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Chem. Mater. 2003, 15, 4200-4204
Low-Temperature Preparation of High-Temperature Nickel Germanides Using Multilayer Reactants Jacob M. Jensen,† Sochetra Ly, and David C. Johnson* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97405 Received May 1, 2003. Revised Manuscript Received July 30, 2003
Nickel germanide thin films were prepared using nickel-germanium multilayers as reactive precursors. Over the composition range xNi ) 0.24-0.42, high-temperature germanides (δ-Ni5Ge2, -Ni5Ge3, and Ni3Ge2) form under conditions where the phases are thermodynamically unstable. δ-Ni5Ge2 forms over a broader range of composition than has been observed previously. The three high-temperature phases accessible by this synthetic route are observed to share related structure types, which are related to the structure of amorphous nickel germanium alloys with similar composition. The ability to selectively prepare metastable phases under mild conditions has important implications with respect to the search for new materials for use in integrated devices.
Introduction An extraordinary need for the development of new materials exists in integrated device applications.1 Decreasing device dimensions drive development of new materials which will maintain suitable electrical and mechanical properties in the face of future miniaturization. Historically, the palette of materials available for device processing has been limited to thermodynamically stable compounds, owing to the high (>1000 °C) temperatures encountered during device fabrication. In recent years, however, shrinking feature sizes have forced the tightening of thermal budgets to lower temperatures. The trend toward milder process conditions enables the exploration of metastable compounds as candidates for application in next generation microelectronic technology. Strategies for the preparation of metastable solids have been primarily directed toward bulk synthetic methods. While these techniques have met with considerable success, they are by and large incompatible with methods for microelectronic processing. For example, conventional “energize and quench” methods often produce environmental conditions which are outside of a restricted thermal budget.2 Gentler preparative methods such as hydrothermal synthesis3 and molten salt methods4 are generally unsuitable for thin film preparation; the compatibility with standard microelectronic fabrication tool sets is questionable. Some investigation has been given to ion-assisted methods of metastable thin film preparation,5 though radiation damage remains a key concern. Other methods of metastable thin film preparation, such as direct vapor deposition of alloys, provide limited rational control over the nature of the products. * Corresponding author. † Intel Corp., Hillsboro, OR. (1) Peters, L. Semicond. Int. 2001, 24, 61-66. (2) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1-184. (3) Demezeau, G. J. Mater. Chem. 1999, 9, 15-18. (4) Deloume, J. P.; Durand, B. Molten Salt Forum 1998, 5-6, 485488.
Work in our laboratory has focused on the low-temperature preparation of binary and higher phases using compositionally modulated multilayers as reactive precursors.6,7 This synthetic strategy provides several potential advantages for preparing metastable materials in integrated devices. First, this method employs a tool set similar to that already found in microelectronic fabrication facilities. Second, the formation of unwanted phases is suppressed in that the desired phase forms directly from the amorphous intermediate. Third, this procedure enables the preparation of metastable phases under conditions compatible with device processing windows. Herein, we report the application of this technique to the controlled preparation of nickel germanides. The nickel-germanium system provides an ideal system for examining the synthetic possibilities of our preparative technique in that distinct low- and high-temperature phases are known over a wide range of compositions. Considerable attention has been given previously to nickel germanide thin film preparation, owing to the potential application of nickel germanides as electrical contacts in compound semiconductor devices.8-13 The thin film techniques employed therein are limited to the preparation of low-temperature germanides, such as Ni2Ge or NiGe. When we use our preparative technique, three of the five high-temperature nickel germanides (5) Dhar, S.; Som, T.; Kulkarni, V. N. J. Appl. Phys. 1998, 83, 23632365. (6) Fister, L.; Novet, T.; Grant, C.; Johnson, D. In Advances in the Synthesis and Reactivity of Solids; Mallouk, T. E., Ed.; JAI Press: Greenwich, 1994; Vol. 2, pp 155-234. (7) Novet, T.; Johnson, D. J. Am. Chem. Soc. 1991, 113 (9), 33983403. (8) Tsunoda, Y.; Murakami, M. J. Electron. Mater. 2002, 31, 7681. (9) Murakami, M.; Koide, Y. Crit. Rev. Solid State Mater. Sci. 1998, 23, 1-60. (10) Furumai, M.; Oku, T.; Ishikawa, H.; Otsuki, A.; Koide, Y.; Oikawa, T.; Murakami, M. J. Electron. Mater. 1996, 25, 1684-1694. (11) Wakimoto, H.; Oku, T.; Koide, Y.; Murakami, M. J. Electrochem. Soc. 1996, 143, 1705-1709. (12) Murakami, M. Sci. Technol. Adv. Mater. 2002, 3, 1. (13) Tanahashi, K.; Takata, H. J.; Otuki, A.; Murakami, M. J. Appl. Phys. 1992, 72, 4183.
