COBALT ALKYLAMIDINATES

Cobalt N,N’-alkylamidinates with various substitute radicals have been synthesized by group of R. Gordon in Harvard university. These are volatile, thermally stable, having high reactivity at low deposition temperatures compounds. Some of them are liquid what makes them useful for direct liquid injection MOCVD and also facilitates  evaporation in case of conventional bubblers  (ALD, conventional MOCVD). The compounds are dark blue/green, air-sensitive solids or liquids.  

The variation of melting points of of different cobalt amidinates was explained by total number of flexible joints in the alkyl substituents  (Fig.). The presence of more flexible ethyl and n-butyl groups tends to lower the melting points, while higher melting amidinates possess more rigid tert-butyl and isopropyl substituents.

Synthesis of cobalt alkylamidinates

Cobalt amidinate compounds have been prepared by reaction of lithium salt of the amidine with with cobalt(II) chloride (in THF at -78°C). The intermediate lithium amidinates can be prepared either by adding an alkyllithium to a carbodiimide in THF at -78°C, or by addition of alkylnitrile to an alkyl chloride in the presence of iron(III) chloride (utilizing catalysts based on lanthanides (samarium) is also possible but it proved incapable of attaching sterically bulky tertbutyl sidegroups); the intermediate alkylnitrilium tetrachloroferrate salt is then reacted with a primary amine in the presence of NaOH, neutralized to release the free amidine, which is then coverted to its lithium salt by addition of butyllithium. The products are extracted from the lithium chloride byproduct with pentane or hexane followed by final purification by distillation or sublimation. [1055]

Characterization of cobalt amidinates

Fig. 1. Thermogravimetry of various Co amidinates: 1) Co(tBuNC(Me)NtBu)2 ; 2) Co(iPrNC(Me)NiPr)2 ; 3) Co(tBuNC(nBu)NEt)2 ; 4) Co(tBuNC(Et)NEt)2 ; 5) Co(tBuNC(nBu)NEt)2 . Temp. ramp rate 10 K/min, N2 flow 1 atm, sample size 10 mg.

Cobalt amidinates have been characterised by various analytical techniques including single crystal X-Ray diffraction which revealed monomeric structure with square planar Co coordination in Co(iPrNC(Me)NiPr)2 and Co(tBuNC(Me)NtBu)2 (bulky isopropyl and tert-butyl groups on both N and N’ ends of amidinate ligand are preventing oligomerization). By contrast, the smaller steric bulk of the ethyl group in cobalt bis(N-tert-butyl-N’-ethylacetamidinate) allows it to form a dimer in the solid state (although it appears to be monomeric in solution) and distorted tetrahedric Co coordination. [1055]

Ab initio (density functional theory) calculations indicated that the tetrahedron cobalt configuration in Co(iPrNC(Me)NiPr)2 and Co(tBuNC(Me)NtBu)2 is energetically preferred, the contradictory X-Ray data indicate energetical preference of distorted square planar coordination of Co in solid state related to denser packing effects in crystals.[[iv]]

 [iv] J. Li, J. Wu, Ch. Zhou, B. Han, X. Lei, R. Gordon, H. Cheng, Int. J. Quant.Chem., Vol 109, 756–763 (2009)

The proton NMR spectra of the products have large shifts and broad peaks due to their paramagnetic nature. The number of peaks are consistent with monomeric structures in solution.     

Thermogravimetry and thermal stability of cobalt alkylamidinates

Thermal stability of Co amidinates vs. temperature

Thermal stability of cobalt amidinates was studied by NMR after heating solutions in deuterobenzene sealed in heavy-walled NMR tubes, at temperatures from 100 to 200◦C. Half-lives of cobalt amidinates are presented in Fig.: the most stable compound is Co(tBuNC(Me)NtBu)2. Steric crowding falls in the same order 1 > 2 > 3 > 4 > 5 as the thermal stability): (1) Co(tBuNC(Me)NtBu)2 (Eact (dec) 91.0 kJ/mol); 2) Co(iPrNC(Me)NiPr)2 (Eact (dec)  90.9 kJ/mol); 3) Co(tBuNC(nBu)NEt)2 (Eact (dec)  79.8 kJ/mol);; 4) Co(tBuNC(Et)NEt)2 ;(Eact (dec) 89.8 kJ/mol);  5) Co(tBuNC(nBu)NEt)2)  (Eact (dec)  81.9  kJ/mol).

  Thermogravimetric analyses of the compounds (Fig.) revealed complete evaporation in single steps without measurable residue. [1055]

Vapor pressures and vaporization rates of cobalt amidinates

  Vapor pressures and vaporization rates are high enough for use in vapor deposition at source temperatures below 80◦C, where the lifetimes are over one year. Cobalt amidinates have been successfully used in the vapor deposition of cobalt metal, as well as Co nitrides and oxides. [[i]]

    Vaporization rates of cobalt amidinates measured in 1 atm of flowing nitrogen gas are presented in Fig. The vaporization rate generally decreases with increasing molecular mass (Fig. , data taken at 80 ◦C).

