Mask Transfer System Aids Inspection, Reduces Damage

Alexander E. Braun, Senior Editor -- Semiconductor International, 2/1/2000

Alexander E. Braun,
Senior Editor

As dimensions shrink steadily toward the 100 nm region and below, and semiconductor devices become more complicated and multilayered, it is no surprise that the price of photomasks is skyrocketing, with the cost for plates going beyond the $50,000 region. A corollary of this becomes the handling of photomasks for shipping, incoming inspection, as well as insertion and removal from the stepper.

About 25% of plates that are ruined are damaged as a result of handling, not due to particles flaking off the pellicle or other factors traditionally looked upon as detrimental to the mask's integrity. This primarily occurs when plates are put into and taken out of stepper boxes, and sometimes when they are handled for inspection. If a fab is doing 100 photomask transfers a day -- not an excessive number considering the requirements of current device production -- and experiences a 1% breakage rate, it does not require an Albert Einstein to realize these costs can be staggering.

QC Optics (QCO, Wilmington, Mass.) has introduced a photomask transfer system that is at present unique and is viewed as an approach to total robotic photomask handling. The system would be practical for reticle transportation and inspection, as well as insertion into and out of steppers. It is designed for a minimum handling of the reticles, avoiding human contact with the photomask. This in turn leads to reduced breakage and diminished contamination. The containers are easily cleaned and can be loaded and unloaded manually.

Generally, when a photomask is received from shipping, it is taken out for inspection, to check on its integrity, before being placed into a stepper box. This procedure has worked well, and the danger of damage while going from the shipping container to inspection and then into the stepper box was acceptable. However, now not only have mask costs risen to the level of a small fortune, but with the industry's globalization, companies often find they are using different steppers, perhaps one from SVG and another from ASML. This situation can create a problem if the company has only one kind of reticle storage system for only one type of stepper box. This would require the manual transfer of plates back and forth from different stepper boxes.

QCO and others already had developed systems capable of removing a plate from a stepper box, and loading it automatically into an inspection system (Figure). However, these did not solve the problems of a fab that has both Nikon and Canon plates, and wants to load them into the transfer system. QCO came up with an adaptation to its transfer system, which it calls a "twin CLS" (cassette load station). This enables the system to take a plate from one stepper box and load it into a different stepper box.

The company believes that if the transfer between stepper boxes were fully automated, the 25% reticle damage due to handling could be eliminated. Realistically, this should be considered as a bridge system, in that two or three years from now the industry probably will have standardized a photomask box capable of working on multiple steppers -- such as ASML, Canon, Nikon and others -- enabling plates to be directly shipped in those boxes. However, an industry-wide, complete transition to this standardized container might take as long as seven years. When this takes place, the QCO system also will be able to handle the standard box.

Assuming, that is, that the standard box under consideration does indeed become a standard. •