Scanning Capacitance Microscopy Measurements Research Articles

Dopant activity for highly in-situ doped polycrystalline silicon: hall, XRD, scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM)

Progressing miniaturization and the development of semiconductor integrated devices ask for advanced characterizations of the different device components with ever-increasing accuracy. Particularly in highly doped layers, a fine control of local conduction is essential to minimize access resistances and optimize integrated devices. For this, electrical Atomic Force Microscopy (AFM) are useful tools to examine the local properties at nanometric scale, for the fundamental understanding of the layer conductivity, process optimization during the device fabrication and reliability issues. By using Scanning Capacitance Microscopy (SCM) and Scanning Spreading Resistance Microscopy (SSRM), we investigate a highly in situ doped polycrystalline silicon layer, a material where the electrical transport properties are well known. This film is deposited on a oxide layer as a passivating contact. The study of the nano-MIS (SCM) and nano-Schottky (SSRM) contacts allows to determine the distribution and homogeneity of the carrier concentration (active dopants), especially by investigating the redistribution of the dopants after an annealing step used for their activation. While the chemical analysis by Secondary Ions Mass Spectroscopy (SIMS) quantifies only the dopant concentration in the polycrystalline layer, the comparison with macroscopic characterization techniques as Hall effect measurements, supported with XRD characterization, shows that careful SCM and SSRM measurements can be used to highlight the dopant activation. This analysis gives a complete investigation of the local electrical properties of the passivating contact when the parameters (applied voltages and applied forces) of the AFM nano-contacts are correctly controlled.

Design, modelling, and simulation of a floating gate transistor with a novel security feature

This study proposes a new generation of floating gate transistors (FGT) with a novel built-in security feature. The new device has applications in guarding the IC chips against the current reverse engineering techniques, including scanning capacitance microscopy (SCM). The SCM measures the change in the C–V characteristic of the device as a result of placing a minute amount of charge on the floating gate, even in nano-meter scales. The proposed design only adds a simple processing step to the conventional FGT by adding an oppositely doped implanted layer to the substrate. This new structure was first analyzed theoretically and then a two-dimensional model was extracted to represent its C–V characteristic. Furthermore, this model was verified with a simulation. In addition, the C–V characteristics relevant to the SCM measurement of both conventional and the new designed FGT were compared to discuss the effectiveness of the added layer in masking the state of the transistor. The effect of change in doping concentration of the implanted layer on the C–V characteristics was also investigated. Finally, the feasibility of the proposed design was examined by comparing its I–V characteristics with the traditional FGT.

Conductivity change of defective graphene by helium ion beams

Applying a recently developed helium ion microscope, we demonstrated direct nano-patterning and Anderson localization of single-layer graphene (SLG) on SiO2/Si substrates. In this study, we clarified the spatial-resolution-limitation factor of direct nano-patterning of SLG. Analysis of scanning capacitance microscopy measurements reveals that the conductivity of helium ion (H+)-irradiated SLG nanostructures depends on their geometrical size, i.e., the smaller the H+-irradiated SLG region, the higher its conductivity becomes. This finding can be explained by the hopping carrier transport across strongly localized states of defective SLG.

Nanoscale probing of the lateral homogeneity of donors concentration in nitridated SiO2/4H–SiC interfaces

In this paper, nanoscale resolution scanning capacitance microscopy (SCM) and local capacitance–voltage measurements were used to probe the interfacial donor concentration in SiO2/4H–SiC systems annealed in N2O. Such nitrogen-based annealings are commonly employed to passivate SiO2/SiC interface traps, and result both in the incorporation of N-related donors in SiC and in the increase of the mobility in the inversion layer in 4H–SiC MOS-devices. From our SCM measurements, a spatially inhomogeneous donor distribution was observed in the SiO2/4H–SiC system subjected to N2O annealing. Hence, the effect of a phosphorus implantation before the oxide deposition and N2O annealing was also evaluated. In this case, besides an increased average donor concentration, an improvement of the lateral homogeneity of the active doping was also detected. The possible implications of such a pre-implantation doping of the near-interface region on 4H–SiC MOS-devices are discussed.

Characterization of process-induced defects in SiC MOSFETs by cross-sectional cathodoluminescence

Cross-sectional cathodoluminescence (CL) and scanning capacitance microscopy (SCM) measurements were carried out for silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) to investigate process-induced defects. The DI defect-related line at 426 nm and a broad luminescence at approximately around 430–470 nm, which were produced by ion implantation, were observed in addition to the near-band-edge emission. CL images showed that the densities of nonradiative recombination and DI centers were high near the source region. Moreover, DI centers existed even in the n-drift region located 10 µm from the surface. These results indicate that many types of defects diffuse and interact with each other during annealing even in the area where dopant atoms are not implanted. The annealing process not only activates dopant atoms but also induces the diffusion of unstable native defects and transforms their structure into more thermally stable defects such as DI centers.

