Fabrication Of Microsystems Research Articles

Advances in Parylene Adhesive Bonding for the Realization of Biocompatible Microsystems

Wafer to wafer and chip to wafer bonding technologies are a key enabling processes for the fabrication of microsystems. The presented paper focuses on the latest research results of Parylene adhesive bonding. Given the established wafer bonding processes based on Parylene C in vacuum, research on how the bonding process alters the Parylene material was performed using x-ray diffraction. Doing so, a significant increase of the crystalline fraction of Parylene was observed. In order to further develop the Parylene bonding beyond its state of the art, new process variants were successfully established with advanced features: Wafer bonding using Parylene C at increased pressure; Wafer bonding using Parylene N and Parylene F, respectively, to address a different bonding temperature range; as well as chip bonding using Parylene C in air.

Wet Chemical and Plasma Etching of Photosensitive Glass

Photosensitive glasses for radiation-induced 3D microstructuring, due to their optical transparency and thermal, mechanical, and chemical resistance, enable the use of new strategies for numerous microscale applications, ranging from optics to biomedical systems. In this context, we investigated the plasma etching of photosensitive glasses after their exposure and compared it to the established wet chemical etching method, which offers new degrees of freedom in microstructuring control and microsystem fabrication. A CF4/H2 etching gas mixture with a constant volumetric flow of 30 sccm and a variable H2 concentration from 0% to 40% was utilized for plasma-based etching, while for wet chemical etching, diluted hydrofluoric acid (1% ≤ cHF ≤ 20%) was used. Therefore, both etching processes are based on a chemical etching attack involving fluorine ions. A key result is the observed reversion of the etch selectivity between the initial glassy and partially crystallized parts that evolve after UV exposure and thermal treatment. The crystallized parts were found to be 27 times more soluble than the unexposed glass parts during wet chemical etching. During the plasma etching process, the glassy components dissolve approximately 2.5 times faster than the partially crystalline components. Unlike wet chemical etching, the surfaces of plasma etched photostructured samples showed cone- and truncated-cone-shaped topographies, which supposedly resulted from self-masking effects during plasma etching, as well as a distinct physical contribution from the plasma etching process. The influences of various water species on the etching behaviors of the homogeneous glass and partially crystallized material are discussed based on FTIR-ATR and in relation to the respective etch rates and SNMS measurements.

Paradigm Changing Integration Technology for the Production of Flexible Electronics by Transferring Structures, Dies and Electrical Components from Rigid to Flexible Substrates

Emerging trends like the Internet of Things require an increasing number of different sensors, actuators and electronic devices. To enable new applications, such as wearables and electronic skins, flexible sensor technologies are required. However, established technologies for the fabrication of sensors and actuators, as well as the related packaging, are based on rigid substrates, i.e., silicon wafer substrates and printed circuit boards (PCB). Moreover, most of the flexible substrates investigated until now are not compatible with the aforementioned fabrication technologies on wafers due to their lack of chemical inertness and handling issues. In this presented paper, we demonstrate a conceptually new approach to transfer structures, dies, and electronic components to a flexible substrate by lift-off. The structures to be transferred, including the related electrical contacts and packaging, are fabricated on a rigid carrier substrate, coated with the flexible substrate and finally lifted off from the carrier. The benefits of this approach are the combined advantages of using established semiconductor and microsystem fabrication technologies as well as packaging technologies, such as high precision and miniaturization, as well as a variety of available materials and processes together with those of flexible substrates, such as a geometry adaptivity, lightweight structures and low costs.

Metal Additive Microfabrication of System-in-Package with Embedded Microfluidic Thermal Management for Heterogeneous Integration with Semiconductor Dies

The scaling of components on a single semiconductor chip has enabled the doubling of performance every 2 years as per Moore’s law, and has been possible due to the miniaturization of individual components. A significant decrease in transistor gate length and increase in component density has also introduced an increase in power density. Hence, there is a dire need for embedded thermal management of these systems-on-chips (SoCs) to remove waste heat. In this work, the metal additive microfabrication of a system-in-package (SiP) package with multiple pads for quad flat no-lead (QFN) packaging and a microfluidic channel and pin fin heat sinks for thermal management are presented. This work introduces heterogeneous integration (integration of separately manufactured components into a SiP for improved functionality) of SiPs fabricated using additive microfabrication to clean-room-fabricated integrated circuits (ICs) for improved functionality in terms of the thermal management of high-power density SoCs. The packages have been fabricated via a metal additive manufacturing technique, direct metal laser sintering (DMLS), using 316L stainless steel as the precursor powder. The work also opens doors to the fabrication and integration of metal microsystems using DMLS to complement semiconductor manufacturing and packaging processes. The use of air or water as the coolant for the packaging devices decreased the thermal resistance from 5.25°C/W for the conventional packages to 2.02°C/W for a flow rate of 84 mL/min when subjected to a constant heat flux. The average surface temperature also decreased from 45.61°C to 29.23°C for the same parameter. This article details the material characteristics and thermal performance of the metal additively microfabricated SiPs.

