Electron beam lithography for Nanofabrication

Abstract

La litografía por haz de electrones (Electron Beam Lithography, EBL) se ha consolidado como una de las técnicas más eficaces que permiten definir motivos en el rango nanométrico. Su implantación ha permitido la nanofabricación de estructuras y dispositivos para su uso en el campo de la nanotecnología y la nanociencia.<br/>La EBL se basa en la definición de motivos submicrónicos mediante el rastreo de un haz energético de electrones sobre una resina. La naturaleza de los electrones y el desarrollo the haces extremadamente finos y su control preciso establecen la plataforma ideal para los requerimientos de la Nanofabricación. El uso de la EBL para el desarrollo de un gran número de nanoestructuras, nanodispositivos y nanosistemas ha sido, y continúa siendo, crucial para las aplicaciones de producción de máscaras, prototipaje o dispositivos discretos para la investigación fundamental. Su éxito radica en la alta resolución, flexibilidad y compatibilidad de la EBL con otros procesos de fabricación convencionales. <br/>El objetivo de esta tesis es el avance en el conocimiento, desarrollo y aplicación de la EBL en las areas de los micro/nanosistemas y la nanoelectrónica. El presente documento refleja parte del trabajo realizado en el Laboratorio de Nanofabricación del Instituto de Microelectrónica de Barcelona IMB-CNM-CSIC durante los últimos cinco años. Debido a la falta de experiencia previa en el IMB en la utilización de la EBL, ha sido necesario el desarrollo y consolidación de una serie de procesos, lo que ha condicionado parcialmente la investigación, tal y como recoge la memoria.<br/>Entre los aspectos relevantes compilados en esta tesis, en cuanto a innovación tecnológica, cabe destacar diversos avances en procesos tecnológicos basados en la EBL. Una nueva resina de tono negativo ha sido caracterizada y disponible para su uso en nanofabricación. La optimización de la EBL se ha llevado a cabo mediante métodos de corrección del efecto de proximidad. Se ha establecido el proceso de integración de estructuras nanomecánicas en circuitos CMOS, así como la fabricación de dispositivos basados en nanotubos de carbono. En concreto, el primer FET basado en un sólo nanotubo de carbono fabricado en España. Finalmente, la compatibilidad y viabilidad de los métodos de fabricación basados en haces de partículas se ha estudiado mediante el análisis del efecto de los haces de partículas cargadas sobre dispositivos. Por otro lado, esta memoria no sólo contiene la descripción de los principales resultados obtenidos, sinó que pretende aportar información general sobre procesos de nanofabricación basados en haces de electrones para ser utilizados en futuras investigaciones de este area.

Electron beam lithography (EBL) has consolidated as one of the most common techniques for patterning at the nanoscale meter range. It has enabled the nanofabrication of structures and devices within the research field of nanotechnology and nanoscience. <br/>EBL is based on the definition of submicronic features by the scanning of a focused energetic beam of electrons on a resist. The nature of electrons and the development of extremely fine beams and its flexible control provide the platform to satisfy the requirements of Nanofabrication. Use of EBL for the development of a wide range of nanostructures, nanodevices and nanosystems has been, and continues to be, crucial for the applications of mask production, prototyping and discrete devices for fundamental research and it relies on its high resolution, flexibility and compatibility with other conventional fabrication processes.<br/>The purpose of this thesis is to advance in the knowledge, development and application of electron beam lithography in the areas of micro/nano systems and nanoelectronics. In this direction, this memory reflects part of the work performed at the Nanofabrication Laboratory of the IMB-CNM. Since there was no previous experience on EBL at CNM, the need for developing a set of processes has determined partially the work. <br/>The variety of topics that concern to nanoscience and nanotechnology is enormous. Chapter 1 briefly sintetizes nanoscale related aspects. This section aims to frame the contents of this thesis, coherently. Also for completeness, it is intended to address the specific subjects under discussion or contained in the following chapters and it is based or oriented to the experimental results that will be presented.<br/>Chapter 2 is a general overview of the EBL technique from the point of view of the system and the physical interaction of the process. In particular, the characteristics of the SEM and specifications of the lithographic capabilities of the system that is used are presented.<br/>In chapter 3, irradiation effect on resists is studied. The chemical behaviour of different polymeric materials is correlated with theoretical simulations for two types of resists: methacrylic based positive resists and epoxy based negative resists. The first is used for validation of the modelization and to describe the general performance of EBL on different conditions. The second covers the experiments oriented to establish the performance parameters of a new resist and comparison with another existing negative electron beam resist. Proximity effect correction concludes with the correlation of theory and experimental results for both types of resists, positive and negative.<br/>Chapter 4 is an example of the fabrication and optimization of a micro/nanosystem for sensing at the nanoscale. In particular, nanoresonators are developed with two approaches (EBL and FIB) and enhanced response is achieved by their integration on CMOS circuitry.<br/>Chapter 5 presents carbon nanotube (CNT) based devices that are realized and implemented for applications in nanoelectronics and sensing. First, different fabrication approaches for contacting CNTs are discussed. Then, the results of electrical characterization of the devices are presented. Finally, technology development for the use of these devices for sensing is established.<br/>The last chapter embraces all the previous sections and pays attention to the effect of electron beam on the devices. In particular, electron induced effect is studied on nanomechanical structures integrated in circuits and CNT based devices, in order to evaluate EBL based fabrication, SEM characterization or more fundamental aspects. Advanced characterization techniques are used together with simulations, both assessing a deeper understanding of the results. Electrical measurements and AFM based techniques are used to characterise the effect of the electron irradiation by changes in their performance characteristics, charging, surface potential imaging, etc.<br/>Main results and solved challenges are summarized in the conclusive chapter 7 that finishes with this document.