FIRAT A new AFM probe for fast imaging

FIRAT: A new AFM probe for fast imaging, material characterization, and single molecular mechanics § F. Levent Degertekin § G. W. Woodruff School of Mechanical Engineering § Georgia Institute of Technology Funding sources: NSF CAREER award, NIH

Outline § Atomic Force microscopy (AFM) background § Force-sensing Integrated Readout and Active Tip (FIRAT) probe structure for AFM § Integration to commercial AFM system § Fast imaging with FIRAT § Experimental setup and initial results § Quantitative surface characterization with FIRAT § Time resolved interaction force (TRIF) mode operation § FIRAT structures with improved dynamics and sensitivity § Application to biomolecular measurements § Conclusion and future work

Atomic Force Microscope • Uses microcantilevers as force sensors • Sharp tip determines image resolution • Optical lever detection used to measure cantilever deflection • Piezo tube moves sample or cantilever in x-y-z • Controller keeps the cantilever deflection or oscillation constant while scanning in X-Y plane 2µm Appl. Nanostructures • AFM is one of the most widely used tools in nanotechnology • Topographic and functional imaging of nanoscale structures • Metrology of IC structures, hard disk drive surface inspection • Measurement of biomolecular forces, material properties

Some Limitations of AFM § Imaging Speed § Bulky piezoactuators are slow § Integrated piezo or magnetic actuators can be complex § Material characterization § Slope detection leads to tip rotation § Point force measurements are somewhat slow for simultaneous topography imaging § High Q of cantilever masks tipsample interaction forces during tapping mode imaging § Array implementation § Parallel biomolecular measurements § Parallel imaging, nanofabrication Vibration spectrum of an AFM cantilever

FIRAT Probe Structure § New AFM probe structure: Sharp tip on micromachined membrane/beam § Integrated optical interferometer for tip displacement detection § Phase sensitive grating § Low-noise, robust interferometer § Integrated electrostatic actuator fast tip actuation § Imaging speed limited by membrane dynamics (fo ~ up to 10 MHz) § Force-sensing Integrated Readout and Active Tip FIRAT

Diffraction Based Optical Displacement Detection Gaussian aperture w 0=9µm, λ=850 nm, 2µm grating period d = /2 y ( m) § Non-moving diffraction grating on transparent substrate § Backside illumination: Reflected diffraction pattern § Reflector displacement changes the intensity of diffraction orders § Photodetectors at fixed locations are used to detect intensity variations § Interferometric sensitivity achieved in a small volume d = /4 d = /8 x ( m) Normalized intensity 0 0. 5 1

Displacement Sensitivity § Reflection order intensities I 0 α cos 2(2πd/λ), I 1 α sin 2(2πd/λ) Output for small deflections Δx + d 0 For d 0=nλ/8 (n odd) • Shot noise limit MDD: √ q. IR/(4 IR/λ) • ~3 x 10 -5Å/√Hz with 450µW laser power on detector is demonstrated Ø Several diffraction orders can be detected to reduce laser intensity noise Ø Electrostatic actuation is used to optimize sensitivity Ø Several methods have been devised to address range limitation

Device Fabrication Surface Micromachining On Quartz Substrate Devices for use in air: § Aluminum membrane (0. 7 -1µm thick) § Aluminum grating (0. 15µm) § PR sacrificial layer (1. 5µm) Membrane array (100µm diam. ) Back side Originally developed as microphone arrays!!

Device Fabrication - Tips § Focused Ion Beam assisted deposition on membranes § Platinum and Tungsten tips with 50 -70 nm tip radius § Image resolution, attractive force levels depend on tip geometry § Batch fabrication of silicon devices seems feasible § Cantilever processes with few extra steps are needed § Silicon and diamond tip integration is underway Platinum Tip On Aluminum Membrane 10µm Tungsten Tip On Nitride Membrane

Device Fabrication – FIRAT chips § Quartz chips mechanically cut using dicing saw § Fully cut for operation in air § Halfway cut for fluid cell Electrodes § Alignment limits the sample geometry FIRAT chip with Al m. / Pt tip Wafer edge Immersion chip with dielectric m. / W tip Etch hole to be sealed 100µm

