Integral Field Spectropolarimetry (IFS) makes feasible the simultaneous measurement of a 2D field with the use of an Integral Field Unit (IFU). Slicer-based IFUs rearrange an input 2D Field of View (FoV) to an output long-slit that is used to feed a standard spectrograph. The spatial resolution reachable with this technique is determined by the slicer width. As a technology demonstrator for a future EST instrument development, here we describe an IFU for the GREGOR solar telescope. It is an evolution from the first IFU installed at GREGOR with 100 μm width slicer mirror. A novel technique was applied to produce a 35 μm-width metallic image slicer, which has been manufactured in collaboration with NAOJ and Canon Inc. for the SOLARNET project. The IFU transforms an input FoV of 4.4" x 2.1" into 16 mini-slits rearranged in two parallel output long-slits to feed the GRIS infrared spectrograph for simultaneous 2D field spectropolarimetric measurements in several wavelengths. A collimator and camera mirror concept was applied to control the pupil and stray light contamination, leading to a telecentric configuration with a 1:1 magnification. Because of the thin dimensions of the slicer mirrors, diffraction effects are not negligible. An study on the impact of the diffraction effects on the whole IFU and the spectrograph, has been carried out. Another important issue that has been taken into account in the design is the individual mini-slit tilt induced by the compact multi-mirror design. Different solutions were considered and the final design is presented.