Vac Sci Technol Research Articles

A modified numerical method for the accurate calculation of molecular flow transmission probabilities and density distributions of cylindrical tubes

Accurate numerical calculations of molecular flow transmission probabilities and density distributions of tubes are important to the benchmark problems of Monte Carlo solutions, dynamic expansion vacuum gauge calibration systems, and molecular beam formation studies. Although Nawyn and Meyer [published by van Essen and Heerens. J Vac Sci Technol 1976; 13:1183] have solved cylindrical tube problems by using the numerical method based on Clausing's equations, perhaps the calculated results still lack sufficient accuracy. In this paper, we propose a modified method that could achieve calculation accuracies of transmission probabilities as high as 10−11–10−13 in the range of L′/R ≤ 100 (where L′ is the tube length, R is the tube radius), which are more accurate than the results recently reported by Mohan et al. [J Vac Sci Technol A 2007; 25:758] and Gómez-Goñi et al. [J Vac Sci Technol A 2003; 21:1452].

Mold Fabrication for 3D Dual Damascene Imprinting

Previously, a damascene process based on nanoimprint lithography has been proposed (Schmid G M, et al. in J Vac Sci Technol B 24(3) 1283, 2006) to greatly reduce the fabrication steps of metal interconnection in integrated circuit. For such a process to become a viable technique, a mold having two pattern levels with precise alignment between them must be fabricated first. To this end, this work demonstrates a “self-aligned” fabrication process where the two pattern levels would be perfectly aligned if ignoring the noise during e-beam writing. The process is based on one EBL on a bi-layer resist stack, with the sensitivity for the top layer much higher than that of the bottom layer, which enables separate pattern transfer of the two pattern levels. Using ZEP-520A and poly(dimethylglutarimide) (PMGI) resists, we fabricated pillars having a diameter of 150 nm sitting on ridges having a width of 1.5 μm, which can be used to create via-holes and trenches for IC interconnect by nanoimprint lithography. The current process can also find applications in other areas that require two-level patterning with precise alignment between them.

Numerical study of a single blade row in turbomolecular pump

Research on turbomolecular pumps (TMPs) based on the analysis of single blade row has been carried out by a number of investigators Kruger CH, Ph.D. Thesis, Massachusetts Institute of Technology; 1960; Katsimichas S, Goddard AJH, Lewington R, Oliveira de CRE. J Vac Sci Technol A 1995;13:2954; Schneider TN, Katsimichas S, de Oliveira CRE, Goddard AJH. J Vac Sci Technol A 1998;16:175; De Simon M. Vacuum 1990;41:2021; Chu JG, Hua ZY. J Vac Sci Technol 1982;20:1101; Amoli A, Ebrahimi R, Hosseinalipour SM. Vacuum 2004;72:427; Sawada T. Bull Jpn Soc Mech Eng 1973;16:993; Joong-Sik Heo, Hwang Young-Kyu. Vacuum 2001;56:133; Sawada T. Bull Jpn Soc Mech Eng 1979;22:362; Amoli A, Hosseinalipour SM. Vacuum 2004;75:361; Sheng Wang, Hisashi Ninokata. Prog Nucl Energy 2005;47:664, but those calculations were either limited to free-molecule flow, or failed to obtain good results in the transition regime. Furthermore, they were usually focused on pumping performance and did not report detailed flow field characteristics. In this study, a three-dimensional direct simulation Monte Carlo (DSMC) code adopting an accurate intermolecular collision model, i.e., a generalized soft sphere (GSS) model, has been developed to simulate the single blade row of a one-stage TMP from free-molecule flow to transition flow without any geometrical simplification. Molecular velocities and position equations are deduced under a rotating frame with consideration for the Coriolis and centrifugal acceleration, and the number of sample molecules is tested for sensitivity. The validity of this study and its accuracy are verified through comparison with previous DSMC results and experimental data, and the deterioration of the maximum pressure ratio in the transition flow regime and the influence of the intermolecular collision model on the maximum pressure ratio calculation are analyzed with the aid of transmission probability. Pumping performance under differing geometrical parameters (clearance between the blade tip wall and the housing wall, and the spacing–chord ratio) is investigated, and detailed flow field characteristics, including the number of molecule–surface collisions, temperature and density distributions, particle traces of single gas components and gas mixtures are all obtained under zero pumping speed.

