Maria Oliva

Multilevel blazed gratings in resonance domain: an alternative to the classical fabrication approach

In this paper we present a novel technological approach for the fabrication of multilevel gratings in the resonance domain. A coded chromium mask is used to avoid alignment errors in electron beam lithography, which typically occur within the standard multistep binary micro-optics technology. The lateral features of all phase levels of the grating are encoded in a single chromium mask. The final profile of the structure is obtained by selective etching process for each level. This new technological method is applied for the fabrication of two different three-level gratings in resonance domain. The corresponding optical response as well as structural characterizations are presented and discussed. In particular, a first order diffraction efficiency of 90% is demonstrated for a grating period twice the wavelength at normal incidence.

Highly efficient three-level blazed grating in the resonance domain

We designed, fabricated, and characterized three-level transmission gratings in the resonance domain with reduced shadowing losses based on a three-wave interference mechanism. A new technological approach allows for fabrication of homogeneous and large area multilevel gratings without spurious artifacts. To our knowledge, the measured efficiency of 86% exhibits the largest value yet reported for a multilevel transmission grating in the resonance domain close to normal incidence.

Multilevel pattern generation by GaN laser lithography: an application to beam shaper fabrication

The new GaN lasers represent a unique combination of compactness, reliability, energy efficiency, and short wavelength. With respect to the previous state of the art in direct laser write lithography, based on gas lasers, this is resulting in a breakthrough, and is opening the way to real desktop micropatterning. The field of diffractive optics can immediately benefit by the availability of a new breed of pattern generators, based on such sources, mainly for fast turnaround device development. This paper presents the technical advantages involved in the use of 405 nm GaN lasers for one-step multilevel patterning. Beam modulation, exposure control and overall process strategy are discussed. In order to evaluate the effectiveness of the new solution, a sample fabrication of beam shapers is also presented.

Highly efficient broadband blazed grating in resonance domain

A resonance domain blazed grating, composed of an effective medium structure and a subsequent mode conversion layer, is designed, fabricated, and characterized. Due to the demanding high aspect ratio geometries, a technological approach for multilevel structures, assisted by atomic layer deposition technology, has been developed. The measured efficiency of about 90% exhibits the largest value yet reported for a multi-level fused silica transmission grating in the resonance domain, operating at non Littrow mounting, close to normal incidence.

Blistering during the atomic layer deposition of iridium

The authors report on the formation of blisters during the atomic layer deposition of iridium using iridium acetylacetonate and oxygen precursors. Films deposited on fused silica substrates led to sparsely distributed large blisters while in the case of silicon with native oxide additional small blisters with a high density was observed. It is found that the formation of blisters is favored by a higher deposition temperature and a larger layer thickness. Postdeposition annealing did not have a significant effect on the formation of blisters. Finally, changing purge duration during the film growth allowed us to avoid blistering and evidenced that impurities released from the film in gas phase were responsible for the formation of blisters.

Smart technology for blazed multilevel gratings in resonance domain

The design and the fabrication of a multilevel blazed grating in resonance domain for first order high efficiency applications are presented. The design shows that a 3 phase level grating is sufficient to achieve efficiency of 90% in the minus first diffraction order. The standard technology for the fabrication of multilevel grating consists in multistep electron beam lithography and reactive ion beam etching of the grating profile into the fused silica substrate. Typical fabrication errors of this technology approach, e.g. misalignment, reduce the theoretical reachable efficiency of the grating. Two new technological approaches were investigated to avoid these typical fabrication errors and to improve the multi level fabrication process. The designed grating has been fabricated by three different technological solutions and the geometrical characterization as well as the diffraction performance are presented and discussed.

Effects of metallic nanoparticle arrays in Si solar cell structures

Metallic inclusions in layered structures can have noticeable effects onto scattering and absorption due to the coupling of the external electromagnetic field and local charge oscillations. These effects are strongly related to both the geometry of the individual particle as well as to the array structure. Having in mind the efficiency improvement of silicon solar cells due to plasmonic effects, we report on the modeling and the fabrication of periodic arrays of metallic nanoparticles on planar substrates. Different characterization techniques as atomic force microscopy (AFM), scanning electron microscope (SEM) and optical measurements are applied which provide particular information with respect to the fabricated structures, each. Special emphasis is placed on the clarification of the dominant features of the optical characterization by detailed numerical analysis. This allows identifying significant modes of the planar geometry which is complemented by the nanostructures, whose interplay with the radiation field does establish changes of the absorption in the silicon layer, finally. These findings may be helpful for optimization and clarification of specific details of technology, later on.

Optical design including characteristics of manufactured nanostructures

Micro- and nanostructures enable specific optical functionalities, which rely on diffractive effects or effective medium features, depending on pattern dimension and wavelength. Performance predictions of optical systems which make use of nanostructured materials require having an accurate description of these materials ready to hand within the optical design. At the one hand, nanostructure characteristics which result from rigorous electromagnetic modeling can be used for the optical design. At the other hand, manufactured nanostructures may deviate from their idealized geometry, which will affect the performance of the optical system, wherein these artificial structures will be used. Thus, detailed optical characterization of the micro- or nanostructure functionality is prerequisite for accurate optical design and performance prediction. To this end, several characterization techniques can be applied depending on the scope of the optical design, finally. We report on a general route to include all accessible and required optical information about the nanostructured material within a corresponding model of the nanostructure as a specific optical component which can be used within a ray-trace engine, finally. This is illustrated by a meta-material with asymmetric transmission properties in some more detail.

Nano-optical gratings for integrated laser interferometer arrays

A wafer-to-wafer inspection of microsystems is a promising approach for a parallel and thus high speed characterization in particular for MEMS and MOEMS. The probing wafers aligned above the M(O)EMS wafer consists of arrays of micro-optical interferometers for the shape-, deformation-, resonance-characterization. In this contribution we show that binary nano-optical gratings in the resonance domain are ideal elements to build up such integrated interferometers. By means of symmetry considerations the gratings requirements for an ideal operation of the interferometer are determined. Utilizing the resonance behaviour of such nano-structures an interferometer operation close to the theoretical limits is reachable. A modal analysis reveals the basic effects causing the special responses of the gratings. The gratings are fabricated by electron-beam lithography and accompanied technology.

AFM characterization of large area micro-optical elements

We discuss AFM (Atomic Force Microscopy) characterization in terms of critical dimension and depth for large area micro-optical elements. Results are shown and discussed in comparison with other techniques, such as SEM (Scanning Electron Microscopy) for CD measurements and FIB (Focused Ion Beam)-SEM characterization for the structure profile.