Laser technologies for micro and nanofluidics systems

AbstractAmong the various micro and nanofabrication technologies, ultrafast laser processing is a promising tool for the fabrication of micro/nanofluidics due to its flexibility, versatility, high fabrication resolution and three‐dimensional fabrication capability. In this article, we review the progress made to date by companies working with ultrafast laser processing for the fabrication of micro and nanofluidics systems.

Experimental Characterization of a Microfluidic Device Based on Passive Crossflow Filters for Blood Fractionation

The separation of red blood cells (RBCs) from blood plasma and the analysis of individual RBCs are of great importance, as they provide valuable information regarding the health of their donor. Recent developments in microfluidics and microfabrication have contributed to the fabrication of microsystems with complex features to promote the separation and analysis of RBCs. In this work, the separation capacity of a multi-step crossflow microfluidic device was evaluated by using a blood analogue fluid made by Brij L4 micelles and human RBCs separated from whole blood, suspended in a solution with hematocrits (Ht) of 0.5 and 1%. All the samples collected at the outlets of the device were experimentally analyzed and compared. The absorbance spectrum was also measured for the prepared blood samples. The results indicate that the tested blood analogue fluid has exhibited a flow behavior similar to that of blood. In addition, the optical absorbance spectrophotometry revealed that it was possible to evaluate the separation efficiency of the microfluidic device, concluding that the concentration of cells was lower at the most lateral outside outlets of the microchannel due to the cumulative effect of the multiple cross-flow filters.

Large Stepwise Discrete Microsystem Displacements Based on Electrostatic Bending Plate Actuation

We present the design, fabrication, and experimental characterization of microsystems achieving large and stepwise discrete displacements. The systems consist of electrostatic bending plate actuators linked in a chain with increasing electrode gaps to allow a stepwise system displacement. A derived analytic transfer function permits to evaluate the influence of the system components on both the total and the stepwise system displacement. Based on calculation and simulation results, systems featuring 5, 8, 10, 13, and 16 steps are modeled and fabricated using a dicing-free SOI-fabrication process. During experimental voltage- and time-dependent system characterization, the minimum switching speed of the electrostatic actuators is 1 ms. Based on the guiding spring stiffness and the switching time, step-by-step and collective activations of the microsystems are performed and the system properties are derived. Furthermore, we analyze the influence of the number of steps on the total system displacement and present 16-step systems with a total maximum displacement of 230.7 ± 0.9 µm at 54 V.

Test Structure for Measuring the Selectivity in XeF2 and HF Vapour Etch Processes

