JP5218954B2

JP5218954B2 - SQUID microscope

Info

Publication number: JP5218954B2
Application number: JP2007153650A
Authority: JP
Inventors: 秀夫糸崎; 忠之林; 実立木
Original assignee: National Institute for Materials Science
Current assignee: National Institute for Materials Science
Priority date: 2007-06-11
Filing date: 2007-06-11
Publication date: 2013-06-26
Anticipated expiration: 2027-06-11
Also published as: JP2008304411A

Description

本発明は、金属材料や無機材料、さらには、半導体素子や半導体材料など、各種の素子や材料からなる試料について、高い空間分解能をもって磁気量を計測できるＳＱＵＩＤ顕微鏡に関し、より詳しくは超伝導量子干渉素子を備えたＳＱＵＩＤ顕微鏡に関するものである。 The present invention relates to a SQUID microscope that can measure a magnetic quantity with high spatial resolution for a sample made of various elements and materials such as a metal material, an inorganic material, and a semiconductor element and a semiconductor material, and more specifically, a superconducting quantum interference. The present invention relates to a SQUID microscope equipped with an element.

ＳＱＵＩＤ顕微鏡は、超伝導量子干渉素子を用いた高感度磁気顕微鏡であり、ＳＱＵＩＤヘッドの構造によって数μｍから数１００μｍ前後の空間分解能を有し、各種素子や材料の計測や観察に使用されている（例えば、非特許文献１参照）。
特許文献１には、ＳＱＵＩＤ顕微鏡が開示されており、被測定試料平面に対して磁性材料からなる針であるプローブを垂直方向に配置している。特許文献１では、プローブが被測定試料に直接接触することで、空間分解能を高めている。 The SQUID microscope is a high-sensitivity magnetic microscope using a superconducting quantum interference device, and has a spatial resolution of several μm to several hundred μm depending on the structure of the SQUID head, and is used for measurement and observation of various elements and materials. (For example, refer nonpatent literature 1).
Patent Document 1 discloses a SQUID microscope, in which probes, which are needles made of a magnetic material, are arranged in a vertical direction with respect to a sample plane to be measured. In Patent Document 1, the spatial resolution is increased by the probe directly contacting the sample to be measured.

特開２００４−１２１８９号公報JP 2004-12189 A Ｊ．Ｒ．Ｋｉｒｔｌｅｙｅｔａｌ．， “ＳＣＡＮＮＩＮＧＳＱＵＩＤＭＩＣＲＯＳＣＯＰＹ”，Ａｎｎｕ．Ｒｅｖ．Ｍａｔｅｒ．Ｓｃｉ．，Ｖｏｌ．２９，ｐｐ．１１７−１４８，１９９９J. et al. R. Kirtley et al. "SCANNING SQUID MICROSCOPY", Annu. Rev. Mater. Sci. , Vol. 29, pp. 117-148, 1999

しかしながら、上記特許文献１では、被測定試料を破損しないようにプローブを接触させてはいるが、破損し易い被測定試料などの測定においては、プローブと被測定試料を非接触で測定することが望まれている。
さらに、従来のＳＱＵＩＤ顕微鏡では、プローブの先端と被測定試料を数μｍ以下に近づけることは困難であるため、数μｍ以下の高い磁場空間分解能が得られないという課題がある。
また、微弱信号の信号対雑音比を高めるためには、測定に周波数フィルタを用いることが有効であるが、従来のＳＱＵＩＤ顕微鏡では被測定試料の磁場分布と走査速度の関係でフィルタの帯域が制限され、最適な周波数フィルタを選択することが困難であり、信号対雑音比の高い信号を得るのが困難であった。
本発明は、上記課題に鑑み、試料とプローブとが接触することなく、かつ、信号対雑音比の高い磁場信号が得られるＳＱＵＩＤ顕微鏡を提供することを課題とする。 However, in Patent Document 1, the probe is brought into contact with the sample to be measured so as not to be damaged. However, in measuring a sample to be measured that is easily damaged, the probe and the sample to be measured may be measured in a non-contact manner. It is desired.
Furthermore, in the conventional SQUID microscope, since it is difficult to bring the tip of the probe and the sample to be measured close to several μm or less, there is a problem that high magnetic field spatial resolution of several μm or less cannot be obtained.
In order to increase the signal-to-noise ratio of a weak signal, it is effective to use a frequency filter for measurement. However, in the conventional SQUID microscope, the filter band is limited by the relationship between the magnetic field distribution of the sample to be measured and the scanning speed. Therefore, it is difficult to select an optimal frequency filter, and it is difficult to obtain a signal with a high signal-to-noise ratio.
In view of the above problems, an object of the present invention is to provide a SQUID microscope in which a magnetic field signal with a high signal-to-noise ratio can be obtained without contact between a sample and a probe.