 [i] R. G. Gordon, “ALD or CVD of Pore-Sealing, Barrier, Adhesion and Seal Layers for Interconnects”, Short Course at the Advanced Metallization Conference,Albany,NY, USA, (2007), published on CDROM, University of California at Berkeley Extension, Engineering, Berkeley, CA, USA, 2007

Cobalt bis(N,N’-diisopropylacetamidinate [613,1034 ,[ii]] , cobalt bis(N,N-ditert-butylacetamidinate [1034] are solids. Recently synthesized asymmetric Co(tBuNC(R)NEt)2 (R = Me, Et and n-Bu) are liquid at room temperature [[iii]]

Cobalt amidinates tests as MOCVD precursors

Cobalt amidinates Co(iPrNC(Me)NiPr)2 and Co(tBuNC(Me)NtBu)2 were tested for thin film deposition and found thermally stable and reactive enough to be used as vapor sources for the ALD of Co metal as well as cobalt (II) oxide CoO.[614]

Cobalt bis(N,N’-diisopropylacetamidinate Co(iPrNC(Me)NiPr)2 ( Co(iPr-MeAMD)2)

      Cobalt bis(N,N'-diisopropylacetamidinate) (Co(iPr-MeAMD)2) was synthesized by methathesis reaction of CoCl2 and Li(N,N'-diisopropylacetamidinate) (synthesized in situ from diisopropylcarbodiimide and LiMe)  (the reaction performed in 1:1 mixture of Et2O and THF as solvent). The crude product was recrystallization in hexanes at -30°C, dark green crystals of pure Co(iPr-MeAMD)2 were obtained (yield 77%). Co(iPr-MeAMD)2 has melting point 72°C and very volatile (sublimes at 40°C/ 50 mTorr). Elemental analysis: calculated for C.H.N.Co: C, 56.29; H, 10.04; N, 16.41. Found: C, 54.31; H, 9.69; N, 15.95.

     The crystal structure of Co(iPr-MeAMD)2 was determined by single crystal XRD (Fig. ).  It was determined that Co(iPr-MeAMD)2 molecule is monomeric with two amidinate ligands arranged in a distorted tetrahedral environment about each Co atom, with average Co- N distance 2.012(8) Ả and Co-N-C-N four-membered rings planar with an imposed mirror plane.[i]

[i] R.G. Gordon, B.S. Lim, US Patent US7737290B2, «Atomic layer deposition using metal amidinates » , https://patents.google.com/patent/US7737290B2/en , https://patentimages.storage.googleapis.com/99/5e/7f/e20606f8f5f3ad/US7737290.pdf

Co(iPr-MeAMD)2 for metal Co films by ALD

     Co(iPr-MeAMD)2 was applied as precursor for the ALD deposition of metallic Co films on Si/SiO/WNx substrates and fused silica capillary tube (20 µm inner diameter): the vaporisation temperature of cobalt bis(N,N'-diisopropylacetamidinate) precursor was 75°C, the substrate temperature was 300° C), H2 was used as reducing agent. In each cycle, Thee dose of cobalt precursor was 4x10 moles/cm (the exposure to Co precursor was 1x105 Langmuirs/cycle), whereas the dose of hydrogen was 9x107 moles/cm (and the exposure to H2 was 2x107 Langmuirs/cycle. The films of pure Co metal was deposited (5x1016 atoms/cm thick or 8x10-8 moles/cm thick, according to RBS). Cobalt film extended to at least 60 diameters (i.e. an aspect ratio >60) into the hole of the fused silica capillaries, as determined by optical microscopy, demonstrating excellent step coverage achieved by cobalt ALD process using Co(iPr-MeAMD)2 as precursor.

Co(iPr-MeAMD)2 for CoO films by ALD

      Cobalt bis(N,N'-diisopropylacetamidinate) Co(iPr-MeAMD)2 (combined with water vapour H2O as oxygen source) was applied as precursor for ALD growth of CoO layers films on Si/SiO/WNx substrates at deposition temperature 300° C, Co precursor vaporisation temperature 75°C/ 0.15 Torr. Uniform smooth films of cobalt (II) oxide with composition approximately CoO were obtained.[i]

[i] R.G. Gordon, B.S. Lim, US Patent US7737290B2, «Atomic layer deposition using metal amidinates » , https://patents.google.com/patent/US7737290B2/en , https://patentimages.storage.googleapis.com/99/5e/7f/e20606f8f5f3ad/US7737290.pdf

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