Methods of forming microelectronic structure are provided. The methods comprise the formation of T-shaped structures using a controlled undercutting process, and the deposition of a selectively etchable composition into the undercut areas of the T-shaped structures. The T-shaped structures are subsequently removed to yield extremely small undercut-formed features that conform to the width and optionally the height of the undercut areas of the T-shaped structures. These methods can be combined with other conventional patterning methods to create structures having extremely small feature sizes regardless of the wavelength of light used for patterning.