JP3922926B2 - Ion beam current intensity measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for measuring ion beam current intensity with high accuracy without blocking a beam.
[0002]
[Prior art]
Japanese Laid-Open Patent Publication No. 7-135999 discloses a method for accurately measuring the ion beam current intensity without blocking the beam. However, in this method, the space where the ion beam flows, the magnetic field collecting means for collecting the magnetic field, and the superconducting elements sensitive to the collected magnetic field have to be magnetically shielded from the external magnetic field. As a result, there has been a problem that the ion beam current intensity measuring apparatus becomes very large. This will be described in detail below. For example, the magnetic field produced by a 1 nA beam current is on the order of 10 ^-15 T at a point 20 cm away from the beam center. On the other hand, the environmental magnetic field including geomagnetism is on the order of at least 10 ^-5 T, so the background is about 10 orders of magnitude larger. As a magnetic shielding means for attenuating the external magnetic field, there is a method of enclosing with a high permeability material. However, the magnetic shielding using permalloy, which is generally used, can attenuate the external magnetic field by 1 to 2 digits at most, so it is impossible to measure the beam current intensity of 1 nA while reducing the background. Superconductivity can be considered as a magnetic shielding means that significantly attenuates the magnetic field. However, since the superconducting magnetic shield needs to be completely surrounded by the superconductor, the ion source and the device for accelerating the ions must be surrounded by the superconductor together with the space where the ion beam is flowing. Therefore, there is a problem that the apparatus becomes very large.
[0003]
As a method for solving this problem, a method of measuring an ion beam current of several μA or less with nA order measurement accuracy is disclosed in Journal of the Physical Society of Japan Vol. 54, No. 1, 1999. This method is characterized in that a superconducting magnetic shield having a narrow gap is provided between a space in which an ion beam flows and a detection coil that collects a magnetic field, as compared with Japanese Patent Application Laid-Open No. 7-135099. The superconducting magnetic shield with a narrow gap structure described in the Journal of the Physical Society of Japan Vol. 54, No. 1, 1999 greatly attenuates the external magnetic field, but does not attenuate the magnetic field generated by the ion beam current. There is a feature to pass through. Because of this feature, the part surrounded by the superconductor is only the detection coil, the superconducting element, and the superconducting circuit around it, so that the size of the apparatus can be greatly reduced.
[0004]
However, this method has a problem that the measurement range is as narrow as about 3 μA or less. When it is desired to measure a beam current of 3 μA or more, the measurement accuracy is lowered, or when it is desired to perform measurement while maintaining the accuracy, there is generally only a method using a measuring means such as a Faraday cup for blocking the beam.
[0005]
[Problems to be solved by the invention]
There has been a demand for providing a method capable of measuring a beam current in a wide measurement range from several tens of nA to several tens of mA without interrupting the beam while maintaining high measurement accuracy of several nA to several μA.
[0006]
In order to solve the above-described problems, a beam current intensity measuring apparatus according to the present invention includes a detection unit that detects a magnetic field corresponding to an ion beam current, a magnetic flux transmission unit that transmits magnetic flux to the measurement unit, and an ultrasonic sensor that is sensitive to the transmitted magnetic flux. Magnetically shields the measurement unit having a feedback coil that feeds a feedback current so as to cancel the change of magnetic flux passing through the conduction element and the superconducting element, and the detection unit, the magnetic flux transmission unit, and the measurement unit from the external space including the space where the ion beam flows A magnetic shielding unit having a gap made of a superconductor, and a circuit for controlling the amount of magnetic flux transmitted to the measuring unit by canceling the magnetic flux induced by the ion beam current at one or both of the detecting unit and the magnetic flux transmitting unit. at least have a trans of the magnetic flux transmission part includes a transformer input coil connected to the detecting portion, and a coil connected to the measurement section, a coil for canceling the change in magnetic flux Characterized in that it comprises et al.
