Atomic layer deposition (ALD) was originally developed in the 1970s to process wide band-gap semiconducting II-VI sulfides as well as insulating oxides for thin film electroluminescent (TFEL) displays. ALD is now a mature technique which has found other applications as well, e.g. in producing catalysts and sensors. Currently, ALD is experiencing a breakthrough as a method-of-choice in the semiconductor industry where so-called high-k insulating oxides can be processed by ALD as very thin overlayers with precise thickness control.
The dielectric constants for conventional dielectrics such as SiO2, silicon nitride and alumina are 3.9, 7, and 9, respectively, but much higher k-values (in the range of 15 to 30) can be obtained with the oxides of Y, La, Hf and Zr. We have recently demonstrated that all these high-k oxides can be processed in a surface-controlled ALD mode exploiting various precursor approaches each having its own advantages. Thus, for instance, completely oxidized and high density films can be obtained by the use of ozone, while low processing temperatures and high growth rates are characteristic for the use of true organometallics, viz. cyclopentadienyl compounds.
When choosing the precursor chemistry, the impurities of the films and reactions at the silicon interface need also to be considered requiring advanced analytical characterization by XPS, TOF-ERDA, X-TEM and other surface-sensitive and high-resolution techniques. Finally, electrical measurements are used to evaluate the dielectric properties.