- Introductory comments
- Historical Background
- Development of instrumentation
A large number of microbeam analytical techniques have been developed for the analysis of solid materials. These techniques use sharply focused incident beams of particles or energy to determine chemical or isotopic composition on the microscopic scale (beam diameters ranging from about 100 pm to 50 µm). The table below lists the more familiar techniques.
Most techniques are non-destructive, but several result in sample damage, usually in the form of pitting where material has been removed for analysis by mass spectrometry. For example, laser-ablation ICP-MS produces sample pitting, as does SIMS where low-energy ions are used to ablate the sample. Nondestructive techniques are more numerous. Many rely on characteristic x-rays produced from the sample to determine chemical composition. As indicated in the table, there are several ways to produce x-rays from the sample, including bombardment by incident x-rays, electrons, and protons.
This course will focus on effects that are produced by electron bombardment (in red below). Using these effects to examine geological materials one may (among other things) allow one to:
- Identify minerals; this is especially useful for opaque and micrometer-sized grains;
- Determine phase compositions, which are required for gothermometry and geobarometry calculations;
- Document chemical zoning within minerals for petrologic, growth, and diffusion studies; and
- Locate rare phases, such as zircon, monazite, and badellyite, which often have distinctive chemical compositions.
Before proceeding, it will be useful to define the units we will be using in the following discussions. All units (with one prominent exception) will be in the International System of Units (abbreviated SI from the French language name Le Système international d’unités). The SI system was developed in 1960 from the meter-kilogram-second (mks) system and uses a series of prefixes that are attached to seven base units that are nominally dimensionally independent:
- meter (m), length
- kilogram, (kg), mass
- second (s), time
- ampere (A), electrical current
- kelvin (K), temperature
- mole (mol), amount of substance
- candela (C), luminous intensity
Other units are derived from these seven base units; for example, a pascal (Pa) is 1 N/m2, a Newton (N) is 1 kg m/s2. Prefixes are attached to base units to denote multiples (or fractions):
- giga (G), 109
- mega (M), 106
- kilo (k), 103
- centi (c), 10-2
- milli (m), 10-3
- micro (µ), 10-6
- pico (p), 10-9
- nano (n), 10-12
One non-SI unit commonly encountered when using x-rays is the Ångstrom (Å), which is 1 x 10-10 m. X-ray wavelengths are typically 1 to 100 Å (0.1 to 10 nm) and visible light has wavelengths from 400 nm (4000 Å) for violet light to 750 nm (7500 Å) for red light. Useful spacial units of measurement include: micrometers (µm), which are sometimes called “microns,” which are appropriate for the sizes of mineral grains, interaction volumes, and beam diameters; nanometers (nm), which are used in nanotechnology; and picometers (pm), which are appropriate for atomic and ionic radii.
The English physicist Sir William Crookes (1832-1919) created a vacuum tube around.1875, which he used to study the conductivity of gases. The glass tube contained negative and positive electrodes across which high-voltage electrical currents were applied. Although he did not know it, these “cathode ray” tubes also produced x-rays. Crookes unsuccessfully sought the cause of repeated fogging of photographic plates that he had stored nearby.
William Crookes and the “Crookes Tube. In a Crookes tube, a negatively biased electrode, called the cathode, emits cathode rays (electrons) which accelerate toward the anode. Many cathode rays miss the anode and instead strike the glass end of tube, causing it to fluoresce.
Crookes found that cathode rays (electrons) travel in straight lines. He also discovered that certain minerals would glow if placed in the tube. This is the phenomenon of cathodoluminescence (production of visible light due to electron bombardment), which we will consider in detail later. Different minerals yielded different colors: calcite (red), apatite (yellow), willemite (bright green), scheelite (bright blue), dolomite (brown), and magnesite (violet).
Mineral Tube. When activated the minerals sample (calcite in this case) glows as it is struck by electrons. Images source: http://www.crtsite.com/page7.html.
