Standard Cleanroom Processes Research Articles

Conformable neural interface based on off-stoichiometry thiol-ene-epoxy thermosets

Off-stoichiometry thiol-ene-epoxy (OSTE+) thermosets show low permeability to gases and little absorption of dissolved molecules, allow direct low-temperature dry bonding without surface treatments, have a low Young's modulus, and can be manufactured via UV polymerisation. For these reasons, OSTE+ thermosets have recently gained attention for the rapid prototyping of microfluidic chips. Moreover, their compatibility with standard clean-room processes and outstanding mechanical properties make OSTE+ an excellent candidate as a novel material for neural implants. Here we exploit OSTE+ to manufacture a conformable multilayer micro-electrocorticography array with 16 platinum electrodes coated with platinum black. The mechanical properties allow conformability to curved surfaces such as the brain. The low permeability and strong adhesion between layers improve the stability of the device. Acute experiments in mice show the multimodal capacity of the array to record and stimulate the neural tissue by smoothly conforming to the mouse cortex. Devices are not cytotoxic, and immunohistochemistry stainings reveal only modest foreign body reaction after two and six weeks of chronic implantation. This work introduces OSTE+ as a promising material for implantable neural interfaces.

High‐Performance Monolayer MoS2 Field‐Effect Transistors on Cyclic Olefin Copolymer‐Passivated SiO2 Gate Dielectric

AbstractTrap states of the semiconductor/gate dielectric interface give rise to a pronounced subthreshold behavior in field‐effect transistors (FETs) diminishing and masking intrinsic properties of 2D materials. To reduce the well‐known detrimental effect of SiO2 surface traps, this work spin‐coated an ultrathin (≈5 nm) cyclic olefin copolymer (COC) layer onto the oxide and this hydrophobic layer acts as a surface passivator. The chemical resistance of COC allows to fabricate monolayer MoS2 FETs on SiO2 by standard cleanroom processes. This way, the interface trap density is lowered and stabilized almost fivefold, to around 5 × 1011 cm−2 eV−1, which enables low‐voltage FETs even on 300 nm thick SiO2. In addition to this superior electrical performance, the photoresponsivity of the MoS2 devices on passivated oxide is also enhanced by four orders of magnitude compared to nonpassivated MoS2 FETs. Under these conditions, negative photoconductivity and a photoresponsivity of 3 × 107 A W−1 is observed which is a new highest value for MoS2. These findings indicate that the ultrathin COC passivation of the gate dielectric enables to probe exciting properties of the atomically thin 2D semiconductor, rather than interface trap dominated effects.

A Peristaltic Micropump Based on the Fast Electrochemical Actuator: Design, Fabrication, and Preliminary Testing

Microfluidic devices providing an accurate delivery of fluids at required rates are of considerable interest, especially for the biomedical field. The progress is limited by the lack of micropumps, which are compact, have high performance, and are compatible with standard microfabrication. This paper describes a micropump based on a new driving principle. The pump contains three membrane actuators operating peristaltically. The actuators are driven by nanobubbles of hydrogen and oxygen, which are generated in the chamber by a series of short voltage pulses of alternating polarity applied to the electrodes. This process guaranties the response time of the actuators to be much shorter than that of any other electrochemical device. The main part of the pump has a size of about 3 mm, which is an order of magnitude smaller in comparison with conventional micropumps. The pump is fabricated in glass and silicon wafers using standard cleanroom processes. The channels are formed in SU-8 photoresist and the membrane is made of SiNx. The channels are sealed by two processes of bonding between SU-8 and SiNx. Functionality of the channels and membranes is demonstrated. A defect of electrodes related to the lift-off fabrication procedure did not allow a demonstration of the pumping process although a flow rate of 1.5 µL/min and dosage accuracy of 0.25 nL are expected. The working characteristics of the pump make it attractive for the use in portable drug delivery systems, but the fabrication technology must be improved.

