Polyimides are polymeric materials with outstanding thermal, chemical, and mechanical properties. For this reason, they find applications in several engineering sectors, including aerospace, microsystems, and biomedical applications. For realizing 3D structures made of polyimides, 3D printing is an attractive technique because it overcomes the limitations of polyimide processing using conventional manufacturing techniques such as molding and subtractive manufacturing. However, current polyimide 3D printing approaches are limited to realizing objects with the smallest dimensions of the order of a few hundred micrometers. 3D printing of polyimide objects featuring sub-micrometer resolution using two-photon polymerization by direct laser writing is demonstrated here. A negative photosensitive polyimide is applied that is widely used in microsystems applications. To demonstrate the utility of this polyimide 3D printing approach and the compatibility of the 3D objects with operation at elevated temperatures, a micro-hotplate is 3D printed and characterized at operating temperatures of above 300 °C.

3D printing of MEMS devices could enable the cost-efficient production of custom-designed and complex 3D MEMS for prototyping and for low-volume applications. In this work, we present the first micro 3D-printed functional MEMS accelerometers using two-photon polymerization combined with the evaporation of metal strain gauge transducers. We measured the resonance frequency, the responsivity, and the signal stability over a period of 10 h of the 3D-printed accelerometer.

3D printing by two-photon polymerization (TPP) is a well-established manufacturing approach for realizing 3D polymer structures at the micro- and nanoscale. However, an important shortcoming of 3D printing by two-photon polymerization is that it is extremely challenging to print 3D objects with long overhanging features, which severely limits the application space of this technology. Here, we introduce a methodology for 3D printing by two-photon polymerization that allows the realization of 3D objects with long overhanging structures that cannot be printed using conventional printing strategies. Our methodology combines different printing approaches for realizing the overhanging structure, including locally adjusted printing block sizes and a mix of the Shell & Scaffold and Solid printing modes. As a result, objects with long overhanging parts can be printed without the need for added support structures. Using this approach, we demonstrate successful printing of overhanging cantilevers with a quadratic cross-section of 50 µm x 50 µm and lengths of up to 1000 µm. Thus, our printing modality substantially extends the capabilities and application space of 3D printing by two-photon polymerization and removes current design limitations regarding 3D printed objects with long overhanging structures.

The functional area of silicon-based microelectromechanical systems (MEMS) devices often occupies only a fraction of the actual silicon area of the chip. As the chip cost directly scales with the total chip area, there is an incentive to reduce the chip to the smallest possible size. However, handling such diminutive devices poses challenges that industry-standard packaging cannot solve. Here, the world's smallest spray nozzle chip for drug delivery to the lung is manufactured and packaged and how magnetic assembly combined with microfluidic glue fixation can overcome this barrier for diminutive MEMS devices is demonstrated. The spray nozzle chips have a circular footprint with a diameter of 280 µm and feature a nickel coating on their conical sidewall, allowing magnetic manipulation. The chips are assembled and sealed into plastic substrates using a three-step gluing process guided by capillary action and activated by heat. Assembly speeds of up to 147 chips per minute are demonstrated and fabrication to packaging and functional operation of this device is shown for the target application.

Microelectromechanical system (MEMS) devices, such as accelerometers, are widely used across industries, including the automotive, consumer electronics, and medical industries. MEMS are efficiently produced at very high volumes using large-scale semiconductor manufacturing techniques. However, these techniques are not viable for the costefficient manufacturing of specialized MEMS devices at low- and medium-scale volumes. Thus, applications that require custom-designed MEMS devices for markets with low- and medium-scale volumes of below 5000-10,000 components per year are extremely difficult to address efficiently. The 3D printing of MEMS devices could enable the efficient realization and production of MEMS devices at these low- and medium-scale volumes. However, current micro-3D printing technologies have limited capabilities for printing functional MEMS. Herein, we demonstrate a functional 3D-printed MEMS accelerometer using 3D printing by two-photon polymerization in combination with the deposition of a strain gauge transducer by metal evaporation. We characterized the responsivity, resonance frequency, and stability over time of the MEMS accelerometer. Our results demonstrate that the 3D printing of functional MEMS is a viable approach that could enable the efficient realization of a variety of custom-designed MEMS devices, addressing new application areas that are difficult or impossible to address using conventional MEMS manufacturing.

The out-of-plane integration of microfabricated planar microchips into functional three-dimensional (3D) devices is a challenge in various emerging MEMS applications such as advanced biosensors and flow sensors. However, no conventional approach currently provides a versatile solution to vertically assemble sensitive or fragile microchips into a separate receiving substrate and to create electrical connections. In this study, we present a method to realize vertical magnetic-field-assisted assembly of discrete silicon microchips into a target receiving substrate and subsequent electrical contacting of the microchips by edge wire bonding, to create interconnections between the receiving substrate and the vertically oriented microchips. Vertical assembly is achieved by combining carefully designed microchip geometries for shape matching and striped patterns of the ferromagnetic material (nickel) on the backside of the microchips, enabling controlled vertical lifting directionality independently of the microchip's aspect ratio. To form electrical connections between the receiving substrate and a vertically assembled microchip, featuring standard metallic contact electrodes only on its frontside, an edge wire bonding process was developed to realize ball bonds on the top sidewall of the vertically placed microchip. The top sidewall features silicon trenches in correspondence to the frontside electrodes, which induce deformation of the free air balls and result in both mechanical ball bond fixation and around-the-edge metallic connections. The edge wire bonds are realized at room temperature and show minimal contact resistance (<0.2 Omega) and excellent mechanical robustness (>168mN in pull tests). In our approach, the microchips and the receiving substrate are independently manufactured using standard silicon micromachining processes and materials, with a subsequent heterogeneous integration of the components. Thus, this integration technology potentially enables emerging MEMS applications that require 3D out-of-plane assembly of microchips.

Holes through silicon substrates are used in silicon microsystems, for example in vertical electrical interconnects. In comparison to deep reactive ion etching, laser drilling is a versatile method for forming these holes, but laser drilling suffers from poor hole quality. In this article, water is used in the silicon drilling process to remove debris and the shape deformations of the holes. Water is introduced into the drilling process through the backside of the substrate to minimize negative effects to the drilling process. Drilling of inclined holes is also demonstrated. The inclined holes could find applications in radio frequency devices.

Tunneling junctions are pairs of electrodes separated by gaps of a few nanometers that allow electrons to tunnel across the gap. Tunneling junctions are of great importance for applications such as label-free biomolecule sensing and single molecule electronics, but their fabrication remains difficult and laborious. In this paper, we present a simple 2-stage process for the fabrication of tunneling junctions consisting of electrode pairs made of gold (Au). This is achieved by combining a novel methodology for fabricating crack-defined Au nanowires at wafer-scale with a constant voltage, feedback-free electromigration procedure to form tunneling nanogaps free of debris.