Whether in semiconductor manufacturing, the display and solar industry, in optics and laser technology or in medical technology, space or high-energy research - there is always one aspect in the foreground: maximum cleanliness! Practically everything that happens here takes place under cleanroom conditions, since every particle, every foreign molecule can have a negative impact on the quality of the results.

The continuously increasing demands to avoid foreign substances in the form of particles or outgassing also makes it more and more complex for VAT to achieve a new Ultra Clean Vacuum (UCV) level. The goal of the UCV approach is to minimize particle and outgassing dynamics in high or ultra-high vacuum. Two aspects are involved. One is the nature and dynamics of the air/gas molecules remaining in the vacuum, and the other is the size of the particles that can be tolerated in the process.

For the between 100,000 and 100 million gas molecules per cm3 remaining in space at an ultra-high vacuum, it is currently defined in the UCV approach that these should not exceed an atomic mass of 45u (or 45 amu). This limits the molecules to the classical air constituents such as nitrogen, oxygen, carbon dioxide and water. In addition to the mass, however, the reactivity of the remaining molecules is also important. I.e., oxygen, for example, as a very reactive element, is rather unpopular and should, if possible, no longer be present.

In ultra-high vacuum, the number of remaining molecules is reduced to such an extent that they collide more frequently with the chamber walls than with each other. Almost all molecular interactions therefore take place on the surfaces in the chamber. These surfaces are thus given special importance in the generation of UCV conditions. In this context, the chamber surfaces also include the sealing surfaces that result from the various inlets and outlets of a chamber. For the surfaces, their outgassing behavior is generally important. The higher the outgassing rate, i.e. the more water molecules adhering to the metal walls of a vacuum chamber, for example, are released, the higher the dynamics within the chamber, since for each new molecule another must be removed in order not to change the vacuum level or the number of molecules in the volume. A high dynamic also means high thermodynamics, which can have a negative effect on the processes taking place in the vacuum. In addition to the outgassing rate of the surfaces, however, extremely small leakages as well as reflux effects during evacuation also contribute to increased dynamics in ultrahigh vacuum.

Particles are the other big issue in ultra clean vacuum. Particles are introduced as residues from the production process of the vacuum components used or are generated by wear processes, for example on dynamic seals. These particles are usually bound to the surfaces of the chambers, but can be released by thermodynamic processes as well as by shocks or vibrations and then attach themselves to the processed or examined products in the chamber, which can have negative effects on the examination and processing results.

For particles, therefore, a limit size of 1 micrometer is currently defined as tolerable for a UCV environment. Particles larger than 1 micrometer should therefore no longer be found. However, strategies must also be developed for the smaller particles to minimize their release.  

In order to meet the Ultra Clean Vacuum requirements in all areas of production, VAT has set up its organization in such a way that tasks can be processed across all areas and adapted production processes can be developed and implemented more quickly. VAT designs particle management solutions for optimized manufacturing, assembly, logistics and installation processes in order to reduce or completely eliminate system and plant contamination even before commissioning.

Today, repeated ultrasonic cleaning in a clean water bath (> 12 MΩ/cm) with suitable cleaners in clean room class 6 DIN EN ISO 14644-1 or better and drying with particle-free air are standard in manufacturing. Assembly always takes place in a cleanroom environment in accordance with the specified cleanroom class. All vacuum systems supplied are always flooded with particle-filtered air or particle-filtered gas (nitrogen) after leak testing and protected in dust-tight double packaging for transport and storage. However, this is only one aspect in the UCV approach.

The example of microchip manufacturing illustrates the challenges for particle management: the transfer valves involved in the manufacturing process close and open the process chambers in the individual manufacturing steps. They are therefore subject to a continuous alternation between movement and standstill phases. Each valve movement triggers changes in the physical and also chemical conditions in the process chambers. These can range from slight changes in the temperature or flow behavior of the process gases to strong effects caused by vibrations, turbulence or even condensation effects. Such strong effects can release or generate particles. These particles can originate in the process itself, but can also be emitted by the transfer valves themselves, e.g. by an overstressed elastomer seal. Even in small numbers and sizes, such particles can damage the microchips produced, thus increasing the reject rate and, in the worst case, jeopardizing the economic viability of production.

