Surface Chemistry

 

What is a Surface?  Where is it?

 

 

 

Surface Chemistry    (defined by “Encyclopedia.com”)

Wolfgang Pauli once stated that “the surface was invented by the devil,” illustrating the complexity and difficulty of studying the surfaces of materials. This prompts a fundamental question: What is the surface of a material? The simplest definition is that the surface is the boundary at which the atoms that make up one material terminate and interface with the atoms of a new material. If the surface is considered to be just the outermost layer of atoms of a material, then it comprises on average only 1015atoms per square centimeter (1 square centimeter equals 0.155 square inch), as compared to the bulk of the material, which consists of approximately 1023 atoms per cubic centimeter. Surface chemistry is important in many critical chemical processes, such as enzymatic reactions at biological interfaces found in cell walls and membranes, in electronics at the surfaces and interfaces of microchips used in computers, and the heterogeneous catalysts found in the catalytic converter used for cleaning emissions in automobile exhausts.

The development of modern surface chemistry did not begin until the early 1960s as the tools needed to detect the small numbers of surface atoms relative to the bulk atoms (predominately through electron-based spectroscopies) became available. Almost thirty years later, the study of surface chemistry received another boost with the development of a new class of tools called scanned probe microscopies, which provide the ability to view the chemical changes of surfaces under different environmental conditions. Such tools were the first to allow for the direct three-dimensional mapping of positions of atoms at surfaces. These techniques changed the view of surfaces by offering scientists and engineers the ability to directly examine and modify surface chemistries at the atomic and molecular levels.

 

Surface Chemistry    (defined by “Wikipedia.org”)

History of Surface Chemistry

The field of surface chemistry started with heterogeneous catalysis pioneered by Paul Sabatier on hydrogenation and Fritz Haber on the Haber process.[3] Irving Langmuir was also one of the founders of this field, and the scientific journal on surface science, Langmuir, bears his name. The Langmuir adsorption equation is used to model monolayer adsorption where all surface adsorption sites have the same affinity for the adsorbing species and do not interact with each other. Gerhard Ertl in 1974 described for the first time the adsorption of hydrogen on a palladium surface using a novel technique called LEED.[4] Similar studies with platinum,[5] nickel,[6][7] and iron [8] followed. Most recent developments in surface sciences include the 2007 Nobel prize of Chemistry winner Gerhard Ertl‘s advancements in surface chemistry, specifically his investigation of the interaction between carbon monoxide molecules and platinum surfaces.

Chemistry at Interfaces

Surface chemistry can be roughly defined as the study of chemical reactions at interfaces. It is closely related to surface engineering, which aims at modifying the chemical composition of a surface by incorporation of selected elements or functional groups that produce various desired effects or improvements in the properties of the surface or interface. Surface science is of particular importance to the fields of heterogeneous catalysis, electrochemistry, and geochemistry.

Catalysis

The adhesion of gas or liquid molecules to the surface is known as adsorption. This can be due to either chemisorption or physisorption, and the strength of molecular adsorption to a catalyst surface is critically important to the catalyst’s performance (see Sabatier principle). However, it is difficult to study these phenomena in real catalyst particles, which have complex structures. Instead, well-defined single crystal surfaces of catalytically active materials such as platinum are often used as model catalysts. Multi-component materials systems are used to study interactions between catalytically active metal particles and supporting oxides; these are produced by growing ultra-thin films or particles on a single crystal surface.[9]

Relationships between the composition, structure, and chemical behavior of these surfaces are studied using ultra-high vacuum techniques, including adsorption and temperature-programmed desorption of molecules, scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. Results can be fed into chemical models or used toward the rational design of new catalysts. Reaction mechanisms can also be clarified due to the atomic-scale precision of surface science measurements.[10]

Electrochemistry

Electrochemistry is the study of processes driven through an applied potential at a solid-liquid or liquid-liquid interface. The behavior of an electrode-electrolyte interface is affected by the distribution of ions in the liquid phase next to the interface forming the electrical double layer. Adsorption and desorption events can be studied at atomically flat single crystal surfaces as a function of applied potential, time, and solution conditions using spectroscopy, scanning probe microscopy[11] and surface X-ray scattering.[12][13] These studies link traditional electrochemical techniques such as cyclic voltammetry to direct observations of interfacial processes.

