DIELECTRIC BREAKDOWN STUDIES ON `BUILT-UP' FILMS OF SOME LONG CH AIN COMPOUNDS

Abstract

The mechanism of dielectric breakdown phenomena in thin insulator films has been a subject of many theoretical and experimental investigations for the last many years. These studies have been greatly enhanced because of their role in the development of a variety of miniaturized solid-estate devices; such as 'diodes', 'triodes', switching devices and capacitors etc. A number of mechanism and theories have been proposed from time to time but they, by no means provide a wholly satisfactory and adequate explanation of the various breakdown phenomenon in all details. Much of the recent work on breakdown behaviour of thin films relate to the thickness dependence of breakdown field which is evidently very important from device applications point of view and is also a crucial aspect of the theories. With these considerations in view, the present work which describes a first detailed and systematic study of such thickness dependence of d, c. and a. c. breakdown field in thin 'built-up' films of barium salts of a few long chain fatty acids such as palmitic, margaric, stearic and behenic,was undertaken. Recently, these films have indeed been • shown to be very promising for making some dielectric devices. These films are very suitable for the present studies because their thicknesses are closely controllable in low ranges and are accurately known and studies are possible on even upto a 25 A° thick film. These films have the added advantages of high dielectric strength, uniformity, reproducibility, high stability and are relatively easy to deposit, requiring a very simple apparatus. Great care is, however, needed for obtaining films free from gross defects (chapter V). D. C. breakdown characteristics have been studied of the 'built-up' films of barium palmitate, margarate, stearate and behenate having monolayers l-80 (Chapter VI). Mainly, two breakdown events, i, e. 'onset breakdown' and 'maximum breakdown' have been distinguished. The onset breakdown field has been found to be a power dependent function of film thickness varying as d -4 where d is the film thickness and. 0C = 0.5 (approx.) for all the films in the range of 10-80 layers and of = 1. 0 (approx, ) in the ultra-thin range of 1-10 layers. The results in the relatively higher thicknes's range (10-80 layers) closely conform to the well known electronic breakdown theory of Forlani and Minnaja,which is based on an electron ionization avalanche mechanism and predicts 0C = 0. 5. The results thus provide a strong quantitative support to this theory. The thickness dependence in ultra-thin range can perhaps be explained (as suggested by Dr V. K, Srivastava) if, the boundary scattering of the electrons from the film surface, which becomes significant in low thickness range, are properly taken into account (sec, 6. 2). In the light of the results obtained, as done by many other workers also, it may safely be concluded that the electron 111 avalanche and tunnel injection m echanism which form the basis of F•M theory offer a realistic approach to the problem of breakdown. Of course, the breakdown mechanism of 'built..up' films seem to be electronic rather than thermal, Incidentally, it is now almost generally accepted that the principal mechanism involved in the electrical break-down is the formation of an electron avalanche and the F-M theory adequately explain many of the results. Another aspect of breakdown e. g. 'maximum breakdown voltage', accompanied by large scale destruction through sparks, studied in the present work, is also important from device applications point of view, because it determines the practically important 'ultimate dielectric strength' of the film. Such studies have been carried out on all the 'built.-up' films having layers (16..80) and their V...J characteristics in the 'non-destructive' phase have been obtained, It is observed that the visibleClestruction'of a film comm ences at a voltage of about 25 volts irrespective of the film thickness and material used and the film finally gets largely destroyed at the 'maximum breakdown voltage', This destruction has been illustrated through transmission photo-micrographs, As the mechanism of destruction at breakdown is not yet known in all details and no adequate theoretical approach is available, no attempt has been made here to interpret the above results theoretically. However, a qualitative explanation has been provided to differentiate between 'maximum breakdown voltage' encountered in the iv present studies and that observed by Klein et al. The 'single.hole' or 'propagating' types of breakdown as suggested by Klein, have not been observed presumably, because of the films, when carefully prepared have high structural perfection and are almost free from gross defects or "weak..spots" (sec. 6. 2), The breakdown studies under alter nating field conditions, which are also very important from device application point of view, have also been made for 'built-up' barium stearate films, as a function of thickness (range 100 A° « 1500 A°) at a fixed frequency of 30 KHz and as a function of frequency (range 10 KHz - 200 KHz) at a film thickness of 500 A° (chapter VII). The first breakdown event which is particularly important in an actual device has been studied here and this breakdown voltage is found to be increasing with increasing film thickness and frequency. Surprisingly, thickness dependence of the type Fab Ll ""C (with oC= 0, 68) which is very near to the one (,(= 0, 5) given in F.M c, theory is found in the case of a, c, studies also. An increase in the breakdown strength has been observed with increasing frequency in the high frequency range which contradicts the prediction of thermal breakdown theory. Therefore, it may be concluded that the breakdown process involved here, which could possibly be thermal in nature, is not so. Although, the results obtained could not be interpreted because of the lack of an a, c. electronic breakdown theory, the results may be very useful in a, c. device applications of the potentially important films, The following is the subject matter of the thesis which has been arranged in seven chapters: Chapter I Different theories regarding breakdown conduction have been reviewed and categorised in two principal theories viz. 'electronic' and 'thermal* breakdown theories. The former, which is relevant to the present studies, has been discussed in detail. Chapter II The experimental investigations by others on breakdown phenomena on thin evaporated film systems, particularly thickness dependence behaviour, has been reviewed. The review has been categorised as 'electronic', 'thermal' and 'destructive' breakdown studies. Previous studies of break-down conduction in 'built-up' films have also been reported. Chapter III The surface tension, phenomena, the information about 'monolayer' formed at water.air interfaces, the mechanism of monolayer spreading, non-occurrence of polymolecular film and some fundamental information about the individual molecules have been discussed in this chapter. Chapter IV The nature, structure and properties of 'built-up' films studied and Blodgett...Langmuir technique of building up these films by transferring monolayers on to slides have beendiscussed. Past studies and the recent ones for vi thickness determination of these films have been given. Chapter V The experimental details of thermal evaporation of Al, sandwich fabrication, selection and cleaning of the substrate and electrical measurement methods have been given in this chapter. The advantages of tbuilt.upt films, which have stimulated the interest in their studies, have also been discussed. Chapter VI The results of a detailed and systematic d. c. breakdown studies on sbuiltoupt films of barium palmitate, margarate, stearate and behenate have been given. Discussion of the results in terms of the known theories has also been provided. Chapter VII The results concerning a. c, breakdown studies of built-up' barium stearate films and their interpretation are given in this chapter, Various graphs and photomicrographs concerning the results in Chapter VI and VII have also been attached in the thesis,