Surface Analysis Techniques and Instruments
What is Surface Analysis?
Surface Analysis is the study of physical and chemical phenomena that occur at the interface of two materials. Some surface analysis practical applications areas are heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, and adhesives. Ultra High Vacuum (UHV) instrumentation is extremely essential for these applications. The most common UHV Surface Analysis techniques are XPS: X-ray Photo electron Spectroscopy, AES: Auger Electron Spectroscopy, ToF-SIMS: Time of Flight Secondary Ion Mass Spectroscopy. Physical Electronics, a leader in the manufacture of UHV Surface Analysis instrumentation delivers high end instrumentation in all the above three Surface Analysis areas with the PHI 700 AES system, the VersaProbe and Quantera XPS systems and the NanoTof SIMS systems.
Surface Analysis Equipment
High performance Scanning Auger Nanoprobe with less than 8 nm Auger spatial resolution is based on PHI’s unique CMA design. Proven charge neutralization technology extends the application of AES to many non-conductive materials.
High performance scanning XPS microprobe with 7.5 µm spatial resolution provides high sensitivity large and micro-area spectroscopy, inorganic and organic sputter depth profiling, and the highest productivity XPS platform available.
PHI’s patented scanning XPS microprobe technology is now available on a versatile multi-technique surface analysis platform.
Surface Analysis Techniques
Xray Photoelectron Spectroscopy (XPS) Surface Analysis
Electron Spectroscopy for Chemical Analysis (ESCA) Surface Analysis
XPS surface analysis (also known as ESCA) is an essentially non-destructive surface analysis technique that can be used to determine the composition of the outermost atomic layers of a solid material. With the exception of hydrogen, all elements can be detected. The average detectability limit is approximately 0.1 atom percent. The average depth of surface analysis is approximately 50Å. Both elemental identification and chemical speciation are possible with careful surface analysis of the obtained binding energy information.
XPS surface analysis is accomplished by flooding the sample with x-rays of a known energy (typically Al Kα 1486.6ev). Absorption of these x-rays by the sample causes photoelectrons to be emitted. The kinetic energy of the emitted photoelectrons is measured with an electron spectrometer. Binding energy is determined from the following equation: BE = hυ - KE where BE is the binding energy associated with the emitted photoelectron, hυ is the energy of the x-rays being used and KE is the measured kinetic energy of the emitted photoelectron. The binding energy associated with a peak is then used to establish it's elemental identity and chemical state.
The incoming x-rays penetrate microns into the surface of the sample. However, the emitted photoelectrons, because of their low kinetic energy (less than 2000ev), can only travel a short distance without being scattered by colliding or interacting with other atoms and losing energy. This short distance is referred to as the escape depth of the electron. Escape depths range from 5 to 100Å, depending upon the kinetic energy of the photoelectron. Photoelectrons that are close enough to the surface to escape without loss of energy will be detected as photoelectron peaks. Those photoelectrons that lose energy before leaving the sample surface will add to the background of the spectrum. This escape depth limitation makes XPS a surface analysis technique with an average depth of surface analysis of approximately 50Å.
Quantification is possible with the use of elemental sensitivity factors to compensate for the transmission function of the spectrometer and the change in photo-ionization cross-sections from element to element. These sensitivity factors have been determined empirically and found to be in agreement with the current theoretical models for quantification of XPS data.
Elemental and in some cases chemical depth profiling is possible with the use of an inert gas sputter ion gun. The ion gun removes material exposing a new surface to be analyzed. By sequentially sputtering and taking XPS surface analysis data, a compositional depth profile may be generated. If the chemical matrix of the sample is not damaged by the sputter ion beam, chemical information may also be obtained as a function of depth.
If the sample is insulating, an electrical charge may develop on the surface of the sample causing all of the peaks to shift to a higher binding energy. To correct for this charging phenomena we reference all the peaks to a peak of known energy. Typically carbon is used for a reference because hydrocarbons are present on most surfaces. This charge correction method allows us to obtain useful chemical state information from insulating samples.