On the air damping of flexible microbeam in free space at the free-molecule regime

Predicting air damping is crucial in the design of high Q microelectromechanical systems. In the past, air damping of rigid microbeam in free space at molecular regime is usually estimated using the free molecular model proposed by Christian (Vacuum 16:175–178, 1966), air damping of flexible microbeam is estimated using the model proposed by Blom (J Vac Sci Technol B 10:19–26, 1992). The relation between the two models is Q Blom = 3Q Christian. In this paper, a general proof is presented that shows the Christian’s model is valid for the air damping of flexible microbeam in free space at molecular regime. By comparing with the experimental results available in the literatures (Blom et al. in J Vac Sci Technol B 10:19–26, 1992; Yasumura et al. in J Micromech Syst 9:117–125, 2000), we conclude that the Christian’s model is the best choice in predicting the air damping of flexible microbeam in free space at the molecular regime.

Cleaning stainless steel for use in accelerators—Phase 1

Particle accelerators such as the SRS at Daresbury Laboratory generally require pressures lower than 10 −9 mbar for satisfactory stored electron beam lifetimes. To achieve this requires that particular attention be given to the cleanliness of the stainless-steel vacuum vessels and other components of the vacuum system. Pre-installation cleaning is an important step in the process. Currently, trichloroethylene is employed at Daresbury Laboratory as the primary cleaning solvent of choice, having been chosen as the outcome of earlier studies of the effects of various cleaning techniques on outgassing and electron- stimulated desorption from stainless steel [Herbert JD, Reid RJ, Groome AE. J Vac Sci Technol A 1994; 12(4): 1767; Herbert JD, Reid RJ. Vacuum 1996; 47(6–8): 693]. However, in light of the re-classification of trichloroethylene by the European Union as a class two carcinogen, with consequent stringent and expensive safety precautions required in order to continue using it, a suitable, more economical, replacement is sought after. This paper, details the assessment of different cleaning agents against the two criteria important to particle accelerator vacuum systems. The criteria being room temperature thermal outgassing rates and electron-stimulated desorption yields. A variety of different cleaning agents, ranging from halogenated solvents to aqueous-based cleaning agents have been tested. A comparison of the performance of a selection of such cleaning agents is presented and a satisfactory alternative to trichloroethylene is proposed.

The barrier height inhomogeneity in identically prepared Au/n-GaAs Schottky barrier diodes

We have prepared small Au/n-GaAs Schottky barrier diodes (SBDs) using e-beam lithography (EBL), and obtained their effective barrier heights (BHs) and ideality factors from current–voltage (I/V) characteristics, which were measured using a conducting probe atomic force microscope (CP-AFM). Although the diodes were all identically prepared, there was a diode-to-diode variation: the effective BHs ranged from 0.795eV to 0.836eV, and the ideality factor from 1.025 to 1.101. Lateral homogeneous BHs were computed from the observed linear correlation between BH and ideality factor using the method of Schmitsdorf et al. [Schmitsdorf RF, Kampen TU, Mönch W. J Vac Sci Technol B 1997;15(4):1221]. These homogeneous BHs were also obtained from the fit to the experimental I/V characteristics of the current through a “patchy” diode. From our model, the barrier height in the patches and their diameter could be determined. It are however the homogeneous BHs which should be used to make theories of the physical mechanisms responsible for the Schottky barrier height of the metal–semiconductor combination considered.

