Ultrahigh Vacuum Region Research Articles

Ultra-High Vacuum Cells Realized by Miniature Ion Pump Using High-Efficiency Plasma Source.

In recent years, there has been significant interest in quantum technology, characterized by the emergence of quantum computers boasting immense processing power, ultra-sensitive quantum sensors, and ultra-precise atomic clocks. Miniaturization of quantum devices using cold atoms necessitates the employment of an ultra-high vacuum miniature cell with a pressure of approximately 10-6 Pa or even lower. In this study, we developed an ultra-high vacuum cell realized by a miniature ion pump using a high-efficiency plasma source. Initially, an unsealed miniature ion pump was introduced into a vacuum chamber, after which the ion pump's discharge current, depending on vacuum pressures, was evaluated. Subsequently, a miniature vacuum cell was fabricated by hermetically sealing the miniature vacuum pump. The cell was successfully evacuated by a miniature ion pump down to an ultra-high vacuum region, which was derived by the measured discharge current. Our findings demonstrate the feasibility of achieving an ultra-high vacuum cell necessary for the operation of miniature quantum devices.

A planar quadrupole device for transmitting and trapping ions in high vacuum.

Hybrid mass spectrometers combine multiple mass analyzers to achieve optimal performance in terms of tandem mass spectrometry, high mass resolving power, and mass measurement accuracy for studying highly complex samples. As a result, the need for transport, trapping, and control of ion kinetic energies is critical for the successful integration of multiple mass analyzers and hybrid instrument operation. In addition, transportation of ion populations between two physically distinct locations can result in time-of-flight (TOF) discrimination against ions with widely disparate m/z values, compromising full mass spectral performance. In this work, we demonstrated a new ion guide, referred to as a planar quadrupole (PQ) ion guide, composed of two parallel printed circuit boards (PCB) that allow radiofrequency (RF) and direct current (DC) voltages to be combined to enable both axial transport and trapping of ion populations in the ultrahigh vacuum region of the mass spectrometer. As compared with a conventional multipole ion guide, the PQ ion guide showed comparable performance in ion m/z values, signal-to-noise, and intensity and effectively reduced mass discrimination caused by TOF effects. A PQ device was developed with two PCBs and simulated with SIMION 8.1. Electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry instrumentation were used for the testing of PQ performance. .In this work, we demonstrated a planar quadrupole (PQ) ion guide composed of two parallel PCB plates. The PQ enables both axial ion transport and trapping of ion populations throughout the ion transfer process from a LTQ to an ICR cell. As compared with a conventional multipole ion guide, the PQ showed comparable ion transmission efficiency and effectively reduced mass discrimination caused by TOF effects. The PQ is a simple design that can be implemented for ion transmission and trapping on virtually any mass spectrometer.

Nano-optomechanical Resonators for Sensitive Pressure Sensing.

Nanomechanical sensors made from suspended graphene are sensitive to pressure changes. However, these devices typically function by obtaining an electrical signal based on the static displacement of a suspended graphene membrane and so, in practice, have limited sensitivity and operational range. The present work demonstrates an optomechanical Au/graphene membrane-based gas pressure sensor with ultrahigh sensitivity. This sensor comprises a suspended Au/graphene membrane appended to a section of hollow-core fiber to form a sealed Fabry-Pérot cavity. In contrast to conventional nanomechanical pressure sensors, pressure changes are monitored via resonant sensing with an optical readout. A miniature pressure sensor based on this principle was able to detect an ultrasmall pressure difference of 1 × 10-7 mbar in the ultrahigh-vacuum region with a pressure range of 4.1 × 10-5 to 8.3 × 10-6 mbar. Furthermore, this pressure sensor can work over an extended pressure range of 7 × 10-6 mbar to 1000 mbar at room temperature, outperforming commercial pressure sensors. Similar results were obtained using both the fundamental and higher-order resonant frequencies but with the latter providing improved sensitivity. This sensor has a wide range of potential applications, including indoor navigation, altitude monitoring, and motion detection.

Low-cost, high-performance nonevaporable getter pumps using nonevaporable getter pills

Nonevaporable getter (NEG) pumps are widely used for maintaining a clean ultrahigh vacuum (UHV) of ≤10−8 Pa because of their high pumping speeds for hydrogen (H2) and active gases in the UHV region. In addition, they are oil free, evaporation free, sputtering free, sublimation free, magnetic field free, vibration free, economical, compact, lightweight, and energy saving. In the present paper, the authors report a new NEG pump which is composed of commercial 60 NEG pills (ϕ10 × 3 mm; 70 wt. % Zr, 24.6 wt. % V, and 5.4 wt. % Fe), titanium parts, a DN 40 conflat flange, and a tantalum heater. The NEG pills are vertically and radially aligned around the heater to maximize the effective area for pumping. After activation at 400 °C for 30 min, the pumping speeds of the NEG pump were measured with the orifice method. Pumping speeds of 140–130, 200–140, 190–130, and 35–17 l/s were estimated for H2, CO, CO2, and N2 gasses, respectively, in a pumped-quantity range of 0.01–0.1 Pa l. Since the NEG pump is composed of a heating unit and a NEG module, the pumping speeds can be improved by increasing the number of NEG modules. These NEG pumps are favorable alternatives to sputtering ion pumps or titanium sublimation pumps.

