HAFNIUM (IV) ALKOXIDES
The use of hafnium (IV) alkoxides [Hf(OR)4] in the deposition of HfO2 thin films is limited. The reason for this is oligomeric nature of the majority of [Hf(OR)4] complexes, which leads to their limited volatility. Hafnium alkoxides form oligomers [Hf(OR)4]x due to the tendency of Hf(IV) atoms to expand their coordination sphere to six, seven or eight [23]. In order to inhibit oligomerization, bulky, sterically hindered ligands such as tert-butoxide have been employed, resulting in the volatile mononuclear alkoxide [Hf(OtBu)4], which has been used for the MOCVD of HfO2 [751]. However, this precursor contains an unsaturated four-coordinate metal centre, and the tert-butoxide ligand undergoes catalytic hydrolytic decomposition in the presence of trace water [[i]]. As a consequence, this complex is extremely air- and moisture- sensitive, which limits its shelf life and makes it difficult to handle and use in MOCVD.
The sterically hindered ligand 1-methoxy-2-methyl-2-propanolate (OCMe2CH2OMe, mmp) has been shown to inhibit the oligomerization of Zr and Hf alkoxides which has allowed to synthesize the monomeric complexes [Hf(OtBu)2(mmp)2] [14], and [Hf(mmp)4] [15], which were successfully applied as precursors for MOCVD films. The hafnium metal centre in Hf(mmp)4 is six coordinate and is less susceptible to attack by water or oxygen due to the steric protection offered by the mmp ligand. [15]
[i] D.C. Bradley, Chem. Rev., 89, 1317 (1989)
Hafnium tetrakis(1-methoxy-2-methyl-2-propanolate) [Hf(mmp)4]
Hafnium tetrakis(1-methoxy-2-methyl-2-propanolate) [Hf(mmp)4] was characterized by single-crystal XRD, its crystal structure was determined. The complex is volatile and was successfully applied as a precursor for the growth of HfO2 films by MOCVD and ALD [[i]]
[i] A.C. Jones, H.C. Aspinall, P.R. Chalker, R.J. Potter, K.Kukli, Antti Rahtu, Mikko Ritala and Markku Leskelä, J. Mater. Chem., 2004, 14, 3101-3112; DOI: 10.1039/B405525J (see Gd(thd)3(tetraglyme), « Crystal structure of a volatile hafnium alkoxide precursor, Hf(OCMe2CH2OMe)4, used to deposit HfO2 by MOCVD and ALD. »
[Hf(OtBu)2(mmp)2] for HfO2 by MOCVD
[Hf(OtBu)2(mmp)2] dissolved in octane was applied as precursor for the growth of thin HfO2 films on (100)Si/SiO2 substrates by liquid-injection MOCVD. HfO2 layers of thickness below 10 nm were grown (with thickness precise control by accurate control of the volume of injected precursor solution); films were characterised by XRR, AFM, attenuated total reflection IR spectroscopy (ATR). Films grown <350°C were X-Ray amorphous and polycrystalline at growth temperature ≥ 360 °C, except for ultrathin films. The main crystalline phase was monoclinic, as determine by ATR. Smooth surfaces for both amorphous and polycrystalline films were grown by AFM. Relative dielectric permittivity ϵr =19 was obtained, in agreement with monoclinic HfO2 phase; EOT measured in MOS structures depended linearly on film thickness. Films with a physical thickness of 3.1 nm had leakage current densities ~1×10–4 A cm–2. Fixed charges in the oxide (Qox < 1×1011 cm–2) as well as the interfacial charge density (Dit peak = 8×1010 eV–1 cm–2) were reduced by rapid thermal annealing under NH3. [[i]]
[i]C. Dubourdieu, E. Rauwel,C. Millon,P. Chaudouët, F. Ducroquet,N. Rochat, S. Rushworth, V. Cosnier, Chemical Vapor Depositon, Volume 12, Issue 2-3, March, 2006 , Pages 187–192, DOI: 10.1002/cvde.200506397, “Growth by Liquid-Injection MOCVD and Properties of HfO2 Films for Microelectronic Applications”
Hafnium bis(tert-butoxide) bis (methoxymethylpropanolate) [Hf(OtBu)2(mmp)2]
[i] K. Kubli et al., Chemical Vapor Deposition (2003), “ALD of HfO2 films using Hf(OtBu)2(mmp)2 and water”.
[ii] J. Kim et al., J. Electrochem. Soc., 152 (4) F45–F48 (2005), “Atomic Layer Chemical Vapor Deposition and Electr ical Character ization of Hafnium Silicate Films”.