[0007]
The detection unit has a configuration in which a superconducting wire is wound around a soft magnetic core for a plurality of turns. The magnetic flux transmission unit may be simply connected to the detection unit and the measurement unit with a superconducting wire, but it is also possible to use a transformer to adjust the inductance of the detection unit and the measurement unit. Is desirable because it can be optimized. The measurement unit includes a superconducting magnetic flux quantum interferometer that functions as a magnetic field sensor. Further, a washer coil is provided in the magnetic flux transmission section, and a structure is employed in which a current is fed back to the feedback coil so that the change in magnetic flux passing through the superconducting magnetic flux quantum interferometer is zero. The superconducting magnetic flux quantum interferometer and the coil and feedback coil provided so as to make the change of magnetic flux penetrating the superconducting magnetic flux quantum interferometer zero are configured on the chip, and as a SQUID chip, biomagnetism measurement, etc. Widely used. The magnetic shield part is a superconducting magnetic shield made of a superconductor arranged so as to surround the ion beam current, and has a shape having a narrow gap sandwiched between two doughnut-type superconductor plates. The attenuation factor of the external magnetic field can be changed as desired according to the ratio between the inner diameter and the outer diameter of the donut-type superconductor plate and the number of gaps. The gap selectively enters the magnetic shield without attenuating only the magnetic field generated by the ion beam. Niobium or lead can be used as the material for the superconducting magnetic shield. For example, a circuit that cancels the magnetic flux of one or both of the detection unit and the magnetic flux transmission unit and limits the amount of magnetic flux transmitted to the measurement unit is provided with a transformer in the magnetic flux transmission unit, for example. This can be realized by constructing a circuit in which a current corresponding to a change in the voltage generated at both ends of the feedback resistor is fed to the transformer by the feedback current passed through the magnetic flux transmission unit so as to cancel the change. Alternatively, it can be realized as a detection coil with a structure in which a superconducting wire is wound around the core of the soft magnetic body so that the beam current induces a magnetic flux in the opposite direction to the magnetic flux induced in the core of the soft magnetic body of the detection coil. it can.
[0008]
With the above apparatus configuration, even when the ion beam has a current of several μA or more, the magnetic flux can be canceled by the magnetic flux transmission unit or the detection unit, so that a large current does not flow through the SQUID chip. The current that cancels the magnetic flux corresponds to the ion beam current intensity. The measurement accuracy is basically determined by the S / N ratio of the change in the feedback current fed back so that the change in the magnetic flux passing through the superconducting magnetic flux quantum interferometer in the SQUID chip becomes zero.
[0009]
The configuration does not allow a large current to flow through the SQUID chip, so the measurement range can be expanded. Since the current for canceling the magnetic flux corresponds to the ion beam current intensity, an output proportional to the ion beam current intensity can be obtained. The measurement accuracy is determined by the sensitivity to the output beam current and the noise level, but according to the present invention, the sensitivity is the feedback current fed back to null the flux change through the superconducting flux quantum interferometer in the SQUID chip. Since it is determined by the sensitivity to the beam current, highly sensitive measurement is possible. In addition, the noise level is a low noise level equivalent to the conventional one using superconducting magnetic shielding. As described above, it is possible to provide a method that can maintain a high measurement accuracy of several nA to several μA and can measure the beam current in a wide measurement range of several tens of nA to several tens of mA without blocking the beam.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The configuration of the apparatus according to the present invention will be described below. However, this is one of the embodiments.
(Basic configuration of the apparatus according to the present invention)
The configuration of an ion beam current intensity measuring apparatus according to the present invention will be described with reference to FIG.
[0011]
The detection unit used as the detection coil 1 was a soft magnetic core in which a superconducting wire was wound for 4 turns. The dimensions of the soft magnetic core are an inner diameter of 250 mm, an outer diameter of 320 mm, and a height of 30 mm. The soft magnetic core was made of an amorphous material having a small coercive force and a large magnetic permeability.
