Research and monitoring of surface
properties with nano-scale resolution

TECHNICAL

I. Principles of Scanning Probe Microscopy (SPM)
II. Construction of NanoScan measuring system
III. Cantilever and tips

Principles of Scanning Probe Microscopy (SPM)

Scanning probe microscopy appeared at the beginning of 80th and by the present time has developed into the field of material science and physics of surface in its own right.

The common functional principle of SPM is monitoring of interaction of a very sharp tip with the studied surface. Depending on the type of SPM, different characteristics of a contact are checked: mechanical interaction (atomic force microscope), tunnel or electrical current between the tip and the surface (tunnel or resistive microscope), magnetic interaction (magnetic force microscope), capacity (capacity microscope), etc. Various modifications of constructions and function principles of atomic force microscopes (AFM) or scanning force microscopes. The term 'atomic force' emphasizes the ability of these microscopes to measure forces on the level of atomic interference. The sensitive element (probe) of AFM is an elastic cantilever with a sharp tip placed at its free end. When the tip contacts the surface, the cantilever bends under repulsion or attraction forces. These functional modes are called contact and non-contact ones, respectively. Measuring the bend of a cantilever, it is possible to determine the forces acting between the tip and the surface, and the relative height of the surface relief (Z coordinate) in the contact point. Those functional modes of AFM, in which the probe vibrates with some frequency in the normal direction to a surface of the specimen, are also used widely. In this case, the changes of values of amplitude, frequency or oscillation phase shift of the probe provide an information about the contact of the probe with the surface; in some cases the statistically averaged bend can be used for that. The so-called 'tapping mode', which means tapping of a surface with a tip, is the mostly used mode of this kind.

At present, a number of methods exist for measurement of the bend and probe vibration parameters. Optical schemes, such as interferometric and deflective, are the most prevalent. The capacity sensors and the piezoelectrical ones have to be marked, as well. Resolution of such registering devices can reach 0.01 nm. Presently, commercially produced probes have bending stiffness within the range 0.1 to few hundreds N/m. High sensibility of probes and resolution of registering devices allows AFM to measure forces 10^-6 to 10^-12 N, which makes them basically different from regular profile measuring devices.

The great majority of SPMs are apparatus of the horizontal scanning type. In the scanning process, one or several parameters of interaction of the probe tip with the surface are observed. In most of SPM functional modes, one of the controlled parameters (i.e. the regulation parameter) is kept constant by means of moving the probe in the normal direction to the surface. As this takes place, the height of the surface relief is traced. The use of precision piezoceramics scanners permits to move the probe about the surface with an accuracy of hundredth of nanometer.

Sensitivity of probes and precision of scanners have allowed to receive SPM surface images with maximum horizontal resolution about 0.05 nm and vertical resolution up to 0.01 nm. This is the basic advantage of SPMs in comparison with optical microscopes. By their resolution, SPMs are not worse than electron microscopes. Besides, SPMs permit to measure a large range of relief heights with a high resolution. Simplicity and multi-purpose usage of SPMs gives them significant advantages compared to electron microscopy, not only in scientific research, but also in technological applications. In most cases, SPMs do not require vacuum. This makes the preparation of the apparatus for work faster, and testing process easier. Moreover, new areas of use of these apparatus have appeared recently. They can be applied to study the surface properties, for micromodification and nanolithography. Application of SPM in biology and medical science is also of great interest.

The use of SPM (in particular, AFM) for investigation of mechanical properties of surfaces and for diagnostics of materials with the heterophase structure progressively extends since the end of 1980th. This use of SPM is based on the fact that the character of interaction of the probe tip and the studied surface depends on the mechanical properties of this surface. Methods of examination of mechanical properties with SPM are developing in two directions:

1. Scanning of the 'load-unload curves' and nanoindentation. These methods permit to determine such 'bulk' characteristics of materials as, for example, the Young modulus, and hardness in a very thin near-surface layer (with thickness from units to hundreds of nanometers). Besides, from the 'load-unload curves', the chemical, electrical, and adhesive properties of surfaces can be determined, as well as the capillary and Van der Waals forces, etc.
2. Obtaining of the pseudo-relief image, which correlates with mechanical properties of the surface, in the process of scanning the surface (i.e. mapping of mechanical properties). Analogous methods can be used to examine not only the structure of the surface, but also the distribution of components in composite materials by analysis of a cut or a section.

