Principles and Applications of Helium Ion Microscopy
Principles and Applications of Helium Ion Microscopy
- Larry Scipioni, Director of Applications Research, Carl Zeiss SMT, Inc
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- MA 01960,
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Materials and life science research is continually pushing toward finer scales for characterization in order to understand and manipulate properties. The failures that begin at the nano-scale, for example, often determine the material strength limit at the macro-scale. Biological functions, too, are defined by chemical and structural processes occurring at the nano-scale. Observation and measurement requirements are now sometimes exceeding what can be obtained with electron microscopy. The helium ion microscope offers benefits in resolution, material contrast, charge control, and surface sensitivity, providing unique image information that complements or exceeds that from an SEM. We will introduce here the technology of this instrument and show some relevant recent results.
The two key technology attributes of this tool are the ion source and the nature of the beam interaction with the sample being imaged. The ORION Plus helium ion microscope now being manufactured by Carl Zeiss SMT is the product of a generation of research on gas field ion emission. Others have described the ion source in detail [1,2], so we touch upon it only briefly here. The source consists of a sharpened needle held at high positive voltage and low temperature in the presence of helium gas. Field ionization allows the creation of a very bright ion beam at the tip of the needle, as described next. A special source formation process creates a tip having just three atoms (the "trimer") at the very apex. The electric field can become intense enough over these atoms that gas atoms impinging on the surface are stripped of an electron and emitted as ions. The cryogenic temperature at the tip increases the density of available gas atoms. The ORION source formation process also provides that just the trimer atoms get to share most of this supply of gas. Selecting one of these atoms as the source delivers a beam with a source size below an Angstrom and a brightness of exceeding 5×109 A/cm2-sr – an order of magnitude beyond even a cold field electron emitter.
The ion beam is then transmitted through a two lens electrostatic ion optical column onto the sample surface. The beam landing energy is typically 25 – 35 keV; beam currents from 0.1 pA up to about 10 pA are used. The column can produce a focused probe with a spot size of about 0.25 nm. This number is determined by optical calculations and also verified experimentally, as discussed below. This exciting result represents a three-fold improvement compared to SEM performance, with chamber-residing samples or two-fold over sample-in-lens SEM.
Image signal acquisition is accomplished with three different detectors. The tool has an Everhardt-Thornley detector for collection of secondary electrons. This can be used in conjunction with an electron flood gun when imaging insulating samples. There is a multichannel plate for collection of backscattered ions, creating a signal with atomic number contrast and also sample charging immunity. Finally, there is a new option for an energy proportional analysis of backscattered helium. Such a detector allows for the collection of edge spectra akin to Rutherford backscattering spectroscopy. This is not discussed further here.
But perhaps the most intriguing and useful aspects of the microscope arise from the nature of the interaction of the beam with the sample. These interactions produce the secondary particle signals that provide the image information, and the unique physics of the interaction of moderate energy helium ions with material surfaces yield sample information that is novel in several aspects. A good way to understand these phenomena and their utility for microscopy applications is to study images taken with the tool. The remainder of this brief article will show several images and describe their salient features.
The single most critical parameter in nanotechnology imaging is resolution. Here the ORION shines especially bright. Figure 1 shows an image of an asbestos fiber, whose edge is seen going left to right across the image. This particular image was analyzed with an edge finding algorithm that measures the 25 – 75% rise distance across multiple edges. This widely used metric for probe size measurement reports a value of 0.25±0.06 nm. This is a world-record for spatial resolution in secondary electron (SE) imaging. It should be noted that this material has an exceptionally sharp edge, so other material systems may not show this level of resolution. Such results do make clear what they tool is capable of, though, since an image-based measurement like this integrates all sources of mechanical and electromagnetic noise, making for a true representation of performance.
Figure 1: Image of the edge of an asbestos (crocidolite) fiber resting on a holey carbon grid. SE mode image, 200 nm field of view.
Material and surface contrast are two other key parameters in creating a useful image. Surface sensitivity is obtained in the helium ion microscope as a by-product of the low energy of the SEs that it creates. These have an escape depth of just a few nanometers and do not create undesirable type-II SEs when exiting the sample. There is no need to reduce the beam energy – and lose probe size performance – to see topology with nanometer level sensitivity. Material contrast is also excellent because of a high SE yield , which also varies well with the target material. Figure 2 is an image of a polycrystalline platinum surface. All the micro-faceting in the grains show clearly, as well as the high number of asperities on the surface. In addition, there is notable grain orientation contrast. This large degree of topological information cannot be approached in a SEM, even with the newest "XHR" technologies.
Figure 2: SE mode image of a polycrystalline platinum surface at a 19 µm field of view.
