Wentao Qin

Image-Based Nanocrystallography in Future Aberration-Corrected Transmission Electron Microscopes

Since the crystallographic phase and morphology of many materials changes with the crystal size in the one to hundred nanometer range and the potential technological applications of nanoparticles are enormous, a need arises to determine the crystallography of nanoparticles individually. Direct space high- resolution phase-contrast transmission electron microscopy (TEM) and atomic resolution Z-contrast scanning TEM when combined with goniometry of direct and/or reciprocal lattice vectors offer the possibility of developing dedicated nanocrystallography characterization methods for such small nanoparticles. Although experimentally feasible for cubic nanocrystals with lattice constants larger than 0.4 nm in contemporary high-resolution TEMs with modest tilt range, image-based nanocrystallography by means of transmission electron goniometry has so far only been employed by a few specialists worldwide. This is likely to change in the future with the availability of aberration-corrected TEMs. The reasons why this change is likely to happen are outlined in this paper.

Measuring Local Thickness Through Small-Tilt Fringe Visibility

Specimen thickness measurements are often limited to analyzing one region at a time, and by the size of the electron probe. With increased availability of lattice fringe data in phase and z contrast images, information on how fringes change with tilt is also more accessible. We discuss how such data can provide thickness information on specimen regions only nanometers on a side, provided they are thin enough for lattice imaging. Using micrographs 1/3 micron across, many regions can be analyzed with only a few images.

Oxide impurities in silicon oxide intermetal dielectrics and their potential to elevate via-resistances.

Silicon oxide used as an intermetal dielectric (IMD) incorporates oxide impurities during both its formation and subsequent processing to create vias in the IMD. Without a sufficient degassing of the IMD, oxide impurities released from the IMD during the physical vapor deposition (PVD) of the glue layer of the vias had led to an oxidation of the glue layer and eventual increase of the via resistances, which correlated with the O-to-Si atomic ratio of the IMD being ~10% excessive as verified by transmission electron microscopy (TEM) analysis. A vacuum bake of the IMD was subsequently implemented to enhance outgassing of the oxide impurities in the IMD before the glue layer deposition. The implementation successfully reduced the via resistances to an acceptable level.

Goniometry of Direct Lattice Vectors Supporting Students' Comprehension of Crystallographic Core Concepts and Demonstrating Image-Based Nanocrystallography

We are of the opinion that students of an introductory materials science and engineering course should gain a thorough understanding of crystallographic core concepts by applying them quasi-experimentally in computer simulation sessions that run parallel to the lectures. Software simulations of goniometry of direct lattice vectors in a transmission electron microscope (TEM) will serve two purposes at once: to introduce students to practical aspects of electron microscopy and support their comprehension of crystallographic core concepts. We use the programming software Matlab and Java (Jmol applets) on a PC platform for the creation of software simulations that demonstrate this methodology and complement already existin g software sim ulations. The newly created software is used in classroom demonstrations of an introductory materials science and engineering course at Portland State University and will become freely accessible over the internet. This software will also support and promote image-based nanocrystallography in TEM.

Application of FIB/SEM and TEM to Bit Failure Analyses in SRAM Arrays

Many microelectronic chips contain embedded memory arrays. A single SRAM bit-cell contains several transistors. Failure of any of the transistors makes the entire bit-cell inoperable. Dual-beam Focused Ion Beam (FIB) combines the slicing capability of FIB with in-situ SEM imaging. The combination offers unparalleled precision in looking for root causes of failures in microelectronic devices. Once a failure site is located, an FIB lift-off method can be used to prepare a TEM sample containing the area of interest. Further structural, elemental information can then be acquired from the failure site. We report here analyses of single and multiple bit failures in SRAM arrays carried out using FIB/SEM, and in two cases TEM imaging and EDS/PEELS. Root causes of bit failures including remnant seed-layer metal between stacked vias have been identified.

Oxide Reduction in Advanced Metal Stacks for Microelectronic Applications

Aluminum and copper are widely used for microelectronic interconnect applications. Interfacial oxides can cause device performance degradation and failure by significantly increasing electrical resistance. Interfacial oxide layers found in Al/Ta and Ta/Cu metal stacks were studied using Transmission Electron Microscopy (TEM) combined with Energy Dispersive Spectroscopy (EDS) and Parallel Electron Energy Loss Spectroscopy (PEELS). The analysis indicates that the observed interfacial oxide layers, Al2O3 and mainly Ta2O5, result from spontaneous reductions of Ta oxide and Cu oxide, respectively. Thermodynamics enables interpretation of the results.

Root‐Cause Analysis and Statistical Process Control of Epilayers for SiGe:C Hetero‐Structure Bipolar Transistors

The SiGe:C hetero‐structure bipolar transistor (HBT) has turned into a key technology for wireless communication. This paper describes various critical analytical techniques to bring up and maintain the SiGe:C epi‐process. Two types of analysis are critical, (1) routine monitoring SiGe base and Si cap thickness, doping dose, Ge composition profile, and their uniformity across the wafer; and (2) root‐cause analysis on problems due to non‐optimized process and variation in process conditions. A transmission electron microscopy (TEM) technique has been developed allowing a thickness measurement with a reproducibility better than 3 A. Charge‐compensated low‐energy secondary ion mass spectrometry (SIMS) using optical conductivity enhancement (OCE) allows a Ge composition measurement to a required precision of 0.5 at. %.