SCM and SIMS investigations of unintentional doping in III‐nitrides

AbstractCross‐sectional scanning capacitance microscopy measurements of unintentionally doped model structures for InGaN quantum wells, GaN barriers and AlInN electron‐blocking layers showed n ‐type conductivity for the In‐containing layers. Secondary ion mass spectrometry indicated that oxygen impurities are the likely source of the electron density in the model layers. The n ‐type conductivity as well as the oxygen impurity level increases to ∼1018 cm‐3 for AlInN lattice‐matched to GaN. These results suggest that the background electron concentration due to oxygen impurities in indium‐containing layers needs to be considered in the design and theoretical modelling of device structures. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct nano-patterning of graphene with helium ion beams

Helium ion microscopy (HIM) was used for direct nano-patterning of single-layer graphene (SLG) on SiO2/Si substrates. This technique involves irradiation of the sample with accelerated helium ions (He+). Doses of 2.0 × 1016 He+ cm−2 from a 30 kV beam induced a metal-insulator transition in the SLG. The resolution of HIM patterning on SLG was investigated by fabricating nanoribbons and nanostructures. Analysis of scanning capacitance microscopy measurements revealed that the spatial resolution of HIM patterning depended on the dosage of He+ in a non-monotonic fashion. Increasing the dose from 2.0 × 1016 to 5.0 × 1016 He+ cm−2 improved the spatial resolution to several tens of nanometers. However, doses greater than 1.0 × 1017 He+ cm−2 degraded the patterning characteristics. Direct patterning using HIM is a versatile approach to graphene fabrication and can be applied to graphene-based devices.

Development of a 60 $\mu{\rm m}$ Deep Trench and Refill Process for Manufacturing Si-Based High-Voltage Super-Junction Structures

A unique and novel, 60 μm deep trench and refill process for manufacturing Si-based Super-Junction device structures for high-voltage applications beyond 600 V is discussed on the following pages. We combine an etching-process with a DCS-HCl epitaxial growth method to achieve a homogenous refilling of the generated deep-trench structures with oppositely charged dopants. Utilizing numerical process simulations, we demonstrate the advantage of the trench and refill technological approach as compared to the more established multiple-epitaxy and implantation manufacturing method. In order to experimentally validate the homogeneity of our refilling procedure, we perform secondary electron potential contrast as well as nanoscaled scanning capacitance microscopy measurements on our fabricated micro-structures.

Two‐dimensional profiling of carriers in terahertz quantum cascade lasers using calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

The distribution of charge carriers inside the active region of a terahertz (THz) quantum cascade laser (QCL) has been measured with scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). Individual quantum well-barrier modules with a 35.7-nm single module thickness in the active region of the device have been resolved for the first time using high-resolution SSRM and SCM techniques at room temperature. SSRM and SCM measurements on the quantum well-barrier structure were calibrated utilizing known GaAs dopant staircase samples. Doping concentrations derived from SSRM and SCM measurements were found to be in quantitative agreement with the designed average doping values of the n-type active region in the terahertz quantum cascade laser. The secondary ion mass spectroscopy provides a partial picture of internal device parameters, and we have demonstrated with our results the efficacy of uniting calibrated SSRM and SCM to delineate quantitatively the transverse cross-sectional structure of complex two-dimensional terahertz quantum cascade laser devices.

A look underneath the SiO2/4H-SiC interface after N2O thermal treatments

The electrical compensation effect of the nitrogen incorporation at the SiO2/4H-SiC (p-type) interface after thermal treatments in ambient N2O is investigated employing both scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). SSRM measurements on p-type 4H-SiC areas selectively exposed to N2O at 1150 °C showed an increased resistance compared to the unexposed ones; this indicates the incorporation of electrically active nitrogen-related donors, which compensate the p-type doping in the SiC surface region. Cross-sectional SCM measurements on SiO2/4H-SiC metal/oxide/semiconductor (MOS) devices highlighted different active carrier concentration profiles in the first 10 nm underneath the insulator–substrate interface depending on the SiO2/4H-SiC roughness.The electrically active incorporated nitrogen produces both a compensation of the acceptors in the substrate and a reduction of the interface state density (Dit). This result can be correlated with the 4H-SiC surface configuration. In particular, lower Dit values were obtained for a SiO2/SiC interface on faceted SiC than on planar SiC. These effects were explained in terms of the different surface configuration in faceted SiC that enables the simultaneous exposition at the interface of atomic planes with different orientations.

