Thin-film three-axis magnetometer and squid detectors for use therein

A magnetometer is prepared by depositing three thin-film SQUID magnetic field detectors upon a substrate. Two of the detectors incorporate stripline SQUID detectors deposited at right angles to each other, to measure the orthogonal components of a magnetic field that lie in the plane of the substrate. The third detector uses a planar loop SQUID detector that measures the component of the magnetic field that is perpendicular to the substrate. The stripline SQUID detectors have thin-film base and counter electrodes separated by an insulating layer which is at least about 1 micrometer thick, and a pair of Josephson junctions extending between the electrodes through the insulating layer.

BACKGROUND OF THE INVENTION 
This invention relates to devices for measuring magnetic fields, and, more 
particularly, to a sensitive thin-film magnetometer that is planar yet can 
measure the three orthogonal components of a very small magnetic field. 
A magnetometer is a device that measures the presence and magnitude of a 
magnetic field, and, in the case of a vector magnetometer, the direction 
of the magnetic field. In a conventional magnetometer, an input loop (or 
coil) of an electrical conductor is placed into a varying magnetic field. 
The magnetic flux passing through the loop creates a responsive movement 
of electrical charge. The resulting electrical current is measured, and 
the magnetic field is calculated from the measured electrical current. 
It is usually important to understand both the direction and the magnitude 
of the magnetic field. A magnetic field can be visualized as a vector 
having orthogonal components along three axes. To characterize the 
magnetic field completely, the magnetometer may have either three separate 
loops oriented perpendicular to the axes of interest, or a single loop 
whose orientation can be changed until it is perpendicular to the magnetic 
field. In the case of the three-axis magnetometer the components of the 
magnetic field are individually measured and the total magnetic field can 
be calculated, and in the case of the single movable loop the total field 
is measured and the components can be calculated. 
In some areas of technology it is desirable, but difficult, to measure the 
local character of a magnetic field with accuracy and very high spatial 
resolution. For example, where the magnetic field is emitted from a solid 
body it is desirable to measure the magnetic field at various locations 
adjacent the solid body in order to understand its origin. Such 
circumstances arise in biomagnetometry, the study of magnetic fields 
arising from the human body, and in nondestructive testing, the study of 
the integrity of a body from external measurements that do not adversely 
influence the body. 
To achieve a particular spatial resolution in such circumstances, the 
magnetometer loop or loops must be made with a lateral dimension on the 
same order of size as the desired spatial resolution. To achieve good 
accuracy, the magnetometer loop must be placed as closely as possible to 
the body whose magnetic field is to be measured. One inherent limiting 
factor in such measurements is that, as the loop is made smaller so that 
it can achieve good spatial resolution and be placed close to the surface, 
the current detector must be made more sensitive because the loop receives 
a lower magnetic flux per unit time than does a larger loop. Also, as the 
loop or loops are made smaller, it becomes progressively more difficult to 
controllably and reproducibly orient the loop or loops adjacent the body 
to be measured. The components of the magnetic field parallel to the 
surface of the body are particularly difficult to measure, because the 
loops must be oriented with their normal vectors parallel to the surface. 
There is a continuing need for a magnetometer that achieves both high 
sensitivity and high spatial resolution for magnetic fields originating in 
a body. Such a magnetometer must be able to measure all three components 
of the magnetic field vector. The present invention fulfills this need, 
and further provides related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides a three-axis magnetometer that is planar, 
and is conveniently deposited upon a substrate as a thin-film device. The 
magnetometer can be placed within a few thousandths of an inch of the 
surface of the body whose magnetic field is to be measured. The lateral 
dimension of the magnetometer relative to each of the three components of 
the magnetic field is on the order of a few thousandths of an inch or 
less. The magnetic field detector is one of the most sensitive detectors 
known. As a result of these factors, the magnetometer is highly effective 
in measuring the three components of a small magnetic field or small 
variations in a magnetic field emanating from a body, with extremely high 
spatial resolution. 
