We demonstrate the use of femtosecond laser micromachining for ablating macro-sized cavities in crystalline silicon. The method employed is laser milling in which the focused laser beam is raster scanned over the area to be removed. We report the achievement of very high volume ablation rates for the cavity of up to 8.48x10<sup>6</sup> μm<sup>3</sup> s<sup>-1</sup>. To achieve such high rates, we make use of a high average power fiber laser source of 1030 nm wavelength and variable per pulse energy of up to 100 μJ. By carefully controlling the process variables such as pulse energy, repetition rate and scanner speed, the tradeoffs between micromachining quality and ablation rate are quantified. The developed process is applied on Siliconon-Insulator (SOI) wafers for improving performance of RF devices. By making use of laser removal and an additional step of selective silicon etch using XeF<sub>2</sub>, handler silicon is removed completely under RF circuits such as SP9T switch. The local removal of silicon under such circuits completely eliminates the losses and non-linearities caused by the coupling of RF signals to the semi-conducting substrate. Small-signal and large-signal RF measurements are performed before and after substrate removal to quantify the performance gain. The obtained performance after substrate removal is better than specialized RF-SOI substrates such as trap-rich SOI. This is of practical significance for next generation wireless technologies like 5G which operate at higher frequencies with stringent specifications. The proposed method is also potentially useful for fabricating membrane based devices in SOI technology such as pressure sensors.