Proceedings of the International Conference on Atom-Probe Tomography and Microscopy (APT&M 2018)

Abstract

On the 40 anniversary of the achievement of surface-lattice atomic resolution by FIM (October 11, 1955), the author wrote about his witnessing of the event in Erwin W. Mueller’s laboratory. The present paper describes the author’s further thinking and perspective of the event and its consequences on the occasion of the 50 anniversary of the introduction of the Atom Probe FIM. That event, many years ago, was hugely significant in view of the later developments of related techniques that led to today’s Atom Probe Tomography and Microscopy meeting. Twenty-two years have elapsed since the “Recollections of Erwin Mueller’s Laboratory: (1951-1956)” (1) was published describing the excitement at the happening of lattice atomic resolution by FIM. The author clarified some details of that paper in a subsequent publication (2), and now has revisited the original paper, mostly with the eyes of a reader, so to speak, rather than the eyes of the author. 1: Comments on the Original Text At the time of the magnificent event discussed in (1), when FIM provided the first atomically resolved map of the specimen surface atoms, no other microscopy existed that could do that or even “image” single atoms. Some 28 years would pass before another microscopy, STM, claimed to image a surface atomic array (3). In 1970, Albert Crewe developed a High-Resolution Scanning Electron Microscope with which he could “see” single atoms of uranium and thorium on a carbon film (4). However, no imaging of an atomic lattice was demonstrated. A human analogy is: I can see stars in the night sky. Does that mean that I have telescopic eyesight? No, for example among other things I cannot distinguish between a single star and a binary pair. It may seem mundane, but the terms “imaging atoms” and “seeing atoms” are anthropomorphic exaggerations. No microscope, arguably other than the reflection optical microscope actually images or sees objects in human terms. Either directly or indirectly, they produce images interpreted as positions of atoms relative to other atoms. Perhaps “mapping”, Field Ion Atomic Mapping would be a more accurate, though a more mundane description. In the original Recollections paper (1) the author described the mood in the laboratory immediately after E.W. Mueller’s first viewing the FIM image of a fully resolved surface atom array, as one of “unprecedented awe and joy”. We all understood that no person had ever established the reality of atomicity of solid matter. Atomicity had been indirectly surmised for centuries, ever since the thinking of Democritus and the x-ray work of Sir Lawrence Bragg. Now the magnificent was unexpectedly upon us! Unbounded joy prevailed at the supposed certainty that our professor would soon win a Nobel Prize. FIM actually directly projects a map of the ionization probability of the specimen surface, which looks like an array of atoms. STM and HRTEM, produce so-called atomically Proceedings of Atom Probe Tomography & Microscopy (APT&M) 2018, Washington, DC, June 10–15, 2018 NIST SP 2100-03 15 Allan J. Melmed Recollections Revisited: the Magnificent and the Mundane Distinguished Papers resolved images only with the aid of computers or other kinds of “image processing”, such as hand-drawn embellishments (3). Raw STM data, for example, is a series of individual lines, which are the recorded response of the measurement of voltage needed to maintain constant tunneling-electron current (3, 5). And HRTEM actually images columns of atoms, not just surface atoms. A section in the original recollections paper (1) discussed “The paramount importance of field evaporation”. In perspective now, field evaporation was indeed extremely important, not only for FIM per se but also was crucial for the later development of atom probes. In contrast to thermal evaporation, field evaporation can be implemented at all temperatures, and is local to the part of the specimen that is subjected to a high electric field. This capability is unique to FIM and Atom Probes. The other student present with the author (1) just outside the room where Kanwar Bahadur showed Erwin W. Mueller the first image of a surface array of atoms was Russell D. Young. In retrospect, Russell Young should be recognized much more than he has been. He did the research on field electron energy distribution that later led to the use of field-emission point sources for various forms of atomic-resolution microscopy and he developed the first instrument similar to the later STM (5). This was recognized by the Nobel Prize committee, and in now-mundane fact, Russell D. Young was cited in the background information for the official 1986 Nobel Prize awarded (6) to Binnig, Rohrer and Ruska, for his closely related work (1970) developing the instrument he called the Topografiner (5)! The author was a colleague of Russell Young at NBS (NIST) during the time the Topografiner was developed. All of us in the surface science lunch group were in awe when we learned of the new instrument – it was truly magnificent! We were amazed that a field-electron tip was being used in a completely new way. Then, when the 1986 Nobel Prizes were announced, we were astonished that Russell Young was not included in the Nobel Prize recipients! Later it was leaked that Young was not included because he did not envision his Topografiner achieving atomic resolution. Clearly this was suspect, so suspect. After all, Ernst Ruska did not envision his electron microscope achieving atomic resolution! 