Later variations on the mineral tube used colored phosphors to produce dramatic displays. In the eponymous Crookes flower tube, copper flowers were covered with different phosphors with rotating vanes made of mica plates situated above them. When the tube is activated, the cathode rays turn the vanes, resulting in a moving shadow on the flowers.
Flower Tube. When activated the flowers painted with phosphors glowed in different colors. Images source:http://www.crtsite.com/page7-3.html.
In 1858, Julius Plücker (1801-1868) and Johann Wilhelm Hittorf (1824-1914) discovered that cathode ray beams could be deflected by a magnetic field. They used a modified Crookes tube, with an insert consisting of an aluminum sheet covered with phosphor to allow observation of the electron paths.
Deflection Tube. A magnet brought near the tube causes the cathode rays (electrons) to deflect from a straight path. Images source: http://www.crtsite.com/page7.html.
In 1869, Hittorf demonstrated that (in the absence of a magnetic field) cathode rays go in a straight line and are blocked by metal. He used a movable Maltese Cross arrangement as shown below. When the cross was down, the glass face of the tube emitted a green glow, which faded over time. When the cross was up, intercepting the cathode rays, its shadow was visible on the end of the tube.
Maltese-Cross Tube. When activated the metal cross intercepts the cathode rays casting a shadow on the tube’s end. Images source (modified): http://www.crtsite.com/page7.html.
The German scientist Philipp Lenard (1832-1947) added a thin aluminum window to the basic Crookes tube. The window’s foil was thick enough to maintain the vacuum inside the tube, yet thin enough to allow the cathode rays to pass out of it. Lenard concluded from subsequent experiments that cathode rays propagated through air for distances of less than about 100 cm, but that in a vacuum they traveled for several meters without being weakened. He published these and other early findings were in 1894. During one of his experiments, the fluorescence from a dissipated cathode ray caused a paper that had been soaking a barium platinocyanide solution to glow. Lenard had unknowingly discovered the first evidence for x-rays, but failed to investigate the strange phenomenon further. Lenard later claimed that he, rather than Roentgen, should be honored as the discoverer of x-rays. Lenard made significant contributions to the field of physics; however, he was a fervent Nazi and condemned Albert Einstein and other individuals with Jewish backgrounds.
Schematic diagram of the Lenard Window. Image source (p. 109): http://nobelprize.org/nobel_prizes/physics/laureates/1905/lenard-lecture.pdf.
Finally, J. J. Thompson confirmed that cathode “rays” were electrons in 1897, also documenting that they travel slower than light and transport negative electricity.
A few months after Lenard published his results, on 8 November 1895, Wilhelm Conrad Röntgen (re)discovered x-rays at the University of Würzburg in Germany. Röntgen was using ~40 keV electrons to bombard inert gas in tubes, when he noticed that a screen across the room that was coated with barium platinocyanide (BaPbCN) began to glow. He placed a deck of playing cards and a two inch book between the tube and the screen and discovered the rays from the tube penetrated these materials. He called them “x-rays” after the algebraic symbol of the unknown, x. While holding a lead pipe up to the rays, he noticed that the bones of his fingers were shadowed on the screen. On December 22nd, Roentgen decided to show his wife what had been occupying his time and he took an x-ray image of her hand.
Röntgen flanked by x-ray photographs of this wife Bertha’s hand (left) and von Kölliker’s hand (right). Sources: http://www.deutsches-museum.de/sammlungen/ausgewaehlte-objekte/meisterwerke-ii/roentgen/ (left and center), http://www.vmas.kitasato-u.ac.jp/radiology/CALS/von-Kolliker.htm (right)
Röntgen also experimentally demonstrated these features of x-rays:
- produced from the fluorescence part of the wall of the discharge tube
- traveled in straight lines
- were not deflected by magnetic fields
- were absorbed more by denser metals
- were scattered when passing through a body
- caused fluorescence on many materials
- and could ionize gases.
