Scanning Spreading Resistance Microscopy Measurements Research Articles

Dopant Activation Depth Profiling for Highly Doped Si:P By Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM)

Heavily n-doped epitaxially grown Si layers are of great importance for source/drain (S/D) application in advanced node nMOS devices. For contact resistivity reduction, the dopant activation level is very important. Various techniques are being used to evaluate dopant activation in Si:P layers. Among these two methods are Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM). SSRM uses an atomic force microscope equipped with a hard conductive probe that is scanned in contact mode on the cross-sectioned sample’s surface and measures spreading resistance. Measured resistance values as a function of depth are converted into resistivity and carrier concentration depth profiles using calibration measurements and conversion relationships. DHEM provides depth profiles of mobility, resistivity and carrier concentration through a semiconductor layer by making successive sheet resistance (Rs) and Hall voltage measurements using Hall effect/Van der Pauw techniques, as the electrically active thickness of the layer is reduced through successive oxidation steps. Controlled oxidation is achieved by electrochemical anodization. Data collected can then be processed to yield the depth profiles. In this contribution we have carried out SSRM and DHEM measurements on Si:P epi layers subjected to different processing conditions including annealing and ohmic contact fabrication. Secondary Ion Mass Spectrometry (SIMS) was used to measure the total (active + inactive) dopant profiles through the films. Effects of these processes on dopant diffusion and activation were studied and the results from DHEM and SSRM were compared.In-situ phosphorus (P) doped Si epi-layers were grown over 300mm diameter boron doped monitor wafers. While one set was kept as the reference, a second set was treated by a spike-annealing process at 1000 °C. In a third set a Ti/TiN contact fabrication process was carried out and the contact was removed before analysis. In the fourth set contact process was applied to the spike annealed wafer before removal of the Ti/TiN layers. Bulk sheet resistance measurements were made using 4-point probe (4PP) for all the samples. SSRM measurements were carried out at IMEC. Cross-shaped Van der Pauw test-patterns were formed on 8mmx8mm areas on the samples and DHEM measurements were performed at ALP. Figure below shows the dopant depth profiles obtained by SIMS and the carrier concentration profiles from DHEM and SSRM techniques for samples D02 (as deposited Si:P) and D03 (spike annealed). One can make some general observations from the data in this figure. The total dopant concentration as measured by SIMS is ~ 1.4E21/cm3. There is only a small difference in the total dopant distribution profiles (SIMS) between the as deposited sample and the spike annealed sample. However, the spike annealed sample shows much higher dopant activation as measured by DHEM. Carrier concentration is ~2.5x higher in sample D03 (~5E20/cm3) compared to sample D02 (~2E20/cm3). Activation levels measured by SSRM, however, are lower for both samples, and the peak carrier concentration value increases only slightly upon spike annealing, going from ~2E20/cm3 in sample D02 to ~2.2E20/cm3 in sample D03. DHEM clearly indicates the sharp interface between the p-type substrate and the n-type epi-layer and its depth calibration agrees well with the expected thicknesses of the epi layers. The tail of the SSRM data is much more graded. Data from other samples will be presented and discussed in the final manuscript. Figure 1

Local Enhancement of Dopant Diffusion from Polycrystalline Silicon Passivating Contacts.

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

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.

Light-assisted scanning probe microscopy characterization of the electrical properties of AlGaN/GaN/Si heterostructures

