Ion Assisted Deposition

As coating specifications and tolerances become more challenging, you may find yourself unable to meet customers’ needs with a standard evaporative coater. With this in mind, many evaporative coating facilities are looking into Ion-Assisted Deposition (IAD) to provide added value to their existing equipment. The benefits of IAD are plentiful, and ion beam sources can be surprisingly easy to use, too!

This article describes the benefits of IAD such as improved adhesion, stoichiometry, and defect mitigation. IAD also plays a key role in improvements such as durability, damage threshold, and reducing spectral shifting. In the “Handbook of Plasma Processing Technology”, J.J. McNally[1] discusses several types of IAD and their benefits in detail, including the basic physical reasons for each idea. For users new to the idea of Ion Beams, this chapter provides a great deal of clarification and can be very valuable.

A Physical Description

     A typical evaporation process requires low-energy particles of a desired material to adhere to the substrate. The energy that the particles have is low – in the 0.1eV range. Due of the low energy state, the particles tend to form crystalline structures (columns) which are somewhat brittle and have large numbers of defects and pores. Once the film is exposed to atmosphere, water vapor naturally migrates into those pores causing a spectral shift and further weakening the coating.
Evaporation also suffers from poor adhesion. Any particulate contamination on the substrates before deposition weakens the coating bonds, and can lead to flaking. Contamination can also be in the form of native oxide layers. Achieving oxide stoichiometric films is challenging with evaporation and may require either extremely high oxygen flow, reduced evaporation rates, or an acceptance that the stoichiometry will not be ideal.

Ion Beam Solution

     An ion beam source directs high-energy oxygen ions at the substrate. These incoming ions have far greater energy than the evaporate (on the order of 100 – 200eV), and upon striking the substrate will deposit this energy into the existing layers of the coating. Importantly, the energy is high enough to embed the oxygen ions down several nanometers into the coating, providing a dose of reactive gas to regions which may not have been fully oxidized before being covered by evaporate. Further, when the energy of the incoming ion is deposited into the coating, surface atoms from the coating are liberated to move and shift along the surface. This helps create an amorphous solid as opposed to a crystalline one, removing the columnar structure that promoted water absorption and creating a more dense overall structure. The absence of defects and better stoichiometry also reduce light absorption, increasing the damage threshold and overall performance of the coating.
As an added benefit, an ion beam source can be directed at the substrates in vacuum conditions before the coating process begins to pre-clean the surfaces removing any native oxides and particulates. For some cases, ion beam energy can be increased to provide surface texturing. Both methods promote adhesion of the evaporate.

Citations

     Don’t take our word for it – let decades of research by field experts speak for itself! Below are a few example papers describing research performed by field experts toward characterizing the exact benefits of ion assisted deposition:

Sites et. al. investigated the value of ion beam pre-clean on the substrates using Argon ions and lower current densities[2]. The results of their experiments showed much higher substrate adhesion of the coating – in fact “[b]oth the single-layer and three-layer coatings exceeded the limit of our adherence tester (10,000 psi or 6×107 N/m2).” Under adhesion testing, their substrates usually failed before the coating.

In their paper, Martin, et. al. tested several common films including TiO2 and SiO2 with and without ion assist in their deposition process[3]. Their results showed “very large changes in the spectral transmittance curves” when the substrates were exposed to air with a 50% relative humidity after reactive evaporation coating. The same coatings showed an increase in refractive index of well over 2%. Substrates deposited with IAD using Argon showed high absorption, but substrates deposited with oxygen IAD showed “much lower absorption losses … and a very large reduction in moisture absorption, indicating very high packing density.” The spectral transmittance of these substrates also changed less than the sensitivity of their instrumentation (less than 0.2%) after exposure to moisture. Multilayer coatings showed similar reductions in absorption and spectral transmittance shift.

Similarly, Al-Jumaily et. al. tested spectral shift, index of refraction, and extinction coefficient for evaporated and IAD-deposited films[4]. Their results showed consistently better extinction coefficients under IAD, and when the samples were bombarded with Oxygen instead of Argon, better transmittance / absorption as well. They also explain some of the basic physics behind why IAD is best when using Oxygen as the ionized gas. Finally, their results showed a 12% improvement in the packing density of the IAD-deposited coatings.

Research performed by Demiryont et. al. demonstrated the importance of reaching the ideal stoichiometry of thin films to optimize your index of refraction and obtain the minimum absorption coefficient[5]. Fully-oxidized (stoichiometric) films had significantly better n-values, and as much as 3 orders of magnitude lower k-values than cermet or suboxide films. In a field where a 1% variation can make or break an optic, the importance of 1000 times lower absorption cannot be overstated.

These publications demonstrate the value of appropriate ion-assisted deposition to the overall performance of an optical coating. Clearly adding energy (and energetic oxygen) to a coating during the deposition process is critical for minimizing defects, eliminating spectral shift, and optimizing stoichiometry.

Let us assist you with your assist selection!

[1] McNally, J.J., “Ion Assisted Deposition”, Handbook of Plasma Processing Technology, S.M. Rossnagel, J.J. Cuomo, W.D. Westwood, Ch. 20, pp 466-482
[2] Sites, J.R., Gilstrap, P., Rujkorakarn, R., “Ion Beam Sputter Deposition of Optical Coatings”, Optical Engineering Vol. 22 Issue 4, pp. 447-449 (1983)
[3] Martin, P.J., Macleod, H.A., Netterfield, R.P., Pacey, C.G., Sainty, W.G., “Ion-Beam-Assisted Deposition of Thin Films”, Applied Optics, Vol. 22 No. 1 (1983)
[4] Al-Jumaily, G.A., Edlou, S.M., “Optical Properties of Tantalum Pentoxide Coatings Deposited Using Ion Beam Process”, Thin Solid Films, Vol. 209 pp. 223-229 (1992)
[5] Demiryont, H., Sites, J.R., Geib, K., “Effects of Oxygen Content on the Optical Properties of Tantalum Oxide Films Deposited by Ion-Beam Sputtering”, Applied Optics, Vol. 24 No. 4 (1985)