AVS2014 Session VT-TuM: Gas Dynamics, Modeling, and Pumping Systems

The excellent cooling capabilities of the electrostatic chuck enable high power plasma etching of modern semiconductor devices. In order to maintain a uniform temperature across the silicon wafer, a thin layer of helium is inserted between the wafer and chuck. Some of this gas leaks out at the wafer edge, and the resulting flow of helium can lead to pressure drops that compromise the heat transfer uniformity of the chuck. Fluid dynamics modeling of the helium distribution is often used in the design phase to ensure uniform pressure under various scenarios.

Because of the small gaps and low pressures, the gas behind the wafer is in the transitional or molecular flow regime. Modeling electrostatic chuck designs then requires using very computationally expensive methods such as Direct Simulation Monte Carlo (DSMC). In this paper, a simplified modeling approach is developed that allows the pressure distribution to be modeled as a two-dimensional conductance problem. This is done by extending the conductance calculations used in one-dimensional vacuum piping networks. The accuracy of the method is compared to molecular flow modeling. The method is then used to model some chuck configurations.

Electrospray ionization (ESI) is a versatile method for creating gas phase ions from solution while maintaining the native chemical functionality of the solute. Using this method, functional bio- and macromolecular thin films can be produced for use in biosensors, scaffolding for tissue generation, photovoltaics and other emerging fields of research. The Macromolecular Patterning System, designed and constructed at the University of South Florida (USF), utilizes ESI as a material source to create such films. The system is comprised of three differential pumping stages, each containing custom designed electrodes used to define the trajectory of the ions. The focus of this study is on the first of the three stages which contains a radio frequency (RF) ion funnel. Computational fluid dynamics (CFD) simulations of the air flow into this chamber were performed and coupled with simulations calculating the generated electric field. Using the ion trajectory simulation software SIMION, the flight paths of ions within this first chamber were calculated. Experiments were then performed to test the results of the simulations. A “randomization parameter” based on the turbulence kinetic energy of the CFD simulations was used to model the time-varying component of the flow velocity yielding a result that closely matched the system. Variation of electrode voltages in the physical apparatus yielded similar results to those obtained from the simulations. Most significantly, the overall trend and peak values of ion transmission were accurately predicted from the simulations.

The Vacuum Surfaces and Coatings (VSC) Group at CERN is involved in several large projects, either at CERN or in collaboration and/or support of other laboratories. The design of new vacuum chambers and components is taking a considerable amount of time of many physicist and engineers in the VSC. New accelerators are being designed, either for fabrication and installation in a short time or as part of the European strategy for future accelerators. Two extreme examples are: 1) The ELENA project (Extra-Low Energy Accelerator ring) a ~ 30 m circumference 100 keV anti-proton decelerator aimed at increasing the anti-proton production from the existing AD machine (Antiproton Decelerator ring). ELENA will require an average pressure better than 3E-12 Torr. ELENA is under design and construction now, with expected start of commissioning in 2016. On the other side of the spectrum are the TLEP and HE-LHC machines, which are part of the Future Circular Colliders lepton-lepton and hadron-hadron (FCC-ee, FCC-hh) versions of colliders aiming at greatly improving the production of Higgs and W/Z0 bosons, and top quarks (FCC-ee), and raising the center-of-mass energy in the 50+50 TeV range (FCC-hh). Such conceptual machines would require circumferences in an unprecedented 80~100+ km range. The FCC-ee versions would generate of the order of 50 MW of synchrotron radiation (SR) for 175+175 GeV electron-positron beams, and the FCC-hh would generate as well a considerable amount of SR in the 4~5 keV critical energy range, thanks to 16-20 tesla superconducting magnets. The already approved High-Luminosity upgrade of the LHC (HL-LHC) and this FCC program will require a thorough upgrade of the injector chain, composed of some accelerators which are today exceeding the 50 year mark. In parallel, there are accelerators like the HIE-ISOLDE upgrade of the ISOLDE machine, aimed at post-acceleration of radioactive ion-beams, and new-concept experiments like the AWAKE plasma-acceleration project. In order to tackle all these projects, the VSC group has decided to develop several numerical analysis tools, namely test-particle montecarlo (TPMC) codes and the electrical-network analogy (ENA) (implemented via the Ltspice freeware). This paper will briefly describe the various codes developed (Molflow+ for molecular flow, SYNRAD+ for SR, and McCRYO-T for radiative heat exchange) and the ENA approach. It will then show some examples of the application of these codes to CERN projects and also comparison and benchmarking with results published in the gas dynamics field and dedicated experiments carried out at CERN.

References

1. F. Sharipov, Numerical simulation of rarefied gas flow through a thin orifice. J. Fluid Mech. Vol.518, pp. 35-60 (2004).

2. F. Sharipov and J.L. Strapasson, Ab initio simulation of rarefied gas flow through a thin orifice. accepted in Vacuum.

Keywords: DSMC, rarefied flow, particle contamination, load lock

Non Evaporable Getter (NEG) pumps are commonly used when large pumping speeds for H2 and active gases (i.e., H₂O, O₂, CO, CO₂) are required. The NEG pumps are very small, lightweight, with reduced magnetic interference, cause no vibration, and consume negligible power. Thanks to these qualities NEG pumps are widespread in many UHV applications.

Nevertheless, some factors still limit an even wider diffusion of NEG pump technology, mainly related to the topics of particle release and gas evolution during activation. Indeed, as a precaution, NEG pumps are generally not used in application requiring particle-free environment. Only recently, measurements performed at a large accelerator facility have shown the compatibility of St172^® alloy sintered getters in particle free environments, i.e RF cavities, after suitable treatments. Also, it is well known during getter activation hydrogen as well as other physisorbed species are desorbed causing pressure increase up to the 1e-6 mbar range. In some applications this is seen as a problem due, for example to constraints in pumping efficiently hydrogen away from long and narrow chambers.