Nondestructive testing system using a SQUID

In order to enhance the sensitivity of a nondestructive testing system, a pair of superconducting coils are disposed in the same plane such that a current flowing through the respective coils when exposed to a uniform magnetic field cancels out. As a result of this configuration, the detection coils are immune to noise, offset fields or other uniform ambient phenomena. In one embodiment, the nondestructive testing unit includes a plurality of detection coils, a SQUID having a pair of connectors for connection to the detection coils, a probe for supporting the detection coils and the SQUID in a coolant, a cryostat for supporting the probe and for keeping the coolant constant, a controller for processing a signal transmitted from the SQUID, and a display device for displaying the result of the processing. At least two detection coils are disposed in the same plane, are directly connected to the SQUID and are integrated on a semiconductor substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to a nondestructive testing system using a 
superconducting quantum interference device (SQUID) to be applied to 
high-sensitivity magnetic sensors. 
As described in the Patent JP 1-245149, conventional nondestructive testing 
systems using a SQUID have been composed of a magnet meter and detection 
coils, in which a detection coil detects the magnetic field in the 
direction that is vertical to the SQUID and the sample, and the detection 
coils are, such as first- or higher-order vertical direction derivative 
coils. FIG. 2 shows the configuration of the nondestructive testing system 
using first-order bobbin-type derivative coils. As shown in FIG. 2, SQUID 
4 comprises a superconductive closed circuit u1 having two Josephson 
junctions 42, 43, an input coil 44, a feedback coil 45, each of which are 
connected thereto, and a detection coil 5 which detects the magnetic field 
that is vertical to the coil surface is connected to said input coil 44. 
Conventional nondestructive testing systems have had severe drawbacks in 
that background noise, possibly generated by a difference in level or a 
failure in uniformity or a problem with the state of the weld on the edge 
or surface of the sample, is significantly large and has caused problems 
in the detection of any small, weak signal given off by defects or 
scratches. 
Furthermore, when measurement is made by applying an electric field and a 
magnetic field to the sample, offset noise is given off by the resulting 
magnetic field of the applied electrical field and the applied magnetic 
field. This noise has been known to bury the small, weak signal given off 
by micro-defects or scratches. 
SUMMARY OF THE INVENTION 
The object of the present invention is to offer a simple configuration, 
high-sensitivity nondestructive testing system which reduces background 
noise and/or offset noise generated by the applied magnetic field or by 
the surface and shape of the sample. 
In conventional nondestructive testing systems, spatial resolution has been 
improved by minimizing the diameter of the detection coil, conventional 
sensitivity has been assured by increasing the number of turns, and the 
efficiency of magnetic flux transmission from said detection coils to the 
SQUID has been maximized by equalizing the inductance of the detection 
coil and the SQUID's input inductance. This provides conventional 
nondestructive testing systems with high sensitivity characteristics. 
However, the (equivalent) area, wherein the magnetic flux intersects, 
increases in proportion to the number of turns, but the inductance of the 
detection coil increases in proportion to the square of the number of 
turns. Thus, the inductance of the detection coil is restricted to the 
value of the SQUID's input inductance. In fact, detection coils ranging 
from one millimeter to several tens of millimeters have been used. To 
accurately detect scratches or defects in a submillimeter order, detection 
coils of a submillimeter order have been found to be required. The purpose 
of the present invention is to offer a nondestructive testing system which 
is capable of detecting small, weak defects, scratches, or deterioration, 
identifying and detecting adjacent defects which have not so far been 
detected, with highly-improved spatial resolution. 
In order to solve the problems described above, the present invention is 
configured in such a way that two or more detection coils are aligned on 
the plane, wherein the base line which connects the center of each 
detection coil is parallel to the surface or the axis of the sample, and 
connected so that the direction of the current generated when said 
detection coils are positioned in the uniform magnetic field is negated. 
Said coils detect only the change on the plane that is vertical to the 
magnetic field. 
