Every coating company in the world needs to worry about the ever-present pinhole. From liquid coating processes like car paint, to thin-film vacuum techniques, pinholes are often the source of problems in a coating.
Pinholes, as the name implies, are small voids in the coatings. In liquid-processing methods they can be caused by poor wetting of the surface or bubbles in the solution, while in the vacuum processes used to make optical filters, they are most often caused by particulates on the surface of the glass that later fall off of the surface, leaving a void. All vacuum processes are susceptible to pinholes- even the cleanest semiconductor processes.
Anatomy of a pinhole - under the microscope
1. The "hole" is the actual void area where there is no coating. Light travels unimpeded through this hole and can cause diffraction effects.
2. The "shadow" is the region around the particle that was partially blocked from coating by the existence of a particle. Because the film thickness is less than designed, the coating does not work properly in this region. The thickness gradient in this region often causes a "rainbow" effect in reflection.
The true pinhole region encompasses both of these areas, so the "effective pinhole" is larger than the actual void. If you were to look at a pinhole under a microscope, you would see something like the examples below.
Viewing the pinhole with transmitted white light accentuates the "hole" (or void) which becomes immediately obvious, but so does the effect of the film gradient in the shadowed region, where other wavelengths preferably make it through the filter (green regions).
Affect of pinhole on filter performance
In a collimated beam, or in an intensity measurement-
Pinholes have an effect on the optical density of the part. A filter that is designed with OD 12 blocking over a 10 mm diameter clear aperture (CA) will only achieve OD 8 with a single pinhole 1 micron in diameter. The table below describes the largest single circular pinhole allowed that would result in a given OD over a set diameter of CA.
|OD||5 mm||10 mm||15 mm||20 mm||25 mm||30 mm||45 mm||60 mm|
Of course, a number of smaller pinholes can also achieve the same OD degradation. In short, the pinhole specifications dictate the amount of blocking that can be practically achieved in a filter. Omega's standard pinhole specification is <25 microns- appropriate for a 25 mm filter at OD 6. Tighter pinhole specifications are available with an increase in inspection time and lower yields, resulting in significantly higher prices.
Positioned at the detector in an imaging system-
When a filter is placed very close (or directly onto) a camera, or in a conjugate image plane in the optical train, the pinhole causes artifacts in the image. It can cause a bright region in a portion of the image, and also scattering into neighboring pixels from diffraction. These are the most demanding situations for pinhole specifications and require costly microscope inspections of each part.
Mitigating pinholes in the coating process
There are two common strategies used to reduce the number of pinholes in a coating.
- Geometry inside the coating chamber (coating "up") can reduce pinholes, but not completely eliminate them due to electrostatic attraction between the small particles and glass plates.
- Double-side coating, or redundant coatings, eliminate most of the effects of pinholes. The likelihood of a pinhole on one side of a part lining up with a pinhole on the other side is very small.
If you're not sure what your pinhole spec should be, please contact us before you order so we can walk you through the thought process!