Etch selectivity between layers is an important parameter in the fabrication of microelectronics and microsystems. This is particularly true in the case of isotropic gas/vapour etching methods used to release free standing structures through the selective etching of sacrificial layers. Commonly used structural materials have been reported to be largely inert when exposed to a given vapour etchant, indicating high selectivity when measured against typical sacrificial layers. However, there is growing evidence that these structural layers are actually etched at an enhanced rate if they are located in the proximity of the sacrificial layer being removed. Hence, removal rates given in the literature, which have resulted from measurements of layers that have been etched in isolation, can no longer be trusted to characterize critical etch processes in device fabrication. In this paper, a test structure is reported that enables a far more appropriate determination of the etch selectivity between sacrificial and structural materials. The method is demonstrated with the two most common vapour etch processes. Firstly, the XeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> vapour etch of a polysilicon sacrificial layer located above a silicon nitride structural layer, and secondly, the HF vapour etch of silicon dioxide placed above a silicon nitride structural layer. Both test structure datasets are presented. The polysilicon and silicon nitride layers, etched with XeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> show a selectivity of 5:4. The silicon dioxide and silicon nitride layers etched with HF, show a selectivity of 6:1 to 8:1.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process that is used in the fabrication of electronic, optical and mechanical microsystems. The chemical bonds are providing an intimate surface contact and strong structural stiffness that is for this reason also suitable for large assemblies. Together with the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems.The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double Super Polished and near perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component.We want to use such optical components for a ~3m diameter segmented optical system that would be part of the Athena observatory, European Space Agency's (ESA) next L-class mission, scheduled to launch in 2032. Athena will observe the hot and energetic Universe and answer two big scientific questions: How does ordinary matter assemble into the large-scale structures we see today? and how do black holes grow and shape the Universe?The scientific objectives of Athena require the largest X-ray optic ever flown. Silicon Pore Optics (SPO) is the selected technology that produces the building blocks for Athena's modular mirror. The segmented Athena optic will be made of about 700 X-ray mirror modules all co-aligned to a common focus, with radii of curvature varying from 0.25 m to 1.25 m. Furthermore, SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research.While considerable progress has been made on the performance of the optic, we are also upscaling towards an automated production line including mirror manufacturing, coating, cleaning, stacking and stack integration into mirror modules. The complete production line features many metrology steps to ensure quality and high yield. From the start, the development process has looked not just at the production of a single sample of high quality but at all the processes eventually required for the production of a complete system, and this in an effort to ensure that issues (mechanical, thermal, environment, supply chain etc) are identified early and addressed in a manner compatible with the core goal of producing high quality optics.In this paper we will discuss the challenges and present solutions that we have addressed. Cleanliness assessment, and bond strength enhancement are for instance two current topics of active development.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process used in the fabrication of electronic, optical and mechanical microsystems. Chemical bonds are providing an intimate surface contact and strong structural stiffness that is, for this reason, also suitable for large assemblies. Based on the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems. The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double-sided super polished and nearly perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools, the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component. We want to use such optical components for a ~3 m diameter segmented optical system that would be part of the Athena observatory.

Temporal-contrast imperfections as drivers for ultrafast laser modifications in bulk silicon

This paper uncovers the role of the temporal contrast in achieving internal modifications in semiconductors with ultrafast lasers and explores how it may influence three dimensional fabrication of monolithic silicon microsystems.

Integrated sensing and delivery of oxygen for next-generation smart wound dressings

Chronic wounds affect over 6.5 million Americans and are notoriously difficult to treat. Suboptimal oxygenation of the wound bed is one of the most critical and treatable wound management factors, but existing oxygenation systems do not enable concurrent measurement and delivery of oxygen in a convenient wearable platform. Thus, we developed a low-cost alternative for continuous O2 delivery and sensing comprising of an inexpensive, paper-based, biocompatible, flexible platform for locally generating and measuring oxygen in a wound region. The platform takes advantage of recent developments in the fabrication of flexible microsystems including the incorporation of paper as a substrate and the use of a scalable manufacturing technology, inkjet printing. Here, we demonstrate the functionality of the oxygenation patch, capable of increasing oxygen concentration in a gel substrate by 13% (5 ppm) in 1 h. The platform is able to sense oxygen in a range of 5–26 ppm. In vivo studies demonstrate the biocompatibility of the patch and its ability to double or triple the oxygen level in the wound bed to clinically relevant levels.

A Novel Lab-on-a-Chip Microdevice for Study the Effectiveness of Electrochemotherapy

