Atomic layer deposition (ALD) of complex oxide thin films for integration into semiconductor devices: Precursor engineering and process optimization

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The ever decreasing size of semiconductor devices enforces the exchange of silicon oxide based layers by higher-k: complex oxide layers in novel integrated structures. Complex oxides are going to replace Si02 layers in different areas of memory as well as logic devices. Considering dynamic random access memory devices (DRAMs) as an example the key point is to sustain a constant cell capacitance of 25 fF at even decreased lateral dimensions. These technological challenges can be achieved by the atomic layer deposition (ALD) of extremely thin (few tens of a nanometer) higher-k films with uniform thickness and homogeneous stoichiometry into three dimensional (3D) structures with high aspect ratio. Other applications of complex oxide thin films which require the low deposition temperature and the homogeneity of ALD cover the fields of alternative gate oxides, coatings on carbon-nanotubes (CNTs), ferroelectric thin films for piezoelectric applications, and insulating layers in resistive switching elements (RRAMs) which are projected to become the new universal memory. In this project we aim to evaluate tailored ALD processes for the deposition of complex oxide thin films which should be possible by a dedicated engineering of specially designed ALD precursors. The deeper understanding of the key-factors which characterize optimized ALD precursors, the interrelafion of the precursors in a multi-component oxide ALD process, and the design of different element precursors with the same ALD window are major challenges for qualifying ALD as a standard deposition technique for complex oxides. The project will concentrate on two classes of complex oxides which are explored as third generation dielectrics: 1) higher-k alkaline earth titanates, zirconates, and hafnates (e.g. (Ba,Sr)Ti03, Ba(Ti,Zr)03, and Ba(Ti,Hf)03), and 2) stabilization of special structural (i.e. higher-k) phases of group 4 metal oxides by incorporation of rare earth elements (specific examples Hf02:La203, Y203:Zr02). One of the main challenges of ALD is the choice and the availability of suitable precursors with appropriate properties in terms of being volatile, stable in the gas-phase, but reactive on the substrate and growing surface. These are quite different criteria to those of standard metal organic chemical vapour deposition (MOCVD). Therefore one objective of this project is to develop precursors for ALD of higher-k oxides with suitable ligand design or modification. Novel precursors will be developed mainly using metal amides in combination with chelating ligands like guanidinates, amidinates, malonates, cyclopentadienyls and their respective derivatives. The precursors will be tested and characterised in ALD and liquid injection (LI)- ALD processes, and optimized precursor combinations with respect to ALD window and growth rate for example, will be used to grow multi-component oxide films in state-of-the-art ALD reactors. One dedicated aim is to control the films microstructure i.e. amorphous and polycrystalline or cubic and monoclinic by choice of precursors and deposition conditions. As the current technologies and the emerging nanotechnologies move towards adopting the nano-scale thin film growth techniques like ALD, a multidisciplinary approach is required for the success of this research project. The strong scientific backgrounds of both the partners are complementary to each other in terms of precursor development and fabrication of thin film device structures. This should pave the way for realisation of the proposed project which is directly relevant for development of new materials and processes for modern day technology.