Master photomasks feature the complete image or in technical terms the array of the final wafer, which is exposed in a single exposure. According to calibration masks experts, the resolution is somewhat limited to about 1um-2 um but a lot of emerging technologies this is more than enough as throughput is already achieved.
Probably the most important step in the photolithography process is called mask alignment. For those not in the know, a photomask is a square glass plate that contains a patterned film on one section or side. This is then aligned with a wafer for the purpose of transferring the pattern onto the wafer surface. It is important that each photomask achieve alignment with the pattern.
Once the mask has successfully been aligned with the pattern on the surface of the wafer, the photoresist is then exposed via the pattern present using high intensity ultraviolet light. There are three primary exposure techniques for the process and these are: contact, proximity and lastly projection.
In this phase of the process, the resist-coated silicon wafer is then brought into physical contact with that of the glass photomask. It is held on a vacuum chuck and the complete assembly goes up until contact of the mask with the wafer is achieved. The issue with contact printing is that debris could get trapped in between the resist and the mask resulting mask damage that results in faults in the pattern created.
This process is quite similar to the contact printing approach with one exception: tiny gap measuring between ten to twenty five microns is maintained between the wafer and the mask during exposure. The purpose of the gap is to lessen mask damage. About 2 to 4 micron resolution are achievable with this type of printing method.
In this process, mask damage is completely avoided. As described above, the image of the patterns on the mask is projected onto the resist-coated wafer. To get the high resolution needed, only a small section of the mask is imaged which is then scanned or stepped over the surface of the wafer. Projection masks make use of pellicles and once the mask is certified free of faults or defects, the pellicle provides an additional layer of protection to make it free from any additional defects for the entire lifespan of the mask unless for some reason it acquires some damage.