2: Comments on the original micrographs. The room temperature helium and neon FIM micrographs (figs. 4 and 5) in the original Recollections paper (1) were presented to clearly establish the fact that Mueller’s 1951 microscope inherently had the magnificent capability of surface atomic lattice resolution. Importantly, it should be noted that these microscope images and micrographs were recorded with the aid of image intensification, not available in 1951. They were obtained using very sharp specimen tips, due to the fact that room temperature resolution degrades with increasing tip radius, more noticeably than at lower temperatures. These images, as all FIM images (7), are projections from a curved surface (the specimen) onto a flat screen or detector. In order to understand them, one should first recognize that an FIM image is a projection from a small curved surface, a point specimen. Then recognize that a single FIM image shows an atomic arrangement representing the atoms located in an outermost spherical envelope of surface atoms and that this envelope is very thin, only about 0.3 nm thick. Due to the shape of FIM specimen (7), the projection arises from about a 60 degree solid angle. To help visualize Proceedings of Atom Probe Tomography & Microscopy (APT&M) 2018, Washington, DC, June 10–15, 2018 NIST SP 2100-03 Distinguished Papers Allan J. Melmed Recollections Revisited: the Magnificent and the Mundane 16 this, Erwin Mueller had a cork ball model of an FIM specimen apex with phosphor paint used to indicate the surface “atoms” that would be seen (in a darkened room) as a simulated FIM image. It is also important to realize that the dark regions may or may not be populated by atoms (most of them are, for crystalline specimens). By very carefully controlled field evaporation (usually using pulsed voltage), dark regions can be explored for atomic content (7). 3: Some Overall Perspectives Revisiting the Recollections paper, an interesting perspective appears. It is ironic that a major feature of FIM is the fact that imaging happens without a lens, that is there is direct projection from a sharp point. It is ironic that this feature also was responsible for the very limited appeal of FIM to researchers around the world! It became clear that FIM studies would always involve the need for the investigator to make a sharp point for every investigation. In FIM, the specimen and the illumination source are inexorably combined. Once the specimen is made, the die is cast, so to speak. Another perspective is the exquisite simplicity of the original (and many later) FIM instruments compared to various other microscopes. The original FIM was a glass vessel, with a conductive coated on the inside, with the illumination source (positive ions) the specimen itself, and a phosphor screen (or later an image intensifier) to display the image. The author has conducted and published research in FEM, FIM and APFIM through the years and interacted with the scanning probe community, manufacturing sharp points of many materials (8) for almost 25 years. This experience provides a broad perch from which to look back at the history of it all, leading to Atom Probe Tomography (APT). The author had an especially deep ponder reflecting on the question of evolution vs creation in the historical path from FEM to APT (a question often asked in another context!) and was profoundly affected by Isaac Newton’s famous statement. Newton stated that “If I have seen further than others, it is by standing on the shoulders of giants”, at first slightly modified by the author to “If I have seen further than others, it is by standing on the shoulders of others”. Upon further contemplation, the author modified it to “If I have seen further that others, it is by thinking about the work of others”. The historic path from FEM to APT, was a series of significant developments with magnificent milestones along the way, some very creative and some evolutionary. In the 1930’s several researchers were studying the field electron emission from tungsten tips, related to vacuum tubes. R.P, Johnson and W. Shockley (9) were studying field electron emission from thin wires, projecting the emission pattern onto a cylindrical phosphor screen. Immediately after this, Erwin Mueller published his first FEM paper (10), a significant, if incremental advance in microscopy, with a sharp point instead of a thin wire as the electron emission source. Definitely evolution! Then t