MEMS fabrication processes
In the Microactuators lab, we have developed know-how in various fabrication processes, which extend beyond classical MEMS fabrication techniques. We combine rapid-prototyping fabrication methods such as laser structuring and CNC-milling with classical MEMS processes and high-volume fabrication processes like wire-bonding or hot-embossing. On the one hand such “hybrid processes” are often necessitated by the multi-scale nature of the devices that we develop and on the other hand, they often simply allow a fast and energy-efficient fabrication. For the characterization of our prototypes, we frequently use optical techniques. While we also use commercially available systems, i.e. white light interferometry and microscopes, we also use and continuously advance our own custom solution for optical 3D profilometry. Together with the rapid prototyping, this allows us fast design iterations and process optimization.
Laser micro-structuring | Dr. Angelina Müller
Laser micro-structuring for rapid prototyping is a core focus of the fabrication technologies at our chair. We use a UV marking laser with a wavelength of 355 nm, pulse duration of 3-200 ns and a mean power of 2 W. The spot size of 15 µm allows us to structure on scales just above the typical scales of classical and novel MEMS processes. This technique is, however, much faster, flexible and cost-efficient. Thanks to the development of suitable pulse sequences, we can structure most organic and inorganic materials, with the exception of a few polymers and highly UV-transparent glass types. In particular, we can also, to some degree, structure selectively and on sensitive materials for example to minimize the heating of piezo materials, to structure thin layers or to UV-expose photoresists for hybrid processes.
Because of the versatility of this technique, we use it in most of our research activities for various applications, for example to structure piezo sheets and their electrode or ultra-thin glass sheets for adaptive optics, test structures for energy harvesters or new rapid prototyping methods for micro-optics . Furthermore, we are able to quickly build tools for fabrication and assembly of precision mechanics, for example vacuum chucks and casting molds or opto-mechanical components like lens mounts and apertures.
Mill/ Rapid prototyping | Binal Bruno
The Microactuators lab features a fully equipped mechanical workshop. Alongside a manual lathe and mill it also includes a precise closed loop 3-axis CNC mill. This enables us to rapidly produce custom-designed small and medium sized parts from various materials, such as metals, thermoplastics, thermosets and composite materials.
This includes, but is not limited to, casting molds, mounts for measurement equipment or housings. An own workshop also makes us independent from third party companies and gives us the possibility to use non-standard and experimental materials and processes not available from contractors or the IMTEK internal workshop.
Wirebonding/ 3D microcoil fabrication | Dr. Ali Moazenzadeh
Automatic wirebonding is a state-of-the-art, CMOS-compatible, cost-effective, and back-end process, intended to create electrical interconnections in semiconductor chip packaging. Although it is a serial process, it provides high throughput and is therefore widely used in industrial applications. At the Laboratory of Microactuators, we utilize the wirebonding fabrication technology in conjunction with traditional MEMS and precision mechanics processing, thus achieving wafer scale, and batch fabrication of miniaturized 3D microcoils. Going beyond the usual contacting applications, wirebonders allow, in general, the high-precision shaping of micrometre-sized wire at a high speed, for example around a prefabricated yoke or a magnetic core to wind coils.
When compared to coils fabricated with traditional microfabrication techniques such as successive metallization layers followed by electroplating, wirebonding is faster and the metal quality is higher, in addition to the greater flexibility and higher aspect ratios. The process is fully compatible with standard microelectronic manufacturing and therefore, enables the direct integration of coils into a given electronic circuit. The process is highly flexible for numerous applications, because wires of various materials, such as copper or gold are available in different diameters in the range of 25–50 µm, with or without insulation layer. The wirebonded microcoils were successfully tested and utilized in several applications such as the MRI/NMR sensors, microtransformers, microlevitation, energy harvesting, and THz metamaterials.
Molding and hot embossing | Roland Lausecker
We apply and develop various techniques to shape thermoplastic and thermosetting materials fast and precisely. We use molding and hot embossing as efficient methods to process amongst other materials PDMS, polyurethanes, PMMA, epoxies and also biomaterials such as shellac or PLA achieving optical surface qualities. These lab-scale fabrication methods do not require cleanroom facilities and enable, if combined with in-house rapid-prototyping of custom machined molds and embossing stamps, short iteration cycles but at the same time the option for high-volume fabrication. Applications range from microfluidic systems and sample handling systems for nuclear magnetic resonance imaging/spectroscopy to the fabrication of tunable lenses. Our processing experience and collection of compatible materials and material combinations grows steadily with the research projects that have their own requirements, but is also shared between projects from different fields.
Green MEMS fabrication methods | Roland Lausecker
Besides their economic success and their applicability to the fabrication of various MEMS products, conventional MEMS fabrication methods inflict a substantial environmental impact. With the project “Green MEMS”, we aim to develop sustainable fabrication methods, establishing both renewable materials and energy-efficient fabrication methods in the MEMS world. As they are cheap, renewable and non-toxic, our research focuses especially on the application of materials that are usually used by the food industry. Moreover, several of them possess favorable material properties, e.g. low glass transition temperatures that allow for energy-efficient machining at low process temperatures using state-of-the-art processes such as hot embossing. Additionally, the incorporation of food materials like gelatin or carbohydrates to MEMS fabrication methods enables novel application fields within the life sciences to be addressed, as biodegradability and biocompatibility are common properties of many food materials. In the whole project, the environmental impacts of established and newly developed fabrication methods and materials are assessed and optimized.
3D Topography measurement | Florian Lemke
We use and continuously improve our custom-built 3D profilometer that can dynamically measure topographies and thickness profiles of all types of surfaces (diffuse, reflective, transparent and thicknesses). This uses the combination of a moveable x-y-stage with a measurement area of 100x100mm² with different optical distance sensors which we choose for our particular measurement needs. Our different confocal sensors achieve a resolution down to 1.55 µm in the transverse directions and a resolution of 12 nm in z-direction or a measurement range up to 20 mm. For fast measurements of actuated samples we select one of our high-speed triangulation sensors with sampling rates as low as 2.55 µs. To measure actuated samples we have integrated our setup into the driving circuitry of our devices, which enables full 3D dynamic measurements up to 50 kHz. For example we can observe the full step-response surface deformations or resonance modes of piezo-actuated adaptive lenses or high speed deformable mirrors, or we can perform high-precision (quasi-)static measurements of fine and steep structures or long-term creep and drift.