The term "pellicle" is used to mean "film,"
"thin film," or "membrane." Beginning in
the 1960s, thin film stretched on a metal frame, also known
as a "pellicle," was used as a beam splitter for optical
instruments. It has been used in a number of instruments to
split a beam of light without causing an optical path shift
due to its small film thickness. In 1978, Shea et. al. at IBM
patented a process to use the "pellicle" as a dust
cover to protect a photomask or reticle(hence will all be called
"photomask" in the rest of this chapter) In this chapter,
the word "pellicle" will be used only to mean a "thin
film dust cover to protect a photomask".
Overview of Pellicles
||The pellicle is used primarily for two purposes
- to increase die yield and reduce overall p handling,
i.e. cleaning and inspection. It is a thin film stretched
on a frame used to protect a photomask from particle contamination.
Today, the pellicle has become an integral component in
the manufacturing process for most IC manufacturers and
high-resolution projection photolithography systems used
in the manufacturing of thin film magnetic reading heads,
LCD flat panels, micro electromechanical system (MEMS),
Use of a Pellicle
||During the printing process the image of
any particle on the pellicle film will be out of focus
on the wafer plane and therefore has only a blurry shadow
which has a minimal effect on the photomask's image on
a wafer. Without pellicle protection, a photomask can
easily get a particle on its surface and form a distorted
image on the wafer, creating a defect on a chip. Before
a time when pellicle was used, a photomask would require
daily cleaning and inspection. Consequently, the photomask
would easily become contaminated from environment or damaged
from cleaning process, resulting in a low die yield and
high replacement cost.
The use of a pellicle in an optical projection system
is illustrated below.
Figure 1: The Use of a Pellicle
||Once a pellicle is properly
attached on the photomask, the surface that is covered
by the pellicle is free from future outside particle contamination.
The original quality of the photomask can therefore be
preserved. Now only a brief inspection of the pellicle
film and photomask surface is required to insure the quality
of the photomask.
ISO 9001-2000 Quality
As a leader in quality with an ISO 9001-2000 certification,
MLI's quality assurance
system is governed by 3 principles:
- Customer Satisfaction
- Continuous Improvement
- Quality through Process
In ensuring a rigid quality control
process, each pellicle is assigned a serial number and can
be easily traced throughout the entire production process.
In addition to effective tracking methods, processes and testing
methods that have been refined over years of experience have
resulted in top quality pellicle products for nearly two decades.
With a Class 1 cleanroom
and years of continuous improvements in manufacturing, MLI
has developed a top-rated process for pellicle production.
Refinements in automation, process enhancements, cleanroom
upgrades, and state-of-the-art manufacturing equipment have
resulted in producing pellicles with the utmost cleanliness.
In addition, every component of a pellicle (such as the glue,
mounting adhesive, frame anodization and coating) has been
continuously improved over the years.
With a relentless focus on cleanliness, MLI has incorporated
the continuous improvement of packing and handling techniques
into its daily operations along with a rigorous quality control
process. Preventing particle contamination with proper shipping
and handling procedures are crucial steps in preserving the
integrity and cleanliness of pellicles received by customers.
MLI's patented pellicle package designed with an interior
adhesive coating captures and holds any loose particles generated
during shipping. The inside of the pellicle is also sealed
with an adhesive coating to better prevent particle contamination.
In addition, MLI has developed a simple packaging and mounting
system that eliminates hand contact with the pellicle during
the handling and mounting process.
To ensure that MLI pellicles stay
clean, a fluoro-polymer anti-reflective coating (FP ARC) is
used during the production process. With low surface energy,
the coating not only improves transmission but also allows
particles to be easily blown off the pellicle membrane.