Onium Salt Photoacid Generators Research Articles

UV-Induced Cationic Ring-Opening Polymerization of 2-Oxazolines for Hot Lithography.

The cationic ring-opening polymerization (CROP) of 2-oxazolines gives polymers with unique characteristics arising from its polyamide backbones and structural versatility. Up to now, poly(2-oxazoline)s were obtained by classical thermal polymerization methods not aiming for application in bulk curing of structural polymers. We introduce the cationic photopolymerization of 2-oxazolines at elevated temperatures for the direct UV-induced curing of materials with exclusive chemical and structural particularities. After efficient photoinitiation via onium salt photoacid generators (PAGs), the immanent low-rate propagation is crucially promoted by thermal energy input to the ring-opening reaction. In simultaneous thermal analysis (STA), photo-DSC, and (thermo)mechanical analyses we investigated the UV-induced CROP of 2-oxazolines in a temperature range of 100-140 °C and show the exceptional potential of the introduced photopolymers. Furthermore, we applied the photopolymerizable system in Hot Lithography, a stereolithography-based 3D printing technology at elevated temperatures.

Crosslinkable photoacid generators for ultrahigh loading in epoxide functionalized molecular resists

A series of ionic onium salt photoacid generators (PAGs) functionalized with epoxide or phenolic groups capable of participating in the cationic polymerization of epoxides have been synthesized and investigated for use in ultrahigh PAG loaded negative tone epoxide functionalized molecular resists. An epoxide functionalized PAG [tris(4-glycidylether-3,5-dimethylphenyl)sulfonium hexafluoroantimonate] was found to be capable of use at loadings up to 100 mol. % in an epoxide functionalized resist [1,1,2,2-tetrakis(4-(oxiran-2-ylmethoxy)phenyl)ethane (4-Ep)] without inhibiting its ability to form fully crosslinked features, and was able to resolve 50 nm features under 100 keV electron-beam lithography at PAG loadings exceeding 50 mol. %. A phenol functionalized PAG [tris(4-hydroxyphenyl)sulfonium hexafluoroantimonate] was similarly capable of use at loadings up to 50 mol. % in 4-Ep without inhibiting cross-linking, and was able to resolve both 50 and sub-50 nm features at PAG loadings exceeding 30 mol. % using 100 keV electron-beam lithography. Initial results from this study indicate that the functionalized PAGs can enable higher PAG loadings in these negative tone resists than was previously possible; however, increasing PAG loading beyond at least 30 mol. % has a negative impact on sensitivity when using 100-keV high energy e-beam exposures.

Driving Force Dependence of Electron Transfer from Electronically Excited [Ir(COD)(μ-Me2pz)]2 to Photo-Acid Generators.

We report the rates of electron transfer (ET) reactions of electronically excited [Ir(COD)(μ-Me2pz)]2 with onium salt photoacid generators (PAGs). The reduction potentials of the PAGs span a large electrochemical window that allows determination of the driving force dependence of the ET reactions. Rate constants of ET from electronically excited [Ir(COD)(μ-Me2pz)]2 to onium PAGs are determined by the reaction driving force until the diffusion limit in acetonitrile is reached.

Electrically curable double-layer polymer resist for dynamic nanoscale lithography.

A double-layer polymer resist composed of a top electrically curable resin layer with onium salt photo-acid generators and a bottom ionic conductive polymer transfer layer has been developed for dynamic nanoscale electric lithography. By applying an electric potential on the resist from conductive patterns on a mask, the acid generators under the conductive patterns are electrolyzed into proton acid, which consequently cross-links the cationically polymerizable resin resist. With the double-layer resist, nanopatterns can be generated with a sub-50 nm resolution. By applying different electric potentials on the individual conductive patterns on the mask, the transferred nanopatterns can be modified dynamically.

Applications for Photoacid Generation in Photoimaging and Nanotechnology

The development of highly efficient onium salt photoacid generators has provided many new opportunities for the design of novel imaging systems. This communication focuses on some of the more recent breakthroughs in imaging technology that are being used in microelectronic and optoelectronic applications. More recently, onium salt photoinitiators have also found applications in the area of nanotechnology.