Adaptation to Commercial AFM FIRAT vs. Veeco (commercial) Cantilever holder § § Laser stereo lithography is used to fabricate the holder with specific angle to use the 11º standard optics FIRAT chip oriented to use first diffraction order on the photodetectors Nearly plug-and-play with the Veeco Dimension and Multi. Mode system with minimal additional electronics Readily adaptable to other commercial systems Imaging head with FIRAT on AFM

Experimental Setup § § § Used Dimension AFM with custom holder and added electronics Integrated electrostatic actuator of FIRAT used as the only Z actuator fast tapping mode imaging (lower loop) with RMS detector BW ~12 k. Hz Direct interaction force imaging (upper loop) for topography and material properties with Dimension’s RMS detector and controller Digitizing interaction force signal at PD output (A) while using upper loop FIRAT actuator is also used as equivalent of tapping piezo A

Fast Tapping Mode Imaging With FIRAT 16 x 512 pixel images 1 Hz X-Y scan by piezo tube 5 Hz control oscillation 20 Hz Z motion by Active Tip § FIRAT probe used with commercial AFM system § FIRAT tapping frequency 600 k. Hz § Sample: 20 nm high calibration grating § 60 Hz scan rate – limited by X-Y scanner § Surface topography limited by interference curve § A digital controller based system is being built 60 Hz FIRAT line scans Cantilever line scans

Direct Measurement of Interaction Forces § § FIRAT Substrate oscillated, tip displacement recorded No tip-sample interaction No signal Zero background Transient signals recorded with high bandwidth of the membrane Every tap provides a dynamic force curve

Simultaneous Topography and Time Resolved Interaction Force (TRIF®) Measurement 2. 5 nm thick Pt GT logo on silicon Topography True constant force tapping mode imaging TRIF signal Individual tap (TRIF) signals • Slope: Stiffness • Tap shape inversion: Elasticity • Attractive peaks: Adhesion, capillary hysteresis • Background: Long range forces • Contact time

Quantitative Characterization TRIF™ Signals § Contact mechanics and adhesion hysteresis models are used to fit the digitized tap signals § Tip remains normal to the sample during interaction § Fast nanoindenter with high resolution imaging capability Simulated vs. measured signals on PR and Si Measured signals on polymers samples and Si • Polymer samples courtesy of Prof. Ken Gall (Ga. Tech MSE)

Extracting material properties (silicon)

Extracting PEGDMA properties

Mapping a feature of TRIF § Material property imaging independent of topography § Adhesion force peak is used as imaging parameter Cr Topography Al Al Cr Al Peak attractive force Al Si Cr Al Si

Mapping properties of CNT over Si TEM image 1. 5 um • Internal details seen in the TEM image of a CNT is observed in stiffness map Adhesion energy (m. J/m 2) Stiffness (N/m) Contact time (%) • CNT samples courtesy of Prof. Sam Graham (Ga. Tech ME)

Modeling of Device Dynamics § The membrane results in complicated frequency response not ideal for AFM applications (Because it is a microphone!!) § Finite element method is used to model squeeze film stiffening and damping effects for the complex membrane structure with vent holes § Simple linear model also accurately predicts the device behavior § Using validated model, structures with desired characteristics can be designed

Other FIRAT Structures § Cantilever, clamped-clamped beams can be more suitable for different applications § The electrodes can be driven to provide tip motion in 3 -D § Interplay between device spring constant, stiffness of air in the gap and damping determines the frequency response § Ideal device: § Flat response until resonance frequency (f 0 ~ 1 MHz) § Reasonable Q (loss) for low thermal noise limited force resolution § Reasonable Q (4 -20) for fast settling time

Clamped-beam (Bridge) Devices § Aluminum bridge devices: 60µm long 20µm or 40µm wide, 0. 8µm thick. Air gap thickness 2. 5µm § Targeted Q values 4 -15, spring constants in the 10 -40 N/m range, resonance frequency ~ 1 MHz to result in 100 k. Hz imaging bandwidth § Also fabricated devices with 7µm gap for large actuation range Top view of a bridge device

Measured Response for Fast Imaging FIRAT § Simple model predicts device response § Ideal response for fast tapping mode imaging – flat below resonance § Thermal noise less than 1 n. N over 1 MHz bandwidth Calculated response Optically measured response

