Franz Selbmann

This study aims to establish and optimize a process for the fabrication of 3D microstructures of the biocompatible polymer Parylene C using hot embossing techniques. The different process parameters such as embossing temperature, embossing force, demolding temperature and speed, and the usage of a release agent were optimized, utilizing adhesive micropillars as a use case. To enhance compatibility with conventional semiconductor fabrication techniques, hot embossing of Parylene C was adapted from conventional stainless steel substrates to silicon chip platforms. Furthermore, this adaptation included an investigation of the effects of the hot embossing process on metal layers embedded in the Parylene C, ensuring compatibility with the ultra-thin Parylene printed circuit board (PCB) demonstrated previously. To evaluate the produced microstructures, a combination of characterization methods was employed, including light microscopy (LM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). These methods provided comprehensive insights into the morphological, chemical, and structural properties of the embossed Parylene C. Considering the improved results compared to existing patterning techniques for Parylene C like plasma etching or laser ablation, the developed hot embossing approach yields a superior structural integrity, characterized by increased feature resolution and enhanced sidewall smoothness. These advancements render the method particularly suitable for diverse applications, including but not limited to, sensor optical components, adhesive interfaces for medical wearables, and microfluidic systems.

Gas sensors based on micro‐electromechanical systems (MEMS) offer advantages such as a broad spectrum of potentially sensitive materials and analytes, easy miniaturization and integration, high sensitivity, and low costs. This paper introduces a novel MEMS sensor platform utilizing a suspended gate field effect transistor (SGFET) transducer. In this approach, the flexible gate membrane of the SGFET is coated with a sensitive material exhibiting responsive swelling behavior. For the proof of concept, kraft lignin hydrogel is chosen as a biorenewable material for humidity sensing. A precision dispensing technique is used to deposit kraft lignin hydrogel on the SGFETs. The sensor measurements yield reversible shifts in the sensor's output current of up to 9% in response to 5000 ppm water vapor. The results successfully demonstrate the feasibility of this new sensing platform.

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.

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.

The polymer Parylene combines a variety of excellent properties and, hence, is an object of intensive research for packaging applications, such as the direct encapsulation of medical implants. Moreover, in the past years, an increasing interest for establishing new applications for Parylene is observed. These include the usage of Parylene as a flexible substrate, a dielectric, or a material for MEMS, e.g., a bonding adhesive. The increasing importance of Parylene raises questions regarding the long-term reliability and aging of Parylene as well as the impact of the aging on the Parylene properties. Within this paper, we present the first investigations on non-accelerated Parylene C aging for a period of about five years. Doing so, free-standing Parylene membranes were fabricated to investigate the barrier properties, the chemical stability, as well as the optical properties of Parylene in dependence on different post-treatments to the polymer. These properties were found to be excellent and with only a minor age-related impact. Additionally, the mechanical properties, i.e., the Young’s modulus and the hardness, were investigated via nano-indentation over the same period of time. For both mechanical properties only, minor changes were observed. The results prove that Parylene C is a highly reliable polymer for applications that needs a high long-term stability.

Traditional methods of wire bonding do not work well on liquid printed contact layers and soft substrate materials. Laser soldered wire bonding i.e., SB²-WB constitutes a novel approach to this problem as it works without the need for high pressures, temperatures or vibration [1].In this work we qualify different conductive layers of printed, plated, and vapor coated materials containing Au, Cu and Ag with regard to their suitability for forming a stable and reliable interface with a SAC_305 laser jetted solder bump, which is the basis for the laser soldered wire bonding process (SB²-WB). As printing technologies, we have chosen stencil- and screen-printing, alternatively gold was deposited by sputtering and patterned by subsequent lithography and wet-chemical etching. As substrates, we selected Polyethylene-naphthalate (PET) Teonex Q51, Parylene and Glass. Au wire and Ag ribbons were used for forming the wire bonds. Investigations on printed layers have been focused on correlation between paste receipt, printing technology related layer thickness, paste-related post-treatment and compatibility to laser assisted solder ball bumping i.e., SB²-Jet.Using a shear-tester, the mechanical load-capacity of the formed solder joints was measured, and the corresponding fracture modes were inspected using an optical microscope. The characteristic of the interfacial layers, material bulk textures and other metallurgical properties was analysed using SEM-FIB, EDX, X-ray and optical microscopy.Moreover, we demonstrated the forming of laser soldered wire bonds on two demonstrators showing different pad metallization, contact form and substrate bulk material.Finally, the future prospects of intended feasibility studies for laser soldered wire bonding, with structures consisting of paste and ink are highlighted.

Wafer bonding is a crucial process for fabricating microsystems. Within this study, the polymer parylene was used to establish a low-temperature adhesive wafer bonding process for wafers of 150 and 200 mm diameters. The bonding process was investigated for silicon and glass wafers with different additional coatings including silicon dioxide, silicon nitride, aluminum, and parylene C. Important process parameters such as bonding temperature and time were also investigated and the parylene adhesive was analyzed in detail with respect to its dimensions and type. The performance of the parylene bonding was characterized in different aspects, including mechanical tests, cross-sectional scanning electron microscopy, infrared light transmission, and different hermeticity tests. The reliability of the parylene bonded compounds was also investigated with respect to constant loading, mechanical shocking, and thermal cycling. As a result, the parylene bonding is feasible with various materials and shows high tensile and shear strengths of up to 35 MPa and 80 MPa, respectively. Hermeticity was excellent, with a helium leakage rate lower than 10^‒7 mbar∙l s⁻¹. The parylene bonded compounds were proven to feature high reliability. Finally, application of the superior properties of the parylene bonding was demonstrated with respect to the fabrication of different three-dimensional structures.