One of the tasks facing VAT Particle Management is to optimize the closing mechanisms and closing movements of transfer valves in such a way that, ideally, no detectable particles are activated or released.

The seals of a transfer valve, which guarantee the defined tightness and dampen closing movements, are another critical area. The elastomers usually used here contain, for example, plasticizers that can be released under certain conditions. In addition, seals that are subjected to the repeated stress of dynamic compression and relaxation movements can release minute elastomer particles as a result of aging. For this reason, VAT is working on the use of seal compounds that have greater durability and resistance in the manufacturing process. This results in elastomer seals that, unlike conventional static seals, are optimized for the dynamic requirements of transfer valves and offer significantly better particle performance.

Redefining boundaries with applied research

VAT is researching both entirely new solutions and promising ways to improve the performance of established processes. For example, potential vibrations triggered by the closing and opening movement of valves are already minimized by braking at the end of the movement - similar to an elevator that gently slows down its movement when reaching a floor. VAT is working to perfect this technology. With dynamic motion profiles, optimized particle performance is guaranteed even during very short opening and closing cycles with high acceleration and braking speeds.

Post-assembly testing in the Ultra Clean Vacuum division is always carried out to the degree of fineness specified by the customer. In this process, VAT often operates at the limit of what is technically feasible in terms of testing accuracy. A good example of this are requirements that come, for example, from high-energy research, with projects such as the LHC of the Cern, ITER, XFEL or SLAG of the Jefferson and Fermilab, but also from the semiconductor sector of manufacturers such as ASML. ASML is currently the only company in the world to offer Extreme Ultraviolet Lithography (EUV), which can be used to fabricate today's finest chip structures based on shorter wavelengths of light in the 13.5 nm range. With EUV, ASML is pushing the physical limits of its predecessor Deep Ultraviolet (DUV) process. While DUV lithography did not require vacuum conditions, the EUV process needs a very pure UHV environment, an ultra clean vacuum. ASML is thus currently defining the standards for pure vacuum for the field of industrial semiconductor manufacturing.

In principle, UCV requirements include not only the components produced and their starting materials, but also all components that come into contact with the respective component during production - from cleaning agents to transport and protective packaging. None of these elements should prove to be a potential source of particle and outgassing emissions.

VAT is researching the use of improved raw materials to further reduce outgassing effects, for example. Directly related to this is the development of innovative processing methods to create surfaces that are more inert to particle buildup or specifically favor it (getter principle) in order to specifically bind particles or individual molecules and keep them away from the process. The early detection of particle-critical operating conditions is also on the agenda. With the use of state-of-the-art sensor technology, VAT is setting an example in the direction of Industry 4.0. The latest VAT vacuum valve developments can provide targeted warnings if there is a deviation from the defined operating state, the self-compensation options are exhausted and there could therefore be a risk of increased emissions.

With the VAT Particle Test Center in San Jose, California, and the VAT Laboratory in Haag, Switzerland, VAT is specifically driving developments in this area. Here, the particle performance of VAT valves is checked in detail and calibrated to customer requirements. This includes testing the valves specifically in selected plant components, such as in the "wafer path" from the load lock to the transfer chamber to the process chamber.

In-house research enables VAT to keep pace with the high innovation speed of its customers and offer solutions at the same cycle rate. An important part of this is also the continuous know-how-based exchange with customers and suppliers. This accelerates and optimizes adaptation to changing, increasing requirements. With its constantly growing knowledge, VAT can now anticipate certain customer needs in many areas in advance and thus react very flexibly to requirements. In this way, both existing and new customers benefit directly from VAT's ever-expanding expertise.

This makes VAT the global leader in the industry when it comes to valve solutions in the ultra clean vacuum sector, while at the same time putting its own claim into practice: Customized high purity vacuum solutions for every customer and every requirement!