The magnetic flux transmission unit is composed of a transformer input coil 3, a coil 4 connected to the measurement unit, a transformer in which a coil 5 for canceling a change in magnetic flux is wound around a soft magnetic core, and a superconducting wire connecting the detection unit and the measurement unit. Composed. The transformer input coil 3 is connected to the detection coil 1. The coil 5 for canceling the change in magnetic flux is a coil for canceling the magnetic flux induced in the soft magnetic core of the transformer by the current flowing through the transformer input coil 3. The soft magnetic core of the transformer was made of the same material as the core of the detection coil 1. The dimensions of the soft magnetic core of the transformer were an inner diameter of φ10 mm, an outer diameter of φ12.5 mm, and a height of 5 mm. The transformer input coil 3 has 100 turns, the coil 4 connected to the measurement section has 15 turns, and the coil 5 for canceling the change in magnetic flux has 10 turns.
[0012]
The measurement unit was a superconducting circuit using DC SQUID as a superconducting element. The superconducting circuit SQUID 9, SQUID input coil 8, feedback coil 6, and washer coil 7 are formed on a SQUID chip 18.
[0013]
The superconducting magnetic shield surrounds the detection unit, the magnetic flux transmission unit, and the measurement unit. The enclosed part is shown in a range 19 surrounded by a superconducting magnetic shield. However, it was not a completely enclosed structure, but a structure having a gap in the part surrounding the detection unit. The gap width was 0.5 mm. Niobium was used as the material for the superconducting magnetic shield. The detection unit, magnetic flux transmission unit, measurement unit, and superconducting magnetic shield were placed inside the cryostat and cooled to liquid helium temperature.
[0014]
The ion beam current intensity measuring device according to the present invention described above is characterized in that, unlike the prior art, a coil is provided for canceling a change in magnetic flux by providing a transformer in a magnetic flux transmission part outside the SQUID chip 18.
(Example of apparatus configuration and measurement mechanism according to the present invention)
FIG. 2 shows a more specific configuration example of the ion beam current measuring apparatus according to the present invention. The measurement mechanism will be described with reference to FIG.
[0015]
A state where a beam current I _B 2 having a certain intensity passes through the detection coil 1 is defined as an initial state. The initial value of the beam current I _B 2 is measured by the Faraday cup 26 and sent to the computer 21. Corresponding to the value, a voltage υ _C 24 is output from the signal output unit 23 to cause a current to flow through the resistor R _C 25, and the voltage υ _C 24 is input to the beam current value display unit. Further, a signal for setting the value of the variable resistor R _B 17 is output from the signal output unit 23 to the control unit 20 of the variable resistor R _B 17, and the variable resistor R _B has a value corresponding to the initial value of the beam current I _B 2. 17 switches. By applying the voltage υ _C 24, a feedback current I _F 28 flows. The feedback current I _F 28 at this time is the ratio of the voltage υ _C 24 and the resistance R _C 25, and I _F 28 = υ _C / R _C. On the other hand, the beam current I _B 2 induces a detection current I _P 27 in a superconducting closed circuit formed by the detection coil 1 and the transformer input coil 3. The detection current I _P 27 and the feedback current I _F 28 flow to the transformer input coil 3 and the coil 5 for canceling the change in magnetic flux, respectively, and induce magnetic flux in the soft magnetic core of the transformer. Here, the feedback current I _F 28 can be set so varied by voltage upsilon _C 24 as the magnetic flux of the internal transformer soft magnetic core becomes zero. The voltage υ _C 24 is set such that values corresponding to various initial values of the beam current I _B 2 are determined in advance by experiment and can be automatically set by the computer 21. Thus, in the initial state, the sum of the magnetic fluxes that the detection current I _P 27 and the feedback current I _F 28 induce in the soft magnetic core of the transformer is balanced with zero, and at this time, the current flows in the coil 4 connected to the measurement unit. Not induced. Therefore, the SQUID input current I _T 29 does not flow through the SQUID chip 18.
