Originally posted by joe shmo
I added that information in regards to leakage and sealing that you guys were discussing. It is the mechanical reality of the pump/system relationship. In reality things are less than ideal becuase there is a "real system" coupled to a "real machine". They each have behavioral characteristics to consider for the model. I was also bringing that up becaus ...[text shortened]... rce (via pressure) provided by the chuck (or he could just put a pressure guage in the chamber).
I specifically used 50% vacuum, not perfect vacuum because that is what I read on other vacuum chucks, where perfect in terms of physical pressures and such is 28 inches of mercury and so 50% is 14 inches of mercury or about 7 PSI which I converted to metric. That's all I did, convert to metric, I just wanted to show my work.
If my numbers are wrong please show me. It's just that the numbers I generated roughly agree with what we feel when we try to separate the substrate from the chuck.
I am bringing this up because I designed a chuck with an o ring instead of channels so the force would not depend on surface defects like scratches and such that would reduce the force due to leakage.
I did find the surfaces to be very smooth, one a very smooth alumina substrate about 210 by 120 mm and I took pains to make the chuck surface smooth and scratch free by using a fine diamond grit polishing tool.
I can see how good that is by the amount of photo resist fluid getting sucked under the substrate. At first go, before polishing, it was terrible, photo resist getting into the controlling vacuum solenoid which jammed up the little magnetic plunger that opens to vacuum and closes. That after a filter that should capture most of the photo resist getting into the vacuum tubing.
After polishing the leakage went way down, it was obvious when we turned off the vacuum and pulled the substrate loose for the next step in processing, the amount of photo resist under the substrate was at least 90% less. It got gradually worse after a number of substrates were processed presumably because the alumina is much harder than the aluminum chuck and would inevitably scratch that surface and thus reduce the effective holding force and contribute to solenoid contamination, which requires disassembly and cleaning with acetone to put it back in operation.
My O ring will stop that completely or at least let us run ten times longer without solenoid contamination.
At least putting out these questions illustrated where I was going wrong calculating the downwards clamping force.
At first I didn't understand why I was only seeing maybe a kg of upwards force on the substrate to cause separation, and it needs a good deal of force when the substrate is spinning at 3000 or more RPM. When the vacuum gives way due to leakage or whatever, bad pump, the substrate flies off the chuck with great force, enough to penetrate the metal outer shell which looks like it has been hit twenty times with shrapnel!
So now I understand the surface area of the channels times the percentage of vacuum compared to atmosphere is what gives the downwards holding force. The channels are only a mm wide and maybe 100 mm long so that would be about 100 mm square to calculate the downwards force.
When I install the new chuck with the o ring the area will be much greater since it will be around 115 X 95 mm or so, giving a force of more like 100 kg holding to the chuck which will mean the substrate will never fly off again even if the vacuum goes to 10% or so instead of my estimate of 50%.
The o ring will also ensure the two surfaces will be separated by a small distance, maybe 100 microns or so which will mean small scratches will not effect the overall holding force.
My only concern there is if there is too much separation the downwards force will break the substrate which as I said is alumina but only 1 mm thick and of course extremely brittle.
The question I have to answer is if the separation forced by the distance inherent in an o ring is sufficient to cause a bending force sufficient to break the alumina substrate.
I have a back up plan for that if needed though.