Ultraviolet and vacuum ultraviolet transparent polymer compositions and their uses Number:6,770,404 from the United States Patent and Trademark Office (PTO) owispatent Disclosed are partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths from 187 to 260 nanometers.<H compositions, transparent, Ultraviolet, and, 6770404.html, Patent, pto, 6770404

Title: Ultraviolet and vacuum ultraviolet transparent polymer compositions and their uses

Abstract: Disclosed are partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths from 187 to 260 nanometers.

Patent Number: 6,770,404 Issued on 08/03/2004 to Wheland, et al.

Inventors: Wheland; Robert Clayton (Wilimington, DE), French; Roger Harquail (Wilmington, DE), Zumsteg, Jr.; Fredrick Claus (Wilmington, DE)
Assignee: E. I. du Pont de Nemours and Company (Wilmington, DE)

Appl. No.: 10/111,442

Filed: April 24, 2002

PCT Filed: November 16, 2000

PCT No.: PCT/US00/31560

PCT Pub. No.: WO01/37043

PCT Pub. Date: May 25, 2001

Current U.S. Class: 430/5 ; 526/253

Current International Class: G03F 1/14 (20060101); G03F 7/09 (20060101); G03F 7/004 (20060101)

Field of Search: 526/253 359/350 430/5

References Cited [Referenced By]

U.S. Patent Documents


5000547	March 1991	Squire
5188873	February 1993	Delannoy
5286567	February 1994	Kubota et al.
5344677	September 1994	Hong
5502132	March 1996	Sugiyama et al.
5614287	March 1997	Sekiya et al.
6335408	January 2002	Russo et al.

Foreign Patent Documents


416 528	Sep., 1990	EP
0416528	Mar., 1991	EP
5948766	Mar., 1984	JP
1241557	Sep., 1989	JP
7295207	Nov., 1995	JP
WO 9822851	May., 1998	WO
WO 9836324	Aug., 1998	WO

Primary Examiner: Wu; David W.
Assistant Examiner: Hu; Henry S.

Parent Case Text

This application claims the benefit of Provisional Application No. 60/166,037, filed Nov. 17, 1999.

Claims

What is claimed is:

1. An ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) .ltoreq.1 at wavelengths from 187-260 nm comprising: amorphous vinyl copolymers of CX.sub.2.dbd.CH.sub.2, wherein X is --F or --CF.sub.3 and up to 25 mole % of one or more monomers CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d where the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d enters the copolymer in approximately random fashion or 40 to 60 mole % of one or more monomers CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d in the case where the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d enters the copolymer in approximately alternating fashion where each of R.sup.a, R.sup.b, and R.sup.c is selected independently from H or F and where R.sup.d is selected from the group consisting of --F, --CF.sub.3, --OR.sub.f where R.sub.f is C.sub.n F.sub.2n+1 with n=1 to 3, --OH (when R.sup.c.dbd.H), and Cl (when R.sup.a, R.sup.b, and R.sup.c.dbd.F), with the proviso that not all of R.sup.a-d can be F.

2. An ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) .ltoreq.1 at wavelengths from 187-260 nm comprising amorphous vinyl copolymers selected from the noun consisting of CH.sub.2.dbd.CHCF.sub.3 and CF.sub.2.dbd.CF.sub.2 ; CH.sub.2.dbd.CFH and CF.sub.2.dbd.CFCl; CH.sub.2.dbd.CHF and CClH.dbd.CF.sub.2, wherein the molar ratio of monomers ranges from approximately 1:2 to approximately 2:1; perfluoro(2-methylene-4-methyl-1,3-dioxolane) and perfluoro(2,2-dimethyl-1,3-dioxole); perfluoro(2-methylene-4-methyl-1,3-dioxolane) and vinylidene fluoride in any ratio that gives an amorphous composition; perfluoro(2-methylene-4-methyl-1,3-dioxolane) with tetrafluoroethylene in any ratio that gives an amorphous composition; and the homopolymer of perfluoro(2-methylene-4-methyl-1,3-dioxolane).

3. The UV transparent material of claims 1 or 2 wherein the transparent material exhibits A/micrometer .ltoreq.0.8 at wavelengths from 187-260 nm.

4. The UV transparent material of claim 3 wherein the UV transparent material exhibits A/micrometer .ltoreq.0.2 at wavelengths from 187-260 nm.

5. The UV transparent material of claim 4 wherein the V transparent material exhibits A/micrometer .ltoreq.0.1 at wavelengths from 187-260 nm.

6. The UV transparent material of claim 1 wherein the polymer is an approximately alternating copolymer having 40-60 mole % CX.sub.2.dbd.CH.sub.2 where X is --F or --CF.sub.3, and having 60 to 40 mole % monomers of the structure CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d where each of R.sup.a, R.sup.b, and R.sup.c is selected independently from H or F and where R.sup.d is selected from the group consisting of --F, --CF.sub.3, --OR.sub.f where R.sub.f is C.sub.n F.sub.2n+1 with n=1 to 3, --OH (when R.sup.c.dbd.H), and Cl (when R.sup.a, R.sup.b, and R.sup.c.dbd.F), with the proviso that not all of R.sup.a-d can be F.

7. The UV transparent material of claim 6 wherein the copolymer is a hexafluoroisobutylene/trifluoroethylene copolymer wherein the molar ratio of the monomers is from approximately 60:40 to approximately 40:60.

8. The UV transparent material of claim 6 wherein the copolymer is a hexafluoroisobulylene/vinyl fluoride copolymer wherein the molar ratio of monomers is from approximately 60:40 to approximately 40:60.

9. The UV transparent material of claim 1 wherein the copolymer is an approximately random copolymer having >75 mole % CX.sub.2.dbd.CY.sub.2 where X is --F or --CF.sub.3, and Y is H, and having <25 mole % monomers of the structure CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d where each of R.sup.a, R.sup.b, and R.sup.c is selected independently from H or F and where R.sup.d is selected from the group consisting of --F, --CF.sub.3, --OR.sub.f where R.sub.f is C.sub.n F.sub.2n+1 with n=1 to 3, --OH (when R.sup.c.dbd.H), and Cl (when R.sup.a, R.sup.b, and R.sup.c.dbd.F), with the proviso that not all of R.sup.a-d can be F.

10. The UV transparent material of claim 9 where the copolymer is >75 mole % vinylidene fluoride and <25 mole % hexafluoropropylene.

11. The UV transparent material of claim 9 wherein the copolymer is 60-40 mole % vinyl fluoride and 40-60 mole % chlorotrifluroethylene.

12. Pellicles comprising the UV transparent material of claims 1 or 2.

13. Anti-reflective coatings comprising the UV transparent material of claims 1 or 2.

14. Optically clear glues comprising the UV transparent material of claims 1 or 2.

15. Light guides comprising the UV transparent material of claims 1 or 2.

16. Resists comprising the UV transparent material of claims 1 or 2.

17. The resists of claim 16 further comprising solubility responsive monomers.

18. Transmissive optical elements comprising the UV transparent material of claims 1 or 2.

19. A copolymer composition comprising poly(hexafluoroisobutylene:trifluoroethylene) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % trifluoroethylene having A/micron <1 at 187-260 nm.

20. An amorphous copolymer composition comprising poly(hexafluoroisobutylene:vinyl fluoride) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % vinyl fluoride having A/micron <1 at 187-260 nm.

