2D-mapping of dopant distribution in deep sub micron CMOS devices by electron holography using adapted FIB-preparation

Transmission electron microscopy (TEM) is a widely used tool for analysis of very large scale integrated (VLSI) semiconductor devices. As a special TEM-feature, off-axis electron holography obtains information about the electrical characteristics of a specimen, which are connected to the dopant concentration in the bulk material. Compared with conventional TEM, application of electron holography for dopant profiling demands a higher quality of specimen preparation, e.g. in terms of thickness homogeneity. Since preparation by means of focused ion beam (FIB) has become an industrial standard for TEM-investigations, its facilities are investigated for meeting the high holographic demands. It turned out that, besides many advantages like precision and speed, the use of FIB for preparation introduces new specific problems, e.g. it is hardly possible to visualize doped areas of semiconductors on a classical, thin FIB specimen. Additionally, some artifacts of FIB-preparation have no great importance for normal TEM analysis, but do significantly influence the results of holographic analysis. In order to satisfy the higher demands of preparation for holography, a special procedure for FIB-preparation has been newly developed.     

Electron holographic mapping of two-dimensional doping areas in cross-sectional device specimens prepared by the lift-out technique based on a focused ion beam

Recently, electron holography has been successfully applied to analyze two-dimensional (2D) dopant distribution in semiconductor devices with high resolution and high sensitivity. The preparation of proper specimens is a fundamental step for the practical application of electron holography in the semiconductor industry. Therefore, it is important to explore a reliable and quick specimen preparation method. In our current work, we have tried to use the lift-out technique based on a focused ion beam, to fabricate cross-sectional CMOS device specimens for electron holographic observation. Using the lift-out technique, specimens with a large area and uniform thickness can be prepared directly from integrated circuit wafers in a very short time. Specimens with a complex and unknown dopant distribution were examined using off-axis electron holography. In the reconstructed phase images, the different 2D doping areas in a CMOS device, such as source, drain, well and substrate, were revealed successfully. The advantages and disadvantages of the technique are discussed.     

Quantitative Analysis of Dopant Distribution and Activation Across p-n Junctions in AlGaAs/GaAs Light-Emitting Diodes Using Off-Axis Electron Holography

Off-axis electron holography has been used to measure the electrostatic potential profile across the p-n junction of an AlGaAs/GaAs light-emitting diode with linearly graded triangular AlGaAs barriers. Simulations of the junction profile showed small discrepancies with experiment when the nominal dopant concentrations of Si and Be impurities were used. Revised simulations reproduced the measurements reasonably using reduced dopant levels that reflected the efficiency of dopant activation. Band-edge diagrams simulated with the nominal and revised dopant concentrations were also compared in terms of the effect that activation efficiency had on the AlGaAs barrier shape and carrier transport. It is concluded that electron holography measurements combined with modeling offer device designers and growers a helpful tool for analyzing and confirming doping profiles in complex heterostructures.      

GaInAsP/InP双异质结液相外延片的p-n结偏位

用扫描电镜电子束感生电流法研究了GaInAsP/InP双异质结液相外延片的p-n结偏位问题。认为Zn沾污是偏位的主要原因之一。用控制Zn的掺入量或用Mg作p型掺杂剂均可制得正常的p-n结。用电化学c-v法测试了部分样品,并与制管后发射光谱进行比较,结果相同。     

Advanced FIB sample preparation techniques for high resolution TEM investigations of HEMT structures

In this paper advanced sample preparation techniques based on focused ion beam (FIB) optimized for TEM investigation of high electron mobility transistor (HEMT) structures are presented. It is shown that the usage of an innovative in-situ lift-out method combined with X2 window and backside milling techniques as well as live thickness control and end point detection can significantly improve the quality of electron transparent samples required for high resolution TEM investigations. This advanced preparation flow is evaluated and demonstrated at GaN HEMT structures for atomic resolution TEM investigation.     