He submitted the first paper describing his work, Uber eine neue Art von Strahlen, to the Würzburg Physical Medical Society on 28 December 1895. He gave a public lecture on 23 January 1896, after which he made a photograph of the hand of Alfred von Kölliker, a famous anatomist. Kölliker proposed that the newly discovered rays be called Röntgen’s Rays and x-rays are still called Röntgenstrahlen in Europe. Röntgen received the first Nobel Prize in Physics in 1901 for his discovery. He donated the prize money (then about $40,000) to the University of Würzburg.
In addition to Lenard, there were others who unknowingly had observed the effects of x-rays before Röntgen. In the United States, A. W. Goodspeed (1860-1943) and William Jennings (1860-1945) made an accidental photograph of coins stacked with photographic plates using x-rays from a Crookes tube on 22 February 1890 in Philadelphia. However, unlike Lenard, neither claimed priority for discovery of x-rays, noting that they had ignored the plates until Röntgen’s announcement caused them to review the photographs.
Duplicate of Goodspeed and Jenning’s 1890 photograph. Source: http://www.learningradiology.com/museum/discoveryhome.html.
X-rays rapidly became the latest rage: the public was fascinated with this unknown phenomenon. People were especially amazed at seeing through human flesh; however, they were commonly ill at very ease with the observation because bones were associated with death. Indeed, people occasionally fainted when first they saw an x-ray image! Thomas Edison capitalized on the American public’s intoxication with the x-rays and announced in March 1896 that he would be the first to photograph the living human brain; however, he never accomplished this feat. American physicians quickly recognized the value of x-rays for examining bone fractures and locating foreign objects in the body. The first diagnostic x-ray was taken at Dartmouth Hitchcock Hospital in 1896; Gilman Frost and his Dartmouth physicist brother, Edwin, used x-ray imaging to help set a boy’s broken arm.
Humorous Postcard and Poster. The card, titled “Beach idyll á al Röntgen,” is dated 20 August 1900. A poster for a British Exhibition reflects the sentiment of the times. Sources: (left) http://www.gamlavykort.nu/artiklar/rontgen1900.htm, (right, p. 18) http://www.slac.stanford.edu/pubs/beamline/25/2/25-2-assmus.pdf.
Another major figure in the investigation of the properties of x-rays was Charles Glover Barkla (1877-1944). He demonstrated in 1905 that x-rays could be polarized, thus having properties identical to visible light. He thus eliminated the possibility that x-rays might be a type of longitudinal wave (like sound) rather than a transverse wave.
In 1909, Barkla discovered what are now termed characteristic x-rays. He found that under certain conditions, bombarding an anode with electrons yielded emergent x-rays with one strong homogeneous component and a constant absorption coefficient. Barkla also showed that the absorption coefficient decreased with increasing atomic weight of the anode material, i.e., x-rays from materials with higher atomic weights were more penetrative. Since no one could yet measure the wavelengths (or frequencies) of the x-rays, Barkla measured the absorption of this radiation by directing it through a 0.01 cm-thick layer of aluminum and measuring how much of the beam was absorbed. Barkla later discovered that his homogeneous x-rays were, in fact, heterogeneous. He wrote: “…. the radiations from Sn, Sb, I … emit … radiation of variable penetrating power.”
Plots of his results reveal two monotonic curves, one for the lighter elements (from Cr to I) and one for the heavier ones (Sn to Bi), with some elements (e.g., Sn and Sb) showing two absorption values. At first he labeled the two curves with by letters B and A, but in 1911 changed his notation to letters in the middle of the alphabet, remarking that “[t]he letters K and L are, however, preferable as it is highly probable that series of radiations both more absorbable and more penetrating exist.”
George Glover Barkla and his Plot of “K” and “L” Series X-rays. Image sources: (left) http://www.chemteam.info/AtomicStructure/Barkla-Graph-1909.GIF, (right)http://www.nndb.com/people/549/000099252/charles-glover-barkla-1-sized.jpg.
