AVS 66 Session VT-TuP: Vacuum Technology Poster Session

One of the biggest problems in surface analysis of gas sensors is the pressure gap; a conventional surface analytical tool needs high vacuum (10^-4 Pa), while gas sensors are usually employed in atmospheric pressure (10⁵ Pa).This problem has been partially overcome by recent operando techniques for surface characterization. In such a operando measurements, either effective differential pumping or a pressure separation technique, which is often called a high pressure cell, are typically utilized. With those techniques, surface analytical tool can be operated in (ultra-) high vacuum while keeping the pressure in the vicinity of a sample (near) atmospheric pressure. Those techniques are obviously useful to analyze gas sensing mechanism on surfaces. However, a high-speed evacuation system is expensive and a membrane for separating pressure limits applicable analytical techniques.

Besides above-mentioned operando measurements, a dynamic high pressure (DHP) technique has been proposed to analyze a device surface in operating condition. Briefly, it is a technique of pulsed-gas injection to a sample surface. The technique seems attractive considering economical cost and possible wide range of applicability. However, the number of reports concerning DHP is quite limited, and thus, it is not clear whether DHP is useful for surface analysis of gas sensors.

In the present study, we have developed a pulsed gas injection system of pure air combined with an ultra-high vacuum chamber and a fast pressure transducer. The pressure at the sample position reached at about 10³ Pa and 10⁴ Pa with pulse width of 10 ms and 100 ms, respectively, with the inlet pressure of 1 MPa. The background pressure was below 10^-2 Pa with pulse width of 10 ms except for the duration of 1 s after the gas injection. We further developed a gas sensor measurement system combined with the pulsed gas injection system. In our preliminary experiment using a W-ZnO thin film gas sensor, we successfully observed substantial change of electric resistance with introducing the pulsed pure air by using a lock-in technique.

Modifying the properties of surfaces has become essential to obtain a desired function in various ultra high vacuum (UHV) systems. Among such techniques, Ti-Zr-V non-evaporable getter (NEG) coating, originally developed at CERN^1,2) and being widely applied to particle accelerators, is one of the most promising functional coatings, as it provides high effective pumping speeds, low outgassing rates, and low secondary electron yields. Since these desirable properties are beneficial in any UHV systems, there has been an increasing demand for its widespread availability. Furthermore, NEG coating is expected to maintain UHV conditions in power-less situations; for example, its application to electron microscopes might enable long-sustained transportation and quick recovery to UHV. For these practical applications to general vacuum systems, we have started a fundamental study on NEG coatings, where the vacuum properties are measured by a build-up method and the durability of the pumping capacity is examined by repetitive cycles of air-exposure and activation. In order to establish a technique to deposit high-performance films on various vacuum chambers by magnetron sputtering, the coated surfaces are characterized by scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). The test tubes used in the build-up experiment are made of 304 stainless steel and measures 50 mm in diameter and 300 mm in length. One tube is coated with 0.7um Ti-Zr-V films and the other is uncoated. After 24 hours of stopping the sputter ion pump (SIP), the pressure in the uncoated tube increased from 2E-8Pa to 3E-6Pa, while the increase was suppressed from 6E-9 Pa to 1E-8Pa in the coated tube. Even after an additional build-up for 10 days, the coated tube was maintained under UHV conditions (7E-6Pa), and the pressure was recovered to 5E-8Pa in 5 hours after switching on the SIP. A comparison by residual gas analysis after the 24-hour build-up showed that the NEG coating improved 360-times for CO and 100-times for H₂. These results suggest a feasibility of the transportation of UHV systems without electricity. The presentation will include preliminary results of film characterization by the surface analyses, as well as pumping properties of the NEG coating.

References

1) C. Benvenuti et al., Vacuum 60 (2001) 57.

2) P. Chiggiato and P. Costa Pinto, Thin Solid Films 515 (2006) 382.

Jefferson Lab been using a DC sputtering system to coat the beampipe with a Ti-Zr-V non-evaporable getter coating for 20 years. A similar system has also been used to sputter coat large diameter chambers with ultimate pressures aproaching XHV(1). The approaching upgrade of the Jefferson Lab CEBAF injector will use NEG coated beampipes of varied diameters for the first 30 meters of the machine. We describe the improvements that are being made to the sputtering system to improve stability, increase monitoring capabilities, and improve NEG film adhesion and morphology.

1) M.L. Stutzman, P.A. Adderley, A.A. Mamun and M. Poelker, J. Vac. Sci. Technol. A 36 031603 (2018).

