I am not so sure atomic weight it the right answer. In fact, I wouldn't even consider atomic weight one of the parameters. Diffusion is just the motion of molecules results in a change in density of the sample until it reaches equilibrium. To me, temperature and pressure would best way to speed up or slow down this process. Because this company has clean rooms perhaps they are referring to diffusion as it relates to optics which depends on lens shape and tepidity of liquid or cleanliness of air ect. I think I am OVER THINKING IT and getting bogged down in what i think the company is looking for, as opposed to the simple mathematical theory. The most important mathematical parameter is the diffusion coefficient. That is my final answer. Yes. Diffusion Coefficient.

While the molecular weight does play a role, the most basic answer would have been the difference in concentration across the boundary layer and the time given to diffuse.

the answer should not be based on what you know from college but from the companies perspective. there are 4 things which impacts diffusion (company level); species, energy, dose and projection angle. least impact of all is projection angle.