In 1912-13, R. T. Beatty demonstrated that electron bombardment of an anode produced two types of x-rays: the characteristic x-rays described by Barkla and a complete continuous spectrum of energies (continuum radiation), which is also called bremsstrahlung. He noted that production of characteristic X-rays required incident electrons with a specific minimum energy: below this energy only continuum x-rays were produced. Beatty also discovered that the depth of x-rays production by electron bombardment from within the anode was very small (<10 mm).
Max von Laue (1879-1960), a junior colleague of Röntgen, considered passage of waves of light through crystalline arrangement of particles and realized that short wavelength electromagnetic rays, such as X-rays, should cause diffraction or interference phenomena in crystals. In 1912, he and his lab assistant, Walter Friedrich and doctoral student, E. Paul Knipping, confirmed diffraction of x-rays by a systematic crystal. Von Laue worked out a mathematical formulation for this behavior and published this discovery.
Laue Diffraction Method. Image source:http://www.matter.org.uk/diffraction/x-ray/laue_method.htm
Von Laue took diffraction as positive proof that X-rays are electromagnetic radiation (waves), not particles. However, he was forced to argue that the x-rays hitting the crystals had to contain only certain wavelengths to account for missing diffracted beams. He received the Nobel Prize in 1914 for his work. X-ray diffraction is a workhorse method for determining crystal structure. Indeed, x-ray work by Rosalind Franklin (1920-1958) was key in determining the structure of DNA.
Max von Laue and the First Diffraction Pattern. Image sources: (left) W. Friedrich, P. Knipping, & M. von Laue, Interferenzen-Erscheinungen bei Röntgenstrablen, Sitzungsberichte der mathematisch-physikalischen Klasse der K. B. Akademic der Wissenschaften zu München (1912), Heft II, fig 1., (right) http://nobelprize.org/nobel_prizes/physics/laureates/1914/laue.gif
In 1896, after William Henry Bragg learned of W. K. Röntgen’s discovery of x-rays, he set about producing the new radiation. On 13 June 1896, he photographed his son’s broken elbow using primitive equipment. William Bragg was proponent of the idea that x-rays were particles, not waves. He supported his view with the evidence that x-rays could ionize gases and engaged in controversy with Barkla, who advocated the wave theory. In retrospect, both were right, and the source of their differences arose largely from the fact that Bragg was studying high energy (hard) x-rays, whereas Barkla was focused on low-energy (soft) x-rays. William Bragg developed an x-ray spectrometer that allowed many crystals to be analyzed by precisely measuring diffraction angles, but perhaps hisgreatest accomplishment was developing an x-ray detector based on ionization of gas. Detectors of this type are still used to day in many applications.
Bragg Spectrometer. Labels: L, lead box; A, B, D, slits; C, crystal; I, ionization chamber; V’, vernier of ionization chamber; K, earthing key; E, electroscope; M, microscope. Image sources: (left, modified)http://www.leeds.ac.uk/library/spcoll/bragg-notebook/images/fig2a_large.jpg, (right) http://nobelprize.org/nobel_prizes/physics/laureates/1915/wh-bragg.jpg
In August 1912, W. H. Bragg’s son, William Lawrence Bragg (1890-1971), who had just graduated from Cambridge, realized that the pattern of spots in the Laue diffractogram could be explained by reflection of waves from crystal planes. Lawrence Bragg used his father’s spectrometer to observe the diffraction of what would now be called Pt-Lα X-rays by a NaCl crystal. He published his results in 1913, The diffraction of short electromagnetic waves by a crystal, in the Proceedings of the Cambridge Philosophical Society. In it, Lawrence concluded that salt consisted of a three dimensional lattice of Na+ and Cl– ions. For some time after, chemists still refused believe that NaCl contains no NaCl molecules (just alternating array of Na+ and Cl– ions)! The collaboration between father and son led many to believe that William had initiated the research, a fact that upset Lawrence and haunted him throughout his life. The Braggs shared the 1915 Nobel Prize in Physics.
