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A G R E AT E R M E A S U R E O F C O N F I D E N C E
Current
Source
Voltmeter
Figure 1. Four-Point Collinear Probe
Instrumentation and
Configuration
= [/ln2] * [V/I] * t * k
Techniques for Measuring where: = volume resistivity (ohm-cm)
High Resistivity and Hall
V= measured voltage (volts)
I = source current (amperes)
t = sample thickness (cm)
Voltage of Semiconducting k = a correction factor based on the
ratio of the probe to wafer di-
Materials
ameter and on the ratio of wafer
thickness to probe separation1.
van der Pauw Resistivity
Measurements
Mary Anne Tupta, Keithley Instruments, Inc. The van der Pauw method involves ap-
plying a current and measuring voltage us-
T
ing four small contacts on the periphery of
HE resistivity and Hall mobility of conductor measurements are the four-point a flat, arbitrarily shaped sample of uniform
semiconducting materials are fun- collinear probe method and the van der thickness. This method is particularly useful
damental properties investigated Pauw method. There are variations in the for measuring very small samples because
during product and process devel- instrumentation depending on whether the geometric spacing of the contacts is unim-
opment. For example, the resistiv- material has high or low resistivity. The fo- portant. Effects due to a sample's size, which
ity of a semiconductor device is primarily cus of this article is on instrumentation and is the approximate probe spacing, are irrel-
dependent on bulk doping and can affect measurement techniques for high resistivity evant.
capacitance, series resistance, and threshold semiconductor material. Resistivity derivation. Using this meth-
voltage. Therefore, accurate measurements od, the resistivity can be derived from a total
of these properties are essential. Measure- Four-Point Collinear Probe Method of eight measurements that are made around
ment techniques and instrumentation affect for Resistivity the periphery of the sample with the configu-
the level of accuracy and difficulty in con- The most common way of measuring the rations shown in Figure 2. Once all the volt-
ducting these tests. resistivity of a semiconductor material is by age measurements are taken, two values of
using a four-point collinear probe. This tech- resistivity, A and B, are derived as follows:
Instrumentation Issues nique involves bringing four equally spaced A = [/ln2] * [fAt] * [(V1