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gh 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 9 surface sensitivity edit xps detects only electrons that have actually escaped from the sample into the vacuum of the instrument in order to escape from the sample a photoelectron must travel through the sample photo emitted electrons can undergo inelastic collisions recombination excitation of the sample recapture or trapping in various excited states within the material all of which can reduce the number of escaping photoelectrons these effects appear as an exponential attenuation function as the depth increases making the signals detected from analytes at the surface much stronger than the signals detected from analytes deeper below the sample surface thus the signal measured by xps is an exponentially surface weighted signal and this fact can be used to estimate analyte depths in layered materials chemical states and chemical shift edit high resolution spectrum of an oxidized silicon wafer in the energy range of the si 2p signal the raw data spectrum red is fitted with five components or chemical states a through e the more oxidized forms of si sio x x 1 2 appear at higher binding energies in the broad feature centered at 103 67 ev the so called metallic form of silicon which resides below an upper layer of oxidized silicon exhibits a set of doublet peaks at 100 30 ev si 2 p 1 2 and 99 69 ev si 2 p 3 2 the fact that the metallic silicon signal can be seen through the overlayer of oxidized si indicates that the silicon oxide layer is relatively thin 2 3 nm attenuation of xps signals from deeper layers by overlayers is often used in xps to estimate layer thicknesses and depths the ability to produce chemical state information i e the local bonding environment of an atomic species in question from the topmost few nanometers of the sample makes xps a unique and valuable tool for understanding the chemistry of the surface the local bonding environment is affected by the formal oxidation state the identity of its nearest neighbor atoms and its bonding hybridization to the nearest neighbor or next nearest neighbor atoms for example while the nominal binding energy of the c 1 s electron is 284 6 ev subtle but reproducible shifts in the actual binding energy the so called chemical shift analogous to nmr spectroscopy provide the chemical state information citation needed chemical state analysis is widely used for carbon it reveals the presence or absence of the chemical states of carbon in approximate order of increasing binding energy as carbide c 2 silane si c h 3 methylene methyl hydrocarbon c h 2 c h 2 c h 3 ch 2 and c h c h amine c h 2 nh 2 alcohol c oh ketone c o organic ester c oor carbonate c o 3 2 monofluoro hydrocarbon c fh ch 2 difluoro hydrocarbon c f 2 ch 2 and trifluorocarbon ch 2 c f 3 to name but a few citation needed chemical state analysis of the surface of a silicon wafer reveals chemical shifts due to different formal oxidation states such as n doped silicon and p doped silicon metallic silicon silicon suboxide si 2 o silicon monoxide sio si 2 o 3 and silicon dioxide sio 2 an example of this is seen in the figure high resolution spectrum of an oxidized silicon wafer in the energy range of the si 2 p signal instrumentation edit the main components of an xps system are the source of x rays an ultra high vacuum uhv chamber with mu metal magnetic shielding an electron collection lens an electron energy analyzer an electron detector system a sample introduction chamber sample mounts a sample stage with the ability to heat or cool the sample and a set of stage manipulators the most prevalent electron spectrometer for xps is the hemispherical electron analyzer it has high energy resolution and spatial selection of the emitted electrons sometimes however much simpler electron energy filters the cylindrical mirror analyzers are used most often for checking the elemental composition of the surface they represent a trade off between the need for high count rates and high angular energy resolution this type consists of two co axial cylinders placed in front of the sample the inner one being held at a positive potential while the outer cylinder is held at a negative potential only the electrons with the right energy can pass through this setup and are detected at the end the count rates are high but the resolution both in energy and angle is poor electrons are detected using electron multipliers a single channeltron for single energy detection or arrays of channeltrons and microchannel plates for parallel acquisition these devices consists of a glass channel with a resistive coating on the inside a high voltage is applied between the front and the end an incoming electron is accelerated to the wall where it removes more electrons in such a way that an electron avalanche is created until a measurable current pulse is obtained citation needed laboratory based xps edit in laboratory systems either 10 30 mm beam diameter non monochromatic al k α or mg k α anode radiation is used or a focused 20 500 micrometer diameter beam single wavelength al k α monochromatised radiation monochromatic al k α x rays are normally produced by diffracting and focusing a beam of non monochromatic x rays off of a thin disc of natural crystalline quartz with a 1010 orientation the resulting wavelength is 8 3386 angstroms 0 83386 nm corresponding to a 1486 7 ev photon energy aluminum k α x rays have an intrinsic full width at half maximum fwhm of 0 43 ev centered at 1486 7 ev e δ e 3457 citation needed for a well optimized monochromator the energy width of the monochromated aluminum k α x rays is 0 16 ev but energy broadening in common electron energy analyzers spectrometers produces an ultimate energy resolution on the order of fwhm 0 25 ev which is the ultimate energy resolution of most commercial systems under practical conditions high energy resolution settings produce peak widths fwhm between 0 4 and 0 6 ev for various elements and some compounds for example in a spectrum obtained for one minute at 20 ev pass energy using monochromated aluminum k α x rays the ag 3 d 5 2 peak for a clean silver film or foil will typically have a fwhm of 0 45 ev citation needed non monochromatic magnesium x rays have a wavelength of 9 89 angstroms 0 989 nm which corresponds to a photon energy of 1253 ev the energy width of the non monochromated x ray is roughly 0 70 ev which is the ultimate energy resolution of a system using non monochromatic x rays citation needed non monochromatic x ray sources do not use any crystal to diffract the x rays allowing all primary x rays lines and the full range of high energy bremsstrahlung x rays 1 12 kev to reach the surface the ultimate energy resolution fwhm when using a non monochromatic mg k α source is 0 9 1 0 ev which includes some contribution from spectrometer induced broadening citation needed synchrotron based xps edit a breakthrough has been brought about in the last decades by the development of large scale synchrotron radiation facilities here bunches of relativistic electrons kept in orbit inside a storage ring are accelerated through bending magnets or insertion devices like wigglers and undulators to produce a high brilliance and high flux photon beam the beam is orders of magnitude more intense and better collimated