Thomas M. Wallis

Calibrated nanoscale capacitance measurements using a scanning microwave microscope

A scanning microwave microscope (SMM) for spatially resolved capacitance measurements in the attofarad-to-femtofarad regime is presented. The system is based on the combination of an atomic force microscope (AFM) and a performance network analyzer (PNA). For the determination of absolute capacitance values from PNA reflection amplitudes, a calibration sample of conductive gold pads of various sizes on a SiO(2) staircase structure was used. The thickness of the dielectric SiO(2) staircase ranged from 10 to 200 nm. The quantitative capacitance values determined from the PNA reflection amplitude were compared to control measurements using an external capacitance bridge. Depending on the area of the gold top electrode and the SiO(2) step height, the corresponding capacitance values, as measured with the SMM, ranged from 0.1 to 22 fF at a noise level of ~2 aF and a relative accuracy of 20%. The sample capacitance could be modeled to a good degree as idealized parallel plates with the SiO(2) dielectric sandwiched in between. The cantilever/sample stray capacitance was measured by lifting the tip away from the surface. By bringing the AFM tip into direct contact with the SiO(2) staircase structure, the electrical footprint of the tip was determined, resulting in an effective tip radius of ~60 nm and a tip-sample capacitance of ~20 aF at the smallest dielectric thickness.

Calibrated nanoscale dopant profiling using a scanning microwave microscope

The scanning microwave microscope is used for calibrated capacitance spectroscopy and spatially resolved dopant profiling measurements. It consists of an atomic force microscope combined with a vector network analyzer operating between 1–20 GHz. On silicon semiconductor calibration samples with doping concentrations ranging from 1015 to 1020 atoms/cm3, calibrated capacitance-voltage curves as well as derivative dC/dV curves were acquired. The change of the capacitance and the dC/dV signal is directly related to the dopant concentration allowing for quantitative dopant profiling. The method was tested on various samples with known dopant concentration and the resolution of dopant profiling determined to 20% while the absolute accuracy is within an order of magnitude. Using a modeling approach the dopant profiling calibration curves were analyzed with respect to varying tip diameter and oxide thickness allowing for improvements of the calibration accuracy. Bipolar samples were investigated and nano-scale de...

Near-field microwave microscope measurements to characterize bulk material properties

The authors discuss near-field scanning microwave microscope measurements of the complex permittivity for bulk dielectric (fused silica), semiconductor (silicon), and metal (copper). The authors use these measurements to test existing quasistatic theoretical approach to deembed the bulk material properties from the measured data. The known quasistatic models fit the measured data well with parameters for silicon (es=11.9, σSi=50S∕m) and fused silica (es=3.85, tanδ=1.0×10−4). However, for copper (with σCu=5.67×107S∕m), apart from quasistatic coupling, an additional loss of 12Ω is needed to fit the data.

Frequency-selective contrast on variably doped p-type silicon with a scanning microwave microscope

We report on frequency-dependent contrast in d(S11)/dV measurements of a variably doped p-type silicon sample in the frequency range from 2 GHz to 18 GHz. The measurements were conducted with a scanning microwave microscope. The measurements were done at selected frequencies while varying the DC tip voltage. The measured d(S11)/dV signal shows a maximum for doping concentrations (NA) of 1015 cm−3−1016 cm−3 at 2.3 GHz. As the microscope operating frequency is increased, this maximum sequentially “switches” through the regions of increasing dopant concentration, displaying a maximum for NA of 1017 cm−3−1018 cm−3 at 17.9 GHz. The frequency dependent “switching” is attributed to the physics of tip-to-sample interaction, particularly as related to the frequency-dependent local surface resistance and the depletion capacitance that control the RC time constant of tip-to-sample interaction. This provides a unique platform for local, frequency-selective, spatially resolved microwave spectroscopy of semiconducting ...

Einstein–de Haas effect in a NiFe film deposited on a microcantilever

A method is presented for determining the magnetomechanical ratio g′ in a thin ferromagnetic film deposited on a microcantilever via measurement of the Einstein–de Haas effect. An alternating magnetic field applied in the plane of the cantilever and perpendicular to its length induces bending oscillations of the cantilever that are measured with a fiber optic interferometer. Measurement of g′ provides complementary information about the g factor in ferromagnetic films that is not directly available from other characterization techniques. For a 50nm Ni80Fe20 film deposited on a silicon nitride cantilever, g′ is measured to be 1.83±0.10.

A Framework for Broadband Characterization of Individual Nanowires

A framework for broadband characterization of individual nanowires (NWs) is discussed in this letter. Specifically, on-wafer multiline thru-reflect-line (TRL) measurements, finite element modeling, and specially fabricated test structures with both extremely high and low impedances are jointly used to validate the feasibility of both measurements and modeling for characterizing small components. The test structures are designed as coplanar waveguide (CPW) devices with 100 nm and 250 nm diameter platinum (Pt) NWs. Though it is not possible to distinguish between the conductivity of the wire and contact resistances, we determine a range for conductivity and contact resistance over wide microwave bandwidth by minimizing the standard deviation between the measurements and full-wave modeling.

Wideband measurement of extreme impedances with a multistate reflectometer

We present a technique for accurate wideband measurements of one-port devices with extreme impedances. Our technique uses a reflectometer with variable parameters (states) to obtain redundant measurements of the extreme impedance device. We process these measurements using statistical techniques that allow us to exploit the redundancy in order to increase the measurement bandwidth and reduce the measurement uncertainty. We demonstrate our technique for a simple setup containing a power splitter and an unknown variable reference impedance connected to one of its arms and an unknown extreme-impedance device connected to its other arm. The variable reference impedance is realized as either a set of mechanical standards or an electronically tunable impedance. Measurement results show that the repeatability of the reference impedance values is essential for achieving increased accuracy.

Microwave Near-Field Imaging of Two-Dimensional Semiconductors

Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS2 and n- and p-doped WSe2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects.

A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured across a sample over a frequency range of 1 GHz - 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S(11) minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se(2) (CIGS) sample. The S(11) and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.

Electrical Characterization of Photoconductive GaN Nanowires from 50 MHz to 33 GHz

The electrical response of two-port photoconductive GaN nanowire devices was measured from 50 MHz to 33 GHz. The admittance of the nanowire devices showed an increase on the order of 10% throughout the measured frequency range after exposure to steady ultraviolet illumination. Two different two-port microwave network models were used to extract microwave circuit parameters in the photoconductive and dark states. After illumination, the GaN nanowire devices showed a measurable increase in shunt capacitance and decreases in both the contact and nanowire resistances.