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Structure and properties of thin-film oxide dielectrics modified by carbon


Yu. V. Sakharov - Ph.D. (Eng.), Аssociate Professor, Department of Physical Electronics, Tomsk State University of Control Systems and Radioelectronics

The study of porous dielectric films has been set a new impulse of late as a result of substantially enlarged sphere of their practical use. Such films can be used both in microelectronics as insulating stuff with low dielectric permittivity, and they can be used in photonics as anti-reflection coating in optoelectronic devices . The porous dielectric films can be also used as basic material for receiving nanomembranes and selective gas-sensing, sensor devices. A number of methods of receiving porous dielectric structure were worked out – anodizing, zol-gel method, matrix template synthesis are among them. All the enumerated methods are chemical which makes it difficult to use them when producing micro – and nanoelectronics devices.
The purpose of the given research is to work out the integral schemes of the formation methods of porous films of oxide materials basing on the technological modes of forming films with their structural and electrophysical properties.
Experiment and measurement methods
The basis of the suggested method is the self-organization principle proceeding in the plasma glow discharge which is formed by magnetron spattering source, compound spattering targets Si:C (graphite), Ta:C (graphite) Nb:C(graphite), Ti:C(graphite) being its cathode. The graphite area on the compound spattering target expressed in percentage - Sc varied in such a case, which resulted in changing the pore quantity and size. Spattering was done in the oxygen atmosphere with the pressure in an evacuated vessel equal to 410–1 Pa. Such are the conditions under which dielectric films of the silicon dioxide (SiO2) and Ta pentoxide (Ta2O5), Nb pentoxide (Nb2O5) and Ti dioxide (TiO2) are received, and injecting carbon is to make a sound porous structure [5]. The given method was pat-ented earlier and it was used for receiving the films SiO2 with low dielectric permittivity. This method, however, is supposed to be applied to other oxide films used in micro – and nanoelectronics, in Ta2O5, Nb2O5, TiO2 in particular. The pore formation in this process is explained by gaseous compounds СО or СО2 which on educing, make the film friable forming in it open pores and gas inclusions.
The thickness of the dielectric films which were researched in the electrophysical operations was 100 nm. The films Al, about 100 nm thick, were used as electrodes at electrical measurements. These films were made by means of thermal evaporation in vacuum. Con-densing structures Al-SiO2-Al, Al-Ta2O5-Al, Al-Nb2O5-Al, Al-TiO2-Al were formed like matrixes with the active area of 1х1 mm on the ceramized substrates of 60480,6 mm in size.
The determination of the pore quantity and size was done by means of electrochemical copper jumping. The width of the Tauc gap (Et) was defined by the extrapolation (αE)1/2 dependence on the photon energy E in the range of the wave lengths of 200-1100 nm. The spectral dependence of the film absorption index (α) was defined by transmission and reflection spectrums with the help of spectrometer USB2000. The determination of the thickness and dielectric film refraction index was stated by means of a spectral ellipsometric complex. Scanning microscope was used to explore the surface of the films, and also an atom-and-force microscope Certus Optic U with the combined optical microscope. The microanalysis was done with the help of the Bruker Quantax 50 EDX microanalyzer as a part of an electron microscope Hitachi TM-1000. The spectral analysis of the researched films was done by using an IR –spectrometer in the range of the frequencies of 500 – 5000 sm-1.
1. The experiments proved that the carbon injection into the process of the formation of the films SiO2,Ta2O5, Nb2O5,TiO2 leads to the formation of self-organized porous structure. The size and density of the pores is determined by the quantity of the injected carbon.
2. Electrical and optical parameters of the films SiO2,Ta2O5, Nb2O5,TiO2 are largely defined by the porosity and they have similar tendencies in some intervals, however, the general dependence type is stated by the chemical properties of the spattered material, itself.
3. Common tendencies in the change of electrophysical properties and in the surface structure of the films SiO2,Ta2O5, Nb2O5,TiO2 with the carbon injected into them, make it possible to assume that analogous changes will be developed in other oxide dielectrics which are formed in the plasma of the glow discharge, though the qualitative dependence type will be different.
4. The formation of the porous structure contributes to the rise of the selective adsorption capacity of the researched dielectrics, mainly, owing to capillary condensation in mesapores and also stimulated adsorption, which can serve the basis of creating gas-sensing sensor devices.

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