Nondestructive one-dimensional scanning capacitance microscope dopant profile determination method and its application to three-dimensional dopant profiles

Quantitative two-dimensional (2D) dopant profiling of a gatelike structure is achieved by scanning capacitance microscopy (SCM) on a cross-sectioned polished silicon wafer. The gatelike structures consist of heavily implanted n+ regions separated by a lighter doped n region underneath a 0.56 μm gate. The SCM is operated in the constant change capacitance mode while scanning with a 37 nm radius tip. The 2D SCM data are converted to dopant density through a physical model of the SCM/silicon interaction. The model parameters are adjusted so that the SCM dopant profile far from the gate edge fits the vertical secondary ion mass spectrometry (SIMS) profile. A 15% error in average accuracy is found between SCM and SIMS profiles evaluated over the dopant range of 1020–1017 cm−3. The same model parameters are used for all points in converting the 2D SCM data, indicating that the accuracy of the full 2D result should be comparable to that of the vertical profile. A direct comparison of the 2D SCM and 2D TSUPREM4 results is made for the first time. The agreement is generally good, but there are some notable differences.

Quantitative dopant profile measurements are performed on a nanometer scale by scanning capacitance microscopy (SCM). An atomic force microscope is used to position a nanometer scale tip at a semiconductor surface, and local capacitance change is measured as a function of sample bias. A new feedback method has been demonstrated in which the magnitude of the ac bias voltage applied to the sample is adjusted to maintain a constant capacitance change as the tip is scanned across the sample surface. A quasi-1D model is used to extract dopant density profiles from the SCM measurements. The inverted SCM dopant profiles are compared with profiles obtained by process simulation and secondary ion mass spectroscopy measurement. Good agreement was found between the SCM measured profile and the lateral profile predicted by SUPREM 4 over the concentration range from 1017 to 1020 cm−3.

Scanning capacitance microscopy (SCM) is a powerful tool for two-dimensional (2D) dopant/carrier profiling. Currently noise limits the accuracy of 2D dopant profiles obtained by SCM. In an effort to reduce noise, a systematic analysis of different SCM noise sources is provided. The main noise sources during SCM measurements are capacitance sensor noise and oxide–semiconductor surface induced noise. For adequate tip size, the dominant noise in SCM measurements is caused by variations in the quality of surface. On as-polished surfaces, nonstationary noise is observed. This noise is likely caused by the variations in the density of oxide traps. Tip induced charging of these traps and local variations or fluctuations in discharge time during SCM imaging cause the noise level and noise pattern to be different from image to image. Heat treatment under ultraviolet irradiation or in a hydrogen ambient is found to be an effective way to reduce or even eliminate this type of SCM noise. Stationary surface noise is mostly created by the variations in the oxide thickness. This type of noise correlates with topographic roughness and is very consistent during SCM measurements. By reducing the topographic roughness, the stationary surface noise may be reduced to the level of ∼10−2 of the depletion SCM signal for typical experimental conditions. It is shown that the capacitance sensor noise depends on the capacitance sensor tuning parameters and under proper conditions can be reduced to a negligible level for standard probe tips used in SCM measurements.

A scanning capacitance microscope (SCM) combines a differential capacitance measurement with an atomic force microscope (AFM). The AFM controls the position and contact force of a scanning probe tip, and a sensor simultaneously measures the capacitance between that tip and the sample under test. The capacitance sensor can detect very small (≈10 −21 F) changes in capacitance. The sensor is electrically connected to an AFM cantilevered tip that has been coated with metal to make it electrically conductive. As the SCM/AFM tip is scanned across the sample surface, simultaneous images of topography and differential capacitance are obtained. Currently, the most common application of SCM is imaging the carrier concentration variations (also known as dopant profiles) qualitatively within transistors that are part of silicon integrated circuits (ICs). Another application that has encouraged the development of commercial SCMs is the need for quantitative two‐dimensional (2‐D) carrier profiling of silicon transistors. This article reviews the history, operating principles, and applications of SCM. Special emphasis is placed on measuring quantitative 2‐D carrier profiles in silicon using SCM. This article also reviews implementation and applications of the scanning capacitance spectroscopy (SCS) technique, the intermittent contact mode of SCM (or IC‐SCM), and two other techniques that can also be classified as scanning capacitance probes: the scanning Kelvin probe microscope (SKPM), when operated at twice its drive frequency, and the scanning microwave microscope (SMWM).

Evaluation of scanning capacitance microscopy sample preparation by focused ion beam

Scanning capacitance microscope (SCM) images, and the two-dimensional (2D) dopant profiles extracted from them, show poor reproducibility from laboratory to laboratory. Major factors contributing to SCM image variability include: poor sample surface and oxide quality, excess carrier generation from stray light, reduced sensor dynamic range from stray capacitance, and use of nonoptimal SCM operating voltages. This article discusses the sources of SCM image variability, how they affect the measured SCM images, and possible approaches for mitigating their effects. Recommended procedures for extracting quantitative 2D are discussed. Finally, a set of informal research materials is introduced consisting of a complementary metal-oxide-semiconductor transistor pair, an identical pair without metallization, and a pair of transistor-like structures with the conductivity type of the source/drains reversed. These structures are intended for use with the FASTC2D software to help improve laboratory-to-laboratory dopant profile reproducibility.

