Lithography: ML3 MicroWriter Direct Write Machine
The Durham Magneto Optics ML3 MicroWriter direct write machine allows the user to do "mask-less" photolithography. The image is projected onto the substrate. There is no minimum size for the substrate. It uses optical focusing using a yellow filtered white LED light. The actual exposure is done by a 385nm LED UV light source reflecting off of a DLP chip with an array of 1 million moving cantilever reflectors. That image is projected down through microscope optics onto the substrate. The light intensity is controlled by turning each pixel on and off for a certain duty cycle during that particular exposure time. The machine stitches multiple images together to create larger exposure areas. It continually focuses to be able to compensate for bowed or somewhat tilted samples. The machine has four selectable lenses to write in four different pixel sizes: 5-micron, 2-micron, 1-micron, and .6-micron. The most commonly used is 1-micron. The machine uses a ground glass substrate holder. There is no vacuum chuck. It relies on friction, and it is programmed not to make any sudden movements, so most of the time there is no risk of the sample moving around during exposure. For larger double-polished wafers that may tend to glide around too easily, a small drop of water placed on the bottom of the wafer works to hold the wafer in place.
385 nm works for most g-line and i-line or broad-band photoresists. It will also work for SU-8, but since the wavelength isn't ideal, the exposure dose must be increased by a certain percentage compared to what you would use at 365nm.
If everything is perfect exposure-wise, and you're using a high-quality photoresist with good contrast, it is possible to get features as small as the pixel size. But you shouldn't assume that you can get to that size without a lot of work. It is easy to get to maybe twice the pixel size without too much effort. If you are trying to push the machine to its limits though, plan on spending a lot of time experimenting with the dose, development time, pre-bake temperatures, spin quality, etc.
With this machine we have Stanford-wide use of a software called Clewin. This is a professional mask-drawing program. If you'd like a link to download this software to your personal computer, send me an email at email@example.com. The machine works in the CIF format. Clewin writes in the CIF format. But other formats can also be used. The machine recognizes several other common formats. It can also write in "native" mode using bit map mask designs.
In order to become a qualified user of the ML3 Direct Write Machine, you need to follow each of these steps in the order as listed here:
- Complete the process to become a lab member of SNSF and follow the instructions to activate a Badger account.
- Complete the SNSF COVID-19 Authorization protocol and receive notification of authorization.
- Complete the process to become a Flexible Cleanroom user.
- IMPORTANT: Review this document if you are considering to use the instrument!
- ML3 Video Training Tutorial (Part 1)
- ML3 Video Training Tutorial (Part 2)
- Arrange for training by contacting Tom Carver.