Patent Application
20050202252
The disclosure relates to polymers, and more particularly to polymers useful as pellicles in photolithography.
Pellicles are membranes used during lithography. A pellicle is placed a desired distance from either the front side or the backside of a mask. Pellicles may be used to block particles that are in the focal plane from reaching the mask or reticle surface. Any particles on the pellicle surface are out of the focal plane and hence should not form an image on the wafer being exposed. A pellicle is a thin transparent layer stretched over a frame above the surface of a mask or reticle. Typically the pellicle is transparent to laser light. Applied laser energy will depend on pellicle and resist transmission. For example, critical dimensions of the printed resist features are very sensitive to the variation dose of laser energy. A 2% difference in dose can result in 10% variation in critical dimensions.
The disclosure provides pellicle materials that have comparable durability and transmissibility as that of CYTOP and are readily available. PVDF can serve as a CYTOP replacement for 193 nm lithography. The optical transmission of PVDF at 193 nm, measured for 1 μm thick film, is equal to 95.5% of CYTOP. PVDF is also soluble in organic solvents and can be used for spin on technology for the generation of pellicles. PVDF's durability at 157 nm is comparable with that of CYTOP and can be further improved by fluorination, purification, and internal stress relief. Accordingly, a pellicle system comprising a PVDF pellicle composite/copolymer material is described. Furthermore, the use of 157 nm wavelength irradiation has proven important in some photolithography techniques. CYTOP shows poor transmissibility and durability when used at shorter wavelengths (e.g., 157 nm). This disclosure further provides pellicle materials having improved durability and transmissibility at 157 nm wavelength irradiation.
Pellicles are used as a photomask protective cover in the projection printer or wafer/mask stepper process to increase the yield of the process. The pellicle is a thin transparent membrane adhered to a frame, which guards a photomask or reticle from harmful particle contamination.
In the lithographic industry, ultraviolet rays of wavelengths: 248 nm and 193 nm are used as exposure light, and with fining of patterns. Far-ultraviolet rays, vacuum ultraviolet rays, electron beam (EB), X-rays, and the like, which have shorter wavelengths, have been used as exposure lights. KrF excimer laser beams having wavelengths of 248 nm, ArF excimer laser beams having wavelengths of 193 nm, and F2 laser beams having wavelengths of 157 nm are being used and are expected to be useful for the formation of fine patterns.
Most pellicle polymers are useful at wavelengths of 193 nm but degrade rapidly at shorter wavelengths (e.g., at 157 nm). Irradiation of polymer pellicles causes pellicle structural degradation that depends on the irradiation dose and wavelength (or energy) of irradiation. For example, irradiation of pellicles made from CYTOP (an amorphous, soluble perfluoropolymer) or Teflon AF polymer (polytetrafluoroethylene amorphous fluoropolymer) with 157 nm in the range from 1 to 100 J/cm2 causes a drop of transmission by as much as 100%. In addition, relying on the transmission properties of CYTOP or TAF at a particular irradiation dose is not practical.
Furthermore, the dose should be uniform over the surface of a pellicle and wafer and should not change during the life of the pellicle. Deviations in transmission by less than 1% can typically be adjusted by an appropriate increase in exposure time (typically through automated adjustments) to take into account loss of transmission. For deviations above 1%, a process lithography engineer needs to make time consuming calculations. If the change in the pellicle transmission is not adequately corrected a change of critical dimensions (CD) on the exposed wafer will occur. This change depends, in part, on the resist thickness, absorption, type, and the like.
Disclosed are co-polymer composition comprising PVDF and an amorphous fluoropolymer. Amorphous fluoropolymers are known in the art and include, but are not limited to, materials comprising a cyclic fluorocarbon oxygen-containing polymer, a polyimide linear fluoropolymer, perfluorinated polyethers, and combinations thereof. PVDF, fluorinated PVDF, and non-fluorinated PVDF can be used in the co-polymer composition. As described herein pellicles comprising such PVDF-amorphous fluoropolymer are provided by the disclosure.
PVDF comprises two hydrogen and two fluorine atoms in its structure (see scheme 1). Further fluorination can improve both transmission and durability of the fluorinated PVDF (see,
Scheme 1 shows the structure of PVDF.
Optical tests performed using PVDF Kynar polymer film are shown in
In addition to PVDF being useful as a pellicle material at a wavelength of 193 nm, fluorinated PVDF having improved pellicle characteristics is provided. The disclosure provides fluorinated PVDF having improved optical transmission and durability compared to a PVDF having a monomeric structure as set forth in Scheme 1, particularly at wavelengths shorter than 193 nm (e.g., 157 nm). Accordingly, PVDF that has been subject to further fluorination may also serve as a polymer pellicle at shorter wavelenghths either alone or as a copolymer material.
