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APPLIED PHYSICS REVIEWS—FOCUSED REVIEW Secondary JOURNAL OF APPLIED PHYSICS 97, 051101 ͑2005͒ APPLIED PHYSICS REVIEWS—FOCUSED REVIEW Secondary electron contrast in low-vacuum/environmental scanning electron microscopy of dielectrics ͒ ͒ Bradley L. Thiela and Milos Tothb Polymers and Colloids Group, Cavendish Laboratory, Department of Physics, Madingley Road, University of Cambridge, Cambridge, CB3 0HE, United Kingdom ͑Received 28 July 2003; accepted 4 October 2004; published online 16 February 2005͒ Low vacuum scanning electron microscopy ͑SEM͒ is a high-resolution technique, with the ability to obtain secondary electron images of uncoated, nonconductive specimens. This feat is achieved by allowing a small pressure of gas in the specimen chamber. Gas molecules are ionized by primary electrons, as well as by those emitted from the specimen. These ions then assist in dissipating charge from the sample. However, the interactions between the ions, the specimen, and the secondary electrons give rise to contrast mechanisms that are unique to these instruments. This paper summarizes the central issues with charging and discusses how electrostatically stable, reproducible imaging conditions are achieved. Recent developments in understanding the physics of image formation are reviewed, with an emphasis on how local variations in electronic structure, dynamic charging processes, and interactions between ionized gas molecules and low-energy electrons at and near the sample surface give rise to useful contrast mechanisms. Many of the substances that can be examined in these instruments, including conductive polymers and liquids, possess charge carriers having intermediate mobilities, as compared to metals and most solid insulators. This can give rise to dynamic contrast mechanisms, and allow for characterization techniques for mapping electronic inhomogeneities in electronic materials and other dielectrics. Finally, a number of noteworthy application areas published in the literature are reviewed, concentrating on cases where interesting contrast has been reported, or where analysis in a conventional SEM would not be possible. In the former case, a critical analysis of the results will be given in light of the imaging theory put forth. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1861149͔ TABLE OF CONTENTS IV. IMAGING IN LOW VACUUM: THE ROLE OF IONS..................................... 8 A. Space charge........................... 9 I. INTRODUCTION............................ 1 B. Electron-ion recombination............... 10 II. ELECTRON MICROSCOPY IN A LOW 1. Spatial filtering....................... 11 VACUUM.................................. 2 2. Energy filtering...................... 11 A. The instrument......................... 2 C. Schottky effect......................... 11 B. Electron-gas scattering in a low-vacuum D. Charge balance......................... 11 SEM................................. 3 E. Other ion effects........................ 12 1. Gas cascade amplification.............. 3 V. APPLICATIONS............................ 12 2. Scattering of the primary beam.......... 4 A. Ferroelectric domains.................... 13 III. SECONDARY ELECTRON EMISSION FROM B. Electronic devices....................... 13 DIELECTRICS............................. 5 C. “Charge contrast imaging”................ 13 A. Intrinsic emission in the absence of 1. High-vacuum contrast................. 14 charging............................... 5 2. Electron flux density.................. 14 B. Physical processes of charging and effects 3. Pressure and anode bias dependence..... 14 on electron emission..................... 7 D. Electronic polymers..................... 15 C. The role of charge traps.................. 8 E. Liquids............................... 16 D. Time dependent behavior................. 8 F. X-ray microanalysis..................... 16 VI. SUMMARY AND OUTLOOK................ 17 a͒ Present address: College of Nanoscale Sciences and Engineering, I. INTRODUCTION University at Albany, Albany, NY 12203; electronic mail: [email protected] ͒ A major driving force behind the development of b Present address: FEI Company, 1 Corporation Way #2, Peabody, MA 01960. environmental/low-vacuum scanning electron microscopes 0021-8979/2005/97͑5͒/051101/18/$22.5097, 051101-1 © 2005 American Institute of Physics 051101-2 B.L. Thiel and M. Toth J. Appl. Phys. 97, 051101 ͑2005͒ ͑SEMs͒ was to enable secondary electron ͑SE͒ imaging of for both high- and low-vacuum conditions. It is more accu- uncoated insulators. In actuality, these microscopes do not rate to describe these operating conditions as a regime in stop charging processes, but rather stabilize them and simul- which the effects of charging on electron imaging are mini- taneously eliminate many of the