Electron Beam Induced Conductivity Research Articles

Model improvements to simulate charging in scanning electron microscope

Background: Charging of insulators is a complex phenomenon to simulate since the accuracy of the simulations is very sensitive to the interaction of electrons with matter and electric fields. Aim: In this study, we report model improvements for a previously developed Monte-Carlo simulator to more accurately simulate samples that charge. Approach: The improvements include both modeling of low energy electron scattering by first-principle approaches and charging of insulators by the redistribution of the charge carriers in the material with an electron beam-induced conductivity and a dielectric breakdown model. Results: The first-principle scattering models provide a more realistic charge distribution cloud in the material and a better match between noncharging simulations and experimental results. The improvements on the charging models, which mainly focus on the redistribution of the charge carriers, lead to a smoother distribution of the charges and better experimental agreement of charging simulations. Conclusions: Combined with a more accurate tracing of low energy electrons in the electric field, we managed to reproduce the dynamically changing charging contrast due to an induced positive surface potential.

Electron beam induced conductivity in polymethyl methacrylate, polyimide, and SiO2 thin films

Electron beam induced conductivity (EBIC) is one sensitive parameter that controls the charging of insulating materials under electron beam irradiation. In an earlier work [J. Vac. Sci. Technol. B 21, 2638 (2003)], we reported the measurement of EBIC in polymethyl methacrylate (PMMA) and thermal SiO2 thin films using an external bias method. The thin films under test were sandwiched between a silicon substrate and a metal electrode. One important observation is that the exposed region in the PMMA resist is ohmic regardless of the bias polarity. However, the EBIC in the metal-thermal oxide-silicon structure is highly asymmetric under opposite bias polarities. A model involving the internal emission of secondary electrons was proposed to interpret the asymmetric EBIC in thermal oxide. In this study, we extend the EBIC measurements to deposited SiO2 thin film and another polymeric material, polyimide. By putting the deposited oxide between symmetric metal electrodes, we validated the conjecture of the internal secondary electron emission from silicon substrate into thermal oxide when silicon is used as the bottom electrode. The results on deposited oxide also implicate the close dependence of EBIC in oxide on the property of Si∕SiO2 interface. On the other hand, under the same exposure condition, polyimide thin film is 50 times more conductive than PMMA, probably as a result of their different molecular structures.

Lateral conductivity in GaAs/InAs quantum dot structures

Lateral conductivity effects have been investigated in self-organised InAs quantum dot (QD) structures grown in a GaAs matrix with different cap layers. Current-voltage (I-V), capacitance-voltage (C-V), DLTS, capacitance and conductance frequency dependence, fast defect transient (FDT), and electron beam induced conductivity (EBIC) measurements were applied. The conductivity in the QD plane decays within a distance of 10 microns. The capacitance transients are dominated by the local QD-plane transversal conductivity and by the free carrier transport, in the cap layer. The nonequilibrium free carrier created by electron beam excitation develop a potential barrier at macroscopic distances from the electrical contacts.

Electron beam induced conductivity in poly(methylmethacrylate) and SiO2 thin films

Electron beam charging and discharging of insulating samples is greatly influenced by electron beam induced conductivity (EBIC). Most prior studies on EBIC focused on inorganic materials; measuring EBIC in organic resist was hampered by high extrinsic leakage and low induced conductivity. We describe here EBIC in thin (1 μm) films of poly(methylmethacrylate) (PMMA) and thermal SiO2. The measurements were made using the external bias method, with the insulating thin film sandwiched between a metal electrode and a highly doped silicon substrate. Under the same exposure conditions, we found the induced conductivity in PMMA to be almost two orders of magnitude less than that in SiO2. The induced current in PMMA responds linearly to the bias, regardless of the polarity, but the induced current in SiO2 is surprisingly dependent on not only electron beam energy, but also on bias polarity. When the top metal electrode is biased positively and the electron beam has high enough energy to penetrate the electrode and the oxide, the results indicate that an internal emission of electrons from the silicon substrate into the oxide contributes substantially to the total measured current.

Direct assessment of recombination noise in semiconductors using electron beam-induced conductivity.