In accordance with the invention, a three-axis planar magnetometer 
comprises a substrate; and a first thin film magnetic field detector 
deposited upon the substrate, the first magnetic field detector measuring 
a first component of a magnetic field that lies in the plane of the 
substrate; a second thin film magnetic field detector deposited upon the 
substrate, the second magnetic field detector measuring a second component 
of the magnetic field that lies in the plane of the substrate and is 
non-colinear with the first component of the magnetic field; and a third 
thin film magnetic field detector deposited upon the substrate, the third 
magnetic field detector measuring a third component of the magnetic field 
that is perpendicular to the plane of the substrate. More generally, a 
three-axis planar magnetometer comprises a substrate; and means for 
detecting three non-coplanar components of a magnetic field, the means for 
detecting being a thin film that is substantially coplanar with and 
supported upon the substrate. 
In one preferred approach to the present invention, the magnetometer 
utilizes Superconducting QUantum Interference Devices, known in the art by 
the acronym "SQUID", as detectors. When cooled to the superconducting 
temperature range of their components, SQUIDs are highly sensitive 
detectors of electrical current flows, and thereby magnetic fields. 
Techniques are known for fabricating SQUIDs as thin-film, planar devices 
on a substrate by microelectronic fabrication procedures, producing SQUIDs 
that are only a few thousandths of an inch in transverse dimension. 
The three components of a magnetic field are measured by employing two 
different types of SQUID detectors. A planar loop SQUID detector is 
sensitive to a component of a magnetic field perpendicular to the plane of 
the loop, and such a SQUID detector can therefore be used to measure the 
component of the magnetic field perpendicular to the plane of the 
substrate upon which the loop is deposited. 
A stripline SQUID detector is sensitive to a component of a magnetic field 
having a particular orientation within the plane of the SQUID detector, 
and such a SQUID detector can therefore be used to measure one of the 
components of the magnetic field lying within the plane of the substrate 
upon which the stripline SQUID detector is deposited. A second stripline 
SQUID detector oriented at an angle (preferably 90 degrees) to the first 
stripline SQUID detector is used to measure the other of the components of 
the magnetic field lying within the plane of the substrate upon which the 
stripline SQUID detector is deposited. 
In accordance with this aspect of the invention, a stripline thin-film 
SQUID detector operable at a preselected temperature to measure magnetic 
fields lying in the plane of the thin film comprises a thin-film base 
electrode made of a material that is superconducting at the preselected 
temperature; a thin-film counter electrode overlying the base electrode 
and spaced at least about 1 micrometer therefrom, the counter electrode 
being made of a material that is superconducting at the preselected 
temperature; an insulating layer between the base electrode and the 
counter electrode; and two Josephson junctions extending between the base 
electrode and the counter electrode, through the insulating layer. 
The magnetometer of the invention not only measures the three components of 
a magnetic field, but does so with high spatial resolution and 
sensitivity. It can be used in situations where the magnetometer must be 
placed close to the body that produces the magnetic field, in order to 
achieve the high resolution. The magnetometer can be fabricated using 
known microelectronic deposition, patterning, and material removal 
techniques, as a planar device on the surface of a substrate. Other 
features and advantages of the invention will be apparent from the 
following more detailed description of the preferred embodiments, taken in 
conjunction with the accompanying drawings, which illustrate, by way of 
example, the principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with a preferred embodiment of the invention, a three-axis 
planar magnetometer comprises a substrate; and a first stripline SQUID 
detector deposited upon the substrate; a second stripline SQUID detector 
deposited upon the substrate and oriented at an angle to the first 
stripline SQUID detector; and a planar loop SQUID detector deposited upon 
the substrate. 
