Atomic layer deposition of TiO₂ and Al‐doped TiO₂ films on Ir substrates for ultralow leakage currents

Abstract

Rutile structured TiO 2 , which has a dielectric constant value ~100 is attracting considerable interest as an alternative dielectric material for sub-30 nm technology dynamic random access memory capacitors [1][2][3][4]. Even though it is not the thermodynamically stable phase at the typical ALD temperature (<300 °C), in-situ formed thin RuO 2 layer induce the formation of rutile TiO 2 films on Ru substrates by atomic layer deposition (ALD) when using O 3 , O 2 or N 2 O plasma as the oxygen sources [1,2,5,6]. In addition, doping Al ions into the rutile TiO 2 (ATO) decreases the leakage current of the capacitor remarkably because the Al ions as an acceptor lower the Fermi level of the films by compensating for the n-type carrier in the films [7,8]. The minimum equivalent oxide thickness (t ox ) with a stable low leakage current density (J < 10 -7 A cm -2 at 0.8 V) as low as ~0.8 nm and ~0.5 nm, respectively, have been achieved on Ru substrates [7]. On the other hand, further decreases in t ox result in a very high J level. This has been attributed to the low work-function of Ru (~4.7 eV), even though RuO 2 has a higher work-function of ~5.0 eV. The interface between the TiO 2 (or ATO) and RuO 2 is believed to have a sufficient Schottky barrier when the RuO 2 has bulk properties. On the other hand, it has been reported that the workfunction of an extremely thin metal is largely influenced by contact with another metal layer [9,10]. Considering the very thin thickness (<2 nm) of the interposed RuO 2 layer, it is highly likely that the work-function of that layer would be lower than that of the bulk value.Therefore, IrO 2 /Ir could be an interesting electrode material system to study because the work-function of IrO 2 is only ~4.2 eV, whereas that of Ir is as high as ~5.3 eV. Furthermore, IrO 2 has an identical ion arrangement to rutile TiO 2 . This structural compatibility can lead to the formation of rutile TiO 2 films, even on Ir substrates, if the IrO 2 layer exists at the interface.It is expected that the leakage current performance of the dielectric film on the IrO 2 /Ir electrode would be degraded compared to RuO 2 /Ru if the bulk work-function of IrO 2 is realized in the structure. On the other hand, the performance would be improved if the influence of the backing Ir electrode dominates the electrode properties of the thin IrO 2 layer.Therefore, in the present study, TiO 2 and ATO films were grown on 100-nm-thick Ir substrates instead of Ru substrates using an ALD method where the O 3 was adopted as the oxygen source. The structural and electrical properties of the dielectric film and intervening IrO 2 layer at the interface were examined.TiO 2 and ATO films were grown in a travelling-wave type reactor (CN-1 Co., Plus-100) by ALD at a wafer temperature of 250 °C using titanium tetrakis isopropoxide (TTIP) and trimethlyaluminum (TMA) as the Ti and Al TiO 2 and Al-doped TiO 2 (ATO) films were grown on Ir substrates by atomic layer deposition using O 3 as the oxygen source. With increasing O 3 feeding time, th...

Impact of Bimetal Electrodes on Dielectric Properties of TiO2 and Al-Doped TiO2 Films

“…It was recently reported that a very thin conducting layer interposed between a thicker metal electrode and an insulator layer does not realize the bulk work function of the thin conducting layer. 12 It is likely that the deterioration in the leakage properties of the ATO film on thinner Ru/TiN results from lowering the work function of the Ru layer through the influence of the lower TiN electrode.…”

“…13,14 Ir also forms IrO 2 , of which structure is also rutile forming the rutile phase TiO 2 on top, during the ALD of TiO 2 or ATO. 12 However, forming interfacial IrO 2 requires an extended O 3 pulse time, which is too long in order to make the ALD process practical. The Pt does not form any oxide, which is structurally compatible with the rutile TiO 2 .…”

“…The crystalline structure and the dielectric constant of the ATO films are identical on both (RuO 2 )/Ru/TiN electrodes, suggesting that the difference in the leakage properties of the ATO films is not from the dielectric film itself. It was recently reported that a very thin conducting layer interposed between a thicker metal electrode and an insulator layer does not realize the bulk work function of the thin conducting layer . It is likely that the deterioration in the leakage properties of the ATO film on thinner Ru/TiN results from lowering the work function of the Ru layer through the influence of the lower TiN electrode.…”

“…Very thin Ru films (2–7 nm) were grown on the Pt and Ir electrodes by ALD for the formation of rutile structured TiO 2 with a high dielectric constant. It was reported that continuous and smooth thin Ru films grow well on metallic surfaces by ALD due to the fluent nucleation of the Ru films on the top. , Ir also forms IrO 2 , of which structure is also rutile forming the rutile phase TiO 2 on top, during the ALD of TiO 2 or ATO . However, forming interfacial IrO 2 requires an extended O 3 pulse time, which is too long in order to make the ALD process practical.…”

Recent advances in the understanding of high-kdielectric materials deposited by atomic layer deposition for dynamic random-access memory capacitor applications

“…The deposition of TiO 2 on RuO 2 is not the only case for crystallinity coherency of dielectric with the electrode. As the same manner, IrO 2 , MoO 2 , and SnO 2 also can induce the rutile-phased TiO 2 thin film during the ALD process [50,75,76,77]. Consequently, using a substrate that has crystal structure coherency is one of the most effective ways of acquiring high dielectric constants in deposited dielectric thin films.…”