本発明は、上記課題を解決するために、例えば図１に示すように、本発明１のＳＱＵＩＤ顕微鏡は、プローブ（３）と被測定試料（６）との間隔を非接触状態に保つ制御部（９）と、被測定試料に所定周波数の振動を付与する加振機構（７、８）と、この振動状態での被測定試料の振動周波数と同じ周波数の磁気信号のみを検出するロックイン検出機構（２１）とが設けてあり、前記制御部においては、プローブの先端と被測定試料との間隔をその間に生じるトンネル電流にて計測し、この電流値が所望の値となるようにプローブと被測定試料との間隔を制御する、超伝導量子干渉素子を備えるＳＱＵＩＤ顕微鏡である。
そして、前記制御部は、前記被測定試料を走査すると共に、プローブの先端と被測定試料の表面との距離が一定となるように、加振機構により被測定試料をその高さ方向に制御して、当該高さ方向の移動量により被測定試料の表面形状を計測するステップと、加振機構を高さ方向に所定間隔だけ移動し、表面形状を計測したときよりも、プローブの先端と被測定試料の表面との間に所定間隔を付加して設けるステップと、加振機構に対して、所定周波数の参照信号を印加して、プローブの先端と被測定試料の表面とが非接触状態を保つ振幅であって、前記所定周波数で、その高さ方向に振動させるステップと、上記表面形状を計測したときの加振機構の高さ方向の移動量を加えて、再度同一走査ライン上を走査してＳＱＵＩＤ顕微鏡部により当該走査ライン上の磁気信号を検出するステップと、検出した磁気信号に対して、ロックイン増幅器によって参照信号と同一の周波数成分を持つ交流信号を選択的に増幅するステップとを有し、プローブの先端と被測定試料の表面とを接触させることなく被測定試料に記録されている磁気信号を測定することを特徴とする。 In order to solve the above-described problem, the present invention provides, for example, as shown in FIG. 1, the SQUID microscope of the present invention 1 is a control unit that keeps the distance between the probe (3) and the sample to be measured (6) in a non-contact state. (9), an excitation mechanism (7, 8) for applying vibration of a predetermined frequency to the sample to be measured, and lock-in detection for detecting only a magnetic signal having the same frequency as the vibration frequency of the sample to be measured in this vibration state A mechanism (21), and the controller measures the distance between the tip of the probe and the sample to be measured by a tunnel current generated between the probe and the probe so that the current value becomes a desired value. It is a SQUID microscope equipped with a superconducting quantum interference device that controls the distance from the sample to be measured .
The control unit scans the sample to be measured and controls the sample to be measured in the height direction by an excitation mechanism so that the distance between the tip of the probe and the surface of the sample to be measured is constant. Thus, the step of measuring the surface shape of the sample to be measured by the amount of movement in the height direction and the tip of the probe and the object to be measured are moved more than when the vibration mechanism is moved by a predetermined distance in the height direction and the surface shape is measured. A step of providing a predetermined interval between the surface of the measurement sample and a reference signal having a predetermined frequency is applied to the vibration mechanism so that the probe tip and the surface of the sample to be measured are not in contact with each other. The step of oscillating in the height direction at the predetermined frequency and the amount of movement of the vibration mechanism in the height direction when the surface shape is measured is added, and the same scanning line is scanned again. By SQUID microscope Detecting a magnetic signal on the scanning line, and selectively amplifying an AC signal having the same frequency component as the reference signal by a lock-in amplifier with respect to the detected magnetic signal, The magnetic signal recorded on the sample to be measured is measured without contacting the tip and the surface of the sample to be measured.

