Omkaram Nalamasu

Low-cost, low-loss microlens arrays fabricated by soft-lithography replication process

This letter describes a soft lithographic approach for fabricating low-cost, low-loss microlens arrays. An accurate negative reproduction (stamp) of an existing high-quality lens surface (master) is made by thermally curing a prepolymer to a silicone elastomer against the master. Fabricating the stamp on a rigid backing plate minimizes distortion of its surface relief. Dispensing a liquid photocurable epoxy loaded to high weight percent with functionalized silica nanoparticles into the features of relief on the mold and then curing this material with UV radiation against a quartz substrate generates a replica lens array. The physical and optical characteristics of the resulting lenses suggest that the approach will be suitable for a range of applications in micro and integrated optics.

Evaluation of cycloolefin-maleic anhydride alternating copolymers as single-layer photoresists for 193-nm photolithography

We have developed a fundamentally new class of photoresist matrix resins for use in 193 and 248 nm lithography based on cycloolefin-maleic anhydride alternating copolymers. When used in three-component formulations with cholate-based dissolution inhibitions (DIs) and conventional photoacid generators, these copolymers afford positive-tone resists with potential sub-0.25 micrometer image fidelity. The resists exhibit high contrast (3 - 5.5) and high sensitivity (3 - 5 mJ/cm2 at 248 nm, depending on exact formulation) with low loadings (ca. 1 wt%) of triphenylsulfonium salt photoacid generators. These formulations are sufficiently transparent to be used at 193 nm without further modification.

Radiation chemistry of polymeric materials: novel chemistry and applications for microlithography

In the last two decades, major advances in fabricating very large scale integration (VLSI) electronic devices have placed increasing demands on microlithography, the technology used to generate todays integrated circuits. In 1970, state-of-the-art devices contained several thousand transistors with minimum features of 10-12 μm. Today, they have several million transistors and minimum features of less than 0.3 μm. Within the next 10-15 years, a new form of lithography will be required that routinely produces features of less than 0.2 μm. Short-wavelength (deep-UV) photolithography and scanning and projection electron-beam and X-ray lithography are the possible alternatives to conventional photolithography. The consensus candidate for the next generation of lithography tools is photolithography using 193 nm light. At this wavelength, the opacity of traditional materials precludes their use, and major research efforts to develop alternative materials are currently underway. Notably, the materials being developed for these short UV wavelengths are demonstrating compatibility with the more advanced electron-beam technologies. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes, eg adhesion, solubility and RF plasma etching stability.

Preliminary lithographic characteristics of an all-organic chemically amplified resist formulation for single-layer deep-UV lithography

When used in conjunction with a nitrobenzylester photoacid generator, poly(t-butoxycarbonyloxy-styrene-sulfone) deep-UV resist films exhibit high contrast, good resolution and linewidth stability. Use of overcoat materials dramatically reduce the surface inhibition problems, improve the latent image stability (time delay) and enhance the sensitivity by isolating the resist surface from environmental contaminants that react with the photogenerated acid. The photospeed of the all organic CAMP formulation is lower compared to the arsenate based system but can be improved by using more aggressive PEB conditions. Coded, 0.35 micrometers l/s pairs could be resolved in 1 micrometers thick resist films at a dose of 20-30 mJ/cm2. The exposure latitude is approximately equals 25% for 0.5 micrometers features, upon exposure with a GCA prototype deep-UV exposure tool with a NA equals 0.35 and 5x reduction optics. This paper will discuss the resolution, depth-of-focus, exposure latitude and processing characteristics obtained during the evaluation of this chemically amplified resist.

