An Èchelle-grating specifically developed for the space borne measurement of earth’s atmospheric CO2-concentration is presented. It has a line density of ~170L/mm and is manufactured on a thick crystalline silicon substrate using electron beam lithography. The echelle profile is realized using a highly anisotropic chemical etching process, which stops on the <111> crystallographic planes of the crystal. The established manufacturing process allows perfect linear grating facets with negligible corner rounding of the profile. The important property of showing a low polarization sensitivity of the diffraction efficiency is achieved by a special design and technology for applying the gold coating of the grating which intentionally leaves one grating facet uncoated.
A compact lightweight AR headset architecture is presented employing a semi-transparent Fresnel mirror combiner. As compared to a grating combiner this design approach minimizes diffractive color aberrations by working in a high order blazed grating regime. Two different technologies, gray scale lithography as well as anisotropic wet-chemical etching of silicon wafers were evaluated for manufacturing of the Fresnel element. A first monocular demonstrator was assembled and analyzed analytically, numerically as well as experimentally. A crucial point in the manufacturing of the Fresnel combiner element was the high degree of surface shape accuracy of the working Fresnel edges as well as the embedding polymer layer to ensure optical imaging quality. Currently, only the wet-chemical etching technology could provide sufficiently good results. Two main reasons for aberrations could be traced down: grating effects of the Fresnel structure and asymmetric refraction at the embedding layer. As a result, different kinds of color aberration will be generated. Additionally, the asymmetric refraction causes different types of distortion. Two approaches for compensating color aberrations could be derived: optical pre-compensation of the lateral color caused by the embedding layer as well as optimizing the Fresnel structure to minimize grating effects.
Surface relief gratings are well-established elements for high power laser applications, e.g. ultra-short pulse compression. A binary submicron period profile, realized by e-beam lithography and reactive ion beam etching in a dielectric material, is utilized for nearly one-hundred percent diffraction efficiency. Because these gratings are manufactured without any replication techniques, a high wave front accuracy and a low stray light background can be achieved. Spectroscopic applications require additional properties, i.e. a larger spectral bandwidth and Off-Littrow operation. We present new approaches for surface relief gratings realized either via multi-level staircase profiles or exploiting sub-wavelength features. The RVS spectrometer grating in ESA’s GAIA mission is a prominent example where these techniques are already in use. The current contribution focuses on the results achieved during a pre-development performed for the MOONS instrument intended to operate at VLT.
We report about our latest achievements to realize the diffraction gratings during the development activities for a future Earth observation high resolution spectrometer studied by ESA. The gratings are manufactured by electron beam lithography on fused silica substrates. The optical performance is considerably increased by applying a dedicated high refractive index coating to the grating structure using atomic layer deposition (ALD). Thus, we were able to achieve diffraction efficiencies larger than 75% averaged over both linear polarizations states, i.e. TE and TM. At the same time, the polarization sensitivity is well below 10% in both cases. Finally, the diffraction gratings for the SWIR-1 spectral channel were bonded on a massive prism substrate in order to realize a GRISM element. This process was achieved by direct fused silica bonding performed under atmospheric pressure within special mechanical equipment designed and constructed particularly for this purpose.
Electron beam lithography becomes attractive also for the fabrication of large scale diffractive optical elements by the use of the character projection (CP) technique. Even in the comparable fast variable shaped beam (VSB) exposure approach for conventional electron beam writers optical nanostructures may require very long writing times exceeding 24 hours per wafer because of the high density of features, as required by e.g. sub-wavelength nanostructures. Using character projection, the writing time can be reduced by more than one order of magnitude, due to the simultaneous exposure of multiple features. The benefit of character projection increases with increasing complexity of the features and decreasing period. In this contribution we demonstrate the CP technique for a grating of hexagonal symmetry at 350nm period. The pattern is designed to provide antireflective (AR) properties, which can be adapted in their spectral and angular domain for applications from VIS to NIR by changing the feature size and the etching depth of the nanostructure. This AR nanostructure can be used on the backside of optical elements e.g. gratings, when an AR coating stack could not be applied for the reason of climatic conditions or wave front accuracy.
The fabrication of complex nano-optical structures for plasmonics, photonic-crystals, or meta-materials on application
relevant areas by electron-beam lithography requires a highly parallel writing strategy. In case of periodic pattern as they
are found in most of the mentioned optical elements this can be achieved by a so called character projection writing
principle where complex exposure pattern are coded in a stencil mask and exposed with a single shot. Resulting shotcount
and writing time reductions compared to standard Variable-Shaped-Beam exposures can be in the order of
100...10000. The limitation in flexibility by using hard-coded exposure shapes can be overcome by implementing the
character projection principle with a highly precise motorized aperture stage capable of carrying several 1000 different
apertures. Examples of nano-optical elements fabricated with the new character projection principle are presented.
Gratings with binary and blazed profiles and periods in the low micron and sub-micron range define a class
of microstructures with a huge application potential. We present a mask based photolithographic fabrication
method for these demanding grating geometries. It combines the advantages of electron beam lithography and
holographic exposure, which are superior homogeneity, high resolution and pattern flexibility on one hand, and
a fast, large aerial exposure with the option for smooth profiles on the other hand. This is accomplished by the
use of an electron beam written phase mask which contains a very homogeneous pattern of diffractive features
and is used for a full-field exposure in a proximity mask aligner. The key for the beneficial use of the technology
is the proper design of the phase mask surface profile which can have a binary or multilevel geometry. Since the
patterns to be exposed are periodic, this is also the case for the phase mask which allows calculating their physical
light transmission with exact methods like rigorous coupled wave analysis. An optimization algorithm has been
developed which can find mask geometries that synthesize a desired complex aerial image in the proximity
distance of choice. Aerial images offering e.g. high resolution features, phase shifts, and tilted propagation
directions can be realized that way. This technology has been successfully used to fabricate e.g. binary gratings
of very high quality with a period of 800 nm as well as blazed gratings with a period of 3 μm.
