What happens to the substrate kept in high vacuum inside the Vacuum Chamber for a long time more than 5hrs? a) Will the contamination on the substrate removed at high vacuum? b) Any change in the surface of the substrate? c) Any other phenomena. Please give your answer.
For clarity, vacuum is the antithesis of pressure. Higher vacuum is lower pressure, so the term high pressure vacuum, to me, means a poor vacuum. I will later refer to lower pressures as better vacuum. On with the answers...
I would not recommend using your vacuum system to remove any contaminants on your substrates. Proper cleaning protocols are required for all substrates to be considered "clean". H2O, IPA, acetone and other solvents will easily volatilize in reduced pressure, but many other contaminants will not (this depends on the vapor pressure of the contaminant). Heat may be used to volatilize some other contaminants but you risk contamination of the chamber by these chemicals which can cause problems later in the process, e.g. during metal deposition when some parts of the chamber may be hot. Keep in mind that when a molecule desorbs from one surface, it may adsorb on another before it gets pumped out. It can take thousands of cycles to get a molecule pumped out once your mean free path is larger than you chamber. This is a reason for gas purging with N2, He, or Ar.
Changes in substrate surface chemistry will depend on the substrate. For substrates like SiO2 or ITO, there will be no change in the material but H2O and VOC's will be removed from the surface which does change the reactivity of the surface. As stated by Saim above, substrates that oxidize at atmospheric pressure will see a reduction in oxidation rate at lower pressures which increases their surface reactivity with other chemical species that you may introduce as a coating.
Whether or not you ever reach any equilibrium of gasses in your chamber will depend on how you pump on the chamber and how many gas sources or leaks you have.
Thank you professors for the answers. I mistakenly wrote high pressure vacuum.. It is high vacuum.. I corrected it.
Prof. Ron Reiserer explained the case in a comprehensive way, so I just want to add some (hopefully helpful) hints:
- If your pumping system is using (ancient :) ) diffusion pumps, the surface of your sample would be contaminated with carbon. Try to use as short pumping cycle as possible;
- A simple formula, well known among technicians: 2 hours=removal of 1 monolayer of water. If adhesion in your samples depends on water, you should pump at least for 2 hours to remove first monolayer of water. Usually each sample mounted from ordinary air is covered with two monolayers of water, so 2 more hours may be necessary.
For removal of contamination,only high vacuum is not sufficient.Baking in inert atmosphere is also required.
Is there any change to the "getter" material when it adsorbs materials to increase and maintain a hard vacuum? I am interested if its resistance or color changes in any way.
Regards,
Joe Rostron
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