Bragg’s Law and the Structure of NaCl. Image source (right): http://nobelprize.org/nobel_prizes/physics/laureates/1915/wl-bragg.jpg
While studying scattering of X rays by crystals in 1913, the Braggs noticed a different distinctive pattern of peaks appeared when using different anodes to produce X-rays. The peaks formed due to constructive interference of the characteristic X-rays, and different peaks resulted when different incident x-rays were used. They formulated Bragg’s Law to explain the relationship between wavelength, crystal structure, and incident angle. Bragg’s Law is routinely applied in wavelength-dispersive spectrometry (WDS) wherein characteristic x-rays are detected at specific angles due to constructive interference using crystals with known structures.
Early work with diffracting crystals (such as that by Henry Moseley described below) used a simple flat crystals, but it was later shown that curved crystals would provide higher peak intensities by better “focusing” the diffracted X-rays. In 1931, H. H. Johann showed that simply bending a crystal could yield a better focus, However, bending resulted in some peak broadening (due to the production of crystal defects during bending) and asymmetry of the focus point. In 1933, T. Johannson proposed grinding the surface of a bent crystal to achieve a perfect focusing; however, the machining difficulties of such a procedure are tremendous and most modern x-ray work is done using Johann crystals. Thin systematic crystals are bend by affixing them to a metal surface machined to the desired curvature.
The apparent conflict between those who considered x-rays particles and those who considered them waves reflects the manner in which they were studied. All electromagnetic energy and particles, depending upon the method of examination, show either wave-like behavior (diffraction) or particle-like behavior (discrete energies, ionization, scattering, photoelectric effect). In 1924, Louis-Victor de Broglie (1892-1987) formulated the de Broglie hypothesis, arguing that matter also has a wave-like nature. He related wavelength, λ, and momentum, p:
where λ = the particle’s wavelength, h = Planck’s constant, p = the particle’s momentum, m = the particle’s rest mass, v = the particle’s velocity, and c = the speed of light in a vacuum. For example, consider the wavelength associated with a baseball (0.15 kg), thrown at 90 mph (40.2 m/s). At subrelativistic speeds:
This wavelength is considerably smaller than the diameter of a proton (about 10-15 m) and approaches the Planck length (1.61 x 10-35 m). The wave-like properties of this baseball are too small to be observable. In contrast, electrons with much smaller mass and much higher velocities, display significant wavelengths. The wavelength of 10 keV electrons is 12.3 x 10-12 m, permitting diffraction to be observed.
Electron Interference. High energy electrons are sent through the slits one at a time, but still build up an interference pattern like that expected of waves (time elapses a → b → c → d). Image source: (right)http://www.hqrd.hitachi.co.jp/em/emgif/fig2.gif
Wave behavior has been documented for larger objects; the diffraction of C60 fullerenes was reported by researchers from the University of Vienna in 1999. Fullerenes have an atomic mass of about 720 amu; their de Broglie wavelength is 2.5 x 10-12 m. The diameter of the molecule is about 400 times larger. In 2003, the same group demonstrated the wave nature of C60F48, a fluorinated buckyball (1600 amu). Larger/heavier molecules have also shown interference, but there may be an upper limit at the Planck mass (21.7651 micrograms).
Image source: http://abyss.uoregon.edu/~js/images/wave_particle.gif.
In late 1913, Henry Gwyn Jefferys Moseley constructed a x-ray spectrometer using a potassium ferrocyanide, K4Fe(CN)6·3H2O, diffracting crystal. Moseley had been working on radioactivity since 1910, but decided to study x-ray diffraction since it was the hottest new field in physics. With his new wavelength-dispersive spectrometer, he measured the wavelengths of characteristic K-series x-rays produced from different anode materials (Ca to Zn). His results documented that the ordering of the wavelengths of x-ray emissions for the elements coincided with ordering by atomic number. This relationship is called Moseley’s Law and will be discussed in the section on spectrometry.