本発明１によれば、また、被測定試料を交流参照信号で振動させて、参照信号と同一の周波数成分の磁気信号のみ選択的にロックイン検出することで、被測定試料の磁場分布や走査速度に影響を受けずに、信号対雑音比の高い信号を得ることができる。
さらに、ＳＱＵＩＤ顕微鏡部により観察される被測定試料とＳＱＵＩＤ顕微鏡部のプローブの先端との距離が、プローブと被測定試料に流れるトンネル電流により制御されるので、非接触で、従来のＳＱＵＩＤ顕微鏡の１０分の１から１００分の１、即ち、サブμｍの高い磁場空間分解能を得ることができる。
According to the first aspect of the present invention, the sample to be measured is vibrated by the AC reference signal, and only the magnetic signal having the same frequency component as that of the reference signal is selectively lock-in detected. A signal with a high signal-to-noise ratio can be obtained without being affected by the speed.
Furthermore , since the distance between the sample to be measured observed by the SQUID microscope unit and the tip of the probe of the SQUID microscope unit is controlled by the tunnel current flowing between the probe and the sample to be measured, it is non-contact and the conventional SQUID microscope A high magnetic field spatial resolution of 1/10 to 1/100, that is, a sub-μm can be obtained.

以下に、本発明の実施例について図面を参照して説明する。
図１は本発明のＳＱＵＩＤ顕微鏡の構造を模式的に示す断面図である。
ＳＱＵＩＤ顕微鏡は、ＳＱＵＩＤ顕微鏡部１と走査トンネル顕微鏡部２とから構成される。ＳＱＵＩＤ顕微鏡部は、その先端に磁性材料からなるプローブ３を備えたＳＱＵＩＤヘッド部４と、ＳＱＵＩＤの検出した磁気信号を電気信号に変換する磁束電圧変換回路（磁束ロック回路）５と、ロックイン検出機構である特定の周波数を持つ交流信号を選択的に増幅し検出するロックイン増幅器２１と、信号被測定試料６を載置する微動ステージ７と、微動ステージを載置した粗動ステージ８と、微動ステージ７および粗動ステージ８を制御する制御部９と、加振機構である微動ステージ７または粗動ステージ８を特定の周波数で振動させるための参照信号を発生させる参照信号発生器２０と、を含み構成されている。さらに、プローブ３と被測定試料６との位置を目視で観察するための光学顕微鏡部を備えてもよい。上記のＳＱＵＩＤ顕微鏡部１は、除振台上に設置されることが好ましく、さらに、ＳＱＵＩＤヘッド部４が磁気シールドされて外部磁場の影響を受けないようにすることが好ましい。 Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a sectional view schematically showing the structure of the SQUID microscope of the present invention.
The SQUID microscope is composed of a SQUID microscope unit 1 and a scanning tunneling microscope unit 2. The SQUID microscope section includes a SQUID head section 4 having a probe 3 made of a magnetic material at its tip, a magnetic flux voltage conversion circuit (magnetic flux lock circuit) 5 that converts a magnetic signal detected by the SQUID into an electric signal, and lock-in detection. A lock-in amplifier 21 that selectively amplifies and detects an AC signal having a specific frequency, which is a mechanism, a fine movement stage 7 on which the signal sample 6 is placed, a coarse movement stage 8 on which the fine movement stage is placed, A control unit 9 that controls the fine movement stage 7 and the coarse movement stage 8, a reference signal generator 20 that generates a reference signal for vibrating the fine movement stage 7 or the coarse movement stage 8 that is the vibration mechanism at a specific frequency, and It is comprised including. Furthermore, you may provide the optical microscope part for observing the position of the probe 3 and the to-be-measured sample 6 visually. The SQUID microscope unit 1 is preferably installed on a vibration isolation table, and the SQUID head unit 4 is preferably magnetically shielded so as not to be affected by an external magnetic field.

次にＳＱＵＩＤヘッド部４を詳細に説明する。
ＳＱＵＩＤヘッド部４は真空容器１０とＳＱＵＩＤ顕微鏡部内のＳＱＵＩＤ１１を冷却するための冷媒容器１２と、プローブ３を固定した窓部１３と、から構成されている。
上記ＳＱＵＩＤヘッド部４の冷媒容器１２は、冷媒容器１２の下部から図面下方に向かって、ＳＱＵＩＤ１１を冷媒容器１２に支持するための支持部１４と、この支持部１４に固定されるＳＱＵＩＤ１１と、から構成されている。 Next, the SQUID head unit 4 will be described in detail.
The SQUID head unit 4 includes a vacuum container 10, a refrigerant container 12 for cooling the SQUID 11 in the SQUID microscope unit, and a window unit 13 to which the probe 3 is fixed.
The refrigerant container 12 of the SQUID head unit 4 includes a support part 14 for supporting the SQUID 11 on the refrigerant container 12 from the lower part of the refrigerant container 12 toward the lower side of the drawing, and a SQUID 11 fixed to the support part 14. It is configured.