Such methods of measurement of mechanical properties by means of SPM are characterized by high spatial resolution (from sub-microns to nanometers), sensibility and comparative simplicity. Their application is important for investigation and technological control of thin films, ultra-dispersed hard alloys, and novel composite materials. Nevertheless, at the moment the wide use of such techniques is hindered by the complexity and comparatively high prices of the equipment, and by absence of methods of quantitative analysis and interpretation of the obtained results.

Construction of NanoScan measurement system

1. General layout of the system
"NanoScan" measuring system includes a Control computer, Control hardware and Measuring head.

1.1 Control computer
The control computer is a regular PC-compatible with Windows 2000/XP operation system. Its functional task is to control the operation regimes of the apparatus and process the measured data.

1.2 Control hardware
The control hardware consist of the following parts:

• The apparatus control unit: PC Card ADCARD-V01-pcmcia which is inserted in an PCMCIA socket into the notebook or PC Card adapter;
• PCI or ISA-to- PC Card adapter;
• the video capture card which is inserted into a PCI socket on the motherboard of the control computer or USB Video input device (for notebook);
• peripheral part of hardware located inside the measuring head.
• The PC Card are connected with the measuring head by a cable.

Fig.1

1.3 Measuring head

The measuring head has cylindrical shape and includes a pan, lid, and vibroisolated platform. A schematic drawing of the measuring head of NanoScan SPM with the lid taken off is shown in Fig.1. The peripheral part of the control hardware is placed inside the pan. The Positioning system (piezodrive), consisting of the XY- and Z-scanners, is located on the vibroisolaited platform, as well as the Probe and Specimen visualization system, all covered by the shroud. The specimen is placed inside the head directly on the XY-scanner supports; or, if the specimen size does not permit that, it has to be placed into the special holder. The probe approaches the investigated surface from below. The lid serves to defend the sample, probe, and systems of positioning and visualization against mechanical, thermal, acoustical and other external influences.

2. Destination and characteristics of the measuring head components

2.1 Vibroisolation system
The measuring head is equipped with a two-step system of seismic interference suppression. The suppression factor for industrial vibrational noises is ~ 10^-7, which allows operation in regular laboratory conditions without additional vibration countermeasures.

2.2 Positioning system
Positioning system represents a piezodrive and is comprised of an XY-scanner and Z-scanner. The XY-scanner provides horizontal movement of the investigated object. The Z-scanner moves the probe vertically. The piezodrive can function in two modes: Step motor (executes macropositioning) and Micropositioner.

In step motor mode, the XY-scanner permits to position an investigated object relative to the probe and, when necessary, to get topologically connected images of extensive objects. Its characteristics are:

• Movement distance of the investigated object up to 20 mm.
• Step size 10 µm to 0.1 µm (defined by the user).

The Z-scanner, while in step motor mode, performs fast delivering the probe to the surface, and moving it off for a distance ~10 mm when the examined object has to be changed.

Micropositioner (scanning) mode is used during the sample scanning process. In this mode, the XY-scanner has the following characteristics:

• Maximal scanning window 100x100 µm;
• Step size down to 0.1 nm.

In scanning mode, checking of relief height in the Z direction is done by the probe cantilever.

• Scanning range up to 15 µm
• Step size down to 0.1 nm

During surface indentation, the construction of the probe allows to develop the force higher than 20 g.

2.3 Specimen visualization system
Specimen visualization system allows visual checking of the relative positions of the probe and the studied sample. It comprises of a miniature video camera, optical system and lights. The system parameters are:

• Window 2x3 mm;
• Resolution 300x400 television lines;
• Focal depth 1 mm.

2.4 Probe
The probe performs a direct checking of contact of the tip with the investigated sample surface during measurements. It represents piezoceramic bimorphous tuning-fork fastened to a metallic holder. The holder is inserted into the nest of the Z-scanner. The tip is fixed at the end of the top cantilever of the tuning-fork.

Cantilever and tips

Cantilever:
Piezoceramic bimorphous tuning-fork fastened to a metallic holder. The tip is fixed at the end of the top cantilever of the tuning-fork.

Diamond tips:

• Bulk diamond tips are grown by a HPHT technique
• Synthetic diamond tips
• Semiconductor diamond tips
• Nominal tip radius of curvature < 50 nm
• Tip apex angle 134°

Cantilever with bulk diamond tip attached	FIB image of tip apex

Ultrahard fullerite C₆₀ tips:
Unique tips made of ultrahard fullerite C₆₀ (patented) which are harder than diamond. The hardness measurements of superhard materials, including hardest facet of diamond (111), are possible with use of these tips.

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