[SUNNY_physics]

Hi everybody.

My goal is to design a process that, with no automated machinery (I'm using a mechanical pipette), would allow for extremely uniform coverage of a hydrophilic surface with a self assembled colloidal layer.

I am using a polymer suspension in water. Currently I'm taking a small microliter size portion of this stock suspension, diluting it by half and dripping it onto the surface of a substrate cut into exactly 1 cm2 squares. Then I let it dry.

Here is the problem. It is NOT uniform because the polymers are concentrated along the drying front; as water evaporates, the polymers do not, and get concentrated, yet diffusion of the polymers in water is much slower than evaporation (by my observations), and they get stuck on the edges.

I'm thinking of adding additives such as detergents or other polymers, changing temperature, or changing humidity, but would like a mathematical model to help guide my experiment. This is important because the theory of fabrication is a major component of the project.

how do I define the conditions such that the sample is dry and only the polymer remains? how do I calculate the distribution of the polymers on the surface? Is there an analytical model?

If no one can help directly, can anyone point me to a useful book to look at? I'd really like someone to point out a good introduction book to diffusion and mass transport, and would be even better if it was from a chemistry/physics point of view, rather than an engineering one.

[Enthalpy]

You might try to imitate the process that deposits single-molecular layers one at a time: dip the item in a liquid on which a mono-layer of material floats. Except that you'd use a thicker layer of your polymer if desired, instead of a single-molecular layer. What thickness do you want?

Did you check what the semiconductor industry does? They need uniform layers of polymers as well, often polyimide, but increasingly exotic polymers for opto applications.

Long ago we spinned silicon wafers and put a drop of thick liquid (MMA?) at the centre; viscosity determined a thin layer (a few µm) whose uniformity was good enough for us. Less solvent and a layer thin from the beginning is probably its answer to the polymer migrating during evaporation.

[SUNNY_physics]

thank you greatly!

the thing is, the semiconductor industry uses just polymer dissolved in an organic solvent to use as a resist. i'm using the polymers in a colloidal form as a lithography mask to create periodic nanostructures; the polymer colloidal particles are pretty big (hundreds of nm), and since its a mask, it should be a monolayer of colloid particles.

i've seen articles about the dip method, i will try it out. i'll also try the spincoating method with a relatively more concentrated layer. from what i've read, almost every group working on this method uses surfactants, so that might be an issue (our lab is not very well funded; we just got our first XRD).

another thing is that since this is part of my thesis project, and i'm a physics student, i also need to write about the theory behind the fabrication process. would Bird's "Transport Phenomena" be a good place to get up to speed? or would more specialized books like "Intermolecular and Surface Forces" be better?

[curiouscat]

another thing is that since this is part of my thesis project, and i'm a physics student, i also need to write about the theory behind the fabrication process. would Bird's "Transport Phenomena" be a good place to get up to speed? or would more specialized books like "Intermolecular and Surface Forces" be better?

Bird's is a great book. Although, it won't tell you much on practical fab details. But if you want to model these processes it's a great starting point but you'll need  a specialized book after.

[SUNNY_physics]

thank you very much. i may have to scale down my ambitions to just write, qualitatively, about order of magnitude estimations through the theory, but would definitely need to somehow include theoretical details into my thesis.

[curiouscat]

thank you very much. i may have to scale down my ambitions to just write, qualitatively, about order of magnitude estimations through the theory, but would definitely need to somehow include theoretical details into my thesis.

Not at all. Fab modelling is very well developed. Lot of detailed math and specialized equations exist. You must read and use those.

Only point is if you start in one of those books without knowing any basic transport theory the going could get a little tough.

[SUNNY_physics]

Where do I get a book beyond the introductory level, or should I go straight for review articles?

Also, I'm not using any industrial machinery; any models involving industrial machinery is not useful for my specific purposes. Basically, I have access to a hot plate, a fume hood, a sputtering system (with RF power supply) and analytical instrumentation. I have a feeling that the highly controlled conditions of most fabs is NOT going to work for my specific case.

I mostly need the model to guide the experiments I can do within a highly limited budget. The keyword I'm searching for, "Nanosphere lithography modeling", is not displaying much. All the articles I'm getting for "semiconductor fab modeling" comes up with highly specific problems about the industrial machinery or production scheduling.

[curiouscat]

Where do I get a book beyond the introductory level, or should I go straight for review articles?

I'd go for a book before you hit the articles. Easier to understand.

[SUNNY_physics]

never mind. the key word search was too obvious. "Self Assembly." Plenty of articles about this at an understandable level.

the fab books were not helpful.