Development of mechanical reliability testing techniques with application to thin films and piezo MEMS components

This work focuses on the development of a method for probing the mechani- cal response of thin film materials based on miniature tensile testing. A number of mechanisms that may compromise the performance and potentially limit the operational lifetime of MEMS devices which incorporate functional ferroelectric ceramics were also identified and investigated. Reliability of piezo MEMS com- ponents was studied at a wafer and at a device level through the development of appropriate techniques based on miniature tensile testing, time- resolved mi- cro RAMAN spectroscopy and laser Doppler vibrometry. Micro tensile testing was further used for the extraction of the elastic properties of various thin film materials. A miniature tensile stage was developed in common with DEBEN UK for the mechanical characterization of functional thin film materials like PZT and ZnO ceramics, which are commonly used in MEMS fabrication. The stage is of- fered with a piezo electric motor which can be fitted with interchangeable heads. These can be combined with di.erent types of mounting jaws, enabling both con- ventional tensile testing and compression testing to be performed. Strains and displacements were measured in- situ using an optical, non destructive method based on CCD imaging. The elastic constants of polymer (LCP), LCP-Au bi- layers and electroplated Ni were defined in good agreement with the literature. However yield of successfully released ceramic samples was rather poor so a col- laboration with IMTEK at Germany was established. Using their facilities batch processing of a large number of wafers was possible. Cont/d

Analysis and Fabrication of MEMS Tunable Piezoelectric Resonators

Piezoelectric MEMS resonators are being used with increased frequency for many applications, operating as frequency sources in sensors, actuators, clocks and filters. Compensation for the effects of manufacturing variation and a changeable environment, as well as a desire for frequency-hopping capabilities, have brought forth a need for post-process tuning of the resonant frequency of at these devices, in particular clocks and filters manufactured at the MEMS scale. This work applies a shunt capacitor tuning concept to three different types of piezoelectric MEMS resonators: bending beam devices, surface acoustic wave devices, and film bulk acoustic wave devices, in order to solve this tuning need across a wide range of the frequency spectrum (single Kilohertz to tens of Gigahertz). Questions about how the material and design parameters of these resonators affect the resonant frequencies and tunability of the devices are further discussed for each of the designs. In addition to the theoretical modeling, the fabrication steps necessary for processing the piezoelectric MEMS bending devices, specifically utilizing PZT thin films and an interdigitated design, are developed. Results of many fabrication trials are discussed, and finalized process plans for fabricating quality thin film PZT and PZT interdigitated devices are provided

An Integrated Gas Sensing System Based on Surface-Functionalized Gallium Nitride Nanowires with Embedded Micro-Heaters