Partially hydrogenated poly(vinyl phenol) based photoresist for near UV, high aspect ratio micromachining

A negative resist platform based on a low Mw epoxy novolac polymer, partially hydrogenated poly(vinyl phenol)s and suitable onium salt photoacid generators is presented and evaluated for high aspect ratio micromachining. The high content of hydroxyl groups in this resist system allows development in standard aqueous base solutions (TMAH 0.26 N). The effects of material parameters such as the degree of poly(vinyl phenol) hydrogenation and the onium salt hydrophobicity on the development process are examined. The cross-linked areas of this resist can be easily removed by commercial wet photoresist strippers after processing. Experimental data demonstrate a promising lithographic performance with an aspect ratio of 7:1 for 5 μm closely spaced lines under exposure at 365 nm.

Strippable aqueous base developable negative photoresist for high aspect ratio micromachining

The patterning of high aspect ratio structures in ultra thick photoresist films is extremely challenging. Photoresists used should provide low absorption in order to achieve vertical sidewalls and easy stripping after the pattern transfer process. In the current work, a novel resist platform is presented and applied for lithographic evaluation in film thickness up to 60 μm. The resist formulation consists of a mixture of an epoxy novolac oligomer, partially hydrogenated poly(4-hydroxystyrene) and onium salt photoacid generators. Since the resist is rich in hydroxyl groups the unexposed regions can be developed in standard aqueous base solutions (TMAH, 0.26 N). Additionally since the required crosslink density for solubility change is limited, the crosslinked regions can be easily removed by commercial wet photoresist strippers. First experimental data revealed promising lithographic performance with resolution down to 20 μm with an aspect ratio 3:1.

Fundamental studies of dissolution inhibition in poly(norbornene-alt-maleic anhydride) based resins

The dissolution inhibition mechanism for tert-butylcarboxylate (e.g. tert-butyl cholate) dissolution inhibitors and onium salt photoacid generators (PAG's) were examined in terpolymers of poly(norbornene-maleic anhydride-acrylic acid) (P(NB/MA/AA)). For tert-butyl carboxylates, increasing hydrophobicity increased the dissolution inhibition ability. Dissolution promotion tracked with the number of carboxylic acid moieties and the hydrophobicity of carboxylic acids moieties released upon acidolytic cleavage of the tert-butyl carboxylate. For onium salt PAG's, increasing the hydrophobicity and size of fluorinated anions decreased dissolution inhibition.

Application of a Novel Aqueous Base Developable Resist in Micromachining.

An increasing number of nowadays advanced lithographic technologies require photoresist film thickness of at least 50μm. The patterning of high aspect ratio structures in ultra thick photoresist films is extremely challenging and the photoresists should provide low absorption in order to achieve vertical sidewalls and easy stripping after the pattern transfer processing. In the current work a novel resist platform is presented and applied in the case of film thicknesses up to 65μm. The resist formulation consists of a mixture of an epoxy novolac oligomer and poly(hydroxy styrene) and onium salt photoacid generators. Since the resist is rich in hydroxyl groups the unexposed regions can be developed in industrial aqueous solutions (TMAH 0.27N). Additionally since the required crosslink density is limited, the crosslinked regions can be easily removed by commercial wet photoresist strippers. First experimental data reveal a promising lithographic performance with resolution down to 30μm.

Fundamental Studies of Molecular Interactions and Dissolution Inhibition in Poly(norbornene-alt-maleic anhydride)-Based Resins

The progress of developer base into films of terpolymers of norbornene−maleic anhydride and acrylic acid was shown to be a percolation process with a critical site concentration of x(c) = 0.084, implying that every acrylic acid site in the terpolymer makes 12 monomer units of the polymer water compatible. Using these terpolymers, the dissolution inhibition mechanism for two types of common additives, tert-butyl carboxylate (e.g., cholates, cyclohexanecarboxylates, and malonates) dissolution inhibititors and onium salt photoacid generators (PAG's), were examined. Additionally, the dissolution promotion mechanism of carboxylic acids released from acidolysis of tert-butyl esters was investigated. For a wide range of cholates and other tert-butyl carboxylate derivatives, increasing hydrophobicity is a good predictor of increased dissolution inhibition. The molar dissolution promotion ability of a carboxylic acid increases with increasing number of both carboxyl and hydroxyl ester functional groups. For onium salt PAG's, a decrease in dissolution inhibition occurred when increasing the hydrophobicity and size of counteranions. Rather than being tied to hydrophobicity, large changes of dissolution inhibition in this class of materials appear to correlate with their ability to interact with carboxyl groups.