Henry Moseley and X-ray Spectra. Moseley in the lab and aligned photographs of X-ray spectra from different anodes. Note impurity lines in the Co and Ni spectra and the unidentified Zn line in the spectrum of brass. Images sources: (left) http://photos.aip.org/history/Thumbnails/moseley_henry_d9.jpg, (right) http://www.geology.wisc.edu/~johnf/g777/Moseley.jpg
This was a very significant discovery! Prior to Moseley’s work, atomic numbers were considered semi-arbitrary, based on atomic masses but reordered when necessary to put an element in an appropriate place in the periodic table. For example, cobalt and nickel had been assigned atomic numbers of 28 and 27, respectively, based on their chemical properties, since they have nearly identical atomic mass (58.933 vs. 58.693 amu). Moseley’s experiments showed that the order of these elements should be reversed. Similarly, Moseley’s work revealed that there were gaps in the atomic number sequence at numbers 43, 61, 72, and 75; he suggested that these would be filled by undiscovered elements. After this groundbreaking work, which was done in Manchester, Moseley moved to Oxford in 1914 and continued his experiments by studying the L-spectra of heavier elements. These too showed the wavelength-atomic number correlation.
Moseley’s Results. The linear relationship between atomic number and the square root of frequency is Moseley’s Law. Notice in this diagram that the lighter elements have two distinctive characteristic x-ray K-lines plotted and the heavier have up to three L-lines. Image source: http://www.physics.ox.ac.uk/history.asp?page=Exhibit12
However, with the onset of war, Moseley enlisted in the Royal Engineers and was killed on the Gallipoli Peninsula (western Turkey), shot through the head while telephoning an order on 10 August 1915. He was just 27. Belatedly, recognizing this great loss to science, the British army changed its policy in World War II, no longer allowing scientists to enlist for combat.
In 1923, Georg Charles von Hevesy proposed using x-rays (rather than electrons) to excite characteristic x-rays from a sample and use diffraction to identify its constituent elements. This technique is called x-ray fluorescence (XRF). Later he introduced a method of analysis based on neutron bombardment (neutron activation); this method yields better detection limits than x-ray analysis with fluorescent x-rays. In 1923, he and Dirk Coster discovered the element hafnium in x-ray spectrum emitted from zircon. He received the 1943 Nobel Prize in Chemistry not for this discovery, but for his work on the use of isotopes as tracers in studying chemical processes. Hevesy was the first to apply the radioactive tracer technique to biology, and he later used it in medical research.
Von Hevesy and X-ray Fluorescent Spectra. Typical wavelength dispersive XRF spectrum; note the systematic increase in atomic number toward the left. Image sources: (left)http://www.radiochemistry.org/nuclearmedicine/pioneers/images/hevesy.jpg, (right) http://en.wikipedia.org/wiki/File:XRFScan.jpg.
Hans Busch showed theoretically in a 1927 paper that a coaxial magnetic field produced by an electric coil should focus a beam of electrons. He also predicted that the focal length of such a magnetic electron lens could be changed continuously by varying the coil current. This theory was confirmed in 1929 by Ernst Ruska (1906-1988) at the High Voltage Institute, Berlin, under the direction of Max Knoll (1897-1969). In 1931, Ruska constructed a magnetic lens of the type that has been used in all magnetic high-resolution electron microscopes since then. Further work, conducted with Knoll, led to the construction in 1933 of a transmitted-electron microscope (TEM) that gave much better definition than a light microscope. Ruska’s 1934 Ph.D. thesis investigated the properties of electron lenses with short focal lengths.
