Realism in nanosensing: hard-won insights from the trenches Michael Roukes Robert M. Abbey Professor of Physics, Applied Physics, & Bioengineering Co-Director, Kavli Nanoscience Institute California Institute of Technology [email protected] http://nano.caltech.edu Abstract: Nanosensors have the unprecedented capability to interact with absolute numbers of analytes down to the single molecular scale. The susceptibility of nanosensors to external perturbation, given their size, is unprecedented -- and this essential physics offers exceptional new opportunities. Indeed, we are here to explore, harness, and even celebrate these attributes! As a archetypal case study of nanosensor physics, I will focus upon resonant NEMS sensors. I will describe the immense increase in responsivity obtained with device size reduction that yields, for example, atomic mass resolution. As an archetypal case study of nanosensor physics, we’ll focus upon resonant NEMS sensors. I will describe the immense increase in responsivity obtained with device size reduction that yields, for example, atomic mass resolution. We will explore several noise mechanisms and backaction effects imposing ultimate physical limits to the sensitivity, and how matching considerations make it extremely challenging to embed these devices in practical measurement systems. The unique properties of nanodevices, however, are not a panacea that absolves us from the need to contend with long-standing, overarching challenges posed by real-world sensing applications. From about a decade and a half’s work focused on metamorphosing nanosensors into nanosystems through large-scale integration, nature has taught us some hard lessons. In my talk I will share several of these, using examples from NEMS and other sensor systems. Among them are: (a) any stand-alone sensor is unlikely to be up to the full dynamic range of challenges posed in the real world; (b) complex combinations of real-world analytes are unlikely to be resolved by any “one-stop” generic approach; (c) arrays of sensors can often give enhanced performance over individual devices, but achieving such gains may require exceptionally careful attention to device variances; (d) careful choices of platform materials are essential to insure a rapid transitioning into large-scale integration and production en masse. Although these sobering considerations may potentially force an initial downselection from amongst the panoply of exciting possibilities of emerging nanosensors; learning from the past, and considering real- world (not academic laboratory) conditions, will accelerate realization of practical nanosensor systems. 22 March 2012 ML Roukes © Caltech 2012 2 single-molecule NEMS-MS AK Naik, MS Hanay, WK Hiebert, XL Feng, and ML Roukes, Nature Nanotechnology 4, 445-449 (2009) 0 background fluctuations -5 BSA -10 single molecule (kHz) adsorption events -15 (precipitous jumps) -20 -25 -30 Frequency Shift z -35 y x -40 0 100 200 300 400 500 600 700 800 900 Time (s) NAIK, Akshay HANAY, Selim HIEBERT, Wayne M.L. Roukes Staff Physicist3 © Caltech/CEA-LETIGrad Student 2011 Postdoc 22 March 2012 the promise Nanosensors have the unprecedented capability to interact with absolute numbers of analytes down to the single molecular scale. The susceptibility of nanosensors to external perturbation, given their size, is unprecedented -- and this essential physics offers exceptional new opportunities. Indeed, we are here to explore, harness, and even celebrate these attributes! 22 March 2012 ML Roukes © Caltech 2012 4 responsivity, noise, backaction, mismatch 22 March 2012 ML Roukes © Caltech 2012 5 physics: responsivity, noise, backaction, mismatch As an archetypal case study of nanosensor physics, we’ll focus upon resonant NEMS sensors. I will describe the immense increase in responsivity obtained with device size reduction that yields, for example, atomic mass resolution. We will explore several noise mechanisms and backaction effects imposing ultimate physical limits to the sensitivity, and how matching considerations make it extremely challenging to embed these devices in practical measurement systems. 