Surface microscope apparatus

Abstract: An atomic force microscope is provided for sensing displacement of a cantilever (1) based on a scanning tunneling microscopy. Concretely, the present invention comprises a first microscope having a cantilever (1) for sensing atomic force or magnetic force and an STM (Scanning Tunneling Microscope) functioned as a second microscope for sensing tunneling current and keeping the tunneling current constant. It results in the microscope being able to carry out atomic force microscopy and tunneling microscopy without changing a single STM tip (1b) and to control the minimal force between the sample (3) and the tip (1b) to be constant.

Description: The present invention relates to a surface microscope apparatus of the kind referred to in the pre-characterizing portion of patent claim 1. Such a surface microscope apparatus is known from EP-A-223 918. As a method for observing fine structure on a sample surface, there has been traditionally provided the method designed to sense displacement of a cantilever with a tunneling probing tip. The displacement of the cantilever results from minimal force (repulsion or attraction) exerted between a sample and a probing tip located at the tip of the cantilever. The method serves to keep the minimal force constant by sensing the displacement of the cantilever with the tunneling tip. This method has been traditionally discussed in Journal of Vacuum Science Technology A6 (1988) pp. 275 to 278. This method takes the steps of locating the cantilever between the probing tip of a tunneling microscope and a sample, locating the cantilever and the tunneling tip close to a tunneling region, locating the sample and the probing tip provided at the tip of the cantilever to an allowable region of sensing the atomic force, and scanning the sample on an X-Y plane. In carrying out the steps, with the tunneling tip, the method serves to sense the displacement of the cantilever resulting from the atomic force exerted between the sample and the probing tip provided at the tip of the cantilever, resulting in being able to obtain a three-dimensional image of a sample surface. The sample, the cantilever and the tunneling tip are mounted to a piezoelectric device or a fine positioning system, which serves to adjust, control and scan the positions of those elements for the purpose of proper observation. The traditional method has made it possible to, when setting the system, remove the tunnelling tip from the sample and the tunneling tip and to use the cantilever as a tunneling microscope for checking the performance of the piezoelectric device or the like, but impossible to use any other means except the atomic force microscope. The foregoing prior art has merely offered a simple method such as atomic force microscopy or magnetic force microscopy. The foregoing prior art has been discussed to take the steps of sensing the displacement of the cantilever as tunneling current with the tunneling tip (referred to as an STM tip) located at the back of the cantilever, controlling the position of the sample toward the cantilever for keeping the tunneling current constant, and measuring surface topography or the like. EP-A-296 871 discloses a scanning tunnelling microscope incorporating a light source. The purpose of the light source is to modify the conductivity of photoconductors. An optical microscope is provided for coarse positioning of the sample. The resolution of STM and optical microscope being quite different, it is evident that the optical microscope can only serve for such auxiliary purposes. It will never be able to supply additional information on a point of the sample which is being scanned by the STM. US-A-4 798 989 relates to a sample moving mechanism for a scanning tunnelling microscope (STM) which is arranged so that the sample may also be subjected to electron microscopy, particularly scanning electron microscopy. STM and electron microscope (SEM) are completely separate devices. They can only be applied to a given sample one after the other (col. 1, lines 64-68). However, the state of the sample may change from one imaging process to the next, due to adsorption or other secular phenomena, or to electron bombardment in the electron microscopy process itself. Therefore, two images obtained by two different microscopy techniques in the device need not relate to identical states of the sample, making interpretation difficult. EP-A-223 918 discloses a surface microscope apparatus comprising a cantilever including a probing tip formed on an end portion thereof for sensing a convex and a concave on surface of a sample which is to be arranged facing said probing tip; a tunneling tip and a current sensor for detecting a displacement of said cantilever, and atomic force imaging means for obtaining an image of atomic force in accordance with information from said detecting means. The cited EP-A-223 918 discloses an AFM having a cantilever and a probe tip