Essential maintenance: Leak Detection

Faebian Bastiman

After performing routine maintenance (Essential Maintenance: The Maintenance Cycle) and pumping down your system to the mid-high 10-8 mBar range (Essential Maintenance: Pump down) you are almost ready to bake. Almost!

First of all you will need to fire up your trusty quadrupole mass spectrometer (QMS) or residual gas analyser (RGA) and sample the vacuum. Here I recommend the (Pfeiffer Prisma) though in this post I will use its predecessor the trail blazing Balzers Prisma. Set up your analyser as per the user manual and start a 100 atomic mass unit (amu) scan. You will (hopefully) obtain a scan similar to that shown in Figure 1.

 RGA spectra

Figure 1: Pre-bake out RGA scan of a III-As system

What are all those little peaks telling us? Well they represent the chemical fingerprint of the vacuum. Each element is easily recognisable by its individual mass number; however compounds require some knowledge of the prominent peaks and the relative intensity. To do that you can use the values in Table 1.

QMS/RGA cracking patterns
Name Formula Mass peak 1 peak 2 peak 3 peak 4 rel sens
m/z % m/z % m/z % m/z %
1 acetone C3H6O 58 43 100 58 33 15 20 42 6 3.6
2 Acetylene C2H2 26 26 100 25 20 24 6 13 6 1
3 air 28 100 32 27 14 6 16 3 1
4 ammonia NH3 17 17 100 16 80 15 8 14 2 1.3
5 argon Ar 40 40 100 20 16 36 1 38 1 1.2
6 benzene C6H6 78 78 100 77 19 52 16 51 15 5.9
7 boron trichloride BCl3 117 81 100 83 64.7 35 28.7 1
8 carbon dioxide CO2 44 44 100 16 9 28 8 12 7 1.4
9 carbon monoxide CO 28 28 100 12 5 16 2 14 1 1.05
10 carbon tetrafluoride CF4 88 69 100 50 12 19 7 31 5 1
11 diborane B2H6 27 26 100 27 97.4 24 89.6 1
12 DP oil DC 705 78 100 76 83 39 73 43 58 1
13 DP oil FOMBLIN 69 100 20 28 16 16 31 9 1
14 DP oil PPE 446 446 100 77 79 51 29 39 10 1
15 ethane C2H6 30 28 100 27 33 30 26 26 23 2.6
16 ethanol C2H5OH 46 31 100 45 39 27 24 29 24 3.6
17 ethylene C2H4 28 28 100 27 63 26 61 25 11 3.6
18 Fomblin oil 69 100 20 28 16 16 1
19 Freon 12 CCl2F2 104 85 100 87 32 50 16 2.7
20 Freon 13 CClF3 104 69 100 85 28 35 15 50 15 1
21 helium He 4 4 100 0.14
22 hydrogen H2 2 2 100 1 10 0.44
23 hydrogen chloride HCl 36 36 100 38 32 35 17 37 5 1.6
24 hydrogen sulphide H2S 34 34 100 32 44 33 42 1 5 2.2
25 isopropyl alcohol C3H7OH 60 45 100 43 17 27 16 29 10 1
26 krypton Kr 84 84 100 86 31 82 21 83 21 1.7
27 methane CH4 16 16 100 15 85.8 14 15.6 13 8 1.6
28 methanol CH3OH 32 31 100 32 66.7 29 64.7 28 6 1.8
29 neon Ne 20 20 100 22 9.9 21 0.3 0.23
30 nitrogen N2 28 28 100 14 5 29 1 1
31 oxygen O2 32 32 100 16 11.4  34 0.4 1
32 phosphine PH3 34 34 100 33 33.1 31 32.1 2 1 2.6
33 pump oil 500 57 100 43 73.3 55 72.7 41 33 1
34 silane SiH4 32 30 100 31 78 29 29 28 27 1
35 silicon tetrafluoride SiF4 104 85 100 86 5.2 28 4 33 4 1
36 sulphur dioxide SO2 64 64 100 48 50 32 10 16 5 2.1
37 Trichloroethane C2H3Cl3 132 97 100 99 64 61 58 26 31 1
38 Trichloroethylene C2HCl3 130 95 100 130 89 132 85 60 65 1
39 water H2O 18 18 100 17 21 16 2 1
40 xenon Xe 132 132 100 129 98 131 79 134 39 3

Table 1: QMS/RGA cracking patterns for common MBE vacuum species

Before we go into detail analysing the spectra, the first thing to do is to check the most prominent peak. The peak at 18 is almost an order of magnitude higher than the peaks around 2 and 28 and only the peaks around 16, 17 and 91 come close. From table 1 we can deduce that the family of peaks at 16, 17 and 18 are in fact water. Since we have just opened our system to atmosphere we would naturally expect water to be the most abundant species in the system. What about the other peaks?

