Atomic layer deposition of amorphous hafnium-based thin films with enhance thermal stabilities
The continuous scaling of microelectronic devices requires high permittivity (high-k) dielectrics to replace SiO₂ as the gate material. HfO₂ is one of the most promising candidates but the crystallization temperature of amorphous HfO₂ is too low to withstand the fabrication process. To enhance the film thermal stability, HfO₂ is deposited using atomic layer deposition (ALD), and incorporated with various amorphizers, such as La₂O₃, Al₂O₃, and Ta₂O₅. The incorporation is achieved by growing multiple ALD layers of HfO₂ and one ALD layer of MO[subscript x] (M = La, Al, and Ta) alternately (denoted as [xHf + 1M]), and the incorporation concentration can be effectively controlled by the HfO₂-to-MO[subscript x] ALD cycle ratio (the x value). The crystallization temperature of 10 nm HfO₂ increases from 500 °C to 900 °C for 10 nm [xHf + 1M] film, where x = 3, 3, and 1 for M = La, Al, and Ta, respectively. The incorporation of La₂O₃, and Ta₂O₅ will not compromise the dielectric constant of the film because of the high-k nature of La₂O₃, and Ta₂O₅. Angle resolved X-ray photoelectron spectroscopy (AR-XPS) reveals that when the HfO₂-to-MO[scubscript x] ALD cycle ratio is large enough (x > 3 and 4 for La and Al, respectively), periodic structures exist in films grown by this method, which are comprised of repeated M-free HfO₂ ultrathin layers sandwiched between HfM[subscript x]O[scubscript y] layers. Generally, the film thermal stability increases with thinner overall thickness, higher incorporation concentration, and stronger amorphizing capability of the incorporated elements. When the x value is low, the films are more like homogeneous films, with thermal stabilities determined by the film thickness and the amorphizer. When the x value is large enough, the periodically-repeated structure may add an extra factor to stabilize the amorphous phase. For the same incorporation concentration, films with an appropriately high periodicity may have an increased thermal stability. The manner by which the periodic structure and incorporated element affect thermal stability is explored and resolved using nanolaminates comprised of alternating layers of [scubscript y]HfO₂ and [xHf + 1M] × n, where y varied from 2 to 20, x varied from 1 to 2, and n varied from 4 to 22.