Designing a robust vacuum connection that can deliver UHV or XHV leak rates, withstand high temperature bake-outs and minimize hardware/bolting size can be difficult to achieve. The design process is complicated even further with Aluminum flanges or when a large diameter or shaped seal is required.

This presentation will review UHV metal sealing concepts and will detail a seal option that significantly reduces seating load by concentrating contact stress to a small area machined into the seal surface. This combination of load concentration and material selection allows for a helium tight seal with much less load than a traditional metal seal without damaging the corresponding flanges and hardware. The resilient seal loading mechanism also allows the seal to perform well in temperature cycles or bake-outs.

There are many factors to consider when comparing the overall suitability of different quadrupole-based gas analyzers for a given application. These can be categorized into two main areas, inlet/interface suitability and quadrupole mass analyzer suitability.

The suitability of the quadrupole mass spectrometer determines very important figures of merit such as precision, stability and detection limit. The quadrupole mass spectrometer includes the ionization method, the transmission characteristics, and the quality of the driving electronics.

Unfortunately, these figures of merit are often misrepresented in the commercial literature and it’s this confusion which we seek to address and clarify in this document by making a direct comparison between two different classes of quadrupole analyzers; a typical 6mm rod diameter RGA type instrument typical of many currently on the market, and a higher performance 19mm rod diameter quadrupole analyzer, typical of high end analytical analyzers used in research and industry. We compare these with nominally identical inlet/transfer conditions, so that only the mass spectrometer performance is under consideration. In doing so, we present a direct comparison as it relates the various figures of merit and attempt to remove some of the mystery surrounding confusing analyzer specifications so that potential users of this powerful analytical technique may query manufacturer specifications and therefore make more informed decisions.

The specifications that we will discuss are:

- Detection Limit (minimum and maximum detectable concentration)

- Speed of Analysis (measurement speed and response time)

- Analysis Precision (repeatability of measurements)

- Analysis Stability (long-term instrument stability)

- Dynamic Range (comparison of largest and smallest detectable signals)

We will study the above by assessing and comparing the performance of two instruments, the MAX300-CAT and the MAX300-LG. The MAX300-CAT is typical of the high-end RGA based gas analyzers, based upon 6mm quadrupole rod technology, whereas the MAX300-LG is a higher performing analyzer based on 19mm quadrupole rod technology and high-performance electronics.

Conventional production of microchannel plates (MCPs) produces linear channels. Our simulations promise a significant improvement in time resolution by changing the geometry of the channels from straight to a zig-zag (Z) configuration, which drives the electrons to land on a specific surface in each gain stage, yielding higher time resolution. The sensitivity is improved as well by controlling the outer shape of the pores; replacing the circular pore pattern with hexagon or square will increase the open area ratio for the same MCP diameter. 3D printing MCPs not only enables this Z channel approach, but is a cost-effective method to produce charged particle detectors.

3dMCPs were created using a 70-micron resolution stereolithography printer. Both linear and Z channels were printed with 30:1 length to diameter (l/d) ratio. Graphite and other dopants have been used to change the resistivity of the printed material. In principle, the 3dMCP can then be coated for secondary emissions using ALD technology. Optical microscopy of the 3dMCP and its cross-section show that good uniformity can be attained throughout once the process was optimized.

To facilitate low vacuum flow testing of the 3dMCPs, a microcontroller based multi-vacuum gauge has been constructed. This device fits on a 2.75" Conflat flange with a built-in pressure gauge and OLED display. The gauges chosen are a Pirani pressure sensor calibrated with a capacitance manometer. To digitize the signals and establish serial connections between the electrical parts, the Trinket M0 MCU was used with a built-in 12-bit ADC and a reference voltage of 3.3v. I have also designed an enclosure that could easily attach to a flange.

An apparatus was built to measure the gain and pulse output characteristics of the MCPs. TauZero (τ0) consists of an ultrahigh vacuum chamber with 2kV electron and 20kV Ga⁺ guns directed at multiple test 3dMCPs. The turbo pumped vacuum system is vibrationally isolated. τ0’s electronics suite includes a 6 ½ digit DMM, 16 GHz digital oscilloscope, spectrum analyzer, a time-to-digital converter with 12ps resolution, and a digitizer with pulse height analysis software.

One of the main concerns regarding 3d printed materials is whether the plastic used will contaminate a high vacuum system. 3dMCPs were outgassed in a vacuum chamber while monitoring with a residual gas analyzer to collect data during the outgassing process. Vacuum did not damage the 3dMCPs and a drop of volatiles over 24 hours was measured. We concluded that the 3dMCPs are high vacuum compatible.