Glow discharge techniques for conditioning high-vacuum systems

Abstract

A review is given of glow discharge techniques which are useful for conditioning vacuum vessels for high vacuum applications. Substantial development of glow discharge techniques has been done for the purpose of in-sttu conditioning of the large ultrahigh vacuum systems for particle accelerators and magnetic fusion devices. In these applications the glow discharge treatments remove impurities from vessel surfaces in order '"o minimize particle-induced desorption coefficients. Cleaning mechanisms involve a mixture of sputtering and ion-(or neutral) induced desorption effects depending on the gas mixture (Ar/Oj vs. H 2 ) and excitation method (DC, RF, and ECR). The author will review the methodology of glow discharge conditioning, diagnostic measurements provided by residual gas and surface composition analysis, and applications to vessel conditioning and materials processing. The ability of a glow discharge to enhance the chemical reactivity of the constitutive gases has given rise to numerous technical applications.Materials processing techniques involving glow-discharge-enhanced deposition 1 or etching 2 '^ processes have become essential for microcircuit fabrication.The subject of this review concerns a les3 well-known application of glow discharges: the cleaning and surface modification of vacuum vessels. Glow discharge cleaning (GDC) techniques have been developed primarily for the surface preparation of large vacuum vessels used with accelerator and magnetic fusion devices. However, GDC techniques are not limited to these historical applications and are useful for the surface preparation of any size vacuum vessel or constituent vacuum components used in ultrahigh vacuum (UHV) or vacuum systems requiring extreme cleanliness.In this review the historical development of GDC techniques is briefly described (Section 2). In Section 3, typical experimental arrangements are described. The major part of the review (Section M) is focus«c on the results of basic GDC studies and example applications from magnetic fusion devices and storage rings. These studies used quantitative residu..l gas and surface analysis measurements that illustrate the basic mechanisms and effectiveness of surface cleaning when GDC is applied to commonly used vacuum materials. Historical Development of GDCThe use of glow discharges for surface cleaning can be traced to studies This arrangement subjects the vessel wall to the ion bombardment which, as the later studies to be described have shown, removes surface and near-surface impurities from the vessel.The GDC technique was given its first wide exposure to the vacuum technology community with the development of Ar and Ar/0 2 GDC techniques^-12for the surface treatment of the first of several large proton storage rings'at CERN in the mid-1970's. Th-? techniques developed for the Intersecting Storage Ring (ISR) at CERN were ..ater adopted for use in other more recently constructed storage rings."" * Storage rings have several unique and stringent vacuum requirements. The operating pres...