Gregory M. Wallraff

Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries.

Li-air batteries have generated enormous interest as potential high specific energy alternatives to existing energy storage devices. However, Li-air batteries suffer from poor rechargeability caused by the instability of organic electrolytes and carbon cathodes. To understand and address this poor rechargeability, it is essential to elucidate the efficiency in which O2 is converted to Li2O2 (the desired discharge product) during discharge and the efficiency in which Li2O2 is oxidized back to O2 during charge. In this Letter, we combine many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes. We find that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge, whereas both cathode and electrolyte instabilities are observed upon charge at high potentials.

Deep-ultraviolet interferometric lithography as a tool for assessment of chemically amplified photoresist performance

The precise control of the exposure step provided by interferometric photolithography facilitates studies of chemically amplified resist physics, chemistry, and functional properties that are difficult using more conventional exposure techniques. We describe here the design and operating characteristics of a deep-ultraviolet interferometric lithography tool designed specifically for the study of high resolution chemically amplified resists. We provide an example of its use to evaluate resist response to controlled variations in aerial image contrast.

Chemical and physical aspects of the post-exposure baking process used for positive-tone chemically amplified resists

Chemically amplified (CA) resists are in widespread use for the fabrication of leading-edge microelectronic devices, and it is anticipated that they will see use well into the future. The refinement and optimization of these materials to allow routine imaging at dimensions that will ultimately approach the molecular scale will depend on an improved in-depth understanding of the materials and their processing. We provide here an overview of recent work in our laboratory on the chemical and physical processes that occur during post-exposure baking (PEB) of positive-tone CA resists. Our results provide a clearer understanding of how this critical step in the lithographic imaging process will affect extendibility of the CA resist concept to nanoscale feature sizes.

Determination of coupled acid catalysis-diffusion processes in a positive-tone chemically amplified photoresist

Acid diffusion during postexposure baking is viewed to be a limiting factor in the extension of lithography using chemically amplified resists to formation of nanoscale features. Quantification of thermally activated reaction-diffusion kinetics in these materials is therefore an important step in understanding the extendability of this class of resist systems. Previous investigations have addressed this issue, however there is poor agreement among them, and too few data exist in the literature to allow the systematics of the effect of polymer, photoacid generator, added base or other resist components on the diffusion process to be understood. We describe in this article a combined experimental and modeling protocol that is designed to elucidate the chemistry and physics of the reaction-diffusion process. Because it is physically based, not phenomenological, it provides a means of developing a set of predictive, mutually comparable data that will allow new insights to be developed into the nanoscale behav...

Supercritical fluid processing: a new dry technique for photoresist developing

Supercritical fluid (SCF) technology is investigated as a dry technique for photoresist developing. Because of their unique combination of gaseous and liquid-like properties, these fluids offer comparative or improved efficiencies over liquid developers and, particularly carbon dioxide, would have tremendous beneficial impact on the environment and on worker safety. Additionally, SCF technology offers the potential for processing advanced resist systems which are currently under investigation as well as those that may have been abandoned due to problems associated with conventional developers. An investigation of various negative and positive photoresist systems is ongoing. Initially, supercritical carbon dioxide (SC CO2) as a developer for polysilane resists was explored because the exposure products, polysiloxanes, are generally soluble in this fluid. These initial studies demonstrated the viability of the SCF technique with both single layer and bilayer systems. Subsequently, the investigation focused on using SC CO2 to produce negative images with polymers that would typically be considered positive resists. Polymers such as styrenes and methacrylates were chemically modified by fluorination and/or copolymerization to render them soluble in SC CO2. Siloxane copolymers and siloxane-modified methacrylates were examined as well. The preliminary findings reported here indicate the feasibility of using SC CO2 for photoresist developing.

Resolution of the nuclear and electronic contributions to the optical nonlinearity in polysilanes

We present a method of resolving the nuclear and electronic contributions to the third‐order susceptibility χ3 in a phase conjugation experiment. The technique is applied to octylmethylpolysilane yielding χ3 (electronic)=(1.8±0.5)×10−12 esu and χ3(nuclear)=(1.1±0.4)×10−12 esu at 532 nm.

Effect of resist components on image spreading during postexposure bake of chemically amplified resists

The ultimate feature size achievable using a chemically amplified resist is determined by chemical and physical processes occurring during the post-exposure bake process. Using a combined experimental-modelling procedure we previously have developed a physically accurate, predictive description of coupled deprotection and diffusion in poly(p- tert-butyloxycar-bonyloxystyrene) (PTBOCST) resist containing a diaryliodonium perfluorobutanesulfonate salt as photoacid generator (PAG). In the present work we extend that study to quantify the impact of anion size and of added base on resist reaction diffusion kinetics. Our results show that both short and long range mobility of the PAG anion influence image spreading; the small triflate counterion leads to acid diffusion larger by a factor of 9 - 70 than that observed with the larger perfluoro-butanesulfonate counterion. The addition of tetra-n-butylammonium hydroxide leads to an overall suppression of image spreading in the exposed resist. This effect can be analyzed quantitatively using a proportional neutralization model, which reveals that base addition can lead to an overall sharpening of the developable latent image of deprotection even in the absence of acid diffusion.

Solution photochemistry of poly(dialkylsilanes): a new class of photoresists

polysilanes r epresent a new class of polymeric materials of considerable theoretical and practical i nterest. Although formally saturated, they absorb in the near UV and undergo rapid photodegradation . After a review of the present understanding of the e lectronic structure of poly(dialkylsi1a nes) as a function of backbone conformation, we summarize their photophysics and describe recent advances in the understanding of their solution

Extendibility of chemically amplified resists: another brick wall?

The chemically amplified resist concept, first described two decades past and originally targeted for the 1000 nm device generation, has proved to have remarkable versatility. The semiconductor industry has come to rely on the properties of CA resists to achieve high resolution, high aspect ratio imaging accompanied by the high throughput that stems from their catalytic imaging mechanism. As the industry maps the evolution of lithographic technology to the 20 nm regime, it is appropriate to review the factors that control the performance of CA resists, and examine whether the traditional evolutionary path of materials refinement will provide materials capable of supporting device manufacturing at those dimensions. The impacts of image blur, line-edge roughness and shot noise on the ability to image CA resists at nanoscale dimensions will be discussed.

Polymer design for 157-nm chemically amplified resists

Based on UV measurements at 157nm of in-house fluoropolymers we have selected (alpha) -trifluoromethylacrylate and norbornene bearing a pendant hexafluoroisopropanol group as our building blocks for 157nm resist polymers. Polymers consisting of these repeat units have an optical density/micrometers of 3 or below at 157nm. We have found that the (alpha) -trifluoromethylacrylate derivatives conveniently undergo radical copolymerization with norornenes, which has provided a breakthrough in preparation of our 157nm resist polymers. This approach offers flexibility and versatility because an acidic moiety or acid-labile group can be placed in either acrylate or norbornene repeat unit. Other platforms of interest include all acrylic, all-norbornene, and acrylic-styrenic polymers.