Abstracts of Magazine FUJITSU 2010-1 (VOL.61, NO.1)

Special Issue : Analysis Technologies for Robust Fujitsu Products

• High-Precision Evaluation of Ultra-Shallow Impurity Distributions by Secondary Ion Mass Spectrometry

As complementary metal oxide semiconductor (CMOS) processes evolve and device structures become finer, semiconductor manufacturers are having to form transistor gates with thin films of just a few nanometers and ultra-shallow junctions of 40 nm or less. For better reliability and performance, these device structures must be controlled at the nanometer level. This requires the nanometer-level analysis and evaluation of the distribution of elements in thin films and the distribution of impurities in ultra-shallow junctions. Secondary ion mass spectrometry (SIMS) is ideal for analyzing compositions and impurity distributions. Although the latest developments in SIMS equipment do enable nanometer-level element distribution analysis, this analysis method works by exploiting complex physical phenomena, so it is important to optimize the analysis conditions in order to obtain the true distribution. In this paper we discuss the latest achievements in our study of analysis conditions where high precision is obtained by considering factors that degrade precision in the SIMS analysis of element distributions across surface regions ranging from a few nanometers to a few tens of nanometers.

• Two-Dimensional Carrier Profiling by Scanning Tunneling Microscopy and Its Application to Advanced Device Development

A high-resolution two-dimensional (2-D) carrier profiling technique has been required to optimize the dopant profile around the source/drain and extension region in the transistor to enhance the electrical characteristics when scaling the gate length down to less than 50 nm. At Fujitsu Microelectronics Limited, high spatial resolution of about 1 nm has been achieved by scanning tunneling microscopy to enable the 2-D carrier profiling technique to be applied to the development of scaled transistors beyond the 90-nm technology node. The dependence of the 2-D carrier profile on process conditions is consistent with that of the electrical characteristics. On the basis of such profiles, the dopant profile in the scaled transistor has been optimized. The technique also enables an evaluation of dopant distribution fluctuations that cause variability in transistor performance. The carrier profile around the extension region was found to depend on the gate line edge roughness. From the measured results, various methodologies for suppressing transistor performance variability have been proposed.

• Application of Scanning Nonlinear Dielectric Microscopy to Measurement of Dopant Profiles in Transistors

We present results obtained when using scanning nonlinear dielectric microscopy (SNDM) to measure dopant profiles in transistors. Secondary ion mass spectrometry (SIMS) measurements of an epitaxial multilayer film on a standard sample and SNDM measurements of the sample surface showed that it was possible to obtain a uniform concentration region with a thickness of approximately 4-5 µm in each layer. We were able to obtain an SNDM signal with a one-to-one correspondence to the dopant quantity. In real devices, we were able to obtain dopant concentration profiles as two-dimensional images by applying calibration curves from the standard sample to cross-sectional observations of n- and p-channel transistors.

• Atomic-resolution Imaging and Analysis with Cs-corrected STEM

Advances in nanotechnology and electronic device miniaturization are making atomic-level control of structure and composition increasingly important. To promote research and development in this field, it is essential to develop technology for measuring the structure, composition, and properties of materials and devices at an atomic resolution. Scanning transmission electron microscopy (STEM), which is a high-spatial-resolution imaging instrument, combined with analysis equipment is being used in diverse fields such as R&D and product development for atomic-level analysis. However, conventional STEM equipment suffers from probe size limitations and a drop in electron-beam current due to the effects of spherical aberration (Cs) in the magnetic lens. Recently, though, spherical aberration correction technology has been developed, and mounting the Cs corrector on commercial STEM equipment has been achieved. This technology is proving to be exceptionally effective in enabling imaging and analysis at even higher resolutions. In this paper, we describe the principles for increasing resolution by applying Cs-correction technology and present examples of atomic-resoluved measurements by STEM method.

• Auger Electron Spectroscopy Analysis of Nonconductive Materials and Deep Interfaces

Auger electron spectroscopy is useful for analyzing the compositions of tiny impurities or thin films and for profiling multilayers or metallic junction interfaces. However, it is usually difficult to analyze nonconductive materials because the primary electron beam causes electrical charging of the sample. When we want to obtain sputter depth profiles of layers deep in the sample, precise evaluation is difficult due to surface roughness and atomic mixing. To solve these problems, we prepared thin samples by the focused ion beam technique and performed Auger electron spectroscopic analysis. We were able to measure nonconductive materials without any influence of electrical charging and to obtain sputter depth profiles of deep layers with less deterioration of depth resolution.

