Methods and Tehniques in Surface Science Prof. Dumitru LUCA “Alexandru Ion Cuza” University, Iasi, Romania

Surface imagistics and topography The most widespread techniques for surface imaging and morphological characterization

SPM - Scanning probe microscopy A comparison between the main applications of SPM

Atomic force microscopy - AFM Laser Specimen XYZ piezoelectric scanner Photodiode matrix Mirror Tip Cantilever 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.

Atomic force microscopy - AFM 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 (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.

A contact mode image of a laser ablation deposited TiO2 surface. AFM in contact mode. Constant height Less frequently used technique. Allows for the determination of surface topography via surface scanning by the tip. There are mainly, 3 operation modes: 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 contact mode. Constant force An image of the surface of a rf magnetron sputtered TiO2 film.

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

AFM 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 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 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. Adhesion 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. Spreading 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 Mode The TM image of a working surface of a stamping device for CD manufacturing.

AFM 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 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) 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. .