In order to solve the above problems, the present invention is also 
configured in such a way that said detection coils are directly connected 
to the SQUID and both are integrated on a semiconductor substrate. Because 
the nondestructive testing system configured above detects only the 
differential amount (strength) of the magnetic flux which intersects two 
or more detection coils or loops, the magnetic flux which intersects two 
or more coils or loops becomes nearly equal. Consequently, said 
nondestructive testing system can eliminate noise, such as offset noise. 
Said device can also effectively attenuate (damp) background noise and 
other noise, whose differential amount is small and whose variation is 
slow. 
Integrating said detection coils and the SQUID increases the spatial 
resolution without reducing the transmission efficiency of the magnetic 
flux, and micro scratches or defects can be detected with high accuracy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention in accordance with drawings are 
explained below. 
FIG. 1 shows an embodiment of the present invention comprising coolant 1 
which creates a superconductive status, cryostat 3 which maintains coolant 
1 and supports a probe 2, SQUID 4 as well as detection coil 5, each of 
which are attached to said probe, a driver 6 which is electrically 
connected to SQUID 4, drives SQUID 4 and measures signal output, and a 
signal processor 7 which analyzes and displays said output signal. 
FIG. 3 shows an embodiment wherein two detection coils of equal area are 
attached, 51 and 52. The two detection coils are aligned on the same plane 
with one end of the first detection coil 51 connected to one end of the 
second detection coil 52, and the other end of both detection coils are 
each attached to a connector on SQUID 4. The direction of the current 
generated when the first detection coil 51 and the second detection coil 
52 are positioned inside the uniform magnetic field is negated by SQUID 4. 
In this type of configuration, when each value of the magnetic field, 
which intersects each detection coil is the same, the current is offset, 
nothing is generated, and thus no signal is detected. However, when the 
value of the magnetic field differs, a current is generated in proportion 
to the differential of the magnetic field intersecting the coils. Said 
current is transmitted to SQUID 4, and becomes a detection signal 
corresponding to the differential of the magnetic field, and it is output 
to the processor. 
In this embodiment, 1-loop type coils have been used in first detection 
coil 51 and second detection coil 52. However, when the area and the 
number of turns are the same, coils with a plural number of turns can also 
be used. FIG. 4 shows another embodiment wherein detection coil 5 is 
composed of 2 detection coils. That is, first detection coil 51 and second 
detection coil 52, whose area is equal, are aligned on the same plane, one 
end of first detection coil 51, one end of second detection coil 52, and 
one end of SQUID 4 are connected, the other end of first detection coil 
51, the other end of second detection coil and the other end of SQUID 4 
are connected, the direction of the current to be generated when first 
detection coil 51 and second detection coil 52 are positioned inside the 
uniform magnetic field becomes equal, and said current flows inside the 
closed circuit to be established by first detection coil 51 and second 
detection coil 52. With this configuration, when the value of the magnetic 
field which intersects each detection coil is the same, said current does 
not flow through SQUID 4 and no signal is detected. However, when the 
value of the magnetic field differs, the current that is in proportion to 
the differential amount of the magnetic field which intersects each 
detection coil flows through SQUID 4. Said current is transmitted to SQUID 
4 and becomes the detection signal which corresponds to the differential 
amount of the magnetic field, and said signal is output to the processor. 
In this embodiment, 1-loop type coils have been used in first detection 
coil 51 and second detection coil 52. However, when the area and the 
number of turns are the same, coils of a plural number of turns can also 
be used. FIG. 5 shows an embodiment wherein detection coil 5 is composed 
of 3 detection coils. That is, first detection coil 51, second detection 
coil 52, whose area is equal, and detection coil 53, whose area is the 
same as that of said two detection coils, but whose number of turns are 
twice as many as that of said two detection coils, are aligned on the same 
plane, one end of first detection coil 51 and one end of third detection 
coil 53 are connected, the other end of third detection coil 53 and one 
end of second detection coil 52 are connected, the other end of first coil 
51 and one end of SQUID 4 are connected, the other end of second detection 
coil 52 and the other end of SQUID 4 are connected, and the direction of 
the current to be generated when first detection coil 51, second detection 
coil 52, and third detection coil 53 are positioned inside the uniform 
magnetic filed is the same direction relative to SQUID 4 for first 
detection coil 51 and second detection coil 52, and is the opposite 
direction relative to SQUID 4 for third detection coil 53. By establishing 
this configuration, said current offset, as described above is not 
generated, and no signal is detected when the value of the magnetic field 
which intersects each detection coil is the same. However, when the value 
of the magnetic field differs, the current that is in proportion to the 
difference between differential amounts of the magnetic field, which 
intersects first detection coil 51 and third detection coil 53, and the 
differential amount of the magnetic field which intersects second 
detection coil 52 and third detection coil 53, namely, second derivative 
value in the uniaxial direction of the spatial magnetic distribution is 
generated. Said current is transmitted to SQUID 4 and becomes a detection 
signal which corresponds to the second derivative value of magnetic field, 
then said signal is output. 