A study of cleaning agents for stainless steel for ultra high vacuum use. Part 2: ESD results

Synchrotron Light Sources must operate in the Ultra High Vacuum (UHV) region (10 −9 mbar) in order to obtain satisfactory stored electron beam lifetimes. To achieve this requires meticulous attention to cleanliness. Previous work has been reported (J D Herbert et al., J Vac Sci Technol, A12, 1767 (1994)) in which various methods of chemical cleaning of contaminated stainless steel samples were shown to produce surfaces with suitably low thermal outgassing rates. The dominant source of gas production in a light source is, however, photon stimulated desorption (PSD). Therefore any satisfactory cleaning process must produce surfaces with a suitably low coefficient of PSD, i.e. the number of gas molecules emitted per incident photon. In this article results for the analogous process of electron stimulated desorption (ESD) for surfaces cleaned as previously reported (J D Herbert et al., J Vac Sci Technol, A12, 1767 (1994)), are presented. These results have permitted the establishment of a satisfactory routine cleaning process that is suitable for light sources.

Defect etching of bulk grown Hg 1− xCd xTe in different solutions

Samples for the investigation of microstructure were cut from Hg 0.8Cd 0.2Te ingots grown by the CRA method (Casting Recrystallization Annealing). The most commonly used etchant for the determination of microstructure has been the Polisar solution [Polisar et al, lzv Vuzov Fiz, 6 , 81 (1968)] (12 ml HNO 3, 5 ml HCl, l CH 3COOH, 1 ml H 2O, 0.02 ml Br 2). With this solution in our p-type material the existence of precipitate particles up to a density of 10 12 cm −3 has been found. This is in good agreement with the data in the literature [Anderson et al, J Vac Sci Technol, 21 (1982)]. Comparative investigations of microstructural properties of our Hg 0.8Cd 0.2Te were performed in a new defect etchant proposed by Hähnert and Schenk [ J Crystal Growth, 101 , 251 (1990)] [1 vol part conc HF, 1 vol part chromic acid, 1 vol part conc HCl, diluted with (H 2O)]. This etchant produces etch pits on all crystallographic orientations of surfaces and improves the revelation of grain boundaries, subgrain boundaries and other defects. In this paper a comparison of surfaces etched in both solutions is reported.

Collisional and chemically guided processes in low energy ion beam oxidation

It has been shown [Todorov and Fossum, J Vac Sci Technol, B6, 466 (1988)] that ultra-thin ( d ≈ 50 Å) oxide films can be grown on silicon surfaces at reduced substrate temperatures using oxygen-containing low energy ion beams ( E < 200 eV). The oxide film evolution with increasing fluence can be explained by a general model based on the competing processes of oxygen incorporation and sputtering. The delivery of the oxidizing species to the lower interface of the growing film is by means of implantation and radiation-enhanced diffusion. It is demonstrated that the ion range distributions calculated on the basis of ballistic atomic transport are insufficient to explain the experimental data. Analytical calculations and dynamic computer simulation are used to investigate the role of ballistic and chemically guided atomic transport and to assess the importance of radiation-enhanced mobility and reactivity.

Optical simulation of the beam flux distribution from molecular beam epitaxy effusion sources

On the basis of their previous work, the authors present new results on optical simulation of effusion molecular beams generated by MBE cells of cylindrical and conical shapes, with inclined surfaces of evaporated liquid charges. The obtained results show that application of a conical cell gives a molecular beamd with an intensity distribution with respect to the cell axis which is more symmetric than in the case of a cylindrical cell. The experiments presented in this paper have shown that the beam distributions obtained by the analytical calculations of Yamashita et al [ Jpn J Appl Phys, 26 , 1192 (1987)] for the same effusion cells are probably incorrect, while the relevant beam distributions obtained by Adamson et al [ J Vac Sci Technol, 87 , 487 (1989)] for the cylindrical cells with Monte Carlo calculations seem to be similar to the optical simulation data presented in this paper.