真空容器１０の底部、即ちＳＱＵＩＤヘッド部４の中心には、開孔部が設けられており、ＳＱＵＩＤ１１が開孔部の内部に位置するように配置される。真空容器１０の中央底部にはＳＱＵＩＤ１１に被測定試料６からの磁束を伝達するプローブ３と、このプローブ３を固定するための窓部１３とが配置されている。プローブ３の上端部はＳＱＵＩＤ１１の中心部に近接して結合している。 An opening is provided at the bottom of the vacuum vessel 10, that is, the center of the SQUID head portion 4, and the SQUID 11 is disposed inside the opening. A probe 3 for transmitting magnetic flux from the sample 6 to be measured to the SQUID 11 and a window portion 13 for fixing the probe 3 are disposed at the center bottom of the vacuum vessel 10. The upper end portion of the probe 3 is coupled close to the central portion of the SQUID 11.

上記真空容器１０及び冷媒容器１２は、非磁性材料からなることが好ましく、ステンレスなどの非磁性鋼材、アルミニウムなどを用いることができる。
ＳＱＵＩＤ１１を冷媒容器１２に支持するための支持部１４は、円柱形状などを有し、ＳＱＵＩＤ１１を冷却する。この支持部１４の材料は、非磁性材料で、かつ、熱伝導性のよい材料を用いることができる。この材料には、サファイアを用いることができる。
このＳＱＵＩＤ１１としては、例えば、液体窒素温度で動作する高温超伝導材料で製作されたＤＣ−ＳＱＵＩＤを用いることができる。 The vacuum vessel 10 and the refrigerant vessel 12 are preferably made of a nonmagnetic material, and a nonmagnetic steel material such as stainless steel, aluminum, or the like can be used.
The support part 14 for supporting the SQUID 11 on the refrigerant container 12 has a cylindrical shape or the like, and cools the SQUID 11. The material of the support portion 14 can be a non-magnetic material and a material with good thermal conductivity. Sapphire can be used for this material.
As this SQUID 11, for example, a DC-SQUID made of a high-temperature superconducting material that operates at a liquid nitrogen temperature can be used.

プローブ３は、先端の半径が数十ｎｍ〜数十μｍの針であり、高透磁率材料を用いて製作することができる。高透磁率材料としてはパーマロイを用いることができる。このプローブ３の先端は、後述する窓部１３の中心孔に挿入され、プローブ３の先端が、ＳＱＵＩＤヘッド部４の内部から外部へ貫通している。 The probe 3 is a needle having a tip radius of several tens of nanometers to several tens of micrometers , and can be manufactured using a high magnetic permeability material. Permalloy can be used as the high magnetic permeability material. The tip of the probe 3 is inserted into a center hole of a window portion 13 described later, and the tip of the probe 3 penetrates from the inside of the SQUID head portion 4 to the outside.

窓部１３は、非磁性材料からなり、プローブ３を貫通させる孔を有し、プローブ３が窓部に真空機密を保つように固定される。この窓部１３の材料としては、ガラスなどを用いることができる。このため、被測定試料６に近接して所定の距離に配設したプローブ３の先端付近の磁束が、プローブ３を通って真空容器１０内部のプローブ３の他端へと導かれ、ＳＱＵＩＤ１１により磁束が計測される。
真空容器１０と冷媒容器１２とは最上部で接続され、真空容器１０と窓部１３とも真空機密が保たれるように接続される。冷媒容器１２は、冷媒１５を保持するための断熱容器である。真空容器１０に接続される真空装置により、所定の真空度となるように真空排気されてもよい。冷媒１５としては、液体窒素、液体ヘリウムなどを用いることができる。また冷媒１５の代わりにヘリウムなどを用いた冷凍機を用いてもよい。 The window portion 13 is made of a nonmagnetic material, has a hole through which the probe 3 passes, and the probe 3 is fixed to the window portion so as to keep a vacuum secret. As the material of the window portion 13, glass or the like can be used. For this reason, the magnetic flux near the tip of the probe 3 arranged at a predetermined distance in the vicinity of the sample 6 to be measured is guided to the other end of the probe 3 inside the vacuum vessel 10 through the probe 3, and the magnetic flux is generated by the SQUID 11. Is measured.
The vacuum vessel 10 and the refrigerant vessel 12 are connected at the uppermost portion, and the vacuum vessel 10 and the window portion 13 are also connected so that vacuum confidentiality is maintained. The refrigerant container 12 is a heat insulating container for holding the refrigerant 15. A vacuum device connected to the vacuum vessel 10 may be evacuated to a predetermined vacuum level. As the refrigerant 15, liquid nitrogen, liquid helium, or the like can be used. A refrigerator using helium or the like may be used instead of the refrigerant 15.

