What is a pellicle and do I need one?
A pellicle is a transparent membrane supported by a frame.
This protects the surface of the photomask, as well as ensuring that any particles that settle on the pellicle membrane are out of the focal plane of the mask surface, thus reducing the impact of these particles.
There are many hundreds of different types of pellicle, with different frame shapes, membranes and other options such as vented frames, or tacky internal surfaces.
An important aspect of the pellicle is the type of membrane as these may be designed for specific exposure wavelengths or a range of wavelengths. The equipment manufacturer will have specifications for valid pellicles for any given system.
In general master photomasks for use with projection alignment systems, Ultratech 1X steppers, and more modern steppers (6"x6"x250' reticles) have pellicles on the coated side of the photomask; whereas older steppers (5"x5"x90' or 6"x6"x120' reticles) tend to normally have pellicles on both sides.
The use of pellicles helps to improve the yield in a production environment, but they do add to the cost of the photomask, and some users prefer not to use them in a prototype environment.