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Ultrapure Water for Electronics.

tratamiento de agua ultrapúra
Ósmosis inversa industrial para tratamiento de agua

Ultrapure water treatment for electronics.

Ultrapure water undergoes special treatments that give it specific qualities such as an electrical resistivity greater than 18.2 MΩ-cm at 25°C, a neutral or slightly acidic pH, and an absence of particles and microorganisms, among others.

Due to its high level of purity, this type of water is inert, so it does not react with other substances.

Ultrapure water has applications in industries where high consistency and reliability is required to avoid undesirable reactions, because although purified water has undergone treatment processes, it is likely to contain very low concentrations of contaminants that can interfere in certain processes in industries such as pharmaceuticals, scientific research and the production of electronic equipment.

In the electronics industry, ultrapure water is mainly used in semiconductor manufacturing and other processes, such as the production of touch screens, LCD or OLED technology and storage devices.

The presence of impurities in these applications would compromise results, yields and quality standards, underlining the imperative need for this type of water in highly specialized and precise fields.

This water quality is achieved through advanced techniques such as reverse osmosis, deionization, filtration and ultraviolet light treatment. If the water is not treated correctly with the proper equipment and processes, the products will suffer from contaminations that will affect their final quality.

Particulate contamination

Suspended solids can remain on silicon wafers, which interferes with the functionality of integrated circuits, obstructing the operation of electrical connections or creating incorrect and unwanted conduction paths.

In photolithography processes for microchip production, the presence of solids could cause defects on the pattern by blocking ultraviolet light or acting as nucleation centers for undesired chemical reactions.

If the particles are conductive or semiconducting, they can create leakage paths between interconnects or transistors, resulting in short circuits or changes in resistance characteristics.

They can also cause scratches on the wafer surface during the cleaning process or during processing, damaging the circuit structures or creating faulty spots that affect the integrity of the microcircuit.

To avoid the presence of suspended particles, cartridge filters are used that are of durable construction such as polypropylene fiber cartridges that can maintain the initial flow pressure. In cartridge filters, solids are physically retained on the surface area of the filter media. Over time, as the filter cartridge fills with retained particles, its ability to effectively retain contaminant particles diminishes. Therefore, it is necessary to replace the cartridge periodically to maintain optimum performance.

Chemical contamination

In the water there are organic elements that if not removed can adhere to the silicon wafers causing undesired chemical reactions that can alter the electrical characteristics of the devices.

The presence of organic layers can decrease adhesion between deposited thin films and silicon wafers, leading to device peeling and failure. While the presence of chlorine can cause uncontrolled oxidation of sensitive materials, especially metals used in integrated circuits. Chlorine and chloramines can be corrosive to metal components used in manufacturing equipment and can affect the integrity of metallic thin films in devices.

The existence of contaminants can lead to an increase in the inspection, reprocessing or discarding of defective wafers due to premature failures, therefore the use of activated carbon is recommended so that, based on its chemical characteristics, it adsorbs unwanted organic and chemical contaminants.

This process takes advantage of the large porous surface of activated carbon to adsorb and retain a wide range of organic compounds, as well as chlorine and chloramines. The adsorption of chlorine from water is necessary to prevent this compound and its by-products from reaching the reverse osmosis and demineralization systems, which are of great importance for obtaining ultrapure water. In the case of osmosis, this chemical can damage the membrane making it less efficient in retaining salts and minerals, while in softeners the resins can be affected by disinfection by-products that can alter the ion exchange process.

Magnesium and calcium contamination

Water with high concentrations of minerals such as magnesium and calcium is unsuitable for use in the electronics industry as it can clog the fine patterns and structures of chips and other components, since silicon wafers require high cleanliness parameters in order not to affect the functionality of microcircuits.

For example, chemical vapor deposition (CVD) and etching processes require strict control of the environment to avoid the inclusion of impurities. Calcium and magnesium ions can interfere with these processes and alter the properties of the deposited or etched materials.

It should not be forgotten that these ions can precipitate and form incrustations in the machinery and production equipment, as well as in the subsequent water treatment processes that allow ultrapure water to be obtained.

For calcium and magnesium control, the water passes through a softener containing cation resin. The resin acts when a solution containing dissolved ions passes through it, causing the ions in the solution to exchange with the ions in the resin selectively, according to their relative charges and affinities.

Control of salts, minerals and other contaminants

Reverse osmosis is a process that helps maintain consistent water quality by removing a broader spectrum of contaminants. Without this step, variability in water quality could be higher, affecting reproducibility and consistency in the production of electronic components.

Although the water has undergone treatments that remove sediments, organic compounds and minerals, for its application in electronics, filtration at the molecular level is required to ensure that there are no such contaminants. This is achieved through reverse osmosis, which produces a permeate low in salts.