Quantitative scanning capacitance microscopy analysis of two-dimensional dopant concentrations at nanoscale dimensions

We have applied the scanning capacitance microscopy (SCM) technique of twodimensional (2-D) semiconductor dopant profiling to implanted silicon cross sections. This has permitted the first direct comparison of SCM profiling scans to secondary ion mass spectroscopy (SIMS) depth profiles. The results compare favorably in depth and several readily identifiable features of the SIMS profiles such as peak concentration and junction depth are apparent in the SCM scans at corresponding depths. The application of dopant profiling to two dimensions is possible by calibrating the SCM levels with the one-dimensional (1-D) SIMS data. Furthermore, we have subsequently simulated the SCM results with an analytic expression readily derivable from 1-D capacitance vs voltage capacitance-voltage theory. This result represents a significant breakthrough in the quantitative measurement of 2-D doping profiles.     

A review of ULSI failure analysis techniques for DRAMs. Part II: Defect isolation and visualization

In this paper the basic techniques for defect isolation and visualization used in physical failure analysis of trench technique dynamic random access memories (DRAMs) are reviewed. The methods described are state-of-the-art for DRAM failure analysis down to 0.14 μm feature size and beyond. In addition to defect isolation and defect visualization from the front side of a die, the backside preparation approach is reviewed. Beginning with basic sample preparation techniques including mechanical polishing, wet and dry etching and focused ion beam (FIB) applications advantages and disadvantages of various techniques are discussed. In the second section of the paper different types of optical microscopes are covered as well as scanning and transmission electron microscopes. The imaging capabilities of the FIB systems are included in this section. Finally, some applications of scanning probe techniques especially for dopant measurements and thin oxide characterization are described.     

Two-dimensional dopant imaging of silicon carbide devices by secondary electron potential contrast

Dopant profiling and two-dimensional imaging of device cross-sections are relevant activities for the development of new power semiconductors and especially of silicon carbide devices. Unlike silicon, SiC presents extreme physical properties, which make the common electrical dopant profiling techniques either poorly reliable or non viable. Recently, Secondary Electrons Potential Contrast maps produced by Scanning Electron Microscopy (SEM) emerged as a powerful tool for two-dimensional quantitative dopant imaging. In this work SEPC is applied for the first time to characterize fully functional power devices with particular focus on MOSFET and JFET. The main component of the SEPC arises from the difference in the built-in potential between differently doped regions. This produces a stray electric field irradiating from the surface of the semiconductor sample, which either accelerates or retards the low-energy secondary electrons depending on their emission site. The use of a SEM equipped with a dedicated energy filter for secondary electrons enhances the observed contrast, which is already particularly intense in wide-bandgap semiconductors, where the density of intrinsic carriers is very low. In the case of SiC, the contrast is sufficiently intense to resolve both p-n and unipolar junctions.     

Side-milling technique of preparing device cross-sections for electron holography based on a focused ion beam micro-sampling system

A new milling technique based on a focused ion beam (FIB) microsampling system is proposed to avoid the curtaining effect, commonly occurring in other FIB milling methods, in order to obtain a crosssectional device specimen with uniform thickness can be obtained for electron holographic observation.     

Two-dimensional dopant profiling in semiconductor devices by electron holography and chemical etching delineation techniques with the same specimen

The electron holography and chemical etching delineation techniques were successfully employed to assess two-dimensional (2D) dopant profiles in semiconductor devices. The results obtained from both techniques with the same specimen were precisely compared and discussed in order to evaluate the performance limits of these techniques. It was demonstrated that both techniques are very effective in obtaining reliable 2D dopant profiles in nanodevice.     