Rapid lipid bilayer membrane formation on Parylene coated apertures to perform ion channel analyses

We present a chip design allowing rapid and robust lipid bilayer (LBL) membrane formation using a Parylene coated thin silicon nitride aperture. After bilayer formation, single membrane channels can be reconstituted and characterized by electrophysiology. The ability for robust reconstitution will allow parallelization and enhanced screening of small molecule drugs acting on or permeating across the membrane channel. The aperture was realized on a microfabricated silicon nitride membrane by using standard clean-room fabrication processes. To ensure the lipid bilayer formation, the nitride membrane was coated with a hydrophobic and biocompatible Parylene layer. We tested both Parylene-C and Parylene-AF4. The contact angle measurements on both Parylene types showed very good hydrophobic properties and affinity to lipids. No precoating of the Parylene with an organic solvent is needed to make the aperture lipophilic, in contradiction to Teflon membranes. The chips can be easily placed in an array utilizing a 3D printed platform. Experiments show repetitive LBL formation and destruction (more than 6 times) within a very short time (few seconds). Through measurements we have established that the LBL layers are very thin. This allows the investigation of the fusion process of membrane proteins i.e. outer membrane protein (OmpF) in the LBL within a few minutes.

Amorphous Silicon Self‐Rolling Micro Electromechanical Systems: From Residual Stress Control to Complex 3D Structures

In surface micromachining, the control of the residual stress of the films is crucial, as excessive stress can cause buckling, cracking, and peeling of the materials and fracture of the devices. However, the gradients of stress across the thickness of individual films or the stress mismatch in composite structural layers can be exploited to obtain highly complex 3D structures via self‐assembly, self‐bending, self‐positioning, or self‐rolling processes. Herein, the thin‐film residual stresses of amorphous silicon, TiW, AlSiCu, Cr, Au, and SiO2 are assessed using polymer substrates and tuned. Then, a stress‐mismatched bi‐layer of amorphous silicon (tensile) and TiW (compressive) is used in the fabrication of complex 3D self‐rolling micro electromechanical systems (MEMS) structures. The freestanding regions of the fabricated MEMS have a characteristic radius of curvature of ≈100 μm. Current‐induced thermal actuation of a suspended plate is demonstrated. The microfabrication process and materials used are compatible with standard cleanroom processing and with a variety of substrates. In the future, amorphous silicon photodiodes, photoconductors, or other semiconductor devices can be incorporated on complex MEMS fabricated by self‐rolling/self‐bending processes and applied to localized optical and electrical sensing.

A high bandwidth microelectromechanical system-based nanopositioner for scanning tunneling microscopy

Limited Z-axis bandwidth of piezotube scanners employed in conventional Scanning Tunneling Microscopes (STMs) has been a major limiting factor in achieving high scan speeds in STM applications. Slow Z-axis dynamics of typical piezotube scanners combined with the weight of the STM tip/tip holder assembly, that the scanner has to carry, substantially limit the achievable Z-axis bandwidth in both imaging and lithography modes. To tackle this issue, we propose a high bandwidth microelectromechanical-system-based nanopositioner to be integrated into an existing STM scanner. The device is designed to replace the STM tip and fine Z-positioning mechanisms in the conventional STM setup, while providing an order of magnitude higher bandwidth in Z axis. The device is microfabricated using double silicon-on-isolator technology, and standard cleanroom processes. Experiments show that tunneling current between the device tip and a highly ordered pyrolytic graphite sample can be successfully established and maintained in air using the proposed device in a feedback loop. Results indicate that the proposed device uniquely combines a very high resolution and a large stroke with a substantially larger Z-axis bandwidth compared to that of conventional STM piezotube scanners, enabling higher scanning speeds in STM operations.

A passive two-way microfluidic device for low volume blood-plasma separation

Blood-plasma separation is one of the main concerns for Point-of-Care devices seeking to retain cells of interest or analyze plasma biomarkers. The present work describes a novel passive device based on size-exclusion method. Starting from a particulate-trap-like filter, numerical simulations in ANSYS Fluent were performed in order to optimize the geometry for blood-plasma separation. The final layout was then embedded inside a microfluidic channel and the related master produced with standard cleanroom processes. A 1 mm thick Cyclic Olefin Copolymer wafer was chosen for hot embossing replication, since Cyclic Olefin Copolymer is a transparent, solvent-resistant and medical-grade material. Moreover, the adopted process is easily scalable to industrial levels. The filtering efficiency was finally assessed with a 4 μm fluorescent beads dispersion, mimicking the erythrocytes average diameter of 8 μm. No fluorescent particles were observed downstream of the filter after filtration, thus all the beads were retained by the optimized filtering structure.

