X-ray photoelectron spectroscopy is a widely used method of determining the chemical composition of a surface. X-rays impinge upon a sample and ionize atoms, releasing core-level photoelectrons. The escaping photoelectron’s kinetic energy limits the depth from which it can emerge, giving XPS its high surface sensitivity and sampling depth of a few nanometers. Photoelectrons are collected and analyzed by the instrument to produce a spectrum of emission intensity versus electron binding energy. Peak areas at nominal binding energies can be used to quantify elemental composition, and small shifts in these binding energies (chemical shifts) provide powerful information about sample chemical states and short-range chemistry. XPS is suitable for the analysis of conductors and insulators such as polymers (see example below).
OUR XPS INSTRUMENTATION
One workhorse instrument in the Surface Analysis Facility is the VG Scientific 220i-XL imaging multitechnique surface analysis system. The system is pictured on the front of this brochure and includes a monochromatic high-flux microfocused Al X-ray source for high-resolution, high-sensitivity work; a magnetic immersion lens for high collection efficiency and stable charge compensation; and a unique detector system for spectral and imaging data acquisition. An Al/Mg twin anode source is also available. Quantitative peak-area ratios for the three types of carbon in polyethylene terephthalate (PET) polymer can be seen below. Spectral resolution is on the order of 0.75 eV for the carbonyl C1s component of PET, and considerably higher for conductors.
XPS spectrum of polyethylene terephthalate (PET) polymer (30kcps, 2min acquisition).
XPS IMAGING
The instrument can also rapidly collect XP images with a lateral resolution of approximately 1 micron and sufficient spectral resolution to discern, for example, carbidic and graphitic carbon domains in a few minutes of acquisition time. All image pixels are collected in parallel with a large time advantage over serial imaging methods. The combination electrostatic-magnetic lens system provides higher spatial resolution than an electrostatic-only system. Small-spot spectral analysis down to approximately 20 microns is available.
XPS image of a carbon fiber with a SiC exterior and a graphite interior.
AUGER ELECTRON SPECTROSCOPY AND MICROSCOPY
The multitechnique system includes a LaB6 electron gun as an excitation source for Auger. This gun can also be used for 200-nm-resolution SEM. In Auger, an incident primary electron creates an excited ion near the surface which decays by the emission of a secondary Auger electron, whose kinetic energy is measured. As in photoelectron spectroscopy, the escaping Auger electron’s kinetic energy limits the depth from which it can emerge, giving AES its high surface sensitivity and few nanometer sampling depth. Auger electron spectra can be acquired from a selected area mapped out in an SEM image of the sample (e.g., from within a rectangle, along a line, or at points; at right an AFM cantilever is seen poised above a patterned surface; three analysis points are selected). Auger images or maps can also be generated for specific elements with approximately 200-nm resolution. Auger finds its greatest strengths in the analysis of inorganic materials not susceptible to electron-beam damage.