Many material and biological samples are highly insulating, so a solution must be provided for imaging them stably. The use of conductive coatings is quite common, but this has the effect of masking any fine surface details. Even with coating, some systems remain a challenge to image. Figure 3 shows collagen fibers from the knee joint of a mouse. Even with a 2 nm coating of Au-Pd, SEM imaging could not be stabilized. Using the low energy electron flood gun in the ORION it was possible to obtain a high quality image that shows great structural detail of the banding in the fibers, which is linked to their performance. One can even see the grains of the coating. Our development efforts are now turning toward imaging uncoated samples, and results will be published soon on this topic.
Figure 3: SE mode image, utilizing charge control, of collagen fibers. 1.1 µm field of view. Sample courtesy of Dr Claus Burkhardt, NMI, Reutlingen, Germany.
Surface sensitivity is also helpful in exploring the properties of other systems. Figure 4 shows a CaF2 globule on the surface of a tooth. Erosion that occurs in the hydroxyl apatite (the substrate in this case) is reversed by fluoride ions from the CaF2. Thus the ability to study these two surfaces together with high fidelity can aid such research. The image shows great surface sensitivity and resolution in support of such investigations. In addition, the long depth of focus in the microscope allows both the surface of the globule and the enamel substrate to be imaged clearly at the same time.
Figure 4: Surface of human tooth. SE mode image at 3 µm field of view. Sample is carbon coated. Courtesy of Dr Frank Altmann, Fraunhofer Institute, Halle, Germany.
Another area we are now exploring is the imaging of beam sensitive samples. Many types of plastics, for example, suffer depolymerization under electron irradiation and subsequent volatization of material, leading to significant sample erosion. We compare the imaging of Delrin between the ORION and a SEM in Figure 5. The SEM was operated at 0.75 keV beam energy in order to minimize damage, and a charge control system was used, since the sample was uncoated. The SEM image, on the right in this montage, shows significant pitting all across the field of view. The sample remained unaltered in the ORION. This is critical in getting reliable data for material strength testing. Charge control and image resolution are superior as well in the ORION.
Figure 5: Montage showing comparison between helium ion imaging (left) and SEM (right) of Delrin plastic. 5 µm field of view, SE mode, in both images.
Our final example helps to highlight the Rutherford backscattered imaging (RBI) mode of the helium ion microscope. This can be used to enhance material contrast and also to avoid charging effects. The image in Figure 6 shows the surface of a thin film solar cell taken in RBI mode. Along the top is the edge of a patterned aluminum line, while the majority of the image shows the anti-reflective coating that is over the active silicon area. This layer consists of particles of TiO2 embedded in a resin. In SE mode this area looked completely featureless, but in RBI a wealth of data is available. Titania particles at the surface are the bright white dots, having relatively higher atomic number contrast. The more diffuse lighter areas are indicating some type of surface variations. This image is completely immune to surface charging since the outgoing particles are mostly neutral helium atoms.
Figure 6: RBI mode image of a thin film solar cell. 10µm field of view. Courtesy of Dr Frank Altmann, Fraunhofer Institute, Halle, Germany.
This brief tour through some images representative of ORION capabilities have given, the author hopes, a clearer picture of what this new technology is able to accomplish. We continuously explore new applications at this phase of the tool’s development along the way to establishing the ORION in the charged particle microscopy landscape.
Many thanks are extended to the customers who provided some of these samples and also to the engineers at Carl Zeiss who captured the images shown here.
-  J. Morgan, et. al., Microscopy Today, 14 (4), p.24 (2006).
-  B. Ward, et. al., J. Vac. Sci. Technol. B, 24 (6) p.2871 (2006).
-  D.C. Joy et. al., Microsc. Microanal. 12(Supp 2), p.1146CD (2006).
- Figure Captions
- Figure 1: Image of the edge of an asbestos (crocidolite) fiber resting on a holey carbon grid. SE mode image, 200 nm field of view.
- Figure 2: SE mode image of a polycrystalline platinum surface at a 19 µm field of view.
- Figure 3: SE mode image, utilizing charge control, of collagen fibers. 1.1 µm field of view. Sample courtesy of Dr Claus Burkhardt, NMI, Reutlingen, Germany.
- Figure 4: Surface of human tooth. SE mode image at 3 µm field of view. Sample is carbon coated. Courtesy of Dr Frank Altmann, Fraunhofer Institute, Halle, Germany.
- Figure 5: Montage showing comparison between helium ion imaging (left) and SEM (right) of Delrin plastic. 5 µm field of view, SE mode, in both images.
- Figure 6: RBI mode image of a thin film solar cell. 10µm field of view. Courtesy of Dr Frank Altmann, Fraunhofer Institute, Halle, Germany.