Plasma-enhanced atomic layer deposition of barium titanate with aluminum incorporation

Abstract

Plasma-enhanced atomic layer deposition (PEALD) of ultrathin (∼7 nm) slightly Ti-rich Ba_xTi_yO_z (BTO) films with different Al-doping concentration ([Al]/([Al] + [Ba] + [Ti]) = 0 to 22 at%) was studied. In particular, the effects of Al-doping in BTO on compositional, crystallographic and electrical properties were investigated. Previously, BTO films with a Ti cation composition, [Ti]/([Ba] + [Ti]) = ∼60 at% was reported to be advantageous for crystallization, resulting in superior dielectric properties. These Ti-rich BTO films, however, suffered from high leakage currents, necessitating the change in its crystalline structure as well as elemental composition. By incorporating Al₂O₃ into the BTO films, the leakage current can be controlled, where the BTO films with an Al-doping concentration of 12 at% showed a leakage current reduced by one order of magnitude compared to un-doped BTO (i.e., ∼10⁻⁷ to ∼10⁻⁶ A/cm² at +1.6 V) without a significant drop of the dielectric constant (43,un-doped to 40, Al-doped).

Introduction

Dielectric films with high dielectric constants (i.e., high-k) and low leakage currents are essential for the applications of information storage devices (e.g. dynamic random access memory (DRAM)) [1]. As the feature size shrinks to enable a higher degree of storage capacity, DRAM industries demand next-generation high-k materials over conventional dielectric materials based on ZrO₂, HfO₂, and Ta₂O₃ which have already approached their intrinsic limits in material properties. In that regard, M-doped TiO₂ (M = Al, Hf) [2], [3], [4] and perovskite-type A_xTi_yO_z (A = Sr, Ba, (Ba,Sr)) have been newly highlighted because i) crystallized TiO₂ intrinsically possesses high dielectric constants (ε) (anatase, 30–40; rutile, 83–100) [5], ii) incorporation of other cations such as Sr, Ba, and La further increases ε values by formation of perovskite structures, and iii) the reduction in leakage current is highly feasible by incorporation of these high band gap materials (BaO, 4.8 eV; La₂O₃, 5.8 eV; SrO, 6.5 eV; Al₂O₃, 8.8 eV) [4], [6], [7].

Atomic layer deposition (ALD) offers unique features for developing high quality ultra-thin dielectric films compared to other deposition techniques. The self-limiting nature of ALD enables the deposition of pin-hole free and conformal layers, even suitable for deep-trench structures which have been widely used in the microelectronics industry [8]. In particular, O₂ plasma-enhanced ALD (PEALD) has proved its capability to deposit ultrathin BaTiO₃ films as well as modulating its crystallinity [9], [10]: O₂ PEALD is an energy-enhanced variant of ALD which utilizes oxygen plasma species as an oxidizer instead of water vapor widely used for conventional thermal ALD (T-ALD) [11]. Plasma generators using microwave plasma, electron cyclotron resonance plasma, or RF-driven inductively-coupled plasma (ICP) create O₂ radical species which renders greater flexibility in processing conditions and a wider range of materials to use in comparison to the conventional T-ALD [11]. Previously, we reported the deposition of ultra-thin Ba_xTi_yO_z (BTO) films with different Ba/Ti ratios using a commercialized PEALD station equipped with the RF-ICP generator, the FlexAL system (Oxford Instruments) [10]. Interestingly, BTO films with higher Ti content than Ba showed larger dielectric constants compared to other Ba-to-Ti stoichiometries (while other deposition conditions were kept the same) [10]. One plausible explanation for the higher dielectric constants is that Ti-rich BTO films are more likely to induce crystallized grains inside of the thin films (under the same plasma power and duration), eventually beneficial for improving throughput. The Ti-rich BTO film, however, suffers from higher leakage currents due to the existence of grain boundaries formed during the crystallization process [4].

One of the promising approaches to minimize leakage currents as well as sustaining the intrinsic benefits of Ti-rich BTO films, described above, is the doping of higher band gap (E_g) material into the thin films. Previously, Sn₂O₃ doping into BTO (>0.4 wt%) showed an anomalous increase in electrical resistivity, resulting in a highly insulating dielectric [12]. Another oxide, Al₂O₃ with the same oxidation number of +3 as Sn₂O_3, proved its effectiveness to reduce leakage currents when it was doped into TiO₂ [4], ZrO₂ [13], [14], [15], [16], and HfO₂ [13]. Especially, the concept of Al₂O₃ incorporation into ZrO₂, widely known as the ZrO₂/Al₂O₃/ZrO₂ (ZAZ)-type dielectric has been realized in DRAMs down to 45 nm pitch size due to the following benefits: ZAZ achieves a moderate dielectric constant (ε = ∼39) while obtaining a low leakage current (2.11 × 10⁻⁶ A/cm² at +2 V) by controlling crystallization to a moderate amount, i.e., intermixing of crystalline and amorphous phases, which suppresses the formation of pathways for electrical leakage. However, even for increasingly integrated DRAM architectures where enlarged electrodes based on the concept of trench-type capacitors will no longer be used, alternative materials with higher dielectric constants (ε > 40) should be utilized [17].

In this study, we demonstrate the PEALD of Al-doped BTO thin films with high dielectric constants (up to ∼43) and the ability to tune the dielectric constant as well as electric leakage currents by i) modulating the amount of Al-doping and ii) facilitating oxygen plasma as a post-treatment. Based on the observations of the changes in crystallinity and electronic structures, we conclude that Al-doping plays a crucial role in controlling the crystallinity, where a trade-off between dielectric constants and leakage currents exists, similar to the ZAZ dielectric case. Despite this trade-off, Al-doping into BTO showed considerable reduction in leakage currents with a minimal sacrifice of the dielectric constants.