Microfabrication and Integration Using Sub-Picosecond Laser Pulses and Magnetic Assembly
Time: Fri 2020-04-03 10.00
Subject area: Electrical Engineering
Doctoral student: Miku Laakso , Mikro- och nanosystemteknik
Opponent: Assistant Professor Massimo Mastrangeli, Delft University of Technology (TU Delft)
Supervisor: Professor Frank Niklaus, Mikro- och nanosystemteknik
Microfabricated devices and systems have many exciting applications such as accelerometers for triggering the launching of airbags in cars, gyroscopes for sensing the rotations of mobile phones, and micromirror arrays for controlling light reflection in digital light projectors. These devices are currently produced using semiconductor manufacturing techniques, which are suitable for large volumes of mostly planar structures. However, they have limited economic viability for products with lower volumes, and they also constrain the three-dimensional (3D) structuring of the microdevices. Therefore, there is a need for new manufacturing techniques that are economically viable even for smaller volumes and allow truly 3D microdevice designs. To address this problem, this thesis presents developments in microfabrication and integration using two main methods: (1) The usage of sub-picosecond laser pulses for locally adding and modifying material and (2) the usage of an external magnetic field to handle fragile micrometric objects in order to assemble them into their target locations. These two methods are used for six main applications out of which four involve packaging and integrating microsystems, one involves the manufacturing of 3D microstructures, and one involves directly patterning microstructures on a surface.
A key technology in the packaging and integration of microsystems, and a focus area of this thesis, is the manufacturing of through-substrate vias. They are used as electrical interconnections through device and package substrates. They allow smaller packages, which is a requirement, for example, for the Internet of Things where different types of microsensors and actuators are placed in our everyday environment. The first application related to the manufacturing of through-substrate vias is laser drilling of through-silicon holes. Laser drilling allows holes to be created where traditional etching methods might be uneconomical or unpractical. Laser drilling also allows the drilling of tilted holes, which can improve the radio-frequency performance of the vias. The second application is the magnetic assembly of metal conductors into holes in a glass substrate. Glass substrates have several benefits over silicon substrates, such as lower radio-frequency losses, but the production of through-glass vias is challenging due to the difficulty of creating regular holes through the glass. The magnetic assembly allows metal conductors to be placed into the holes in glass independent of the hole shape. This could lead to wider use of glass with its excellent properties as a packaging substrate for microsystems. The third application is through-substrate vias for high-temperature environments. These vias are manufactured by magnetically assembling metal conductors with low thermal expansion into holes in a silicon substrate. The low thermal expansion leads to reduced stresses at elevated temperatures. This could allow using through-substrate vias to reduce package sizes even in demanding high-temperature environments found, for example, in the space industry.
The fourth and last application related to the packaging and integrating microsystems is the vertical assembly of microchips using an external magnetic field. Microsystem fabrication is focused on in-plane structures, but some applications require or would benefit from out-of-plane structures. Examples of such applications are a biosensor placed inside a microneedle inserted into tissue or flow sensors bending in the flow. Manufacturing the out-of-plane structures on the same substrate with other structures requires complicated manufacturing techniques and occupies a large surface area. When using the vertical assembly process, the out-of-plane structures can be manufactured on a separate substrate using standard microfabrication techniques, and the out-of-plane structures can then be assembled afterward in a vertical orientation on a receiving substrate.
Manufacturing of 3D microstructures is not trivial using the standard micromanufacturing techniques. Free-form 3D printing of submicrometric features is possible using two-photon polymerization, but the material properties of polymers are not comparable to those of silica glass. This thesis demonstrates 3D printing of silica glass with submicrometric features using sub-picosecond laser pulses. This new 3D freedom in micromanufacturing could be used, for example, in building more complicated micro-opto-electro-mechanical systems.
Directly patterning microstructures on a surface is possible by exposing the surface to laser pulses. These structures can affect the optical and wetting properties of the surfaces. More specifically, periodic ripple structures can act as diffraction gratings, altering the optical reflection properties of the surface. Exposure to sub-picosecond laser pulses can also cause chemical changes on the surface, and these changes can potentially affect the reflection properties. This thesis demonstrates that the chemical changes indeed affect the reflection properties, and this information could be used when manufacturing ripple patterns, for example, for security markings or for decorative use.