Bridge Device with FIB Tip § Platinum tip built on 60µmx 40µm, 0. 8µm thick aluminum beam § Gap thickness 4. 5µm, k ~ 40 N/m, Q~ 4 -5

Structures for Enhanced Sensitivity § The membrane substrate gap can be converted to a Fabry. Grating fingers Perot cavity (metal) § Reflective part of membrane is made of alternating stack of silicon oxide-silicon nitride quarter wave layers § With 5 -6 pairs slope increases by 10 x, shot noise remains the same displacement sensitivity in the 10 -5Å/√Hz levels with 60μW laser power § Low dynamic range, but very high transient force sensitivity Membrane (metal+dielectric mirror) Dielectric mirror number of dielectric stack increases (2, 4, 6, 8)

Experimental Verification of Enhanced Sensitivity § § Fabricated device with 5. 5 pairs of dielectric layers Four-arm structure Non-contact measurement of 3 -D forces in vacuum and fluids Measured Finesse factor of 10 -14 Calculated Finesse factor of ~40

Bio-application: Single Molecule Force Spectroscopy § AFM is used to measure binding forces between molecules and inside a single molecule linked to effectiveness of drugs § Many measurements need to be performed to form a statistical model P. Hinterdorfer et. al. , PNAS, 93, (1996) Daniel Muller, ICNT 2006

Parallel Force Spectroscopy Not individually actuated No force control § § Most parallel molecular force spectroscopy measurements require individually controlled force probes Individually actuated cantilevers can be complex to build, can limit the type of cantilever to be used Individual actuator on cantilever Complex structures FIRAT based solutions § Functionalized, elecrostatically actuated membranes conform to cantilever array, force measurement is performed by moving membranes § Accurate, parallel detection of membrane displacement with integrated detector accurate control of force and molecule extension “Locally actuated sample surface” Transparent substrate

Immersion Device Fabrication Transparent substrate Deposit and pattern the diffraction grating 20/80 nm Ti/Au Top view Spin and pattern the polymer sacrifical layer ~3 μm of film Deposit and pattern The bottom insulator: 0. 1 µm Si. N/Si. O 2 The top electrode: 5/80 nm Ti/Au The top insulator: 1. 5 µm Si. N/Si. O 2 Bottom view Array with separate actuation electrodes: Decompose polymer layer at 440 °C Fabricated both nitride and soft parylene membranes: Spring constant ranging 20 -1000 N/m

AFM-FIRAT Combination § An experimental tool for single molecular force measurements § Allows for comparison of both methods, calibration of membrane spring constants etc. Conventional AFM head 10 x optical camera PD array Motorized vertical position control for AFM head FIRAT , regular AFM cantilever Laser Position adjustment knobs

FIRAT Based Devices – Initial Results § § Integrated electrostatic actuator moves the membrane in vertical direction Integrated optical interferometer measures displacement with high resolution Nitride and parylene membranes have been coated with PEI cushion and functionalized with desired proteins Electrical isolation ensures properation in buffer solutions AFM Molecular system used in experiments cant il ever V Diffracted beam Incident laser beam Collaborator: Prof. Cheng Zhu (BME/ME)

Experiment with Piezo Actuation § § § Membrane is passive sample Piezo substrate Commercial AFM piezo is used to Drive: ON move the cantilever for conventional molecular pulling Adhesion-rupture events recorded Bond rupture 0. 01 N/m AFM Cantilever multiple ruptures

Experiment with Membrane Actuation § § Piezo driver turned OFF (very small motion) The membrane is driven with 5 Hz triangular signal to provide tip contact Continuous tip contact tip follows periodic membrane motion with some piezo drift Piezo Drive: OFF V Force measured by the cantilever (+ve tip pushed up, -ve tip pulled down) No adhesion Adhesion/ rupture Adhesion/Rupture No adhesion

Array of Membrane Sensors § § Soft membranes can be used for both force sensing and tip actuation Cantilever is eliminated High force sensitivity along with soft mechanical structure: 10 fm/√Hz * 1 N/m 3 p. N with 1 Hz-100 k. Hz BW Recently demonstrated < 10 fm/√Hz down to 1 -2 Hz on FIRAT system Polymer membranes with actuation capability are being fabricated