• Materials Analysis Using Synchrotron Radiation and Neutron Beam

To develop high-performance and highly reliable products that do not contain hazardous substances, Fujitsu uses many materials evaluation techniques to check products. Within these techniques, we have been developing analysis methods using synchrotron radiation and a neutron beam. These special "probes" have been provided at national facilities. In this paper, we describe the status of the SUNBEAM consortium organized by 13 industrial companies at the SPring-8 synchrotron radiation facility by looking at case examples of materials analysis. In addition, we introduce a new field of materials analysis based on an x-ray free electron laser constructing at the SPring-8 and a neutron beam provided by the high flux proton accelerator complex J-PARC (Japan Proton Accelerator Research Complex).

• Radiation Measurement Technologies for Evaluating Soft Errors

Two radiation measurement technologies for estimating and lowering the occurrence rate of "soft errors" are introduced in this report. Radiation such as fast neutrons (which originate in cosmic ray) and α-rays (which are generated by the materials composing LSIs) cause incorrect operations—known as soft errors—in computer systems. Since soft errors undermine the reliability of mission-critical products such as backbone servers, it is necessary to estimate and reduce their occurrence rate. To enable experimental measurements of the dose rates of neutron and α-ray, we have developed and utilized a vacuum α-ray tracking method—for measuring the amount of α-rays with high sensitivity—as well as a cosmic ray neutron spectrometer. With these two technologies, it has become possible to measure the radiation dose rate in low α-ray emitting materials, which has been impossible until now, and the neutron dose rate in arbitrary environments such as mountainous regions and to estimate the soft error occurrence rate accurately. Moreover, these technologies make it possible to select materials with low α-ray emissivity and choose environments with a low neutron dose rate, thereby contributing to improvements in computer system reliability.

• Device and Materials Analysis by Transmission Electron Microscopy

For advanced devices consisting of fine complicated structures controlled on the nanometer scale, there have recently been requirements for layer control and high functionalization for the device materials. Therefore, nanoscale analysis of such devices is indispensable to obtain high performance and high reliability. In this paper, we present nanometer-level device and materials analysis techniques that use transmission electron microscopy (TEM), which provides high spatial resolution. First, we describe a three-dimensional technique based on electron tomography for observing device structures controlled on the nanometer scale. Next, we describe a technique for visualizing magnetic and electric fields in devices on the nanometer scale by electron holography. Then, we describe a crystal structure and composition analysis technique for nanometer-scale multilayers that uses high-resolution TEM observation and electron energy-loss spectroscopy. Finally, we describe a technique for directly observing the atomic arrangement of ordered alloys and heteroepitaxial interface structures by high-angle annular dark-field scanning TEM.

• Compact Sensor for Environmental Monitoring

This report describes a simple means of detecting trace amount of substances in the atmosphere. In the fabrication processes for semiconductor large scale integrated circuits (LSIs), production defects occur if gas phase contaminants in the atmosphere stick to the wafer being processed. To overcome this problem, we have developed and are currently applying a contamination sensor that uses a quartz crystal microbalance (QCM) to detect such contamination sources. The detection system using this sensor has high sensitivity and is well suited to tracking the time-varying concentration of an atmospheric contaminant. Investigating the behavior of contaminants in a semiconductor LSI fabrication plant makes it possible to identify contamination sources and eliminate them. Therefore, this sensor system should be effective in lowering production costs and improving product quality. Moreover, the sensor is compact, so it can be easily installed, for example, at various locations around a plant and in the wafer container. Besides being compact, the QCM controller is easy to use and offers outstanding portability: it simply needs to be plugged into the universal serial bus (USB) port of a device such as a notebook personal computer. As a result of these features, we expect that this sensor will not only be used in semiconductor fabrication plants but also be applied to other kinds of environmental measurement.

