Robert Stephenson

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

Evaluating probes for “electrical” atomic force microscopy

The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes p...

Contrast reversal in scanning capacitance microscopy imaging

We have investigated the quantification properties of scanning capacitance microscopy (SCM) by using two dedicated test structures and highlight the response of SCM to changes in dopant density. Our results indicate that contrast reversal occurs and that the SCM output is not always a monotonically increasing signal with decreasing dopant density. Two epitaxially grown staircase structures covering the doping ranges 1014–1020 cm−3 p type and 5×1014–5×1019 cm−3 n type were produced for this study as the turning point in the response function typically occurs at a doping level of around 1017 cm−3. Through the use of a simple simulation model we see that contrast reversal is expected due to a relative shift between the dC/dV curves for different doping levels. The onset of contrast reversal can be adjusted by changing the dc sample bias leading to a shift in the operating position of the SCM, and the significance of this point will be discussed here.

Nonmonotonic behavior of the scanning capacitance microscope for large dynamic range samples

The phenomenon of contrast reversal in scanning capacitance microscopy (SCM) imaging will be discussed, taking into account the implications for samples which contain both p-type and n-type dopants. Experiments show that a monotonic change in SCM output versus dopant concentration for large dynamic range samples (1014–1020 cm−3) is dependent on the applied dc bias. Incorrect adjustment of this parameter can lead to contrast reversal in the SCM images causing a problem for conversion algorithms and dopant quantification. Simulation results demonstrating this feature will be presented. In addition, the appearance of shifting bands which are common in voltage-dependent measurements of p-n junctions complicates the data interpretation and extraction of length measurements within an image, such as the effective electrical channel length in the case of transistor characterization. The significance of these problems with respect to the SCM response curve will be discussed.

Practicalities and limitations of scanning capacitance microscopy for routine integrated circuit characterization

We have imaged several n-type metal-oxide-semiconductor transistors with different source and drain architectures to assess the feasibility of extracting useful figures of merit, such as the effective channel length of a device, from the data. By varying the dc bias on the sample we observe a shift of the junction position in the image and consider how best to interpret a set of voltage dependent images produced for a single sample. Careful attention is paid to the effects of surface variation from sample preparation and tip wear during an experiment by considering the scanning capacitance microscopy signal in the substrate as a function of applied dc bias.

New aspects of nanopotentiometry for complementary metal–oxide–semiconductor transistors

Nanopotentiometry is a scanning probe microscopy (SPM) technique providing insight in the actual working behavior of semiconductor devices under operation. In nanopotentiometry, a conductive SPM tip is used as a voltage probe in order to measure the distribution of the electrical potential on the cross section of an operating device. The information thus provided is complementary to carrier profiling and is a method for the calibration of device simulations. The suitability of alternative SPM techniques for studying ultrashallow devices under operation is examined. Measurements have been carried out in deep submicron complementary metal–oxide–semiconductor devices. The impact of the changes in the doping profile on the potential distributions has been explored using simulations and experimental results. Due to further improvements in sample preparation and measurement methodology, the response of the devices to variable bias conditions could be studied in more detail. The formation of the conductive chann...

Fabrication and use of metal tip and tip-on-tip probes for AFM-based device analysis

Different techniques based on the atomic force microscope (AFM) have been developed in the last few years for the electrical characterization of semiconductor devices. The quality of these measurements strongly depends on the tip which should not only have a small radius of curvature but also a high electrical conductivity. Therefore, the choice of metal as tip material is obvious. We have developed a process scheme for the fabrication of pyramidal metal tips which are integrated into a silicon cantilever. This paper discusses this process in detail and shows how the transition was made from prototyping to batch friction using standard 150 mm silicon wafer technology. Results are presented concerning the application of such probes for two-dimensional carrier profiling of InP and silicon structures using scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). A novel tip configuration called tip-on-tip has also been developed. This concept looks promising for future applications. We demonstrate how such a tip-on-tip configuration can be realized.