Interfacial phenomena in vacuum-encapsulated micro and nano-structures [electronic resource]
Keywords: Mechanical Engineering

In the micro and nano-scale regimes, the physical phenomena stemmed from interfaces are becoming non-negligible. These phenomena are important in micro and nanoelectromechanical systems (M/NEMS) as reliability issues such as fatigue, adhesion, friction, and charging, are highly influenced by their surface conditions. However, the understandings of interfacial phenomena in micro and nano-structures are currently limited to specific materials and environments, which prevents the improvement in performance of M/NEMS devices. This dissertation aims to enhance and understand the surfaces of M/NEMS through three individual works. The contributions of each work include the development of reproducible fabrication processes that incorporate novel materials, design of test structures, and material characterization. The first part of this dissertation presents bulk and surface micromachining processes to fabricate freestanding structures formed by atomic layer deposition (ALD). Incorporating U-shape trenches, freestanding structures that have > 4000:1 aspect ratios are successfully fabricated with ~12-nm thick ALD platinum (Pt) and aluminum oxide (Al2O3) films. The structures fabricated by the developed processes can provide noteworthy combinations of electrical, thermal, and mechanical properties that will be useful for many applications. With ALD-grown freestanding structures that are fabricated by these technologies, the electrical and thermal conductivities of ALD Pt films of thickness 7.3, 9.8, and 12.1 nm are measured at 50-320 K. Conductivity data for the 7.3-nm film are reduced by 77.8% (electrical) and 66.3% (thermal) compared to bulk values due to the electron scattering at interfaces and grain boundaries. The experimental Lorenz numbers of ALD Pt films exceed the bulk value due to phonon conduction. Finally, based on the developed processes and measured material properties, microbolometer pixels and arrays made of ALD Pt/Al2O3 films are demonstrated. The thermal time constants of the fabricated microbolometer pixels with 50 x 50- and 25 x 25-[Mu]m absorbers are 1.97 and 0.43 ms respectively, which are [greater than] 7.6 times smaller than conventional VOx microbolometers. The noise equivalent temperature difference of the 50 x 50-[Mu]m bolometer is 116 mK, assuming f/1.0 optics and negligible 1/f noise. The second part reports on the first study of fatigue in single crystal silicon (SCS) and Si-SiO2 composite micromechanical resonators fabricated in an extremely clean and controlled environment using an epi-seal encapsulation technology. This packaging technology provides a unique opportunity to investigate controversial issues in silicon fatigue since the surfaces of devices are not exposed to air, oxygen, or other residues that might complicate the initiation and observation of fatigue. Fatigue experiments are conducted on 42 devices over 10^9 actuation cycles with various dynamic loadings ranging from 0.2 to 4.0 GPa at 29-32°C and from 0.2 to 1.2 GPa at 273-290°C. However, no device failure due to the high cyclic fatigue has been observed. This is also true for Si-SiO2 composite. SCS devices that were annealed in hydrogen at [greater than] 925°C do not show fatigue up to 2.5 x 10^9 cycles with 4.0-GPa stress amplitude for the < 110> direction and 2.1 x 10^9 cycles with 1.5-GPa stress amplitude for the < 100> directions at room temperature. Si-SiO2 composite devices could withstand 5 x 10^9 cycles with 2.5-GPa stress amplitude. Although the experimental results could not verify the fatigue mechanism of silicon, these results could be applicable to ensure the reliability of silicon M/NEMS devices. The last part describes the fabrication and characterization of a polycrystalline 3C silicon carbide (poly-SiC) thin film encapsulation process. In this fabrication technique, devices are sealed with a nominally 2-[Mu]m poly-SiC and the device layer is simultaneously coated with a nominally 0.2-[Mu]m poly-SiC thin film. Device characterization includes the measurement of the resonant frequencies and quality factors of double-ended tuning fork micromechanical resonators, which have Si-SiC composite beam structures. Experimental results show that the pressure inside the packaging can be controlled from 447 Pa to 15.5 kPa with a 400°C annealing process. The frequency drifts of the encapsulated resonators are less than the frequency noise level measured over 29 days at 84.6°C ± 0.1°C, which suggests that the poly-SiC thin film packaging technique can offer hermetic packaging for various applications in M/NEMS including inertial sensors. In addition to the packaging performance, the temperature coefficient of Young's modulus for poly-SiC is derived from the resonant frequency changes with temperature. Reduction of the quality factor due to the poly-SiC coating, predicted in the theoretical model, is confirmed by measurements.