Chamber Background Research Articles

Phase evolution in boron nitride thin films prepared by a dc-gasdischarge assisted pulsed laser deposition

Boron nitride (BN) thin films were deposited on a Si (111) substrate using ArF pulsed excimer laser radiation for ablating a hexagonal BN (hBN) target and a de-gasdischarge for activating N 2. The present phases in the films were determined by Fourier transformation infrared spectroscopy (FTIR). The results show that BN thin films containing largely cubic BN (cBN) phase on Si(111) substrates were successfully prepared by this de-gasdischarge assisted pulsed laser deposition (PLD). The effects of the de-gasdischarge and the chamber background gas on the formation of the cBN thin films were analyzed. The reasons for the phase evolution from hBN to cBN induced by the de-gasdischarge are discussed. The results suggested that the de-gasdischarge has two effects on the formation of cBN thin films. On one hand, it activated N 2 and caused the formation of nearly stoichiometric BN films. On the other hand, it caused the activated ions to bombard the substrate and induced the formation of the cBN phase. The dual beam ion source assisted PLD method is proposed to be the most promising method in preparing cBN films.

Reactive ion etching of GaSb and GaAlSb using SiCl4

Reactive ion etching of GaSb and GaAlSb using pure SiCl4 was investigated. Etching rate and etching profiles were characterized as functions of working pressure, chamber background pressure, flow rate, and power density. The etching rate and profile of metal organic chemical vapor deposited-grown GaSb and GaAlSb thin films are strongly dependent on background pressure and applied power. These interesting characteristics can be applied to control selective or nonselective etchings for device fabrication. Etching profiles exhibited a high degree of anisotropy and smooth surface morphologies.

Numerical and experimental investigations of low-density supersonic jets of hydrogen

Low-density flow of molecular hydrogen from a small nozzle is studied using numerical and experimental techniques. The conditions in the nozzle indicate that non-equilibrium effects will significantly influence the flow. Therefore, the numerical analysis is undertaken using a Monte Carlo approach. The experimental studies employ spontaneous Raman scattering. Comparisons of the measured data and computed results are made for total number density, rotational temperature, and for the number density of the first rotational level. The numerical results are found to be quite sensitive to the rotational relaxation rate, and a strong degree of thermal non-equilibrium is observed at the exit plane of the nozzle. Comparisons between experiment and analysis permit estimation of the rotational relaxation rate for hydrogen. Investigations are also conducted for expansion of the supersonic jet into a finite back pressure. The interaction of the plume with the chamber background gas is found to form shock waves in both the simulations and experiments. This phenomenon is investigated further by increasing the background pressure. Direct comparison of the simulation results and experimental measurements is very favourable.

Fast, high temperature and thermolabile GC—MS in supersonic molecular beams

This work describes and evaluates the coupling of a fast gas chromatograph (GC) based on a short column and high carrier gas flow rate to a supersonic molecular beam mass spectrometer (MS). A 50 cm long megabore column serves for fast GC separation and connects the injector to the supersonic nozzle source. Sampling is achieved with a conventional syringe based splitless sample injection. The injector contains no septum and is open to the atmosphere. The linear velocity of the carrier gas is controlled by a by-pass (make-up) gas flow introduced after the column and prior to the supersonic nozzle. The supersonic expansion serves as a jet separator and the skimmed supersonic molecular beam (SMB) is highly enriched with the heavier organic molecules. The supersonic molecular beam constituents are ionized either by electron impact (EI) or hyperthermal surface ionization (HSI) and mass analyzed. A 1 s fast GC—MS of four aromatic molecules in methanol is demonstrated and some fundamental aspects of fast GC—MS with time limit constraints are outlined. The flow control (programming) of the speed of analysis is shown and the analysis of thermolabile and relatively non-volatile molecules is demonstrated and discussed. The tail-free, fast GC—MS of several mixtures is shown and peak tailing of caffeine is compared with that of conventional GC—MS. The improvement of the peak shapes with the SMB—MS is analyzed with the respect to the elimination of thermal vacuum chamber background. The extrapolated minimum detected amount was about 400 ag of anthracence- d 10, with an elution time which was shorter than 2s. Repetitive injections could be performed within less than 10 s. The fast GC—MS in SMB seems to be ideal for fast target compound analysis even in real world, complex mixtures. The few seconds GC—MS separation and quantification of lead (as tetraethyllead) in gasoline, caffeine in coffee, and codeine in a drug is demonstrated. Controlled HSI selectivity is demonstrated in the range of 10 1 to 10 4 anthracene/decane which helped to simplify the selective analysis of aromatic molecules in gasoline. The contribution of SMB to the operation of the fast GC—MS is summarized and the compatibility with conventional GC having a megabore column is shown. Splitless injections of 100 μL sample solutions for trace level concentration detection is also presented (with a conventional GC).