Quantitative two-dimensional (2D) dopant profiling for 0.25 micron CMOS technology was demonstrated using secondary ion mass-spectrometry (SIMS) and scanning capacitance spectroscopy (SCS). P-well and N-well dopant profiles measured by 2D SIMS are in agreement with TSUPREM4 (TS4) simulations. TS4 was pre-calibrated to the specific technology using 1D SIMS and electrical data. 2D SIMS, TS4 and SCS show agreement on the well boundary location. SCS is an advanced form of scanning capacitance microscopy (SCM) devised for pn-junction delineation. 2D SIMS dopant profiling with lateral resolution of 10 nm (limited by the lithography pixel size) and sensitivity of 1/spl times/10/sup 17/ cm/sup -3/ was realized on commercial equipment.

The differential capacitance C(Vg) = dQM/dVg in a metal-oxide-semiconductor structure introduces the silicon capacitance Cs(ΨS) = −dQS/dΨS depending on the surface band-bending ΨS at the oxide–semiconductor interface. In order to calculate the dependence of Cs on the gate voltage Vg, we propose in this paper a simple numerical method, based on first-order Taylor expansions, to inverse the explicit equation Vg = f(ΨS). This method is then applied to calculate the analytic differential capacitance of the scanning capacitance microscope (SCM) in all conditions relative to physical parameters of SCM such as oxide thickness, doping profiles and probe erosion. It results in a competitive tool for SCM users to evaluate the theoretical values of capacitance and differential capacitance in all configurations.

In this work, cross-sectional scanning capacitance microscopy (SCM) is used to investigate InGaAs/InP quantum wells. It is demonstrated that SCM is indeed capable of detecting the electrons in the quantum wells and that the SCM signal shows a systematic trend for the different wells. Clear dips in the dC/dV signal are observed at the InGaAs quantum wells and imply carrier densities higher than the surrounding barriers. It is also shown that the depletion regions in the barriers adjacent to the wells can be resolved. The results show that geometric tip-averaging effects significantly influence the imaging of electrons in quantum wells and limit the lateral resolution.

In the present work, recent improvements in sample preparation and a tip quality evaluation are reported. A new method for cross-sectional surface preparation has been developed. The sample is heated at a temperature of 200–300C while being exposed to ultraviolet irradiation. This additional surface treatment improves dielectric layer uniformity, signal to noise ratio, and C-V curve flat band offset. The performance of three types of tips are also compared. It is shown that heavily doped silicon as well as worn tips are often depleted by the applied bias voltage causing errors in the Scanning Capacitance Microscope (SCM) measured dopant profile. When these effects are tested for and eliminated, SCM quantitative profiling over a 5-decade range of dopant density is demonstrated.

Nanowire transistors are typically undoped devices whose characteristics depend strongly on the injection of carriers from the electrical contacts. In this letter, we fabricate and characterize SiGe nanowire transistors with an n-p-n doping profile and with a top gate covering only the p-doped section of the nanowire. For each device, we locate the p-segment with scanning capacitance microscopy, where the p-segment position varies along the channel due to the stochastic nature of our dropcast fabrication technique. The current-voltage characteristics for a series of transistors with different gate positions reveal that the on/off ratios for electrons is the highest when the gated p-type section is closest to the source contact, whereas the on/off ratios for holes is the highest when the gated p-type section is closest to the drain contact.

Dopant profile measurements in ion implanted 6H–SiC by scanning capacitance microscopy

Cross-section analysis of electric devices by scanning capacitance microscope

Scanning microwave impedance microscopy (sMIM) is an emerging technique that can provide detailed information beyond that of conventional scanning capacitance microscopy (SCM), and other electrical scanning probe microscopy (SPM) techniques, for the investigation and failure analysis (FA) of semiconductor devices. Integration of new dielectric materials at lower levels of the device structure with the need for quantification of dielectric and dopants in semiconductor devices with sub-micron spatial resolution pushes the practical boundaries of typical atomic force microscopy (AFM) electrical modes. sMIM can measure both linear and non-linear materials (insulators and doped semiconductors, respectively) simultaneously. sMIM has a linear response to log k (dielectric number) and log N (doping concentration) making it an ideal method for providing quantitative measurements of semiconductor devices over a large range of values. This work demonstrates an example of a practical application of sMIM for quantitative measurement of the dopant concentration profile in production semiconductor devices. A planar dopant calibration sample is used to calibrate the sMIM prior to performing the measurements on an “unknown” production device. We utilize nanoscale C-V data to establish a calibration curve for both n- and p-type carriers and apply the calibration curve to an “unknown” device, presenting the measurements in units of doping concentration.

Single-grain Silicon Technology for Large Area X-ray Imaging