Cycloheptatrienyl-Cyclopentadienyl Heteroleptic Precursors for Atomic Layer Deposition of Group 4 Oxide Thin Films

Abstract

Atomic layer deposition (ALD) processes for the growth of ZrO2 and TiO2 were developed using novel precursors. The novel processes were based on cycloheptatrienyl (CHT, -C7H7) – cyclopentadienyl (Cp, -C5H5) compounds of Zr and Ti, offering improved thermal stability and purity of the deposited oxide films. The CpMeZrCHT/O3 ALD process yielded high growth rate (0.7–0.8 Å/cycle) over a wide growth temperature range (300–450 °C) and diminutive impurity levels in the deposited polycrystalline films. Growth temperatures exceeding 400 °C caused partial decomposition of the precursor. Low capacitance equivalent thickness (0.8 nm) with low leakage current density was achieved. In the case of Ti, the novel precursor, namely CpTiCHT, together with ozone as the oxygen source yielded films with low impurity levels and a strong tendency to form the desired rutile phase upon annealing at rather low temperatures. In addition, the thermal stability of the CpTiCHT precursor is higher compared to the usually applied ALD precursors of Ti. The introduction of this new ALD precursor family offers a basis for further improving the ALD processes of group 4 oxide containing thin films for a wide range of applications.

Titanium dioxide thin films by atomic layer deposition: a review

“…Notably, for the Ti(NMe2)3(CpMe)/O2 plasma process formation of the rutile structure was observed at temperatures as low as 200 o C for films deposited on Si substrates, by applying substrate biasing [786]. For the Ti(Cp)(CHT)/O3 process, the rutile content progressively increases starting from 300 o C being higher than the anatase content above 325 o C [791]. For amidinate-based processes the crystallization onset is similar such that the films fabricated using Ti(O i Pr)3(N i Pr-Me-amd) and H2O reactants crystallize in the anatase phase at 250-375 o C [776], while use of O3 as the oxygen source increases the onset of the crystallization slightly to 275 o C [777].…”

“…The main driver for this development is the deposition of rutilestructured TiO2 and SrTiO2 thin films, which are interesting high-k materials for dynamic random access memories (DRAM). [775], Ti(OMe)3(CpMe)/O3 [767], Ti(OMe)3(CpMe5)/O3 [767,778], Ti(OMe)3(CpMe5)/O2 plasma [775,784], Ti(NMe2)3Cp/O3 [767], Ti(NMe2)3(CpMe5)/O3 [767], Ti(NMe2)3(CpMe)/O2 plasma [787,802], Ti(NMe2)3(CpN)/O2 plasma [785], Ti(CHT)Cp/O3 [791].…”

“…[785] Ti(CHT)Cp with O3. Ti(CHT)Cp is different from the common Ti precursors in that instead of having four ligands it has only two, such that the metal center is surrounded by one η 5 -coordinated cyclopentadienyl ligand and one η 7 -coordinated cycloheptatrienyl ligand; the oxidation state of Ti has been debated but IV+ is considered the most likely one [791]. The ALD process with ozone shows GPC values around 0.35 Å/cycle at 300 o C with an increasing trend with increasing temperature in the range of 250-450 o C; precursor decomposition manifests itself at 400 o C, but it likely starts at around 350 o C. For films grown at 300 o C the carbon content is below 1 at%.…”

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

“…In a recent study, Niinistö et al introduced cycloheptatrienyl ligands in novel solid Cp‐CHT compounds for the ALD of TiO 2 and ZrO 2 (Table ) . These precursors do not react with water and require ozone as the oxygen source.…”

Comparative study on the use of novel heteroleptic cyclopentadienyl-based zirconium precursors with H2O and O3 for atomic layer deposition of ZrO2

“…19 Also, the properties of the Cp ligands can easily be tuned by varying the number and composition of the substitutes on the ring. Examples of heteroleptic precursors with Cp ligands are RCpZr(NMe2)3 (R=H, ethyl or methyl), 20 RCp2ZrMe2 (R=H or methyl) 21 and MeCpZrCHT (CHT=cycloheptatrienyl) 22 . Many of the heteroleptic precursors have been reported to exhibit self-limiting growth at 300 °C , including the CpZr(NMe2)3 that is widely used in industry 23 .…”