72-100 Electron Multiplier Supply Notes

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The 72-100 electron multiplier supply is used on older Physical Electronics (PHI) Auger electron spectroscopy systems to provide voltage to the cylindrical mirror analyzer (CMA) and secondary electron detector (SED) electron multipliers.

The 72-100 comprises a digitally controlled 0 to +4000V variable switching supply, a 0 to 300V fixed supply, and a voltage to frequency monitoring circuit. The digital control from the PC software is converted to an analog output which drives a switching supply.  

The AES and SED 72-100 board addresses are set by changing the segments on SW2 as follows;

72-100 switch settings
Switches set up for AES SW2 segment 3 closed

Changing the switch settings allows you to swap the 72-100 boards to confirm whether or not there is a problem. For example, if you think that your SED 72-100 board has a problem, you can swap it out with the CMA 72-100 after first changing the CMA 72-100 address to SED (SW2 segment 4 closed). If the SED image works after swapping the boards, then you know that your SED 72-100 board does have a problem.

Once you confirm that your 72-100 is not functioning properly there are a some simple things that you can try.   ***CAUTION – high voltages (up to 6kV) are present on this board, refer servicing to qualified personnel who have been trained to work safely with high voltage.***

Note that the CMA 72-100 is controlled by AugerScan software and the SED 72-100 is controlled by AugerMap software.   You can also use the AugerScan diagnostic menu to send commands to the board per the 72-100 manual calibration procedure. 

Make sure that the card rack power is OFF and then extend the 72-100 board.   Use the high voltage extender (the one with red wires on the bottom section).

72-100 board mounted on extender card

For testing the AES 72-100 board, you can first check the analog output from the DAC to see if you are getting the proper control voltage. Note that the card rack power needs to be ON when measuring voltages.  For the high voltage supply, measure from the left side of R 32 with respect to the left side of CR 3.   

Meter ground left side of CR 3
Meter POS left side of R 32

With the electron gun supply off, set up and acquire a survey in AugerScan. The DAC output voltage should ramp up from 0 to about 5 volts as the multiplier voltage is being set by AugerScan.  Since there is no electron beam current, the AES electron multiplier voltage will default to the PC (pulse count) voltage which is typically 1800 to 2200V. 5 volts on the DAC output corresponds to 2000V on the multiplier supply POS output.

If the DAC voltage output is correct, then the problem is probably related to the switching supply transistors.  With the 72-100 board on the bench (card rack power OFF when removing or inserting the 72-100), test the diodes and transistors for shorts.   Note that two legs on all of the power transistors will show as shorted because those points are connected to the switching transformers.

One common problem that causes the HV output to be zero is that Q7 and Q8 are defective, even though they may check out OK with the diode test.  If the DAC voltage is correct but you do not have any high voltage output, then try replacing Q7 and Q8 with the correct 2N5337 transistor.  Some 72-100 boards have 2N3725 transistors instead of 2N5337 transistors installed. The 2N5337 can dissipate 6 watts of power but the 2N3725 can only dissipate 3 watts. The lower power rating of the 2N3725 can result in poor switching characteristics.

Q7 and Q8 are located under the black high voltage cover. It is held in place with 3 screws. Remove the cover and you will see Q7 and Q8.

Remove the high voltage cover
Q7 and Q8

Assuming that replacing the transistors solved the issue, you can adjust the high voltage by adjusting R7 to match the software.  For example, if the AES PC voltage is 2000 V then adjust R7 for 2000V between the POS and NEG cables. Note that you need to use a high voltage probe when measuring the POS output voltage as most DVMs are limited to 1000 V or less.

R2 is the 300V adjustment and you measure that voltage from the NEG cable center pin to the outer cable or vacuum chamber.

The easiest place to connect your high voltage probe to measure the AES high voltage is between the center pin of the J6 HV POS IN cable to the 96A/B / VF4 preamp and the center pin of the NEG cable on the CMA electron multiplier.

When testing the SED 72-100 then you need to control the SED voltage in the Auger Map TV dialog box.  As you change the SED voltage in AugerMap you should see the DAC output voltage change.  2000V would correspond to about 5V on the DAC output. For the SED high voltage, measure between the center pins on the 97 SED preamp POS and NEG cables. Set the SED voltage in Auger Map to 2000V and then adjust R7 on the SED 72-100 board for 2000V. Remember that you need to use a high voltage probe when measuring the SED and CMA POS output voltages.

Regarding the V/F circuitry on the 72-100 board, those components are not used and so can be removed if your board has them installed in sockets. Some 72-100 boards do not have those ICs installed at all, but others do and they may be soldered in.  If they are soldered in just leave them as is; it is not worth the effort to pull them off the board if they are soldered in place.  The ICs which can be removed are:

U1, U2, U3, U4, U5, U6 and U9.

