bias | RBD TechSpot

The 9103 USB picoammeter is often equipped with a +90 V bias option which improves the accuracy of electron and ion current measurements (by reducing the number of low energy secondary electrons that are generated by the beam from leaving the target).

The +90 V bias comprises two 45V batteries in series which are located inside the 9103 chassis. This blog post will explain how to test and replace the batteries in a 9103 picoammeter.

1. Connect a DVM (digital volt meter) to the 9103 Input BNC connector. A BNC to double banana cable works well. Set the DVM to DC volts.

2. In Actuel (the 9103 software), select the input Grounded and bias On.

3. Sample the current.

4. When the bias ON is checked there will be about +90V DC on the input of the 9103.

5 .The input impedance of most DVMs when measuring DC voltage is 10 meg ohms. The two 45 volt batteries should total 90 to 95V DC. The bias voltage divided by the input impedance of the DVM will equal the current. In this case the voltage of the two 45 volt batteries totaled 94V and the current was 9.414 uA.

9.4 uA in Actuel

94 V DC on DVM

6. It is recommended that the bias batteries be tested every 6 months and replaced when the voltage drops below 80 volts. It is normal for the batteries to wear out over time and with use. Once the bias voltage drops to less than 50 V the effectiveness of preventing secondary electron emission is greatly reduced, which in turn reduces the accuracy of electron and ion beam current measurements.

The RBD part number for the 45 V battery is BAT-45-213.

Whether you have an RBD 9103 USB picoammeter or an older Keithley with a PHI model 78 bias box, you should test the batteries as part of your preventive maintenance procedure and replace them as needed.

To replace the batteries in a 9103:

Unplug the 9103 USB power and input cables.

Using the Torx wrench that was included with the 9103, remove the screws from the front and rear 9103 chassis covers.

Slide the board out from the front of the 9103. You will need to rotate the back cover to feed it in. The back cover has a ground wire that is attached to the 9103 board. Also note which groove the 9103 board is in as you will need to put it back in the same groove.

Remove the battery support bracket (white plastic).

Carefully remove the old batteries.

Install the new batteries.You may need to adjust the contacts on the batteries to get them to fit onto the board snaps more easily.

Reattach the battery support bracket.

Carefully slide the back cover and board back into the chassis. Make sure that you put the board back in the same groove that it came out of. If the front cover does not line up with the chassis then you are not in the correct groove.

Reattach the screws to the front and back covers. Do not over tighten the screws!

Once you have installed the new batteries, test the voltage. You should have 90 to 95 Volts.

9103 bias batteries

This post explains the 9103 picoammeter bias modes and also explains the concept of “floating”.

There are 4 picoammeter bias modes:

No bias – the current source is connected directly to the picoammeter input

Internal – a low noise DC supply (two 45 V DC batteries in series) provides a bias between the current source and the input of the picoammeter. This application is typically used for providing a +90V bias to a target in a vacuum chamber in order to prevent secondary electrons from leaving the sample. Adding the 90V bias results in a more accurate measurement for both electron and ion current. It is also possible to use regular 9V batteries and so lower internal bias voltages are possible. Batteries are used for the internal bias as they are very clean with no ripple.

External –

There are two external bias options for the 9103, both are limited to 600 V DC.

9103EXTBIASPR has a BNC connector on the back of the 9103 that connects to the customer’s voltage source which needs to be isolated from ground (floating).

9103DBPR (shown below) has two BNC connectors on the front panel of the 9103 and the 9103 floats on the customer’s ground referenced DC voltage source.

The 9103DBPR is the most common external bias option as most voltage sources are referenced to ground (not isolated).

Two BNC connectors are used to float the 9103 up to as much as +/- 600V DC with an external low noise voltage source (provided be the user).    The voltage source is connected to the HV (signal ground) BNC connector and the 9103 and the current source (DUT) is connected to the Input BNC connector.

The maximum voltage that can be applied in this mode is 600V DC (that is the rating of the BNC input on the 9103 USB picoammeter).

External Bias Dual BNC

Floating – In this mode the picoammeter signal ground is raised from near chassis ground up to a value as high as +/- 5000V DC.   If you were to measure between the signal input and the signal ground reference the voltage would be low (under 1V typically). However measuring the DC voltage from the signal reference to chassis ground the voltage would be high, the floating supply value.

When floating the 9103 USB picoammeter there are some safety considerations. First of all, the input to the 9103 is changed from a BNC to either a SHV or MHV connector (user specified). These connectors are rated for up to 5kV. The recorder output on the back of the 9103 also floats. For example if you apply a floating voltage of 1000V the recorder output would also measure 1000V with respect to chassis ground. So if you are using the recorder output and also floating the 9103 the meter that you are measuring the recorder output with also needs to be isolated from ground. Finally, to prevent voltage spikes a current limit resistor is placed between the floating supply and the 9103 signal ground.   The floating mode is typically used to measure the collector output of an electron multiplier or a Faraday cup.

The maximum float voltage with the latest rev 9103 is +/- 5000 V DC.   If your 9103 was purchased before January 2019 then it can only float up to 1500V DC maximum.

The floating mode cannot be used with either the internal or external bias modes. However you really would not need either of those options as you can float the 9103 up to get what ever bias you need. For example, let’s say you want to measure an ion beam that is at 3000V DC and you wanted to retard the ions by 200V. You could simple set the bias supply to +3200 V DC and you would be retarding the 3000 V beam by 200V DC.

For all modes of operation the chassis ground of the 9103 USB picoammeter should be connected to the ground on the PC or chamber.

If you would like more detailed information please contact us via email at the link in the upper right hand corner of our website at www rbdinstruments dot com.