Methods and Tehniques in Surface Science

Prof. Dumitru LUCA

“Alexandru Ion Cuza” University, Iasi, Romania

Surface imagistics and topographySurface imagistics and topography

The most widespread techniques for surface imaging and morphological characterization

SPM - Scanning probe microscopySPM - Scanning probe microscopy

A comparison between the main applications of SPM

Atomic force microscopy - AFMAtomic force microscopy - AFM

Specimen surface is scanned by the cantilever tip attached to the cantilever.

The system is able to detect and measure forces in the nN range using the optical lever.

Laser

Specimen

XYZ piezoelectric scanner

Photodiode matrix

Mirror

Tip

Cantilever

Contact mode

Interaction between individual atoms of the tip and the surface.

The theory is very complex (Coulomb forces and/or forces induced by polarization).

Strong interactions which may affect the physical condition of the surface.

Used for morphological caracterization of the surface. Non-contact mode

The tip-to-sample distance is maintained constant in the 2-30 nm range.

The theory is quite simple, since only there are only the interaction between an atom in the apex and entire specimen as a whole.

Coulomb, magnetostatic, van der Waals forces (2 - 4 orders of magnitude smaller than in the case of contact mode).

Tapping (resonant) mode

A combination of the previous two modes. It is based on the damping of the cantilever oscillations upon approaching the tip to the surface. Scratching the sample is thus avoided.

Atomic force microscopy - AFMAtomic force microscopy - AFM

In principle, the AFM reminds of a record player

transducer or the stylus profilometer.

AFM incorporates, however, many more technical

refinements allowing atomic scale resolution:

1. Phase sensitive detection (lock-in)

2. Sensitive cantilevers

3. Sharper tips

4. Posibility of positionning the tip under high spatial

resolution conditions.

5. Feedback loop for force control.

Atomic force microscopy (AFM)Atomic force microscopy (AFM)

Less frequently used technique. Allows for the determination of surface topography via surface scanning by the tip. There are mainly, 3 operation modes:

AFM in contact mode. Constant heightAFM in contact mode. Constant height

A contact mode image of a laser ablation deposited TiO2 surface.

An image of the surface of a rf magnetron sputtered TiO2 film.

AFM in AFM in contact mode. Constant forcecontact mode. Constant force

AFM in contact mode. Error contact modeAFM in contact mode. Error contact mode

A contact mode image of a laser ablation deposited TiO2 surface.

AFM in contact mode. Force imagingAFM in contact mode. Force imaging

It is used to scan flat surfaces. This is done in a

more precise and rapid way by eliminating

the feedback loop. z = const.

Applications in polymer physics, semi-

conductors, composite materials and so on.

AFM in contact mode. Force imagingAFM in contact mode. Force imaging

Distinguish regions with different

values of static friction coefficients.

Allows for taking sharp contour

images of any surfaces.

Can be used in association with

other AFM techniques for more

complete characterization.

Applications in semi-conductor and

polymer physics, mass storage

devices, surface contamination, nano-

tribology.

AFM in contact mode. Force imagingAFM in contact mode. Force imaging

Force-distance characteristics are plotted.

The vertical component of the tip-to surface force is derived in the contact mode.

Application: characterization of

catalist,

semiconductors,

polymers,

thin films,

mass storage devices.

AFM in contact mode. AFM in contact mode. Adhesion force imagingAdhesion force imaging

Information is derived concerning the adhesion features of the sample.

Sample surface is scanned. The F(d) plots are used to get a 2-D map of the forces for which the SNAP BACK occurs.

AFM in contact mode.AFM in contact mode. Spreading resistance modeSpreading resistance mode

Here, a sharp tip is used to find, for

example, the dopant concentration in the

sample.

A relatively strong force is ensured so as to

punch the native oxide film which usually

covers the surface (ex. Si).

By using a conductive cantilever, the local

conductivity of the surface is mapped.

It is currently used in association with an

other conventional AFM imaging technique.

AFM in Tapping ModeAFM in Tapping Mode

The TM image of a working surface of a stamping device for CD manufacturing.

AFM in tapping mode. Phase modeAFM in tapping mode. Phase mode

Measurements are performed in

association with other conventional

imaging techniques.

Maps of various surface physical

quantities (such as composition,

adhesion, friction, viscoelasticity) are

drawn.

Useful information for a wide range

of application (chemistry, biologie,

physics, engineering etc.)

Kelvin SPMKelvin SPM

In this case a voltage is applied

between the tip and the sample to get

information between the tip and the

sample and subsequently the superficial

distribution of the electric potential.

The method allows for the localization

and identification of failure causes of

multi-material, multi-level devices.

Magnetic Force Microscopy (MFM)Magnetic Force Microscopy (MFM)

This technique is used for basic and applied research to obtain the image of ferro-magnetic thin films, magnetic disk and tapes, permanent magnets and soft magnetic materials.

.