Philip D. Rack

 UT office (865) 974-5344
ORNL office (865) 241-1598
fax (865) 974-4115

Combinatorial Thin Film Synthesis for Rapid Materials Discovery

One of the cornerstone pieces of equipment in my laboratory is a multisource rf magnetron sputtering system capable of depositing thin film metals, semiconductors, and insulating materials.  The system consists of a 20” diameter stainless steel chamber pumped with a 550lps turbomolecular pump.  The system is equipped with a load-lock chamber that can currently accommodate up to 6” substrates and has a routine base pressure ~ 1x10-8 Torr.  The sputtering system has 4-2” sources (scalable to 5) with in-situ source tilting capabilities which allows thin film materials (metals, semiconductors, and insulators) to be deposited in a combinatorial or layered fashion.  The sources have a variable magnetron assembly for balanced, unbalanced and magnetic materials modes and are powered by 500 Watt rf power supplies and matching networks.  Each source has a localized gas delivery for optimized deliver of the inert sputter gas.  In addition, the substrate holder assembly has a localized gas injection for preferential delivery of reactive gases to the substrate for reactive sputtering.  The compositions as a function of position can be uniform by rotating the substrate, or a deliberate composition gradient can be introduced by depositing onto a stationary substrate.  By varying the individual source powers and tilt angles, large composition gradients can be realized across a single wafer leading to rapid materials discovery.  To enhance/modify the thin film quality the substrate is equipped with a heater (up to 800oC) and bias sputtering capabilities.  Figure 1 shows a schematic of the multi-source sputtering system and a digital photograph of the system while depositing a combinatorial ternary Fe-Cr-Ni thin film sample. 
Figure 1.  a) Schematic of the multi-source rf-magnetron sputtering system and

digital photograph of the sputtering system while depositing a combinatorial ternary phase diagram of Fe-Ni-Cr.

 This method has been used to explore Y2O3:Gd [1,2] and Y3Al5O12:Gd [3,4] ultraviolet emitting materials, Y3Al5O12:Cr [5] temperature sensor materials, Mo-W [6] high temperature electrodes, Zr-Cu-Al [7] bulk metallic glass alloys, Fe-Ni-Cr [8,9] phase diagrams, and Ni [10] and Cu-Ni  [10] for carbon nanofiber catalysts.  

 Vertically Aligned Carbon Nanofiber Catalyst Development

Spatially controlled carbon nanofiber arrays are grown by plasma-enhanced chemical vapor deposition (PECVD) at pre-defined binary alloy catalyst sites. The nanofiber tip radius and fiber shape must be precisely controlled for stable electron field emission applications and intracellular probe arrays. A combinatorial Cu-Ni thin film was used as a catalyst/substrate for PECVD carbon nanofiber growth and yielded a rich variety of carbon nanofiber tip architectures strongly dependent on thin film composition.  To investigate the effect that the Cu-Ni composition has on the carbon nanofiber morphology, copper and nickel were co-sputtered to form an alloy CuxNi1-x ranging from 0.2<x<0.8 along a 10cm long silicon substrate.  Figure 2 shows a series of scanning electron micrographs of the carbon nanofiber morphology as a function of composition which reveals at ~70% copper, a high radius of curvature (~ 20nm) nanofiber results.  These fibers are currently being explored as possible intracellular probing devices and will be investigated for their field emission properties.  This combinatorial approach was successful in rapidly determining a new alloy catalyst composition, whereas a standard serial approach would have required many catalyst and nanofiber growth runs.   Figure 2.  Scanning electron micrographs along of cabon nanofibers with varying copper concentration in the catalyst alloy prepared via the combinatorial thin film sputtering technique. 

