Atomic Layer Deposition of Highly Conductive Metals

Description:

TECHNOLOGY AREA(S): Sensors 

OBJECTIVE: Atomic Layer Deposition (ALD) techniques have established the ability to grow conformal, defect free films over large areas, atomic layer by atomic layer. While many dielectric, semiconductor, and metal materials have been deposited with ALD, the metals with the highest electrical conductivity have not been demonstrated in a reproducible manufacturing environment. The objective of this solicitation is to demonstrate ALD deposition of a very thin (<10 nm thick), highly conductive, continuous layer of silver, copper, gold, or aluminum on a dielectric substrate. 

DESCRIPTION: Atomic layer deposition (ALD) is used extensively in the semiconductor industry for the growth of high permittivity, ultra-thin dielectrics [1]. In addition to precise control of the film thickness, ALD provides conformal deposition on extremely high aspect ratio geometries [2]. This combination of features has motivated research in other nontraditional applications of ALD, in particular electromagnetic designer surfaces consisting of multilayers of different materials for specific applications [3]. For example, optical filters composed of multilayers of dielectrics with a large contrast in the index of refraction have been fabricated for bandpass filters and antireflection coatings [4]. The ability to coat arbitrary surface geometries with ultrathin films and laminates will allow for specified electromagnetic properties from the visible to microwave and has enormous potential for military and commercial applications. While ALD has been very successful at depositing nearly one hundred different materials it has been difficult to deposit metals having the highest electrical conductivity. The significant problem is the nucleation sites on the surface in which the metal deposition process starts with small metal islands. These islands grow in size as the deposition process continues and eventually the islands coalesce at the percolation threshold and the metal film experiences a huge increase in the conductivity. Ultrathin films of silver, copper, gold, and aluminum have a percolation threshold on the order of 10 nm for traditional sputter, thermal, and electron beam deposition techniques. While post annealing dielectric films at high temperatures tends to increase the uniformity of the films, annealing has a negative result on metal films due to the surface tension of metals [5]. Metal/dielectric multilayers have been used to make what has been termed transparent metals [6, 7]. The photonic band gap approach to metal/dielectric multilayers allows for a specific passband to be opened at a desired frequency range and for all other regions of the spectrum to be blocked. This type of material has wide ranging application for laser protection, sensor protection, and microwave shielding while retaining the ability to have high transparency in a spectral region of choice. The ability to achieve extremely high transparency depends on the ability to make continuous metal films of 10 nm thickness or less. For applications in the visible, silver and gold are the preferred metals due to the low losses in that spectral range. Copper and aluminum work well for longer wavelengths. Of these four metals, gold is the most robust to environmental factors and contamination. Oxide and sulfide formation can be problematic for copper, silver, and aluminum and these issues will need separate attention in the ALD process. There has been some success in ALD deposition of copper especially on metallic surfaces [8]. However, depositing copper on an oxide surface has nucleation problems similar to other techniques such as sputtering [9]. Recently, innovative surface chemistry in conjunction with plasma assisted ALD was demonstrated to produce gold films on borosilicate substrates [10]. 

PHASE I: Demonstrate the ability to grow a single continuous film of silver, copper, gold, or aluminum on a dielectric substrate with a percolation threshold of less than 10 nm thickness. The measured properties of the film should include optical transmittance, four point probe conductivity, and direct measurement of the film thickness. 

PHASE II: Demonstrate the ability to grow a multilayer metal/dielectric laminate containing at least 3 metal layers that have individual thicknesses of 10 nm or less. The measured properties of the film should include optical transmittance, four point probe conductivity, and microwave transmittance, and a direct measurement of the film thickness. 

PHASE III: Demonstrate a working ALD system that can deposit single or multilayer metal/dielectric films onto dielectric substrates including 3D printed materials for applications in filtering, shielding, conductive surfaces, and electromagnetic signature control. 

REFERENCES: 

1: S.M. George, "Atomic Layer Deposition: An Overview," Chem. Rev., 110, p. 111 (2010), DOI: 10.1021/cr900056b110.

2: G. Pardon, H. Gatty, G. Stemme, W. van der Wijngaart and N. Roxhed, Al2O3 dual layer atomic layer deposition coating in high aspect ratio nanopores," Nanotechnology, 24, p. 11 (2013).

3: D. Riihel, M. Ritala, R. Matero, M. Leskel, "Introducing atomic layer epitaxy for the deposition of optical thin films," Thin Solid Films, 289, p. 250 (1996), DOI:10.1016/S0040-6090(96)08890-6.

4: A. Szeghalmi, M. Helgert, R. Brunner, F. Heyroth, U. G'sele, and M. Knez, "Atomic layer deposition of Al2O3 and TiO2 multilayers for applications as bandpass filters and antireflection coatings," Applied Optics, Vol. 48, p. 1727 (2009).

5: R. J. Warmack and S. L. Humphrey, "Observation of two surface-plasmon modes on gold particles, Phys. Rev. B 34, 2246 (1986).

6: M.J. Bloemer and M. Scalora, "Transmissive properties of Ag/MgF2 photonic band gap," Appl. Phys. Lett. 72, 1676 (1998

7: M. Scalora, M. J. Bloemer, A. S. Pethel, J. P. Dowling, C. M. Bowden, and A. S. Manka "Transparent, metallo-dielectric, one-dimensional, photonic band-gap structures," J. Appl. Phys. 83, 2377 (1998).

8: L.C. Kalutarage, S.B. Clendenning, and C.H. Winter, "Low-Temperature Atomic Layer Deposition of Copper Films Using Borane Dimethylamine as the Reducing Co-reagent," Chem. Mater., 26, p. 3731 (2014), DOI: 10.1021/cm501109r.

9: Z. Li, A. Rahtu, and R.G. Gordon, "Atomic Layer Deposition of Ultrathin Copper Metal Films from a Liquid Copper(I) Amidinate Precursor," Journal of The Electrochemical Society, 153, p.787 (2006).

10: M.B.E. Griffiths, P.J. Pallister, D.J. Mandia, S.T. Barry, "Atomic layer deposition of gold metal," Chem. Mater. 44 (2016).

 

KEYWORDS: Atomic Layer Deposition, Ultrathin Film, Transparent Metal, Metal/dielectric Multilayers, Thin Film Laminates, Nucleation, Metal Island Film 

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