Four-terminal measuring device that uses nanotube terminals

Details Four-terminal measuring device that uses nanotube terminals Four-terminal measuring device that uses nanotube terminals

2001-07-13

2003-10-21

Le, N. (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S715000

Reexamination Certificate

active

06636050

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a four-terminal measuring device which measures a very small resistance or very small impedance of an object under test, and more specifically relates to a four-terminal measuring device which can measure extremely small regions of an object under test, or which can measure an object under test of extremely small size, by using nanotube terminals.
2. Prior Art
Conventionally, in order to measure the resistance Rx of an object under test, a method has been used in which a voltage is applied across both ends of the object under test, the current I flowing through the object under test and the voltage V across both ends of the object under test are measured, and then the resistance Rx is calculated using the formula Rx=V/I. A problem that has arisen in such cases is a generation of measurement error caused by the resistance of connecting wires such as lead wires, etc. and the contact resistance between the connecting terminals and the object under test.
FIG. 6
shows a conventional two-terminal measuring device which is used to measure the DC resistance of an object under test. Connecting wires
32
and
34
such as lead wires used for measurement, etc. are connected to connecting terminals C
1
and C
2
on both ends of the object under test
30
, and a voltmeter
36
, ammeter
38
and DC power supply
40
are connected to these connecting wires
32
and
34
. The measured resistance Rm of the object under test is determined from the measured voltage V and current I as Rm=V/I=Rc
1
+Rx+Rc
2
+Rc, where Rx is the resistance of the object under test
30
, Rc
1
and Rc
2
are the resistances of the connecting wires
32
and
34
, and Rc is the contact resistance.
However, it is seen that the connecting wire resistances Rc
1
and Rc
2
and contact resistance Rc are admixed in this measured resistance Rm as errors. From the ratio of Rm/Rx, the error rate is given by (Rc
1
+Rc
2
+Rc)/Rx; accordingly, it is seen that the error rate increase abruptly as the resistance Rx of the object under test becomes very small. Consequently, the two-terminal measurement method is not suitable for the measurement of low resistance.
As a result, a four-terminal measuring device has been developed as a device for measuring low resistances.
FIG. 7
is a circuit diagram of a four-terminal measuring device. Here, elements that are the same as in
FIG. 6
are labeled with the same reference numeral, and only those elements that differ will be described. The connecting terminals C
1
and C
2
are respectively split in two, so that current terminals Ci
1
and Ci
2
and voltage terminals Cv
1
and Cv
2
are provided. Current connecting wires
42
and
44
and voltage connecting wires
46
and
48
are connected to these terminals as lead wires used for measurement. The resistances of the current connecting wires
42
and
44
are designated as Ri
1
and Ri
2
, and the resistances of the voltage connecting wires
46
and
48
are designated as Rv
1
and Rv
2
.
The effect as described in the following is obtained as a result of the provision of the current terminals Ci
1
and Ci
2
and voltage terminals Cv
1
and Cv
2
. The current I flowing in the direction indicated by the arrow flows through the ammeter
38
, current connecting wire
42
, object under test
30
, current connecting wire
44
and DC power supply
40
, and thus returns to the starting point. Since the internal resistance of the voltmeter
36
is extremely large, there is almost no branching of the entreat I into the voltmeter
36
; instead, the current flows “as is” into the object under test
30
. Accordingly, the current I flowing through the object under test
30
may be viewed as being measured by the ammeter
38
.
Meanwhile, since the internal resistance of the voltmeter
36
is far greater than the resistances Rv
1
and Rv
2
of the voltage connecting wires
46
and
48
, the voltage drops caused by the voltage connecting wires
46
and
48
can be more or less ignored. Thus, the voltage V that is measured by the voltmeter
36
may be viewed as the voltage across both ends of the object under test
30
.
Accordingly, the resistance Rx of the object under test
30
can be derived with good precision according to V/I from the current I and voltage V measured by the four-terminal measuring device. By changing from two terminals to four terminals, it becomes possible to ignore the connecting wire resistance and contact resistance. Thus, a four-terminal measuring device is an effective means to measure low resistances.
Such a four-terminal measuring device is certainly an effective device for measuring low resistances in oases where the object under test
30
is of a certain size or greater.
FIG. 8
shows a resistance measurement diagram for a plate-form object under test in which a four-terminal measuring device is used. The constant-current power supply
43
is constructed by means of a DC power supply
40
and a constant-current resistance
41
. The tip ends of the current connecting wires
42
and
44
(which are lead wires) are caused to contact the current terminals Ci
1
and Ci
2
of the plate-form object under test
30
, and the tip ends of the voltage connecting wires
46
and
48
are caused to contact voltage terminals Cv
1
and Cv
2
, which are disposed between the abovementioned terminals Ci
1
and Ci
2
. Then, the resistance Rx of the object under test between the voltage terminals Cv
1
and Cv
2
is derived according to V/I. In this example, the current terminals Ci
1
and Ci
2
and voltage terminals Cv
1
and Cv
2
refer to the contact points where the current connecting wires
42
and
44
and voltage connecting wires
46
and
48
contact the object under test
30
.
The current connecting wires
42
and
44
and voltage connecting wires
46
and
48
are lead wires whose tip ends are caused to contact the object under test
30
. Accordingly, as the size of the object under test
30
becomes smaller, it becomes necessary to reduce the size of the lead wires. Even though the points of current terminals Ci
1
and Ci
2
shift somewhat in the right or left direction of the object under test
30
, a constant current I flows between the voltage terminals Cv
1
and Cv
2
. Accordingly, there is no great problem even if the current terminals Ci
1
and Ci
2
are in a state of surface contact with the surface of the object under test
30
. However, because of precision requirements, the voltage terminals Cv
1
and Cv
2
must be single points in order to give both ends of the resistance that is being measured.
In other words, it is necessary to reduce the size of the tip ends of the voltage connecting wires. However, in conventional measuring devices, there are naturally limits to this size reduction. For example, even if the tip end of a metal needle is subjected to a sharpening treatment by means of electrolytic polishing, etc. or to an etching treatment using a semiconductor technique, it is difficult to reduce the size of the tip end to a nano-scale size. Meanwhile, as ultra-high densities have been developed in semiconductor techniques, objects under test have become progressively smaller. Today, for example, attempts have been made to manufacture nano-size electronic circuits, and resistance measurements and impedance measurements in the nano-region have become an absolute necessity. With conventional four-terminal measuring devices, it is absolutely impossible to handle such extremely small resistance measurements.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a four-terminal measuring device that measures, with a use of extremely small nanotube, extremely small resistances or extremely small impedance values with good precision even in cases where the object under test itself is extremely small or the region being measured is extremely small.
More specifically, the present invention relates to a four-terminal measuring device that uses nanotube terminals and i

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