[0016]
When the intensity of the beam current I _B 2 changes from the initial state, the detection current I _P 27 changes in proportion to the amount of change. A change in the beam current I _B 2 is ΔI _B 2, and a change amount of the detection current I _P 27 is ΔI _P 27. [Delta] I _P 27 can be expressed as _{ΔI P 27 = α · ΔI B} 2 in proportion to [Delta] I _B 2. Due to ΔI _P 27, the sum of magnetic fluxes induced in the soft magnetic core of the transformer changes from a balanced zero state to a finite amount of magnetic flux proportional to ΔI _P 27. As a result, a SQUID input current I _T 29 is induced in a superconducting closed circuit formed by the coil 4, the washer coil 7 and the SQUID input coil 8 connected to the measurement unit. The SQUID input current I _T 29 flows through the SQUID input coil 8 and attempts to change the amount of magnetic flux passing through the SQUID 9, but the feedback current _{If f} 30 flows through the feedback coil 6 so that the amount of magnetic flux passing through the SQUID 9 does not change. The feedback current I _f 30 generates a voltage across the feedback resistor R _A 13. This is defined as an output voltage υ _A 14. The output voltage υ _A 14 is input to the electronic circuit 15.
[0017]
Here, an embodiment in which the electronic circuit 15 includes an amplifier 31 as shown in FIG. 3 will be described. The output voltage υ _A 14 is amplified κ times from the amplifier 31 and output as the output voltage υ _B 16. Due to the generation of the output voltage υ _B 16, the current υ _B / R _B flows through the variable resistor R _B 17 and is added to the feedback current I _F 28. If the amount of change of the feedback current I _F 28 is ΔI _F 28, ΔI _F 28 is υ _B / R _B. If ΔI _F 28 is expressed using the output voltage υ _A 14 instead of the output voltage υ _B 16, ΔI _F 28 = κ · υ _A / R _B. Since further _{υ A = I f 30 · R} A, expressed as _{ΔI F 28 = κ · I f} 30 · R A / R B.
ΔI _F 28 induces a magnetic flux in the coil 5 for canceling the change of the magnetic flux. The magnetic flux induced by ΔI _P 27 in the transformer input coil 3 is canceled by the induced magnetic flux. The current value of ΔI _F 28 required to cancel the magnetic flux induced by the soft magnetic core of the transformer by ΔI _P 27 is the number of turns of the transformer input coil 3 and the coil 5 for canceling the change of the magnetic flux as N3 and N5, respectively. Then, ΔI _F 28 = (N3 / N5) · ΔI _P 27. In this embodiment, (N3 / N5) = 10. Recalling that ΔI _P 27 is proportional to ΔI _B 2, ΔI _F 28 = (N3 / N5) · α · ΔI _B 2, and ΔI _F 28 is proportional to ΔI _B 2. The output voltage upsilon _B 16 than _{υ B = ΔI F 28 · R} B, is _{υ B = (N3 / N5)} · α · R B · ΔI B 2. Therefore, an output proportional to the change ΔI _B 2 of the beam current I _B 2 can be obtained by measuring the output voltage υ _B 16. On the other hand, the relationship between ΔI _B 2 and I _f 30 is ΔI _B 2 = {(κ · R _A / R _B ) / ((N3 / N5) · α)} · I _f 30. Conventionally, the upper limit of the measurement range was when the output voltage υ _A 14 reached 15 V. For example, when the feedback resistance R _A 13 is set to 4.7 kΩ, the upper limit of measurement is I _f 30 of about 3.2 μA or less. The change in beam current I _B 2 that can be measured when I _f 30 is about 3.2 μA or less was about 2.5 μA. On the other hand, in the present invention, by increasing (κ · R _A / R _B ) / ((N3 / N5) · α), the upper limit of ΔI _B 2 that can be measured when I _f 30 is about 3.2 μA or less is set. Can be big. Although it depends on the self-inductance and the number of turns of the detection coil, transformer input coil 3 and coil 5 for canceling the change of magnetic flux, (N3 / N5) · α is about 1/100, feedback resistance R _A 13 is 4.7 kΩ, variable In the present example in which the resistance R _B 17 is 1.5 MΩ and κ is 1, the upper limit of ΔI _B 2 that can be measured when I _f 30 is about 3.2 μA or less is about 1 mA. In other words, the measurement range can be expanded about 400 times.
