Scanning Electron Microscope

Scanning Electron Microscope - High Vacuum System

I didn't think to get a picture of the SEM's high vacuum system in its original condition, but the problems were all internal, so a picture wouldn't have been very helpful. This is a picture of the disassembled system, spread out on a work bench.

Here is a drawing (to scale) of how the parts go together, o-rings are labeled with their aerospace standard AS568A size.

OK, just spent ~$300 on o-rings, tubing, tubing clamps, etc. Let's put it back together. Here are the parts for the bonnet and valve actuator assembly. The new anti-rotation shaft and a new lead screw to indicator screw pin (the original snapped when I tried to remove the jammed poppet by turning the handle) can be seen to the left of the lead screw.

Pack bearings, coat threads and o-rings with Krytox(RTM) GPL grease and assemble: bearing on lead screw shaft, shaft and bearing into bonnet, o-ring and bearing over shaft, indicator screw over shaft and pin in place, indicator nut screwed on, followed by indicator sleeve, gear/knob secured to indicator screw with three set screws, anti-rotation shaft pressed into bonnet (with a wee bit of teflon tape to hold it securely) and the bonnet seal o-ring pressed into groove.

Next, o-rings lubed and installed in upper half of valve body, bonnet screwed onto the upper half of the valve body, upper and lower poppets joined with screws and o-ring, joined poppets slipped into upper body section over the anti-rotation shaft and actuator lead screw threaded into the upper poppet.

Final step (of valve assembly), lube and install o-rings in lower half of valve body, slide upper half into lower, install split ring in groove of upper half, slide flange over split rings, bolt flange to lower body half, align indicator with poppet and screw indicator sleeve to bonnet.

Moving on, here are the parts of the up-to-air solenoid valve (except for the solenoid itself, which is wired to the microscope column and will be installed later). Except for the o-ring seals, this valve seems to be in pretty good shape, probably because it is a normally closed valve that is only energized briefly during sample loading and hasn't had much wear.

Up-to-air solenoid valve assembled.

Vacuum gauge and connector parts.

Vacuum gauge assembled to holder.

High vacuum manifold with o-rings, up-to-air valve and vacuum gauge installed.

High vacuum valve ready to be connected to vacuum system support plate (with water-cooled baffle attached), high vacuum manifold assembly, rough vacuum line and rough vacuum to diffusion pump outlet line. Also, I have connected a filter to the inlet of the up-to-air valve.

Assembled vacuum system (except for diffusion pump and anti-suck-back valve). The upper and lower column connectors will be finessed into place after this assembly is mounted in the column frame.

Here is the vacuum system mounted on the column frame and connected to the column. The mounting plate was shimmed with five nylon washers at each mounting point, in order to get the system at the right height (partially for the alignment of the high vacuum manifold to column connector pieces, but fine tuned to allow meshing of the gears between the valve and the automated valve drive system, more on that later).

Here is another view, showing the top connection to the column.

And here is the bottom connection to the column (mostly hidden by the column frame).

This picture shows the diffusion pump in the middle of an oil change. According to the plaque on the pump, it requires 40 cc of oil. Draining recovered 60cc of oil (Dow-Corning 704 silicone fluid), which was full of black flakes. The oil was filtered leaving a clear, slightly yellow, fluid of which 40 cc was returned to the pump. I have an almost full, 500 cc, bottle of fresh 704 fluid (left foreground) and thought about replacing the pump oil altogether, but I'm cheap and this stuff is ~ $120 a bottle... Also in the picture, you can see a small piece of blue, polyurethane, tubing, from which I made a set of bushings to align the mounting screws in the flange and the mounting screws, each with a set of six nylon washers to properly shim the pump relative to the water-cooled baffle.

Diffusion pump oiled, jets installed, flange gasket lubed, bushed, shimmed and bolted into the vacuum system, over-temp limit switch installed and heater power cord connected.

Next is the anti-suck-back valve. Here are the parts. I opened the pilot solenoid valve (lower left, just above the grease) and found that it was in pretty bad shape. This is a three-way valve, with a poppet that has two seals. Both seals had compression-set dimples from contact with the orifices. The bonnet seal was a flat gasket molded to the bonnet and couldn't be replaced. The air side of this valve was normally open, so in use, this valve was continuously energized (and probably quite hot) accelerating wear.

Anti-suck-back valve drawing, the numbers indicate o-ring sizes (aerospace standard AS568A).

Assembled.

Installed in the vacuum logic chassis.

The vacuum logic chassis mounted on the instrument frame.

Let's hook it all together. First, the roughing line to the diffusion pump. Because the clearances are all so tight in this system, the manufacturer decided to use thin wall tubing, with internal support. I've kept to that theme, but used heavier internal springs. The old connecting piece is shown to the left of the new parts. You can see that over time the old tubing collapsed down onto the spring, drastically constricting the flow path. The new tubing is plasticized PVC, 5/8" ID X 7/8" OD and the spring is 302 stainless steel, 5/8" OD.

Here it is installed.

The same method was used for the rough vacuum lines between the main valve and anti-suck-back valve and between the anti-suck-back valve and the rough pump. The tubing is plasticized PVC, 1" ID X 1.25" OD and the spring is hard-drawn steel, 1" OD. Here is a pic of the line between the valves.

Also, the line to the vacuum pump and the manifold connecting the pump, tubing adapter and thermocouple vacuum gauge. The TC gauge should come in handy during leak testing.

