Introduction

electrical electric resistance is a basic corporeal property that quantifies a material ‘s resistance to current flow ; it is the reciprocal of conduction. The electric resistance of a material depends upon respective factors, including the material dope, process, and environmental factors such as temperature and humidity. The electric resistance of the substantial can affect the characteristics of a device of which it ‘s made, such as the series immunity, doorway voltage, capacitor, and other parameters
Determining the electric resistance of a material is common in both inquiry and fabrication environments. There are many methods for determining the electric resistance of a material, but the technique may vary depending upon the character of substantial, magnitude of the resistor, form, and thickness of the substantial. One of the most coarse ways of measuring the electric resistance of some sparse, flat materials, such as semiconductors or conductive coatings, uses a four-point collinear probe. The four-point probe proficiency involves bringing four equally separated probes in contact with a material of stranger immunity. A DC current is forced between the outer two probes, and a voltmeter measures the voltage deviation between the inside two probes. The electric resistance is calculated from geometric factors, the source stream, and the electric potential measurement. The instrumentation used for this test includes a DC current reservoir, a sensitive voltmeter, and a fourpoint collinear probe .
To simplify measurements, a single musical instrument, the Model 2450 SourceMeter® Source Measure Unit ( SMU ) Instrument, can be used. This instrument can generator and measure both current and voltage and can be configured to display immunity or electric resistance. The Model 2450 enables the exploiter to choose a test current over many decades ( from picoamps to amps ), arsenic well as measure voltage with resolution in the microvolt range. The model 2450 is pictured in Figure 1 connected to a Signatone four-point collinear probe. This shape is set up to measure the electric resistance of a semiconductor wafer .

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Figure 1. Model 2450 SourceMeter SMU Instrument with the Lucas/Signatone Corporation (Gilroy, CA) Model SP4 4-Point Probe Head and S-302 Test Stand measuring the resistivity of a sample.

This application note explains how to perform electric resistance measurements on materials using the Model 2450 and a fourpoint collinear probe .

The Four-Point Collinear Probe Method

The four-point collinear probe technique involves bringing four equally separated probes in touch with a material of unknown underground. The probes, mounted into a probe head, are lightly placed in the center of the wafer as shown in the electric resistance test lap in Figure 2 .

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Figure 2. Four-point probe resistivity test circuit.

The two outer probes, 1 and 4, are used for sourcing stream. The two inside probes, 2 and 3, are used for measuring the result voltage drop across the surface of the sample .
The bulk or bulge electric resistance ( ρ ) is calculated as follows :

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where :
ρ = the book electric resistance ( Ω-cm )
V = the voltage measured between probes 2 and 3 ( electric potential )
I = the magnitude of the source current ( amps )
thymine = the sample thickness ( centimeter )
kilobyte = a correction factor based on the proportion of the probe to wafer diameter and on the proportion of wafer thickness to probe separation
For some materials such as reduce films and coatings, the tabloid resistor, or come on electric resistance, is determined alternatively, which does not take the thickness into account. The sheet resistance ( σ ) is calculated as follows :

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where :
σ = the sheet electric resistance ( Ω/square or just Ω )
note that the units for sheet resistance are expressed in terms of Ω/square in order to distinguish this number from the measured resistance ( V/I ) .

Using the Kelvin Technique to Eliminate Lead and Contact Resistance

Using four probes eliminates measurement errors due to the probe resistance, the spreading electric resistance under each probe, and the contact electric resistance between each metallic probe and the semiconductor material. Figure 3 is another representation of the four-point collinear probe apparatus that shows some of the lap resistances.

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Figure 3. Test setup showing circuit resistances.

The RL terms represent the test lead immunity. RC represents the liaison electric resistance between the alloy probe and the semiconductor device corporeal. The liaison electric resistance can be several hundred to a thousand times higher than the resistance of the sample material, which is represented by RS .
Notice that the current flows through all the resistances in the first and fourth determined of leads and probes, a well as through the semiconductor device material. however, the electric potential is only measured between probes 2 and 3. Given that between probes 2 and 3, the current merely flows through RS2, only the electric potential drop ascribable to RS2 will be measured by the voltmeter. All the other unwanted run ( R.L ) and contact ( RC ) resistances will not be measured .

