deposition of conformal copper and nickel films from

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Device quality copper and nickel films were depositedonto planar and etched silicon substrates by the reductionof soluble organometallic compounds with hydrogen insupercritical carbon dioxide solution. Exceptional stepcoverage on complex surfaces and complete filling of highaspect ratio, sub-100 nanometer - wide features wereachieved. Nickel was deposited at 60°C by the reduction ofbis(cyclopentadienyl)nickel and copper was depositedfrom either Cu(I) or Cu(II) compounds onto the nativeoxide of silicon or metal nitrides with seed layers attemperatures up to 200°C and directly on each surface attemperatures above 250°C. The latter approach providesa single-step means for achieving high aspect ratio featurefill necessary for Cu interconnect structures in futuregenerations of integrated circuits.

The increasing complexity and decreasing dimensions ofdevices for microelectronic, system-on-a-chip, data storageand other applications are placing stringent demands ondeposition technologies that to date have not been fullysatisfied. These include reductions in the thermal budgetduring fabrication, mitigation of the negative environmentalimpact of current processes, conformal coverage of complexsurfaces, and complete filling of narrow, high aspect ratiostructures (1). The difficulties associated with achieving thelatter two requirements are particularly severe. For exampleto keep pace with the historical increase in processor speedpopularly referred to as Moore’s Law, copper interconnectstructures for integrated circuits must drop below 100 nm inwidth with aspect ratios that exceed 3 after 2005. Accordingto the Semiconductor Industry Association’s InternationalTechnology Roadmap for Semiconductors (ITRS), however,there are no viable means for Cu deposition at thesedimensions (1).

We have developed a new technique, called chemical fluiddeposition (CFD), that can meet each of the criteria outlinedabove for interconnect technology in particular and devicefabrication in general. CFD involves the chemical reductionof soluble organometallic compounds in supercritical carbondioxide (sc CO2) to yield high-purity deposits. Hydrogen,which is miscible with sc CO2, is typically used as a reducingagent. Recently, we demonstrated the utility of this approachfor the deposition of platinum, palladium, gold and rhodiumfilms on planar surfaces (2–4). These reports indicate thatpure, continuous films can be deposited at low temperatures(40° to 80°C) using precursors that produce comparativelybenign effluents. While this advance addressed the issues ofthermal budget and environmental acceptability, planar filmssimilar to those deposited in these examples can be obtainedby conventional methods including chemical vapor deposition(CVD) and line-of-sight techniques such as sputtering. Here,we report the deposition of device quality copper and nickel

films for use in interconnect and data storage applications,respectively. For copper, we developed a cold-wall reactor inwhich metal deposition occurs exclusively on the heatedsubstrate. Moreover, we demonstrate that the conditions usedyield unprecedented step coverage that can not be achievedvia CVD or physical vapor deposition techniques underpractical conditions and can provide filling of sub-100 nm,high aspect ratio features in a single step.

The benefits of chemical fluid deposition are aconsequence of the physicochemical properties ofsupercritical fluids (SCFs), which lie intermediate betweenthose of liquids and gases. Consequently, CFD can be viewedas a hybrid of solution plating and vapor phase techniques.Table 1 compares the deposition conditions for each. Likeelectroless plating, CFD is solution based, but the transportproperties of SCFs, which are more akin to those of a gas,afford distinct advantages that are typically associated withchemical vapor deposition including low viscosity, rapiddiffusion and the absence of surface tension. Theseproperties, when combined with the miscibility of H2 with scCO2, alleviate the sluggish mass-transfer typical of liquidphase reductions and promote conformal coverage. Theenabling distinction between CFD and CVD is the mode ofprecursor transport. In CVD, the limited volatility of suitableprecursors, including organometallics, leads to low vaporphase concentrations and mass transfer limited reactions thatpreclude uniform depositions. In CFD, precursorconcentrations in sc CO2 solution are up to three orders ofmagnitude greater.

