Scanning Microwave Microscopy: Unique Method Enables Measurements at the Nanoscale
A scanning microwave microscope consists of an Agilent AFM interfaced with an Agilent vector network analyzer . A microwave signal is sent directly from the network analyzer and transmitted through a resonant circuit to a conductive AFM probe that is in contact with a sample being scanned. The probe also serves as a receiver to capture the reflected microwave signal from the contact point. By directly measuring the complex reflection coefficient from the network analyzer, known as the S11 parameter, the impedance of the sample at each scanned point can then be mapped simultaneously with the surface topography. A half-wavelength impedance transformer is placed directly across a 50Ω load to form a matched resonance circuit. Combined with enhanced peak detection, this circuit significantly increases the dynamic range and sensitivity of the impedance measurement. With a superimposed low-frequency modulation, changes in the capacitance as a result of depletion of carriers in a semiconductor sample, or dC/dV, can also be obtained at the same time with an AC mode controller containing digital lock-in amplifiers.
SMM measurement is done at a fixed microwave frequency. However, a wide range of excitation frequencies (2 GHz to 20 GHz) is available to ensure that SMM users can select the optimal frequency to maximize the signal-to-noise ratio and achieve the best sensitivity. A special dopant profile measurement module (DPMM) is available to extend the capabilities of SMM mode to provide calibrated, absolute measurement of dopant densities, critical for advanced device physics studies.
For dopant profile measurement, the microwave signal from the VNA is divided into two parts within the DPMM. The first part is amplified and used as the local oscillator (LO) signal for the dC/dV mixer. The second part is amplified and sent through the main arm of the coupler to the AFM probe tip, where a low-frequency (radio frequency, RF) signal is also applied to the tip from an external source (e.g., a function generator).
Due to changes in the capacitance of the sample induced by the RF signal, the microwave signal is reflected and modulated at a rate equal to the RF frequency. The reflected, modulated microwave signal is then divided into two parts as well. The first signal is amplified and directed to the DPMM internal mixer, where it is mixed with the LO signal and demodulated. This demodulated signal is further processed using a lock-in amplifier for dC/dV amplitude and phase signal, which give the information of the dopant and its concentration in the sample. The second part of the reflected microwave signal is amplified and delivered to the VNA receiver, and is used to measure the capacitance of the sample. Therefore, SMM is capable of obtaining capacitance and dC/dV images simultaneously.
SMM Imaging of Dopant Structures of Semiconductor Devices
In the case of semiconductor devices, the mobile charge carrier in the doped region can either accumulate or deplete in the vicinity of a contact electrode depending on the DC bias applied [2, 3]. By applying an AC voltage, Vac, around a fixed working potential, Vdc, a change of capacitance dC in response to the modulation voltage Vac will be introduced . Vdc is usually chosen at that point where the C-V curve has the largest slope, which is around the flatband voltage. In this way, maximum sensitivity is achieved. Under these conditions, a higher dC/dV value corresponds to low carrier concentrations, while a lower dC/dV value corresponds to higher carrier concentrations. It is also clear that for both p- and n-type Si, dC/dV is the same in magnitude but of opposite sign (positive for p-type, negative for n-type) for identical carrier concentrations. Therefore, the modulation index of the reflected microwave signal (i.e., the magnitude and phase of the modulated signal measured by SMM) can be used to characterize the structure and type of dopant in semiconductor devices.
As stated earlier, SMM mode can be utilized to obtain capacitance and dC/dV images simultaneously. Capacitance (VNA amplitude) and dC/dV images of an integrated circuit (IC) sample are presented in fig.1 . The capacitance image, fig.1a, clearly reveals differences in the varied shallow doping that constructs devices within the active areas. Dramatic contrast difference in the dC/dV phase image, fig.1b, confirms the presence of sub-surface highly doped layers. A vertical line profile across the phase image (as indicated by the green line in fig.1b) is displayed in fig.1c. The measurement shows that the different bands in the IC are 180 degrees in phase mode, which suggests that those two different regions of the sample are dominated by opposite charge carriers, in correspondence to the existing bands of n-type buried layer (NBL) and p-type buried layer (PBL) under the active components. The circuitry map is also overlaid atop the VNA amplitude image to show the regions of the NBL and PBL. A small difference in capacitance can be seen between the two regions.
SMM Imaging of Highly Doped Marker Layers in GaN on Sapphire
Here, SMM is used to investigate the quality of GaN (gallium nitride, a III-V compound semiconductor with a wide bandgap) films and the origin of unintentionally doped regions in the films grown on a sapphire substrate.
Fig.2 shows SMM topography, a capacitance map, a dopant density map of the film cross-section, and a line profile across the dopant density map . The sapphire substrate is located on the left edge of the dataset; the wafer surface would be farther towards the right but it is not within the scan range shown. The topography shows a step from the substrate to the GaN layers, several steps within the GaN, and some undefined contaminants at the right edge. The capacitance map shows some contrast at the substrate-film interface and a regular pattern of bright and dark lines towards the wafer surface.
In SMM dopant density maps, undoped as well as highly doped material yields lower signal and is seen as darker regions. The dark regions comprise the sapphire substrate, the highly doped marker layers, and the undoped GaN layers. The bright features are regions with a low (lower than the highly doped layer, but higher than the undoped layer and the substrate) density of charge carriers. This is the case in the regular stripes towards the wafer surface. Between the layers dubbed "undoped" a layer of highly doped material was grown. Both materials have little dC/dV signal and show dark. Due to diffusion of carriers from the doped into the undoped layers a low density of carriers is present at the boundaries between the doped/undoped region and thus shows a high dC/dV signal. The straight regular stripes indicate that the film growth of these layers was regular and smooth.
Between the substrate and the smooth layers is another region of doped material. This region was unintentionally doped during the growth process. In this region, there are one to three dark bands meandering from left to right. The bands are highly doped marker layers. They mark the position of the growth surface at those times when dopant material was introduced. In the unintentionally doped region the doped layers show a strong fluctuation, indicating a rough surface during growth . Furthermore, SMM dopant density maps of larger regions (not shown here) appear to support the assumption that inclined surfaces are crucial for the growth of unintentionally doped material .
Additional SMM Applications
Scanning microwave microscopy can be used on metals, semiconductors, dielectric materials, ferroelectric materials, insulators, and even biological materials to measure a variety of properties associated with slight variations in the electromagnetic interactions of different components of a sample with the incident microwave signal, either statically or dynamically. SMM mode is capable of mapping a material’s properties at a resolution ultimately limited by the sharpness of the probe.
For instance, SMM can be utilized to study the dielectric properties of organic thin films, including self-assembled monolayers (SAMs) of organic molecules. When carefully calibrated, SMM can detect small capacitance differences across the organic film in the attofarad range . With the knowledge of the film thickness from a material of known relative dielectric constants, the effective contact area between the tip and the sample can be estimated based on established models. This estimate of the effective contact area can then be applied to measure dielectric constants or film thickness of other films, using the same probe.
Another important area in which the use of SMM mode is rapidly proving invaluable to researchers is the study of novel materials such as graphene. The outstanding versatility and precision of SMM can be expected to provide great utility for many future applications as well.