Permanent reduction of dissipation in nanomechanical Si resonators by chemical surface protection

We report on mechanical dissipation measurements carried out on thin (∼100 nm), single-crystal silicon cantilevers with varying chemical surface termination. We find that the 1–2 nm-thick native oxide layer of silicon contributes about 85% to the friction of the mechanical resonance. We show that the mechanical friction is proportional to the thickness of the oxide layer and that it crucially depends on oxide formation conditions. We further demonstrate that chemical surface protection by nitridation, liquid-phase hydrosilylation, or gas-phase hydrosilylation can inhibit rapid oxide formation in air and results in a permanent improvement of the mechanical quality factor between three- and five-fold. This improvement extends to cryogenic temperatures. Presented recipes can be directly integrated with standard cleanroom processes and may be especially beneficial for ultrasensitive nanomechanical force- and mass sensors, including silicon cantilevers, membranes, or nanowires.

Bulk-Aluminum Microfabrication for Micro Fuel Cells

We present a simple method for microfabricating microfluidic devices by wet etching of bulk aluminum wafers. Aluminum wafers are identical to silicon wafers in terms of dimensions and can easily be processed in standard clean room processes like lithography, etching, and thin film deposition. Aluminum thin film wet etching in phosphoric acid-based etchants is well established. In this paper, it is extended to much greater depths than before. 80 μm deep structures have been fabricated. A hydrogen-fueled micro fuel cell has been microfabricated from bulk aluminum. These fuel cells achieved very high current densities of 1.1 A cm-2 and power density up to 228 mW cm-2. Considering the simplicity of bulk aluminum micromachining, these results are very encouraging.

Facile Fabrication of Single‐Crystal‐Diamond Nanostructures with Ultrahigh Aspect Ratio

A robust and facile approach for making single-crystal-diamond MEMS and NEMS devices is presented. The approach relies entirely on commercial diamond material and standard cleanroom processes. As an example, batch fabrication of cantilever beams of thickness down to 45 nm and aspect ratios exceeding 2000:1 is demonstrated.

In Situ Tuning of Focused-Ion-Beam Defined Nanomechanical Resonators Using Joule Heating

Nanomechanical resonators have a huge potential for a variety of applications, including high-resolution mass sensing. In this paper, we demonstrate a novel rapid prototyping method for fabricating nanoelectromechanical systems using focused-ion-beam milling as well as in situ electromechanical characterization using a transmission electron microscope. Nanomechanical resonators were cut out of thin membrane chips, which have been prefabricated using standard cleanroom processing. We have demonstrated the fabrication of double-clamped beams with feature sizes down to 200 nm using a fabrication time of 30 min per device. Afterwards, the dynamic and structural properties of a double-clamped beam were measured after subsequent Joule heating events in order to ascertain the dependence of the internal structure on the Q-factor and resonant frequency of the device. It was observed that a change from amorphous to polycrystalline silicon structure significantly increased the resonant frequency as well as the Q-factor of the nanomechanical resonator. Aside from allowing detailed studies of the correlation between internal structure and nanomechanical behavior on an individual rather than a statistical basis, the combination of a short turnaround time and in situ nonlithographic tuning of the properties provide a flexible approach to the development and prototyping of nanomechanical devices.

Design and development of a novel paddle test structure for the mechanical behavior measurement of thin films application for MEMS

A new technique was developed for studying the mechanical behavior of thin films on substrate applications for micro-electro-mechanical system (MEMS). The test structure was designed on novel “paddle” cantilever beam coated thin film specimens with dimensions of a few hundred to 50 nm. This beam has a triangle shape that provides a uniform plane strain distribution. Standard clean room processing was used to prepare the paddle sample. The experiment can be operated using the electrostatic force to deflect the “paddle” cantilever beam and measure the mechanical response of the sample with surface deposited thin film. A capacitance measurement is used to observe the deflection of the cantilever plate on the other side of the sample with respect to the electrostatic force on the one side. The measured strain was then converted through this capacitance measurement to conduct mechanical behavior studies on the coated thin film. Both system performance experiments and calculations were studied to verify the design concepts. The residual thin film stress measurements were performed and compared with the calculated results from three different forces exerted on the “paddle” cantilever beam, including the force due to the film, compliance force, and electrostatic force.

