Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 115

heads, accelerometers, gyroscopes, and other sensors as well as actuators. Based on CMOS (comple-
mentary metal-oxide-semiconductor) technology, freestanding polysilicon structures were fabricated by
etching a sacrificial layer. This technique laid the foundation for the so-called silicon surface micro-
machining technology, which was widely adopted in the industry for making accelerometers, gyroscope,
and comb-drive actuators. The most famous applications of surface micromachining technologies are
accelerometers made by analog devices and digital mirror display made by Texas Instruments.

4.1.1 Basic technologies
4.1.1.1 Photolithography
The batch fabrication for the majority of microdevices is based on photolithography, a technology
adapted from microelectronics. The different lithography techniques include photolithography, elec-
tron lithography, X-ray lithography, and ion lithography [2] , of which photolithography and X-ray
lithography for LIGA 1 are the most relevant techniques for the fabrication of micromixers. Since
photolithography requires a mask to transfer patterns to a substrate, this technique and almost all other
microtechniques are limited to the fabrication of two-dimensional structures. There is little control
over the third dimension. The pattern of microstructures is transferred through the mask to a photo-
sensitive emulsion layer called photoresist. The mask is a transparent glass plate. The patterns are
made of a metal layer, such as chromium, to block light. A mask printed on a plastic transparency film
by high-resolution laser printer is popular in the microfluidics community due to its low cost and fast
prototyping. The relatively large size of microfluidic components, such as micromixers, allows the use
of this low-cost mask.
The photolithography process consists of three basic steps: positioning, exposure, and develop-
ment. In the first step, the mask is positioned laterally to a substrate, such as a silicon wafer. The
substrate is coated with a resist, which will carry the pattern after the subsequent exposure step. After
lateral positioning, the distance between the mask and substrate is adjusted. The exposure step
transfers the pattern on the mask into the photoresist layer. Energy from the exposure source, such as
ultraviolet (UV) light or X-ray, changes the properties of exposed photoresist. In the development step,
unexposed negative resist is dissolved, while the exposed area remains due to crosslinking. In contrast,
exposed positive resist is etched away in the developer solution.
According to the relative position between the mask and the photoresist layer, photolithography is
categorized as contact printing, proximity printing, and projection printing. In contact printing and
proximity printing, the mask is brought close to the substrate. The resolution b of proximity printing is
determined by the wavelength l and the distance s between the mask and the photoresist layer [2] :

5

p

b ¼ 1 : l s : (4.1)

Contact printing and projection printing can reach a resolution on the order of 1 m m. Due to the gap s ,

proximity printing has a lower resolution on the order of several microns.
The resolution of a projection printing system can estimated as

l
2NA

b ¼ (4.2)

1

German acronym of “Lithographie, Galvanoformung, Abformung.”