Questions and Answers from Customers using Jandel Four Point Probing Equipment

Note: Question 1 and 2 shown below are in reference to Jandel's previous model model, the RM3 Test Unit, which has been replaced by the RM3-AR Test Unit which has an auto-ranging feature which can calculate for the user the best choice of input current based upon the material being measured. The balance of the questions refer to the long discontinued, RM2 Test Unit (four point probe electronics). The RM3-AR Test Unit reads-out directly in ohms-per-square (or can be toggled to read-out in mV) and is provided with PC software at no additional cost. The RM3-AR also has a greater measurement range on both ends of the spectrum and it includes onboard NVM memory which can store up to 50 measured values.

Q1.  I understand that the RM3 Test Unit can read out directly in ohms-per-square for use when measuring thin films (sheet resistance), but how do I measure thick materials that are measured in ohms-cm (volume resistivity)? What current level do I select for a particular material?

A.  The RM3 Test Unit includes PC software which will calculate that for you. However, if you want to use the RM3 Test Unit without a PC, the millivolt value displayed on the front of the RM3 will allow that to be calculated. When making an ohm-cm measurement, ideally you will want to use a current which will simplify the math.

The formula is 2 x pi x s x V/I where s is the spacing between each of the needles in cm. If you use a probe with tip spacing of 1.591mm (~same as 62.6 mils), it makes the math easier since 0.1591 is 1/(2 x pi). Therefore we would have: Resistivity = V/I

This means that if 1mA current is used, then the measured voltage value (in millivolts) = the resistivity of the sample in ohms-cm.

If you want to measure on the 'High' range it may be that the voltage will be too high to measure. In this case you could try 100uA and the mV result would need multiplying by 10. If the voltage value is quite low (maybe 9mV or so) you could increase the current to 10mA and then the mV result could be divided by 10 to give the resistivity (higher currents can sometimes offer more stable results).

Q2.  Could you please give more information on how the Jandel RM3 test unit calculates the resistivity? Is it merely an implementation of one of the equations in the 4-point probe manuals depending on the needle arrrangement used and whether it is a thin film or bulk material?

A.   When measuring sheet resistance (which calculations can be used for most homogeneous silicon wafers, depending on the thickness), the RM3 Test Unit forces a constant current across the outer two probes and then the resulting voltage is read across the inner two probes. If a constant current of 4.5324 milliamps is used, then the resulting voltage in millivolts is numerically equivalent to the sheet resistance value in ohms-per-square. Some materials require a greater or lesser current level and an appropriate adjustment must be made to the voltage read so that the value will be correct in ohms-per-square. For example, if you needed to use 10 times the current, then the voltage would be 10 times higher and it would need to be divided by 10 to be the correct value in ohms-per-square. If you need to calculate bulk resistivity from a sheet resistance measurement, the value in ohms-per-square can be multiplied times the thickness of the material in centimeters to arrive at the bulk (or volume) resistivity value which is expressed in "ohms-cm". For example, if you measured a thin film that was 100 microns thick, and the sheet resistance value was 20 ohms-per-square, then the volume resistivity for that material would be 0.2 ohms-cm (0.01 x 20). The RM3 Test Unit includes PC software which prompts you for information about what you are measuring (bulk materials, wafers, thin films) and then it calculates the volume resistivity or sheet resistance for you. The RM3 will also read out directly in ohms-per-square (or millivolts) without the need to use the software. If you are measuring materials that are thicker than 62.5% of the spacing between two of the probes (after which sheet resistance would need more than 1% correction) then the calculations (using the RM3 software) will take into account the probe tip spacing since this becomes a volume resistivity measurement expressed in ohms-cm. The RM3 Test Unit is a specialty instrument designed for the four point probe measurement, and it includes an accurate current source and a sensitive volt meter with preset current levels, software and other features which make the four point probe measurement easily accomplished. The software applies the basic four point probe equations giving it the ability to calculate sheet resistance or bulk resistivity by taking into account some user inputs such as material thickness and probe tip spacing.

