The patterns required to build micros-and nanosystems are created by lithography.
When using UV lithography, the desired computer designed patterns are transferred to a shadow mask which is either manufactured by third parties or fabricated using Danchip's mask writing tool.
From this shadow mask the pattern is transferred to the actual wafer by bringing the shadow mask in contact with the a wafer coated with photosensitiv polymer also called photoresist.
The mask wafer assembly is then illuminated with UV light. Where the UV light reaches the photoresist, the resist becomes soluable in the so-called resist developer (positive photoresist). By removing the soluable (= exposed) parts of the resist in the developer bath, the mask pattern is transferred to a resist pattern on the wafer.
These step defines the actual microstructured patterns on the wafer. From here the process can be continued by removing or depositing a range of materials to create the actual features.
Often there is more than one mask step required to obtain the desired pattern. Alignment next mask to the already existing patterns on the wafer is critical. For some devices it is necessary to create geometries on both the back and front side of the wafer. Since the top geometries of i.e. a MEMS device has to be aligned with the bottom geometries, an exact alignment from the structures from back to front of the wafers is crucial.
The alignment and exposure of the wafers is done in a so-called aligner tool.
DTU Danchip has three aligners located in two different lithography sections. In 2006 a semiautomatic aligner, capable of handling 6" wafers in a cassette to cassette system, has been installed together with the semiautomatic spin-coater in the new lithography section.
Lithography is one of the most demanding processes with regards to cleanliness of the air in the room as well as sensitivity to vibrations. Any dust particles on mask, wafer or aligner optics, will block the light on certain areas of the wafer and reduce yield of the complete process. Vibrations will smear out fine structures and cause a significant loss of transfer fidelity. Even slight temperature changes will cause errors from one mask to the next due to thermal extension of the mask and wafer.
DTU Danchip's cleanroom has been designed to meet the requirements for precision lithography. The additional and newly installed lithography section is running in a class 10 cleanroom. The temperature is stabilized to plus/minus 1 degree C. See our full lithography equipment list here.