• Evaluation and Analysis Technologies for Printed Wiring Board Materials

The printed wiring board (PWB) is a vital component of electronic equipment. Today, with product diversification and a shorter development cycle being dominant trends, it is essential to develop PWBs that satisfy performance and reliability requirements as quickly as possible. It has therefore become important to correctly select and appropriately use PWB materials that suit objectives from among the many commercially available materials. Over the last ten years, Fujitsu has been independently evaluating insulation materials for PWB use and has been comparing those results with the results of evaluating PWBs themselves while performing tests and accumulating data. We are currently developing evaluation technologies for determining whether new insulation materials can achieve the performance demanded of PWBs used in a variety of Fujitsu products. Among these technologies, we introduce ones that target thermomechanical properties, heat resistance, and transmission properties and mention their future directions.

• Interconnection Evaluation Technology for Printed Wiring Boards

As a developer of world-class products including server and network devices, Fujitsu recognizes the printed wiring board (PWB) as a core component among the various components of those products. One basic element supporting high-quality PWBs is through-hole interconnection reliability. Existing methods for evaluating interconnections typically involve temperature cycle tests that subject the PWB to low and high temperatures. We have developed an evaluation technique that applies current to interconnections and wiring patterns to heat the PWB's interior and generate a temperature rise. This technique can apply a temperature load closer to actual conditions than temperature cycle tests can, enabling evaluation in one-fifth the time. In this paper, we introduce this new through-hole interconnection evaluation technique.

• Simulation Method for Connector Packaging

There is a growing demand for technology that can analyze and test contact reliability with high accuracy in high-pin-count flat packaging such as sockets for central processing units (CPUs) and application specific integrated circuits (ASICs) and in card edge connectors for modular products. We have developed simulation technology for visualizing the behavior of individual contacts, which cannot be obtained empirically. This technology lets engineers test connections in connector packaging and clarify their behavior, enabling the mechanism behind stable contacts to be determined from the desktop without the need to perform numerous experiments. This paper introduces two key examples of applying this technology. First, for land grid array (LGA) connections having a high mating pressure, we clarified the behavior of individual contacts and tested long-term connection reliability while taking into account the package and system (board) displacement over time. Next, for connector packaging for module boards that must secure a mechanical board while also achieving electrical stability in the same contacts, we tested connection safety by clarifying the mechanism of module-board slippage due to vibrations during equipment transport.

• Ion Implantation Profile Analysis for Accurate Process/Device Simulation
  —Ion Implantation Profile Database Based on Tail Function—

For ion implantation, which is a standard technology for doping impurities into very large-scale integrated circuit (VLSI) processes, accurate prediction of the dopant profiles is indispensable. We need an analytical function that expresses a vast amount of experimental data to construct an ion implantation profile database. The profiles for arbitrary ion implantation conditions can be generated by interpolating or extrapolating the parameters of the analytical function. Indeed, some analytical functions have been developed. The simple Gaussian is extended to the joined half Gauss, Pearson, and dual Pearson, which is the standard function used in commercial simulators. However, we realized that the function has difficulties from the standpoint of a unique parameter set and proposed a tail function. On the basis of this tail function, we constructed a vast ion implantation database. Furthermore, with the addition of only one parameter, our database can be used to predict the amorphous layer thickness.

• Ion Implantation Profile Analysis for Accurate Process/Device Simulation
  —Prediction of Ion Implantation Profile Based on Quasi Crystal Extended LSS Theory—

Ion implantation profiles are sometimes required even when where the corresponding experimental data is nonexistent or poor. Although a Monte Carlo simulation such as TRIM can provide such profiles, tracing many ion trajectories takes a long time. We have developed an extended Lindhard-Scharff-Schiøtt (LSS) theory that provides almost the same profile as Monte Carlo simulation in a much shorter time, several orders of magnitude shorter, so data can be obtained almost instantaneously. Since this extended LSS theory cannot accommodate profiles in crystalline substrates, we further developed an empirical model that expresses crystal-related phenomena and linked it to the LSS theory. This theory, called the quasi crystal extended LSS theory (QCLSS), is vital for predicting profiles for any combination of ion and substrate. We applied this theory to ion implantation profiles in Si_1-x Ge_x substrates and established the corresponding database for arbitrary values of content ratio x.