Cross-section analysis of electric devices by scanning capacitance microscope

The development of methods to measure electrical properties, which are suitable to directly yield the desired carrier distributions on a nanometer scale has greatly benefited from the development of scanning probe technology over the last decade. Scanning Probe Microscopes (SPMs) offer inherent two-dimensionality and have been shown to have applications ranging from Magnet force to electro-chemistry. We have used an SPM in contact mode to simultaneously measure topography (and therefore physical structure) and capacitance variations (due to an applied bias) of various electronic materials such asdoped silicon, poly silicon, SiC, and III-V materials.The technique aptly named Scanning Capacitance Microscopy (SCM) takes advantage of the fact that electrical carrier response to an applied electric field is largely dependent upon the local carrier concentration. Using a high 'Q' GHz resonant circuit, SCM measures capacitance variations due to an applied bias between the metalized nano-probe tip and semiconductor sample during scanning. Since thesevariations are directly related to dopant (carrier) concentration, the SCM generates a two-dimensional image with contrast corresponding to near-surface variations in carrier density. Because the measurement is done with an extremely sharp probe, we have been able to resolve features as small as lOnm, corresponding to attofarad (le-18 farad) capacitance changes.

In this chapter, we discuss recent developments in the characterization of polarization doped wide bandgap semiconductor (WBS) heterojunctions with an emphasis on characterization by scanning probe microscopy (SPM). Wide bandgap semiconductors, such as SiC and GaN, have a wide variety of applications in electronic and optoelectronic devices [1–6]. The operation of devices made from these materials is dominated by the polarization properties of III-N and SiC semiconductor crystals. For example, polarization doped 2 dimensional electron gas (2DEG) channels in undoped GaN/AlGaN and SiC polytypic heterojunctions have been fabricated with current densites in excess of 1A/mm. Other effects, such as surface charge instability stemming from polarization, offer interesting potentials and challenges.While some of these effects may be studied using more conventional techniques such as currentvoltage (IV) and capacitance-voltage (CV) techniques (described in sections 2 and 3), others require probes capable of investigation on the nanometer scale. Transmission electron microscopy (TEM) can be used to investigate such polarizationinduced phenomena with atomic resolution, but involves destructive and laborious sample preparation. SPM techniques offer powerful, non-destructive tools to effectively and conveniently probe electronic materials and devices. Since the discovery of scanning tunneling microscopy (STM) in 1981 by Binnig and Rohrer [7], the field of SPM has advanced to encompass a large number of variations including, (i) STM, (ii) Atomic force microscopy (AFM), (iii) Electrostatic force microscopy (EFM), (iv) Scanning capacitance microscopy (SCM), (v) Scanning conduction microscopy (SCM), (vi) Magnetic force microscopy (MFM), (vii) Piezoelectric force microscopy (PFM), and (vii) Near field scanning optical microscopy (NSOM). Of the above modes, the first five modes have been extensively used for semiconductor characterization [8–15]. For WBS characterization, especially III-N, AFM, EFM and SCM modes are the most important due to the strong polarization of III-N and SiC. In the latter half of this chapter, we focus on the characterization of III-N by EFM operated in

In this article, we present a new one-dimensional (1D) dopant profile determination method, which extends to the quantitative three-dimensional (3D) dopant profile extraction. This nondestructive method, which is different from the common scanning capacitance microscopy (SCM) measurement/dopant extraction, can potentially measure real metal–oxide–semiconductor field-effect transistor devices having 3D structure. Through SCM modeling, we found that the depletion layer in silicon was of a form of a spherical capacitor with the SCM tip biased. Two-dimensional (2D) finite differential method code with a successive over relaxation (SOR) solver has been developed to model the measurements by SCM of a semiconductor wafer that contains an ion-implanted impurity region. Then, we theoretically analyzed the spherical capacitor and determined the total depleted-volume charge Q, capacitance C, and the rate of capacitance change with bias dC/dV. It is very important to observe the depleted carriers’ movement in the silicon layer by applying the bias to the tip. So, we calculated the depleted-volume charge, considering different factors such as tip size, oxide thickness, and applied bias (dc+ac), which have an influence on potential and depletion charges. Finally, we developed a 1D inversion algorithm to convert the SCM output (dC/dV) into real dopant concentration, comparing the SCM signal output with the calculated dC/dV. Using the inversion modeling, we have quantitatively extracted the 1D dopant profile from the SCM dC/dV vs V curves.