disposed on the cantilever for detecting the movement of the cantilever, but it discloses neither structure nor material of the probe tip. It is an object of the present invention to provide a surface microscope which allows to measure the surface topography of the sample as well as a thickness of an insulating film existing on the sample surface and an electron state under the film using the tunneling current flowing between the tip and the sample. This object is accomplished with a surface microscope as claimed in claim 1. Dependent claims are directed on features of preferred embodiments of the invention. Another issue placed on the surface microscope is to obtain properties of interface. In order to carry out the foregoing object, the surface microscope of the invention comprises an auto-approach system for the cantilever and the STM tip, a control system for keeping positional relation between the cantilever and the STM tip constant, an STM tip holding system, an STM tip retreating system for setting a desired minimal force value, an auto-approach system for the sample and the tip located at the tip of the cantilever, a system for controlling the position of either one of the sample and the cantilever and the STM tip combined with each other, a tip located at the tip of the cantilever and composed of a magnetic material, said tip is coated with a conductive film if necessary, and a system for measuring an insulating film thickness, an electron state and a tunner barrier existing on the sample surface based on a tunneling current flown between the probing tip and the sample. In operation, the auto-approach system for the cantilever and the STM tip is used for sensing the position of the cantilever to which no force is exerted (free condition). The control system serves to keep the tunneling current flowing between the STM tip and the cantilever constant. The tunneling current has to be constant for accurately sensing the position with a piezoelectric device. This is because it is necessary to continue the position sensing during a desired waiting time until creep of the piezoelectric device and thermal drift of the sample are made stable. The STM tip holding system and the STM tip retreating system for setting a desired minimal force value serve to stop the operation of sensing the position of the cantilever, hold the STM tip at the position, retreat the STM tip by a desired distance, and secure it. The retreating distance corresponds to a setting variation of the cantilever. The setting minimal force can be defined on a spring constant of the cantilever. The auto-approach system for the sample and the tip located at the tip of the cantilever and the system for controlling the position of either one of the sample and the combination of the cantilever and the STM tip serve to move either one closer to the other one and control the position between both so as to keep the tunneling current flowing between the cantilever and the STM tip to be a desired value. It results in allowing the sample and the probing tip located at the tip of the cantilever to come closer to each other and controlling the tunneling current value, that is, the minimal force value to be constant, thereby being able to sense the sample surface. Under the constant tunneling current, by raster-scanning either one of the sample and the probing tip located at the tip of the cantilever, it is possible to measure the surface topography with the same method as STM. For the purpose of knowing how the sample made of a magnetic material is magnetized, it is necessary to compose the tip located at the tip of the cantilever from the magnetic material or coating the tip with the condutive film. That is, the repulsive force is measured as a convex and the attractive force is measured as a concave. For the sample having an insulating film coated on the surface, the magnitude of the tunneling current flowing between the probing tip located at tip of the cantilever and the sampel depends on the thickness of an insulating film and an interface between the sample and the insulating film. Hence, it is possible to measure a thickness of an insulating film and properties of interface between the sample and the insulating film based on the change of the tunneling current. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing an essential portion of a microscope according to an embodiment of the invention; Fig. 2 is a top view showing the essential portion of the microscope in case of carrying out the atomic force microscopy; Fig. 3 is a block diagram schematically showing a control circuit for a microscope according to the invention; Figs. 4A to 4D are views showing basic construction of a surface microscope according to the invention and the operating sequences of the surface microscope; Fig. 5 is a view showing concrete arrangement of the surface microscope for realizing the operating sequence shown in Fig. 4A; and Fig. 6 is a block diagram showing the basic arrangement of the surface microscope according to another embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereafter, an embodiment of the invention will be described with reference to Fig. 1. Fig. 1 is a perspective view showing an essential portion of a scan type microscope including a cantilever moving system. 