1. Could you explain the significance of your article to the non-specialist? 

Atomic layer deposition (ALD) is a novel self-limiting film growth method distinguished from chemical vapour deposition (CVD) by the alternate exposure of precursor species. ALD was originally developed to process electroluminescent (EL) displays but presently, because of continued scaling down of semiconductor devices, ALD is considered the only feasible technique to produce very conformal, ultrathin films in the nanometer range at the relatively low growth temperatures allowed. New dielectric materials with sufficiently high permittivities (high-k) are needed as dielectrics to replace SiO₂ and these materials can be produced by ALD through advanced precursor chemistry. The oxides of Zr, Hf and rare earths are among the candidates for high-k materials but in order to further increase the permittivity and at the same time decrease the leakage current and hysteresis, ternary oxides in amorphous state are being considered as next generation dielectrics. We have developed two ALD routes to ternary yttrium scandate, which exhibits excellent electrical properties while remaining amorphous even at 800ºC.

 

2. What has motivated you to conduct this work? 

The ALD research group at the Helsinki University of Technology (TKK), Laboratory of Inorganic and Analytical Chemistry, led by Professor Lauri Niinistö, has over 20 years of experience in ALD technology starting from the early development of EL materials in the l980s. During the past 10 years or so the focus has been on materials needed for microelectronic and other devices, e.g. high-k dielectrics or materials for gas sensors. As precursor chemistry is the key to good materials by ALD, considerable emphasis has been placed on developing and testing new precursor combinations, especially the organometallics like beta-diketonates with ozone and cyclopentadienyl compounds with water. While developing the precursor chemistry and processing the thin films, the precursors and resulting materials have been characterized by a comprehensive set of techniques for chemical composition, structural and functional properties.

The expertise gained over the years prompted us to combine two binary processes for high-k rare earth oxides, viz. those for Sc₂O₃ and Y₂O₃, in order to develop an ALD process for amorphous ternary material with enhanced permittivity.

 

3. Where do you see this work developing in the future? 

Based on the success of the present work, new organometallic precursors will be used to develop ALD processes also for other rare earth scandates and related phases as well as to characterize them for electrical and other properties.

 

4. Are there any particular challenges facing future research in this area? 

Although many ALD processes have been developed for materials needed by the semiconductor industry, viz. metals, nitrides and oxides, there is a constant need for new precursors and ALD processes based on them. Depending on the precursors employed, problems exist which must be overcome: low film growth rate, poor precursor thermal stability and film impurity levels. In particular, the number of suitable ALD precursors for the heavier rare earths is quite limited.