
mobility can be calculated for planar FETs using
Lgm
(3)
μFE =
W Cox Vds
where Cox is the gate capacitance per unit area and gm is the
maximum value of the transconductance, determined from the
numerical derivative of the transfer characteristics. As shown
in Table I, we observed no statistically significant difference
between the TMAH- and Cl2 -processed devices, although
both exhibited less degradation than the CF4 RIE-processed
devices.
IV. C ONCLUSION
We have demonstrated the use of anisotropic wet orientationdependent etching with TMAH as a method of producing
bioFETs with high-quality surfaces and, consequently, superior electrical characteristics as compared to plasma-etched
surfaces. Our results lead us to conclude that, as bioFET
devices are downscaled further and the surface-to-volume ratio
increases, the particular etch process used will be critical in determining the density of surface states and, ultimately, the noise
performance. We have also demonstrated that TMAH-etched
NW devices have considerably lower subthreshold swing than
devices produced by two common plasma processes suggesting
lower surface state densities and, consequently, higher sensitivity when such devices are employed as chemical or biological
sensors.
617
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