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. 1 Paul Scherrer Institut, Villigen PSI, Switzerland 2 University of Basel, Department of Physics, Basel, Switzerland 1 K. Bedner *, 1 V. A. Guzenko, 2 A. Tarasov, 2 M. Wipf, 1 R. A. Minamisawa, 2 O. Knopfmacher, 2 D. Just, 2 R. L. Stoop, 2 J. Brunner, 2 W. Fu, 1 C. David, 2 M. Calame, 1 J. Gobrecht, 2 C. Schönenberger MOTIVATION RESULTS 1 10 100 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 S V (V 2 rms /Hz) f (Hz) 4.3MΩ 9.2MΩ 15.4MΩ 25.8MΩ 39.6MΩ 90.7MΩ V bg =-4V V ds =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 V bg V ref V lg V V sd A A 0 200 400 600 800 1000 0 10 20 30 40 50 60 70 W top (nm) HfO 2 Al 2 O 3 sensitivity (mV/pH) Sensitivity -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 V ref (V) W (top) =100nm W (top) =1μm pH = 4, 7, 10 G (S) V sd =0.1V V bg =0V HfO 2 Transfer Curves (subthreshold regime) Wbottom Wtop W si de silicon SiO2 signal : transfer curves shift with changing 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 V ref (V) at 20nS pH V ds =0.1 V, V bg =0 V HfO 2 61 mV/pH pH Response (subthreshold regime) pH-response : linear, negligible hysteresis, ideal sensitivity comparison of different nanowire widths: transfer curves have to be normalized by W eff =2W side +W top -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0.0 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) V ref (V) HfO 2 V ds = 0.1V V bg = 0V Transfer Curves (linear regime) W eff = 2W side + W top 0 200 400 600 800 1000 0 1 2 3 4 5 6 7 HfO 2 10nm HfO 2 20nm Al 2 O 3 20nm -g m / W eff (S/Vm) W top (nm) V ds = 0.1V V bg = 0V Transconductance g m normalized transconductance g m is similar for different nanowire widths: sensitivity is indepen- dent of nanowire widths ideal sensitivity of 60mV/pH at 300K sensitivity indendent of nanowire widths same sensitivity for Al 2 O 3 and HfO 2 sensitivity is a surface property K at pH mV V pH pH q T k V pzc B 300 / 60 ) ( 3 . 2 max = ∆ - - = ∆ α V G gm ∆ ∆ = ∆V ∆G gm INFLUENCE OF THE NANOWIRE WIDTH •V sd drives current through the nanowire •V bg modulates the conductivity •V lg is swept to measure transfer curves •V ref measures the potential in the liquid analyte: pH buffer solutions -0.5 0.0 0.5 1.0 1.5 2.0 0 2 4 6 8 V sd =0.1V NW 1 NW 2 G (μS) V ref (V) V bg =-6V pH 7 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 width =1μm G (S) 120mV/dec Al 2 O 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: - Al 2 O 3 and HfO 2 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 liquid electrode source contacts drain contact liquid channel backgate contact PMMA metal silicon silicon oxide resist metal ALD oxide epoxy HfO2, Al2O3 100 nm 5μm lithography wet and dry etching ion implantation of contacts ALD deposition contact metallization 40μm 100μm 10x10mm bonding, microfluidic functionalization SU-8/PDMS • 48 nanowires per sample • nanowire width: 100nm – 1μm • nanowire length: 6μm