GaP LPE半导体材料的扫描电子显微镜研究

本文论述了用扫描电子显微镜研究GaP LPE半导体材料,二次电子像用于分析样品的表面形貌,电子束感生电流像(EBIC)用于显示p-n结的位置,定量EBIC用以确定少子扩散长度和表面复合速度等重要参量。     

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 computerized signal acquisition system for the quantitative evaluation of EBIC and CL data in a SEM

Electron beam-induced current (EBIC) and cathodoluminescence (CL) are widely used methods to obtain information about recombination properties of semiconducting materials and their defects on a micrometer length scale. In this article a computerized SEM (scanning electron microscope) setup is described, which enables us to perform simultaneous measurements of several signals and automatic temperature-dependent measurements. As examples for the performance of this system we present results obtained by simultaneous EBIC/CL experiments, allowing a reconstruction of the defect geometry. In a second example, the temperature dependence of the EBIC contrast is analyzed, introducing the method of EBIC spectroscopy.     

Advanced semiconductor diagnosis by multidimensional electron-beam-induced current technique

We present advanced semiconductor diagnosis by using electron-beam-induced current (EBIC) technique. By varying the parameters such as temperature, accelerating voltage (V(acc)), bias voltage, and stressing time, it is possible to extend EBIC application from conventional defect characterization to advanced device diagnosis. As an electron beam can excite a certain volume even beneath the surface passive layer, EBIC can be effectively employed to diagnose complicated devices with hybrid structure. Three topics were selected to demonstrate EBIC applications. First, the recombination activities of grain boundaries and their interaction with Fe impurity in photovoltaic multicrystalline Si (mc-Si) are clarified by temperature-dependent EBIC. Second, the detection of dislocations between strained-Si and SiGe virtual substrate are shown to overcome the limitation of depletion region. Third, the observation of leakage sites in high-k gate dielectric is demonstrated for the characterization of advanced hybrid device structures.     

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.     

Low angle boundary migration of shot‐peened pure nickel investigated by electron channeling contrast imaging and electron backscatter diffraction

Study on recrystallization of deformed metal is important for practical industrial applications. Most of studies about recrystallization behavior focused on the migration of the high‐angle grain boundaries, resulting in lack of information of the kinetics of the low angle grain boundary migration. In this study, we focused on the migration of the low angle grain boundaries during recrystallization process. Pure nickel deformed by shot peening which induced plastic deformation at the surface was investigated. The surface of the specimen was prepared by mechanical polishing using diamond slurry and colloidal silica down to 0.02 μm. Sequential heat treatment under a moderate annealing temperature facilitates to observe the migration of low angle grain boundaries. The threshold energy for low angle boundary migration during recrystallization as a function of misorientation angle was evaluated using scanning electron microscopy techniques. A combination of electron channeling contrast imaging and electron backscatter diffraction was used to measure the average dislocation density and a quantitative estimation of the stored energy near the boundary. It was observed that the migration of the low angle grain boundaries during recrystallization was strongly affected by both the stored energy of the deformed matrix and the misorientation angle of the boundary. Through the combination of electron channeling contrast imaging and electron backscatter diffraction, the threshold stored energy for the migration of the low angle grain boundaries was estimated as a function of the boundary misorientation.     

Local variability in PTC thermistor grain boundary structures

Scanning electron microscopy-based conductive mode (CM) microscopy, using the remote electron beam-induced current configuration, was carried out on a positive temperature coefficient of resistance thermistor at temperatures below and above the Curie temperature, T(C). Below T(C), when the thermistor is in a low resistance state, no strong CM contrast was observed. Above T(C) the thermistor grain boundaries become highly resistive and significant CM contrast formed owing to three mechanisms: all of the areas that were studied showed resistive contrast, but in addition some grain boundaries showed additional contrast due to electron beam-induced current, the origin of which was consistent with the presence of a back-to-back Schottky barrier structure at the grain boundary. Other grain boundaries exhibited additional contrast owing to beta-conductivity, which suggests a slightly different n-i-n grain boundary structure at these interfaces. These results suggest that electrically active grain boundaries with different structures coexist within the thermistor.     

Segregation of solute elements at grain boundaries in an ultrafine grained Al-Zn-Mg-Cu alloy

The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) at 200 °C was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size.     

Quantitative measurements of charging in a gaseous environment

Charge accumulation in insulating or semiconducting samples due to electron beam irradiation is one of the key problems in electron microscopy. One of the most promising techniques for reducing the severity of such charging is to surround the sample with a low-pressure atmosphere of a gas. The charging behavior of a number of materials, surrounded by a variety of gases, has been determined to identify the important factors which control charging under these conditions. The magnitude of the surface potential was deduced from an analysis of x-ray spectra from the surface. The relationship between surface charge, gas pressure, and gas type are measured, and the charging reduction efficiency (CRE) is compared.     