10.1021/cm030385v CCC: $25.00 © 2003 American Chemical Society Published on Web 10/28/2003
Preparation of Ni Ge’s Using Multilayer Reactants
(δ-Ni5Ge2, Ni3Ge2, ′-Ni5Ge3) found on the equilibrium phase diagram are directly accessible at temperatures below 200 °C from multilayers of designed composition. These phases are formed in conditions under which they are metastable with respect to one or more low-temperature phases and are stable to annealing temperatures of 400 °C or higher. While the suitability of these phases for a specific application has not yet been evaluated, the ability of our synthetic method to access high-temperature phases under moderate conditions broadens the palette of materials available for use in integrated device applications. Experimental Section Ni-Ge multilayers were prepared by electron beam evaporation in a custom-built multisource deposition chamber operating at or below 1 × 10-6 Torr. Deposition rates were monitored in situ via a quartz crystal microbalance. Precise tooling factors were determined by comparing QCM film thickness measurements with thicknesses determined by X-ray reflectivity. Ni shot (2.0 cm, 99.95+%) or Ni rod (1/2′′, 99.5%+) and Ge shards were used as source materials. Multilayers were deposited onto Si(100) substrates approximately 4 × 4 cm; in all cases Ni was the first layer deposited. The native oxide was left intact to prevent reaction between the multilayer and substrate. Samples were annealed in a nitrogen atmosphere (1-2 ppm O2). X-ray reflectivity measurements were performed on a Philips X’Pert MRD diffractometer. The incident beam was collimated with a 1/32° divergence slit and a parabolic multilayer mirror. The exit beam was conditioned with a parallel plate slit assembly, graphite monochromator, and a 0.1-mm exit slit prior to collection by a scintillation counter. Data near the direct beam (2θ ) 0.0-1.5°) were collected at 20 kV, 10 mA to prevent saturation of the detector. Data above 1.5° were collected at 45 kV, 40 mA. Structural parameters (i.e., repeating unit thickness) were extracted from raw reflectivity data using REFS Mercury reflectivity simulation software (Bede Scientific, Ltd.).14 Parameters determined using this software were checked using “by hand” analytical methods. X-ray diffraction measurements were carried out on the instrument as described above. The optical configuration is as used for X-ray reflectivity, with a 1/4° divergence slit and the 0.1 exit slit removed. X-ray diffraction was carried out in a grazing angle geometry; the incident beam was fixed at ω ) 0.4° while scanning the detector. This incident angle was found to best attenuate the diffraction from the substrate. Electron probe microanalysis (EPMA) was carried out on a Cameca SX-50 Microprobe. Intensities of Ni KR, Si KR, Ge LR, and O KR lines were collected on separate wavelength dispersive spectrometers (WDS) using gas flow proportional detectors with P-10 gas. Data were collected at three different accelerating voltages (10, 15, and 20 keV). Raw intensities were corrected by procedures detailed by Donovan et al.15,16 Quantitative elemental analysis was obtained by modeling the film using StrataGEM thin film composition analysis software.17 The presence of oxygen in the films was observed to be