The level of internal noise of the transistors, diodes, and other semiconductor components limits the successful design of any low noise electronic system. All types of noise, namely, Johnson, 1/f, and so forth, are generated due to activity of crystalline defects such as vacancies, dislocations, and others. The intensity of the electron scattering and recombination processes, inflicted by defects (traps), controls the level of noise. Dependent on the dynamic operation condition of semiconductor devices, such as external biases and level of current injection, the traps will generate certain type and level of noise. Material growth or device processing technologies could introduce all kind of defects. Therefore, characterization of the semiconductor wafer in the early stages of processing (at least before packaging) could help to predict the level of noise due to the type and density of defects present on the wafer. Sorting out bad semiconductor chips could save money and effort in the radio frequency design of low-noise circuits. This current study focuses on 1/f noise modeling, which involves most powerful generators of noise and linear defects, named dislocations. The study also examines the possibility of assessing this noise by quantitative electron beam-induced conductivity (EBIC) measurements. These defects could be found in the bulk as well as at the epitaxial interfaces of a semiconductor device. The nanoscale size of these defects makes the scanning electron beam an instrument of choice for the proposed study. Conventional EBIC produces images of the defects, where contrast is proportional to the recombination rate at the site of a defect. Since contrast is measured as a fraction of one percent, the relative nature of contract value precludes quantitative measurements of the recombination rate, thus making quantitative assessment of 1/f noise impossible. In our model, using the Boltzman continuity equation, the recombination-generation processes per unit of length of a dislocation was defined for two operational conditions of EBIC, namely, for low and high intensity of an electron beam. The experimental technique of the quantitative measurement of carrier recombination (Mil'shtein 2001) consists of taking two EBIC scans along the selected defect at two different beam intensities, digitally subtracting the first scan from the second one and normalizing the result to the size of the electron range. The value of the recombination rate, extracted from the model, could then be used to predict the level of 1/f noise in the tested semiconductor sample.

A study of electron beam-induced conductivity in resists.

The charging of polymeric resist materials during electron beam irradiation leads to significant problems during imaging and lithography processes. Charging occurs because of charge deposition in the polymer and charge generation/trapping due to formation of electron-hole pairs in the dielectric. The presence of such charge also results in the phenomena of electron beam-induced conductivity (EBIC). Electron beam-induced conductivity data have been obtained for three commercial e-beam resists under a variety of dose rate and temperature conditions. From the observed values of induced conductivity under varying conditions significant information about the generation of electron-hole pair and the transport of charge in the resist can be obtained. Three electron beam resists, EBR900, ZEP7000, and PBS are examined by an external bias method. The difference in resist chemistry is considered to play the role in the initial state EBIC behaviors among three resists even though the way that it affects the behaviors is not clear. A comparison of the power consumption comparison is proposed as a measure to give a preliminary estimate of the carrier concentration and carrier drift velocity differences among the resists. A simple single trap model with constant activation energy is proposed and provides good agreement with experiment.

EBIC investigation of defects in SOI materials using a Schottky diode and buried oxide capacitor

Silicon-on-insulator (SOI) is a promising technology for the fabrication of ultra large scale integration circuits as thin silicon layers of good crystalline quality are now available which permit substantial enhancements of device performance. However, the fabrication of ultra thin films suitable for CMOS applications often reveals some crystal defects which must be characterised to understand their consequences on device performances. Such defects were already extensively studied from a crystal point of view using preferential etching techniques. In this work, the non-destructive electron beam induced conductivity (EBIC) technique was successfully used to study dislocations and stacking faults in SOI films obtained by direct bonding. The presence of a buried oxide in the SOI structure prevented any direct current path between the collection junction formed on the active layer and the back ohmic contact. It is shown that collection using a Schottky junction in series with the buried oxide capacitor yields a differentiation of the EBIC signal. The latter may be digitally integrated to retrieve the image with a substantially improved signal over noise ( S/N) ratio. The variation of the EBIC signal as a function of the scanning rate was also investigated. This technique permitted the observation of stacking faults and dislocations in the active layer simultaneously with defects in the buried oxide and/or at the silicon-oxide interface.

