I wasn't sure where to ask a question about spin casting but I thought I would start with the polymer people. I am currently an undergraduate research assistant, and part of my task involves spinning ultra-thin layers of liquid pdms (
I've done a little spin coating but I'm by no means an expert. Coincidently I have also used PDMS - in my experience it is useful to use a 'program' of spin speeds. Depending on the viscosity of solution you begin with, you can start out with a slow spin speed to distribute the solution a little and then ramp up the spin rate. This will obviously take a little experimentation but certainly using a single high spin rate did not work in my case. How do you intend to asses the uniformity of your coating (down to 5nm ) across the wafer?
I will try to start using a program of spin speeds, (maybe by starting out with ~300 rpm for 10s, and moving up to 6000 rpm in steps or something),
It is interesting that you said the single high spin rate did not work in your case, so that gives me even more incentive.
As for assessing the thickness, I have available an ellipsometer that I have been using that can give nanometer resolution. I will also note that our university's material science center is about to purchase a very expensive optical profiliometer, and maybe they will figure out soon how to measure films less than a micron thick with it.
I believe we also have one of those machines that is used to determine the flatness of large diameter objects (optical lenses, etc. greater than 20cm diameter), but I can't remember the name of that machine, nor have I ever used it.
The spin coater is currently in a fume hood, but I can smell it slightly after using it a lot. So I am very interested in hearing about safer silicon fluids that can be used as solvents. Do you have any suggestions?
Actually I was quoting OSHA PEL (permisible exposure level) but ACGIH (American Conference of Government and Industrial Hygenists) TLV (Threshold Limit Value) is 50 ppm for skin contact. US, Canadian and EC chemists must be familiar with these things because they guide workplace exposure. Frank
I haven't done much spin coating for the past ten years, but I'll take a crack at helping...
As GillianH commented, I'd agree that a single high speed spin will result in your excess material giving you an inconsistent profile.
A useful "rule of thumb", which was demonstrated to me by an experienced photolithographer, was to start your spin at an initial RPM one order of magnitude lower than your final spin (in your case, 600 RPM), and spin long enough for the liquid to wet out the full surface of your part. Next, ramp up to ~50% of your final spin (~1500 RPM), and dwell there for a time equal to your first spin. Ramp up to your final spin (6000 RPM), and your thickness will be unifrom, and proportionate to the final spin speed.
The 600 RPM spin is to get the coating uniformly spread out, and throw the gross excess of material you don't need. The second spin throws even more material off, and is also a dwell state to get the liquid further along on its shear-thinning profile. You should have the last (fastest) spin remove the least amount of material. Experimenting with the intermediate spin speeds will give you the widest range of results in final coating profile.
A cheap way to get fewer surface defects is to bag your spinner (or tape up PE film over the spin area), and hook it up to a source of low pressure (5-10 psi) CDA. The air pressure will be somewhat efficient at keeping exterior particulates out. It won't solve everything, but if you cannot get into a clean room, it'll help. Also very important, keep the spin bowl clean!!! With your volatile system, the sidewalls of the spin bowl can quickly become a source for "particulate" defects; as you throw low viscosity PDMS solution, it can strike the higher solids PDMS on the bowl, and cause splatter back onto the spinning substrate. A small viscid glob of coating striking a spinning low viscosity film will give nearly the same defect as a hard particulate settling on the surface. If you share the spin coater with other students and researchers, the problem gets progressively worse, as you have little liklihood knowing what was spun before your work.
The shear forces at the outside edges of your part are different than those at the center. Since your fluid is non-newtonian (pseudoplastic, thixotropic), the different shear rates are affecting the uniformity of yoru final coating. You have to accomodate your solution's rheology; if the fluid needs time (and/or a ramp up of shear rate) to thin out, give it that time... If you wanted to get fancy, see if you can get a rheological profile of your PDMS solution (if your uni has a materials science or polymer science group, they will have a good rheometer), and get a grad student from that group to help you interpret the results. While its not strictly needed, I've yet to meet a professor who didn't appreciate an excess of data ;-)