Chem. Mater. 2003, 15, 2969-2976 2969 Highly Conformal Thin Films of Tungsten Nitride Prepared by Atomic Layer Deposition from a Novel Precursor Jill S. Becker, Seigi Suh, Shenglong Wang, and Roy G. Gordon* Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, 02138 Received December 4, 2002. Revised Manuscript Received May 19, 2003 Highly uniform, smooth, and conformal coatings of tungsten nitride were synthesized by atomic layer deposition (ALD) from vapors of a novel precursor, bis(tert-butylimido)-bis- t (dimethylamido)tungsten, ( BuN)2(Me2N)2W, and ammonia at low substrate temperatures (250-350 °C). This tungsten precursor is a low-viscosity, noncorrosive liquid with sufficient volatility at room temperature to be a vapor source for ALD. These vapors were alternately pulsed into a heated reactor, yielding up to 0.1 nm of tungsten nitride film for every cycle, with no initial delay or induction period. The films were uniform in thickness along the 20-cm length of the deposition zone, as determined by scanning electron microscopy. Successful depositions were carried out on all substrates tested, including silicon, glass, quartz, glassy carbon, stainless steel, aluminum, gold, and copper. The films are shiny, silver- colored, and electrically conducting. All of the films showed good adhesion to the substrates, were acid-resistant, and did not oxidize over time. The stoichiometry of the WN films was determined to be 1:1 by Rutherford backscattering spectrometry. The films were amorphous as-deposited, as shown by X-ray diffraction and high-resolution transmission electron microscopy. 100% step coverage was obtained inside holes with aspect ratios greater than 200:1. Annealing for 30 min at temperatures above 725 °C converted the WN to pure, polycrystalline tungsten metal. WN films as thin as 1.5 nm proved to be good barriers to diffusion of copper for temperatures up to 600 °C. ALD of copper onto the surface of the WN produced strongly adherent copper films that could be used as “seed” layers for chemical vapor deposition (CVD) or electrodeposition of thicker copper coatings. I. Introduction are currently the most intensively studied barrier materials. There are many requirements to be met by Shrinking dimensions in integrated circuits without a diffusion barrier. The process demands and desired increasing device delay times has forced a move away properties of a diffusion barrier are listed as follows:3,4 from aluminum toward copper interconnects.1 Copper (1) Prevention of copper diffusion; (2) effective barrier has a lower resistivity than aluminum, 1.7 versus 2.7 thickness <5 nm; (3) resistivity < 500 × 10-6 Ω-cm; (4) µΩ-cm, respectively (bulk values). This property of halide-free; (5) excellent step coverage in high aspect copper enables the signal to move faster by reducing ratio holes and trenches; (6) chemical mechanical pol- the RC (resistance × capacitance) time delay. Another ishing (CMP) compatible; (7) deposition temperatures major advantage of copper is its superior resistance to under 350 °C (due to the thermal instability of low-k electromigration, a common reliability problem with materials); (8) good growth and adhesion on, and lack aluminum lines.2 Narrower copper metal lines can carry of reaction with (a) SiO and low-k insulators, (b) Si N , the same amount of current as aluminum. This means 2 3 4 SiC, or other etch-stop and barrier materials, and (c) that a greater density of interconnections can be Cu; (9) amorphous structure; (10) crack- and pinhole- achieved. Copper interconnects also require a very free films. thin and efficient barrier to prevent the copper from Titanium nitride may fail to meet all these demands diffusing into neighboring silicon or dielectric layers, due to its polycrystalline nature, allowing leakage along causing device failure. A diffusion barrier is also needed grain boundaries.5 Another candidate, sputtered tan- as a passivating layer to protect the copper interconnect talum nitride, is amorphous and conductive, but sput- from corrosion and oxidation and to promote the adhe- tering deposition methods can lead to poor surface sion of copper. Transition metal nitrides, such as coverage of high aspect ratio structures such as trenches titanium nitride, tantalum nitride, and tungsten nitride (3) Semiconductor Industry Association International. International * To whom correspondence should be addressed. E-mail: gordon@ Technology Road map for Semiconductors 2001 edn, http://public.itrs- chemistry.harvard.edu. .net/. (1) Awaya, N.; Ohno, K.; Arita, Y. J. Electrochem. Soc. 1995, 142, (4) Elers, K.; Saanila, V.; Soininen, P. J.; Li, W.; Kostamo, J. T.; 3173. Haukka, S.; Juhanoja, J.; Besling, W. F. A. Chem. Vap. Deposition (2) Nucci, J.; Neves, H.; Shacham, Y.; Eisenbraun, E.; Zheng, B.; 2002, 8, 149. Kaloyeros, A. MRS Symp. Proc. 1993, 309, 377. (5) Park, K.; Kim, K. J. Electrochem. Soc. 1995, 142, 3109. 