On-Chip Mechanics as a Contribution to the 4H-SiC Technology Platform

Silicon carbide (SiC) is a premier platform for next-generation quantum technologies. The combination of high-quality single-crystalline 4H-SiC wafers and the material's exceptional mechanical properties form an outstanding set of possible applications. In particular, SiC possesses the lowest intrinsic mechanical damping (Akhiezer damping limit) of any known material - a property that remains largely unexplored. To fully leverage these outstanding mechanical properties, this thesis introduces a monolithic fabrication strategy that forms the desired 3D-structures monolithically out of the single-crystal wafer. This approach diverges from traditional dry-etching methods by relying on a sophisticated electrochemical etching (ECE) process. This strategy uses doping contrast to selectively remove p-SiC regions while preserving n-SiC areas. Through ion implantation, etchable volumina are defined: aluminum for p-type doping as etchable region (p-SiC) and nitrogen for inert, n-type doping (n-SiC). The dissolution of SiC relies on the availability of positive charge carriers (holes, h+), supplied through a continuous current flow via ohmic contacts on the p-SiC. ECE is carried out in the dark, without employing the common above-bandgap UV light seen in photoelectrochemical etching. The resulting free-standing, high-quality monocrystalline n-SiC layer is used as the device layer to structure nanomechanical resonators, such as cantilevers, bridges, and membranes. Such fabricated devices exhibit high-quality surface characteristics on the (nominally) unetched top n-SiC layer. Its upper surface maintains an almost pristine surface roughness, with root-mean square (rms) values below 0. 4 nm. Furthermore, the lower surface - which was previously connected to the p-SiC layer before electrochemical etching - demonstrates rms surface roughnesses below 4 nm. This low surface roughness is particularly remarkable given the extensive fabrication procedures. The monolithic devices remain remarkably shape-stable up to 1550°C (with surface reconfigurations preferably along the m-planes at higher temperatures), confirming compatibility with high-temperature processing like epitaxial graphene growth. Quite remarkably, the ECE fabrication strategy yields essentially stress-free resonators with a residual tensile pre-stress below 10 MPa (less than 2. 5 x 10^-5 strain), which is crucial for color-center-based quantum technologies. Monolithic cantilevers and bridges exhibit excellent room-temperature mechanical quality factors (Q) up to 2 x 10⁵, approaching the thermoelastic limit at eigenfrequencies exceeding 10 MHz. The reduction of Q caused by atomic layer etching (ALE), which is employed here to fine-tune the resonator's eigenfrequency by reducing its thickness, is shown to be restored by high-temperature annealing. Large monolithic membranes up to 220 µm in diameter reach quality factors approximately 50 percent of the theoretical thermoelastic damping limit at frequencies above 5 MHz. A central contribution of this work is the precise adjustment of 4H-SiC resonator properties achieved by applying controlled tensile stress through a chip-bending method using the squeezable nanojunction (SNJ) setup. This provides an outstanding degree of freedom for post-fabrication fine-tuning of both eigenfrequencies and quality factors, which are typically constrained by manufacturing tolerances. Here, this technique explicitly demonstrates the contrasting effects of stress application versus material removal: controlled stress permits reversible adjustment of the mechanical quality factor and eigenfrequency (increase under tension, decrease under compression), whereas thinning the resonator via dry-etching offers a more restricted, irreversible frequency reduction. It highlights the flexibility and utility of the chip-bending setup for optimizing 4H-SiC nanomechanical resonators. In the context of controlled stress application, COMSOL simulations confirm a constant and homogeneous stress distribution within bent bridge resonators, thereby validating the use of the simplifying Euler-Bernoulli beam (EBB) model to determine tensile stress from eigenfrequency measurements conducted by laser Doppler vibrometer (LDV). The measurements allow the first eigenfrequency of a bridge resonator (110x10 µm) to be fine-tuned in the range from 540 kHz to 1400 kHz - a 2. 6-fold increase. Its quality factor experiences a five-fold boost under a maximum stress of 228 MPa. However, stress decreases with both distance to the metal indenter tip (stress source) and a decrease in beam length, as seen in a 52x7 µm bridge, where tensile stress reaches up to 190 MPa, boosting its eigenfrequency by 50 percent and its quality factor is enhanced by 2. 1 times. Additionally, the mechanical bending deformation of the SiC substrate in the SNJ was measured using the Topography Measurement Software (TMS) mode, a white light interferometry technique. Overall, the experimental data (TMS for substrate bending and LDV for eigenfrequency and quality factor measurements) and the theoretical predictions, including COMSOL simulations and the EBB theory, consistently align within a 13 percent margin. This work identifies ECE as a vital technology for scalable, high-performance SiC hybrid systems in quantum applications, combined with precise stress control.