21. A copolymer composition of claim 19 or 20 having A/micron <0.8 at 187-260 nm.

22. A copolymer composition of claim 21 having A/micron <0.2 at 187-260 nm.

23. A copolymer composition of claim 22 having A/micron <0.1 at 187-260 nm.

24. The ultraviolet transparent material of claim 1 wherein X is F.

25. The use of the polymer of claim 10 as an adhesive.

26. The use of the polymer of claim 10 as an adhesive in attaching a pellicle to a photomask.

27. The use of the polymer of claim 10 as an adhesive in attaching a pellicle to a frame.

28. The material of claim 1 or claim 2 wherein the wavelength is 187 to 199 nm.

Description

FIELD OF THE INVENTION

This invention concerns partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths from approximately 187 nanometer to 260 nanometers.

TECHNICAL BACKGROUND OF THE INVENTION

The semiconductor industry is the foundation of the trillion dollar electronics industry. The semiconductor industry continues to meet the demands of Moore's law, whereby integrated circuit density doubles every 18 months, in large part because of continuous improvement of optical lithography's ability to print smaller features on silicon. The circuit pattern is contained in the photomask, and an optical stepper is used to project this mask pattern into the photoresist layer on the silicon wafer. Current lithography is done using 248 nm light; lithography with 193 nm light is just entering early production. Alternate methods of lithography that do not use visible or ultraviolet light waves, i.e., the next generation lithographies utilizing X-rays, e-beams or EUV radiation have not matured sufficiently that they are ready to be adopted for production. As use of this new technology develops, there remains a continuing need for improved materials with higher transparencies and greater resistance to radiation damage.

Certain fluoropolymers have already been identified in the art as useful for optical applications such as light guides, anti-reflective coatings and layers, pellicles, and glues. Most of this work has been done at wavelengths above 200 nm where perfluoropolymer absorption is of little concern.

WO 9836324, Aug. 20, 1998, Mitsui Chemical Inc., discloses the use of resins consisting solely of C and F, optionally in combination with silicone polymers having siloxane backbones, as pellicle membranes having an absorbance/micrometer of 0.1 to 1.0 at UV wavelengths from 140 to 200 nm. Data in the literature, together with applicant's measurements, for fluoropolymers (see Table 1 below) demonstrate that, at least at 157 nm, C and F fluoropolymers have absorbances much larger than A/.mu.=0.1 to 1 as claimed by WO 9836324.

WO 9822851, May 28, 1998, Mitsui Chemicals, Inc., claims the use of photodegradation-resistant, tacky polymers that immobilize dust particles when coated on the inside of a pellicle frame. These tacky materials have compositions consisting largely of low molecular weight --(CF.sub.2 --CXR) copolymers in which X is halogen and R is --Cl or --CF.sub.3. Higher molecular weight polymers such as poly(perfluorobutenyl viny ether), poly[(tetrafluoroethylene/ 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole)], poly(tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride), poly(hexafluoropropylene/vinylidene fluoride), or poly(chlorotolyl fluoroethylene/vinylidene fluoride) are added as a minor component to improve creep resistance. It should be noted that all exemplifications of this technology were with poly(chlorotrifluoroethylene) as the low molecular weight adhesive agent and that retention of tackiness after extended UV degradation (illustrated only at 248 nm), not transparency, was the only demonstrated advantage of the claimed formulations.

Japanese Patent 07295207, Nov. 10, 1995, Shinetsu Chem. Ind Co, claims double layer pellicles combining Cytop CTXS (poly(CF.sub.2.dbd.CFOCF.sub.2 CF.sub.2 CF.dbd.CF.sub.2)) with Teflon.RTM. AF 1600 for greater strength.

U.S. Pat. No. 5,286,567, Feb. 15, 1994, Shin-Etsu Chemical Co., Ltd., claims the use of copolymers of tetrafluoroethylene and five membered cyclic perfluoroether monomers as pellicles once they have been made hydrophilic, and therefore antistatic, by plasma treatment.

European Patent 416528, Mar. 13, 1991, DuPont, claims amorphous fluoropolymers having a refractive index of 1.24-1.41 as pellicles at wavelengths of 190-820 nm. Copolymers of perfluoro(2,2-dimethyl-1,3-dioxole) with tetrafluoroethylene, chlorotrifluoroethylene, vinylidene fluoride, hexafluoropropylene, trifluoroethylene, vinyl fluoride, (perfluoroalkyl)ethylenes, and perfluoro(alkyl vinyl ethers) are cited.

Japanese Patent 01241557, Bando Chemical Industries, Ltd., Sep. 26, 1989, claims pellicles usable at 280-360 nm using (co)polymers of vinylidene fluoride (VF.sub.2,), tetrafluoroethylene/hexafluoropropylene (TFE/HFP), ethylene/tetrafluoroethylene (E/TFE), TFE/CF.sub.2.dbd.CFORf, TFE/HFP/CF.sub.2.dbd.CFORf, chlorotrifluoroethylene (CTFE), E/CTFE, CTFE/VF.sub.2 and vinyl fluoride (VF).

Japanese Patent 59048766, Mar. 21, 1984, Mitsui Toatsu Chemicals, Inc., claims the use of a stretched film of poly(vinylidene fluoride) as having good transparency from 200 to 400 nm.

Many of the fluoropolymers cited in the references above are noticeably hazy to the eye because of crystallinity and would therefore be expected to scatter light to a degree unsuitable for high light transmission and the accurate reproduction of circuit patterns. Poly(vinylidene fluoride), poly(chlorotrifluoroethylene), poly(tetrafluoroethylene/ethylene), commercially available poly(tetrafluoroethylene/hexafluoropropylene) compositions, and poly(ethylene/chlorotrifluoroethylene) are all such crystalline, optically hazy materials. More recent references have thus been directed at Cytop.TM. and Teflon.RTM. AF because they combine perfluorination with outstanding optical clarity, solubility, and a complete lack of crystallinity. Cytop.TM. and Teflon.TM. are less than ideal, however, because the difficulty of the their monomer syntheses make them extremely expensive.

It is an object of the present invention to overcome the difficulties associated with the prior art by providing partially fluorinated and fully fluorinated polymers that are substantially transparent to ultraviolet radiation at wavelengths between 187 and 260 nanometers, especially at 193 nanometers and/or 248 nanometers.

SUMMARY OF THE INVENTION

This invention provides An ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) .ltoreq.1 at wavelengths from 187-260 nm comprising amorphous vinyl copolymers of CX.sub.2.dbd.CY.sub.2, wherein X is --F or --CF.sub.3 and Y is H and 0 to 25 mole % of one or more monomers CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d where the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d enters the copolymer in approximately random fashion, or 40 to 60 mole % of one or more monomers CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d in the case where the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d enters the copolymer in approximately alternating fashion where each of R.sup.a, R.sup.b, and R.sup.c is selected independently from H or F and where R.sup.d is selected from the group consisting of --F, --CF.sub.3, --OR.sub.f where R.sub.f is C.sub.n F.sub.2n+1 with n=1 to 3, --OH (when R.sup.c.dbd.H), and Cl (when R.sup.a, R.sup.b, and R.sup.c.dbd.F). Another useful embodiment within the present wavelength range is 187 to 199 nm.

This invention also provides an ultraviolet transparent material exhibiting an absorbance/micron (A/micrometer) .ltoreq.1 at wavelengths from 187-260 nm comprising amorphous vinyl copolymers of CH.sub.2.dbd.CHCF.sub.3 and CF.sub.2.dbd.CF.sub.2 ; CH.sub.2.dbd.CFH and CF.sub.2.dbd.CFCl; CH.sub.2.dbd.CHF and CClH.dbd.CF.sub.2, wherein the ratio of monomers ranges from approximately 1:2 to approximately 2:1; perfluoro(2-methylene-4-methyl-1,3-dioxolane) and perfluoro(2,2-dimethyl-1,3-dioxole); perfluoro(2-methylene-4-methyl-1,3-dioxolane) and vinylidene fluoride in any ratio that gives an amorphous composition; perfluoro(2-methylene-4-methyl-1,3-dioxolane) with tetrafluoroethylene in any ratio that gives an amorphous composition; and the homopolymer of perfluoro(2-methylene-4-methyl-1,3-dioxolane).