Introduction Lab-on-a-Chip systems are innovative tools which can be used in the field of life sciences. They find applications e.g. in single cell analysis or cytotoxicity tests [1]. Despite the growing popularity of the use of microsystems in biological research, there is a lack of microsystems that may be used in the electrochemotherapy (ECT) studies. ECT is a antitumor therapy, based on an application of electroporation (EP) during standard chemotherapy (CT). EP uses external electric field to form hydrophilic pores in the cells membrane. Electropores are additional migration pathway for molecules which enhanced their delivery into cells [2]. ECT allows to use lower concentration of drug and reduces side effects in comparison to standard chemotherapy [3]. We develop a Lab-on-a-Chip microsystem for cell electroporation that could be used to examine the effectiveness of chemotherapy as well as for evaluation the effectiveness of electrochemotherapy. Microsystem fabrication The microsystem is made of polydimethylsiloxane (PDMS) and glass. Casting method was used to obtained the microchannels and microchambers in PDMS layer. Pairs of gold electrodes were arranged parallelly along the microchannel with microchambers at the distance of 2 mm. The microsystem allows simultaneous culturing of normal and tumor cells. There are four rows of microchambers for each cell line: I - cells not exposed to compound or electric field (control), II - electroporated cells not exposed to compound (control for EP), III – cells electroporated with compound (simulating condition of ECT), IV - cells incubated with compound (simulating condition of CT). In this way, it is possible to evaluate and compare the effectiveness of two types of therapeutic procedures. Method The microchip was sterilized using 70% ethanol and UV radiation. After that, the cell suspension was introduced using a peristaltic pump at a speed of 3.5 µl/min. After 24 h of incubation, the cells medium (control) and the solution of the test compound were respectively introduced into the microsystem, nextly the cells were electroporated. Cell observations were performed using an inverted fluorescence microscope. In addition, fluorescence intensity measurements of the introduced molecules were carried out using a multi-well plate reader. The AlamarBlue test was performed to determine cell viability. For this purpose, a 10% AlamarBlue solution was introduced into the microsystem, than fluorescence intensity was measured at λex=558 nm and λem= 585 nm. Results and Conclusions To determine the optimal electroporation parameters (pulse length, their number, voltage) preliminary experiments were led using propidium iodide (PI). Tests were carried out for two skin cell lines: normal HaCaT and tumor A375. Two sets of parameters were examined: 1 pulse 10 ms and 8 pulses 0.1 ms, each in three voltage variants: 150, 180 and 200V. Cell viability after electroporation was determined. It was found that there were no significant changes in the cell viability after electroporation with the voltage lower than 200V. The efficiency of PI delivery into cells was confirmed by microscopic observation as well as determined by fluorescence intensity measurements. Significantly lower PI level inside cells (at the level of 30%) using 8 pulses 0.1 ms for both cell lines was observed. The best efficiency of PI delivery (about 90%) was observed when 1 pulse of 180V lasting 10 ms was applied. In addition, cell morphology was observed and cells parameters such as: shape factor, sphericity, convexity and elongation were determined. It was confirmed that the electroporation of cells does not change their morphology. Based on the obtained results it was concluded that the optimal electroporation conditions for HaCaT and A375 cell lines are: 1 pulse 10 ms 180V.

Competing Nonlinear Delocalization of Light for Laser Inscription Inside Silicon with a 2- µ m Picosecond Laser

Recent advances in ultrafast laser 3D writing offer direct fabrication of in-chip silicon microsystems, but this technique remains extremely challenging, because of propagation nonlinearities that limit energy localization inside narrow-band-gap materials. This study examines the complex energy flux and subsequent unusual modification of silicon due to intense picosecond pulses from a thulium-doped fiber laser. The best conditions for reliable data inscription deep inside silicon are identified, which is a critical step toward engineering solutions to this problem. Also, the wavelength used here points to facing the next challenges associated with materials of even narrower band gap.

Atomic Layer Deposition for Membranes, Metamaterials, and Mechanisms.

Bending and folding techniques such as origami and kirigami enable the scale-invariant design of 3D structures, metamaterials, and robots from 2D starting materials. These design principles are especially valuable for small systems because most micro- and nanofabrication involves lithographic patterning of planar materials. Ultrathin films of inorganic materials serve as an ideal substrate for the fabrication of flexible microsystems because they possess high intrinsic strength, are not susceptible to plasticity, and are easily integrated into microfabrication processes. Here, atomic layer deposition (ALD) is employed to synthesize films down to 2 nm thickness to create membranes, metamaterials, and machines with micrometer-scale dimensions. Two materials are studied as model systems: ultrathin SiO2 and Pt. In this thickness limit, ALD films of these materials behave elastically and can be fabricated with fJ-scale bending stiffnesses. Further, ALD membranes are utilized to design micrometer-scale mechanical metamaterials and magnetically actuated 3D devices. These results establish thin ALD films as a scalable basis for micrometer-scale actuators and robotics.

