V. F. Cherstkov

Novel hydrofluorocarbon polymers for use as pellicles in 157 nm semiconductor photolithography: fundamentals of transparency

With the advent of 157 nm as the next photolithographic wavelength, there has been a need to find transparent and radiation durable polymers for use as soft pellicles. Pellicles are � 1 mm thick polymer membranes used in the photolithographic reproduction of semiconductor integrated circuits to prevent dust particles on the surface of the photomask from imaging into the photoresist coated wafer. Practical pellicle films must transmit at least 98% of incident light and have sufficient radiation durability to withstand kilojoules of optical irradiation at the lithographic wavelength. As exposure wavelengths have become shorter the electronics industry has been able to achieve adequate transparency only by moving from nitrocellulose polymers to perfluorinated polymers as, for example, Teflon 1 AF 1600 and Cytop TM for use in 193 nm photolithography. Unfortunately, the transparency advantages of perfluorinated polymers fail spectacularly at 157 nm; 1 mm thick films of Teflon 1 AF 1600 and Cytop TM have 157 nm transparency of no more than 38 and 2%, respectively, with 157 nm pellicle lifetimes measured in millijoules. Polymers such as ‐[(CH2CHF)xC(CF3)2CH2]y‐, or ‐(CH2CF2)x[2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole]y‐ with chains that alternate fluorocarbon segments with either oxygen or hydrocarbon segments frequently show >98% transparency at 157 nm, if amorphous. These polymers are made from monomers, such as vinylidene fluoride (VF2) and hexafluoroisobutylene, which themselves exhibit good alternation of CH2 and CF2 in their structures. In addition, we find that ether linkages also can serve to force alternation. In addition, we find that fluorocarbon segments shorter than six carbons, and hydrocarbon segments less than two carbons or less than three carbons if partially fluorinated also promote 157 nm transparency. We also find that even with these design principles, it is advantageous to avoid small rings, as arise in the cyclobutanes. These results suggest a steric component to transparency in addition to the importance of alternation. Upon irradiation these polymers undergo photochemical darkening and therefore none has demonstrated the kilojoule radiation durability lifetimes required to be commercially attractive. This is likely because these exposure lifetimes require every bond to absorb � 10 photons, each photon having an energy roughly twice common bond energies. We have studied intrinsic (composition, molecular weight) and extrinsic (trace metals, impurities, environmental contaminants, oxygen, water) contributions to optical absorption and photochemical darkening in these polymers. Studies of photochemical darkening in model molecules illustrate the dynamics of photochemical darkening and that appreciable lifetimes can be achieved in fluorocarbons. To a first approximation the polymers that have lower 157 nm optical absorbance also tend to show the longest lifetimes. These results imply that quantum yield, or the extent to which the polymer structure can harmlessly dissipate the energy, can be important as well. # 2003 Elsevier Science B.V. All rights reserved.

Electrophilic isomerization of fluoroaliphatic oxygen-containing compounds

An unusual electrophilic cyclization of fluorine-containing carbonyl compounds and α-oxides was discovered. Upon the action of SbF5, perfluorinated ketones, diketones, and α-oxides isomerize to oxolanes. This reaction proceeds with the obligatory participation of the terminal CF3 group.

Catalytic synthesis of polyfluoroolefins

A catalytic synthesis of polyfluoroolefins was developed proceeding from polyfluorochlorocarbons with the use of industrially produced nickel-chromium catalyst. Three ways of the catalytic synthesis of fluoroolefins were implemented: the cleavage of vicinal chlorine atoms from polyfluorochlorocarbons, the replacement of vinyl chlorine atoms by hydrogen in fluorochloroolrfins, and the reductive dimerization of polyfluorochlorocarbons containing a trichloromethyl group. The condition of a prolonged operation of the nickel-chromium catalyst was found consisting in the application of quartz for absorption of the hydrogen fluoride formed as a side product.

Substitutive sulfotrioxidation of branched perfluoroolefins

We have studied the insertion of SO3 into the C-F bond of perfluoroolefins under conditions of electrophilic catalysis. It was shown that sufficiently high allylic mobility of the fluorine atoms is only preserved when in the allylic triad, in addition to the substituted fluorine; also not less than three fluorine atoms are present in the α and γ positions.

ESR study of the free radicals formed in the photolysis of α-ketofluorosulfates

ESR spectroscopy was used to establish that the UV irradiation of α-ketofluorosulfates containing an OSO2F group at a tertiary carbon atom gives radicals formed upon cleavage of the C-OSO2F bond. If the OSO2F group is attached to a primary or secondary carbon atom, we observe ordinary photodecomposition with cleavage of the acyl—carbon bond. The ESR spectra of the new radicals were studied. Replacement of fluorine by the OSO2F group in the trifluoromethyl radical leads to compression of the radical site. Branched α-oxoperfluoroalkyl radicals are not heteroallylic and the unpaired electron is localized largely on the carbon atom. The [(CF3)2CF]2ĊC(O)CF3 radical is stable in the air.

Fluoroaliphatic esters of fluorosulfonic acid. 5. Conjugate radical fluorosulfatohalogenation of higher fluoroolefins

A study has been made of the electrochemical fluorosulfation of perfluoro-2-alkenes in the presence of halogens, leading to the formation of fluoroaliphatic vicinal fluorosulfatohalides. The reaction proceeds as a conjugate radical addition of peroxydisulfuryl difluoride and the halogen at the double bond of the olefin; the ratio of the resulting regioisomers is determined by steric factors.

Electrophilic acylation of fluoroolefins by perfluorinated ?,?-unsaturated carboxylic acid fluorides

Abstractα,β-Unsaturated perfluorocarboxylic acid fluorides react with fluoroolefins in the presence of SbF5 under mild conditions to give vinyl ketones.

Fluoroaliphatic esters of fluorosulfonic acid. 2. Reaction of bis(fluorosulfato)perfluoroalkanes with cesium fluoride

Abstract2,3-Bis(fluorosulfato)perfluoroalkanes split under the action of CsF in the absence of solvents, giving a mixture of α-fluorosulfatoperfluoro ketones, perfluoroalkene sulfates, and perfluoro α-diketones. The occurrence of these reactions in solutions results mainly in the formation of oxides of the corresponding fluoroolefins or products of their conversions. The reactions carried out are the first examples of nucleophilic substitution at a secondary carbon atom in a perfluorinated saturated chain.

Substitutive sulfotrioxidation of unsaturated and araliphatic organofluorine compounds

ConclusionsReaction of SO2 or SO3/[B2O3] with terminal fluoroolefins or fluoroalkylbenzenes gives the corresponding fluoroalken-2-yl fluorosulfates and fluorobenzyl fluorosulfates.

ELECTROCHEMICAL PERFLUOROALKOXYLATION OF AROMATIC COMPOUNDS

New approaches to the electrochemical synthesis of aryl perfluoroalkyl ethers based on a) the electrooxidation of perfluorocarboxylic acid on a consumable PbO2 anode and b) the simultaneous anodic oxidation of the “aromatic compound-alkaline perfluoroalkoholate” system on a platinum electrode were proposed.