Scanning probe microscope

Abstract: A scanning probe microscope comprising: a probe 1 equipped with a probe tip at its front end, a vibration application portion consisting of a piezoelectric vibrating body 2 and an AC voltage-generating portion 3, a vibration-detecting portion consisting of a quartz oscillator 4 and a current/voltage amplifier circuit 5, a coarse displacement means 6 for bringing the probe close to a surface of a sample, a sample-to-probe distance control means consisting of a Z fine displacement element 11 and a Z servo circuit 12, a two-dimensional scanning means consisting of an XY fine displacement element 13 and an XY scanning circuit 14, and a data processing means 15 for converting a measurement signal into a three-dimensional image. The probe 1 is held to the quartz oscillator 4 by resilient pressure.

Description: The present invention relates to a scanning probe microscope utilising a quartz oscillator to control the position of a probe. Known methods for position control in a scanning probe microscope include a method consisting of detecting a tunnelling current, a method consisting of detecting evanescent light, and a method consisting of detecting an atomic force. One form of scanning probe microscope making use of a tunnelling current for the control of a probe is a scanning tunnelling microscope (STM). One form of scanning probe microscope in which evanescent light is employed for the control of a probe is a photon STM. However, limitations are imposed on samples capable of being measured. Therefore, principal applications lie in an atomic force microscope (AFM), where an atomic force is used to control the position of a probe, and in a near-field scanning optical microscope (NSOM). One method of detecting an atomic force consists of detecting displacements of a probe by means of laser light. Another method makes use of variations in the current generated by a quartz oscillator. A scanning probe microscope in which laser light is used to detect displacements of a probe is disclosed, for example, in Japanese Patent Unexamined Publication No. 50750/1994, entitled, "Scanning Microscope Including Force-Detecting Means", by Robert Erik Betzig. An example of a scanning probe microscope in which a quartz oscillator is used to detect displacements of a probe is disclosed in App]. Phys. Lett . 66(14), 1995, pp. 1842-1844, by Kaled Karai et al. These instruments are outlined below. Fig. 2 is a schematic view of the prior art "scanning probe microscope using laser light". The tip of an optical fiber 310 is machined into a tapering form 70. A sample stage 20 is placed on an XYZ stage 50. A sample 30 is set on the sample stage. The optical fiber probe 70 is vibrated parallel to the sample surface, using a fine displacement element 40. A horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. To measure the state of vibration of the probe 70, laser light (not shown) used for position control is directed at the tip, and the shadow of the probe 70 is detected by a lens 90 and a photodetector 80. The distance between the sample surface and the tip of the probe is controlled, using the fine displacement element 40, so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilising this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. The sample surface is raster-scanned, using the XYZ fine displacement element 40. In this way, the topography of the sample surface can be measured on the order of nanometers. Fig. 3 is a schematic view of main portions of the prior art "scanning probe microscope using a quartz oscillator". Indicated by 400 is an optical fiber probe, and indicated by 410 is a quartz oscillator. The optical fiber probe is bonded to the quartz oscillator with adhesive. The quartz oscillator is made to resonate, using a piezoelectric device (not shown) for vibrations. As the tip of the probe approaches the sample, a horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. The state of vibration of the quartz oscillator is measured by measuring electric charge generated by a piezoelectric effect of quartz. The distance between the sample surface and the tip of the probe is controlled, using a piezoelectric scanner (not shown) so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilising this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. The sample surface is raster-scanned, using an XYZ fine displacement element (not shown). In this manner, the topography of the sample surface can be measured on the order of nanometers. Such a priort art is disclosed in DE 195 10368 The prior art scanning probe microscope described above has the following disadvantages. In the scanning probe microscope using laser light, the laser light is directed at the sample surface near the tip of the optical probe, and an image (shadow) of the tip