This invention further provides pellicles, anti-reflective coatings, optically clear glues, light guides and resists comprising the UV transparent material provided above.

This invention further provides copolymer compositions comprising poly(hexafluoroisobutylene:trifluoroethylene) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % trifluoroethylene and copolymer compositions comprising poly(hexafluoroisobutylene:vinyl fluoride) with 40-60 mole % hexafluoroisobutylene and 60-40 mole % vinyl fluoride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 describes the 157 nm transmission T of a pellicle in units of % as a function of the 157 nm absorbance, in units of inverse microns, of the polymer for an absorbance range from 0.4 to 0.0. The effects of thin film interference in the pellicle membrane are neglected in this calculation.

FIG. 2 describes the absorbance in units of inverse microns for Teflon.RTM. AF 1601 (Sample 7a) and Cytop.TM. (Sample 13) versus wavelength lambda (.lambda.) in units of nanometers.

FIG. 3 describes the index of refraction n and the extinction coefficient k determined for a Teflon.RTM. AF 1601 film (Sample 7b) of thickness 2146 angstroms on a silicon substrate versus wavelength lambda in units of nanometers determined by VUV spectroscopic ellipsometry.

FIG. 4 describes the spectral transmission in absolute units versus the wavelength lambda in units of nanometers for a pellicle of Teflon.RTM. AF 1601 designed as an unsupported tuned etalon with a film thickness of 6059 angstroms. The interference fringes of the tuned etalon are clearly visible as a function of wavelength.

FIG. 5 describes the spectral reflectance in absolute units versus the wavelength lambda in units of nanometers for a pellicle of Teflon.RTM. AF 1601 designed as an unsupported tuned etalon with a film thickness of 6059 angstroms. The interference fringes of the tuned etalon are clearly visible as a function of wavelength, and a minimum in the pellicle reflectance is seen at 157 nm which contributes to the maximized pellicle transmission at this lithographic wavelength.

FIG. 6 describes transmission of a tuned etalon pellicle film of Teflon.RTM. AF 1601 at a lithographic wavelength of 157 nm as a function of the pellicle film thickness. The oscillations in the pellicle transmission with thickness arise due the thin film interference fringes in the film and give rise to pellicle transmission maxima and minima. The optimum tuned etalon pellicle design will correspond to the film with sufficient mechanical integrity and a thickness such that the transmission is at a maxima. Still as can be seen, pellicles designed from this material have substantially lower transmissions than the target transmission for a 157 nm pellicle.

FIG. 7 describes the absorbance in units of inverse microns for Teflon.RTM. AF 1200 (Sample 8), Teflon.RTM. AF 1601 (Sample 7a), and Teflon.RTM. AF 2400 (Sample.sub.-- 5)_versus wavelength lambda (.lambda.) in units of nanometers. Notice the dramatic decrease in the absorbance/micron as the PDD content of the polymer increases and the TFE content of the polymer decreases from 52% to 32% to 11%, and therefore the lengths of any (CF.sub.2).sub.n runs in the polymer decreases.

FIG. 8 describes the absorbance in units of inverse microns for TFE:HFP (Sample 14) and TrFE:HFP (Sample 12) versus wavelength lambda (.lambda.) in units of nanometers. The presence of the HF carbons in the CF.sub.2.dbd.CFH monomer interrupts extended CF.sub.2 runs. This effect can also be understood as the absorption maxima of TFE:HFP polymer shifts to shorter wavelengths in the TrFE:HFP polymer.

FIG. 9 describes the absorbance in units of inverse microns for VF.sub.2 :PDD (Sample 2), VF.sub.2 :HFP (Sample 1), HFIB:TrFE (Sample 3) and HFIB:VF (Sample 4) versus wavelength lambda (.lambda.) in units of nanometers.

FIG. 10 describes the index of refraction n and the extinction coefficient k determined for a HFIB:VF film (Sample 4a) of thickness 14,386 angstroms on a silicon substrate versus wavelength lambda in units of nanometers determined by VUV spectroscopic ellipsometry.

FIG. 11 describes the spectral transmission in absolute units versus the wavelength lambda in units of nanometers for a pellicle of HFIB:VF designed as an unsupported tuned etalon with a film thickness of 3660 angstroms. The interference fringes of the tuned etalon are clearly visible as a function of wavelength.

FIG. 12 describes the spectral reflectance in absolute units versus the wavelength lania in units of nanometers for a pellicle of HRIB:VF designed as an unsupported tuned etalon with a film thickness of 3660 angstroms. The interference fringes of the tuned etalon are clearly visible as a function of wavelength, and a minimum in the pellicle reflectance is seen at 157 nm which contributes to the maximized pellicle transmission at this lithographic wavelength.

FIG. 13 describes transmission of a tuned etalon pellicle film of HFIB:VF with an absorbance per micron of 0.022 and an index of refraction of 1.5 at a lithographic wavelength of 157 nm as a function of the pellicle film thickness. Note that for pellicle film thicknesses up to 3660 angstroms, the maximum pellicle transmission is above the target specification of 98%.

FIG. 14 describes transmission of a tuned etalon pellicle film of a polymer with an absorbance per micron of 0.01 and an index of refraction of 1.5 at a lithographic wavelength of 157 nm as a function of the pellicle film thickness. Note that for pellicle film thicknesses up to 8371 angstroms, the maximum pellicle transmission is above the target specification of 98%.

FIG. 15 describes the absorbance in units of inverse microns for 5:6 TFP:TFE (Sample 17), HFIB:VF (Sample 18), 5:2 VF2:PFMVE (Sample 19), 7:5 VF2:PFPVE (Sample 21) and 79:21 VF2:HFP (Sample 22) versus wavelength lambda (.lambda.) in units of nanometers.

FIG. 16 describes the absorbance in units of inverse microns for 1:1 PDD:TrFE (Sample 9), 13:10 VF2:PFMVE (Sample 20), 2:5:2 HFP:PFMVE:VF2 (Sample 23), 10:7 HFIB:VF (Sample 24) and 6:5 PDD:PFMVE (Sample 25) versus wavelength lambda (.lambda.) in units of nanometers,

FIG. 17 describes the absorbance in units of inverse microns for 20:11 VF:ClDFE (Sample 26), 1:2 PDD:VF2 (Sample 27), 1:1 HFIB:VA (Sample 32), PMD (Sample 33) and PMD:PDD (Solution 34) versus wavelength lambda (.lambda.) in units of nanometers.

FIG. 18 describes the absorbance in units of inverse microns for 1:1 CTFE:VF (Sample 6b), 5:2 VF2:TrFE (Sample 10), 10:23 PDD:CTFE (Sample 11), 5:4 VF2:CTFE (Sample 15) and 1:1 PMD:TFE (Sample 18) versus wavelength lambda (.lambda.) in units of nanometers.

FIG. 19 describes the absorbance in units of inverse microns for 5:8 PDD:VF2 (Sample 29) and 41;37:22 HFIB:VF:VF2 (Sample 19) versus wavelength lambda (.lambda.) in units of nanometers.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides fluoropolymer compositions and their use in certain electronic applications.