The following Q&A's refer to questions asked about the now discontinued RM2 Test Unit, however, some of the information applies to Jandel's current equipment as well.

Q3.  Is this statement correct?....By setting the current to 453 microamps, I'm putting that amount of current through two of the probe tips. The other 2 tips are then reading a voltage value which is the value that shows up in the RM2 output unit as a mV reading?

A.  That is correct - on our equipment the outer two probes are used for current, and the inner two are used for voltage measurement. We tend to set the RM2 (if the samples are suitable) to deliver a current of 4.5324 milliamps (453.24 on the x10 setting). This means that the measured voltage drop displayed in millivolts is numerically equal to the sheet resistance in ohms per square.

Q4.  If I used a different current input, I would then need to calculate sheet resistance from the formula, right?

A.  Correct.

Q5.  Compliance is set at 25V currently. How do you determine what to set this at?

A.  Generally it would be a difficult sample if more than 25V was required (however, the RM3-AR Test Unit has 40 volts available to drive the measurement). A number of machines by manufacturers such as Keithley have a compliance voltage of around 6V which often is not sufficient. On occasion if a current cannot be driven through the sample the red light under "compliance voltage" on the RM2 will glow. It is possible that a higher compliance voltage (i.e. moving the knob to 50V) will help. There is no problem with leaving the compliance voltage set at 50V at all times.

Q6.  On the Jandel RM2 Test Unit, what is the "Filter" feature? If this improves the reading, when would you ever want to shut it off?

A.  The filter can slow readings where a small current is required. At times it is absolutely essential, however with some samples which give stable readings speed of readings can be improved by not using the filter.

Q7.   On the Jandel RM2 Test Unit, regarding the "200mV / 2000mV" buttons - Does this change the reading range that I could possibly see as an output voltage or is there another purpose for the buttons?

A.  It merely changes the range so that you can get an optimum readout. At times readings could be, for instance, 976mV. This would not show on the 200mV range, but would show on the 2000mV range. Conversely a reading of 10.34mV would be better displayed on the 200mV setting.

Q8.  On the Jandel RM2 Test Unit, regarding the X1 / X10 buttons--When is it beneficial to switch these buttons? I thought X10 decreases the current by a factor of 10...why can't I just change it on the current output push-button switches?

A.  When set to "x1", the current is as displayed on the push button switches. As expected, the x10 button increases the current by a factor of 10. Although current of 4.5324mA makes it easy to translate the voltage directly into ohms-per-square, some samples - for instance those with very high resistance - cannot have measurements made with such a high current. We have at times had to measure samples using only 0.01 microamps current. These buttons increase the range of current available, but when possible 4.5324mA is fine (453.24 x 10).

Q9.  This is what happens when I go to probe my sample. The probe needles touch the sample as the micro-switch is first conacted. The arm that lowers the probe head moves down approximately 1/8" with no change in the needles/head but the switch itself becomes fully made. Last, it appears as if the probe head cover lowers until the probe head housing contacts the sample while the needles are pushed into the probe head. Is this how it should work with a correct height setting?

A.  Exactly right. The purpose of the perspex 'pad' is to prevent the metal part of the probe head (which while anodised, could cause a problem) touching the sample. It also acts as a height guard. The allocated probe load is set at the point where the needles are retracted into the probe body to the level of the pad. The function of the microswitch (which as you have seen, activates when the probe is already in contact with the sample, and deactivates before the needles leave the sample) is to prevent current flowing during the time when the probe is making/leaving contact with the sample. Current flowing at this point could cause 'sparking' which would shorten the lifetime of the probe if not damage it permanently.

Q10.   How do you adjust the needle load? (It's fine as is but I'm just curious because it sounds like it is adjustable feature.)