Backside probing of flip–chip circuits using electrostatic force sampling

A non-contact scanning probe microscopy technique for measuring high-frequency voltage waveforms from the backside of a flip-chip mounted integrated circuit is presented. The signals on interconnects on the frontside of the die are accessed by mechanical thinning and focused ion beam milling through the backside of the die. A scanning force microscopy micromachined probe is placed inside the focused ion beam hole so that it is in close proximity to the circuit measurement point. Internal circuit voltage waveforms are measured by using the scanning probe in a non-contact mode and sensing the local electrostatic force on the tip of the probe. The instrument currently has a 3 GHz bandwidth and a capacitive loading on the test point of less than 1 fF. The output waveforms from ring oscillator flip-chip test circuits are measured.     

Characterisation of dopants distribution using electron holography and FIB-based lift-off preparation

Electron holography, a special method based on TEM is capable of visualizing the dopant distribution in a structured semiconductor by detection of differences in the inner electrostatic potential. There are some special requirements on sample preparation for successful application, which can be fulfilled by Focussed Ion Beam technique in combination with mechanical and chemical treatments. The application of Lift-Out-technique in a Dual-Beam tool allows target preparation under very restricted circumstances. Results show the feasibility of the method for p-channel FET as well as for n-channel FET. There are hopeful chances for a quantitative treatment.     

Examination of electrostatic potential distribution across an implanted p-n junction by electron holography

In the manufacture of semiconductor microelectronic devices, a p-n junction is formed usually by implanting a high concentration of impurity into a less heavily doped region and then heat annealing. A Si/Si p-n junction test sample has been made following the above practical process and thinned for electron holographic observation by using argon ion-milling. From the reconstructed phase image, the phase shift induced by potential drop across p-n junction can be seen clearly. To characterize quantitatively this potential drop, the mean inner potential V0 of silicon was measured precisely by electron holographic method. By measuring 25 different crystalline silicon spheres with diameter ranging from 40 to 170 nm, an average result of V0 = 12.16 +/- 0.83 V was obtained. By using this V0 value, a quantitative measurement yields the potential drop approximately 0.70 V, which is reasonably consistent with expected Si/Si junction parameter. The thickness of electric dead layer in depletion region produced from this measuring is approximately 20 nm on each sample surface.     

Nanoscale Analysis of a Hierarchical Hybrid Solar Cell in 3D

A quantitative method for the characterization of nanoscale 3D morphology is applied to the investigation of a hybrid solar cell based on a novel hierarchical nanostructured photoanode. A cross section of the solar cell device is prepared by focused ion beam milling in a micropillar geometry, which allows a detailed 3D reconstruction of the titania photoanode by electron tomography. It is found that the hierarchical titania nanostructure facilitates polymer infiltration, thus favoring intermixing of the two semiconducting phases, essential for charge separation. The 3D nanoparticle network is analyzed with tools from stochastic geometry to extract information related to the charge transport in the hierarchical solar cell. In particular, the experimental dataset allows direct visualization of the percolation pathways that contribute to the photocurrent.     

Local damage free Si substrate ultra thinning for backside emission spectral analysis using OBPF for LSI failure mode detection

In this study, we focused on an emission spectral analysis using OBPF, since emission spectral analysis is possible even with weak emissions. We also developed Si substrate local damage free thinning by ablation laser processing, and alkali solution wet etching for Si backside emission spectral analysis. The emission spectral analysis using an OBPF was effective for estimating the LSI semiconductor device failure mode and was able to classify the three failure modes of: gate oxide thin film leakage, P-N junction leakage and nMOSFET saturation current (Idsat) where gate floating occurs. Furthermore, we were able to estimate the failure mode, including metal/metal line short mode, from power approximation formula: Y = aX^b of a photon count increase rate by rising applied voltage at a PEMs observation. Each failure mode has it’s own coefficient “b” value. These two techniques allow a much more precise estimation of the representative failure mode of LSI semiconductor devices. Next, we developed damage free and large area local Si substrate thinning for backside emission spectral analysis at an isolated point. This technique uses a 266 nm DUV pulse laser ablation process and Si substrate crystal anisotrophic wet etching by KOH alkali solution. We achieved a damage free thinning area of approximately 2.6 × 2.6 mm². In addition, we developed a very precise, nondestructive calculation method for Si substrate with thickness of less than 2.3 μm by combining the interference fringe of equal thickness with an optical microscope, and an SEM image from depth of primary electron penetrations. The emission spectral analysis using OBPF from Si substrate backside became possible as an addition to surface analysis by combining thinning techniques with thickness calculations. We succeeded in estimating the failure mode by backside emission spectral analysis using these techniques.     