undesirable artifacts associ- mized. In fact, significant amounts of charge of both signs ated with charging. When imaging conditions are carefully may be present simultaneously, as will be discussed in detail. controlled, dynamic charging processes can be exploited to The present review summarizes recent efforts to explain reveal spatially resolved information on the electronic inho- exactly how stable imaging conditions are achieved. It then mogeneities in dielectric substances. As this technique ma- goes on to provide a description of the SE contrast mecha- tures, it is emerging that complex interactions between high- nisms and imaging considerations relevant to working in a and low-energy electrons, the weakly ionized environmental low-vacuum environment. The discussion on contrast gas, and the specimen can give rise to contrast mechanisms mechanisms is divided roughly into three parts: ͑i͒ SE gen- which are potentially useful and unique to these instruments. eration and intrinsic SE emission from dielectrics ͑i.e., in the This paper reviews and summarizes recent progress towards absence of charging͒, ͑ii͒ information present in SE images understanding the various phenomena involved in low- due to localized charging, and ͑iii͒ contrast mechanisms re- vacuum imaging and then uses this foundation to interpret sulting from interactions between ionized gas molecules and several examples of “anomalous” contrast reported in the low-energy electrons at/near the specimen surface. A few literature. select applications are then reviewed, where contrast effects There is an unfortunate tendency in the literature to dis- are particularly interesting. Finally, we note that for many of tinguish high-vacuum, low-vacuum, and environmental SEM the materials that can be examined only in low-vacuum in- solely as pressure regimes. This often leads to confusion with struments ͑e.g., liquids͒ the charge carriers have intermediate trademarked names and acronyms used by manufacturers mobilities, as compared to those of metals and common solid ͑e.g., operating an ESEM® in low-vacuum mode͒. For clar- insulators. This property can give rise to electron-flux- ity, the definition of terms used throughout this review is density-dependent contrast, which may be useful for the given here. The term “low vacuum” is applied when the gas characterization of electronic inhomogeneities. Although this performs an electronic role, such as charge stabilization or technique has not yet been developed in earnest, a few strik- signal amplification. The term “environmental SEM” is used ing examples are reviewed that highlight one of the unique specifically in situations where the gas primarily performs a capabilities of low-vacuum microscopy. thermodynamic role, such as preventing the evaporation of This review concentrates on peer-reviewed literature. liquids from the specimen or initiating chemical reactions. However, much of the academic discussion has taken place Naturally, the two functions are not mutually exclusive, nor in scientific meetings. In particular, the proceedings of the are their pressure ranges. Alternatively, in the language of Microscopy Society of America’s annual meeting, Micros- physical chemistry, low vacuum is appropriate when the ab- copy and Microanalysis, contain reports on a wide variety of solute gas pressure matters, and “environmental” applies to applications. For the most part, these proceedings are ex- conditions where the partial pressure of gas is relevant. tended abstracts that report anomalous contrast effects or are Within this review, the term low vacuum is also used when progress updates on efforts to understand various phenom- generic issues are being discussed such as electron-gas inter- ena. Both categories are encompassed by the refereed papers actions or practical/engineering considerations. cited here. Most of the earlier work and references can be The simplified description of low-vacuum imaging put found in the numerous papers authored by Danilatos, cited forward in most literature is as follows: If a small amount of throughout this review. A particularly good compilation of gas is fed into a SEM specimen chamber, positive ions are early applications can be found in a dedicated issue of Mi- created via collisions between electrons and gas molecules. croscopy Research and Technique.7 That same volume also When the ion current reaching the specimen exactly offsets contains a very extensive bibliography of early literature the rate of negative charge accumulation, charge balance compiled by Danilatos.8 Finally, two reviews of general ap- conditions are said to exist. Expressions describing each of plications with an emphasis on polymeric, hydrated, and liq- these contributions can be summed together to provide an uid materials have
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