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Anatomy

 

Anatomy of a Pellicle
There are several key features to a pellicle that allow it to properly perform its function. A typical pellicle is shown in Figure 1.


Figure 1: Cross Section of a Micro Lithography, Inc. (MLI) Pellicle

A few of these features are the following:

Film
Film in a pellicle provides a physical barrier to prevent outside contamination, i.e., particles or vapor outgassing, from contaminating the photomask surface. At the same time, because it is thin it provides an optical path with minimum focus and transmission distortion. >

Manufacturing Process
Dip-coating, chemical vapor deposition and spin-coating have been used to create the pellicle film. Currently, most pellicle film is produced by the spin-coating process.2,3 In 2003, a pellicle as big as 582 mm x 348 mm for LCD photomask has been produced with spin coating process. The pellicle film can also be coated with anti-reflective materials to give it suitable anti-reflective properties. The anti-reflective coating process can be done by spin-coating or vacuum deposition with low refractive index materials. Fluoropolymers which are used for anti-reflective coating or for DUV film, create a low energy surface and can make it easier to remove particles from the pellicle surface.

Transmission and Material
Transmission depends on the film thickness, type of anti-reflective coating, and light absorption of film material and the light wavelength used by the wafer aligner or wafer stepper. Nitrocellulose was the film material initially used and can be used for g-line (436 nm) or i-line(365nm) wafer steppers and wideband projection wafer aligners. However, this material begins to absorb just below 350 nm and cannot be used below 350 nm. Cellulose esters, such as cellulose acetate and cellulose acetate butyrate, have good transmission above 300nm while amorphous per-fluoropolymer materials, such as Teflon AFR (DuPont ?) or CytopR (Asahi Glass Co. Ltd.) can be used for 248 nm and 193 nm steppers. An example of the different types of film and transmission curves is shown below in Table 1 and Figure 2, respectively.
Material
Part
No.
Thickness
(£gm)
Double-Sided,
AR Coating
Transmission
Wavelength
(nm)
NC
122**
1.40
Yes
99% (min)
365, 436
CE
201**
1.40
Yes
99% (min)
365, 436
FC
603***
0.81
No
99% (min)
248, 365,436
 
703***
0.54
No
99% (min)
193, 248, 365
 
NC = Nitrocellulose MLI U.S. Patent # 4,759,990.
CE = Cellulose Ester MLI U.S. Patent # 5,339,197
FC = Fluorocarbon Polymer MLI U.S. Patent # 5,772,817
Table 1: MLI's Film Specifications


Figure 2: Typical Transmission Curves

The film material must have the proper uniformity, mechanical strength, optical transmission, and cleanliness to allow continuous replication of the photomask image onto the wafer surface. Specifically, a few necessary characteristics are as follows:
  • Transmission Uniformity - Since most film is generated from spin-coating, uniformity is from the center of the pellicle film to the edge.
  • Mechanical Strength - The film and glue adhesion must be able to withstand certain air pressure from a nitrogen or air blow-off gun with a 2mm or larger opening at all angles. For fluoropolymers used for Deep-UV film or anti-reflective coating, it is very difficult to find a suitable glue to bond the film to frame due to the film's low surface energy. Therefore, the glue developed for this purpose can sometimes show only a marginal strength and limited life time of adhesion strength. Adhesion strength, i.e. adhesion of glue on the frame, should be checked with each vendor's pellicle.
  • Usage Life - The pellicle lifetime can vary greatly, depending on pellicle materials and the light source of the wafer stepper or wafer aligner, i.e. light source wavelength, intensity, light filter used. All material components in a pellicle are subject to UV light degradation, oxidation degradation, and outgassing; and should be considered as having a limited lifetime.


