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

http://spm.phy.bris.ac.uk/

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 http://spm.phy.bris.ac.uk/

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

http://spm.phy.bris.ac.uk/

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

http://www.ntmdt.ru/SPM-Techniques/SPM-Methodology/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

http://stm1.phys.cmu.edu/stm/si5x5s.gif

http://www.d.umn.edu/~jmaps/stm1.html

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

http://molebio.iastate.edu/~p_haydon/nsom.html

SNOM Cont.

SNOM– Uses a very small

aperture– Keeps the specimen

in the near field regime

– Resolution is determined by the aperture diameter

http://spm.phy.bris.ac.uk/

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?