And here is the automated valve drive installed. In order to make sure that the drive and valve lined up properly, I set the valve to the shut position manually and then powered up the system with the drive also set to the shut position (gratifyingly causing the motor to drive the gear to the proper position) before screwing the drive into place.

I powered up the SEM, and hit the "rotary pump" switch, which (although my pump is not powered via the SEM, as was originally intended) was necessary to supply power to the vacuum logic chassis and close the ASB valve pilot solenoid. I then turned on the pump and found that the system wouldn't even pump down well enough for the TC gauge to register on scale. Time to back up a few steps. I disconnected the tubing from the pump to the ASB valve at the valve and plugged it with a rubber stopper. The TC gauge then came on scale and eventually bottomed out at a little less than 200 microns (the pump is not improving with age). I reconnected the pump to the valve, disconnected the tubing running from the valve to the roughing connection of the 3-way, high vacuum, valve and plugged the open ASB valve port with the stopper. In this configuration, the TC gauge again failed to come on scale. This made me suspect the pilot solenoid, which was now too hot to touch. After shutting everything down, I removed the pilot solenoid valve and plugged the two connections between it and the ASB valve (the o-ring fitting with a sawed off screw, the shank of which was the right diameter to seal against the o-ring, and the small tube with a greased rubber cap). In this configuration, the pump could evacuate the ASB valve below 200 microns. The stopper was removed and the ASB valve reconnected to the tubing from the roughing line of the 3-way valve.

Time to try out the main valve. The SEM and mechanical pump were turned on and the valve control "operate" button pressed. This caused the valve to be driven from the shut position (see drawing below) to the rough position. In this setting, the rough line (connected to the back port, indicated by the dashed circle) is opened to the high vacuum manifold (right port) and the microscope column. Both the diffusion pump inlet (bottom port) and the diffusion pump rough line (left port) are closed, isolating the diffusion pump.

With the 3-way valve in the rough position and the mechanical pump running, the pressure indicated by the TC gauge slowly came down to ~ 400 microns, but wouldn't go lower. Also, the SEM vacuum gauge (a Pirani style device) only barely came on scale. At first, I thought this was due to a leak somewhere in the high vacuum system, but then it occurred to me that the problem might lie with the ASB valve. Originally, with the three-way solenoid valve acting as a pilot operator for the ASB valve, in the energized state the o-ring (bottom left in the drawing below) and tubular pilot (gray circles on the "to pump" port) ports are connected to one another allowing the space behind the poppet to be evacuated by the pump and the force of the spring to push the poppet into that space, connecting the two main ports. In the de-energized state, the three-way solenoid allows air into the space behind the poppet, pushing the poppet closed against the spring force, while isolating the tubular port and leaving both main ports under vacuum (pretty much guaranteeing that oil will suck back from the pump to the ASB valve). With those two ports simply plugged off and all ports at atmospheric pressure, the poppet is held in the open position by the force of the spring (left side of drawing). When the pump is turned on and the pressure falls in the main ports, the air pressure trapped behind the poppet can force the poppet to partially close and restrict the flow between the SEM and pump.

To test this theory, I replaced the plugs with a by-pass, made from a short length of 5/32" OD X 0.015" wall 304 stainless steel tubing (to fit the o-ring port) a length of 5/32" ID X 1/16" wall flexible PVC tubing, a length of 8-32 threaded nylon rod (to act as a flow restriction) and a couple of tubing clamps. The idea is that the flow restrictor will allow the space behind the poppet to pump out during operation and the poppet to open fully, while preventing an in-rush of air to the vacuum pump during shut down, when I manually break the o-ring connection to allow air in to close the poppet and vent the vacuum pump (more or less negating the need for an anti-suck-back valve). At first, the TC gauge would not register on scale, then I greased the join between the two pieces of tubing, added another tubing clamp to that join and cranked the clamps down with pliers (two notches past finger tight). If this works, I'll order a tubing tee and manual valve to add to the bypass, so I don't have to disassemble it for shut down.

Hmmm... the column is no longer under vacuum. The microscope column has two seals that are routinely made and broken, an o-ring at the top for the electron gun and a molded gasket at the bottom for the sample chamber door. I replaced the e-gun o-ring and removed, cleaned, greased and re-installed the chamber door gasket. The column probably contains more seals, but they are static and it's not obvious how to get at them. Also, the chamber door contains a number of motion feed-throughs for moving the stage (linear travel along three axis and rotation about two axis) and I worry about the integrity of their seals, but, again, it's less than obvious how to access them and I'm sure I don't want to pull apart the stage mechanism (it makes your average mechanical watch look simple by comparison).

Circulating cooler

Sorry, a couple more things first. Ordered an 1/8" npt ball valve, 0.170" tubing tee, 1/8" npt air filter and 1/8" npt to 0.170" tubing adapter to spruce up the bypass on the anti-suck-back valve. Here is a pic. The ball valve and air filter are on the other end of the tubing which exits the picture at lower left and sit on the console for easy access.

As seen here.

The other thing is that I have removed the thermocouple gauge and flanged cross from the mechanical vacuum pump, removing four extraneous connections from the system. Here is a view of the back of the instrument, with the new vacuum connection. Also seen are the circulating cooler and the giant wall-wart (for reasons known only to them, the manufacturer designed the instrument to operate with a 100 volt input thus necessitating the extra large adapter).

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