The article proposes a new methodology that combined light-assisted Scanning Surface Potential Microscopy (SSPM), Scanning Spreading Resistance Microscopy (SSRM) and Scanning Capacitance Microscopy (SCM) techniques for the extended nanoscale characterization of the electrical properties of AlGaN/GaN/Si heterostructures. Two measurement regimes were used to investigate the influence of surface defects on local electrical properties of samples: surface mapping with additional light excitation and one-point spectroscopy which enabled the accurate investigation of energetic transitions associated with a specific surface structure. The influence of two types of surface defects existing in the AlGaN/GaN/Si samples grown by metalorganic chemical vapour deposition epitaxy on the two-dimensional electron gas (2DEG) formation process on the AlGaN/GaN interface was explained. In the case of the sample with the SiN nanomasking layer added to the growth scheme, the areas of negative charge associated with defects created the localised, isolated islands that did not influence the process of electric field formation in the AlGaN/GaN system and had no significantly detrimental impact on the electron transport. The presence of surface defects in the sample without the SiN layer caused the disturbance of the 2DEG electrical properties not only in the place of the defect’s presence but also at some distance away from it. In that case, the photovoltage and photoconductivity spectra indicated the existence of a disordered structure of the AlGaN layer, near and away from the defects. Additionally, it was clarified that the characteristic electronic transition at 3.0 eV related to a deep trap state existing in the AlGaN barrier was localised solely at the surface site of the defects. It was observed during the SSRM measurements that both types of defects created a leakage current paths with prolonged conductivity (up to several seconds), which effect was not reported earlier, that provide an original insight into the origins of the long relaxation time effects existing in the AlGaN/GaN heterostructures.

Understanding the effect of confinement in scanning spreading resistance microscopy measurements

Scanning spreading resistance microscopy (SSRM) is a powerful technique for quantitative two-and three-dimensional carrier profiling of semiconductor devices with sub-nm spatial resolution. However, considering the sub-10 nm dimensions of advanced devices and the introduction of three-dimensional architectures like fin field effect transistor (FinFET) and nanowires, the measured spreading resistance is easily impacted by parasitic series resistances present in the system. The limited amount of material, the presence of multiple interfaces, and confined current paths may increase the total resistance measured by SSRM beyond the expected spreading resistance, which can ultimately lead to an inaccurate carrier quantification. Here, we report a simulation assisted experimental study to identify the different parameters affecting the SSRM measurements in confined volumes. Experimentally, the two-dimensional current confinement is obtained by progressively thinning down uniformly doped blanket silicon on insulator wafers using scalpel SSRM. The concomitant SSRM provides detailed electrical information as a function of depth up to oxide interface. We show that the resistance is most affected by the interface traps in case of a heterogeneous sample, followed by the intrinsic resistance of the current carrying paths. Furthermore, we show that accurate carrier quantification is ensured for typical back contact distances of 1 μm if the region of interest is at least nine times larger than the probe radius.

Fabrication and Analysis of Vertical Thin Poly-Si Channel Transfer Gate Pixels for a 3-D CMOS Image Sensor

This paper reports the process integration and fabrication characteristics of a vertical thin poly-Si channel (VTPC) transfer gate (TG) pixel, which is one of the candidates for future 3-D CMOS image sensor (CIS) applications. The proposed process integration can effectively suppress grain boundary formation at the interface between the photodiode and the poly-Si channel by solid phase epitaxial growth (SPEG) mechanism. Furthermore, dopant diffusion characteristics for source-drain junction formation in polySi substrates are investigated using secondary ion mass spectroscopy (SIMS) depth profiles under various process conditions. In addition, combining SIMS results with Id-Vg curve and scanning spreading resistance microscopy measurements, we explained the discrepancy in diffusion characteristics between the bulk and the thin-film poly-Si. Finally, by using optimized SPEG conditions and adopting the proposed VTPC-TG pixel structures in a commercial 5-Mpixel CIS image sensor product, we successfully-verified the improvement in image quality.

Nanoscale Electrical Degradation of Silicon-Carbon Composite Anode Materials for Lithium-Ion Batteries.

High-performance lithium-ion batteries (LIBs) are in increasing demand for a variety of applications in rapidly growing energy-related fields including electric vehicles. To develop high-performance LIBs, it is necessary to comprehensively understand the degradation mechanism of the LIB electrodes. From this viewpoint, it is crucial to investigate how the electrical properties of LIB electrodes change under charging and discharging. Here, we probe the local electrical properties of LIB electrodes with nanoscale resolution by scanning spreading resistance microscopy (SSRM). Via quantitative and comparative SSRM measurements on pristine and degraded LIB anodes of Si-C composites blended with graphite (Gr) particles, the electrical degradation of the LIB anodes is visualized. The electrical conductivity of the Si-C composite particles considerably degraded over 300 cycles of charging and discharging, whereas the Gr particles maintained their conductivity.

