Coating processes – Electrical product produced – Metal coating
Reexamination Certificate
2002-03-21
2004-12-14
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Metal coating
C427S125000, C427S226000, C427S229000
Reexamination Certificate
active
06830778
ABSTRACT:
TECHNICAL FIELD
This invention relates to electrical conductors, and in particular to a process of forming thin-film metal conductors on a substrate by directly printing thereon metal chelate inks and decomposing the inks.
BACKGROUND ART
Thin metal films have a wide variety of applications ranging from interconnects in semiconductor device manufacture, including particle based contacts to photovoltaic semiconductors, to the optical tailoring of glass monoliths and to gas permeable membranes in separations technology. As a result, conventional processes have looked toward optimizing process design and in the synthesis of new inorganic, metal-organic and organometallic compounds specifically for use as thin film precursor materials. Optimization desirably includes providing high purity films of acceptable conductivity while eliminating conventional processing steps in order to reduce costs. It is also desirable to eliminate the photolithography and mask preparation steps used in screen printing and vacuum application, both of which are not conformal. In particular, inks which are amenable to low temperature deposition, such as ink jet printing, screen printing and other direct write approaches, are desirable in order to eliminate the use of costly vacuum application systems. Low temperature deposition is also desirable in the formation of semiconductors, particle-based contacts to photovoltaic semiconductors, and in spray printing on conformal substrates, such as flexible circuit boards, because high-temperature sintering cannot be performed due to thermal limitations associated with the underlying layers. For example, the thermal treatment of a Ni contact onto a ZnO conducting layer, as the top layer in a CuInSe
2
(“CIS”) solar cell, is limited to ~200° C. for 2 minutes because of the thermal instability of the underlying solar cell device. It has also been found that when a 1,2-propanediol slurry of Ni powder is deposited onto a conducting ZnO film and annealed in air at 200° C. for 2 minutes, the resultant Ni contact becomes crumbly in structure and is not electrically conductive. Moreover, the demand for improved performance in integrated circuits has led to integration of an increasing number of semiconductor devices on chips of decreasing size. This has been achieved by scaling down the device feature size, while increasing the number of interconnect layers. As a result, the topography has become much more severe with each successive device generation. In addition, as metal linewidths shrink, device speed is expected to be limited by the interconnect performance.
Copper is a widely applied electronic material with low bulk resistivity of ~2 &mgr;&OHgr;·cm. Many direct write approaches, to the formation of copper conductors, are limited by impurity phase formation. For example, copper (II) carboxylate analogs to Ag(neodecanoate) produce copper oxide when heated to decomposition in air. Most Cu(II) precursor chemistries, however, require relatively high-temperatures (e.g., Cu(hfa)
2
yields Cu at 340-400° C.) or subsequent processing in the presence of a reducing species (e.g., hydrogen gas) to produce metallic layers. The chemistry of Cu(I) complexes, as chemical-vapor-deposition precursors to Cu films, has also been evaluated for use in the next-generation ultra-large scale integrated circuits. Copper (I) complexes, based on Cu(hfa).L (where hfa-hexafluoroacetylacetonate and L=CO, phosphine, alkene, or alkyne), have been shown, conventionally, to produce copper films by chemical vapor deposition (“CVD”) at low temperature (100-150° C.) with resistivities approaching that of bulk copper.
The CVD process using aerosol precursors for the application of metal alloy thin films has been described for low temperature deposition on a variety of substrates. In Xu, C., et al.,
Chem. Mater
. 1995, 7, 1539-1546, the CVD of Ag—Pd, Cu-Pd, and Ag—Cu alloys using aerosol precursor delivery over a range of preheating temperatures (70-80 ° C.) and substrate temperatures (250-300° C.) is disclosed. There, the precursors (hfac)Ag(SEt
2
), (hfac)Cu
1
(1,5-COD), Cu(hfac)
2
, Pd(hfac)
2
and Pd(hfac)
2
(SEt
2
), dissolved in toluene with 10% H
2
in Ar as carrier gas, are used in the CVD process, a combination, which the authors claim provides advantages over traditional methods. These advantages include higher deposition rates, the ability to transport thermally sensitive compounds, and the reproducible deposition of binary materials. See also, Jain, A., et al.,
J. Vac. Sci. Technol
. B 11(6), Nov/Dec. 1993, 2107-2113 ((CVD of copper on both SiO
2
and W from (hfac)CuL, where hfac=1,1,1,5,5,5-hexafluoroacetylacetonate, and L=1,5, COD or vinyltrimethylsilane (VTMS)); and Norman, J. A. T., et al.,
Journal De Physique
IV, Colloque C2, suppl.
Au Journal de Physique
II, Vol. 1, September 1991, 271-277 ((CVD of the volatile liquid complex Cu
+1
(hexafluoroacetylacetonate) trimethylvinylsilane, [Cu
+1
(hfac)TMVS])).
In an aerosol-assisted CVD process, the precursor is first dissolved in a solvent. The solution is passed through an aerosol generator, where micron-sized aerosol droplets are generated in a carrier gas and are transported into a preheating zone where both the solvent and the precursor evaporate. The precursor vapor reaches the heated substrate surface where thermally induced reactions and film deposition takes place. This method may be employed on a variety of mask-based substrates. However, distinct disadvantages of the CVD process have heretofore included a necessity to mask the substrate for deposition, the resulting large-grain-microstructures (e.g., 0.1 &mgr;m to 0.6 &mgr;m) of the film, and the fact that the deposition rate is precursor evaporation-rate limited in the sense that the deposition rate is related to the partial pressure of the precursor which is fixed by the vapor pressure. These limitations and others, such as the inefficient use of expensive precursor materials, inherent in the CVD process, render it incapable of forming linewidths in the range of 130 &mgr;m, or less, at high deposition rates without increasing the deposition temperature.
Screen printing using metal powders and metallo-organic decomposition (MOD) compounds has also been used for metallization. For example, U.S. Pat. No. 5,882,722 describes the use of screen printable metal powders and MOD products to print thick films at low temperature. The thick films are formed of a mixture of metal powders and metallo-organic decomposition (MOD) compounds in an organic liquid vehicle in a two-step screen-print and heat process. The mixtures contain a metal flake with a ratio of the maximum dimension to the minimum dimension of between 5 and 50. The vehicle may include a colloidal metal powder with a diameter of about 10 to about 40 nanometers. The concentration of the colloidal metal in the suspension can range from about 10% to about 50% by weight. The MOD compound begins to evaporate at a temperature of approximately about 200° C. and then consolidation of the metal constituents and bonding to the substrate is completed at temperatures less than 450° C., in a time less than six minutes.
Direct printing, using a spray or ink jet process, however, necessitates the formulation of inks which are substantially different from those formulations which are currently used in screen printing applications. Unlike screen printable inks, the viscosity of these inks must be at or near that of water, in order to permit printing with piezoelectric or thermal ink jet systems and to prevent agglomeration of the ink on the substrate. There is also no need to have the printed line be free standing or for the ink to include binders, and the like. Volatility of these inks should also be low enough to prevent consequential solvent loss at low temperature but high enough to be readily lost when applied at the substrate temperature.
The use of metal organic precursor materials, either with or without metallic particles, for use in a process to directly write conducting metal layers or gri
Curtis Calvin J.
Ginley David S.
Schulz Douglas L.
Midwest Research Institute
Talbot Brian K.
White Paul J.
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