Experimental Phase Diagram Determination

Equilibrium and non-equilibrium phase diagram determination is typically a very labor intensive process requiring the preparation and characterization of numerous alloy samples.  To rapidly identify the equilibrium and non-equilibrium phase diagram of Cr-Ni-Fe, ternary libraries were prepared by co-sputtering Cr, Fe, and Ni on a single-crystal sapphire substrates.  Subsequent to the depositing five samples, four of the samples were annealed at 200, 400, 600, and 800C.   Structural and compositional maps of the alloys were produced using synchrotron radiation and simultaneous detection of 2D diffraction patterns and x‑ray fluorescence spectra.  Figure 3 shows the phase diagram evolution for this ternary alloy as a function of annealing temperature.  Good agreement between the measured and calculated phase diagram is demonstrated which illustrates the utility of this approach for materials development.  As-deposited a very non-equilibrium structure exists with a metastable a-manganese simple cubic structure (shaded area).  When the alloy is annealed the grain size increases (as determined with the Scherrer formula) and the phases evolve into the thermodynamically stable phase. 

  Figure 3.  Phase diagram evolution for Fe-Ni-Cr ternary alloy. 

 

References

  1. J.D. Fowlkes, P.D. Rack, J.M. Fitz-Gerald, “Ultraviolet emitting (Y1-xGdx)2O3-z thin films deposited by rf magnetron sputtering:  structure – property – thin film processing relationships” Thin Solid Films, (in press).

  2.  J.D. Fowlkes, P.D. Rack, J.M. Fitz-Gerald, Ultraviolet emitting (Y1-xGdx)2O3-z thin films deposited by rf magnetron sputtering:  Combinatorial modeling, synthesis, and rapid characterization, Thin Solid Films, Vol. 510, no. 1-2, pp. 68-76 (July 2006).

  3. Yuepeng Deng, Jason D. Fowlkes, Philip D. Rack, James M. Fitz-Gerald, Thin Film rf Magnetron Sputtering of Gadolinium Doped Yttrium Aluminum Garnet Ultraviolet Emitting Materials, Journal of Optical Materials Vol. 29 pp.183-191 (November 2006).

  4. Yuepeng Deng, Jason D. Fowlkes, James M. Fitz-Gerald, Philip D. Rack, Combinatorial Thin Film Synthesis of Gd-doped Y3Al5O12 Ultraviolet Emitting Materials, Applied Physics A. 80 no. 4, pp. 787-789 (February 2005). 

  5. Yuepeng Deng, Y. Guan, Philip D. Rack, Combinatorial Synthesis and Parameter Optimization of Chromium-doped Yttrium Aluminum Garnet Thin Film Sputtering, Thin Solid Films, vol 29, no 2-3, pp. 183-91 (Novemeber 2006). 

  6. Seung-Ik Jun, Anatoli V. Melechko, Timothy E. Mcknight, Michael L. Simpson, Philip D. Rack “Electrical and microstructural characterization of molybenum tungsten electrodes using a combinatorial thin film sputtering technique” Journal of Applied Physics Vol. 97 059046 pp. 1-6 (March 2005).

  7.  Yuepeng Deng, Yinfeng Guan, Jason D. Fowlkes, S.Q. Wen, George M. Pharr, Fengxiao Liu, Peter K. Liaw, C.T. Liu, Philip D. Rack, A combinatorial thin film sputtering approach for synthesizing and characterizing ternary ZrCuAl metallic glasses, Intermetallics (in press). 

  8. A. Rar, J. Frafjord, Jason D. Fowlkes, E. D. Specht, P. D. Rack, M. L. Santella, E.P. George, and G. M. Pharr, “PVD Synthesis and High-Throughput Property Characterization of Ni- Fe- Cr Alloy Libraries” Journal of Measurement Science & Technology, Vol 16, pp. 46-53 (January 2005). 

  9.  E.D. Specht, P.D. Rack, A. Rar, G.M. Pharr, E.P. George, J.D. Fowlkes, H. Hong, and E. Karapetrova, “Metastable Phase Evolution and Grain Growth in Annealed Nanocrystalline Cr-Fe-Ni Films” Thin Solid Films (in press). 

  10. K. L. Klein, A. V. Melechko, P. D. Rack, J.D. Fowlkes, H. M. Meyer, and M. L. Simpson, “Cu-Ni composition gradient for the catalytic synthesis of vertically aligned carbon nanofibers” Carbon Vol. 43, Iss. 9, pp. 1857-1863 (August 2005).