In summary, XPS is a surface analysis technique that can provide: elemental identification, chemical speciation, accurate quantification, and depth distribution information.
Auger Electron Spectroscopy (AES) Surface Analysis
Auger Electron Spectroscopy (AES) surface analysis is a fast, non-destructive analytical technique used to determine the elemental composition of the top few atomic layers of a surface or exposed interface in a solid material. AES surface analysis can detect all elements except hydrogen and helium and can provide semiquantitative information with an average detection limit of 0.1 to 1 atomic percent. Modern Auger instruments with field emission electron sources can provide characterization of sample features as small as 100Å.
Auger Electron Spectroscopy surface analysis is performed under ultra-high vacuum conditions, using an electron beam typically in the 3 to 25keV range. The Auger process begins with electron bombardment of the sample material causing the ejection of an inner shell electron to form a vacancy. A second electron from a higher shell fills this inner shell vacancy. This is an energy gain process. This energy can then cause the ejection of a third electron, referred to as an Auger electron. A Frenchman, Pierre Auger, first described this process in 1925. The energy of the escaping Auger electron is analyzed by an electron spectrometer. Since each element has a unique set of Auger peaks, the resulting spectrum provides a fingerprint of the probed surface. The kinetic energy of the Auger electrons is typically between 40 and 2500 eV. In this energy regime, electrons have escape depths of approximately 5-100Å. This shallow escape depth gives Auger its surface sensitivity.
Scanning Auger Microscopy (SAM) is accomplished by scanning an electron beam across the surface of a sample while measuring resultant electron signals. This process generates Secondary Electron Microscope (SEM) images, Backscattered Electron (BSE) images, and Auger maps. SEM images, which provide a topographic view of the sample by detecting low energy electrons emitted from the surface, are used to locate specific areas for more detailed study. BSE images, involving higher energy electrons, which have undergone scattering processes before escaping from the sample, reveal atomic number contrast and crystallographic information. Auger maps, obtained by measuring the emitted Auger electron intensity while scanning the electron beam, reveal the lateral distribution of elements across the sample surface.
Additional information can be obtained by using inert gas ions to sputter etch the sample surface. Such sputter etching erodes the sample in a controlled manner and is typically performed with argon ions. A sputter depth profile, which plots the Auger signal as a function of sputter time, shows elemental concentration as a function of depth into the material. This surface analysis technique is particularly effective for thin film analysis and for solving materials problems which require identifying the thickness of a thin surface layer, the composition of a thin film deposit, the presence of interdiffusion between thin films, or the presence of contamination at an interface between two layers.
While AES surface analysis is normally used for analyzing conductive solids, it is also possible to analyze inorganic oxides. An advantage of Auger analysis over Energy Dispersive X-ray Spectroscopy (EDS) analysis is that light elements such as B, C, N, O and F can easily be detected. Auger surface analysis is also more appropriate for submicron particles since the analyzed volume of material with Auger is much less than that for EDS.
The nomenclature used to identify the various Auger peaks involves listing the 3 electron shells involved in the formation of transition. For example, for low atomic number elements, the most probable transitions occur when a K-level electron is ejected by the primary electron beam, an L-level electron drops into the vacancy, and another L-level electron is ejected. This is referred to as a KLL Auger electron.
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS)Surface Analysis
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is an analytical surface analysis technique that uses a high-energy, primary ion beam to probe the surface of a solid material. The transfer of momentum from the primary ions to the solid results in the ejection of ions. Greater than 90% of these secondary ions originate from the outermost 1-2 layers of the solid thus defining TOF-SIMS surface analysis as an extremely surface sensitive technique. Once desorbed from the surface, the ions are extracted into a TOF mass spectrometer where their masses are determined with high accuracy. The resulting mass spectrum allows for unambiguous identification of chemical moieties, typically in the 1-1500 amu range, that can be correlated to the original surface structure. Moreover, by using a finely focussed ion beam it is possible to record the lateral distribution of species across a surface with micron to sub-micron spatial resolution. Lastly, a third dimension can be added by using high primary ion fluxes. This allows atomic and diatomic species to be monitored as a function of depth.
