Aqueous Base Solubility Research Articles

High Sensitivity, Positive Tone, Low-k Polynorbornene Permanent Dielectric for Electronics Packaging

Permanent dielectric materials are necessary in microelectronic devices to separate electronic components. Low dielectric constant (low-k) materials are especially important because they electrically isolate on-chip, chip-to-chip, chip-to-package, or on-package interconnections. Polymers are widely used for these purposes, as they offer a low stress, easily processed alternative to inorganic materials. Additionally, permanent low-k polymeric materials can often be formulated to be directly patternable by photolithography. Currently, most positive tone, permanent dielectrics utilize the diazonaphthoquinone-based photochemistry, which provides relatively low sensitivity (D100 > 300 mJ/cm2) and low contrast (< 2.5). In order to pattern very thick films or high aspect ratio structures, materials with greatly improved lithographic properties are needed. This can be accomplished with a well-known photochemistry known as chemical amplification. In this case, a photo-generated acid catalyzed a deprotection reaction that leads to the solubility switch of the film. The low loading of the photoactive compound provides high sensitivity and high contrast films. However, cross-linking of these positive tone films after patterning is also desirable, and the presence of the strong acid makes cross-linking difficult. To this end, our lab has developed a functional chemistry for chemically amplified photo-patterning and subsequent cross-linking. Of particular interest is the low dielectric constant, ca. 2.2 to 2.8 depending on composition, without added porogens. This was achieved on a polynorbornene backbone with pendent tert-butyl ester and fluoroalcohol moieties. Films of this polymer are cast with small loadings of a photoacid generator (PAG). These films are originally insoluble in aqueous base. Exposure of the PAG to ultraviolet light produces a strong acid that catalyzes the deprotection of the tert-butyl ester. This deprotection results in a carboxylic acid, increasing the solubility of the film in aqueous base. A film with 3 parts per hundred PAG was measured to have a D100 of 8.09 mJ/cm2, and contrast of ≥ 14.2, significant improvements over existing positive tone dielectric values. After photolithography, the unexposed regions of the dielectric are thermally cured for 2 hours at 250°C. During this step, the PAG thermally forms the acid catalyst and deprotects the tert-butyl ester. The resulting carboxylic acid undergoes a Fischer esterification reaction with the alcohol functionality to form a covalent cross-linked network. It was found that the monomer ratio of the tert-butyl ester and the fluoroalcohol had a significant effect on the mechanical and electrical properties. Increasing the fluoroalcohol content decreased the dielectric constant, likely because of the high carbon-fluorine bond content. However, increasing the fluoroalcohol decreased modulus and hardness. This is likely due to the fact that the carboxylic acid will readily cross-link with another carboxylic acid to form an anhydride, confirmed by infrared spectroscopy. A polymer with 60 mol% fluoroalcohol was measured to have a dielectric constant of 2.78, a modulus of 2.95 GPa, and a hardness of 243 MPa. These values are appropriate for use as a permanent dielectric material. In order to further improve electrical and mechanical properties, a third monomer was incorporated into the polynorbornene backbone. This third monomer has an alkyl pendent group which does not participate in photolithography or cross-linking. This monomer addition does not come without challenges, namely in solubility. The alkyl monomer lacks an acidic proton and lowers solubility in aqueous base. The physical properties of this dielectric will be presented. Further, the challenges associated with this chemistry will be addressed in an effort to formulate a robust dielectric with improved lithographic, mechanical, and electrical properties.

Aqueous developable dual switching photoresists for nanolithography

AbstractPhoton‐mediated switching of polymer solubility plays a crucial role in the manufacture of integrated circuits by photolithography. Conventional photoresists typically rely on a single switching mechanism based on either a change in polarity or, molecular weight of the polymer. Here we report a photoresist platform that uses both mechanisms. The molecular weight switch was achieved by using a poly(olefin sulfone) designed to undergo photo‐induced chain scission. The polarity switch was achieved using pendant groups functionalized with o‐nitrobenzyl esters. These are hydrophobic photosensitive‐protecting groups for hydrophilic carboxylic acids. On irradiation, they are cleaved, making the polymer soluble in aqueous base. Importantly, the resists do not contain photoacid generator, so do not suffer from problems associated with acid diffusion that are detrimental to pattern fidelity. The 193 nm photochemistry of polymer thin films was followed using grazing angle attenuated total reflectance Fourier transform infrared spectroscopy, variable angle spectroscopic ellipsometry, and measurements of solubility in aqueous base. The nanoscale patterning performance of the polymers was also assessed using 193 nm interference lithography and electron‐beam lithography. The implications of using dual switching mechanisms are discussed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Synthesis, characterization and properties of polycarbonate containing carboxyl side groups

AbstractDiphenolic Acid, DPA [bis(4‐hydroxyphenyl)pentanoic acid] can be made from cellulose‐rich waste. The t‐butyl ester was converted to homo‐ and copolycar‐ bonates (with bis‐phenol‐A, BPA). Deblocking the ester yielded polycarbonates with pendent carboxyl groups that exhibit all the properties of polyelectrolytes and retain solubility in aqueous base without degradation for long periods.

