Semiconductor characterization by scanning ion beam induced charge (IBIC) microscopy

The acronym IBIC (ion beam induced charge) was coined in the early 1990s to indicate a scanning microscopy technique which uses MeV ion beams as probes to image the basic electronic properties of semiconductor materials and devices. Since then, IBIC has become a widespread analytical technique to characterize materials for electronics or for radiation detection, as testified by more than 200 papers published so far in peer-reviewed journals. Its success stems from the valuable information IBIC can provide on charge transport phenomena occurring in finished devices, not easily obtainable by other analytical techniques. However, IBIC analysis requires a robust theoretical background to correctly interpret experimental data. In order to illustrate the importance of using a rigorous mathematical formalism, we present in this paper a benchmark IBIC experiment aimed to test the validity of the interpretative model based on the Gunn’s theorem and to provide an example of the analytical capability of IBIC to characterize semiconductor devices.     

Ion beam induced charge imaging of charge transport in CdTe and CdZnTe

Ion beam induced charge (IBIC) imaging is a powerful technique for quantitative mapping of the charge transport performance of wide bandgap semiconductor materials. In this paper we present results from a study of electron and hole mobility-lifetime product and drift mobility in CdTe:Cl and CdZnTe, which are semiconductor materials used for radiation detector applications. IBIC imaging has been used to produce mobility-lifetime product maps in CdTe:Cl and CdZnTe, revealing the influence of extended defects and tellurium inclusions and assessing the large area response uniformity of the materials. The recent extension of this method in the form of digital time-resolved IBIC is also discussed and time of flight maps are presented which give quantitative images of electron and hole drift mobility.     

Ion beam induced charge characterisation of a silicon microdosimeter using a heavy ion microprobe

An ion beam induced charge (IBIC) facility has been added to the existing capabilities of the ANSTO heavy ion microprobe and the results of the first measurements are presented. Silicon on insulator (SOI) diode arrays with microscopic junction sizes have recently been proposed as microdosimeters for hadron therapy. A 20 MeV carbon beam was used to perform IBIC imaging of a 10 μm thick SOI device.     

Charge sharing in multi-electrode devices for deterministic doping studied by IBIC

Following a single ion strike in a semiconductor device the induced charge distribution changes rapidly with time and space. This phenomenon has important applications to the sensing of ionizing radiation with applications as diverse as deterministic doping in semiconductor devices to radiation dosimetry. We have developed a new method for the investigation of this phenomenon by using a nuclear microprobe and the technique of Ion Beam Induced Charge (IBIC) applied to a specially configured sub-100 μm scale silicon device fitted with two independent surface electrodes coupled to independent data acquisition systems. The separation between the electrodes is comparable to the range of the 2 MeV He ions used in our experiments. This system allows us to integrate the total charge induced in the device by summing the signals from the independent electrodes and to measure the sharing of charge between the electrodes as a function of the ion strike location as a nuclear microprobe beam is scanned over the sensitive region of the device. It was found that for a given ion strike location the charge sharing between the electrodes allowed the beam-strike location to be determined to higher precision than the probe resolution. This result has potential application to the development of a deterministic doping technique where counted ion implantation is used to fabricate devices that exploit the quantum mechanical attributes of the implanted ions.     

IBIC analysis of CdTe/CdS solar cells

This paper reports on the investigation of the electronic properties of a thin film CdS/CdTe solar cell with the Ion Beam Induced Charge (IBIC) technique. The device under test is a thin film (total thickness around 10 μm) multilayer heterojunction solar cell, displaying an efficiency of 14% under AM1.5 illumination conditions. The IBIC measurements were carried out using focused 3.150 MeV He ions raster scanned onto the surface of the back electrode. The charge collection efficiency (CCE) maps show inhomogeneous response of the cell to be attributed to the polycrystalline nature of the CdTe bulk material.Finally, the evolution of the IBIC signal versus the ion fluence was studied in order to evaluate the radiation hardness of the CdS/CdTe solar cells in a view of their use in solar modules for space applications.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Study of basic mechanisms of semiconductor devices using ion beam induced charge (IBIC) collection

