Atomic Layer Deposition of SrTiO₃ Thin Films with Highly Enhanced Growth Rate for Ultrahigh Density Capacitors

Abstract

The ever-shrinking dimensions of dynamic random access memory (DRAM) require a high quality dielectric film for capacitors with a sufficiently high growth-per-cycle (GPC) by atomic layer deposition (ALD). SrTiO 3 (STO) films are considered to be the appropriate dielectric films for DRAMs with the design rule of ∼20 nm, and previous studies showed that STO films grown by ALD have promising electrical performance. However, the ALD of STO films still suffers from much too slow GPC to be used in mass-production. Here, we accomplished a mass-production compatible ALD process of STO films using Ti(O-i Pr) 2 -(tmhd) 2 as a Ti-precursor for TiO 2 layers and Sr( i Pr 3 Cp) 2 as a Sr-precursor for SrO layers. O 3 and H 2 O were used as the oxygen sources for the TiO 2 and SrO layers, respectively. A highly improved GPC of 0.107 nm/unit-cycle (0.428 nm/supercycle) for stoichiometric STO films was obtained at a deposition temperature of 370 °C, which is ∼7 times higher than previously reported. The origin of such high GPC values in this STO films could be explained by the partial decomposition of the precursors used and the strong tendency of water adsorption onto the SrO layer in comparison to the TiO 2 layer. The STO film grown in this study also showed an excellent step coverage (∼95%) when deposited inside a deep capacitor hole with an aspect ratio of 10. Owing to the high bulk dielectric constant (∼ 146) of the STO film, an equivalent oxide thickness of 0.57 nm was achieved with a STO film of 10 nm. In addition, the leakage current density was sufficiently low (3 Â 10 À8 Acm À2 at þ0.8 V). This process is extremely promising for fabrication of the next generation DRAMs.

Improved Initial Growth Behavior of SrO and SrTiO3 Films Grown by Atomic Layer Deposition Using {Sr(demamp)(tmhd)}2 as Sr-Precursor

“…13 After these findings, the authors' group has focused on a high-temperature (>350°C) ALD process by adopting precursors that can withstand high temperatures, such as Ti(O i Pr) 2 (tmhd) 2 as a Ti-precursor. 1,2,14 In contrast, a research group in Interuniversity Microelectronics Centre (IMEC) focused on the low temperature ALD (∼250°C) process to make use of the facile ALD reactions between the Sr( t Bu 3 Cp) 2 and Ti(OMe) 4 with H 2 O, where Bu and Me correspond to butyl and methyl groups, respectively, which required PDA at a higher temperature to crystallize STO films. 4,15,16 Similar to the current authors' case, such high-temperature PDA induced micro-and nanocracking problems; as a result, they introduced a double-layer structured ALD process.…”

Atomic layer deposition of strontium titanate films from Sr(iPr3Cp)2, Ti[N(CH3)2]4 and H2O

“…Ideally the Sr ion is adsorbed to the surface by chemically bonding to an OH group under splitting off one Cp ring, where all propyl groups remain on the Cp ring. Because depositions take place at the lower temperature limit for the precursor decomposition [36], it is possible that after adsorption of the Sr( i Pr 3 Cp) 2 molecule on the OH-terminated surface, bonds do not break between the center Sr atom and the remaining Cp ring, where an activation energy of E a = 2.58 eV is required, but instead, at least partially, only the lower-energy bond (E a = 1.48 eV) [30] between the nearest neighboring alkyl carbon and the next nearest neighboring alkyl carbon to the Cp ring opens. This could lead to a hydroxyl termination of the nearest neighboring alkyl carbon to the Cp ring.…”

“…1. The weak bond between the i Pr 3 Cp ligand and the Sr ion with a binding energy of E b = 2.58 eV guarantees a much higher growth rate or growth per cycle (GPC) of STO than other Sr-precursors like Sr(tmhd) 2 , with a binding energy between Sr and tmdh ligand of E b ≈ 4.94 eV [30]. Also it has a high vapor pressure above 300°C [36], which ensures the fluent delivery of the precursor into the ALD reactor.…”

Stoichiometry variation for the atomic layer deposition of SrxTiyOz from Sr(iPr3Cp)2, Ti[N(CH3)2]4 and H2O

“…Just recently, also cyclopentadienyl (Cp) based precursors like Cp*Ti(OMe) 3 [18] and (CpMe 5 )Ti(OMe) 3 [19] are described for the ALD of SrTiO 3 . Precursors used for the SrO half cycle are also based on either diketonates like Sr(rhd) 2 [13,17] and Sr(tmhd) 2 [20] or Cp ligands [10,12,15,21].…”

“…Ozone enables a higher growth per cycle but yields the formation of carbonates [14]. On the other hand, Cp ligands guarantee a much higher growth rate or growth per cycle (GPC) because of the weak bond between the Cp ligand and the Sr ion (E b = 2.58 eV for i Pr 3 Cp ligand [12]), which is much lower than the binding energy between Sr and diketonate ligands (E b ≈ 4.94 eV for tmhd (tetramethylheptanedionate) ligand [20]). Hence, ligands are removed easily during the chemisorption of the Sr( i Pr 3 Cp) 2 molecules.…”