# Four Point Probe ```Four Point Probe
Overview
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What is Four Point Probing
How the system works
Simple Calculations behind Four Point
Probing
Methods and Procedures for using Four Point
Probing
Why is it important
Conclusion
What is Four Point Probing
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Four Point Probing is a
method for measuring
the resistivity of a
substance.
How the system works
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In order to measure the
resistivity of a substance,
four points of contact must
be made with the probe and
the substance.
Current goes through the
two outer probes, and the
difference in voltage is
measured between the two
inner probes.
Through this process the
resistance can be
calculated.
Simple Calculations
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If the spacing between the probe points is
constant, and the conducting film thickness
is less than 40%, and the edges of the film
are more than 4 times the spacing distance
from the measurement point, the avg
resistance of the substance is given by…
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Rs= k x V/I
For bulk resistivity, the sample is much
larger than the spacing of the probes, the
sheet resistance is defined as the resistance
equal to the bulk resistivity divided by the
sample thickness. The equation is given
by…
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Rs= P/t
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Terms
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Rs = Resistivity of the
substance
K = constant for the
average resistivity that is
equal to 4.53
P = Resistance
t = thickness of the
substance
Procedures and Methods for
determining the resisitivity of a
sample
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One way to determine
the resistivity of a
sample is to use a
machine.
There are may
machines that can do
that and they range in
the thousands of
dollars.
One of those machines
is the Pro-4.
The Pro-4
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The Pro-4 is one of the methods used to
measure the resistivity of a sample no
matter what the thickness of the sample
is.
The Pro-4 uses and is connected to a
computer and power source/ voltmeter.
The Pro-4 is controlled by a computer for
easy use.The computer automatically
controls the Keithley 2400
(current/voltage source) series and steps
through a number of current settings to
find the ideal current for accurate
readings. A V/I measurement is taken
and recorded. The system uses the Dual
Configuration test method of ASTM
Standard F84-99 to compensate for
errors in probe spacing and errors
caused by proximity to the edge of the
conducting layer. Proper use of the
standards and the calibration procedure
insures the specified system accuracy of
better than 1%
Procedures for using the Pro-4
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1. Click on the tab that says auto test
2. Then insert the required information
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3. Position the sample dead center on
the mounting table.
4. Then move the sample to the required
location
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All of this information can be found out the
samples packaging label.
Usually you start in the middle, then move
it to the left, then top, then the right, and
finish up with the bottom.
After it is finish, values of currents,
resistivity, difference in voltage, and other
values will be given.
The program will also calculate if the
given substance pass or failed the test of
resistivity.
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In order to pass, the substance must
resistivity must be uniform throughout the
entire substance. The resistivity can be off
only in the range specified by the user.
Usually is 3% to 10% range.
Why is it important?
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Resistivity is a particularly important
semiconductor parameter because it can be
related directly to the impurity content of a
sample.
The purity of a substance is an important
factor when it comes manufacturing or
engineering certain projects.
Conclusion
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Four Point Probing is an important method of
calculating the resistivity of a substance.
Four Point Probing is very useful and needed
in the micro electronics world.
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