Grayscale lithography creates three-dimensional microstructures with height gradients in a low-contrast positive phooresist (i.ei series ma-P 1200G). The exposure depth is precisely controlled by modulating the laser intensity. The latter is converted into exposure depth and subsequently into resist topography on the microscale.
The system tools use a process called 8-bit grayscale. The design is exposed to the photoresist eight times with eight different intensities, where the smallest bit is worth 0.39 % of the base dose. The next bit is worth double the bit to its right until the 8th bit, which itself is worth 50 % of the base dose.
The combination of these 8 bits, corresponding to 8 exposed designs, will create a 2.5D design. For example, here is a potential stair design that we could try to expose.
There are many different possibilities for the machine/operator to expose this design. Here is a possibility of a bits combination to create the various doses and the corresponding designs.
The layer exposed with a dose corresponding to the first bit covers the whole surface. The next 3 bits should not be exposed on the outer parts of the design. Finally, bits 5 to 8 are exposed only in the center.
Training :
We are offering a training of ~30-60min to any person trained on the MLA and who wants to use the grayscale capabilities of our MLA-2.
Before the training, you can already look at the following document created by HIMT.
The information in the next section are here to give you a few more information on the process and have an overview of what will be done.
In practice :
- Create your design in DXF or Bitmap (8bit) format. Each layer or color will correspond to a dose (i.e. a depth/height).
- Determine the link between thickness after development and dose :
You can use the following design to determine the thickness obtained by 256 levels with the Filmetrics F54 : CMi Greyscale DoseTest
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- MaP1275-G : 1500RPM, SB = 15min@105°C, Relaxation = 20min, Laser 405nm – 100%, Dev. time = 9min
- MaP1215-G : 3000RPM, SB = 90s@105°C, no Relaxation, Laser 405nm – Filter10%, Dev. time = 90s
- MaP1275-G : 1500RPM, SB = 15min@105°C, Relaxation = 20min, Laser 405nm – 100%, Dev. time = 9min
- Create the dose map :
Each layer will be assigned a bit (which corresponds to a dose). This assignment is quite restrictive on which doses are possible. Here is how it work :
- The operator determine a Base Dose, let’s take 1024 as an example.
- The MLA assigns a dose to each bit in the following manner :
Step = Base dose / 8bit = 1024 / 256 = 4
Max dose = Base dose = 1024
- In X-Convert (the conversion software), when importing a DXF-Greyscale file, you will be able to define for each layer which bit you want to assign them (i.e. which dose) → the Graytable
You will be able to define this manually or by importing a Graytable (i.e. similar to a text file).
- To characterize your results, SEM and optical profiler (e.g. Bruker Contour-X) are suitable options.
- Then it is a matter of iteration to make the design you want. Good luck 🙂