Evaluation of the pumping performance of combinations of Ti–Zr–V–Hf alloy nonevaporable getter coatings with different morphologies, activation protocols controlled at cryogenic temperatures

Nonevaporable getter (NEG) coatings were originally proposed for use in vacuum chambers and specifically particle accelerators where they provide distributed pumping and minimize thermal, photon, and electron (and other particle) stimulated desorption and outgassing.^1–3 In addition to the established use within high energy physics applications, it is envisaged that this technology may be increasingly applied to other UHV/XHV applications. Different alloy coatings and morphologies are comprehensively described in the literature. The primary aim of the study reported here was to compare the pumping performance of the several different coating types described below.

It is possible to use a single element coating, such as Ti, Zr, and Hf; however the temperatures required to achieve full activation of the coatings are relatively high >300 °C. This may limit the use of these coatings to steel substrates. Lower activation temperatures (180–250 °C) compatible with aluminum substrates can be obtained using binary and tertiary alloys and, in particular, the tertiary alloy Ti–Zr–V is a commonly deployed solution.⁴ These alloys also provide relatively higher sticking probabilities. Quaternary alloy coatings may also be considered. One such alloy widely referenced in the literature is Ti–Zr–V–Hf,⁵ which has an activation temperature ≤160 °C and the highest recorded sticking probability of all readily available alloy coatings.⁶ 

A commonly adopted method for deposition of coatings is the use of magnetron sputtering from a wire target. It has been demonstrated that coatings prepared using alloy target wires rather than twisted single element wires produce an NEG coating with an improved uniform composition and reduced activation temperature.⁷ The thickness of the films prepared by magnetron sputtering from a wire target can commonly range from 0.1 up to 2 μm thickness, but thicker films in the range of 5 μm may also be developed. It has been demonstrated by studies on NEG coatings with thicknessess from 0.1 to 1.0 μm that for H₂, the initial sticking probability (as per ASTM standard F798-97) is almost directly proportional to thickness.⁸ For CO, the initial sticking probability does not depend on coating thickness, however, the sorption capacity is almost proportional to coating thickness. A reasonable inference is that even after allowing for the fact that CO adsorption needs a single surface site, whereas N₂ and CO₂ occupy more sites on the surface, the general trends for CO should apply to similar chemically active gases reacting at the getter surface, including the common residual vacuum components CO₂, O₂, and N₂. In the same study, it was also shown that the thinner the coating the stronger the observed degradation of the coatings’ pumping performance over several activations.

The morphology of the coating is also an important consideration, and this is influenced by the deposition conditions.⁹ The first configuration is a dense coating, which gives a uniform surface coverage and acts as a barrier between the bulk wall material and the vacuum; this is predicted¹⁰ to be the best for reducing outgassing. The second configuration is a columnar coating structure, which due to geometric differences provides a larger surface area and more open structure, which leads to higher sticking probability and terminal capacity. However, due to the more open structure, this configuration is predicted to provide a coating less effective in reducing outgassing. The third configuration, referred to as dual coating(s) in the following, is an initial dense coating on top of which a columnar structure coating is formed. This configuration is expected to provide the best performance in terms of both suppressing outgassing and maximizing the sticking probability and terminal capacity.

The work described in this report was carried out using NEG coated samples supplied by the Vacuum Solutions Group of AESTeC, STFC Daresbury Laboratory, UK. These NEG coated samples were prepared as internal coatings on CF63 210 mm length straight stainless steel (304L) tubes by magnetron sputtering. Alloy wire targets were employed with an equal atomic percent of each element. The resulting film thicknesses are estimated to be in order of 5 μm. Details of these samples are shown in Table I.

Initial measurements were made for H₂ and N₂ of sticking probabilities as a function of activation temperature. For N₂, the terminal capacity was also measured. Second, a study was carried out to investigate whether activation of Ti–Zr–V–Hf at 140 °C was achieved, through an assessment of the sticking probability as a function of the activation time at this temperature. Finally, an investigation into the behavior and performance of an activated Ti–Zr–V–Hf coated tube cooled to liquid N₂ temperature was conducted.