Geochemistry

Geologic phenomena such as iron cycling and soil contamination are controlled by the interfaces between minerals and their environment. The atomic-scale structure and chemical properties of mineral-solution interfaces are studied using in situ synchrotron X-ray techniques such as X-ray reflectivity, X-ray standing waves, and X-ray absorption spectroscopy as well as scanning probe microscopy. For example, studies of heavy metal or actinide adsorption onto mineral surfaces reveal molecular-scale details of adsorption, enabling more accurate predictions of how these contaminants travel through soils[14] or disrupt natural dissolution-precipitation cycles.[15]

Physics

Surface physics can be roughly defined as the study of physical interactions that occur at interfaces. It overlaps with surface chemistry. Some of the topics investigated in surface physics include friction, surface states, surface diffusion, surface reconstruction, surface phonons and plasmons, epitaxy, the emission and tunneling of electrons, spintronics, and the self-assembly of nanostructures on surfaces. Techniques to investigate processes at surfaces include Surface X-Ray Scattering, Scanning Probe Microscopy, surface enhanced Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS).

Surface Analysis techniques

The study and analysis of surfaces involves both physical and chemical analysis techniques.

Several modern methods probe the topmost 1–10 nm of surfaces exposed to vacuum. These include X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy, thermal desorption spectroscopy, ion scattering spectroscopy, secondary ion mass spectrometry, dual polarization interferometry, and other surface analysis methods included in the list of materials analysis methods. Many of these techniques require vacuum as they rely on the detection of electrons or ions emitted from the surface under study. Moreover, in general ultra high vacuum, in the range of 10−7 pascal pressure or better, it is necessary to reduce surface contamination by residual gas, by reducing the number of molecules reaching the sample over a given time period. At 0.1 mPa (10−6 torr) partial pressure of a contaminant and standard temperature, it only takes on the order of 1 second to cover a surface with a one-to-one monolayer of contaminant to surface atoms, so much lower pressures are needed for measurements. This is found by an order of magnitude estimate for the (number) specific surface area of materials and the impingement rate formula from the kinetic theory of gases.

Purely optical techniques can be used to study interfaces under a wide variety of conditions. Reflection-absorption infrared, dual polarisation interferometry, surface enhanced Raman and sum frequency generation spectroscopies can be used to probe solid–vacuum as well as solid–gas, solid–liquid, and liquid–gas surfaces. Multi-Parametric Surface Plasmon Resonance works in solid-gas, solid-liquid, liquid-gas surfaces and can detect even sub-nanometer layers.[16] It probes the interaction kinetics as well as dynamic structural changes such as liposome collapse[17] or swelling of layers in different pH. Dual Polarization Interferometry is used to quantify the order and disruption in birefringent thin films.[18] This has been used, for example, to study the formation of lipid bilayers and their interaction with membrane proteins.

X-ray scattering and spectroscopy techniques are also used to characterize surfaces and interfaces. While some of these measurements can be performed using laboratory X-ray sources, many require the high intensity and energy tunability of synchrotron radiation. X-ray crystal truncation rods (CTR) and X-ray standing wave (XSW) measurements probe changes in surface and adsorbate structures with sub-Ångström resolution. Surface-extended X-ray absorption fine structure(SEXAFS) measurements reveal the coordination structure and chemical state of adsorbates. Grazing-incidence small angle X-ray scattering (GISAXS) yields the size, shape, and orientation of nanoparticles on surfaces.[19] The crystal structure and texture of thin films can be investigated using grazing-incidence X-ray diffraction (GIXD, GIXRD).

X-ray photoelectron spectroscopy (XPS) is a standard tool for measuring the chemical states of surface species and for detecting the presence of surface contamination. Surface sensitivity is achieved by detecting photoelectrons with kinetic energies of about 10-1000 eV, which have corresponding inelastic mean free paths of only a few nanometers. This technique has been extended to operate at near-ambient pressures (ambient pressure XPS, AP-XPS) to probe more realistic gas-solid and liquid-solid interfaces.[20] Performing XPS with hard X-rays at synchrotron light sources yields photoelectrons with kinetic energies of several keV (hard X-ray photoelectron spectroscopy, HAXPES), enabling access to chemical information from buried interfaces.[21]

