1" sensitivity surface cleaning processes

It is now known that the quantum process of photodesorption does occur and that it is surface-material sensitive. For clean metals it is an extremely inefficient process if it occurs at all99-101). Quantum efficiencies appear to be of the order of < 10 9 molecules/photon, with corresponding cross sections of the order of < 10"27 m2. Photodesorption from semiconductors appears to be a very efficient process with quantum efficiencies approaching 10-2 molecules/photon and cross sections as high as 10-21 m2. Photodesorption is material dependent and therefore the material must be specified as well as the photon flux as a function of wavelength before estimates of the contribution to overall desorption processes can be made. Additionally the gaseous species present in the system which may adsorb and contribute to photodesorption must also be known. 

Electrode Pretreatment. There is ample evidence that the rate of electron transfer at a solid electrode is sensitive to the surface state and previous history of the electrode. An electrode surface that is not clean usually will manifest itself in a voltage-sweep experiment to give a decrease in the peak current and a shift in the peak potential. Various pretreatment methods have been employed to clean or activate the surface of electrodes the process is intended to produce an enhancement of the reversibility of the reaction (i.e., produce a greater rate of electron transfer).97 This activation or cleaning process may function in two ways by removing adsorbed materials that inhibit electron transfer and by altering the microstructure of the electrode surface. 

In this review the various modes of SIMS and examples of their applications are discussed. SIMS depth profiles are widely used to study dopant profiles and Intermetallic diffusion. The extreme surface sensitivity and low concentration detection limits of SIMS make It useful for Investigation of substrate and metallization cleaning processes. SIMS elemental Imaging Is also used In contamination studies. The ability of SIMS to provide Isotopic Information has allowed elegant mechanistic studies. The Identification and determination of the relative abundance of various molecular or elemental species by SIMS Is applicable to the development characterization and understanding of microelectronic processing. The capability of SIMS In the area of quantitative analyses Is also discussed. 

Another problem is connected with sputtering and heating of the surface. Surfaces in UHV are initially covered in crud that is associated with prior exposure to the atmosphere. This normally consists of several monolayers of oxide and adsorbed hydrocarbons that need to be cleaned away by argon ion bombardment to permit analysis of the sample by a surface sensitive technique. This process of sputter cleaning can change the surface composition and/or structure and is not always successful at removing all contamination. 

Plasma cleaning Dry process (no solvents or chemical solutions) active oxygen plasma is an aggressive cleaner for organic residues effective in removing adhesive bleedout Electron-free plasma is required to avoid damage to sensitive components cleaning surfaces with plasma prior to adhesive cure enhances bleedout... 

SCF cleaning alternative is especially useful for extremely sensitive and complex assemblies. At temperatures above 31°C and pressures above 73 bar, carbon dioxide transforms into a supercritical phase. Supercritical carbon dioxide reveals an extremely low surface tension. Consequently, the wetting of small gaps and complex assembly architectures can be achieved. However, the contamination that should be removed has to be nonpolar, and the compounds should be resistant to high pressures. Due to these basic limitations and the high costs of this cleaning process, this process does not play an important part in the electronic industry. 

AES can be used to analyze the top 2-20 atomic layers and in conjunction with ion beam sputtering, can be used for depth profiling. The process must be undertaken in UHV 1.3 X 10 Nm and surfaces must be sputter cleaned by directing a beam of ions such as Ar at 0.5 5 keV onto the surface. This cleaning process can also erode the sample and expose surface structure. AES can analyze areas as small as 100 mn and up to 2.5 cm and is most sensitive when analyzing elements with low atomic number. However, it cannot detect H2 or He. Resolution is down to 250 nm (cf SEM at 100 mn). 

Cleaning and Cleanliness. Improper handling procedures and improper selection and application of solder paste and wave-solder fluxes and their associated cleaning processes can cause ionic residues to be left on the board that result in low surface insulation resistance. Low SIR values can cause failures in and of themselves for some sensitive circuits and in other cases set up the conditions for further corrosion that eventually result in short circuits. Sodium and potassium ions and halide ions are the most commonly quoted culprits for these failures. The major source of sodium and potassium ions is handling, i.e., fingerprints. The primary sources of halide ions are soldering fluxes. 

Depending on the energy tico of the incident photons, valence band states and even core level electrons can be excited. UPS is a surface-sensitive technique since electrons have a very short inelastic mean free path, Xi, which depends on the kinetic energy Ek, and has a minimum value of 0.5 nm for T k 100 eV. The leading edge of the valence band is taken as the VBM or HOMO maximum and has to be referred to which has to be determined from a clean inorganic metal surface. Those electrons with k > 0 are removed from the sample and transmitted to the detector. The fundamental equation of the photoemission process is (Einstein, 1905) ... 

Because decontamination occurs after buildings have been evacuated and first responders have treated any casualties, it allows for some time lapse before decontamination begins. With some planning, damage can be minimized and the efficacy of the decontamination process can be maximized. For example, sensitive equipment, electronics, valuable artwork, and personal objects will require chemicals that are less harsh than those used to clean air ducts and walls. The lack of time sensitivity also allows the choice of a decontaminant that may have a longer reaction time, but that is more suitable for the surface or the ambient conditions suoh as humidity and temperature. Thus, it is best to obtain a suite of deoontamination methods. 

---