The objectives of the proposed research are to model, optimize, and control variable frequency microwave (VFM) curing of polymer dielectrics. With an increasing demand for new materials and improved material properties, there is a corresponding demand for new material processing techniques that lead to comparable or better material properties than conventional methods. Presently, conventional thermal processing steps can take several hours. A new thermal processing technique known as variable frequency microwave curing can perform the same processing steps in minutes without compromising the intrinsic material properties. Current limitations in VFM processing include uncertain process characterization methods, lack of reliable temperature measuring techniques, and the lack of control over the various processes occurring in the VFM chamber. Therefore, the proposed research addressed these challenges by: (1) development of accurate empirical process models using statistical experimental design and neural networks; (2) recipe synthesis using genetic algorithms; (3) implementation of an acoustic temperature sensor for VFM process monitoring; and (4) implementation of neural control strategies for VFM processing. and #8194;

The benefits of microwave technology in materials processing is well documented and researched. It offers many potential advantages over conventional processing such as rapid heating, faster processing times and more consistent product quality. However the actual implementation of this technology has been lacking and the benefits have gone largely unrealised. This is due largely in part to the non-uniform heating obtained in multimode cavities in conventional microwave processing. Recently, a new processing method dubbed the Variable Frequency Microwave (VFM) Technique has been developed to overcome the inherent problems associated with conventional microwave processing. By sweeping through a bandwidth of frequencies, the limitations observed in conventional processing, and specifically the problem of heat uniformity, are avoided. With the increase in research activities in alternative processing methods for new and current materials that will provide better product quality as well as time and cost savings, the VFM technique has the potential to rejuvenate interest in microwave processing. This thesis documents the research work undertaken on the VFM technique with emphasis on its characterization, optimisation and implementation to suitable applications in particular in the upcoming area of Microfabrication. A commercial Variable Frequency Microwave with an operating bandwidth of 2.5-8.0 GHz was investigated through modelling and experimental work to determine the energy distribution within a multimode cavity and to provide an insight of the mechanisms of the method. Modelling was found to be an efficient and cost-effective tool to simulate VFM and to examine the reported advantages of this new technique. Results obtained confirm the superiority of the VFM method over the conventional fixed-frequency processing showing a marked improvement in the heating uniformity achieved. Quantitative analysis of the three major VFM parameters that influence heat uniformity - Sweep Rate, Bandwidth and Central Frequency - indicate that although slight variation in heat uniformity was observed when changing these parameters, these variations are only small which implies that the VFM technique is quite insensitive to changes in the parameters making it quite a robust system. An analytical model of the Variable Frequency Microwave technique was developed and it was found that the heating uniformity could be further optimised using a sweep rate that varies as the inverse of the frequency squared (weighted-sweep). In this study, VFM Technique was successfully extended to the Micro-Electro- Mechanical Systems (MEMS) industry as an alternative method for the processing of a polymer system - negative-tone SU8 photoresist - which is gaining widespread use in Microfabrication. The VFM method was compared to conventional hotplate curing as well as a new hybrid curing method introduced in this work and the product quality assessed optically and by thermal analysis. Results from this work indicate that the Variable Frequency Microwave technique is a viable alternative to the conventional cure currently used in practice. With proper optimisation of the VFM parameters, VFM was found to provide samples that are comparable or better than conventionally cured samples in terms of properties and microstructure quality. Using the VFM method, enhancement in cure rates and drying rates, which are described by others as microwave effects, were observed and investigated. A significant increase on the degree of cure of up to 20% greater than conventional cure was observed when VFM was utilized and an apparent enhancement in solvent evaporation in the thin SU8 films observed. Experiments undertaken show that microwaves irradiation can enhance diffusion rates of cyclopentanone in the SU8 system by approximately 75-100%. The findings signify that SU8 curing at lower temperatures or rapid curing are possible and long drying times could be reduced significantly thus alleviating many of the problems associated with conventional thermal curing. Outcomes of this study demonstrate the ability of the new VFM technique to provide uniform heating which is essential for materials processing. Its application to the emerging field of Microfabrication exhibits its unique advantages over conventional curing methods and establishes itself to be a versatile and robust processing tool. The experimental observations made under microwave irradiation are further proof of the existence of specific microwave effects which is one of the most debatable topics in the Microwave processing field. A mechanism based on the Cage Model by Zwanzig [1983] was put forward to explain the increase in transport rates.

As the world moves towards eliminating lead from consumer products, the microelectronics industry has put effort into developing lead-free solder paste. The major drawback of lead-free solder is the problems caused by its high reflow temperature. Variable frequency microwave (VFM) processing has been shown to allow some materials to be processed at lower temperatures. Issues addressed in this study include using VFM to reduce the solder reflow temperature, comparing the heating rate of different size solder particles, and comparing the reliability of VFM reflowed solder versus conventionally reflowed solder. Results comparing the effect of particle size on the heating rate of solder showed that the differences were negligible. This is due in part to the particle sizes overlapping. Many lead-free solder pastes reflow around 250℃. Results indicate that when using the VFM, lead-free solder paste will reflow at 220℃. The reliability of solder that was reflowed using the VFM at the reduced temperature was found to be comparable to solder reflowed in a conventional manner. Based on these findings, VFM processing can eliminate the major obstacles to making lead-free solder paste a more attractive option for use in the microelectronics industry.

The lithium disilicate (LS2) glass system provides the basis for a large number of useful glass-ceramic products. Microwave processing of materials such as glass-ceramics offers unique benefits over conventional processing techniques. Variable frequency microwave (VFM) processing is an advanced processing technique developed to overcome the hot spot and the arcing problems in microwave processing. In general, two main questions are addressed in this dissertation: 1. How does microwave energy couple with a ceramic material to create heat? and, 2. Is there a "microwave effect" and if so what are the possible explanations for the existence of that effect? The results of the present study show that VFM processing was successfully used to crystallize LS2 glass at a frequency other than 2.45 GHz and without the aid of other forms of energy (hybrid heating). Crystallization of LS2 glass using VFM heating occurred in a significantly shorter time and at a lower temperature as compared to conventional heating. Furthermore, the crystallization mechanism of LS2 glass in VFM heating was not exactly the same as in conventional heating. Although LS2 crystal phase (Orthorhombic Ccc2) was developed in the VFM crystallized samples as well as in the conventionally crystallized samples as x-ray diffraction (XRD) confirmed, the structural units of SiO4 tetrahedra (Q species) in the VFM crystallized samples were slightly different than the ones in conventionally crystallized samples as the Raman spectroscopy revealed. Moreover, the observed reduction in the crystallization time and apparent temperature in addition to the different crystallization mechanism observed in the VFM process both provided experimental evidence to support the presence of the microwave effect in the LS2 crystallization process. Also, the molecular orbital model was successfully used to predict the microwave absorption in LS2 glass and glass-ceramic. This model was consistent with experiments and indicated that microwave-material interactions were highly dependent on the structure of the material. Finally, a correlation between the Fourier transform infrared reflectance spectroscopy (FTIRRS) peak intensities and the volume fraction of crystals in partially crystallized LS2 glass samples was established. / Ph. D.