Analysing the spectra in figure 1 from left to right, we can first of all see the prominent H and H2 peaks that are caused by the stainless steel (SS) outgassing into the vacuum. The exact height of these hydrogen peaks varies for system to system and largely relate to how the MBE system was heat treated when it came off the production line. Often this involves heating the system to 250°C for 400 hours then 350°C for 120 hours causing the SS to take on a “golden” tinge. After the water peaks discussed above, the next peak is at 28 and corresponds to N2, as is to be expected from the multiple N2 flushes we used during the pump down. Notice that the O2 peak at 32 amu is very low, but together with the peaks at 28 and 14 we can say there is some air in the vacuum chamber. But how much air do we have in the chamber? Do we have a leak? Well let me finish the spectra analysis first and then I will answer those questions… the peaks at 37.5 and 45.5. are probably double ionised As and AsO (thanks to Scott for his comment), and the peaks at 75 and 91 are the single ionised As and AsO (as to be expected on a III-As system). Some of the As has also formed arsine-type compounds AsH and AsH3 with masses of 76 and 78 respectively. The peak at 44 is CO2 which was soluable in the water vapour at atomspheric pressure and is now dissociating. CO2 has a corresponding peak at 12 and adds to the peaks at 16 and 28. The other small peaks at 15, 19, 20, 25, 26 and 30 are various simple hydrocarbons formed from the interaction of the numerous C and H ions. All in all a very clean pre-bake vacuum.

Back to the oxide: Well from table 1 we can see that air has an O2 peak that is 27% of the N2 peak in the “air” spectrum. Remember figure 1 is on a log scale so we would expect the O2 peak to be much higher (~2.4 x 10-10 A) for a N2 peak at 9 x 10-10 A to represent purely air. Clearly the O2 peak is at around 5 x 10-11 A and thus the N2 peak is formed predominantly from pure N2 (and 8% CO2) gas. From the relative peak heights we can estimate there is in fact roughly 5x as much N2 as there is air. The air is actually two orders of magnitude lower than the water vapour, so we can conclude that there is no significant leak, at least no leak above 10-10 mbar.

What if the situation were reversed? If the air peak had been dominant and the O2 peak had been in the mid 10-10 A range, we could only conclude we had a small leak in the 10-9 mbar range. If the background pressure had been in the low 10-7 mbar range and the “air” spectra was an order of magnitude higher, we could only conclude we had a more significant leak in the 10-8 mbar range. What to do?

Well first of all do some simple checks and work through a process of elimination. Check the pump exhaust fittings, particularly the turbo pump to roughing pump connections and the TSP ceramics. Close the various gate valves and try to determine on which chamber the leak is present. Spray some IPA onto any suspicious looking flanges (where the gasket looks unevenly compressed) and look for pressure and current spikes in the ion gauge and RGA spectra. You can find a guide that details the flange nut sequence required for leak free flange in MBE maintenance: how to tighten a CF flange. If none of these work you will need to utilise He leak detection. To do this set the RGA to 4 amu for He detection only, turn off your air conditioning so it does not waft the He around the MBE system and with a narrow aperture pipe and very low flow rate of He systematically spray every area of the machine. Once you have identified a likely origin of the leak wrap a plastic bag around it and test both inside and outside the bag to see if you can “enhance” and “supress” the He signature.

In summary, because the water vapour pressure is higher than the N2, and the N2 is higher than the “air” and because our pressure is in the high 10-8 mBar range we can conclude that this system has no significant leak. After baking to 140°C for 48 hours (plus 8 hour ramp up and ramp down times) we get the RGA spectra shown in Figure 2. Note this was immediately after baking, whilst the system was at around 40°C with no water or LN2 cooling attached. The ion gauge pressure was in the high 10-10 mBar range.

post bake crop

During the baking the water vapour rapidly evaporated and was swept out through the pumps with the other temperature assisted highly mobile species. The result is a spectra that is almost identical to Figure 1, except that each peak is reduced by 1-2 orders of magnitude. So we cannot conclude that  the hydro-carbons and oxygen are “gone”, they are simply beneath the sensitivity of the detector (that is to say they are now in very dilute quantities). What do we have left in the vacuum? Well the most dominant peak is now the H2 (2) closely followed by As (75, 37.5) and water (18, 17, 16) then N2 (28),  CO2 (44, 28, 16, 12) and AsO (91). On the one hand there is an argument for baking the system for longer, since every additional day at high temperature would see more of the contaminants removed from the system.  On the other hand since the rate of removal of contaminants falls exponentially with time the “time-purity trade off” results in a most efficient bake out time between 16 to 72 hours. The system will continue to “clean up” through the first week of operation and the contaminants will experience another 1-2 orders of magnitude reduction once the LN2 cooling is turned on. And so now it is time to perform the many little post-bake tasks that get the system ready to grow.

3 thoughts on “Essential maintenance: Leak Detection

  1. Hello Dr. Bastiman. I have been following your blog for the last few months and have been really learning a lot. I am currently a graduate student in Texas working on III-V MBE, so this blog is very pertinent to what I do.

    In regards to your discussion of the various peaks on the RGA spectrum, I would suppose that the peak at 37.5 is indicative of As, especially since you are working on III-As MBE. Since the ion detector is measuring the mass-to-charge ratio, doubly-ionized As would show up at a mass of 37.5 amu.

    Regards, Scott

    • Thank you kindly Scott. I think you are right, how else could something have a non-integer amu? I’ve given the post a second pass and added a bit more detail.

      Faebian

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s