Introduction to Imaging Techniques in the HIM
The helium ion microscope (HIM), as the name implies, is primarily an imaging tool. This chapter serves as an introduction to imaging with the HIM and explores the various ways this is implemented by first describing the numerous imaging signals and contr
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Introduction to Imaging Techniques in the HIM Stuart A. Boden
Abstract The helium ion microscope (HIM), as the name implies, is primarily an imaging tool. This chapter serves as an introduction to imaging with the HIM and explores the various ways this is implemented by first describing the numerous imaging signals and contrast mechanisms available and then giving an overview of some practical HIM imaging techniques. Several examples from the literature are used to illustrate the important imaging modes including high resolution secondary electron imaging, backscattered ion imaging, ionoluminescence imaging and imaging with transmitted or reflected ions. Key concepts such as ion channeling, charge neutralization and utilizing the large depth of field are introduced, setting the scene for the subsequent chapters in this section that focus on particular aspects of HIM imaging.
6.1 Introduction The primary use of the helium ion microscope is as a high resolution surface imaging tool with similar applications to the more established scanning electron microscope (SEM). Over recent years, the application space for the HIM has expanded into the territory of focused ion beam (FIB) tools and some remarkable nanoengineering capabilities have been demonstrated. These will be covered later in the book (see Part IV). The focus for this chapter will be on the imaging capabilities of the HIM. In the HIM, helium atoms are ionized in the high electric field concentrated around three tungsten atoms (the ‘trimer’) at the tip of source. This creates three ‘beamlets’, one of which is directed, by tilting the gun housing, down the column, through an aperture and various beam steering and focusing optics to be brought to focus on the surface of a sample in the main chamber. The atomic size and high brightness of the helium ion source together with the large momentum (and so smaller de Broglie wavelength) of helium ions compared to electrons, means that the resultant probe size on the sample is smaller than what can be achieved with a focused electron beam in a scanning electron microscope. This, coupled with the S.A. Boden (✉) Electronics and Computer Science, University of Southampton, Highfield, Southampton, UK e-mail: [email protected] © Springer International Publishing Switzerland 2016 G. Hlawacek and A. Gölzhäuser (eds.), Helium Ion Microscopy, NanoScience and Technology, DOI 10.1007/978-3-319-41990-9_6
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reduced lateral and back-scattering of the beam as it enters a sample leads to a small interaction volume and hence high resolution images. Furthermore, the beam divergence angle is 5–10 times smaller than in an SEM, leading to a larger depth of field and so the ability to capture in sharp focus highly structured three dimensional (3D) surfaces. Charge neutralization can be achieved through the use of an integrated electron flood gun so that the benefits of high resolution and large depth of field can be realized in images of insulating samples. This chapter begins with a survey of the different signals generated b
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