Atomic Layer Deposition of Group 4 and 5 Transition Metal Oxide Thin Films : Focus on Heteroleptic Precursors

The atomic layer deposition process (ALD) is an alternative to the chemical vapour deposition (CVD) method that is universally appreciated for its unique advantages such as excellent repeatability, conformity and thickness control at the atomic level. ALD precursor chemistry has mainly been based on homoleptic compounds such as, but not limited to, metal halides, alkylamides or alkoxides. However, these precursors have drawbacks such as possible halide contamination and low thermal stabilities with respect to the alkylamides and alkoxides. Consequently, heteroleptic precursors were investigated as alternatives to the existing homoleptic counterparts, which have led to the development of several advantageous processes. Nevertheless, no thematic review dedicated to heteroleptic precursor and their properties exists and it seems that no coherent strategy has been adopted for the development of heteroleptic precursors. This thesis gives a brief description of ALD and presents studies on the deposition of thin films of groups 4 and 5 metal oxide films using ALD. A description of the general ALD properties of homoleptic precursors in addition to a review on the thermal ALD of groups 4 and 5 metal oxides from heteroleptic precursors is provided. Trends in the properties of heteroleptic ALD precursors based on a literature review and new experimental data are discussed. Several novel heteroleptic compounds were evaluated for the ALD of thin films of TiO2, ZrO2, Nb2O5 and Ta2O5. The characteristics of these processes were evaluated and the film properties of these oxides were investigated by means of various characterization approaches. The effects of oxygen source, water or ozone, on the film growth characteristics and properties of ZrO2, Nb2O5, and Ta2O5, were also investigated. Mixed alkoxide-alkylamide and alkoxide-amidinate titanium compounds are liquid at room temperature. They are highly volatile, have excellent reactivity towards water and have high thermal stability. The deposited films exhibit high purity and conformability on high aspect ratio substrates. The growth of thin films of ZrO2 from a heteroleptic alkylamide-guanidinate zirconium precursor was notable in that the films grew in the high permittivity cubic phase and the ozone-based process had a high growth rate. Thin films of VOx were deposited from the homoleptic vanadium alkylamide precursor,vanadium tetraethylmethylamide. It was found that the structure and oxidation state of the films could be tuned from an amorphous mixture of VO2 and V2O5 to crystalline VO2 or V2O5. This was accomplished by simply exposing the films to heat treatment in different atmospheres, namely air or N2. Finally, alkylamide-imide precursors were investigated for the ALD of Nb2O5 and Ta2O5 thin films. These precursors are liquid at room temperature, and exhibited high thermal stabilities compared with the earlier known niobium and tantalum ALD precursors. The alkylamide-imide precursors studied had high volatility and excellent reactivity towards both water and ozone. The deposited films were smooth, uniform, and contained only low amounts of impurities.Atomikerroskasvatus (Atomic Layer Deposition, ALD) on ohutkalvojen kasvattamisessa käytettävä menetelmä, jonka ensisijaisia etuja ovat erinomainen toistettavuus ja konformaalisuus. Lisäksi menetelmällä kasvatettavien ohutkalvojen paksuutta voidaan kontrolloida atomitasolla. ALD-menetelmän lähtöaineina on käytetty tavallisesti homoleptisiä yhdisteitä. Homoleptisten lähtöaineiden käyttämisessä on kuitenkin haittapuolia, kuten esimerkiksi ohutkalvoihin mahdollisesti jäävät halidiepäpuhtaudet. Muista homoleptisten lähtöaineiden haittapuolista voidaan mainita homoleptisten alkyyliamidi- ja alkoksidilähtöaineiden heikko terminen pysyvyys. Homoleptisten lähtöaineiden korvaamista heteroleptisillä lähtöaineilla on tutkittu, mikä on johtanut useiden hyödyllisten prosessien kehittämiseen. Heteroleptisten yhdisteiden käyttämisestä ALD-lähtöaineina ei ole kuitenkaan tehty kattavaa tutkimusta, eikä uusien lähtöaineiden kehittämiselle ole tiettävästi luotu johdonmukaista suunnitelmaa. Tässä väitöskirjassa esitellään lyhyesti sekä ALD-menetelmän yleisiä ominaisuuksia että homoleptisten lähtöaineiden hyödyntämistä ohutkalvojen kasvattamisessa ALD:n avulla. Heteroleptisten lähtöaineiden hyödyntämisestä ryhmien 4 ja 5 metallioksidiohutkalvojen kasvattamisessa esitetään kattavampi katsaus. Katsaus koostuu sekä kirjallisuuslähteistä että uusista kokeellisista tuloksista. Kokeelliset tulokset koostuvat TiO2, ZrO2, Nb2O5 ja Ta2O5 ohutkalvojen kasvattamisessa käytettävien uusien heteroleptisten lähtöaineiden ominaisuuksista, sekä uusia lähtöaineita hyödyntävien kasvatusprosessien kartoittamisesta. Kasvatettujen metallioksidiohutkalvojen ominaisuuksia tutkittiin usealla eri menetelmällä. Lisäksi vertailtiin kahden eri happilähteen, veden ja otsonin, käyttämisen vaikutusta TiO2, ZrO2, Nb2O5 ja Ta2O5 ohutkalvojen kasvatusominaisuuksiin sekä itse metallioksidiohutkalvojen ominaisuuksiin

[Zr(NEtMe) 2 (guan-NEtMe) 2 ] as a Novel Atomic Layer Deposition Precursor: ZrO 2 Film Growth and Mechanistic Studies

[Zr(NEtMe)2(guan-NEtMe2)2], a recently developed compound, was investigated as a novel precursor for the atomic layer deposition (ALD) of ZrO2. With water as the oxygen source, the growth rate remained constant over a wide temperature range, whereas with ozone the growth rate increased steadily with deposition temperature. Both ALD processes were successfully developed: the characteristic self-limiting ALD growth mode was confirmed at 300 °C. The growth rates were exceptionally high, 0.9 and 1.15 Å/cycle with water and ozone, respectively. X-ray diffraction (XRD) indicated that the films were deposited in the high-permittivity cubic phase, even when grown at temperatures as low as 250 °C. Compositional analysis performed by means of X-ray photoelectron spectroscopy (XPS) demonstrated low carbon and nitrogen contamination

Heteroleptic Cyclopentadienyl-Amidinate Precursors for Atomic Layer Deposition (ALD) of Y, Pr, Gd, and Dy Oxide Thin Films

Thin films of rare-earth (RE) oxides (Y₂O₃, PrO_<i>x</i>, Gd₂O₃, and Dy₂O₃) were deposited by atomic layer deposition from liquid heteroleptic RE­(<i>ⁱ</i>PrCp)₂(<i>ⁱ</i>Pr-amd) precursors with either water or ozone as the oxygen source. Film thickness, crystallinity, morphology, and composition were studied. Saturation was achieved with Gd₂O₃ when O₃ was used as the oxygen source at 225 °C and with Y₂O₃ with both oxygen sources at as high temperature as 350 °C. The growth rates were 0.90–1.3 Å/cycle for these processes. PrO_<i>x</i> was challenging to deposit with both oxygen sources but with long, 20 s purges after the water pulses uniform films could be deposited. However, saturation was not achieved. With Dy₂O₃, uniform films could be deposited and the Dy­(<i>ⁱ</i>PrCp)₂(<i>ⁱ</i>Pr-amd)/O₃ process was close to saturation at 300 °C. The different oxygen sources had an effect on the crystallinity and impurity contents of the films in all the studied processes. Whether ozone or water was better choice for oxygen source depended on the metal oxide material that was deposited