A dedicated test rig, shown in Fig. 1, was designed following the technique described by Malyshev et al.¹¹ to provide a method for the measurement of the sticking probability and terminal capacity of the coated samples at pressures typical for UHV applications. Consequently, the aim was to measure the sticking probability and terminal capacity at test gas pressures ≈1 × 10⁻⁹ mbar. This required a base pressure dominated by H₂ within the measurement chamber of <1 × 10⁻¹⁰ mbar along with individual partial pressures <1 × 10⁻¹¹ mbar for CO, CO₂, H₂O, O₂, and N₂. To achieve this base pressure both the gas injection chamber and the measurement chamber were pumped with an Edwards nEXT85H turbomolecular pump (TMP), during bakeout, NEG coating activation, and performance measurement. Both TMPs were in series with a further TMP backed by an Edwards nXDS15i scroll pump, providing a backing pressure to the system-attached TMPs <1 × 10⁻⁵ mbar.

The heating cycle for bakeout and activation following the installation of a new sample tube is shown in Fig. 2. At the end of the final activation heating cycle, additional cooling fans directed at the sample tube were turned on to rapidly cool the sample to <25 °C. This cooling stage was typically ≈1 h and the gas injection for the sticking probability tests was started ≈1.5 h after the activation heating finished.

For sticking probability measurements, a test gas was introduced into the gas injection chamber (as shown in the left-hand side of the chamber in Fig. 1) to achieve a target pressure of 2 × 10⁻⁹ mbar in the test measurement chamber (as shown in the right-hand side of the chamber in Fig. 1). The two chambers are separated by a 2 mm length, 1.5 mm diameter orifice. The pressure in both chambers was recorded using Leybold IoniVac IE514 extractor gauges, and the sticking probability and pumped gas quantity are calculated based on the measured pressure values as described in Sec. II B. The initial sticking probability was the value measured 180 s after the first injection of gas into the gas injection chamber. The terminal capacity is taken as the capacity at which the sticking probability reaches 5% of the initial value.

For all samples successive measurements were made at increasing activation temperatures; from 140 °C up to a maximum of 300 °C. The first experiment was made with H₂. Following the completion of each H₂ sticking probability experiment a 12-h activation step was conducted before carrying out a further test with N₂. The shorter second activation time following the H₂ test was made with the assumption that any residual H₂ adsorbed on the surface would either diffuse into the bulk or undergo desorption during this time and not influence the N₂ sticking probability. During this second activation time, only the coated section of the sample was heated. For a given coated tube sample, no further venting or bakeout of the test rig was performed. At the end of the N₂ measurements, the gas injection was stopped, and the chambers were allowed to recover to their first base pressures; the next 24-h activation at an increased temperature was then conducted.

It has been suggested that some level of activation can be achieved for Ti–V–Zr–Hf coatings at temperatures as low as 150 °C and significant levels of activation may be achieved with Ti–V–Zr coatings at 160 °C using extended activation times.^6,12 To investigate the feasibility of activation of Ti–V–Zr–Hf at 140 °C using extended activation times, additional measurements were conducted with a single tube sample; tube C. Before each activation cycle, the tube and chamber was purged with N₂ to atmospheric pressure and then re-evacuated. The bakeout and activation were then carried out with an increased activation time prior to conducting sticking probability measurements of H₂.

Some works have been reported on the investigation of the behavior of activated NEG coatings at cryogenic temperatures.^13,14 These works indicate that the sticking probability of H₂ and CO for a NEG coating is expected to increase as the coating is cooled down to cryogenic temperatures. This was investigated using a single sample, tube C, which was activated at 200 °C for 24 h and then cooled down to ambient laboratory temperature. A measurement of the H₂ sticking probability commenced, and the sample was further cooled by immersion in LN₂ for 2.5 h, then the flow of LN₂ was stopped and the sample was allowed to return to room temperature. A control measurement was made with a nonactivated NEG coated (also tube C). The temperature at the external surface of the sample tube was measured with a thermocouple, and an assumption was made that the NEG coating was at the same temperature as the measured surface value.

A greater accuracy in measurement can be achieved using test particle Monte Carlo (TPMC) simulations to characterize the relationship between the pressures at the point of measurement and the sticking probability of the NEG coated tube regions.¹⁶ A measurement was taken of the pumping speed of the TMP (according ISO 21360-1 5.2 orifice method) attached to the test measurement chamber, and this value was applied to TPMC simulations of the test rig conducted in the Molflow + program to relate the pressures measured by the extractor gauges to the sticking probability of the coated surface of the tube. These simulations assume that only the region of the tube between the flanges is activated and the sticking probability is constant across this region. The relationship between the pressure function, Υ, and the sticking probability, α, along the activated surface of the coated tube sample is shown in Fig. 3.

The results for the initial sticking probability of H₂ and N₂ for all coated samples as a function of activation temperature are shown in Figs. 4 and 5. The results for the N₂ terminal capacity for all coating samples as a function of activation temperature are shown in Fig. 6.

