Atomic force microscopy (AFM) is a key tool for nanotechnology research and finds its principal application in the determination of surface topography. However, the use of the AFM tip as a probe of electrical properties allows enormous insights into material functionality at the nanoscale. Hence, a burgeoning suite of techniques has been developed to allow the determination of properties such as resistivity, surface potential and capacitance simultaneously with topographic information. This has required the development of new instrumentation, of novel probes and of advanced sample preparation techniques. In order to understand and quantify the results of AFM-based electrical measurements, it has proved important to consider the interplay of topographic and electrical information, and the role of surface states in determining a material's electrical response at the nanoscale. Despite these challenges, AFM-based techniques provide unique insights into the electrical characteristics of ever-shrinking semiconductor devices and also allow us to probe the electrical properties of defects and self-assembled nanostructures.

https://iopscience.iop.org/article/10.1088/0034-4885/71/7/076501/pdf

Advances in AFM for the electrical characterization of semiconductors

▪ Abstract The scanning capacitance microscope (SCM) provides a direct method for mapping the dopant distribution in a semiconductor device on a 10 nm scale. This capability is critical for the development, optimization, and understanding of future ULSI processes and devices. The basic elements of the SCM and its application to nanometer scale metal oxide semiconductor (MOS) capacitor measurements are described. Experimental SCM methods are reviewed. Basic measurements show that nanometer scale capacitance-voltage relations are understood. High-quality probe tips and surfaces are critical for obtaining accurate measurements of two-dimensional dopant profiles. Quantitative modeling of SCM measurement is described for converting raw SCM data to dopant density. An inverse modeling method is presented. Direct comparison between secondary ion mass spectroscopy (SIMS) and SCM-measured dopant profiles are made. Quantitative junction measurements and models are discussed and images of small transistors are presented.

Two-dimensional dopant profiling by scanning capacitance microscopy

An overview of the existing two-dimensional carrier profiling tools using scanning probe microscopy includes several scanning tunneling microscopy modes, scanning capacitance microscopy, Kelvin probe microscopy, scanning spreading resistance microscopy, and dopant selective etching. The techniques are discussed and compared in terms of the sensitivity or concentration range which can be covered, the quantification possibility, and the final resolution, which is influenced by the intrinsic imaging resolution as well as by the response of the investigated property to concentration gradients and the sampling volume. From this comparison it is clear that, at present, none of the techniques fulfills all the requirements formulated by the 1997 Semiconductor Industry Association roadmap for semiconductors [National Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 1997)]. Most methods are limited to pn-junction delineation or provide a semiquantitative image of the differen...

Status and review of two-dimensional carrier and dopant profiling using scanning probe microscopy

Emerging trends in surface metrology

A doping approach for p-type ZnO is reported which is reproducible and long-time stable For p-type doping the zinc oxide layers were doped simultaneously with nitrogen and arsenic in metal organic vapor phase epitaxy The conductivity type of the layers was investigated by scanning capacitance microscopy, a technique based on local capacitance-voltage analysis (C-V) with submicron spatial resolution Depending on the growth parameters, largely extended p-type domains were observed, surrounded by n-type regions The differences in local conductivity type are directly correlated to the topography as measured with atomic force microscopy revealing p-type for smooth, two-dimensional surfaces and n-type signals in the case of three-dimensional island growth or structural defects, ie, microcracks or surface pits

Local p-type conductivity in zinc oxide dual-doped with nitrogen and arsenic

Scanning capacitance spectroscopy (SCS), a variant of scanning capacitance microscopy (SCM), is presented. By cycling the applied dc bias voltage between the tip and sample on successive scan lines, several points of the high-frequency capacitance–voltage characteristic C(V) of the metal–oxide–semiconductor capacitor formed by the tip and oxidized Si surface are sampled throughout an entire image. By numerically integrating dC/dV, spatially resolved C(V) curves are obtained. Physical interpretation of the C(V) curves is simpler than for a dC/dV image as in a single-voltage SCM image, so that the pn junction may be unambiguously localized inside a narrow and well-defined region. We show SCS data of a transistor in which the pn junction is delineated with a spatial resolution of ±30 nm. This observation is consistent with the conclusion that SCS can delineate the pn junction to a precision comparable to the Si depletion width, in other words, the actual size of the electrical pn junction. A physical model to explain the observed SCS data near the pn junction is presented.

Michael Gribelyuk

Papers

Scanning capacitance spectroscopy: An analytical technique for pn-junction delineation in Si devices