Ground-based telescopes can achieve diffraction-limited images when equipped with adaptive optics (AO). A major limitation of AO is the small field of view, which is due to the limited isoplanatic patch size. Nevertheless, conventional long-slit spectrographs cannot sample the entire AO-corrected field of view in a single exposure. However, equipped with a modern, large detector array, the Integral Field Unit (IFU) technique will allow a 3-dimensional (3-D) data cube to be recorded simultaneously over the entire AO corrected field of view, with a conventional long-slit spectrographs. We are building a state-of-the-art image slicer IFU for the National Solar Observatory's (NSO) McMath-Pierce Solar Telescope (McMP). This will be the first time that an advanced image slicer IFU is used for 3-D spectroscopy and polarimetry at a solar telescope. The IFU consists of 25 slices that will sample a 6.25" x 8" AO corrected field of view simultaneously, and produces a 200" long slit for diffraction-limited 3-D spectroscopy and polarimetry. This IFU 3-D technique will provide the most high spatial, high temporal resolution with high throughput for solar spectroscopy and polarimetry. This is critical for state-of-the-art spectral diagnosis of solar velocity and magnetic fields. We discuss the design, construction, and testing of this new IFU.