1 denotes a cantilever having a probing tip 1b. The cantilever 1 is mounted on the cantilever moving system 5 through a lever base 4. The cantilever moving system 5 employs an inchworm structure for obtaining large movement and keeping submicron or less positioning accuracy. As shown, the cantilever moving system 5 includes the lever base 4, piezoelectric devices 5a, 5b, terminals 6a, 6b, and clamps 7a, 7b. 2 denotes a tunneling tip located above a stage 12b. The tunneling tip 2 is mounted to a scanner 8 composed of piezoelectric devices. The scanner is mounted onto a coarse moving system 10 including the elements numbered 10 or later. The tunneling tip side unit is located on a stage 12c. 3 denotes a sample which is mounted onto a scanner 9 composed of piezoelectric devices. The scanner 9 is mounted onto a coarse moving system 11 including the elements numbered 11′ or later. The sample side unit is located on a stage 12a. Both of the stages 12a and 12c are allowed to move relatively with respect to the X and the Y directions. The stages 12a and 12c thus help to select a desired spot of the sample and arrange the relative positional relation among the sample 3, the cantilever 1 and the tunneling tip 2 in concert with the coarse moving systems 10 and 11. The cantilever moving system 5 is allowed to move in the X direction by opening or closing the clamps 7a, 7b and expanding or shrinking the piezoelectric devices 5a, 5b. In case of carrying out the atomic force microscopy, the cantilever 1 is traveled in a manner to allow itself to locate just between the tunneling tip 2 and the sample 3. Then, with the coarse moving systems 10 and 11, the distance between the tunneling tip 2 and the cantilever 1 and between the sample 3 and the cantilever 1 are reduced in a manner to allow the atomic force microscopy to be carried out. In case of carrying out the tunneling microscopy, with the cantilever moving system 5, the cantilever 1 is moved retreated to such a region as allowing the tunneling tip 2 to smoothly come closer to the sample 3. The control and scanning of the position of the tunneling tip 2 and the sample 3 are carried out by the scanner 8 and 9 composed of piezoelectric devices. It results in being able to obtain a scanning microscope image. Fig. 2 is a top view showing the essential portion of the microscope shown in Fig. 1 in case of carrying out the atomic force microscopy. The cantilever 1 is provided between the tunneling tip 2 and the sample 3. By flowing current between the tunneling tip 2 and the cantilever 1, with the tunneling microscopy, it is possible to measure how the cantilver 1 is displaced or control the Z-axis position of the sample 3 in a manner to keep the displacement of the cantilever 1 constant through a feedback circuit. Fig. 3 is a block diagram showing an essential circuit included in the microscope. The description will be directed to the atomic force microscopy and then the tunneling microscopy. In Fig. 3, 20 denotes a current sensor for sensing current flowing between the probing tip 2 and the cantilver 1. In order to measure the displacement of the cantilever, the switches 21 and 22 are turned to the side of a feedback circuit I 23 as shown in Fig. 3. Then, the scanner 8 serves to move the tip 2 in the Z-axis direction in a manner to keep the tunneling current constant, that is, maintain the constant interval between the tip 2 and the cantilever 1. The scanner 9 and an X-Y scanning circuit 29 serve to scan the material on the X-Y plane. The displacement of the cantilever 1 corresponds to that of the tip 2, which is sent to a memory 25 together with an X-Y scanning signal. It results in allowing a display 26 to represent a concave and convex image on the surface of the sample 3. In order to keep the displacement of the cantilever 1 constant, the switches 21 and 22 are turned to the opposite side to that shown in Fig. 3 so that the tip 2 is made fixed. A feedback circuit II 24 serves to control the Z-axis position of the sample 3 so as to keep the current flowing between the tip 2 and the cantilever 1 constant, that is, keep the displacement of the cantilever 1 constant. The Z-axis control signal given by the feedback circuit II 24 is represented as a convex and concave image on the surface of the sample 3 on the display 26. Figs. 4A to 4D are graphs showing basic operating sequences of a surface microscope according to the invention. In Fig. 4A, the cantilever 1 stays in the free condition. The current sensor 20 serves to sense the tunneling current flowing between the cantilever 1 and the STM tip 2 as a result of a bias voltage 30 applied therebetween. At a