NANO 225 Micro/Nanofabrication Characterization: Scanning Probe Microscopy 1

History Scanning Tunneling Microscope invented in 1982 by, Binning, Rohrer, Gerber and Weibel 2 Binning and Rohrer won Nobel prize in 1986 AFM developed in 1986 by Binning, Quate and Gerber

Hierarchy of Techniques Scanning Probe Microscopy 3 Scanning Tunneling Microscopy Atomic Force Microscopy Contact Mode Tapping Mode Non-contact Mode

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STM 5 Feedback loop keeps current constant therefore d is constant Sample must conduct electricity Capable of detecting atomic scale defects I t ~ Ve -cd

AFM Contact Mode 6 Scan tip along surface Tip contacts sample through adsorbed fluid layer Maintains constant cantilever deflection (force) using a split photo diode Samples can be in liquid state Works in ambient conditions Hook’s law F=-kx k limits sensitivity (want low k)

AFM Tapping Mode 7 Cantilever oscillates at or below resonance frequency Maintains constant RMS of tip movement amplitude Position of scanner stored to create image Tip must breakthrough water layer without getting stuck Also works in ambient or liquid

AFM Non-Contact Mode 8 Tip oscillates but does not touch sample above resonance frequency The resonant frequency is decreased by van der Waals forces Position of scanner used to map 3-D plot of surface

Silicon Nitride Probe Spring Constant (k) 0.58, 0.32, 0.12, 0.06 N/m (1) Nominal Tip Radius of Curvature nm Cantilever Lengths 100 & 200μm Cantilever Configuration V-shaped Reflective Coating Gold Sidewall angles35° on all 4 sides 9 (1)Calculated spring constant values are based on the 0.6μm silicon nitride thickness; however, this value can actually vary from 0.4μm to 0.7μm. Thickness is cubed in the spring constant calculation, thus, actual values can vary substantially.

Silicon Probe 10

AFM Imaging and Tip Shape 11 The radius of curvature of the tip limits the resolution of the image that can be taken

AFM Imaging and Tip Shape 12 The probe cannot image a sidewall that is steeper than the angle of the tip Silicon nitride probe

AFM Imaging and Tip Shape 13 Silicon probe

Surface Roughness Measurements 14 R z : average difference in height between the five highest peaks and five lowest valleys relative to the mean plane Image R a : average of the absolute values of the surface height deviations measured from the mean plane Image R q : Root mean square average of height deviations taken from the mean plane. NanoScope Software 6.13 User Guide, Section 6.4.3

Four Point Probe 15 University of California EECS 143 Manual Constant current in two outer probes Voltage measurement on two inner probes eliminates contact resistance Four point technique required for precision resistance measurements

Four Point Probe 16 University of California EECS 143 Manual Thin film case Current flow is restricted to a thin film t<<s t

Four Point Probe 17 University of California EECS 143 Manual Bulk resistivity case Current flows through bulk material s<<t t