Process-safe high-purity cleaning: Why rinsing determines success or rejection

During rinsing, it is decided whether high-purity components meet the required cleanliness limits or whether recontamination after cleaning causes rejects. Three factors are particularly decisive:

– Purified water quality directly at the Point of Use (particles, organics/TOC, conductivity, pH).
– Minimal carryover between process stages and defined rinsing procedures.
– System technology and circuit hygiene: suitable materials, well-designed piping, and continuous monitoring.

The following article explains the background of these levers and shows why rinsing in high-purity component cleaning becomes a system-critical process step.

Media as a limiting factor

In high-purity component cleaning, cleanliness requirements are stricter than in any other industry: permissible upper limits of the chemical composition of the component surface in atomic percent, low outgassing rates, and particulate-free in the submicron range. The reason for these high demands stems from the extreme vacuum conditions in which the components are used; for example, in EUV lithography, space technology, or mass spectrometers for analytics.

In these applications, cleanliness becomes a system feature. This means that the entire system must be designed to achieve high limits and exclude recontamination. This particularly applies to the media used, as each contact medium must also meet the high limits to avoid violating specifications. Otherwise, the component cannot be properly cleaned or recontamination of the component may occur.

For ambient air and process air, the limits are generally manageable through HEPA/ULPA filtration and cleanroom technology. However, process water is more complex. During rinsing, it must not carry contaminants such as particles or organics onto the component, otherwise recontamination and component rejects are at risk.

What does "rinsing" mean?

Rinsing is not simply "washing off" cleaner, but essentially diluting the transported liquid film on the component surface with rinse water. Through dilution, residual contamination is simultaneously removed and carried away. This makes rinsing a matter of mass transport: the goal is to reduce the contamination concentration from stage to stage to below the specified limits (both particulate and film/organic).

In short: the component can only be as clean as the last rinse.

Key factors for good rinsing quality

The key factors for good rinsing quality are water quality at the Point of Use and minimal carryover.

The required water quality, as outlined above, depends on the relevant cleanliness requirements. Often, so-called ultrapure water (UPW) is used in the final rinses, although standardized definitions for ultrapure water are not widespread in the European Economic Area, and terminology is sometimes used inconsistently.

The water quality again depends on the raw water quality of municipal water. Depending on hardness, conductivity, and salts present, pretreatment with activated carbon filtration, particulate filtration, de-manganization, or de-icing may be necessary.

Subsequently, permeate (low-salt water) is typically separated from concentrate via reverse osmosis using pressure and a semi-permeable membrane.

For very high demands, further treatment often follows, such as mixed-bed ion exchange or alternatively electrodeionization (EDI), which works continuously without regeneration chemicals. Depending on requirements, additional stages like UV disinfection for microbial reduction or degassing may be added.

The water quality is ultimately determined and monitored via parameters such as conductivity, pH, or TOC value.

Through all this modern water treatment technology, excellent water quality can be guaranteed at the end of a water treatment system. However, it is even more crucial to maintain water quality at the Point of Use, i.e., in the tank or chamber of the cleaning system.

System technology as an enabler: material selection, carryover, circuit hygiene

An advanced system technology is required here to provide good water quality at the Point of Use through minimal carryover, appropriate material selection, and possibly circuit hygiene. Because the rinsing quality is only as good as the system that supports it.

The design and construction of the cleaning system must be geared toward minimizing carryover. This includes the consistent separation of media circuits: each tank has its own piping system, filters, and pump. Additionally, the piping layout must be flow-optimized to prevent water residue accumulation. Often, carriers and racks are neglected. These must also be designed according to the principle "form follows function" so that they have no crevices or large surfaces where water residues can settle.

Depending on the system type, there are other structural features; for example, in immersion cleaning systems, defined overflow and baffle plates between tanks must be observed. Draining times and shaking of carriers support the prevention of carryover. In chamber systems, an optimized chamber drainage, complete emptying, and chamber cleaning between steps are beneficial.

Additionally, the water quality should be monitored in the cleaning system, i.e., at the Point of Use, with appropriate sensors.

The choice of suitable materials and surfaces is also crucial. Due to better material compatibility, V4A stainless steels (e.g., 1.4404, 1.4571) should be used instead of V2A steels, and welding quality should be ensured. Brass should be completely avoided. PP and PVDF can also be suitable for piping.

Practical example: Design of cleaning systems for high-purity rinsing processes

As a practical implementation example, BvL Surface Technology GmbH mentions cleaning systems that can be designed for high rinsing quality at the Point of Use, among other things through separate media circuits, suitable material selection, and process-monitoring of water quality. Exemplary are the NiagaraUP chamber cleaning system and the AtlanticTR immersion cleaning system. Sample cleaning runs can be performed in the company's own laboratory. Additionally, a purified water system (EnviroFALK) is available for testing (parameters: conductivity 0.04 µS/cm; TOC 13.58 ppb).