The topographical structure of grain boundaries in tungsten

This paper discusses the observation of a variety of topographical defects in grain boundaries in tungsten as a function of thermomechanical treatment. It is shown that, for both cold worked and crept specimen material, the boundary topography is highly complex, whereas for secondary recrystallized specimen materials the boundaries approximate to that expected for an ‘equilibrium’ configuration. However, no evidence for any period step structure was observed in this specimen material although, in this and all specimen states, grain boundary dislocations were frequently observed.     

Parameters and mechanisms governing image contrast in scanning electron microscopy of single-walled carbon nanotubes

Image formation of single-walled carbon nanotubes (SWNTs) in the scanning electron microscope (SEM) is peculiarly sensitive to primary electron landing energy, imaging history, sample/substrate geometry, electrical conductivity, sample contamination, and substrate charging. This sensitivity is probably due to the extremely small interaction volume of the SWNTs' monolayered, nanoscale structures with the electron beam. Traditional electron beam/bulk specimen interaction models appear unable to explain the contrast behavior when directly applied to SWNTs. We present one systematic case study of SWNT SEM imaging with special attention to the above parameters and propose some physical explanations for the effect of each. We also demonstrate that it is possible to employ voltage biasing to counteract this extrinsic behavior, gain better control of the image contrast, and facilitate the interpretation of SWNT images in the SEM.     

Characterization of defects in semiconductors by combined application of SEM(EBIC) and SDLTS*

Besides the characterization of the geometrical structure of defects in semiconductors by TEM the estimation of their electrical activity is of importance. SEM(EBIC) and SDLTS (scanning deep level transient spectroscopy) are especially suitable for this purpose; they allow the inspection of electronic properties with a spatial resolution in the micron-range. On the one hand, SEM(EBIC) yields information on the recombination efficiency of defects in the crystal volume adjacent to a pn junction or a Schottky barrier; on the other hand, SDLTS enables the detection to be carried out of the distribution and the energetic levels of deep level defects lying in the space charge region. Accordingly, the combined application of these techniques is very promising for investigating physical processes implying an inhomogeneous incorporation of deep level defects in semiconductor crystals. In comparison to the widely used SEM(EBIC) technique SDLTS has only rarely been applied, a fact that is due to the high detection sensitivity necessary for measuring capacity transients. The application of a highly sensitive (10^—6 pF) micro-computer-controlled SDLTS system in combination with a conventional EBIC system allows a reliable inspection of semiconductor materials and devices, based on A₃B₅ compounds and on silicon. A typical application of the above technique is the investigation of the impurity distribution around extended crystal defects, like dislocations and precipitates, to study their gettering activity.     

Scanning electron microscopy studies of double-layer silicon structures prepar by epitaxy and direct bonding

The electrical properties of multilayer structures obtained by direct bonding of silicon wafers and epitaxial growth have been investigated. The measurements were made by scanning electron microscopy (SEM) in either secondary electron or electron beam-induced current (EBIC) regime, using cross sections of the structures with a p-n junction formed in the subsurface region of the active layer. The measurements of defect recombination activity were made using Schottky diodes formed on the active layer surfaces. Parasitic p-n junctions in some samples under a small direct voltage have been observed and the reason for the appearance of such parasitic junctions has been established. Two types of defects with different distribution densities and amplitudes of EBIC contrast have been detected.     

An atomistic study of grain boundary stability and crystal rearrangement using molecular dynamics techniques

The stability of grain boundaries (GBs) and the dynamic behavior of atoms in the boundary region are investigated from an atomistic standpoint. Symmetric and non-symmetric GBs are constructed using an fcc configuration, and the GB energy is calculated as a function of the misorientation angles using a Lennard-Jones-type interatomic potential. Several specific angles are revealed to exhibit cusp-shaped low values. The effect of atomic relaxation at the GB is then simulated, showing a decrease in the GB energy. Changes in the morphology of a grain embedded in a bulk single crystal are also simulated. Using both a square-grain and a circular-grain model, the following results are obtained. In models with small misorientation angles, the grain changes orientation and the GB vanishes. When the orientations are initially stable, no change in the grain is observed. However, in models with non-stable orientations, local stabilization occurs by a rearrangement of the atoms around the GB, and the shape of the grain is transformed. Finally, a similar simulation is carried out at a high temperature, and this reveals that grain contraction occurs even in models that are stable at a low temperature, and that the grain eventually disappears.     

Effect of electron beam-induced deposition and etching under bias

Electron-beam-induced deposition (EBID) and etching (EBIE) provides a simple way to fabricate or etch submicron or nanoscale structures of various materials in a direct-write (i.e.nonlithographic) fashion. The growth rate or the etch rate are influenced by many factors such as beam energy, beam current, temperature of the substrate material, pressure of the chamber, and geometry of the gas injector etc. The mechanism of EBID and EBIE involves the interaction of the incident electron beam or emitted electron from the target material. The role of these electrons is still not completely understood although the contribution of low energy secondary electrons (SE) has been assumed to be the dominant contributor of EBID and EBIE based on its overlap with the dissociation cross section. We have studied the growth and etching phenomenon under various biasing conditions to investigate how low voltage biasing of the substrate affects secondary electron trajectories and subsequently modifies electron-beam-induced deposition and etching.     

