Scanning Probe Microscopy Colin Folta Matt Hense ME381R 11/30/04

Outline Background and History AFM –MFM –EFM –SThM STM SHFM SNOM

Background First scanning probe microscope invented in 1981 by Binning and Roher Wide range of applications –Topography/Atomic Structure –Magnetic/Electric fields –Surface temperatures

Branches of Scanning Probe Microscopy

Operation Scanning probe microscopes operate by detecting the deflection in the cantilever Modern scanning probe microscopes use a split photo diode to detect the deflection

Atomic Force Microscopy (AFM) Most widely used branch of scanning probe microscopy Operates by measuring the interaction force between the tip and sample

AFM Operation Modes Contact Mode –Tip remains in the repulsive regime of the inter- molecular force curve Tapping Mode –Tip is oscillated at a high frequency –Deflections in the oscillations are observed Non-Contact Mode –Tip is oscillated outside of the repulsive regime

Image Defects Broadening –Occurs when feature is roughly the same size as the radius of curvature –Side wall of tip comes into contact before the tip itself Compression –The forces involved actually change the shape of the specimen (ex. DNA)

Image Defects Cont. Aspect Ratio –Steep walled features become distorted –The tip can not follow a perfectly vertical wall

Magnetic Force Microscopy (MFM) Coated with a magnetic covering Two modes of operation –Non-vibrating for larger magnetic fields –Vibrating for weaker fields that require a greater sensitivity

MFM Cont. Uses a two pass technique –First pass finds topography of sample –Second pass finds the magnetic field On the second pass tip is kept at a constant height Magnetic_Force_Microscopy_MFM/text45.html

Electrostatic Force Microscopy (EFM) A bias is used to create an electrostatic field between the tip of the probe and the sample Two uses –Determine which regions are conducting and which are insulating –Determine the electric potential at different points

Scanning Tunneling Microscopy (STM) Electrons are transferred between the tip and the sample due to overlapping orbitals –A net transfer can be sustained by applying a voltage across the gap Change in current is a result of a change in the tip-sample separation

STM Modes of Operation Constant Current –Maintain a constant tunneling current by adjusting the separation Constant Height –Maintain a constant height and measure the current change

Scanning Thermal Microscopy (SThM) Thermocouple is placed on the tip of the probe Combined with AFM, SThM can associate thermal properties with surface features By heating the tip ~30K higher than the sample, local thermal conductivity can be determined Thermocouple can be used conventionally to measure temperature distribution along the sample

Scanning Near Field Optical Microscopy (SNOM) Typical optical microscopes –Limited by the Abbe diffraction barrier –Resolution equal to one half of the wavelength of the light

SNOM Cont. SNOM –Uses a very small aperture –Keeps the specimen in the near field regime –Resolution is determined by the aperture diameter

Shear Force Microscopy (ShFM) Probe oscillates parallel to the specimen Oscillation changes because of Van der Waals interactions. Topography can be determined from these changes. Advantages –More rigid set up –“Jump to contact” problem is almost eliminated Disadvantages –Can be very difficult to set up –Probe tip is very hard to reproduce reliably

Questions?