When a scientist is looking to determine the topography of a material, atomic force microscopy (AFM) has been the go-to imaging technique for many years. While AFM is a powerful technique, it has also given rise to a series of other useful methods that use the principles of AFM alongside the principles of other processes, where the product becomes more significant than the sum of its parts.
In this article, we look at one of these techniques known as scanning capacitance microscopy (SCM) which uses the principles of AFM and scanning probe microscopy (SPM) to measure variations in capacitance between the sample and the tip.
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What is Scanning Capacitance Microscopy (SCM)?
Scanning capacitance microscopy (SCM) is a technique that takes operational principles from both atomic force microscopy (AFM) and scanning probe microscopy (SPM), and is used to generate a topographic map of a sample alongside its corresponding capacitance image. It relies on the location principles of AFM, alongside the scanning principles of SPM, to produce information about a materials electrical capacitance.
There are many different variations of AFM that only differ by the type of tip, how the tip interacts with the sample, and the information generated from this interaction. SCM is no different and uses a metal coated probe tip to scan the surface, to map the localized differences in capacitance, charge carrier concentration and dopant profile of a material. It is a technique which is mostly associated with semiconductors, particularly metal-oxide-semiconductors (MOS), as the oxygen atoms play a vital role in the measurement process.
How SCM Works
SCM is a contact mode technique composed of two different sets of principles. The AFM principles are standard throughout the many different variations and are used in SCM to locate the position of a capacitance measurement by deflecting a laser beam off the cantilever onto a position-sensitive photodiode (PSPD). While SCM still uses a sharp silicon tip, it is coated with a metallic thin film. The most common metal coatings for tips used in SCM are Pt/Ir and Co/Cr.
The SPM-type principles are where SCM differentiates itself from conventional AFM. Even though it is a contact technique, it also a non-destructive method. It also provides an image of both the topography and variations in capacitance across the surface with a high spatial resolution.
The system itself is composed of the metal tip attached to a cantilever beam, a highly sensitive capacitive sensor, and the usual AFM components. When the tip is set to scan the sample, a voltage is passed through it. The current applied can be either an alternating current (AC) or a direct current (DC).
This then makes the tip act as one electrode while the surface of the material acts as a second electrode. Any oxides on the surface of the material act as the dielectric material and the tip acts as the ground. This turns the surface into a capacitor.
Two capacitors are formed in series. The first being from the insulating oxide layer and the second is from the depletion layer close to the oxide-silicon interface.
The measured capacitance signal is dependent on many factors. The capacitance is determined using the thickness of the oxide material, the thickness of the depletion layer and the applied voltage between the tip and the surface. As such, measuring the changes in capacitance can yield a large amount of information. Some of the important information that can be obtained include the contact area between the tip and the surface, the properties of the oxide material and the concentration of charge carriers within the semiconductor material.
There are cases when there are no oxide groups present in the sample. In these circumstances, the interface between the sample, and the tip acts as a Schottky contact. The interactions then become more complex and the current flows between the two capacitor plates.
In these situations, the electrons accumulate at the surface of the material, and when the voltage on the gate turns negative, the electrons move away from the material. This causes a depletion of carriers and causes a drop in the capacitance. This more complex mechanism is the reason why SCM is often used with oxide-based semiconductor materials.
As it is closely related to two widely used techniques, there are few different ways in which SCM can be used. One of the most common applications is for the characterization of semiconductor materials, and it fills the gap where other high-powered microscopy techniques have been ineffective. It can be used to measure the dopant levels, the extent of defects, charge carrier concentrations and the oxide gate properties in MOS devices.
Outside of these common areas, SCM is also a powerful technique for the determination of a material’s local dielectric properties and a materials resonant electronic structure. It has also been known to image two-dimensional electron gases under cryogenic conditions and a magnetic field.