time, the current sensor 20 serves to drive a Z-axis piezoelectric device of the scanner 8 through the STM constant current servo circuit 23 so as to allow the STM tip 2 to come closer to the cantilever 1 until the tip 2 reaches a tunneling region. When the tip 2 reaches the tunneling region, the tunneling current flows between the tip 2 and the cantilever 1. The constant current servo circuit 23 serves to compare the tunneling current value with a reference value, supply a signal (Z₁ signal) for driving the Z-axis piezoelectric device of the scanner 8, and control the Z-axis position of the tip 2 so as to keep the tunneling current constant. After stabilizing the overall system in the constant tunneling current, the tip 2 is temporarily held. Then, as shown in Fig. 4B, for setting the minimal force (displacement of the cantilever) to a predetermined value, a predetermined amount of driving signal 31′ is supplied to the Z-axis piezoelectric device of the scanner 8. The tip 2 is moved backward in the Z-axis direction by a predetermined distance ΔZ from the position at which the tip 2 is held. The tip 2 is held at the position again. These operations allow a desired minimal force value to be preset. Next, as shown in Fig. 4C, the sample 3 is pushed against the tip 1b located at the tip of the cantilever 1 until the tunneling current flows through the tip 2 again. At this time, the tip 2 serves as a mechanically-positioned electric switch. When the tunneling current starts to flow through the tip 2 again, as shown in Fig. 4D, in response to a signal (Z₂ signal) sent from the constant current servo circuit 32, the Z-axis piezoelectric device of the tripod type scanner 9 supporting the sample is driven to control the Z-axis position of the sample for keeping the tunneling current constant (the same constant value as that set in the state shown in Fig. 4A). In this state, the X- and Y-axis piezoelectric devices of the sample scanner 9 serve to drive the sample on the X-Y plane so that the probing tip 1b located at the tip of the cantilever 1 is allowed to raster-scan on the sample surface. It results in being able to obtain the surface topography of the sample provided when the tunneling current keeps a predetermined minimal force value. The coarse moving system for the STM tip 2 and the sample 3 may be provided, through it is not shown in Figs. 4A to 4D. Further, the cantilever 1 and the STM tip keeping the predetermined positional relation between them are moved closer to the sample 3 in concert by the coarse moving system. Or, it may be possible to carry out the X-Y raster scanning and the servo operation done on the constant minimal force value using the combination of the cantilever 1 and the STM tip 2, without departing from the spirit of the invention. Fig. 5 is a block diagram showing the concrete arrangement of the surface microscope for realizing the operating sequences shown in Figs. 8A to 8D. An inchworm 36 for supporting a sample scanner 9 and an auto-approach inchworms I 37 and II 38 are provided on a table 35 and are served as a coarse moving system. On the inchworm II 38 are provided a cantilever 1, an STM tip scanner 8 and an STM tip 2. The inchworms I 37 and II 38 serve to execute the operating sequences shown in Figs. 4A to 4C. The control system includes a control system for presetting a desired minimal force, a control system for servo-controlling a desired minimal force, and a system 39 for obtaining surface topography. The minimal force presetting control system comprises a constant current servo circuit I 40, a hold circuit 41, a minimal force presetting circuit 42, an adding circuit 43, a high-votlage amplifier 44, and an auto-approach circuit 45. When the operation shown in Fig. 4A is carried out, the hold circuit 41 is set to an off mode in response to a signal 46 for setting whether or not any voltage is held. Then, the auto-approach circuit 45 serves to drive the inchorm I 37 so that the STM tip 2 is allowed to come closer to the cantilever 1 staying in the free condition. At a time, the inchworm I 37 serves to servo-control the Z-axis position of the STM tip 2 in a manner to keep the tunneling current flowing between the STM tip 2 and the cantilever 1 constant. When a certain amount of tunneling current flows between the STM tip 2 and the cantilever 1, the servo-control system serves to slowly move a tripod 8 to push the STM tip 2 against the cantilever 1. The constant current servo circuit I 40 slowly changes its output from a maximum voltage to a predetermined one for the purpose of controlling the inchworm I depending on the changing output. The control of the inchworm I results in realizing the state shown in Fig. 4A. The operating mode shown in Fig. 4B is carried out as follows. The hold circuit 41 is set to an on mode in response to an on/off signal 46 for the hold circuit 41 so that the hold circuit can fix a servo voltage. An adding circuit 43 serves to add the fixed servo voltage