Electron beam lithography of molecular glass resist films prepared by physical vapor deposition

The progress of our information society in this current era depends mostly on the improvement of the computing speed of microprocessors. Therefore semiconductor industries realize further developments in processor architecture but also in the patterning process which is utilized for the manufacturing of microprocessors. Of great importance is thereby the advancement to smaller feature sizes. However, the state of the art exposing technique 193 nm immersion lithography with applied multi-patterning gradually reaches its limits in the ongoing miniaturization. But for the continuation of miniaturization of integrated circuits in the long term, a further progression in the processing steps as well as the development of further improved materials are essential. Favored exposure methods of the next generation lithography are extreme ultraviolet lithography or multi-beam electron beam lithography, as these techniques provide the opportunity of patterning smaller feature sizes. In this thesis electron beam lithography, physical vapor deposition as alternative resist preparation technique and the materials development for this film preparation technique are covered. In the first chapter of this thesis the characterization of the utilized electron beam exposure tool but also the establishment of a processing procedure are in the focus. The second chapter covers the investigation of physical vapor deposition (PVD) as an alternative resist film preparation method. Encouraged by the promising result utilizing PVD as film preparation technique, the synthesis of applicable positive tone molecular glass resist materials was addressed in the third chapter. Finally in the fourth chapter a completely new patterning concept based on the salt formation between a base and the acid of an exposed photoacid generator was established, combinatorially optimized and high resolution features were demonstrated. The first chapter of this thesis covers the characterization of the utilized electron beam lithography tool Zeiss Leo1530 equipped with the lithography control unit Raith Elphy Plus and the establishment of processing procedures. The investigation on crucial device parameters and their influence on the pattern quality are the main objects as well as the establishment of an optimized working procedure to prevent irregularities. Even though the utilized exposure tool is not a dedicated electron beam lithography tool, it allows the realization of beam diameters down to 2.1 nm for the acceleration voltage of 20 kV under optimized conditions. This allows the realization of feature elements in the nanometer scale. The continuous work with this exposure tool during this thesis lead to the identification of external and internal tool as well as software parameters crucial for the achieved patterning quality. The most important identified impact factors are the beam adjustment, the acceleration voltage, and the electrostatic beam blanker. The beam adjustment is a key issue for the achievable feature quality and has to be performed properly. Using a less adjusted beam means the beam diameter is increased and the beam shape might be elliptic. These two deviations both affect the maximal realizable resolution. The utilized acceleration voltage defines the penetration depth of the electrons and thus the number of backscattered electrons and affected area. These backscattered electrons increase the exposure dose of the exposed area and also expose the surrounding of the actual patterns. This effect can be an issue concerning the formation of residues in these surrounding areas. Theoretical simulations of primary electrons’ pathways proved the influence of the acceleration voltage on affected area and on the local density of backscattered electrons in dependence of the distance to the beam impact position. The electrostatic beam blanker has its advantages in the avoidance of unintended exposure of the non-blanked beam during movement between different positions on the substrate as well as during the exposure process of the features itself. But during the thesis, also a big disadvantage of the installed electrostatic beam blanker was identified. As soon as it is turned on, the electron beam is affected by a 50 Hz fluctuation, resulting in periodical deflections of the electron beam. These deflections lead to reduced image sharpness in the imaging mode of the scanning electron microscope. They also have an impact on the obtained features written by the lithographic exposure process and result in the formation of periodic defects by utilizing a very sensitive non-chemically amplified resist system. For electron beam exposed chemically amplified resists in contrary, no such defects were identified, which is obviously a consequence of diffusion of the acid utilized in such systems for the chemically amplification reaction. Additionally, during the research done for this thesis, the processing procedure for electron beam lithography with this tool and the thereby utilized write field designs were continuously developed further and improved to overcome the identified issues where possible. With the latest generation of these write fields a toolbox is now available for the investigation of the resists’ sensitivity, for the performing of an exposure dose gradient in a write field with only 100 µm edge length, as well as for the investigation of the resolution capability of novel resists. The second chapter focuses on the investigation of physical vapor deposition as alternative film preparation technique in lithography. In the curse of this the influences on resist characteristics of a chemically amplified resist were investigated, which result from two different film application methods, namely the standard spin coating process in comparison to the alternative PVD technique. In the standard spin coating process the whole film is formed at once by solvent evaporation. The PVD technique in contrary is a solvent-free vacuum process, whereby the evaporated material deposits molecule by molecule on the substrate. This results in a continuous increase in film thickness and a statistically controlled distribution of the different materials for multi-component films. The statistical deposition prevents the formation of material aggregates in a multi component system, which can be an issue in the spin coating process due to possible segregation or phase separation. Published simulations show an impact of photo acid generator aggregates in chemically amplified resists. Such aggregates can result in decreased sensitivity, increased line edge roughness and decreased resolution. For this thesis the resist performance of resist films prepared by the different film application techniques spin coating and PVD was compared for the first time. Therefore a literature known negative tone chemically amplified molecular glass resist was adapted for PVD processing. The resist was investigated systematically on the resist characteristics dissolution behavior and resist sensitivity as well as on overall lithographic performance for both film application techniques. The investigations lead to the conclusion that the solvent-free PVD results in a distinct increase in sensitivity. This increased sensitivity is also demonstrated by higher responsiveness for the formation of residues due to backscattered electrons. This effect can be explained by the individual molecule by molecule deposition step resulting in a more homogeneous distribution, especially of the photoacid generator. To support this promising result of increased sensitivity