Presently, copper interconnects are fabricated using a twostep, dual damascene process. First, a continuous Cu seedlayer is deposited within the feature using physical vapordeposition (PVD). This seed layer is then used as a cathodefor electrolytic Cu deposition, which fills the trench. BecausePVD (sputtering) is essentially a line of sight technique,difficulties are encountered when attempting to deposituniform seed layers in confined geometries. In principle,CVD could be used for this purpose, but to date it has notbeen successfully integrated. A primary obstacle is precursorvolatility constraints, which lead to low vapor phaseconcentrations, mass transport limited depositions, andultimately poor step coverage.

Copper was deposited during CFD by the reduction of CO2

solutions of either Cu(I) or Cu(II) organometallic compoundsin a high-pressure cold wall reactor (5). The substrate wasplaced on a resistively heated stage and maintained at anelevated temperature relative to the bulk solution. Atsubstrate temperatures of 200°C or less, deposition by H2

reduction of the Cu(II)(betadiketonates), copper (II)bishexafluoroacetylacetonate [Cu(hfac)2] and Cu(II)tetramethylheptanedionate [Cu(tmhd)2] occured only on theheated stage and was selective for metal surfaces or metaloxide surfaces seeded with catalytic clusters over the bare

Deposition of Conformal Copper and Nickel Films from Supercritical Carbon DioxideJason M. Blackburn, David P. Long, Albertina Cabanas, James J. Watkins*

Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA.

*To whom correspondence should be addressed. E-mail: [email protected]


oxide. For example, H2 reduction of 0.7 wt% Cu(hfac)2

solutions in CO2 at substrate temperatures between 175° and200°C and a pressure of 200 atm proceeded readily on Nifilms or on silicon wafers seeded with small Pd clustersdeposited by CFD, but not on the native oxide of a Si wafer.This approach affords opportunities for selective film growththrough control of the spatial distribution of the seed layer.

In all cases, the deposited films are continuous, highlyreflective, and essentially free of impurities. X-rayphotoelectron spectroscopy (XPS) indicates insignificantcarbon or fluorine contamination of a Cu film deposited onnickel from Cu(hfac)2 (Fig. 1A). High purity is facilitated bythe high solubility of the ligand by-products in sc CO2. Infact, fluorine nuclear magnetic resonance (NMR) analysis ofthe reaction products isolated from the reactor effluent revealsa single peak at ~77 ppm that corresponds tohexafluoropentanedione, which suggests the reductionproceeds cleanly at 200°C with no detectable ligandfragmentation. In addition to promoting film purity, intactdesorption of precursor decomposition products providesopportunities for ligand recovery and recycle.

Although deposition from Cu(hfac)2 yields high purityfilms, the presence of fluorine is undesirable due toenvironmental and device performance concerns. Becausetransport in CFD occurs in solution, non-flourinatedprecursors that exhibit insufficient vapor pressure for use inCVD are suitable for CFD. For example, deposition fromCu(tmhd)2 in H2/CO2 solutions proceeded readily to yieldpure Cu films. Secondary ion mass spectrometry (SIMS)analysis of such a film deposited onto Pd-seeded, etched Si ata substrate temperature of 200°C reveals C and Oconcentrations of less than 0.2 atomic percent (at%) in thebulk of the film. Further analysis reveals that the high reagentconcentrations and favorable fluid-phase transport propertiesinherent to CFD facilitate conformal deposition. Figure 2Ashows a scanning electron microscope (SEM) image of thefilm cross-section that demonstrates excellent step coveragein narrow (~100 nm by 800 nm) trenches. The sample cross-section was prepared using a focused ion beam (FIB).

It is highly desirable to eliminate the seed layers anddeposit high-purity, conformal films in a single step.Presently, no such single-step process exists for Cuinterconnect metallization. We deposited Cu films using CFDdirectly onto the native oxide of Si at 250°C and onto Cudiffusion barriers such as TiN at slightly lower temperaturesby H2 reduction of Cu(tmhd)2. SIMS analysis of a 380-nm-thick film deposited from Cu(tmhd)2 at 225°C onto TiNindicates that C and O concentrations in the bulk of the filmsare less than 0.5 at%. The resulting film had low resistivity (~2.0 microohm-cm) after annealing, which approaches that ofbulk copper (1.7 microohm-cm) and is well below the upperlimit for interconnect structures specified by the ITRS (2.2microohm-cm) (1).