In general, the coating samples exhibit the pumping characteristics predicted. The use of a dense coating was measured to have the lowest sticking probability. The columnar coating on its own gave an increased sticking probability. The dual coating, combining a dense layer overlayed with a columnar layer, provides the highest sticking probability.

For a given coating structure and activation temperature, the Ti–Zr–V–Hf alloy is seen to provide the highest sticking probability, closely followed by the Ti–Zr–V alloy with the single element Zr providing the lowest sticking probability. At 250 °C, the Ti–Zr–V–Hf and Ti–Zr–V coatings are approaching full activation, whereas it is necessary to heat the single element Zr at a higher temperature to achieve a comparable sticking probability. In terms of general trends, the variation in the terminal capacity with coating structure and material closely follows the trends in variations observed for the sticking probability.

It is to be noted that a systematic error = 5 × 10⁻⁵ applies to all measured sticking probabilities, and this defines the smallest measurable sticking probability. For capacity measurements, we must also consider the uncertainty associated with the absolute pressure measured by the ion gauges from which we ascribe a 15% measurement error.

The results for the measured sticking probabilities following activation of tube C at 140 °C for an extended time are shown in Fig. 7. Although some evidence of partial activation is observed, even after an extended activation period of 7 days, the sticking probability is at least 1 order of magnitude below that recorded for the same tube activated at 250 °C for 24 h.

The results showing the variation in sticking probability of tube C activated at 200 °C for 24 h during cooling to liquid nitrogen temperatures and subsequent warming to room temperature are shown in Fig. 8. This shows an initial large increase in sticking probability as the coating is cooled from room temperature to 77 K. This increase is transitory.

As the sample is warmed up to room temperature, an initial decrease in the apparent sticking probability is observed, which was seen to drop to an apparently negative sticking probability. This may suggest an initial thermally driven desorption of H₂, from the surface of the coating as the sample warms. As the sample is further warmed, the sticking probability increases and approaches the original room temperature value.

Figure 9 shows the “control” measurement with the inactivated NEG coating, clearly showing no pumping. In addition, analysis of the data recorded from the RGA corresponding to the test data shown in Figs. 8 and 9 confirmed that within the measurement chamber, H₂ is the dominant gas species with a partial pressure ≈2 × 10⁻⁹ mbar and contributions from all other gas species up to a mass charge ratio, m/z 200 are below 10⁻¹¹ mbar. Based upon these two observations, we conclude that the behavior we observe in Fig. 8 is solely due to changes in the sticking probability of H₂.

The initial increase in sticking probability agrees with previous works,^13,14 however, neither of these works report a transient nature for this. This transient nature is not fully understood by the authors. Further investigations into the behavior at LN₂ temperatures are required to characterize the observed transient nature of the change in sticking probability.

The range of Ti, V, Zr, and Hf NEG coatings with different modular structures provided results consistent with expectations from other published data. For the dual coating structure with the same activation temperature, the Ti–Zr–V–Hf alloy was seen to provide the highest sticking probability, followed by the Ti–Zr–V alloy and then single element Zr.

At 250 °C, the Ti–Zr–V–Hf and Ti–Zr–V coatings approached full activation, whereas for the single element Zr, a higher temperature of 300 °C was required to achieve a comparable sticking probability.

For the Ti–Zr–V–Hf alloys, the dense coating gave the lowest sticking probability followed by the columnar dual coating, while combining a dense layer overlayed with a columnar layer, it gave the highest sticking probability. For activation temperatures increased from 140 to 300 °C, the sticking probability increased by a factor of ∼100 for H₂ and N_2. The terminal capacity for N₂ increased by a factor of up to 1000 for the same increase in temperature.

For the same coatings with an activation temperature of 140 °C, the H₂ sticking probabilities increased by a factor of 2 when the activation time increased from 1 to 7 days and that of N₂ increased by a factor of 6 in the same period. The significant increase in sticking probability for H₂ at LN₂ temperatures is in line with expectations reported in the literature. The transient nature of this change is not fully understood and further investigations into the behavior at LN₂ temperatures are required to characterize the observed transient nature of the change in sticking probability.

The authors would like to thank Reza Valizadeh, Andrew Vick, Anthony Gleeson, and Oleg Malyshev of the Vacuum Solutions Group of AESTeC, STFC Daresbury Laboratory, UK, for the preparation of the NEG coated tube samples

The authors have no conflicts to disclose.

Paul Smith: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (lead). Sam Lodge: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). Andrew Chew: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.