of the probe is detected from the reflected light in order to detect the shear force. Therefore, the amount of reflected light is readily affected by the topography of the sample surface and by the reflectivity. Hence, it is difficult to measure the amplitude of vibration, and it is difficult to precisely measure the surface topography. Furthermore, it is not easy to align the laser light and so data reproducibility has posed problems. In the scanning probe microscope using a quartz oscillator, the portion where the quartz oscillator and the optical fiber are adhesively bonded together tends to be a microscopic region (e.g., a square region about 100 µm square). It is difficult to perform the bonding operation. Furthermore, the characteristics of the quartz oscillator device are easily affected by the amount of adhesive, the hardness, the location at which they are bonded, and other factors. Thus, it is difficult to obtain an oscillator sensor with high reproducibility. For these reasons, it has been difficult to use the instrument in industrial applications. When the probe is replaced, the quartz oscillator must also be replaced. This gives rise to an increase in the cost. In addition, reproducible surface topography measurement has been impossible to perform. JP 05180616 shows a probe being pressed by a leaf spring against a fixing member, so as to be easily exchangeable. A scanning probe microscope in accordance with the present invention has a probe equipped with a probe tip at its front end, a vibration application portion consisting of a piezoelectric vibrating body and an AC voltage-generating portion, a coarse displacement means for bringing the probe close to a surface of a sample, a sample-to-probe distance control means consisting of a Z fine displacement element and a Z servo circuit, a two-dimensional scanning means consisting of an XY fine displacement element and an XY scanning circuit, and a data processing means for converting a measurement signal into a three-dimensional image wherein the probe is held to the quartz oscillator by resilient pressure and is either a scanning tunnelling microscope tip or an atomic force microscope cantilever. This structure provides a scanning probe microscope capable of measuring surface topography with high reproducibility. Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view of a scanning probe microscope generally in accordance with the present invention; Fig. 2 is a schematic view of the prior art scanning probe microscope using laser light; Fig. 3 is a schematic view of the prior art scanning probe microscope using a quartz oscillator; Fig. 4 is a schematic view of Embodiment 1 of a scanning probe microscope in accordance with the present invention; Fig. 5 is a schematic view of Embodiment 2 of a scanning probe microscope in accordance with the present invention; and Fig. 6 is a schematic view of Embodiment 3 of a scanning probe microscope in accordance with the present invention. With reference to figure 1, a scanning probe microscope generally in accordance with the present invention comprises a probe 1, a vibration application portion consisting of a piezoelectric vibrating body 2 and an AC voltage-generating means 3, a vibration-detecting portion consisting of a quartz oscillator 4 and a current/voltage amplifier circuit 5, a coarse displacement means 6 for bringing the probe close to the sample surface, a sample-to-probe distance control means consisting of a Z fine displacement element 11 and a Z servo circuit 12, a two-dimensional scanning means consisting of an XY fine displacement element 13 and an XY scanning circuit 14, and a data processing means 15 for converting a measurement signal into a three-dimensional image. A resilient body 16 produces spring pressure that holds the probe 1 to the quartz oscillator 4. When the probe vibrating horizontally is brought close to the sample surface, a shear force acts on the tip of the probe. This reduces the amplitude of the vibration. The probe and the quartz oscillator are secured together by spring pressure and thus operate as a unit. Therefore, the decrease in the amplitude of the vibration of the probe results in a decrease in the amplitude of the vibration of the quartz oscillator. This in turn reduces the output current, which is detected by the current/voltage amplifier circuit. The distance between the sample and the probe is controlled with the Z fine displacement element and the Z servo circuit to maintain the output current from the quartz oscillator constant. In this way, the tip of the probe is kept at a constant distance from the sample surface. Under this condition, the probe is scanned in two dimensions across the sample plane to produce a three-dimensional image. The distance between the probe and the sample is controlled by the