SAES experience in NEG coating of narrow gap insertion devices and small diameter chambers for accelerators

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Winfred Preston
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1 SAES experience in NEG coating of narrow gap insertion devices and small diameter chambers for accelerators Gero Bongiorno*, Paolo Manini, Enrico Maccallini, Fabrizio Siviero, Tommaso Porcelli, Stefano Raimondi SAES Getters S.p.A. Viale Italia 77, Lainate (Milan) Italy

2 Outline Narrow gap insertion devices : status of the activity at SAES The new challenges : < 10 mm diameter NEG coated tubes Status of the R&D activity Results of the characterizazion of the coated chambers Critical issues and challenges Next steps Conclusions 2

3 Narrow gap IDs : SAES experience SAES getters has been pioneering NEG technology since the 60s, developing a variety of NEG alloys, getter related technologies and NEG solutions which have found wide use in a variety of industrial and research applications (X ray tubes, lamps, sealed off devices, accelerators, purifiers for the semiconductor industry ect). About 15 years ago SAES added, through a licence agreement with CERN, the NEG coating technology to its product portfolio as this was nicely complementing the product family based on lumped NEG pumps (e.g. Capacitorr pumps). Since then a substantial R&D and NEG coating activity has been done in our company to : industrialize the process explore the feasibility of NEG coating of complex shape chambers, including narrow gap insertion devices. 3

Trends in narrow gap chambers The first large synchrotron project using relatively small section chambers was the SOLEIL ring, which was the first light source to adopt NEG coating extensively (> 55%

Depending on the required film specifications, 2 or even 3 cathodes were used in the assembly.

4 Trends in narrow gap chambers The first large synchrotron project using relatively small section chambers was the SOLEIL ring, which was the first light source to adopt NEG coating extensively (> 55% of the ring circumference). For SOLEIL ring chambers, cathode wires could be suitably positioned inside the cross section of the chambers as this was reasonably large. Depending on the required film specifications, 2 or even 3 cathodes were used in the assembly. Spacing among cathodes was modeled to ensure that the thickess distribution was as close as possible to user s requirements. Using three cathodes was for example possible to deposit more film at the edges of the chamber (e.g 1 micron) and less in the central part (e.g 0,5 micron), thus minimizing possible impedance effects. 4 Courtesy of SOLEIL

Trends in narrow gap chambers In parallel with SOLEIL, several other facilities (DLS, MAX, NSRRC, PSI ) started to ask for NEG coating of insertion devices with

Also the section length was accordingly reduced (from 70 mm to 35 mm, typically) The combination of both effects make the coating much more difficult as Inserting

5 Trends in narrow gap chambers In parallel with SOLEIL, several other facilities (DLS, MAX, NSRRC, PSI ) started to ask for NEG coating of insertion devices with gaps in the range between 12 and 9 mm. In the following years gaps have decreased in size down to 7 mm. Also the section length was accordingly reduced (from 70 mm to 35 mm, typically) The combination of both effects make the coating much more difficult as Inserting and mounting the cathodes is more critical the plasma volume is pretty much reduced which may cause plasma instabilities and/or dis uniformities unless properly addressed. Plasma adjustments may require higher working pressures 5

6 Typical thickness variation in a narrow ID A typical thickness distribution is showed in the chart below for the ID section given in the picture. Average film thickness is about 0,6 micron, however thickness can move from a 0,3 micron minimum to a 1,5 micron high, depending on the area. This is unavoidable as there is no way to prevent sputtered atoms to deposit and grow a film closer to the emitting source (the cathode). Use of three cathodes can mitigate the issue and smooth somewhat the thickness distribution. Cathode Cathode 1,75 1,5 thickness (µm) 1,25 1 0,75 0,5 0, half perimeter lenght (cm) 6

7 Typical thickness variation in a narrow ID Coating thickness 30% 100% 3 Cathode wires 7

8 Typical thickness variation in a narrow ID Cathode Cathode Cathode 8

9 What is achievable, main limitations IDs down to 8mm gap and up to 6m lenght have been coated in our facility. IDs 7mm up to 2,5 m long have been also successfuly processed. The sputtering setup is well established so we do not expect major issues even in coating longer chambers. However, the following important points have to be considered : The use of three cathodes is almost impossible when the section is smaller than 50 mm. Space for inserting them in a controlled and reliable way is very small. Cathodes at the edges may be just too close to the chamber, making the sputtering process unstable. This is especially true for long IDs ( e.g. 4 6 m) where the possibility for the cathodes assemblies to move a few mm along the whole lenght cannot be ruled out. With two cathodes you have to be aware that you have more thickness variation along the section profile. Cleaning of long IDs is also more critical as the flowing of reactants and chemicals is more dfficult 9

10 What is achievable, main limitations Based on this, we can consider as a rule of thumb that a practical limit for NEG coating with the present cathode technology may be an elliptical ID with section of 7mmx30 mm Lenght of 6 m It is probably still possible to go a bit further down, however the users have to accept a more drastic thickness variation along the section ( e.g. 0,2 micron 2,5 micron) Elliptical sections: same perimeter, same ammount of deposited material Elliptical section 60mm x 11,5mm Elliptical section 62mm x 6mm Acronym 09/10/

Small diameter pipes Since a few years now, pipes with internal diameter as small as 4 mm are increasingly considered as insertion devices for high emittance light sources.