Synthesis and characterization of boron doped diamond/β-SiC composite films

Boron doped diamond/β-SiC composite films with a conductive diamond phase separated by the insulating β-SiC phase are fabricated by the microwave plasma chemical vapor deposition process. By manipulating the gas phase composition during the film deposition, the boron incorporation and diamond/β-SiC ratio in the composite film are well controlled. Scanning electron microscopy, transmission electron microscopy, Raman and X-ray diffraction measurements are carried out to study the structural composition of the films. It is observed that the introduction of boron during the growth process does not affect the independent growth of diamond and β-SiC crystals with high crystal quality. Scanning spreading resistance microscopy measurements confirm the presence of the conductive diamond phase and the insulating β-SiC phase in the film. The observed differences in the conductivities between diamond and β-SiC are explained by the different boron acceptor levels in diamond and β-SiC crystals. Cyclic voltammetry measurements are carried out to study the electrochemical property of the films. Our results demonstrate that boron doped diamond/β-SiC composite films are a good candidate for electroanalysis applications whereby exploiting diamond's high chemical and dimensional stability as well as its excellent electrochemical properties.

Nanoscale study of MoSe2/poly(3-hexylthiophene) bulk heterojunctions for hybrid photovoltaic applications

Hybrid solar cells are in sustained development among the photovoltaic (PV) technologies. Molybdenum diselenide (MoSe2) is a promising alternative semiconductor for hybrid PV due to its absorption in the near-infrared spectrum, its good carrier transport and its ability to form 2D structures. Scanning Spreading Resistance Microscopy (SSRM) is used for the electrical characterization of crystalline Nb-doped MoSe2. An n-type nature of doping is demonstrated, with local accumulation of dopants in the crystal when substituting Mo atoms by Nb in a 0.01% stoichiometric ratio. On the basis of the SSRM measurements, a band structure for the probe-sample system is proposed. Blends are also prepared with the p-type semiconducting polymer poly(3-hexylthiophene) (P3HT) and the photo-electrical properties of those Nb-doped MoSe2:P3HT systems are studied at the nanoscale by PhotoConductive-Atomic Force Microscopy (PC-AFM).

Probing at Nanoscale Underneath the Gate Oxides in 4H-SiC MOS-Based Devices Annealed in N<sub>2</sub>O and POCl<sub>3</sub>

In this paper a comparative study of the impact of N2O and POCl3 annealing on the SiO2/SiC system is presented, combining nanoscale electrical characterization of SiC surface doping by scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) to the conventional capacitance-voltage (C-V) and current-voltage (I-V) measurements on MOS-based devices. A significant reduction of the interface states density (from 1.8×1012 to 5.7×1011 cm-2eV-1) and, correspondingly, an increase in the carrier mobility (from 19 to 108 cm2V-1s-1) was found moving from N2O to POCl3 annealing. Furthermore, SSRM measurements on bare p+-type SiC regions selectively exposed to N2O and POCl3 at high temperature provided the direct demonstration of the incorporation of N or P-related donors in the SiC surface, leading to a partial compensation of substrate acceptors during N2O treatment and to an overcompensation during POCl3 annealing. Finally, cross-sectional SCM profiles performed on epitaxial n-doped 4H-SiC with 45 nm SiO2 (subjected to post deposition annealing in the two ambients) allowed to quantify the active donors concentrations associated to P or N incorporation under the gate oxide, showing almost a factor of ten higher doping (4.5×1018cm-3 vs 5×1017cm-3) in the case of P related donors.

