Dario L. Goldfarb

Aqueous-based photoresist drying using supercritical carbon dioxide to prevent pattern collapse

A supercritical drying process was developed to eliminate the capillary forces naturally present during normal drying of photoresist materials. Supercritical carbon dioxide (scCO2), organic solvents and surfactants were used to prevent the collapse of high-aspect-ratio structures fabricated from aqueous-based photoresist. Nondistorted resist lines were patterned with this process with aspect ratios of at least 6.8. Water rinsed resist structures cannot be dried directly with scCO2 due to the low solubility of water in the supercritical phase. In our process we introduced the replacement of the aqueous rinse by n-hexane mediated by a compatible surfactant. The surfactant allowed to incorporate the aqueous phase into micellar microdomains in the organic phase while keeping the interfacial tension at values close to zero. Noncollapsed supercritically dried structures were rewet in n-hexane or water and dried using nitrogen at atmospheric pressure. Under these conditions, the patterns were collapsed as a resu...

Effect of thin-film imaging on line edge roughness transfer to underlayers during etch processes

For the patterning of sub-100 nm features, a clear understanding of the origin and control of line edge roughness (LER) is extremely desirable, from a fundamental as well as a manufacturing perspective. With the migration to thin photoresists coupled with bottom antireflective coating (ARC)-hardmask underlayers, LER analysis of the developed resist structures is perhaps an inaccurate representation of the substrate roughness after the etch process, since those underlayers can play a significant role in increasing/decreasing linewidth variations during the image transfer process and hence can impact the device performance. In this article, atomic force microscopy is used to investigate the contribution of the imaging resist sidewall topography to the sidewall roughness of the final etched feature in thin photoresists, ARC, and hardmasks. Resist systems suitable for 248 and 193 nm lithography as well as fluorine-containing resists were processed using N2-H2 or fluorocarbon plasma etch. It is shown that the ...

Electrochemical Protection of Thin Film Electrodes in Solid State Nanopores

Solid state nanopores are a core element of next-generation single molecule tools in the field of nano-biotechnology. Thin film electrodes integrated into a pore can interact with charges and fields within the pore. In order to keep the nanopore open and thus functional electrochemically induced surface alteration of electrode surfaces and bubble formation inside the pore have to be eliminated. This paper provides electrochemical analyses of nanopores drilled into TiN membranes which in turn were employed as thin film electrodes. We studied physical pore integrity and the occurrence of water decomposition yielding bubble formation inside pores by applying voltages between -4.5 and +4.5 V to membranes in various protection stages continuously for up to 24 h. During potential application pores were exposed to selected electrolyte-solvent systems. We have investigated and successfully eliminated electrochemical pore oxidation and reduction as well as water decomposition inside nanopores of various diameters ranging from 3.5 to 25 nm in 50 nm thick TiN membranes by passivating the nanopores with a plasma-oxidized layer and using a 90% solution of glycerol in water as KCl solvent. Nanopore ionic conductances were measured before and after voltage application in order to test for changes in pore diameter due to electrochemical oxidation or reduction. TEM imaging was used to confirm these observations. While non-passivated pores were electrochemically oxidized, neither electrochemical oxidation nor reduction was observed for passivated pores. Bubble formation through water decomposition could be detected in non-passivated pores in KCl/water solutions but was not observed in 90% glycerol solutions. The use of a protective self-assembled monolayer of hexadecylphosphonic acid (HDPA) was also investigated.

Electrochemical Characterization of Thin Film Electrodes Toward Developing a DNA Transistor

The DNA-Transistor is a device designed to control the translocation of single-stranded DNA through a solid-state nanopore. Functionality of the device is enabled by three electrodes exposed to the DNA-containing electrolyte solution within the pore and the application of a dynamic electrostatic potential well between the electrodes to temporarily trap a DNA molecule. Optimizing the surface chemistry and electrochemical behavior of the device is a necessary (but by no means sufficient) step toward the development of a functional device. In particular, effects to be eliminated are (i) electrochemically induced surface alteration through corrosion or reduction of the electrode surface and (ii) formation of hydrogen or oxygen bubbles inside the pore through water decomposition. Even though our motivation is to solve problems encountered in DNA transistor technology, in this paper we report on generic surface chemistry results. We investigated a variety of electrode-electrolyte-solvent systems with respect to their capability of suppressing water decomposition and maintaining surface integrity. We employed cyclic voltammetry and long-term amperometry as electrochemical test schemes, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning, as well as transmission electron microscopy as analytical tools. Characterized electrode materials include thin films of Ru, Pt, nonstoichiometric TiN, and nonstoichiometric TiN carrying a custom-developed titanium oxide layer, as well as custom-oxidized nonstoichiometric TiN coated with a monolayer of hexadecylphosphonic acid (HDPA). We used distilled water as well as aqueous solutions of poly(ethylene glycol) (PEG-300) and glycerol as solvents. One millimolar KCl was employed as electrolyte in all solutions. Our results show that the HDPA-coated custom-developed titanium oxide layer effectively passivates the underlying TiN layer, eliminating any surface alterations through corrosion or reduction within a voltage window from -2 V to +2 V. Furthermore, we demonstrated that, by coating the custom-oxidized TiN samples with HDPA and increasing the concentration of PEG-300 or glycerol in aqueous 1 mM KCl solutions, water decomposition was suppressed within the same voltage window. Water dissociation was not detected when combining custom-oxidized HDPA-coated TiN electrodes with an aqueous 1 mM KCl-glycerol solution at a glycerol concentration of at least 90%. These results are applicable to any system that requires nanoelectrodes placed in aqueous solution at voltages that can activate electrochemical processes.