In the last few decades, significant improvements have been made in gas sensor technologies. Metal-oxide sensors have been used for low-cost detection of combustible and toxic gases. However, hurdles relating to sensitivity, stability and selectivity still remain. Recently, nanotechnology has helped tremendously through the introduction of nano-engineered materials like nanowires and nanoclusters. Nanowire sensors have much better sensitivity as compared with thin-film devices due to the larger detecting surface-to-volume ratio. But clearly, improvements are still needed. For real-world applications, selectivity between different classes of compounds, such as combustible and toxic gases, is highly desirable. An ideal chemical sensor should distinguish between the individual analytes from a single class of compounds. For example, in detection of benzene or toluene, a good sensor will not be disturbed by other aromatic compounds present in the environment. This is a huge challenge for semiconductor based metal-oxide sensors, such as TiO2, SnO2, Fe2O3 and ZnO, which have inherent non-selective surface adsorption sites. Recently, a new class of nanowire-nanocluster (NWNC) based gas sensors has gained interest. This type of sensor represents a new method of functionalizing the surface for selective adsorption and detection. The adjustable sensitivity can be achieved by tuning the density, size or composition of the nanoparticles that decorate the nanowires. These advantages make the NWNC sensors a good alternative to conventional thin-film sensors. So far, research into NWNC sensors has demonstrated the potential in sensing many important classes of compounds. However, most of these NWNC devices require elevated working temperatures. They also have long response/recovery times and must function in an inert atmosphere. All these limitation will be the obstacles in real-world usage for domestic, environmental or industrial applications. And finally, the sensors thus developed must be manufacturable. That is, they must be batch fabricated with high yield. To remedy these problems, my thesis was divided into the following tasks, 1. Develop dry etching techniques to fabricate horizontally aligned GaN nanowires (NW), combining these techniques with wet etching treatment for surface damages removal. I call this a “top-down approach” using a subtractive process that fabricates NWs from thin-films and adding sensitive nanocrystals after the initial NW definition. This is to be compared to the additive “bottom-up” nanowire growth by MBE/HVPE/Sol-gel, in which NWs are grown, harvested from the growth surface and subsequently re-attached to a new surface. The top-down approach enhances the yield and homogeneity of the NW and it is mass-production oriented. 2. Study the metal-oxide nanoclusters (NCs) deposition method by physical vapor deposition (PVD) and rapid thermal annealing (RTA) for TiO2, SnO2, WO3, Fe2O3, etc. Develop the metal nanoparticle deposition method by PVD for Au, Ag, Pt, Pd, etc. 3. Study the crystalline phases and gas adsorption sites formed by the method and establish a database connecting metal-oxide bonding sites with different target chemicals. 4. Utilize Si doped n-type and unintentionally doped GaN nanowires functionalized with different metal-oxide and metal-oxide/metal composite nanoclusters to create a series of highly selective and sensitive gas sensing nanostructure devices. 5. Develop a low-cost micro-heater (MH) for local high temperature generation with low power consumption. This allows the rapid chemical desorption cycles as we anticipate frequently re-use or reset of the sensor. It also enables the use of these NWs in high temperature sensor applications. 6. Integrate the NW, NCs and MH into one working sensor, and integrate multiple types of gas sensors on a single chip. The chip can simultaneously sense many types of gases without interference. In this study, the potential of multicomponent NWNC based sensors for developing the next-generation of ultra-sensitive and highly selective chemical sensors was explored. We have achieved uA and nA levels of baseline detector current and we have shown that low UV illumination enhances sensitivity for some cases. These sensors have low power consumption making them suitable for portable devices

Synthesis of N-doped broken hollow carbon spheres and inorganic-organic hybrid perovskite materials for application in photovoltaic devices

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for degree of Master of Science in ChemistryThe mandate for renewable energy sources to replace the current reliance on fossil fuels as a primary energy source has recently attracted a lot of research interest. The research has also focussed on bringing the technologies that take into consideration the goal of reducing environmental pollution. Consequently, approaches using photovoltaic (PV) technologies have been a promising arena to tackle the problem facing energy sources. Recently, more focus has been placed on improving the power conversion efficiency (PCE) of PV devices, such as organic and/or organic-inorganic hybrid perovskite solar cells. Therefore, in this work two different materials were applied in two independent PV devices, namely organic and/or organic-inorganic hybrid perovskite solar cells. One study employed nitrogen doped broken hollow carbon spheres (N-bHCSs), with an aim of enhancing the electronic properties of the P3HT:PCBM active layer of an organic photovoltaic (OPV) solar cell. N-bHCSs were successfully synthesized using a horizontal chemical vapour deposition method (H-CVD) employing a template-based method and the carbon was doped using in-situ and ex-situ doping techniques. Pyridine, acetonitrile and toluene were used as both carbon and nitrogen precursors. The dispersity of the SiO2 spheres (i.e. templates) was found to play a role on the breakage of the N-bHCSs. Incorporation of the N-bHCSs into the P3HT:PCBM active layer was found to enhance the charge transfer and this led to less recombination of photogenerated charges in the interface between the donor and acceptor. The current-voltage (I-V) characteristics of the ITO/PEPOT:PSS/P3HT:PCBM:N-bHCSs/Al solar cell devices revealed an increased chargetransport distance due to increased electron density by n-type doping from the N-bHCSs. The second study employed the organic-inorganic hybrid perovskite (CH3NH3PbI3) material as a light harvesting layer in an ITO/PEDOT:PSS/CH3NH3PbI3/PC6BM/Al solar cell device. Initially, the device parameters were optimised to obtain the best performing device. These include parameters such as the degradation of the hybrid film as a function of time and air exposure. A rapid degradation was seen on the device after 24 h of air exposure which was accompanied by the decrease in the PV performance of the device. The degradation was visually seen by the formation of crystal grains (i.e. “islands”) on the perovskite film.GR201

