Atomic Layer Deposition of High-Permittivity Yttrium-Doped HfO[sub 2] Films

Abstract

Yttrium-doped HfO 2 films were grown by atomic layer deposition via alternating HfO 2 and Y 2 O 3 growth cycles. Precursors used were ͑CpMe͒ 2 Hf͑OMe͒Me or Hf͑NEtMe͒ 4 and ͑CpMe͒ 3 Y together with ozone. The 5-8 nm thick HfO 2 :Y films were amorphous in as-deposited state and crystallized as high-permittivity cubic/tetragonal polymorphs upon annealing. The best combination of low leakage current of 10 −7 A/cm 2 at 1 V and high capacitance was achieved with the films grown from Hf͑NEtMe͒ 4 , with yttrium content being about 6-7 atom %. The highest permittivity values measured for these films reached 30.Hf-based high-permittivity ͑high-k͒ oxides, especially those deposited by atomic layer deposition ͑ALD͒, have been extensively researched for complementary metal oxide semiconductor devices, and recently they have been adapted in production of the most advanced microprocessors. 1 Capacitor dielectrics in dynamic random access memory devices also have demanding requirements for capacitance equivalent oxide thickness ͑CET, also t ox is used͒ and leakage current, 2,3 and among others, 4,5 Hf-based materials are strong candidates there too.As the permittivity of HfO 2 strongly depends on its crystallographic phase, it is of utmost importance to obtain HfO 2 films in cubic or tetragonal polymorphs, whose permittivity values exceed 30, instead of the low ͑ϳ18͒ permittivity monoclinic phase. 6,7 One solution for obtaining the higher permittivity phases is to incorporate small amounts ͑Ͻ20 atom %͒ of other elements into the host HfO 2 film. 8 HfO 2 has been doped with Si, 9 Al, 10 N, 11 Sr, 12 Gd, and Er, 13 as well as Dy, 13,14 Sc, 14 and Y. 15,16 For example, permittivity values of over 25 have been reported for sputtered yttrium-doped HfO 2 ͑HfO 2 :Y͒ films with low yttrium content of 4 atom %. 15 However, the key technology for dielectric film growth is ALD, recognized by the semiconductor industry to be capable of growing ultrathin films with perfect conformality and thickness accuracy. 17,18 The most important factor in developing a successful ALD process is the choice of precursors. For the multicomponent oxide film growth, such as HfO 2 :Y, feasible processes for both metal oxides, HfO 2 and Y 2 O 3 , should be available. The growth temperature can strongly affect the dielectric properties due to variations in the film purity and crystal growth intensity. Because in an ALD process the precursor decomposition should be avoided, the applicable temperature window often remains narrow.In this study, high-k HfO 2 :Y films were grown by ALD. In order to compare the effect of precursor chemistry and growth temperature, an alkylamido-type precursor, Hf͑NEtMe͒ 4 ͑TEMAHf͒, was employed at a temperature of 275°C, while a cyclopentadienyl-type precursor, ͑CpMe͒ 2 Hf͑OMe͒Me, was applied at 350°C. TEMAHfbased processes are known to suffer from thermal decomposition of the metal precursor at temperatures around 300°C, 19 whereas ͑CpMe͒ 2 Hf͑OMe͒Me exhibits self-limited growth even at 400°C. 20 For yttrium doping, a precursor behav...

Ferroelectricity in yttrium-doped hafnium oxide

“…[9][10][11][12][13][14][15][16][17] However, considering the occurrence of ferroelectricity in Y:HfO 2 thin films, a simple monoclinic=cubic phase mixture or intermediate tetragonal phases seem contradictory, since neither the cubic=tetragonal nor the monoclinic phases in HfO 2 possess ferroelectric space groups. 29 A close examination of the GI-XRD diffractograms presented in Fig.…”

“…7 Analogous to the well-known yttrium stabilized zirconia (YSZ), 8 successful stabilization of the cubic phase in epitaxial, as well as polycrystalline Y:HfO 2 thin films has been presented by several authors. [9][10][11][12][13][14][15][16][17] Reported doping levels for complete stabilization of the cubic phase in HfO 2 range from 4 to 17 mol% YO 1.5 admixture, thereby achieving k-values of about 18 to 32.…”

Growth and phase stabilization of HfO2 thin films by ALD using novel precursors

“…Increasing the deposition temperature causes crystallization which is further enhanced at 350 1C. The tetragonal or cubic reflection (1 1 1) is visible [7,24], but with considerably lower intensities than the monoclinic reflections. The other applied precursor, CpHf(NMe 2 ) 3 yields identical diffractograms (not shown).…”

“…It is important to note that a mixture of monoclinic and cubic or tetragonal phases tends to develop very early already in asdeposited HfO 2 [24], even in the thickness range of 4-10 nm, most relevant to the microelectronic applications. In addition, post- deposition heat treatment may further induce recrystallization, but unfortunately still not driving the film into a single phase [24].…”

“…It is important to note that a mixture of monoclinic and cubic or tetragonal phases tends to develop very early already in asdeposited HfO 2 [24], even in the thickness range of 4-10 nm, most relevant to the microelectronic applications. In addition, post- deposition heat treatment may further induce recrystallization, but unfortunately still not driving the film into a single phase [24]. Furthermore, it is also important to note that HfO 2 should not be stabilized in the thermodynamically stable monoclinic polymorph, but in the cubic or tetragonal phase due to their higher dielectric constant [2].…”