High-purity Cu films were also deposited in a single stepvia CFD of Cu(I) compounds of the general type (β-diketonate)CuLn, where L is a Lewis base, and n is 1 or 2.These compounds are known to undergo thermaldisproportionation during CVD to yield Cu metal and thecorresponding Cu(II)(β-diketonate)2 without the addition ofan external reducing agent (Eq. 1) (6)

2(β-diketonate)Cu(I)Ln →Cu(0) + Cu(II)(β-diketonate)2 + 2 Ln(1)

We deposited high-purity Cu films onto the native oxide of Siwafers as well as Cu diffusion barriers including TiN byhydrogen-assisted reduction of Cu(hfac)(2-butyne) in CO2

solution at substrate temperatures of 225°C and a pressure of200 atm. Here the disproportionation reaction provides apathway for the initial Cu deposits, which in turn yield activesurfaces for H2 reduction of Cu(hfac)2 generated by thedecomposition of the Cu(I) precursor. This scenario allowsfor higher deposition efficiency relative to that of thedisproportionation reaction alone as Cu(hfac)2 generated insitu is also reduced to yield Cu. XPS analysis of filmsdeposited from Cu(hfac)(butyne) yields results similar to thatfor Cu(hfac)2. SIMS analysis indicates F contamination is ofthe order of 1 at%, while C and O concentrations are less than0.5 at%. Figure 2B is a FIB SEM image of a Cu filmdeposited onto an etched Si wafer from a CO2 solution ofCu(hfac)(butyne) at 225°C. Step coverage is exceptionaldespite the presence of a number of trenches that areconsiderably more narrow at the entrance (<100 nm) than themidpoint. The trench asymmetry is a consequence of waferpreparation prior to metallization. The depositions are not yetoptimized, but resistivity of films deposited fromcopper(I)(hfac)(2-butyne) in initial experiments is about 2.7microohm-cm, which is typical of films deposited by CVD.At substrate temperatures of 200°C or less we find thedeposition is selective for metal surfaces including Pt, Pd, andNi over the native oxide of Si wafers.

The deposition of Ni is of interest for the fabrication ofcontacts in microelectronic devices and for data storageapplications. CFD deposition of Ni from sc CO2 solutions ofbis(cyclopentadienyl)nickel (NiCp2) proceeds readily at 60°Cand 200 atm in the presence of H2 over Ni, Pd or Pt films.Low temperature plating on non-catalytic surfaces such aspolymers or metal oxides is accomplished by seeding thedeposition.

We deposit Ni in batch or continuous hot-wall reactors at60°C using two strategies (7, 8). The first involves the use ofa Pd-seed layer in the same manner as used for the Cudepositions. In the second, cosolutions of NiCp2 and an easilyreduced organopalladium compound such as 2-methylallyl(cyclopentadienyl)palladium(II) [CpPd(C4H7)] areprepared in stoichiometric ratios ranging from 0.005:1 to0.1:1 (Pd:Ni). The reduction of CpPd(C4H7) proceedsimmediately in CO2 upon the addition of H2 (9) to yieldcatalytic clusters that serve as nucleation sites for thereduction of NiCp2. Once the incipient metal surface isformed, growth of the Ni film proceeds readily. This scenariois confirmed by composition analysis by XPS depth profiling,which reveals a Pd-rich Ni deposit at the substrate-metal filminterface that is overlain by a pure Ni film. At temperaturesabove 120°C, Ni can be deposited non-selectively byreduction of NiCp2/CO2 solutions without the need for a seedlayer or co-reduction with an organopalladium reagent. In allcases, the reduction of NiCp2 proceeds to completion to yieldonly the relatively benign, hydrocarbon decompositionproducts and nickel metal. The metal coatings prepared byCFD are reflective, continuous and free of contamination(10).

