Scanning Spreading Resistance Mode (SSRM)

The Scanning Extended Resistance Microscope (SSRM) extends the tip/sample current range that the TUNA application module can withstand to measure the surface resistance of the sample. SSRM can measure the resistivity (reciprocal of conductivity) of various conductivity samples from insulation to semiconductor to metal. This mode is based on contact mode technology and is used to map changes in the majority carrier concentration in doped semiconductors.

The geometry of the tip determines the lateral resolution, which is roughly equal to the tip radius. With the tip at virtual ground, an optional bias voltage V DC is applied between the conductive tip and the sample. When scanning in contact mode, the current range of the logarithmic current amplifier is 10 pA to 0.1 mA, which can sense the current I flowing through the sample. Calculate resistance according to Ohm's law: R = V DC / I. The resistivity of a material (in ohm-meters) is defined as the resistance multiplied by the cross-sectional area divided by the length of the conductive path made of the material. By maintaining a constant force between the tip and the sample, the topography map and the current image can be generated simultaneously, so that the local topography is directly related to the electrical characteristics.