Utilizing the Charging Effect in Scanning Electron Microscopy
Zhang, Hai-Bo, Feng, Ren-Jian, Ura, Katsumi, Science Progress
The charging effect of an insulating specimen from electron beam (e-beam) irradiation may be utilized to facilitate imaging in the scanning electron microscope (SEM). This has been confirmed by a great deal of experimental work during the last three decades. Particularly, recent investigations indicate that even located underneath insulating thin films that a low energy e-beam cannot penetrate, conductors not biased and overlay marks, are observable through a novel imaging pattern, charging contrast. Unlike conventional SEM contrasts, which usually reflect surface characteristics, the dynamic charging contrast can reveal information of underlying structures without any external exciting signal. The authors consider that this kind of charging contrast arises from the different redistribution rates of secondary electrons returning to the surface under the surface local field of the charged specimen. The charging contrast has the prospect of extending the SEM application and forming new testing methods matched with the fast development of integrated circuits.
Keywords: Scanning electron microscope (SEM), image contrast, insulator, charging effect, surface local field, secondary electron, redistribution, integrated circuit measurement
Different kinds of signals carrying specimen information may be stimulated when an accelerated electron beam (e-beam) irradiates the specimen. These signals include transmission electrons (TE), backscattered electrons (BSE), secondary electrons (SE), Auger electrons, X-ray and so on. The electron microscope (EM) mainly uses TEs, SEs and BSEs to observe and analyze the structure, and the topographic and compositional characteristics of the specimen. The German scientists Ruska and Knoll designed the first EM in 1931. This EM was a transmission EM (TEM), which produced the electron image with a 17-time magnification. Since then, the EM had been greatly developed and become an essential scientific instrument in many fields of science and technology. Ruska was therefore awarded the Nobel Prize for physics in 1986. For the detailed history of the EM, some review articles can be recommended (1-4.)
As one type of EM, the scanning electron microscope (SEM) is based on SE or BSE imaging and has advantages of the relatively simple specimen preparation and a high resolution available for larger specimens. The origin of the SEM can be traced back to 1930s after the birth of the TEM (3). In 1938, a German scientist, von Ardenne, reputed as the true father of the SEM, proposed the principles underlying the SEM as we know them today and built the first genuine SEM by adding deflection coils to a TEM. Although Knoll proposed the basic principle of the SEM in 1935, Zworykin et al. of the RCA laboratory developed the SEM that achieved a resolution of about 50nm in 1942. It is worth mentioning here that Oatley, a pioneer of the SEM, with his group in Cambridge University had carried out much work to raise the SEM performance from the end of World War II to 1965, laying the foundations of the modern commercial SEM (5). In this period, Smith built a properly engineered SEM for the Paper Institute of Canada in 1960 so that the SEM became a practical instrument. At last, the Cambridge Instrument Company produced the first commercial SEMs with the trade name "Steroscan" in 1965. From then on, the SEM began to be widely used all over the world. Moreover, during the recent two decades, the SEM instrumentation has further advanced owing to the development and application of the field emission gun (FEG) (4). The highest resolution of the commercial SEM with a FEG (FESEM) can now reach 0.6 nm. There have also emerged some new types of SEM, such as the environmental SEM (ESEM) (6) working in the low vacuum condition, and the low voltage SEM (LVSEM) (7) which gives a high performance with the accelerating voltage below 5 kV.
An SE image of the specimen is formed by scanning a focused e-beam over the specimen surface. …