Ruska and Knoll’s Electron Microscope. The first electron microscope was a transmitted electron instrument shown here in a wonderful “mad-scientist” photograph (Knoll is at left). Image source:http://www.microscopy.ethz.ch/history.htm
The major limitation of their electron microscope was that the electrons could not pass through thick specimens. Thus it was impossible to utilize the instrument to its full capacity until methods of producing thin specimens were developed. The diamond knife and ultra-microtome were invented in 1951. Ruska was awarded half of 1986 Nobel Prize for Physics; the other half divided between Heinrich Rohrer and Gerd Binnig for their invention of the scanning tunneling microscope (STM).
The earliest known paper presenting the concept of a scanning electron microscope was by Knoll (1935). Subsequently, in 1938, Manfred von Ardenne (1907-1997) constructed a scanning transmission electron microscope (STEM) by adding scan coils to a transmission electron microscope. These used magnetism to systematically moved a beam of focused electrons over a small area. The first “micrograph” ever produced was of a ZnO crystal imaged at an operating voltage of 23 kV at a magnification of 8000 times. The spatial resolution was between 50 and 100 nm. The micrograph contained 400 x 400 scan lines and took 20 min to record, because the film had to be mechanically scanned in synchronization with the beam. The instrument also had a viewing CRT, but it was not used to record the image. Ardenne’s Berlin laboratory was bombed in 1944 and he never returned to SEM development.
Von Ardenne and the Scanning TEM. The scan coils (blue star) are located between the electrostatic lenses (red stars). Image sources: (left, modified) http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/images/fig4.jpg; (right) http://media-2.web.britannica.com/eb-media/80/5280-004-8A73DA34.jpg.
The first, true scanning electron microscope (SEM) was developed and described in 1942 by Russian immigrant Vladimir Kosmo Zworykin (1889-1982), James Hillier and R. L. Snyder, working at the Radio Corporation of America (RCA) Laboratories in the United States. Zworykin also invented a cathode-ray tube he called the “kinescope” in 1929, which was the precursor to modern television picture tubes; he is thus sometimes called the “father of television.”
The instrument consisted of an inverted column (electron gun at the bottom), three electrostatic lenses with electromagnetic scan coils placed between the second and third lenses. A photomultiplier tube detected scintillations on a phosphor screen caused by incident secondary electrons. This detector was an early version of the combination of phosphor and photomultiplier that Everhart and Thornley developed nearly twenty years later. The electron gun was located at the bottom so that the specimen chamber was at a comfortable height for the operator, but this arrangement had the disadvantage that the specimen could fall down into the electron column! The instrument achieved a resolution of about 50 nm, but this figure was considered unexciting compared to results achieved by the rapidly developing STEM, and further development lapsed.
Vladimir Zworkyin and a Schematic of the first SEM. Note that the column is inverted with the electron gun located at the bottom. Image sources: (left) http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/images/fig7.jpg, (right) http://media-2.web.britannica.com/eb-media/80/5280-004-8A73DA34.jpg
The early history of the SEM is very well described at the Cambridge University, Department of Engineering SEM site. The following summary is based on the information there. In the late 1940s, Charles Oatley at Cambridge decided that another look at the SEM might be worthwhile. He decided that “Zworykin … had shown that the scanning principle was basically sound” and believed that improvements in electronics that had resulted from work during the war would allow better results. Specifically, Oatley thought that the RCA detector had a too low efficiency and thus that the images were noisy in spite of the long recording time. Oatley selected Dennis McMullan to build an SEM as his Ph.D. project! McMullan first completed a 40 keV electrostatically focused TEM, which had been begun by another PhD student, which he then converting into an STEM. Finally, he converted the STEM into an SEM by adding scan coils, an electron multiplier detector and a long persistence cathode-ray tube.
Schematic Diagram and Photograph of McMullan’s SEM1. Note that the electron gun is at the top of the column. Image sources: (left): http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/mcm.htm. (right)http://www2.eng.cam.ac.uk/~bcb/cwo2.htm.