to a predetermined voltage preset by the minimal force presetting circuit 42. The addition allows the STM tip 2 to be moved backward by a predetermined distance ΔZ so that the force exerted between the sample 3 and the tip 1b located on the cantilever 1 is allowed to be set as repulsive force (kΔZ). K is a spring constant of the cantilever 1 and is preferably about 1 N/m. Assuming that ΔZ is 10 nm, for instance, the set minimal force (repulsive force) is 10⁻⁸N. Herein, for setting the predetermined minimal force, the Z-axis piezoelectric device of the tripod 8 is moved backward. As another means, however, it may be possible to employ a retreating function of the auto-approach inchworm I. That is, the tip may be moved backward only by ΔZ using the piezoelectric device for driving the inchworm I 37 in the Z-axis direction. In the foregoing state, a switch 47 is turned on so that the current sensor 20 supplies a current output to the constant current servo circuit II 48 for operating the constant current servo system. The auto-approach circuit 45 serves to drive the auto-approach inchworm II 38 in a manner to preset a variation of ΔZ corresponding to a predetermined minimal force between the STM tip 2 and the cantilever 1. In the variation-preset state, the probing tip 1b located at the tip of the cantilever 1 is allowed to come closer to the sample 3. When the tip 1b receives repulsive force off the sample 3, the cantilever 1 is flexed toward the STM tip 2, thereby allowing the tunneling current to be flown between the cantilever 1 and the STM tip 2. Then, as the piezoelectric device for driving the inchworm II 38 is expanded, the piezoelectric device for z-control of the sample scanner 8 slowly reduces the expansion to a proper value so that the sample scanner 4 can obtain the surface topography of the sample. At this time, the constant current servo circuit II 48 and the high voltage amplifier 49 are constantly operative so that they control the Z-axis position of the sample 3 for keeping the tunneling current value constant. With the foregoing methods, the operations shown in Figs. 4C to 4D are realized. Moreover, the surface topography can be obtained by the surface topography obtaining system 39 having a memory 25, a two-dimensional scanning system 29 and a display 26. In operation, the two-dimensional scanning system 29 supplies a signal 34 for driving the X- and Y-axes piezoelectric devices of the sample scanner 9. With the piezoelectric devices being driven, the probing tip 1b is allowed to relatively raster-scan on the sample surface. The constant current servo circuit II 48 thus changes its output signal depending on the convexes and concaves formed on the sample surface. It means that the output signal represents the convex and concave information corresponding to the X and Y scanning signal. The output signal is stored in the memory 25 for the purpose of obtaining the topographic structure. After obtaining the topographic structure, the topographic structure is read out of the memory 25 and is sent to the display 26 on which a three-dimensional image is displayed on the topographic structure. As mentioned above, the present invention provide a highly accurate and reliable repulsive type atomic force microscope. More excellent microscope system can be realized by using a computer for controlling the aforementioned procedures. The foregoing description about the embodiment has concerned with the repulsive type atomic force microscope. By providing the STM tip 2 in opposite to the tip 1b of the cantilever 1 (sample 3), however, the embodiment may apply to the attractive force. Fig. 6 shows another concrete arrangement of the invention. Under the state shown in Fig. 4D (constant minimal force) this arrangement makes it possible to measure the surface topography of the sample as well as a thickness of an insulating film existing on the sample surface and an electron state under the film using the tunneling current flowing between the tip 1b and the sample 1. The arrangement is designed to coat a conductive film 120 on the tip 1b and the cantilever 1, provide an electric power supply 123 between the conductive film 120 and the sample 3, and sense the tunneling current flowing between the tip 1b and the cantilever 1 with a current sensor 20. To measure how a thickness of the insulating film formed on the surface of the sample 3 is changed, a log converter 124 is used. Further, a sensing channel 129 is also provided for measuring the tunneling current itself. For selecting either one of the log converter 124 and the sensing channel 129, a switch 125 is provided for storing the selected data in a memory A126. The surface topography of the sample, on the other hand, can be obtained by storing a Z signal 128 sent from a constant current servo system 32 in a memory B127. The foregoing mechanisms and control systems make it possible to measure the surface topography of the insulating film and how a thickness