PDMS & microfluidics: Soft-lithography definitions

When preparing the PDMS mixture, a curing agent must be added to the PDMS base monomer. Because the two components must be stirred vigorously, many air bubbles appear.

Bubbles may also appear when pouring the PDMS solution on the surface of the master. Because the microfluidic device could be useless, it is essential to try to minimize the formation of air bubbles. A common option to clear air bubbles is to place the PDMS mixture in a desiccator connected to a vacuum line.

Photolithography shares fundamental principles with film photography as well as techniques used to make printed circuit boards. Basically, photolithography aims at transferring a geometric pattern from a photomask to a light sensitive material such as SU-8 photoresist. It is required for fabricating the SU-8 master that enables the replication of PDMS-based microfluidic devices.

A photomask transfers a specific pattern onto an underlying SU-8 photoresist layer when it is exposed to a UV light source. Depending on resolution needs, a photomask can be created with different materials and methods.

• For a limited resolution, the photomask may be a laser printed overhead transparency (1200 dpi or 250 µm resolution)

• A high resolution print can be obtained on thin polymer transparency (56 000 dpi or 10 µm resolution)

• A resolution down to 600 nm (420 000 dpi) can be achieved with a laser quartz print in chromium deposited on the quartz

The mask controls the shape and lateral dimensions of the pattern transferred to the SU-8 film.

During the fabrication of the master mold, the SU-8 photoresist must be heated several times. The terms soft-baking, post-exposure baking and hard baking are employed. They refer to different stages of the photolithography process and involve different heating times and temperatures.

Coating is one of the first steps of the photolithography process. It is intended to deposit a uniform film of SU-8 photoresist over the surface of a silicon wafer (i.e., a substrate). The thickness of the SU-8 layer spread on the surface of the wafer typically controls the height of the structures that will be imprinted in the microfluidic chip.

Exposure is used for the fabrication of the SU-8 master during the photolithography process. It takes place when the wafer has been uniformly coated with SU-8. A photomask is then positioned on top of the SU-8 layer. This latter is then exposed to ultraviolet (UV) radiations that travel through the photomask.

Generally speaking, a photoresist (or resist) is a light sensitive material. Basically, it is a mixture of organic compounds held together in a solvent solution. A photoresist can be positive or negative depending on how it responds to an ultraviolet (UV) light source:

• With a positive photoresist, the areas of the photoresist exposed to the light source through a mask become more soluble. When developed, the exposed photoresist is dissolved.
• On the contrary, with a negative photoresist such as SU-8 (Microchem Corp.), the regions exposed to the UV radiations harden. When developed, the exposed portions remain on the wafer.