Characterisation of graphene electrodes for microsystems and microfluidic devices

Fabrication of microsystems is traditionally achieved with photolithography. However, this fabrication technique can be expensive and non-ideal for integration with microfluidic systems. As such, graphene fabrication is explored as an alternative. This graphene fabrication can be achieved with graphite oxide undergoing optical exposure, using optical disc drives, to impose specified patterns and convert to graphene. This work characterises such a graphene fabrication, and provides fabrication, electrical, microfluidic, and scanning electron microscopy (SEM) characterisations. In the fabrication characterisation, a comparison is performed between traditional photolithography fabrication and the new graphene fabrication. (Graphene fabrication details are also provided.) Here, the minimum achievable feature size is identified and graphene fabrication is found to compare favourably with traditional photolithography fabrication. In the electrical characterisation, the resistivity of graphene is measured as a function of fabrication dose in the optical disc drive and saturation effects are noted. In the microfluidic characterisation, the wetting properties of graphene are shown through an investigation of the contact angle of a microdroplet positioned on a surface that is treated with varying fabrication dose. In the SEM characterisation, the observed effects in the previous characterisations are attributed to chemical or physical effects through measurement of SEM energy dispersive X-ray spectra and SEM images, respectively. Overall, graphene fabrication is revealed to be a viable option for development of microsystems and microfluidics.

Gas molecule modulated ionic migration through graphene oxide laminates

Up to date, an increasing number of carbon-based nanomaterials have provided routes for the fabrication of smart and high-performance analytical microsystems. Graphene oxide (GO) as one promising carbon-based material has aroused much interest due to the moderate GO water dispersibility and reactive sites for further modification. In this study, free-standing laminar GO membrane was obtained on the tip of glass nanopipette by electrophoresis. Scanning electron microscope (SEM), optimal microscope and electrochemical method were used to determine the assembly process, respectively. Stable and obvious rectification change shows that gas molecule can modulate ion migration owing to the change of charge density between GO sheets. Such results may pave the way for development of gas differentiation membrane, and study on fundamental mechanism.

Repulsive magnetic field–assisted laser-induced plasma micromachining for high-quality microfabrication

Surface micro-/nanostructures are widely used in the fabrication of various functional microsystems. Laser-induced plasma micromachining can greatly improve surface quality in terms of recast layers and thermal defects compared with laser direct writing. Magnetic field has the ability to constrain plasma diffusion and can ensure the stability of laser-induced plasma processing. This paper compares the effects of laser direct–writing processing and laser-induced plasma processing of single-crystal silicon at the micro-/nanoscale, and emphatically analyzes the material removal mechanism of repulsive magnetic field–assisted laser-induced plasma micromachining. It is shown that the volume of the laser-induced plasma was constrained under the influence of Lorentz force, a high-quality smooth microgroove without thermal defects was obtained, and its line width was reduced by 30%.

Material selection for Micro-Electro-Mechanical-Systems (MEMS) using Ashby's approach

A key aspect in design optimization of a product or a system is the selection of materials that best meet the design needs, ensuring maximum performance and minimum cost. Ashby's approach, originally introduced for macro-systems and products, has been very successfully employed for Micro-Electro-Mechanical-Systems (MEMS)/micromachined sensors, actuators and devices. This paper presents a comprehensive review and critical analysis of MEMS material selection studies using Ashby's approach reported in the literature during the last two decades. Performance and Material Indices derived for various microsystems and MEMS devices have been summarized. Moreover, all MEMS materials reported in the literature and the most suitable materials proposed for a variety of MEMS systems and devices have also been consolidated. A material selection case study utilizing micro-scale properties of 51 MEMS compatible materials has been presented to demonstrate that the use of different materials' bulk properties is not the best choice for MEMS materials selection. This paper will serve as a reference guide and useful resource for researchers and engineers engaged in the design and fabrication of various microsystems and MEMS sensors, actuators and devices.

Liquid PMMA: A High Resolution Polymethylmethacrylate Negative Photoresist as Enabling Material for Direct Printing of Microfluidic Chips

Polymethymethacrylate (PMMA) is one of the most important thermoplasts and a commonly used material in microsystem fabrication, for example, microfluidics owning mainly to its optical transparency, biocompatibility, low autofluorescence, and low cost. However, being a thermoplastic material PMMA is typically structured using industrial replication techniques making PMMA unsuitable for rapid prototyping. The fact that neither material nor processing technique can be directly transferred from laboratory to industrial state makes the research‐to‐business conversion often extremely difficult in microfluidics since material properties have a major impact on the final system behavior. This paper presents “Liquid PMMA,” a fast curing viscous PMMA prepolymer which can be used as a negative photoresist and directly structured using ultraviolet or visible light with tens of micron resolution and smooth surfaces. Using this technique microfluidic chips in PMMA can be fabricated within minutes. The cured Liquid PMMA parts show the same high optical transparency, low autofluorescence, and surface properties like commercial PMMA. In this way, microfluidic chips can be rapidly developed and optimized on the laboratory scale in the same material which is later on used on the industrial scale.