A blog on the repair, operation and calibration of surface analysis systems and components including electron spectrometers, sputter ion guns and vacuum related hardware. Click on the Index tab below to see a list of all posts. Visit our website at http://www.rbdinstruments.com
Some of the older PHI electronic units have a type of heat sink with a built-in spring tab (shown in the picture below) that forces the back of a transistor into the heat sink. Over time, the spring tab can lose tension (most likely due to heat induced metal fatigue) and then the transistor no longer connects to the heat sink, eventually resulting in the transistor failing due to overheating.
For units such as the 32-100 Electron Multiplier Supply, overheated transistors are often the cause of multiplier voltage output problems.
For this blog post we will look at a 32-100 electron multiplier supply with no output on the CMA high voltage output. The problem was isolated to a bad TIP120 transistor which shorted out and melted because the back of the transistor separated from the heat sink over time.
In addition to replacing the TIP120 transistor, we also modified the heat sink to ensure a good contact with the transistor.
To modify the heat sink, you need to first remove the defective transistor and then remove the heat sink. You will need a hot soldering iron as the heat sink has enough mass that it will drain away some of the heat from the soldering iron. You can use a solder sucker or some solder braid to remove the solder from the heat sink contacts.
Once the heat sink as been removed, break off the spring tab.
Next, drill a small hole in the back of the heat sink where the indent is located. We used a 9/64″ drill bit since we needed to clear a 6-32 screw and lock nut.
Put some heat sink compound or conductive tape on the back of the replacement transistor. This is necessary to ensure good thermal transfer from the transistor to the heat sink.
Use a screw and lock nut to attach the transistor to the heat sink. Make sure that the transistor is centered in the heat sink. In this case we also added another small heat sink to the back of the original heat sink to add some additional cooling for the transistor.
Next, insert the transistor leads into the holes on the board and insert the heat sink into the larger holes in the board. Solder the heat sink and the transistor leads. Cut the excess leads from the transistor and remove any excess flux from the board.
Now that we have replaced the transistor and improved the transistor to heat sink contact, the 32-100 should perform well for many years.
Since we were replacing the one defective TIP120 transistor, we also replaced the one for the SED supply as well (and modified its heat sink) as a preventive measure. In this case we could not add the extra small heat sink due to a tight clearance to the nearby transformer. Even so, the improved contact to the heat sink will provide improved heat transfer from the transistor and result in improved reliability.
Need help with your older PHI (Physical Electronics) surface analysis system electronics (or optics)? Contact us at rbdinstruments.com
The raster size potentiometers that are used in the 11-065 ion gun control are coarse single turn potentiometers that make it difficult to accurately reproduce raster sizes.
By replacing the single turn potentiometers with 10-turn 1% potentiometers, the accuracy of the raster sizes is greatly improved. This will improve the repeatability of your sputter rates when changing raster sizes. One turn of the raster size potentiometer will now equal 1 mm instead of 10 mm.
When you replace the 25 kΩ 1-turn deflection potentiometers in the 11-065 with 25 kΩ 10-turn potentiometers, note that CCW = black and CW = solid brown. The other wires are the wipers.
You will also need to replace the one turn raster size knobs with a 10 turn vernier dial.
TIP: When removing the old potentiometers you can simply cut the tabs off the old ones and then insert and solder the tabs into the new potentiometer. That is much easier than un-soldering the wires from the old potentiometers.
It is very important that the CW, CCW and wipers wires match up with the old potentiometers. Otherwise the raster sizes might be backwards (0 = full raster and 10 = zero raster.
If your 11-065 controller ever needs repair and you send it to RBD Instruments, we will ask you if you would like us to add this update to your 11-065 as part of the repair.
If you are new to UHV vacuum chambers and how to create a seal using copper gaskets when mounting optics, this blog post has some useful tips.
In many cases, installing the copper gaskets that are used to seal flanges on UHV vacuum chambers is as simple as placing the copper gasket in the knife edge recess. If the flange is facing down, then the gasket can be placed on the optics part being installed and the gasket will stay in place.
But in cases where the flange is perpendicular to the floor, the gasket will not stay in place on its own because the gravitational constant will prevent the gasket from staying in place. In other words, the gasket will fall.
There are a few ways to install a gasket onto the flange. But first, you need to make sure you’ve removed the old gasket properly.