走査トンネル顕微鏡部２は、プローブ３と被測定試料６間にトンネル電流を流すためのバイアス電圧源１６と、トンネル電流を高感度で検出するための電流検出器１７と、トンネル電流の大きさにより微動ステージ７を制御する制御部９とからなる。この場合の制御部９はＳＱＵＩＤ顕微鏡部１の制御部９とを兼ねて図示している。 The scanning tunneling microscope unit 2 includes a bias voltage source 16 for passing a tunnel current between the probe 3 and the sample 6 to be measured, a current detector 17 for detecting the tunnel current with high sensitivity, and a magnitude of the tunnel current. The control unit 9 controls the fine movement stage 7. The control unit 9 in this case also serves as the control unit 9 of the SQUID microscope unit 1.

本発明のＳＱＵＩＤ顕微鏡は以上のように構成されており、次にその動作について説明する。
先ず、被測定試料６の観察すべき箇所にプローブ３の先端が移動するように、その位置が制御される。
次に、被測定試料６をＸ方向に走査する。このとき走査トンネル顕微鏡部２により被測定試料６の表面とＳＱＵＩＤ顕微鏡部１のプローブ３の先端との間にバイアス電圧源１６から電圧を印加して一定のトンネル電流が流れるように、すなわち被測定試料６の表面とプローブ３の先端との間の距離が一定となるように、微動ステージ７のＺ軸が制御され、被測定試料６の高さ情報が得られる。
次に、上記高さ情報をもとに、プローブ３の先端を被測定試料６の表面から一定の距離を離して再度同一走査ラインを走査してＳＱＵＩＤ顕微鏡部１により走査ライン上の磁場信号を検出する。この走査の際に参照信号発生器２０で発生させた特定の周波数、振幅の参照信号をもとに微動ステージ７または粗動ステージ８を振動させ、被測定試料の磁気信号に交流変調をかける。変調された磁気信号は参照信号と同じ周波数を持つので、信号をＳＱＵＩＤ１１およびＦＬＬ回路５で電気信号に変換後、ロックイン増幅器２１で参照信号と同じ周波数成分を選択的に増幅、検出することによって、雑音成分と区別された信号対雑音比の高い磁気信号を得ることができる。つぎに、Ｙ方向にステージを移動し、上記を繰り返して２次元の磁場分布像１８を得る。好ましくは、被測定試料の表面形状像１９と磁場分布像１８の双方とを描出できることが望ましい。 The SQUID microscope of the present invention is configured as described above, and the operation thereof will be described next.
First, the position of the probe 3 is controlled so that the tip of the probe 3 moves to a position to be observed on the sample 6 to be measured.
Next, the sample 6 to be measured is scanned in the X direction. At this time, a constant tunnel current flows by applying a voltage from the bias voltage source 16 between the surface of the sample 6 to be measured and the tip of the probe 3 of the SQUID microscope unit 1 by the scanning tunneling microscope unit 2, that is, the sample to be measured. The Z-axis of fine movement stage 7 is controlled so that the distance between the surface of sample 6 and the tip of probe 3 is constant, and height information of sample 6 to be measured is obtained.
Next, based on the height information, the tip of the probe 3 is separated from the surface of the sample 6 to be measured by a predetermined distance, and the same scanning line is scanned again. The magnetic field signal on the scanning line is generated by the SQUID microscope unit 1. To detect. The fine movement stage 7 or the coarse movement stage 8 is vibrated based on a reference signal having a specific frequency and amplitude generated by the reference signal generator 20 during this scanning, and AC modulation is applied to the magnetic signal of the sample to be measured. Since the modulated magnetic signal has the same frequency as the reference signal, after the signal is converted into an electric signal by the SQUID 11 and the FLL circuit 5, the lock-in amplifier 21 selectively amplifies and detects the same frequency component as the reference signal. Thus, it is possible to obtain a magnetic signal having a high signal-to-noise ratio that is distinguished from the noise component. Next, the stage is moved in the Y direction, and the above is repeated to obtain a two-dimensional magnetic field distribution image 18. Preferably, it is desirable that both the surface shape image 19 and the magnetic field distribution image 18 of the sample to be measured can be drawn.