11 Small diameter pipes Since a few years now, pipes with internal diameter as small as 4 mm are increasingly considered as insertion devices for high emittance light sources. There is therefore a growing interest in the scientific accelerator community in understanding the coating process and feasibility. As of now we have coated and characterized stainless steel pipes with diameter of 10 mm 8mm 6mm Pipes with 4 mm diameter have been prepared and are being tested in these weeks. An update of this activity is provided in the following 11

12 Small diameters pipes: 10mm Pipes lenght 2m, internal diameter 10mm, stainless steel Plasma could be ignited without major issues and coating process was smooth with coating parameter recorded and stable The pipes were cut and opened and visually and SEM inspected to check for uniform presence of the film Covered area: 100% Thicknessmeasurementwere carriedoutbyeds analysis Average thickness about 1 micron (± 30%) 12

13 Small diameters pipes (10mm): chemical composition uniformity Nominal composition Experimental Average Ti V Zr 30 %at 40 %at 30 %at 29,8 %at 40,0 %at 30,2 %at 13

14 Small diameters pipes (10mm): SEM analysis The coated film is quite conformal and follows the surface roughness of the substrate. Clean substrate surface Few microns thick NEG film 14

15 Tipical morphology on Si substrates (reference) The coated film is quite conformal and follows the surface roughness of the substrate. As a reference this is the morfology of a NEG film (1,5 micron thick) deposited on a flat silicon wafer. NEG film on Si substrate Si substrate 15

16 Small diameters pipes: 8mm Pipes lenght 2m, internal diameter 8mm, stainless steel Also in this case plasma ignited without issues and the coating process was smooth with coating parameter recorded and stable The pipes were cut and opened and visually and SEM inspected to check for uniform presence of the film Covered area: 100% Thickness measurement were carried out by EDS analysis. Average thickness about 1 micron (± 40%) 16

17 Small diameters pipes (8mm): chemical composition uniformity Nominal composition Experimental Average Ti V Zr 30 %at 40 %at 30 %at 25,3 %at 46,3 %at 28,4 %at 17

18 Small diameters pipes (8mm): SEM analysis Film thickness: 1,5 m 18

19 Small diameters pipes (8mm): ASTM tests ASTM* tests have been carried out on 20 cm long coated pipes. These tests were not aimed at measuring the pumping speed (due to the pipe conductance limitation) but rather to assess the getter capacity for CO. Measured capacity values are in line with previous SAES measurement (see next slides). More tests are in progress. 19 *ASTM F798 97

20 SAES historical data on NEG coating capacity for CO SAES internal technical report (2006) 20

21 Small diameters pipes: 6mm Initial tests on 6 mm stainless steel pipes, 2m lenght have showed a number of issues related to the mounting of the cathodes and the stabilization and uniformity of plasma conditions. To segregate the different factors we have reduced the test pipe length to 1m. Plasma was ignited succesfully, even though higher plasma pressure were required. First ignition with the fresh cathode was more tricky than the following ones. It seems that cathode ageing (removal of the oxide layer) helps in ignite the plasma. As in previous cases also these chambers were tested for : Film uniformity across the whole chamber length Covered area: 100% Film chemical composition and thickness uniformity. Thickness was comprised between 0,8 and 1,8 micron (average value 1,3 micron ± 30% ) Chemical composition was close to the nominal values 21

22 Small diameters pipes (6mm): chemical composition uniformity Average thickness about 1,3 micron (± 30%) Nominal composition Experimental Average Ti V Zr 30 %at 40 %at 30 %at 30,5 %at 40,3 %at 29,2 %at 22

23 Small diameters pipes (6mm): SEM analysis Stainless steel substrate 1.8 m thick NEG film 23

24 Small diameters pipes: 4mm (work in progress) Pipes have been prepared and are now ready for coating. We will start with 1 m long samples and we will then increase the length. On these pipes in addition to the standard characterization we also plan to measure the Kr outgassing properties. We expect in fact that due to the higher gas pressure needed, Kr may be trapped in the film and released later during the machine operation, which would be detrimental to the electron beam. 24

25 Summary and conclusions Narrow gap IDs have become quite popular in the past 10 years in light sources. Even though far from trivial, reliable and consistent NEG coating of IDs up to 6 m and with gaps in the order of 7 mm is currently achieved in our facility. Almost 100 IDs have been delivered by SAES in the past 10 years which are currently being used in most of the new generation machines worldwide. Chemical composition uniformity and thickness distribution is ensured by a stringent manufacturing protocol and process control. R&D activity on small diameter pipes is also in progress. First results with pipes diameter of 6 mm indicate that homogenous coating through out the whole pipe length is possible at least up to 1 m length. NEG coating thickness can vary by within ± 40%, however there may be room from improvements as the process will be made more robust. In all cases, chemical composition was close to the nominal composition and within the acceptable range. Tests on 4mm pipes are in progress and we expect first results shortly. 25

26 Summary and conclusions (2) Coating 4mm pipes is not an easy walk as many issues have to be optimized. In particlular : Adjust the plasma parameters and pressure to the given geometry (length, curvature ) Use high applied magnetic fields to ensure the plasma discharge is kept stable and running during the whole coating process Design the cathode assembly in the proper way, to avoid shorts allowing at the same time enough space for the pumping and plasma discharge volume. Use of very thin cathodes is key with this regard. Even though twisted multiple cathodes have been so far working, the use of a single alloy wire might be beneficial. More considerations on this aspect are ongoing. Finally, as the pressure of the sputtering discharge has to be necessarily increased, a special focus must be given to Kr outgassing. Kr outgassing measurement as well as pumpig speed tests on small diameter pipes are planned in the forthcoming months 26

27 Thank you for your attention

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