FIG. 1 illustrates a three-axis magnetometer 20 with detector means 22 
supported upon a planar substrate 24. (Also shown in FIG. 1 is a set of 
superimposed orthogonal x-y-z axes, which are provided to aid in the 
discussion of the embodiment but are not themselves a part of the 
invention. In the figure, the x and y axes lie in the plane of the 
substrate 24 at right angles to each other and the z axis is perpendicular 
to the plane of the substrate 24.) The detector means 22 preferably 
includes a SQUID detector that is operated at a temperature below its 
superconducting transition temperature T.sub.c. The substrate 24 is 
therefore preferably cooled during operation to a temperature below the 
superconducting transition temperature of the material used to construct 
the detector means 22. The substrate 24 is preferably a conventional 
electronic microcircuit substrate such as a piece of silicon or sapphire 
(aluminum oxide) having a thickness of from about 0.008 to about 0.020 
inches. Sapphire is most preferred as the substrate material, because of 
its good thermal conductivity. 
The substrate 24 is typically mounted on the outer surface of a vessel 25 
that contains a cryogenic fluid such as liquid helium (boiling point 4.2K) 
for a conventional low T.sub.c superconductor, or liquid nitrogen (boiling 
point 77K) for a high T.sub.c superconductor, as may be appropriate for 
the selected materials of construction of the detector means 22. The 
detector means 22 is cooled to below its T.sub.c by conduction through the 
substrate 24. 
The detector means 22 includes three Superconducting QUantum Interference 
Device ("SQUID") detectors, two of which are stripline SQUID detectors and 
the third a planar loop SQUID detector. (In FIG. 1, the SQUID detectors 
are indicated in a schematic form, and their operation will be discussed 
in more detail subsequently.) A first stripline SQUID detector 26 is 
deposited upon the substrate 24 and oriented parallel to the y-axis. The 
first stripline SQUID detector 26 is sensitive to magnetic field vector 
components 27 parallel to the x-axis. A second stripline SQUID detector 28 
is deposited upon the substrate 24 and oriented parallel to the x-axis. 
The second stripline SQUID detector 28 is sensitive to magnetic field 
vector components 29 parallel to the y-axis. A planar loop SQUID 30 is 
deposited upon the substrate 24 with its loop extending in the x and y 
directions. The planar loop SQUID 30 is sensitive to magnetic field vector 
components 31 parallel to the z-axis. Because one SQUID detector measures 
each magnetic field component, the three detectors are used together and 
form a basic trio unit 32. 
FIG. 2 illustrates another three-axis magnetometer 34, having at least two 
of the trio units 32 (of SQUID detectors 26, 28, and 30), and preferably a 
plurality of the trio units 32. (Since the stripline SQUID detectors 26 
and 28 are essentially linear units, they are depicted schematically in 
FIG. 2 as straight lines for ease of illustration. Since the planar loop 
SQUID detectors 30 are essentially planar square loops in one form, they 
are depicted schematically as squares.) These trio units 32 are arranged 
in an ordered array across the face of the substrate 24. 
This arrayed form of the SQUID detectors permits the spatial variation of a 
magnetic field to be determined without moving the three-axis magnetometer 
relative to the body bring measured. The SQUID detectors are deposited and 
supported on the surface of the substrate, and can therefore be placed 
very close to another object that is emitting the magnetic field of 
interest. The SQUID detectors can be fabricated with dimensions and 
spacings on the order of one to a few thousandths of an inch. The 
combination of sensitivity of the SQUID detectors, small dimensions of the 
SQUID detectors, small spacings of the SQUID detectors, and proximity of 
the substrate surface to the object being measured all contribute to high 
resolution, high sensitivity measurements of all three components of the 
magnetic field. 
The structure of a stripline SQUID detector 40 suitable for the present 
application is illustrated in perspective view in FIG. 3, and in detailed 
elevational view in FIG. 4. This configuration of stripline SQUID detector 
is preferably used for the first stripline SQUID detector 26 and the 
second stripline SQUID detector 28. 
