Atomic layer deposition of high permittivity oxides: film growth and in situ studies

Antti Rahtu
Abstract & Cover

In this thesis, several oxide atomic layer deposition (ALD) processes were studied.  The main focus was on insulating materials to be used for microelectronic devices. At the  moment Al2O3, ZrO2, HfO2, rare earth oxides and their aluminates and silicates are the  most promising materials for metal oxide semiconductor field effect transistors  (MOSFETs). In MOSFETs the gate insulator should be deposited in a way that no more  than one or two monolayers of SiO2 would form during the whole device fabrication.  SrTiO3 and BaxSr1-xTiO3 are the most promising materials for dynamic random access  memories (DRAM).  A new method was developed for depositing oxide thin films on silicon by using  alkoxides as oxygen sources. Several binary and mixed oxides could be deposited. Of these  oxides Al2O3 was examined more closely and found to grow on silicon without a SiO2  interface layer. ZrxTiyOz thin films deposited using this method had a permittivity of 45 -  65 and a leakage current of 10-4 A/cm2 at 0.2 MV/cm.  A detailed understanding of thin film growth mechanism is important. A  quadrupole mass spectrometer (QMS) - quartz crystal microbalance (QCM) in situ  characterization system was developed in this study. QMS monitors the gas phase while  QCM the mass changes on the surface. By combining these two methods reaction  mechanisms can be examined in real time during the ALD growth in flow-type reactor  conditions. The main drawback of QCM is its temperature sensitivity and therefore  methods for compensating this effect were developed.  Reaction mechanisms were studied in various oxide processes such as Al2O3, TiO2,  ZrO2, ZrxTiyOz, and SrTiO3. Different titanium alkoxides were studied as ALD precursors.  The alkoxide group was found to have a strong effect on the ALD reaction mechanism and  the stability of the precursor against thermal decomposition. The oxygen source in all  binary processes studied was D2O and thus the main reaction byproduct was the deuterated  ligand while in the metal alkoxide - metal halide process two reaction paths were found.  In the SrTiO3 process at 250 oC the surface had no effect on the growth rate or  reaction mechanism, while at 325 oC especially the TiO2 surface catalyzed the reactions.  The change in the growth mechanism was attributed to the formation of crystalline SrTiO3  phase at 325 oC instead of a solid mixture of two oxides at 250 oC. In addition, in situ  methods showed a possibility to monitor the metal ratio of ternary films.  Key words: Atomic layer deposition, Oxide, Reaction mechanism, Quadrupole mass  spectrometry, Quartz crystal microbalance  

Source of Information
FinALD40 exhibition material,
University of Helsinki, Department of Chemistry, Laboratory of Inorganic Chemistry
(Helsinki, Finland)
Other notes
Oksana: here term ALD is used, ALE mentioned for historical note only
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