Glass and Optics



Glass and many types of optics hav ebeen cleaned.  Cleaned objects include glass, uncoated substrates, coated glass, lenses, all kinds of optics, diamond turned optics, Si and Ge optics, mirrors, telescopes mirrors, gratings (carefully), diamond turned optics, and more.


Coated and Uncoated Glass 
We have studied several items within this category including cleaning coated and uncoated glass, quartz, and also compared solvent cleaning to snow cleaning. Overall, with optics, substrates, and other glass objects, cleaning is effective with proper attention to processing parameters.  Coated optics pose minimal problems.  If a thin film is properly deposited on a glass substrate, it will stay on the substrate.


Cleaning Dirty Filters 
Carbon dioxide has been used extensively to clean glass and optical surfaces. This includes removing contamination before applying optical coatings, cleaning surfaces before or after assembly, and many other applications. One of the first examples we published (see Sherman - 1991), was the removal of extensive debris from neutral density and bandpass filters. These filters, all one inch in diameter and about a 1/4" thick, were loaded with dirt and grease from handling after being used in a laboratory. We cleaned the surfaces with CO2 Snow. The surface compositions, as found by XPS and given below, and optical inspections, indicated effective cleaning. Visually, they were "clean" and this was verified by examination by bright field and dark field images.

XPS Surface Analysis Data for Filters


Coated optics will remain coated if the deposited layer is properly bonded to the substrate.  This means that almost all thin film deposited layers will stay if the substrate was cleaned properly.  Damage is not expected for the overlayer either.  Kodak, in a 1998 patent, showed that a thin gold layer on glass had the same reflectivity after CO2 cleaning as before cleaning.  Detailed RMS analysis by AFM showed that the overall smoothness was better after CO2 cleaning.  This result is expected in that CO2 cleaning will only remove physically adsorbed species, not chemically bound species.  


A few glasses are thermally sensitive and in our experience glass substrates or overlayers did not break.  However, there are exceptions.  We have seen issues with potassium phosphate, lithium niobinate, and a couple other glasses such as NPK52.  Generally, if a material has a bad reputation for thermal shock, it may not be a good material for CO2 cleaning.  However, magnesium fluoride thin films stay intact without damage while bulk fluoridesamples can show damage immediately.  

Stain Removal

CO2 Snow Cleaning can remove stains from glass.  This includes solvent stains, water stains and even Sharpie markings.  This procedure relies upon the freeze fracture mode and is not always quick.  It is imperative that proper thermal input be applied before and during cleaning, in that glass will freeze immediatley.  Below is an image of a silcion wafer with Sharpie faces on them and after CO2 snow cleaning, there are just two.  Solvent stains are easier, and water spots are inconsistent.  

Another example is an X-ray Optic that was cleaned on the left side.