Fluoropolymers absorptivity

Amorphous polymers characteristically possess excellent optical properties. Unlike all the other commercially available fluoropolymers, which are semicrystalline, Teflon AF is quite clear and has optical transmission greater than 90% throughout most of the UV, visible, and near-IR spectrum. A spectrum of a 2.77-mm-thick slab of AF-1600 is shown in Figure 2.5. Note the absence of any absorption peak. Thin films of Teflon AF have UV transmission greater Ilian 95% at 200 mm and are unaffected by radiation from UV lasers. The refractive indexes of Teflon AF copolymers are shown in Figure 2.6 and decrease with increasing FDD content. These are the lowest refractive indexes of any polymer family. It should be noted that the abscissa could also be labeled as glass transition temperature, Tg, since Tg is a function of the FDD content of the AF copolymer. Abbe numbers are low 92 and 113 for AF-1600 and AF-2400. 

In general, fluoropolymers possess the unique combination of high thermal stability, chemical inertness, unusual surface properties, low dielectric constants and dissipation factors, low water absorptivities, excellent weatherability and low flammabilities. Therefore there appears to be an ever-increasing market for fluoropolymers in spite of their relatively high cost [211,212],... 

Fluoropolymers have received much attention for applications in various fields because of their excellent properties (1) such as high thermal stability, high chemical stability, low adhesion, biological suitability, low frictional resistance, and transparency at vacuum ultraviolet (VUV) region (2). Because of the sufficient transparency at 157 nm, fluoropolymers are used as a polymer for F2 and F2 immersion lithography for semiconductor industry (2). Polymethacrylates containing bi- and/or tri-alicyclic structures, which are major polymers in ArF resist (3), are too absorptive at 157 nm and cannot be used as a polymer in F2 and F2 immersion resist (2). 

As described in detail later, the polymers for use in 157 nm lithography universally contain fluorine because only fluoropolymers provide low enough absorption at 157 nm. The acid group of choice is hexafluoroisopropanol, which has a pKa similar to that of phenol, as mentioned earlier. Several platforms are available (Fig. 41) 1) tetrafluoroethylene-norbornene,2) 2-trifluoromethyl-acrylate-norbornene, 3) 2-trifluromethylacrylate-styrene, 4) 2-trifluoromethy-lacrylate-vinyl ether, 5) all-norbornene, 6) methacrylate, and 7) cyclopolymers. 

Candidates and design rules for 157 nm resist polymers were sought by measuring VUV absorption of existing polymers including fluoropolymers or... 

Resistance to stress cracking can be reduced by the absorption of substances that chemically resemble the polymer and will plasticize it. In addition, the mechanical strength will be reduced. Halogenated chemicals, particularly those consisting of small molecules containing fluorine or chlorine, are especially likely to be similar to the fluoropolymers and should be tested for their effect. 

We have measured die absorptiai coefficients at 157 nm of many polymers including mine than 400 fluwopdymm. Figure 1 diows the histograms of fluOTopolymers and non-flucnrqiolymas. The average absorption coefficient of fluon lymo is ca. 2.3 pm, which is smaller than 6.3 pm of non-fluoropolymers. The fluorination makes polymers more tran iarent by 4 pm. ... 

Figure 2 shows the details of Suwc olymer bar chat in Figure 1. They are classified into fluOTopolymo incorpOTating fluorine at their backbones and side-chains. The main-chain-fluorinated polymers have die good optical property transparent than side-chain-fluorinated polymers. Some fluoropolymers have the good absorption coefficient less than 1 pm, which can not be achieved by fluorination at side-chains. Tetrafluoroethylene (TFE) is one of good monomers for backbone-flucainated polymers. Figure 3 shows the TFE-contoit dependence of absorption coefficient of TFE-nwb( Tiene (NB) cqiolymers. The more TFE-content ino eases, the more transparent the copolymer becomes. Note diat die TFE polymers have the potential of the absorption coefficient less than 1 pm. ...

Figure 4 shows the dependence of optical absorption coefficients of the fluoropolymers on the fluorine concentration. Fluoropolymers generally become mwe transparent with increasing fluorine content. The second ordinate, or the 50%T thickness , shows the film thickness at the bottom of which the light intensity decreases to half the incident intensity of 157-nm light. Some FPl... 

Figure 4. The dependence of absorption coefficient on fluorine contents in fluoropolymers. FPl open dots, FP2 closed dots. Others x.

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