Martha I. Sanchez

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.

Polyimides derived from nonaromatic monomers: synthesis, characterization and potential applications

A number of mixed aromatic/cycloaliphatic as well as fully nonaromatic polyimides have been prepared. Whereas all the poly(amic acids) derived from nonaromatic diamines involved salt-formation during the initial stages of the polymerization, the majority of these eventually formed homogeneous, highly viscous polymer solutions. Only in a few select cases involving all nonaromatic monomers traditional solution polymerization was unsuccessful. The polyimide derived from hexafluoroisopropylidene diphthalic anhydride (6FDA) and trans- 1,4-diaminocyclohexane (DACH) yielded films with tough mechanical properties, a glass transition temperature of ∼360°C, good solvent resistance, and a low dielectric constant of 2.6. Thermal stability of this polyimide as determined by thermal gravimetric analysis in both air and nitrogen was quite good, exhibiting a weight loss of only 0.07 wt%/h at 350°C under isothermal conditions in nitrogen. However, mechanical properties as a function of thermal aging in both air and nitrogen indicated a maximum use temperature of only 350°C under inert conditions and less than 300°C in the presence of oxygen.

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.

Carbon nanotube scanning probe for profiling of deep-ultraviolet and 193 nm photoresist patterns

The continual scaling down of complementary metal–oxide semiconductor feature size to 100 nm and below necessitates a characterization technique to resolve high-aspect-ratio features in the nanoscale regime. We report the use of atomic force microscopy coupled with high-aspect-ratio multiwalled carbon nanotube (MWCNT) scanning probe tip for the purpose of imaging surface profile of photoresists. MWCNT tips of 5–10 nm in diameter and about a micron long are used. Their exceptional mechanical strength and ability to buckle reversibly enable resolution of steep, deep nanoscale features. Images of photoresist patterns generated by 257 nm interference lithography as well as 193 nm lithography are presented to demonstrate MWCNT scanning probe tips for applications in metrology.

High-temperature polyimide nanofoams for microelectronic applications

Abstract Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term ‘nanofoam’. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block, leaving pores of the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials were investigated as well as from a number of thermally labile polymers. The foams were characterized by a variety of experiments including TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and low dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

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.

Liquid immersion lithography: evaluation of resist issues

We address in this report a set of key questions tied to the implementation of liquid immersion lithography, from the perspective of the resist materials. We discuss the broad question of whether chemically amplified resists are capable of achieving the spatial resolution that ultimately will be required for the most advanced immersion scenario. Initial studies undertaken using model 193 nm resist materials provide some insight into how an aqueous liquid immersion process can affect the resist material.

Aerial image contrast using interferometric lithography: effect on line-edge roughness

Interferometric lithography affords the unique ability to independently control dose, pitch and aerial image contrast during photolithographic exposure. In this report, we describe the use of a deep-UV interferometric lithography exposure tool to study the impact of aerial image contrast on resists imaging properties. A wide range of high resolution resists materials was surveyed, including positive- and negative-tone systems, chemically amplified and conventional diazonaphthoquinone imagining chemistries, and aqueous- and solvent-developed systems. In all cases, resist line- edge roughness was observed to increase as aerial image contrast was decreased, though the precise behavior varied with resist material. Polymer molecular weight was systematically varied with resists materia. Polymer molecular weight was systematically varied in a negative- tone chemically amplified resist formulation. The results indicate that molecular weight is a significant factor influencing the magnitude and type of line-edge roughness at low aerial image contrast.

Patterning ∼20nm half-pitch lines on silicon using a self-assembled organosilicate etch mask

Lines of ∼20nm half-pitch were generated on silicon surface using a self-assembled organosilicate nanostructure. A mixture of a poly(styrene-b-ethylene oxide) (PS-b-PEO) with an organosilicate precursor that is selectively miscible with PEO was used to create lamellar phase whose orientation was controlled perpendicular to the surface by tuning the surface energy of substrates. Thermal cross-linking of the organosilicate precursor followed by thermal decomposition of the PS-b-PEO leaves a robust organosilicate line pattern of sublithographic length scales on the surface. Line patterns on silicon substrate were created by transferring this self-assembled pattern into the underlying silicon substrate using anisotropic plasma etching.

Polyimide Nanofoams For Low Dielectric Applications

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term “nanofoam”. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block leaving pores the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials, were investigated as well as a number of thermally labile polymers. The foams were characterized by a variety of experiments including, TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.