Carrier profiling in Si-implanted gallium nitride by Scanning Capacitance Microscopy

To face silicon limits, gallium nitride (GaN) exhibits major interests for optoelectronics and power electronic devices. Nevertheless, several challenges have to be overcome, with local doping by ion implantation as a major one. It requires development of reliable characterization tools able to provide electrical information with nanoscale resolution. In this work, Atomic Force Microscopy (AFM) combined to its Scanning Capacitance Microscopy (SCM) mode was used for surface damages quantification and nanoscale dopant profiling. GaN samples have been implanted with Si in order to obtain a box-like profile and annealed above 1000°C under nitrogen with AlN protective cap layer. SCM measurements have led to reliable and quantitative dopant electrical activity measurements thank to calibration sample. Moreover, a good agreement, in terms of depth and shape, has been obtained between SCM and SIMS profiles. This work has evidenced that a high activation rate of implanted Si can be achieved using rapid thermal annealing.

Capacitive Imaging of Graphene Flakes on SiO2 Substrate

We used scanning capacitance microscopy (SCM) for the local electrical imaging of graphene flakes on a SiO2 substrate. As a result of analyzing the dependence of the SCM measurements on the area of thin graphite nanoislands together with the observed difference in contrast of SCM images related to the graphene layer's thickness, we have concluded that the SCM measurements can selectively image high-conductivity few-layer graphene (FLG) flakes on an insulating substrate without having to fabricate external electrical contacts on the graphene. Our technique is a simple way to explore the conductive properties of low-dimensional systems on an insulating substrate with nanoscale resolution.

Nano-scale Characterization of Ultra-thin Dielectrics/Conductive Films through Scanning Capacitance Microscopy Studies

Scanning capacitance microscopy (SCM) techniques to investigate the local electronic properties of the ultra-thin films are presented. In the first part, we show the application of SCM experiments for the high-permittivity (high-k) dielectric thin films. The static capacitance (dC/dZ) images and spatially resolved dC/dZ versus sample bias spectra have revealed the charge distributions within high-k dielectrics. Moreover, the bias stress examination has clearly imaged the charge-trapped region in the high-k stacked dielectrics. In the second part, we have demonstrated that the conductance variation ascribed to the graphene thickness can be probed through SCM measurements. The UHF field transmitted from SCM probe tip induces a local accumulation of carriers just beneath the tip. This causes carrier diffusion over the effective biased area of Seff on graphene films, which can be detected as the increment of dC/dZ signal intensity. These results indicate that our SCM technique provides a valuable method to explore the electronic characteristics of low-dimensional materials with nanoscale resolution.

(Invited) Electrical Scanning Probe Microscopy Techniques for the Detailed Characterization of High-k Dielectric Layers

The use of scanning probe microscopy (SPM) techniques for the electrical characterization of high-k dielectric layers is reviewed, focusing on conductive atomic force microscopy (cAFM) and scanning capacitance microscopy (SCM). It is demonstrated exemplarily for ZrO2 high-k layer stacks that cAFM enables a detailed analysis of the morphological and electrical layer characteristics. Film morphologies can be determined even for surfaces where topography mapping can not detect distinct surface roughness like grain structures. For as-deposited amorphous layers, the current distribution is random whereas increased leakage currents at grain boundaries can be observed for nano-crystalline samples. The dominant current mechanisms through the layers can be determined from current voltage curves at distinct local points. Comparison with conventional techniques proofs the findings by cAFM. The quantitative interpretation of SCM measurement results often suffers from parasitic capacitances and light induced effects. Local thickness variation of dielectrics, however, can be detected very sensitively.

Study of the Electronic Structure of Individual Free-Standing Germanium Nanodots Using Spectroscopic Scanning Capacitance Microscopy

High spatial resolution spectroscopic scanning capacitance microscopy (SCM) measurements at room temperature based on the differential capacitance (dC/dV) versus probe tip-to-substrate bias characteristics, is directly used to characterize the electronic structure of a pyramidal and an ellipsoidal shaped germanium (Ge) nanodot (with physical sizes smaller than the Bohr exciton radius of Ge), among the arrays of Ge nanodots fabricated on a highly doped silicon substrate using an anodic alumina membrane as an evaporation mask. The ability to study the individual nanodots under the probe tip circumvents and isolates the statistical disorders encountered when studying large ensembles of size-dispersed nanodots. It is demonstrated that the spectra of the positive dC/dV peaks are reflective of the energetics of electron trapping in the quantized energy states inside the Ge nanodots. Furthermore, the spectroscopic SCM dC/dV profile is observed to be substantially influenced by the nanodot shape where the pyramidal Ge nanodot shows three distinct shells which are interpreted as the s-like ground state, the first excited p-like state and the second excited d-like state. On the other hand, the elliptical deformation of the ellipsoidal Ge nanodot breaks the shell structure which significantly affects its electronic structure. Using the spacing between the dC/dV peaks from the spectroscopic SCM measurements, an analytical expression is derived to calculate the energy separation between the different energy states in the Ge nanodots.

