Effect of Post Metallization Annealing for La 2 O 3 Thin Film

any recovery of VFB shift, but shifted VFB toward more negative side. Hence,this assumption does not seem to be correct. Another is the reactionbetween metal upper electrodes and rare earth oxides. In the case ofaluminum upper electrodes which realized a recovery of VFB shift, it isnatural to consider that aluminum oxides (Al2O3) or lanthanum aluminate(LaAlO3) was formed if there were some reactions between aluminum upperelectrodes and rare earth oxides. And this assumption seems to becomemuch more reasonable that these reactions formed aluminum oxides, notlanthanum aluminate. This assumption is well substantiated by three factsobtained by this investigation. One is a decrease of capacitance value afterPMA with aluminum upper electrodes compared to one of PDA. It is wellknown that aluminum oxides have the lower dielectric constant (8~10)compared to lanthanum aluminate (20~25) and rare earth oxides (15~30).The second is the fact that PMA with gold upper electrodes increased thecapacitance value in contrast to the case of PMA with aluminum upperelectrodes. The third is an effect of a recovery of VFB shift for Gd2O3 by PMAwith aluminum as described in sub-section 3.3.4. This result also greatlysupports this assumption. Therefore, it is conceivable that the formation ofaluminum oxides decreased the capacitance value. Another, and this is themost significant part, is the fact that aluminum oxides have the property ofshifting VFB toward positive side. Aluminum oxide has been widelyinvestigated as the one of the most promising and realistic next generationgate insulator material and this VFB shift toward positive side has beencommonly reported. And these reactions might occurred not only in thefilms but also at the interface between lanthanum oxides and aluminum, andthat reaction could increase the own film thickness. These assumptionsexplained up to this point is illustrated in Figure 4-3.Finally, it can be suggested that the formation of higher qualityaluminum oxides between rare earth oxides which have high dielectricconstant such as laminate structure could realize the zero flat band shiftwith preserving higher capacitance value.- 96 -