Although conventional computer technology made a huge leap forward in the past decade, a vast number of computational problems remain inaccessible due to their inherently complex nature. One solution to deal with this computational complexity is to highly parallelize computations and to explore new technologies beyond semiconductor computers. Here, we report on initial results leading to a device employing a biological computation approach called network-based biocomputation (NBC). So far, the manufacturing process relies on conventional Electron Beam Lithography (EBL). However we show first promising results expanding EBL patterning to the third dimension by employing Two-Photon Polymerization (2PP). The nanofabricated structures rely on a combination of physical and chemical guiding of the microtubules through channels. Microtubules travelling through the network make their way through a number of different junctions. Here it is imperative that they do not take wrong turns. In order to decrease the usage of erroneous paths in the network a transition from planar 2-dimensional (mesh structure) networks to a design in which the crossing points of the mesh extend into the 3rd dimension is made. EBL is used to fabricate the 2D network structure whereas for the 3D-junctions 2PP is used. The good adaptation of the individual technologies allows for the possibility of a future combination of the two complementary approaches.
In this paper, the results of the successful fabrication as well as the optical backscattering characterization of single plasmonic gold dipole nanoantennas on a SiO2/Si layered substrate are shown. The nanoantennas were designed for a scattering resonance in the NIR range. In contrast to usually used glass substrates, a six inch Siwafer with a thermally oxidized SiO2 layer in combination with an electron beam lithography lift-off fabrication process has been used for the sake of compatibility with microelectronics fabrication processes. In order to achieve high structural resolutions, a bilayer resist system with different exposure sensitivities was realized. In a second step, the entire resist thickness of 540 nm was reduced to 150 nm in a single layer. The SiO2 thickness was chosen in a way that the optical near-field interactions of the nanoantennas with the silicon substrate are decoupled. The SEM characterization of the fabricated structures shows precise nanoantenna geometries with low edge roughness in the case of the bilayer resist system. The aspect ratio of the fabricated nanoantenna structures is slightly decreased compared to the desired value of five. Depending on the applied e-beam exposure dose, an increase of the structural cross-section, i.e. critical dimension of the dipole width, was observed. Furthermore, the single resist layer introduces some structuring issues. The spectral behavior of the nanoantenna structures was investigated with an optical confocal broadband backscattering measurement setup allowing the spectral characterization of single nanoantenna structures. The developed numerical models helped to understand the impact of the manufacturing imperfections providing improved designs.
We present nanostructured reflectors as alternative for well-known alternating layer stack reflectors for Fabry-Pérot Interferometers (FPI) for the use in miniaturized spectrometry systems. The addressed FPI is part of an online monitoring system for specific molecules by Surface Enhanced Raman Spectroscopy (SERS). Key part is the tunable FPI with nanostructured reflectors, which is fabricated with MEMS and NEMS technologies. Nanostructured Photonic Crystal (PhC) and Sub-Wavelength Grating (SWG) reflectors are developed. The PhC reflectors consisting of 400 nm thin moveable LP-CVD Si3N4 membranes with nanostructured holes realize an aperture of 1 mm with high reflectivity in the VIS range. The SWG reflectors are realized as nanostructured aluminum polygons on 150 nm thin LP-CVD Si3N4 membranes. The challenge in manufacturing of the PhC and SWG structures on 50 μm thin predefined silicon membrane areas is the thin wafer handling, because they are very fragile and tend to warp under their own weight. Further challenges such as delamination of the NIL-stamp from the wafer and eBeam resist homogeneity on the deflected thin silicon membranes for nanostructure replication as well as residual free resist layers for the followed RIE process and the match of the used Nanoimprint, 1:1 and eBeam lithography processes for the different layers have to be considered. The manufacturing and characterization of both alternative reflectors for prospective integration in VIS-FPIs on 6" wafers is described.
Further developments of miniaturized spectrometry systems require tunable Fabry-Pérot-Interferometers (FPI). A main part of the FPI is the reflector, which is usually realized as a stack of alternating dielectric layers with high and low refractive index. To achieve high reflectivity adequate material property homogeneity for each layer is needed. For stacks with larger number of layers not only the integration with MEMS processes is challenging. Particularly stack structuring and the achievement of process compatibility to moveable MEMS structures are important.