A change of wave form of current transients induced by a single heavy ion was investigated around a pn junction with 8 μm width and 10 μm length as a function of the ion incident position. Three pn junctions were made on a 3 μm thick Si epilayer (1 × 10¹⁶/cm³) grown on Si substrate and were in a line along an aluminum electrode with 10 μm spacing between the adjacent junctions. The elements of a pn junction array were irradiated with a 1 μm diameter 15 MeV C⁺ heavy ion microbeam spacing steps by 3 μm. At a bias voltage of − 10 V, 148, 91, and 54 fC were collected at the pn junction center, and at 3 μm and 4 μm from the edge of the electrode, respectively. Internal device structure was examined by IBIC (ion beam induced current) method by using a 2 MeV He⁺ ion microbeam. From the IBIC spectrum and the IBIC image, the charge collected from the open space by the diffusion process was observed in addition to the charge collected from the depletion layer of the pn junction.     

A characterisation of electronic properties of alkaline texturized polycrystalline silicon solar cells using IBIC

In this study, electronic properties of p-type alkaline texturized polycrystalline silicon solar cells were investigated using ion beam induced charge (IBIC) analysis. With this technique, quantitative information on electronic diffusion lengths and average electronic capture cross sections of lattice defects generated by high energy protons were obtained. Angular-resolved IBIC analysis was used to quantify the electronic diffusion lengths. For this purpose, the experimental data were fitted using a simulation based on the Ramo-Shockley-Gunn (RSG) theorem and the assumption of an abrupt pn-junction. In order to determine the average electronic capture cross section of proton-induced lattice defects, the loss of charge collection efficiency (CCE) was plotted vs. the accumulated ion fluence. As will be demonstrated, a simple model based on charge carrier diffusion and Shockley-Read-Hall (SRH) recombination is able to fit the CCE loss well. Furthermore, spatially and energetically highly resolved IBIC-maps of grain boundaries were recorded. A comparison with PIXE-maps shows that there is no correlation observable between CCE variations at grain boundaries and metallic impurities within the PIXE detection limits of a few ppm. On the contrary, there is an evident correlation to the morphology of the sample’s surface as was observed by comparing IBIC-maps and SEM-micrographs. These local CCE fluctuations are dominated by the interplay of charge carrier diffusion processes and the sample surface morphology.     

Spatially resolved imaging of charge collection efficiency in polycrystalline CVD diamond by the use of ion beam induced current

Diamond based detectors have potential applications in high energy physics experiments. These detectors can be fabricated from synthetic Chemical Vapour Deposited (CVD) polycrystalline diamond films. Previously it has been shown by the Turin group and their coworkers in Zagreb that it is possible to investigate the electrical characteristics of high quality polycrystalline CVD diamond films by Ion Beam Induced Current (IBIC). The present work describes IBIC images obtained using 2 MeV He⁺ irradiation of 250 μm thick polycrystalline diamond films through a thin gold surface contact layer biased positively relative to the grounded rear surface of the film. In contrast to previous experiments the present spectra of collected charge display a clearly defined peak from the induced charge. Images obtained by separating these spectra into different regions of interest allow the identification of regions in the sample of different charge collection efficiency. In particular the presence of some grains in which no charge collection appears possible and the reduction in charge collection efficiency at the grain boundaries is evident.     

Recent applications of nuclear microprobe techniques to microelectronics

Applications of nuclear microprobe techniques to microelectronics, in particular, two- or three-dimensional analysis of integrated circuit structures using micro RBS (Rutherford Backscattering) and IBIC (Ion Beam Induced Current) measurements are discussed. SEU (Single Event Upset) mapping and IBIC measurements in SRAMs (Static Random Access Memories) and DRAMs (Dynamic Random Access Memories) are reviewed together with charge carrier collection simulation and transient IBIC measurements. Importance of applications of microprobes to new microelectronic structures such as SOI (Silicon-on-Insulator) devices for future hand-held information systems are also discussed. Possible applications of SEU measurement as alternative experimental procedures to cosmic ray neutron strikes are discribed.     