Electrical/electronic effects of titanium and iron impurities in EFG and FZ solar cell silicon: SPV/EBIC analysis

In this paper results on surface photovoltage (SPV) and electron beam induced conductivity (EBIC) studies of edge-defined film-fed growth (EFG) and floating zone (FZ) silicon solar cell materials (both p-type) are presented. A systematic comparison based on minority carrier diffusion length and carrier recombination is made between: (i) samples contaminated with Ti and/or Fe, (ii) samples gettered by phosphorous diffusion, and (iii) as-received samples. Deep level transient spectroscopy (DLTS) measurements, together with the iron-boron (FeB) pairing kinetics [1] have successfully been used to detect the presence of Fe in the samples. Even though this process is effective in revealing Fe impurities in p-type FZ silicon it is evidently not suitable for Fe identification in p-type EFG silicon. Ti, like Fe, is found to be a prominent lifetime-limiting metallic impurity in both EFG and FZ silicon. Phosphorous diffusion is proven to be an effective external gettering technique for fast-diffusing impurities such as Fe, but not for Ti.

High-field EBIC by computer controlling

Abstract High field avalanching in thin insulator and semiconductor layers is investigated by means of electron beam induced conductivity (EBIC). A digital scanning electron microscope DSM 960 provides a precise electron beam system for quantitative EBIC measurements in metal-insulator-semiconductor structures. A remote microscope control has been performed for automatic measurements and data processing. We obtain the avalanche coefficient α as a function of field strength F = (10 5 –10 7 ) V/cm. Moreover we get information on the energy-range-relation for electrons in the thin layer structures, the inner secondary electron (SE) generation and the related creation energy for SE.

A microbeam analytical characterization of diamond films

Diamond films prepared in an ASTeX plasma deposition system and the Heriot-Watt UHV compatible microwave deposition system have been characterized by a range of techniques including electron microscopy, electron diffraction, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry. XPS studies have concentrated on a comparison of the valence band structure, the X-ray excited Auger peaks, and the characteristic loss region of the carbon l s photoelectron peak. AES work has involved a study of the structure of the C KVV Auger peak. Microtwinning and grain structure in the diamond films has been studied by TEM. In EELS the fine structure at the carbon K-edge has been studied to give information on the form of carbon bonding. Reflection electron diffraction has been used to determine the interplanar spacings of the films. The interface formed between the diamond and the silicon substrate has been studied by electron beam induced conductivity (EBIC) in the SEM. The film morphology has also been studied in the SEM.

A study of the interaction of silver with amorphous chalcogenide films in the Scanning Electron Microscope

The diffusion of silver in amorphous chalcogenides is the basis for high-resolution lithographic applications. Previous studies of the diffusion of silver on contact with chalcogenide films has been studied by Auger depth profiling and the effects of photodoping on chemical bonding have been studied by x-ray photoelectron spectroscopy. Electron lithographic effects have been studied in the transmission electron microscope.The objective of the investigation described here has been to determine the degree of diffusion of silver in the amorphous chalcogenides, As2S3, As2Se3, GeS and GeSe when these films are in contact with thin silver films. The silver distribution has been determined by x-ray microanalysis of film cross-sections in the scanning electron microscope (SEM). Electron beam induced conductivity (EBIC) at points in these films has also been investigated.

Charge collection ion microscopy: Imaging of defects in semiconductors with a positive ion microbeam

The new technique of Charge Collection Ion Microscopy (CCIM) for investigating defects in semiconductors is described. It utilizes the SUNY/Albany ion microbeam and is based on the collection efficiency of electron-hole (e-h) pairs produced by individual ions penetrating the sample. Compared to the technique of electron beam induced conductivity (EBIC), CCIM produces more damage per projectile but has the compensating advantage that more e-h pairs are created. Analysis of individual pulses rather than an average current therefore is necessary and practicable. Illustrative results are presented for Schottky diodes which have been locally damaged by ion bombardment.

Electron-beam-induced conductivity at high electric fields in GaAs

EBICO (electron-beam-induced conductivity) — which is similar in principle to photoconductivity — is found to be a valuable tool in investigating minority carrier diffusion at high electric fields. Due to the very local excitation of e-h pairs by the electron beam (approx. 1 μm width) it is possible to scan the EBICO-signal from the vicinity of the contacts into the bulk of the sample. In this case recombination (at the contacts and in the bulk) as well as drift governs the structure of EBICO. By comparison of the observed scans with model calculations, it is possible to determine the diffusion length and lifetime of minority carriers (holes) as a function of the applied electric field. In the range of 50–1300 V/cm we find a decrease of the lifetime from about 0.6 to 0.2 ns for our samples of GaAs.

