Chemical corrosion protection of optical components using atomic layer deposition.

Very thin films deposited using atomic layer deposition (ALD) on aluminum mirrors showed extraordinary resistance toward concentrated alkali solutions, various chemical etchants, and solvents. Aluminum mirrors with no surface protection dissolved immediately in 24% aqueous KOH. Mirrors protected with 30 nm electron-beam deposited silica film were fully removed in 30 min. Mirrors with 10, 30, and 120 nm films grown by ALD required, respectively, 10, 15, and 60 h of exposure to the alkali for the mirror to be fully removed. Likewise, large-scale hydrogen bubbling was observed immediately upon immersing mirrors coated with electron-beam deposited silica in aqueous KOH, whereas no visible hydrogen bubbling was detected for the mirror protected by ALD silica. For mirrors protected by 120 nm ALD silica, a 2 h soak in various acid and solvent media produced no discernible change.