Thickness mapping of thin dielectrics with emission microscopy and conductive atomic force microscopy for assessment of dielectrics reliability

Thickness mapping of thin dielectrics, one of the most important parameters for semiconductor reliability monitoring, is investigated by means of the Fowler-Nordheim (FN) current distribution. Local thinning in otherwise homogeneous dielectrics, like MOS gate oxide or EEPROM tunneling oxide, is accompanied by a sharp increase of the FN-current at these sites. The spatially resolved detection of measurable FN-currents is performed with two different methods: Firstly, Emission Microscopy (EM) with one of the most sensitive detectors commercially available yields images of photoemission due to increased FN-current. These can cover large areas but lateral resolution is limited to about 1 micron. The second method we employed uses an Atomic Force Microscope (AFM) to measure FN-current directly through a conductive tip. This yields far better resolution of only several nanometers for the FN-current distribution and the surface topography can also be recorded simultaneously. By the latter method it was possible to detect thickness decreases in the nanometer range over several nm width at the boundary between MOS gate oxide and field oxide of ill-processed wafers. As this measurement procedure is applied directly on top of the oxide it is capable of in-line monitoring and reliability control for thin dielectrics.