Peter De Wolf

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.

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.

Dopant characterization round-robin study performed on two-dimensional test structures fabricated at Texas Instruments

The lack of a two-dimensional (2D) dopant standard and hence a priori knowledge of dopant distribution makes it impossible to unambiguously judge accuracy of any experimental or theoretical effort to characterize silicon doping in two-dimensions. Recently a strong progress has been made in quantitative scanning capacitance microscopy (SCM), scanning spreading resistance microscopy (SSRM), secondary electron (SE) and transmission electron microscopy (TEM) doping profiling. Several research groups have claimed an ability of quantitative 2D dopant characterization. A round-robin study involving various analytical techniques and comprehensive numerical simulations should help to evaluate an accuracy of available quantitative techniques and set some helpful standard for further development. We report on a world-wide round-robin study performed on CMOS 2D test structures fabricated at Texas Instruments (TI). Seven research groups, which represent an advanced SCM, SSRM and TEM dopant profiling, participated in t...