Fabrication and Sensing Properties of Nanowire FET Arrays

NanowireSensor RTD 2009

Nanowire field effect transistor (FET) arrays are developed as a platform for an ion and (bio)-chemical sensor that enables an electrical and label-free detection of charged particles. The sensing mechanism is based on the modification of the conductance through the silicon nanowire due to the adsorption of the target ions or molecules. To enable the specific detection of (bio-) analytes, the surface of the nanowire has to be functionalised with appropriate receptors.

1Paul Scherrer Institut, Villigen PSI, Switzerland2University of Basel, Department of Physics, Basel, Switzerland

1K. Bedner*, 1V. A. Guzenko, 2A. Tarasov, 2M. Wipf, 1R. A. Minamisawa, 2O. Knopfmacher, 2D. Just, 2R. L. Stoop, 2J. Brunner, 2W. Fu, 1C. David, 2M. Calame, 1J. Gobrecht, 2C. Schönenberger

MOTIVATION RESULTS

1 10 10010-12

10-11

10-10

10-9

10-8

10-7

SV (

V2 rm

s/Hz)

f (Hz)

4.3MΩ 9.2MΩ 15.4MΩ 25.8MΩ 39.6MΩ 90.7MΩ

Vbg

=-4V

Vds

=0.09V

1/f

width=100nm

pH 7

R

OUTLOOK

Analysis of noise measurements:- signal-to-noise ratio- detection limit- operating range- influence of the nanowire width

Vbg

Vref

Vlg

V

Vsd

A

A

0 200 400 600 800 10000

10

20

30

40

50

60

70

Wtop

(nm)

HfO2

Al2O

3

sens

itivi

ty (

mV

/pH

)

Sensitivity

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.510-10

10-9

10-8

10-7

10-6

10-5

10-4

Vref

(V)

W(top)

=100nm

W(top)

=1µm

pH = 4, 7, 10

G (

S)

Vsd

=0.1V

Vbg

=0V

HfO2

Transfer Curves(subthreshold regime)

Wbottom

Wtop Wside

silicon

SiO2

signal: transfer curves shift withchanging pH value

3 4 5 6 7 8 9 10 11

-1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

W(top)

=100nm

W(top)

=150nm

W(top)

=200nm

W(top)

=200nm

Vre

f (V

) at

20n

S

pH

Vds

=0.1 V, Vbg

=0 V

HfO2

61 mV/pH

pH Response (subthreshold regime)

pH-response: linear, negligiblehysteresis, ideal sensitivity

comparison of different nanowirewidths: transfer curves have tobe normalized by Weff=2Wside+Wtop

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.0

0.5

1.0

1.5

2.0

2.5

3.0 W(top)

=100nm

W(top)

=1µm

pH = 4, 7, 10

G/ W

eff (

S/m

)

Vref

(V)

HfO2

Vds

= 0.1V

Vbg

= 0V

Transfer Curves(linear regime)

Weff = 2Wside+ W top

0 200 400 600 800 10000

1

2

3

4

5

6

7

HfO2 10nm

HfO2 20nm

Al2O

3 20nm

-gm /

Wef

f (S

/Vm

)

W top

(nm)

Vds

= 0.1V

Vbg

= 0V

Transconductance g m

normalized transconductance gmis similar for different nanowirewidths: sensitivity is indepen-dent of nanowire widths

ideal sensitivity of 60mV/pH at 300K sensitivity indendent of nanowire widths same sensitivity for Al2O3 and HfO2

sensitivity is a surface property

KatpHmVV

pHpHq

TkV pzc

B

300/60

)(3.2

max =∆

−−=∆ α

V

Ggm ∆

∆=

∆V

∆G

gm

INFLUENCE OF THE NANOWIRE WIDTH

• Vsd drives current through the nanowire• Vbg modulates the conductivity• Vlg is swept to measure transfer curves• Vref measures the potential in the liquidanalyte: pH buffer solutions

-0.5 0.0 0.5 1.0 1.5 2.00

2

4

6

8

Vsd

=0.1V

NW 1 NW 2

G (

µS)

Vref

(V)

Vbg

=-6V

pH 7

10-10

10-9

10-8

10-7

10-6

10-5

width =1µm

G

(S

)

120mV/dec

Al2O

3

Transfer Curves

• reproducible electrical characteristics• negligible hysteresis• no leakage currents

EXPERIMENT SUMMARY

• successful fabrication of silicon nanowire sensors: - nanowire widths: 100nm to 1µm

• ideal pH-sensitivity : - Al2O3 and HfO2 as detection surface

• sensitivity is independent of nanowire width (subthreshold & linear regime) feasible to use nanowire FET arrays for multiplexed detection

Measurement Set-up

SENSOR FABRICATION

liquidelectrode

sourcecontacts

draincontact

liquidchannel

backgatecontact

PMMA

metal

silicon

silicon oxide resist

metalALD oxide

epoxy

HfO2,Al2O3

100 nm

5µm

lithography wet and dry etching ion implantationof contacts

ALD deposition contact metallization

40µm

100µm

10x10mm

bonding, microfluidicfunctionalization

SU-8/PDMS

• 48 nanowires per sample• nanowire width: 100nm – 1µm• nanowire length: 6µm