As an alternative to the alternating layer stack reflector, nanostructured photonic crystal (PhC) reflectors indicate equivalent performance by using only one layer leading to a minimized reflector complexity. This contribution presents a novel PhC reflector consisting of a 400 nm thin moveable nanostructured LP-CVD Si3N4 membrane realizing an aperture of 0.5 mm and 1 mm for reflectivity in the VIS range. Manufacturing of the reflectors is done on 6" wafers. The array of nanostructures is designed as 1 mm circular dies consisting of 436 nm wide holes with 545 nm pitch. The circular dies are arranged in an 8 x 8 matrix on the wafers with 7.5 mm pitch. Manufacturing and integration of the PhC reflectors into MEMS is realized by eBeam and nanoimprint lithography (NIL) nanostructure replication on 50 µm thin pre-etched Si membranes combined with further dry and wet etching processes. The fabricated PhC reflectors showed 424 nm wide holes and a pitch of 549 nm with a measured reflectivity above 90 % in the spectral range from 557 to 589 nm and a maximum reflectivity of 99 %.
Technologies for the 3D integration are described within this paper with respect to devices that have to retain a specific minimum wafer thickness for handling purposes (CMOS) and integrity of mechanical elements (MEMS). This implies Through-Silicon Vias (TSVs) with large dimensions and high aspect ratios (HAR). Moreover, as a main objective, the aspired TSV technology had to be universal and scalable with the designated utilization in a MEMS/CMOS foundry. Two TSV approaches are investigated and discussed, in which the TSVs were fabricated either before or after wafer thinning. One distinctive feature is an incomplete TSV Cu-filling, which avoids long processing and complex process control, while minimizing the thermomechanical stress between Cu and Si and related adverse effects in the device. However, the incomplete filling also includes various challenges regarding process integration. A method based on pattern plating is described, in which TSVs are metalized at the same time as the redistribution layer and which eliminates the need for additional planarization and patterning steps. For MEMS, the realization of a protective hermetically sealed capping is crucial, which is addressed in this paper by glass frit wafer level bonding and is discussed for hermetic sealing of MEMS inertial sensors. The TSV based 3D integration technologies are demonstrated on CMOS like test vehicle and on a MEMS device fabricated in Air Gap Insulated Microstructure (AIM) technology.
This paper describes the application of a micromachined resonator to verify the vacuum pressure and sealing of cavities in micromechanical components. We use an electrostatic driven and capacitively sensed bulk silicon resonator fabricated by Bonding and Deep Reactive Ion Etching (BDRIE) technology. The resonator operates at the first fundamental frequency. The damping is used as a degree of the pressure. Transversal comb structures act as squeeze film damping sources. Post-processing gap reduction substructures are used to increase the damping in the vacuum pressure range. This method makes it possible to observe the pressure over the time of smallest gas volumes by monitoring the damping of integrated micro mechanical resonant structures. Therewith it is possible to estimate the hermetic sealing quality of the closed sensor package. A transfer curve with a logarithmic characteristic is measured.
An adhesive bonding technique for wafer-level encapsulation of high aspect ratio microstructures (HARMS) is presented. The adhesive material is spin coated on a cap wafer and structured prior to bonding. Thus sealed cavities of variable height are created in the bonding layer. SU-8 negative photoresist is used as the adhesive material in combination with miscellaneous surface materials: silicon, silicon dioxide and aluminum. The influences of the bonding process parameters - bonding pressure, bonding temperature and process time - as well as the SU-8 layer properties on the bond strength and the homogeneity of the bond have been investigated. To evaluate the process conditions the shear strength of the bond has been measured according to the ASTM standard D 1002 for adhesive bonds. Each bond interface was tested by 32 test specimens of 10 by 10 mm2 side length. With optimal process conditions shear strength of 19.2, 23.3 and 21.3 MPa have been obtained for silicon, silicon dioxide and aluminium respectively. The application of the selective adhesive bonding technique has been successfully demonstrated by encapsulating different types of single crystal silicon inertial sensors.
This paper presents a new process flow for the fabrication of Air gap Insulated Microstructures (AIM) with strengthened interconnection beams based on standard single crystal silicon wafers. The main focus on the new development was set on the attributes of reliability and fatigue. As a result of our investigations, the interconnection beams were identified as weakest point in the system. To improve the quality of the beams, several material stacks with well defined properties were tested in order to find a suitable material stack for the interconnection beams instead of pure aluminum. The new process flow enables the use of layered structured beams without loosing any of the advantages of the AIM technology and also without increasing the number of masks.
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