by Tim Nunney AL
Using XPS and a Multi-Mode Ion Source to Understand Advanced Layered Materials
Used in a wide range of products and devices, complex, multi-layered materials have traditionally been based around metallic or oxide layers. Now, with the drive toward lightweight and less expensive components, polymer-based materials are being used more frequently, particularly in areas like display technologies, biomedical devices and energy genera- tion. Understanding how these layers interact with each other and the environment is a critical part of the development cycle. Because these in- teraction zones are typically just a few nanometers thick, surface-sensitive techniques are required that can profile through the material to access subsurface interfaces.
X-ray photoelectron spectroscopy (XPS) is widely used to study the surface and layer chemistry of materials. The surface sensitivity of XPS provides a powerful analytical platform to probe ultrathin layers. The informa- tion obtained in a standard XPS experiment comes from within the top 10 nanometers of a material, roughly 20–30 atomic layers, the region that can impact such properties as electrical performance or adhesion. It also gives direct access to quantitative, chemical state characterization, as op- posed to purely elemental information.
Beyond the surface, the interaction of individual layers can have a pro- found effect on a device’s performance. As an example, reflection of the infrared component of sunlight by architectural glass is controlled by several layers of metals and oxides applied to the outer layer of the glass sheet. This stack is usually around 100–200 nm thick. A process known as depth profiling is used to analyze these types of materials, and involves interleaving cycles of ion beam etching with spectroscopy to build up a layer-by-layer picture of the sample structure. Most XPS instruments include an ion gun that produces monatomic argon ions (Ar+
) for this
purpose. A range of beam energies are usually available, typically from a few hundred eV up to a few keV, allowing operators to choose etch rates depending on the thickness of the layers to be analyzed. Spectra are collected after each cycle of material removal by ion milling to generate a depth profile, typically displayed as atomic concentration, which shows the variation in the chemistry with depth into the surface. Depths up to a few microns can be investigated using this approach.
The process works particularly well for most inorganic materials, and keeps the chemical structure intact as layers are removed by ion bombardment. However, the types of layer structures that are now being developed for applications such as biosensors, display screens and energy generation
AMERICAN LABORATORY 12
depend on mixtures of polymers, metals and oxides. Polymers and some oxides can be damaged by interaction with a monatomic ion beam, changing the material’s chemistry and distorting test data.
The key to minimizing damage to a polymer system during depth profil- ing is to reduce the energy going into the surface. With a monatomic beam, any energy not used to eject material from the surface generally penetrates into the surface, breaking bonds and damaging the remaining material. This damage is typically just greater than the XPS information depth, so spectra become representative of the damaged surface rather than the real surface. It is possible to minimize the damage zone by reduc- ing the beam energy, but below a threshold of around 50 eV, it no longer has an effect on the surface.
An ion source that sputters the sample surface using large, singly charged gas clusters was developed to address this. The monatomic and gas clus- ter ion source (MAGCIS) for Thermo Scientific XPS instruments (Thermo Fisher Scientific, East Grinstead, U.K.) uses both Ar+
and Arn + (50 ≤ n ≤ 2000)
gas cluster sputtering, enabling both surface cleaning and depth profiling of the growing class of advanced materials built from mixtures of organic and inorganic compounds. The technique helps to negate the low-energy sputtering threshold by making the projectile much heavier. By using a weakly bound cluster of several thousand gas atoms, material can still be removed, and the beam energy can spread across the whole cluster. Upon impact, the cluster removes surface material, but also breaks apart, minimizing penetration of the projectile into the surface, and instead disperses the energy in the beam laterally. Such a low energy per atom reduces damage to the remaining surface so that the resultant spectra represent the sample’s actual chemistry.
The examples below illustrate the process and its ability to switch be- tween monatomic ion mode and gas cluster ion mode.
Organic electronics: field effect transistor Polymer-based, or organic, electronics meet the need for the increas-
ingly sophisticated, yet low-cost, components required for a wide range of applications, from consumer goods to portable devices. XPS and gas cluster depth profiling can be useful in understanding the manufacturing processes of such components.
Using the depth profiling technique described above, XPS can moni- tor the composition of a coated or layered material to greater depths
MARCH 2016
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