The tube-cleaning process is a critical part of preparation. I find that using the first cleaning step to remove metallic impurities works well. If you were to HF etch first, you may etch the glass around the metallic impurity, but not necessarily remove it. If you only HF etch, some metallic impurities may remain. Here is my approach to the tube-cleaning process:

  • For the first step, aqua regia can be used (an acid mix of hydrochloric acid nitric acid, and water). A typical acid cleaning cycle is 30-60 minutes with aqua regia to remove metallic impurities, followed by a DI water rinse. You may want to adjust the time depending on your process.
  • The next step is to etch the glass surfaces, cleaning the inside and outside. Most MCVD operators use hydrofluoric acid. A 15% concentration for 30-60 minutes is a good starting point. You may want to adjust the time and concentration for your process.
  • Follow that etch cycle with a final DI rinse and dry with nitrogen to prevent water spotting.

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