- Near Field Scanning Optical Microscopy - Electrostatic Force Microscopy - Magnetic Force Microscopy Yongho Seo Near-field Photonics Group Leader Wonho Jhe Director School of Physics and Center for Near-field Atom-photon technology, Seoul Nation University in South Korea
Scanning probe Microscopy NSOM (near-field scanning optical microscopy) EFM (electrostatic force microscopy) MFM (magnetic force microscopy) Optical Microscope : low resolution diffraction limit real time Scanning Probe Microscope nano-scale resolution Slow scanning Quartz Crystal Resonator probe - self oscillating - self sensing - chip and simple design
High Frequency Dithering Probe for Shear Force Detection Bimorph, tuning fork Low dithering frequency 10 ~ 100 khz Slow response time ~ 100 ms High Frequency Quartz Crystal Resonator Thickness Shear mode 2 ~ 100 MHz dithering frequency fast response time ~ 1 ms High frequency (rf) dithering fast scanning Small dithering amplitude (<< 1 nm) high lateral resolution Large Signal voltage (> 0.1 V) high signal to noise ratio
Quartz Crystal Resonators Z-cut Tuning fork AT-cut QCR BT-cut trident QCR Low Frequency (10 khz) Flexural Mode k = 10 4-10 5 N/m High Frequency (rf) Thickness Shear or Extensional mode k = 10 5-10 6 N/m
QCR based NSOM Shear mode NSOM - 2 MHz dithering frequency - make a hole to insert optical fiber tip - easy to replace tip - increased the stability - high Q-value > 10 3 Perforated QCR probe
QCR probe Feedback Scheme Function Generator high frequency dithering tip induced signal Phase detection Tube scanner Simple design Low cost No lock-in amp
High Frequency Dithering Shear Force Microscopy Topographic Image of CD surface Total time : 20 s Amplitude mode Large dithering amplitude Phase mode White dots : dust
Schematics for high speed NSOM Near field detection scheme Laser Diode : 650 nm PMT : Optical Signal measurement Reflection mode
Fastest Scanning NSOM Image Near field Scanning Probe Microscopy Image of grating surface in reflection mode Total time : 0.5 s Total time : 0.5 s 7x7 mm 2 1x1 mm 2
Electrostatic Force Microscopy Tuning Fork (32.768 KHz) L = 2.2 mm, t = 190 mm, w = 100 mm k = 1300 N/m. The tip is electrically shorted to an electrode.
Force Sensitivity of Tuning Fork Force sensitivity (k/qf) -1/2 Si Cantilever f = 10-100 khz k = 1-100 N/m Q = 10 2-10 3 ~ 10 nm dithering Quartz Tuning Fork f = 10-100 khz k = 10 4-10 5 N/m Q = 10 3-10 5 smaller than 1 nm dithering In this experiment, L = 2.2 mm, t = 190 mm, w = 100 mm k = 1300 N/m. Q = 1800, f = 32 khz
Electrochemical Etching Reduce the diameter of Co and Ni wire Pt H 3 PO 4 Co, Ni D = 100 mm 10 mm
Make tip and Attach it to the tuning fork -Attach the wire to the tuning fork and make a tip -use home-made micromanipulator Pt Silver paint Co, Ni H 3 PO 4 Tuning fork
PZT Thin Film PZT thin films (Zr/Ti = 20/80) by INOSTEK Inc. and Crystalbank The property requirements of PZT thin films for high quality nano storage devices : smooth surface roughness high piezoelectric properties even in the case of very thin films long term stability and reliability
Electrostatic Force Microscopy Approach Curve with Bias voltage Tip PZT Pt Bias voltage applied between the tip and Pt substrate
Electrostatic Force Microscopy Minimum Detectable Capacitance due to thermal noise 2 x 10-20 F Frequency shift due to surface charge
Electrostatic Force Microscopy Tuning Fork based EFM - polarization images After poling of square area Line drawing 7 x 7 mm 2 0.9 x 0.9 mm 2 -long time stable -High resolution (50 nm)
Electrostatic Force Microscopy Tuning Fork based EFM - polarization images 4 x 4 mm 2 7 x 7 mm 2
Magnetic Force Microscopy MFM contrast - magnetic force gradient between tip and sample Force gradient Frequency shift Phase shift Magnetic force - very weak force (~pn) Lift mode - keep constant gap between tip and sample (~10 nm) - to avoid the strong short range topographic contrast
Magnetic Force Microscopy Approach Curve Shear force Approach Withdraw attractive force high S/N ratio high frequency Sensitivity < 3 mhz
Magnetic Force Microscopy Tuning Fork : Co tip attached L = 2.2 mm, t = 190 mm, w = 100 mm spring constant, k = 1300 N/m (smallest one commercially available)
Magnetic Force Microscopy Advantage of the shear mode MFM -Perpendicularly recorded sample and longitudinally polarized tip
Magnetic Force Microscopy (a) shear mode, Co tip, perpendicular 100 Mb hard disk (b) shear mode, Co tip, parallel dithering (c) shear mode, Ni tip (d) tapping mode 30 x 30 mm 2 30 x 30 mm 2 30 x 30 mm 2 30 x 30 mm 2
Magnetic Force Microscopy Tuning Fork based MFM : height and amplitude dependency 13 x 3 mm 2 3 x 1 mm 2
Magnetic Force Microscopy High resolution Tuning Fork based MFM 1 Gbit/inch 2 hard disk Dithering Amplitude : 20 nm lift height : 50 nm Spatial resolution : 50 nm 2 x 2 mm 2
Atomic Layer of HOPG with trident QCR (1MHz) Atomic layer (3Å ) 160 x160 nm 2
NSOM, EFM, and MFM using Quartz Crystal Resonator In summary, Quartz Crystal Resonator based NSOM, -high resonance frequency, and small dithering amplitude. -facilitates high-speed scanning -obtained atomic scale AFM Tuning fork based EFM and MFM - EFM : obtained with high resolution, for the first time. - MFM : shear mode MFM; improved resolution. - Tuning fork : Force sensitive SPM sensor Published results : Y. Seo, J.H. Park, J.B. Moon and W. Jhe, Appl. Phys. Lett. 77 4274 (2000). Y. Seo, Wonho Jhe, Rev. Sci. Instrum. 73 (2002).