What are acceptable partial pressure levels during deposition?

I would like to ask you if you can provide reference values for the acceptable partial pressure levels that one would typically recommend in thin film deposition.

We are working with thin film deposition on silicon wafers with a thermal oxide, we have done RGA analysis of the process chamber (evaporation tool) and I have included the base pressure RGA analysis.

From this we can conclude a corresponding water partial pressure of around 1*10-6 Torr, which corresponds to about a monolayer of water molecule impact pr. sec. this sounds high, but on the other hand it very difficult to relate this to actual oxidation issues for e.g. the Al or Ti layers that we deposit other metals onto or for that matter the impact on the adhesion to the oxide due to this.

Water would provide the following components:                                                                                                                                           

Component:                m/z:                                  Partial pressure in chamber:             

(H2O)+                         18                                        7*10-7 Torr                                    

(HO)+                           17                                        5*10-7 Torr                           

As all originate from water the sum would indicate the partial pressure of water in the chamber:            

(H2O + HO)                Sum:                                      1.3*10-6 Torr

(O)+                              16                                        1*10-7 Torr                           

(H)+                               1                                          NA                                       

At a partial pressure of 1*10-6 Torr one monolayer hit the surface pr. second

If you could provide some reference RGA levels that you have experience with, which do provide good results for thin film adhesion then this would be very helpful.

ANSWER

My experience is primarily with web coaters which generally carry in huge quantities of water and hence I do not have particular levels of water vapour background that are or are not acceptable for deposition onto wafers.  However there are some general comments that might help.

The quality and purity of the coating does not just depend on the background pressure but it also depends on the deposition rate.  The aluminium metallization of polymer webs typically requires the system to be pumped out into the 10^-6 mBar range & when the deposition starts the pressure typically rises into the 10^-4 mBar region.  If we take the pressure to be 1 x 10^-4 mBar the time to form a monolayer is only 0.01s.  Typically the polymer passes through the aluminium deposition zone in 0.1s (or less) and so there would be 10 monolayers of contaminants arriving at the same time as the aluminium arrives.   At the typical metallizing thickness this represents a contamination of approximately 1% -2%.  For slower deposition processes this contamination level will be higher.

So if you know what the acceptable purity of your coating is you can calculate the minimum deposition rate you require to have the appropriate arrival rate of coating to contaminant.  It will depend on the deposition source what the deposition rate is. With evaporation sources being many times faster than sputtering sources.  This gets more complicated if you are also requiring a minimum density to the coatings as evaporated coatings will be less dense because of the lower energy of the deposition process. If additional energy is added the density of the coating will increase but the deposition rate will fall.

The adhesion is a separate matter.  Often surfaces are plasma cleaned to make sure the surface is chemically clean. Aluminium deposition can have improved adhesion if there is oxygen at the interface.  Again sputtered deposition gives higher adhesion because of the higher energy deposition that can remove the poorly adhered    material which evaporation tends not to do. 

If you want to speed up the outgassing of water from surfaces during the pumpdown then using either a plasma or a UV light source can put energy into surfaces that will encourage the outgassing.

Thus a simple measure of the partial pressure of the system may not be only part of the information you need. In aluminium metallizing the partial pressures are largely unknown. The base pressure and deposition pressure is often used to provide information about when the system needs to be cleaned as a dirty system takes longer to pump down and the pressure rise on starting deposition can be higher too for a dirty system.

So although I have not been able to directly answer your question I hope the response is of some use.