Certain abbreviations used throughout the specification are tabulated: TFE tetrafluoroethylene HFP hexafluoropropylene VF vinylfluoride CTFE chlorotrifluoroethylene VF.sub.2 vinylidene fluoride HFIB hexafluoroisobutylene TrFE trifluoroethylene PDD 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3- dioxole PMD Perfluro(2-methylene-4-methyl-1,3- dioxolane) Teflon .RTM. AF 2400 89:11 PDD:TFE Teflon .RTM. AF 1601 68:32 PDD:TFE Teflon .RTM. AF 1200 48:52 PDD:TFE ClDFE 1-chloro-2,2-difluoroethylene PPVE perfluoro(propyl vinyl ether) PMVE perfluoro(methyl vinyl ether) VOAc vinyl acetate VOH vinyl alcohol TrP 3,3,3-trifluoropropene Fluorinert .RTM. FC-75 Electronic fluid manufactured by 3M, believed to approximate perfluoro(butyl tetrahydrofuran) Fluorinert .RTM. FC-40 Electronic fluid manufactured by 3M, believed to approximate perfluoro(tributylamine) Vazo .RTM. 56 WSP Initiator manufactured by DuPont, 2,2'- bis(2-amidino-propane)dihydrochloride DSC Differential scanning calorimetry

Perfluoro-(2,2-dimethyl-1,3-dioxole) and perfluorodimethyldioxole are terms commonly used in the art as variant designations of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole, the abbreviation of which is widely known as PDD, as noted above.

Absorption maxima for selected hydrocarbon and fluorocarbon compounds are shown in Table 1.

TABLE 1 Comparison of UV Absorption Maxima for Hydrocarbons and Fluorocarbons WAVELENGTH OF ABSORPTION MAXIMUM C.sub.n H.sub.2n+2.sup.1 C.sub.n F.sub.2n+2 n = 1 143 nm & 128 nm n = 2 158 nm & 132 nm n = 3 159 nm & 140 nm 119 nm.sup.1 n = 4 160 nm & 141 nm 126 nm.sup.1 n = 5 161 nm & 142 nm 135 nm.sup.1 n = 6 162 nm & 143 nm 142 nm.sup.1 n = 7 163 nm & 143 nm n = 8 163 nm & 142 nm n = 172 161 nm.sup.2 .sup.1 B. A. Lombos, P. Sauvageau, and C. Sandorfy, Chem, Phys. Lett., 1967, 42. .sup.2 K. Seki, H. Tanaka, T. Ohta, Y. Aoki, A, Imamura, H. Fujimoto, H. Yamamoto, H. Inokuchi, Phys. Scripta, 41, 167(1990).

As can be seen from the table, UV absorption maxima move to longer wavelengths as chain length increases for both hydrocarbons and fluorocarbons. Fluorocarbon chains (CF.sub.2)n absorb at 157 nm somewhere between n=6 (142 nm) and n=172 (161 nm) while hydrocarbon chains (CH.sub.2)n absorb at 157 nm as early as n=2. Clearly then fluorocarbon chains are more resistant than hydrocarbon chains to UV absorption and not surprisingly the industry has been moving increasingly towards perfluorination when seeking high UV transmission. But, as long as chain lengths still offering acceptable transparency can never exceed (CH.sub.2).sub.1 or (CF.sub.2).sub.6, perfectly transparent polymers at 157 nm and somewhat longer wavelengths would seem precluded. Consistent with this, V. N. Vasilets, et al., J. Poly. Sci, Part A, Poly. Chem., 36, 2215(1998) for example report that poly(tetrafluoroethylene/hexafluoropropylene), [poly(TFE/HFP)], (Teflon.RTM. FEP), shows strong absorption and photochemical degradation at 147 nm. Similarly we find that 1:1 poly(hexafluoropropylene:tetrafluoroethylene) is highly absorbing at 157 mm (Table 2, A/micron=3.6@157 nm).

Polymers play a critical role in lithography in multiple areas: one is the polymer pellicle which is placed over the mask pattern to keep any particulate contaminants out of the photomask object plane, thereby ensuring that the lithographic imaging will be defect free. The pellicle is a free standing polymer membrane, typically 0.8 micrometers in thickness, which is mounted on a typically 5 inch square frame. The pellicle film must have high transparency or transmission of light at the lithographic wavelength for efficient image formation and must neither darken nor burst with prolonged illumination in the optical stepper. Pellicles for current lithographic wavelengths utilize pellicles with >99% transmission, through exploitation of polymers with very low optical absorption combined with thin film interference effects. Typically the electronics industry likes to see greater than 98% transparency over an exposure lifetime of 75 million laser pulses of 0.1 mJ/cm.sup.2, or a radiation dose of 7.5 kJ.

A pellicle transmission of 98% corresponds to an absorbance A of approximately 0.01 per micrometer of film thickness. The absorbance is defined in Equation 1, where the Absorbance A per micron of film thickness, is defined as the base 10 logarithm of the ratio of the substrate transmission divided by the transmission of the Sample consisting of the polymer film sample on its substrate, this quantity divided by the polymer film thickness. ##EQU1##

In this manner the Absorbance A has units of inverse microns (or 1/micron, where a micron, is a micrometer or um of polymer film thickness. The absorbance/micron of polymer films discussed here were measured for polymer films spun coated on to CaF.sub.2 substrates using standard methods. The VUV transmission of each CaF.sub.2 substrate was measured prior to the spin coating of the polymer film. Then the VUV transmission of the polymer film on that particular CaF.sub.2 substrate was measured, and using the measured film thickness (reported in Table 2) and equation 1, the values of the absorbance/micron for the polymers, as a function of wavelength was determined, and the value of the absorbance/micron for a wavelengths of 157, 193, and 248 nm is tabulated in Table 2. For some materials, two films, of differing thicknesses are presented in the Table 2, and Absorbance/micron values for each film are also presented.

The VUV transmission of the CaF.sub.2 substrates and the polymer films on the CaF.sub.2 substrates were measured using a VUV spectrophotometer using a laser plasma light source, a sample chamber capable of both transmission and reflectance measurements, a 1 meter monochromator and a sodium salicylate phosphor coated 1024 element photodiode detector. This is discussed in greater detail in R. H. French, "Laser-Plasma Sourced, Temperature Dependent VUV Spectrophotometer Using Dispersive Analysis", Physica Scripta, 41, 4, 404-8, (1990) which is incorporated herein by reference.

The absorbance per micron of a polymer will determine the average transmission of an unsupported pellicle film made from that polymer. In FIG. 1, the 157 nm transmission T of a pellicle in units of % as a function of the 157 nm absorbance, in units of inverse microns, of the polymer for an absorbance range from 0.4 to 0.0 is shown. The effects of thin film interference in the pellicle membrane are neglected in this calculation. The results for pellicle films of thicknesses ranging from 0.2 microns to 1 micron are shown, and demonstrate that for any particular polymer, the pellicle transmission can be increased, through the use of a thinner pellicle film thickness. This approach to increasing the pellicle transmission has a limited range of utility, since the pellicle film is an unsupported polymer membrane and must have sufficient mechanical strength and integrity. These mechanical requirements suggest the use of polymer with relatively high glass transition temperature T.sub.g and polymer film thicknesses of 0.6 microns or greater. From FIG. 1 it can be seen that a target absorbance per micron of a polymer for pellicle applications is <0.02 abs./micron at 157 nm.

In FIG. 2 the absorbance in units of inverse microns for Teflon.RTM. AF 1601 and Cytop.TM. versus wavelength lambda (.lambda.) in units of nanometers is shown. At 157 nanometers, the absorbance/micron for Cytop has a value of 1.9/micron while Teflon.RTM. AF 1601 has an absorbance of 0.42/micron, approximately 5 times lower than Cytop. In Example 1 we will demonstrate that even the 157 nm absorbance of Teflon.RTM. AF 1601 of 0.42/micron is too high for application as a pellicle for use at 157 nm.