Simulations of scanning electron microscopy imaging and charging of insulating structures

We present a three‐dimensional simulation of scanning electron microscope (SEM) images and surface charging. First, the field above the sample is calculated using Laplace's equation with the proper boundary conditions; then, the simulation algorithm starts following the electron trajectory outside the sample by using electron ray tracing. When the electron collides with the specimen, the algorithm keeps track of the electron inside the sample by simulating the electron scattering history with a Monte Carlo code. During this phase, secondary and backscattered electrons are emitted to form an image and primary electrons are absorbed; therefore, a charge density is formed in the material. This charge density is used to recalculate the field above and inside the sample by solving the Poisson equation with the proper boundary conditions. Field equation, Monte Carlo scattering simulation, and electron ray tracing are therefore integrated in a self‐consistent fashion to form an algorithm capable of simulating charging and imaging of insulating structures. To maintain generality, this algorithm has been implemented in three dimensions. We shall apply the so‐defined simulation to calculate both the global surface voltage and local microfields induced by the scanning beam. Furthermore, we shall show how charging affects resolution and image formation in general and how its characteristics change when imaging parameters are changed. We shall address magnification, scanning strategy, and applied field. The results, compared with experiments, clearly indicate that charging and the proper boundary conditions must be included in order to simulate images of insulating features. Furthermore, we shall show that a three‐dimensional implementation is mandatory for understanding local field formation.     

Monte Carlo simulation of CL and EBIC contrasts for isolated dislocations

Electron beam-induced current (EBIC) and cathodoluminescence (CL) are widely used to investigate semiconductor materials and devices, particularly to obtain information on the recombination properties and the geometry of defects. This report describes a simple formulation of CL and EBIC contrasts based on the Born approximation of excess carrier density in the presence of a pointlike defect. Quantitative interpretation of the CL and EBIC images is often difficult because of a lack of accurate theory treating simultaneously both the details of the electron beam penetration in the semiconductor and the generation of EBIC and CL signals. To overcome this difficulty, the Monte Carlo approach to the phenomenon of the electron beam penetration in solids has been developed to calculate the CL and EBIC signals during a simulation of the electron trajectory. Results for an inclined dislocation in GaAs are presented.     

Site-specific atomic scale analysis of solute segregation to a coincidence site lattice grain boundary

A site-specific method for measuring solute segregation at grain boundaries in an Aluminum alloy is presented. A Σ7(Σ7=38°〈1 1 1〉) grain boundary (GB) in an aluminum alloy (Zr, Cu as main alloying elements) was evaluated using site-specific Local Electrode Atom Probe (LEAP). A sample containing a Σ7 GB was prepared by combining electron backscatter diffraction (EBSD) and focused ion beam (FIB) milling to locate the GB of interest and extract a specimen. Its composition was determined by LEAP, and compared to a general high angle GB (HAGB). Zr was the only alloying element present in the Σ7 GB, whereas the general HAGB contained both Cu and Zr. This site-specific LEAP method was found to be an accurate method for measuring GB segregation at specific GB misorientations. The method has advantages over other methods of measuring chemistry at GBs, such as spectroscopy, in that GB structure can be assessed in three dimensions.     

SEM and TEM observations of emitter-collector pipes in bipolar transistors

The EBIC mode of the SEM has been used to identify the sites of emitter—collector pipes in diffused npn bipolar transistors. After thinning, the same devices were studied in the TEM at 1 MV. A one-to-one correlation was found between the sites of the pipes and the arms of complex dislocations originating from the emitter diffusion-induced dislocation network and looping down into the collector. The white EBIC contrast at pipes was often accompanied by adjacent black contrast due to the recombination of electrons at parts of the dislocation in the base region.     

Crystallographic analysis of facets using electron backscatter diffraction

Applications of electron backscatter diffraction (EBSD), also known as backscatter Kikuchi diffraction in the scanning electron microscope (SEM) are first and foremost microtexture and grain boundary misorientation analysis on a single polished section in the specimen. A more subtle and revealing approach to analysis of these data is to use EBSD to probe the orientations of planar surfaces, i.e. facets, which bound crystals. These surfaces include: • grain or phase boundaries • fractures • cracks It is of great interest to know the crystallography of such facets since it provides a key to understanding the physical properties of them.
As far as investigation methodology is concerned, surfaces or facets associated with polycrystals are of two types: exposed or unexposed. Exposed facets, such as a fracture surface, can be viewed directly in the SEM, whereas unexposed facets, such as a grain boundary, are usually revealed as an etched trace on a polished surface. Photogrammetric methods can be used to obtain the positional orientation of an exposed facet, and the crystallographic orientation is obtained either directly from the surface or by indirect sectioning. Calibrated sectioning is required to obtain the equivalent parameters for an internal surface. The present paper compares the methods for obtaining and interpreting the crystallography of facets, with illustrations from several materials.