ANÁLISIS DEL COSTO COMPUTACIONAL

In this final chapter the general conclusions and their implications are discussed particularly with respect to future avenues of research and applications. This chapter will also be used to speculate somewhat on further developments necessary to continue the present rapid rate of progress in the field of thin gate dielectrics. There are essentially two main parts to this thesis. These are the development of a new and possibly useful oxide growth technology and the application of SE and MASE in conjunction with other techniques to the analysis of thin oxides on Si.

In chapter 1 the requirements for future gate dielectrics were discussed. The decrease in gate oxide thickness means that both fabrication and characterisation become increasingly difficult. The nature of thin oxides was proposed to be somewhat different from thick fully relaxed layers. The high temperatures used in present conventional processing are believed to be incompatible with device scaling trends. Thus the success of this study can be partly gauged in the progress made in addressing these issues.

Initially studies of near UV oxidation proved that insufficient growth was induced to be technologically useful. However, it was found that deep UV was capable of inducing oxide growth at very low temperatures. From these studies the UV/ozone scheme of oxidation was proposed and investigated. It was demonstrated to be effective at temperatures less than 550° C. UV /ozone oxidation with the present arrangement (with a low

pressure Hg lamp) has several advantages. These are its simplicity, scalablity (i.e. wafers of any size can be oxidised by just increasing the areas of the lamp grid and heater) and relatively low cost in comparison to high temperature systems which are more energy intensive. Additionally, for larger wafers the reduced thermal stresses due to lower temperature processing may be particularly beneficial. The final oxide thickness grown in 1 hour at 500° C in 1% ozone was equivalent to a thermal oxidation temperature of 775 °C. In fact, the growth of very thin oxides (<5nm ) was rapid but oxide growth at temperatures below 500° C appeared to be self-limiting (as in the case of native oxides). The thickest oxides grown were around 8nm. During the period of experiments the efficiency of the process was improved considerably, leading to faster growth rates and thicker oxides. This suggests that further growth rate increases should be possible by improving the experimental arrangement. However, to improve the growth rate it is important to understand the controlling mechanisms so that the process variables can be effectively manipulated.

The oxide growth mechanism was also studied and Cabrera-Mott (logarithmic) type kinetics identified. Growth was found to be a function of the substrate temperature, the UV output of the lamp and the ozone concentration in the ambient above the oxidising surface. A mechanistic description of the possible route to oxide formation was given in section 2.4. The same methods to test the mechanisms of growth can also be used to enhance growth rates. A simple extension of the trend of increasing the UV flux and ozone concentration (followed through the experimental period) would seem the best method of inducing faster reaction rates. The mechanism proposed points to the different effects of UV and ozone in the reaction. The Hg lamp acts as a combined source of both, however, it is feasible to use separate sources for UV and ozone. Commercial ozone generators are capable of producing higher concentrations which can be excited by a separate lamp. The lamp can be placed external to the chamber to avoid the overheating effects observed in the present

experimental arrangement. Additionally by using lamps of differing wavelength output it may be possible to determine the photoexcitation mechanism. It was perceived that photoexcitation of the Si surface played an important part especially once oxide thickness exceeded 3-4nm. However, the main UV emission line (254nm) of the lamp was strongly absorbed by the ozone (see Fig.2.3). Thus a U V source emitting radiation outside the strong ozone absorption band centred near 255nm which is still capable of exciting the Si surface (emitting electrons into the oxide) is likely to induce faster growth. Alternatively, if the mechanism is not surface driven and is dominated by the presence of atomic species in the ambient, growth should be restricted. Such an experiment would reveal more precisely the nature of the reaction. Hence a growth system of the type shown schematically in Fig.5.1 can be envisaged. The main difficulty lies in obtaining a UV emitting source which has the desired attributes. These are, the emission of photons over the desired wavelength range with a high energy density and a large area uniform flux. Very recently lamps with all these features have been reported in the literature by Eliasson et al. (1988, 1989). The principle of operation is to discharge gases such as those typically used in excimer lasers (for example KrF, XeCl and Xe) to produce an incoherent UV output. An important advantage of this type of lamp is that the output wavelengths can be varied by changing the gas. Indeed the excimer discharge lamps are more efficient and produce higher flux densities than low pressure Hg sources.

A better understanding of the growth kinetics is only possible by applying an in-situ oxide thickness measurement technique. This remains a very difficult task experimentally and theoretically because of the growth system and possible density variations in thin oxide layers. An in-situ spectroscopic ellipsometer with a very short measurement time would be vital for such a study (an instrument meeting these requirements is sold by SOPRA but at a typical cost of 100K pounds).

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