Applications of TEM imaging,analysis and electron holography to III-nitride HEMT devices

This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ～2.1 × 10¹³/cm² have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.     

Characterization of DLC films by EELS and electron holography

Thickness measurements of diamond-like carbon (DLC) films by electron energy-loss spectroscopy (EELS) and electron holography are discussed. In order to evaluate the thickness by EELS and electron holography, the mean free path for inelastic scattering and mean inner potential of DLC films were determined precisely, respectively. It is found that both the mean free path for inelastic electron scattering and the mean inner potential are sensitive to the preparation methods, namely the density of DLC films. The present work has demonstrated that thickness measurement by EELS is available to DLC films thicker than 20 nm, while electron holography can be applied to thinner films ( approximately 5 nm). Furthermore, close relations are observed between the density of DLC films and the energy-loss spectra.     

Backside spectroscopic photon emission microscopy using intensified silicon CCD

A typical solution for backside analysis of photon emissions from semiconductor integrated circuits has been the InGaAs detector. It takes advantage of the transparency of the silicon material to wavelengths within its spectral sensitivity regime. However, it has been recently demonstrated that light spectral information extraction is more reliable when using silicon CCDs (SiCCDs) assisted by proper backside bulk Si thinning of the device. In this paper we demonstrate further improvement of the photon emission spectrum acquisition using the solid-state intensified silicon CCD (SI-CCD). Using the presented solution it is possible to significantly reduce the analysis time, expand the detectable spectral regime as well as improve the spectral wavelength resolution.     

Application of the equivalent circuit model for semiconductors to the study of Au-doped p-n junctions under forward bias

The Green-Shewchun method of solution is applied to the transmission line circuit model of Sah to obtain the forward current-, capacitance-, and conductance-voltage characteristics of semiconductor p-n junctions. Numerical solutions are obtained for diffused dopant impurity profiles and several position dependent concentration profiles of gold recombination centers to illustrate the variation of the reciprocal slope parameter m in the dc current, exp (qV/mkT). A new behavior of m = 2 is observed for many decades of current in the low-level range when the recombination centers are concentrated at the edge of the space-charge layer as expected from ion implantation. The theoretical calculations are compared with experimental forward current-, conductance-, and capacitance-voltage data from 10 to 10⁶Hz and 77 to 300 K. Excellent agreements are obtained without adjustable parameters for boron and gold diffused p⁺-n silicon diodes from low to high injection levels. A twenty-five fold increase of the steady-state hole lifetime from low to high injection level is both observed and predicted. Agreements are also obtained for phosphorus- and gold-diffused n⁺-p silicon diodes from low to intermediate injection levels where the steady-state electron lifetime is nearly constant and controlled by electron capture into the positively charged gold donor centers.     

应用逆算子方法定量分析线性缓变p－n结

逆算子方法是一类新的求解强非线性问题的非数值方法．本文采用此类方法分析线性缓变ｐ－ｎ结．先把分析问题表述为一维非线性Ｐｏｉｓｓｏｎ方程，再应用逆算子方法求解该强非线性常微分方程，并采用Ｍａｔｈｅｍａｔｉｃａ软件推导其近似解析解，还对求得的近似解作了误差分析研究．模拟计算结果较为精确、可靠，基本上实现了线性缓变ｐ－ｎ结的定量分析，有助于更深入地定量研究ｐ－ｎ结的物理机理．此项研究表明，逆算子方法具有一定的优越性，它将为半导体器件的数值分析开辟一条新的途径．