We report the use of multilayer graphene (MLG) grown by chemical vapor deposition (CVD) approximately 75 layers (26 nm) thick configured in a simple electrochemical device. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI-] (ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]]) resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm−1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm−1 ), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI− ion and the associated change in the free carrier density (Δn = 1021 cm−3). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔEF= 1.1 eV. The thermal emissivity of the MLG device is also substantially modulated over the range from 7 – 14 µm. Using this device, have demonstrated free-space optical communication that utilizes ambient Planck radiation from a warm body and modulates the emitted intensity. We present an experimental demonstration achieving error-free communications at 100 bits per second over laboratory scales. We have also developed an AC Raman technique for measuring the Raman spectra during the intercalation−deintercalation half cycles in a multilayer graphene (MLG) device operating at 0.2−10 Hz. Raman spectra taken just 200 ms apart show the emergence and disappearance of the intercalated G band mode at around 1610 cm−1 . By integration of over hundreds of cycles, a significant Raman signal can be obtained. The intercalation/ deintercalation is also monitored with thermal imaging via voltage-induced changes in the carrier density, complex dielectric function ε(ω), and thermal emissivity of the device. “Dynamic Study of Intercalation/Deintercalation of Ionic Liquids in Multilayer Graphene Using an Alternating Current Raman Spectroscopy Technique” Zhi Cai, Haley Weinstein, Indu Aravind, Ruoxi Li, Sizhe Weng, Boxin Zhang, Jonathan L. Habif, and Stephen B. Cronin. Journal of Physical Chemistry Letters, 14, 7223 (2023).“Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions” Zhi Cai, Indu Aravind, Haley Weinstein, Ruoxi Li, Jiangbin Wu, Han Wang, Jonathan Habif, and Stephen Cronin. Journal of Applied Physics. 132, 095102 (2022).“Harvesting Planck radiation for free-space optical communications in the long-wave infrared band“ Haley A. Weinstein, Zhi Cai, Stephen B. Cronin, and Jonathan L. Habif, Optics Letters. 47, 6225-6228 (2022). Figure 1

The scanning Kelvin probe force microscope (SKPM) is a member of the scanning probe microscope (SPM) family, and was derived from the non-contact atomic force microscope (NCAFM) technique. The contact potential difference (CPD) originating from the work function difference between the tip and sample surfaces can be measured using SKPM with simultaneous observation of the topography image. Using SKPM the surface of semiconductor device has been observed to measure the dopant concentration as the CPD difference in two dimensions. Most of the SKPM measurements are acquired in the atmosphere. The lateral resolution is achieved less than 100 nm.On the other hand, Scanning capacitance microscope (SCM) of contact mode is now most popular method to measure the dopant concentration in semiconductor device. However, the lateral resolution of SCM is limited, because of the fact that the contact mode cannot achieve a true atomic resolution, the fact that the capacitance is measured through the insulating layer on surface, and the fact that the tip and sample is damaged by the high electric field to be applied between the tip and sample. So SCM never achieves the atomic level resolution.

We used scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) to determine the drift hole mobility in nanometer thick Si∕Si0.75Ge0.25∕Si quantum wells (QWs), from the carrier concentration profiles (obtained by SCM) and the local resistivity values (obtained by SSRM). A relevant decrease of the hole mobility at room temperature was observed when reducing the QW width from 10 nm down to 1 nm. This effect has been explained in terms of the increasing role of surface scattering at the Si∕SiGe interface for lower QW widths.

Nanolithography and nanoindentation of tantalum-oxide nanowires and nanodots using scanning probe microscopy

In this paper, we present our recent applications of scanning capacitance microscopy (SCM) on specific devices with sampling window as small as 100nm. The dopant related root causes were successfully identified on those devices fabricated with 90nm CMOS technology. The key step in our approach is the development of a sample preparation technique that allows us to precisely x-section through a transistor without being affected by focused ion beam (FIB) artifacts. FIB was used to mark the area of interest with high precision, but it did not expose the devices of interest. Optical microscope and atomic force microscope (AFM) were used to inspect the mechanically polished surface, thus avoiding beam effects from FIB or SEM. In the first application, a doping anomaly was identified in a PFET poly gate, in a single bit failed SRAM cell. In the second application, an asymmetry of a PWell implant profile in a window of 150nm was identified as the cause of leakage in a capacitor array. Our approach may be applied to other scanning probe microscopy (SPM) techniques in the same category, i.e., scanning spreading resistance microscopy (SSRM) or scanning microwave microscopy (SMM).

Evaluation of scanning capacitance microscopy sample preparation by focused ion beam

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.

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.

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.

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).

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Scanning capacitance microscopy (SCM) is based on a contact-mode variant of atomic force microscopy, which is used for imaging two-dimensional carrier (electrons and holes) distributions in semiconductor devices. We introduced a method of quantification of the carrier concentration by experimentally deduced calibration curves, which were prepared for semiconductor materials such as silicon and silicon carbide. The analytical procedure was circulated to research organizations in a round-robin test. The effectiveness of the method was confirmed for practical analysis and for what is expected for industrial pre-standardization from the viewpoint of comparability among users. It was also applied to other electric scanning probe microscopy techniques such as scanning spreading resistance microscopy and scanning nonlinear dielectric microscopy. Their depth profiles of carrier concentration were found to be in good agreement with those characterized by SCM. These results suggest that our proposed method will be compatible with future next-generation microscopy.