Biomedical Engineering Reference
In-Depth Information
the developing results including the soft bake time and temperature;
degree of exposure; developer concentration; the time, temperature,
and method for development. Over or under development will cause
problems such as poor dimension control or removal of excess
photoresist intended for protection.
Development includes wet (e.g., immersion, spray, puddle) and
dry (plasma) techniques. The most common and low-cost method
is immersion. To process that, the developer solution is poured into
a chemical resistant container, and then the wafer with patterned
photoresist is immersed in the solution for a speciic time, followed
by rinsing with water. This could be a simple and effective way to
develop the relative large patterns with feature size in the range
of several microns. For small patterns, the surface tension of
developing solution will prevent the chemicals from penetrating
into small trenches. In practical operation, users should care the
developer chemicals becoming diluted through multiple uses. Room
temperature luctuation can result in the changes in developing
rate. Agitation is helpful in developing process to make soluble
photoresist more uniform and allow the developer easily penetrates
into small openings on patterned photoresist. The agitation can be
achieved by mechanical stirring or activating ultrasonic waves in the
developer.
Hard bake is usually treated after the developing step, and use
the similar equipment and methods as soft bake. Nominal process
temperature is from 130 to 200°C for 30 min in a convection oven.
The purpose is to achieve good adhesion of the photoresist to the
wafer surface. The heat makes the photoresist contained liquid
evaporate and further polymerized, therefore improving its etch-
resistant properties.
Finally, the development results are inspected to determine if the
wafer is passed for further process, such as etching or deposition,
otherwise, the rejected wafer will be striped the patterned
photoresist and re-processed from the cleaning step again.
6.2 SoftLithography
Sensor systems with micro-components created with micro-
molding methods are now common due to the use of micro-injection
molding and hot embossing technologies to produce large numbers
of miniature luid control components and systems. Micro-injection
 
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