Ion-beam-induced-charge characterisation of particle detectors

Ion-beam-induced-charge collection (IBIC) in a nuclear microprobe has been used to characterise detectors for the measurement of particles over a median energy range (100 keV–1 MeV). Three standard detector devices have been studied: a PIPS detector with a buried (ion-implanted) junction structure, a Schottky barrier junction device and a PN-junction photodiode. A 2.0 MeV focussed helium ion beam was used to probe the active area of each device with a spatial resolution 1–2 μm, to quantify the thickness of the dead layer, the charge collection response and the reduction in charge collection efficiency induced by ion-beam damage.     

Empirical approach to the description of spectral performance degradation of silicon photodiodes used as particle detectors

The spectral deterioration of Hamamatsu S5821 silicon photodiodes for ion types and energies frequently used in Ion Beam Analysis was investigated. Focused proton beams with energies 430 keV and 2 MeV were applied to generate radiation damage via an area selective ion implantation in unbiased diodes at room temperature. The variations of spectroscopic features were measured “in situ” by Ion Beam Induced Current (IBIC) method as a function of fluence, within the 10⁹–5 × 10¹² ion/cm² range and diode bias voltages, between 0 and 100 V.An empirical model has been developed to describe the radiation damage. Equations are derived for the variations of the normalized peak position and peak width. The derived empirical equations are physically correct, as far as they account for the superposition of the influence of charge carrier trapping by native and radiation-induced defects and for the effect of charge carrier velocity saturation with electric field strength, as well.     

离子束诱导电荷显微术的现状与发展趋势

离子束诱导电荷显微术（IBIC）是核子微探针显微成像技术的又一新发展，它具有低束流（fA量级），高效率的特点，已被广泛应用于半导体材料和微电子材料研究中。本文简述了离子束诱导电荷显微术（IBIC）的原理和实验方法，综述了IBIC 研究的现状和进展。     

Imaging of radiation detectors properties by IBIC

Ion Beam Induced Charge (IBIC) technique was used for imaging different properties of several radiation detectors. Small lateral dispersion of 3–5 MeV proton microbeam allowed us to investigate a scanned detector as an array of independent charge sensitive detectors. The detector response function at each point was separately analyzed. The mean and the median detected proton energy, as well as standard deviation, was calculated and connected to the local value of the detector collection efficiency and resolution. The progressive change (increase or decrease) of the collection efficiency with the time was studied. IBIC was also applied for imaging the incomplete charge collection layer in Si(Li) X-ray detectors.     

Charge collection efficiency mapping of interdigitated 4H–SiC detectors

The Ion Beam Induced Charge (IBIC) technique was used to map the charge collection efficiency (CCE) of a 4H–SiC photodetector with coplanar interdigitated Schottky barrier electrodes and a common ohmic contact on the back side.IBIC maps were obtained using focused proton beams with energies of 0.9 and 1.5 MeV, at different bias voltages and different sensitive electrode configurations (charge collection at the top Schottky or at the back Ohmic contact).These different experimental conditions have been modeled using a two-dimensional finite element code to solve the adjoint carrier continuity equations and the results obtained have been compared with experimental results. The excellent agreement between the simulated and experimental CCE maps allows an exhaustive interpretation of the charge collection mechanisms occurring in pixellated or strip detectors.     

Thermal solid phase epitaxial growth and ion-beam induced crystallisation of Ge ion implanted layers in silicon

Solid phase epitaxial growth (SPEG) of amorphous SiGe layers in Si has been investigated. The amorphous layers were formed by 40 keV ⁷⁴Ge⁺ ion implantation in Si(100) single crystals with doses giving 22 at.% Ge at the maximum of the ion implanted distribution of Ge. SPEG of the amorphous layers was achieved by either thermal SPEG or a combination of thermal SPEG and ion-beam induced crystallisation (IBIC). The crystal quality of the layers was investigated by Rutherford backscattering spectrometry and transmission electron microscopy. Fully crystallised SiGe alloy layers were obtained by annealing in a furnace at 550°C for 60 min or at 850°C for 20 min. However, the SiGe alloy layers contain extended defects formed at the relaxation of the built-in strain in the alloy layer. When the combination of thermal SPEG and IBIC was used for the SPEG very few of these defects were formed.     

