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 –
- Set the analyzer auger energy to the end of the range (1030 eV in this case).
- 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.
- 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.
- 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.