Standard High Vacuum Research Articles

Treatment of surfaces with low-energy electrons

Electron-beam-induced deposition of various materials from suitable precursors has represented an established branch of nanotechnology for more than a decade. A specific alternative is carbon deposition on the basis of hydrocarbons as precursors that has been applied to grow various nanostructures including masks for subsequent technological steps. Our area of study was unintentional electron-beam-induced carbon deposition from spontaneously adsorbed hydrocarbon molecules. This process traditionally constitutes a challenge for scanning electron microscopy practice preventing one from performing any true surface studies outside an ultrahigh vacuum and without in-situ cleaning of samples, and also jeopardising other electron-optical devices such as electron beam lithographs. Here we show that when reducing the energy of irradiating electrons sufficiently, the e-beam-induced deposition can be converted to e-beam-induced release causing desorption of hydrocarbons and ultimate cleaning of surfaces in both an ultrahigh and a standard high vacuum. Using series of experiments with graphene samples, we demonstrate fundamental features of e-beam-induced desorption and present results of checks for possible radiation damage using Raman spectroscopy that led to optimisation of the electron energy for damage-free cleaning. The method of preventing carbon contamination described here paves the way for greatly enhanced surface sensitivity of imaging and substantially reduced demands on vacuum systems for nanotechnological applications.

Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04eV. The second process, slow tritium outgassing with a time constant of 200–300h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60years.

Cr, Mo and W used as VPO promoters in the partial oxidation of n-butane to maleic anhydride

The present work addresses the issue of the promoter effect produced by group VIB elements added to the basic VPO formulation using different methods and loads in order to obtain information linking the catalytic behavior of the solids with their physicochemical properties. The unpromoted VPO precursor (VOHPO 4·1/2H 2O) was prepared from V 2O 5 reduced with an alcoholic mixture and 100% H 3PO 4. Cr, Mo or W were either impregnated on the VPO precursor or co-precipitated during the precursor synthesis. The precursors containing varying loads of the promoter were activated following a standard procedure in the same flow micro-reactor where the catalytic performance was evaluated. The precursors and the catalysts were characterized by X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). Acetonitrile was used as a probe molecule to test the strength of the Lewis acid sites monitored by FTIR. The amount of chemisorbed acetonitrile was measured in a standard high vacuum system. The addition of the promoters invariably increased the catalytic activity but in every case there was an optimum load to achieve the best selectivity. The origin of this maximum could be ascribed to the right balance between the presence of very strong Lewis acid sites and the development of V 5+ isolated sites in the matrix of the vanadyl pyrophosphate, by far the major crystalline phase present in the catalyst. The three group VIB promoters affect the balance between acid and redox properties but increase the overall activity in all cases.

Ultrahigh vacuum apparatus for quartz crystal microbalance measurements in the temperature range 4–400 K

We describe an apparatus for friction experiments on molecularly thin films carried out with the quartz crystal microbalance technique (QCM) in ultrahigh vacuum (UHV) and at a temperature as low as 4 K. Through the implementation in the UHV chamber of standard cryogenic procedures, the temperature of the quartz can be controlled with a resolution better than 5 mK below 10 K. The main chamber is provided with a stainless steel jacket that allows a quick change of the various temperature inserts (cryocooler head, liquid nitrogen insert, Peltier-cell stage, etc.) that span the working range 4–400 K, without ever breaking the vacuum. The crystal mounting is compatible with UHV conditions and with the Omicron standard and guarantees a good thermal and electrical contact over the entire temperature range. Once inserted in the copper sample holder, the quartz frequency and amplitude stabilities are found to be as good as the best recorded values achieved with QCMs mounted in standard high-vacuum, liquid bath cryostats. This apparatus can also be used for general QCM investigations in surface science, like the determination of film growth in deposition and sputter processes, the stoichiometry of submonolayer oxide films, and the measurement of adsorbate sticking probability.

A “LEGO” Hybrid Photon Detector – assembled from standard mass-produced vacuum components

Motivated primarily by the MAGIC atmospheric Cherenkov telescope project, we have developed a “LEGO” Hybrid Photon Detector (HPD) comprising excellent focusing properties and protection against positive ion feedback. LEGO-HPD is supposed to be assembled from standard high vacuum components, which insures simplicity in the assembly procedure and reliability of operation.

