Generally, single-step cleaning for MLD diffraction gratings for high laser damage resistance is not possible due to the several types of contamination present on the surface that are not removed by any one process. A number of cleaning operations are necessary to remove all contamination from the surface of an MLD diffraction grating. The exact details of the cleaning steps can be influenced by the materials comprising the diffraction grating, and also be influenced by the manner in which these materials have been deposited during the multilayer coating process.
Applicants have demonstrated the following process to be effective for the cleaning of MLD gratings consisting of alternating layers of, for example, HfO2/SiO2 with a grating etched into the top SiO2 layer:
(1) Exposure of a grating surface to aqueous base (5% NaOH solution or commercial RS6 solution available from Cyantek Corporation of Fremont, Calif.), at room temperature in a gentle agitation, e.g. by flowing the aqueous base thereon. Preferably, this is performed for approximately 5 minutes. This removes the bulk of the organic photoresist mask. This step is immediately followed by extensive rinsing with de-ionized (DI) water, and preferably ultrapure DI water.
(2) Exposure of grating surface to aqueous oxidizing acid solution, preferably aqueous oxidizing sulfuric acid solution such as for example Nanostrip™ or Nanostrip™ 2X solution, commercially available from Cyantek Corporation, self-heated by periodic addition of water to 50-70 C., or at room temperature, in a gentle agitation (e.g. flow) process. This is preferably performed for approximately 60 minutes. This removes metallic contaminants and residue organic compounds. Alternatively, a point-of-use oxidizing acid solution, such as for example ‘Piranha’ solution, can be used to similar effect, although not at room temperature. This step is immediately followed by extensive rinsing with ultrapure DI water. It is appreciated that Piranha solution is a mixture of a strong acid (e.g. sulfuric acid, H2SO4) and a strong oxidizing agent (e.g. hydrogen peroxide, H2O2) which produces an extremely energetic solution. However, Piranha solution must be prepared immediately before use, has a very limited shelf life, and cannot be stored in normal closed containers due to an explosive pressure buildup caused by the gradual loss of hydrogen peroxide gas, ergo “point of use.”
(3) Optionally, exposure of grating surface to a weak tetrymethyl ammonium hydroxide solution to neutralize any remaining acidic residue on the surface, flowed by extensive rinsing with ultrapure DI water. This is performed prior to the next step.
(4) Exposure of grating surface to an oxygen plasma “ashing” process using reactive oxygen species generated by RF excitation of an oxygen flow in a vacuum chamber for a period of several hours. This process oxidizes and removes fluorinated hydrocarbon residue.
(5) Exposure of grating surface again to aqueous oxidizing acid solution, (preferably Nanostrip™ or Nanostrip™ 2X solution), self heated by periodic addition of water to 50-70 C., or at room temperature, in a gentle agitation (e.g. flow) process for approximately 60 minutes. This removes metallic contaminants possibly introduced by the ashing process. This step is immediately followed by extensive rinsing with ultrapure DI water.
The above process can be modified as necessary due to the incompatibility of certain MLD coating materials to certain chemistries. For example, if an Al2O3 etch-stop layer is incorporated into the MLD stack, treatment with aqueous base or hot acid is not possible due to dissolution of this layer. The cleaning process is modified by removing the first step (i.e. exposure to aqueous base) and changing steps (2) and (5) to room-temperature Nanostrip™ solution for a longer period of time greater than 60 minutes.
While particular operational sequences, materials, temperatures, parameters, and particular embodiments have been described and or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.
This application claims priority in provisional application filed on Aug. 23, 2006, entitled “Method for Cleaning Diffraction Gratings” Ser. No. 60/839,753, by Jerald A. Britten et al, and incorporated by reference herein.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.
Number | Date | Country | |
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60839753 | Aug 2006 | US |