Published on Web 06/08/2005
Remarkably Volatile Copper(II) Complexes of N,N′-Unsymmetrically
Substituted 1,3-Diketimines as Precursors for Cu Metal Deposition via CVD or
ALD
Kyung-Ho Park* and Will J. Marshall
Materials Science and Engineering, Central Research and DeVelopment, DuPont, Experimental Station,
Wilmington, Delaware 19880-0328
Received February 23, 2005; E-mail: kyung-ho.park@usa.dupont.com
Volatile copper â-diketonates are often regarded as promising
precursors for copper metal deposition via CVD and/or ALD1 for
industrial semiconductor devices of the new generation. For
example, hexafluoroacetylacetonato(trimethylsilylethylene)copper
(1 in Figure 1), or Cupraselect,2 is an important industrial candidate
for copper metal deposition. Another example is 2,2,7-trimethyl-
Figure 1. Cu(I) and Cu(II) precursors for CVD or ALD.
3,5-octanedionatecopper(II), 2, which can also be used in a process
using a supercritical fluid.3 In the modern industry of microelec-
tronics, however, it is increasingly recognized that the presence of
oxygen or halogens in the precursor can be detrimental to the desired
performance, including device efficiency.4 Thus, oxygen- and
halogen-free nitrogen-containing ligands are currently viewed as
an alternative to 1,3-diketones, particularly with metal complex
precursors for microchip interconnect layers. In this regard, Cu
derivatives of 1,3-amidines and 1,3-diketimines should be the very
alternative to the 1,3-diketonate complexes. Gordon et al. have
recently reported5 the synthesis of N,N′-symmetrically substituted
dimeric copper(I) amidinate 3 as a copper precursor in the ALD
process. However, the corresponding Cu(II) derivative cannot be
obtained due to the facile reduction of CuCl2 upon its treatment
with the Li amidinate. Monomeric Cu(I)L(amidinate) cannot be
synthesized either, due to steric factors. As for 1,3-diketimine,
monomeric Cu(I)L(diketiminate) can be synthesized but is usually
unstable especially when aliphatic diketimine is employed, easily
undergoing disproportionation to Cu(0) and Cu(II). As a result,
stable Cu(II) (diketiminate)2 has been focused as a potential copper
precursor in ALD. Some oxygen- and halogen-free metal complexes
of 1,3-diketimines have been prepared.6 However, only metal
complexes of N,N′-symmetrically substituted 1,3-diketimines (4)
have been prepared.
To enhance the volatility of the copper precursor in the copper
deposition process, it was envisioned that N,N′-unsymmetrically
substituted 1,3-diketimines should be more volatile than their
symmetrically substituted counterparts, possibly due to the less
compacted mode of molecular stacking originating from the
unsymmetrical ligand. It is also based on the assumption that the
lower symmetry would change the entropy of vaporization (making
it higher) and possibly making the enthalpy of sublimation lower
by destabilizing the crystal lattice. When a symmetrical molecule
leaves a solid lattice to the vapor (or liquid) phase, the entropy of
vaporization is lowered relative to a comparable unsymmetrical
molecule leaving a solid lattice to the vapor phase. A higher entropy
of vaporization and a lower enthalpy of sublimation would both
contribute toward a higher vapor pressure at constant temperature.
The desired, individual N,N′-unsymmetrically substituted 1,3-
diketimines, however, cannot be prepared by the McGeachin
method.7 We have recently developed a new strategy for the
synthesis of nonhalogenated N,N′-unsymmetrically substituted
aliphatic 1,3-diketimine ligands (5, 6, and 7 in Figure 2) via the
Figure 2. N,N′-Unsymmetrically substituted 1,3-diketimines.
Scheme 1. Synthesis of Cu(II) (Unsymmetrical 1,3-Diketiminate)
reaction of exocyclic enaminoketones with amines or metallo-
enamines with imidoyl thioethers.8
Having prepared unsymmetrical ligands 5-7, we synthesized
their Cu(II) bischelates in up to 96% yield, as shown in Scheme 1
and Table 1.
The new Cu(II) complexes of N,N′-unsymmetrically substituted
1,3-diketimines were characterized by X-ray crystallography, and
some representative structures are given in Figure 3. The structures
exhibited distorted tetrahedral geometries, where dihedral angles
between the two chelating NCuN planes vary in the range of 33.5-
64.3°.
All unsymmetrical Cu(II) diketiminates appeared sublimable
under vacuum without decomposition. As can be seen from the
data in Table 1, Cu(II) complexes with aliphatic substituents are
more volatile than those with aromatic groups (8-c, 8-d, and 9-d).
Within the aliphatic series, unsymmetrical Cu(II) acyclicdiketimi-
nates 8 were more volatile than mono- or dicyclic diketiminate
copper(II) complexes 9 and 10. This might be due to the tethered
ring system contributing to more spatially compacted molecular
stacks, thus reducing the volatility. TGA studies of these Cu(II)
complexes at 500 mTorr were performed, and some representative
TGA curves are given in Figure 4. Remarkably, the higher
molecular weight 8-b (N-i-Bu, N′-Me) was found to be more volatile
than 8-a (N-Et, N′-Me). Thus, within the comparable molecular
weight range, bulkier unsymmetrical substituents seem to play a
more important role to enhance the volatility of diketiminate Cu-
(II) complexes.
The results from isothermal TG analysis proves the effect of
unsymmetry in Cu(II) complexes on their volatility (Figure 5).
9
9330
J. AM. CHEM. SOC. 2005, 127, 9330-9331
10.1021/ja051158s CCC: $30.25 © 2005 American Chemical Society
Park, Kyung-Ho
