SMALL IS BEAUTIFUL

Long time ago in an… island far, far away I had the pleasure of spending some great time in an “Engineers Paradise”.

The place was quiet and you had to remove your shoes even before entering the offices. Sleepers were provided, although of a size a bit too small for me, and furniture was barely essential but everything you needed was available. From a wonderful clean room with the best microfabrication etching tools to an old fashion workshop for precision machining everything was there, including technicians and refined experts with gentle manners and precious suggestions, all immersed in the nature which even offered marvelous sakura blossoming views once per year.

That is the place where I created the second generation of my Microthrusters.

After running an extensive Computational Fluid Dynamics (CFD, also Colors For Directors) simulation I chose the most suitable geometries to produce 1mN Thrust with a decent efficiency of at least 50s Specific Impulse and I designed the profiles for the Lithography Masks.

Then I embarked in the Art of Micro-Nano Lithography, a truly enriching experience.

The simplest task of spinning a photoresist on a silicon wafer required careful timing and speeds so that the polymeric layer had the right uniform thickness across the entire surface. Also the subsequent curing time and temperature had their relevance to the next steps success.

Certainly the most beautiful part of the job was the exposure of the wafer through the mask with UV light, and the back side to front side alignment[1] for the second exposure was an almost magic procedure which I learnt to do on the microscope by searching the markers and carefully turning the stage knobs until the correct position.

And then the wafer had to go inside the Deep Reactive Ion Etching (D-RIE) where I had to use the proper recipe, the only one that could obtain the smooth trenches with high aspect ratio needed for the correct performance of the thrusters.

Eventually, after washing away the unexposed photoresist with a bath that a good friend always likes to remind me would have eaten my bones in few seconds, I used a diamond disc saw to obtain the beautiful micronozzles chips that will later be the core of our microthrusters.

Obviously this did not come right at the first attempt and many wafers where wasted with horrible results before we defined all the correct recipes.

So, you can imagine the satisfaction at the end of the process, when we could take the Scanning Electron Microscope (SEM) pictures that confirmed the quality of the job.

But somehow more satisfactory was to celebrate the beauty of the whole activity by having a moth and a mosquito witnessing the size of our devices.

At that time I had no idea that a few years later, a further improved version of those thrusters, made in another… island far, far away, will be maneuvering our satellite POPSAT in Space !

This is how I started to appreciate how “Small is Beautiful”, a fact that goes well beyond Micro-Nano Technology… but this is already a story for another time.

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[1] Of course that refers to the second lithography, after the first side was already completed