JPH0555646A - High sensitivity magnetic field detector - Google Patents

Abstract

PURPOSE:To provide a high sensitivity by increasing a voltage change with respect to an input magnetic field of a DC-SQUID. CONSTITUTION:A plurality of Josephson junctions of a Josephson device 1 are connected in series with each other. Since the output voltages are increased as much as the number of the junctions connected in series, the change amount of the output voltage with respect to an external magnetic field is increased to improve a sensitivity.

Description

Detailed Description of the Invention

[0001]

This invention relates to a high-sensitivity electromagnetic sensor,
DC drive type superconducting quantum interference device (DC Superc) applied to ammeter, displacement meter, high frequency signal amplifier, etc.
onducing Quantum Interface
The following is a circuit of a "rance device" (abbreviated as DC-SQUID).

[0002]

2. Description of the Related Art A conventional circuit is shown in FIG. The Josephson element 1 consisted of one Josephson junction. DC-
When the SQIUD is supplied with a bias current 17 and a magnetic field is applied from the input coil 4 and the feedback modulation coil 6, the output voltage changes in the cycle of the magnetic flux quantum (Φo). Change in output voltage △
V is the critical current of the Josephson device 1, the gap voltage is Ic and Vg, and the shunt resistor 2 is Rs.
V <Rs Ic <Vg. When the value of ΔV is small, the amount of change in voltage with respect to the external magnetic field is small, and the sensitivity is poor. Moreover, since the dynamic range is also reduced, it becomes difficult to detect a signal by lock-in. As a result, accurate measurement was not possible.

[0003]

In the circuit of FIG. 2, only one Josephson junction is connected to each end of the washer coil, so that the output voltage ΔV is the voltage for one Josephson junction. There wasn't. When the value of ΔV is small, the amount of change in voltage with respect to the external magnetic field is small, and the sensitivity is poor. Moreover, since the dynamic range is also reduced, it becomes difficult to detect a signal by lock-in. As a result, there is a problem that accurate measurement cannot be performed.

Therefore, an object of the present invention is to solve the above-mentioned conventional problems and to provide a DC-SQ having a large output voltage.
To get the UID.

[0005]

In order to solve the above-mentioned problems, according to the present invention, a large number of Josephson junctions, which used to be one, are connected in series to increase the output voltage.

[0006]

In the magnetic field detection circuit configured as described above, the change amount of the voltage with respect to the external magnetic field is ΔV <RsIc for one Josephson junction, so that a plurality of Josephson junctions should be connected in series. For example, the voltage is increased by adding the number of Josephson junctions.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of a first embodiment according to the present invention.
In the conventional DC-SQUID, the Josephson device 1 is composed of one Josephson junction, but in the present invention, a plurality of them are connected in series. Therefore, the conventional output ΔV is tens of μ
Although only about V was output, for example, if 100 units are connected in series, an output of several mV can be obtained and the signal can be detected easily.
That is, when the value of ΔV is large, the amount of change in voltage with respect to the external magnetic field is large, and the sensitivity is improved. Moreover, since the dynamic range is increased, signal detection by lock-in becomes easy. Further, since the capacitance component of the Josephson junction is reduced and the resonance frequency and the resonance voltage of the washer coil 3 and the Josephson element 1 are increased, the SQUID operation is not affected and highly accurate measurement is possible. The present invention is particularly effective because it is necessary to detect a weak magnetic field such as a magnetoencephalography in the biometric measurement.

FIG. 3 shows a second embodiment of the present invention, which is a circuit in which a damping coil 5 is inserted in parallel with the washer coil 3 of the first embodiment. Generally, the highly sensitive DC-SQUID, which is used for biometrics, has a large washer coil inductance and a small change in output voltage. Therefore, insert a damping resistor 5 to increase the output voltage and ensure stable operation. There is.