72-100 board with no V/F ICs installed
72-100 with V/F ICs removed from sockets

Finally, the 1K ohm filter resistors on the 72-100 mother board can fail (burn and become open).  In that case the 72-100 board may be working properly, but the voltage will not get out to the cables.  You can ohm out the resistors on the motherboard to see if that is an issue.

72-100 R1 R2
72-100 R1 R2

72-100 R1 R2

And here is the schematic that shows both resistors:

72-100 mother board schematic
72-100 mother board schematic

If you can’t repair the 72-100 board yourself, RBD Instruments provides a 72-100 repair service and loaner 72-100 boards. 

Cylindrical mirror analyzer fringe field termination ceramics

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Physical electronics (PHI) cylindrical mirror analyzers ( CMA) use fringe field termination ceramics to reduce the fringe fields from the end of the cylinders.

Abstract an early patent:

Field termination plates for cylindrical electron analyzers are provided wherein the plates are constructed of an insulative material coated on the interior surface with a high resistance, electrically conducting coating. Spaced concentric rings of relatively high conductivity material in electrical contact with said coating are provided; the rings providing equi-potential regions on the plates, thereby minimizing field fringing near the ends of the cylindrical tube electron analyzer.

The PHI CMA (cylindrical mirror analyzer) utilizes conical and flat termination ceramics to eliminate electrostatic edge effects between the inner and outer cylinders. These ceramics are essentially gold rings with resistors in between that divide the outer cylinder sweep voltage down in even steps. The result is a very high throughput and even energy distribution of the Auger electrons. If a CMA has a poor contact on a termination ceramic, the results are noisy data and poor energy linearity.

Single pass AES CMAs have just 2 terminating ceramics, a conical at the front of the CMA and a flat at the base. Double Pass AES/XPS CMAs have 3 terminating ceramics, a conical at the front, a center and a base.

The conical and flat ceramics are essentially resistors and so the total resistance between the inner and outer cylinders add up like this:

The table below lists the values on the combined resistances of the older PHI CMAs.

Values are in Meg ohms

If the VM or IC to OC (inner to outer cylinder) resistances are off significantly in your CMA then you probably have a contact issue between a conical or flat (base) ceramic between the outer cylinder or the inner cylinder. Sometimes the resistances of the conical ceramic can be off due coating caused by years of sputter depth profiling.

A poor electrical contact can result in high background counts or extremely high noise levels in the data due to arcing. If you suspect that your CMA has a contact issue with a terminating ceramic then you will need to tear down your CMA to where you can add improve the electrical contact by adding some thin copper or gold shim foil between the suspect ceramic and cylinder. If you need some guidance on how to do that, please contact RBD Instruments.

The pictures below show the conical and flat ceramics from a 25-120A CMA. You can see the gold rings and also the thin film resistors.

Low-Cost High-Potter

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Occasionally Ion Pumps can short out from the flakes that accumulate over the years because of using titanium sublimation pumps (TSP).   An old trick is to use a high potter (high voltage potential generator) to create an arc which can sometimes (if you are lucky) burn off the short.   High potting can work well for ion pumps, ion guns and practically any electrical feedthrough on a vacuum chamber where flakes or deposition build up is an issue.

High potting will only work on small flakes as the current generated is low (100uA max).  But the voltage generated is extremely high at 20,000 to 50,000 volts.  Think of this as a small Van der Graaf generator where you can direct the arc towards the electrical feedthrough on the vacuum chamber. The voltage from the arc is applied to the shorted electrical feed though and will work it’s way to the short inside the vacuum chamber and if you are lucky, will melt the short.

High potting does not always work but when it does it buys you some time before you need to vent the chamber and perform the necessary maintenance to fix the underlying shorting issue.

Back in the 80s it was common to see high potters in labs that had vacuum chambers for analytical instruments or deposition systems. These days you do not see high potters as often due to more stringent electrical safety standards.   Used properly a high potter is perfectly safe.  Although it is a little bit exhilarating to be pointing a device that has a 50,000-volt arc coming out of the end.  😊

Electro-Technic Products, Inc. provides an assortment of high frequency generator testers that work well as high-potters (high voltage potential) to melt away small particles and flakes that can cause shorts in high and ultrahigh vacuum applications.  In particular, these are useful for high-potting shorted ion pumps and ion guns.

We prefer the BD-10AS as it has an ON-OFF switch.  As with all high voltage, only qualified personnel should use this device.  Check with your organizations’ electrical safety officer to make sure that your organization allows high potter use before purchasing a high potter.

For more information or to place an order, contact Electro-Technic at http://www.electrotechnicproduct.com/pinhole.html

Or by phone at 773-561-2349