Medium Energy Ion Scattering       

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Medium Energy Ion Scattering - overview


Medium Energy Ion Scattering (MEIS)
High resolution analysis of thin films and surfaces

Matt Copel ( mcopel@us.ibm.com)

Continued downwards scaling of transistors suggests that soon we will need to develop dielectrics that are only a few atomic layers thick. Characterizing these structures is a major challenge. MEIS is a technique that allows us to probe film composition with sub-nanometer depth resolution. Since MEIS uses the same physics as Rutherford BackScattering (RBS) it is easy to interpret. But because it uses lower energy ions, the depth resolution is greatly improved.

At IBM, MEIS is being used to look at unusual dielectrics, like ZrO2, Al2O3, La2Si2O7 and other metal oxides. Someday these may provide a high capacitance replacement for silicon dioxide gate dielectrics. But before we can use any of these exotic substances, we have to look at some materials properties. For example, we need to know whether these materials react with silicon, and whether they are stable enough to survive CMOS fabrication. MEIS is a powerful tool for looking at these problems. Here are some details:

Detector

detector image We use an electrostatic energy analyzer with a channel plate detector. The analyzer disperses ions with different energies and angles across the exit slit, where they impinge on the channelplate, a charged particle detector which gives out a charge cloud with 106 electrons for every incoming ion. (Channelplates are more commonly used for night vision technology.) The charge cloud exits from the channel plate array onto a two-dimensional 'backgammon' collector. The collector divides the charge between the four outputs (labeled A, B, C, and D). From the ratio of these outputs, we compute the position of the charge cloud, and use this to find the energy and scattering angle.

Sample Data

sample spectrum The 2d collector allows us to get both scattering angle and energy at the same time, so our spectra wind up looking a little different from conventional RBS. This is a 2d display of the data, showing scattering angle (x-axis) and energy (y-axis) of backscattered ions. The colors show the intensity of backscattered ions. The bright bands correspond to ions backscattered from La, Si and O.

It is easier to look at an energy distribution, by taking a cut through the 2d data parallel to the y-axis. In the energy distribution, we can see peaks corresponding to each of the bright bands, containing information on the depth distribution of the different species in the sample. This spectrum tells us that the sample consists of a 4.6 nm La2Si2O7 film on an 1.3nm SiO2 layer.

sample spectrumNow we can learn something about La based dielectrics. This sample was made by depositing lanthanum oxide on a Si substrate, and oxidizing it. During the oxidation step, Si reacted to form La2Si2O7. We conclude that La oxide is unstable during oxidation, but the resulting compound is a more stable dielectric. This is the type of fundamental materials reaction that is easily investigated with MEIS, and is an important step in learning how to create alternative dielectrics.

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