Glass is in general brittle and therefore usually cannot sustain large deformation. Recent advances in glass material development as well as micro-mechanical designs, however, are set to defy the conventional wisdom through the demonstration of flexible integrated photonics that can be bent, twisted, and even stretched without compromising its structural integrity and optical performance. In this paper, we review the latest progress in this emerging field, and discuss the rational material and mechanical engineering principles underlying the extraordinary flexibility of these photonic structures. Leveraging these design strategies, we demonstrated bendable chalcogenide glass waveguide circuits, flexible glass waveguide-integrated nanomembrane photodetectors, and stretchable glass photonics.

A new twist on glass: A brittle material enabling flexible integrated photonics

Silk fibroin is an excellent candidate for biomedical implantable devices because of its biocompatibility, controllable biodegradability, solution processability, flexibility, and transparency. Thus, fibroin has been widely explored in biomedical applications as biodegradable films as well as functional microstructures. Although there exists a large number of patterning methods for fibroin thin films, multilayer micropatterning of fibroin films interleaved with metal layers still remains a challenge. Herein, we report a new wafer-scale multilayer microfabrication process named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials (e.g., metal and dielectrics) with high-precision microscale alignment. To the best of our knowledge, our AMoS process is the first demonstration of wafer-scale multilayer processing of both silk fibroin and metal micropatterns. In the AMoS process, aluminum deposited on fibroin is first micropatterned using conventional ultraviolet (UV) photolithography, and the patterned aluminum layer is then used as a mask to pattern fibroin underneath. We demonstrate the versatility of our fabrication process by fabricating fibroin microstructures with different dimensions, passive electronic components composed of both fibroin and metal layers, and functional fibroin microstructures for drug delivery. Furthermore, because one of the crucial advantages of fibroin is biocompatibility, we assess the biocompatibility of our fabrication process through the culture of highly susceptible primary neurons. Because the AMoS process utilizes conventional UV photolithography, the principal advantages of our process are multilayer fabrication with high-precision alignment, high resolution, wafer-scale large area processing, no requirement for chemical modification of the protein, and high throughput and thus low cost, all of which have not been feasible with silk fibroin. Therefore, the proposed fabrication method is a promising candidate for batch fabrication of functional fibroin microelectronics (e.g., memristors and organic thin film transistors) for next-generation implantable biomedical applications.

Wafer-Scale Multilayer Fabrication for Silk Fibroin-Based Microelectronics.

Experimental and numerical investigations on the tensile behavior of 3D random fibrous materials at elevated temperature

Investigations on the compressive behavior of 3D random fibrous materials at elevated temperatures

Cantilever arrays are employed to increase the throughput of imaging and manipulation at the nanoscale. We present a fabrication process to construct cantilever arrays with nanotips that show a uniform tip–sample distance. Such uniformity is crucial, because in many applications the cantilevers do not feature individual tip–sample spacing control. Uniform cantilever arrays lead to very similar tip–sample interaction within an array, enable non-contact modes for arrays and give better control over the load force in contact modes. The developed process flow uses a single mask to define both tips and cantilevers. An additional mask is required for the back side etch. The tips are self-aligned in the convex corner at the free end of each cantilever. Although we use standard optical contact lithography, we show that the convex corner can be sharpened to a nanometre scale radius by an isotropic underetch step. The process is robust and wafer-scale. The resonance frequencies of the cantilevers within an array are shown to be highly uniform with a relative standard error of 0.26% or lower. The tip–sample distance within an array of up to ten cantilevers is measured to have a standard error around 10 nm. An imaging demonstration using the AFM shows that all cantilevers in the array have a sharp tip with a radius below 10 nm. The process flow for the cantilever arrays finds application in probe-based nanolithography, probe-based data storage, nanomanufacturing and parallel scanning probe microscopy.

https://iopscience.iop.org/article/10.1088/0957-4484/23/13/135301/pdf

Cantilever arrays with self-aligned nanotips of uniform height.

This paper proposes a microfabrication process for the reliable release of SiO2 beam structures. These structures are intended to be utilized in SiO2 photonic MEMS. A major fabrication challenge is the release of thick (>10 ?m) SiO2 structures with high yield. A single mask process is developed based on temporary reinforcement of the SiO2 structure. A supporting layer of Si functions as a reinforcing layer during etching and release, thereby enabling a high fabrication yield. Furthermore, the process allows to create structures of which the final Si support thickness is configurable from tens of micrometers to zero, thereby providing additional design freedom. The fabrication process is tested on a silicon wafer with a ?15 ?m thick thermal oxide layer. The obtained suspended structures are mechanically characterized. Two deformation effects can be distinguished: a curvature of the beam and a slope at the base of the beam. These effects are caused by the compressive mean stress and the gradient stress in the thermal SiO2. The curvature of the SiO2–Si beams corresponds to a concave downward profile while the SiO2 beams without supporting Si reveal a small curvature in the opposite direction (concave upward). The slope at the base is approximately ?0.5° for the SiO2 beams and between ?0.5° and 0° for the SiO2–Si beams. The acquired bending stiffness of long SiO2 beams is in the newton per meter range (e.g. 0.8 N m?1 for a cantilever measuring 1000 ?m in length and 13 ?m in width).