In this embodiment, 1-type turn coils have been used in first detection 
coil 51 and second detection coil 52, and 2-turn type coils have been used 
in third detection coil 53. However, when the volume of the area and the 
number of turns of the first detection coil 51 and the second detection 
coil 52 are the same, and the volume of the area and the number of turns 
of the third detection coil 53 are twice as many as those of said two 
coils 51 and 52, coils of a plural number of turns can also be used. 
FIG. 6 shows an embodiment, wherein detection coil 5 is magnetically 
connected to SQUID 4. SQUID 4 is configured by a superconductive closed 
circuit 41 having two Josephson junctions 42 and 43, an input coil 44 and 
a feedback coil 45 connected thereto. Detection coil 5 in accordance with 
the present invention is connected to input coil 44, and thereby the 
detected signal is transmitted to SQUID 4. 
FIG. 7 shows an embodiment, wherein detection coil 5 is directly connected 
to SQUID 4. SQUID 4 is configured by a superconductive closed circuit 41 
having Josephson junctions 42 and 43, and a feedback coil 45 connected 
thereto, and a detection coil 5 in accordance with the present invention 
is connected directly to said superconductive closed circuit 41 so that 
detection coil 5 forms a part of superconductive closed circuit 41. The 
signal detected by detection coil 5 is directly transmitted to a 
superconductive closed circuit 41 in a form of current signal, and thereby 
detected by SQUID 4. 
FIG. 8 shows an embodiment, wherein first detection coil 51 and second 
detection coil 52 are connected to SQUID 4 and both are integrated on a 
semiconductor substrate. In this embodiment, in order to further increase 
the signal transmission efficiency between detection coil 5 and 
superconductive closed circuit 41 in the embodiment illustrated in FIG. 7, 
detection coil 5, which has so far been connected to SQUID 4, and 
superconductive closed circuit 41, are united, integrated to form a 
superconductive loop having two Josephson junctions 42 and 43. 
Superconductive loops 8 consisting of two loops with the same area are 
configured on the same plane. Said two loops are symmetrical and opposite 
as if the one end were twisted 180.degree. (like 8-shaped), wherein one 
end of said loop 8 is secured. When said superconductive loop 8 is 
positioned inside the uniform magnetic field, magnetic flux intersects in 
such a direction that individual opposite current generates in two loops. 
Because of this, there is no magnetic flux which actually intersects the 
whole loop, and, accordingly, no signal is detected. However, since the 
different amount of magnetic flux intersects two loops when said 
superconductive loop 8 is positioned inside the gradient magnetic field, 
for instance, the magnetic flux corresponding to the difference intersects 
(on) the whole of the loops, and the signal in proportion to the spatial 
difference of the gradient magnetic field in the distance between two 
loops is detected. 
FIG. 9 shows another embodiment, wherein first detection coil 51 and second 
detection coil 52 are connected to SQUID 4, and they are integrated on a 
semiconductor substrate. This is another embodiment, wherein the shape of 
superconductive loop 8 is different from that illustrated in FIG. 8. 
However, the effect is the same. In this embodiment, said superconductive 
loop 8 is configured by two superconductive loops having the same area. 
Said loops are configured symmetrically and on the same plane, interposing 
two Josephson junctions 42 and 43 which are aligned in series. 