7445. Wall conditioning and leak localization in the Advanced Toroidal Facility: R A Langley et al, J Vac Sci Technol, A8, 1990, 3058–3062

7445. Wall conditioning and leak localization in the Advanced Toroidal Facility: R A Langley et al, J Vac Sci Technol, A8, 1990, 3058–3062

7159. A further study of the state of residual stress in TiN films made by physical vapor deposition methods: AJ Perry, J Vac Sci Technol, A8, 1990, 3186–3193

7159. A further study of the state of residual stress in TiN films made by physical vapor deposition methods: AJ Perry, J Vac Sci Technol, A8, 1990, 3186–3193

7586. Characterization of high purity GaAs films grown by molecular beam epitaxy from a solid As cracker: R Chow et al,J Vac Sci Technol,B8, 1990, 163–167

7586. Characterization of high purity GaAs films grown by molecular beam epitaxy from a solid As cracker: R Chow et al,J Vac Sci Technol,B8, 1990, 163–167

7581. Investigation of low growth temperature AlGaAs and GaAs using metal-insulator-semiconductor diagnostic structures: A C Campbell et al,J Vac Sci Technol,B8, 1990, 305–307

7581. Investigation of low growth temperature AlGaAs and GaAs using metal-insulator-semiconductor diagnostic structures: A C Campbell et al,J Vac Sci Technol,B8, 1990, 305–307

7664. The relation between lift-off of photoresist and the surface coverage of trimethylsiloxy groups on silicon wafers: a quantitative time-of-flight secondary ion mass spectroscopy and contact angle study: J J Ponjeé et al,J Vac Sci Technol, B8, 1990, 463–466

7664. The relation between lift-off of photoresist and the surface coverage of trimethylsiloxy groups on silicon wafers: a quantitative time-of-flight secondary ion mass spectroscopy and contact angle study: J J Ponjeé et al,J Vac Sci Technol, B8, 1990, 463–466

7473. Modeling of inhomogeneous film deposition and target erosion in reactive sputtering: S Kadlec et al,J Vac Sci Technol, A8, 1990, 1560–1565

7473. Modeling of inhomogeneous film deposition and target erosion in reactive sputtering: S Kadlec et al,J Vac Sci Technol, A8, 1990, 1560–1565

7681. Characterization of interfacial failure in SiC reinforced Si 3N 4 matrix composite material by both fiber push-out testing and Auger electron spectroscopy : J I Eldridge et al,J Vac Sci Technol, A8, 1990, 2101–2106

The influences of interfacial reaction on tensile strength of continuous SiC fiber (SCS-6, Textron Specialty Materials) embedded in the Ti-48 at% Al matrix consisting of γ-TiAl and α2-Ti3Al compounds were studied. Due to the heat-treatment at 1423 K for various times, four reaction layers were formed; (A) degenerated layer in the periphery of the fiber in which pitts were observed for prolonged heat-treating time, and (B) TiC, (C) Ti2AlC and (D) Ti5Si3 layers in the matrix side. The interfacial debonding between layer (B) and the originally coated C-rich layer observed in the fracture surface of composite samples suggests that the premature fracture of the layers (B)-(D) is not a main reason for the reaction-induced reduction in fiber strength in composites. The tensile strength of the fibers extracted from the composite samples, in which only the layer (A) was contained (the other reaction layers (B)-(D) were removed during extraction), was reduced with increasing heat-treatment time. The reduction could be attributed mainly to the premature fracture of the layer (A) and the pits on its surface.

7194. Film thickness distribution control with off-axis circular magnetron sources onto rotating substrate holders: comparison of computer simulation with pratical results: S Swann et al,J Vac Sci Technol, A8, 1990, 1299–1303

7194. Film thickness distribution control with off-axis circular magnetron sources onto rotating substrate holders: comparison of computer simulation with pratical results: S Swann et al,J Vac Sci Technol, A8, 1990, 1299–1303

7151. Development of an aluminum alloy valve for an extremely high vacuum system: K Itoh et al, J Vac Sci Technol, A8, 1990, 2836–2839

7151. Development of an aluminum alloy valve for an extremely high vacuum system: K Itoh et al, J Vac Sci Technol, A8, 1990, 2836–2839

7699. Backstreaming of a perfluorinated polyether pump oil—an X-ray photoelectron spectroscopy study: R Sherman and J Vossen,J Vac Sci Technol, A8, 1990, 3241–3243

7699. Backstreaming of a perfluorinated polyether pump oil—an X-ray photoelectron spectroscopy study: R Sherman and J Vossen,J Vac Sci Technol, A8, 1990, 3241–3243