The optical properties (index of refraction, "n" and extinction coefficient, "k") are determined from variable angle spectroscopic ellipsometry (VASE) at three incident angles covering the wavelength range from 186-800 nm, corresponding to an energy range of 1.5-6.65 eV, in combination with VUV ellipsometry (VUV-VASE) measurements performed at a single angle of incidence from 143-275 nm, corresponding to an energy range of 4.5-8.67 eV. The polymer films were spin coated onto a silicon substrate. The VASE ellipsometers were manufactured by J. A. Woollam Company, 645 M Street, Suite 102, Lincoln, Nebr. 68508 USA. Optical constants were fit to these data simultaneously, using an optical model of the film on the substrate. See generally, O. S. Heavens, Optical Properties of Thin Solid Films, pp. 55-62, Dover, N.Y., 1991, incorporated herein by reference.

From knowledge of the spectral dependence of optical properties, the transmission of a pellicle film of arbitrary thickness can be calculated by using an optical model for the unsupported pellicle film, at a specific polymer film thickness, and then calculating the pellicle film transmission and reflectance. In this manner the pellicle film thickness can be optimized such that the pellicle will exhibit a thin film interference maximum in the transmission spectrum at the desired lithographic wavelength. These transmission maxima occur for various film thicknesses, determined from the index of refraction of the polymer, the lithographic wavelength of interest and the film thickness. The transmission maxima of a properly tuned etalon pellicle film occur where the reflectance of the pellicle film exhibits minimum in the reflectance, and correspond to minimizing the pellicle's reflectance and maximizing its transmission at the lithographic wavelength. The relationship between the extinction coefficient k and the absorption coefficient a and the absorbance per micron A is given in Equation 2, where lambda is the wavelength of light. This relationship is useful in comparing results of absorbance measurements and ellipsometry measurements. This relationship for A is exact if light scattering in the polymer film (as may occur due to crystallinity of the polymer), thin film interference effects, and surface scattering effects are minimized. ##EQU2##

Polymeric materials with very low absorbance/micron or extinction coefficients and low values of the index of refraction also have very important applications as anti-reflection coatings and optical adhesives. A low absorbance material, as taught here in can be used to reduce the light reflected from the surface of a transparent substrate of a relatively higher index of refraction. This decrease in the reflected light, leads to a concomitant increase in the light transmitted through the transparent substrate material. This anti reflective coating effect of these low absorbance/micron materials can be seen in the results for VF.sub.2 :PDD, VF.sub.2 :HFP, HFIB:TrFE, and HFIB:VF where the absorbance of the polymers on CaF.sub.2 substrates for the case of very thin films, exhibits negative absorbance/micron. This corresponds to an increase of 157 nm light transmission through the polymer film on the CaF.sub.2 substrate as compared to the light transmission through the bare CaF.sub.2 substrate. For VF.sub.2 :HFP, HFIB:TrFE and HFIB:VF we have measured much thicker polymer films (also listed in Table 2) in which this antireflective coating effect is not seen, where the absorbance/micron of the polymers is seen to be positive and very small.

Polymers such as these can also be used as adhesives to join optical elements together, and since they have low optical absorbance/micron and low values of the index of refraction, they serve to reduce the reflectance of light at the air/substrate interfaces among the optical elements, and serve to direct more of the transmitted light from one optical element into subsequent optical elements in the system.

The materials of the present invention are useful in the manufacture of transmissive optical elements, such as lenses and beam splitters, for use in the vacuum UV region.

These materials may also be used as elements in a compound lens designed to reduce chromatic aberrations. At present only CaF.sub.2 and possibly hydroxyl free silica are viewed as having sufficient transparency at 157 nm to be used in transmissive focussing elements. It is also commonly known (e.g., see R. Kingslake, Academic Press, Inc., 1978, Lens Design Fundamentals, p. 77) that by using a second material of different refractive index and dispersion, an achromatic lens can be created. A Sellmeier fit to the data shown in FIG. 10 for HFIB:VF as described in Example 4 shows it to have a refractive index of 1.4942 and a dispersion of 0.00220 nm.sup.-1 at 157 nm. A similar fit to the index of refraction data for CaF.sub.2 from Edward D. Palik, Handbook of Optical Constants of Solids II, p. 831, Academic Press, Inc., Boston, Mass. (1991) and French shows it to have an index 1.5584 and a dispersion of 0.00234 nm.sup.-1 at 157 nm. Thus, by using one of these materials in conjunction with CaF.sub.2, it is expected that an achromatic lens can be constructed from this and other similar materials described in this application.

An additional area in which polymers play a critical role is as the photosensitive photoresist which captures the optical latent image. In the case of photoresists, light must penetrate the full thickness of the resist layer for a latent optical image, with well defined vertical side walls to be produced during optical imaging which then will produce the desired resist image in the developed polymer. When used as a resist at 157 nm, a polymer can have a considerably higher absorption coefficient of A<.about.2-3 per micrometer of film thickness, if the resist thickness is limited to about 2000 .ANG..

WO 9836324 discloses carbon/fluorine polymers such as poly(tetrafluoroethylene/hexafluoropropylene) as pellicle membranes having absorption A/.mu. of 0.1 to 1 for use at 140 to 200 nm. The data in Table 1 above, derived from literature sources, Vasilets' report of high absorption and photodegradation of poly(hexafluoropropylene/tetrafluoroethylene) at 148 nm [V. N. Vasilets, et al, J. Poly. Sci., Part A, Poly. Chem., 36, 2215 (1998), and the data in Table 2 which combines additional literature data with applicant's data, casts doubt on this disclosure. For example, poly(tetrafluoroethylene:hexafluoropropylene), polymer 14 of Table 2, shows a relatively strong A/.mu. of 3.9 at 157 nm that fails not only the industry goals for pellicles (A/.mu. <0.01) but also the much looser goals for resists (A/.mu. <2-3). Japanese Patent 072952076 claims bilayer membranes of Cytop.TM. and Teflon.RTM. AF 1600 as pellicle films. At 157 nm Cytop.TM. has an A/.mu. of 1.9 (polymer 13, Table 2) and Teflon.RTM. AF 1600 an A/.mu. of 0.4 (polymer 7, Table 2). This is not surprising considering the data of Table 1 that shows significant UV light absorption at 160 nm whenever there are more than about 6 CF.sub.2 groups connected in a chain. Indeed, W. H. Buck and P. R. Resnick report that Teflon.TM. AF 1600 has more than 30% of its CF.sub.2 units present as (CF.sub.2)n runs with n>6 consistent with its A/.mu.=0.4 [J. Scheirs, Modern Fluoropolymers, John Wiley, New York, 1997, Chapter 22, page 401]. Unless the interactions responsible for such absorption can be broken up, it would seem impossible to find a carbon based polymer completely transparent at wavelengths shorter than about 160 nm.

Table 2 below lists absorbance/micrometer (A/.mu.) at 157, 193, and 248 nm for partially and fully fluorinated polymer films that have been spin coated on CaF.sub.2 crystals. Polymers are listed in order of increasing absorption. In some cases, more than one sample of various polymers have been prepared within an Example. Therefore, Table entries are identified by both Example number (first column) and sample number. In addition, reference to Figures displaying spectra of various polymers are cross referenced in the Table.