このとき、被測定試料６の表面と、ＳＱＵＩＤ顕微鏡部１のプローブ３の先端との距離は、例えば０．０１μｍと制御することができる。
また、被測定試料６の表面の形状を計測し、最も高い試料表面とプローブ３の先端と距離を設定し、微動ステージ７の高さを一定にしてＸ方向の走査ができることが望ましい。
これにより、本発明のＳＱＵＩＤ顕微鏡においては、ＳＱＵＩＤ顕微鏡部１により観察される被測定試料６とＳＱＵＩＤ顕微鏡部１のプローブ３の先端との距離が走査トンネル顕微鏡部２により制御され、所定の距離に保持されることで高い空間分解能を得ることができる。
本発明のＳＱＵＩＤ顕微鏡によれば、被測定試料として、磁性を有する金属材料、無機材料、半導体材料など、あるいは各種の素子や材料からなる試料について、高い空間分解能をもって磁気量を計測することができる。例えば、磁気材料の非破壊検査、磁区構造の観察や、集積回路等の各種電気回路における配線の欠陥検査などに使用することができる。 At this time, the distance between the surface of the sample 6 to be measured and the tip of the probe 3 of the SQUID microscope unit 1 can be controlled to 0.01 μm , for example.
Further, it is desirable that the shape of the surface of the sample 6 to be measured is measured, the distance between the highest sample surface and the tip of the probe 3 is set, and the height of the fine movement stage 7 is kept constant to perform scanning in the X direction.
Thereby, in the SQUID microscope of the present invention, the distance between the sample 6 to be measured observed by the SQUID microscope unit 1 and the tip of the probe 3 of the SQUID microscope unit 1 is controlled by the scanning tunneling microscope unit 2 to a predetermined distance. A high spatial resolution can be obtained by being held.
According to the SQUID microscope of the present invention, a magnetic quantity can be measured with a high spatial resolution for a sample made of a metallic material, an inorganic material, a semiconductor material, etc., or various elements or materials having magnetism as a sample to be measured. . For example, it can be used for nondestructive inspection of magnetic materials, observation of magnetic domain structures, and inspection of wiring defects in various electric circuits such as integrated circuits.