Radio-Frequency Ionization of Organic Compounds for Mass Spectrometry Analysis

A new ionization technique: A radio-frequency signal was used to ionize neutral organic molecules in the ultrahigh-vacuum region of a Fourier transform ion cyclotron resonance mass spectrometer. Radio-frequency ionization (RFI) yielded signal/noise (S/N) ratios roughly six times higher than those generated by conventional 70 eV electron impact ionization (EI).

Comparison between Monte Carlo and analytical calculation of the conductance of cylindrical and conical tubes

The accurate calculation of transmission probabilities of ducts in the molecular flow regime has become a need of vacuum standards on the ultrahigh vacuum region. The usual approach is to simulate a molecular flow of molecules with a Monte Carlo method, because of the difficulties that arise trying to find a solution of the Clausing integral equation in a system of a given geometry. As modern computers increase their speed, the accuracy of Monte Carlo methods is getting higher. It is therefore important to compare simulation results with data obtained by other methods to check that pseudorandom numbers generated are uniformly distributed and not correlated. We have applied both a Monte Carlo method and numerical methods to solve the Clausing integral equation for cylindrical and conical tubes. Both Monte Carlo and numerical methods are consistent within estimated uncertainties. In the case of conical ducts, we obtain values which agree very well with values found in the literature and with uncertainties several orders of magnitude smaller. In the case of cylinders, very accurate transmission probabilities are obtained solving numerically the Clausing integral equation. Comparing with other values found in the literature using a variational method, our values are near the upper bound and quite far away from the lower bound.

In situ pressure measurements in small gettered volumes

In modern small optoelectronic devices like field emitter displays, miniature cathode ray tubes (CRTs), channel photomultipliers, etc., the vacuum requirements are much more stringent than in conventional electron beam devices. As there should be a pressure in the ultrahigh vacuum region and the volume is only a few cm3, a direct measurement is not feasible and is often estimated on the basis of the expected pumping speed of the getter. The present study was arranged to investigate the pressure in small CRTs (25 cm3) during a period of several months, namely after the conventional pumping and bakeout procedure, immediately after the activation of Ba getters and after the accumulation of some months. All the CRTs were equipped by a spinning rotor gauge ball. Two barium getter sizes were studied: St15/AM/O/9.5 and St15/AM/O/5, both made by SAES. After the evaporation by the prescribed procedure the pressure did not drop, but increased from p∼1×10−5 mbar up to p∼1×10−3 mbar, showing that the pumping speed was completely suppressed by forming of a nongetterable gas. When the same experiments were repeated inside identical glass bulbs connected with a valve to a quadrupole mass spectrometer, a formation of methane was observed. The initial rate just after the activation was Q∼10−8 mbar l s−1, but even after several hours it was still as high as Q∼10−9 mbar l s−1. By switching-on the cathode heater, methane was pumped by the getter after a precedent cracking procedure. The virtual pumping speed was directly related to the heater power, but independent of the getter area. Therefore, within the cathode warm-up period, methane was “pumped” and did not represent a harmful gaseous contaminant in a small electron beam device with a thermionic cathode.

Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.

Coupling an optical trap to a mass separator

The efficient coupling of a magneto-optical trap to a mass separator is being developed to undertake high-precision electroweak interaction measurements in a series of radioisotopes. The use of ion implantation and subsequent heated-foil release is being pursued as a suitable way of introducing radioactive samples into the ultra-high vacuum region of an optical trap without gas loading. In this paper we discuss the layout of the mass separator, the coupling to a magneto-optical trap, and the implantation and release scheme.

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.

Ultrahigh vacuum in the semiconductor industry

Vacuum processing equipment designed for high-end commercial silicon production does not operate in the ultrahigh vacuum region (10−9–10−12 Torr). However, for reasons of reducing process and gas impurities to the sub-part per billion level and increasing product throughput, UHV technology is required and is being incorporated into production equipment. This article reviews progress in the field of extremely clean device processing systems. It highlights progress in pump design, chamber design, operational procedures, and materials developments that have made today’s clean processes possible and suggests directions for improvements.

コンダクタンス変調法による超高真空での真空ポンプの排気速度測定

The conductance modulation method has three advantages useful for the measurement of pumping speed in ultrahigh vacuum region. First, the flow rate of gas for the speed measurement is not required if the rate is constant. Secondly, the absolute value of pressure in the test dome is also not necessary if the sensitivity of pressure gauge is unchanged during the measurement. Thirdly, the effects of the positions of pressure gauge and gas inlet in the test dome are relatively small.The orifice system composed of three discs is used as the variable conductance which has two definit values of conductance determined by the Monte Carlo calculation. One of the two values can be selected from outside of the ultrahigh vacuum chamber. The pumping speed of a combination pump made of a sputter ion pump and a titanium getter pump can be successfuly measured down to 10-7 Pa for nitrogen and hydrogen.