Processing and characterization of ultralow-dielectric constant organosilicate

A class of nanoporous organosilicate has been developed that can reach an ultralow-dielectric constant (k⩽2). In this approach a series of triblock polymers, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO), are used as sacrificial materials in silicon based spin-on matrices, poly(methylsilsesquioxane) (MSQ) to generate pores when heated above 400 °C. To improve the electrical and mechanical properties, different parameters, such as temperatures, heating rates, polymer molecular weights, copolymer compositions, and the molecular weight of MSQ prepolymers, were studied. Heating temperatures and ramping rates have no large effect on dielectric constants and dielectric breakdown strength. However, at a higher temperature, 500 °C, the mechanical properties are found stronger than those from 400 °C. A higher molecular weight of MSQ prepolymer will also lead to higher mechanical properties of the final porous MSQ, while no temperature effect is observed. Block copolymers with different ...

193 nm single layer resist strategies, concepts, and recent results

Matrix resins used in conventional resists are not suitable for use at 193 nm due to their opacity. Hence new materials that are functionally similar to but structurally different from novolac and poly(hydroxy styrene) are required for 193 nm lithography. We report on the use of alternating copolymers of cycloolefins with maleic anhydride as effective 193 nm matrix resins, with or without dissolution inhibitors based on polyfunctional cholates, for 193 nm lithography. Due to their structural diversity, the required high transparency and etch stability, compatibility with industry standard 0.262 N tetramethyl ammonium hydroxide (TMAH) can be built into the polymer by conventional free redical polymerization techniques. A correlation between the molecular properties of the resist components (matrix resin, dissolution inhibitor, photoacid generator, and base additive) and resist lithographic performance parameters is illustrated. The formulations containing dissolution inhibitors currently show 0.13 μm line/...

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 platfo...

Recent advances in 193 nm single-layer photoresists based on alternating copolymers of cycloolefins

We report on our recent investigations on the formulation and processing of 193 nm single layer photoresists based on alternating copolymers of cycloolefins with maleic anhydride. Resists formulated with cycloolefin copolymers are compatible with 0.262 N tetramethylammonium developers, have excellent adhesion, sensitivity, etch resistance and thermal flow properties. The effect of polymer structure and composition, dissolution inhibitor structure and loading as well as the effect of the photoacid generator on the resist dissolution properties was investigated. Based on the results high contrast formulations were evaluated on a GCA XLS (NA equals 0.53, 4X reduction optics) deep-UV stepper to exhibit 0.27 micrometer L/S pair resolution with excellent photosensitivity. Based on the dissolution properties and a spectroscopic examination of the resist, we have designed materials that show less than 0.17 micrometer L/S pair resolution with 193 nm exposures. In this paper, the formulation methodology is detailed and the most recent results upon both with 248 and 193 nm irradiation are described.

Application of Plasmask R resist and the DESIRE process to lithography at 248 nm

Plasmask resist and its associated dry development process called DESIRE constitute one product which has incorporated surface imaging for lithography at 436 and 366 nm. Based on its performance at these wavelengths, Plasmask 150U resist is predicted to have a resolution limit of ∼0.3 μm at 248 nm with a 0.38 NA lens. Using best focus exposures of 1.5‐μm thick films we have examined a variety of factors that influence the resolution and sensitivity of Plasmask 150U resist at 248 nm. These include standard processing steps as well as silylation with hexmethyldisilazane, plasma development conditions and equipment, and various descum techniques. Processing without a descum step resolved 75° profile, 0.3 μm line and space patterns at high exposure doses, but always afforded a grassy residue. Use of Ar sputter etching to remove ∼2200 A from unexposed areas prior to plasma development afforded vertical profile 0.3‐μm resolution patterns with no grassy residue.

Resist outgassing as a function of differing photoadditives

The effect of different photoadditives in high and low activation energy resist resins on resist outgassing during lithographic exposure was studied by quartz microbalance and gas chromatography/mass spectroscopy techniques. The resist outgassing was analyzed both qualitatively and quantitatively and structure-property relationships were developed between resist outgassing and the molecular structure of photoacid generators and additives. The photoadditives examined include, aryl iodonium perfluoroalkylsulfonates, triarylsulfonium perfluoroakylsulfonates, photogenerators of sulfamic acids, 2-nitrobenzyl PAGs and doxyl derivatives.