[0018]
Next, an embodiment in which the electronic circuit 15 includes an integrator 32 as shown in FIG. 4 will be described. The integrator 32 increases the output voltage υ _B 16 with a certain time constant when the output voltage υ _A 14 as an input signal exceeds a threshold value. When the output voltage υ _A 14 is within the threshold value, the constant output voltage υ _B 16 is output as it is. When the output voltage υ _A 14 falls below the threshold, the output voltage υ _B 16 is dropped with a certain time constant. The generation of the output voltage υ _B 16 causes the current υ _B / R _B to flow through the variable resistor R _B 17 and be added to the feedback current I _F 28. As in the previous section, ΔI _F 28 is ΔI _F 28 = υ _B / R _B , and ΔI _P 27 is transferred to the transformer input coil 3 by the magnetic flux induced in the coil 5 for canceling the change in magnetic flux by ΔI _F 28. The induced magnetic flux is canceled out. ΔI _F 28 = (N3 / N5) · ΔI _P 27, and since ΔI _P 27 is proportional to ΔI _B 2, ΔI _F 28 = (N3 / N5) · α · ΔI _B 2. The output voltage υ _B 16 is υ _B = (N3 / N5) · α · R _B · ΔI _B 2. Therefore, an output proportional to the change ΔI _B 2 of the beam current I _B 2 can be obtained by measuring the output voltage υ _B 16. Here, the measurement accuracy can be determined by setting a threshold value. For example, if the threshold value of the output voltage upsilon _A 14 is set to ± 500 mV, which is about ± 100 nA in terms of the beam current I _B 2, to measure the accuracy of the beam current I _B 2 at about ± 100 nA. At this time, the measurement range is basically not limited, and the magnitude of the output voltage υ _B 16 is determined by setting (N3 / N5) · α · R _B. For example, when (N3 / N5) · α is about 1/100 and the variable resistance R _B 17 is 150 kΩ, an output voltage υ _B 16 of 15 V is obtained for a change of ΔI _B 2 of 10 mA. That is, the 10 mA beam current I _B 2 can be measured with an accuracy of about ± 100 nA. Further, for example, if the threshold value of the output voltage υ _A 14 is set to ± 50 mV, this is about ± 10 nA in terms of the beam current I _B 2. Assuming that (N3 / N5) · α is about 1/100 and the variable resistor R _B 17 is 1.5 MΩ, an output voltage υ _B 16 of 15 V is obtained for a change of ΔI _B 2 of 1 mA. That is, the 1 mA beam current I _B 2 can be measured with an accuracy of about ± 10 nA. In this way, by changing the threshold value of the output voltage υ _A 14 and the setting of the variable resistor R _B 17, the measurement accuracy and the output voltage υ _B 16 can be adjusted appropriately, and the measurement range of the beam current I _B 2 is Basically there are no restrictions.
[0019]
The intensity of the beam current I _B 2 is obtained by adding the beam current I _B 2 and the change [Delta] I _F 28 in the initial state. In practice, the beam current value display unit 22 calculates the sum I _F 28 + ΔI _F 28 of I _F 28 = υ _C / R _C and ΔI _F 28 = υ _B / R _B. Then, the beam current I _B 2 is calculated from the relationship between I _F 28 and the beam current I _B 2 determined separately in the experiment.
[0020]
By the way, even if the coil 5 for canceling the change of the magnetic flux is wound around the detection coil 1, the same effect can be obtained.
(Comparative example)
FIG. 5 shows a configuration example of a conventional beam current measuring apparatus. Conventionally, the configuration is such that a magnetic flux is induced in the washer coil 7 by flowing a feedback current through the feedback coil 6 provided in the SQUID chip 18 and the magnetic flux induced in the detection coil 1 by the beam current I _B 2 is canceled. . FIG. 2 shows a measurement example. When the measurement accuracy was about 2 nA, a beam current I _B 2 of about 3 μA or less could be measured with good linearity, but no more could be measured. As described above, the conventional configuration has a problem that the linearity of the output cannot be maintained when the preamplifier 11 exceeds a certain voltage and a current exceeding a certain value flows through the circuit in the SQUID chip 18. Therefore, there is a problem that the preamplifier 11 can measure only the beam current intensity that can be handled at a certain voltage or less. When it is desired to measure a larger beam current I _B 2, it is necessary to reduce the output sensitivity to the beam current I _B 2. However, measurement accuracy decreases due to a decrease in sensitivity. FIG. 6 also shows the results when the sensitivity is about 1/5 compared with the above. At this time, the beam current I _B 2 of about 16 μA or less could be measured with good linearity. On the other hand, the measurement accuracy decreased to about 10 nA.
