Abstract:
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.

Description:
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&#39;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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a configuration of the nondestructive testing system of the invention; 
     FIG. 2 shows a configuration of the conventional nondestructive testing system; 
     FIG. 3 is an explanation chart of the detection coil for the nondestructive testing system of the invention; 
     FIG. 4 is an explanation chart of the detection coil for the nondestructive testing system of the invention; 
     FIG. 5 is an explanation chart of the detection coil for the nondestructive testing system of the invention; 
     FIG. 6 is an explanation chart of the detection coil for the nondestructive testing system of the invention; 
     FIG. 7 is an explanation chart of the detection coil for the nondestructive testing system of the invention; 
     FIG. 8 is an explanation chart of the SQUID for the nondestructive testing system of the invention; 
     FIG. 9 is an explanation chart of the SQUID for the nondestructive testing system of the invention; 
     FIG. 10 shows a configuration of the nondestructive testing system of the invention; 
     FIG. 11 is a drawing showing nondestructive testing using the nondestructive testing system of the invention; 
     FIG. 12 shows a measurement result using conventional nondestructive testing system of the invention; and 
     FIG. 13 shows a measurement result using the nondestructive testing system of the present invention. 
    
    
     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° (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 μm-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.