Modern physical analysis methods include scanning-tunneling microscopy (STM) and a family of methods descended from it, including atomic force microscopy. These microscopies have considerably increased the ability and desire of surface scientists to measure the physical structure of many surfaces. For example, they make it possible to follow reactions at the solid–gas interface in real space, if those proceed on a time scale accessible by the instrument.[22][23]

References

  1. ^ Prutton, Martin (1994). Introduction to Surface Physics. Oxford University Press. ISBN 978-0-19-853476-1.
  2. ^ Luklema, J. (1995–2005). Fundamentals of Interface and Colloid Science. 1–5. Academic Press.
  3. ^ Wennerström, Håkan; Lidin, Sven. “Scientific Background on the Nobel Prize in Chemistry 2007 Chemical Processes on Solid Surfaces“ (PDF).
  4. ^ Conrad, H.; Ertl, G.; Latta, E.E. (February 1974). “Adsorption of hydrogen on palladium single crystal surfaces”. Surface Science. 41 (2): 435–446. Bibcode:1974SurSc..41..435C. doi:10.1016/0039-6028(74)90060-0.
  5. ^ Christmann, K.; Ertl, G.; Pignet, T. (February 1976). “Adsorption of hydrogen on a Pt(111) surface”. Surface Science. 54 (2): 365–392. Bibcode:1976SurSc..54..365C. doi:10.1016/0039-6028(76)90232-6.
  6. ^ Christmann, K.; Schober, O.; Ertl, G.; Neumann, M. (June 1, 1974). “Adsorption of hydrogen on nickel single crystal surfaces”. The Journal of Chemical Physics. 60 (11): 4528–4540. Bibcode:1974JChPh..60.4528C. doi:10.1063/1.1680935.
  7. ^ Christmann, K.; Behm, R. J.; Ertl, G.; Van Hove, M. A.; Weinberg, W. H. (May 1, 1979). “Chemisorption geometry of hydrogen on Ni(111): Order and disorder”. The Journal of Chemical Physics. 70 (9): 4168–4184. Bibcode:1979JChPh..70.4168C. doi:10.1063/1.438041.
  8. ^ Imbihl, R.; Behm, R. J.; Christmann, K.; Ertl, G.; Matsushima, T. (May 2, 1982). “Phase transitions of a two-dimensional chemisorbed system: H on Fe(110)”. Surface Science. 117 (1): 257–266. Bibcode:1982SurSc.117..257I. doi:10.1016/0039-6028(82)90506-4.
  9. ^ J.-H. Fischer-Wolfarth, J.A. Farmer, J.M. Flores-Camacho, A. Genest, I.V. Yudanov, N. Rösch, C.T. Campbell, S. Schauermann, H.-J. Freund, “Particle-size dependent heats of adsorption of CO on supported Pd nanoparticles as measured with a single-crystal microcalorimeter”, Phys. Rev. B 81 (2010) 241416
  10. ^ M. Lewandoski, I.M.N. Groot, S. Shaikhutdinov, H.-J. Freund, “Scanning tunneling microscopy evidence for the Mars-van Krevelen type mechanism of low temperature CO oxidation on an FeO(1 1 1) film on Pt(1 1 1)”, Catalysis Today181 (2012) p. 52-55
  11. ^ A.A. Gewirth, B.K. Niece, “Electrochemical applications of in Situ Scanning Probe Microscopy”, Chem. Rev. 97 (1997) p. 1129-1162
  12. ^ Z. Nagy, H. You, “Applications of surface X-ray scattering to electrochemistry problems”, Electrochimica Acta 47 (2002) p. 3037-3055
  13. ^ Gründer, Yvonne; Lucas, Christopher A. (2016-11-01). “Surface X-ray diffraction studies of single crystal electrocatalysts”. Nano Energy. 29: 378–393. doi:10.1016/j.nanoen.2016.05.043. ISSN 2211-2855.
  14. ^ J.G. Catalano, C. Park, P. Fenter, Z. Zhang, “Simultaneous inner- and outer-sphere arsenate adsorption on corundum and hematite”, Geochim. et Cosmochim. Acta 72 (2008) p. 1986-2004
  15. ^ M. Xu, L. Kovarik, B.W. Arey, A.R. Felmy, K.M. Rosso, S. Kerisit, “Kinetics and mechanisms of cadmium carbonate heteroepitaxial growth at the calcite (10-14) surface”, Geochim. et Cosmochim. Acta 134 (2014) p. 221-233
  16. ^ Jussila, Henri; Yang, He; Granqvist, Niko; Sun, Zhipei (5 February 2016). “Surface plasmon resonance for characterization of large-area atomic-layer graphene film”. Optica. 3 (2): 151. doi:10.1364/OPTICA.3.000151.
  17. ^ Granqvist, Niko; Yliperttula, Marjo; Välimäki, Salla; Pulkkinen, Petri; Tenhu, Heikki; Viitala, Tapani (18 March 2014). “Control of the Morphology of Lipid Layers by Substrate Surface Chemistry”. Langmuir. 30 (10): 2799–2809. doi:10.1021/la4046622. PMID 24564782.
  18. ^ Mashaghi, A; Swann, M; Popplewell, J; Textor, M; Reimhult, E (2008). “Optical Anisotropy of Supported Lipid Structures Probed by Waveguide Spectroscopy and Its Application to Study of Supported Lipid Bilayer Formation Kinetics”. Analytical Chemistry. 80 (10): 3666–76. doi:10.1021/ac800027s. PMID 18422336.
  19. ^ Renaud, G.; Lazzari, R.; Leroy, F. (2009). “Probing surface and interface morphology with Grazing Incidence Small Angle X-Ray Scattering”, Surf. Sci. Rep. 64 p. 255-380
  20. ^ Bluhm, H.; Hävecker, M.; Knop-Gericke, A.; Kiskinova, M.; Schlögl, R.; Salmeron, M. (2007). “In Situ X-Ray Photoelectron Spectroscopy Studies of Gas-Solid Interfaces at Near-Ambient Conditions”, MRS Bulletin 32 p. 1022-1030.
  21. ^ Sing, M. et al.. “Profiling the Interface Electron Gas of LaAlO3 / SrTiO3Heterostructures with Hard X-Ray Photoelectron Spectroscopy”, Phys. Rev. Lett.102 176805.
  22. ^ Wintterlin, J.; Völkening, S.; Janssens, T. V. W.; Zambelli, T.; Ertl, G. (1997). “Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction”. Science. 278 (5345): 1931–4. Bibcode:1997Sci…278.1931W. doi:10.1126/science.278.5345.1931. PMID 9395392.
  23. ^ Waldmann, T.; et al. (2012). “Oxidation of an Organic Adlayer: A Bird’s Eye View”. Journal of the American Chemical Society. 134 (21): 8817–8822. doi:10.1021/ja302593v. PMID 22571820.

