Imaging sub-nanometer atomic steps with a compact field emission scanning electron microscopeb
Imaging Sub-Nanometer Atomic Steps with a Compact Field Emission Scanning Electron Microscope
- Jim Rynne, Novelx
- 3746 Mt. Diablo Blvd,
- Suite 100,
- Tel. +1 925 962. 889 x304
- scanning electron microscopy,
- crystalline materials,
- low-voltage electron channeling contrast imaging,
- field emission
Nanoscale metrology of material surfaces is an important capability for a wide variety of applications. Scanning electron microscopes (SEMs) are convenient tools for these measurements since they provide very high spatial resolution in the plane of the sample and allow visualization of the third (i.e. out of plane) dimension. Simply the presence or absence of certain topographic features is often sufficient for many applications.
Topographic Mode Imaging with the Novelx mySEM
Surface features at the nanoscale can be enhanced using a variety of techniques. Lowering the primary beam accelerating voltage to 1 kV decreases the penetration depth of the beam (typically 30-40 nm) and accentuates surface features which may otherwise be masked at higher voltages. Operating the electron detector in a differential or "topographic" mode, where the common background signal is subtracted from the image, also improves surface contrast. With crystalline materials, using the relatively recent technique of electron channeling contrast imaging (ECCI), vertical resolution can be further improved resulting in the ability to image individual dislocations, atomic steps and other defects on or near the surface.
Poly-type 6H-SiC polished wafers were imaged using a Novelx mySEM at low voltage (0.5 -1.2 kV) in topographic mode. Both backscattered and secondary electrons were collected to enhance the signal-to-noise ratio. Even without specialized hardware, the sensitivity of the technique due to electron channeling was sufficiently high to clearly resolve sub-nanometer steps on the surface of the wafer. Atomic force microscopy was used to verify the step height.
A unique attribute of the mySEM is a quad-segmented microchannel plate (MCP) detector assembly located directly below the objective lens. The detector collects backscattered electrons and, depending on the bias of the front plate, accepts or rejects secondary electrons. By operating the detector in a differential mode, an effect similar to positioning a light source at glancing incidence can be created. An example of a topographic mode image of a 20-nm thick holey carbon sample and a comparison to secondary electron image is shown in Figure 1.
Figure 1: Comparison of secondary mode (left) to topographic mode (right). The sample consists of a 20-nm thick carbon film on a standard copper TEM grid. The holes are 1.2 um in diameter on a 2.5 um pitch. The four quadrant collector plate used to collect the images is also shown. The topographic image is formed by combing the four segments as (A+D)-(B+C) or (A+B)-(C+D).
Electron Channeling Contrast Imaging
Representative micrographs of the SiC samples, imaged in topographic mode by the Maboudian Research Group at the Center of Integrated Nanomechanical Systems (COINS) at UC Berkeley, are shown in Figure 2a and 2b. Several surface features are evident, including, point defects and terraced planes. Close examination of the terraces shown in Figure 2a reveal an additional fine structure as evident in Figure 2b. These fine features were visible in both backscatter and secondary-plus-backscatter topographic modes, but invisible in any of the non-topographic modes. An AFM image of the same positions determined the features were atomic steps with heights of ~0.8 nm.
Figure 2: (a) Hillocks, terraces and defects on 6H-SiC sample. (b) The fine structure on the terraces was determined to be sub-nm atomic steps. Images provided courtesy of the Maboudian Lab, COINS, UC Berkeley.
The contrast mechanism for sub-nm resolution could be inferred by considering the contrast mechanisms listed in Table 1. By process of elimination, channeling contrast was determined to be responsible for the contrast at the atomic terrace edges.
Table 1: Contrast mechanisms in SEMs. E-T is the Everhart-Thornley type detector, SSD is a solid-state detector and FS is forward-scattered electrons. A-B is the topographic mode where the signal in the left channel of a detector is subtracted from the signal on the right side.
Previous TEM studies of atomic steps on Si(111) concluded that strain at the edges of steps produced image contrast. The topographic mode of the mySEM could detect this strain and as a consequence produce channeling contrast. Using low-voltage electron channeling contrast imaging (LVECCI) at normal incidence, sub-nm steps in 6H-SiC were resolved. The particular geometry of the mySEM system combined with a high brightness thermal field emission (TFE) source and a quad-segmented detector allowed imaging of steps, defects, voids and hillocks with no additional hardware.
The Novelx mySEM pictured below is a compact field emission SEM that offers capabilities previously only available in more expensive and much larger conventional field emission SEMs. The mySEM provides sub-10 nm resolution and several low-voltage imaging techniques that are being used in a wide variety of materials science and life sciences applications.
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