Atif Imtiaz

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...

A novel STM-assisted microwave microscope with capacitance and loss imaging capability.

We report a new technique of scanning capacitance microscopy at microwave frequencies. A near field scanning microwave microscope probe is kept at a constant height of about 1 nm above the sample with the help of scanning tunneling microscope (STM) feedback. The microwaves are incident onto the sample through a coaxial resonator that is terminated at one end with a sharp tip (the same tip is used to conduct STM), and capacitively coupled to a feedback circuit and microwave source at the other end. The feedback circuit keeps the source locked onto the resonance frequency of the resonator and outputs the frequency shift and quality factor change due to property variations of the sample. The spatial resolution due to capacitance variations is congruent with 2.5 nm. The microwave microscope is sensitive to sample sheet resistance, as demonstrated through measurements on a doped silicon sample. We develop a quantitative transmission line model treating the tip to sample interaction as a series combination of capacitance and sheet resistance in the sample.

Nanometer-scale material contrast imaging with a near-field microwave microscope

The authors report topography-free material contrast imaging on a nanofabricated boron-doped silicon sample measured with a near-field scanning microwave microscope over a broad frequency range. The boron doping was performed using the focus ion beam technique on a silicon wafer with nominal resistivity of 61Ωcm. A topography-free doped region varies in sheet resistance from 1000Ω∕◻ to about 400kΩ∕◻ within a lateral distance of 4μm. The qualitative spatial resolution in sheet resistance imaging contrast is no worse than 100nm as estimated from the frequency shift signal.

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.

Effect of tip geometry on contrast and spatial resolution of the near-field microwave microscope

The near-field scanning microwave microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips and found them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1to12.6μm. Tips with larger rsphere show good signal contrast [as measured by the frequency shift (Δf) signal between tunneling height and 2μm away from the sample] with NSMM. For example, the tips with rsphere=8μm give signal contrast of 1000kHz compared to 85kHz with a tip of rsphere=0.55μm. However, large rsphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sa...

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 ...

Near-field microwave microscopy on nanometer length scales

The Near-field scanning microwave microscope (NSMM) can be used to measure ohmic losses of metallic thin films. We report on the presence of an interesting length scale in the probe-to-sample interaction for the NSMM. We observe that this length scale plays an important role when the tip-to-sample separation is less than about 10 nm. Its origin can be modeled as a tiny protrusion at the end of the tip. The protrusion causes deviation from a logarithmic increase of capacitance versus a decrease in the height of the probe above the sample. We model this protrusion as a cone at the end of a sphere above an infinite plane. By fitting the frequency shift of the resonator versus height data (which is directly related to capacitance versus height) for our experimental setup, we find the protrusion size to be 3–5 nm. For one particular tip, the frequency shift of the NSMM relative to 2 μm away saturates at a value of about −1150 kHz at a height of 1 nm above the sample, where the nominal range of sheet resistance...

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.

High frequency characterization of a Schottky contact to a GaN nanowire bundle

A two-port GaN nanowire (NW) device with one Schottky contact and one Ohmic contact was characterized up to 10 GHz using on-wafer microwave measurements. In addition to the measurement of the broadband response, two additional applications of microwave measurements are introduced: (1) the capability to distinguish a Schottky-type contact from an Ohmic contact based on the reflected broadband signals (S11 and S22) and (2) the measurement of a capacitance voltage (CV) curve for a Schottky contact to a bundle of a few NWs. The junction capacitance of the Schottky contact is determined at various bias voltages by fitting the broadband response with a microwave circuit model. The carrier concentration is estimated from the resulting CV curve to be 5.3×1018/cm3 and the Schottky barrier height is estimated to be 0.89 eV.