For the previous version, the size of the motor was not well controlled, because of the limitation of the mechanical cutting. We expected to make the size of the motor around 50µ. However, the Leica-crytomicrotome can't reach that accuracy. So we had to search for a more sophisticated and accuracy method to produce motor.
One good candidate was the photolithography photoetching method with the precision of 2μm that was accurate enough for our motor. The remaining problem was how to make the asymmetric biotin-coated surface. We designed several improvements to achieve the one-side coating, based on the traditional process of the photolithography photoetching method. The main steps of motor production are listed as follows:
Step 1: Photoresist (SU-8) Spin Coating
- Typical contaminants must be removed prior to photoresist (SU-8) coating which is represented by red color in the figure above.
- Adhesion promoters are used to assist resist-coating.
- Ideally, no water is allowed on wafer surface.
- Wafer is held on a spinner chuck by vacuum. Resist is coated to uniform thickness by spin coating.
- Resist thickness is 1-2 mm.
Step 2: Alignment and Exposure
- For simple contact, proximity, and projection systems, the mask is the same in size and scale as those of the printed wafer pattern.
- Projection systems give the ability to change the reproduction ratio. Adjusting to 10:1 reduction allows larger size patterns on the mask, which is more robust to mask defects.
- Normally requires at least two alignment mark sets on opposite sides of wafer or stepped region.
- We use "deep ultraviolet", which is produced by excimer lasers, as light source.
Step 3: Dry Etch
- Dry Etching is an etching process that does not utilize any liquid chemicals or etchants to remove materials from the wafer. Only volatile byproducts are generated in the process.
- In this project, we use chemically reactive gases to consume silicon.
- Dry etching may be accomplished by any of the following choices:
- Chemical reactions using chemically reactive gases or plasma to consume the material
- Physical removal of the material, usually by momentum transfer
- Combination of both physical and chemical removal method
Step 4: Photoresist (SU-8) Spin Coating
Note: Please refer to Step1.
Step 5: Alignment and Exposure
Note: Please refer to Step2.
Step 6: Silver Plating
Note: A silver coating is plated onto the “primary motor”, which is around 50μm thick.
Step 7: Photoresist Removal (Stripping)
- The aim is to eliminate photoresist(SU-8) .
- We use hydrofluoric acid to remove the photoresist (SU-8). While it is extremely corrosive and difficult to handle, it is technically a weak acid. It can react with SiO2 and SU-8 and dissolve them, but it cannot react with silver (Ag). We thus coat one side of the motor with silver and leave the other side uncoated. At the same time, the substrate SiO2 has also been removed.
Step 8: Biotin Binding
Biotin can only bind on the silver (Ag) side, while the other side (Si) will have no biotin.
Previously, we have designed four kinds of motor with different shapes, which are shown in the figure below. The red lines in the figure represent the biotin-coated sides.
Our final design is shown below. The red lines in the figure represent the biotin-coated sides.
The force exerted by microorganisms on the motor is proportional to R. The rotational motility of the motor is proportional to 1/R3.The smaller size allows a larger angular speed.