[0021]
【The invention's effect】
As described above, by using the ion beam current intensity measuring apparatus according to the present invention, a high measurement accuracy of several nA to several μA is maintained, and a wide range of several tens of nA to several tens of mA without blocking the beam. The beam current can be measured in the measurement range.
[Brief description of the drawings]
1 is a block diagram of an ion beam current measuring apparatus according to the present invention. FIG. 2 is a block diagram of an ion beam current measuring apparatus according to the present invention. FIG. 3 is a diagram showing an example 1 of an electronic circuit 15. FIG. FIG. 5 is a diagram showing a configuration of a conventional beam current measuring apparatus. FIG. 6 is a diagram showing a measurement example of a conventional beam current measuring apparatus.
1 Detection coil 2 Beam current I _B
3 Transformer input coil 4 Coil connected to measuring section 5 Coil for canceling magnetic flux change 6 Feedback coil 7 Washer coil 8 SQUID input coil 9 SQUID
10 DC power supply 11 Preamplifier 12 Integrator A
13 Feedback resistor R _A
14 Output voltage υ _A
15 Electronic circuit 16 Output voltage υ _B
17 Variable resistance R _B
18 SQUID chip 19 Range surrounded by superconducting magnetic shield 20 Control unit 21 of variable resistance R _B Computer 22 Beam current value display unit 23 Signal output unit 24 Voltage υ _C
25 Resistance R _C
26 Faraday cup 27 Detection current I _P
28 Feedback current _IF
29 SQUID input current I _T
30 Feedback current _If
31 Amplifier 30 Integrator B

Claims (10)

A detection unit that detects a magnetic field corresponding to the ion beam current, a magnetic flux transmission unit that transmits magnetic flux to the measurement unit, a superconducting element that is sensitive to the transmitted magnetic flux, and feedback so as to cancel changes in the magnetic flux passing through the superconducting element A measurement unit having a feedback coil for passing a current, a magnetic shielding unit having a gap made of a superconductor that magnetically shields the detection unit, the magnetic flux transmission unit, and the measurement unit from an external space including a space through which the ion beam flows, and an ion beam current the induced magnetic flux either one of the detection portion and the magnetic flux transmission part, or at least have a circuit for controlling the magnetic flux amount to be transferred to the measuring unit to cancel both by,
The transformer of the magnetic flux transmission unit is an ion beam current intensity measuring device including a transformer input coil connected to the detection unit, a coil connected to the measurement unit, and a coil for canceling a change in magnetic flux . 2. The ion beam current intensity measuring apparatus according to claim 1, wherein the detection unit has a configuration in which a superconducting wire is wound around a core of a soft magnetic material for a plurality of turns. 2. The ion beam current intensity measuring apparatus according to claim 1, wherein the magnetic flux transmission section includes at least a transformer in which a superconducting wire is wound around a soft magnetic core. 3. The ion beam current intensity measuring apparatus according to claim 2, wherein the detecting unit controls the amount of magnetic flux transmitted to the measuring unit by causing a current to flow through a coil for canceling a change in magnetic flux. 5. The change of the current passed through the coil for canceling the change of magnetic flux corresponds to the change of the feedback current passed so as to cancel the change of the magnetic flux passing through the superconducting element. Ion beam current intensity measuring device. 6. The ion beam current intensity measuring apparatus according to claim 5, wherein a change in the ion beam current intensity is measured by measuring a change in a current passed through a coil for canceling a change in magnetic flux. 2. The ion beam current intensity measuring apparatus according to claim 1, wherein the superconducting element is a superconducting magnetic flux quantum interferometer. Claim 1 An ion implantation apparatus comprising the ion beam current intensity measurement apparatus described. Claim 1 An electron beam exposure apparatus comprising the ion beam current intensity measuring device described above. Claim 1 An accelerator comprising the described ion beam current intensity measuring device.

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