Editing X-ray photoelectron spectroscopy (section)

==Measurement==
[[File:wide.jpg|thumb|350px|Wide-scan or survey spectrum of a somewhat dirty silicon wafer, showing all elements present. A survey spectrum is usually the starting point of most XPS analyses. It allows one to set up subsequent high-resolution XPS spectra acquisition. The inset shows a quantification table indicating the atomic species, their atomic percentages and characteristic binding energies.]] 
A typical XPS spectrum is a plot of the number of electrons detected at a specific [[binding energy]]. Each element produces a set of characteristic XPS peaks. These peaks correspond to the [[electron configuration]] of the electrons within the atoms, e.g., 1''s'', 2''s'', 2''p'', 3''s'', etc. The number of detected electrons in each peak is directly related to the amount of element within the XPS sampling volume. To generate atomic percentage values, each raw XPS signal is corrected by dividing the intensity by a ''relative sensitivity factor'' (RSF), and normalized over all of the elements detected. Since hydrogen is not detected, these atomic percentages exclude hydrogen.

===Quantitative accuracy and precision===
XPS is widely used to generate an empirical formula because it readily yields excellent quantitative accuracy from homogeneous solid-state materials. Absolute quantification requires the use of certified (or independently verified) standard samples, and is generally more challenging, and less common. Relative quantification involves comparisons between several samples in a set for which one or more analytes are varied while all other components (the sample matrix) are held constant. Quantitative accuracy depends on several parameters such as: [[signal-to-noise ratio]], peak intensity, accuracy of relative sensitivity factors, correction for electron transmission function, surface volume homogeneity, correction for energy dependence of electron mean free path, and degree of sample degradation due to analysis. Under optimal conditions, the quantitative accuracy of the atomic percent (at%) values calculated from the major XPS peaks is 90-95% for each peak. The quantitative accuracy for the weaker XPS signals, that have peak intensities 10-20% of the strongest signal, are 60-80% of the true value, and depend upon the amount of effort used to improve the signal-to-noise ratio (for example by signal averaging). Quantitative precision (the ability to repeat a measurement and obtain the same result) is an essential consideration for proper reporting of quantitative results.

===Detection limits===
Detection limits may vary greatly with the cross section of the core state of interest and the background signal level.  In general, photoelectron cross sections increase with atomic number. The background increases with the atomic number of the matrix constituents as well as the binding energy, because of secondary emitted electrons. For example, in the case of gold on silicon where the high cross section Au4f peak is at a higher kinetic energy than the major silicon peaks, it sits on a very low background and detection limits of 1ppm or better may be achieved with reasonable acquisition times. Conversely for silicon on gold, where the modest cross section Si2p line sits on the large background below the Au4f lines, detection limits would be much worse for the same acquisition time. Detection limits are often quoted as 0.1–1.0 % atomic percent (0.1% = 1 [[parts per thousand|part per thousand]] = 1000 [[parts per million|ppm]]) for practical analyses, but lower limits may be achieved in many circumstances.

=== Degradation during analysis ===
Degradation depends on the sensitivity of the material to the wavelength of X-rays used, the total dose of the X-rays, the temperature of the surface and the level of the vacuum. Metals, alloys, ceramics and most glasses are not measurably degraded by either non-monochromatic or monochromatic X-rays. Some, but not all, polymers, catalysts, certain highly oxygenated compounds, various inorganic compounds and fine organics are. Non-monochromatic X-ray sources produce a significant amount of high energy Bremsstrahlung X-rays (1–15 keV of energy) which directly degrade the surface chemistry of various materials. Non-monochromatic X-ray sources also produce a significant amount of heat (100 to 200&nbsp;°C) on the surface of the sample because the anode that produces the X-rays is typically only 1 to {{convert|5|cm|0|abbr=on}} away from the sample. This level of heat, when combined with the Bremsstrahlung X-rays, acts to increase the amount and rate of degradation for certain materials. Monochromatised X-ray sources, because they are farther away (50–100&nbsp;cm) from the sample, do not produce noticeable heat effects. In those, a quartz monochromator system diffracts the Bremsstrahlung X-rays out of the X-ray beam, which means the sample is only exposed to one narrow band of X-ray energy. For example, if aluminum K-alpha X-rays are used, the intrinsic energy band has a FWHM of 0.43 eV, centered on 1,486.7 eV (''E''/Δ''E'' = 3,457). If magnesium K-alpha X-rays are used, the intrinsic energy band has a FWHM of 0.36 eV, centered on 1,253.7 eV  (''E''/Δ''E'' = 3,483). These are the intrinsic X-ray line widths; the range of energies to which the sample is exposed depends on the quality and optimization of the X-ray monochromator. Because the vacuum removes various gases (e.g., O<sub>2</sub>, CO) and liquids (e.g., water, alcohol, solvents, etc.) that were initially trapped within or on the surface of the sample, the chemistry and morphology of the surface will continue to change until the surface achieves a steady state. This type of degradation is sometimes difficult to detect.

===Measured area===
Measured area depends on instrument design. The minimum analysis area ranges from 10 to 200 micrometres. Largest size for a monochromatic beam of X-rays is 1–5&nbsp;mm. Non-monochromatic beams are 10–50&nbsp;mm in diameter. Spectroscopic image resolution levels of 200&nbsp;nm or below has been achieved on latest imaging XPS instruments using synchrotron radiation as X-ray source.

===Sample size limits===

Instruments accept small (mm range) and large samples (cm range), e.g. wafers. The limiting factor is the design of the sample holder, the sample transfer, and the size of the vacuum chamber. Large samples are laterally moved in x and y direction to analyze 
wider  area. {{Citation needed|date=June 2015}}

=== Analysis time ===
Typically ranging 1–20 minutes for a broad survey scan that measures the amount of all detectable elements, typically 1–15 minutes for high resolution scan that reveal chemical state differences (for a high signal/noise ratio for count area result often requires multiple sweeps of the region of interest), 1–4 hours for a depth profile that measures 4–5 elements as a function of etched depth (this process time can vary the most as many factors will play a role). The time to complete a measurement is generally dependent on the brilliance of the X-ray source.<ref>{{cite journal |doi=10.1088/1742-6596/430/1/012131|doi-access=free |title=On the relation between X-ray Photoelectron Spectroscopy and XAFS |date=2013 |last1=Mårtensson |first1=N. |last2=Söderstrom |first2=J. |last3=Svensson |first3=S. |last4=Travnikova |first4=O. |last5=Patanen |first5=M. |last6=Miron |first6=C. |last7=Sæthre |first7=L. J. |last8=Børve |first8=K. J. |last9=Thomas |first9=T. D. |last10=Kas |first10=J. J. |last11=Vila |first11=F. D. |last12=Rehr |first12=J. J. |journal=Journal of Physics: Conference Series |volume=430 |issue=1 |page=012131 |bibcode=2013JPhCS.430a2131M }}</ref>