Polymers for microlithographic applications: new directions and challenges

Advances in the fabrication technologies associated with electronic devices have placed increasing demands on microlithography, the technology used to generate today's integrated circuits. Within the next few years, a new form of lithography will be required that routinely produces features of less than 0.1 μm. As the exposing wavelength of light decreases to facilitate higher resolution imaging, the opacity of traditional materials precludes their use; and major research efforts to develop alternate materials are underway. As a current example, lithography tools utilizing 193 nm light are now being introduced into the manufacturing environment. Through understanding of materials structure and its relationship to device process requirements and performance, a new class of cyclo-olefin based polymers was designed for these applications. In particular, alicyclic monomers such as norbornene are readily copolymerized with maleic anhydride and substituted acrylates to afford a wide range of alternative matrices that exhibit transparency at the exposing wavelength and aqueous base solubility. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes. Further reduction in exposing wavelength to 157 nm introduces new challenges in polymer materials design. Efforts to address those challenges will be discussed.

Aqueous base development and acid diffusion length optimization in negative epoxy resist for electron beam lithography

A new aqueous base developable, chemically amplified negative resist based on epoxy chemistry is evaluated for high-resolution, high-speed e-beam lithography. This resist is formulated using partially hydrogenated poly(hydroxy styrene) and epoxy novolac polymers. Degree of hydrogenation controls the aqueous base solubility and microphase separation phenomena. Reduction of edge roughness compared to the pure epoxy systems is observed whereas the absence of swelling phenomena allows lithography up to 100 nm regime and a sensitivity of 4–8 μC/cm2 at 50 keV. The diffusion coefficient has been evaluated both from high-resolution line and dot exposures and it is found to be 5×10−14 cm2/s for the optimal thermal processing conditions selected.

Aqueous base developable epoxy resist for high sensitivity electron beam lithography

Aqueous base developable, chemically amplified negative resists, based on epoxy chemistry are introduced and evaluated for high resolution, high speed e-beam lithography. These resists are formulated using partially hydrogenated poly(hydroxy styrene) and epoxy novolac polymers and they do not suffer from thermal instability of unexposed regions during processing. Degree of hydrogenation controls the aqueous base solubility and micro phase separation phenomena. Reduction of edge roughness compared to the pure epoxy systems is observed whereas the absence of swelling phenomena allows lithography up to 100 nm regime.

Materials Challenges and Alternatives for Advanced Photolithographic Patterning: From 193 to 157 nm and Beyond

AbstractIn the last decade, major advances in fabricating electronic devices have placed increasing demands on microlithography, the technology used to generate today's integrated circuits. Within the next few years, a new form of lithography will be required that routinely produces features of less than 0.1 μ. As the exposing wavelength of light decreases to facilitate higher resolution imaging, the opacity of traditional materials precludes their use; and major research efforts to develop alternate materials are underway. Through understanding of materials structure and its relationship to device process requirements and performance, cycloolefin based polymers provide for sub-0.1 μm imaging capability using 193 nm exposure. Alicyclic monomers such as norbornene are readily copolymerized with other units to afford a wide range of alternative matrices that exhibit transparency at the exposing wavelength and aqueous base solubility. Further reduction in imaging wavelength necessitates renewed research to define alternative materials platforms. Materials transparency is the key issue to be addressed for 157 nm or EUV lithography. Novel polymer architectures including fluorinated polymers will be required to effect sufficient transparency coupled with requisite solubility, sensitivity, contrast etching resistance, shelf life and purity. Each of these issues will be discussed from the perspective of polymer materials chemistry.

Resist design concepts for 193 nm lithography: Opportunities for innovation and invention

Photolithography using 193 nm radiation is the leading candidate for the manufacture of 0.18–0.13 μm design rule devices. The optical absorption of materials such as novolacs, and functionalized poly(hydroxystyrenes) and styrene-acrylate copolymers which are the matrix materials of choice for G line, I line, and 248 nm lithography is significantly higher than one at 193 nm making them too opaque to be useful at this shorter wavelength. The opacity of the current photoresists at 193 nm requires innovation in designing alternative materials and processes to realize the full potential of 193 nm (ArF) lithography. From a materials standpoint, this challenge must be addressed by new chemistries and process schemes capable of providing resists with the aqueous base solubility, etching resistance, resolution, photospeed, and process latitude required for large-scale manufacturing. In addition, regulatory constraints on volatile organic chemical emissions have spurred efforts to design revolutionary resist platforms that address these concerns while providing the performance required for sub-0.18 μm device manufacture.

Synthesis of Cycloolefin−Maleic Anhydride Alternating Copolymers for 193 nm Imaging

A series of novel cycloolefin−maleic anhydride copolymers have been prepared and evaluated for 193 nm imaging applications. Free radical induced copolymerization of norbornene and maleic anhydride affords a hydrolytically robust alternating copolymer. Aqueous base solubility can be induced via incorporation of acrylic acid and/or acrylate esters that can be cleaved to afford the parent acid via acidolysis. The proportion of acrylate in the resulting terpolymers is a linear function of the starting monomer ratio. These terpolymers are thermally stable and hydrolytically robust. Due to their aqueous base solubility and UV transparency, they have potential in high resolution imaging applications. Sub-0.18 μm imaging has been demonstrated upon 193 nm pattern-wise exposure of selected materials.