use of a quartz oscillator as described above. This dispenses with a laser normally used for position control such as in a scanning probe microscope employing laser light. In addition, the problem of inaccurate data due to variations in the position of the laser light and variations in the amount of reflected light can be circumvented. The spring pressure of the resilient body anchors the probe to the quartz oscillator. In the prior art probe microscope using a quartz oscillator, data would be affected by the manner in which they are adhesively bonded. In exchanging the probe, it is only necessary to replace the probe. In consequence, the same quartz can be used. The reproducibility of the measurement conditions and the reproducibility of data can be enhanced. Moreover, the replacement of only the probe gives rise to lower cost. In addition, the adhesive bonding that is difficult to perform is made unnecessary. Consequently, the instrument is made very easy to handle. Fig. 4 is a schematic view of Embodiment 1 of a scanning probe microscope in accordance with the invention. The described embodiment provides a scanning probe microscope capable of controlling the ambient conditions around the sample. A quartz oscillator 4 and a piezoelectric oscillator 2 are bonded to a quartz oscillator holder 25 with adhesive. A PZT device in the form of a flat plate is used as the piezoelectric oscillator. A quartz oscillator used for a clock or watch is used as the aforementioned quartz oscillator. When an AC voltage is applied to the PZT device, it vibrates, forcing the quartz oscillator to vibrate. If the vibration frequency is made coincident with the resonant frequency (e.g., 32.7 kHz), the quartz oscillator resonates. The piezoelectric effect induces an electric charge on the electrodes of the quartz oscillator. The resulting current is detected by a current/voltage amplifier circuit. Since a current proportional to the amplitude of the vibration of the quartz oscillator is produced, the state of the vibration of the quartz oscillator can be measured from the detected current. A cylindrical PZT scanner, a laminated PZT plate, or other structure may be used as the piezoelectric oscillator, as well as the PZT plate. All of them are embraced by the present invention. Furthermore, quartz oscillators used in applications other than clocks and watches may be used as the quartz oscillator herein. A probe 1 is held to the quartz oscillator by the spring pressure of a resilient body 16. The probe may be prepared by chemically etching a tip of tungsten and machining it into a tapering form. The probe can be made of metals in this way. Alternatively, it is possible to use a cantilever of silicon or silicon nitride, an optical fiber, or a glass pipette machined into a tapering form to fabricate the probe. This is embraced by the present invention. The tapering method may include mechanical polishing and heating-and-elongating processing, as well as chemical etching. It is possible that a magnetic film is deposited on the probe tip to make a magnetic force-sensing probe. In addition, it is possible that a film of gold or platinum is formed to make a conductive probe. All of them are embraced by the present invention. A leaf spring made of a stainless steel is used as the resilient body. Since the sensitivity of the quartz oscillator to forces is high, it is desired that the spring constant of the resilient body be small. In the present embodiment, a cantilever spring having a thickness of 100 µm, a width of 1 mm, and a length of 10 mm is used. Alternatively, the resilient body may be a leaf spring of phosphor bronze or various kinds of rubber such as silicone rubber. All of them are embraced by the present invention. Furthermore, the body may be held by making use of the resilience of the probe itself. This is also embraced by the invention. Where the body is held by spring pressure, this pressure is measured, utilising the oscillating characteristics of a quartz oscillator, i.e., Q-value. Where the probe is not held, the Q-value of the quartz oscillator is about 3000, for example. Where the probe is held with a spring, the Q-value is less than 500. A Q-value preferable for the scanning probe microscope is approximately 100 to 400. The spring pressure is adjusted so that the Q-value falls within this range. The quartz oscillator holder 25 is held to XYZ fine displacement elements 11 and 13. A cylindrical piezoelectric device in which X-, Y-, and Z-axis scanners are combined into a unit is used as each fine displacement element. Alternatively, a piezoelectric scanner in which Z-axis is separate from X- and Y-axes and electrostrictive devices may be used as the fine displacement elements. These are embraced by the present invention. Other possible structures include piezo-stages, stages