Software for automatization of IEA-analysis of plasma-laser spectra

A software has been developed in order to automatize the ion energy analyzer (IEA) spectra analysis of laser-generated plasmas.A Nd:Yag laser operating at an intensity of the order of 10¹⁰ W/cm², 9 ns pulse width and energy of the order of 600 mJ, has been employed to irradiate different metallic targets (Al, Ti, W) and to produce plasma pulses. The ion emission from the plasma is monitored through an IEA instrument permitting time-of-flight (TOF) measurements to determine the ion energy distributions as a function of the charge state.The software program consists in two sections. The first one permits to identify the IEA ion peaks corresponding to different charge states as a function of the theoretical TOF values. The second section permits to plot the ion velocity and energy distributions as a function of the charge state. The obtained distributions are fitted using the “Coulomb-Boltzamnn shifted” function approach through the “Peakfit” code. The fit of the experimental data permits to estimate the equivalent plasma temperature and the average energy shift of the distributions as a function of the ion charge state.     

Monte Carlo analysis of a lateral IBIC experiment on a 4H-SiC Schottky diode

The transport properties of a 4H-SiC Schottky diode have been investigated by the ion beam induced charge (IBIC) technique in lateral geometry through the analysis of the charge collection efficiency (CCE) profile at a fixed applied reverse bias voltage.The cross section of the sample orthogonal to the electrodes was irradiated by a rarefied 4 MeV proton microbeam and the charge pulses have been recorded as function of incident proton position with a spatial resolution of 2 μm.The CCE profile shows a broad plateau with CCE values close to 100% occurring at the depletion layer, whereas in the neutral region, the exponentially decreasing profile indicates the dominant role played by the diffusion transport mechanism.Mapping of charge pulses was accomplished by a novel computational approach, which consists in mapping the Gunn’s weighting potential by solving the electrostatic problem by finite element method and hence evaluating the induced charge at the sensing electrode by a Monte Carlo method. The combination of these two computational methods enabled an exhaustive interpretation of the experimental profiles and allowed an accurate evaluation both of the electrical characteristics of the active region (e.g. electric field profiles) and of basic transport parameters (i.e. diffusion length and minority carrier lifetime).     

Research of Si and GaAs diode structures by Ion Beam Induced Charge (IBIC) collection

A Si pn junction diode and a GaAs Schottky diode were prepared for studying the basic mechanism of charge collection followed by high energy charged particle incidence in order to improve the resistance against single event upset. A 2 μm wide and 20 μm long rectangular Al electrode attached to a circular Al electrode with a 50 μm diameter was made on a 2.5 μm thick epilayer (minority carrier density 2 × 10¹⁵ /cm³). Both a Schottky electrode of Al (5 μm × 110 μm) and two ohmic electrodes of AuGe/Ni (110 μm × 110 μm) were made on a 2 μm thick epilayer (7.3 × 10¹⁵ /cm³) grown on a semi insulator GaAs substrate (1 × 10⁷ Ω cm). The internal device structure was examined by the IBIC (Ion Beam Induced Charge) method using a 2 MeV He⁺ ion microbeam. IBIC images clearly show an Al electrode, the SiO₂, and an epilayer. These results were then used to improve the qualities of the test diodes.     

Recent results in ion beam induced charge microscopy: Unconnected junction contrast and an assessment of single contact IBIC

The recent development of Ion Beam Induced Charge Microscopy (IBIC) promises to deliver a powerful imaging tool for the analysis of active regions in microelectronics devices. The high penetration power (47 μm for 2 MeV protons in Si) allows direct high resolution access to buried structures, a feature not available to the well established EBIC (Electron Beam Induced Current) technique. As multi-level designs become more prevalent this deep penetration is a significant advantage. We report several results: firstly, that contrast is present in IBIC images from junctions that are not directly connected to the preamplifier. The contrast from unconnected junctions vanishes if these junctions are short circuited. Secondly, we discuss the degradation of IBIC images under prolonged irradiation for protons and He⁺ ions. Finally, we discuss the feasibility of single connection IBIC imaging. We demonstrate that IBIC imaging is possible with only one connection to the device substrate.