以下、実施例によって本発明をさらに詳細に説明する。
実施例に用いた本発明のＳＱＵＩＤ顕微鏡は、図１の構成であり、ＳＱＵＩＤには高温超伝導ＤＣ−ＳＱＵＩＤを用い、液体窒素で冷却している。被測定試料６として、２μｍ間隔の幅の磁気信号を記録した磁気記録媒体を用いる。プローブ３の先端と被測定試料６との間に１Ｖのバイアス電圧を印加する。このときプローブ３の先端と被測定試料６との間に流れるトンネル電流を電流検出器１７により検出し、トンネル電流の値が０．１ｎＡになるよう微動ステージ７をＺ軸方向に移動する。次に、トンネル電流を常に０．１ｎＡに、すなわちプローブ３の先端と被測定試料６の表面との距離が一定となるように微動ステージ７をＺ軸方向に制御し、微動ステージ７をＸ軸方向に走査する。この走査中の微動ステージ７のＺ軸方向への移動量により被測定試料６の表面形状が計測される。次に、バイアス電圧を０Ｖとしてトンネル電流を流さないよう制御し、微動ステージ７をＺ軸の負の方向に１１００ｎｍ移動し、表面形状を計測したときよりも、プローブ３の先端と被測定試料６表面との間に１１００ｎｍの距離を設ける。次に、微動ステージのZ軸方向に１１５Ｈｚの参照信号を印加し、ステージをZ軸方向に周波数１１５Hz、振幅２０００ｎｍで振動させながら、上記表面形状を計測したときの微動ステージ７のＺ軸方向への移動量を加えて、同一走査線をＸ軸方向に走査し、磁気信号をＳＱＵＩＤ１１およびＦＬＬ回路５により検出したのち、ロックイン増幅器２１によって参照信号と同一の周波数成分を持つ交流信号を選択的に増幅する。これにより、プローブ３の先端と被測定試料６の表面とを接触させることなく被測定試料６に記録されている磁気信号を高感度に測定する。次に、微動ステージ７をＹ方向に移動し、次の走査線の表面形状と磁気信号を測定する。上記を繰り返して、２次元の被測定試料６の表面形状像１９と磁場分布像１８を得る。本実施例では、プローブ３の先端と被測定試料６の表面との距離は最短で１００ｎｍ程度となり、実施例で得られる磁気記録媒体からの磁気信号には０．５μｍ、つまり、サブマイクロンでの磁気変化が確認できる。
次に、比較例について説明する。実施例と同じ被測定試料として、２μｍ間隔の幅の磁気信号を記録した磁気記録媒体を用いる。従来の光学顕微鏡を用いたＳＱＵＩＤ顕微鏡を比較例とした場合。測定においては、プローブの先端と被測定試料との距離は５μｍに設定する。本比較例で得られる磁気記録媒体の磁気信号からは５μｍの幅の磁気変化が確認できる。つまり、分解能が５μｍと低い。これから、実施例の場合には、水平方向の分解能が比較例の１／１０に改善される。また、実施例とほぼ同じ構成で、ロックイン検出法を用いない方法を比較例とした場合では、ロックイン検出法を用いた実施例の場合、信号対雑音比が一桁以上改善される。
本発明は、上記実施例に記載のＳＱＵＩＤ顕微鏡に限定されることはなく、被測定試料に応じてＳＱＵＩＤの構造やプローブ構造などは、特許請求の範囲に記載した発明の範囲内で種々の変形が可能であり、それらも本発明の範囲内に含まれることはいうまでもない。 Hereinafter, the present invention will be described in more detail with reference to examples.
The SQUID microscope of the present invention used in the examples has the configuration shown in FIG. 1, and a high-temperature superconducting DC-SQUID is used as the SQUID, which is cooled with liquid nitrogen. As the sample 6 to be measured, a magnetic recording medium on which magnetic signals having a width of 2 μm are recorded is used. A bias voltage of 1 V is applied between the tip of the probe 3 and the sample 6 to be measured. At this time, the tunnel current flowing between the tip of the probe 3 and the sample 6 to be measured is detected by the current detector 17, and the fine movement stage 7 is moved in the Z-axis direction so that the value of the tunnel current becomes 0.1 nA. Next, the fine movement stage 7 is controlled in the Z-axis direction so that the tunnel current is always 0.1 nA, that is, the distance between the tip of the probe 3 and the surface of the sample 6 to be measured is constant. Scan in the direction. The surface shape of the sample 6 to be measured is measured by the amount of movement of the fine movement stage 7 in the Z-axis direction during scanning. Next, the bias voltage is set to 0 V so as not to flow the tunnel current, the fine movement stage 7 is moved 1100 nm in the negative direction of the Z axis, and the tip of the probe 3 and the sample 6 to be measured 6 are compared with the case where the surface shape is measured. A distance of 1100 nm is provided between the surface. Next, a reference signal of 115 Hz is applied in the Z-axis direction of the fine movement stage, and the stage is vibrated in the Z-axis direction at a frequency of 115 Hz and an amplitude of 2000 nm, while the surface shape is measured in the Z-axis direction of the fine movement stage 7. After the same scanning line is scanned in the X-axis direction and the magnetic signal is detected by the SQUID 11 and the FLL circuit 5, the lock-in amplifier 21 selectively selects an AC signal having the same frequency component as the reference signal. Amplify to. Thereby, the magnetic signal recorded on the sample 6 to be measured is measured with high sensitivity without bringing the tip of the probe 3 into contact with the surface of the sample 6 to be measured. Next, fine movement stage 7 is moved in the Y direction, and the surface shape and magnetic signal of the next scanning line are measured. By repeating the above, a surface shape image 19 and a magnetic field distribution image 18 of the two-dimensional sample 6 to be measured are obtained. In this embodiment, the distance between the tip of the probe 3 and the surface of the sample 6 to be measured is about 100 nm at the shortest, and the magnetic signal from the magnetic recording medium obtained in the embodiment is 0.5 μm , that is, submicron. Magnetic change can be confirmed.
Next, a comparative example will be described. A magnetic recording medium on which magnetic signals having a width of 2 μm are recorded is used as the sample to be measured as in the example. When a SQUID microscope using a conventional optical microscope is used as a comparative example. In the measurement, the distance between the probe tip and the sample to be measured is set to 5 μm . A magnetic change with a width of 5 μm can be confirmed from the magnetic signal of the magnetic recording medium obtained in this comparative example. That is, the resolution is as low as 5 μm . From this, in the case of the example, the resolution in the horizontal direction is improved to 1/10 of the comparative example. Further, when the comparative example is a method that does not use the lock-in detection method with the same configuration as that of the example, the signal-to-noise ratio is improved by an order of magnitude or more in the case of the example using the lock-in detection method.
The present invention is not limited to the SQUID microscope described in the above embodiment, and the SQUID structure and the probe structure are variously modified within the scope of the invention described in the claims depending on the sample to be measured. Needless to say, these are also included in the scope of the present invention.