using parallel stages, tripod-type piezoelectric devices in which one-axis piezoelectric devices are mounted on X-, Y-, and Z-axes, respectively, and laminar piezoelectric scanners. All of them are embraced by the present invention. A coarse displacement means 6 is used to bring the probe close to a sample 17. A coarse displacement means consisting of a stepping motor and a speed-reduction gear, a rough motion screw, or a linear guide is used as the above-described coarse displacement means. Other examples of the coarse displacement means may consist of a Z stage to which a stepping motor is added. A further example includes a stage using piezoelectric devices. For instance, an inchworm mechanism or Z stage may be combined with a piezoelectric device. All of them are embraced by the present invention. The sample is held in a vacuum, using a vacuum chamber 18. In this way, the sample can be retained in a vacuum. The vacuum chamber may be provided with a gas inlet port, and the sample may be exposed to an inert gas or reactive gas. This is also embraced by the present invention. Because of the structure described above, the surface topography can be measured with high reproducibility on the order of nanometers. Fig. 5 is a schematic view of Embodiment 2 of a scanning probe microscope in accordance with the present invention. The described embodiment is a scanning near-field optical microscope capable of heating and cooling the sample. Light emitted by a laser light source 19 is amplitude-modulated periodically by an optical modulator 27 consisting of an acoustooptical (AO) modulator. Alternative optical modulators include an electro-optic modulator (EO modulator) using an electric field and mechanical modulators in which an optical chopper is rotated by an electric motor. All of them are embraced by the present invention. The modulated laser light is introduced into the probe 1 by a coupling 21. The optical waveguide probe is held to the quartz oscillator 4 by the spring pressure of the probe itself. The light is directed at the sample 17 from the aperture at the tip of the probe. Light reflected off the sample is gathered by a lens 8 via a lens 7, mirrors 23, 22, and an optical window 24. The light is then split into two beams travelling in two directions by a half-mirror 31. The split light beams are measured by a photodetector 9 and a CCD camera 29. In some cases, the half-mirror may be replaced by a dichroic mirror. To secure sufficient amount of light, it is possible to omit the mirrors. The light detected by the photodetector 9 is measured with high S/N, using a lock-in amplifier. The resulting signal is converted into a three-dimensional image by a data processing means 15. The measured region on the sample surface is moved, using an XY stage 26 for the sample. A piezoelectrically driven stage is used as this XY stage. Alternatives include an XY stage in which a stepping motor is combined with an XY stage. This is also embraced by the present invention. A cryostat is used to cool the sample. Since a cooling mechanism relying on helium gas flow was provided, the sample temperature was cooled down to liquid helium temperature in a short time (about 30 minutes). Other means for cooling the sample include (i) cooling a metal such as copper and bringing it into contact with the metal in vacuum to cool the sample and (ii) using a mechanical refrigerator making use of adiabatic expansion of liquid helium. All of them are embraced by the present invention. A heater 32 was used to heat the sample. The heater consisted of a wire of manganin wound around a sample stage of copper. Heating is done by controlling the current fed to the heater. Other examples of the heater include tungsten wire, carbon thin film, and manganin thin film. All of them are embraced by the present invention. Using the structure described thus far, it is possible for laser light to be directed at the sample surface from an aperture which is less than the wavelength of the optical waveguide probe while varying the sample temperature from the low temperature of liquid helium temperature to a high temperature. The reflected light was gathered by lenses and detected by the photodetector. The surface topography could be measured with high reproducibility on the order of nanometers by scanning the optical waveguide probe across the sample plane. At the same time, the distribution of reflected light within the sample plane could be measured with high resolution less than the said wavelength. Fig. 6 is a schematic view of Embodiment 3 of a scanning probe microscope in accordance with the present invention. The described embodiment is a scanning probe microscope capable of holding a sample in a liquid. A sample 17 is held in a liquid cell 33 part filled with a liquid 34. This embodiment is similar