of the insulating film is changed or the surface topography of the film. In case different kinds of atoms exist on the surface, like a scanning tunneling spectroscopy (STS), it is necessary to take the steps of moving the tip 1b or the sample 3 for scanning in X- and Y-axis direction, temporarily holding it at each measuring point, changing a voltage of the electric power supply 123 for measuring change of the tunneling current depdnding on the change of the voltage, and measuring how electrons are arranged on the surface of the sample. The method of measuring a thickness of the insulating film may employ an equation (∼ΔI/I) based on the change of the tunneling current except the log conversion method. In either case, the present invention has a feature of measuring the thickness of the insulating film based on the change of the tunneling current. Concretely, it is possible to use an output of the divider 102. With the means for keeping the minimal force constant and for measuring the tunneling current using the tip 1b, as described above, the surface and interface of the insulating film are measured. An another means, it may be possible to employ heat (temperature), light and sound wave for measuring the surface and interface of the insulating material in place of using the tunneling current without departing from the spirit of the invention. Further, by radiating a laser beam or a ray to the sample 3, it may be also possible to measure the electronic structure on the sample 3. If the tip lb of the cantilever 1 is made of a magnetic material, it is possible to measure how magnetization is distributed on the sample 3. Further, by minutely vibrating the cantilever 1 toward the sample, it is possible to sense the change of the tunneling current and measure a variation (k₁ΔIn(I)) or (k₂ΔI/I) of the log conversion for sensing a tunneling barrier amount, wherein k₁ and k₂ are proportional constants. It results in being able to obtain an image of a tunneling barrier and measure the density distribution of impurities contained in a semiconductor. For the measurement, the cantilever is supported by the piezoelectric device and the measurement is allowed by vibrating the piezoelectric device. The STM tip remains held in the measurement. The aforementioned system is an analog system, but it may be a digital system using a computer. The digital system can offer the same result. Further, the digital system makes it easier to perform the sequence control, thereby making it possible to display the surface topography from various points of views and provide a wide range of utilities. The present invention may employ a computer system without departing from the spirit of the invention. The foregoing embodiment has used the tunneling current, but it may use an electric-field emitting current. When the sample topography is measured, it is also possible to measure a thickness of the insulating film, how atoms are arranged on the interface or electrons are arranged on the material, and the surface topography of the tunneling barrier, thereby making it possible to measure different kinds and density of elements included on the interface. It results in being able to elucidate impurity contamination on the interface and observe the impurity density distribution.

Claims: A surface microscope apparatus comprising: a cantilever (1) including a probing tip (1b) formed on an end portion thereof for sensing a convex and a concave on the surface of a sample (3) which is to be arranged facing said probing tip (1b); a tunneling tip (2) and a current sensor (20) for detecting a displacement of said cantilever (1); atomic force imaging means for obtaining an image of atomic force in accordance with information from said detecting means; characterized by an electrode formed by coating the probing tip (1b) with a conductive film (120); a voltage source (123) for applying a voltage between said cantilever (1) and said sample (3) causing a tunnelling current to flow through said electrode; tunnelling image processing means for generating an image from said tunnelling current. A surface microscope apparatus according to claim 1, characterized in that said tunnelling image processing means comprises film thickness converting means for converting said tunnelling current into information on the thickness of an insulator on said sample (3). A surface microscope apparatus according to claim 2, characterized in that said film thickness converting means comprises logarithmic converting means. A surface microscope apparatus according to one of claims 1 to 3, comprising a sample support (9) for mounting said sample thereon.

Inventors: HASEGAWA TSUYOSHI HITACHI DAIY, HONDA YUKIO, HOSAKA SUMIO, HOSOKI SHIGEYUKI, ICHIKAWA MASAKAZU, HASEGAWA, TSUYOSHI, HITACHI DAIYON KYOSHINRYO B208, HONDA, YUKIO, HOSAKA, SUMIO, HOSOKI, SHIGEYUKI, ICHIKAWA, MASAKAZU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01J37
IPC codes: G01N23, H01J37, G01N27, G01Q30, G01Q60