Hafnium tetrakis(tert-butoxide) Hf(OtBu)4
Hafnium tert(butoxide) Hf(OtBu)4 has been applied as precursor for the in situ deposition of ultra thin HfO2 films by rapid thermal CVD at 400°C, using O2 as oxidiser and N2 as carrier gas, which were applied as dielectric layers for the preparation of TaN/HfO2/Ge gate stack having EOT of 12.9Å and exhibiting excellent leakage current density of 6mA/cm2 @Vg=1V and interface state density (Dit) of 8x1010/cm2-eV. [[i]]
[i] W. P. Bai, N. Lu, J. Liu, A. Ramirez, D. L. Kwong, D. Wristers, A. Ritenour, L. Lee, D. Antoniadis, http://sauvignon.mit.edu/fitz/papers/2003/Bai%20VLSI%202003.pdf , “Ge MOS Characteristics with CVD HfO2 Gate Dielectrics and TaN Gate Electrode”
Comparison of [Hf(OtBu)4] and [Hf(OtBu)2(mmp)2]
The comparison of thermal characteristics of [Hf(OtBu)4] and [Hf(OtBu)2(mmp)2] was done by AirLiquide (Fig.) [19] Hafnium tert-butoxide exhibits clean evaporation, while Hf(OtBu)2(mmp)2 shows more complex behaviour including simultaneous evaporation and partial decomposition
Hafnium tetrakis (2-methyl-3-butene-2-olate) Hf(OCMe2-CH=CH2)4
Hafnium tetrakis (2-methyl-3-butene-2-olate) Hf(OCMe2-CH=CH2)4 is volatile and has been considered as potential ALD precursor for the growth of HfO2 thin films
Hf(OCMe2-CH=CH2)4 has been synthesized by reaction of HfCl4 and 2-methyl-3-butene-2-ol in the presence of EtNH2 in hexane, first below −10° C, then RT and finally at 50° C; the solution was filtered and the filtrate was concentrated and distilled under reduced pressure; . A colorless transparent liquid was obtained as a fraction boiling at 79-80°C/ 0.11-0.12 Torr. (Yield: 35.4%), and characterized by elemental analysis and 1 H-NMR. [[i]]
Vapor pressure of the complex has been determined, it obeys the Clapeyron equation : Log P (Torr)=6.83−2641/ T ( K )
[i] S. Wada et al, US 2008/0187662, Aug 7, 2008 , METAL ALKOXIDE COMPOUND, MATERIAL FOR FORMING THIN FILM,AND METHOD FOR PRODUCING THIN FILM
Hafnium 3-Methyl-3-pentoxide Hf(mp)4
Hafnium 3-Methyl-3-pentoxide Hf(mp)4 , or Hf(OCMeEt2) is a liquid at room temperature and has a moderate vapor pressure comparable to that of hafnium tert-butoxide, Hf(OtBu)4, and a lower residual weight (<10%) in TGA analysis.
Hf(mp)4 was applied as single-source precursor (with no additional oxygen source) for the deposition of HfO2 layers on Si substrates by CVD; The deposition rate was 2.7 nm/min at 400 °C, the activation energy was 68.1 kJ/mol - higher than with other hafnium alkoxide and hafnium amide precursors. The deposition mechanism was determined – it was clearly demonstrated that HfO2 layers are grown via β-hydrogen elimination processes of the Hf(mp)4 single precursor, according to gas chromatography/mass spectroscopy and NMR studies of the products of thermal decomposition collected during the growth of HfO2 layers,. The absence of other decomposition mechanisms and/or recombination processes was confirmed by the negligible carbon incorporation into the deposited HfO2 layers, as was found by the XPS and depth-profiling AES studies. The morphology (by SEM), crystallinity (by XRD), and electrical properties (by capacitance−voltage and CV measurements) of the grown HfO2 films were determined. [[i]]
[i] Taek S. Yang, Ki-Seok An, Eun-Joo Lee, Wontae Cho, Hong S. Jang, Sun K. Park, Young K. Lee, Taek-Mo Chung, Chang G. Kim, Sungmoon Kim, Jin-Ha Hwang, Choongkeun Lee, Nam-Soo Lee, and Yunsoo Kim, Chem. Mater., 2005, 17 (26), pp 6713–6718, DOI: 10.1021/cm050662t, « Chemical Vapor Deposition of HfO2 Thin Films Using the Novel Single Precursor Hafnium 3-Methyl-3-pentoxide, Hf(mp)4“
Hafnium tetrakis[2-(4,4-dimethyloxazolinyl)-propanolate] Hf(dmop)4
Hafnium tetrakis[2-(4,4-dimethyloxazolinyl)-propanolate] Hf(dmop)4 has been applied as precursor for the growth of thin films of HfO2 at substrate temperature 350-650 °C by liquid injection metal-organic MOCVD. The crystal structure of Hf(dmop)4 has been determined by single-crystal XRD, and was demonstrated to be a six-coordinate monomeric complex. [Hf(dmop)4] is volatile and is significantly less reactive to air and moisture than simple Hf alkoxide complexes such as [Hf(OtBu)4]. The HfO2 layer deposited at ≤400 °C were demonstrated to be X-Ray amorphous XRD, but at >450 °C substrate temperatures above, the films became monoclinic (α-HfO2) with a growth temperature-dependent fiber texture. The grown HfO2 layers were non-stoichiometric and contained residual C (2.1 – 11.6 at.-%) and N (0.4 – 1.9 at.-%) impurities, according by AES. [[i]]
[i] Y. F. Loo, R. O'Kane, A. C. Jones, H. C. Aspinall, R. J. Potter, P. R. Chalker, J. F. Bickley, S. Taylor, L. M. Smith, Chemical Vapor Deposition, Volume 11, Issue 6-7, pages 299–305, July, 2005 , DOI: 10.1002/cvde.200506384, “ Deposition of HfO2 Films by Liquid Injection MOCVD Using a New Monomeric Alkoxide Precursor, [Hf(dmop)4]”
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