Low Energy Electron Microscopy

M.S. Altman

Department of Physics
Hong Kong University of Science and Technology

Low Energy Electron Microscopy
Imaging Principle
Real-time Imaging
Spatial Resolution
Depth sensitivity
Operation conditions of LEEM

Low Energy Electron Microscopy
     Low energy electron microscopy (LEEM) utilizes low energy, elastically backscattered electrons to image surfaces with high spatial and temporal resolution[1]. Advantages of LEEM over other surface imaging techniques are: (a) real-time imaging capability, (b) several unique contrast mechanisms for image formation, (c) operation under extreme conditions. LEEM is a powerful tool for studying the dynamic and static properties of surfaces and thin films, including growth and decay, phase transitions, reactions, structure amd morphology, magnetism, and more.
Imaging Principle

     LEEM is fundamentally an emission microscopy. In conventional emission microscopy, electrons are emitted by some external stimulus, e.g., photo, thermal, or secondary emission. In LEEM, electron emission is stimulated by a coherent (collimated, monoenergetic) electron beam at or near normal incidence angle. First of all, an electron beam is generated by the cathode in the electron gun and accelerated up to the microscope potential, typically 20 keV. The illumination optics, consisting of a sequence of lenses and apertures, is then used to form a collimated electron beam suitable for illumination of the sample. Before reaching the sample, the beam must pass through a magnetic sector field that deflects the beam typically by 60 degrees. The magnetic sector field serves the important purpose of separating the beam that is reflected from the surface (green in Fig.1) from the incident beam (red in Fig.1). The incident electron beam is focused by the illumination optics to a point in the back focal plane of the objective lens (Fig.1), which is the heart of the LEEM. The incident and reflected beam that has undergone no momentum transfer parallel to the surface are overlapping in the objective region. The objective lens acts on both the incident and reflected electron beams and serves a dual purpose in both instances. In the objective, the incident beam is simultaneously focused to a parallel beam and decelerated to the desired incident energy that is given by the bias between the sample and the electron gun cathode. For crystalline samples, the backscattered beam will be confined to several specific angles - the Bragg angles - by diffraction. These electrons are then reaccelerated by the objective up to the microscope potential and focused to points in the back focal plane. This forms a diffraction pattern in the back focal plane. The reflected beam is then deflected by the sector field into the imaging column, which consists of a number of lenses. An image is formed by selecting one of the diffracted beams with a contrast aperture in the imaging column and magnification of this beam onto the microchannel plate image intensifier and phosphor screen. So-called bright field imaging is performed when the (00) or specular beam is used to form an image. The electrons in this beam have undergone no momentum transfer parallel to the surface. Tilted bright-field imaging also employs the (00) beam, but with a slight tilt of the illumination from normal incidence. Finally, dark-field imaging utilizes a non-specular fractional or integer order diffraction beam for imaging. Alternatively, the entire diffraction pattern may be viewed. This capability of imaging and diffraction allows a direct comparison of real and reciprocal space features.

Real-time Imaging [Top]
     Three important aspects of LEEM permit real-time imaging at video rates. First, LEEM is a non-scanning technique. That is, every point in the region of the surface that is being imaged is illuminated simultaneously and every point in the image is detected in parallel. Secondly, the reflected intensity for very low incident energies can be very high, reaching as much as 50% of the incident intensity for incident energies below 10 eV. For comparison, reflected intensities are typically one to three orders of magnitude smaller at incident energies, 50 - 300 eV, that are used in conventional low energy diffraction. Finally, the reflected intensity is confined to the diffraction angles for crystalline samples. The combination of parallel illumination/detection and high reflected intensity confined within small angles of specific directions by diffraction is favorable for fast imaging acquisition. Although the reflected intensity at higher energies or for non-crystalline sample may not allow imaging at video rates, images may still be obtained at slower rates by signal integration, provided that ccd cameras with suitably low noise are used.

Spatial Resolution [Top]
     The spatial resolution of LEEM is determined by the aberrations of the objective lens and by diffraction at the contrast aperture. At very low energy, LEEM is diffraction limited. The resolution of LEEM is primarily limited otherwise by chromatic aberrations. Calculations for an ideal homogeneous acceleration field [2] indicate that the ultimate resolution of LEEM would be on the order of 2 nm. Real objective lenses are only approximations of the ideal homogeneous acceleration field. The LEEM at HKUST employs a magnetic objective lens and has a spatial resolution of about 5-7 nm depending upon the incident energy. Phase contrast mechanisms, which are described below, give LEEM atomic resolution perpendicular to the surface.

Depth sensitivity [Top]
     According to the "universal curve" for electron mean free paths, the minimum penetration electron depth of just a few Å occurs at an incident energy of about 50 eV. The mean free path increases at lower energies appropriate for LEEM imaging. At very low energies, i.e., below 10 eV, the penetration depth may even approach 100 Å. This indicates that LEEM is generally sensitive to the topmost several atomic layers, but that deeper layers may also be probed by using very low incident energies. This fact is attested to by measurements of the quantum size effect in the reflection of low energy electrons [3,4], which demonstrate that interfaces that are buried at least 9 atomic layers below the surface can be detected.

