Resist processing cleanliness

Stoneware is used mainly for storage, transportation and processing, where its great resistance to corrosion is important. As it can be glazed and is completely non-reactive, it is also used in situations where cleanliness is important, as in the food industry. 

Device defects stem from hardware, processes, and materials. Hardware-related defects include mask defects and contamination in the exposure environment. Those cauused by processing are determined by the cleanliness of the process and the number of processing steps. Material-related defects are caused by particulate matter in the resist or to the formation of unwanted insoluble particulate matter after the resist is coated and patterned. In summary, defects are generally caused by dirt and/or polymer particles, and great care must be exercised in eliminating unwanted contamination at every step in the lithographic process. 

As in all processing steps, cleanliness of the exposure hardware is of paramount importance. Any particle that lands on the resist prior to exposure, will shield the film underneath the particle from the exposing radiation and give rise to opaque spots in the case of positive resist, or pinholes in the case of negative resists. Particulate contamination is especially troublesome with electron beam and ion beam systems where the probability of a particle landing on a substrate is increased relative to other techniques because of the much longer exposure times involved. 

The control of corrosion in pharmaceutical product processes is largely managed through the use of stainless steel. Rust-free surfaces and cleanliness issues to prevent product contamination have been the primary corrosion concerns. Resistance to mildly aggressive cleaning solutions and saline solutions and the potential for under deposit or crevice corrosion present the most severe service conditions. The high standards of cleanliness necessary for pharmaceutical processes favor the mitigation of corrosion. 

The scale or fouling resistances represent a necessary safety factor that increases the surface of the heat exchanger. This enables the full process duty requirements to be attained between cleaning periods. When an exchanger is first placed in operation there is no dirt or scale on the tubes consequently, the overall resistance consists of the two film and the tube wall resistances. During operation, dirt or scale accumulates on the surface of the tubes and the overall heat transfer rate decreases as the dirt buildup increases. The rate of this scale or dirt depends on the cleanliness or fouling tendencies of the process fluids. 

In addition to drinking water and environmental applications, water purity is critical to many industries. Conductivity detectors are used in semiconductor and chip fabrication plants, to monitor cleanliness of pipelines in the food and beverage industry, to monitor incoming water for boilers to prevent scale buildup and corrosion. Any process stream with ions in it can be analyzed by conductometry. Conductivity detectors are part of commercial laboratory deionized water systems, to indicate the purity of the water produced and to alert the chemist when the ion-exchange cartridges are exhausted. The detector usually reads out in resistivity theoretically, completely pure water has a resistivity of 18 MO cm. 

The new apparatus for automating this whole process in a 96 well format is shown disassembled in Figure 9.7A.The three metal pieces are made from anodized aluminum. The bottom plate sets the correct micro-titer plate footprint. The middle piece holds the glass vials in place. The top piece has holes appropriate for a 96-well autosampler. Tapered glass vials are used for their cleanliness, acid resistance, and volume efficienqr. A Teflon insert between the middle metal piece and the lips of the vials prevents vial breakage when the device is sealed with a Teflon/silicone membrane and screw clamps. Assembled, the apparatus has a standard %-well plate footprint, ready to be used as illustrated in Hgure 9.1 B, in commercial autosamplers. 

The main barrier to PI within the sector was that the food industry was innovative in products, but not innovative in processes. As well as institutional barriers in the sector, a number of technical hazards can be identified. Surface deposits (fouling) are perhaps a greater problem in the food and drink sector than elsewhere, as cleanliness is essential. Fouling, of course, leads to inefficiencies due to increased thermal resistance and/or pressure drop. It is therefore important that, in selecting PI plant for applications here, accessibility for cleaning, preferably CIP (cleaning in place), is available. It is sometimes difficult to incorporate CIP in PI plant, however, and care must be taken in selecting appropriate solutions. 

For processes utilizing potentially corrosive rosin based fluxes, simple test options to determine cleanliness such as resistance of solvent extract (ROSE) tests can be used. Other more significant tests may be required to determine the residue properties of organic residue fluxes, such as those commonly used during hot air solder leveling (HASL) of boards.The most common of these are ion chromatography tests that can characterize the residuals and their potential hazards to the product. 

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. 

In 2003, a process qualification study was initiated with a test laboratory to qualify and validate the assembly process using Surface Insulation Resistance (SIR) electrical performance and Ion Chromatography testing on a test board. The study used IPC Class 3 level performance per IPC ANSI J-STD OOlC. The analysis was conducted using ionic cleanliness evaluation by Ion Chromatography IPC-TM-650, method 2.3.28 and SIR characterization IPC-TM-650, method 2.6.3.3A. 

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