to Embodiment 1 except for the vacuum chamber. This structure makes it possible to measure the surface topography of the sample, even when the sample is immersed in a liquid, with high reproducibility on the order of nanometers. As described thus far, embodiments of the present invention comprise: a probe 1 equipped with a probe tip at its front end, a vibration application portion consisting of a piezoelectric vibrating body 2 and an AC voltage-generating portion 3, a vibration-detecting portion consisting of a quartz oscillator 4 and a current/voltage amplifier circuit 5, a coarse displacement means 6 for bringing the probe close to a surface of a sample, a sample-to-probe distance control means consisting of a Z fine displacement element 11 and a Z servo circuit 12, a two-dimensional scanning means consisting of an XY fine displacement element 13 and an XY scanning circuit 14, and a data processing means 15 for converting, a measurement signal into a three-dimensional image. The probe 1 may be held to the quartz oscillator 4 by spring pressure of a resilient body 16. As described above, the distance between the probe and the sample is controlled, using the quartz oscillator. This dispenses with the position controlling laser which would normally be used in a scanning probe microscope using laser light. In addition, the problem of inaccurate data due to variations in the position of the laser light and due to variations in the amount of reflected light can be circumvented. The spring pressure of the resilient body anchors the probe to the quartz oscillator. In the prior art probe microscope using a quartz oscillator, data would be affected by the manner in which they are adhesively bonded. In exchanging the probe of the described embodiments, it is only necessary to replace the probe itself. In consequence, the same quartz can be used. The reproducibility of the measurement conditions and the reproducibility of data can be enhanced. Moreover, the replacement of only the probe gives rise to lower cost. In addition, the adhesive bonding that is difficult to perform is made unnecessary. Consequently, the instrument is made very easy to handle. In this way, a scanning probe microscope with high reproducibility can be accomplished.

Claims: A scanning probe microscope having a probe (1) equipped with a probe tip at its front end, a vibration application portion consisting of a piezoelectric vibrating body (2) and an AC voltage-generating means (3), a vibration-detecting portion consisting of a quartz oscillator (4) and a current/voltage amplifier circuit (5), a coarse displacement means (6) for bringing the probe close to a surface of a sample, a sample-to-probe distance control means consisting of a Z fine displacement element (11) and a Z servo circuit (12), a two-dimensional scanning means consisting of an XY fine displacement element (13) and an XY scanning circuit (14), and a data processing means (15) for converting a measurement signal into a three-dimensional image, wherein the probe (1) is held to the quartz oscillator by resilient pressure; wherein the probe is either a scanning tunnelling microscope tip or an atomic force microscope cantilever. A scanning probe microscope as claimed in claim 1, wherein the probe is a cantilever of a magnetic force microscope consisting of a magnetic substance. A scanning probe microscope as claimed in claim 1, wherein the probe is a conductive cantilever, and wherein there is further provided means for detecting an electric potential or current on the surface of the sample. A scanning probe microscope as claimed in any of claims 1 to 3, wherein there is further provided at least one of: means for cooling the sample, means for heating the sample, means for holding the sample in a vacuum, means for holding the sample in an inert gas or reactive gas, and means for holding the sample in a liquid. A scanning probe microscope as claimed in any of claims 1 to 3, wherein there is further provided a sample stage capable of moving a measurement position on the surface of the sample. A scanning probe microscope as claimed in any of claims 1 to 3, wherein there is further provided means for illuminating the surface of the sample with light. A scanning probe microscope as claimed in claim 6, wherein there are further provided means for collecting and detecting light transmitted through the sample, light reflected off the surface of the sample, or light emitted from the surface of the sample. A scanning probe microscope as claimed in claim 7, wherein there are further provided means for periodically modulating light and a lock-in detecting means.

Inventors: TOMITA EISUKE, TOMITA, EISUKE
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1998
Main classification: G01B7
IPC codes: G01N37, G01B21, G01B7, G01Q60, G01Q30, G01Q10