Highly sensitive Scanning Capacitance Microscope
Agilent Technologies, US
scanning capacitance microscope, AFM, SPM, semiconductor doping profile
Summary—Emerging nanotechnology and biotechnology are in direct need of metrology tools with better than 10 –nm spatial resolution that are capable of imaging surfaces and nanoembeded structures. Scanning local probe microscopy (SLPM) techniques have advanced our knowledge of surface and materials at atomic scales. These tools include the scanning tunneling microscope (STM), atomic force microscope (AFM), scanning capacitance microscope (SCM), magnetic force microscope (MFM), scanning thermal microscope (SThM), and scanning near field optical microscope (NSOM). The focus of this paper is to build a comercially vaible scanning capacitance microscope (SCM) which is compatible with an atomic force microscope (AFM) for imaging of embedded nanostructure and observe the kinetics of melcular binding. The goals of the design are to use commercially available conductive AFM tips, and to incorporate it into an existing AFM system. The design is to have a couple of orders of magnitude more sensitivity than existing comercially availble SCM,in orde to enable the use of 10- nm and smaller tips. Also the new instrument ia capable of operating as both SCM and AFM, enabling the characterization of both the surface and material properties such as the doping profile of silicon chips. Capacitance Measurement Challenge The design challenge is to measure the minute changes in capacitance ( 0.0001 Ff or better). This is complicated due to the fact that the conductive tip parasitic capacitance and the changes of this parasitic due to proximity to the substrate can make the measurement rendered useless.We used an inovative sheilding technique to eliminate the effect of the said parasitic capacitance. Also, measuring the change of the tip capacitance with existing impedance measuring equipment such as vector network analyzer is impossible. The technique used here is to build a resonator structure in conjunction with the tip that can operate in both transmission and reflection mode, therefore giving the vector network analyzer the ability to resolve the minute changes of the tip to ground impedance (capacitance). This approach measures the capacitance at the resonance and the higher order resonance frequencies of the coaxial resonator, enabling the user to measure the capacitance at multiple frequencies. This capability was not available up to now. A highly sensistive SCM using the said technique was built.A SRAM memory chip without the final metallization was scaned using the said instrument. This substarte is consisits of a bare silicon with the regions of different P and N doping concentrations. The scan was obtained using a coaxial half a wavelength cable resonator and a 50 Ohm shunt resistor to ground. This device was connected as a reflection mode to the Agilent's Vector Network analyzer,integrated with an MFP 3D AFM from Asylum Research. We measured the changes in the capacitance between the tip and silicon to ground, this corresponds to the minute changes in doping concentrations with an unprecedented sensitivity and resolution. This was also achieved without use of(existing state of the art) the DC modulation method (dc/dv) and locking amplifier to increase the measurement sensitivity. The instrument can also measure and observe the kinetics of melcular binding in real time.
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Nanotech 2007 Conference Program Abstract