 

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Semiconductor Fabrication Chemistry

The surface cleaning and preparation of wafers in the Back End Of the Line (BEOL) processes are now critical steps in the production of Integrated Circuits (IC). Over the last twenty years Surface Chemistry Discoveries’ staff has developed and successfully commercialized a number of products that are now in use in many of the largest fabrication plants around the world.

In many cases surface cleaning and surface preparation are separate, distinct process steps, each with its own requirements. Surface cleaning usually requires removal of surface contamination of various kinds including metals such as sodium, or copper from various surfaces of structures such as vias, metal runners and dielectrics.

On the other hand, surface preparation is intended to provide a suitable surface for deposition and/or plating of the next layer. Therefore, the surface preparation product used must allow for rapid uniform deposition or plating without generating any voids. We have the products and expertise to provide you with formulations that allow for the cleaning of metal surfaces and provide a desirable surface roughness while passivating the metal. Our products have been proven to prevent time delayed defect growth by effectively passivating certain soft metal surfaces.

The cleaning of various substrates (wafers) that include metals (aluminum, copper, tungsten, tantalum, tin, etc.) in the IC industry has led to the development of cleaning formulations that include corrosion inhibitors. A number of our staff members were part of the original team of scientists who initiated the successful use of various corrosion inhibitors in formulated products for use in the semiconductor industry.

Surface Chemistry Discoveries offers superior cleaning products that also protect exposed metals from corrosion. We also know how to protect the various dielectrics, such as SiO2 and Organo-Silicate Glass (OSG, used as substrates).