Previous work has shown that photomask blank flatness as well as photomask patterning and pelliclization all play an
important role in finished photomask flatness. Additional studies have shown that pellicle mounting techniques,
pellicle adhesives, frame flatness and shape and pellicle mounting tools play a role as well. It has become clear that
frame flexibility, coupled with frame mounting surface flatness and shape are the principal factors influencing the
pellicle effect on the mask distortion. Pellicle suppliers have begun to evaluate various polymers as potential
replacements for the standard aluminum frame in current use. The elasticity of the frame adhesive has also been adjusted
to evaluate its effect on the pellicle influence on mask flatness.
This paper describes some joint evaluations between IBM, Toppan and ShinEtsu, performed to determine the effect of
pellicle frame composition,, mount surface flatness, adhesive elasticity and adhesive surface flatness on the distortion of
photolithography masks. It demonstrates that polymer pellicle frames with more flexible adhesive improve finished
mask flatness approximately the same amount as reducing the total frame standoff height of aluminum frames with
The two possible 157nm-pellicle membrane materials are discussed. Currently polymers have considerable absorption at 157nm. The F2 laser durability evaluation was carried out for the polymer with the least absorption. The polymer membrane suffered severe damage after short irradiation. What is the most important for the polymer membrane is to reduce the absorption at 157nm in order to increase the transmission and to improve the durability. Modified fused silica is the material for mask substrate for F2 laser lithography. This fused silica shows good laser durability, but stringent tolerances are required when the fused silica plate is applied for pellicle membrane. Thickness uniformity and bending of plate are discussed.
We have developed pellicle for ArF excimer laser lithography. Especially we have studied light resistivity against ArF excimer laser. We have selected fluoropolymer for ArF pellicle material because of its high transmission against deep uv light. Transmission of the pellicle film at wavelength (lambda) equals 193 nm is over 99.5% at peak value from sinusoidal transmission spectrum. Lifetime of our pellicle film against the ArF excimer laser irradiation is estimated for total exposure energy 70,000 J/cm2. Number of (phi) 300 mm wafer processed within the lifetime becomes 480,000 wafers from the total exposure energy 70,000 J/cm2. Degradation mechanism of pellicle film caused by the ArF excimer laser irradiation has been investigated. The degradation mechanism is interpreted as following. Pellicle film is first etched from its surface by the ArF excimer laser irradiation. This etching of the pellicle film causes the film thickness reduction and roughens the surface of the pellicle film. Thus the transmission is reduced. The pellicle film material, which is fluoropolymer, however, has not changed on chemical basis. Fluorination of the pellicle film material, i.e., fluoropolymer, has improved its light resistivity against ArF excimer laser. Further fluorination of pellicle film material is expected to improve more its light resistivity against ArF excimer laser.
The effect of solvent existence in the photolithographic environment on pellicle is studied. Solvent induces photoreaction and affects pellicles when absorptive solvent at exposure light wavelength exists in the photolithographic environment. The photoreaction depends greatly on exposure intensity. Oxide deposition is dominant at weaker intensity and that has a worse influence on pellicles than thickness reduction at strong intensity. It is necessary to keep the concentration of solvent in the photolithographic environment at low level to prevent the photoreaction of solvent.
This paper describes development of the pellicle for KrF excimer laser photolithography. The components of KrF excimer pellicle should have high light resistance. Our pellicle membrane consists of amorphous perfluoropolymer. Since this material provides that scattering and absorption of light at deep-UV wavelength region are very small, transmissivity is high at that wavelength and light resistance is strong against KrF excimer laser. Membrane bond and reticle adhesive consist of silicone resin so that the light resistance of them is high. And the bond strength of our pellicle is very high despite the use of fluoro-polymer membrane. This is because the bond consists of silicone resin which contains fluorocarbon-group. Strict particle suppression is required for the excimer pellicle. We have covered the frame with UV resistant fluoropolymer for the purpose to reduce the possibility of particle generation. This treatment has suppressed the particle generation during transportation. Membrane cutting has been achieved by melt-cutting method. The prominency of this method is melting the membrane with heat and cutting it with no contact with pellicle frame, so that the membrane edge becomes smooth and pellicle frame has not been damaged.