Characterisation and optimisation of the variable frequency microwave technique and its application to microfabrication

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-ElectroMechanical 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.