Deposition of Ni onto patterned Si wafers is highlyconformal. Figure 3 shows SEM micrographs of filmsdeposited within trenches of etched Si wafers from solutionsof NiCp2 using the continuous-feed hot-wall depositionprocess. Figure 3A is a FIB SEM image of a film depositedonto a Pd-seeded Si wafer at 60°C and 180 atm. The narrowseams in the 100 nm wide by 800 nm deep trenches reveal


that film growth occurs uniformly from both sides of thetrench. Figure 3B is a SEM image of a fracture cross-sectionof a Ni film grown on a substrate containing 75-nm trenches.In this example, two of the trenches were not etchedcompletely during fabrication of the test wafer and leftfeatures as narrow as 45 nm (far right), which are filled uponNi deposition. The deposited films can be grown to virtuallyany thickness in the flow reactor. For example, we havegrown 2.5-µm-thick Ni films that uniformly fill 450 nm widetrenches in etched Si wafers.

Although we have focused on the utility of CFD for devicefabrication, the use of non-fluorinated precursors eliminatesemission problems typically associated with CVD and the useof CO2 as the deposition medium obviates the need foraqueous plating baths used for electrolytic and electrolessplating that generate large amounts of contaminatedwastewater. This is particularly acute for nickel plating: dueto large volumes and environmental and human toxicity,nickel is one of 17 high priority pollutants targeted by theUnited States EPA for reduction under its 33/50 program(11).

References and Notes1. Semiconductor Industry Association, “1999 International

Technology Roadmap for Semiconductors”(Semiconductor Industry Association, San Jose, CA 1999).

2. J. J. Watkins, T. J. McCarthy, U.S. Patent 5,789,027(1998).

3. J. J. Watkins, J. M. Blackburn, T. J. McCarthy, Chem.Mater. 11, 213 (1999).

4. D. P. Long, J. M. Blackburn, J. J. Watkins, Adv. Mater. 12,913 (2000).

5. All Cu depositions were conducted in a dual flange cold-wall reactor in batch mode. The stainless steel reactor hasa total volume of 70 ml and contains a resistively heatedstage. Precursor and a single substrate were loaded into thereactor. The vessels were sealed, purged and charged withCO2 using a high-pressure syringe pump. The substratewas heated to the desired temperature (typically 200° to250°C) and the reactor walls were maintained at a lowertemperature (typically 60° to 80°C). Film deposition wasinitiated by the addition of excess of H2 using a small,pressurized manifold (70 ml). The quantity of H2 admittedinto the vessel was calculated using the pressure-dropmeasured in the manifold.

6. M. Hampden-Smith, T. Kodas, in The Chemistry of MetalCVD, T. Kodas, M. Hampden-Smith, Eds. (VCH, NewYork, 1994).

7. All batch depositions were conducted in 17 ml high-pressure stainless steel reactors. A single substrate (~1 by7 cm) and a known amount of precursor were loaded atambient conditions. The vessels were sealed, purged withCO2, weighed, placed in a thermal bath, and allowed toequilibrate to the desired temperature. CO2 was then addedto the desired pressure via a high-pressure syringe pump.Film deposition was initiated by the addition of excess ofH2 using a small, pressurized manifold (3.6 ml). Thequantity of H2 admitted into the vessel was calculatedusing the pressure-drop measured in the manifold.

8. Continuous depositions were conducted in a 1.3 cm i.d. by10.2 cm long hot-wall reactor operating at 60°C and 180atm. 3 computer-controlled high-pressure syringe pumpswere used to meter reagent and carrier gas streams to thereactor. The precursor feed stream was either admitteddirectly to the reactor or was diluted with pure CO2

upstream of the reactor inlet. Deposition was initiated bymetering a ~2 mol% solution of H2 in CO2 into the vesselat the desired flow rate. After dilution by all streams, theconcentration of precursor at the reactor inlet wasapproximately 0.2 wt.%. The reactor effluent was directedto an activated carbon bed before venting. Prior to Nideposition, the substrates were seeded by Pd deposition viahydrogenolysis of CpPd(C4H7) in CO2 solution.

