HAFNIUM TETRAKIS(DIALKYLAMIDES)

The metal alkylamide complexes [Hf(NR2)4] have been shown to be promising precursors for the CVD deposition of HfO2 films [[i], [ii]]. Commonly available Hf complexes with amido ligands are tetrakis(dimethylamido)hafnium (TDMAH), tetrakis(ethylmethylamido) hafnium (TEMAH) and tetrakis(diethylamido)hafnium (TDEAH). Successful ALD of HfO2 [, [iii]].and Hf3N4 [736] from such hafnium amides has been previously reported

[i] Y.Ohshita, A. Ogura, A. Hoshino, S. Hiiro, H.Machida, J. Cryst. Growth, 233, 292 (2001)

[ii] Y. Ohshita, A. Ogura, A. Hoshino, S. Hiiro, T. Suzuki, H. Machida, Thin Solid Films, 2002, 406, 215.

[iii] A. Despande et al., J. Vac. Sci. Tech.,vol A (2004), 22(5), 2035–2040,“Atomic layer deposition and characterization of hafnium oxide grown on silicon from tetrakis(diethylamino)hafnium and water vapour”.

Hafnium tetrakis-dimethylamide Hf(NMe2)4 (TDMAH)

Fig. Comparison growth rates of HfO2 from [Hf(mmp)4] (•) and [Hf(NMe2)4]2 (■)

Although hafnium tetrakis-dimethylamide Hf(NMe2)4  is a liquid at RT, its molecular structure in solid state could be determined by single-crystal XRD at low temperatures. It is dimeric in solid state with two [Hf(NMe2)4] units joined by two bridging NMe2 groups. (Table ) The average Hf-N bond distances are slightly shorter then the corresponding bonds in the zirconium analogue [735] The Hf-Hf distance of TDMAH is slightly shorter than the zirconium analogue as well, and is consistent with a non-bonding interaction. The hafnium metal centre is only five coordinate, which makes it highly susceptible to reactions with oxygen or water. The relatively low coordination number of five at the Hf centre makes the compound difficult to handle without suitable inert atmosphere techniques, and decreases it shelf life. [[i]]

A comparison of [Hf(NMe2)4]2 with alkoxide precursor [Hf(mmp)4] wascdone by J.L. Roberts; it was shown that [Hf(NMe2)4]2 allows to deposit oxide films at lower temperatures and over a wider temperature range than [Hf(mmp)4].[764] Both compounds give oxide growth rates over 0.3 μm/h at moderate evaporator temperatures. For [Hf(mmp)4] the oxide growth rate increases at 300 - 400°C, (kinetic control). The onset of HfO2 growth with [Hf(NMe2)4]2 occurs ~100°C lower than for [Hf(mmp)4] with kinetic control region at 200 - 300°C. The oxide growth rate for [Hf(mmp)4] reaches a maximum in the region 400 - 500°C, corresponding to a narrow region of diffusion-controlled growth from a fully decomposed precursor. For the [Hf(NMe2)4]2 precursor, the region of diffusion-controlled growth occurs over a much wider temperature range (300-500°C). At temperatures of 550°C and higher the oxide growth rate decreases for both precursors, due to their depletion in the gas phase and on the reactor walls due to radiative heating from the substrate. Both precursors give HfO2 films with a columnar structure.

[i] J.L. Roberts, P.A. Williams, A.C. Jones, P. Marshall, P. R. Chalker, J.F. Bickley, H.O. Davies, L.M. Smith, Mat. Res. Soc. Symp. Proc., Vol. 745, 2003

Hafnium tetrakis-ethylmethylamide Hf(NEtMe)4 (TEMAH)

Hf(NMeEt)4 for Hf3N4 by ALD

Hafnium amides: Hf(NMeEt)4, and for comparison Hf(NMe2)4, Hf(NEt2)4, have been used for ALD growth of thin films of insulating Hf3N4 at low substrate temperatures (150-250 °C). [736] The films could be deposited with 100% step coverage in deep trenches (aspect ratio 1:40).