The stripline SQUID detector 40 includes a base electrode 42 with an 
integral attachment lead 44, and a parallel, spaced-apart counter 
electrode 46 with an integral attachment lead 48. The electrodes 42 and 
46, and their leads 44 and 48, are formed by microelectronic techniques 
such as deposition and patterning. The electrodes 42 and 46 are formed 
from a material that is superconducting at a selected temperature of 
operation. In the preferred embodiment, the electrodes 42 and 46 are 
formed of niobium metal. 
Separating the electrodes 42 and 46 is a layer 50 of an insulating 
material. The preferred insulating material is a polymer such as a 
polyimide dissolved in a solvent, available commercially from duPont Corp. 
as PI-2555, applied as a spin-on insulation. The important advantage of 
using a polymer insulation layer 50 is that it can be made quite thick. 
Many conventional microelectronic insulating materials, such as silicon 
monoxide, cannot be practically applied in thicknesses as great as one 
micrometer or more. 
Extending between the electrodes 42 and 46 are two Josephson junctions 52, 
illustrated in general view in FIG. 3 and in more detail in FIG. 4. Each 
Josephson junction 52 includes a first metallic layer 54, which is 
preferably aluminum about 60-80 Angstroms thick, an insulating layer 56, 
which is preferably aluminum oxide about 20 Angstroms thick, and a second 
metallic layer 58, which is preferably aluminum about 20 Angstroms thick. 
The performance of the SQUID depends upon the thickness of the insulating 
layer 56, so that the thickness may be varied but is typically about 20 
Angstroms. A superconducting layer 60 of the same material as the 
electrodes 42 and 46 is deposited over the second metallic layer 58 to 
adjust the height of the Josephson junction to match the thickness of the 
insulation layer 50. 
The electrodes 42 and 46 and the Josephson junctions 52 define an input 
coil 62, indicated in the figures by a dashed line in the form of a 
rectangle. The plane of the rectangle is perpendicular to the plane of the 
electrodes 42 and 46, and also perpendicular to the x-axis, which is 
perpendicular to the line joining the Josephson junctions 52. Magnetic 
flux lines, indicated schematically by vectors 64 parallel to the x-axis, 
induce a current flow through the input coil 62. This current flow is 
detected by the Josephson junctions 52, permitting detection of the 
magnetic flux component lying in the plane of the electrodes 42 and 46. 
The distance between the electrodes 42 and 46 is about 4 micrometers in 
the preferred embodiment, which is about 1/6 of one-thousandth of an inch. 
Thus, the stripline SQUID detector 40 is a substantially planar device, 
which can be placed quite close to the surface of a magnetic field source 
and still make measurements of the magnetic field component that lies 
parallel to that surface. 
Thus, the stripline SQUID is essentially a linear, single dimension device, 
as shown in FIGS. 1-3. It length is typically about 200 micrometers (or 
about 0.008 inches), its width is typically about 50 micrometers, and its 
height between the base electrode and the counter electrode is typically 
about 4 micrometers in the form used in the magnetometer of the invention. 
The stripline SQUID must have a spacing between the electrodes 42 and 46 
(i.e., thickness of the insulating layer 50) of at least about 1 
micrometer. Two considerations are important in determining the minimum 
spacing between the electrodes. The first is that the maximum capacitance 
between the electrodes 42 and 46 must be less than the capacitance of the 
Josephson junctions 52. For a typical junction capacitance of 10.sup.-12 
Farad, an electrode length of 100 micrometers, an electrode width of 20 
micrometers, and an insulator dielectric constant of 3 .epsilon..sub.0, 
the absolute minimum thickness is about 0.05 micrometers. However, this 
thickness is impractical to make and allows insufficient flux coupling 
into the device. For a useful device, the lower limit of spacing between 
the electrodes 42 and 46 is set by the maximum desired equivalent field 
noise. To reduce the noise below 1 pT/(Hz).sup.1/2, a minimum spacing of 
about 1 micrometer is required. If the spacing is less, then the 
sensitivity of the SQUID to magnetic fields lying in the plane of the 
electrodes is simply too small to be useful. There is no upper limit on 
the spacing between the electrodes, as increasing the spacing increases 
the sensitivity of the SQUID. As the spacing becomes larger, however, the 
device is less planar in character. A typical practical upper limit of the 
thickness is about 20 micrometers, or just under 1 mil. In this thickness, 
the device remains substantially planar, yet is able to detect magnetic 
fields parallel to the plane of the device. 
