Developers are designed to remove exposed areas of positive-tone resists and unexposed areas of negative-tone resists without leaving any residuals. The respective other areas of the wafer are ideally not attacked by the developer and remain with a film thickness identical to the initial value.
Different procedures can be performed for development:
In the simplest case, the wafer (the substrate) is fixed or hold in a beaker (e.g. by tweezers) and gently agitated until the exposed (unexposed in the case of negative resists) areas are dissolved. During production processes, often 25 wafers are developed simultaneously in one container and moved mechanically in the developer bath, mostly by shaking up and down. After completion of the development process, substrates are have to be thoroughly rinsed with DI-water and dried afterwards. For this purpose, pressurized air may be used, or the wafers (in respective containers) may be spin-dried.
Exposed wafers are placed on a spin chuck, set in rotation and sprayed with developer using air pressure. This procedure provides a more intense development procedure than immersion development since fresh developer is added again and again. Spray development therefore requires (approximately 2-3 times) less time than immersion development. Immediately after development, DI-water is sprayed onto the wafer, followed by spin-drying of the wafer.
Exposed wafers are again mounted on a spin chuck and developer solution is carefully poured onto the wafer (automatically, using a dispense system) until the wafer is completely covered and surface tension keeps the developer on the wafer and prevents it from flowing downwards. The wafer is then gently rotated half a turn to the left, after a short pause again to the right and so forth until all structures are developed. These turns are important to move the developer, which is subsequently spun off. Wafers are then rinsed with DI-water and spin-dried.
Interrupting the development procedure
An interruption of the development step is possible, but this step differs for aqueous-alkaline and solvent-based developers. The various dissolution processes are described in the respective sections for stoppers (see Stoppers). A solvent-based development (mostly for PMMA e-beam resists) may be interrupted several times. If the development is continued after a while, development features will still be the same. If the optimum development regime still has to be experimentally determined, a development in several steps is an advantageous strategy.
For aqueous-alkaline developers however (photoresist, negative-tone e-beam resists), an interruption is not recommended. In this case, still undissolved polyelectrolytes (cresolates) are deposited on the surface and form a thin layer which reduces the development rate when this step is continued. As a consequence, an accurate reproducibility of results will no longer be given.
Overview Process Chemicals- Developer