Fluorination of PVDF improved optical properties, such as durability and transmission as shown in
Of particular interest are pellicles comprising PVDF copolymers comprising PVDF (fully-, partially, and non-fluorinated) and (i) a cyclic fluorocarbon oxygen-containing polymer, (ii) a polyimide linear fluoropolymer, (iii) perfluorinated polyethers, or (iv) composites of any of (i)-(iii).
The use of PVDF copolymers in pellicle optimizes pellicle synthesis, improves pellicle plasticity and improves optical properties, to name a few advantages. PVDF has flexible macromolecular chains such that insertion the blocks of copolymer into a rigid polymer membrane makes the membranes less rigid and more “soft”. High rigidity of polymer films can cause film breakdown during pull-out of thin polymer membranes during spin coating. Inserting PVDF blocks (including PVDF blocks that have been further fluorinated) into the original polymer structure can improves optical properties of the pellicles as well as pellicle synthesis.
The PVDF polymers and copolymers are useful as pellicles for a number of reasons. For example, PVDF copolymers have (i) improved optical properties including high transmission percentages at 157 nm due to the presence of alternating CF2—CH2 segments breaking sigma sigma conjugation of C—C bonds, and (ii) better durability than homopolymers due to the introduction of a linear portion in the polymer backbone. The linear component allows for free radical propagation along the main chain delaying polymer backbone or main chain breakdown.
A pellicle is typically produced by using a solution of the fluorine-containing polymer. Any solvent can be used so long as it dissolves the polymer. Common solvents include fluorine-containing solvents in which the polymer is highly soluble. For example, common solvents may include polyfluoroaromatic compounds such as perfluorobenzene, pentafluorobenzene and 1,3-bis(trifluormethyl) benzene. Polyfluorotrialkylamine compounds such as, for example, perfluorotribuylamine and perfluortripropylamine are also useful. In addition, polyfluorocycloalkane compounds such as perfluoroocyclohexane are useful as well as polyfluorocyclic ether compounds (e.g., perfluoro (2-butyltetrahydrofuran)).
A pellicle membrane is formed from a layer of polymer on a substrate by any number of methods such as, for example, roll coating, casting, dip coating, spin coating, water casting, or die coating. The thickness of the pellicle is usually selected to be in a range from about 0.01 to 50 μm. Typical substrates may include silicon wafer, quartz glass, or the like, having a smooth surface.
Pellicles are typically manufactured using spin-on technology. As such, pellicle polymers combine high optical clarity at certain wavelengths and solubility. CYTOP and Teflon AF (TAF) are commonly used pellicle materials that possess high optical clarity and good solubility as a consequence of their amorphous morphology attributed to their cyclic structure. Although fluoropolymers show high optical transparency, many fluoropolymers are not soluble in organic solutions. Thus, most fluoropolymers are not applicable to spin-on of the polymer solutions. As such, an appropriate alternative material for pellicle manufacturing should combine high optical clarity, durability and solubility in the organic solutions. Pellicles comprising PVDF satisfy this need. Certain solvents such as methyl ethyl ketone (MEK) are acceptable for spin coating of PVDF copolymers on most surfaces of relevance, for film thicknesses typical of present-day commercial applications.
Accordingly, a method comprising coating a fluorine-containing polymer and PVDF copolymer composition on a substrate is provided by the disclosure. The method comprises coating on a substrate a fluorine-containing polymer and PVDF copolymer composition dissolved in a solvent, and drying the solvent such that a thin film of the fluorine-containing polymer and PVDF copolymer is formed on the substrate. As described herein various amorphous fluorine containing polymer materials are known in the art. The coating of the copolymer material can be performed in any number of ways including roll coating, dip coating, spin coating, water casting, and die coating. Using the methods provided, a pellicle film can be easily generated and used for lithography purposes.
Fluorination of PVDF can be accomplished using a number of techniques. For example, post-formation fluorination of polymer materials useful as pellicles is provided by the disclosure. Physical and chemical modifications of PVDF and polymer materials contribute to the durability and optical properties of the materials and polymers, particularly at shorter wavelengths. The polymer surface characteristics (including end atoms) have a strong influence on the final product's physical and chemical properties.
An aspect describes improving PVDF pellicle and polymer characteristics such as durability and optical transmission by surface treatment techniques. Various surface modification techniques may include, for example, chemical treatment; flame treatment; coronas; low pressure plasmas; IR, UV, X-ray and gamma-ray irradiation; electron and ion beam bombardment; ozone exposure; plasma treatment; and others.
A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.
