Lithography: ML3 MicroWriter Direct Write Machines

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Overview

We have two Durham Magneto Optics ML3 MicroWriter direct write machines. They allow the user to do “mask-less” lithography. A UV image is projected onto the substrate. There is no minimum size for the substrate. The maximum size is 8” x 8” square, but a substrate that large would take many hours to write. It uses optical focusing using yellow filtered white LED light.

Both machines have high powered LED light sources, but one machine has a 365 nm light source, while the other has a 385 nm light source. The UV light source, in conjunction with a digital light processor (DLP) chip with an array of 1 million moving cantilever reflectors, projects an image 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 machines stitch multiple images together to create larger exposure areas. The ML3’s continually focus to be able to compensate for bowed or somewhat sloped surfaces. Both machines have four selectable lenses to write in four different resolutions: 5-micron, 2-micron, 1-micron, and .6-micron (using 3x, 5x, 10x, and 20x lenses).

The 365 nm machine uses a ground glass substrate mounting chuck, and the 385 nm machine uses a smooth glass substrate mounting chuck. There is no vacuum chuck. It relies on friction, and it is programmed to not make any sudden movements, so usually 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 very small drop of water placed on the bottom of the wafer works to hold the wafer in place.

Both 385 nm and 365 nm work fine for most g-line and i-line or broad-band resists. 385 nm will expose SU-8, but very slowly. Anyone working with SU-8 should use the 365 nm one, which exposes SU-8 about 15x faster than the 385 nm one, since SU-8 is narrow band with a steep drop off above 365 nm.

Users working with transparent substrates will usually want to use the 385 nm ML3 because light traveling through the chip will fan out and dissipate, rather than be reflected back up through the substrate like what the white ground glass chuck in the 365 nm machine does.

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 resolution rating of the lens you’re using. But you shouldn’t assume that you can get features that small without a lot of work. It is much easier to get features that are 2 or 3 times larger than the resolution you’re using. But if you are trying to push the machine to the limits though, plan on spending time coming up with the optimum dose, and possibly experimenting with defocus, baking time/temperature and development times, spinning technique, etc. Occasionally slight modifications to the mask design will help achieve optimum results.

With these machines we have Stanford-wide use of a mask-writing software called CleWin. Contact: carver@stanford.edu for more info. The machine writes in the CIF format. Other formats can be converted to .cif using CleWin. The machine can also write using bitmap files, and is capable of 256 levels of grey scale.

IMPORTANT: Review this document if you are considering to use the instrument!