Removing and Installing Copper Gaskets On Vacuum Chambers

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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.

Removing Gaskets

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.

6" long nose Vice Grip needle nose pliers

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.

Pry up gasket

Two resources for removing gaskets:

Removal of metal UHV gaskets; Journal of Vacuum Science and Technology 12, 654 (1975)

A UHV-Gasket Removal Tool; Published March 1, 1983

Installing New Gaskets

Method 1: Gasket Clips

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.

Gasket Clip

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.

Hold the gasket

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.

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.

Knife edges

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.

Tape on copper gasket
6 inch CF flange
Gasket with tape

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.

UHV aluminum foil

RBD Instruments provides copper gaskets for less than what most other vacuum supply companies charge. –

Tightening a CF con flat copper gasket

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.

Star pattern method of tightening bolts on a CF flange

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.

AugerScan – Electron Multiplier Voltage Logic

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The AugerScan software that is used on older Physical Electronics (PHI) AES and XPS surface analysis systems provides for control of data acquisition electronics such as analyzer controllers and electron multiplier supplies.  This blog post will explain how AugerScan automatically sets the electron multiplier voltage when acquiring data.

On Auger Electron Spectrometer (AES) systems there are two modes of signal collection.  V/F (voltage to frequency) is the analog detection mode and is used for electron beam currents of 100 nA or more. The model 96A (or 96B) V/F preamplifier converts the current that flows through the electron multiplier into a voltage which is then converted to a frequency so that the computer interface can read in the signal.  Zero to 500 nA of current through the electron multiplier corresponds to zero to 1 M cps of signal. V/F preamps were developed back in the early 80s when high resolution A/Ds (analog to digital) converters were very expensive and V/F preamps were a lower cost alternative.  V/F preamps also replaced lock-in recorder preamplifiers which were used on the very early AES spectrometers.

The other AES detection mode is pulse count (PC).   In pulse count mode an amplifier discriminator is used to count low currents (think of it as counting individual electrons).  Pulse count mode is used with electron beam currents of 100 nA or less. 

XPS systems are always operated in some type of pulse count mode – single channel, position sensitive detector or multi-channel detector. The detector currents are much lower in XPS than in AES.

Auto EMS is the AugerScan software feature that controls the electron multiplier voltage.  The first step in setting up the Auto EMS is to determine the electron multiplier plateau voltage.  The plateau voltage sets the upper limit that the electron multiplier will be set to.

 For a given current electron current, as voltage is applied to an electron multiplier more electrons are generated and counted by the detector.  More electron multiplier voltage translates into more counts until the multiplier gets into a mode where more electron multiplier voltage does not generate more electrons, it flat-lines.   That region is called the electron multiplier plateau.   The plateau region can span several hundred volts and only when the multiplier voltage is increased further do the counts start coming up again.  The higher count rate at very high multiplier voltages is saturation and you do not want to operate the multiplier in this range as the electron enriched material in the multiplier can become depleted very quickly.  The correct setting for pulse count mode would be about 50 to 100V past the knee where the signal has plateaued.

The figure below shows an AES multiplier plateau voltage of 1700V.

Before you acquire an AES gain curve you must first set the target current to 10nA.

Also make sure that your V/F preamp model (96, 96A/B, VF4) is selected in the AES Hardware Properties dialog box and then press the Preamp Defaults button in the AES Multiplier Properties dialog box. The V/F set points values are different depending on your systems V/F preamp.  The very old 96 V/F preamp has a maximum count rate of 100 k cps, the 96A/B has a maximum count rate of 1 M cps and the V/F4 has a maximum count rate of 4 M cps.

Next, open the AES Multiplier Properties dialog box and check the Auto EMS box and the Pulse Count Input box.

Press the Acquire Gain Curve button and a message will pop up to confirm that you have the beam current set to 10nA –

Press the Yes button and then the EMS gain curve will acquire and display the counts vs. voltage.  The cursor will display what it thinks is the correct multiplier plateau (Pulse Count) voltage value.  You can change it a little bit if desired, but it should be about 50 to 100V past the knee where the counts plateau.  