Pressure generation in the ultrahigh vacuum region: G Grosse and G Messer, Physikalisch Technische Bundesanstalt, Institut Berlin, Abbestr. 2-12, D 1000 Berlin, 10, FRG

Pressure generation in the ultrahigh vacuum region: G Grosse and G Messer, Physikalisch Technische Bundesanstalt, Institut Berlin, Abbestr. 2-12, D 1000 Berlin, 10, FRG

Aluminum alloy ultrahigh vacuum chamber for molecular beam epitaxy

An aluminum alloy molecular beam epitaxy chamber has been constructed and its vacuum properties tested. Prior to the construction an oil-free lathing technique was developed and was applied to the inner surface of the chamber, which resulted in an extremely low outgassing rate of the order of 10−11 Torr 1/s cm2 before bakeout and less than 1×10−13 Torr 1/s cm2 after a 100 °C, 45-h bakeout. This low outgassing rate of the material enabled the chamber to be pumped down from atmospheric pressure to the order of 10−10 Torr in 23 h of evacuation without bakeout and in only 6 h with bakeout. With this short pumpdown time and its great reduction in bakeout temperature and time together with its reduced weight and the low cost, the aluminum alloy ultrahigh vacuum system provides us with a remarkably convenient tool to access the ultrahigh vacuum region.

Quantitative measurement of the sticking coefficient for oxygen on nickel

The initial sticking coefficient and the saturation coverage in the chemisorption region for oxygen on evaporated nickel films, polycrystalline nickel foils and various nickel single crystals, have been determined quantitatively. The oxygen coverage was measured by calibration of the oxygen Auger signal with a quartz crystal microbalance. For absolute pressure measurements in the ultrahigh vacuum region, the ionization gauge was calibrated with a gas friction manometer. On very rough surfaces the value of the initial sticking coefficient is nearly 1, whereas on the flat Ni(111) plane it only reaches the value 0.12. Interestingly on Ni(111), the adsorption probability increases with increasing coverage up to 0.2 monolayers of oxygen. An adsorption model involving non-zero dissociation probability at an occupied adsorption site is used to explain this unusual behaviour.

Surface Reactions on Pd Studied With a Hydrogen Sensitive MOS-Structure and Photoelectron Spectroscopy

During the last couple of years catalytic reactions on Pd surfaces have been studied at our department by means of a hydrogen sensitive Pd-MOS structure and work function measurements. These studies have been performed at atmospheric pressures with Ar and O2 as carrier gases. We have now extended this type of measurements to the ultra-high vacuum (UHV) region and also combined them with both UV and, in certain cases, X-ray photoelectron spectroscopy (UPS, XPS) studies. With this technique we are able to detect about 0.001 of a monolayer of hydrogen, and changes in rate or equilibrium constants corresponding to changes in an energy coordinate of less than 10 meV. Furthermore, results from a Cd contaminated Pd-MOS structure indicates that the ability of Pd to dissociate H2 is related to a microscopic parameter and not to any general metal-like parameter.

Performance of mechanical vacuum pumps in the molecular flow range

An important and remarkable property of oil-sealed rotary vane mechanical vacuum pumps has been demonstrated. Such pumps retain their positive displacement pumping mechanism at partial pressures approaching the ultrahigh vacuum region. Pumping speed measurements for helium, based on partial pressure, have been made at an inlet pressure as low as 10−6 Pa with an ultimate partial pressure of helium near 10−8 Pa. Experiments conducted with a nominal 3 l/s two-stage pump (Varian 200) demonstrate that a constant pumping speed is maintained at very low helium concentrations as long as the presence of helium at the discharge of the pump is minimal, i.e., as long as the maximum compression ratio for helium is not exceeded. Similar results may be expected with other gases except the pump oil vapor or species outgassing from the pump.

Pumping behavior of sputter ion pumps

The ultrahigh vacuum requirements of ISABELLE are obtained by pumping stations, each consisting of a 1000 l/s titanium sublimation pump for active gases (N2, H2, O2, CO, etc.), and a 20 l/s sputter ion pump for inert gases (methane, noble gases like He, etc.). The combination of the alarming production rate of methane from titanium sublimation pumps (TSP) and the decreasing pumping speed of sputter ion pumps (SIP) in the ultrahigh vacuum region (UHV) leads us to investigate this problem. In this paper, we first describe the essential physics and chemistry of the SIP in a very clean condition, followed by a discussion of our measuring techniques. Finally, we present our measured methane, argon, and helium pumping speeds for three different ion pumps in the range of 10−6 to 10−11 Torr. The virtues of the best pump are also discussed.

New method for calibrating manometers in high- and ultrahigh-vacuum regions

New method for calibrating manometers in high- and ultrahigh-vacuum regions

Improvements in the use of a field emission microscope to measure hydrogen pressures in the ultra high vacuum region

Improvements in the use of a field emission microscope to measure hydrogen pressures in the ultra high vacuum region