https://iopscience.iop.org/article/10.1088/0960-1317/25/10/105003/pdf

Fabrication and characterization of suspended beam structures for SiO2 photonic MEMS

This paper proposes and tests a design of electro-thermal bimorph actuators for alignment of flexible photonic waveguides fabricated in 16 µm thick SiO2. The actuators are for use in a novel alignment concept for multi-port photonic integrated circuits (PICs), in which the fine alignment is taken care of by positioning of suspended, mechanically flexible waveguide beams on one or more of the PICs. The design parameters of the bimorph actuator allow to tune both the initial relative position of the waveguide end-facets, and the motion range of the actuators. Bimorph actuators have been fabricated and characterized. The maximum out-of-plane deflection of the bimorph actuator (with 720 μm-long poly-Si) can reach 18:5 μm with 126:42mW, sufficient for the proposed application.

/pdf/bimorph-actuators-in-thick-sio-2-for-photonic-alignment-4d8ipgtuw3.pdf

Bimorph actuators in thick SiO 2 for photonic alignment

This paper presents a new alignment concept for the alignment of multichannel photonic intergrated circuits (PICs) using flexible photonic waveguides on one of the PICs that are positionable by integrated micro electro mechanical system (MEMS) actuators. The concept aims for high precision and high degree of assembly process automation. The proposed concept includes pre-alignment of both PICs on a common substrate followed by fine-alignment using the on-chip flexible waveguides and MEMS functionality. This paper introduces the alignment approach and reports on the development and fabrication of suspended and mechanically flexible photonic waveguides. Single suspended waveguide beams and suspended arrays with two and four coupled parallel waveguide beams of different lengths (250 ?mto 1000 ?m) and different widths (18 ?mto 34 ?m) are designed and fabricated. After fabrication, waveguide beam fracturing is observed. The fabrication process has been extended by an additional under-etching step in order to reduce beam fracturing. The static out-of-plane deflection of the fabricated devices follows a specific profile with a dominating upward curvature resulting in a measured maximum out-of-plane deflection of 2% of the length. The beam stiffness of the fabricated devices is measured and proves to be within the available force of microactuators.

/pdf/suspended-photonic-waveguide-arrays-for-submicrometer-3u2lu7mvnq.pdf

Suspended photonic waveguide arrays for submicrometer alignment

This paper reports on the progress related to a multichannel photonic alignment concept, which aims to achieve submicrometer alignment of the waveguides of two photonic integrated circuits (PICs). The concept consists of two steps: chip-to-chip positioning and fixing provide a coarse alignment after which waveguide-to-waveguide positioning and fixing result in a fine alignment. For the waveguide-to-waveguide alignment, mechanically flexible waveguides are used. Positioning of the waveguides is performed by integrated MEMS actuators. The flexible waveguides and the actuators are both integrated in one of the PICs. This paper reports on the fabrication and the mechanical characterization of the suspended waveguide structures. The flexible waveguide array is created in a PIC which is based on TriPleX technology, i.e. a silicon nitride (Si 3 N 4 ) core encapsulated in a silicon dioxide (SiO 2 ) cladding. The realized flexible waveguide structures consist of parallel cantilevered waveguide beams and a crossbar that connects the free ends of the waveguide beams. The fabrication of suspended structures consisting of a thick, i.e. 15 µm, TriPleX layer stack is challenged by the compressive mean stress in the SiO 2 . We have developed a fabrication method for the reliable release of flexible TriPleX structures, resulting in a 96% yield of cantilever beams. The realized suspended waveguide arrays have a natural out-of-plane deformation, which is studied using white light interferometry. Suspended waveguide beams reveal a downward slope at the base of the beams close to 0:5_. In addition to this slope, the beams have a concave upward profile. The constant curvature over the length of the waveguide beams is measured to range from 0:2 µm to 0:8 µm. The profiles measured over the length of the crossbars do not seem to follow a circular curvature. The variation in deflection within crossbars is measured to be smaller than 0:2 µm.

/pdf/mechanically-flexible-waveguide-arrays-for-optical-chip-to-vfoqz95xnn.pdf

Tjitte-Jelte Peters

Papers

Cantilever arrays with self-aligned nanotips of uniform height.

Fabrication and characterization of suspended beam structures for SiO2 photonic MEMS

Bimorph actuators in thick SiO 2 for photonic alignment

Suspended photonic waveguide arrays for submicrometer alignment

Mechanically flexible waveguide arrays for optical chip-to-chip coupling