Because reverse osmosis is a hyperfiltration due to the small opening of its pores (0.0001 to 0.001 μm) it can retain residual ions, low molecular weight organic molecules and minerals that may still be present in the water. This is thanks to a high pressure applied to a semi-permeable membrane that provides values of total dissolved solids in the water of 10 to 15 ppm (parts per million).

The use of this equipment is a highly relevant step to obtain water with the necessary conductivity and resistivity specifications required for the production of electronics.

Conductivity and resistivity of ultrapure water

Ions are charge carriers in aqueous solutions. The more ions that are present in water or other solvent, the greater the liquid's ability to conduct electricity. This is because free ions move toward opposing electrodes when an electric field is applied, thus facilitating current flow. If water with residual ions is used in manufacturing, these ions could create unintended conductive paths, leading to short circuits or electrical faults.

Resistivity is the inverse of conductivity. When ions increase the conductivity of a solution, they simultaneously decrease its resistivity. A high ion concentration implies that the solution has a low resistivity and is more conductive.

Water used in cleaning and rinsing processes during the manufacture of electronic devices must be free of ions that can contaminate semiconductor surfaces or circuit substrates. Even minute amounts of contaminants remaining after reverse osmosis treatment can cause defects in microscopic patterns or thin layers of semiconductor material.

To achieve the required water quality, a mixed ion exchange resin treatment process is used. These resins are composed of type 1 strong base anions and strong acid cations that work by removing ionic impurities from the water through an ion exchange process that reduces the dissolved solids value from 10 to 15 ppm of the osmosis water to practically 0.

Mixed resins for ions in water

Mixed resins are essentially a combination of cation and anion exchange resins in a single bed. By passing water through a bed of mixed resins, complete demineralization can be achieved, as positive and negative ions are removed simultaneously. The result is high-purity water with a resistivity close to 18.2 MΩ-cm at 25 °C, which is the standard for ultrapure water.

Ultrapure water has a very low conductivity, which means that it contains very few dissolved ions that can conduct electricity. This is critical in the electronics industry because the presence of ions can lead to corrosion of components and the creation of unwanted conductive paths that affect the electrical behavior of devices.

Strong acid cation resin

Strong acid cation resins have a high affinity for cations such as Na+, Ca2+, Mg2+, and other alkali metals. These resins are in their H+ (proton) form and when they come in contact with water, they exchange their H+ ions for the cations present in the water.

The resin must be regenerated with chemicals to maintain its performance.

Strong-base type 1 anion resins

Strong base type 1 anion resins are effective in removing anions such as Cl-, SO4^2-, NO3^- and other unwanted anions. These resins work by exchanging their OH^- ions for anions present in water.

The resin must be regenerated with chemicals to maintain its performance.

Electrodeionization (EDI)

Electrodeionization systems combine the capabilities of ion exchange from resins that serve as ion conductors, using a mechanism that benefits from the application of a direct electric current. This current facilitates the migration of ions, with cations moving toward the cathode and anions toward the anode. Ion-selective membranes, arranged between the resins, allow only specific types of ions to pass through, thus facilitating the effective separation of contaminants from the water. As ions are moved into and through the resins, protons (H+) and hydroxyl groups (OH-) are released into the water, which combine to form pure water.

A distinctive feature of EDI systems is their ability to continuously regenerate the resins. During the electrodeionization process, the resins are constantly regenerated by the electric current itself, thus avoiding the need for periodic chemical interventions. This continuous regeneration process is possible because hydrogen ions and hydroxyl groups generated locally in the resins exchange ions with ionized contaminants (such as cations and anions), restoring the exchange capacity of the resin and keeping it active for the deionization process.

An EDI type system can be installed in the final part of the ultrapure water treatment process after reverse osmosis. Electrodeionization can replace mixed resin beds for demineralizing water, as they still provide water with high resistivity and purity for electronics applications. The use of an EDI system avoids the use of chemicals for resin regeneration, optimizing operation and reducing the costs and environmental impact associated with handling chemicals.

Benefits of ultrapure water treatment for electronics

  • Removal of ionic contaminants: Ultrapure water has a low enough ion concentration to prevent short circuits, corrosion and other failures in electronics applications, as even small amounts of ions can cause contamination problems in integrated circuits and other devices.
  • Correct resistivity, conductivity and purity: Ultrapure water has extremely low conductivity, meaning that it does not conduct electricity significantly, which is ideal for rinsing and cleaning electronic components without generating damage on circuitry.
  • Reduced defects and increased reliability: The purity of ultrapure water contributes to the reduction of defects in the manufacture of electronic components. The presence of impurities in the manufacturing process can lead to defects in the circuitry, which in turn can result in device failure.

More information:

What is EDI Electrodeionization?

Other sources

Ultrapure water

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