Precursors as enablers of ALD technology: Contributions from University of Helsinki

The review focuses on ALD precursors of selected elements such as alkaline earth (Mg, Ca, Sr, Ba), group 4 metals, bismuth, silver, iridium, selenium, tellurium and antimony. These elements are needed in different high tech applications but are challenging for ALD. Their precursor design needs careful balancing between volatility, thermal stability and reactivity-the key properties of ALD precursors. The extensive studies showed that cyclopentadienyl based precursors of alkaline earth metals are versatile ALD precursors which react with both water and ozone forming oxide. Group 4 ALD chemistry has been studied very widely and many good precursors have been found for the oxide ALD. From a bunch of different compound types studied the most promising ALD precursor for bismuth is Bi(OCMe2iPr)3 which shows stable ALD process with water at 150-250 °C. The success in depositing noble metal films by ALD can be attributed more to the reactant rather than the metal precursor. Ru, Pt, Ir, Rh and Os films can be deposited from various organometallic and metal organic precursors using O2 as the other precursor. Typically temperatures above 225 °C are needed. Using O3 as a reactant films can be deposited at lower temperatures. Noble metal oxides are obtained below approx. 200 °C and metallic films above that. By supplying both O3 and H2 as consecutive pulses, noble metal films can be deposited well below 200 °C. For silver phosphine stabilized carboxylato and β-diketonato complexes are thermally stable enough enabling hydrogen plasma enhanced ALD of silver metal films. Alkylsilyl compounds of selenium and tellurium are versatile ALD precursors for metal selenide and telluride films when combined with metal chloride precursors. The use of alkylsilyl compounds is not limited to group 16 elements but can also been used for group 15.