Many improvements were incorporated in subsequent SEM2, SEM3, and SEM4 models. In 1952, K. C. A. Smith developed a way to efficiently detect low-energy secondary electrons (previously images were made using high-energy electrons), allowing vastly improved surface imaging. In 1955, T. E. Everhart devised a new electron detector; the “Everhart-Thornley” detector, which is still used today. Magnetic lenses replaced electrostatic lenses on the SEM4 in 1961. Eventually, Oatley persuaded Associated Electrical Industries (AEI), a company that manufactured both transmission electron instruments and electron probe microanalyzers, to take an interest in the SEM. The first four production models made by Cambridge Instruments, sold under the trade name “Stereoscan”, were delivered in 1965.
The first electron microprobe was built in 1944 by James Hillier (1915-2007) and R. F. Baker at the Radio Corporation of America (RCA) Laboratories at Princeton, NJ, built an electron microprobe by combining an electron microscope and an energy-loss spectrometer. Electron energy-loss spectrometry is very good for light element analysis and they were able to obtain spectra of C-Kα, N-Kα and O-Kα radiation from a collodion film. In 1947, Hillier patented the idea of using an electron beam to produce analytical X-rays, but never constructed a working model. His proposed design used Bragg diffraction from a flat crystal to select specific X-ray wavelengths and a photographic plate as the x-ray detector. Unfortunately, RCA had no interest in further developing a microprobe, and the concept lay fallow.
Hillier’s Electron Microprobe. a) Schematic of microprobe constructed. b) Patented design of the unconstructed microprobe. Note that the electron gun is at the bottom of the column in both designs. Image sources: (left) http://www.geology.wisc.edu/~johnf/g777/Hillier-EELS.jpg, (center) http://www.geology.wisc.edu/~johnf/g777/Hillier_xtal.jpg, (right) http://www.cedmagic.com/mem/whos-who/hillier-james.html
It wasn’t until 1948-1950 that Raimond Castaing (1921-1999), supervised by André Guinier, built the first electron microprobe (“microsonde electronique”) at the University of Paris. He was apparently unaware of Hillier’s ideas and patents and worked independently. The resulting instrument produced an electron beam diameter of 1-3 mm with beam current of ~10 nA and used a Geiger counter as an x-ray detector. In 1950, Castaing added a fully-focused Johannson quartz crystal between sample and detector to permit wavelength discrimination and an optical microscope to view the point of beam incidence.
Castaing’s 1951 Ph.D. thesis laid the foundations of the theory and application of quantitative analysis by electron microprobe. He recognizing that x-ray intensities from materials of unknown composition measured relative to a pure element could be used as a first approximation for quantifying the chemical composition of the specimen. Castaing soon realized that x-ray generation in multi-element specimens was complex. For example, the interaction volume from which x-rays are produced varies with composition, as do the path lengths for the x-rays leaving the specimen. These complications are known as atomic number (Z) and absorption (A) effects. The x-ray emission from a sample is further compounded by a secondary fluorescence effect (F), due to generation of secondary x-rays by absorption of primary x-rays within the specimen. Castaing also discussed effects such as instrumental drift and x-ray background due to the continuum x-ray spectrum. Because of the enormous breadth of his contributions to the field, Castaing is considered the “father” of electron microprobe analysis.
Schematic diagram of Castaing’s microprobe and photograph of the MS85 instrument. Note that the electron gun is at the top of the column. Image sources: (left) <unknown>, (center) https://fr.m.wikipedia.org/wiki/Fichier:Microsonde_castaing_MS85.jpg, (right) <unknown>.
Cameca (France) produced the first commercial microprobe, the MS85, in 1956, based on Castaing’s design. In the late 1950s and 1960s, many researchers built “homebrew” microprobes to be used for semiconductor and other materials analysis. For example, D. Wittry at CIT, built a microprobe for his 1957 thesis. He and his advisor, P. Duwez, translated Castaing’s thesis from the French. Later in his distinguished career, Castaing was involved in the development of the ion microprobe, which uses a focused beam of ionized atomic nuclei to ablate material from a sample for analysis using a mass spectrometer.
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