AR Coating
An anti-reflective coating (ARC) on a pellicle can improve the transmission and its uniformity over the entire pellicle. The ARC on a pellicle also makes the transmission less sensitive to the thickness variations of the base film. In early 1983, MLI began research on the use of inorganic ARCs such as magnesium fluoride, calcium chloride and lithium chloride. However, we discontinued that research because of process and environmental instability.
After 1983, instead of using an inorganic ARC, MLI pioneered the use of fluooro-polymer ARCs (FP ARC), and has since invented many types of FP ARCs for pellicles. These include a multiple-layer FP ARC that has a very high transmission of more than 99.5% with a uniformity of 0.1%, even with a film thickness of 2.85 μm ± 0.2 μm.

Frame
The frame is used to support the film and to be bonded on a photomask. It must be mechanically rigid, flat and stable and create no contamination and easy for inspection. The material is typically of a black, anodized aluminum alloy.

Frame Coating
The current frame manufacturing process creates a pellicle frame that has a very rough, irregular surface. Coating is therefore used to give the pellicle frame a smoother surface. Hidden particles on the irregular surface are sealed by the coating, while the coating allows for easier detection of particles on the frame surface. Coating is often used in conjunction with an adhesive or liquid-like material to catch possible airborne particles. Without coating on the pellicle frame, particles can potentially hide in frame crevices and eventually fall on the pellicle film and/or photomask. It is important that the coating have no adverse affects on the pellicle. For example, the liquid coating must be UV resistant and have minimal outgassing to avoid any transmission loss of the film and condensation or crystallization on the photomask surface, respectively. The figures 3 and 4 below show frame surfaces of an MLI pellicle with and without coating.


Figure 3: Scanning electro microscope picture of a frame without coating


Figure 4: Scanning electro microscope picture of a frame with coating

Vent Hole and Filter
During air shipment, a pelliclized photomask is subjected to significant air pressure differentials, causing the volume of air under the pellicle film to expand or contract.5 This can cause the film to damage or even break. Consequently, a vent hole, i.e., breath hole, in the pellicle frame was developed to equalize the air pressure differentials inside and outside the pellicle film during air shipment. A vent hole with a cap screw was first used by Intel in the early 1980s for shipment of an Ultratech wafer stepper reticle from one factory to another at different attitude. Then filter on the vent hole was introduced by Kasunori Imamura in a patent.

Although a vent hole with a filter can equalize the pressure from inside to outside of a frame, the hole itself is always a potential place for hidden particles even when the wall of the hole is coated with a pressure sensitive adhesive. Therefore it is recommended unless it is necessary to ship on air or used at different attitude, it is not recommended to use a vent hole with filter.

In addition, with the introduction of a single-layer cast pressure-sensitive adhesive which has a tight seal on a photomask or a reticle, a fast mounting process can trap some air and cause a bulge of the film. A frame with a vent hole with filter can eliminate this problem.

With the concern of environmental outgassing and photomask container outgassing, especially for DUV photolithography, one has to get enough outgassing data in deciding if a vent hole with a filter is right for the process because the filter can connect the inside of a pellicle to possible outside vapor contamination.

Mounting Adhesive or Gasket
. The mounting adhesive or gasket is used to bond the pellicle to the photomask and is pre-applied on the frame with a release liner. There are two different types of adhesives that have been used - a carrier and non-carrier adhesive. An adhesive with a carrier is a double-sided coated pressure-sensitive acrylic or rubber adhesive with a polyurethane foam, vinyl foam, or solid carrier. Foam adhesives were used widely in the early stages of pellicle manufacturing. Adhesives without a carrier can be applied to the frame from a one-layer transfer tape or cast in place on the frame from hot melt, UV-cured or emulsion pressure sensitive adhesives. Both types of adhesives have a thickness that varies anywhere from about 0.10 mm to 0.80 mm.


Figure 5: MLI #17 Non-Carrier Adhesive vs. #13 Carrier Adhesive

Backside Cover
The backside cover was first introduced by MLI in 1982 and now is widely used in the field. The purpose of a backside cover is to seal the inside of the pellicle against airborne particles during transportation. This cover is then removed before the pellicle is applied to the photomask.