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

Atomic and subnanometer resolution in ambient conditions by atomic force microscopy

The review will describe the various scanning probe microscopy tips and cantilevers used today for scanning force microscopy and magnetic force microscopy. Work undertaken to quantify the properties of cantilevers and tips, e.g. shape and radius, is reviewed together with an overview of the various tip–sample interactions that affect dimensional measurements.

https://iopscience.iop.org/article/10.1088/0022-3727/41/10/103001/pdf

Aspects of scanning force microscope probes and their effects on dimensional measurement

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.

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

We report a study of the repeatability as limited by instrumental imaging distortions in scanning-probe microscope (SPM) measurements of the heights of nominal 44 and 88 nm steps in calibration artifacts. By imaging the same series of locations on different days, we are able to distinguish sample variations from variations originating in the imaging process. In particular, the value and repeatability of the measured step heights are found to depend upon the algorithm used to infer the step height from the SPM image. The three general approaches tested are: a manual single-point method, which represents the most commonly used practice in the SPM community; a histogram-based method, which is available in commercially available SPM image-analysis software; and the polynomial step-function fit (PSFF), which explicitly removes image bow and sample tilt. Factors related to variations in the sample measured such as image size and location on the sample cause up to 10% variations in the step-height measurements. ...

Vertical metrology using scanning-probe microscopes: Imaging distortions and measurement repeatability

Characterization of a high selectivity polysilicon chemical mechanical polishing (CMP) is studied in this report. The process is developed for the fabrication of two-dimensional secondary ion mass spectrometry (2D SIMS) test chip, in which a silicon trench is filled with polysilicon and polished back in an effort to create a flat surface. In this application, the polysilicon surface roughness and the step height between polysilicon and c-Si substrate must be minimized. Atomic force microscopy shows that the CMP process reduces polysilicon surface roughness. The step height between polysilicon and c-Si substrate can be minimized by choosing the proper thickness for the polish-stop layer (e.g., silicon nitride) over the c-Si surface. We found that the step height between the polysilicon and the c-Si surface is relatively sensitive to the opening of the trench, but not sensitive to slurry selectivity and total polishing time. We speculate that deformation of the polishing pad is critical for the trench recess. In addition, we discuss important issues of characterizing the trench recess. Using a histogram of height, the high and low modes of the distribution are sometimes used to identify the trench recess. We showed that this approach results in various offsets from the peak-to-valley height.

http://jes.ecsdl.org/content/146/3/1158.full.pdf

Elisa U

Papers

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

Vertical metrology using scanning-probe microscopes: Imaging distortions and measurement repeatability

Control of Polysilicon Surface Topography in a High Selectivity Chemical Mechanical Polishing Process

Vertical metrology using scanning-probe microscopes: Imaging distortions and measurement repeatability