Oxidation of silicon and non-equilibrium segregation of boron during redistribution

Abstract

The oxidation of silicon and the diffusion of boron into siliconhave been investigated together with device manufacturing proceduresin the microelectronics laboratory at the University of New SouthWales .A number of silicon wafers were oxidised at a fixed temperature forvarious times following thorough chemical cleaning and the resultant oxidethicknesses were measured using a Tolansky microscope. Fromthe experimental results an equation relating the oxide thickness tothe oxidation time (at 1100'C) was derived using the method of leastsquares. The experimental oxidation curve is in agreement with otherworks .In an attempt to characterise the diffusion of boron in silicon afurther four wafers were subjected to boron predepositions anddrive-ins, the drive-ins being performed in oxygen. Sheet resistivitymeasurements, made with a four point probe after the predepositions,agree with general trends established elsewhere. The redistributionof boron during thermal oxidation was then examined and three boronconcentration profiles were obtained. These profiles indicated thatsurface depletion of boron had occurred as a result of the thermaloxidation, the degree of surface depletion being dependent on thedrive--in conditions.To determine the concentration profiles each wafer was subjected to anetch, weigh and resistivity measurement sequence from which the boronconcentration as a function of depth into the silicon was calculated.The methods used were systematically refined as the work proceededand all have been documented in the appendices.Finally, the theory of the redistribution of boron during oxidationwas examined in conjunction with a closed form solution to theredistribution problem by Huang & Welliver (Ref.11).Their diffusion model was found to be inaccurate and a refinement wasmade with the introduction of a reaction-rate constant, resulting ina mathematical model which appears to describe the redistribution ofboron more correctly.In conclusion, the thesis contains(i) successful step-by-step procedures for many solid-statedevice production process(ii) a silicon oxidation characteristic showing the relationshipbetween oxide thickness and oxidation time at 1100'C(iii) the results of boron predepositions and drive-ins includingboron concentration profiles(iv) a modification to the model for the redistribution of boronduring thermal oxidation as described by Huang & Welliver