In TOF-SIMS surface analysis, the desorption/ejection of secondary ions from a surface is initiated by a primary ion that impinges on a surface at high angles of incidence. The momentum transfer from the primary ion to the solid initiates a ‘collision cascade’ within the solid, much like a microscopic billiards game. A portion of this momentum is redirected back toward the surface resulting in the ejection of atomic and molecular ions. The emitted secondary ions are extracted into a TOF analyzer by applying a potential between the sample surface and the mass analyzer.
Secondary ion detection sensitivities are maximized by using a pulsed, primary ion source (≤ 1 ns) in conjunction with a TOF analyzer. Once extracted into the mass spectrometer, the secondary ions travel through a field free region where they separate in time, based on their mass to charge ratio (KE=1/2mv2). For each primary ion pulse, a full mass spectrum is obtained by measuring the arrival times of the secondary ions at the detector and performing a simple time to mass conversion. Chemical images are generated by collecting a mass spectrum at every pixel (256 x 256) as the finely focussed, primary ion beam is rastered across the sample surface.
Sample integrity is preserved during analyses by operating under “static” conditions. This mode dictates that ≤ 1x1012 ions/cm2 impinge on a surface ensuring that ≤ 1 percent of the characterized region is consumed.
Because of its surface sensitivity and non-destructive nature, TOF-SIMS surface analysis has been utilized to characterize a variety of conducting and non-conducting surfaces. Common applications include metals, lubricants, semiconductors, pharmaceuticals, biomaterials, plastics/polymers, glasses, ceramics, catalysts, coatings/thin films and paper.
Lastly, shallow depth profiling can be performed by TOF-SIMS surface analysis. A low energy ion gun is operated in DC mode for sputtering with high depth resolution. Data acquisition is accomplished using the same or a second ion gun operated in pulsed mode. Depth profiling by TOF-SIMS surface analysis allows monitoring of all species of interest simultaneously, and with high-mass resolution to remove any interferences.
One of the Techniques in Surface Analysis is Dynamic Secondary Ion Mass Spectrometry (SIMS)
In D-SIMS surface analysis, as a solid material is bombarded (sputtered) by a primary beam of energetic ions, a variety of species are emitted from the sample surface. These include neutrals, excited state species, electrons, photons, and both positive and negative secondary ions. It is these secondary ions which comprise the analytical signal for the Secondary Ion Mass Spectrometry (SIMS) experiment.
In the Dynamic SIMS surface analysis mode of operation the sample surface is rapidly eroded by a primary beam of ions in order to generate as many secondary ions per unit time as possible. The secondary ions emitted from the surface are energy filtered and mass separated so that the number of ions at any given mass can be counted. The surface analysis technique is typically used for depth profiling where the intensities of specific elements with depth are monitored. Survey spectra can also be acquired. These spectra show the intensities of all the elements in the periodic table and are used to monitor dopants and impurities on the surface and in the bulk of materials and also at interfaces between layers. At any given time, the depth of analysis is only about 20Å, making it an ideal tool for interface surface analysis.
In order to maximize the secondary ion yield for different elements, reactive primary ion beams are typically used to etch the sample. An oxygen beam is used to enhance positive secondary ions and cesium is most often used to enhance negative secondaries.
The surface analysis technique is capable of detecting extremely small concentrations of elements in a wide range of materials and compounds. Detection limits in the ppm range are routinely achieved, and for certain elements, the limits may reach the ppb range. Unfortunately, unless standards are available, quantification for these dopants and impurities can be very difficult. This is due to the fact that secondary ion yields vary by orders of magnitude depending on the matrix being sputtered. For example, the positive secondary ion yield for Si in a pure Si matrix is approximately 3 orders of magnitude less than for Si in a SiO2 matrix. Si and GaAs are two semiconductor substrate materials in which many impurity standards are commonly available.
The lateral resolution of the surface analysis technique is, at best, a few μm, however, areas a few hundred μm square are typically bombarded with the primary beam to ensure uniformity of the center of the sputter crater, from where the secondary ion signal is acquired.