Infrared absorber for pyroelectric detectors

A quarter-wavelength absorber structure was developed for pyroelectric detector applications in the 2.5–3.5μm range. The absorber consisting of Pt and plasma-enhanced chemical vapor deposited SiO2 layers was fabricated using standard clean-room processes. It exhibited a minimum reflectance of 2%.

Batch fabrication of nanotubes suspended between microelectrodes

We report a fabrication method, which uses standard UV-lithography to pattern the catalyst for the chemical vapour deposition (CVD) of suspended double clamped single walled carbon nanotubes. By using an aqueous solution of Fe(NO 3) 3 the patterning of the catalyst material onto microelectrodes can be done with a simple lift-off process with standard photolithographic resist. An applied electric field is sustained between the microelectrodes during CVD to guide the nanotube growth. Comparison with simulations shows that the location and the orientation of the grown carbon nanotubes (CNT) correspond to the regions of maximum electric field, enabling accurate positioning of a nanotube by controlling the shape of the microelectrodes. The CNT bridges are deflected tens of nm when a DC voltage is applied between the nanotube and a gate microelectrode indicating that the clamping through the catalyst particles is not only mechanically stable but also electrical conducting. This method could be used to fabricate nanoelectromechanical systems based on suspended double clamped CNTs depending only on photolithography and standard Cleanroom processes.

Technology for Fabrication of Nanostructures by Standard Cleanroom Processing and Nanoimprint Lithography

Sub-micron structures are routinely fabricated by electron beam lithography (EBL). However EBL is a time consuming and costly technology. We present a technology for fabrication of nanostructures by standard UV-lithography and thermal nanoimprint lithography (NIL). NIL-stamps with sub-30 nm patterns are fabricated by standard micrometer resolution cleanroom processing, i.e. UV-lithography, reactive ion etching and thermal oxidation, and the pattern is transferred to a polymer thin film on a substrate by NIL. Subsequently the patterned polymer film is used either as a direct etching mask to transfer the pattern to the substrate or as a metal lift-off mask. This way we have demonstrated the fabrication of sub-100 nm nanochannels in silicon oxide and sub-50 nm gold lines on silicon.

Miniaturizable Si-based electro-optical modulator working at 1.5 μm

Optoelectronic devices are considered the innovative element for the next generation of microelectronic integrated circuits. For this purpose, both active and passive devices—extremely miniaturized—must be implemented. We fabricated and electro-optical Si-based light intensity modulator working at 1.5 μm using a bipolar mode field-effect transistor integrated within a Si rib waveguide. The principle of operation is the light absorption by a plasma of free carriers that can be opportunely moved inside or outside of the device optical channel by properly changing the control bias. The devices, only 100 μm long, were fabricated using epitaxial Si wafers and standard clean room processing. The optical characterization at 1.48 μm in static conditions shows a modulation of ∼90% while the dynamic electrical characterization provides a switching time of ≈10ns (foreseen modulation frequency of hundreds of MHz). A modulation depth above 25% is observed for modulation frequency up to 300 kHz.

Poly(dimethylsiloxane) microfluidic flow cells for surface plasmon resonance spectroscopy

A fabrication technique has been developed that uses a novolak-based positive photoresist, SPR-220, for fabrication of channels in poly(dimethylsiloxane) (PDMS) that are up to 61 μm deep. Like methods relying on SU-8 (an epoxy-based negative photoresist), the new method enables rapid design and fabrication of microfluidic devices but avoids possible incompatibilities with standard cleanroom processes. The utility of devices fabricated with this technique is demonstrated for use with surface plasmon resonance spectroscopy (SPR). Two microfabricated flow cells having volumes of 336 and 73 nl were used to detect an analyte in bulk solution and deposition of proteins on a surface.