Scanning spreading resistance microscopy for failure analysis of nLDMOS devices with decreased breakdown voltage

Scanning Spreading Resistance Microscopy (SSRM) is successfully applied to investigate failing nLDMOS test devices that exhibit a lowered break down voltage (BVDSS) in electrical test. Cross-sectional, two-dimensional maps of the local sample resistivity from fail and reference (pass) devices reveal significant differences of the dopant concentration in individual, specific regions. This important information enables unambiguous identification of the root cause of the device failure to be dopant related. Furthermore, from a set of hypothesis, which explains the failed electrical test, SSRM results confirm exactly one and rule out the other. These are two important steps towards root cause identification. Since a relative comparison of fail and pass SSRM scans is sufficient for this failure analysis, an extensive data calibration for the absolute dopant concentration by means of additional SSRM measurements on test samples with known dopant concentration is not required. The ability of SSRM to prove or disprove miscellaneous fail hypothesis even without data calibration makes this method a very powerful tool for analysis of dopant related failure types.

Surface transport properties of Fe-based superconductors: The influence of degradation and inhomogeneity

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors.

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.

Structural and electrical characterizations of n-type implanted layers and ohmic contacts on 3C-SiC

In this work, non-intentionally doped cubic silicon carbide (3C-SiC) epilayers grown on (1 0 0) silicon substrates were implanted using nitrogen (N), phosphorus (P) implantations or their co-implantation (N&P). After annealing from 1150 to 1400 °C, Secondary Ion Mass Spectroscopy (SIMS), Atomic Force Microscopy (AFM), Fourier Transformed InfraRed spectroscopy (FTIR), Scanning Spreading Resistance Microscopy (SSRM) and Scanning Transmission Electron Microscopy (STEM) analysis were performed. Specific contact resistances ( ρ c) of Ti/Ni ohmic contacts were determined using Circular Transfer Length Method (c-TLM) patterns. Our work shows that co-implantation, experimentally investigated for the first time in 3C-SiC, is not beneficial for the doping efficiency. According to the silicon substrate, the post-implantation annealing is limited to 1400 °C. Consecutively to this limit, the total recovering of the lattice does not seem to be possible, whatever are the implanted species. Moreover, as the crystal damages increase when increasing the atomic mass of the implanted species, a comparative study using SSRM measurements proved that, for the same post-implantation annealing treatment, the resistivity of implanted layers depend on the doping species. As a consequence, the lowest ρ c value (2.8 × 10 −6 Ω cm 2) has been obtained (using Ti/Ni 25/100 nm pattern) for a 1400 °C–30 min annealing consecutively to the nitrogen implantation. This value is among the best values obtained on implanted 3C-SiC layers in the literature. Furthermore, for this annealing temperature, a doping activation close to 100% has been evaluated by quantitative SSRM technique which evidences that an efficient dopant activation could be done. The high activation rate obtained on n-type implanted 3C-SiC and the low specific contact resistance achieved with Ti/Ni are very promising for electronic device fabrication.

Conductive diamond tips with sub‐nanometer electrical resolution for characterization of nanoelectronics device structures

AbstractOver the past decade, boron‐doped diamond tips have become the ultimate choice for electrically characterizing microelectronics devices using scanning probe methods such as scanning spreading resistance microscopy (SSRM). Although nanometer‐scale electrical resolution has been demonstrated, the development of a reliable probe process remained a challenge. Therefore, we did develop in this work solid diamond tips with sub‐nanometer electrical resolution and integrated them into metal cantilevers using a peel‐off approach. It is shown that the ultra‐high tip resolution is achieved by diamond nanocrystals protruding from the apex of the diamond pyramid. The yield for sub‐nanometer probes is 20–30% in air and 40–60% in vacuum. This paper describes the fabrication scheme, discusses probe characterization, and shows SSRM measurements obtained with these probes. Our probes are routinely used for SSRM measurements and current efforts are focusing on increasing the yield for sub‐nanometer tips further.

