A method for planarizing a microelectronic substrate. In one embodiment, the microelectronic substrate includes an insulating portion having at least one aperture that is empty or at least partially filled with a sacrificial material. The method can include pressing a planarizing medium having small abrasive elements against the microelectronic substrate and moving at least one of the microelectronic substrate and the planarizing medium relative to the other to remove material from the microelectronic substrate. In one aspect of the invention, the abrasive elements can include fumed silica particles having a mean cross-sectional dimension of less than about 200 nanometers and/or colloidal particles having a mean cross-sectional dimension of less than about fifty nanometers. The smaller abrasive elements can reduce the formation of cracks or other defects in the insulating material during planarization to improve the reliability and performance of the microelectronic device.