Revolutionary and evolutionary resist design concepts for 193 nm lithography

The intense absorption of aromatic molecules at 193 nm severely limits the use of conventional matrix aromatic resins such as novolac, poly vinyl phenol for 193 nm lithography. This paradigm shift in resist design provides opportunities for new chemistries and process schemes to provide the required aqueous base solubility, etch resistance, resolution, photospeed and process latitude. In addition, regulatory constraints on the volatile organic emissions (VOC) also provide opportunities to design revolutionary resist schemes that not only address the lithographic performance requirements but also alleviate the environmental safety and health (ES&H) aspects of resist technology.In this paper, we will analyze the several resist options available for 193 nm lithography and provide results for evolutionary single layer, bilayer and revolutionary “all-dry” plasma polymerized methyl silane (PPMS) resist schemes. For single layer schemes, we have synthesized several co- and terpolymers with cycloolefins, maleic anhydride and acrylates (acrylic, methacrylic acids and esters) and have used the protected polymers as matrix resins in three component systems with a photoacid generator (PAG) and dissolution inhibitor(s) (DI). Alternately, we have used the unprotected terpolymer with DI's and PAG in three component systems. The lithographic results for single layer and bilayer resist materials were obtained at 193 nm using a 0.55 NA Nikon or 0.56 NA ISI (Integrated Solutions Inc.) small field exposure systems. Single layer resist materials showed at least 0.16 μm l/s pair resolution and modulation down to 0.14 μm l/s pairs using a formulation and process optimized at 248 nm.We have also evaluated the performance of P(SI-CMS) (poly(trimethyl silyl methyl methacrylate-co-chloromethyl styrene), a negative e-beam resist, at 193 nm and have obtained 0.25 μm l/s pair resolution without much optimization. Structure-activity relationships between the polymer properties and lithographic performance for this system have been identified.The results obtained for PPMS, a plasma deposited all-dry resist technology, on the 193 nm Micrascan (0.50 NA) were also very encouraging. Again using a non-optimized process, we have obtained at least 0.15 μm l/s pair resolution in a bilayer scheme.

Preparation and properties of Si‐containing copolymer for near‐UV resist. III. Maleimide/silylstyrene copolymer

AbstractPoly(maleimide‐alt‐p‐trimethylsilyl‐α‐methylstyrene) (α‐PMTMMS) and poly(male‐imide‐alt‐p‐trimethylsilylstyrene) (PMTMSS) were synthesized for positive/negative near‐UV resists in the containing of diazonaphthoquinone sulfonate (DNS). The electron‐rich styrenic monomers tend to undergo alternating copolymerization with an electron‐poor maleimide monomer. The chain‐stiffening effects of the maleimide and α‐methylstyrene group were responsible for high thermal stabilities. A higher glass transition temperature of 226°C was found in α‐PMTMMS. Lithographic positive/negative images were obtained that were stable to temperatures higher than 200°C. Excellent solubility in aqueous base was observed with PMTMSS at 10.3 wt % silicon content and an oxygen‐plasma etching rate of 1:12 compared to hard‐baked HPR–204. © 1993 John Wiley &amp; Sons, Inc.

A novel silicon-containing copolymer for near-u.v. resist

N-(4-hydroxyphenyl)maleimide/allyltrimethylsilane copolymer (PHAMS) was synthesized as a novel near-u.v. resist when containing diazonaphthoquinone sulphonate chloride. The chain-stiffening effect of the cyclic maleimide group was responsible for high temperature stability. A high glass transition temperature of 226° and a thermal decomposition temperature of 418° were found for PHAMS; a well-defined melting point was not observed because of its low crystallinity. The prepared resist could produce high contrast images at dose of 42.4 mJ/cm 2. Microlithographic positive images were obtained stable to temperatures above 200°. Excellent solubility in aqueous base was observed with PHAMS having 9.3 wt% silicon content and an oxygen-plasma etching rate of 1:10.5 compared to that of hard-baked HPR-204 (believed to be a novolac-type resin; baked at 200° for 30 min as a coating on silicon wafer).

A novel approach to inducing aqueous base solubility in substituted styrene-sulfone polymers

A new terpolymer, poly[(tert-butoxycarbonyloxy)styrene-acetoxystyrene-sulfone], is synthesized and spectroscopic evidence for the chemical reactions this polymer is capable of undergoing is provided. The (tert-butoxycarbonyloxy)styrene undergoes acidolysis of the tert-butoxycarbonyl moiety in the solid state in the presence of a strong acid. If this acid is generated by exposure to UV light, this reaction can form the basis for imaging this polymer. The acetoxy group can be cleaved from the acetoxystyrene monomer in aqueous base solution only when sulfone is incorporated into the copolymer in the appropriate amounts. After cleavage by base the resulting polymer is soluble in the base solution only if the tert-butoxycarbonyl group has been removed by acid