New SPM concept for accurate and repeatable tip positioning

In routine scanning probe microscopy (SPM) measurements either the tip is hidden underneath the cantilever and thus the position of the tip is only known with a very low accuracy (>5 μm) or a high aspect-ratio tip is used which is visible but is very fragile. Furthermore, for electrical applications, visible tips are simply coated with a thin layer of active material that wears fast in case of high pressure applications such as scanning spreading resistance microscopy (SSRM). In this work we present a novel approach for fabricating SPM probes with tips that can be located with sub-micrometer accuracy. A fully-integrated batch fabrication process was developed and used to make low aspect-ratio tips entirely made of hard metal (TiN) with an uncertainty on the tip position below 200 nm. We evaluated our fabricated tips in tapping mode topography atomic force microscopy (AFM) and SSRM analysis. A reference sample of SrTiO 3 was used for tip evaluation in tapping mode AFM and SSRM measurements were performed on an n-type GaAs staircase test structure with different doping levels. From the topography measurements we calculated a tip radius below 40 nm for our fabricated tips and we were able to resolve all of the different layers in the GaAs reference sample.

Characterizing the Two-Dimensional Doping Concentration inside Silicon-Nanowires Using Scanning Spreading Resistance Microscopy

AbstractThe characterization of doped regions inside silicon nanowire structures poses a challenge which must be overcome if these structures are to be incorporated into future electronic devices. Precise cross-sectioning of the nanowire along its longitudinal axis is required, followed by two-dimensional electrical measurements with nanometer spatial resolution. The authors have developed an approach to cross-section silicon nanowires and to characterize them by scanning spreading resistance microscopy (SSRM). This paper describes a cleaving- and polishing-based cross-sectioning method for silicon nanowires. High resolution SSRM measurements are demonstrated for epitaxially grown and etched silicon nanowires.

Ultrashallow junctions formed by C coimplantation with spike plus submelt laser annealing

A leading candidate for the formation of the ultrashallow junctions needed for Lg⩽45nm devices is the combination of coimplantation of a diffusion-retarding species such as carbon with a high temperature, millisecond annealing process after the conventional spike annealing. C coimplantation with B+ for p-type metal-oxide semiconductor and P+ for n-type metal-oxide semiconductor combined with conventional spike annealing produces reduced junction depths and improved dopant activation and profile abruptness, compared to similar implants without the coimplanted species. Addition of submelt laser annealing may further improve junction activation, but the dominant impact is gate depletion reduction, in that way, delaying the need to introduce metal gates. Devices show that the overlap capacitance is reduced, consistent with the shallower junction depths and reduced lateral diffusion. The improved dopant activation manifests itself in reduced series resistance and improved Ion values. Finally, scanning spreading resistance microscopy measurements confirm the shallow junction depths and reduced lateral diffusion.

Electrical Properties of Single Carbon Nanofibers Grown on Tips of Scanning Probe Microscope Cantilevers by Ion Irradiation

The Ar ion irradiation method has a high potential for the batch fabrication of single carbon nanofibers (CNFs) onto the tip of cantilevers, and is known to have excellent mechanical characteristics. In this study, we analyzed the electrical properties of ion-induced CNFs. For this purpose, current–voltage (I–V) properties were measured using an atomic force microscope (AFM). Commercial-type Si probes (without CNFs) showed a typical diode characteristic. By contrast, Si probes with ion-induced CNFs (CNF probes) showed a metallic characteristic similar to that of Pt-coated Si probes. The Pt-coated Si probes were completely damaged at an applied voltage of 5 V, whereas the CNF probes retained their metallic I–V characteristic and no increase in resistance was detected even at 10 V. Electrical properties were further investigated by scanning spreading resistance microscopy (SSRM) measurements. Compared with electroconductive diamond probes, which are commonly used for SSRM measurements, CNF probes were superior in the sensitivity for measuring surface roughness owing to both the small radial curvature of the tip and a high aspect ratio. Thus, it was believed that the CNF probes are quite promising as electroconductive probes.