FIG. 4 shows an example of a method of connecting the Josephson element 1 and the shunt resistor 2 in series. In the present invention, the operation is the same even if a shunt resistor is connected in parallel to each Josephson junction in addition to FIGS. Therefore, it is possible to replace the Josephson element 1 of FIGS. 1 and 3 with the circuit of FIG. FIG. 5 is an example of a plan view when actually manufacturing the circuit of the second embodiment (FIG. 3) of the present invention for detecting a DC-SQUID with high sensitivity. The manufacturing method is to pattern the thin film by a photolithography process. Since the Josephson element 1 increases the inductance of the washer coil 3, it is better to cover the ground plane 7 in an overlapping manner in order to eliminate the influence of the inductance. ..

FIG. 6 shows the Josephson device 1 of the circuit of FIG.
Fabrication when changing the shunt resistor 2 part to the connection shown in Fig. 4.
FIG. FIG. 7 is a cross-sectional view of FIG. 6A-A ′.
A manufacturing example will be described with reference to FIG. First Josephson element
The ground plane 7 under 1 is made of superconducting material
After attaching the interlayer insulating film 11, the DC-SQUID is
After depositing the resistance film of the resistance 2 and the damping resistance 5,
The resistance film is insulated by the insulation film 11, and the resistance is set to the design value.
To do. Examples of resistive films include Mo, MoN, Pd, Au,
There are metals such as Cu, Al, Pd and Ti, all of which are spa
It can be deposited by a sputtering method or vapor deposition. Here, Mo is DC mug
Photolithography process after depositing 100 nm by Netron sputtering
Then, pattern to the design size. Etch of Mo
Both wet and dry etching are possible.
An example of wet etching is the method using nitric acid.
It CF is an example of dry etching_Four And CF_Four And acid
Reactive Ion Etching (RI
There is E). CF here_Four With a mixture of oxygen and oxygen
Mo is etched by RIE. Interlayer insulating film 11
Is SiO₂ , SiO, Si, MgO and the like. None
It can be deposited by a putter, vapor deposition, CVD or the like. The deposited film thickness is resistance
From 1.5 times the resistance film so that the film is completely insulated
Double the amount. RF magnetron sputtering here
And SiO₂ Is deposited to 150-200 nm, and photolithography is performed.
The contact is made with the resistance film 5 in the process. Si
O₂ Etching is both wet and dry etching
Is possible. An example of wet etching is hydrofluoric acid.
There is a method using a mixed solution of. An example of dry etching
Then CF _Four And CHF₃ Reaction using mixed gas of oxygen and oxygen
There is a characteristic ion etching (RIE). CHF here
₃ SiO by RIE using mixed gas of oxygen and oxygen₂ D
Touch.

Next, the lower electrode 10, the barrier layer 9, and the upper electrode 8
In order to manufacture the Josephson device 1 made of, each layer is deposited, and the upper electrode 8 and the barrier layer 9 are etched by a photolithography process. An example of the Josephson device 1 is Nb / Al-o.
In addition to the xide / Nb structure, NbN / MgO / NbN, N
There are various structures such as b / Si / Nb, Nb / Nb-oxide / Nb, but here, Nb / Al-oxide / N.
b structure is sputter deposited. An example of deposition is shown below. 1
Ar gas was introduced into the reaction chamber that was evacuated to a high vacuum of 0 ^-5 Pa or less, and the pressure of 0.2 to 4 Pa was applied to the Nb of the lower electrode 10.
The film is deposited by DC magnetron sputtering. The film thickness is 20
Deposit from 0 to 300 nm. After the introduction of argon gas was stopped and the reaction chamber was evacuated to a high vacuum of 10 ⁻⁵ Pa or less again, argon gas was introduced and Al was added at a pressure of 0.2 to 4 Pa.
C magnetron sputter and deposit 1 to 20 nm.
Here, there is no problem even if the gas is not evacuated to a high vacuum before the Al sputtering. The reaction chamber is evacuated to a high vacuum of 10 ⁻⁵ Pa or less, and oxygen gas or a mixed gas of oxygen and argon is introduced to adjust the pressure to a set value, and the Al surface is oxidized to form AlOx.
A barrier layer 9 of / Al is formed. The reaction chamber was evacuated to a high vacuum of 10 ⁻⁵ Pa or less, and the upper electrode 8 was re-applied under the above Nb deposition conditions.
Of 100 to 300 nm is deposited. Next, the upper electrode 8 and the barrier layer 9 are etched by a photolithography process to form the Josephson device 1. The etching method generally uses dry etching using plasma. Nb of the upper electrode 8
Is subjected to reactive ion etching (RIE) using CF ₄ or a mixed gas of CF ₄ and oxygen. Al of the barrier layer 9 is removed by wet etching using acid or RIE using Ar gas. Here, the barrier layer 9 may not be etched.