of PVD prepared resist films, several approaches were pursued in order to reduce the responsiveness for the formation of residues due to backscattering. The investigation of the approaches was performed highly efficiently by ternary combinatorial libraries of exposure dose, post exposure bake temperature, and development time. As first approach a higher acceleration voltage of 100 kV was applied for exposure instead of the formerly used 20 kV. The higher acceleration voltage successfully decreased the amount of formed residues due to a lowered density of backscattered electrons around the exposed features. However, the effect of the higher acceleration voltage was not sufficient to completely inhibit residue formation. Thus in a second approach a polymeric bottom layer was applied between the silicon substrate and the resist film. The bottom layer reduces the amount of formed backscattered electrons reaching the resist film and thus decreases the amount of formed residues. The effect of a bottom layer on the number of backscattered electrons was additionally supported by theoretical simulations of primary electrons’ pathways. Applying both approaches, clear 100 nm patterns were observed utilizing the high sensitive PVD prepared resist film. This fundamental investigation demonstrated for the first time that the application of the solvent-free film coating technique PVD is not only an alternative high quality film preparation technique but rather improves the resist performance by homogeneous material blending. This novel interesting scientific finding has been published in Advanced Functional Materials . The third chapter covers the synthesis and characterization of positive tone chemically amplified resist materials utilizable for PVD. This topic was encouraged by the promising results described in chapter two, where negative tone chemically amplified resist systems had featured higher sensitivity due to film application by PVD. To transfer this result to positive tone chemically amplified resists, PVD suitable materials had to be identified. However, no such materials are literature known. Their properties on the one hand must feature a cleavable group, which has to withstand the thermal stress of the evaporation process without decomposition. On the other hand it must show a stable amorphous phase with a glass transition temperature (Tg) higher than or at least in the range of the post exposure bake temperature. It is especially the required thermal stability that excludes the industrial used polymeric resist materials. Promising candidates are protected molecular glass materials with suitable acid cleavable groups. The here presented investigation focused on the functional groups carbonate esters and carboxylic acid esters, which both are known as cleavable groups from industrial applied polymeric resists. The carbonate ester class was investigated in cooperation with the workgroup of Prof. Ober, where molecular glass materials with tBoc protection groups were synthesized. The thermal characterization showed a decomposition temperature of around 160 °C of the tBoc protection group regardless of the utilized phenolic core. This low decomposition temperature allowed a successful PVD processing only for a very low molecular weight material. However, such materials missed the required glass transition temperature criteria. The investigation of synthesized ester based materials showed decomposition temperatures of around 210°C for tert-butylester, and even 250 °C for an isobornyl ester. Thus the ester function is thermal more stable and more promising for a PVD processible positive tone chemically amplified resist. The thermal characterization of the synthesized materials showed that an ether linking group dramatically decreases the glass transition temperature and thus disqualifies the synthesized positive tone resist material with this functional group. Most promising turned out to be the direct esterification of a carboxylic acid core with a secondary or tertiary alcohol like isoborneol. But further synthetic efforts are needed to achieve fully operational PVD processible positive tone molecular glass resists. In summary, for the first time positive tone chemically amplified resist materials were investigated on their PVD processability, which allowed the identification of the ester based material class as the most promising one. The fourth chapter covers the investigation of a novel physical vapor depositable resist material concept based on exposure induced salt formation. The background for this topic is the fact that the semiconductor industry is seeking for resist systems, which are capable of realizing sub 20 nm feature sizes. Beside the chemical modification of existing resist types also the investigation of completely new resist material concepts are in the focus to fulfill this demand. With this target in mind a completely new patternable material concept based on salt formation was combinatorially investigated and optimized in this chapter of this thesis. It is based on the formation of ionic interactions between a base molecule and an acid, which is released by the exposure of the photoacid generator. The induced ionic interactions change the polarity of the material, which serves as development contrast. The proof of principle for such acid-base-resist systems was processed with a carbazol derivative as base component and a non-ionic photoacid generator. The formation of the salt was verified by infrared spectroscopy and the produced development contrast was investigated by quartz crystal microbalance measurements. A developer screening demonstrated the applicability of developers of a wide polarity range. The optimized resist performance considering resist sensitivity and feature quality was observed utilizing cyclohexane. The investigation on different base materials demonstrated a strong dependence of the resist sensitivity on the applied base, with beneficial performance for bases with high basicity and many amino functions. The presented combinatorial investigation was combined with a polymeric bottom layer investigation to prevent residue formation due to backscattering. The material screening showed that the optimized acid-base-resist consists of 1,1'-Binaphthalene-2,2'-diamine and the mixture of two isomeric photoacid generators applied on a Poly-(2-vinyl-pyridine) bottom layer. Further more detailed screenings on thicker bottom layer and the additivation of the cyclohexane developer with a nonionic surfactant allowed the improvement of feature quality by reducing the amount of residues in between the patterned line features. Finally the identified resist was combinatorially optimized to investigate the potential of this new resist material concept. The prepared combinatorial library covered gradients of interacting processing variables as composition, development time and exposure dose, which were applied in a specific layout which allowed both a fast and precise investigation of the resolution potential in the region of interest within the same library. In the optimized sector, high resolution 40 nm line features with the relatively low line edge roughness of 6.4 were observed. This achievement demonstrates the enormous potential of this novel resist material concept on an acid-base-resist, especially as the high resolution pattern was written utilizing a non-dedicated 20 kV electron beam lithography tool.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Kolb, Tristan

Contributors dc:contributor

• Schmidt, Hans-Werner

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1953/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1953