Resolution of the field ion microscope

In a point projection microscope, the image magnification is given by 

The spot size at the screen due to the lateral motion in two opposite directions, vt, of the ions for a tip of spherical shape is therefore 

Let us now consider the increase in the spot size due to the effect of Heisenberg s uncertainty. When a particle is confined to pass through a small space of width Ay, at the tip, the uncertainty in the tangential component of the momentum of the particle is of the order of hi2 Ay, and the corresponding velocity component is h/2M Ayt. Thus the spread of the spot size at the screen by this uncertainty alone is 

We then minimize the spot size by letting d(Ays)/d(Ay,) = 0 this gives Ays = 2(t]htlM)m. Therefore the optimum resolution after replacing V() by xF0rt, is 

To estimate the broadening of spot size by the thermal velocity of the ions, let us assume that the image gas atoms immediately before ionization have a Maxwell-Boltzmann velocity distribution with an effective temperature T which is very close to the tip temperature Tt. If n(y) dy represents the number of ions arriving at the screen between y and y + dy, we have 

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Fig. 2.31 (a) Calculated resolution of the field ion microscope using He as the image gas. The resolution depends on the tip radius as well as the equilibrium temperature of the gas atoms just prior to field ionization. 

The field ion microscope is perhaps the simplest of all atomic resolution microscopes as far as mechanical and electrical designs are concerned. The atomic resolution microscopes, at the present time, include also different types of electron microscopes,1 the scanning tunneling microscope (STM)2 and the atomic force microscope (AFM)2 Before we discuss the general design features of the field ion microscope it is perhaps worthwhile to describe the first field ion microscope,3 and a very simple FIM4 which can be constructed in almost any laboratory. The first field ion microscope, shown in Fig. 3.1, is essentially a field emission microscope5 except that it is now equipped with a palladium tube with... 

The field ion microscope (FIM) has been used to monitor surface self-diflfiision in real time. In the FIM, a sharp, crystalline tip is placed in a large electric field in a chamber filled with Fie gas [14]. At the tip. Fie ions are fonned, and then accelerated away from the tip. The angular distribution of the Fie ions provides a picture of the atoms at the tip with atomic resolution. In these images, it has been possible to monitor the diflfiision of a single adatom on a surface in real time [15]. The limitations of FIM, however, include its applicability only to metals, and the fact that the surfaces are limited to those that exist on a sharp tip, i.e. difhision along a large... 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

E. W. Muller recently developed an ingenious modification of the field emission microscope, based on the field ionization of hydrogen (1,12). The device consists of an ordinary field emission tube containing hydrogen at a pressure of about 10 mm. When the tip is made the anode, adsorbed hydrogen in the form of ions can be pulled off at fields of the order of 2 X 10 v./cm. A pattern, corresponding to the electron emission, but with greatly increased resolution ( 3A.) results. 

Low energy electron diffraction is the dominant diffraction method for studying adsorption structures. It gives information in many ways complementary to that obtained from the field ion microscope (i). Brief comparison of LEED with field ion microscopy (FIM) is instructive because these two high resolution methods of finding surface atom positions differ greatly in their actual and potential applications for study of catalysis. 

IPM can be used simultaneously with RBS (Rutherford backscattering spectrometry), NRA (nuclear reaction analysis), PIXE (particle induced X-ray emission) or PIGE (particle induced gamma ray emission). More specialized examples include the field ion microscope (FIM), which gives better then atomic resolution in the study of high melting point materials. 

MiiUer, E.W (1956) Resolution of the atomic structure of a metal surface by the field ion microscope. Journal of Applied Physics, 27,474-476. 

Tsong, T, T. (1979). Quantitative atom-probe and field ion microscope studies at atomic resolution. Direct Imaging of Atoms in Crystals and Molecules Nobel Symposium 47, The Royal Swedish Academy of Sciences, 7-15. 

The atom-probe field ion microscope is a device which combines an FIM, a probe-hole, and a mass spectrometer of single ion detection sensitivity. With this device, not only can the atomic structure of a surface be imaged with the same atomic resolution as with an FIM, but the chemical species of surface atoms of one s choice, chosen from the field ion image and the probe-hole, can also be identified one by one by mass spectrometry. In principle, any type of mass analyzer can be used as long as the overall detection efficiency of the mass analyzer, which includes the detection efficiency of the ion detector used and the transmission coefficient of the system, has to be close to unity. 

Brenner et al. reported an atom-probe field-ion microscope study of decomposition in an Fe-Cr-Co alloy (see Fig. 18.11) [23]. The atom probe allows direct compositional analysis of the peaks and valleys of the composition waves. It is probably the best tool for verifying a spinodal mechanism in metals, because the growth in amplitude of the composition waves can be studied as a function of aging time, with near-atomic resolution. In spinodal alloys, there is a continuous increase in the amplitude of the composition waves with aging time. On the other hand, for a transformation by nucleation and growth, the particles formed earliest generally exhibit a compositional discontinuity with the matrix. 

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