Next, the lower electrode 10 of the Josephson device 1 and the
Patterned superconducting film by photolithography process
Then, the feedback modulation coil and the input coil are formed. Etch
Generally, dry etching with plasma is used as the etching method.
use. Nb of the lower electrode 10 is CF_Four And a mixed gas of oxygen
Use plasma etch or reactive ion etching (R
IE) The etching here is performed on the upper electrode 8.
Unlike etching, increasing the amount of oxygen for isotropic etching
The resist film around the pattern to oxygen.
Make a shaving taper shape. As a specific example, plasma etch
CF with a ringing device _Four Gas with 10% oxygen added to
With a pressure of 133 Pa and power of 50 w
Is.

Next, after depositing the interlayer insulating film 11, a contact hole is opened in a photolithography process, a superconducting film is deposited, and a washer coil 3 and a counter electrode are formed in the photolithography process. Examples of superconducting films include Nb, NbN and Pb-I.
In some cases, n and Pb-In-Au are deposited by sputtering or vapor deposition. Here, in the same manner as the Josephson junction electrode, the Nb film was subjected to 400-60 by DC magnetron sputtering.
Deposit 0 nm. Prior to deposition, the substrate is reverse sputtered with Ar gas into a superconducting contact. After that, a washer coil 3, a counter electrode and other wiring portions are formed by a photolithography process. The etching is performed by plasma etching as in the case of the lower electrode 10.

The magnetic field detection circuit of the present invention can be manufactured as described above. The order of each layer can be changed and can be changed if the circuit does not change. FIG. 8 shows an example of a system for highly sensitive magnetic field detection using the DC-SQUID of the present invention, which is a FLL (Flux Loc) that has been conventionally used.
ked Loop) circuit. A detection circuit is superconductingly connected to the input coil 4 depending on the purpose of measurement.

[0015]

As described above, the present invention provides the DC-S.
By connecting a plurality of QUID Josephson junctions in series, the output voltage with respect to the magnetic field can be increased.
This has the effect of increasing the sensitivity of the C-SQUID.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a first embodiment of a DC-SQUID of the present invention.

FIG. 2 is a circuit diagram of a conventional DC-SQUID.

FIG. 3 is a circuit diagram of a second embodiment of the DC-SQUID of the present invention.

FIG. 4 is a circuit diagram of a Josephson junction series connection.

5 is a plan view of the circuit (entity) of FIG. 3. FIG.

FIG. 6 is a plan view of the circuit (entity) of FIG.

7 is a cross-sectional view taken along the line AA ′ of FIG.

FIG. 8 is a block diagram of a FLL circuit using the present invention.

[Explanation of symbols]

 1 Josephson Element 2 Shunt Resistor 3 Washer Coil 4 Input Coil 5 Damping Resistor 6 Feedback Modulation Coil 7 Ground Plane 8 Upper Electrode 9 Barrier Layer 10 Lower Electrode 11 Interlayer Insulation Film 12 Head Amp 13 Lock-in Detector 14 Oscillator 15 Integral Amplifier 16 Superconducting transformer 17 Bias current

Claims (1)

[Claims] 1. A direct current composed of an input coil magnetically coupled to the washer coil and a feedback modulation coil by connecting a Josephson element and a shunt resistor connected in parallel to both ends of the washer coil to form a superconducting ring. In the drive-type superconducting quantum interference device, the Josephson device has a configuration in which a plurality of Josephson junctions are connected in series, and a high-sensitivity magnetic field detection circuit.