Characterization of Nanomaterials for Thermal Management of Electronics

Recently, there has been a growing interest in flexible electronic devices as they are light, highly flexible, robust, and use less expensive substrate materials. Such devices are affected by thermal management issues that can reduce the device’s performance and reliability. Therefore, this work is focused on the study of the thermal properties of nanomaterials and the methods to address such issues. The goal is to enhance the effective thermal conductivity by adding nanomaterials to the polymer matrix or by structural modification of nanomaterials. The thermal conductivity of copper nanowire/polydimethylsiloxane and copper nanowire/polyurethane composites were measured and showed more than threefold enhancement compared to the thermal conductivity values of the neat polymers. Furthermore, identical heat sources were used on the neat polymer as well as the composite samples, and the resulting thermal images were taken, which showed that the resulting hot spot was significantly less severe for the composite sample, demonstrating the potential of copper nanowire/polymer composite as a substrate for flexible electronics with better heat spreading capability. In addition, the thermal properties of cellulose nanocrystals-poly (vinyl alcohol) composite films with different structural configurations of cellulose nanocrystals (such as isotropic and anisotropic configurations) were investigated as an alternative to commonly used petroleum-based materials for potential application in the thermal management of flexible electronic devices. Also, the in-plane thermal conductivity of the anisotropic composite film was as high as ~ 3.45 W m-1 K-1 in the chain direction. Moreover, the composite films showed ~ 4-14 fold higher in-plane thermal conductivity than most polymeric materials used as substrates for flexible electronics. A high degree of cellulose nanocrystal orientation and the inclusion of poly (vinyl alcohol) were the reasons for such improvements. In addition, thermal images showed that the cellulose nanocrystals-poly (vinyl alcohol) composite films had better heat dissipation capability compared to the neat poly (vinyl alcohol) films, indicating its potential application for flexible electronic devices. In another study, thermal properties of nanodiamond films obtained through a solution-based directed covalent assembly were studied as a low-cost and greener alternative to the nanodiamond films grown via chemical vapor deposition method for thermal management of electronics. The results obtained showed cross-plane thermal conductivity as high as 3.50 +/- 0.54 W m-1 K-1 for nanodiamond film of 139.1 +/- 19.5 nm thick. Such a low cross-plane thermal conductivity value can be attributed to higher porosity and poor interface quality compared to that of the nanodiamond films grown via chemical vapor deposition method. Hence, there is still more room for improvement for such nanodiamond films. The above chapters were focused on the study of the thermal properties of various types of nanomaterials for thermal management of electronic devices. In the next chapter, a technique for the fabrication of a device, that is capable of performing characterization nanomaterials was presented. In this work, suspended beam microdevices for electrothermal characterization of nanomaterials were fabricated through a standard photolithography technique that is less time-consuming, less expensive and much simpler than the methods used by other research groups in the past. The agreement of the measured in-plane thermal conductivity of the suspended central silicon nitride bare bridge with the literature validated the microdevice, setup, and the experimental procedure. Furthermore, these microdevices can be used to measure other important thermoelectric properties of nanomaterials such as the Seebeck coefficient, electrical conductivity, and thermoelectric figure of merit

Investigation of advanced light trapping concepts for plasma-deposited solid phase crystallised polycrystalline silicon thin-film solar cells on glass

Ph.DDOCTOR OF PHILOSOPH

Fiber-Optic Temperature Sensor Using a Thin-Film Fabry-Perot Interferometer

A fiber-optic temperature sensor was developed that is rugged, compact, stable, and can be inexpensively fabricated. This thin-film interferometric temperature sensor was shown to be capable of providing a +/- 2 C accuracy over the range of -55 to 275 C, throughout a 5000 hr operating life. A temperature-sensitive thin-film Fabry-Perot interferometer can be deposited directly onto the end of a multimode optical fiber. This batch-fabricatable sensor can be manufactured at a much lower cost than can a presently available sensor, which requires the mechanical attachment of a Fabry-Perot interferometer to a fiber. The principal disadvantage of the thin-film sensor is its inherent instability, due to the low processing temperatures that must be used to prevent degradation of the optical fiber's buffer coating. The design of the stable thin-film temperature sensor considered the potential sources of both short and long term drifts. The temperature- sensitive Fabry-Perot interferometer was a silicon film with a thickness of approx. 2 microns. A laser-annealing process was developed which crystallized the silicon film without damaging the optical fiber. The silicon film was encapsulated with a thin layer of Si3N4 over coated with aluminum. Crystallization of the silicon and its encapsulation with a highly stable, impermeable thin-film structure were essential steps in producing a sensor with the required long-term stability