The micro images will be printed in microscope slides like this one.
This slide is 60 mm x 26 mm, however, the actual images will not be larger than 1 mm x 1mm.
The thickness of this slide is around 0.2 mm, i.e. about twice as thick as an average human hair.
Titanium (Ti)/gold (Au) coating
The slides are coated with a very thin metal film. In this example the metals were titanium and gold. Only the gold layer is visible, however, the titanium is necessary to improve the adhesion of gold, which doesn’t naturally adhere well to glass.
The total Ti/Au film thickness is roughly 125 nm, which is about 30 times thinner than spider silk. The metals were deposited in an electron-beam evaporator (shown below). The metal deposition process in the evaporator is very slow, but it provides extremely accurate control of the film thickness.
Ti/Au film patterning – photoresist
In order to print a gold shape on the microscope slide, the Ti/Au film must be patterned, and in order to pattern the Ti/Au, we need to create a mask with the design we want to transfer onto the metal.
We use photoresist, a UV light-sensitive resin. In the photo, a few millilitters of photoresist are applied on the surface of the Ti/Au...
Ti/Au film patterning – photoresist spinning
... and then spun at 4000 rpm in order to distribute the photoresist evenly. The resulting layer of photoresist has a thickness of 1.5 mm (almost a third of the thickness of spider silk).
Ti/Au film patterning – photoresist baking
Then it goes into the oven for baking at 90oC. This prepares the photoresist for exposure to UV light.
Ti/Au film patterning - photomask
The picture shows a photomask. It is a quartz glass with chrome patterns which will be transferred to the photoresist. We routinely design photomasks, but manufacture is outsourced from external companies.
Exposure to UV light through the photomask will define the regions of photoresist that are exposed to light and those that remain dark. The exposed photoresist becomes ‘weak’ and can be easily dissolved in developer.
Ti?au film patterning-UV exposure
Exposure to UV light through the photomask.
Ti/Au film patterning – develop
The photoresist is then developed. The patterns will become visible after a few seconds.
The image shown below is a microscope capture of the resulting photoresist pattern on top of the gold surface.