10.1021/cm021772s CCC: $25.00 © 2003 American Chemical Society Published on Web 06/20/2003 2970 Chem. Mater., Vol. 15, No. 15, 2003 Becker et al. and vias due to shadowing.6 Chemical vapor deposition vapor from one precursor. Then any excess unreacted (CVD) TaN produces better step coverage, but its vapor from that precursor is pumped away. Next, a electrical resistance is too high.7 vapor dose of the second precursor is brought to the Tungsten nitride is a promising barrier material for surface and allowed to react. This cycle of steps can then copper metallization because of its refractory nature and be repeated to build up thicker films. One particularly excellent thermal, chemical (acid-resistant), and me- important aspect of this process is that the ALD chanical properties.8 Compared to the other transition reactions must be self-limiting, in that only a certain metal nitrides, WN has the lowest electrical resistivity maximum thickness can form in each cycle, after which (single crystal). Copper forms no known compounds with no further deposition occurs during that cycle, even if tungsten nitride, indicating its stability as a barrier.9 excess reactant is available. Because of this self-limiting WN can also be deposited in amorphous form, which character, ALD reactions produce coatings with highly means that it has no grain boundaries and no fast uniform thicknesses. Uniformity of ALD film thick- diffusion pathways for copper. WN can also be deposited nesses extends not only over flat substrate surfaces but smooth and crack- and pinhole-free, again preventing also into narrow holes and trenches. This ability of ALD other pathways for Cu to diffuse through to the dielec- to make conformal films is called “good step coverage.” trics surrounding it. The rate of CMP of WN is a close Coatings of WN made by ALD from WF6 and NH3 match to that of copper, whereas the rate of TaN have good step coverage.19 A disadvantage of this removal by CMP is much slower, requiring an additional process is that WF6 and/or its reaction byproduct, HF, 10 CMP step with a different formulation. WN has other attacks substrates made of Si or SiO2. Also, this process potential applications, such as providing a barrier can leave the WN surface with a fluorine residue that between polycrystalline silicon and tungsten for gate may impede adhesion of copper to the surface.4 In electrodes and forming electrodes for Ta2O5 capacitors. particular, adhesion of Cu deposited by ALD or CVD is WN can also act as an adhesion promoter for copper and often considered to be poor in part because of fluorine for tungsten. contamination at the interface between the tungsten WN has been made by reactive sputtering11-13 and nitride and the copper. by CVD methods,14-16 but the uniformity of film thick- In the present work, we studied the deposition of WN ness inside narrow features (“step coverage”) is not barrier films by ALD using a novel precursor, bis- t expected to be adequate for use in future microelectronic (tert-butylimido)-bis(dimethylamido)tungsten, ( BuN)2- devices having narrow features with high aspect ratios. (Me2N)2W, and ammonia at low substrate temperatures WN has also been made by metal-organic chemical (250-350 °C). The basic properties of these films were vapor deposition (MOCVD) from the precursor bis(tert- investigated, as well as their performance as a barrier t butylimido)-bis(tert-butylamido)tungsten, ( BuN)2W- to the diffusion of copper. t (NH Bu)2, where it was used as a single-source precursor 17 to grow WxN films. These films contained carbon due II. Experimental Section to thermal decomposition of the precursor and were not General Procedures. All reactions and manipulations conformal. were conducted under a pure nitrogen atmosphere using either The shrinkage of integrated circuits puts great de- an inert atmosphere box or standard Schlenk techniques. mands on thin film processing techniques. Atomic layer Solvents were dried using an Innovative Technology solvent deposition (ALD), characterized by excellent conformal- purification system. Reagents were used as-received from ity, and thickness control, is receiving attention for Aldrich. NMR spectra were recorded with a Bruker AM-500 applications in the deposition of diffusion barriers, spectrometer. Cryoscopic measurements of molecular weight nucleation layers, and high-k dielectric materials.18 ALD were carried out by temperature measurements using a thermistor in a paraxylene solution cooling in an ice bath. (also known as atomic layer epitaxy) is a process for A. Synthesis of Tungsten Precursor. The tungsten depositing thin layers of solid materials from two or precursor, bis(tert-butylimido)-bis(dimethylamido)tungsten- t more vapor precursors. The surface of a substrate onto (VI), ( BuN)2(Me2N)2W, is readily synthesized from com- which film is to be deposited is exposed to a dose of mercially available reactants, by the following three chemical reactions: (6) Rossnagel, S.; Nichols, C.; Hamaguchi, S.; Ruzic, D.