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Coating Process Tutorial

There are currently a number of different processes used for the manufacture of optical thin films, the most common of these being traditional thermal or electron beam evaporation, ion assisted deposition (IAD) and ion beam sputtering (IBS).  If properly utilized, each of these techniques has value, and possesses its own unique advantages and limitations.  Unfortunately, many coating manufacturers use only one of these methods, and therefore attempt to make it appear that their single approach is uniquely advantageous for all applications.  REO understands that this is not the case, and employs all of these coating technologies, using each where necessary to deliver the precise combination of performance, physical characteristics and price that our customers require. 

While we don’t believe that our customers should have to become expert in thin film coating methods, we do feel that informing the market will help people to make better decisions and cut through the hype.  To that end, this section provides a brief overview and comparison of various coating technologies.

	Wavelength Range			Mechanical & Environmental Durability	Laser Damage	Scatter & Absorption	Precision *	Cost
	< 266 nm	266 nm - 5 µm	> 5 µm
Evaporative	good	good	good	poor	good	poor	poor	good
IAD	fair	good	fair	fair	good	fair	fair	fair
IBS	poor	good	poor	good	good	good	good	fair

* The ability to precisely control deposited layer characteristics and therefore reliably meet even difficult performance targets.

E-beam and Thermal Evaporation

Traditional electron-beam and thermal evaporation are the most widely employed methods for producing thin films because of their simplicity and relatively low cost of implementation.  Here, a coating material is heated either resistively (for metals) or through electron beam bombardment (for dielectrics) within a high vacuum chamber until it vaporizes.  This vapor then streams away from the source and recondenses on all surfaces that are in a line of sight with the source. 

This method of evaporation is a relatively low energy process, and, as a result, the dielectric films it produces are porous, of relatively low density, and exhibit a columnar structure. Typically, the substrate is heated to several hundred degrees Celsius during coating to mitigate this effect, but it is by no means eliminated. 

The problem with these porous films is that they can subsequently absorb moisture, which changes the refractive index of the layer(s).  This, in turn, means that coating performance parameters (such as center wavelength) can shift in use with changes in ambient temperature and humidity.  Low density also limits mechanical durability to some extent, although these films can typically meet most of the MIL-SPEC durability and environmental requirements. 

Primary advantages:

• Low cost
• Works with coating materials from the deep UV through the infrared

Primary disadvantages:

• Heat cycling during processing can limit substrate choice and introduce internal stress
• Lower environmental stability and mechanical durability

Ion Assisted Deposition (IAD)

IAD is a variant of the electron-beam evaporation process which adds a high energy ion beam that is directed at the part to be coated.  These ions act almost like an atomic sized hammer, producing a higher film density than can be achieved with purely by evaporation alone.  The ion beam can also be used to pre-clean or etch the surface of the substrate, which can improve film adhesion. 

The result of higher coating density is improved mechanical durability, greater environmental stability and lower scatter than films produced using just electron beam evaporation.  The amount of ion assist can be smoothly varied from zero up to its maximum level for each layer individually, which also gives the process tremendous flexibility.  In particular, it enables the intrinsic stress of a coating to be modified during deposition, in some cases changing the overall film stress from tensile to compressive.  This can help to maintain substrate surface figure, especially when depositing thick infrared coatings.  It also extends the wavelength range over which the process can be used.  For example, while IAD is not compatible with some of the commonly used materials in the infrared, it can be used solely on the outermost layer to yield an overall coating with superior durability. 

Primary advantages:

• Enhanced density provides good compromise between cost, spectral stability and durability
• Can be selectively employed from the UV through the IR
• Doesn’t require heating the substrate which broadens material choices

Primary disadvantages:

• Higher scatter and loss than IBS

Ion beam sputtering (IBS)

In this technique, a high energy ion beam bombards a target, typically composed of a metal or oxide, causing target atoms or molecules to sputter off.  These particles then stream away from the source and are then deposited on to the substrates.  A low pressure of oxygen is usually present in the chamber to act as a reactant for the creation of oxides from metal targets, or to re-oxidize any free atoms dissociated by the sputtering process when using oxide targets. 

The high energy of the ion beam sputtering process results in extremely uniform, high density, completely amorphous films with excellent adhesion to the substrate.  This translates into high environmental stability and mechanical durability.  Furthermore, the surface roughness of the deposited layers is very low, even into the sub-angstrom level, which can yield visible and infrared with a combined scatter and absorption of less than 1 ppm. 

The ability to precisely control the ion beam sputtering process makes it possible to hit performance targets with greater precision than evaporative techniques.  Thus, it is a good choice for producing steep edge filters, very broad band mirrors, and multi-wavelength mirrors and AR coatings.

The biggest drawback of ion beam sputtering is that it only works with a limited range of materials, typically metal oxides.  Most of these aren’t as transmissive below 266 nm or above about 5 µm. 

Primary advantages:

• Delivers the ultimate in stability and durability
• Lowest possible absorption and scatter of any coating technology
• Doesn’t require heating the substrate which broadens material choices
• Enables accurate process control resulting in high precision coatings

Primary disadvantages:

• Not applicable in the deep UV or far IR
• Higher cost than evaporative due to longer cycle times