Remote Plasma ALD of SrTiO[sub 3] Using Cyclopentadienlyl-Based Ti and Sr Precursors

Abstract

Remote plasma atomic layer deposition ͑ALD͒ of SrTiO 3 films with different ͓Sr͔/͓Ti͔ ratios is reported, employing Star-Ti ͓͑pentamethylcyclopentadienyl͒trimethoxy-titanium, ͑CpMe 5 ͒Ti͑OMe͒ 3 ͔ and Hyper-Sr ͓bis͑trisisopropylcyclopentadienyl͒-strontium with 1,2-dimethoxyethane adduct, Sr͑ i Pr 3 Cp͒ 2 DME͔ precursors and O 2 plasma. The as-deposited films were amorphous, but crystallized after post-deposition anneal above 500°C. For annealed SrTiO 3 films with ͓Sr͔/͓Ti͔ = 1.3 and a thickness of 50 nm, a high dielectric constant k Ͼ 80 and low leakage current of ϳ10 −7 A/cm 2 at 1 V were obtained. It is demonstrated that changes in the composition and microstructure are apparent in the optical dielectric function of the SrTiO 3 films, as obtained by spectroscopic ellipsometry.Next-generation dynamic random access memories with an equivalent oxide thickness ͑EOT͒ ഛ 0.35 nm ͑Refs. 1 and 2͒, and high-density metal-insulator-metal ͑MIM͒ capacitors with specific capacitance Ͼ500 nF/mm 2 for automotive and decoupling applications, require ultrahigh permittivity dielectric films ͑preferentially, k Ͼ 100͒ that are conformally deposited in high-aspect ratio structures. Among the perovskite oxide materials with their high dielectric constant for the crystalline films, 3,4 strontium titanate ͓SrTiO 3 ͑STO͒, bulk k ϳ 300, Ref. 3͔ is of particular interest because of its relatively low crystallization temperature between 500 and 600°C ͑Ref. 5͒ which is compatible with the process integration schemes limited to a maximum temperature of 600°C. 4 Due to the stringent requirements on the conformality of Ͼ95% step coverage for aspect ratios Ͼ20-30, targeted thickness of 5-50 nm, and low process temperature, atomic layer deposition ͑ALD͒ is considered as the method of choice for the synthesis of these SrTiO 3 films for such MIM capacitors ͓requirements formulated by MaxCaps Consortium ͑2009͔͒.Various ALD processes for SrTiO 3 using different Ti and Sr precursors have already been reported. They share the approach of alternating the ALD processes of TiO 2 and SrO in a certain ratio to allow for the control of the film properties of the ternary oxide, such as composition, crystallization temperature, and dielectric constant. 5-12 The challenge for the STO ALD processes is to optimally tune the material properties within the ALD process windows of TiO 2 and SrO. For example, the process temperature is commonly chosen as high as possible to yield the material with lowest impurities levels and best electrical performance. The maximum deposition temperature allowed is, however, limited by the thermal stability of the Ti and Sr precursors employed.A new class of ALD Sr precursors with cyclopentadienyl ͑Cp͒ ligands were introduced by Vehkamäki et al. 6,7 These Cp-based precursors allowed for the thermal ALD with H 2 O at reasonably high temperatures ͑ Ͻ 300°C͒ while providing an adequate vapor pressure. Recently, also Cp-compounds of titanium with improved thermal stability have become available. However, these precursors are not suitable...

Plasma-Assisted Atomic Layer Deposition of SrTiO3: Stoichiometry and Crystallinity Studied by Spectroscopic Ellipsometry

“…These values deviate from those reported previously by Langereis et al for the home-built ALD-I reactor, 22 in which the GPC for TiO 2 and SrO were 0.054 and 0.051 nm/cycle, respectively. Differences in GPC between these two setups have previously been reported for plasma-assisted ALD process of Ta 2 O 5 , 25 and were ascribed to the differences in the reactor designs.…”

Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges

“…In order to obtain SrTiO 3 dielectric films with the best electrical performance, the highest possible deposition temperature within the shared temperature window should be chosen. 195,327 The fact that plasma-assisted ALD can lead to improved material properties has also been demonstrated by results on metallic TiN films. 233,237 The combination of TiCl 4 with an H 2 /N 2 plasma yielded thin TiN films with an excellent resistivity and low impurity levels.…”

Substrate-biasing during plasma-assisted atomic layer deposition to tailor metal-oxide thin film growth

“…3 , purity >99%, donated by Air Liquide] was used as the precursor. 22 The bubbler was heated to 78 C, and Ar bubbling was used. During processing, all dose and purge times were set to 5 s. TiO 2 films of approximately 30 nm thick were deposited at 300 C. Several techniques were used for the characterization of the thin films properties.…”