When you remove the old gaskets, you want to be very careful not to nick the knife edge on the flange when removing the old copper gasket because a nicked knife edge often results in a vacuum leak. And, nicked knife edges are difficult and sometimes very expensive to repair.
Usually the gaskets will come off easily with a minimum amount of force. For those gaskets that are pressed in tightly and very hard to remove, I have found that using a long-nosed vice grips locking pliers works quite well.
You can adjust the gap on the pliers so that it firmly clamps down on the gasket, then simply bend the pliers so that the leverage will pop the gasket off the flange. It works every time and most importantly, it follows the number one rule of flanges – protect the knife edge. You can use a screwdriver to pry up the edge of the gasket, but if the screwdriver slips, you risk damaging the knife edge.
The first method is to use a gasket clip. Gasket clips hold the copper gasket to the flange via a spring action. They line up with the flange’s leak check groove and hold the gasket in an area that is just past the knife edge of the flange.
Here is a link to the gasket clips that Ideal Vac provides:
Gasket clips work well most of the time. Sometimes they will not work due to geometry limitations with other nearby flanges or optics.
If you do not have gasket clips, there are some other ways to mount a copper gasket to a horizontal flange.
Method 2: Elongate the Gasket
The second method is to elongate the gasket. This works well for larger copper gaskets, such as those for 10-inch and 8-inch flanges. It works with smaller gaskets as well, but you will need to drop them from a higher distance from the floor.
For an 8-inch gasket, hold the gasket about 1 foot above the floor. Hard concrete floors work best. This technique will not work on carpet.
Drop the gasket on its edge and it will hit the floor and bounce back up. You need to catch it when it bounces back up. If you don’t catch it and it falls to the floor, that is OK. You will just need to clean the gasket off with some isopropanol or methanol.
It takes a little bit of practice to get the correct height from which you are dropping the gasket. But the general rule is that the smaller the gasket, the higher the height. I have used this technique on 10-inch to 4.5-inch gaskets with good results. 10-inch flange gaskets should be dropped from about 6 inches. 4.5-inch gaskets should be dropped from about 18 inches. 2.75-inch gaskets are more difficult as they are harder to deform than the larger gaskets.
When you insert the gasket into the flange, you need to press it into the knife edge recess. The slight elongation will act like a spring and the gasket will stay in place.
Method 3: Cellophane (Scotch) Tape
Which brings us to the third method for mounting copper gaskets – cellophane (Scotch) tape.
You can use Scotch tape to mount the gasket as long as the tape is just barely on the gasket. Since the tape is mounted outside the knife edge, then it is OK if any of the tape stays on the gasket because it will be on the air side of the knife edge.
The pictures below show the knife edge cuts on common gasket sizes. For most copper gaskets (2.75- to 10-inch flange sizes) the knife edge is approximately .100 inches from the outside edge of the gasket. For the very small 1.33” flange, it is .050 inches from the gasket’s outside edge.
The procedure is to place the tape to where it is barely on the gasket (outside the knife edge region) and then very lightly touch the flange. You can use 2 or 3 sections of tape as needed.
Once you get the optics part mounted to the flange with just a few nuts or bolts to hold it in place, pull the tape straight up and away from the flange. Usually all of the tape will come out. But if any small piece of tape is left behind, it will not matter since it is on the air side of the knife edge and will not have an effect on the vacuum.
Rotate the flange slightly to make sure that the copper gasket is properly seated before you tighten the bolts.
And don’t forget that if you are up to air for awhile and need to cover a flange or optics component with aluminum foil, you should use a UHV foil such as All Foils. UHV foil does not have an oil coating on it like foil from the super market does.
There are 2 schools of thought on tightening copper CF gaskets. The most prevalent is to use a star pattern where you crisscross the bolts that you are tightening as shown in the drawing below.
The other method is to simply tighten in a circle pattern (which is the method that I prefer). The trick with this method is that you need to use very small increments of increased torgue as you move from one bolt to the next, otherwise you can over-tighten one section of the gasket or possibly bend the flange out of shape which would result in a leak. But, going in a circle is easier to keep track of which bolt is next.
Here are some links to other posts on the subject of how to tighten a CF copper gasket, so that you can make your own decision on which way is best. The top link has some information on recommended torgue for different size flanges.