Press the OK button and the Pulse Count Settings voltage will be set.    The Pulse Count Settings voltage will be the upper limit of the electron multiplier voltage.

Let’s look at the logic for how AugerScan sets the AES electron multiplier voltage in a Survey.  The V/F setpoint for surveys is typically set to 800 K cps which is also 80% of the maximum 1 M cps for a 96A/B V/F preamp.

AES surveys are typically acquired starting at 30 eV and ending at 1030 or 2030 eV depending on where the Auger peaks of interest are.  For this example, we will use a 30 to 1030eV survey.

The background in AES rises as a function of the kinetic energy as shown by the red line.  The higher the eV, the higher the counts. This means that the highest kinetic energy is typically also the highest count in the survey. However, in some cases (as in the survey below) there can be an auger peak that is near the end of the survey which has a higher count rate than the very end of the survey.   For this reason, the survey V/F setpoint is set to 80% of the maximum counts in the V/F mode.  Otherwise, the electron multiplier would saturate if there is a large peak near the end of the survey. Saturation presents itself as a straight line at the top of the survey.

The logic for setting the multiplier voltage for a survey is as follows –

  1. Set the analyzer auger energy to the end of the range (1030 eV in this case).
  2. Start ramping up the electron multiplier voltage while monitoring the count rate. The starting voltage, volts per step and time per step are the values in the Multiplier Properties dialog box. In this case the starting voltage is 500, the volts per step is 5 and the time per step is 25ms.
  3. If the count rate gets to 80% of the maximum V/F value (800 K cps for the 96A/B V/F preamp) then that multiplier voltage is set and used for the survey and the input is set to VF1.
  4. However, if the multiplier voltage reaches the Pulse Count Settings voltage (1700) before the count rate reaches 800 K cps, then the pulse count input is selected, and the electron multiplier voltage is set to 1700 volts.

To summarize the logic – If there is a lot of signal the multiplier voltage will be lower, and the detection mode will be V/F.  If the amount of signal is lower, then the multiplier voltage will be higher.  But when the multiplier voltage reaches the Pulse Count Settings voltage value, then the multiplier voltage will be set to the Pulse Count Settings value and the detection mode will be PC1 (amplifier discriminator).

For AES multiplexes and depth profiles, the center of the highest kinetic element being acquired is used for the V/F setpoint.  The V/F setpoint values for multiplexes and depth profiles are lower than in surveys.  For depth profiles there needs to be more room for the counts to come up without saturating the electron multiplier because counts for some elements will increase as the sample is sputtered. 

For an elastic peak alignment, the beginning of the sweep is used for the analyzer energy since there are essentially no counts after the elastic peak.  The elastic peak (alignment) setpoint value is typically 30% (300 K cps for the 96A/B) of the max V/F counts.  If your elastic peak clips, lower the alignment setpoint to 20%.  

The electron multiplier logic is only applied to AES.    For XPS, the multiplier is always set to the pulse count (plateau) value.

To run an XPS gain curve, turn on the X-ray source to the power that you normally run at (typically 300 watts) and select an analyzer energy of 500.  The pass energy can be set from 50 to 100eV.  The input is set in the Hardware Properties dialog box.   Double pass CMAs are set to PC1, 5100 XPS systems are set to PC1, 5400 XPS systems are set to PSD, and 5600 to 5800 XPS systems are set to MCD.

Acquire the gain curve and AugerScan will automatically set the multiplier voltage to the plateau voltage to about 100V past the knee.  Sometimes if the counts are too high or too low AugerScan may not pick the correct voltage in which case you can move the cursor to the correct value and then set the Multiplier voltage by pressing the OK button.

For all XPS acquisitions the electron multiplier is set to the plateau voltage which is entered in the Multiplier section of the XPS Multiplier Properties Dialog box as shown above.   There is no input selection logic for XPS acquisitions, the input is always set to pulse count (single channel), PSD (position sensitive detector) or MCD (multichannel detector) input depending on the specific XPS detector that your XPS analyzer has.