Microactuators for Fast Imaging in Liquids § Compatible with existing AFMs § FIRAT membrane is used as active sample holder § Devices with > 100 k. Hz BW have been fabricated and tested § Fast Z-motion provided by the FIRAT membrane § Built-in displacement detector for closed loop operation § Suitable for molecular imaging in fluids Optically measured frequency response in liquid

Conclusion § A new type of AFM probe tip has been developed A fully integrated FIRAT probe § Integrates electrostatic actuation with interferometric detection in a compact manner § Suitable for fast topographic imaging § Provides sensitive broadband frequency response for direct measurement of time resolved interaction forces § Tip motion normal to the sample simplifies quantitative analysis similar to nanoindentation § Operation in fluids and application to force spectroscopy has been demonstrated § Structure is suitable for miniaturization and array implementation PD integrated 9 x 9 membrane array Membrane. Sensor array 1 cm

Challenges & Future Work § FIRAT structures with integrated sharp tips § More complex than cantilevers § Silicon tips are feasible, cost can be an issue § Better sample handling with required accurate alignment § Current/future work: § § § Commercialization Fast imaging: Improved range and robustness Parallel molecular force spectroscopy Silicon tip fabrication Unique probe structures for specific applications ….

Acknowledgments § Graduate students and postdocs: Guclu Onaran, Hamdi Torun, Dr. Mujdat Balantekin, Dr. Krishna Sarangapani, Wook Lee, Byron Van Gorp, Dr. Jemmy Sutanto, Dr. Will Hughes (MSE), Brent Buchine (MSE), Rasim Guldiken, Zehra Parlak § Prof. Calvin F. Quate (Stanford University) § Prof. Cheng Zhu § Prof. Z. L. Wang, Dr. Paul Joseph § FIB facility, Mi. RC at Georgia Tech

Sample preparation § Cantilever tips are coated with Anti-L-selectin m. Ab DREG 56 (100 µg/ml) § Membranes are coated with § 100 ppm polyethylenimine (PEI) solution to reduce non-specific adhesions § 5 μl lipid vesicle solution

Probe–sample interaction modeling • Modeling the membrane with a spring • Finding rest position y. B 0 from minimum force in advancing phase ~

Force F as a function of distance d Assuming BCP contact mechanics model and a spherical tip of radius R d 0 : intermolecular distance, E* : effective tip-sample elasticity, : surface energy

Extracting material properties Elasticity ~ Surface energy (advance) ~ Surface energy (recede)

FIRAT for Material Characterization § Broadband, damped response for clean tap signals § Device geometry and gap adjusted for desired response Measured tap signals

FIRAT-AFM Comparison AFM “FIRAT” Fast X-Y scan by piezo Fast Z motion by “Active tip” • Active tip with integrated actuator • Piezo-tube scanner • Membrane dynamics limit Z-scan speed • Slow moving piezo tube limits Z-scan speed • Z-motion 100 x fast compared to piezo tube • Not integrated, bulky • Device volume ~ 200µm x 5000µm • Fast X-Y scan • Optical lever detection • Requires re-alignment for every new cantilever • Integrated Interferometric detection • 10 -100 x more sensitivity smaller impact force time consuming • Fixed laser-detector location Simple, fast • Optical interference from sample probe change • Cantilever with tip • Direct measurement of tip-sample interaction • Large background signal without tip-sample • Fast imaging of adhesion, elasticity, subsurface interaction Fast imaging of adhesion, features elasticity not possible • Measurement of 3 -D forces • Array implementation is difficult • Integrated structure is suitable for arrays

Research in Degertekin Lab § Micromachined integrated acousto-optic devices § Biomimetic microphones for hearing aids (NIH) § Optical microphone –novel signal processing integration (Catalyst Foundation) § Capacitive micromachined ultrasonic transducers (CMUTs) for intravascular imaging for cardiology applications § Forward looking arrays (Boston Scientific Corp. ) § Side looking arrays with CMOS electronics (NIH) § Atomic Force microscopy § Quantitative material characterization (NSF) § Parallel single molecule force spectroscopy (NIH)