Ultra-high vacuum scanning probe microscopy by STM and AFM
Researcher: Daniel Marconi
Keywords: scanning probe microscopy, STM, contact AFM, non-contact AFM, ultra-high vacuum
Keywords: scanning probe microscopy, STM, contact AFM, non-contact AFM, ultra-high vacuum
Scanning probe microscopy, AFM and STM, is a technique for mapping at the nanoscale the physico-chemical surface properties of a material, such as relief, electrical conductivity, elastic modulus, magnetization, chemical composition, thus obtaining a picture of how these properties are distributed on the surface of the sample under study.
Depending on the physical or chemical properties being mapped, scanning probe microscopy can be of several types. Two important scanning scanning microscopy techniques are Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM). Each has different versions of functionality, and there is even a scanning technique that combines STM and AFM called Qplus.
AFM scanning microscopy offers sub-nanometer resolution and, depending on the specific version used, can either record the surface topography of the sample to be studied or map certain mechanical properties of the surface (elasticity, hardness, coefficient of friction).
The operating principle of the AFM technique is based on the physical, mechanical interaction between a very sharp AFM tip, made of silicon or silicon nitride, and the surface of the sample to be studied, the tip having a radius of curvature of several nanometers. The AFM tip is attached to the end of a cantilever. There are three main versions of the AFM technique: AFM contact, AFM noncontact, AFM tapping mode.
In contact AFM mode, the cantilever presses the AFM tip against the surface to be studied. As the AFM tip scans the sample surface, the sample will also be moved vertically to keep the position of the laser beam spot reflecting off the cantilever tip constant. This means that the AFM tip’s pressing force on the surface will be constant, thus the topography of the sample will be recorded as a multitude of linear profiles forming a three-dimensional image.
In noncontact AFM mode, the cantilever is placed in a vibrating state in the immediate vicinity of its resonant frequency. The AFM tip, as it is brought into the vicinity of the surface to be scanned, will sense an interaction with the sample surface, which will cause the cantilever’s resonant frequency to change. Thus a difference will occur between the excitation frequency of the cantilever and the resonant frequency of the cantilever, and if this difference is kept constant during the scan, the interaction between the tip and the surface is constant, which means that the AFM tip will faithfully scan the surface relief.
The multitude of linear scan profiles thus recorded will form a three-dimensional image of the topography under investigation. The noncontact AFM mode is used when there is an adhesion force between the AFM tip and the surface to be studied, which would make it difficult to scan in contact AFM mode due to the high friction forces between the tip and the sample.
The oscillation amplitude of the AFM tip in the noncontact mode is typically a few nanometers (< 10 nm) and can go down to a few picometers, with the resolution provided being better the smaller the oscillation amplitude.
The tapping AFM mode is similar to the noncontact AFM mode but the oscillation amplitude of the AFM peak is substantially higher, about 200 nm. Also, the AFM tip will make contact with the scanned surface at each oscillation at the position of minimum negative elongation. This AFM technique is suitable for those samples which cannot be scanned by the contact AFM method and which exhibit a topography with large vertical variations at a high inclination slope.
The principle of operation of scanning tunneling microscopy (STM) is based on the quantum phenomenon in which the wave properties of electrons allow them to tunnel beyond the surface of a solid into regions of space that are forbidden to them according to the rules of classical physics. The sharp tip of a tungsten needle is positioned a few ångstroms from the surface of the sample, and a small voltage applied between the tip and the surface causes the electrons to tunnel. As the tungsten tip scans the surface, it records variations in the tunneling current, and this information can be processed to provide a topographic image of the surface. The topographic images of the surface are collected in one of two modes: in constant height mode, where changes in the tunneling current are mapped directly, while in constant current mode the voltage controlling the height of the tip is recorded and the tunneling current is maintained at a preset level.
Areas of application: R&D in electronics, biology, medicine, materials science
Systems: Ultra-high vacuum scanning probe microscopy is mainly used for samples that have been fabricated under such conditions, avoiding taking them out into the ambient atmosphere to avoid contamination. There may also be situations where the sample of interest has been manufactured in ambient atmosphere and, for microscopy measurements, needs to be introduced into the ultra-high vacuum facility to benefit from a preliminary surface decontamination process by thermal degassing.
Industries: STM and AFM microscopy allows the confirmation of the required surface quality, in terms of topography correlated with physico-chemical properties, for devices finding applications in the microelectronics and optoelectronics industry.
The Department of Molecular and Biomolecular Physics has a modern STM and AFM scanning microscopy facility (Omicron, Germany).
The facility has an ultra-high vacuum enclosure at a level of 10-10 mbar, provided by a pump system including a turbo-molecular pump, an ion pump and a titanium sublimation pump.
Particularly good resolution: the image obtained can have a resolution about 1000 times better than that characteristic of optical microscopy, where light diffraction limits the resolution to a value comparable with the wavelength of the optical radiation used.
Nanolithography method: AFM and STM microscopy can be used to create small structures on the sample surface, either by indentation of the surface (contact AFM) or by local melting (STM).
Preserves sample purity: ultra-high vacuum scanning probe microscopy protects the surface under study from potential contaminants in the ambient atmosphere.
Easy access to auxiliary manufacturing equipment: the ultra-high vacuum scanning probe microscopy facility is coupled to an Molecular Beam Epitaxy thin films deposition facility with argon ion beam sample treatment facilities and facilities for coating the sample with a thin protective layer of a suitable material of choice.
The price for the measurement of a sample by ultra-high vacuum scanning probe microscopy starts from a minimum of 500 lei and depends on the specific characteristics of the required measurement.