Development of the acoustics laboratory at Digital Equipment Corporation

Development of the acoustics laboratory at Digital Equipment Corporation began in late 1970’s. The goal was laboratory capabilities to support noise control engineering as part of computer product engineering. Technical and business requirements were identified and validated. Labs in other industries were visited. The best space available in the plant to construct the lab was at ground level, requiring a tight-fit design, then excavation, blasting, and innovative materials handling during construction. Absorption adjustments in the 200 cubic meter reverberant room and draw-away tests in the hemianechoic room resulted in qualification to ISO 3742 and ISO 3745, respectively; and chamber background levels have been quite acceptable. Instrumentation, computer tools, and lab practices have been continually enhanced since then for component and product data acquisition, diagnostics, data basing, reporting, product design computation and consultation, and installation noise prediction. The lab continues to achieve its objectives today in this competitive and rapidly evolving industry.

Development of a long life, 2 kHz repetition rate X-ray preionizer

A simple and rugged X-ray preionizer system has been developed that is capable of sustained operation at the 2 kHz level with a demonstrated lifetime of2.5× 108 shots and a potential lifetime of >109 shots. At the heart of the system is an advanced form of corona plasma cathode. Cathode wear is minimised by utilising adsorbates from the dielectric surface and not cathode material to form the surface discharge from which the electron beam is extracted. The preionizer chamber background pressure (which is directly proportional to the sticking rate of adsorbates) is found to be of crucial importance in maintaining good cathode emission at high repetition rates. The device emits a rectangular X-ray beam, 5 cm×40 cm, with an X-ray risetime of 20 ns, a FWHM of 60 ns, and a peak energy of 95 keV. Ionization levels of >107 cm−3 in 1 atm of Ne could be generated after the X-rays had passed through 2 mm of aluminium and 1 mm of steel. Good beam uniformity and reproducibility at 2 kHz were shown to be maintained with the aid of a segmented electrode charge collection chamber. The system should be especially suitable for preionizing lasers containing a strongly electron attaching gas mixture.

Multielement proportional chamber — a study of background in the Gran Sasso underground laboratory

We describe the counting and background characteristics of the multielement proportional chamber used for the search on two-neutrino and neutrinoless double beta decay of 136Xe in the Gran Sasso underground laboratory. Enriched xenon to 64% in the 136Xe isotope and natural xenon cleaned by ultracentrifugation were filled up to 9.5 bar and counted for 6210 and 5120 h, respectively. The reduction of chamber background using suitable software triggers is discussed in detail.

Oxygen and carbon incorporation in low temperature epitaxial Si films grown by rapid thermal chemical vapor deposition (RTCVD)

Using SIMS analysis, we have measured oxygen and carbon concentrations in epitaxial Si films grown between 550 and 900° C. The films were grown by rapid thermal chemical vapor deposition from SiH4 as well as several different SiH2Cl2 sources. We have found that at low deposition temperatures (∼750° C or lower), oxygen incorporation is first dictated by source gas impurities and then by residual chamber gases. For the case of SiH2Cl2, which can have substantial oxygen content due to its reactivity with H2O, oxygen concentrations of about 1020 cm-3 are typical at low deposition temperatures. SiH4, however, can be obtained in higher purity, and oxygen concentrations of 1018 cm-3 can be realized at low temperatures. At higher deposition temperatures (750-900° C), SiO volatilizes, leaving the films grown from all sources with low oxygen concentrations, typically less than 5 × 1017 cm-3. Carbon incorporation is much less of a problem since it is present to a lesser extent both in the chamber background and in the source gases. Carbon levels less than or equal to 1018 cm-3 can be obtained at all deposition temperatures greater than about 650° C. The performance ofp/n junctions is shown to degrade significantly for junctions grown below 850° C. We conclude that for growth of long lifetime Si films in the temperature range <800° C, that low residual H2O partial pressures (<10-10 Torr) are desired. Therefore, CVD chambers should be loadlocked and also capable of base pressures as low as about 10-9 Torr.

