Specialty Metal Films Co.



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Specialty Metal Films Co., LLC
 

      Metalized Infrared Windows 
 SMFCo is
 ITARS registered 
Germanium, Silicon, Sapphire, Fused Silica, others
 
 

Created by Confocal Sputtering Source Configuration.........

Confocal sputtering is a configuration of the sputtering chamber to provide for more uniform coatings on non-planar substrates, whereby the sputtering source(s) are strategically "aimed" at a rotating workpiece that has a complex shape.  Most of our window customers want two surfaces to be solderable, the face and the vertical edge.  

Direct sputtering, where the sputtering source is parallel with the optical face, is fine for coating that face, but may not be the optimal configuration for the best coverage of the vertical edges of the optic. In the direct sputtering configuration, the vertical edges of the window will receive 1/3 to 1/2 of the metal that the face receives, on average.  We say "on average" because the deposit on the edge may also be tapered, from top to bottom. So, the 3000 angstroms of solderable metal that you have on the face of your windows, may only be 1000 angstroms on the edge, and depending upon your soldering conditions, that thickness could pose a problem.  That's where the confocal configuration comes in. With the sputtering source now aimed, or focused towards the vertical edge, the metal distribution ratio between the optical face and the vertical edge becomes much more favorable.

 

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Fused Silica windows with metals deposited on edge surface only

The aimed source eliminates the glancing angle problem, resulting in better adhesion, more uniform growth structure, denser films, and significantly better metals distribution. We have improved the ratio of metal thickness on the face to the thickness on the edges from 3:1 to 2:1. So, that 3000Å to 1000Å angstrom situation that we talked about above is now 3000Å to 1500A. So what does this all mean? It means that windows coated with a 3000Å thickness of Platinum on the face perimeter, and 1500Å on the vertical edge, will survive a 240°C solder pot test (Sn63) for over 2 minutes without dewetting. That is a very aggressive test.

What metals do I need?

Typically, the solderable metal stack consists of three layers,

 

  •       The adhesion, or base layer. This layer is usually Chromium, or Titanium, and is the foundation of the stack.
  •     The solderable layer is next. This layer is often Nickel,  or Platinum.
  •     The protective layer is almost always Gold. 
 

 
Platinum or Nickel, which is better for your application?
 
Both metals solder quite well, but Platinum tends to be somewhat more user friendly, in our opinion. 
A major difference between the two metals is the dissolution rate by the solder.  Tin rich solders will immediately consume, or scavenge, the gold layer, and will next begin to dissolve the underlying layer of Ni or Pt. Nickel dissolves in molten tin at a rate of around three times that of Platinum. If your process requires relatively long dwell times in the liquidus phase, then Platinum is a better choice as your solderable layer. Or, if you think that you may want or need to reflow the solder to correct for a misaligned optic or component, then platinum lends itself much better to this situation than nickel, at least in our experience.
 
 
 Most certainly there is a cost difference. Platinum does come at a premium, but it is not prohibitively more costly than nickel.
Our high volume, commercial use customers generally use nickel. Our military and special use customers use platinum.