Electrical Simulation of the Scanning Capacitance Microscopy Imaging of the PN Junction with Semiconductor Probe Tips

Scanning capacitance microscopy (SCM) has enabled the imaging of two-dimensional doping profiles of very small transistors. Initial SCM imaging was performed using metal-coated probe tips with a diameter of 30-50nm. The limited resolution achievable with these tips due to their size prompted us to investigate micro-machined silicon cantilever tips with a smaller tip diameter. Electrical simulations of a pn junction structure probed with semiconducting tips indicate that image improvements resulting from semiconductor nature of the silicon tips are possible, along with resolution advances. The tip becomes "active" in the imaging process as its charge distribution, along with that of the sample, changes in response to a bias voltage. The interaction of the tip and sample capacitance-voltage (C-V) responses improves image contrast and decreases the bias voltage dependence of the pn junction locations. SCM images of a 60nm gate length n-MOS device, obtained using a boron doped silicon tip, demonstrate these effects.