TABLE 2 Absorbance per micron (A/.mu.m) for Selected Fluoropolymers Figure #, Spin Example Sample Structure A/.mu.m A/.mu.m A/.mu.m Thick Speed # # Name 157 nm 193 nm 248 nm Tg Tm Angstroms Rpm FIG. 9 1a 79:21 0.015 0.005 0.003 -22.degree. C. 69,800 3 k Ex. 5 1b CF.sub.2.dbd.CH.sub.2 :CF.sub.2.dbd.CFCF.sub.3 (-0.3) (-0.15) (-0.1) (1641) 6 k 79:21 VF2:HFP FIG. 9 2 1:1 -0.04 0.02 0.08 ND.sup.7 2097 6 k CF.sub.2.dbd.CH.sub.2 :PDD.sup.2 1:1 VF2:PDD FIG. 9 3a 3:2 (CF.sub.3).sub.2.dbd.C 0.012 0.005 -0.001 93.degree. C. 12146 3 k Ex. 2 3b .dbd.CH.sub.2 :CF.sub.2.dbd.CFH (-0.05) (0.03) (0.01) (1500) 6 k 3:2 HFIB:TrFE FIG. 9 4a 3:2 0.027 0.020 0.008 56.degree. C. 14386 3 k 4b (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CFH (-0.04) (0.009) (0.03) (2870) 6 k FIG. 10 4c 0.022 Ex. 3 3:2 HFIB:VF FIG. 7 5 89:11.sup.4 0.007 -0.06 -0.06 240.degree. C. 2133 6 k PDD.sup.2 :CF.sub.2.dbd.CF.sub.2 TAF2400 PDD:TFE FIG. 18 6a 1:1 0.129 -0.073 -0.037 1850 Ex. 21 6b CF.sub.2.dbd.CFCl:CH2.dbd.CHF (0.388) (0.016) (0.006) (17644) 1:1 CTFE:VF FIG. 2,7 7a 68:32.sup.5 0.42 0.02 0.01 160.degree. C. 3323 6 k Ex. A, 7b PDD.sup.2 :CF.sub.2.dbd.CF.sub.2 (0.35.sup.#) (2146) TAF1601 PDD:TFE FIG. 7 8 48:52.sup.6 0.64 0.004 -0.001 120.degree. C. 4066 6 k Ex. PDD.sup.2 :CF.sub.2.dbd.CF.sub.2 TAF1200 PDD:TFE FIG. 16 9 1:1 0.03 -0.004 -0.001 150.degree. C. 7688 PDD.sup.2 :CF.sub.2.dbd.CFH 1:1 PDD:TrFE FIG. 18 10 5:2 0.924 0.188 0.083 ND.sup.7 98.degree. C. 4500 CH.sub.2.dbd.CF.sub.2 :CF.sub.2.dbd.CFH 147.degree. C. 5:2 VF2:TrFE FIG. 18 11 10:23 1.44 0.018 0.046 1903 6 k PDD.sup.2 :CF.sub.2.dbd.CFCl 10:23 PDD:CTFE FIG. 8 12 2:98 1.37 0.143 -0.02 179.degree. C. 1389 6 k CF3CF.dbd.CF2:CF2.dbd.CFH 2:98 HFP:TrFE FIG. 2 13 Poly(CF.sub.2.dbd.CFOCF.sub.2 CF.sub.2 CF.dbd.CF.sub.2) 1.9 0.02 0.02 108.degree. C. 5595 6 k Ex. A Cytop FIG. 8 14 1:1 3.9 0.086 0.073 .about.30.degree. C. 1850 6 k CF.sub.3 CF.dbd.CF.sub.2 :CF.sub.2.dbd.CF.sub.2 1:1 HFP:TFE FIG. 18 15 5:4 5.6 0.27 0.12 99.degree. C. Ex. 26 CF.sub.2.dbd.CH.sub.2 :CF.sub.2.dbd.CFCl 5:4 VF2:CTFE FIG. 18 16 .about.1:1 1.17 -0.015 0.07 64.degree. C. 2207 6 k Ex. 27 PMD.sup.3 :TFE FIG. 15 17 5:6 0.149 0.008 -0.00085 9.degree. C. 41413 1.5 k Ex. 7 CF.sub.3 CH.dbd.CH.sub.2 :CF.sub.2.dbd.CF.sub.2 5:6 TFP:TFE FIG. 15 18 47:53 0.005 -0.00082 -0.002 18.degree. C. 9239 1.5 k Ex. 8 (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CHF 47:53 HFIB:VF FIG. 15 19 5:2 0.016 0.006 0.004 -32.degree. C. 72750 Ex. 9 CH.sub.2.gradient.CF.sub.2 CF.sub.3 OCF.dbd.CF.sub.2 5:2 VF2:PMVE FIG. 16 20 13:10 0.034 0.015 0.018 -29.degree. C. 25970 Ex. 10 CH.sub.2.dbd.CF.sub.2 :CF.sub.3 OCF.dbd.CF.sub.2 13:10 VF2:PMVE FIG. 15 21 7:5 0.028 -0.003 -0.00074 -32.degree. C. 29874 Ex. 11 CH.sub.2.dbd.CF.sub.2 :CF.sub.3 CF.sub.2 CF.sub.2 OCF.dbd.CF.sub.2 7:5 VF2:PPVE FIG. 15 22 79:21 0.014 -0.002 -0.00056 -22.degree. C. 13000 Ex. 12 CH.sub.2.dbd.CF.sub.2 :CF.sub.3 CF.dbd.CF.sub.2 79:21 VF2:HFP FIG. 16 23 10:27:63 0.008 -0.00048 -0.00045 ND.sup.7 316500 Ex. 13 CF.sub.3 CF.dbd.CF.sub.2 :CF.sub.3 OCF.dbd.CF.sub.2 :CH.sub.2.dbd.CF.sub.2 10:27:63 HFP:PMVE:VF2 FIG. 16 24 59:41 -0.013 -0.016 -0.011 115.degree. C. 7500 Ex. 14 (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CF.sub.2 59:41 HFIB:VF FIG. 16 25 6:5 0.209 0.006 0.013 133.degree. C. 12450 Ex. 15 PDD.sup.2 :CF.sub.3 OCF.dbd.CF.sub.2 6:5 PDD:PMVE FIG. 17 26 20:11 0.226 0.036 0.003 ND.sup.7 9461 Ex. 16 CH.sub.2.dbd.CHF:CF.sub.2.dbd.CHCl 20:11 VF:ClDFE FIG. 17 27 1:2 0.009 0.003 -0.00030 52.degree. C. 82000 PDD.sup.2 :CH.sub.2.dbd.CF.sub.2 1:2 PDD:VF2 28 2:1 96.degree. C. PDD.sup.2 :CH.sub.2.dbd.CF.sub.2 2:1 PDD:VF2 FIG. 19 29 5:8 0.018 0.010 0.000057 59.degree. C. 38298 PDD.sup.2 :CH.sub.2.dbd.CF.sub.2 5:8 PDD:VF2 Ex. 19 30 5:3:1 48.degree. C. (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CF.sub.2 :CH.sub.2.dbd.CHF 5:3:1 HFIB:VF2:VF FIG. 19 31 41:37:22 0.016 -0.010 -0.002 71.degree. C. 5289 Ex. 19 (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CHF:CH.sub.2.dbd.CF.sub.2 41:37:22 HFIB:VF:VF2 FIG. 17 32 1:1 0.350 -0.047 -0.107 90.degree. C. 1350 Ex. 23 (CF.sub.3).sub.2 C.dbd.CH.sub.2 :CH.sub.2.dbd.CH(OH) 1:1 HFIB:VA FIG. 17 33 Poly[perfluoro(2-methylene-4- 0.603 -0.0007 -0.0001 135.degree. C. 14818 Ex. 28 methyl-1,3-dioxolane)] PMD FIG. 17 34 1:1 PMD.sup.3 :PDD.sup.2 0.404 0.006 -0.002 147.degree. C. 12762 Ex. 29 1:1 PMD:PDD Ex. 30 35 54:46 0.085 0.002 0.002 PMD3:CH.sub.2.dbd.CF.sub.2 PMD:VF2 (A/.mu.m) determined from VUV transmission based absorbance measurements, one result, marked by .sup.# was determined from VUV ellipsometry .sup.1 J. Scheirs, editor, Modern Fluoroplastics, John Wiley U Sons, West Sussex, England, 1997, Chapter 28 .sup.2 PDD = 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole .sup.3 PMD = Perfluoro(2-methylene-4-methyl-1,3-dioxolane .sup.4 Teflon .RTM. AF 2400 .sup.5 Teflon .RTM. AF 1601 .sup.7 Teflon .RTM. AF 1200 .sup.7 Monomers were loaded at 3:4 mole ratio. Polymer product not analyzed