than typically produced by anode based sources synchrotron radiation is also tunable over a wide wavelength range and can be made polarized in several distinct ways this way photon can be selected yielding optimum photoionization cross sections for probing a particular core level the high photon flux in addition makes it possible to perform xps experiments also from low density atomic species such as molecular and atomic adsorbates semiconductor manufacturing xps edit semiconductor manufacturing nodes prior to the 90 nm process node tended to use silicon dioxide or oxide nitride oxide stacks for the transistor gate which could be controlled using optical techniques such as ellipsometry 10 for some technology designs beginning with the 90nm process node compositional analysis of films around the gate oxides became critical with the proliferation of plasma nitride sion gate dielectrics during this period it was found that small shifts in the nitride composition of the transistor gate resulted in undesirable shifts to the electrical properties of the devices being manufactured 11 xps metrology tools designed to be used in the factory during the processing of the wafers are well suited for the measurement and control of the composition of these sion films by the 65 nm process node high κ materials were being used regularly in the semiconductor industry and xps tooling in the factory became more commonplace to add capability compared with traditional optical techniques these tools have also found use on thin metal films such as aluminum oxide and titanium nitride to ensure uniform compositions over time as well as to monitor any contaminants both of which may impact the desired electrical characteristics for example titanium to nitrogen ratio or oxygen concentration within an aluminum film while inline semiconductor xps metrology tools share the same basic principles as their laboratory based counterparts these automated tools are more limited in functionality and capability these tools do not have the ability to adjust the beam angle and the beam size is reduced to enable the measurement to take place on small test structures on the wafer further reducing the intensity available due to the high vacuum required for the electron beam and sample chamber throughput measured in wafers per hour is significantly reduced compared with traditional optical based techniques used to measure thickness and is therefore undesirable to use unless compositional analysis is necessary for control more recent iterations of inline semiconductor xps tools have begun to incorporate a secondary x ray fluorescence xrf detector to enable the measurement of films which would normally be too thick for traditional xps tooling because xps tools generate x rays with an aluminum source these are considered low energy xrf tools and are therefore not as capable as the heavy element sources found in traditional xrf tooling both in terms of total depth penetration as well as which materials are possible to measure data processing edit peak identification edit the number of peaks produced by a single element varies from 1 to more than 20 tables of binding energies that identify the shell and spin orbit of each peak produced by a given element are included with modern xps instruments and can be found in various handbooks and websites 12 13 because these experimentally determined energies are characteristic of specific elements they can be directly used to identify experimentally measured peaks of a material with unknown elemental composition before beginning the process of peak identification the analyst must determine if the binding energies of the unprocessed survey spectrum 0 1400 ev have or have not been shifted due to a positive or negative surface charge this is most often done by looking for two peaks that are due to the presence of carbon and oxygen charge referencing insulators edit charge referencing is needed when a sample suffers a charge induced shift of experimental binding energies to obtain meaningful binding energies from both wide scan high sensitivity low energy resolution survey spectra 0 1100 ev and also narrow scan chemical state high energy resolution spectra charge induced shifting is normally due to a modest excess of low voltage 1 to 20 ev electrons attached to the surface or a modest shortage of electrons 1 to 15 ev within the top 1 12 nm of the sample caused by the loss of photo emitted electrons if by chance the charging of the surface is excessively positive then the spectrum might appear as a series of rolling hills not sharp peaks as shown in the example spectrum charge referencing is performed by adding a charge correction to each of the experimentally measured peaks since various hydrocarbon species appear on all air exposed surfaces the binding energy of the hydrocarbon c 1s xps peak is used for the charge correction of all energies obtained from non conductive samples or conductors that have been deliberately insulated from the sample mount the peak is normally found between 284 5 ev and 285 5 ev the 284 8 ev binding energy is routinely used as the reference energy for charge referencing insulators so that the charge correction is the difference between 284 8 ev and the experimentally measured c 1s peak position conductive materials and most native oxides of conductors should never need charge referencing conductive materials should never be charge referenced unless the topmost layer of the sample has a thick non conductive film the charging effect if needed can also be compensated by providing suitable low energy charges to the surface by the use of low voltage 1 20 ev electron beam from an electron flood gun uv lights low voltage argon ion beam with low voltage electron beam 1 10 ev aperture masks mesh screen with low voltage electron beams etc peak fitting edit the process of peak fitting high energy resolution xps spectra is a mixture of scientific knowledge and experience the process is affected by instrument design instrument components experimental settings and sample variables before starting any peak fit effort the analyst performing the peak fit needs to know if the topmost 15 nm of the sample is expected to be a homogeneous material or is expected to be a mixture of materials if the top 15 nm is a homogeneous material with only very minor amounts of adventitious carbon and adsorbed gases then the analyst can use theoretical peak area ratios to enhance the peak fitting process peak fitting results are affected by overall peak widths at half maximum fwhm possible chemical shifts peak shapes instrument design factors and experimental settings as well as sample properties the full width at half maximum fwhm values are useful indicators of chemical state changes and physical influences their increase may indicate a change in the number of chemical bonds a change in the sample condition x ray damage or differential charging of the surface localised differences in the charge state of the surface however the fwhm also depends on the detector and can also increase due to the sample getting charged when using high energy resolution experiment settings on an xps equipped with a monochromatic al k alpha x ray source the fwhm of the major xps peaks range from 0 3 ev to 1 7 ev the following is a simple summary of fwhm from major xps signals citati...