Jens Harhausen

On plasma ion beam formation in the Advanced Plasma Source

The Advanced Plasma Source (APS) is employed for plasma ion-assisted deposition (PIAD) of optical coatings. The APS is a hot cathode dc glow discharge which emits a plasma ion beam to the deposition chamber at high vacuum (p 2 × 10−4 mbar). It is established as an industrial tool but to date no detailed information is available on plasma parameters in the process chamber. As a consequence, the details of the generation of the plasma ion beam and the reasons for variations of the properties of the deposited films are barely understood. In this paper the results obtained from Langmuir probe and retarding field energy analyzer diagnostics operated in the plasma plume of the APS are presented, where the source was operated with argon. With increasing distance to the source exit the electron density (ne) is found to drop by two orders of magnitude and the effective electron temperature (Te,eff) drops by a factor of five. The parameters close to the source region read ne 1011 cm−3 and Te,eff 10 eV. The electron distribution function exhibits a concave shape and can be described in the framework of the non-local approximation. It is revealed that an energetic ion population leaves the source region and a cold ion population in the plume is build up by charge exchange collisions with the background neutral gas. Based on the experimental data a scaling law for ion beam power is deduced, which links the control parameters of the source to the plasma parameters in the process chamber.

Modelling and Simulation of the Advanced Plasma Source

Plasma ion assisted-deposition (PIAD) is a combination of conventional thermal evaporation deposition and plasma-beam surface modification; it serves as a well-established technology for the creation of high quality coatings on mirrors, lenses, and other optical devices. It is closely related to ion-assisted deposition to the extent that electrons preserve quasineutrality of the ion beam. This paper investigates the Advanced Plasma Source (APS), a plasma beam source employed for PIAD. A field enhanced glow discharge generates a radially expanding plasma flow with an ion energy of about 80-120 eV. Charge exchange collisions with the neutral background gas (pressure 0.1 Pa and below) produce a cold secondary plasma, which expands as well. A model is developed which describes the primary ions by a simplified Boltzmann equation, the secondary ions by the equations of continuity and momentum balance, and the electrons by the condition of Boltzmann equilibrium. Additionally, quasineutrality is assumed. The mode...

Investigation on the reproducibility of optical constants of TiO 2 , SiO 2 , and Al 2 O 3 films, prepared by plasma ion assisted deposition

Titanium dioxide, aluminum oxide, and silicon dioxide layers have been prepared by plasma ion assisted electron beam evaporation employing the Advanced Plasma Source (APS). The refractive indices have been determined by spectrophotometry in order to quantify their reproducibility. Standard deviations in the refractive index turned out to be highest for titanium dioxide, and lowest for silicon dioxide. The refractive index reproducibility of titanium dioxide could be improved by replacing the commonly used BIAS voltage control concept by a novel alternative approach concerning ion beam power, termed JE. All these findings are discussed in terms of a model that considers the real oxide film as a binary mixture of a solid fraction with a small amount of pores, within the limits provided by the Wiener bounds.

Investigation of the refractive index repeatability for tantalum pentoxide coatings, prepared by physical vapor film deposition techniques

Random effects in the repeatability of refractive index and absorption edge position of tantalum pentoxide layers prepared by plasma-ion-assisted electron-beam evaporation, ion beam sputtering, and magnetron sputtering are investigated and quantified. Standard deviations in refractive index between 4*10-4 and 4*10-3 have been obtained. Here, lowest standard deviations in refractive index close to our detection threshold could be achieved by both ion beam sputtering and plasma-ion-assisted deposition. In relation to the corresponding mean values, the standard deviations in band-edge position and refractive index are of similar order.

Influence of supersonic ions and nonlocal electron kinetics on the sheath voltage in an expanding plasma

We present the investigation of the sheath potential in an expanding plasma. The properties of the expanding plasma are measured by means of a Langmuir probe. The obtained data is used to calculate the sheath potential and the electron distribution function. We show that the sheath voltage is typically about 40% lower than in a case that neglects supersonic ions and assumes a Maxwellian electron distribution. We explain the magnitude of the measured sheath potential by balancing the ion flux density calculated with an analytical model for the expanding plasma and the electron flux density calculated with the electron distribution function.

Prospects for the enhancement of PIAD processes by plasma diagnostics

The purpose of this paper is to present concepts for an improved control of plasma ion assisted deposition (PIAD) processes which are employed for the production of optical interference coatings. While the well established PIAD technique typically comprises methods for in situ monitoring of thin film properties, there is no detailed knowledge about plasma parameters which are the foundation of magnitude and stability of plasma assistance, however. We adopt optical emission spectroscopy (OES) and active plasma resonance spectroscopy (APRS) and present schemes for controlling radiance and electron density on a batch coater equipped with an Advanced Plasma Source (APS). In a repeatability experiment of a 5-layer quarterwave stack (QWS, SiO2/TiO2), characteristics of two plasma based control schemes are compared to those of a conventional approach. For the conventional process we find systematic drifts and shifts in time traces of monitored plasma paramaters which correlate to properties of the layer stack. By using the novel concepts, stability of plasma paramaters can be improved by a factor of up to 6, while repeatability of in situ QWS transmission is strongly enhanced, exhibiting no spectral shift and minimal variation in reflectivity.

Physics of the Advanced Plasma Source: a review of recent experimental and modeling approaches

The Advanced Plasma Source (APS), a gridless hot cathode glow discharge capable of generating an ion beam with an energy of up to 150 eV and a flux of 1019s−1, is a standard industrial tool for the process of plasma ion-assisted deposition (PIAD). This manuscript details the results of recent experimental and modeling work aimed at a physical understanding of the APS. A three-zone model is proposed which consists of (i) the ionization zone (the source itself) where the plasma is very dense, hot, and has a high ionization rate, (ii) the acceleration zone (of ~20 cm extension) where a strong outward-directed electric field accelerates the primary ions to a high kinetic energy, and (iii) a drift zone (the rest of the process chamber) where the emerging plasma beam is further modified by resonant charge exchange collisions that neutralize some of the energetic ions and generate, at the same time, a flux of slow ions.