Thursday, May 27, 2021

Atomic billiards helps to understand Atomic Layer Deposition

In the beautiful German city of Münster, scientists are playing games on an atomic scale to help ALD developers understand what is going on in their process. Critical ALD parameters, such as the evolution of film closure and thickness with increasing cycle number are determined with a game of billiards at the atomic level. This game is called LEIS (Low Energy Ion Scattering), the most surface specific chemical analysis technique available to the surface scientist.

The fundamental principles behind LEIS are surprisingly simple: In an ultrahigh vacuum chamber, light charged particles (ions) are aimed at the sample where they collide with the atoms in and on the sample. These collisions obey the same laws of physics as collisions between large objects, such as balls. This means that the ions are bouncing (or scattering) back with high speed (or energy) when they collide with a heavy atom and with low energy after a collision with a light atom. The energy of the scattered ions is measured to determine the mass of the surface atoms.

Figure 1: The principle of LEIS: When ions collide with surface atoms, their energy after the collision depends on the mass of the atoms that they collided with. A LEIS spectrum shows the number of returned ions as a function of their energy. This represents the surface concentrations of different elements, sorted after their mass.

LEIS helps to develop and optimize ALD processes


We show an example of a co-operation between scientists at Tascon in Münster and the ALD experts from ASM Microchemistry Oy in Helsinki, Finland where the initial formation of a GaSb film on SiOx was investigated.

Figure 2 shows a set of LEIS spectra for the increasing number of ALD cycles recorded with Neon ions. There are two peaks due to collisions with Gallium (Ga) and Antimony (Sb) atoms in the outermost atomic layer of these samples. Antimony atoms are heavier than Gallium atoms and therefore the peak from collisions with Antimony lies at higher energy than the Gallium peak.


Figure 2: 5 keV 20Ne+ LEIS spectra of increasing cycle numbers of GaSb deposited on SiOx
As one would expect, the amount of Gallium in the outermost atomic layer is increasing continuously with increasing cycle number (from red to purple) showing how the fraction of Gallium increases in the outermost atomic layer. The Antimony behaves differently, though. After increasing initially, its signal goes through a maximum (the green spectrum), and with increasing cycle number the amount of Antimony at the surface decreases. With this valuable information, the ALD expert can optimize the deposition process.

LEIS separately analyzes the outermost atomic layer and the layers below it

As we have seen, LEIS is sensitive to the outermost atomic layer of a sample. The used noble gas ions (Helium or Neon) lose their charge as soon as they enter the material. Since the instrument can only detect ions, the neutral Helium or Neon atoms that collided in deeper layers are not detected. Therefore, the peaks in the spectrum from figure 2 represent Gallium and Antimony at the surface.

Particularly when Helium ions are used, there is a second effect. A Helium atom, that collided in deeper layers, may lose an electron as it leaves the surface. The probability for this re-ionization is small enough to recognize the peaks in the spectra, but large enough to cause an additional signal in the spectrum, as shown in figure 3, a set of spectra recorded with Helium ions from the same GaSb deposition study.

Figure 3: 7 keV 4He+ LEIS spectra of increasing cycle numbers of GaSb deposited on SiOx.
Again, we see the Gallium peak increasing and the Antimony peak going through a maximum. This time, we also see the Silicon (Si) peak decreasing with increasing cycle number, confirming that the substrate is getting covered. But we also see that with increasing cycle number shoulders appear on the left side of the Gallium and Antimony peaks (indicated by the dashed arrows).

These shoulders are caused by collisions from Gallium and Antimony atoms below the surface. The Helium atoms have slowed down while traveling through the sample on their way to and from the colliding atom. The more the shoulder extends to lower energy, the more the atoms have slowed down and the deeper the colliding atom was in the sample. The fact that the shoulders are extending more and more to the left with increasing cycle number shows that the film is getting thicker.

Since LEIS is a quantitative analysis technique, the surface fractions can be determined from the spectra. Figure 4 shows a ternary diagram for the composition of the sample surface with increasing cycle number. It clearly shows that initially Gallium and Antimony are deposited together. But as the film is almost closed, the Gallium deposition starts to dominate.


Figure 4: Ternary diagram showing the surface composition of the samples with increasing cycle number. The colors of the data points correspond to the colors of the spectra.
This example shows the value of LEIS in the study of ALD. Because of its surface specificity and the need for ever thinner films, the role of LEIS in ALD is expected to increase in the coming years.

Acknowledgement

The GaSb films in this study were kindly provided by ASM Microchemistry Oy, Helsinki, Finland.

Guest blog by Rik ter Veen and Karsten Lamann, Tascon GmbH, Münster, Germany

About the authors:

Rik ter Veen and Karsten Lamann are scientists at Tascon GmbH, a service provider and consulting company for the analysis of surfaces, films and interface for over 20 years with two locations in Germany and one in the USA. In addition to LEIS, the subject of this blog, Tascon offers surface and materials analysis with techniques such as ToF-SIMS, XPS and SEM-EDX. If you are interested in their services or have questions about LEIS, or other techniques, the authors can be contacted through their website.

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