The Bohm criterion is studied experimentally in the case of a two ion species plasma. Measurements are carried out in Ar and Ar+He plasmas (PA(r I) approximately 0.1 mtorr, 0< or =P(He)/P(Ar)< or =25, and 0< or =n(+)(He)/n(e)< or =0.5, T(e)< or =2 eV) created in an unmagnetized dc hot filament discharge confined by surface multidipole magnetic fields. Laser-induced fluorescence (LIF) measurements of Ar II ion velocity distribution functions (ivdfs) within the presheath up to the sheath edge show that the ions reach the sheath edge traveling faster than their individual Bohm speed by more than 75%, approaching a speed equal to the ion sound speed of the system.

Abstract-The presheath located near boundaries in weakly ionized plasmas is a rich environment in which charge exchange, and ion-ion streaming instabilities combine to establish the electric fields that accelerate ions to close to the Bohm velocity at the sheath/presheath boundary. Charge exchange sets the presheath scale length in weakly collisional plasma, in which ionization can be neglected. The transition of mobility limited ion flow near the bulk plasma to free fall motion close to the plate for single species plasmas is explored. Measurements in argon-helium multidipole plasmas of plasma potential with emissive probes and ion energy distribution functions with laser induced fluorescence are presented. These data show that the argon ions are speeded up by the presheath electric fields, argon ions are heated, and ion-ion instability is present as ions approach the boundary.Index Terms-Bohm velocity, charge exchange, free fall motion, ion-ion streaming instability, ion velocity, mobility limited flow, presheath, sheath, weakly ionized plasma.

Diode lasers have proved to be a valuable light source for laser-induced fluorescence (LIF) measurements for plasma science since the early 1990s, and they have recently improved the state of the art of measuring ion flow from ion velocity distribution functions (ivdfs) at the sheath -presheath boundary in single and multiple ion species plasmas. In the case of a low temperature two ion species plasma (ArI + HeI), we were the first to show experimentally that ion species may reach the sheath edge flowing at a very different speed than that expected from the single species Bohm Criterion (ArII ions exceed the individual Bohm flow speed by almost a factor of 2 at the sheath edge). Simulation results are found to agree. Diode laser technology relevant to LIF measurements in multiple ion species plasmas is discussed with the aim of addressing outstanding problems in sheath formation in such plasmas. D

Articles you may be interested inFlux and energy analysis of species in hollow cathode magnetron ionized physical vapor deposition of copper Rev. Sci. Instrum. 81, 123502 (2010); 10.1063/1.3504371In situ plasma diagnostics study of a commercial high-power hollow cathode magnetron deposition toolThe quality of physical vapor deposition (PVD) grown barrier/seed interface in Cu interconnect metallization was significantly improved by enhancing Cu nucleation on the Ta barrier surface. This was accomplished through filtering of nonenergetic species from the deposition flux, increasing the fraction of Cu ions, improving metal ion flux uniformity, and minimizing gas ion bombardment of the growing film. The self-sputtering ability of Cu was combined with a magnetically confined high-density plasma in the Novellus hollow cathode magnetron (HCM V R ) PVD source. Spatial profiles of plasma density and temperature, as well as ion flux, metal ion fraction, and ion energy, were measured by planar Langmuir probes, quartz crystal microbalances, and gridded energy analyzers, all located at the wafer level. Multiple criteria, such as seed step coverage and roughness, the seed layer's resistance to agglomeration, and its stability in the plating bath, have been used to evaluate interface quality. As a result, a new and improved Cu PVD process which demonstrates superior stability during subsequent process steps and ensures successful electrofill performance with a more than 50 % reduction in minimal requirement of field thickness as well as sidewall thickness has been developed.

To meet the stringent requirements of interconnect metallization for sub-32 nm technologies, an unprecedented level of flux and energy control of film forming species has become necessary to further advance ionized physical vapor deposition technology. Such technology development mandates improvements in methods to quantify the metal ion fraction, the gas∕metal ion ratio, and the associated ion energies in the total ion flux to the substrate. In this work, a novel method combining planar Langmuir probes, quartz crystal microbalance (QCM), and gridded energy analyzer (GEA) custom instrumentation is developed to estimate the plasma density and temperature as well as to measure the metal ion fraction and ion energy. The measurements were conducted in a Novellus Systems, Inc. Hollow Cathode Magnetron (HCM(TM)) physical vapor deposition source used for deposition of Cu seed layer for 65-130 nm technology nodes. The gridded energy analyzer was employed to measure ion flux and ion energy, which was compared to the collocated planar Langmuir probe data. The total ion-to-metal neutral ratio was determined by the QCM combined with GEA. The data collection technique and the corresponding analysis are discussed. The effect of concurrent resputtering during the deposition process on film thickness profile is also discussed.