Three of the acyclic polymer structures listed in Table 2 show no detectable UV absorption at 157 nm (polymers VF.sub.2 :HFP, HFIB:TrFE, and HFIB:VF). These are copolymers containing either vinylidene fluoride (VF.sub.2) or hexafluoroisobutylene (HFIB). VF.sub.2 and HFIB have a unique structural feature in common. Taken by itself, for example, the hexafluoroisobutylene monomer can not form CH.sub.2 nms longer than CH.sub.2 CH.sub.2 before the run is broken up by a --C(CF.sub.3).sub.2 -- segment. Similarly vinylidene fluoride can not form of CH.sub.2 nms longer than CH.sub.2 CH.sub.2 before they are broken up by a CF.sub.2 or CF.sub.2 runs longer than CF.sub.2 CF.sub.2 before they broken up by a CH.sub.2. That is, CX.sub.2.dbd.CY.sub.2 monomers such as VF.sub.2 and HFIB have the property of being "self-interrupting" when judging the potential for extended interactions between C--F bonds or extended interactions between C--H bonds. The stiff five membered ring of PDD likely has a related effect by forcing conformations unfavorable for interaction. Comparing PDD/TFE copolymers 5, 7, and 8, absorption falls off from 0.6 to 0.4 to 0.0 as TFE content drops from 52 to 32 to 11 mole %. That is, transparency improves as increasing PDD content interrupts (CF.sub.2).sub.n runs. It is anticipated that other perfluorinated ring structures or partially fluorinated ring structures can be found that will serve the same interrupting function as PDD.

Solutions of TFE:HFP and TrFE:HFP were spin coated at spin speeds of 6000 rpm onto CaF.sub.2 substrates to produce polymer films of 1850 angstroms and 1389 angstroms thicknesses respectively. VUV absorbance measurements were then used to determine the absorbance per micron.

The introduction of TrFE in place of TFE demonstrates another example of decreasing the 157 nm absorbance/micron. The absorbance in units of inverse microns for TFE:HFP and TrFE:HFP versus wavelength lambda (.lambda.) in units of nanometers is shown in FIG. 8. The presence of the CHF carbons in the CF.sub.2.dbd.CFH monomer interrupts extended CF.sub.2 runs.

This produces a decrease in the 157 nm absorbance of TFE:HFP of 3.9/micron to an absorbance/micron of 1.37/micron for TrFE/HFP. This effect can also be understood as the absorption maxima of TFE:HFP polymer shifts to shorter wavelengths in the TrFE:HFP polymer.

Polymer relatively transparent at 157 nm tend to be even more transparent at longer wavelengths, including the region of interest here, 187-260 nm. Thus, we define a class of polymers highly transparent (A/.mu. <1, more preferably A/.mu. <0.1) to light of <260 nm made by homo or copolymerizing PDD and CX.sub.2.dbd.CY.sub.2 monomers (X.dbd.--F or --CF.sub.3 and Y.dbd.--H) and optionally other monomers CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d where any one or all of R.sup.a, R.sup.b or R.sup.c, may be H or F and where R.sup.d may be F, --CF.sub.3, --ORf where --Rf is C.sub.n F.sub.2n+1 with n=1 to 3, OH (when R.sup.c.dbd.H), and Cl (when R.sup.a, R.sup.b, and R.sup.c.dbd.F)as needed to introduce solubility or break crystallinity.

When the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d monomer polymerizes in an alternating fashion with PDD or the CX.sub.2.dbd.CY.sub.2 monomer, a higher concentration of the CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d monomer can be tolerated. An example of this is the 1:1 HFIB:TrFE copolymer which had an A/.mu. of -0.05 (Table 2, polymer #3). Random and alternating structures of course are never 100% ideal and some monomers naturally tend to either block or avoid polymerizing with themselves so the limits of 25% CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d for random structures and 50% CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d for alternating structures are approximate. Other highly transparent combinations include .about.2:1 to 1:2 copolymers of CH.sub.2.dbd.CHCF.sub.3 :CF.sub.2.dbd.CF.sub.2, CH.sub.2.dbd.CHF:CF.sub.2.dbd.CFCl, CH.sub.2.dbd.CHF:CClH.dbd.CF.sub.2. Preferred monomers for CX.sub.2.dbd.CY.sub.2 include vinylidene fluoride and hexafluoroisobutylene. Preferred CR.sup.a R.sup.b.dbd.CR.sup.c R.sup.d monomers are ones that introduce an asymmetric center in the polymer chain such as vinyl fluoride, trifluoroethylene, hexafluoropropylene, and chlorotrifluoroethylene so as to increase solubility and break crystallinity. Finally it should be pointed out that making highly transparent structures as defined above does not automatically make a polymer useful for all claimed applications. It has already been pointed out for example that the crystallinity of poly(vinylidene fluoride) precludes its use as a perfectly clear and transparent optical material [that is the failure of poly(vinylidene fluoride) as a pellicle would not be in the absorption but for a physical reason (light scattering)]. As another example, poly(4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole) is too insoluble to easily spin coat as thick films. In the case of poly(4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole) this can be gotten around by coating liquid monomer and polymerizing in place. Films of thickness greater than about 250 nm can be prepared by placing monomer optionally diluted with solvent and/or initiator in the location where the film is desired. Polymerization can be initiated by appropriate physical and/or chemical means which leads to the deposition of polymer, as formed, in the desired location. The result, after subsequent solvent removal, is a film of polymer thicker than can be prepared by customary solvent coating techniques. As a final example, poly[vinylidene fluoride/perfluoro(methyl vinyl ether)] is a tacky gum useful for glues but not useful as a self-supporting pellicle film. As used herein, the term amorphous fluoropolymer means a fluoropolymer that exhibits no melting point when analyzed by Differential Scanning Calorimetry. No melting point means no melting associated thermal event of greater than 1 Joule/gram.

Listing a monomer as a precursor to transparent polymers is not meant to imply that it will either homopolymerize or form a copolymer with any other listed monomer. Hexafluoroisobutylene for example, does not form useful quantities of decent molecular weight homopolymer or copolymerize with tetrafluoroethylene under ordinary conditions. While these materials are being claimed for use at 187 to 260 nm, they also make excellent clear polymers at longer wavelengths, up to 800 nm, and may also be suitable for some applications at still shorter wavelengths. Specifically preferred wavelengths are 245-255 nm, 187-199 nm most preferred.

EXAMPLES

Many of the polymers tested were obtained as commercial samples. Teflon.RTM. AF 1200: DuPont, Wilmington, Del. Teflon.RTM. AF 1600: DuPont Teflon.RTM. AF 2400: DuPont Cytop.TM.: Asahi Glass, poly[perfluoro(butenyl vinyl ether)]

.about.1:1 Poly(hexafluoropropylenetrafluoroethylene) was made by the procedure described in U.S. Pat. No. 5,478,905, Dec. 26, 1995.

Many of remaining polymer compositions are known to the art and were prepared by standard methods. Typically autoclaves were loaded with monomer, solvent and initiator and heated to start polymerization. Most polymerizations were initiated using hexafluoropropylene oxide dimer peroxide 1 (DP) at ambient temperatures.