A new PIXE chamber for highly reproducible and automatized operation

A new PIXE chamber is presented which was designed for a high throughput of samples, ensured by a tight geometry of its components and an automatized analysis cycle. The choice of appropriate materials for chamber walls, components and especially collimators rendering a low intrinsic chamber background is discussed. The chamber enables high analytical reproducibility (< 1%) due to well defined and stable geometric conditions, but nevertheless offering a wide adaptability to various analytical applications: combination of PIXE with complementary ion beam analysis techniques is possible (e.g. RBS under 159 °) as is the variation of beam diameters between 0.5 and 10 mm, the choice of absorber foils and detector entrance collimators, and scanning of the sample.

A simple, controllable source for dosing molecular halogens in UHV

An electrochemical source for the reproducible generation and dosing of halogens in UHV systems is described. This approach minimizes corrosion damage to vacuum hardware and lends itself to the study of both submonolayer and multilayer halogen adsorption. The corrosive action of halogens on ultrahigh vacuum components has necessitated the development of molecular halogen beam sources, which allow in situ generation and dosing of fluorine, chlorine, bromine, and iodine, with a negligible increase in vacuum chamber background pressure. This communication describes the chlorine and bromine sources developed and routinely used in our laboratory. Iodine dosing may be accomplished using a source of similar design. Fluorine, however, because of its highly reactive nature, requires a somewhat different design of source, such as that recently described in detail by Bechtold.1

A Spark Emission Method for Hydrogen Containing Compounds Adsorbed on Aluminum Surfaces

The presence of hydrogen containing compounds as surface contaminants is the main source of porosity or weld defects in the welding of aluminum. In this study on the relationship of surface contamination to weld quality, the objective was to obtain a value that was indicative of the level of contamination that contributed to weld bead porosity. Of the analytical techniques evaluated, a helium—argon shielded high-voltage spark technique using the H-6563-Å line was the most reliable. A special glass-walled, controlled-atmosphere chamber was designed. The upper 1/8 in. tungsten electrode was off center in order to use two lower electrodes. The lower electrodes, a 1/8 in. diam tungsten electrode and a 1 × 1 1/4 × 1/4 in. thick aluminum sample block, were mounted on a synchronous driven rotating stage. This arrangement permitted the sparking of a 225° arc of sample surface in a fixed time. Following cleanup and after loading, the procedure consisted of a 20-min purge, a 1-min W—W spark, a 1-min W—Al spark, and a second 1-min W—W spark. The W—W sparking periods establish chamber background with respect to hydrogen. The H-6563-Å line served as the analytical line and Ar-6752-Å served as the internal standard line. The average of the H/Ar blanks was compared to the sample ratio. There was good correlation between intensity ratio values and weld defect data.

A Propane Bubble Chamber as a Low Level Counter for Tritium and CARBON-14

A bubble chamber has been operated as a counter with the propane filling being highly superheated so that it is sensitive to the low energy (average 6 kev) electrons emitted in tritrium beta decay. Experimental techniques are described using quartz chambers of 10 ml volume operated at 55°C. Three plateaus are observed in a plot of the chamber counting rate against the superheat pressure (superheat pressure equals vapor pressure at 55°C minus expansion pressure). These were due to alpha particles (30-40 psi) recoil protons (50-90 psi) and electrons (140-220 psi). The chamber background counting rate was stable and reproducible for long periods of time. The counting rate was shown to be proportional to the activity in a tritiated propane filling; the detection efficiency was 65% on a plateau from 200 to 220 psi superheat. Cosmic rays could be eliminated from the background counting rate using anticoincidence techniques. The background rate for a 10 ml quartz chamber shielded by 2 of borated paraffin and 8 of iron was 4.5 cpm. We conclude that bubble chambers of somewhat larger size are suitable for the measurement of very low specific activities of such isotopes as carbon-14 and tritium with a sensitivity comparable to the best available gas counting systems.

Steel chamber background, autoabsorption and geometry in Anthropo-Spectrometry

Steel chamber background, autoabsorption and geometry in Anthropo-Spectrometry