AES input tips –

In the older PHI 10-150 AES analyzers there is no PC (pulse count) output, only a V/F output.  When there is no PC output on a CMA then the Pulse Count Settings voltage needs to be set higher than the Max V voltage in the AES Multiplier Properties dialog box.  Since the multiplier voltage will not be set higher than the Pulse Count Settings voltage, the input will always be V/F.

In the Multiplier Dialog box below the max voltage (Max V) is set to 2000, and the Pulse Count Settings voltage is set to 2100.  So, the multiplier voltage will not go past 2000 V and never reach the Pulse Count Settings value.  Since the input is set to V/F1 in the AES Hardware Properties dialog box, the input will stay in V/F mode.  The electron multiplier voltage will still be set to the V/F setpoint value automatically for any acquisition type.

For the Auto EMS to work, the AES Hardware Properties Input must be set to V/F (typically V/F1).  Then the electron multiplier voltage logic will select the input based on the amount of signal.   If the input is set to PC (typically PC1) in the AES Hardware Properties dialog box, then the input will be set to PC and the electron multiplier voltage is set to the Pulse Count Settings value. In that case the electron multiplier voltage is not ramped up to the V/F Setpoints value, it is set to the Pulse Count Settings voltage.  You normally would not want to force the input to PC on an AES analyzer as if the target current is high the electron multiplier could be damaged.

If the Auto EMS box is not checked, then the input is set to whatever is specified in the AES Hardware Properties dialog box and the voltage is set to the Pulse Count Settings value in the AES Multiplier Properties dialog box.   On some of the oldest AES systems the electron multiplier supply is the model 20-075 and the multiplier voltage is set manually. If you have a 20-075 electron multiplier supply, then the Auto EMS must not be checked. 

Auto EMS vs. manual setting of the electron multiplier voltage –

If you have a 32-100 electron multiplier supply, for the Auto EMS logic to work the mode switch must be set to Digital.   You can also control the electron multiplier voltage manually by setting the mode to Analog.   Uncheck the Auto EMS box in the AES Multiplier Properties dialog box and set the input to V/F1 or PC1 in the AES Hardware Properties dialog box depending on whether you are acquiring AES or XPS data.

0 to 10 turns CW on the Analog multiplier knob translates into 0 to 4,000V of electron multiplier voltage, or 400 volts per turn.   Elastic peaks usually need about 1000 volts or 2.5 turns on the CMA analog multiplier potentiometer, and surveys typically need 1200 to 1600 volts or 3 to 4 turns.  When operating the 32-100 in the manual mode, make sure to turn the Mode switch to OFF when not acquiring data.

CMapp for the microCMA Version 1.0 Released

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RBD has released Version 1 of CMapp – the data collection, analysis and control application for the microCMA. Of course, this is not the first version of CMapp available, but we had a set of features in mind for Version 1 that would truly represent the most feature-complete version of CMapp. Of course, this won’t be the last version – we’re already busy adding new features and working on a completely redesigned application for a future release.

Download the latest version here.

Cmapp Version 1

Electron Gun Control Pane

The key new feature in CMapp 1.0 is the addition of the Electron Gun Control Pane, which replaces the dialog window. All of the electron gun controls are always available on-screen in the familiar layout. There’s no longer a need to move or minimize a window in order to view acquisition data.

CMapp Electron Gun Control

Other Features and Changes

CMapp now offers the ability to differentiate and smooth data while acquiring. This is invaluable for getting important peak information in real-time, and removes the need to wait until the end of an acquisition to determine if the parameters are resulting in useful / expected data.

The latest version of CMapp also includes the ability of the edit the Wehnelt setpoints in the Hardware properties menu. Normally these values are factory set, but now they are easier to adjust without putting undo “stress” on the filament by ramping the beam voltage from 2 to 3 kV before having ballpark Wehnelt values.

Lastly, some minor changes have been made to improve the UI when working with Windows 11.