Chinese 中文
Handling

 

A pellicle has to be made clean in a cleanroom area, inspected, stored and shipped in clean box and bag, mounted on the photomask, stored and used for a long time in a photomask box. It is expected that all these steps won't create any more particles or contamination. Actually, many steps of handling and inspection can degrade the cleanliness of the pellicle. Usually, many different sizes and shapes of pellicle have to be handled in one area, making automation a difficult task and human handling the only solution. Even holding a pellicle for inspection is a difficult task and can be a source of contamination. Handling of a pellicle is not a small task. Standardization of pellicle sizes might be a good idea to solve some of our handling problems.

Controlling Static Charge
The importance of controlling static charge during pellicle handling cannot be overemphasized. Given that a pellicle is plastic, e.g. film, release liner, backside cover, it is easy to generate static charge during handling. For example, peeling the release liner or backside cover from the mounting adhesive can generate 2000 volts of electricity and, if not neutralized quickly, can almost instantly attract particles from the surrounding environment onto the pellicle. Also the packaging material such as shipping box, bag, and storage container are all plastic. Handling of these materials in the production line can easily generate electrostatic charge and attract particles to the working area. Therefore, it is crucial to control the electrostatic discharge (ESD) at the working area. Under appropriate air flow conditions, proper antistatic equipment should be used to reduce ESD and allow for a decay time at least 10 seconds for a charging plate monitor to decay from 1000 volts to 100 volts and thus minimize contamination from electrostatic attraction. Without a clean and proper ESD environment, contamination can be generated and lead to a defective photomask, causing them to fail during incoming inspection, outbound inspection, or even after repeated uses. Proper air flow is also needed at the mounting machine and working table to insure cleanliness.

Mounting
Putting a pellicle onto a photomask is called mounting. Ideally, a mounting machine with automatic handling and automatic inspection should do the job. In reality, only the film of a pellicle can be automatically inspected. Mounting operations, peeling release liner or back side cover from a pellicle can create static charge and contamination. Therefore, a simple, easy to keep clean, mounting machine with human inspection before mounting is still a better solution for mounting.

Even force has to be used in mounting the pellicle on a photomask. The mounting fixture can damage the pellicle frame edge and get contaminated. Mounting tools have to be kept clean all the time because cross contamination is possible. It is recommended a strong anti-static environment with good surface air flow on the machine should be used. Although mounting accuracy is typically obtained, proper communication between pellicle manufacturer and equipment designer is necessary for many sizes of pellicles. MLI has invented a process of putting each pellicle on a mounting plate and ships the whole package to the customer.7 The customers don't have to touch the pellicle directly in handling and can put the whole package into a simple mounting machine after an inspection of the pellicle.

Cleaning
If particles are generated onto the pellicle film, blowing may be used in an attempt to remove these particles. Specifically, a filtered, deionized air or nitrogen gun with a needle point blower is preferred. Blowing is only effective in removing large particles and might generate some small particles and contaminate the environment in the clean area if not used carefully.

Cleaning mounting adhesive residue on a photomask after removal the pellicle can be a challenging job in an area using multiple vendors because different vendors use different mounting adhesives. Different vendor may supply different cleaning method or solution.