FIG. 10 is a configuration showing an example of a nondestructive testing 
system in accordance with the present invention. The present invention 
comprises coolant 1 which produces a superconductive status, cryostat 3 
which maintains said coolant and supports a probe 2, SQUID 4 attached to 
said probe 2, a detection coil 5, a driver 6 which is electrically 
connected to SQUID 4, drives SQUID 4, and measures the signal output, a 
movable stage 11 used for sample scanning, a stage controller 12 which 
drives said movable stage 11 and detects its position, a sample 9 having a 
scratch 10 which is placed on said movable stage 11, and signal processor 
which analyzes and displays the output signal from said driver 6 and the 
signal output from said stage controller 12. 
FIG. 11 is an overview showing the positional relationship between 
detection coil 51 and sample 10 illustrated in FIG. 10. Detection coil 5 
is the coil illustrated in FIG. 3 and is aligned so that the face to be 
created by first detection coil 51 and second detection coil 52 and the 
surface of sample 9 are parallel. 
FIG. 12 is an example of measurement results obtained using conventional 
uniaxial embodiment according to sample scanning direction 13 illustrated 
in FIG.11. 
FIG. 13 is an example of measurement results obtained using the present 
uniaxial embodiment according to said sample scanning direction 13 
illustrated in FIG. 11. Change in the magnetic field due to scratch 10 is 
minute and significantly smaller than that of magnetic field which said 
sample 9 itself possesses inherently. FIG. 12 is the result obtained by 
measuring said sample using a conventional instrument that detects the 
amount of the magnetic field in the Z direction and is provided with coils 
illustrated in FIG. 2. In this case, substantial signal to be generated 
from scratch 10 is buried in background noise generated from sample 9. 
This makes it difficult to correctly measure the position and dimensions 
of the scratch. However, when measuring said sample 9 using the instrument 
illustrated in FIGS. 10 and 11, the result is obtained as shown in FIG. 
13. Since the change in the magnetic field due to scratch 10 is 
significantly sharp, compared to that due to sample 9, when the change in 
the magnetic field is detected, a signal corresponding to the change in 
the magnetic field due to scratch 10 is detected by the signal lager than 
that corresponding to the change in the magnetic field due to sample 9. 
2-dimensional nondestructive testing is also possible by using a means 
which activates cryostat 3 and the XY movable stage, in addition to the 
uniaxial movable stage illustrated in FIG. 10. Of course, either the 
sample stage or the probe can be scanned. 
Square-type (or Squared) detection coils have been used in the present 
embodiment. However, the same effect is also obtained by using either 
round or polygonal coils. With respect to SQUID 4, the same effect can be 
obtained by using the RF-SQUID which is configured by the superconductive 
closed circuit including only one Josephson junction. 
As described thus far, the following effects are obtained in accordance 
with the present invention; 
Noise Restriction Effect: Small, weak signals buried by background noise 
can be detected. In the present invention, in order to detect not the 
dimensions, but the amount of change with respect to the magnetic field, 
small signals due to smaller changes in the magnetic field and large 
signals due to larger changes in the magnetic field can be detected. By 
this, background noise due to the magnetism of the Earth for which change 
is slow, and the inherent magnetic field that the sample itself possesses 
are restricted to a minimum, and signals, whose change is sharp and 
prompt, due to scratches, defects and change in composition are detected 
with high accuracy. 
A Decrease in Offset Noise and an Increase in Detection Sensitivity: When a 
current or a magnetic field is applied externally to the sample, or 
measurements are done outside the magnetic shield room, offset noise can 
be decreased and detection sensitivity can be increased. 
Improvement of Spatial Resolution: The spatial resolution for detecting 
micro scratches or defects to be generated in a narrow range of change in 
the magnetic field has been improved. Submicron-order defects can be 
detected using a 5 mm-square chip, onto which a 50 .mu.m-square detection 
coil and SQUID are integrated on a semiconductor substrate. 
Defects such as scratches which exist in a deep position away from the 
surface of the substance being tested can be measured.