The control of very small distances is essential for many applications and alignment procedures in the field of micro
technology, e.g. micro lithography for MEMS or micro optics, where proximity lithography is often used for cost
effective mass fabrication. Also in proximity lithography the requirements, especially for resolution, are increasing
permanently. Recently new techniques have been developed to get sub-micron resolution even for larger distances
between mask and substrate. But then also the proximity distance has to be controlled with sub-micron accuracy. A
passive and an active sensor concept have been developed based on triangulation using diffractive structures. The
required sensing patterns are implemented directly in the photo mask. In the passive gap alignment the distance can be
reconstructed from the resist pattern obtained as a result of a lithographic step in which the diffractive sensor structure is
exposed in the Mask-Aligner. In the active configuration the proximity gap can be controlled already during the
alignment procedure prior to the lithographic exposure. A collimated laser beam irradiates a diffractive structure in the
photo mask, which deflects the beam which will be reflected from the resist coated substrate towards the mask. A second
mask structure, which is placed in a defined lateral distance to the first one, acts then as a ruler for the distance between
mask and wafer and can be observed through the alignment microscope or a camera module.
The design and fabrication of the diffractive structures, the measurement results for the full-wafer proximity distance
distribution according to the passive method, as well as the realization of an active sensor module for mask aligners are
presented in this paper.
A novel technique for the fabrication of high resolution sub-micrometer patterns by diffractive proximity lithography in a
mask-aligner is presented. The technique is based on the use of specially designed diffractive photo-masks. It requires
some small modifications of the mask-aligner, especially for the mask illumination and the settings of the proximity gap
between mask and substrate. The huge potential of this novel technique is demonstrated at the example of structures
having lateral feature sizes in the sub-500nm range printed with mask-to-substrate distances of several ten micrometers.
One and two dimensional grating structures with submicron period have a huge number of applications in optics and
photonics. Such structures are conventionally fabricated using interference or e-beam lithography. However, both
technologies have significant drawbacks. Interference lithography is limited to rather simple geometries and the
sequential writing scheme of e-beam lithography leads to time consuming exposures for each grating. We present a novel
fabrication technique for this class of microstructures which is based on proximity lithography in a mask aligner. The
technology is capable to pattern a complete wafer within less than one minute of exposure time and offers thereby high
lateral resolution and a reliable process. Our advancements compared to standard mask aligner lithography are twofold:
First of all, we are using periodic binary phase masks instead of chromium masks to generate an aerial image of high
resolution and exceptional light efficiency at certain distances behind the mask. Second, a special mask aligner
illumination set-up is employed which allows to precisely control the incidence angles of the exposure light. This degree
of freedom allows both, to shape the aerial image (e. g. transformation of a periodic spot pattern into a chessboard
pattern) and to increase its depth of focus considerably. That way, our technology enables the fabrication of high quality
gratings with arbitrary geometry in a fast and stable wafer scale process.
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.
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.
The half-tone lithography using pixilated chromium masks in a projection stepper is an established technology in micro-optics
fabrication. However, the projection lithography tool is comparably expensive and the achievable lateral
resolution is typically limited. By using pixel diffraction effects, binary and continuous profile lithography with
submicron resolution can be installed on a conventional mask aligner. To achieve this goal the control of both, the
angular spectrum of the illumination and the mask features is essential. We used a novel micro-optics based illumination
system referred as "MO Exposure Optics System" in a SUSS MicroTec MA6 mask aligner for the dedicated shaping of
the angular illumination distribution. In combination with an adapted lithography mask the formation of a desired
intensity distribution in the resist layer is possible. A general mathematic model describes the relation between the
angular spectrum of the mask illumination, pixel size and pitch in the mask, proximity distance and propagated field,
which also includes special cases like Talbot imaging. We show that a wide range of different micro-optical structures
can be optimized by controlling the light diffraction in proximity lithography. Parameter settings were found for
submicron binary pattern up to continuous profile structures with extensions up to several tens of microns. An additional
interesting application of this approach is the combination of binary and continuous profiles in single elements, e.g.
micro lenses with diffractive correction or AR structures. Experimental results achieved for blazed gratings with a period
of 2 microns are presented.
Calcium Fluoride microlens arrays have been produced with the help of an ion milling transfer of photoresist lenses which have been fabricated by two different techniques (i) by a melting resist technique and (ii) gray scale lithography. Major technical problems connected with the ion milling transfer of the photoresist lenses in the CaF2 material are surface roughness enhancement and variation of the lens shape. We studied the ion-induced decomposition of CaF2 and the surface roughness equatio in dependence on different milling conditions. For the microlens patter transfer the ratio of etching rates of the photoresist and the CaF2 have been adjusted by gas admixture of nitrogen to the Ar sputtering gas. The angle dependence of the etching rates leads also to a changing of the lens profile. The effect of ion beam induced surface roughness and the accuracy of the transfer process is discussed.
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