A primary standard high vacuum calibration station for industrial applications

The Metrology Laboratory at Lockheed Martin Missiles & Space in Sunnyvale, CA, has developed a primary standard high vacuum calibration station. Maintaining an in-house capability for calibrating Bayard–Alpert gauge tubes and spinning rotor gauges (SRGs) at the primary level eliminates the need for external sources of calibration. This not only significantly reduces costs but also provides for continuous monitoring of the reference standards used to support research programs and manufacturing processes. A constant volume, pressure drop flowmeter is used to generate known pressures in a vacuum chamber partitioned by an orifice of calculable conductance. Data are presented which show vacuum chamber pressure to be directly proportional to flowmeter pressure drop in the three decades of vacuum spanned by the SRGs (10−3–10−6 Torr). Using a SRG calibrated at the National Institute of Standards and Technology, the proportionality constant is determined and shown to agree well with the constant calculated from system fixed parameters and measured pressure ratios. A new design of flowmeter is discussed. An estimate of when the system will be fully utilized as a primary reference will be made.

Ion beam assisted deposition of Al films on Si

Ion beam assisted deposition (IBAD) was used to grow thin Al films (∼100 nm thick) onto Si substrates at room temperature. Al was deposited by electron gun evaporation, while an Ar+ ion beam bombarded the film during the growth. Ion beam energies of 300, 500, and 650 eV were used, with current densities ranging between 4 and 20 μA/cm2. Polycrystalline Al films have been obtained, whose structural properties were strictly related to the IBAD parameters (ion current and energy). Samples were analyzed by transmission electron microscopy and Rutherford backscattering spectrometry, pointing out the following effects: (1) IBAD produces smaller polycrystalline Al grains with respect to a film evaporated in standard high vacuum conditions; (2) the Al grain size decreases by increasing the ion current or decreasing the ion energy; (3) sputtering yields during IBAD are higher with respect to a postdeposition bombardment process, and show an inverse trend with the ion beam energy. Results have been explained in terms of modification of kinetics of growth induced by IBAD. A simple model is also proposed to take into account the unexpected result about the sputtering yield. Finally, the IBAD capability to modify the microstructure of the Al films was used to improve the coverage of artificially stepped substrates.

Low temperature epitaxial silicon film growth using high vacuum electron-cyclotron-resonance plasma deposition

We report on the growth technique and electrical properties of epitaxial Si films grown at low temperatures using an electron-cyclotron-resonance plasma deposition technique. We have used standard high vacuum apparatus to grow high quality films at 450–525 °C. A critical step in achieving high quality films is an in situ hydrogen plasma cleaning of the wafer before growth. We have systematically studied the influence of ion bombardment during growth by biasing the substrate, and find that the films are crystalline for substrate bias voltages less negative than about −15 V, but become polycrystalline as the magnitude of the negative bias is increased. The crystallinity of the film was measured using Raman spectroscopy. The undoped films are n type with carrier concentrations in the 1016–1017 cm−3 range. The Hall mobilities measured for the films are comparable to values obtained in bulk Si crystals. We can achieve abrupt profiles in carrier concentrations between the heavy doped substrate and the epilayer, with no evidence of diffusion.

Monte Carlo analysis for the appropriate gauge location for an American Vacuum Society standard high vacuum pump speed measurement dome

A Monte Carlo molecular flow simulation was performed on an AVS standard high vacuum pump dome to determine the appropriate location of a pressure gauge to measure the speed of a high vacuum pump. The location was chosen to match the molecular flux at the gauge with the molecular flux at the pump inlet. The resulting measured pumping speed will be equivalent to the pumping speed measured on an infinitely large dome, the so‐called ‘‘intrinsic’’ pumping speed. Simulations were run on straight tubes and found to agree with available literature information. Simulations were run on an AVS standard high vacuum speed measurement dome to determine the appropriate gauge location as a function of pump capture coefficient, gas inlet tube geometry, and pressure gauge tubulation. The appropriate gate location for a 4 in. pump speed dome with a 3/8 in. o.d. tubing gas inlet tube 1 in. long after the inlet tube elbow is 1.05±0.03 R (90% confidence interval). The value was independent of pump capture coefficient, but a slight dependency on inlet tube geometry was found. The effect of gauge tubulation was also examined. Use of the results of the analysis in determining a ‘‘recommended practice’’ for pump speed measurement is mentioned.

Thermal cycling and torque analysis of an ultrahigh vacuum flange

A major design objective of a vacuum assembly in a specialized ultrahigh vacuum exhaust/leak detection processing system was to incorporate replaceable vacuum sealing flange ports. The vacuum processing operation with which the system was to be used required that these ports be capable of repeatable thermal cycling, from ambient to 525±25 °C and be torqued between the range of 17 to 50 ft lb prior to each thermal cycle while guaranteeing a leak-free seal to less than 1×10−12 std cm3/s. This design concept was proven to be feasible through computer analysis and documented experiments. One of the major breakthroughs was the employment of nickel metal gaskets for sealing as opposed to OFHC copper (standard high vacuum gasket material). This nickel gasket was proven to be vastly superior to copper in its sealing reliability with respect to thermal cycling at elevated temperatures. Torque analysis results have shown that these ports could be torqued to as high as 50 ft lb, both at ambient and elevated temperatures, and still be leak free.