A.   There is a red screw in the top of the probe head with a short metal pin protruding. Moving the pin in a clockwise direction will increase the load, and in a counter-clockwise direction reduce the load. The indications at the top of the probe - 'L' and 'H' - are for 'high' and 'low'. The probe as set here at Jandel will be +/-5g of the stated load. While it is not possible to indicate graduations on the probe head, some samples may work better with the load at 'L' (approx. 60g), and some will work better at 'H' (approx. 150g). A medium setting will work for a wide range of materials. In some instances - for example when a probe is being used for silicon substrates - an even higher load is sometimes required. In such a case we sometimes provide a probe head with the load set at 200g and put in place a red cap to prevent adjustment of the load.

Q11.  On p. 11 in the manual, it warns about causing damage to the probe needles if the controller is in FWD mode when the probe head is not touching the specimen. I thought the micro-switch is there to prevent it from trying increase the voltage/take a reading when the probe head is not contacting the sample. Is this true?

A.  This is true. The microswitch is there as a fail-safe. An operator making a series of measurements could forget to set the meter to standby. Any damage from 'sparking' could only occur if the meter was not returned to Standby AND the microswitch was incorrectly set. See the answer to Q7 above.

Q12.  What if I forget to switch the controller to SBY [Standby] mode before raising the probe head off the sample? Will it damage the probe head?

A.  If the microswitch was not properly set and if the current is flowing, then damage can occur from sparking when the probes are either brought down into contact or lifted up out of contact. Setting the microswitch properly prevents this from being an issue. See the answer to Q7 above.

Q13.  How often do you recommend calibrating the RM2 test unit?

A.  Probe heads are 'consumables'. Inconsistent readings may occur which could indicate the probe head is nearing the end of its life. However, at any point where inconsistent readings / unexpected readings are obtained it may be worth calibrating the RM2 to see if this leads to improvement. For 'routine' calibration once a year should be sufficient. We would expect a probe head's life to be in the region of 50000 to 100000 touchdowns, but this can vary depending on the samples being measured.

Q14.   Is the Jandel Lead marked 4PL a special item that you can buy to make the calibration easier or is it just referring to the cable that connects the prober to the test unit? (p. 13, item 3 under equipment required) What is the cost of this?

A.  The equipment should have been supplied with a '4PL' lead - the lead between the RM2 and the probe station. Generally this is marked on the lead but may have been omitted as the indications are really for users of our Universal probe. The Universal probe has the facility to make spreading resistance measurements and has a '4PL' and 'SRL' lead (4-point lead and spreading resistance lead).

Q15.  Is there any way to calibrate the probe head itself? Does this need to be done regularly? How do you know when it's time to replace the actual probe head?

A.  The probe head cannot be calibrated by the user and calibration is not required. Erratic or unexpected readings can indicate that the probe requires replacement/overhaul.

Q16.  How do I determine the proper amount of current to inject into my sample?

A.  This is dependent to some extent on the nature of the sample. If it is a homogenous wafer you need a sufficiently high test current to give a good indication on the voltmeter - say between 20mV and 100mV. If it is a shallow implant or film you should be aware of the possibility of damaging the sample when using a high current. We commonly use 4.5324mA so that the mV reading is numerically equal to the sheet resistance in ohms/square.

Q17.  What problems I might encounter in using four-point probe technique when my silicon sample is highly resistive?

A.  You will need to use only a low current, because the measured voltage will be high. You could find you cannot drive the test current you need because of high contact resistance. This can be due to the 4 point probe itself which needs to make good contact by high pressure. You may need a higher voltage to drive the test current - our equipments have between 40-50V available.

Q18.  For p type and n type silicon wafer, resistivity in the center is higher or resistivity near the edge is higher? Why is that so?

A.  If you measure very near the edge of a uniform wafer there is a need to apply a correction factor, so that even if the measurement appears high it may be OK after correction. Correction factors are available from http://www.fourpointprobes.com/haldor.html Alternatively the wafer may have non-uniform resistivity.

Four-Point-Probes is a division of Bridge Technology. To request further information please call Bridge Technology at (480) 988-2256 or send e-mail to Larry Bridge at: sales@bridgetec.com