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The critical applications in the electronics industry are the production of ultra-pure water for the washing of semi-conductor material (silicon chips), and the cleaning of the chemical fluids (liquid and gaseous) used in their manufacture. The object of an ultra-pure water plant for semi-conductor manufacture is to produce water as close to the theoretical purity as possible. Users of ultra-pure water should take all steps possible to ensure that the filters selected meet the highest standards of quality and performance. Every fluid that comes into contact with integrated circuit surfaces is a potential source of the contamination that will affect yields. For this reason, filtration of these fluids at the point of use is essential to obtain high yields. All chemicals that contact microcircuits should be filtered to a level of at least 0.2um.
As integrated circuit fabrication advances into the very large-scale integration (VLSI) range, the effect of contaminants on yields becomes of paramount importance. In 1977, an impurity level of 10 particles/ml was acceptable, but nowadays a
level significantly less than one particle/ml is sought. Fluids with higher levels than one particle/ml can be very damaging: one defect/cm2 on a VLSI wafer can potentially cause a greater than 80% yield loss per wafer.
The sensitivity to contamination of an integrated circuit increases as the geometries shrink. Particles as small as one tenth of the size of the minimum wafer geometry can cause a defect in the device. This means that, for VLSI geometries, particles of 0.1um are potential source of defects and are therefore of much concern. Particulates in fluids can produce various types of defects in a semi-conductor device, including holes, gaps and bridges, or leave additional elements on diffusion. If particles are caught between oxide layers, they can lead to cracks and eventual failure of the device.
The source of these contaminants is the fluids that come in contact with the surface of the wafer during the various process steps. These fluids include water, reactive chemicals, gases and photoresists, all of which are used in device fabrication. In
these fluids there is a wide variety of undissolved solids (particulates), which include polymeric colloids, colloidal silica and iron, glass particles, fibres and metallic and airborne particles. The levels at which these particulates affect the fabrication of the device depend upon the device itself, its geometry and the chemical process used.
The acceptable level of particulates also depends upon particle size, particle type and the procedure for counting the particles. For example, the large particles that are simple to remove and are easily and accurately countable should not be present at all, while smaller innocuous particles less than 0.2m in size can exist in substantially higher relative numbers without affecting the yield of some devices.
As a rule filters must be sized so that the maximum amount of filtration can be carried out in the smallest possible housing, with the highest surface area per cartridge within the housing. Filter cartridges should have a large surface area, a high
flow rate and a low pressure drop. Another way of stating this requirement to optimize particle and contaminant removal is that the fluid face velocity (volume per unit area filtered) should be minimized. For example, the face velocity for liquids
should be kept to less than 0.8l/s/m2 of membrane area.
The production process for wafers includes marking their surfaces with the circuits required in the form of photoresists, etching the surface where it is not protected by the photoresist, and washing it after every process stage. This wash water is the one that needs to be ultra-pure, and a typical analysis for such a water is given in Table 4.3 .
