The Electrostatic Force Microscope (EFM) and its variants are of interest for the measurement of potential distributions within nanostructures, and for work function measurements of gate metals for next generation CMOS. In phase mode, the EFM measures variations in the cantilever phase with respect to the oscillation drive signal that are due to electric field gradients in the sample. The addition of a voltage feedback loop on the cantilever oscillation amplitude creates a Scanning Kelvin Force Microscope, which nulls the force on the cantilever by applying a voltage equal and opposite to the contact potential difference (CPD) between the tip and sample. If the work function of the tip is known, the work function of the sample can be simply calculated. SKFM has poorer spatial resolution than EFM phase mode because the signal depends on the electric field rather than the electric field gradient. We have determined the optimum data acquisition conditions for spatial resolution and accuracy of CPD measured with SKFM in atmosphere by using various commercially available tips and a specially designed test chip containing up to four different metal layers. The test chip contains metal-oxide-semiconductor (MOS) capacitors for determining gate metal work function from the variation of flatband voltage with oxide thickness; test structures for the measurement of electrical line width; interdigitated electrodes with the opposing electrodes made from different metals; and structures to simulate nanoparticles and nanowires in various combinations of metals. By comparison to measurements on the test structures using tips with known work functions, the effective work functions of tips with unknown work functions can be estimated. A simple computer model was developed that predicts SKFM signal as a function of tip and sample measurement geometries. Qualitative agreement between measured and modeled SKFM signal is seen based on the reported values of tip radius, length and tip metal for various commercial probes; and variations in the SKFM data acquisition conditions.
Proceedings Title: International Conference on Frontiers of Characterization and Metrology for Nanoelectronics
Conference Dates: March 27-29, 2007
Conference Location: Gaithersburg, MD
Pub Type: Conferences
contact potential difference, EFM, electrostatic force microscope, scanning Kelvin force microscope, SKFM, work function