Using Compensation Techniques to Correct for Voltage Offsets

For increased accuracy, a technique to compensate for voltage offsets can be applied. There are two coarse methods of recompense to reduce electric potential offsets : the offset compensation method acting and the current reversal method .
One method acting takes a electric potential measurement at zero amps and then subtracts this value from a take taken at the desire examination current. This method is called counterbalance recompense and is an choice that can be performed automatically in the resistance manner of the Model 2450 .
The early proficiency, the current transposition method, is similar to the early method but offers better signal to noise proportion. This technique cancels electric potential offsets by making two measurements with currents of opposite mutual opposition. When the two measurements are averaged, the voltage offsets are mathematically eliminated from the final result. This proficiency can be performed using software to control the Model 2450 or by downloading a script .
More data on either of these two methods can be found in Keithley ‘s Low Level Measurement Handbook, available at www.keithley.com .

Making Connections to the Four-Point Probe

The four-point probe can be connected to the Model 2450 using either the front panel banana jacks or the rear control panel triax connectors. A circuit diagram showing the Model 2450 connected to the four-point probe is shown in Figure 4. The storm terminals are connected to probes 1 and 4, and the sense terminals are connected to probes 2 and 3. The buttocks dialog box triax terminals should be used for higher electric resistance materials. In this encase, all cables, the probes, and the sample should besides be electrostatically shielded. Some materials are photograph sensitive and would require that the substantial be light shielded during the measurement .

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Figure 4. Connecting the Model 2450 to a four-point collinear probe head.

Configuring the Test

After the probe is connected to the input signal terminals of the Model 2450 and the sample is placed on the mounting chuck, follow these steps to measure the electric resistance of a sample :

  1. On the Model 2450, press the FUNCTION key and select Source Current and Measure Resistance (or Voltage). Choose the desired source current and voltage source limit (compliance) by using the front panel touch buttons. Set the Sense Mode to 4-Wire by pressing the MENU key, Measuring Settings, and then Sense Mode button
  2. Lower the probe head so the pins are in contact with the sample.
  3. Turn on the output of the Model 2450 and observe the reading. If the readings appear unstable, try increasing the test current. Make sure that the voltage drops between Force HI and Sense HI and also between Force LO and Sense LO do not exceed 5V.
  4. Calculate the resistivity from the readings

The electric resistance can be mechanically calculated using either the built-in “ mx+b ” Math Function or by using a script. A handwriting is a list of commands that can be downloaded into the Model 2450. The results of executing a bare script to calculate the electric resistance in ohms/square from the measured resistance are shown in Figure 5. Notice the deliberate electric resistance is displayed in the bottom one-half of the touch screen .

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Figure 5. Model 2450 displaying the measured resistance and resistivity of a sample.

Creating and Downloading Scripts to the Model 2450

To correct for electric potential offsets by performing current reversals or to display the measurements with units of Ω/square or Ω-cm, you can easily create and download a handwriting to the Model 2450. A script is a collection of TSP commands that you can download to the instrument using a USB drive. The scripts are executed from the presence panel and are stored in non-volatile memory. These can be added or deleted from a front gore menu
An model script to perform electric resistance measurementsis listed in Appendix A. In this script, the Model 2450 is programmed to perform a current reversion compensation technique using a user-prompted test current to measure the resistance. then, the surface electric resistance in ohms/square is calculated and displayed on the bottom half of the movement dialog box. The results of executing the script are shown in . Figure 5 .

Steps to Generate, Download, and Execute a Script

here are the simpleton steps to create and execute a script on the Model 2450 :

  1. By using either Notepad or Test Script Builder (TSB) Software, create a script to perform a resistivity measurement using the TSP command set. TSB is a software tool included with the Model 2450 that will enable you to execute the script before you download it to the unit. For your convenience, you can copy the script listed in this application note and modify it.
  2. Save the script as a .tsp file on a USB drive, and insert the USB drive into the Model 2450 front panel USB port.
  3. Once the USB drive is inserted, all .tsp files on it will appear in the MANAGE SCRIPTS window. This can be accessed by pressing the MENU key and then Manage Scripts as shown in Figure 6. Press on the desired USB script and press the ”

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    Figure 6. The Manage Scripts window enables the user to download scripts from a USB drive and store them into non-volatile memory.

  4. Once the script is stored into internal memory, the user can execute the script from the Home Page as shown in Figure 7. At the top of the Home Page, press the “No Script” button to view the available scripts. Press the desired script and the test will automatically execute.
  5. 2001 SPECIFIED CALIBRATION INTERVALS
    Figure 7. Pressing the desired script will automatically execute that script.

Conclusion

The Model 2450 SourceMeter SMU Instrument is an ideal tool for measuring electric resistance on samples using the four-point collinear probe method acting. This SMU instrument can source current and measure voltage or immunity using a four-wire method to prevent the contribute and contact resistor from affecting the measurement. The Model 2450 enables the drug user to specify the quiz current, measure the electric resistance using offset compensation, calculate the electric resistance, and shows the results on the display of the instrument .

Appendix A

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