Fundamental Studies of Dissolution Inhibitors in Poly(norbornene-alt-maleic anhydride) Based Resins.

Using terpolymers of Poly(norbornene-alt-malefic anhydride-co-acrylic acid) [P(NB/MA/AA)], the dissolution inhibition mechanism for two types of common additives in 193nm resist formulations, tert-butylcarboxylate (e.g. tert-butyl cholate) dissolution inhibitors (DI) and onium salt photoacid generators (PAC's) were examined. For simple cholate ester derivatives, increasing interactions with maleic anhydride repeating units paralleled the dissolution inhibition (tert-butyllithocholate>tert-butyldeoxycholate>tert-butylcholate). For a wider range of cholate derivatives, increasing hydrophobicity as measured by logPoct or the cloud point formation in water/acetone mixtures, is a good predictor of increased dissolution inhibition. Increases in dissolution promotion appeared to track with the number of carboxylic acid moieties and the hydrophobicity of carboxylic acid moieties released upon acidolytic cleavage of carboxylate esters. Finally, for onium salt PAC's, increasing the size of fluorinated anions decreased dissolution inhibition. This may be because these anions offer more steric hindrance, which disfavors interactions with the terpolymer matrix. The nature of this interaction may be that of a PAG with carboxylic acid as judged from modeling studies with acetic acid in which increasing strength of the interaction correlated with increased dissolution inhibition (triflate>nonaflate> perflurooctanesulfonate)

Synthesis and photopolymerization of monomers bearing isopropenylphenoxy groups

AbstractThe synthesis of a series of monomers containing isopropenylphenoxy groups was carried out. On irradiation with UV light in the presence of onium salt photoacid generators, these monomers undergo a chain extension reaction consisting of a dimerization followed by a Friedel‐Crafts ring closure which results in the formation of polymers with indane groups in the backbone. Aryl imide‐containing monomers bearing isopropenylphenoxy groups were also shown to undergo facile photoinduced cationic polymerization. The resulting polymers displayed excellent thermal stability. © 1995 John Wiley & Sons, Inc.

T‐BOC blocked hydroxyphenyl‐methacrylates: On the way to quarter micron deep‐UV lithography

Abstractt‐Butyloxycarbonyl (t‐BOC) blocked compounds based on the protection of phenolic groups, e.g. poly‐4‐hydroxystyrene derivatives, Bisphenol A type dissolution inhibitors, or onium salt photoacid generators, have found widespread research interest for photoresist systems with excellent photosensitivity and high resolution power. We have made an extension of this approach using new phenol type polymers. This contribution presents first details on the chemistry of these systems and results of their lithographic evaluation as positive tone photoresists for deep UV applications.

Tetrahydropyranyl protected polyhydroxy-styrene for a chemically amplified deep u.v. resist

Tetrahydropyranyl (TH) protected poly(p-hydroxystyrene) (PHS) was synthesized and its acid-catalysed thermal deprotection utilized in the design of an alkali developable, positive photoresist system incorporating chemical amplification. The solubility of THP-PHS films mixed with the appropriate onium salt photo acid generators in alkaline developers increases upon exposure to deep u.v. radiation and subsequent heating. A resist composed of THP-PHS and bis(tert-butylphenyl)iodonium triflate resolves 0.35 μm line-and-space patterns using a KrF excimer laser (248 nm) stepper (20 mJ cm−2).

New t-BOC blocked polymers for advanced lithographic applications.

t-Butyloxycarbonyl (t-BOC) blocked compounds based on the protection of phenolic groups, e.g. poly-4-hydroxystyrene derivatives, Bisphenol A type dissolution inhibitors, or onium salt photoacid generators, have found widespread research interest for photoresist systems with excellent photosensitivity and high resolution power. We have made an extension of this approach using new phenol type polymers. This contribution presents first details on the chemistry of these systems and results of their lithographic evaluation as positive tone photoresists for UV4, UV3 and especially, UV2 applications.