DP was prepared and used as a 0.05 to 0.2 molar solution in solvents such as Vetrel.TM. XF (CF.sub.3 CFHCFHCF.sub.2 CF.sub.3) or 3M's Performance Fluid PF-5080 (largely perfluorooctane). DP can be made conveniently by either a routine laboratory procedure [Chengue, et al, J. Org. Chem., 47, 2009 (1982)] or on demand by a jet mixer process (U.S. Pat. No. 5,962,746 of Oct. 5, 1999). The reaction mixture was then recovered and the polymer isolated by evaporation or filtration. Polymer compositions were determined by elemental analysis or by NMR. Procedures are given for the preparation of new compositions of matter, and representative procedures are given for previously known compositions.

The polymer films were prepared by spin coating of the polymer solutions onto CaF.sub.2 substrates and then the polymer film on substrate samples were subjected to a post apply bake so as to assure that no residual solvents were remaining in the polymer film. The post apply bake temperature was in the range from 120.degree. C. to 250.degree. C. on a hot plate for two to five minutes or in a vacuum oven overnight. The spin speeds for the samples are listed in Table 2.

Polymer film thicknesses were determined using single or multiple wavelength ellipsometry as discussed below, or by analysis using a Filmetrics Model F20 thin film measurement system (Filmetrics, Inc. 7675 Dagget St., Suite 140, San Diego, Calif. 92111-2255). Film Thicknesses are reported in Table 2. For some materials, two films, of differing thicknesses are presented in the table, and Absorbance/micron values for each film are presented.

Comparative Example A Teflon.TM. AF 1601 and Cytop.TM.

Solutions of Teflon.RTM. AF 1601 were spin coated at spin speeds of 6000 rpm onto CaF.sub.2 substrates to produce polymer films of 3323 angstroms thickness. VUV absorbance measurements were then used to determine the absorbance per micron.

Teflon.RTM. AF 1601 is a 68:32 PDD:TFE polymer which is currently used as a polymer for pellicles designed for use at lithographic wavelengths of 248 nm and 193 nm. The absorbance/micron for 157 nm light is 0.42/microns as determined from VUV absorbance measurements. From FIG. 1 this corresponds to pellicle transmissions below 70% for pellicle films of only 0.2 microns thickness, of course these results do not consider the additional effects of the thin film interference effects which arise from a properly designed pellicle film.

To determine the VUV optical properties of Teflon.RTM. AF 1601, VUV ellipsometry was performed on a Teflon.RTM. AF 1601 polymer sample on a silicon wafer, and the index of refraction and extinction coefficient shown in FIG. 3 was determined. The 157 nm index of refraction for Teflon.RTM. AF 1601 is 1.4251. The 157 nm extinction coefficient determined corresponds to an absorbance/micron of 0.35/micron and is also listed in Table 2.

With these Teflon.RTM. AF 1601 optical properties, and the methods of O. S. Heavens discussed above, one can design the tuned etalon pellicle film, whereby the reflectance of the unsupported pellicle film is minimized and the pellicle transmission is maximized. (An etalon is a thin film in which thin film interference effects such as constructive and destructive interference of the light from the front and back surfaces of the constant thickness film gives rise to optical fringes in the wavelength dependence of the reflectance or transmission of the thin film (Principles of Optics, a book by Max Born and Emil Wolf, Pergamon Press, New York, 6th Edition, copyright 1980, PP 329-333). For a pellicle film thickness of 6059 angstroms, a 157 nm pellicle transmission is 65.7% while the 157 nm pellicle reflectance is 0.4%. The spectral transmission in absolute units versus the wavelength lambda in units of nanometers for a pellicle of Teflon.RTM. AF 1601 designed as an unsupported tuned etalon with a film thickness of 6059 angstroms is shown in FIG. 4. The interference fringes of the tuned etalon are clearly visible as a function of wavelength. The spectral reflectance in absolute units versus the wavelength lambda in units of nanometers for the pellicle of Teflon.RTM. AF 1601 designed as an unsupported tuned etalon with a film thickness of 6059 angstroms is shown in FIG. 5. The interference fringes of the tuned etalon are clearly visible as a function of wavelength, and a minimum in the pellicle reflectance is seen at 157 nm which contributes to the maximized pellicle transmission at this lithographic wavelength. For a 157 nm pellicle film of Teflon.RTM. AF 1601 with a film thickness of 6335 angstroms, the tuned etalon will not be optimized for maximum pellicle transmission and the 157 nm pellicle transmission will be 59.4% while the 157 nm pellicle reflectance increases to 8.1%.

Transmission of a tuned etalon pellicle film of Teflon.RTM. AF 1601 at a lithographic wavelength of 157 nm as a function of the pellicle film thickness is shown in FIG. 6. The oscillations in the pellicle transmission with thickness arise due the thin film interference fringes in the film and give rise to pellicle transmission maxima and minima. The optimum tuned etalon pellicle design will correspond to the film with sufficient mechanical integrity and a thickness such that the transmission is at a maxima. Still as can be seen, pellicles designed from this material have substantially lower transmissions than the target 98% transmission for a 157 nm pellicle.

Pellicles designed from Teflon.RTM. AF 1601 are not able to achieve pellicle transmissions above 98%. Pellicles designed from Cytop.TM., which has a much higher 157 nm absorbance/micron, will have even lower 157 nm pellicle transmissions. This demonstrates that methods are needed to produce polymers with dramatically lower 157 nm absorbance/micron so as to meet the desired 157 nm transmission of 98% for a pellicle film. Therefore we need polymers with substantially lower absorbance/micron.

Samples 7 and 8--Teflon.TM. AF 1200, 1601, 2400

Solutions of Teflon.RTM. AF 1200, 1601, and 2400 were spin coated at spin speeds of 6000 rpm onto CaF.sub.2 substrates to produce polymer films of 4066 angstroms, 3323 angstroms and 2133 angstroms thicknesses respectively. VUV absorbance measurements were then used to determine the absorbance per micron.

The absorbance in units of inverse microns for Teflon.RTM. AF 1200 versus wavelength lambda (.lambda.) in units of nanometers is shown in FIG. 7 for Sample 8. The 157 nm absorbance/micron determined is 0.64/micron. The 193 nm absorbance/micron determined is 0.004/micron. The 248 nm absorbance/micron determined is -0.001/micron.

The absorbance in units of inverse microns for Teflon.RTM. AF 1601 versus wavelength lambda (.lambda.) in units of nanometers is shown in FIG. 7 for Sample 7a. The 157 nm absorbance/micron determined is 0.42/micron. The 193 nm absorbance/micron determined is 0.02/micron. The 248 nm absorbance/micron determined is 0.01/micron.

The absorbance in units of inverse microns for Teflon.RTM. AF 2400 versus wavelength lambda (.lambda.) in units of nanometers is shown in FIG. 7 for Sample 5. The 157 nm absorbance/micron determined is 0.007/micron. The 193 nm absorbance/micron determined is -0.06/micron. The 248 nm absorbance/micron determined is -0.06/micron.

One way to dramatically reduce the 157 nm absorbance of a PDD:TFE polymer is by increasing the percentage of PDD in the polymer. The stiff five membered ring of PDD likely has a similar effect, as VF.sub.2 and HFIB, of being "self-interrupting" when judging the potential for extended interactions between C--F bonds by forcing unfavorable conformations. This is shown in FIG. 7 where the absorbance in units of inverse microns for Teflon.RTM. AF 1200, Teflon.RTM. AF 1601, and Teflon.RTM. AF 2400 versus wavelength lambda (.lambda.) in units of nanometers demonstrate the decreasing 157 nm absorbance/micron for these polymers. Comparing PDD/TFE copolymers Teflon.RTM. AF 1200, 1601, and 2400, absorbance/micron at 157nm falls off from 0.6 to 0.4/micron to 0.01/micron as TFE content drops from 52 to 32 to 11 mole %. That is, transparency improves as increasing PDD content interrupts (CF.sub.2).sub.n runs. It is anticipated that other perfluorinated ring s