Current trends in miniaturized diagnostics indicate an increasing demand for large quantities of mobile devices for health monitoring and point-of-care diagnostics. This comes along with a need for rapid but preferably also green microfabrication. Dry film photoresists (DFPs) promise low-cost and greener microfabrication and can partly or fully replace conventional silicon-technologies being associated with high-energy demands and the intense use of toxic and climate-active chemicals. Due to their mechanical stability and superior film thickness homogeneity, DFPs outperform conventional spin-on photoresists, such as SU-8, especially when three-dimensional architectures are required for micro-analytical devices (e.g. microfluidics). In this study, we utilize the commercial epoxy-based DFP ADEX to demonstrate various application scenarios ranging from the direct modification of microcantilever beams via the assembly of microfluidic channels to lamination-free patterning of DFPs, which employs the DFP directly as a substrate material. Finally, kinked, bottom-up grown silicon nanowires were integrated in this manner as prospective ion-sensitive field-effect transistors in a bio-probe architecture directly on ADEX substrates. Hence, we have developed the required set of microfabrication protocols for such an assembly comprising metal thin film deposition, direct burn-in of lithography alignment markers, and polymer patterning on top of the DFP.

A photolithographic technique based on dry film photoresists for facile and low-cost patterning of microcantilever beams is presented. Dry film photoresists enable instantly homogenous photoresist coatings on flexible and 3D patterned substrate surfaces, represented here by microcantilever beams, which is otherwise challenging if conventional spin-coating of photoresist is utilized. Compared to alternative microtechnologies, such as focused ion beam milling or resist spray coating, our strategy is far less elaborate, fully compatible with routine additive and subtractive microfabrication processes and can be readily scaled. We show specifically microcantilever shape modification by CF 4 reactive ion etching, localized metal deposition in combination with conventional lift-off procedures as well as a utilization of patterned dry film photoresists as permanent microstructural elements. These microstructural elements are in particular flat-ended cylindrical dry resist micropillars created at the freestanding end of the cantilever beam that can be employed as scanning probes. The resist pillars enabled imaging of a 3T3 mouse fibroblast cell culture surface to determine their elastic force constants. Alongside UV-exposure by a conventional mask aligner, we also demonstrate dry film photoresist exposure by contact-free laser lithography eliminating possible substrate damage by photomask contact.

Traditionally, polymeric microcantilevers are assembled by a multitude of process steps comprising liquid spin-coated photoresists and rigid substrate materials. Polymer microcantilevers presented in this work rely instead on commercially available dry film photoresists and allowed an omittance of multiple fabrication steps. Thin, 5 μm thick dry film photoresists are thermally laminated onto prepatterned silicon substrates that contain AFM compatible probe bodies. Partially suspended dry film resists are formed between these probe bodies, which are patterned to yield microcantilevers using conventional photolithography protocols. A limited amount of thermal cycling is required, and sacrificial probe-release layers are omitted as microcantilevers form directly through resist development. Even 1 mm long polymeric cantilevers were fabricated this way with superior in-plane alignment. The general effects of post-exposure bake (PEB) and hardbake protocols on cantilever deflection are discussed. Generally, higher PEB temperatures limit out-of-plane cantilever bending. Hardbake improved vertical alignment only of high-PEB temperature cantilevers, while surprisingly worsening the alignment of low-PEB temperature cantilevers. The mechanism behind the latter is likely explained by complex interactions between the resist and the substrate related to differences in thermal expansion, heat conduction, as well as resist cross-linking gradients. We present furthermore multilayer structures of dry film resists, specifically cylindrical dry film resist pillars on the polymer cantilever, as well as the integration of metal structures onto the polymer cantilever, which should enable in future integrated piezoresistive deflection readout for various sensing applications. Finally, cantilever spring constants were determined by measuring force–displacement curves with an advanced cantilever calibration device, allowing also the determination of both, dry film resist cantilever density and Young’s modulus.

Silicon nanowires are considered promising future biomedical sensors. However, their limited stability under physiological conditions poses a challenge in sensor development and necessitates a significantly improved knowledge of underlying effects as well as new solutions to enhance silicon nanowire durability. In the present study, we deduced the dissolution rates of silicon nanowires under simulated physiological conditions from atomic force microscopy measurements. We correlated the relevant change in nanowire diameter to changes in the electronic properties by examining the I-V characteristics of kinked silicon nanowire p–n junctions. Contact potential difference measurements and ambient pressure photoemission spectroscopy additionally gave insights into the electronic surface band structure. During the first week of immersion, the Fermi level of n-type silicon nanowires shifted considerably to higher energies, partly even above the conduction band edge, which manifested in an increased conductivity. After about a week, the Fermi level stabilized and the conductivity decreased consistently with the decreasing diameter caused by continuous nanowire dissolution. Our results show that a physiological environment can substantially affect the surface band structure of silicon nanowire devices, and with it, their electronic properties. Therefore, it is necessary to study these effects and find strategies to gain reliable biomedical sensors.

Bottom-up grown single-crystalline silicon nanowires (SiNWs) are highly intriguing to build nanoscale probes, for instance for atomic force microscopy (AFM), due to their mechanical robustness and high aspect ratio geometry. Several strategies to build such nanowire-equipped probes were explored but their fabrication is still elaborate, time-consuming and relies partly on single-crystalline substrates. Here, we explore a new strategy to fabricate AFM probes that are equipped with single-SiNW scanning tips. The conceptual evaluation begins with a discussion on the overall design and softness of such probes based on finite-element-method simulations. For the experimental realization, SiNWs were grown by the well-established gold-catalyzed vapor–liquid–solid method employing gaseous monosilane. As-grown SiNWs were subsequently transferred onto flexible membranes and even freestanding AFM microcantilever beams via mechanical nanowire contact printing. Elongation of the deposited nanowires by so-called regrowth was triggered by reusing the original gold catalyst to yield the prospective AFM scanning tip. SiNW-equipped scanning probes were created in this manner and were successfully employed for topography imaging. Although a multitude of challenges remains, the created probes showed an overall convincing performance and a superior durability.