Compared to an optical microscope, the electron microscope achieves far greater resolution and magnification by taking advantage of the wave aspect of electrons. An electron’s wavelength is typically 1/100,000 that of visible light. Resolutions of 50 pm and 10 million-X magnification have been achieved, far better than the 200-mm resolution and 2,000X magnification of a laboratory-grade optical microscope.
The principle behind electron microscope imaging is simple: Illuminate the object under investigation using an accelerated electron beam rather than visible light. Then translate the reflected or transmitted electron beam to create a useful image. Making the whole thing work is complex instrumentation developed over the last nine decades.
Introduced in the early 1930s was the transmission electron microscope (TEM). Conceptually it resembled the cathode ray tube (CRT), which was highly developed at that time. The TEM uses an accelerated and focused high-voltage beam to illuminate the object and create an image in the manner of an optical microscope using light. The electron beam is created by a hot, biased cathode or anode and accelerated by mean of charged grids.
Information is acquired because the electron beam is partly transmitted and partly scattered. The image is magnified and then projected onto a phosphor-coated screen, where it can be viewed or photographed. As in a CRT, the entire process including the mounted specimen must be within a vacuum to prevent the tungsten filament providing the electrons from burning and also to permit the unimpeded flow of electrons.
The first TEMs had limited resolution due to spherical aberration. But currently, this harmful phenomenon is reduced by means of hardware correctors, increasing the resolution to 50 pm, with magnification attaining an astonishing 50 million X. This instrument can image individual atoms.
A problem in TEMs is that specimens can be imaged only when sliced into thin sections so the electron beam can pass through them. Biological samples, for example, must be embedded in a polymer resin to permit sectioning. Semiconductor specimens require elaborate processing.
For this and other reasons, the scanning electron microscope (SEM) soon dominated the electron imaging field. Because SEM images the surface only, the samples it examines needn’t be thin slices. Specimens can be quite tall, with a highly textured surface. In operation, the focused electron beam scans a rectangular section of the object to be imaged, and the acquired information in a modern instrument is digitized to be displayed on a flat screen and/or saved in computer memory.
SEM resolution is less than TEM resolution, but an advantage is depth-of-field imaging of an irregular surface. Additionally, lower vacuum is required so unfixed biological specimens can be readily imaged.
The scanning transmission electron microscope (STEM) combines TEM and SEM features. A thin slice of the specimen is prepared, and the electron beam is directed to pass through it as in the TEM. The image is scanned and rasterized as in the SEM, enabling high resolution. A conventional TEM is frequently built with optional scanning capability so the instrument can function in either mode.
Electron microscopes are often seen in a medical or healthcare context, but they also play a large role in industry, quality control and failure analysis.
The scanning tunneling microscope (STM), invented in 1981 at the Zurich IBM laboratories, is one of several innovations that permit researchers to more accurately visualize individual atoms and molecules and to watch them interact in real time. A limitation, however, is that this powerful new instrumentation works only when the surface to be imaged is at least partially conductive.
STM makes use of quantum tunneling, whereby an electron can penetrate an energy barrier that is more energetic than the electron attempting to cross it. In this technology, a probe with a sharp metal tip, one atom thick, traverses close to the surface of a conductive specimen. A bias relative to the surface is applied to the tip and the current is monitored, indicating the distance at any instant between the tip and the irregular surface. An image is derived, corresponding to the specimen’s surface topology, with single-atom resolution.
A related technology, also mechanical rather than electrical, is atomic force microscopy (AFM). It is suitable for examining less conductive specimens. The methodology uses equipment similar to that of STM, but the defining difference is that the probe actually touches and rides over the surface under examination. Precision is maintained by means of electronically controlled piezoelectric elements.
In addition to imaging, AFM is capable of force measurement and manipulation. To measure separation between the probe and the specimen, AFMs determine the amount of force between them. From this metric, it is possible to ascertain physical properties of the object such as its stiffness at a particular location on its surface.
Varying with time in the scanning process, the amount of force imposed by the sample upon the probe provides a high-resolution, three-dimensional image of the object. Going a step further, the force between tip and sample can actually manipulate the object at an atomic level. Because AFM does not depend upon optics or on an electron beam, spatial resolution is not compromised by aberration, and elaborate preparation is not necessary.
AFM is applicable to solid-state physics and semiconductor science and technology, primarily because the substrate need not be conductive, nor is elaborate sample preparation necessary. AFM in solid-state physics is useful in identifying atoms at the surface, determining interactions among adjacent atoms and characterizing changes resulting from atomic manipulations.
In non-contact atomic force microscopy, the probe tip does not touch the object under investigation. In this mode, there is less tip wear. The probe is oscillated at a high frequency. Because the force caused by the nearby surface reduces the resonant frequency of the probe, continuously measuring that frequency yields a topographic image.
Because the SEM and its variations have much higher resolution than competing technologies, they are the methods of choice in semiconductor research, development, and production-line monitoring. A challenge for SEM use in semiconductors is that low voltages can easily destroy the component under test. Low-voltage measurements tend to be less accurate. Counter-intuitively, low-energy electrons react strongly with low atomic number elements required by semiconductor designs. Also, the low-energy signal is difficult to detect.
An obvious challenge in SEM examination of semiconductors is the damage to the device caused by the electron beam. The measurement process can be invalidated and/or the product may incur undetected damage. Additionally, the probe can cause physical contamination by leaving behind foreign particles.