Confinement effects on the spatial extent of the reaction front in ultrathin chemically amplified photoresists

Sub-100 nm lithography poses strict requirements on photoresist material properties and processing conditions to achieve necessary critical dimension control of patterned structures. As resist thickness and feature linewidth decrease, fundamental materials properties of the confined resist polymer can deviate from bulk values and impact important processing parameters such as the postexposure bake (PEB) temperature. The effects of these confinement-induced deviations on image or linewidth spread have not been explored. In this work, we characterize the resist thickness dependence of the spatial extent of the reaction-diffusion process in a chemically amplified photoresist system under varying processing conditions. Bilayer samples are prepared with a lower layer of a protected polymer (p-tert-butoxycarboxystyrene) and a top layer of a de-protected polymer [poly(4-hydroxystyrene)] loaded with a photoacid generator. After flood exposure, PEB, and development, changes in the thickness of the protected polyme...

Thin film confinement effects on the thermal properties of model photoresist polymers

The demand to print increasingly smaller microelectronic device features means that the thickness of the polymer films used in the lithographic processes must decrease. The thickness of these films is rapidly approaching the unperturbed dimensions of the polymer, length scales at which confinement deviations and dewetting are a significant concern. We combine specular x-ray reflectivity (SXR) and incoherent neutron scattering (INS) to probe the thermal stability and dynamical effects of thin film confinement in poly(hydroxy styrene) (PHS), a polymer used in a majority of the 248 nm deep UV photoresists. PHS forms stable thin films (down to 5 nm) that do not dewet over a wide temperature range on Si surfaces ranging from hydrophilic to hydrophobic. The surface energy has a profound influence on the magnitude of the thin film expansion coefficient, especially above the glass transition, in films as thick as 100 nm. Confinement also appears to suppress the mean-square atomic displacements and the level of an...

Chemically amplified resists resolving 25 nm 1:1 line: space features with EUV lithography

We have investigated a number of key resist factors using EUV lithography including activation energy of deprotection. Our standard high activation resist material, MET-2D (XP5271F), is capable of robust performance at CDs in 40 nm regime and thicknesses above 100 nm. Below 100 nm film thickness, controlling acid diffusion becomes a difficult challenge. We have also developed a low activation resist (XP6305A) which shows superior process window and exposure latitude at CDs in the 35 nm regime. This resist is optimal for 80 nm film thickness. Lastly, we have demonstrated 25 nm 1:1 resolution capability using a novel chemical amplification resist called XP6627. This is the first EUV resist capable of 25 nm resolution. The LER is also very low, 2.7 nm 3&sgr;, for the 25 nm features. Our first version, XP6627G, has a photospeed of 40 mJ/cm2. Our second version, XP6627Q, has a photospeed of 27 mJ/cm2. Our current focus is on improving the photospeed to less than 20 mJ/cm2. The outstanding resolution and LER of this new resist system raises the possibility of extending chemically amplified resist to the 22 nm node.

Probing surface and bulk chemistry in resist films using near edge x-ray absorption fine structure

The performance of chemically amplified photoresists is extremely sensitive to interfacial and surface phenomena, which cause deviations in the pattern profile near an interface. Striking examples include T-topping or closure near the air/resist interface and footing or undercutting near the resist/substrate interface. One focus of our research is to identify mechanisms that cause lithographic patterns to deviate near interfaces. Near edge x-ray absorption fine structure (NEXAFS) is a powerful tool that can be developed and adapted to probe for detailed chemical information near lithographically relevant interfaces. NEXAFS showed that our model resist films exhibited significant surface segregation of the photo acid generator (PAG) at the air interface. The PAG surface mole fraction was 20–70 times greater than the bulk mole fraction and the amount of surface segregation was dependent on the polarity of the polymer. NEXAFS also revealed that the PAG surface fraction was reduced after a postexposure bake. ...

Sub-50nm half-pitch imaging with a low activation energy chemically amplified photoresist

Critical lithographic dimensions are rapidly approaching the sub-50nm regime where there is a concern that image blur due to acid diffusion will impose a practical limit to the resolution of chemically amplified (CA) resists. Although recent EUV and 193- and 157nm immersion interferometric experiments have reportedly resolved line-space arrays with individual dimensions on the order of ∼40nm, smaller nested features are likely to prove problematic. Numerous reports suggest that conventional photoresist performance degrades rapidly at half-pitch dimensions in this range. New approaches to processing and materials development of photoresists will likely be required if the concept of chemical amplification is to be extended to the 32nm node and beyond. In this article we show that through materials choice and proper processing, image blur can be controlled to an extent where dense features below 40nm can routinely be resolved in CA resists. We describe our studies on high-sensitivity resists of differing act...

Fundamental investigation of negative tone development (NTD) for the 22nm node (and beyond)

In this work, we investigate the Negative Tone Develop (NTD) process from a fundamental materials/process interaction perspective. Several key differences exist between a negative tone develop process and a traditional positive tone develop system. For example, the organic solvent dissolves the unexposed material, while the deprotected resist remains intact. This causes key differences in key patterning properties, such as pattern collapse, adhesion, remaining resist, and photoresist etch selectivity. We have carried out fundamental studies to understand these new interactions between developer and remaining resist with negative tone develop systems. We have characterized the dynamic dissolution behavior of a model system with a quartz crystal microbalance with both positive and negative tone solvent developers. We have also compared contrast curves, and a fundamental model of image collapse. In addition, we present first results on Optical Proximity Correction (OPC) modeling results of current Negative Tone Develop (NTD) resist/developer systems.