A microbeam analysis study of some heterojunctions formed with CuInTe 2

Heterojunctions formed with CuInTe 2 have been examined by a range of microbeam analytical techniques. The junctions studied were n-CdS/p-CuInTe 2 and n-Si/p-CuInTe 2. The p-Si/n-CdS heterojunction has also been studied. Microstructural studies of thin films of CuInTe 2 in a scanning transmission electron microscope under the conditions prevailing during heterojunction formation have been made. X-ray microanalysis depth profile studies have been used to determine the degree of cross-diffusion at the heterojunctions. Electron beam induced conductivity (EBIC) in the scanning electron microscope has been used to measure minority carrier diffusion lenghts in each heterojunction.

Compositional and electrical co-characterization of HgCdTe diodes by cross-sectional EBIC, AVC, and AES analysis

Compositional and electrical characterization of Hg1−xCdxTe heterojunction devices has been performed by cross-sectional electron-beam induced conductivity (EBIC), Auger voltage contrast (AVC), and Auger electron spectroscopy (AES). Devices were fabricated from double layer LPE HgCdTe grown on CdTe and CdZnTe substrates. Cleaved diodes were wire bonded to a specimen mount and analyzed in a scanning Auger microscope at 123 K. This is the first reported Auger analysis of HgCdTe at cryogenic temperatures. EBIC analysis located the electrical junction while the contactless AVC also confirmed an electrical junction at the substrate interface. Subsequent AES compositional analysis located and determined interface widths of 1.2 μ and 1000 Å for the substrate and heterojunction interfaces, respectively. Although EBIC is a ‘‘bulk’’ phenomenon and AES and AVC are ‘‘surface’’ phenomena, results from concurrent experiments aid in relating device performance to structural composition.

Lattice defects within grain volumes that affect the electrical quality of cast polycrystalline silicon solar-cell materials

Lattice defect structures within large grains of cast polycrystalline silicon have been investigated using optical microscopy, x-ray topography, and electron-beam-induced conductivity (EBIC) measurements. Optical observations through a series of wafers of an ingot indicate that kinks in high-angle grain boundaries develop by forming straight grain boundaries rather than curved boundaries. It is also observed that a junction of three grain forms a kink in the grain boundary. X-ray topographic observations of lattice defects show that cumulative dislocations are generated at these kinks when the material is subjected to thermal stresses at sufficiently elevated temperatures during solidification. Subsequently the dislocations propagate into the grain volume and form subgrain boundaries. In some cases, they pile up in the coherent {111} twin boundary interfaces. Observed EBIC constrast has shown that both dislocations and subgrain boundaries are electrically active and serve to decrease the effective minority carrier diffusion length.

SEM EBIC INVESTIGATIONS OF ZnO VARISTOR CERAMICS

The electron beam induced conductivity (EBIC) of zinc oxide varistor ceramics is studied in the scanning electron microscope (SEM). It is found that only particular grain boundaries give rise to an EBIC signal and that the signal strength and its linescan profile show strong variation with bias voltage. The experimental results are discussed in terms of Schottky emission of majority carriers across the grain boundary potential barrier.

Electron beam induced conductivity in ‘PET’ and ‘FEP’

Electron Beam Induced Conductivity (EBIC), classified into EBIC (bulk) and EBIC (surface) have been measured in PET and FEP respectively. The peculiar oscillatory nature of the induced gain versus beam energy variations is explained in terms of the spatial distributions of the trapping centres near the surface of the polymers.

Measurement of screened voltages by electron-beam-induced conductivity (EBIC)

A new method is introduced to measure screened voltages by EBIC. Some applications are shown.

Charge collection scanning electron microscopy

This review encompasses the application of the scanning electron microscope to the study and characterization of semiconductor materials and devices by the Electron Beam Induced Conductivity (EBIC) method. In this technique, the charge carriers generated by the electron beam of the microscope are collected by an electric field within the material and sensed as a current in an external circuit. When employed as the video signal of the SEM, this collected current image reveals inhomogeneities in the electrical properties of the material. The technique has been used to determine carrier lifetime, diffusion length, defect energy levels, and surface recombination velocities. Charge collection images reveal the location of p-n junctions, recombination sites such as dislocations and precipitates, and the presence of doping level inhomogeneities. Both the theoretical foundation and the practical aspects of these effects are discussed in a tutorial fashion in this review.