Contrast [Top]
     Contrast mechanisms in LEEM conventionally falls into two categories: diffraction and phase constrast. Diffraction contrast occurs due to local variations of the structure factor. Diffraction contrast arises, for example, in bright-field imaging at normal incidence/exit because of geometric structure differences between different phases (Fig.2). Diffraction contrast is also important in tilted bright-field imaging. A well-known example is the Si(100) (1x2) surface. The surface structure consists of dimer rows which run parallel to either the surface (100) or (010) direction, and is called accordingly (1x2) or (2x1) to reflect the rotational relationship. Because of the bulk stacking, the dimer row direction rotates by 90 degrees on adjacent terraces, which leads to a topographical domain structure. Due to the strong dependence of the reflected intensity upon the incident angle on this highly anisotropic surface, a strong difference of the reflected intensity is realized between the (1x2) and (2x1) domains when the incident beam is tilted slightly in the (100) or (010) directions (Figs. 3, 4). Such a tilt may be parallel to the dimer rows in the (1x2) domains and perpendicular to the rows in the (2x1) domains, or visa-versa.

     Diffraction contrast is also important in dark-field imaging. Dark-field imaging is especially useful for identifying the spatial distributions of coexisting phases or when the small tilt angles used in titled bright field imaging are not sufficient to obtain contrast between rotational variants of low symmetry structures. An example from our work on Cr oxidation on the W(100) surface [5] is shown in Fig.5. In this example, dark-field images of three-dimensional Cr-oxide crystals are formed using the (½ 0) and (0 ½) diffraction spots. The areas in the crystal "light up" when they contribute to the diffraction spot used for imaging. These images clearly show the complex, complementary domain microstructure of the Cr-oxide crystals.

     One important class of LEEM contrasts are those having their origin in the phase of the imaging electron waves. Two important examples of LEEM phase contrast are atomic step contrast [3,6] and quantum size contrast [3,4]. Both of these phase contrasts provide atomic resolution information perpendicular to the surface. LEEM step contrast arises from the interference of the electron waves which are reflected from terraces on opposite sides of a step. The interference is a consequence of the phase shift that stems from the path length difference of the two waves. Since the wavelength of the electrons in the very low energy regime appropriate for LEEM is on the order of the step height, a wide range of phase shifts, and consequently interference conditions, can be sampled by making small changes of the imaging electron energy. A wave-optical model for LEEM step contrast has been developed [3,6]. Step contrast is calculated as the interference of the Fresnel diffracted waves from the terrace edges which meet at a step. This model allows for the routine identification of the step sense, i.e., the up and down-sides of a step, by simple visual inspection. Sample images that show step contrast are shown in Fig.6.

     Quantum size contrast is a vivid manifestation of the quantum size effect (QSE) in electron reflectivity. This QSE is understood classically to be an interference phenomena between the electron waves which are reflected from the surface of a thin film and from its interface with the substrate. A simple quantum mechanical Kronig-Penney model has been developed which accurately predicts the modulation of the reflected intensity as a function of film thickness and incident electron energy that is caused by the QSE [3,4]. A sample image that shows quantum size contrast is shown in Fig.7.

Operation conditions of LEEM [Top]
     LEEM is an ultra-high vacuum instrument, with typical base pressure of 1x10-10 torr and operational pressure under experimental conditions of up to 10-6 torr, or possibly higher. The pressure limitation during experimental operation is imposed by high voltage isolation between the sample and objective lens. LEEM can be operated continuously during deposition of metals or semiconductors or during gas exposure, including reactive gas. Imaging may also be performed over a wide range of temperature. The minimum temperature of operation so far that was achieved at HKUST was 50K, which was limited by the sample cooling apparatus. This is also a world record. LEEM also functions well at higher temperatures. Our experience shows that imaging is possible as high as 1500 K and possibly higher, depending upon the sample characteristics. At HKUST, we have also developed a sample holder with which it is possible to apply up to 100 G magnetic field at the sample position. Deflection of the imaging electron beam by magnetic fields parallel to the surface causes a sever limitation in working with applied magnetic fields in LEEM. Therefore, only magnetic field that is perpendicular to the sample can be applied conveniently in-situ during imaging.


Fig.1 Schematic description of electron trajectories in the LEEM objective lens.

Fig.2 Diffraction contrast in LEEM bright field imaging between the clean Si(111) (7x7) (dark areas) and Si(111)-In (bright areas) surface re-constructions. The image field of view is 6µm.

Fig.3 Tilted bright-field imaging of the Si(100) surface. Alternating (1x2) and (2x1) terraces (appear black and white, respectively). The image field of view is 6 µm.

Fig.4 Slighlty tilted bright-field LEEM image of the Si(100) surface showing monoatomic steps that separate the (1x2) and (2x1) terraces.

Fig.5 Dark-field images of the "(2x1)" Cr-oxide grown on the W(100) surface, and corresponding diffraction pattern.

Fig.6 LEEM Step phase contrast on the Si(111) (7x7) surface. The imaging energy is 42.5 eV. The field of view is 6 µm.


Fig.7 LEEM quantum size phase contrast in a Cu film on a W(110) surface. The imaging energy is 8.4 eV. The image field of view is 6 µm.

References [Top]
1. E. Bauer, Repts. Prog. Phys. 57, 895 (1994).
2. E. Bauer, Ultramicroscopy 17, 51 (1985).
3. C.H. Liu, W.F. Chung, M.S. Altman, Surf. Rev. Lett. 5, 1129 (1998).
4. M.S. Altman, W.F. Chung, Z.Q. He, H.C. Poon, S.Y. Tong, Appl. Surf. Sci. 169-170, 82 (2001).
5. Q.L. Guo, K.L. Man, M.S. Altman (unpublished).
6. W.F. Chung and M.S. Altman, Ultramicroscopy 74, 237 (1998)