9. J. M. Blackburn, D. P. Long, J. J. Watkins, Chem. Mater.12, 2625 (2000).

10. XPS analysis of a film co-deposited at 60°C and 140 atmfrom a CO2 solution of NiCp2 and CpPd(C4H7) (0.2 and0.02 wt.%, respectively) onto glass in a batch reactorreveals Ni peaks at their appropriate binding energies andinsignificant levels of carbon contamination (Fig. 1B).Films of similar purity are obtained when nickeldepositions are performed on Pd-seeded substrates.

11. U.S. EPA, “International Waste MinimizationApproaches and Policies to Metal Finishing” EPA530-R-96-008 (U.S. EPA, 1996).

12. This work was funded by the National ScienceFoundation (CTS-9734177), the David and Lucile PackardFoundation, and Novellus Systems. We also acknowledgeNovellus Systems for assistance with film characterizationand donation of test wafers. Facilities supported by theMaterials Research Science and Engineering Center at theUniversity of Massachusetts were used for depositcharacterization.

6 July 2001; accepted 29 August 2001Published online 13 September 2001;10.1126/science.1064148Include this information when citing this paper.

Fig. 1. XPS analysis of metal films deposited by chemicalfluid deposition conducted after Ar+ sputtering to removeatmospheric contaminants. (A) Analysis of a Cu filmdeposited onto Ni-seeded Si at 200°C by the reduction ofCu(hfac)2 with H2 in CO2 solution in a cold wall reatorreveals Cu peaks at the appropriate binding energies. There isno significant carbon contamination (C 1S binding energy =284 ev). (B) Characterization of a Ni film deposited ontoglass (60°C) by the co-reduction of NiCp2 (0.2 wt.%) with H2

and CpPd(C4H7) (0.02 wt.%) in CO2 solution reveals Nipeaks at the appropriate binding energies. There is nosignificant carbon contamination. Characteristic peaks for Pdat 335 and 340 eV were not observed until additional Ar+

sputtering exposed a Pd-rich region adjacent to the substratesurface.

Fig. 2. (Left) FIB SEM micrographs of Cu films deposited byCFD onto (A) a Pd-seeded test wafer by hydrogen reductionof Cu(tmhd)2 in CO2 solution at a substrate temperature of200°C and a pressure of 207 atm and (B) onto a bare Si testwafer by hydrogen assisted reduction of Cu(hfac)(2-butyne)in CO2 solution at a substrate temperature of 225°C and apressure of 200 atm. Both depositions were conducted in acold-wall high-pressure reactor. Fig. 3. (Right) SEMmicrographs of Ni films deposited onto etched Si wafers bythe hydrogen reduction of NiCp2 in CO2 solution in a hot-wallcontinuos – flow reactor. (A) A FIB SEM image of a filmdeposited at 60°C and 140 atm provides evidence of a narrowseam within the trenches, indicating uniform film growthfrom opposing feature surfaces. (B) An SEM image of afracture cross-section of a Ni film deposited conformally at


60°C and 180 atm on a test wafer containing features asnarrow as 45 nm that were not etched completely duringwafer fabrication (far right).

Table 1. Comparison of reduction media for the deposition ofmetal films.

J. M. Blackburn et al. - p. 1

Table 1.

Parameter Phase

Liquid SCF Gas (CVD)

Density (g/cm3) 1 0.1-1 10–3

Viscosity (Pa · s) 10–3 10–4-10–5 10–5

Diffusivity (cm2/s) 10–5 10–3 10–1

Surface tension (dynes/cm) 20-50 0 0

Precursor conc. (M/cm3) 10–3 10–5 10–8

Deposition temp. (°C) 25-80 40-250 250 +

deposition of conformal copper and nickel films from

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