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.
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|
|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|
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.
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.