Local mapping of interface traps in HfSiO/Si structure by scanning capacitance microscopy using dV/dC signal

Scanning capacitance microscopy (SCM) using a dV/dC signal has been proposed for local mapping of interface traps in an insulator/semiconductor structure. In conventional SCM measurements using a dC/dV signal, it is difficult to accurately analyze the spatial distribution of interface traps from the obtained SCM images because the dC/dV signal is inversely proportional to the interface trap density. In the proposed technique, however, the spatial distribution of the interface traps can be characterized using the dV/dC signal, which is directly proportional to the interface trap density. The effectiveness of the proposed technique has been demonstrated by characterizing the spatial distributions of the interface traps in HfSiO/Si structures before and after H2 annealing. The dV/dC images obtained reveal that the spatial variation in the interface trap density is reduced by H2 annealing. Consequently, SCM measurements using a dV/dC signal have been verified to effectively characterize the spatial distribution of interface traps.

Impact of boron-interstitial clusters on Hall scattering factor in high-dose boron-implanted ultrashallow junctions

The Hall scattering factor rH has been determined for holes in high-dose boron-implanted ultrashallow junctions containing high concentrations of boron-interstitial clusters (BICs), combining scanning capacitance microscopy, nanospreading resistance, Hall effect, and secondary ion mass spectroscopy measurements. A value of rH=0.74±0.1 has been found in reference defect-free fully activated junctions, in good agreement with the existing literature. In the case of junctions containing high concentrations of immobile and electrically inactive BICs, and independently of the implant or the annealing process, the rH value has been found to be equal to 0.95±0.1. The increase in the rH value is explained in terms of the additional scattering centers associated to the presence of high concentrations of BICs.

Formation of Pentacene wetting layer on the SiO 2 surface and charge trap in the wetting layer

We studied the early-stage growth of vacuum-evaporated pentacene film on a native SiO 2 surface using atomic force microscopy and in-situ spectroscopic ellipsometry. Pentacene deposition prompted an immediate change in the ellipsometry spectra, but atomic force microscopy images of the early stage films did not show a pentacene-related morphology other than the decrease in the surface roughness. This suggested that a thin pentacene wetting layer was formed by pentacene molecules lying on the surface before the crystalline islands nucleated. Growth simulation based on the in situ spectroscopic ellipsometry spectra supported this conclusion. Scanning capacitance microscopy measurement indicated the existence of trapped charges in the SiO 2 and pentacene wetting layer.

Scanning capacitance microscopy detection of charge trapping in free-standing germanium nanodots and the passivation of hole trap sites

An array of freestanding germanium (Ge) nanodots was fabricated on a highly doped silicon substrate using an anodic alumina membrane as an evaporation mask. Approximately half of the Ge nanodots exhibit contrast reversal and hole trapping characteristics during scanning capacitance microscopy (SCM) measurements, as shown by a negative peak in the SCM differential capacitance (dC∕dV) versus probe tip-to-substrate bias profile. The disappearance of the negative dC∕dV characteristic peak after forming gas anneal at 450°C show the complete passivation of the hole trap sites by hydrogen. This is a demonstration on the spectroscopic detection of hole trapping in Ge nanodots and hydrogen passivation of hole trap sites using SCM. Partial passivation of electron trap sites after the relatively low temperature forming gas anneal was also observed as compared with complete passivation of hole trap sites. This suggests that hole traps sites are possibly located at or closer to the surface of the Ge nanodots as compared to electron traps which may be located deeper within the Ge nanodots.

Analysis of the Electrical Activation of P<sup>+</sup> Implanted Layers as a Function of the Heating Rate of the Annealing Process

The surface morphology and the electrical activation of P+ implanted 4H-SiC were investigated with respect to annealing treatments that differ only for the heating rate. P+ implantation was carried out in lightly doped n-type epitaxial layers. The implantation temperature was 300 °C. The computed P profile was 250 nm thick with a concentration of 1×1020 cm-3. Two samples underwent annealing at 1400 °C in argon with different constant ramp up rates equal to 0.05° C/s and 40 °C/s. A third sample underwent an incoherent light Rapid Thermal Annealing (RTA) at 1100 °C in argon before the annealing at 1400 °C with the lower ramp rate. The ramp up of the RTA process is a few hundred degrees per second. Atomic Force Microscopy (AFM) micrographs pointed out that the surface roughness of the samples annealed at 1400 °C increases with increasing heating rate and that the critical temperature for surface roughening is above 1100 °C. Independently on the annealing cycle, Scanning Capacitance Microscopy (SCM) measurements showed that the P profiles are uniform over the implantation thickness and have plateau concentration around 9×1018 cm-3 in all the implanted samples. The fraction of P atoms activated as donors is 13% of the total implanted fluence.