3.1.1 Photolithography

Lithography is the most important technique for fabricating microscale structures. Depending on the type of energy beam, lithography techniques can be further divided into photolithography, electron lithography, X-ray lithography, and ion lithography [2]. Photolithography and X-ray lithography for LIGA-technique (see Section 3.3.1.1) are the most relevant techniques for the fabrication of microfluidic devices.

The patterning process with photolithography is limited to two-dimensional, lateral structures. This technique uses a photosensitive emulsion layer called resist, which transfers a desired pattern from a transparent mask to the substrate. The mask is a transparent glass plate with metal (chromium) patterns on it. For microfluidic applications with relatively large structures, a mask printed on a plastic transparency film by high-resolution imagesetter is an option for low-cost and fast prototyping. Photolithography consists of three process steps:

• Positioning process: Lateral positioning of the mask and the substrate, which is coated with a resist, adjusting the distance between mask and substrate.

• Exposure process: Optical or X-ray exposure of the resist layer, transferring patterns to the photoresist layer by changing properties of exposed area.

• Development process: Dissolution (for negative resist) or etching (for positive resist) of the resist pattern in a developer solution.

Generally, photolithography is categorized as contact printing, proximity printing,...