Реферат - To dynamical theory of electrothermal degradation and NDT of defects in metal

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To dynamical theory of electrothermal degradation and NDT of defects in metal-dielectric-metal (MDM) structures
Valentin M. Bogomol’nyi
Moscow Institute of Technology, 141220, Moscow Region, Pushkinsky raion, Cherkizovo-1, Russia
ABSTRACT
On base of the solid state physics and theory of nonlinear oscillations interpretations a development of the thermofluctuation fatique of mechanics theory is formulated. It is shown that electrical damage haves resonance nature. An influence of the electron processes on the first time pre-breakdown stage with mainly of microdefects formation is considered. The proposed theory contents consideration of polarization of the local domains the «cross-pieces» between neighbouring micropores, which formed elementary electrical dipoles. Strong external constant electrical field lead to negative differential resistance of the local dielectric domains with N-or S-type current-voltage-characteristic (CVC) parts and as result to current oscillations and electromagnetic wave radiation from MDM structure (as in Gunn’s diode). On base of A. Puankare’s limit cycles nonlinear oscillations theory it is shown that defects formation leads to selfexciting current oscillations and microwave radiation. This information can be used in thermosense NDT and, that is principal, for elimination of the defects, which arised under fabrication of electronic devices.
Keywords: Electrothermal, resonance, degradation, dielectric, polarization, thermosense
1. INTRODUCTION
Dielectric thin films aging and breakdown is a phenomenon of major technological significance in optoelectronics, micro- and nanoelectronics. Study of the microdefects formation haves a practical interest at first for improvement of industrial electronic devices quality. On other hand the specially used structure defects lead to sensors, diodes and transistors functional parameters enhance1.
Pre-breakdown reversible effects are used in radars as sources of high-frequency radiation (0,1 - 103 GHz), for signals amplification in GHz diapason, where usual transistor not can be used, thermistors, electronic switching2, low-voltaic «cold» cathodes. Reversible breakdown used in «electrical forming» - an known technology process fabrication of the vacuum luminescent screens and origins of high energy electron radiation. By this in space regular local defects (high conducting microchannels, which content low - temperature plasm) are formed in voltage controlled - negative -resistance MDM structures, considered in this work.
Interest to study of electrical degradation connected also with an elaboration of high reliability optoelectronic metal - oxide - semiconductor (MOS) devices on base Si - SiO2, Si - SiN4, which used in computer technique, memory and recording elements. The surface of dielectric, contacting with metal electrode, is the most likely domain, where destruction takes place3,5. Local defects on surface are seats of electrical and temperature field concentrations, where origins of the mechanical destruction and electromagnetic radiation are located.
The main structure defects - dislocations (linear, plane, screw) arise as rule in all electronic devices under their fabrication. For example, distinction between temperature - expansion - coefficients at dielectric surface metalization lead to creation in GaAs density of dislocations - 108cm-2.
When an electrical breakdown event occurs local evaporation of the electrode material can be so that this regions become electrically and mechanically disconnected from the rest structure and a complete series of measurements can be made to renew of devices reliability, physical background of these methods is aim of this work.
B.K. Ridley suggested a breakdown essence in SiO2 based devices, which contents the assumption of the presence of ~100 protuberance at electrode surface. Greatly current injection enhanced at protuberances produces high temperature filaments in which dissociation of ion coupling takes place. The resulting positive ions drift to the cathode, producing a positive feedback connection on the current, which lead to its instability and oscillations4-9.
A.K. Jonsher et al propose a breakdown statistical model, connected with the existence of many point defects5,6. High electrical field lead to charge carriers injection in dielectrics and to generation of cumulative dynamical process of formation of the defects clusters5. Dislocations and micropores can instantly grow by clusters generation into a highly conducting channels (with diameter 40-80) connecting the electrodes.
Accepted mechanism of breakdown involves the further creation of gaseous channels through the dielectric. The channels volume is ~ 10-3-10-5 from common volume of dielectric. High conductance at breakdown is associated just with these channels and not with conduction trough the all rest dielectrics.
The formation of the gaseous channels before a marked change integral conductance of MDM structures is ascribed to energy supply from the external electric field and its storage in the solids by polarization dielectric («memory effect»), heating and collision ionization, trapping charge corriers processes, atomic «displacements», temperature rise and as result broken molecular bonds.
At first time dielectric breakdown occurs with very little «warning» of an increased current preceding the breakdown. By this short pulses of current were observed 5,10,11. Microwave radiation takes place on the first growing part of the current - voltage - characteristic (CVC) and might be not connected with integral volume negative differential resistance mechanism.
Simultaneously with microwave radiation in In Sb crystals, for example, the low-frequency current oscillations (105 - 108c-1) were observed. In electrical circuit is not oscillations arised the same as radiated from dielectrics. From this it is follows, that microwave radiation is not depends on integral volume dielectric properties and defines eigenfrequency properties of defects.
Usually theoretical treatments of the electrical breakdown were concerned with «quasistatic» behavior of high energy electrons12 and ionization avalanches in isolators4,5. The aim of this work a study of coupled dynamic thermoelectron emission currents3,14-19, polarization5,6 and heating processes13 in dielectrics at the first time pre-breakdown stage before of the gaseous channels formation.
Following model of the pre-breakdown stage it is supposed, taking into account the electron injection in dielectric from cathode. If a first injecting electron start from cathode towards anode a «polarization echo» channel («memory track») remains in dielectric. The second and following electrons move in the same «polarization-echo» channel. As result at first time constant current «filaments» arise. Electrical current in dielectrics with high electrical resistance lead to its heating. Owing to temperature rise then the electrical conductivity increases. This selfexciting nonlinear periodic process lead to high heating to and nonlinear N-or S-type CVC. The «polarization echo» - «memory track» collects charges from a relatively large catchment of environment rest dielectric volume, this constitutes the basis of temporal energy effectiveness of breakdown coupled with secondary electron emission from volume of dielectrics into «memory track».
Once the increase of the electrical field and temperature shorten time interval between moving in «memory track» electrons to the «point» and the «memory track» retains sufficient strength channeling becomes positive feedback, which lead to rapid increase of the micropores formation in direction of the force lines of the external electrical field and further conducting channels.
The energy dissipation resulting from environment dielectric polarization fluctuations caused by narrowly focused «beam» of charged particles rapidly leads to electrothermoelastic destruction of dielectrics5,6,13.
As result of secondary electron emission from environment dielectrics the charges are stored on the surfaces of the cavities. External electrical field separates the