Hf(NMeEt)4 for HfO2 by ALD

TEMAH have been used as a precursor for ALD deposition of HfO2 thin films at 160-420°C, using ozone O3 or H2O as co-reactant. Film thickness uniformities <1% and step coverage ~100% for trenches with aspect ratio 40:1 have been achieved. The optimal deposition temperature was 320°C, with atomic ratio Hf:O = 1:2.04. Lower carbon and hydrogen levels were obtained in the films deposited with H2O co-reactant than in the films grown  with O3. [[i]] Other reports are as well confirming hafnium alkylamides and H2O as precursors for HfO2 films with 100%step coverage and excellent electrical properties. [[ii], [iii]]

[i] X. Liu, S. Ramanathan, A. Longdergan, A. Srivastava, E. Lee, T. E. Seidel, J. T. Barton, D. Pang, R. G. Gordon, J.Electrochem. Soc., 152 (3) G213-G219, 2005

[ii] J. H. Lee, Tech. Dig. - Int. Electron Devices Meet., 2002, 9.1.

[iii] R.G. Gordon, D. Hausmann, E. Kim, and J. Shepard, Chem. Vap. Deposition, 9 (2),  73 (2003)

Hafnium tetrakis-diethylamide Hf(NEt2)4 (TDEAH)

A comparison of thermal characteristics of different Hf amide (TDEAH, TEMAH, TDMAH) and alkoxide precursors was done by Air Liquide researchers (Fig., Table)

            TDEAH and TEMAH have relatively low vapor pressure, permitting less vapor density to be introduced into the deposition chamber. Thus, for a comparable vapor density as the other Hf precursors, higher vaporization temperatures will be required for TDEAH and TEMAH. However, an upper temperature limit will be imposed by thermal stability of the precursor and this is where complementary thermal characterization of the precursor is necessary. TDMAH and Hf(OtBu)4 have higher vapor pressures than the other two organometallic precursors examined in this study. From vapor pressure data, these precursors may be better candidates to meet the step-coverage requirements for DRAM deep-trench structures with high aspect ratios since it requires pulse times lasting several seconds of residence time to enable the molecules to reach the bottom of deep trenches. For TDMAH, the downside is in the difficulties associated with handling and storage (low melting point solid that requires freezer storage). [19]

The comparison of TGA patterns of Hf(OtBu)4, TDEAH , TDMAH and TEMAH at atmospheric pressure under open crucible conditions and 10°C/min temperature ramp are presented in Figure . The non-volatile residues for HTB, TDEAH, TEMAH and TDMAH are 2.0%, 7.3%, 2.1% and 3.4% respectively. This may indicate that for increasing temperature and atmospheric pressure, TDEAH has the highest decomposition that occurs before evaporation and may, as a result, entail clogging issues. TDMAH has the lowest 50% mass loss temperature (148°C), but its non-volatile residue was slightly higher than that of HTB and TEMAH, indicative of some partial decomposition even before 148°C. Thermal transition points are assessed using DSC. For TDMAH, it reveals a melting point at about 40°C. Nonvolatile residue amounts using DSC (closed crucible) are higher, e.g. 17% for TDEAH and 10% for TEMAH, as a result of longer evaporation time, allowing the precursor to reach a higher temperature that can lead to decomposition. Hf(OtBu)4 has much lower non-volatile residue even with a closed crucible (<2%) due to its much higher volatility that enables evaporation before decomposition. [19] From TGA data the thermal stability of precursors follows this order: TDEAH < TDMAH < TEMAH~ Hf(OtBu)4~ Hf(OtBu)2(mmp)2

Hf(NEt2)4 for HfSiO by MOCVD

Nitrogen-based combination of liquid at RT precursors Hf(NEt2)4 and Si(NMe2)4 is also suitable for HfSiO deposition [538] A significant growth rate increase (a kind of synergistic behaviour) was observed when Hf precursor was added to Si precursor. As the phenomenon was observed in both precursor systems, the simplest explanation is not the gas phase /surface interactions of precursors, but the activation of oxidizers on the surface by the presence of HfO2 because this is the common element in both processes.

Growth rate of HfSiO with Hf and Si amide system is insensitive to temperature below 500ºC. As temperature increases, SiO2 dep. rate is increasing, while HfO2 dep. rate is decreasing. The contamination with nitrogen was below detection detection (0.5%),  carbon was also mostly ≤detection (0.4-0.5%), maximum 1.2 at% [545]. Cross-sectional TEM studies revealed that interfacial layers were thin (<10Å) both with amide and O-containing precursors and could be graded in composition. (Fig.) [[i]] Interfacial layer with amide precursor system was minimal

 [i] Bevan, et al., SSIC 2001

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