図１本発明の請求項１に記載のＳＱＵＩＤ顕微鏡の構造を模式的示す模式図である。 FIG. 1 is a schematic diagram schematically showing the structure of a SQUID microscope according to claim 1 of the present invention.

Explanation of symbols

１：ＳＱＵＩＤ顕微鏡部
２：走査トンネル顕微鏡部
３：プローブ
４：ＳＱＵＩＤヘッド部
５：ＦＬＬ回路
６：被測定試料
７：微動ステージ
８：粗動ステージ
９：制御部
１０：真空容器
１１：ＳＱＵＩＤ
１２：冷媒容器
１３：窓部
１４：支持部
１５：冷媒
１６：電圧源
１７：電流検出器
１８：磁場分布像
１９：表面形状像
２０：参照信号発生器
２１：ロックイン増幅器 1: SQUID microscope unit 2: Scanning tunneling microscope unit 3: Probe 4: SQUID head unit 5: FLL circuit 6: Sample to be measured 7: Fine movement stage 8: Coarse movement stage 9: Control unit 10: Vacuum vessel 11: SQUID
12: Refrigerant container 13: Window part 14: Support part 15: Refrigerant 16: Voltage source 17: Current detector 18: Magnetic field distribution image 19: Surface shape image 20: Reference signal generator 21: Lock-in amplifier

Claims

A control section (9) for keeping the distance between the probe (3) and the sample to be measured (6) in a non-contact state;
An excitation mechanism (7, 8) for applying vibration of a predetermined frequency to the sample to be measured;
A lock-in detection mechanism (21) for detecting only a magnetic signal having the same frequency as the vibration frequency of the sample to be measured in this vibration state;
Wherein the control unit, the distance between the sample to be measured and the tip of the probe is measured by a tunnel current generated between them, a distance between the probe and the sample to be measured as the current value becomes a desired value A SQUID microscope comprising a superconducting quantum interference device ,
The control unit scans the sample to be measured and moves the sample to be measured in the height direction by the vibration mechanism so that the distance between the tip of the probe and the surface of the sample to be measured is constant. Controlling and measuring the surface shape of the sample to be measured by the amount of movement in the height direction;
Moving the excitation mechanism by a predetermined interval in the height direction and providing the predetermined interval between the tip of the probe and the surface of the sample to be measured, rather than measuring the surface shape; ,
A reference signal having a predetermined frequency is applied to the vibration mechanism, and the amplitude is such that the tip of the probe and the surface of the sample to be measured are kept in a non-contact state at the predetermined frequency and in the height direction. Step to vibrate,
Adding the amount of movement in the height direction of the excitation mechanism when measuring the surface shape, scanning the same scanning line again and detecting a magnetic signal on the scanning line by a SQUID microscope unit;
Selectively amplifying an alternating current signal having the same frequency component as the reference signal by the lock-in amplifier with respect to the detected magnetic signal;
A SQUID microscope for measuring a magnetic signal recorded in the sample to be measured without bringing the tip of the probe into contact with the surface of the sample to be measured.