22 March 2012 ML Roukes © Caltech 2012 6 frequency-shift based mass sensing frequency shift Δ f ~ − ( f0 /2M eff )⋅δ m “gain” added mass responsivity mass resonant response z y x doubly-clamped beam resonator M.L. Roukes 7 © Caltech/CEA-LETI 2011 22 March 2012 minimum resolvable mass is determined by “gain” and noise ∂M eff -1 δ M ≈ δω0 ~ R σ A (τ ) mass ∂ω0 Mass Allan a measure of resolution Responsivity Deviation frequency-fluctuation noise M.L. Roukes 8 © Caltech/CEA-LETI 2011 22 March 2012 Maximizing mass responsivity leads to minimizing the resolvable mass ∂M eff -1 δ M ≈ δω0 ~ R σ A (τ ) mass ∂ω0 Mass Allan a measure of resolution Responsivity Deviation frequency-fluctuation noise large mass responsivity For any complex mechanical mode, modeled as a damped simple harmonic oscillator: ω R ≈− 0 2Meff 1 ∝ huge benefit in 4 scaling downward ! M.L. Roukes 9 © Caltech/CEA-LETI 2011 22 March 2012 size-scaling the minimum resolvable mass: resonant inertial mass sensors quartz crystal microbalances M eff ~1−100mg; δ m ~1μg SAW/FPW sensors (MEMS) M eff ~1μg −1mg; δ m~1ng microcantilevers (MEMS) M eff ~1ng−1μg; δ m ~1fg NEMS M eff ~1fg−1pg; δ m ≤ 1zg M.L. Roukes 10© Caltech/CEA-LETI 2011 22 March 2012 Massive increase in responsivity: zeptogram mass sensitivity ( 1zg = 10-21g ) 0 SiC doubly clamped beam NEMS UHF bridge configuration -200 ( f0 ~ 133 and 190 MHz ) Ultralow noise PLL readout -400 ~100 zg Nano Lett. 6, 583 (2006) -600 NEMS shutter -800 0 Frequency Shift (Hz) -1000 nozzle -500 0 50 100 150 200 250 300 350 -1000 Time (sec) -1500 -2000 133 MHz Mass Frequency Shift (Hz) responsivity -2500 Ya-Tang Yang, Carlo Callegari , 190 MHz ~1Hz/zg Xiaoli Feng, Kamil Ekinci, MLR -3000 0 1000 2000 3000 4000 M.L. Roukes Mass (zeptograms)© Caltech/CEA-LETI 2011 22 March 2012 11 BUT… minimizing frequency-fluctuation noise is also critical !!! ∂Meff -1 δ M ≈ δω 0 ~ R σ A (τ ) mass ∂ω0 Mass Allan a measure of resolution Responsivity Deviation frequency-fluctuation noise frequency resolution optimized phase-locked loop frequency tracking noise processes (f) a log S 012 Frequency f /fc M.L. Roukes 12© Caltech/CEA-LETI 2011 22 March 2012 Real-time zeptogram mass sensitivity ( 1zg = 10-21g ) 0 SiC doubly clamped beam NEMS UHF bridge configuration -200 ( f0 ~ 133 and 190 MHz ) Ultralow noise PLL readout -400 ~100 zg Nano Lett. 6, 583 (2006) -600 NEMS shutter -800 100 0 Frequency Shift (Hz) -1000 nozzle 10 Mass -500 (zg) resolution m 1 ~7 zg 0 50 100 150 200 250 300 350 δ -1000 Equivalent to about Time (sec) 0.1 0 2000 4000 30 Xe-1500 atoms… Time (s) -2000 133 MHz Mass Frequency Shift (Hz) responsivity -2500 Ya-Tang Yang, Carlo Callegari , 190 MHz ~1Hz/zg Xiaoli Feng, Kamil Ekinci, MLR -3000 0 1000 2000 3000 4000 M.L. Roukes Mass (zeptograms)13© Caltech/CEA-LETI 2011 22 March 2012 physics: responsivity, noise, backaction, mismatch As an archetypal case study of nanosensor physics, we’ll focus upon resonant NEMS sensors. I will describe the immense increase in responsivity obtained with device size reduction that yields, for example, atomic mass resolution. We will explore several noise mechanisms and backaction effects imposing ultimate physical limits to the sensitivity, and how matching considerations make it extremely challenging to embed these devices in practical measurement systems. 22 March 2012 ML Roukes © Caltech 2012 14 sources of frequency-fluctuation noise 22 March 2012 ML Roukes © Caltech 2012 15 responsivity, noise, backaction, mismatch # " ! $%" &' ! 22 March 2012 ML Roukes © Caltech 2012 16 physics: responsivity, noise, backaction, mismatch As an archetypal case study of nanosensor physics, we’ll focus upon resonant NEMS sensors. I will describe the immense increase in responsivity obtained with device size reduction that yields, for example, atomic mass resolution. We will explore several noise mechanisms and backaction effects imposing ultimate physical limits to the sensitivity, and how matching considerations make it extremely challenging to embed these devices in practical measurement systems. 22 March 2012 ML Roukes © Caltech 2012 17 Cantilever tales: “Can a gauge factor ever be “too big”? Scaling down: semiconducting nanopiezoresistor issues RF electronics signal Pros: 50Ω >>1MΩ 1. A large gauge factor V. serious impedance Cons: matching problem at device RF & VHF and beyond 1. V. high resistance 2. Scaling exacerbates issues 3. Noisy (Johnson + 1/f) 4. Large temp. coefficients 5. Difficult to fabricate (epi Si) 6. Expensive to produce 19 Technology: self-sensing NEMS resonators 10μm 2μm 500nm 200nm Piezoresistively-detected resonant response from a family of SiC nanocantilevers to a 1nN AC drive signal vs. frequency using metallic piezoresistors, at room temperature in vacuo. Mo Li, HX Tang, ML Roukes, M.L. Roukes Nature Nanotechnology 2, 114 (2007) 20 © Caltech 2010-2012 22 March 2012 Technology: self-sensing NEMS resonators 10μm 2μm 500nm 200nm 3.9×10-15 m/√Hz Piezoresistively-detected resonant response from a family of SiC nanocantilevers to a 1nN Thermomechanical noise spectrum and Lorentzian fit AC drive signal vs. frequency using metallic piezoresistors, at room temperature in vacuo. (red trace) for a 123 MHz nanocantilever measured at room temperature and atmospheric pressure. Mo Li, HX Tang, ML Roukes, M.L.