Linear SQUID structures are known in the art, see, for example, M. W. 
Cromar and P. Carelli, "Low-noise tunnel junction dc SQUID's" Appl. Phys. 
Lett., Vol. 38, No. 9, pages 723-725 (1981). However, conventional linear 
SQUID structures have been designed to have as low an inductance as 
possible, and have not been used as magnetic field detectors. The spacing 
between the electrodes of the linear SQUID of Cromar and Carelli is 350 
nanometers (i.e., 0.35 micrometer), as determined by their silicon 
monoxide insulator technology. As discussed above, this spacing is simply 
too small to permit the direct detection of in-plane magnetic fields due 
to the small resulting loop area. Only by making the spacing between the 
electrodes more than about 1 micrometer, as in the present device, can the 
practical stripline SQUID suitable for direct magnetic field measurements 
be built. 
FIG. 5 illustrates a planar loop SQUID detector preferred for use as the 
detector 30. Such SQUIDs are known in the art and are described, for 
example, in U.S. Pat. Nos. 4,761,611, 4,386,361, and 4,389,612, whose 
disclosures are incorporated by reference. Briefly, such a SQUID includes 
a patterned planar loop 70 formed of a material such as niobium that 
becomes superconducting when cooled below its superconducting transition 
temperature T.sub.c. The loop 70 is interrupted at some location 71, and 
an insulating layer 72 (of a material such as silicon dioxide) and 
overlying superconducting layer 74 (of a material such as niobium) bridge 
between the two sides of the loop. One SQUID contact 76 is to the loop 70, 
and the other SQUID contact 78 is to the superconducting layer 74. 
Josephson junctions 80 are provided between the superconducting layer 74 
and the loop 70, one on each side of the location 71. The Josephson 
junctions are generally as described previously, except that they need not 
be made with a large separation between the layer 74 and the loop 70. Each 
Josephson junction 80 is preferably formed as a layer of aluminum about 30 
Angstroms thick, a layer of aluminum oxide about 20 Angstroms thick, and 
another layer of aluminum about 30 Angstroms thick. The entire planar loop 
SQUID detector 30 is readily formed by deposition, patterning, and etching 
of layers by conventional microelectronic techniques well known in the 
art. The total thickness of the planar loop SQUID detector is typically 
about 5000 nanometers, and the lateral dimensions of the loop 70 are about 
50 micrometers by about 50 micrometers. A magnetic field component passing 
through the loop 70 excites a current flow in the loop, whose presence is 
detected by the Josephson junctions 70. 
SQUID detector electronics for both the stripline and planar loop SQUIDs is 
well known in the art. See, for example, U.S. Pat. Nos. 4,386,361 and 
4,389,612. 
Thus, the present invention provides an important advance in the art of 
magnetic field detectors. The magnetometer of the invention can be 
deposited on a substrate to form a very thin detecting device that can be 
placed close to the object whose magnetic field emissions are to be 
measured. The detectors measure the three orthogonal components of the 
magnetic field, with a spatial resolution that is on the order of 100 
micrometers or less in the preferred approach, but which may become even 
smaller with improvements in SQUID detector design and fabrication 
techniques. 
Although particular embodiments of the invention have been described in 
detail for purposes of illustration, various modifications may be made 
without departing from the spirit and scope of the invention. Accordingly, 
the invention is not to be limited except as by the appended claims.