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Nanofabrication on insulating substrates

AuthorsFreire Fernández, Francisco
AdvisorTeresa, José María de; Sangiao, S.
Issue Date2015
PublisherUniversidad de Zaragoza
AbstractElectron Beam Lithography (EBL) is a fundamental technique used in nanofabrication, allowing the direct writing of structures down to sub-10nm dimensions. Derived from the early Scanning Electron Microscopes (SEM), the technique briefly consists of scanning a beam of electrons accross a surface covered with a resist film sensitive to those electrons and energy is deposited in the desired pattern in the resist film. Depending on the resist nature, positive or negative tone, exposed or unexposed regions are removed in the development step, respectively. Nanoimprint lithography, deep, and extreme ultraviolet lithography are examples of high volume nanoscale patterning technologies which rely on EBL as a basic tool for the creation of the masks and the templates that they use. However, EBL applicability to insulating substrates remains challenging because of surface charging effects. Unlike patterning on conducting substrates where the charge excess is dissipated as the beam passes through the resist, patterning on insulating samples results in charge trapping near the surface. This charge accumulation produces an unbalanced suface potential of the resist that deflects the beam and causes severe pattern distortions. To overcome this problem, the most widely used anti-charging method is to coat the resist with a thin metal or conducting polymer layer to dissipate the charge. Similar to variable pressure scanning electron microscopy, variable pressure EBL has demonstrated the capability of conducting EBL on insulating substrates since the negatively-charged electrons can be balanced by the positive ions created by the electron-gas molecule collisions; yet, the resolution may suffer from the electron scattering by the gas molecules. Another charging dissipation technique is the Critical Energy EBL (CE-EBL), which makes use of the fact that at certain electron energy the number of ejected electrons (secondary and backscattered) is equal to the injected primary electrons, leading to the suppression of the charging effect. In addition, charging distortion can be prevented by carring out the EBL on a thin electron-transparent membrane that traps only a small percentage of the electrons. SEM image of a quartz substrate coated with an Au layer; EBL squares were exposed, only quartz remains in those regions. This image shows the charge accumulation on the insulating areas of the sample. Whereas the Au layer, that is connected to ground, efficiently evacuates the charge excess. In this text we will study the use of charge conducting layers (on top and underneath of the resist layer) and the CE-EBL method to fabricate nano-optical devices such a standard and sub-wavelength metal difraction gratings and subwavelength appertures arrays. The characterization of these devices will be done using a Fourier Transform InfraRed (FTIR) system, which provides transmittance and reflectance measurements. A sub-wavelength grating (SWG), which has a smaller period than the wavelength of an incident light, does not generate higher diffraction orders. When the grating period is much smaller than the wavelength, the grating behaves as a homogeneous layer with effective refractive index between the material index and the surrounding index which depends on the polarization of the incident light.
Applications of SWGs today include antireflection filters and phase plates. The optical properties of subwavelength apertures in metallic films have been the focus of much research activity since the extraordinary optical transmission (EOT) phenomenon. EOT is an optical phenomenon in which a structure containing subwavelength apertures in an opaque screen transmits more light than might naively be expected on the basis of either ray optics or even knowledge of the transmission through individual apertures. Surprisingly, such arrays may, for certain wavelengths, exhibit transmission efficiencies (normalized to the total area of the holes) that exceed unity. In other words, for these wavelengths a periodic array of subwavelength holes transmits more light than a large macroscopic hole with the same area as the sum of all the small holes.
DescriptionTrabajo de Fin de Máster: Física y Tecnologías Físicas, Departamento de Física de la Materia Condensada de la Universidad de Zaragoza.
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